Bowen
Liu†
ab,
Emilia
Emmanuel†
ab,
Tao
Liang
*c and
Bin
Wang
*ab
aCAS Key Laboratory of Nanosystem and Hierarchical Fabrication, National Center for Nanoscience and Technology (NCNST), Beijing 100190, China. E-mail: wangb@nanoctr.cn
bUniversity of Chinese Academy of Sciences, Beijing 100049, China
cHangzhou Institute for Advanced Study, University of Chinese Academy of Sciences, Hangzhou, China. E-mail: liangtao@ucas.ac.cn
First published on 29th May 2023
Technology-driven modern civilization demands new materials as its backbone. Consequently, based on intense research, a promising candidate, diamane, which is a two-dimensional (2D) form of diamond with a bilayer sp3 carbon nanostructure, has been proposed and recently achieved from bi-layer graphene (BLG) or few-layer graphene (FLG) through high-pressure technology or surface chemical adsorption. This material has been reported to possess a tunable bandgap, excellent heat transfer ability, ultralow friction, and high natural frequency, which can be a potential asset for cutting-edge technological applications, including quantum devices, photonics, nano-electrical devices, and even space technologies. In this review, following the history of the development of diamane, we summarize the recent theoretical and experimental studies on diamane in its pristine form and functionalized with substituents (H–, F–, Cl–, and OH–) in terms of atomic structure, synthesis strategies, physical properties, and potential technological applications. Also, the current challenges and future opportunities for the further development of diamane are discussed. As a young material with great potential but limited experimental research, there is still great space for its exploration.
The synthesis routes for diamane can be roughly classified into two major categories, where one route is surface functionalization-induced interlayer carbon atoms bonding in bilayer or multilayer graphene under atmosphere pressure, while the other route is to convert bilayer or multilayer graphene to diamane directly by applying high pressure, similar to the transformation of graphite to diamond. The former route generates functionalized diamane, which is stable with heteroatoms on its surface, were the products may be diamondol (when the topmost surface is covered with –OH groups),10 diamanoid19,36 (crystalline sp3-bonded 2D carbon materials either similar to diamane but composed of more than two layers or with only one outer hydrogenated (or fluorinated) layer regardless of the number of layers) or diamondene14 (a mixture, diamondized by hydroxyl groups and hydrogens) depending on the surface functional atoms and the layers of graphene precursor. In contrast, the latter route can usually be used to obtain clean diamane (CD) with carbon atoms only.
To date, there are only a few reviews in the literature discussing the emerging material diamane. Piazza et al. summarized the progress on diamane and diamanoid thin films obtained via pressure-less synthesis,37 and subsequently Sorokin et al. provided a mini review on the stabilization of diamond and its corresponding structure–property relationships.33 Tiwari et al. and Qin et al. presented reviews on the recent progress of diamanes and diamanoids for emerging techniques.38,39 In this review, we categorize the history of the development of diamane, which has not been provided in previous reviews, and mainly focus on the experimental results associated with its synthesis processes and several key properties obtained by theoretical simulations. By summarizing the exciting works on the synthesis and understanding diamane and its derivatives, this review may help more researchers focus on this rising star material and accelerate its research toward potential applications in optics, electronics, and mechanics.
