Exploring the emerging applications of the advanced 2-dimensional material borophene with its unique properties

Abstract

Borophene, a crystalline allotrope of monolayer boron, with a combination of triangular lattice and hexagonal holes, has stimulated wide interest in 2-dimensional materials and their applications. Although their properties are theoretically confirmed, they are yet to be explored and confirmed experimentally. In this review article, we present advancements in research on borophene, its synthesis, and unique properties, including its advantages for various applications with theoretical predictions. The uniqueness of borophene over graphene and other 2-dimensional (2D) materials is also highlighted along with their various structural stabilities. The strategy for its theoretical simulations, leading to the experimental synthesis, could also be helpful for the exploration of many newer 2D materials.

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M. Bhavyashree

Miss Bhavyashree received her BSc degree (2017) in Physics, Chemistry and Mathematics, and MSc degree (2019) in Physics from Maharani's Science College for Women, Mysore, India. Her research interests are mainly focused on nano composite for field emission applications.

Sachin R. Rondiya

Dr Sachin R. Rondiya obtained his PhD (2017) from Department of Physics, Savitribai Phule Pune University, Pune, India. Later he joined Institute of Nano Science and Technology (INST), Mohali, Punjab, India as Post-Doctoral Fellow (2018–2019). Currently, he is a Post-Doctoral Research Associate in School of Chemistry, Cardiff University, Cardiff, United Kingdom. His research work focuses on developing nanoscale semiconductors for energy conversion and energy storage applications.

K. Hareesh

Dr Hareesh received his PhD (2014) from Mangalore University, India followed by Post-Doctoral Fellowship (2013–2017) from Savitribai Phule Pune University, India and University of Western Australia, Australia. Currently, he is an Assistant Professor in Physics at R.V. College of Engineering, Bangalore, India. His research interest includes 2D-based nanocomposites with transition metal oxides and conducting polymer for energy storage and energy conversion application.

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1. Introduction

Graphene, a 2-dimensional material first invented by Novoselov and Geim,¹ has brought about a new era for the 2D material-based world towards the next-generation electronic devices and energy conversion devices.² Though it is yet to be materialized further, it has triggered the motivation that has led to the discovery of various 2D materials, either predicted theoretically or synthesized experimentally, for many potential electronic applications.³ Moreover, on comparison with the bulk materials, the larger surface area, linear band structures, as well as enhanced quantum confinement effect of monoatomic layered 2D nanosheets,^4,5 have furnished their wonderful properties including high mobility, electronic properties such as metallicity, Dirac–Fermi effect and conductivity, which include AC conductivity, DC conductivity, optical conductivity and ultrahigh thermal conductivity, mechanical properties such as excellent flexibility and high mechanical toughness and magnetic properties such as conductivity.^6,7 These properties have led to the potential applications of 2D nanosheets in energy storage, spintronics, photonics, electronics, sensing, biomedical fields, etc.^8,9 The other 2D compounds shown in Fig. 1(a) include, transition metal dichalcogenides (TMD),^10,11 boron nitrate (BN),¹² black phosphorous¹³ and Xenes [ X= As, B, Ge, P, Sn, Si, Sb].^14–20 The material field has made great strides through the emergence of one of the 2D materials, graphene, due to its ability to form spheres, tubes, sheets and a variety of curious shapes (Fig. 1(b)); however, its zero-band gap has interrupted many existing applications in photodynamic therapy, bio imaging, logic circuits, photonic devices and electronic components.²¹ Hence there is the need for alternative 2D materials that could be more advanced as compared to graphene but with similar excellent properties such as high thermal conductivity, electrical conductivity, elasticity, flexibility, hardness and resistance, along with the ability to generate electricity when exposed to light and less effect towards ionizing radiation, low Joule effects (i.e. heating when conducting electrons) and electricity consumption, however, they must have an opening in their bandgaps. Among the previously studied 2D materials, the single-element materials are much more compatible with the previous technology of semiconductors, i.e., Si and Ge, and are relatively easier to apply in high-quality synthesis and for decomposition and metabolization by the biological systems. For example, one of the monoelemental 2D nanomaterials, black phosphorous, has good biocompatibility and can easily decompose into phosphate, which could be useful for maintaining the physiological activities of DNA and ATP when used as raw materials. Additionally, the high specific area along with different response levels make these single-element 2D materials favourable candidates for various fields.^22–24 Among them, the recent interest is towards the mono layered boron allotrope, i.e. borophene, because, it not only has unique anisotropic, elastic, electronic, optical, superconducting, thermal and transport properties^25–27 but also impresses by its surface properties, electronic conductivity and unique chemical structures^28,29 due to the deficiency of electrons and multiple forms of chemical bonding, with two-electron bonds ranging from two-center to multicenter,^30–32 leading it to form a diversity of crystalline structures on varying the conditions and the method of synthesis. Pmmn borophene, α borophene and β₁₂ borophene are predicted to exhibit different properties, where Pmmn is a Dirac material with novel electrical characteristics and Dirac cones, α is isotropic, whereas β₁₂ is anisotropic,²¹ indicating that any condition for the required applications could be met by regulating and controlling the preparation conditions. As such, this material requires much more exploration.

Fig. 1 (a) Existing homo-elemental and hetero-elemental 2D materials. (Reproduced from ref. 33 with permission from John Wiley and Sons). (b) Different shapes formed by graphene. (Reproduced from ref. 34 with permission from Elsevier).

Boron is the fifth chemical element of the periodic table, which has valence orbitals like that of carbon (Fig. 2(a, e)) and is the only group 13 (i.e., 3A) element with semiconducting properties.³⁵ For this reason, its 2D form, “Borophene”, also possesses similar characteristics to that of graphene (i.e., a 2D allotrope of carbon)^36,37 but has unique polymorphism in contrast to other 2D single-element materials.^38–40 Although boron is one of the nearest neighbours of carbon in the periodic table and has similar physical properties, its structure differs from that of graphene due to the difference in their degrees of buckling in the out-of-plane direction (as observed in Fig. 2(b, c and f, g)).⁴¹ However, for a buckled borophene with triangular lattice, an important role is played by hydrogen as an acceptor⁴² as it includes a system that has a surplus of electrons, and after hydrogenation a linearly dispersed Dirac cone as observed in Fig. 2(h), similar to that of graphene in Fig. 2(d), arises in the resulting borophene.⁴³ For this reason, there is still the need for more research on the allotropes of borophene. However, theoretical simulations from the 1990s have revealed interesting properties related to different low-dimensional borophene allotropes.⁴⁴ Its synthesis in 2015 via the condensation of atoms of hot boron gas on a cool silver surface forced the creation of its hexagonal structure because of the regular silver arrangement. However, the notable proportions of silver and boron (B) atoms have led to the creation of vacancies or holes in the lattice and these vacancy patterns provide unique properties for the borophene crystal.⁴⁵

Fig. 2 Schematic representation of the valence electrons of (a) carbon and (e) boron respectively in their similar orbital. (Reproduced from ref. 27 with the permission of Elsevier), graphene with (b) top view, (c) side view (reproduced from ref. 46 with the permission of American Physical Society), (d) graphene Dirac cone, borophene (f) top view and (g) side view (reproduced from ref. 31, published by IOP Publishing Ltd, licensed under Creative Commons Attribution 3.0) and (h) Dirac cone of borophene (reprinted with permission of ref. 3. Copyright (2020) Wiley-VCH Verlag GmbH).

Unlike all other direct experimentally synthesized traditional materials, the borophene design began through theoretical simulations^47–49 with the predictions of their probable characteristics,^50–52 followed by preparation methods, and then their synthesis in laboratories and their potential applications. This unique strategy is schematically represented in Fig. 3(a),⁵³ and the yearly progress in the publication of borophene papers, is plotted in Fig. 3(b). This emerging development of borophene could be a model for various new materials and a best example for the material genome project (MGI),²¹ which can improve the synthesis efficiency, minimize the research material cost and increase the material design output rate.

Fig. 3 (a) Schematic diagram of the development of borophene. (Reproduced from ref. 54 with the permission of John Wiley and Sons). (b) The number of publications focusing on borophene each year. (From the web of sciences, https://publons.com/publon/).

This paper provides useful information regarding the progress of borophene through previous simulation analyses, experimental confirmation, and different mechanisms for laboratory synthesis. We also report a few of their properties, including the factors that can cause alterations, make comparisons with other 2D materials, and explore their existing applications with updated information to guide researchers in identifying further challenges, and provide opportunities for improvement in various other fields.

2. History of theoretical prediction before experimental confirmation

Until 2015, experimental research on allotropes of nanostructured boron was very sparse due to their toxic and costly precursor (diborane), which was typically suggested.⁴⁵ However, in the 1990s, using computer simulations, a physicist, I. Boustani, first examined the structure of tiny boron clusters on studying their stability and reported that the final structures mostly preferred the quasi-planar or planar form rather than the icosahedral geometry like that of borane.⁴⁸ Computational studies have suggested various structures for the 2-dimensional boron sheet⁵⁰ but in accordance with the general class of quasiplanar clusters of B₇ [Fig. 4(a)], Boustani proposed the aufbau principle (inset of Fig. 8(a))⁴⁷ for constructing nanostructures, which predicted the possibilities of fullerenes of boron (borospherene),⁵⁵ boron nanotubes^30,56,57 and monoatomic sheets of boron (borophene) (Fig. 4(d–f)).⁴⁹ Since the monoatomic sheet of boron was 2-dimensional and as the traditional rule suggests the fundamental name for the 2-dimensional materials to be ‘Xene’, where X is the element, the monoatomic sheet of boron was named as “Boronene”.⁵⁸ Further, after analysing the graphene and its aromatic rings of carbon atoms, a monolayer sheet was introduced for the B atoms as well and was called “Borophene”. However, those sheets gave high stability when enhanced by the out-of-plane distortion^39,59 or by superstructures of vacancies.^30,50 This was because, as a contradiction to graphene, the valency of boron was only three, and it was deficient of a single electron to make sp² bonding and form a hexagonal lattice as graphene, hence a planar triangular lattice was proposed in the beginning.⁶⁰ Later, Boustani noticed that the planar triangular lattice was not stable and put forth a buckled structure, i.e., a distortion along its out-of-plane direction from the model of clustered boron in two layers, which consisted of zigzag chains of boron in the up-down direction as seen in Fig. 4(b and c), where its energy for each atom was 0.03 eV lower than the unbuckled one and 0.7 eV lower than the graphene-like honeycomb lattice.⁶¹

Fig. 4 Boron allotropes: (a) quasi-planar cluster of B₇ (reproduced from ref. 62 with the permission of Elsevier). (b and c) A double-layered boron cluster, side view and top view, respectively (reproduced from ref. 61 with the permission of AIP Publishing). (d) Fullerene of boron (reprinted with permission from ref. 60. Copyright (2019) Wiley-VCH). (e) A boron nanotube (reprinted with permission from ref. 63. Copyright (2008) American Physical Society). (f) Monoatomic sheet of boron (reproduced from ref. 53, Science and Technology review publishing house licensed under Creative Common Attribution 4.0 international).

In 2005, advanced buckled structures were proposed for borophene as seen in Fig. 5(a–d) and it was determined that the p-orbital symmetries of the B atom were not agreeable with the six-fold coordination in the flat borophene triangular lattice. Hence, via σ–π bond mixing, a buckled structure termed buckled {1212} boron sheet (δ₆ borophene) was formed, enhancing the cohesive energy, and when compared with the single-atomic strength of the flat one, it was 0.26 eV stronger.⁶⁴ In δ₆ borophene two different B–B bonds of bond length 1.81 and 1.63 Å were formed, where the height of buckling was ∼0.85 Å.⁶⁵ Further, ab initio structural optimization disclosed that any random lattice of buckled triangles would relax into δ₆ borophene, indicating it as the best structure. Different configurations of borophene including planar reconstructed {1212}, idealized buckled {1212}, icosahedral (bulk boron with α-phase) and few low-symmetry hybrid structures were investigated and among them the most stable structure was the one highlighted in Fig. 5(b), i.e., the buckled {1212}.⁵⁹

Fig. 5 Advanced buckled structures: (a) reconstructed {1212}, (b) idealized and buckled {1212} with the stable one marked in red, (c) icosahedral boron, (d) lower symmetry and (e) few hybrids. Reprinted with permission from ref. 59 Copyright {2007} American Chemical Society.

