Introductory lecture: origins and applications of efficient visible photoluminescence from silicon-based nanostructures

Abstract

A variety of silicon-based nanostructures with dimensions in the 1–5 nm range now emit tunable photoluminescence (PL) spanning the visible range. Achievement of high photoluminescence quantum efficiency (PLQY) relies critically on their surface chemistry passivation and an impressive “tool box” of options have been developed. Two distinct PL bands are dominant. The “S-Band” (red–green emission with Slow microsecond decay) has PLQY that has steadily improved from ∼3% in 1990 to 65 ± 5% by 2017. The “F-Band” (blue–yellow with Fast nanosecond decay) has reported PLQY values that have improved from ∼0.1% in 1994 to as high as ∼90% by 2016. The vast literature on both bands is surveyed and for the S-band, size-structure-PL correlations and selective photo-excitation studies are highlighted. Resonant photoexcitation and single quantum dot studies have revealed the key role of quantum confinement and the excitonic phonon-assisted nature of the radiative transitions. For the F-band, in contrast, specific phenomenological studies are highlighted that demonstrate similar emission without the presence of silicon nanostructures. Low PLQY F-band emission from pure silicon–silica core shell systems is probably associated with oxide-related defects, but ultrahigh PLQY from many lower temperature synthesis routes is likely to be from carbon-based nanostructures or chromophores, not silicon nanostructures. Potential applications for both PL bands include sensing, medical imaging, theranostics, photovoltaics, LED colour converters and nano-thermometry. Emerging “green” synthesis routes are mentioned. If scalability and cost are significantly improved then a number of other proposed uses of ultra-efficient PL from “nano-Si” could become viable in cosmetics, catalysis, security and forensics.

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1. Introduction

1.1. Scope of review

Nanostructuring has introduced an exciting new capability to semiconducting silicon: the ability to emit visible light very efficiently under photoexcitation (photoluminescence (PL)). This property was quickly recognized in the early 1990s as a route to extend the functionality of an already powerful silicon technology. The topic has thus received intense study over the last 30 years, with now thousands of publications. This review focuses on the visible photoluminescence of silicon nanostructures; collating both experimental and theoretical work on most types of nanostructure; providing quantitative performance metrics, particularly on photoluminescence quantum yield (PLQY); photoluminescence wavelength tunability (PLWT) and photoluminescence decay times (PLDT).

There are two dominant PL emissions in the visible (see Fig. 1): the “Slow” (S) band in the red–yellow spectral range with long microsecond decay times (see Section 2) and the “Fast” (F) band in the blue–green with faster nanosecond decay times (see Section 3). In a given nanostructure, one or the other, or both can be present. Often, as shown in Fig. 1, the intensity of the blue emission will often increase with storage time. The origins of such behaviour have received intense study.

Fig. 1 The S-band and F-band photoluminescence of silicon-based nanostructures.¹ (a) Photographs of nanocrystal suspensions under UV illumination. Freshly etched nanocrystals: orange emission. After one day of air exposure: blue emission. (b) Evolution of S-band and F-band spectra with varying air exposure of the times indicated. Adapted with permission from A. Gupta and H. Wiggers, Nanotechnology, 2011, 22, 055707. Copyright IOP Publishing, 2011. All rights reserved.

This review highlights many spectroscopy-based studies and selected phenomenological studies that provide insight into their likely PL mechanisms. The review also covers six application areas of efficient visible PL: sensing, photovoltaics, medical imaging, theranostics, white LEDs and nano-thermometry. It also proposes a number of ways in which the field might further progress, and other application areas that might emerge if cost and scalability issues are surmounted. There is no detailed discussion of nanostructure synthesis techniques and surface chemistry manipulation, as these have already been the focus of many prior reviews that are referenced.

Data on electrical transport, electroluminescence, optical gain, and thereby optoelectronics, are also omitted in order to entirely focus here on the potential of photoluminescent properties. Such a platform property is however often combined with other crucial ones for specific uses, such as biodegradability and biocompatibility for medical applications. These additional properties are covered briefly in the relevant application sections. In summary, this review collates and discusses some of the very large amount of data now available on many types of photoluminescent silicon nanostructures and many applications thereof. It is hoped to be of use both as a broad introduction to the vast literature of this maturing field, but should also stimulate specific areas of discussion and perhaps further research in highlighted topics.

1.2. Electron–hole recombination processes in bulk silicon

When a semiconductor such as silicon is stimulated by light (photoexcitation) it may return to equilibrium by a variety of processes.² Some involve the re-emission of light (photoluminescence or fluorescence – these terms are used interchangeably in the literature on luminescent silicon nanostructures) and are so called radiative transitions. Others redistribute the absorbed energy amongst vibrational modes of the lattice or electronic excitations and ultimately generate heat – so-called non-radiative transitions.

Radiative processes in silicon include recombination from electrons and holes³ free excitons,⁴ biexcitons or excitonic molecules,^5–7 polyexcitons,⁸ electron–hole liquids,^9,10 bound excitons¹¹ and donor–acceptor pair luminescence.^12–14 Free excitons will diffuse through the silicon lattice¹⁵ and can be trapped and emit light at isolated dopants,¹⁶ clusters of dopants,¹⁷ isoelectronic traps¹⁸ and sometimes near extended defects^19–22 such as dislocations and precipitates. Such trapping can occur in the bulk and near surfaces. The majority of these radiative excitonic processes however are only observed at very low cryogenic temperatures, due to the small binding energy of the exciton. Not only that, but their efficiency is also normally low. The efficiency of a radiative process generally depends on the electronic bandstructure of the solid and for crystalline semiconductors the perfection of its lattice and surfaces, since most defects and surfaces promote non-radiative processes. Non-radiative processes²³ in silicon are dominated by Auger recombination²⁴ and recombination at “deep level” defects such as transition metal impurities in the bulk²⁵ or defects at surfaces and interfaces such as that created by ambient or thermal oxidation of silicon.²⁶

1.3. Photoluminescence of crystalline bulk silicon

It is instructive to first survey the PL of bulk Si because this reveals certain “spectroscopic fingerprints” of radiative excitonic recombination processes in this crystalline material. The fundamental indirect energy gap in crystalline bulk silicon, between the top of the valence band and lowest conduction band is 1.170 eV at 0 K, decreasing monotonically to 1.125 eV at 300 K.²⁷ Interband near infrared photoluminescence from silicon was first observed by Haynes and co-workers in the 1950s.³ At room temperature its near bandgap emission consists of a single weak band at ∼1.09 eV (1134 nm) with a full width at half maximum (FWHM) that is about 100 meV.^3,28,29 At cryogenic temperatures recombination becomes excitonic. The exciton in bulk silicon has a Bohr radius of 4.2 nm and a binding energy of only 14 meV.³⁰Fig. 2 shows how the spectra broaden considerably above room temperature and sharpen considerably at cryogenic temperatures, with a spectral position that follows the temperature dependence of the silicon bandgap.

Fig. 2 Normalized interband PL from bulk Si at 35–674 K.²⁹ BE stands for bound exciton, FE for free exciton (see text). Reprinted with permission from V. Alex, S. Finkbeiner and J. Weber, J. Appl. Phys., 1996, 79, 6943. Copyright AIP Publishing.

At low temperatures (<40 K) and in pure silicon, free exciton (FE) recombination totally dominates over bound exciton (BE) recombination (Fig. 3). Transition linewidths are now narrow enough to resolve fine structures corresponding to the TA and TO phonons as shown in Fig. 2 and 3. Two phonon replicas are very weak. Note the extremely weak “zero-phonon” peak I⁰ near 1.16 eV in Fig. 3 where electron and hole recombine without a momentum conserving phonon. The spacings of the features I^TA and I^TO from I⁰ in the spectrum are determined by the corresponding phonon energies. Currently accepted values for bulk silicon are: TA phonon = 18.4 ± 0.2 meV; LO phonon 56.2 ± 1 meV; TO phonon = 58.0 ± 1 meV. The relative ratios of the phonon-assisted peaks are temperature dependent but the TO mode is dominant with approximate values in Fig. 3 being 1 [TO] : 0.1 [LO] : 0.03 [TA].³¹

Fig. 3 Intrinsic free exciton PL from high purity silicon at 26 K.¹⁶ The LO and TO phonon components are not resolved. Reprinted with permission from P. J. Dean, J. R. Haynes and W. F. Flood, Phys. Rev., 161, 711–729. Copyright the American Physical Society, 1967.

Concerning defect-related emission let us first consider the simplest point defects: substitutional donors, like P, As and Sb; interstitial donors like Li; and substitutional acceptors like B, Al, Ga and In. Free excitons migrating through the lattice can bind to the neutral donor or acceptor and binding energies lie in the range 3–50 meV.³¹ When the exciton is localised on the impurity, there is a relaxation of the wave vector selection rule so the zero phonon line is thus much stronger for bound excitons (e.g. In⁰ in Fig. 4) than for free excitons (e.g. I⁰ in Fig. 3).

Fig. 4 30–100 K PL due to bound exciton recombination at indium atoms.¹⁶ Reprinted with permission from P. J. Dean, J. R. Haynes and W. F. Flood, Phys. Rev., 161, 711–729, 1967. Copyright the American Physical Society, 1967.

Rapid progress in identifying many radiative defects began in the late 1970s when Ge photodetectors became available and were orders of magnitude more sensitive than PbS detectors in the near infrared.³¹ The huge increase in resolution enabled isotope shifts in sharp zero phonon line transitions to be studied, thereby unambiguously identifying which impurities were involved in specific vibronic band emission. When combined with uniaxial stress measurements to elucidate the defect symmetry, and correlations with other spectroscopic techniques, detailed models of radiative point defects emerged.³¹

Unfortunately, some luminescent bulk silicon crystals also display broad featureless bands at sub-bandgap wavelengths^19,21,32 providing a challenge for identification that researchers of today’s silicon nanostructures are more than well aware. Fig. 5 shows an example of a broad and featureless band at 0.94 eV attributed to extended defects (hydrogen platelets) in bulk silicon.²¹

Fig. 5 Low temperature PL from plasma etched bulk silicon with high levels of hydrogen.²¹ Reprinted and adapted with permission from H. Weman, B. Monemar, G. S. Oehrlein and S. J. Jeng, Phys. Rev. B, 42, 3109–3112, 1990. Copyright the American Physical Society, 1990.

1.4. Surface passivation chemistries for bulk silicon

Surface or interface “passivation” refers to the removal of electrically or optically active defects and has become increasingly important in silicon solar cell technology due to the cost-driven reduction of cell thickness and corresponding increase in surface to volume ratio. There has been an associated significant effort in understanding the origin of, and minimizing, such non-radiative recombination. That literature is utilized here to identify some of the most useful passivation chemistries that could be applied to luminescent silicon nanostructures, and indeed many, but not all, have already been used. Given that nanostructures have huge surface areas, the importance of minimizing non-radiative surface recombination becomes of paramount importance if high PLQY is to be realized.

Surfaces disrupt the band structure, creating energy states in the bandgap via strained or un-terminated (“dangling”) bonds. These states can capture electrons or holes with capture velocities S_n0 and S_p0 that are governed by the product of carrier diffusion, capture cross-section and defect density, D_it. A surface will have different steady-state carrier concentrations (n_s, p_s) of electrons and holes to those in the bulk, and different to the intrinsic carrier concentration (n_i).

The total recombination at a surface can be characterized by an effective “surface recombination velocity (SRV)” that quantitatively assesses surface or interface passivation:

S_eff = (1/Δn_d) × (n_sp_s − n_i²)/((n_s + n_i)/S_p0 + (p_s + n_i)/S_n0) (1)

where Δn_d is the excess minority carrier concentration that dominates overall recombination rates.³³ What eqn (1) nicely illustrates are the two complementary techniques that have been investigated for reducing surface recombination in bulk silicon. One is “chemical” in the sense of new bonding arrangements of surface dielectric films or organic monolayers and chemical species like hydrogen. These can lower capture rates by having fewer bandgap states (defects) or lowering the capture cross sections of existing defects. They therefore lower the carrier capture velocities S_p0 and S_n0. The other is “physical” in the sense of utilizing surface charge and thereby electric fields to lower the concentration of minority carriers at the surface or interface. This can be achieved by having a raised charge density at the surface or within the interfacial layer that acts to reduce the concentration of one type of carrier at the surface, where most defects reside. They therefore lower n_s or p_s or change their ratio. The lowest values of SRV for bulk silicon have been achieved by utilizing both techniques.³³Table 1 shows SRV values for a range of bulk Si surface and interface chemistries.