Fig. 1 Schematic representation of key works during the development of diamane research. (a) Reproduced with permission.23 Copyright 2009, Springer Nature. (b) Reproduced with permission.10 Copyright 2011, Wiley-VCH. (c) Reproduced with permission.43 Copyright 2012, Wiley-VCH. (d) Reproduced with permission.26 Copyright 2014, the American Chemical Society. (e) Reproduced with permission.14 Copyright 2017, Springer Nature. (f) Reproduced with permission.15 Copyright 2018, Springer Nature. (g) Reproduced with permission.16 Copyright 2019, Elsevier B.V. (h) Reproduced with permission.30 Copyright 2020, Springer Nature. (i) Reproduced with permission.18 Copyright 2020, Elsevier B.V. (j) Reproduced with permission.31 Copyright 2020, Elsevier B.V. (k) Reproduced with permission.17 Copyright 2020, the American Chemical Society. (l) Reproduced with permission.44 Copyright 2021, Elsevier B.V. |
Fig. 2 (a) Atomic structures of (a1) graphane, (a2) diamane, and (a3) diamane II (AA-diamane). (a4) Scheme of the formation of a diamane nucleus in an initial AB-stacked BLG: hydrogen atoms settle from two sides and initiate the “bonding” of carbon atoms located over each other in neighboring carbon layers. Reproduced with permission.23 Copyright 2009, Springer Nature. (b) Atomic structure of the considered non-Janus and Janus diamane nanosheets. Iso-surfaces (set at 0.7) illustrate the electron localization function within the unit cell. Reproduced with permission.49 Copyright 2020, Elsevier B.V. (c) Top and side views for the atomic structure of diamane nanosheets with different functional groups (H, F, and Cl) and layers of carbon atoms (2L, 3L, and 4L). Contours illustrate the electron localization function (ELF) in the unit-cell. Mechanical properties are examined along the armchair (Arm.) and zigzag (Zig.) directions, as shown in the top views. Reproduced with permission.31 Copyright 2020, Elsevier B.V. |
In addition, besides non-Janus diamane, there is a possibility that Janus diamane nano-membranes can be fabricated with the outer surface of BLG functionalized with heteroatoms.49 The generalized gradient approximation (GGA) with Perdew–Burke–Ernzerhof revised for solids (PBEsol)50 was used to investigate the atomic structure and dynamic stability of non-Janus C2H, C2F, and C2Cl diamane and their Janus counterparts of C4HF, C4HCl, and C4FCl, respectively. The top and side views of the geometry-optimized diamane monolayers considered are shown in Fig. 2b. Electron localization function (ELF)51 was used to investigate the type of chemical bond, which was found to be covalently bonded throughout the diamane nanosheets, and the thermal stability was calculated through ab initio molecular dynamics (AIMD) simulations. Furthermore, elastic modulus investigation showed that these nanosheets exhibit an in-plane isotropic elasticity property. Using PBEsol and the hybrid HSE06 functional,52 the electronic band structure was found to exhibit semiconducting features with a direct HSE06 (PBEsol) bandgap. Similarly, the simulation results of the C4HCl Janus diamane structure showed that it has higher thermal conductivity than non-Janus C2Cl. Also, Janus C4HF yields lower and higher thermal conductivity than its non-Janus C2H and C2F counterparts, respectively. Mortazavi et al. used the PBEsol method to describe the atomic structure in functionalized diamane, showing energy-minimized H-, F-, and Cl-diamane (ClD) nanosheets with a graphene-like hexagonal atomic lattice, as shown in Fig. 2c.31 Meanwhile, these structures were described with respect to the functional group and number of carbon layers. The iso-surface densities (ELF = 0.7) around the center of C–C bonds, and also along the functional groups confirm the covalent interactions throughout these 2D materials.31 Phonon dispersions and ab initio molecular dynamics results confirmed that the H-, F-, and Cl-diamane nanosheets exhibit good stability. The extensive theoretical research results provide a comprehensive perspective on the key properties of diamane from stability and mechanics to electronics/optics, and further provide persuasive guidance for experimental exploration.
Nevertheless, insufficient evidence was provided in these reports to claim the production of diamane from BLG instead of diamane/graphene hybrids. Research showed that once the number of graphene layers in the starting FLG is higher than 2–3, the sp2-C to sp3-C conversion happens partially due to the prevalent Bernal stacking sequence. Partially hydrogenated FLG is commonly called diamanoid, as shown in Fig. 3a.19 The first proof of the successful synthesis of diamane from BLG and evidence of UV Raman spectra together with TEM characterization were reported by Piazza et al.18 Micro-Raman mapping was performed before and after (Fig. 3(b1) and (b2)) the hydrogenation process promoted by hot filaments, respectively. The spectrum of BLG showed a normal sharp G peak at around 1582 cm−1 before hydrogenation owing to the extended bonds of all pairs of sp2-C in the graphene sheets. In contrast, significant changes were observed in the Raman spectrum after the hydrogenation process, as follows: (i) the G peak was no longer detected and (ii) only the sp3-C stretching signal appeared; clarifying the full sp2-C to sp3-C conversion, that is, the full conversion of BLG into genuine diamane, at least at the position of the Raman laser spot.