As mentioned previously, the proposed triangular lattice structure, due to the electron deficiency in boron,^63,66 can form multicenter-two-electron bonds for stabilizing the structure by buckling.^30,67 Later, it was found the flat triangular structure had an excess of electrons, while the hexagonal structure was highly unstable due to the shortage of electrons and for decreasing the energy it requires more electrons. Hence, a mixture of hexagonal lattice, i.e., acceptors and trigonal lattice, i.e., donors, was proposed, which gave rise to triangular motifs of borophene having hexagonal holes³⁰ and predicted that the superstructure vacancies could also improve the stability of the previously proposed flat triangular lattice of borophene. Moreover, a greater cohesive energy was noticed for the hexagonal vacant triangular sheet than the previous one without hexagonal holes.⁵⁴ This then gave rise to the density parameter,²¹ based on this, the concentration of hexagonal vacancies could be varied in the motif, where the density parameters were found to be η = 0 and η = 1/3 for triangular and hexagonal sheets, respectively. Depending on the same, various configurations were formed as in Fig. 6(a) but more stability was noted in V_1/9 (η = 1/9) and V_1/7 (η = 1/7) structures, also termed α-borophene and β-borophene, respectively.^21,30,68 Analysis of the calculated band structure of V_1/9, designated α-borophene to be metallic⁶³ with the highest cohesive energy/atom.³⁰ A new borophene structure similar to that of α-borophene was proposed with hexagonal holes in parallel and was denoted as the γ-sheet (B₃₆). This γ-sheet was ultra-stable, especially for the B₁₀₀ cage (highlighted in Fig. 6(b)).⁶⁹ However, in 2014, based on the outcomes of photoelectron spectroscopy and the simulations of theoretical data, there was a first report on the experimental discovery that the B₃₆ boron cluster was quasiplanar with high reliability and consisted of a predominant hexagonal hole and hence it was potentially viable for reproducing sheets of single-atomic layered boron on a larger scale with hexagonal vacancies.⁵⁰

Fig. 6 (a) Various density parameter-dependent configurations with two stable structures marked in red. (Reproduced from ref. 70 with the permission of AIP Publishing). (b) Structural variation with an increase in the number of boron atoms. (Reproduced from ref. 58).

A better understanding of α-borophene was given by Galeev et al.⁷¹ by reporting about its fragments along with chemical bonding, wherein an adaptive natural density partitioning (AdNDP) method was used. As seen in Fig. 7(a) α-borophene was classified with regard to ‘n’center-2 electrons, where n = 2,3 and 4 in σ-bonds and n = 6 and 7 in π-bonds, but there was instability in the borophene structure when the σ : π electron ratio was 0.5.⁷¹ Subsequently, various possible borophene structures, as seen in Fig. 7(b), were constructed by the cluster expansion (CE) method and named B_1−x ⎔_x, with ⎔ as the hexagonal vacancy. Those configurations were different in structure, dependent on x = 0.1–0.15 but they had nearly similar energy formations, especially for borophenes with x = 1/9, 1/8 and 2/15, which were also analyzed as shown in Fig. 7(b). At room temperature, on taking KBT = 0.026 eV, the substrate-independent borophene mostly preferred to have a polymorphic structure.⁷² As seen in Fig. 7(c–f) several complexes of borophene were predicted via the swarm optimization (CALYPSO) code, and the α-sheet was the ground state among the independent borophenes.⁷³ Since the energy difference was very narrow between V_1/8, V_2/15 and α(V_1/9)-borophene, GGA-PBE was less effective for ensuring their relative stability, hence their cohesive energy was calculated using a hybrid functional PBE0,⁷⁴ where α₁-borophene showed 136 meV per atom higher energy than the α-sheet, and β₁-borophene showed 118 meV per atom and 96 meV per atom high stability than the β-sheet and α-sheet, respectively. However, V_1/8 and V_2/15 had approximately the same cohesive energy and were much more stable (i.e., 30 meV per atom) than α-borophene.^30,74,75 Hence, the α-sheet, previously trusted as the ground state, was proven wrong.

Fig. 7 Complexes of borophene: (a) n-center, 2-electron (reproduced from ref. 71 with the permission of the Royal Society of Chemistry), (b) cluster expansion (CE) method (reprinted with permission of ref. 72. Copyright (2012) American Chemical Society), (c–f) CALYPSO code (reprinted with permission from ref. 74 Copyright (2012) American Chemical Society), (g, h) buckled structures with small thicknesses and 2 × 2 × 1 supercell projections along [001] and [100] for (g) Pmmn-boron and (h) Pmmm-boron. (Reprinted with permission from ref. 60. Copyright (2019) Wiley-VCH).

Besides these, two more buckled structures, having non-zero thicknesses, namely Pmmn (Fig. 7(g)) and Pmmm (Fig. 7(h)), were also predicted. The results of the HSE06 (GGA-PBE) calculation showed the ultra-stability of the Pmmn/Pmmm sheet as compared to the α-sheet but their stability relations are yet to be studied. The Pmmn phase having low symmetry with the linearity of the Dirac dispersion similar to graphene was identified to be semi-metallic.³⁹ All these predictions were only for the substrate-independent borophene and all of them showed perfect single-crystallinity. Nevertheless, substrate-supported borophene varied in stability and offered many more new, mixed structures.

The dynamical instability in the proposed freestanding borophene³² motivated many researchers to find various methods for improving their stability, which included surface functionalization and adsorption on substrates.^28,32,76,77 After analyzing the growing mechanism of graphene and silicene on the metal surface, the metal-based substrates were regarded to be a productive way for finding the solution for borophene sheets.⁵⁴ Through theoretical calculations, numerous 2-dimensional boron allotropes in Fig. 8(a) were reported,^36,67,78,79 wherein the computational studies by Liu et al.⁸⁰ proved that they could be extracted by the deposition of boron sources on the substrates of Au, Ag, Cu or metal bromides (MgB₂).⁸⁰ Again, using swarm optimization (CALYPSO), numerous complexes were predicted on the surface of Ag/Au/Cu/Ni and their results in Fig. 8(b) indicated their structural variations at the respective ground states for different metal substrates. Later, on identifying a large difference in the lowest energies of the two structures formed on Cu, V_1/6 sheets, Pmmm were found to be much more stable on Cu.⁸¹ It was also found that resultant 2D boron clusters on Cu(111) contained holes and it was observed that nucleation of the 2D cluster by Cu substrate could easily form hexagonal holes.⁸² This was an essential framework that led to other successive theoretical predictions. Further, based on the first principles calculation via surface structure search and the CE method, the borophene sheets were systematically investigated and resulted in a hexagonal-holed triangular pattern when grown on Cu, Au, Ni and Ag, where icosahedral β₁₂ was not favorable on Ni and Cu but was minimum on Au and Ag, which indicated that those metals could promote the nucleation as represented in Fig. 8(c) within boron sheets and donate extra electrons for stabilizing it.⁸¹ Successively, the predictions were experimentally confirmed initially in the same year (2015) by Mannix et al.,⁴⁵ using Ag(111) as the metal substrate.⁴⁵ Furthermore, corresponding experiments on same substrate showed that it could cause nucleation and donate electrons for stabilization and reported β₁₂ (η = 1/6) to be the ground state.¹⁵ Presumably, under ultra-high vacuum conditions, four phases of borophene, namely the 2-Pmmn phase, β₁₂ phase, χ³ phase and graphene-like honeycomb phase were developed successfully on Al/Ag/Au/Cu(111) substrates,^15,51,53 which are discussed in detail in Section 3.

Fig. 8 (a) Theoretically simulated borophene structures with yearly categorization. (Reproduced from ref. 54 with the permission of John Wiley and Sons). (b) Stability versus hole density under vacuum and on Au, Ag, Cu and Ni substrates with structures formed at their different energy states. (Reprinted with permission ref. 81 Copyright 2015, Wiley-VCH). (c) Schematic representation of boron sheet nucleation over the metal substrate for its stabilization, (reproduced from ref. 83).

3. Synthesis of 2D borophene

The 3D boron bonding configurations,³² accompanied by a large difference in energy as compared with the 2D form,⁸⁴ make the fabrication of borophene a challenging task. Usually, this requires ultralow pressures and sophisticated facilities for its fabrication.³ Even then, on considering the theories, borophene synthesis was possible via the bottom-up approach,^15,18,45,85 top-down approach,^86–88 exfoliation techniques (i.e., sonochemical, liquid phase),^87–89 deposition techniques (i.e., CVD, PVD)^32,90–92 etc. It gave raise to different complexes of borophene, which were either supported by a substrate, or independent (i.e., freestanding) one.⁹³

3.1. Growth of borophene supported on a substrate

Due to the presence of periodic hexagonal holes in borophene or hexagonal boron (highlighted in Fig. 6(b)) within its triangular lattice, theoretically, it was very stable⁶³ when supported on metal substrates such as aluminium Al(111),⁹⁴ silver Ag(111),^15,45 gold Au(111)⁹⁵ or copper Cu(111).^82,96 This was achieved through various fabrication methods such as atomic layer deposition, molecular beam epitaxy, chemical vapor deposition, and so on. The merit of these method includes the chemical purity of the obtained borophene for the further study of the chemical and physical behaviours.

3.1.1. Atomic layer deposition (ALD). Atomic layer deposition, being an ultra-thin film deposition technique with high quality, was the first synthesis technique employed for growing atomically thin borophene sheets on a clean single-crystal surface of inert Ag(111) by Mannix et al.⁴⁵ The sheets were developed by using solid boron as an atomic source, under ultrahigh-vacuum as shown in Fig. 9(a). This resulted in three phases of borophene, namely, stripped phase, homogeneous phase and nanoribbons (Fig. 9(b)), at the substrate temperature of 550 °C. The scanning tunnelling microscopy (STM) results represented in Fig. 9(c–h) also disclosed that at a lower deposition rate, there appeared a flat strip as shown in Fig. 9(b), which consisted of a rectangular lattice with hollow hexagons, whereas a high deposition rate gives a homogeneous phase as in Fig. 9(b). As the temperature increased, the flat phase appeared as a conjugated striped phase in Fig. 9(b) but both phases showed metallic characteristics.⁴⁵ However, the ultra-high vacuum condition provision in laboratories is costly and challenging and the process was time consuming with high material use, as well as energy wastage.

Fig. 9 (a) Schematic representation of the atomic layer deposition of boron on Ag(111), (b) three phases formed at 550 °C (reproduced from ref. 97, published by Taylor & Francis Group, licensed under Creative Common Attribution). (c–h) Series STM topography in large-scale (on left) and dI/dV closed-loop (on right) images for the borophene sheets, where (c and d) shows low coverage of boron, (e and f) medium coverage of boron, (g and h) high coverage of boron, with blue, red and white arrows indicating the striped phase nanoribbon, homogeneous-phase and striped phase of borophene (reproduced from ref. 21, published by Wiley-VCH GmbH, licensed under Creative Common Attribution 4.0 International). (i and j) I–V curve and dI/dV spectra indicating the metallic characteristics of the resulting boron sheets. (Reproduced from ref. 53. Published by Science and Technology Review publishing house, licensed under Creative Commons attribution 4.0 international).