Table 1 SRV values for a range of bulk Si surface and interface chemistries

Si structure	Surface chemistry	Passivation method	SRV (cm s^−1)	References
Bulk Si (111)	Silicon hydride	In situ in HF and other acids	<1 (0.25)	(Yablonovitch 1986)^34
Bulk Si	Silicon hydride	HF-based cleans	<0.05 → 0.1	(Sun 2018)^35
Bulk Si (111)	Native oxide	HF and air exposure (24 days)	>1000	(Nemanik 2006)^36
Bulk Si (100)	Thermal oxide	Annealed silica	<1	(Kerr 2002)^37
Bulk Si	Thermal oxide	Charged annealed silica	<1 (0.44)	(Collet 2017)^38
Bulk Si	Silicon nitride	PECVD	<1 (0.67)	(Wan 2013)^39
Bulk Si	Alumina	Plasma ALD	2–6	(Hoex 2008)^40
Bulk Si	Amorphous Si	PECVD a-Si/SiOx/SiNx	<1 (0.06)	(Bonilla 2016)^41
Bulk Si (111)	Alkyl groups	Chloro-alkylation	<25	(Royea 2000)^42
Bulk Si (111)	Methyl, ethyl, propyl, butyl, phenyl	Chloro-alkylation and 24 days in air	<100 (44–80)	(Nemanik 2006)^36
Bulk Si	Superacid	HF-dip coating	3–10	(Bullock 2016)^43
Bulk Si (100)	Superacid	Pre-clean/etches-SA Soln. Coating	0.3–3	(Grant 2017)^44
Bulk Si (100)	Nafion	Spin coating	1.5	(Chen 2018)^45

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Some observations from surveying this field and the data in Table 1:

• Hydride passivation is effective but susceptible to native oxide growth which can seriously raise non-radiative rates.³⁶

• Organic monolayers with Si–C bonding are also effective, but passivation also deteriorates with extended air exposure.³⁶

• SRV values below 1 cm s⁻¹ are achievable at bulk Si interfaces with optimized a-Si, thermal oxides and nitride coatings. Alumina, titania and silicon carbide films can also passivate silicon surfaces but SRV values are generally a little higher.

• Interface densities are much lower for atomically smooth interfaces.⁴⁶

• Surface/interface charging has not been explored thoroughly versus PLQY of nanostructures.

• Superacids and Nafion can provide very low SRV, at least with bulk Si of moderate surface areas.^43–45 They are yet to be explored with silicon nanostructures.

• SRV values are generally much higher for nanostructured silicon (see Section 5.1).

1.5. Luminescent Si nanostructure diversification

The easily reproduced demonstration of efficient⁴⁷ and wavelength-tunable^47,48 visible emission from porous silicon and silicon nanocrystals around 1990 led to a wide range of silicon nanostructures being explored, together with a variety of synthesis techniques. Fig. 6 illustrates many of these schematically. Each class of nanostructure is at least mentioned in this review, but with emphasis that very much reflects their respective levels of study and development, and also their success in achieving high photoluminescence efficiency.

Fig. 6 Example classes of silicon nanostructures.

A variety of different terms for luminescent silicon nanostructures are used in the literature, so we will start by mentioning them here. The terms silicon “quantum” wells, wires, and dots are the smallest 1D, 2D and 3D confined crystalline silicon structures, respectively, with diameters below that of the Bohr exciton radius (∼4 nm) in bulk silicon (see Section 1.3). Such structures are expected to have bandstructures that are size-dependent due to quantum confinement effects as discussed in Section 1.6. Silicon “nanoparticles”, nanowires, nanopillars or nanorods can often, but not always, have larger diameters in the range 5–100 nm. These can exhibit carrier confinement effects on PL but their bandstructure is similar to that of bulk Si. In the literature both terms are used interchangeably. “Nanoclusters”, for example, often denote the tiniest structures consisting of under 50 atoms. Likewise, “nanosheets” usually refer to free-standing layers of, at most, a few atoms thickness.

The experimental literature is currently dominated by luminescent silicon nanocrystals, porous silicon, silicon nanowires and silicon superlattices, in that order. The theoretical literature is dominated by calculations of silicon quantum wires and silicon quantum dots. Other silicon nanostructures have received some, but not substantial development. Examples of silicon nanostructures where efficient visible luminescence at 300 K is either not reported or not quantified to date include silicon nanoribbons,⁴⁹ nanoshells and nanotubes,⁵⁰ nanosheets,^51–54 molecular clusters,⁵⁵ allotropes⁵⁶ and metastable polymorphs.⁵⁷

1.6. Theory of quantum confinement effects in Si nanostructures

Theoretical studies of how the silicon bandstructure evolves with size and shape in the critical 1–10 nm size range began in the early 1990s and there is now a substantial body of work. I am not aware of a prior review that tries to comprehensively analyze the considerable theoretical literature on silicon nanostructures and will not attempt to here. Numerous publications are provided here and a few concepts and trends are selected where there is a consensus with respect to issues that affect PLQY, PLWT and PLDT of silicon nanostructures.

Starting with the earliest calculations in 1992–1993 (ref. 58–60) and many others, theoretical work has continuously grown over the last 30 years, due to a myriad of potential applications for nano-Si, many of which do not utilize photoluminescence. Studies are grouped in Table 2, not by calculation technique, but by nanostructure morphology and surface/interface chemistry. The aim is to help readers access the available theory most relevant to their specific luminescent nanostructures. A large number of theoretical studies have focused on silicon quantum wires, but the literature on quantum dots is growing. Luminescent porous silicon has been modelled as idealized quantum wires and quantum dots or by introducing an artificial periodicity of holes via supercell techniques.¹⁰⁹ In reality, morphology is variable but often of mixed 2–3D confinement due to undulating wire thickness (see Section 2.1) combined with other geometrical shape variations.¹¹⁰

Table 2 Theoretical literature on different classes of model Si nanostructures

Model nanostructure morphology	Surface chemistry	Size range studied	References
Quantum slabs (free-standing)	Si–Hx	0.3–6.0 nm	61–64
Quantum wells (embedded)	Si–SiOx, Si–CaF2	0.3–6.0 nm	65–69
Quantum wires (free-standing)	Si–Hx	0.8–3.0 nm	70–85
Quantum wires (core–shell)	Si–SiOx	1.0–3.0 nm core	86–88
Quantum wires	Si–CH3	1.0–3.0 nm	89
Quantum dots	Si–Hx	1.0–5.0 nm	74 and 90–94
Quantum dots	Si–SiOx	1.0–5.0 nm	95–99
Quantum dots	Si–organic ligands	1.0–5.0 nm	100–106
Ultrasmall Si clusters	Si–Hx	1–50 atoms	107 and 108

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Surveying the theoretical data of Table 2 there would appear to be a consensus that:

• The silicon bandgap starts to widen significantly for quantum wells (slabs/nanosheets) with widths under 2 nm, quantum wires with diameters under 3 nm and quantum dots with diameters under 5 nm.

• In principle the entire visible range is accessible for quantum dots and wires, with bandgap tunability extending from 1 eV to 3 eV.

• Exciton binding energy is significantly enhanced due to quantum confinement reaching values as high as a few hundred meV for 1.5–3 nm diameter quantum dots or 1–2 nm diameter quantum wires.

• Surface chemistry significantly affects the bandstructure of smaller nanostructures, e.g. oxidation narrows the bandgap of hydride passivated nanostructures.

Table 3 provides some theoretical data for hydride passivated silicon quantum dots, quantifying how the bandgap and strength of exciton binding increase dramatically with decreasing size for 1–5 nm nanocrystals. Note how the predicted radiative recombination rates however, only become comparable with those of direct bandgap semiconductors for the smallest silicon nanocrystals around 1 nm diameter.

Table 3 Theoretical estimates of bandgaps, exciton binding energies and radiative recombination rates for 1–5 nm diameter silicon nanocrystals (quantum dots) with hydride passivation

Size	1 nm	1.5 nm	2.0 nm	2.5 nm	3.0 nm	3.5 nm	4.0 nm	5.0 nm	References
Number of silicon atoms	29	87	191	389	705	1087	>1500	>3000	111
Number of hydrogen surface atoms	36	76	148	254	300	436	>500	>750	112
Excitonic bandgap (eV) (nm) associated spectral range	>3.5, <354, UV	>2.5, <496, blue	2.2, 563, green	1.9, 653, red	1.7, 729, red	1.6, 775, near IR	1.5, 827, near IR	1.35, 918, near IR	112
Exciton binding energy (meV)			∼400	∼300	200	160	125	95	113 and 114
Radiative recombination rate (s^−1)	>10^7	∼10^6	∼10^5	∼10^4	<10^4				115

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Aside from size, shape and surface chemistry, there are other factors that affect bandstructure and therefore PL. There have, for example, been a series of theoretical effects investigating the roles of strain.^116–119 For quantum wires, crystallographic orientation also has a significant effect on bandstructure.^75,77,83,120

1.7. PL quantum yield (PLQY)

For any photoluminescent material or molecular species the efficiency of its radiative processes or “photoluminescence quantum yield” (PLQY) of its luminescence is an important quantitative property that will impact on its likely use. The PLQY depends on direct competition between the specific radiative process and all non-radiative processes:

PLQY (%) = photons emitted/photons absorbed = k_r/(k_r + Σk_nr) (2)

where k_r is the radiative recombination rate and k_nr the non-radiative rate.

Measurement of both PLQY η and luminescence decay time τ_PL enables one to separate the radiative and non-radiative lifetimes of excited states using the simple relation

PLQY = η = τ_PL/τ_r = 1/τ_r/(1/τ_r + 1/τ_nr) (3)

The intrinsic radiative recombination coefficient k_r of bulk c-Si has been estimated to be 2 × 10⁻¹⁵ cm³ s⁻¹ which is about 100 000 times lower than direct bandgap semiconductors like GaAs. Absolute quantum efficiency estimates of interband recombination (see Fig. 2) in standard silicon crystals (wafers) with native oxide surfaces (see Table 1) typically range from 10⁻⁶ to 10⁻⁴. However, eqn (3) emphasizes that if all non-radiative processes can be virtually removed, then PLQY can be quite high. This has been elegantly demonstrated by solar cell specialists (Green and co-workers). In 2003 they showed that in the highest purity float zone silicon with optimized surface passivation, PLQY of near infrared emission could be as high as 6.1 ± 0.9% at 300 K.¹²¹

Accurate measurement of PLQY is not as straightforward as measuring PL decay times and a number of different protocols exist, but they can be divided into comparative ones that utilize photoluminescence standards of known PLQY and absolute ones that directly measure the fraction of absorbed photons that are emitted as photoluminescence or the fraction of photons lost by non-radiative recombination. The latter use integrating spheres to collect all emitted light, or use photoacoustic/calorimetric methods to quantify heat production. Methodology can depend on the physical form of the photoluminescent material (e.g., film versus dry powder versus colloidal solution) and the spectral positions of both excitation and emission. A number of important reviews are available that detail the theory, methodologies, standards, and sources of error involved.^122–127 Of particular relevance here is the review by Valenta on PLQY measurements of nanomaterials.¹²⁷

For comparative methods, the most established standards are quinine sulphate in 0.5 M sulphuric acid, fluorescein in 0.1 M NaOH and rhodamine 6G in ethanol with peak emissions at 451, 515 and 552 nm, respectively. Their PLQY accuracy is better than 4% under well-defined conditions.¹²⁴ These standards are well suited to the spectral positions of the F-band (see Section 3).