Fig. 3 (a), (a1) and (a2) two-side view and (a3) top view of the partially hydrogenated few-layer graphene (FLG) used in DFT calculations with ABBA stacking. Reproduced with permission.19 Copyright 2020, Elsevier B.V. (b) Raman spectra (at 244 nm) of FLG (b1) before and (b2) after hydrogenation. Reproduced with permission.18 Copyright 2020, Elsevier B.V. (c) Initial and optimized geometries for (c1) four, (c2) three, and (c3) two graphene layers. (c4) Spin-dependent electronic dispersion and the density of states (DOS) for the unit cell of the final structure in (c3). Reproduced with permission.10 Copyright 2011, Wiley-VCH. (d) Evolution of the G band with increasing pressure (up to ≈14 GPa) using water as the pressure transmission medium. The respective applied pressure is indicated on the right side of each respective spectrum. Two spectra are shown for each pressure level, one obtained with an excitation laser energy EL = 2.33 eV (green symbols), and the other with EL = 2.54 eV (blue symbols). The solid lines are the Voigt fit to the experimental data. Reproduced with permission.14 Copyright 2017, Springer Nature. |
Similar to hydrogenation, hydroxyl groups can also be used as chemical radicals to stabilize the diamondization of FLG, which is called diamondol, with the topmost of FLG covered with hydroxyl groups and no further modification of the eventual additional underlying graphene layers. Barboza et al. reported how the pressure applied by means of the tip of a scanning probe microscope (SPM) to bilayer and multilayer graphene samples induced a structural change in water medium.10 The amount of pressure-dependent injected charge manipulated with electric force microscopy (EFM) was found to be substantially reduced and this observation was not observed in a dry climate. The diamondization of the top graphene surface was assumed to be due to the hydroxyl group given by the water pressure, and the structure could not be sustained at ambient pressure. The hydroxyl-induced surface was also demonstrated spontaneously in an ab initio calculation. Fig. 3(c1) shows the initial and final configurations of an unconstrained geometry optimization process, where the initial geometry is a four-layer graphene with one hydroxyl group placed above each carbon atom at the top layer sublattices. Fig. 3(c2) and (c3) show the corresponding optimized structures obtained with three- and two-layer graphene in the initial geometry and (c4) represents the spin-dependent electronic dispersion and the density of states (DOS) for the unit cell of the final structure in (c3). Consequently, the structure obtained was proven to have a ferromagnetic insulator phase with various bandgap energies for each spin with regard to the stacking layers. This phenomenon, which was observed as a reversible compression-induced charging inhibition of bilayer and multilayer graphene, presented theoretical and experimental evidence for the room-temperature diamondization of few-layer graphene.
Furthermore, spectroscopic evidence of the formation of a hydroxylated 2D-diamond structure, denoted as diamondene (actually a mixture, diamondized by hydroxyl groups and hydrogens) was provided. Martins et al. conducted experiments with high pressure on the top of BLG in a diamond anvil cell (DAC).14 Water was used as the experimental supplier of hydroxyl or hydrogen atoms and the pressure transmission medium (PTM). BLG was synthesized via chemical vapor deposition and transferred to a Teflon substrate. The substrate did not react with the precursor graphene and could avoid the functionalization of the bottom carbon atoms. Through Raman spectroscopy measurement, it was found that the hybridization of carbon in the sample existed in a mixed state of sp2 and sp3, and an enhancement in the G band occurred as the pressure increased, as shown in Fig. 3d. DFT results showed that diamane was formed at the pressure of 4–5 GPa. The DAC-assisted high pressure experiments induced the as-observed transformation of carbon structures and enabled the investigation of some intrinsic physical properties of diamane, although the obtained structure is reported to be unstable when being exposed to ambient condition, and thus is not a universal material preparation method.