3.1.2. Molecular beam epitaxy (MBE). Borophene was experimentally realized by molecular beam epitaxy (MBE) in 2016, and it can also be grown epitaxially on the substrate Ag(111). The boron sheets were grown in ultrahigh vacuum conditions on Ag(111) by directly evaporating the pure boron using the mechanism of molecular beam epitaxy (MBE), as shown in Fig. 10(a). It was S confirmed that two different 2-dimensional patterns of boron sheets were formed successfully on the Ag(111), namely sheets of β₁₂ (Fig. 10(b)) and sheets of χ³ (Fig. 10(c)). The results revealed a new application in microelectronic appliances and proposed that materials like Au(111), Cu(111) and metal borides can be further employed as the substrates instead of Ag(111), where the MBE method could result in extremely pure/clean borophene.¹⁵ Li et al.⁵¹ and Kiraly et al.⁹⁵ used metal substrates with low surface energy, i.e., Al(111) and Au(111), respectively, to grow the borophene via the MBE method. The obtained honeycomb structural borophene on Al(111) was much more stable than the one grown on Ag(111), which allowed the fabrication of extremely thin films. By logical calculations, it was manifested that a charge of around 1 eV was transferred from the Al to each of the B atoms in the honeycomb borophene, which stabilized the borophene/Al(111) structure in contrast to borophene/Ag(111) because the charge transfer within it was negligible (Fig. 10(d)).⁵¹ In the borophene/Au(111) synthesis procedure as demonstrated in Fig. 10(e), at higher temperatures, the B atoms diffuse into gold and when allowed to cool, those B atoms become separated on its surface, giving rise to boron sheets. Further, on increasing the B concentration, the initially trigonal Au(111) network breaks, resulting in a large-scale borophene island and can extend to several nanometres on increasing the boron dose via electron-beam evaporation.⁹⁵ Although MBE is a typical process for preparing substrate-supported borophene with high uniformity and fewer defects, an alternate method similar to MBE for borophene synthesis over mica⁹⁸ and Ir(111)⁹⁹ was reported via van der Waals epitaxial growth and segregation-enhanced epitaxial growth (schematically shown in Fig. 10(f)), respectively, which resulted in larger single-phased borophene. However, the process was a very slow and laborious method, i.e., with a growth rate of a few microns per hour indicating the suitability of the MBE method for a scientific research laboratory rather than high-volume production. Further, on comparing the atomic structures obtained by ALD and MBE, it was reported to have a small difference in the densities of boron atoms.³ However, the deposition of borophene is limited to small area via both methods, which could make the further fabrication of practical devices a challenging and costly task at the industrial scale.

Fig. 10 (a) Schematic MBE method representation, (Published by GO LOOK IMPORTANTBOOK), (b and c) (from top to bottom) top view, side view and STM image of (b) β₁₂ and (c) χ³ on Ag(111), (reproduced with the permission of ref. 25. Copyright (2019) Wiley-VCH Verlag GmbH) (d) borophene (honeycomb structure) on Ag(111) and Al(111) represented with different colours as mentioned (from top to bottom) top view, side view and the electron density map, (reproduced from ref. 51 with permission of Elsevier). (e and f) Schematic borophene growth on (e) gold substrate, (reprinted with permission from ref. 95. Copyright (2019) American Chemical Society) and (f) iridium substrate with its STM image (reprinted with permission from ref. 100, Copyright (2019), American Chemical Society).

3.1.3. Plasma-enhanced chemical vapor deposition (CVD). Of the modern methods for graphene synthesis, plasma-enhanced CVD is a distinct alternative to exfoliation.¹⁰¹ This method enables various 2D material syntheses at the industrial scale without the requirement of high vacuum levels. Previously, with the help of catalytic substrates, CVD emerged as a convenient method for obtaining large-sized graphene with high-quality, in which graphene was obtained in the deposit form by suitable precursor decomposition when exposed to the substrate by controlling the pressure of the CVD system as represented in Fig. 11(a), which could be later transferred to various target substrates by a polymer-assisted transfer process as seen in Fig. 11(b).⁹¹ As observed for silicene and graphene, CVD growth may improve the size of borophene laterally, resulting in a decrement of density defects along with an increment in borophene grain size,^91,102 which enable the researchers to study the advanced properties of borophene under intrinsic conditions. This method can even enhance the scalability of borophene. Moreover, the thin 2D γ-B film was synthesized by Tai et al.⁹² via CVD using a copper foil substrate (Fig. 11(b–d)). The source was taken as a mixture of powdered boron oxide and pure boron, and hydrogen as the carrier gas. The temperature of the source region (T1) and substrate region (T2) was manipulated independently as 1100 °C and 1000 °C, respectively, in the CVD furnace.⁹² However, a few reports have suggested that borophene growth via the CVD process requires the identification of a precursor for boron and an active catalyst for its decomposition, including the nucleation and growth of the 2D material. Another important fact is the identification of the growth of the non-metallic substrates, which facilitates the characterization of the lateral charge transport.³² This method showed different chemical properties such as resistance to corrosion and high chemical and thermal stability. However, the process was very time consuming and required high-cost equipment and the resulting borophene was unstable against aging and humidity.

Fig. 11 Schematic representation of (a) the CVD graphene system at a controlled pressure, (b) the transfer of the synthesized graphene to the target substrate (reproduced from ref. 91 with permission from the American Chemical Society). (c) Diagram of the furnace with two zones for the growth of borophene on Cu via CVD. (d) Top view and (e) side view of the resultant borophene obtained in (c). (Reproduced from ref. 92 with permission of John Wiley and Sons).

An improvement in the CVD technique was noticed when micrometer-sized borophene was grown over a thick Cu(111) film on sapphire, in which the boron was repeatedly submersed into the Cu at a high temperature, and at low temperature it was recrystallized and resurfaced. Through this technique, the size of the wafer was scalable, copper film could be relinquished and sapphire could also be reused.⁹⁶ Furthermore, an efficient technique namely, van der Waals epitaxy was used for growing borophene on a larger scale via CVD in a furnace with two zones. An additional intermediate of metastable boron cluster was formed by annealing sodium borohydride powder [NaBH₄] (i.e., boron source), the polymer and substrate chosen were fluorophlogopite mica (KMg₃AlSi₃O₁₀F₂)⁹⁸ and polymethyl methacrylate (PMMA), respectively. Recently, CVD was employed for synthesizing borophene on Ni foil to get a semiconductor having a direct band gap with tunable properties.⁹⁸

3.2. Free-standing borophene sheets

Due to the myth that bulk boron forms non-van der Waals crystals,¹⁰³ the fabrication of free boron nanosheets is considered challenging; however, there have been a few recent findings on free-standing (i.e., independent) borophene by methods like exfoliation and the modified Hummers' method, which have completely broken the myths.

3.2.1. Micromechanical exfoliation. Due to a network with complex bonding in boron, there are very few studies on undeviated exfoliation as a technique for the fabrication of borophene. However, borophene exfoliation employing solvents or chemicals resulted in defects/functionalities along with fragmented sheets with uniform layers and high quality.¹⁰³ The borophene synthesis was reported through three steps of a top-down approach (Fig. 12(a)), which included liquid exfoliation twice along with etching through thermal oxidation. At first, the exfoliation resulted in a thick boron nanosheet, which was oxidized to B₂O₃ and then dissolved in water. Subsequently, a boron nanosheet of less than 5 nm thick with 100 nm lateral size was obtained after ultrasonic exfoliation.⁸⁸ However, atomic borophene sheets were easily synthesized by exfoliation through a liquid-phase and proved the borophene oxide reduction as well. The existence of intermediate borophene phases along with β₁₂ (Fig. 12(c)) and χ³ (Fig. 12(d)) was verified by characterization using electron microscopy.⁸⁷ Further, as illustrated by Coleman et al.,⁸⁹ defect-free graphene was synthesized via liquid exfoliation of graphite and was used for a variety of applications. The ultrasonication of graphite powder in a suitable solvent (i.e., dimethylformamide, deionized water, isopropyl alcohol, ethylene glycol, acetone, etc.) gave rise to a stabilized nanosheet. It was recommended that similar liquid-phase exfoliation be employed for the fabrication of defect-free borophene also.

Fig. 12 (a) Schematic micromechanical exfoliation representation by the top-down approach. (b) Electron microscope image of the resulting borophene sheet. (c and d) Zoomed images of the (c) β₁₂ and (d) χ³ phases observed in (b) (reproduced from ref. 53 Published by the Science and Technology Review Publishing House licensed under the Creative Common Attribution 4.0 International).

Mixing the required nanosheet in a suitable solvent is given in terms of a thermodynamic equation:

where ΔH_mix is the mixing enthalpy, V_mix is the mixture volume, T_B is borophene thickness, E_S,S is the solvent surface energy, E_S,B is borophene surface energy and φ_B is the dispersed borophene volume fraction. A small amount of external energy (sonication) would be enough for exfoliation if E_S,S and E_S,B are similar and borophene nanosheet structures were formed due to the relaxation of the developed local strain. They even proved that the atomic interaction of solvent with the borophene sheet is crucial to the process as their experimental trial suggested that acetone could yield the monolayer formation in the case of sonochemical exfoliation on behalf of other solvents.¹ Moreover, this method of synthesis helps in obtaining a larger lateral dimensioned monolayer, bilayer or tri-layer sheets on varying the frequency, time and amplitude of sonication as well as the chosen solvent. However, prolongated sonication can result in borophene dots rather than sheets and slight changes give a very different structure. Hence, the choice of an adequate and suitable solvent along with the sonication parameter is challenging, time consuming and crucial for obtaining the particular borophene sheets.

3.2.2. Modified Hummers' method. The modified Hummers' method is one of the most commonly used methods for the synthesis of high-quality graphene. Introductory pure crystalline, free-standing sheets of large area borophene were obtained extensively by Ranjan et al.⁹³ through the modified Hummers' method when combined with sonochemical exfoliation as shown in Fig. 13. It showed that the potassium permanganate and boron ratio is also an important parameter during exfoliation. Borophene obtained through such a method was suitable for application in the sensing of gas, strain and molecules (i.e., in surface-enhanced Raman spectroscopy). This even disproved the hypothesis that borophene would not exist in free-standing form and showed that a single chemical reaction can confirm the existence of β₁₂ and χ³, accompanied by their intermediate phases; the formation of these phases formation was governed by strain, interlayer interaction and the number of layers.⁹³ Although, the improved oxidation level obtained via the modified Hummers' method improves the product performances, the long-time taken for the complete process along with its tedious purification process revealed the complexity of the modified Hummers' method in borophene synthesis. A comparison of the merits and demerits of each method is given in Table 1.

Fig. 13 Schematic representation of the modified Hummers' method. (Reproduced from ref. 104 published by MDPI, licensed under the Creative Commons Attribution).

Table 1 Comparison of merits and demerits of different methods of borophene synthesis

	Method	Merits	Demerits	References
Substrate supported	Atomic layer deposition	• High-quality ultra-thin film	• Time consuming chemical reaction	45
		• Excellent repeatability	• Very high material and energy waste rate
		• High film density
		• Large area thickness uniformity
		• Atomically flat and smooth surface coating	• Obtaining ultra-high vacuum condition is challenging and costly
		• Low-temperature processing
	Molecular beam epitaxy	• Extremely pure/clean borophene	• Very slow and laborious method. i.e. growth rate few micrins per hour	15, 51 and 95
		• Ultra-thin film
		• High uniformity with less defect
	Plasma enhanced CVD	• Industrial scale synthesis possible	• Time-consuming	32, 91, 92, 96, 98, 101 and 102
		• No requirement of high vacuum level	• Costly equipment
		• High thermal and chemical stability	• Instability to humidity
		• Corrosion-resistant
Freestanding	Micromechanical exfoliation	• Uniform layered high quality	• Slight changes in conditions/parameters cause irreversible changes in the structure	88, 89, 103 and 104
		• Larger lateral dimensions with different layers	• Choice of suitable and adequate solvent is tedious
	Modified Hummers' method	• Oxidation level improved	• Time-consuming process	93
		• Enhanced product performance	• Tedious purification process

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On comparing the overall scalability of the process with that of graphene, it was revealed that an allotrope of carbon, i.e., graphite, being the starting material of graphene, is a non-metallic and tetravalent element, which is the 15th most abundant element in the Earth's crust and the 4th most abundant element in the universe, hence is easily available in a larger quantity and has a wide application. However, boron, the starting material for borophene, occurs on the Earth in a significant concentration known as borate mineral, which is of 100 different types but the most common ones are borax, kernite and ulexite; their cost is ten times higher when compared to that of graphite.

4. Properties

Given the interesting facts about borophene, its promising contribution is suggested for sensing and electronic applications, hence numerous studies have been devoted to understanding borophene's properties and structure. A detailed discussion regarding a few major properties such as high mechanical properties, electronic or ionic conductivity, lattice thermal conductivity, super conductivity, optical properties, atomic adsorption, surface reactivity and high capacity are mentioned in some of the review articles.^3,53,54,105

After understanding the borophene structure as discussed in Section 2 it is important to introduce its properties. The natural 3D boron structure indicates that it is an intermediate of both metals and non-metals, i.e., it is a metalloid and is used commonly in semiconductors.¹⁰⁶ Even then, due to its electron deficiency along with multicenter bonds,^32,76,77 boron is gifted with some interestingly unique desirable properties when it is in its 2-dimensional form, i.e., as borophene. Among the borophene allotropes, 2-Pmmn, χ³ and β₁₂ phases include properties like mechanical flexibility, metallicity, transparency, superconductivity, selectivity, sensitivity, etc.