The physical form of silicon nanostructures can have a pronounced effect on PLQY. For example, for thin solid films light entrapment effects due to total internal reflections and Fresnel transmission at the film–air interface can dramatically lower PLQY, even if the internal quantum efficiency (IQE) is very high. A light extraction efficiency Ex_PL for a thin film on an absorbing substrate has been estimated.¹²⁸ The escape probability of emitted light is strongly dependent on refractive index:

Ex_PL = 1/[n_c(n_c + 1)²] (4)

So, for example, with a solid silicon film of refractive index 3.5 only 1.4% of light escapes directly from the top surface. For silicon nanocrystals embedded in silica films with a refractive index of 1.64, only 8.7% escapes internal reflections. For porous silicon nanostructures, PLQY and IQE values start to get closer as refractive index is lowered. High PLQY pSi structures have porosities in the range 70–90% with refractive indices as low as 1.1. Now more than 20% of light emitted internally can directly escape, lowering its self-absorption. Forming luminescent nanocomposites will often normally lower the perceived brightness due to refractive indices then being higher again.

Table 4 collates the highest PLQY values reported to date for all types of silicon nanostructure. It is clear that the highest PLQY values have been obtained in colloidal quantum dots, quantum dot-based superlattices and porous silicon and so these materials are given more consideration in this review. Amorphous nanoparticles have much lower PLQY than crystalline ones and are therefore not discussed. Molecular size clusters and size-dependent PL from ultrathin quantum wells have not had PLQY quantified to date. Nanotube studies have not reported visible PL.

Table 4 Si nanostructure classes with the highest PL efficiencies (PLQY). For data on the F-band there is considerable uncertainty over the origin of the emission and specifically whether it arises from silicon nanostructures and question marks are included in the table to emphasize this (see Section 3)

Class of nanostructure	Structure details	PL peak position	PL efficiency (PLQY at 300 K) and PL band (F or S)	References
Si QDs (?)	4 nm NPs and fluorescein ligands in colloidal solution	522 nm, 2.38 eV	90% (F)	(Zhong 2019)^129
Si QDs (?)	5 nm NPs and arylamine ligands in colloidal solution	555 nm, 2.23 eV	90% (F)	(Li 2016)^130
Porous silicon nanoparticles	3 nm alkyl-Si NCs in colloidal solution	740 nm, 1.87 eV	68 ± 5% (S)	(Yuan 2017)^131
Si QDs	3 nm alkyl–Si NCs in colloidal solution	638 nm, 1.94 eV	66 ± 8% (S)	(Yang 2015)^132
Si QD-based nanocomposites	4.5 nm alkyl–Si NC in polymer matrix	775 nm, 1.6 eV	65 ± 5% (S)	(Marinins 2017)^133
Si QDs	5 nm methyl–Si NC colloidal solution	775 nm, 1.6 eV	65 ± 6% (S)	(Sangghaleh 2015)^134
Si QDs	4 nm octadecyne–Si NC colloidal solution	789 nm, 1.57 eV	62 ± 11% (S)	(Jurbergs 2006)^135
Si-QD based superlattices	Embedded 4.5 nm NC with 3 nm silica barriers	992 nm, 1.25 eV	50% (S)	(Valenta 2019)^136
Porous Si microparticles	SCD pSi with <3 nm interlinked NCs and 1150 m^2 g^−1	685 nm, 1.81 eV	32% (S)	(Joo 2016)^137
Si nanorods (?)	∼20 nm wide NWs	500 nm, 2.48 eV	15% (F)	(Song 2016)^138
Si nanosheets (?)	Buckled silicon monolayers	500 nm, 2.48 eV	9% (F)	(Ryan 2019)^139
Porous Si nanowires	20–200 nm wide NWs with interlinked 3 nm NCs	740 nm, 1.68 eV	5% (S)	(Gonchar 2012)^140
Solid Si quantum wires	3–4 nm wide NWs	730 nm, 1.70 eV	4.3% (S)	(Lu 2013)^141
a-Si:Ox QDs	Embedded 2 nm NPs in oxide	600 nm, 2.07 eV	1–4% (S)	(Goni 2014)^142
a-Si:H QDs	Embedded 3–5 nm NPs	850 nm, 1.46 eV	<2% (S)	(Anthony 2009)^143
Solid Si quantum wires	5 nm Si NWs	750 nm, 1.65 eV	0.5% (S)	(Irrera 2012)^144
Si quantum wells	1.1–2.7 nm layers with silica barriers	700 nm, 1.77 eV, 912 nm, 1.36 eV	—	(Cho 2007)^145
Si molecular clusters	Octasilacubane (Si8)–butyldimethylsilyl crystal	751 nm, 1.65 eV	—	(Kanemitsu 1995)^55
Si nanosheet nanocomposite	Si6 H6 sheets in polystyrene	510 nm, 2.43 eV	—	(Helbich 2016)^146
Solid Si nanotubes	3–7 nm c-Si tubular walls	—	—	(Tang 2005)^50

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2. S-Band photoluminescence

2.1. Porous Si (2D to 3D confinement)

Historically, relatively efficient (∼3%) S-band emission at room temperature was first observed in high porosity silicon⁴⁷ which is why we first consider this nanostructure, despite its complex range of morphologies which make theoretical modelling a challenge. Freshly etched red-emitting high porosity silicon was found to contain “undulating” quantum wires¹⁴⁷ of below 3 nm average width (Fig. 7b).

Fig. 7 (a) S-band PL tuning via porosity of hydride passivated porous silicon.¹⁴⁸ Reprinted and adapted with permission from M. V. Wolkin, J. Jorne, P. M. Fauchet, G. Allan and C. Delerue, Phys. Rev. Lett., 82(1), 197–200, 1999. Copyright 1999, the American Physical Society. (b) Complex morphology of photoluminescent porous silicon of 78% porosity.¹⁴⁷ Adapted with permission from A. G. Cullis and L. T. Canham, Nature, 353, 335–337, 1991. Copyright 1991, Nature Publishing Group

Efficient S-band emissions are also evident in material where hydride passivation is replaced by native oxides,^137,149 anodic oxides,¹⁵⁰ high temperature oxides³⁰ and organic monolayers.¹⁵¹ All these surface chemistries, with the notable exception of native oxides in some cases (see Table 1) produce low levels of non-radiative surface recombination in bulk Si. In oxide-free material tuning from the near infrared to the blue spectral region (1300 to 400 nm) has been achieved, as shown in Fig. 7a.^148,152 In oxidized material, from the near infrared to the yellow (1300 to 590 nm).¹⁴⁸ Typical PL lifetimes (see Table 5) are in the microsecond range but depend on wavelength and surface passivation quality (SRV) (see eqn (3)).

Table 5 Experimental S-band decay times (PLDT) in porous silicon versus wavelength/energy for hydride or oxide passivation. SCD: SuperCritically Dried, HP: High Pressure

Wavelength (nm)	Energy (eV)	Decay time (μs)	Decay time (μs)	Decay time (μs)	Decay time (μs)	Decay time (μs)	Decay time (μs)
Surface passivation		Native oxide	Native oxide	Hydride	Native oxide	SCD native oxide	HP wet oxide
References		153	154	152	155	137	156
800	1.55		10	20	50	65	100
700	1.77	9	10	12	30	42	50
600	2.07	2.5	5	9	12	20	20
550	2.25	1.5	1	3			10
500	2.48			0.7			2.5
450	2.76			0.1			0.2
400	3.10			0.02

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The data in Table 5 shows variability in PLDT at given wavelengths, due to differing silicon skeleton morphology and interconnectivity, levels of polycrystallinity and surface passivation. Nonetheless, the longest PLDT values of Gelloz 2005 probably reflect primarily the lowest SRV values obtained to date by their wet oxide passivation technique.

2.2. Single quantum wells and nanosheets (1D confinement)

Takahashi et al.¹⁵⁷ and Saeta et al.¹⁵⁸ were the first to report visible emission from single 2D crystalline silicon structures. Both groups used SIMOX wafers, thinning the silicon layer with high precision via thermal oxidation. A red and near infrared emission band (1.65 eV) was observed with an intensity dependent on the well thickness but spectral position varying little. No luminescence was observed unless the quantum well thickness was less than 5–8 nm. PLQY peaked at ∼2 nm thickness but was very low at 10⁻⁴.^158,159 Okamoto et al.¹⁶⁰ reported 2 K PL from SIMOX-derived quantum wells of only 0.6 and 1.6 nm. Emission was again at 1.66 eV. Pauc et al.¹⁶¹ studied PL at 6 K from SIMOX derived wells of 3.9 down to 0.6 nm. A 1.6 eV band appeared for the narrowest wells with an estimated PLQY of only 0.1% at 6 K.

The first really size-tunable emission was achieved by Green and co-workers.¹⁶² They used ELTRAN structures with superior defect densities (<50 cm⁻²), atomically flat interfaces and lower SRVs. Fig. 8 shows TEM data and the 650–950 nm PL spectra as a function of well thickness over the range 1.1–2.7 nm. Their data on processed SIMOX wafers also reported an emission whose peak wavelength was insensitive to QW thickness but correlated instead with oxide thickness. Zhu et al.¹⁶³ studied (110) SIMOX wafers with ∼1–4.4 nm thick wells. Both QC and interface effects were discussed in relation to their 750–800 nm emission. Wagner et al.⁶⁸ also achieved size dependent 75 K emissions over 1.6 to 1.2 eV for 1–4 nm well widths.

Fig. 8 Single silicon quantum well and associated S-band PL for varying well thickness.¹⁶² Reprinted and adapted with permission from E. C. Cho, M. A. Green, J. Xia, R. Corkish, P. Reece, M. Gal and S. H. Lee. J. Appl. Phys., 2007, 101, 024321. Copyright AIP Publishing.

2.3. Superlattices and multilayers (1D to 3D confinement)

Multilayer and superlattice structures have primarily been based on silica passivation,^164–171 but photoluminescent lattice matched Si/CaF₂ superlattices^172–174 have also been achieved. Lockwood and co-workers^164,165 first reported a photoluminescent MBE grown Si/SiO₂ superlattice. The 6 period structure had 1.0–5.4 nm thick Si quantum wells with 1 nm thick oxide barriers. Visible emission was only observed for wells below 3 nm. PL peak energies were tunable over the 1.7–2.3 eV range but PLQY values were not given. A major advance was made by the Zacharias group in a series of papers^171,175,176 using superlattices to exploit 3D rather than 1D confinement. They also dramatically raised PLQY via improved interface passivation, size control and nanocrystal density. Fig. 9 shows a typical structure and the tunability of the PL.¹⁷¹ Valenta et al.¹⁷⁶ provided detailed information on PLQY values, lying in the range 10–20% and doubling when the silica barrier layer thickness was increased from 1 to 3 nm. PLQY variation with temperature was also studied, peaking at 30% at 100 K. Very recently,¹³⁶ PLQY values reported for near infrared emission (1.35 eV/0.97 micron) corresponding to 4.5 nm nanocrystals were as high as 50%.

Fig. 9 Superlattice-based nanocrystals and their S-band photoluminescence.¹⁷¹ Reprinted and adapted with permission from M. Zacharias, J. Heitmann, R. Scholz, U. Kahler, M. Schmidt and J. Blasing, Appl. Phys. Lett., 2002, 80(4), 661–663. Copyright AIP Publishing.