“Top-down” and “bottom-up” approaches have been used to prepare FD in experiments. Experimental proof of the stable FD developed over a wide area at atmospheric pressure was provided by Bakharev et al.30 The experimental process was performed by flowing xenon difluoride (XeF2) gas over a BLG sample on a single-crystal CuNi (111) substrate for several hours at ambient pressure. The reaction between graphene and XeF2 forces the carbon atoms in BLG to bond with fluorine atoms in an sp3 configuration, resulting in the formation of interlayer carbon–carbon bonds in FD. There are two possible structures of the obtained samples, where the first structure in shown in Fig. 4(a1) with sp3-hybridized carbon atoms participating in the interlayer C–C and surface C–F bonds, which is characterized as single diamond-layer (FD), and the second structure (Fig. 4(a2)) with the fluorine atoms bonding on both sides of each sheet where no interlayer linkage happens.30
Fig. 4 Experimental evidence of the fluorinated diamane (FD). (a) Ball and stick models of (a1) FD and (a2) the C2F structure without interlayer C–C bonds. (b) TEM study of FD on CuNi (111) surface. High-resolution cross-sectional TEM images of as-grown pristine graphene bilayer (b1) and sample (b2, b3, and b4) HR-TEM images of FD. Reproduced with permission.30 Copyright 2020, Springer Nature. (c) Schematic illustration of the fabrication process of the BLG device exposure to XeF2 gas. Reproduced with permission.57 Copyright 2021, the American Chemical Society. (d) Magnified TEM image of exfoliated (C2F)n platelet, showing the edge of the platelet. The inset in (d1) shows a magnified image of the edge. (d2) Plot of the profile along the line in (d1) inset, indicating a ∼0.9 nm interlayer distance. Reproduced with permission.44 Copyright 2021, Elsevier B.V. |
Interlayer C–C linkages stabilize the local structure by forming covalent bonds between the F atoms at the interface and bottom layer. In this work, the structural information and transition dynamics were demonstrated by various techniques, and the most direct evidence was given by high-resolution transmission electron microscopy (TEM), as shown in Fig. 4b. An obvious variation in interlayer distance and the formation of C–F bonds on two sides of graphene were observed. Similarly, applying XeF2 gas as the source of fluorine for fluorination, Son et al. demonstrated a method to fabricate single- or double-sided fluorinated BLG by tailoring the substrate interactions, where both the top and bottom surfaces of BLG on the rough silicon dioxide (SiO2) are fluorinated. Meanwhile, only the top surface of graphene on hexagonal boron nitride (hBN) is fluorinated, providing an interesting strategy for the fabrication of FD (Fig. 4c).57 Unlike the above-mentioned “bottom-up” manner, which has a rigid process control and relatively high cost, liquid phase exfoliation as a typical “top-down” process is a simple and versatile means to produce ultrathin layers in large quantities. Chen et al. implemented a new fabrication method with sonication-assisted exfoliation of (C2F)n in solvents to synthesize few-layer FD sheets with a high yield (Fig. 4d).44 The TEM images showed an interlayer space of ∼0.90 nm, in comparison to that of ∼0.34 nm for the pristine graphene layers, suggesting the formation of FD materials with just a few layers. Thus, this liquid exfoliation method can possibly be a future approach to produce large amount diamane materials for potential applications.
Fig. 5 Studies on the preparation of clean diamane materials. (a) Strain dependence of the normalized G-band of the present seven-layered graphene under hydrostatic compression in comparison with that measured in non-hydrostatic tension by four-point bending measurements. Reproduced with permission.58 Copyright 2013, Elsevier B.V. (b) Phase diagram for diamond films with (110) surface. Reproduced with permission.26 Copyright 2014, Elsevier B.V. (c) Schematic showing the diamond anvil cell and the transformation from trilayer graphene to atomically thin CD. (d) Sheet resistance vs. pressure curves for 12-, hexa-, tetra-, tri-, and bilayer graphene measured at room temperature. The solid lines are a guide for the eyes. The pressure dependence of the resistance of graphite is also presented as a reference for comparison (inset). Reproduced with permission.17 Copyright 2020, the American Chemical Society. |
The first practical demonstration of the transformation of BLG into diamond-like ultrahard structures under ambient condition was reported by Gao et al., showing that at room temperature and after nano-indentation through the tip of the atomic force microscopy (AFM) probe, the BLG on SiC (0001) exhibits a transverse stiffness and hardness comparable to diamond and the electrical conductivity shows a reversible drop upon indentation.15 DFT calculations suggested that upon compression, the BLG film transformed into a diamond-like film, producing both elastic deformations and sp2 to sp3 chemical changes. In brief, current studies showed that extremely high pressure is necessary for the fabrication of high-quality CD if there are no additional chemical moieties such as –H and –F to stabilize the dangling bonds on the surface of the carbon layers. However, the expensive instrumental setup, and critically the unstable CD structure when released to atmospheric pressure present challenges for the large-scale fabrication of CD.