4.1. Mechanical properties

The scientific research communities have shown great interest in the mechanical properties of borophene due to its mechanical modulus with ultrahigh value, strong B–B bonds and uniqueness in atomic structures^107–109 and have described that chemical modification, number of layers, temperature and the vacancy defects of borophene also contribute to their unique mechanical properties,¹¹⁰ and its mechanical strength can be determined by a few of the interpretative parameters including lateral size, anisotropy, reduced dimensionality, and disparate polymorphs.

For studying the chemical modulation effect, researchers have been performing physical and theoretical experiments on different borophene forms to find their mechanical coefficients.¹¹¹ Nonetheless, few results have disclosed that there is a decrease in the coefficients of different forms of borophene due to electrochemical fluorination. It has been reported that after the fluorination of 2-Pmmn borophene, two anisotropic structures B₂F and B₄F,¹¹² with high stability were obtained, which showed good thermal, electrical and mechanical conductivity properties and were thus identified as encouraging candidates for electronic devices as semiconductors.¹⁰⁶ Similarly, on hydrogenation of borophene (borophane), the Young's modulus of 2-Pmmn phase along its zigzag and armchair directions decreased to 110.59 and 172.24 N m⁻¹, respectively, indicating an increment in mechanical flexibility.¹¹¹ The reason for the decrease in Young's modulus was investigated and it was proved that on hydrogenation the B–B bonds become elongated from 1.616 Å to 1.941 Å, leading to the bond strength decrement, which reduces Young's modulus. Moreover, critical strain increased along all three directions (zigzag, armchair and biaxial) after the hydrogenation (a comparison of borophane with other 2D material is shown in Fig. 14(c)),^50,111 which maintained its stability until the tensile uni-axial strain along the zigzag and armchair directions was up to 22% and 17%, respectively.¹¹³ Later, the analysis of α₁-borophene and γ-B₂₈ (ref. 92 and 114) showed that boron sheets are able to open a new bandgap after hydrogenation, which can act as a semiconducting material.

Fig. 14 (a) Diagram showing the mechanical flexibility of borophene. (Reproduced from ref. 105 with permission from Elsevier). (b) Comparison of the electronic DOS for the densest borophene (S1) and borophene with an increase in vacancy defect concentration (S2–S5) across the armchair direction with respect to different strains (ε) and at maximum tensile strength (ε_uts). (c) The ratio of overall strain in the zigzag (η_ZZ) and armchair (η_AC) directions for different 2D materials. (Reproduced from ref. 111 with the permission of the Royal Society of Chemistry). (d and e) Variation in Young's modulus across the (d) armchair direction and (e) zigzag direction with an increase in the conc. of vacancies from (S1–S5). (Reproduced from ref. 108 with the permission of the Royal Society of Chemistry).

The study of multi-layer borophene of the 2-Pmmn phase was able to relate the variation in the mechanical properties due to the effect of change in the number of layers, wherein Young's modulus was measured as 136 N m⁻¹, 141 N m⁻¹, 144 N m⁻¹, 158 N m⁻¹ along the zigzag direction, and along the armchair direction it was 338 N m⁻¹, 360 N m⁻¹, 380 N m⁻¹, 397 N m⁻¹ on varying from a four-layered borophene to a monolayered one, respectively, revealing that Young's modulus is maximum for the one with four layers. Hence, borophene exhibits superior mechanical properties similar to black phosphorene on increasing their layers.¹¹⁵ A monolayered borophene is extremely flexible and is indicated as a promising nanomaterial for wearable devices.⁵³

It was found that when boron concentration was 4/27^th of the total borophene concentration, along the zigzag direction, Young's modulus reduced from 180 N m⁻¹ to 87 N m⁻¹ when the temperature was increased from 1 K to 600 K, indicating the inverse proportionality of Young's modulus with temperature.¹¹⁵

The results of mechanical properties seen in Fig. 14(d and e) revealed the inverse proportionality of Young's modulus to vacancy defect along the armchair direction, and Fig. 14(b) revealed the inverse proportionality of borophene density of states to vacancy defect concentration, which also decreased its tensile strength.¹⁰⁸ Borophene with phase β₁₂ and χ³ had a much smaller Young's modulus when compared to the 2-Pmmn phase across the armchair direction, whereas along the zigzag direction, Young's modulus was not much affected by the vacancy defects. Along with this, it was observed that for α sheet, β₁₂ and χ³, the critical strains across both in-plane directions were much larger when compared to the defect-less 2-Pmmn phase borophene. Nevertheless, the atomic displacement method indicates that an increase in hollow hexagons reduces B–B bonds, causes a stiffness change in borophene, and increases the hardness of the material, indicating the effect of hollow hexagons on its mechanical strength.^31,67

4.2. Chemical properties

Focussing mainly on the reactivity of borophene during the chemical reaction, i.e., metallicity, boron ([He]2s²2p¹) located between carbon (nonmetallic) and beryllium (metallic), exhibits both metallicity and non-metallicity due to the presence of 2p electrons along with orbit radius near the 2s state.^41,116 One of its interesting facts is that its 2-dimensional structure shows intrinsically high anisotropic metallic properties⁷ and few of its allotropes with Dirac fermions are also semi-metallic.³⁹ Along with allotropes 2-Pmmn, χ³ and β₁₂, the honeycomb borophene phase also exhibits metallic behavior,¹¹⁷ diverging from its triangular and hexagonal lattice structures.³⁰ These metallic characteristics were predicted theoretically by Zhang et al.⁸¹ and experimentally confirmed through the STM data.¹⁵ The theoretical results¹¹⁸ in which the two bands crossing the Fermi energy as in Fig. 15(c) and the dispersion of the Fermi surfaces E_F = E^BS(k_x,k_y) throughout the Brillouin zone as in Fig. 15(d) are in agreement with the previous studies,^61,119 and revealed that borophenes are metallic. When compared with the spin–orbit couplings of other 2-D materials and graphene, borophene has a negligible spin–orbit coupling and is considered the lightest 2D material showing metallic properties in which there was linear dependence of energy on momentum, i.e., a Dirac metallic material.¹²⁰

Fig. 15 (a) Diagram showing the metallicity of borophene, (reproduced from ref. 105 with permission from Elsevier). (b) DOS with electronic band structure for α₁-2H borophene with different colours for different orbitals of borophene (where blue: total DOS, black: B-s, red: B-p, green: H-s) with respect to the valence band maximum. (Reprinted with permission from ref. 98. Copyright (2021) American Chemical Society). (c) Band structures for the sheets of borophene plotted along high symmetry points; fat bands indicate the sp character of the σ-bond, where the Fermi energy lies at E = 0 and the Γ point is represented by G. (d) The 2-dimensional Fermi surface of the electronic energy (EE) bands crossing the Fermi energy in (c) is represented in red and yellow. (Reprinted with permission from ref. 118, Copyright (2019–2020) American Physical Society). (e) STM image of (blue circle) valley hexagonal boron nitride, (black circle) mesa hexagonal boron nitride, (red square) borophene. (f) dI/dV spectra representing the images marked in (e) in their respective colours. (Reprinted with permission from ref. 99 Copyright (2021) American Chemical society). (g, h) Band spectrum including TDOS and PDOS of χ⁴-borophene with (g) no hydrogen atom and (h) 5 hydrogen atoms. (Reprinted with permission from ref. 121, Copyright (2021) American Chemical Society).

Based on the PBE functional, DOS corresponding to the band structure of α₁-2H borophene indicated in Fig. 15(b) proved its metallicity recently, which was in agreement with the UV-visible and PL results.⁹⁸ Further, a distinct metallic property was observed when borophene formed a heterostructure with any other 2D materials.³²

By the technique of thermal decomposition, a newly introduced borophene on Ir(111) also showed metallic character as specified by the results of the dI/dV curve in Fig. 15(eand f), where the decrement in the Fermi level to form a V-shape is partial evidence for borophene on Ir(111) showing the Dirac-like behavior.⁹⁹ The presumed electronic band structures of χ⁴-5H-borophene indicated in Fig. 15(g and h), disclosed its metallic behavior based on the Density Functional Theory (DFT) through the Vienna ab initio Simulation Pack (VASP),¹²¹ where the metallic behavior is significantly contributed by the p-orbitals, which is comparable to previous reports¹²² indicating χ⁴-5H-borophene to be suitable for employment in the applications of memory devices as well. Moreover, the same can also be identified by the 3 bands (2 in the G–X direction and 1 in the S–Y direction) in the Fermi level.⁷ The metallic structure of a single-layered borophene was due to the overlapping of their electronic bands at the Fermi level⁴⁵ in which the metallic states were predicted to originate due to the appearance of the band gap near the Fermi level as a result of their corresponding 2p_x and 2p_y states.⁵⁴

Chemical modifications like hydrogenation, fluorination, and lithiation can also affect the metallic properties.¹¹⁰ On hydrogenation, the band structure of the 2-Pmmn phase changes to that of a semiconductor with zero bandgap and even in a state free of stress, a metallic band structure is observed in graphene-like borophene, and a biaxial or uniaxial shear strain can cause the transition of the metallic band structure to a semiconducting one (Fig. 16(a–c)).¹²³ The theoretical results of fluorinated borophene were investigated and the energy band structure as shown in Fig. 16(d–f) revealed that B₄F is metallic and B₂F is an indirect semiconductor.¹¹² Metallic characteristics were exhibited by the electronic structures during the lithiation process,¹²⁴ indicating that borophene is an excellent electronic conductor with high ionic conductivity. Moreover, the β₁₂ and χ³ phases when used as an anchoring material in Li–S batteries showed electronic structures with metallic properties during their entire discharging and charging cycles.¹¹⁰ As predicted previously, even the PDOS of the borophene heterostructure with Li proved that the lithiation process does not diminish the metallicity of the entire system completely (Fig. 16(g and h)),¹²⁵ whereas the interactions of halogen atoms or molecules with borophene, do not alter its metallicity.¹²⁶

Fig. 16 (a) Dispersion of phonon for W-borophane (completely hydrogenated borophene) under different shear strains. (b–f) Electronic band structures of W-borophane: (b) with H vacancy, (c) with a dimer (B–H) vacancy. (Reproduced from ref. 123, published by Royal Society of Chemistry licensed under Creative Common Attribution Non-Commercial 3.0 Unsupported license). (d) Pure borophene, (e) B₄F, (f) B₂F, (reproduced from ref. 112, published by ACS Author Choice). (g) Total and partial DOS of borophene, graphene, borophene/graphene. (h) Total DOS of B/G with single Li (reprinted (adapted) with permission from ref. 125. Copyright (2020) American Chemical Society).

These metallic features of borophene are useful for the sensing of various small molecules and gases although it depends primarily on the electronic structure.¹²⁷ Since metallic borophene consists of abundant electrons, it can be used for catalysis as well.¹²⁸ It has been predicted that even the conventional superconductivity mediated by phonon is due to the intrinsic metallicity.³²

4.3. Electrochemical properties

Electrochemical properties are among the most essential for various applications such as energy storage and sensing, and researchers are still finding ways to improve the electrochemical properties of existing nanomaterials that are already used as sensors, as well as finding new efficient materials for the same. The atomic configuration is one of the important parameters that has opened up platforms for the development of sensors and interestingly, in the case of borophene, rather than the usual binding, the existence of additional electron-deficient bonds inspires it towards the sensing properties.³ However, the comparatively low conductance of borophene identified during the previous reports on their electronic properties⁵ is one of the main motives for sensing hydrogen coverage. Moreover, electron mobility along with metallic structure is necessary for exhibiting various sensing properties including photonic, electronic, thermoelectric and thermal sensing for numerous small molecules and gases.¹²⁹ The adsorption of ammonia¹³⁰ and HCN^105,131 is one of the parameters that suggests that B₃₆ can sense ammonia and HCN; there was a variation in the electronic properties at the B₃₆ edges when compared to its other configurations.¹⁰⁵ Recently Wu et al.⁹⁸ reported the photosensitive properties of PMMA-supported borophene through the results of photocurrent versus reverse bias on varying the light power (Fig. 17(b)).⁹⁸ The Fe₃O₄ nanoparticles, when functionalized by borophene to result in χ⁴-borophene, exhibited good sensing properties due to the core–shell structure.¹²¹ Ultrasensitive molecular sensing could be delivered by borophene due to its ridgelines (an excellent molecule anchor) along with light and strain sensitivity.¹⁰³

Fig. 17 (a) Diagram indicating the anisotropic and polymorphous structure of borophene favoring its properties like transparency and superconductivity. (Reproduced from ref. 105 with permission from Elsevier). (b) Variation in the photocurrent vs. reverse bias on PMMA-supported borophene (reprinted (adapted) with permission from ref. 98. Copyright (2021) American Chemical Society). (c) Vertical and (d) horizontal variations in I–V curves for bare borophene (black) and on addition of NO (red). (Reprinted (adapted) with permission from ref. 138. Copyright (2018) American Chemical Society). (e) Band structures and DOS with Fermi-energy aligned initial energy. (Reproduced from ref. 137 with the permission of IOP Publishing Ltd.). (f) Graph of wavelength (λ) and transition temperature (T_c) vs. the vacancy concentration of boron (η). (Reproduced from ref. 70 with permission from AIP Publishing).