2.4. Nanowires and quantum wires (2D confinement)

The first free-standing isolated NWs of widths under 10 nm realized by lithographic etching techniques showed very weak photoluminescence^177–179 or no visible luminescence at all, ascribed to insufficient surface passivation.¹⁸⁰ Using laser ablation to create 13 nm wide ultralong nanowires, and then oxidation to thin the silicon core to 3.5 nm, Feng and co-workers reported red emission of unknown PLQY.¹⁸¹ Complete oxidation of the wires removed the red emission but a distinct green emission band persisted (see Section 3.2). Brongersma and co-workers used catalysed CVD growth to create 20 nm wide nanowires and then oxidation to decrease core widths to below 5 nm. Tunable 800 to 720 nm emission with microsecond decay times was achieved.¹⁸²

Examination of Table 4 reveals that nanowire fabrication, excluding porous silicon, has been less successful to date with regards to achieving the highest PLQY values for S-band emission. Korgel’s group reported a PLQY of 4.3% for 3–4 nm wide solid quantum wires.¹⁴¹ For nanowires we need to distinguish those that are solid silicon from those that have larger diameters but are themselves porous. The latter “porous silicon nanowire arrays” will behave in a similar manner to porous silicon films but with lowered nanocrystal densities and, to date, lower PLQY (see Table 4). Some much larger solid nanowires also have oxidized rough sidewalls containing nanocrystals that generate the visible photoluminescence.¹⁸³

Fig. 10 shows an energy filtered TEM image of ultrathin luminescent solid quantum wires reported recently.¹⁸⁴ These were extracted by-products of a plasma torch spherodization of micro-particulate powder. Once extracted via centrifugation, the various fractions showed PL that shifted from 950 to 680 nm as the average Si core diameter evolved from ∼5 to ∼3 nm.

Fig. 10 Luminescent ultrathin silicon nanowires from plasma synthesis and their S-band 300 K PL at varying levels of surface passivation.¹⁸⁴ Adapted with permission from V. Le Borgne, M. Agati, S. Boninelli, P. Castrucci, M. de Crescenzi, R. Dolbec and M. A. E. Khakani, Nanotechnology, 2017, 28, 285702. Copyright IOP Publishing. All rights reserved.

2.5. Nanoparticles and quantum dots (3D confinement)

S-band PL from visibly luminescent silicon nanocrystals (nanoparticles) synthesized by a broad range of techniques have now received a vast amount of study. Fabrication techniques include plasma-assisted decomposition of silane or silicon tetrachloride; laser pyrolysis of silane; liquid phase synthesis; laser ablation; high energy milling and microemulsion growth. The reviews of Mangolini,¹⁸⁵ Huan et al.¹⁸⁶ and Cheng et al.¹⁸⁷ provide details on the majority of these synthesis techniques. Highlighted here is the recent study of the Ceroni and Korgel Groups using dodecene passivated nanocrystals in colloidal suspension.¹⁸⁸Fig. 11 illustrates the excellent control over size. Fig. 12 shows how spectral position smoothly “blueshifts” with decreasing nanocrystal size right from the 1.1 eV bandgap of bulk silicon (for 9.1 nm average size) to 2.1 eV (1.8 nm average size).

Fig. 11 S-band photoluminescent silicon nanocrystals of controllable size.¹⁸⁸ Adapted with permission from Y. Yu, G. Fan, A. Fermi, R. Mazzaro, V. Morandi, P. Ceroni, D. M. Smilgies and B. A. Korgel, J. Phys. Chem. C, 2017, 121, 23240–23248. Copyright 2007 American Chemical Society.

Fig. 12 S-band 300 K PL from the various-sized nanocrystals of Fig. 11.¹⁸⁸ Adapted with permission from Y. Yu, G. Fan, A. Fermi, R. Mazzaro, V. Morandi, P. Ceroni, D. M. Smilgies and B. A. Korgel, J. Phys. Chem. C, 2017, 121, 23240–23248. Copyright 2007 American Chemical Society.

Table 6 shows the decay times from a number of nanocrystal studies and it is informative to compare values therein with Table 5 (data from porous silicon). For example, at 800 nm, pSi decay times (Table 5) lie in the range 10–100 microseconds; isolated nanocrystals emitting at 800 nm (Table 6) have much longer decay times in the range 80–247 microseconds. It is likely that the internal surfaces of porous silicon nanostructures are more challenging to passivate than isolated or embedded nanocrystals.

Table 6 Experimental S-band decay times (PLDT) in Si nanocrystals. Decay time units are in microseconds, as in Table 5

Wavelength (nm)	Energy (eV)	Ref. 154	Ref. 189	Ref. 190	Ref. 188	Ref. 191	Ref. 192
1000	1.24				600	800
900	1.38		120		300	420
800	1.55	100	80	175	100	200	247
700	1.77	35	40	90	30	100	134
600	2.07	8		40	6	40	69
550	2.25			25			36

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2.6. PLQY of S-band

Table 7 summarizes the remarkable improvements made in raising the PLQY of S-band luminescence alone since 1990. Early values in the range 3–5% probably reflected the lower levels of size control, combined with higher SRV levels. The highly interconnected silicon skeleton of porous silicon facilitates rapid exciton migration to lower bandgap parts of the network that have slower radiative rates.^137,193 Size control is thus key. This, in combination with differing light extraction efficiencies, explains why colloidal pSi nanoparticles typically have higher PLQY than microparticles and films (see Table 7).

Table 7 Improvements in the PLQY of the S-band over time (1990–2020)

Maximum PLQY (%)	Nanostructure	Peak PL wavelength (nm)	Mean size/size range (nm)	Surface chemistry	Fabrication technique	References
68	pSi NPs	740	3	Dodecyl	pSi ablation	(Yuan 2017)^131
66	NP	700	3	Dodecyl	HSQ decomposition	(Yang 2015)^132
65	NP	690	4	Native oxide	Silane pyrolysis	(Potrick 2011)^194
62	NP	789	4	Octadecene	Plasma synthesis	(Jurbergs 2006)^135
62	NP	825	4	High pressure wet oxide	Plasma synthesis	(Gelloz 2019)^192
50	SL	954	4.5	Thermal oxide	PECVD	(Valenta 2019)^136
60	NP	775	3	Dodecyl	HSQ decomposition	(Sanggahelah 2015)^134
59	NP	750	2 to 3	Thermal oxide	Ion implantation and anneal	(Walters 2006)^195
45	NP	827	3.5	Dodecene	HSQ decomposition	(Yu 2017)^188
43	NP	760	1.9 to 2.5	Allylbenzene	HSQ decomposition	(Mastronardi 2011)^196
32	pSi particles	685	<2.5	Native oxide	Anodisation and supercritical drying	(Joo 2016)^137
31	pSi films	735	<2.7	High pressure wet oxide	Anodisation	(Gelloz 2019)^192
30	NP	600–850	2 to 7	Native oxide	Laser pyrolysis	(Ledoux 2002)^200
25	NP	650	1.6	Allylbenzene	HSQ decomposition	(Mastronardi 2011)^196
19	SL	950	4.5	Thermal oxide	PECVD	(Valenta 2014)^176
0.3	SL	1170	12	Thermal oxide	Sputtering	(Zacharias 1998)^197
5	NP	600–800	1 to 2	Thermal oxide	Disilane pyrolysis	(Brus 1995)^198
3	pSi films	700–850	2 to 10	Anodic oxide	Electrochemical etch and oxidation	(Vial 1992)^150
3	pSi films	750–850	2 to 10	Hydride	Electrochemical etching	(Canham 1990–1991)^47

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2.7. Wavelength tuning of S-band via nanostructure size and shape

A wealth of data now exists confirming that S-band spectral position depends on size distribution for a variety of nanostructures: 1.1–2.7 nm quantum wells within oxide;¹⁶² 3–5 nm quantum wires from plasma synthesis;¹⁸⁴ 3–8 nm nanocrystals from disilane pyrolysis;¹⁹⁹ 3.4–4.8 nm nanocrystals from silane pyrolysis;²⁰⁰ 3–8 nm and 4–9 nm nanocrystals embedded in silica via sputtering;^200–202 2–5 nm nanocrystals embedded in silica via ion implantation;²⁰³ 1.5–4 nm nanocrystals embedded in silica via PECVD;²⁰⁴ and hydride passivated porous silicon of varying porosity.^{47,148,152,205–207}

Fig. 13 presents a published and very useful compilation of PL data from 17 studies on nanocrystals made by a variety of techniques²⁰⁸ and effective mass approximation (EMA) fitting of nanocrystal bandgap versus size. There is clearly considerable scatter of quantitative data, partly because both S-band (this section) and F-band (see Section 3) data were used. For the S-band data, another complicating factor are the size-dependent optical absorption coefficients: smaller silicon nanocrystals have lower optical absorption which biases the PL spectra towards larger nanocrystals.¹⁸⁸Fig. 14 illustrates how one parameter (nanocrystal size dispersion) can therefore strongly influence the theoretical EMA modelling of PL tunability versus average size. Good agreement between such theory and experimental data is achieved once this is taken into account.

Fig. 13 Size dependence of photoluminescence for silicon quantum dots.²⁰⁸ Reprinted with permission from X. Liu, Y. Zhang, T. Yu, X. Qiao, R. Gresback, X. Pi and D. Yang. Part. Part. Syst. Charact., 33(1), 44–52, 2016. Copyright 2015, WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim. Some of the featured studies reported F-band data rather than S-band data, as highlighted here.

Fig. 14 Effective mass approximation (EMA) modeling of S-band spectral position versus nanocrystal size for various levels of size polydispersity (standard deviations).¹⁸⁸ Adapted with permission from Y. Yu, G. Fan, A. Fermi, R. Mazzaro, V. Morandi, P. Ceroni, D. M. Smilgies and B. A. Korgel, J. Phys. Chem. C, 2017, 121, 23240–23248. Copyright 2007, American Chemical Society.

2.8. Wavelength tuning of S-band via surface chemistry

The ability to tune PL emissions by surface chemistry passivation, as well as by nanostructure size, would be desirable, particularly for fabrication techniques which offered high volume throughput, but lacked fine tunability of size. Theoretical calculations have shown that the bandstructure of ultrasmall nanostructures such as 1–1.5 nm diameter quantum dots and quantum wires are strongly affected by surface chemistry.^86,95,96 Spectral position of the S-band might then also be tunable by surface chemistry. However direct experimental evidence for extensive S-band tuning is sparse. One study reported that for larger nanocrystals (1.64 eV PL corresponding to ∼3.5 nm nanocrystals) changing from hydride to alkyl (dodecene) passivation had little effect on spectral position.²⁰⁹ Not surprisingly however, PLQY and PLDT were affected, as the SRV will differ. In contrast, a study of amide and alkoxy functionalization relative to alkyl did find a small but measurable effect on spectral position for 3.5–6.4 nm nanocrystals.²¹⁰ In addition, another study using alkynyl(aryl) groups also observed PL shifts (in the range 685–800 nm). They attributed the effect to “surface-state induced bandgap changes”.²¹¹ In instances where chemical tuning “right across the visible range” is claimed for silicon nanocrystals,^212,213 it would be important to investigate whether the blue emission is in fact F-band emission by measuring PLDT as a function of tuning (see Section 3) and therefore of quite different origin to the yellow or red S-band emission. The extent to which the S-band can be tuned over a broad wavelength range via surface chemistry alone needs further investigation.

2.9. Selective photoexcitation of S-band emission

As we saw with many radiative transitions in bulk silicon (Section 1.3), sharp features in PL spectra can allow the determination of precise energies of electronic states, the symmetry of luminescent defects and the phonon energies involved. The large linewidth of the S-band PL obscures the spectroscopic information it contains. Photoexcitation at resonant wavelengths via tunable lasers can selectively excite subsets of the nanocrystal ensemble and lower the effects of inhomogeneous broadening, thereby revealing otherwise hidden spectral features. Starting with the studies of Calcott, Nash and co-workers it has now been investigated extensively with luminescent porous silicon^{198,214–222} but also with silicon superlattice nanocrystals,^223,224 large silicon nanowires,²²⁵ quantum wells²²⁶ and nanocrystals made by various other techniques.^{198,227–230} Initial concerns raised about its applicability to high PLQY porous silicon emission at room temperature and the precise number of phonon features were resolved by 1997.³⁰ Highlighted here in Fig. 15 are S-band resonant photoexcitation data from a range of different silicon nanostructures. All show the TO and TA phonon-related features so characteristic of exciton recombination in crystalline silicon. For the S-band PL from a given nanostructure, at increasing levels of confinement (shorter wavelength emission), the exciton binding energy increases (Table 3) and the no-phonon (NP) to phonon (TO, TA) intensity ratio increases, just as for localized bound excitons in bulk silicon (Fig. 3 and 4).