Fig. 6 Studies on the mechanical properties of diamane. (a) Molecular dynamics simulation of the deformation of the diamane membrane: initial, elastic deformation, and membrane break. Reproduced with permission.23 Copyright 2009, Springer Nature. (b) Experimental indentation curves in 2-L epitaxial graphene (red) and SiC (black). (c) Cross-section profile of residual indents in 2-L graphene, SiC, and 5-L graphene. Reproduced with permission.15 Copyright 2018, Springer Nature. Uniaxial stress–strain responses of H-diamane nanosheets for the uniaxial loading along armchair (d) and zigzag (e) direction. Reproduced with permission.31 Copyright 2020, Elsevier B.V. (f) Young's moduli of D-AB and D-AA samples along the armchair and zigzag directions and fracture strength of the D-AB sample along the armchair direction as a function of temperature. Reproduced with permission.70 Copyright 2021, the American Chemical Society. |
DFT was also adopted by Gao et al. to estimate the mechanical characteristics of multilayer-graphene films on SiC combined with experimental data. The results showed the reversible transformation from BLG to diamond-like structure due to the structural transformation from sp2 to sp3 at room temperature under the pressure of 1–10 GPa, resulting in stiffness and hardness values comparable to that of diamond (Fig. 6b).15 In contrast, no characteristic phase change was observed with graphene films thicker than three to five layers (Fig. 6c). This is the first experimental verification that the mechanical strength of diamane may be comparable to that of bulk diamond through nano-indentation.
The functional components that control the ratio of the sp2–sp3 hybridization state affect the coefficient of the finite interlayer connected domain of carbon, and thus also affect the mechanical characteristics of diamane.69 Bohayra et al. systematically assessed the thickness and functionalization effects on the mechanical properties of diamane nanosheets, as functionalized with H-, F-, and Cl-atoms via DFT simulations.31Fig. 6d and e show the acquired stress–strain responses of diamane nanosheets for the uniaxial loading along the armchair and zigzag edges, respectively. Analysis of the mechanical properties revealed that by increasing the number of carbon atomic layers in diamane, the elastic modulus and tensile strength increased, and the thickness dependency of the elastic modulus and tensile strength became more distinct on going from H to Cl atoms. It is clear that these novel 2D materials yield highly anisotropic tensile behavior, in which along the zigzag direction, they not only show distinctly higher tensile strengths but also keep their load-bearing ability at considerably larger strain levels. The study of the mechanical properties of diamane under tensile and bending deformation showed that the layer stacking sequence has a negligible impact on its mechanical properties.70 These stretching and bending characteristics are different from graphene, which enables a higher opportunity for many other potential uses. Specifically, a similar Young's modulus was found along the zigzag and armchair directions, while a much greater fracture strain/strength was observed along the zigzag direction. Atomic configurations indicate that the diamane fracture is dominated by the propagation of the crack in the zigzag direction, regardless of the direction of the tensile strain. Furthermore, as the temperature increases, the relationship between the fracture strain/strength and temperature may well be outlined by the kinetic fracture theory, and the Young's modulus and the fracture strain/strength were found to decrease (Fig. 6f). These results can facilitate the essential comprehension of the mechanical behavior of diamane, which should benefit its usage in mechanic-related devices, such as mechanically stiff nano-thick elements in nanoelectronics.70
Fig. 7 Electronic band structures and total electronic density of states (DOS) of H-, F- and Cl-diamane nanosheets with different thicknesses from 2L to 4L predicted by the HSE06 functional. Reproduced with permission.31 Copyright 2020, Elsevier B.V. |
Thus far, the electronic structure of diamanes varies sensitively with the type of functional group and nanosheet thickness. Studies revealed that a smaller effective mass of electrons is present in diamane compared with a thick diamond film, and thus the carrier mobility in diamane may be better compared to the thickener diamond film.72 In addition, diamane is also an excellent candidate for electrical use under extreme operating conditions, that is, high voltage, high temperature, and high frequency.55 Exploring new 2D materials with intriguing properties can be inspired by the potential of modifying their electronic structure through functionalization, as well as the immense promise of novel electronic features that may be discovered. The electronic properties of non-Janus C2H, C2F, and C2Cl diamane and their Janus counterparts of C4HF, C4HCl, and C4FCl, were investigated by analyzing their electronic band structure using the PBEsol and hybrid HSE06 functionals.49 As shown in Fig. 8, the diamane monolayers exhibit semiconducting features with a direct HSE06 (PBEsol) bandgap. Interestingly, despite some similarities between the band structures of the Janus and non-Janus monolayers (e.g., band dispersions around the Fermi level), the Janus structures exhibit different bandgap values and absolute band edge positions. The relatively small direct bandgap and highly dispersed valence and conductions bands of Janus C4HCl and C4FCl monolayers, which are associated with very small charge carrier effective masses, make them potential materials for applications in nano-electronics devices.