Although there are rare reports on the sensing property of substrate-supported borophene, the first synthesized independent borophene sheets by Ranjan et al.⁹³ showed its ability to sense the gas, strain and molecules based on the surface-enhanced Raman spectroscopy (SERS). Even though both β₁₂ and χ³ phases of borophene exhibited metallic behavior, only the β₁₂ phase has the ability to sense the molecules with the help of electrical signals due to the increment in the mobility of electrons along the ridgelines.¹³²

Firstly, the sensing property of borophene was identified by the first-principles calculations.¹⁰⁵ Theoretical analysis through DFT calculations have indicated the suitability of borophene particularly for the toxic gas sensing wherein the adsorption of NH₃, NO₂, CO₂, NO and CO gas molecules was observed by analyzing the transport and electronic properties of borophene sheets within the Perdew–Burke–Ernzerhof exchange-correlation functional and generalized gradient approximation using VASP27, and it was observed that the binding energy was low for CO₂ molecules and high for the remaining gases.¹³³ The theoretical results using the Cambridge Sequential Total Energy Package (CASTEP) also revealed that borophene could be an actively good material for the sensing of CO₂ molecules.¹³⁴ Similarly, the adsorption of NO, H₂, N₂, O₂, H₂O, CO and NO on B₃₆ sheet revealed by the DFT results indicated that B₃₆ can detect NO, CO and O₂.¹³¹ The employment of the Minnesota 06 and B97D functionals using DFT predicted the increase in the electrical conductivity in the presence of formaldehyde, which indicated that B₃₆ exhibited the property of sensing formaldehyde.¹³⁵ Furthermore, there have been findings of borophene with electronic and energetic sensitivities for nucleobases (adenine, cytosine, guanine and thymine).¹³⁶

Borophene is highly sensitive for the detection and transmission of electronic signals.¹³⁵ The sensitivity of borophene can be further enhanced for transforming stimuli such as pH and temperature into mechanical, optical and electrical signals when combined with hydrogel-based substrates.⁸¹ Lherbier et al.¹³⁷ proved the improvement in the photosensitivity of borophene through its surface modifications. Also, their high surface volume ratio is one of the favorable properties for detecting gas molecules with low concentration. As observed from the results of I–V characteristic data as shown in Fig. 17(c and d), borophene showed the variation in its electrical properties with the addition of NO; the decrease in band gap and change in the adsorption energy when treated with ethanol also indicated the sensitivity of borophene for ethanol vapor.¹³⁸ Further, the property of gas adsorption for spintronic and gas sensor devices could be improved by embedding borophene with the atoms of transition metals.¹³⁹ Hence, the outstanding sensing properties of borophene has helped it to emerge in various fields.

4.4. Electrical properties

Borophene with a combination of high electrical, ionic, optical and thermal conductivities serves as an outstanding material for various applications.¹¹⁰ Herein, we mainly focus on its electrical and thermal conductivity. For borophene with the 2-Pmmn phase, the electrical conductivity is higher along the armchair direction when compared to that of the zigzag direction.¹²³ Studies on the electron–electron and electron–phonon interactions by state-of-the-art methods showed that electrical conductivity is controlled by electron–phonon interactions³⁸ and as seen in Fig. 17(e), an important role is played by the Fermi velocity for the electrical conductivity when the electron–phonon relaxation time is closer to the Fermi level (EF) and the density of states is at EF.¹³⁷ Moreover, the conduction of electricity is facilitated by the construction of metallic bands in borophene. Along with this, the tendency of borophene to donate electrons motivated the research on halogen atom (F, Cl, Br, I) interactions with borophene, and on analyzing the single halogen migration on borophene surface it was observed that high diffusion barriers which decrease with atomic size were able to alter the anisotropy of electrical conductivity.¹²⁶ The electrical conductivity of the electrode can be further improved when Li/Na atoms are absorbed by borophene, showing its metallic property, which has low open-circuit voltage when treated as the anode material¹⁰⁵ and especially during the complete lithiation process the electrical conductivity of borophene was excellent due to the metallic characteristics observed in its electronic structure.¹²⁴ In a study,¹³⁶ the variation in electrical conductivity was also observed when B₃₆ borophene was in contact with different nitrogenous bases. The boron vacancy and good electronic conductivity of borophene highlight its ultrahigh ionic-conductivity at its 2-Pmmn phase representing its best performance during it discharging and charging cycles.¹²⁴

The thermal conductivity of borophene was reported to be much low due to its anisotropy when compare to other similar isotropic 2-dimensional materials.⁴⁵ The results of borophene grown on Ag(111) showed low thermal conductivity at its lattice, which was strongly anisotropic as the phonon–phonon scattering was very strong and could be further reduced when the length of the lattice was reduced below 300–400 nm.¹⁴⁰ Using DFT, on solving the Boltzmann distribution for phonon frequency, the thermal conductivity of δ₄ borophene after intercalating it with Al was studied and it was found that thermal conductivity increased along the armchair as well as in the zigzag direction, even though the atomic density of intercalated borophene was large. This was because of the grouping of the acoustic branches in borophene after the intercalation, which decreased the phase space of the phonon scattering process. Along with this, even the anisotropy also got tuned.¹⁴¹ Mortazavi et al.¹⁴² calculated the thermal conductivity through the molecular dynamics at non-equilibrium and observed the effects of strain, where the thermal conductivity of borophene increased largely when strained along the armchair direction, i.e., 250% increment and the increase in thermal conductivity was much less when strained along the zigzag direction, i.e., a 20% increment for a strain of 8%.¹⁴²

Recent investigations on the diffusion property of lithium in the heterostructure of borophene/graphene revealed that the overall conductivity of borophene could be further enhanced through the borophene/graphene heterostructure.¹²⁵ The study of the optoelectronic properties of δ₆ and β₁₂ via the perturbation theory and DFT revealed that both allotropes of 2D boron are optically transparent, which strongly depends on the energy and thickness; a weak absorbance was also observed in the visible range. This also introduced borophene as a good material for nano-optoelectronics with tunable monolayers.

4.5. Magnetic properties

Various 2-dimensional materials, especially Dirac materials, are being investigated based on their superconductivity behavior¹⁴³ and it was reported that graphene becomes superconducting when heavier atoms are doped into it.¹⁴⁴ Similarly, borophene being another 2D Dirac material is also reported to show superconductivity properties^{79,116,145,146} due to its low mass along with a powerful electron–phonon coupling. The hexagonal molecular geometry with different critical temperatures favors the superconducting properties of borophene.¹¹⁶ Since the electronic states of the Fermi surface are occupied by both σ and π electrons of borophene, it shows superconductivity. Through analysis of the phonon spectra, electron–phonon coupling and electronic structure, borophenes are reported to disclose phonon-mediated superconductivity at the transition temperature T_c ∼ 10–20 K.⁷² As indicated in Fig. 17(f), a systematic study on the dependence of T_c on the density of hexagonal holes showed a V-shaped curve, where T_c is inversely proportional to the vacancy concentration up to x = 1/9 and later becomes directly proportional, which can be observed in bilayer borophene intercalated with transition metals. It even proved that T_c is high when DOS is large at the Fermi surface.⁷⁰ For the free-standing 2-Pmmn phase, T_c was 19 K due to powerful electron–phonon coupling as a result of the solid Fermi surface and high DOS at the Fermi surface.¹⁴⁷ Other substrate-supported phases of borophene including β₁₂ and χ³ also exhibited superconductivity with a transition temperature of 14 K and 12 K, respectively.¹⁴⁵ Additionally, bilayer borophene intercalated with Mg (BxMgBx), i.e., the Mg–B system, also displayed excellent superconductivity with T_c = 23.2 K and 13.3 K, respectively, for x = 2 and 4.¹¹⁶

The superconductivity can be further tuned for free-standing, substrate-supported, and bilayer metal-intercalated borophene. The T_c of 2-Pmmn borophene can be altered either by the strain or doping carriers, and the in-plane orbital density states close to the Fermi surface relate their impact on T_c, where tensile strain and the doping of holes increases T_c, i.e., T_c of the 2-Pmmn phase increased to 27.4 K and 34.8 K, respectively, while compressive strain and the doping of electrons decreased T_c.¹⁴⁷ However, there was a reduction in T_c to 0.09 K when 0.1e of electrons was doped per boron in the β₁₂ phase.¹⁴⁵ Moreover, the superconductivity for borophene supported by a substrate can also be probed experimentally by inducing suppression through electron doping and with the decrease in the in-plane projected DOS at the Fermi-level, the T_c for the Mg–B substrate also decreased. This superconductivity can also be closely related to the intrinsic structure of the crystal.²⁷

5. Stability of the different isomers of borophene

With an increase in the concentration of vacancies in the 2D allotrope of boron, the evolution of the triangular lattice structure into a hexagonal lattice⁷² is the fundamental principle behind the extensive findings of 4239 isomers of borophene.^68,72 These isomers include both categories of borophenes, i.e., those with hexagonal holes (HHs), as well as with large holes (LHs). However, with stability being an important aspect in experimental synthesis along with production on a large scale, numerous theoretical studies have been proposed in which the high stability in the planar sheets of boron is reported due to the presence of vacancies.⁹⁷ It has been reported that borophene isomers with LHs have high stability at higher vacancy concentrations and slightly lower stability at lower vacancy concentrations when compared to the isomers with only HHs.¹⁴⁸ Few of the theoretical studies reported B₃₆ with a quasi-planar cluster of borophene to have high stability;⁵⁰ there were also predictions on the stabilities of Pmmm, Pmmn, struc-1/8-, snub-, g_1/8-, g_2/15-, α₁- and β₁-sheets.^{39,68,72,74,149} Further, to study the crystal structure stability in detail, it can be differentiated as dynamical stability, thermodynamical stability and mechanical stability.⁹⁷

5.1. Dynamical stability

Two parallel experimental studies reported the successful growth of borophene on a substrate of Ag, which mainly proposed three structures, i.e., stripped borophene, χ³ and β₁₂.^15,45 The dynamic stabilities of different phases of borophene were studied.⁷⁴ Any crystal structure is said to be dynamically stable when the imaginary frequencies of phonons are absent.¹⁵⁰ In Fig. 18(a–c), the absence of imaginary vibration modes for χ³ and β₁₂ indicates their kinetic stability at 0 K. However, striped borophene is dynamically unstable, since the frequency of vibration of striped borophene becomes imaginary along the direction of -X at the longer wavelength limit.^7,45 In support of this, a theoretical study showed the dynamic instability of free-standing striped borophene even when the tensile stress was very high.³¹

Fig. 18 (a–c) Dispersion of phonons with different lines of symmetry for the (a) β₁₂ phase, (b) χ³ phase, (c) striped borophene. (d) Helmholtz free energy varying with the increase in temperature for the β₁₂ phase (black), χ³ phase (red), striped borophene (green). Comparison of (e) Young's modulus and (f) Poisson's ratio for all three phases of borophene (red dots), graphene (green line), boron nitrate (yellow line), silicene (blue line). (Reproduced from ref. 97, published by Taylor & Francis Group, licensed under Creative Common Attribution.)