Fig. 15 Resonant photoexcitation of silicon nanostructures. (a) Large silicon nanowires,²²⁵ (b) freshly etched porous silicon (pSi) and oxidized porous silicon (OpSi),²²⁸ (c) free standing silicon nanocrystals,²³⁰ (d) silicon nanocrystals embedded in oxide-based superlattice.²²⁴ Adapted with permission from Y. Kanemitsu, H. Sato, S. Nihonyanagi and Y. Hirai, Phys. Stat. Solidi, 190, 755–758, 2002; D. Kovalev, H. Heckler, M. Ben-Chorin, G. Polisski, M. Schwartzkopf and F. Koch, Phys. Rev. Lett., 1998, 81(13), 2803–2806; B. Goller, S. Polisski, H. Wiggers and D. Kovalev, Appl. Phys. Lett., 2010, 97, 041110 and J. Heitman, F. Muller, L. Yi, M. Zacharias, D. Kovalev and F. Eichhorn, Phys. Rev. B, 2004, 69, 195309, respectively.

Even more revealing than resonant photoexcitation can be the photoexcitation of isolated individual nanostructures. Single nanostructure spectroscopy can reveal homogeneous linewidths and most previously hidden spectral features, but is experimentally challenging. The first such spectroscopy studies were performed on porous silicon particles in 1998 by Buratto and co-workers.²³¹ Progress has been recently thoroughly reviewed by the Linnros group²³² so only a few comments will be made here that relate to PLQY of nanostructure ensembles and the origin of S-band PL:

• Individual nanocrystals “blink” (periods of no emission under constant photoexcitation) and some are completely “dark”; others are “bright” with near unity IQE. The ensemble PLQY can thus be raised significantly by lowering the density of dark silicon nanocrystals that have a very efficient non-radiative channel(s), by decreasing blinking rates, or by both routes.

• PL linewidths for single nanocrystals are substantially narrower than for the ensemble (typically 100–160 meV at 300 K but values as low as 17 meV are now being reported²³³). Both temperature and the medium surrounding the core have a strong influence.

• Substantial TO phonon involvement was again invoked to explain the emission spectral data.

2.10. Origin of S-band PL

The wavelength tuning via size (Section 2.7), right from the bulk silicon bandgap^201,205 to at least the yellow spectral region, convinced many that quantum confinement was indeed responsible for S-band emission. Resonant photoexcitation experiments (Fig. 15), providing a spectroscopic signature of established silicon phonon-assisted transitions (Section 1.3), convinced many more. Single nanocrystal spectroscopy (Section 2.9) holds the most potential for unravelling the complexity of ensemble nanosystems where size and shape control is still improving, but not yet sufficient to avoid significant inhomogeneous broadening and rapid carrier migration effects.²³⁴ Nonetheless, after many hundreds of studies, there is now substantial evidence, both experimental and theoretical, supporting a quantum confinement model for S-band emission, at least for near infrared to green emission. Agreement between theory and experiment exists for PL position (excitonic bandgap) versus size and shape (Fig. 14), together with strongly wavelength-dependent measured decay times (Tables 5 and 6) and extracted radiative decay times. The reasons for rapid decline in PLQY for yellow to blue S-band emission; the ultimate PL linewidths achievable; the avoidance of “blinking”; improvements in SRV for smaller nanostructures – these are all topics requiring further study.

3. F-band photoluminescence

3.1. Porous Si

Reports of blue–green emission (“fast”-nanosecond decay time and hence “F-band emission”) from porous silicon started to appear^235,236 in 1992. Kovalev and co-workers²³⁷ showed that its intensity could be significantly raised by rapid thermal oxidation and since then it has received a series of studies. It is noteworthy that even moderate PLQY requires oxidation²³⁸ and/or heat treatment of the porous silicon. Freshly oxidized porous silicon often does not display F-band emission and it only emerges after storage in ambient air.²³⁹

3.2. Nanowires

Growth of nanowires via catalyzed laser ablation VLS growth or thermal evaporation often results in short wavelength emission alone.^240–244 A silicon oxide phase is invariably present and often silicon core width greatly exceeds that required for silicon bandgap enlargement (<3 nm). Sometimes there is no silicon phase at all.²⁴⁰ PLQY values have not generally been provided.

3.3. Nanoparticles and quantum dots

For silicon quantum dots prepared by high temperature synthesis, such as silane decomposition, F-band emission also appears after air storage (see Fig. 1).

A very large number of reports of F-band emission of high PLQY (only some are in Table 8) have appeared since the 2013 report of “one-pot aqueous synthesis”²⁴⁵ by He and co-workers. Here, efficient blue–green emission is achieved immediately after reduction synthesis using inexpensive precursors, low temperatures (e.g. <200 °C) and short reaction times. Highlighted here though instead, is the recent reappraisal study of Oliinyk and co-workers.²⁴⁶ They cast doubt on the likelihood of such conditions generating crystalline silicon nanostructures. Importantly, they also report that repeating the synthesis with a control (reducing agent and a silicon-free organic amine) produced remarkably similar photoluminescence. The de Cola group have also repeated such a synthesis under both short (15 minute) microwave irradiation and lengthy (300 minute) hydrothermal conditions.²⁴⁷ After thorough characterization of repeat and control formulations (lacking reducing agent or silicon source) they attribute the blue-emitting product to the formation of silica and carbon dots rather than silicon nanocrystals.

Table 8 Improvements in the PLQY of the F-band over time (1994–2020)

Maximum PLQY (%)	Peak PL wavelength (nm)	Mean decay time (ns)	Mean size of NP component (nm)	Fabrication technique	References
90	522	—	4	Solution synthesis	(Zhong 2019)^129
85	425	—	3.5	Solution synthesis	(Zheng 2019)^249
76	440	7	2.8	Laser ablation in organic solvents	(Wang 2019)^250
62	440	8.3	2	Solution synthesis	(Geng 2018)^251
90	550	3	5.2	Solution synthesis	(Li 2016)^130
50	600	1.0	3	Solution synthesis	(Zhong 2016)^252
82	450	14.3	3	Solution synthesis	(Wu 2015)^253
75	480	31	2.5	Solution synthesis	(Wang 2015)^254
85	520	3.9	3.5	Solution synthesis	(Cho 2014)^255
25	375	—	3	Solution synthesis	(Wang 2011)^256
23	425	—	3.4	Solution synthesis	(Atkins 2011)^257
60	425	—	<4	Ball milling in organic solvents	(Heintz 2009)^258
44	450	—	3	Electrochemical etching and sonication in organic solvents	(Portoles 2009)^259
18	480	<100	1.5	Silane decomposition and etching	(Fojtik 2006)^260
<1	500	—	3.7	Gas evaporation or vacuum sublimation	(Kimura 1999)^261
0.1	470	1	<5	Electrochemical etching and oxidation	(Tsybeskov 1994)^238

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A further reappraisal study²⁴⁸ has now also been conducted on a different wet chemical synthesis route that uses a magnesium silicide precursor developed by the Kauzlarich group. Once again the key control run, where the source of silicon was omitted, led to similar F-band emission to where the silicon precursor was included. These three studies^246–248 highlight the current uncertainty that surrounds the true origin(s) of F-band emission (see Section 3.5).

3.4. PLQY of F-band

Table 7 illustrates the dramatic rise in PLQY for the F-band over the last 5 years. In the first reports of emission from oxidized porous silicon films and microparticles, quantum yield was probably at best 0.1%.²³⁸ Colloidal NP emission prior to 2010 was much better, but in the range 1–60%. Since 2015 however there have been a number of reports of PLQY above 75%, even as high as 90%. The table also emphasizes the dominance of bottom-up solution synthesis techniques in achieving the highest PLQY values. The question is, as discussed in the previous section, to what extent does such high PLQY have anything to do with silicon nanocrystals?

3.5. Selective photoexcitation and “fine structure” of F-band

As with the S-band, selective photoexcitation studies of F-band PL might provide some insight by revealing hidden spectral features. Currently, very few studies have been conducted^262,263 but these did reveal potentially useful fine structures that need further study. However, even under non-resonant excitation, some interesting structures of the emission band have been reported in some instances,²⁶⁴ especially for NPs made by laser ablation in solvents.^265–267 By conducting control experiments where the solvent (e.g. toluene or octene) alone was UV irradiated, there appeared to be a good match of F-band spectra with those where silicon nanoparticles were created. Fluorescent solvent by-products need to be considered in such laser ablation studies.

The study of Kobayashi and co-workers²⁶⁸ is also particularly insightful with regards to the potential influence of nanostructure-adsorbed impurities at low concentrations. They utilized high energy milling to generate 5 nm NPs that were then suspended in hexane and generated a blue–green PL band with numerous sub-peaks. Control spectra of hexane alone, without NPs, yielded very much weaker spectra in the same location (Fig. 16). They attributed the PL to adsorbed dimethylanthracene (DMA), a highly fluorescent impurity in hexane. DMA has been used as a standard to estimate comparative PLQY for blue-emitting silicon nanoparticles.²⁵⁸ Importantly, intentional addition of only a 20 nM solution of DMA in hexane to another DMA-free solvent containing silicon NPs was sufficient for the vibronic structure to appear in spectra. Exciting light absorption via energy transfer from the silicon NP was estimated to raise PL output of DMA in hexane by a factor of about 3000. This study highlights the need for extensive characterization of the purity of potentially luminescent silicon nanostructures.

Fig. 16 Photoluminescence of adsorbed fluorescent organic contaminants on silicon nanoparticles.²⁶⁸ Adapted with permission from T. Matsumoto, M. Maeda and H. Kobayashi, Nanoscale Res. Lett., 2016, 11(7), available via an open access CC-BY license.

3.6. Origin(s) of F-band PL

Studies focused on elucidating the origin of the F-band emission have broadly drawn four proposals to date: that the emission is connected to the core of small silicon NPs;^269–273 radiative silicon–oxide interface states;^1,274,275 defect states in pure oxide;^1,276,277 or contamination of that oxide.^239,278,279

Valenta et al.²⁷⁰, for example, studied F-band emissions from small nanoparticles obtained by electrochemical etching, hydrogen peroxide treatment and filtration. They interpreted the very large spectral shifts of the band under varying photoexcitation wavelengths (270–420 nm) to the behaviour of Si QD core emission, rather than defective oxide states. Data was fitted to theoretical predictions of bandgap for small hydride-terminated nanoparticles. No FTIR data or chemical composition data was presented however, so the level of hydride remaining vs. oxide passivation (known to generate different bandgaps) is unknown.

F-band emission assignment to the oxide phase of oxidized silicon nanostructures is based on a number of observations, not least that it bears similarities to emissions from pure defective silica structures²⁸⁰ and can still be observed when the nanocrystalline silicon phase has been removed.²⁴⁰ It is interesting that high surface area silica, for example, shows similar “aging” and “regeneration” effects for its blue photoluminescence to oxidized porous silicon and nanocrystals from high temperature synthesis, attributed to interactions with water vapour and organic molecules in ambient air.^281–283

Are there specific contaminants that might be responsible? A very recent study²⁸⁴ used pulverized pSi particles to investigate the effects of nitrogen contamination on F-band emission, following the work of Dasog et al.²⁸⁵ reporting significant influence of nitrogen incorporation in nanocrystals made by other routes. The porous Si structures were heavily oxidized (FTIR spectra) and intentionally subjected to environments rich in nitrate and nitrates. They found increased nitrogen levels diminished, rather than induced, blue emission.

The study of Chen et al.²⁸⁶ in 2013 instead provides strong support for a link to carbon contamination as one potential origin of the F-band in silicon nanostructures containing organic molecules that are subjected to oxidation, heat treatments, microwave treatments or UV laser treatments. They calcined mesoporous silica NPs containing organic molecules at 400 °C and observed a 450–475 nm emission band. The fluorescent particles had retained an ordered silica framework with 3 nm pores. When these ∼80 nm mesoporous silica spheres were subsequently soaked in 37% HF to dissolve the silica phase, the blue output was however completely preserved. Under subsequent TEM examination, the residue (see Fig. 17) was found to contain a high density of carbon nanoparticles (∼3 nm diameter). The presence of carbon dots does not show that they are necessarily the origin of the luminescence.^287,288 It could be from molecular-sized organic chromophore by-products.