Fig. 8 Electronic band structures of diamane monolayers computed using the PBEsol (dotted lines) and hybrid HSE06 (continuous lines) methods. Reproduced with permission.49 Copyright 2020, Elsevier B.V. |
In summary, unlike graphene with zero bandgap electronic character and diamond with a stable chemical structure, the adjustable bandgap of diamane depending on the film thickness and the type of functional group make it attractive for nanoelectronics, bandgap engineering in semiconductor devices, chemical nanosensors in nanocapacitors and active laser medium in nanooptics.
Fig. 9 (a) Thermal conductivity as a function of temperature for AA-stacked diamane (D-AA). Reproduced with permission.61 Copyright 2019, The Royal Society of Chemistry. (b) Thermal conductivities of FD-AB (squares) and FD-AA (circles) obtained based on the relaxation time approximation (RTA) (dashed lines) and the conjugate gradient algorithm with preconditioning (CGP) (solid lines) method. The corresponding thermal conductivities in diamane-AB and diamane-AA are also shown for comparison. Reproduced with permission.29 Copyright 2019, AIP Publishing. (c) Estimated lattice thermal conductivities as a function of temperature for non-Janus and Janus diamane. Reproduced with permission.49 Copyright 2020, Elsevier B.V. (d) Optical absorption of diamane nanosheets predicted using the HSE06 method. (e) Absorption coefficient of the Si-doped diamane configurations. The three energy regions divided by the two vertical lines are infrared, visible, and ultraviolet. Reproduced with permission.31 Copyright 2020, Elsevier B.V. |
To understand the impact of functional groups on the thermal transport properties of diamane, Zhu et al.29 investigated the thermal transport properties of FD-AB/FD-AA and compared them to that of diamane (Fig. 9b). The thermal transport in FD is significantly suppressed, that is, the thermal conductivity of FD-AB/FD-AA is reduced by about 82% compared to diamane-AB/diamane-AA, which is reported due to the significant reduction in the contribution of acoustic modes after fluorination. Moreover, Raeisi et al. explored the underlying mechanisms resulting in significant effects of functional groups on the thermal conductivity of diamane nanosheets by employing machine-learning interatomic potentials in obtaining the anharmonic force constants.49 According to the results, the room temperature lattice thermal conductivity of graphene and non-Janus C2H, C2F, and C2Cl diamane and their Janus counterparts of C4HF, C4HCl and C4FCl diamane were estimated to be 3636, 1145, 377, 146, 454, 244, and 196 W mK−1, respectively (Fig. 9c). It is interesting to see the thermal conductivity of the Janus diamanes fall between that of their non-Janus counterparts. Besides, the thermal conductivity of diamane can also be tuned by changing the mass of functional groups alone. Zhang et al. revealed the relationship between the thermal conductivities of diamane-AB/diamane-AA and the mass of hydrogen at 300 K, and found that the thermal conductivity of diamane was maximized at 1960 and 2236 W mK−1 for diamane-AB and diamane-AA, respectively.74 With an increasing mass of hydrogen, the phonons in diamane are subjected to dramatic suppression and the lifetimes of acoustic phonons are appreciably shortened, resulting in a quick reduction in thermal conductivities in diamane. It is worth noting that although the thermal properties of diamanes reported in these studies do not show superiority compared to that of bulk diamond, the giant thermal conductivity together with the modulated wide bandgap and its special 2D geometry hold great promise in electronic semiconductors applications under extreme conditions, e.g., high temperature.