5.2. Thermodynamic stability

Phonons determine the relative stability of the crystal structure at 0 K as well as finite temperature via Helmholtz free energy, describing their thermodynamical stability.¹⁵⁰ The Helmholtz free energies of stripped borophene, χ³ and β₁₂ after including the zero-point energy at 0 K were found to be −6.106, −6.159, −6.145 eV per atom, respectively. These values were altered at higher temperatures, as shown in Fig. 18(d), for an increase in temperature between 0 K and 1000 K. Helmholtz free energy is in the order χ³ < β₁₂ < stripped borophene, which indicates that for a wide range of temperatures, χ³ and β₁₂ are thermodynamically very stable when compared to stripped borophene.⁹⁷

5.3. Mechanical stability

The fulfillment of the Born–Huang criteria is essential to confirm the mechanical stability; the observed elastic constants for all three phases of borophene satisfy the criteria for Born stability,⁹⁷ indicating their mechanical stability. Nevertheless, through Fig. 18(e and f), Young's modulus and Poisson's ratio were compared with the other 2D materials and it was found that the stiffness of χ³ and β₁₂ was lower than in BN and graphene but higher than in silicene.¹⁵¹

Furthermore, updated research on the mono- and double-doping of carbon atoms into β₁₂, related to the resulting decrease in π-electrons, increase in σ-electrons and decrease in coordinated atoms from 4 to 6 in the double doped β₁₂, contributed to a much better understanding of the stability.¹⁵²

6. Advantages over graphene

After the synthesis of borophene, researchers have been characterizing the properties of borophene in order to find its advantages as compared to graphene and other 2-dimensional materials, and have found that borophene is stronger, more flexible, a good conductor of heat and electricity and a superconductor when compared to graphene; all these properties can also be tuned depending on the arrangement as well as the orientation of vacancies in borophene.⁹⁷ Moreover, the electron-deficient behavior of boron causes the formation of borophene with multicenter bonds possessing several desirable unique physical properties.¹⁰⁸ A much lighter, electron-rich borophene is also estimated to be an equivalent competitor of graphene due to its remarkable mechanical properties⁴⁵ along with orientationally controllable properties for various applications.

A theoretical analysis demonstrated that borophene with a hollow hexagon concentration of 1/6 has a bending stiffness that is twice that of graphene and even though the H–H bonds are absent in the boron sheets, they are much stronger than graphene, which makes borophene a better replacement for advancement in the composites.⁶⁷ In the armchair direction, borophene with the phase Pmmm and 2-Pmmn has a much larger Young's modulus (574.61 N m⁻¹ and 398 N m⁻¹) when compared to that of graphene (338.08 N m⁻¹),¹¹⁰ which highlights the interesting mechanical properties of borophene over graphene. Strikingly, although the interaction of H₂ with borophene is similar to that of graphene, its capacity for hydrogen storage is greater than the capacity of graphene^153–155 and the binding energies of gas molecules such as NO₂, NO, CO₂, NH₃ and CO are also stronger on borophene when compared to the reports on other 2D material.^156,157 Hence, borophenes are highly recommended for the detection of toxic gases, as other 2D materials including graphene lack high selectivity and sensitivity due to their zero-band gap.³

Interestingly, theoretical calculations revealed that metallic borophene sheets with an exceptional anisotropic behavior^74,158 are a better conductor of electricity^38,159 and possess better properties for thermal transport¹⁴⁰ when compared to graphene. Moreover, the stability induced by the substrate on borophene frames an intermediate class of covalently bonded, templated sheets having distinct properties along with consistent structures when compared to other previously reported 2D materials.¹⁶⁰

7. An interesting material for numerous applications

Presently, borophene is considered to be a potential material for a range of applications in various fields. Physicists are entranced by the ability of borophene as a sensor for the detection of various kinds of molecules and their atoms.^{133,161–164} Chemists are excited about its catalytic capability,^{152,165–170} and electrochemists are researching borophene as a good anode material for the generation of a better, powerful metal-ion battery.^{4,125,171–173} Borophene also exhibits potential in various other applications.^{5,38,106,174–178}

7.1. Biomedical applications

Borophene as a 2D nanomaterial has attracted significant attention and has been studied widely in the field of biomedicine because of its good flexibility.⁵³ It can be considered one of the promising biomaterials since it shows different physical and chemical characteristics in its different phases, which could be heterogeneously personalized for various biomedicines as well as medical devices.¹⁰⁶ The unstable nature of honeycomb borophene satisfies the biodegradability essential for biomedical applications.⁵³ When combined with metals and semiconductors, borophene showed improvement in the biological applications without negative impacts.⁵⁰

Recent reports indicated that the sequencing of DNA is done in different ways by determining the sequence of the base, which is essential for decoding the human biological code,^179,180 and several studies regarding medicine and biology have been conducted.^181,182 The results of the electron sensitivity of B₃₆ borophene towards 4 nitrogenous bases, cytosine (C), thymine (T), guanine (G) and adenine (A), over DNA showed that borophene when attached to different bases as shown in Fig. 19(a) gave different electrical signals and conductivity, as well as variations in energy and energy gap as shown in Table 2, provoking it to be used in devices for the sequencing of DNA.¹³⁶

Fig. 19 (a) Different nucleobase/B₃₆ complexes {adenine/B₃₆ (A-1, A-2, A-3, A-4), cytosine/B₃₆ (C-1, C-2, C-3), thymine/B₃₆ (T-1, T-2), guanine/B₃₆ (G-1, G-2, G-3, G-4, G-5)} consisting of carbon (white), nitrogen (blue), oxygen (red), boron (purple), hydrogen (yellow) formed after optimization. (Reproduced from ref. 136 with the permission of Elsevier). (b) Fluorescence imaging (FI) of tumors and other organs. (c) Comparison of (FI) for bare Cy5.5 (green) and B-PEG/Cy5.5NSs (brown). (Reproduced from ref. 21, published by Wiley-VCH GmbH, Licensed under Creative Common Attribution 4.0 International).

Table 2 E_ad (adsorption energy in kcal mol⁻¹), E_HOMO (HOMO energy), E_LUMO (LUMO energy), E_g (energy gap of the HOMO and LUMO) in eV for different complexes formed by B₃₆ (Fig. 22) when combined with different nucleobases, and ΔE_g represents the change in the energy gap after the adsorption. (Reproduced from ref. 136 with the permission of Elsevier)

System	Ead	EHOMO	ELUMO	Eg	ΔEg (%)
B36	—	−5.05	−4.01	1.04	—
Adenine/B36-1	−19.7	−4.89	−3.91	1.06	+1.6
Adenine/B36-2	−39.2	−4.37	−3.52	0.86	−17.5
Adenine/B36-3	−57.3	−4.38	−3.51	0.87	−16.3
Adenine/B36-4	−45.5	−4.43	−3.60	0.83	−20.2
Cytosine/B36-1	−38.1	−4.19	−3.49	0.71	−32.0
Cytosine/B36-2	−4.8	−3.81	−3.77	0.05	−95.5
Cytosine/B36-3	−34.7	−4.00	−3.69	0.30	−70.9
Guanine/B36-1	−35.2	−4.45	−3.58	0.88	−15.3
Guanine/B36-2	−43.1	−4.67	−3.80	0.88	−15.6
Guanine-3	−37.3	−4.04	−3.15	0.88	−14.8
Guanine-4	−22.3	−3.98	−3.43	0.54	−47.9
Guanine-5	−20.5	−4.15	−3.39	0.76	−26.6
Thymine-1	4.4	−4.31	−3.57	0.74	28.9
Thymine-2	10.2	−4.27	−3.60	0.67	−35.5

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Tumor therapy is one of the fields that has popularized nanomaterials because tumors exhibit enhanced permeability and retention (EPR) effect; drugs with sizes of 10 to 1000 nm can enrich the tumor site to passively target tumors, reduce side effects and improve efficacy.^183,184 Additionally, drug delivery is more advantageous through borophene as it can make itself stable by easily combining with other groups or elements. Nonetheless, drug delivery often requires special group modification to stabilize the delivery system of the body and for the accurate release of the drug at the tumor site.^185,186 As indicated in Fig. 19(b and c) the fluorescence image of the malignant tumor could be identified by easily differentiating it from that of normal tissue in the case of a tumor mouse injected with PEG-modified Cy5.5 borophene (B-PEG/Cy5.5NSs) when compared to the tumor mouse injected with bare Cy5.5.⁸⁸ This promotes the application of borophene towards bioimaging, which includes photothermal imaging, photoacoustic imaging and fluorescence imaging.²¹

Cancer treatments can be achieved with minimal invasiveness through photothermal therapy, where NIR radiation is used to irradiate the tumor, which kills the tumor cells by increasing their temperature by photothermal conversion.¹⁸⁷ A better therapeutic effect was observed in the case of borophene due to its UV absorbance property, and efficacy in photothermal conversion and photothermal stability; hence borophene was a potential candidate for photothermal tumor therapy.⁸⁸ Furthermore, biomedical performances such as biocompatibility, targeting ability, circulation, and loading capacity can be enhanced by the appropriate functionalization or surface modification. Surface-functionalized Xenes when compared to their pristine form showed enhanced surface reactivity, stability of dispersion and invoked degradation kinetics, which minimized the toxic response and maximized the biological outcomes.¹⁸⁸ Hence, the exfoliation of borophene after modification by PEG-NH₂ (molecular weight = 2000) by powerful electrostatic adsorption, reported a layer with about 33.6% (w/w) of PEG in the resulting material. This could release a better borophene than its bare form with better dispersibility and stability, resulting in agglomeration in the cell culture medium and phosphate buffer saline,⁸⁸ which could be a favorable factor for both theragnostic and therapeutic applications.¹⁸⁸

7.2. Energy storage applications

The energy storage applications include metal-ion batteries, supercapacitors, hydrogen storage devices, solar cells, etc. Most frequently, the energy storage capacity of borophene is theoretically investigated through software simulations of molecular dynamics.¹⁷⁶

7.2.1. Metal-ion batteries. The combination of electrical conductivity with the thermal stability of borophene makes it a favorable material for metal-ion batteries. The applications of the borophene films as a promising anode material for four different metal ion batteries (Li, Na, Mg or Al) were studied with the help of the simulations from the first-principles density functional theory. The investigation was carried out for four different structures of borophene, which predicted exothermic adsorption when the metal-to-substrate ratio was very high and it was not dependent on the type of atom. The flat borophene when used for the storage of Mg atoms resulted in an ultra-high capacity of 2480 mA h g⁻¹ and the capacity of 1640 mA h g⁻¹ and 2040 mA h g⁻¹ was predicted for the storage of Na and Li, respectively (different charging capacities of different structures of borophene (Fig. 20(a)) are given in Table 3). Even higher capacities were predicted to be accessible on increasing the hole density in the structure of flat borophene. The reasonable charge–discharge rates for the batteries were ensured by the predicted diffusion energy barriers of the metal ions on the surface of borophene. All their theoretical results along with the absence of negative voltage in the voltage profile in Fig. 20(b) suggested that all structures of borophene are efficient anode materials for rechargeable ion batteries.¹⁷³

Fig. 20 (a) Top view of borophene sheets with different hole densities (μ) and the metal atoms Li (light blue), Na (dark blue), Mg (green), Al (red) binding their favorable sites to form B₁, B₂, B₃, B₄ respectively. (b) Voltage curve of metal atom coverage for B₁, B₂, B₃, B₄ for its maximum yielding capacity. (Reproduced from ref. 173 with permission from Elsevier). (c–f) Electrolytic properties of borophene supercapacitors. (c) C–V results at different voltages. (d) C–V results at different scan rates. (e) GCD results. (f) Cell capacitance curve (blue) and specific capacitance curve (red) for varying current densities. (Reprinted (adapted) with permission from ref. 194. Copyright {2018} American Chemical Society). (g) Bare α-sheet borophene. (h) Top view and (i) side view of α-sheet borophene decorated with Li atoms (yellow). (j) Top view and (k) side view of H₂ (green) adsorbed α-sheet borophene decorated with Li atoms (yellow). (Reprint (adapted) with permission from ref. 202. Copyright (2009) American Chemical Society). (l) Side view of the H₂ (green) adsorbed α-sheet borophene decorated with Ca atoms (blue). (Reproduced from ref. 154 with permission from Elsevier).