Fig. 17 F-band like photoluminescence and carbon dot contamination of porous silica.²⁸⁶ Adapted with permission from H. Chen, Z. Zhen, W. Tang, T. Todd, Y. Chuang, L. Wang, Z. Pan and J. Xie, Theranostics, 2013, 3(9), 650–657. Copyright IvySpring.

What this does demonstrate however, is that porous silica and oxidized porous silicon nanostructures can act as unintentional “nanoreactors” for carbon dot synthesis. Careful experimentation is required to check that F-band-like emission is not a result of carbon contamination of an initially hydrophobic and organophilic nanostructure. As pointed out many years ago, there is direct evidence of carbon contamination of hydride-terminated porous silicon from a variety of sources: extensive water rinsing used to remove HF residues;²⁸⁹ atmospheric and vacuum storage;¹⁴⁹ plastic wafer and sample container outgassing.²⁷⁹ Low levels of carbon contamination are likely to generate F-band emission that is detectable, but of low PLQY. Intentional carbon impregnation of mesoporous silica (e.g. by glycerol) is now used by the carbon dot scientific community to generate high PLQY²⁹⁰ in the blue–green spectral range. It would be informative to search for carbon dots as well as silicon quantum dots in the now varied protocols for realizing ultrahigh F-band PLQY (see Table 7). It is also noteworthy that the carbon dot research community has similar issues with regards to unintentional fluorescent impurities.^287,288 Without sufficient purification steps, molecular carbon-based chromophores can contribute the majority of the emission from bottom-up carbon dot formulations. Intentional introduction of both silicon and carbon dots into silica has led to white light emission (see Section 4.5), so optimizing PLQY for both S and F-bands can be of use, even if their origins are completely different.

4. PL applications

4.1. Sensing

Very soon after the demonstration of efficient visible photoluminescence from porous silicon⁴⁷ it became apparent that PL output could be sensitive to a range of adsorbates.²⁹¹ This has been the basis of many studies exploiting the photochemical properties of nanoscale silicon to attain sensors for gas and liquid analytes. Prior reviews include those of Sailor et al.,²⁹² Gonzalez et al.²⁹³ and Ji et al.²⁹⁴ Chemical toxicants, pollutants and biomedical analytes have been the primary targets. Both PL quenching^295–299 and enhancements (“photobrightening”) have been reported.^300,301 The vast majority of sensors have utilized PL quenching, which in many cases is reversible, but irreversible quenching has also been considered for single-use sensors.

Fig. 18 illustrates S-band sensing of various explosives with silicon nanocrystals embedded within a convenient paper substrate.

Fig. 18 S-band sensing of explosives.³¹⁴ Reproduced with permission from C. M. Gonzalez, M. Iqbal, M. Dasog, D. G. Piercey, R. Lockwood, T. M. Kiapotke and J. G. C. Veinot, Nanoscale, 2014. Copyright Royal Society of Chemistry.

Quenching can be static (binding), dynamic (collisional), oxidation-induced or deposition-induced.^302,303Table 9 and 10 collate the majority of sensing studies performed together with Limit of Detection (LOD) values and separate them into those that use the S-band (red–yellow PL) from the many recent studies exploiting the F-band (blue–green PL).

Table 9 Sensing with S-band (red–green) photoluminescence

Analyte class	Analyte	Sensing mode	LOD	Silicon nanostructure	References
Environmental contaminants and toxins	Aromatic hydrocarbons	PL quenching	0.5 mM	pSi-hydride	(Fisher 1995)^295
	Sulphur dioxide	PL quenching	440 ppb	pSi-oxide	(Kelly 1996)^304
	Nitrogen dioxide	PL quenching and conductance	5 ppm	pSi multilayer	(Baratto 2002)^305
	Alcohols	PL quenching	40 ppm	pSi-undecanoate	(Holec 2002)^306
	Ethanol	PL quenching and PL decay rates	<620 ppm (vapour)	pSi- native oxide	(Dian 2007)^307
	Alcohols	PL quenching	0.2 μM (vapour)	pSi-undecanoate	(Dian 2010)^308
	Ethanol	PL quenching	380 ppm (vapour)	NP-hydride	(Zhang 2013)^309
	Heavy metals	Immunoassay	10 nM	pSi-enzyme	(Syshchyk 2014)^310
	Copper	PL quenching	1 μM	pSi-alkyl	(Hwang 2016)^311
	Aflatoxin	PL quenching	2.5 pg mL^−1	pSi-Au-protein	(Myndrul 2017)^312
	Metal ion discrimination	PL quenching and peak shift	10 μM	pSi-native oxide	(Jin 2019)^303
Explosives	TNT, DNT	PL quenching	1–2 ppb	pSi-hydride	(Content 2000)^313
	DNB, DNT	PL quenching and PL decay rates	<50 μM	NP-oxide	(Germanenko 2001)^314
	DNT	PL quenching	18.2 ng (solid)	NP-alkyl	(Gonzalez 2014)^315
	DNT	PL quenching	6 ppb (vapour)	NP-alkyl	(Nguyen 2016)^316
Medical analytes	Nitric oxide	PL quenching	2 ppb	pSi-hydride	(Harper 1996)^296
	Viral DNA	PL shift	194 fM	pSi multilayer	(Chan 2000)^317
	Gram negative bacteria	PL shift	1.7 μg	pSi multilayer	(Chan 2001)^318
	Urea	PL quenching	30 μM	pSi and enzyme	(Chaudhari 2005)^319
	Ascorbic acid	PL quenching	2 mM	pSi-oxide	(La Ferrara 2012)^320
	Intracellular Cu	PL quenching	2.5 μM	pSi-carboxyl	(Xia 2013)^321
	AChE inhibitor	PL enhancement	1.25 μg mL^−1	pSi-enzyme	(Saleem 2014)^322
	Human IgG	PL quenching	500 fM	pSi-protein	(Cho 2015)^323
	Dopamine	Ratiometric PL quenching	4 μM	pSi-silica	(Hollett 2019)^302

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Table 10 Sensing with F-band (blue–yellow) photoluminescence

Analyte class	Analyte	Sensing mode	LOD	Silicon nanostructure	References
Environmental contaminants and toxins	Pesticides	PL enhancement	7–200 ng L^−1	NP-enzyme	(Yi 2013)^325
	Mercury	PL quenching	50 nM	NP-amine	(Zhang 2014)^326
	Copper	PL quenching	8 nM	NP-hydride	(Zhao 2014)^327
	Tetracyclines		20–28 nM	NP-amine	(Lin 2015)^328
	Chromium	PL quenching	2.7 μM	NP-dendrimer	(Campos 2015)^329
	Copper		0.5 μM	NP-oxide	(Liao 2016)^330
	Sudan I	PL quenching	39 nM	NP-hexadecylamine	(Jose 2016)^331
	Hypochlorite	PL quenching	10 nM	NP-amine	(Guo 2016)^332
	Chromium, hydrogen, sulphide	PL quenching and restoration	28 nM, 22 nM	NP-amine	(Zhu 2017)^333
	Crystal violet	PL quenching	25 ng mL^−1	NP-amine	(Han 2018)^334
	Ethyl carbamate	RF-ELISA	2.6 μg L^−1	NP-enzyme	(Luo 2018)^335
	Nitrophenol	PL quenching	29 nM	NP-amine	(Han 2019)^336
Explosives	TNT	PL quenching	1 nM	NP-amine	(Ban 2015)^337
Medical analytes	Extracellular pH	PL quenching	pH 4–10	NP-amine	(Feng 2014)^338
	Dopamine	PL quenching	0.3 nM	NP-amine	(Zhang 2015)^339
	Intracellular pH	Ratiometric	pH 4–10	NP-amine	(Chu 2016)^340
	ALP	PL quenching	1 U L^−1	NP-carboxyl	(Li 2018)^341
	Creatinine	PL quenching	0.14 μM	NP-amine	(Meng 2019)^342
	Hemoglobin	PL quenching	40 nM	NP-amine	(Li 2019)^343

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General advantages of fluorimetric (PL) sensing include low background signals and thereby high sensitivity, fast data acquisition and low cost. General challenges include sensor stability, specificity, repeatability, linearity, response and recovery times, and calibration. For Si-based sensors (Tables 9 and 10), approaches to improve specificity have included the immobilization of enzymes;³¹⁹ ratiometric PL spectroscopy;³⁰² combining PL quenching at a single wavelength with peak shifts;³⁰³ or decay time changes.³⁰⁷ With porous Si, multilayer microcavities offer multiparametric sensing.³¹⁷ Recently, pSi-based biosensors have focussed on reflectance-based interferometry rather than photoluminescence-based analysis³²⁴ but as PLQY and more importantly, its stability, continues to improve, there could be a resurgence of interest in exploiting the S-band.

4.2. In vitro and in vivo imaging

Cellular and biomolecular imaging in vitro enables contactless studies of numerous physiological processes. Medical imaging facilitates noninvasive imaging of disease progression in vivo. These are critical tools in clinical diagnostics, physiology and cell biology.

A significant volume of literature now exists on using photoluminescent silicon nanostructures for both in vitro and in vivo imaging. It has been the focus of a few prior reviews^344–348 including a very recent one,³⁴⁹ so only a short overview will be given here.

The S-band emission has been used extensively in vitro as a “fluorescent label” in cell cultures.^349–355 It is also increasingly being evaluated for in vivo use.^357–366

The facile synthesis for achieving very high PLQY of F-band emission has led to a recent surge in its use as a label in vitro and there is now also a significant body of work on this.^367–380 In contrast, in vivo usage of F-band emission is very rare.³⁸¹

S-band emission can be used in vitro and in vivo, whereas F-band emission is generally restricted to in vitro use, because of high tissue absorption of both UV excitation wavelengths and blue–green emission wavelengths. Tissue has two “transmission windows” in the near infrared – 700–900 nm (“NIR 1”) and 1000–1700 nm (“NIR II”), where scattering and absorption of light is lower than adjacent wavelengths.³⁸² The S-band emission of Si nanostructures normally falls within the NIR1 window. Interestingly, with degenerate doping of nanocrystals to shrink their bandgap, room temperature PL output in NIR II has also been achieved, but the PLQY was relatively low at about 2%.³⁸³ An ideal luminescent in vivo probe would have high PLQY with both excitation and emission occurring within these windows. Unfortunately, efficient PLQY of S-band emission necessitates short wavelength 300–450 nm excitation which limits the depth that can be imaged in vivo. For this reason there have been recent studies exploring two-photon excitation of the S-band^384,385 where both excitation and emission target the NIR I window. Sailor and co-workers have recently shown the feasibility of this with 850 nm excitation under two photon microscopy (TPM) and 560–740 nm emission. Although the efficiency of this process with 60 nm pSiNPs was significantly lower than an established TPM molecular probe like rhodamine 6G, the porous silicon probe showed much better photostability, the ability to target tumours and carry drug payloads.³⁶⁶

Gooding and co-workers have also utilized two-photon excitation, in combination with in vitro fluorescence lifetime imaging, to excite F-band emission.³⁸⁶

A wide range of luminescent nanostructured materials are being assessed for in vivo medical imaging³⁸⁷ so I will summarize the perceived merits and challenges of using either the S-band or the F-band PL for biomedical imaging. There are 3 primary drivers for using S-band luminescence in Si nanostructures for in vivo imaging:

(a) Wavelength tunable and now very efficient luminescence output (see Table 7) within tissue window(s);

(b) Time-gated imaging that exploits the microsecond decay times to improve contrast;^357,388,389

(c) Biodegradability^356,390 combined with good in vivo biocompatibility in different body sites.^391–393

Challenges for in vivo use of the S-band include low optical absorption coefficients, the relatively broad FWHM of PL, colloidal stability of the PL in body fluids, and one-photon excitation wavelengths lying outside the tissue windows.