Diamanes have extraordinary optical qualities in addition to electrical and thermal properties. In accordance with the available literature, the optical features of diamane are attributed to its large bandgap, which can be influenced by a number of external factors.71 Its linear optical property was studied quantitatively based on the frequency-dependent complex dielectric function. The results showed that the static refractive index of diamanes (1.01–1.73) are smaller than that of bulk diamond (2.20). The low refractive index represents the faster speed of spread than in bulk diamond and the energy loss would decrease in diamanes, which can be used as optoelectronic sensors.75 Materials made of functionalized diamanes that are clear and bendable can be utilized in nano-photonics devices.76 In addition, the optical properties of diamane nanosheets studied by calculating their frequency-dependent absorption coefficients using the HSE06 functional indicate that diamane and FD nanosheets have no absorbance in the visible light region, but possess remarkable absorbance coefficients in the UV region, while ClD nanosheets are predicted to absorb light in a broad range of the visible region of the light spectrum and phonon dispersions (Fig. 9d). Ab initio molecular dynamics results confirm that the diamane, FD, and ClD nanosheets exhibit good stability.31 Considering their good dynamical and thermal stabilities and moderate direct bandgap, Cl-diamanes may be promising candidates for optoelectronic devices. Besides different functional groups, the optical properties of diamane can also be strongly modulated by various dopants, such as Si (Fig. 9e), P and Li and the introduction of vacancies,75,77 endowing it with potential for application in the fields of highly active photodetectors/photocatalysts. Diamanes are considered to have superior optical capabilities because of their optical uniqueness, wide gap spectra, and numerous resonance peaks in the valent and conductivity bands in the density of states. Particularly, different functional groups can add outstanding optical qualities to this unique material through substitution or functionalization. These qualities are essential for the effective operation of various optoelectronic applications.
Taking the thermal transport properties as an example, as will be readily seen when calculating the physical properties of diamanes with atomistic simulations, there are still some uncertainties and challenges, which give rise to doubts and questions among researchers, mainly due to the following reasons. Size effects: the thickness of atomically thin diamanes is just a few atoms, which has a significant impact on the way heat is transported through the material, leading to size effects that are not well understood to date. Anisotropy: similar to other 2D materials exhibiting anisotropic thermal transport, this anisotropy is challenging to calculate accurately, especially when working with different theoretical models. Interface effects: the thermal transport properties of diamanes are affected by the interfaces they form with other materials, such as substrates and other 2D materials. These interface effects are usually difficult to model and measure, leading to uncertainties in the calculated properties. In addition, there are also some experimental limitations: measuring the physical properties of diamanes is reported to be challenging, and different experimental techniques may give different results. There may also be issues with sample preparation, such as contamination and damage, which can affect the measured properties.
Therefore, although significant progress has been achieved in understanding the physical properties of diamanes, there is a great need for experiments to acquire well-defined available samples beyond the chemical functionalization and high-pressure methods (i). Besides, although the chemical transformation method is already the most commonly used way to prepare diamane-based materials, the realization of completely hydrogenated or fluorinated diamane appears challenging, which is limited by the control of the experimental conditions and the characterization capabilities (ii). Also, more sufficient evidence of the hydrogenation process of diamane should be supplemented. Specially, direct observation through techniques such as transmission electron microscopy has not been achieved due to the fact that the sp2-C to sp3-C conversion was found to be electron sensitive, even for an electron energy as low as 80 keV. New in situ characterization techniques can be employed in the research of diamane-formation processes (iii). With the diverse stacking sequence, composition, layers, and functional groups of graphene, the abundant structure configurations of diamane can be presented. In addition, various physical properties of different diamanes have been forecasted by theoretical simulation, which indeed still need support from persuasive normalization experiments to ascertain the predicted properties. Thus, the clarification of the structure of diamane and summary of the precursor–product–property relationship require extreme works (iv). Although preliminary research on diamane has created a high hope about its suitability for some practical applications, proposing alternative routes for the bulk production of high-quality diamane and investigating its real application possibility remain great challenges to the science community. We hope that this review can provide an in-depth comprehensive reference for further exploration and innovation on diamanes and other new carbon allotropes.
Footnote |
† These authors made equal major contributions to this work. |
This journal is © The Royal Society of Chemistry 2023 |