Table 3 Charging capacity for the storage of different metal ions when different sheets of borophene are taken as the anode. (Reprinted from ref. 173. Copyright (2017) with permission from Elsevier)

Structure	Hexagonal hole density	Charge capacity (mA h g^−1)
		Li	Na	Mg
Buckled	0	1720	1380	1960
B2	1/5	2040	1480	2400
B3	1/6	1980	1550	2330
B1	1/6	1880	1640	2480
B4	1/9	1840	1340	—

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It was observed that the conductivity of the 2-Pmmn phase of borophene was very high with an enormous oxidative process, which allows a charge–discharge process without any dispersion of energy, and hence could serve as a better anode material in Li-ion batteries.^105,106 Moreover, the results from the simulations of ab initio molecular dynamics revealed that the specific capacitance of lithiated borophene was 4 times higher as compared to the graphite anode and the energy barrier was much lower than that of Ti₃C₂, and phosphorene served as the anode material. This suggests that borophene when used as an anode for Li-ion batteries, boosts power and energy density.¹⁸⁹ However, modifications such as stratification, hydrogenation or doping alter their interactions; i.e., a Li-ion battery hydrogenated borophene shows good performances,¹⁷² whereas the level of the stored energy is impacted in the Li–sulphide (LiS) battery when β₁₂ and χ³ phases are introduced into it. χ³ modifies the structure of the LiS battery for the absorption of energy to avoid its structure alteration during charge–discharge cycles and the adsorption energies of Li₂Sn on χ³ are 2.87, 2.53 and 2.67 for n = 8, 6 and 4 respectively, indicating it to be suitable material for anchoring over LiS batteries and although β₁₂ adsorbs just a small amount of energy, it enhances the overall structural stability of LiS. Hence, both defective borophenes enhanced the capacity of retention and suppressed the shuttle effect, which served as excellent anchoring materials for LiS batteries.⁸⁷ A LiS battery with high energy density can be realized by stabilizing the polysulphide shuttle and loading high sulphur and this can be achieved when borophene serves as a host for the sulphur cathode, due to its small deformation, high conductivity and strong adsorption.¹⁷¹

On considering the sodium-ion battery as an example of ultra-high theoretical capacitance,¹⁸⁹ the potentials of two structures, namely the triangular boron layer (B_Δ) and hexagonal vacancies (B_⎔) were investigated by the first-principles calculations. It was observed that the adsorption energy of Na and specific capacitance for B_Δ were −1.731 eV and 248 mA h g⁻¹, respectively, and for B_⎔ they were −1.448 eV and 298 mA h g⁻¹, respectively. Moreover, the energy barrier for B_Δ was just 12 meV, suggesting the extremely fast diffusion and directional anisotropy of sodium in B_Δ. Along with this, the metallic characteristics observed during the sodiation process indicate an outstanding electronic conductivity and suggest that it can efficiently serve as an anode for Na-ion batteries, boosting the power density and energy of the batteries.⁴

Due to the small diffusion barrier with high mechanical strength and specific capacity, borophene was predicted to be a potential anode material but it was unstable without a metal substrate. Hence, to improve its stability, researchers recently proposed a new form of borophene establishing van der Waals heterostructures with graphene (B/G), and observed a high absorbance energy (−2.959 eV) and high specific capacitance (1469.35 mA h g⁻¹), with a small specific capacitance (0.613 eV) of Li theoretically increasing the specific capacitance and decreasing the diffusion barrier to 1763 mA h g⁻¹ and 0.353 eV, respectively, as a result of the changes made by Li on the distance of interlayers in B/G. This demonstrated B/G to be a new anode material in which its electrochemical performances could be controlled by altering the spacing of the interlayers.¹²⁵

7.2.2. Supercapacitors. Supercapacitors are among the modern energy storage applications with remarkable properties and are of low cost with good charge–discharge rates, which can run for lengthy cycles. The supercapacitor performances are affected by the mechanism of charge storage, surface area and electrical conductivity of the electrode material;^190,191 2D materials being ultrathin, with wide surface area and high electrical mobility are suitable.^192,193

Previous studies reported that the supercapacitors consisting of borophene with multi-layers showed good results, i.e., they retained 88% of the initial capacity, even when there was a discharge of more than 6000 cycles¹⁹⁴ it could store a huge amount of energy to serve as a promising biomedical implantable device, especially in rehabilitative robotics.¹⁹⁵ Recent theoretical studies showed that borophene could exhibit better electrochemical performances than graphene, which made it suitable for supercapacitors. The properties include a larger voltage window with high stability (Fig. 20(c)), the CV curves were nearly rectangular even with the scan rate = 100 mV s⁻¹ (Fig. 20(d)),¹⁹⁴ and good capacitance behavior with fast charge–discharge rate (Fig. 20(e)). The specific capacitance was 147.6 F g⁻¹ for 0.3 A g⁻¹ of current density (Fig. 20(f)), and was much higher as compared to bulk boron and carbon-based materials.^196–198

A very recent study on the capacitive properties through the band structures of eight sheets related to borophene revealed that among them, the Fermi levels of seven sheets consisted of Dirac cones and their carriers had higher velocity than graphene. Hence, the larger differential and integrated quantum capacitance with large charge stored at the surface make them superior electrodes for supercapacitors.¹⁷⁴ The ultrasonic exfoliation of α-borophene and further growth into polyaniline: α borophene through electrostatic repulsion was investigated on a nickel foam substrate and it was reported that its specific capacitance was 960 F g⁻¹ and there was 95% retention even after 1000 cycles. This indicated that α-borophene could enhance the performance of polyaniline, making it a promising material for supercapacitors.¹⁷⁵ Thus, all the above results with remarkable features made the supercapacitance of the already existing supercapacitor more advanced when treated with borophene.

7.2.3. Hydrogen energy storage. Considering the environmental issues and finite fuel sources, the development of an efficient and clean fuel is in high demand. Hydrogen is a clean, renewable and abundant energy carrier¹⁹⁹ but its large-scale usage is prevented due to the disadvantage of its safe and dense storage in moderate conditions along with fast loading and unloading.²⁰⁰ Simple storage for hydrogen with its high gravimetric and volumetric density with long life reversible cyclic stability is needed.^169,201 Hence, its storage in molecular form may be more beneficial as compared to the atomic form. Since boron sheets have unique structures with a large distance between the nearest neighbors creating a stable lattice, the molecular hydrogen binds more strongly to boron sheets (0.05 eV) when compared to carbon sheets.²⁰² Numerous DFT calculations were carried out on boron sheets, which revealed their capacity for hydrogen storage.^202–204 Also, through DFT calculations using VASP, an improvement in the capacity of hydrogen storage was observed in the case of planar borophene and the ab initio studies through predicted borophene after doping could be used as one of the potential mediums for hydrogen storage;¹⁵⁴ i.e., when Ca atoms were introduced into boron sheets, the hydrogen storage capacity was improved by 0.20–0.32 eV per H₂ (ref. 154) and it was also observed that the β₁₂ borophene phase gave better absorption sites for Ca as it naturally forms hollow hexagons as shown in Fig. 20(l) with an adsorption capacity of 8.92 wt%.¹⁵⁴ When Ca was introduced into the χ³ phase, the adsorption capacity of H₂ was 7.2%, close to that of graphene.^205,206 A list of hydrogen atom adsorption energies with the capacity of metals decorated on borophene is given in Table 4.

Table 4 Previously reported hydrogen storage capacity in weight percentage for different 2D materials when decorated with different metal atoms along with their respective adsorption energies per H₂ molecule

Decorated metal atom	2D material	Structure	Adsorption energy (eV per H2)	Hydrogen storage capacity (in wt%)	References
Li	Borophene	η = 1/8	0.23	15.26	207
		β12	0.22	10.85	208
		χ^3	10.39	10.79	133
		α-sheet	0.15	10.75	202
		η = 1/7	0.35	9.22	209
		2-Pmmn	∼0.11	6.8	210
	Graphyne	—	0.19	13.0	211
	Graphene	—	0.21	12.8	212
	Silicene	—	0.26	6.35	204
	MoS2	—	0.20	4.80	213
Na	Borophene	2-Pmmn	0.11	9.0	210
		α-sheet	0.07	8.36	202
		χ-ho	0.15	8.28	214
Ca	Borophene	α-sheet	0.19	12.68	154
		β12	0.24	9.5	153
		2-Pmmn	∼0.11	7.6	210
		χ^3	0.23	7.2	153
	Graphyne	—	∼0.2	∼7	215
	BC7	—	0.26	4.96	216
K	Borophene	α-sheet	0.06	2.39	202

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The interaction of a single hydrogen molecule on α-borophene sheet (with adsorption energy of 0.047 eV) was found to be similar to hydrogen interaction with graphene (with adsorption energy of 0.025 eV).²⁰² Moreover, it was observed that the storage capacity and binding energy of H₂ increased when alkali-metals were dispersed on the sheet of boron (the dispersion of Li on boron and binding of H₂ are represented in Fig. 20(g–k)). Among them, the boron-Li system could contain 10.7 weight% of the hydrogen molecule with 0.15 eV binding energy, indicating it to be a good template for storing hydrogen.²⁰² In the previous study, it was observed that Ca could capture 6 hydrogen molecules at 0 K, and when both sides were adsorbed by Ca, its gravimetric density reached 12.68 wt%. Moreover, an increase in temperature along with pressure elevated the performance of hydrogen storage on borophene decorated with Ca.¹⁵⁴

In one of the very recent research projects on the honeycomb borophene oxide, when decorated with different metals (K, Na and Li), the DFT-d₂ (dispersion corrected) simulations showed that a structure of B₂O decorated with Li achieved a high gravimetric density (8.3 wt% H₂) theoretically, and at saturation, the binding energy of two hydrogen molecules when bound by a single Li gave a binding energy of −0.24 eV per H₂. Born-Oppenheimer Molecular Dynamic (BOMD) simulation at different temperatures showed its stability along with reversibility in its binding of H₂ on Li/B₂O or desorption of hydrogen, which introduced a new form of borophene that could show reversibility in the hydrogen storage as well.¹⁵⁵ Hence, various forms of borophene can be used for storing hydrogen in fuel cell equipped vehicles.

7.3. Sensors

The high mobilities of 2D materials at their monolayer thickness have attracted phosphorene, WS₂, MoS₂, BN and graphene towards sensing applications. However, the deficiency of the electronic bandgap in materials like graphene leads to a lack of high selectivity and sensitivity, indicating its unsuitability for the detection of toxic gases.³ However, to improve these deficiencies of 2D materials, borophene with metallic structure and enhanced electron density was introduced for vast gas sensing applications as it can adsorb several molecules of gas due to its large surface.⁹⁵ In particular, it can be efficiently used as a sensor for toxic gases like NO, CO^157,217 and formaldehyde,¹³⁵ as well as non-toxic gases like ethanol¹⁶² and ammonia¹³⁰ along with the detection of hydrogen cyanide.¹³¹ Borophenes were also considered to be suitable gas sensors after analysing the electronic and transport properties of edge-hydrogenated borophene through DFT and non-equilibrium Green's function (NEGF).¹³³

Theoretical electrochemical studies showed that easier charge transfer between the borophene nanosheets leading to the changing conductivity is one of the conditions favourable for chemical sensors and these can be observed through the band structures and DOS spectrum of borophene nanosheets.¹⁶² On considering this, the adsorption of ethanol was observed through the sharp increment in the current through borophene, indicating that it is an ethanol sensor.¹⁶² Moreover, in B₃₆ borophene, electrical signals were generated due to the increase in its electrical conductivity after the adsorption of formaldehyde. This indicated that B₃₆ is suitable for the construction of a formaldehyde sensor,¹³⁵ and its sensitivity towards different concentrations of HCN indicates B₃₆ to be a good sensor for toxic gases. Additionally, it has been reported that for the CO and NO sensors, all fullerenes of boron can be used.²¹⁷ Further, after observing the current–voltage, transmission function and the electronic structure of the 2-Pmmn phase after the adsorption of NH₃, NO₂, NO and CO, it was studied as a sensor,¹³⁸ and a similar phenomenon was observed in the free-standing borophene for a good ammonia sensor.⁹³ Hence the overall results reported different phases of borophenes acting as sensors for the detection of different gasses. Along with these, the semiconductors such as MoS₂ could also be used as gas sensors for NO molecules when used as substrates for the heterostructures of borophene/MoS₂.¹⁶¹ Recently, greater efficiency as a sensor was observed in the β₁₂ and χ³ phase when the direct micromechanical exfoliation technique was employed for their synthesis because by this method, the resulting sheets of borophene were highly crystalline, due to which there was an enhancement in the purity of chemical phase formation, necessary for an ideal sensor.¹⁰³ Moreover, there are other reports on the computational studies on borophene as humidity sensors,¹⁶⁴ metronidazole drug sensor²¹⁸ and many other sensors.