When using the S-band for in vitro cell imaging the primary advantages are tunable emission, the low cytotoxicity of silicon nanoparticles^394–400 and the flexible surface chemistry “toolbox” that has been realized (see Section 5.3).

For in vitro imaging exploiting the F-band emission, the primary advantages are very high PLQY (see Table 8), PL stability (less photobleaching and stable over a large pH range⁴⁰¹) and narrower FWHM of PL than the S-band. Challenges for the F-band include the short excitation and emission wavelengths which overlap with tissue scattering and biological autofluorescence, and current uncertainties with regards to the origin of PL.

It should also be noted that whereas fluorescence labelling is a core technique used in in vitro cell biology, using luminescent agents in vivo is currently very much a developing research tool, rather than a mainstream clinical imaging modality. The latter point has relevance with regards to the likelihood of fast clinical translation of Si NP-based luminescence imaging and hence short-term impact. In this regard, the development of biomodal imaging probes, where photoluminescence imaging is combined with a more clinically established technique, has more potential for clinical translation. Unfortunately, silicon nanoparticles have low X-ray contrast compared to metals. Therefore more complex “nanoconstructs” are needed. There could be, for example, an opportunity for developing porous silicon with entrapped gas for a photoluminescent and biodegradable ultrasonic contrast agent. Table 11 provides examples from the literature – attention to date has been on accessing MRI rather than ultrasonic imaging.

Table 11 Dual imaging with photoluminescent Si nanostructures

Photoluminescence emission	2^nd imaging mode	Silicon nanostructure	References
F-band (540 nm)	MRI	3 nm NPs with iron oxides	(Sato 2011)^402
F-band (430 nm)	MRI	3 nm NPs doped with iron	(Singh 2012)^405
S-band (750 nm)	MRI	170 nm complex with 3 nm NPs and chelated Gd^3+	(Erogbogbo 2012)^361
S-band (500–800 nm)	MRI	pSi and iron oxide NPs	(Xia 2017)^403
F-band (460 nm)	MRI	3.85 nm NPs with ^19F ligands	(Li 2018)^404
F-band (400–510 nm)	MRI	6.6 nm NPs doped with gadolinium	(Li 2018)^381
F-band	PET	2.4 nm NPs with NIR dye and ^64Cu radiolabel	(Singh 2020)^406

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4.3. Theranostics

Theranostic nanomedicine, where nanostructures simultaneously facilitate disease therapy and diagnosis, is currently dominated by drug delivery based therapies.⁴⁰⁷ In particular, the pre-clinical evaluation of pharmacokinetics and the target site and healthy organ accumulation of drugs.⁴⁰⁸ The potential of theranostics with nano-Si was initiated by the pioneering studies of Sailor and co-workers ³⁵⁷ who demonstrated in vivo imaging of S-band and organ targeting. The field has been very recently reviewed³⁴⁹ so only a short perspective is given here.

Whilst for many therapies the luminescence provides only a means to track the nanostructures, the photosensitization of oxygen scales with PLQY.⁴⁰⁹ In contrast to photodynamic therapy, for photothermal therapy one wants to maximize non-radiative processes, so PL is unwanted. Thus luminescent Si nanostructures can themselves provide therapy (photosensitization^409,410) or carry payloads that perform therapy (drugs, radioisotopes, metals³⁴⁹). Luminescent pSi is ideally suited to theranostics^411,412 because of the much higher drug payloads achievable via filling the pore volume, compared to solid nanoparticles where the drug is attached only to the outer surfaces. At 80% porosity for example, drug payloads in pSi can exceed 50 wt% by melt loading techniques and the drug is itself nanostructured by entrapment within 5–50 nm mesopores. Control over pharmacokinetics is achieved via a combination of restricted diffusion and carrier biodegradability. Fig. 19 illustrates the multifunctionality of porous silicon nanoparticles (pSiNP).⁴¹²

Fig. 19 Theranostics with porous silicon nanoparticles.⁴¹² Reproduced with permission from V. Stojanovic, F. Cunin, J. O. Durand, M. Garcia and M. Gary-Bobo, J. Mater. Chem. B, 2016, 4, 7050–7059. Copyright Royal Society of Chemistry.

4.4. Photovoltaics

A major limiting factor in the ultimate efficiency of silicon solar cells is the spectral mismatch between the energy distribution of the incident solar spectrum and the bandgap. An efficient photoluminescent material can act as a spectral converter to raise the efficiency of a solar cell by down-shifting (DS), quantum cutting (QC) or “upconversion” (UC) as shown schematically in Fig. 20.⁴¹³

Fig. 20 Classes of spectral converters.⁴¹³ Reproduced with permission from C. X. Huang, S. Han, W. Huang and X. Liu, Chem. Soc. Rev., 2013, 42, 173–201. Copyright Royal Society of Chemistry.

Visibly photoluminescent silicon can provide “down-shifters”^414–420 or quantum cutters^421–425 for traditional solar cells.

A second, recently emerged potential use is in “luminescent solar concentrators” (LSCs) for building integrated photovoltaics.^426–428 Here, the large Stokes shift between strong UV absorption and S-band emission is of great benefit in minimizing light re-absorption over long distances. Solar concentrators are large plastic plates filled with photoluminescent material (commonly organic dyes) that redirect and concentrate solar energy to small photolvoltaic cells at their perimeter (see Fig. 21). A large fraction of the light can be effectively waveguided in the plastic sheet due to total internal reflections. The tunable bandgaps of silicon nanostructures offer design freedom with regards to aesthetics and optical performance. Challenges for silicon solar concentrator use include low absorptivity, scattering due to nanocrystal agglomeration, scalability of production and above all, cost.

Fig. 21 Luminescent solar concentrator using photoluminescent silicon nanocrystals.⁴²⁶ Reprinted with permission from F. Meinardi, S. Ehrenberg, L. Dhamo, F. Carulli, M. Mauri, F. Bruni, R. Simonutti, U. Kortshagen and S. Brovelli, Nature Photonics., 2017, 11, 177–186. Copyright 2017, Springer Nature.

4.5. Colour converters for white LEDs

White light emitting LEDs of 5 lm W⁻¹ brightness first became commercially available in 1996, and by 2010, brightness had already improved to the 150–250 lm W⁻¹ range. The performance and cost of white LEDs are now competitive for general lighting, automotive, communication and medical uses.⁴²⁹ There are 3 methods for generating white light via an LED: (a) combining blue, green and red LEDs; (b) combing a UV LED with blue, green and red phosphors; and (c) using a blue LED to excite green or yellow and red phosphors. Fig. 22 illustrates methods (b) and (c). The latter approach (c) has received increasing use following the development of ultra-bright blue GaN LEDs in the 1990s.

Fig. 22 White light generation using red, green or blue photoluminescent materials.⁴²⁹ Adapted with permission from J. Cho, J. H. Park, J. K. Kim and E. F. Schubert, Laser Photonics. Rev., 2017, 11(2), 1600147. Copyright John Wiley & Sons.

For general lighting the total power emitted, power efficiency and “whiteness” (colour temperature and rendering) are critical technical parameters. A 60 W incandescent tungsten light bulb, for example, currently has a luminous flux of 1000 lumins (lm) and a luminous efficiency of 17 lm W⁻¹. The colour rendering index (CRI) is a quantitative measure of a light source’s ability to reproduce the colours of objects against natural or reference colours. Perfect performance is 100. Fluorescent lights typically have values in the range 50–98. LEDs are typically around 80. Colour correlated temperature (CCT) quantifies the temperature of an ideal blackbody radiating light of a colour comparable to the light source. Daylight (“cool”) white has a CCT values above 5000 K, neutral white 3000–5000 K, whilst incandescent (“warm”) white corresponds to a CCT value in the range 2500–3000 K.

In 2014 the start-up company Lumisands Inc, USA, reported the first use of visibly photoluminescent Si to generate “warm” white LEDs by method (b), and combined the use with other green and blue phosphors.⁴³⁰ The Si NPs were derived from porous silicon. Surprisingly high PLQY values for the S-band were estimated under blue (405 nm) excitation, especially since the authors reported their particles had “micron-size silicon cores with nanosize Si QD attached”. One would expect significant scattering and re-absorption of red emission with this type of structure. Colour rendering close to incandescent bulbs with a CCT ∼2800 K and CRI ∼95 were reported, without a value for luminous efficiency.

Broad band white PL has been reported from a number of silicon-containing nanostructures: heavily oxidized porous silicon,⁴³¹ oxidized and carbon containing porous silicon,^432,433 carbon doped silicon-rich silicon oxide⁴³⁴ and silicon and carbon co-implanted silica films.⁴³⁵ Ghosh et al.⁴³⁶ have also used method (b) but with white-light emitting Si nanocrystals containing material derived from rice husks (see Section 5.4) whose broad PL (230 nm FWHM) was possibly a combination of the S-band and F-band. In this regard, of particular relevance is the study where laser ablation of organosilica also produced white-emitting nanoparticles.⁴³⁷ Both silicon and carbon nanoparticles were discovered by TEM and are jointly attributed with white light generation. The carbon dots were revealed after HF removal of the silica phase, as used by Chen et al.²⁸⁶ and discussed in Section 3.6.

A study with commercial blue LEDs and porous silicon NPs embedded in an overlying polymer film demonstrated the flexibility of colour tuning by varying Si NP concentrations.⁴³⁸ Luminous efficiency was again not reported. For maximizing brightness, thin down-converting films should have low refractive indices to maximize light extraction efficiencies.¹²⁸

The F-band has also been investigated for this application.^439–441 Solution synthesis of nanoparticles was adapted to achieve broad yellow-green emissions centred at 540 nm under 410 nm excitation from a blue LED.⁴³⁹ The 43.5% EQE of the blue LED was reported to be lowered to 8% for the resulting white LED. A luminance efficiency value of 13 lm W⁻¹ was provided together with CCT of 7982 K and a CRI of 82. In a second study by He’s group, luminescent nanorods achieved a similar luminance efficiency.⁴⁴⁰ In a third study, green emitting NPs with PLQY of 60%, derived from rice husks, were combined with blue and red emitting materials. Under 3 W UV LED excitation the composite film generated white light with a luminance efficiency of 75 lm W⁻¹. Photostability of the green-emitting material was monitored over 1 month (720 hours) and PLQY dropped by 12%.

However, high reliability and a very long operating lifetime (50 000–100 000 hours) is one of the key features of LEDs that have led to their remarkable uptake in recent years. Successful inorganic color-converting phosphors like YAG:Ce have ultrahigh chemical stability that can withstand continuous ∼100 mW mm⁻² radiance from a blue LED. Long-term photostability testing under intense blue/UV excitation is needed for Si nanostructures, as this might be a daunting challenge for this application area.

4.6. Nanothermometry

Measuring temperature with nanometer scale spatial resolution has potential widespread use in electronics, photonics and biomedicine but also a range of other niche areas.⁴⁴² There are diverse methods but using luminescent probes of temperature are one of the most attractive, as they are non-invasive, contactless and easy to use.⁴⁴³ Most PL parameters are dependent to some extent on temperature: intensity (efficiency), spectral position, spectral shape, bandwidth, polarization and decay time. Which PL parameter(s) and which applications have the most potential for Si nanostructures? This topic is only starting to receive attention so it is difficult to gauge. Suggested specific applications to date range from tribology⁴⁴⁶ to cryosurgery.⁴⁴⁴

Researchers at INSA, Lyon, were the first to realize the potential of S-band PL in this regard and they chose tribological studies of mechanical contacts as the target application area.^445–450 Functionalized 4–8 nm porous Si NPs with S-band emission were studied in alkanes and alkenes from 293–393 K. As expected, with increasing temperature, red shifts of PL peak position were universally observed but data analysis revealed that shift magnitude also depended on solvent viscosity. They attributed this to Brownian motion causing energy transfer between nanoparticles of different size.⁴⁵⁰ A consequence is that temperature sensitivity will have a complex dependence on liquid viscosity, NP concentration, NP size distribution and NP passivation. A significant advance was therefore shown by Ferreira and Kortshagen’s groups⁴⁵¹ who demonstrated that with better size control (2.0 ± 0.2 nm) of surface oxidized NPs, exciton migration effects were greatly reduced and the emission thermal dependence mainly follows the shrinkage of the bandgap. A single calibration then suffices for different media.