7.4. Electrocatalytic applications

Borophene being the lightest 2D nanomaterial can serve as a potential catalyst in the reactions for the evolution of hydrogen (HER),^165,167,169 reduction of oxygen (ORR),¹⁶⁶ the evolution of oxygen (OER)¹⁶⁶ as well as for the electroreduction of CO₂ (CRR).^128,178

Borophenes show wide applications in the HER. By the method of first principles, it was predicted that borophene, consisting of a basal plane with numerous active sites and exhibiting metallic conductivity with almost free energy could explore the boron monolayers (BMs) as a promising electrocatalyst for the HER.¹⁶⁵ In the case of the χ³ phase, where the active site is the boron atom with 5-coordinates at S₂ (indicated in Fig. 21(a)), the available free energy as shown in Fig. 21(f) for the adsorption of hydrogen was found to be 0.02 eV, which is much closer to zero when compared to that of platinum¹⁶⁵ with ΔG_H = −0.09 eV (ref. 219) as well as the strong interaction of H atoms with the 2-Pmmn phase (Fig. 21(b)) was observed in borophene due to high surface activity. Moreover, the metallic band structure along with numerous active sites in the basal plane of the χ³ phase indicate its catalytic performance in HER. Similarly, for the β₁₂ phase with a 4-coordinated boron atom as an active site, i.e., at its S₁ site (indicated in Fig. 21(c)), the ΔG_H was only 0.1 eV Fig. 21(f) and a small value of ΔG_H was also observed in the β₁ phase and α-sheet as shown in Fig. 21(g) at its S₁ site and S₂ sites, respectively (Fig. 21(d and e)).¹⁶⁵ Nevertheless, all their catalytic performances were not destroyed by Ag substrates.²²⁰ Further, analysis of the effect of substrate, due to the mismatch in charge transfer and strain, showed that the HER performances were positively affected by the substrate.¹⁶⁵

Fig. 21 Crystal structure of the (a) χ³ phase, (b) 2-Pmmn phase, (c) β₁₂ phase, (d) α-sheet, (e) β₁ phase of borophene with purple and yellow balls indicating the B atoms. (f and g) HER free energy diagram. (Reproduced from ref. 165 with the permission of the Royal Society of Chemistry). (h) Polarization curves measured in H₂SO₄ (0.5 M) for Pt/C, α-MoB, MoB₂, Mo, B and carbon sheet. (i) CV curves, i.e., stability measurements of MoB₂ and β-MoB for the first and 1000^th cycles. (Reproduced from ref. 168 with the permission of John Wiley and Sons). (j and k) Thermodynamic overpotentials of different transition metal-supported borophenes for (j) ORR activity and (k) OER activity of Gibbs free energy. (Reproduced from ref. 166 with the permission of the Royal Society of Chemistry).

Furthermore, 2D sheets of boron when synthesized via chemical vapor deposition over the molybdenum foils formed an ultrathin film of molybdenum bromide (Mo₃B) and showed its excellent performance as a catalyst in the reaction of hydrogen evolution (HER).¹⁶⁷ However, as MoB₂ consisted of subunits of borophene similar to that of graphene, their catalytic activities were measured with a scan rate of 1 mV s⁻¹ for different phases of molybdenum bromide (MoB₂, MoB₂, α-MoB and β-MoB) in a solution of 0.5 M H₂SO₄. In general, the electrocatalytic performances of molybdenum and bromide are very rich when compared to that of carbon. Moreover, the four phases of molybdenum bromide are much better, with a ranking order of MoB₂ > β-MoB > α-MoB > Mo₂B > Mo > B, which indicates the proportionality of electrocatalytic activity with that of the amount of boron as verified from the graph of current density vs. potential in Fig. 21(h).¹⁶⁷ The results indicated in Fig. 21(i) for the 1000th cycle in comparison with the 1st cycle revealed excellent performance as a catalyst in the hydrogen evolution reaction even after long-term cycling as compared to that of molybdenum and bromide and their performances are in the order of MoB₂.^167,168

A series of DFT simulations suggested that a transition metal atom supported by borophane (TM-BH) consisting of vacancies could be efficiently used as an electrocatalyst for the ORR as well as the OER and their corresponding thermodynamic potentials are represented in Fig. 21(j and k). This kind of electrocatalyst can be efficiently used for the technologies of renewable energy.¹⁶⁶ The active reactive sites are generated for the ORR and OER due to the coupling of the transition metal d orbitals with that of the surrounding boron atoms. Among the studied TM-BH the promising electrocatalysts for the ORR were Rh–BH and Fe–BH with an overpotential of 0.47 V and 0.43 V, respectively, whereas an overpotential of 0.24 V was found for Rh–BH, indicating its suitability as an electrocatalyst for the OER.¹⁶⁶ Further, through ab initio predictions, a study on borophene after doping with elemental dopants like lithium, sulphur, phosphorous, nitrogen and carbon also proved that the adsorption energies of oxygen and hydrogen were at the top, which subsequently suggested that doped borophene could be used as the catalyst for the evolution reactions of both oxygen and hydrogen.¹⁶⁹ A very recent article explored the catalytic properties of vacancy-containing, carbon-doped borophene towards the OER. They discussed the role of mono-as well as dual-doping of C in β₁₂-borophene, exploring its effect on the π and σ occupancy and reported that boron atoms deficient in charge reduced the binding of oxygen, which reduced the overpotential for the OER.¹⁵²

Reduction of CO₂ into hydrocarbons electrochemically promotes the usage of CO₂, reducing the greenhouse effect. Practically, it is challenging to choose an ideal gas adsorbent material with good selectivity and appropriate adsorbability.⁵³ More specifically, searching for a highly efficient, metal-free electrocatalyst for the reduction of CO₂ as well as for its storage (energy storage) is still a great challenge. Nevertheless, through a DFT investigation, Qin et al.¹²⁸ reported that the metallic borophene, free of metal, serves as an efficient electrocatalyst for the reduction of CO₂ to CH₄. The corresponding results indicated that the χ³ and β₁₂ sheets, deficient in electrons, can be given an e⁻ to activate CO₂ by breaking the π bond. The resulting small value of the activation barrier (0.98 eV) as well as limiting potential (−0.27 V) indicated that the reduction reaction is kinetically and thermodynamically feasible.¹²⁸ Further, the theoretical study along with the experiments on the electroreduction of CO₂ by studying the properties of TM-β₁₂ as a catalyst showed the stability of this boron monolayer, wherein the initial reduction products for Sc and Ti–Zn were CO and CH₄ with an overpotential of 0.45 V and 0.90 V, respectively, whereas the overpotential for Fe-β₁₂ was 0.45 V, thus revealing the reduction mechanism of CO₂ via TM-β₁₂.¹⁷⁰ However, this was achieved even on α-borophene when doped with different metals, where the order of increase in the free energy was Co- = Fe- < Pd- < Ni- < Rh- < Ru- < Ir- < Os-doped α borophene. This indicated the very energetic reduction of CO₂ on the α-borophene doped with Co and Pd when compared to the HER.¹⁷⁷ Nevertheless, the newly designed borophene consisting of Cu chains supported on it preferably causes the electroreduction of CO₂ in the following path: CO₂ → COOH* → CO* → CHO* → CH₂O* → CH₃O* → CH₃OH.¹⁷⁸

7.5. Optical applications

The features of borophene with its capability of absorption, hybridization, optical modulation²²¹ along with atomic layer thickness²²² and excellent optical nonlinearity²²³ play an important role in photonic and electronic devices. It was reported that in a 2D borophene synthesized via liquid -phase exfoliation, when its nonlinear optical properties were measured through Z-scan, the observed saturable absorber (SA) properties indicated that it could serve as an excellent broadband optical switch that could be used for ultrafast mode-locking in mid-infrared and near infrared laser systems. The results further indicated that borophene could also serve as the SA in passively mode locked lasers, as well as future electro-optical applications²²³. Liu et al.²²² used critical coupling in the visible wavelength region on the simple structure of borophene to boost the interaction of light-borophene. It was reported that the anisotropic behaviour of borophene could enhance the light absorption of the structure by 99.8% when the polarization-dependent absorption is coupled with guided resonance. The results of this mechanism also indicated that on adjusting the lifetime and carrier density of borophene, the absorption behaviour could be tuned, which could be used for future photonic and electronic devices.²²² Recently, it was reported that the buckled borophene δ₆ exhibits layer-dependent hyperpolarizability and optical absorption and the DFT predictions reflected the anisotropic structural characteristics, indicating the transparency with the weak absorbance of the multilayer borophene. The borophene with a non-centrosymmetric structure indicated an increase in the value of non-zero dipole hyperpolarizability with an increase in the number of layers. Hence, the integration of these properties (i.e., optics, electronics and optoelectronics) could make multi-layered borophene a promising candidate in high-performance devices.²²⁴ Zhang et al.²²¹ found that borophene could support the surface plasmon polariton in infrared and visible regimes and hence it could integrate with numerous nanophotonic and optoelectronic devices, which could be further employed as borophene plasmon for the next-generation PoC diagnostic systems.²²¹ Additionally, improvements in the optical and electronic properties were observed in the ionic liquid interface of hybrid borophene/graphene and this was due to the dramatic changes in the static dielectric constant as a new plasmon frequency appeared in the interface.²²⁵

8. Summary and outlook

In a very short period, a new 2-dimensional allotrope of boron with much more advanced properties was predicted theoretically via DFT calculations, as well as extracted experimentally through MBE, plasma-enhanced CVD, modified Hummers' method and micromechanical exfoliation. It included both substrate-supported borophene and an independent one where all borophene phases showed interesting properties such as high mechanical properties, metallic and sensing properties, negative Poisson's ratio, conductivity and super conducting behaviour. These unique superlative properties could be further altered through strain, carrier doping, altering vacancy concentrations, the formation of heterostructures, etc. In this review, the stabilities of different structures were studied with their advantages over other 2D materials and their promising applications. The unique strategy employed by looping the theoretical and experimental results for the material design of borophene rather than direct experimental synthesis has suggested that any element and its compounds could be easily studied in the 2D form to improve the efficiency of synthesis for exploring their desirable properties and discovering new materials.

Despite the advantages of the latest 2D borophene, there are many more challenges for its synthesis and practical application, as shown in Fig. 22. Although there are many theoretical suggestions regarding the properties, only a few have been experimentally confirmed because the requirement of an atmosphere with ultrahigh vacuum, and their polymorphism in 3D form limit the single-phase synthesis of borophene on a large scale. The employed growth methods are very limited and CVD being harsh and of high-cost produces materials with very small surface areas, whereas a large surface area is key for further applications. However, even though a few top-down methods like micromechanical exfoliation are simple and economical, they were unable to produce atomically layered borophene with uniform thickness. Moreover, the experimental realization of independent borophene is specifically a great challenge, as theories suggest that substrates are necessary for the crystallization of the 2D form of boron since 3D boron crystals are not a van der Waals material. However, most commonly, for their growth, the suggested substrates are metals. Since the borophene interaction is very strong with metallic substrates, a few intrinsic properties (i.e., superconductivity) of β₁₂ and χ³ are affected and their transfer from the metal surface is also a challenging task. Moreover, the electrical shorting nature of the chosen substrate, i.e., metal, renders the transport measurements in borophene.

Fig. 22 Synthesis and application problems faced in general by 2D materials (reprinted with permission from ref. 3. Copyright (2020) Wiley-VCH Verlag GmbH).

The rapid progress of borophene is outstanding but the efficient utilization of its high potential is yet to be confirmed. Hence, it can be summarized that along with the growing theoretical suggestions, experimental studies through the large-scale synthesis of borophene on an insulating substrate in high volume are in high demand due to the effective applications. However, conventional techniques employed in the case of graphene could also be used for the synthesis of different phases of borophene, giving rise to a final product with much more advanced properties as compared to graphene and subsequently, many more exotic properties are yet to be experimentally confirmed.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

S. R. R. acknowledges the UK Engineering and Physical Sciences Research Council (EPSRC) for funding (Grant No. EP/S001395/1).

Notes and references

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