Use of the blue–green F-band in thermometry has also been reported.^448,452 The low temperature solution synthesis route⁴⁵² yielded very stable 480 nm emission with also very high 75% PLQY and monoexponential decay, but the PL origin is not established, and hence neither is the biodegradability of the luminescent entity (see Section 3.6). They chose to utilize decay times which linearly decreased from 23 ns at 0.5 °C to 8 ns at 60 °C, yielding a sensitivity of 0.24 ns °C⁻¹ with a thermal resolution under 1 °C. Interestingly, when Ryabchikov and co-workers⁴⁴⁸ compared the thermal sensitivity of Si NCs with carbon NPs (produced by anodization of Si and SiC wafers, respectively) they found spectral shifts to be 3–4 times higher for the Si nanomaterial and of opposite sign to those of carbon dots (−0.18 vs. +0.05 meV K⁻¹, respectively). Future comparative studies of the temperature responses of high PLQY F-band emissions from Si-based formulations and those of established “carbon dots” may thus provide further insight into the origin of the former.

The quantitative thermal sensitivity values reported to date for both S and F-bands are shown in Table 12. It is clear that, at least for the S-band for biomedical uses, the standard figures of merit are currently significantly lower than the highest values (1–5%/K) of other material systems.⁴⁴² Nonetheless, with further improvements of surface passivation, size control and PLQY, the thermal sensitivity should improve, and this combined with having a thermometer system of tunable biodegradability, makes this an interesting area for further development.

Table 12 Thermal sensitivities of S and F-band PL for potential nanothermometry applications

PL band and peak emission (nm)	PL parameter	Temperature range (K)	Thermal sensitivity	References
S (1.65 eV)	Decay time, peak position, intensity	293–393, 293–373	0.1–0.3 μs K^−1 (<0.75%/K), 0.18 meV K^−1, −0.8%/K	(Ryabchikov 2012, 2013)^445–447
F (480)	Decay time	273–333	−0.24 ns K^−1	(Li 2013)^452
F (2.35 eV)	Peak position, bandwidth	293–393	+0.97 meV K^−1, +1.2 meV K^−1	(Ryabchikov 2014)^448
S (630)	Peak position	303–383	−0.9 meV K^−1	(Hajjaji 2014)^449
S (1.7 eV)	Peak position	303–390	<0.052%/K	(Hamza 2015)^450
S (650)	Peak position	13–480	−0.04%/K	(Botas 2016)^451

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5. Summary and future perspectives

5.1. PL efficiency and passivation (stability)

Progress in improving PLQY of both the S-band (Table 7) and F-band (Table 8) over the last 30 years has been spectacular. Photostability of the S-band from derivatized silicon quantum dots is now competitive against many organic dyes with regards to biotagging. For applications like LED’s and photovoltaics where very long term stability is essential, further improvements in passivation seem mandatory. How do the effectiveness of surface passivation chemistries developed by the PV community for bulk Si compare with those used on silicon nanostructures? There are only a few studies that have estimated SRV values for 1 and 2D confinement in relatively large nanostructures that hence showed no S-band emission, as shown in Table 13.

Table 13 Surface passivation (SRV values) for large silicon nanolayers and nanowires

Silicon nanostructure	Surface chemistry	Technique	SRV (cm s^−1)	References
2D Si nanolayer (90–200 nm)	Hydride	1% HF treatment	22.5 → 399 (10 K) air storage	(Coustel 2011)^453
2D Si nanolayer (90–200 nm)	Alkyl groups	Hydrosilylation	62 → 209 (10 K) air storage	(Coustel 2011)^453
Si nanowire (114, 224 nm diameter)	Thermal oxide (7–21 nm thick)	Diluted oxygen and forming gas at 960 °C	20	(Demichel 2010)^454
Si nanowire (25–125 nm diameter)	Thermal oxide (5–10 nm thick)	Diluted oxygen at 1000 °C	10 000–30 000	(Grumstrup 2014)^455
Si nanowire (50–90 nm diameter)	a-Si (10 nm thick)	VLS deposition	4500	(Dan 2011)^456

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As for bulk silicon, hydride and organic monolayer passivation is metastable with SRV values rising with air storage. SRV values for nanowires can be remarkably high, reflecting perhaps challenges in avoiding defects along elongated nanostructures with highly curved interfaces.

5.2. Size control and wavelength tunability

If properties are size-dependent then size control and also metrology of nano-size distribution becomes critical.^457,458 Control over size can be strived for during synthesis, or after synthesis, or a combination of the two. Some of the narrowest size distributions for as-synthesized silicon nanocrystals have been demonstrated with superlattice manipulation,⁴⁵⁹ gas phase laser ablation,⁴⁶⁰ plasma synthesis⁴⁶¹ and micro-emulsion techniques.^462,463 Some data are shown in Table 14. Various methods have also been applied to narrow size distributions of already fabricated luminescent silicon nanostructures (see Table 15). These include gas phase selection techniques,^{460–462,465,466} size selective precipitation,⁴⁶³ size exclusion chromatography¹⁹⁹ and density gradient ultracentrifugation.^196,468,469

Table 14 Size control via synthesis techniques

References	(Tilley 2005)^462, (Rosso-Vassic 2008)^463	(Patrone 2000)^460	(Tan 2011)^265	(Zacharias 2002^171)	(Kothermann 2019)^458	(Gresback 2011)^461
Method	Reverse micelles	Laser ablation (gas phase)	Laser ablation (liquid phase)	Superlattice (reactive evaporation)	Superlattice (PECVD)	Plasma synthesis
Mean nanocrystal size (nm)	1.6–1.8	1.8	2.37	3.4	3.7	5.0
Size variation (SD nm)	0.2	0.2	0.56	0.5	0.7	1.0
PL emission band	F	S	F	S	S	S

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Table 15 Size control via post-synthesis techniques

References	(Littau 1993)^199	(Camata 1996)^464	(Mastronardi 2011)^467	(Miller 2012)^468	(Liu 2016)^208
Method	Size exclusion chromatography	Gas phase differential mobility	Size selective precipitation	Density gradient ultracentrifugation	Density gradient ultracentrifugation
Mean size (nm)	7.5	2.8	1.61	3.60	2.81
Size variation (SD nm)	2.5	0.6	0.26	0.35	0.25
PL emission band	S	—	S	S	S

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The S-band has been tuned over a wide spectral range but currently very high PLQY (>50%) is restricted to the 700–1000 nm red-near infrared region (see Table 7). The F-band has been tuned over a complementary spectral region with very high PLQY (>50%) in the 440–600 nm blue to yellow region (see Table 8).

5.3. Surface chemistry toolbox

The well-developed surface chemistry of bulk silicon⁴⁷⁰ was the platform from which many functionalization routes have been successfully applied to luminescent silicon nanostructures.⁴⁷¹ The availability of metastable hydride surfaces has been both a help and a hindrance in this regard: it has facilitated the grafting of a wide range of organic capping ligands but 100% substitution seems virtually impossible due to steric hindrance issues. The resulting derivatized surfaces then have a mixed chemical nature with the desired Si–C bonding plus residual Si–H bonding. Oxidation is suppressed but not eliminated. Complex surface chemistries are more exploitable in ex vivo uses like sensing than in vivo imaging where clinical translation is needed for success. For applications where long term PL stability is of paramount importance, attention should focus on inorganic coatings that have a proven track record with regards to surface passivation of silicon (cf.Table 1). The challenge is to find scalable techniques that realize full surface coverage of nanostructures with low SRV values (cf.Table 13).

5.4. Scalability and cost of luminescent silicon

Nanostructures are typically more much expensive than bulk materials and more difficult to manufacture at scale. Table 16 summarizes some reported throughputs for silicon nanocrystals. It is evident that the majority of these for luminescent nanocrystals lie in the range 0.1–10 g h⁻¹ (i.e. less than 0.01 kg h⁻¹). Whilst pilot plant production rates of about 1 kg h⁻¹ have been reported, the nanocrystals were not small enough to be photoluminescent.

Table 16 Throughputs of various techniques for fabricating Si nanocrystals. As in Table 4, question marks signify considerable uncertainty regarding origin of F-band PL

Silicon nanostructure	Luminescent?	Fabrication technique	Stated throughput (g h^−1)	References
Nanocrystals, <5 nm	Yes (S-band)	Silane pyrolysis and stain etching	0.02–0.2	(Li 2003)^472
Nanocrystals, 4–9 nm	Yes (S-band)	Silane reduction	0.1–1	(Lacour 2007)^473
Nanocrystals, 100–500 nm	No	Silane reduction in pilot plant	750	(Hulser 2011)^474
Nanocrystals, (?) 2 nm	Yes (F-band)	Microwave assisted chemical synthesis	0.6	(Zhong 2013)^245
Nanocrystals, 2–10 and 50–120 nm	Yes (S-band)	Plasma decomposition of silane	6	(Dogan 2013)^475
Nanocrystals, (?) 4–9 nm	Yes (F-band)	Triethoxysilane reduction	4–12	(Pujari 2018)^476
Nanocrystals, 10–40 nm	No	Silane reduction	22–200	(Kunze 2019)^477

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Exciting progress has been made with silicon nanostructure synthesis by bottom-up high temperature techniques with molecular precursors and conversion synthesis with biogenic silica structures. Continuous belt-furnace production using “silicon accumulator plant” nanostructures⁴⁷⁸ is one very scalable technique for producing kilogrammes of biodegradable porous silicon. However, no efficient S-band emission has been reported to date from biogenic porous silicon. Where luminescent nanostructures have been achieved from plant material it has primarily been restricted to F-band emission^441,479,480 and thus might be from carbonaceous material rather than any silicon nanoparticles also present. A significant goal is thus to achieve tunable S-band output of high PLQY from, for example, bamboo-derived silicon nanostructures. This would open a suite of new high volume and low cost uses (see next section). Dasog’s group have recently demonstrated that high S-band PLQY (∼50%) can be achieved via magnesiothermic reduction techniques.⁴⁸¹ Synthetic sol–gel silica precursors were used. If this can be replicated with inexpensive biogenic silica feedstocks then the cost of visibly luminescent silicon should be significantly lowered in the future.

5.5. Novel applications

If the cost and scalability of luminescent nanosilicon can be lowered substantially then a range of further applications could emerge as commercially viable. In most cases, ultrahigh PLQY and much lower cost would need to be supplemented with another key attribute for that application. I mention a few examples here from the diverse fields of dentistry, cosmetics, environmental remediation, forensics and security:

• Sunlight-driven photocatalyst (e.g. adsorbed toxicant degradation).

• Anti-ageing sunscreens (blue emitters and UV blockers).

• Dental “optical brightener” (tooth whitening formulations use blue emitters).

• Cosmetic skin hue converter (green-emitters used to mask red blemishes).

• Luminescent inks for banknotes (security features).

• Contrast agent in forensics (e.g. luminescent dusting powder).

6. Concluding comments

Remarkable progress has been made in raising the efficiency of visible emission from silicon-based quantum dots, superlattices and porous silicon over the last 30 years, with efficiencies of S-band red emission around 60–70% and green to blue F-band emission around 80–90% being the highest reported values. Surface chemistry plays a pivotal role in determining the efficiency and stability of emission. The widely tunable S-band arises from phonon-assisted exciton recombination within the silicon nanostructure. Emission in the F-band spectral range can have more than one origin, but carbon-based nanostructures and molecular chromophores are looking like the most likely origin of very efficient output. Some low temperature synthesis routes need to be revisited in this regard with more attention given to luminescent nanoparticle characterization and purification. Given the abundance of silica, low toxicity and biodegradability of luminescent silicon nanostructures compared to many other luminescent material nanosystems, their further development for a myriad of applications looks inevitable.

Conflicts of interest

There are no conflicts of interest to declare.

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