Photoluminescence from colloidal silicon nanoparticles: significant effect of surface

Qi Li; Rongchao Jin

doi:10.1515/ntrev-2017-0145

Article Open Access

Photoluminescence from colloidal silicon nanoparticles: significant effect of surface

Qi Li and Rongchao Jin

Published/Copyright: September 5, 2017

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From the journal Nanotechnology Reviews Volume 6 Issue 6

Abstract

Silicon nanoparticles (NPs) have long been regarded as a promising alternative for commercial organic dyes and typical quantum dots (e.g. CdSe) for applications in light emitting, bio-imaging, sensing, etc. The photoluminescence (PL) of Si NPs, since the first observation in the early 1990s, now has shown wide tunability in the PL wavelength ranging from UV to near IR and lifetime from less than nanoseconds to hundreds of microseconds. Meanwhile, the synthetic advances and methods of size separation and surface modification have improved the quantum yield of Si NPs up to 90% and the PL bandwidth down to ~30 nm FWHM. On the other hand, despite more than 20 years of research, it remains controversial in terms of the exact origin and mechanism of the PL from Si NPs. In this review, we intend to relate the structure of Si NPs with their optical properties in the hope of revealing some general, underlying laws of the size/surface-property relationships based on the reported research. Especially, we highlight the latest research progress on the complex influences of surface chemistry, such as the surface nitrogen capping and surface oxidation, which can dramatically alter and enhance the PL properties of Si NPs.

Keywords: quantum confinement; quantum dots; silicon nanoparticles; surface chemistry

1 Introduction

Silicon (Si) is the dominant material in microelectronics industry, and it can be safely said that we are now living in a technological world made possible by Si. However, compared to its widespread use in electronics and photovoltaics, Si has quite limited optical applications as bulk Si is a poor light emitter at room temperature. This weakness arises from the intrinsic indirect bandgap, that is, the lowest point of the conduction band and the highest point in the valence band are located at different wavevectors in the reciprocal space, causing the bandgap optical transition dipole forbidden in the bulk form. It, thus, has long been a dream for researchers in the scientific community to gain light from Si. There is no doubt that Si-based highly efficient light-emitting or amplifying materials will be of great significance for both fundamental scientific research and real-world daily applications. Since Canham’s [1] discovery in the 1990s that Si is capable of emitting light at room temperature when made nanoporous, research on light-emitting Si nanomaterials has since been initiated and progressed significantly over the decades, moving from the initial porous Si to the recent free-standing colloidal Si nanoparticles (NPs) or the so-called Si quantum dots (QDs) [2], [3], [4], [5]. Especially for the free-standing colloidal Si NPs, since being synthesized for the first time, it has been treated as a potentially promising alternative for typical cadmium-based quantum dots and commercial organic dyes in various applications such as light-emitting [6], [7], [8], [9], [10] and bioimaging [11], [12], [13], [14] owing to Si’s intrinsic merits of the least toxicity, low cost, and high abundance [2], [5], [15], [16], [17], [18]. Recently, Si NPs have also been explored as active materials in sensors [19], solar cells [20], [21], photodetectors [22], and diodes [23], as well as catalysts [24], [25].

Some terminology needs to be explained first. Three terms are most used in this area, that is, Si nanoparticles (NPs), Si nanocrystals (NCs), and Si quantum dots (QDs). The scope of Si NPs is the broadest, and particles with dimensions in the nanometer regime (1–100 nm) can be named as “nanoparticles”. On the contrary, the term “Si QDs” has the narrowest scope; only those with dimensions smaller than ~10 nm (the Bohr radius of the bulk Si exciton: ~4.9 nm) are considered to be QDs, and the observed spectrally tunable emission should follow the quantum confinement rule (E_g~1/R², where R is the radius). The term “Si NCs” is named for those NPs with high crystallinity. Nevertheless, under a certain size limit, the concept of crystallinity becomes looser due to an insufficient number of atoms and lack of long-range order. Generally, the prominent peaks of crystalline Si in the X-ray powder diffraction pattern are accepted as a general standard to determine the use of the “nanocrystals” term [26]. It should also be noted that “Si QDs” does not need to be “Si NCs” as amorphous Si NPs can still show significant quantum confinement effects [27].

In the past 20 years, researchers have made great efforts in controlling the size, surface, shape and crystallinity of Si NPs to tailor their optical properties and enhance the performance of devices based on Si NPs. In the first decade, research on nanosilicon was mainly focused on the porous Si, which is now known as Si NPs embedded in the SiO₂ matrix [28], [29]. Since the new century, research attentions are turned to the free-standing colloidal Si NPs due to the latter’s wider application prospects. Until now, different methods for preparing Si NPs have been reported, and the most commonly used methods are, for example, thermal disproportionation of Si-rich oxides [30], [31] and decomposition of silanes using plasma [32], [33] or heat [34], [35]. The thermal disproportionation of Si-rich oxides first creates Si NCs embedded in an oxide host, and the produced Si NCs were then liberated from the silica host by HF etching [30]. These two methods are able to produce Si NPs with high crystallinity, tunable size and relatively “clean” H-Si or C-Si surface. The optical properties of C/H-terminated Si NPs made by these methods are well consistent with the quantum confinement model, which shows a size-dependent PL bandgap with a microsecond PL lifetime. Other commonly used methods to synthesize Si NPs in solution are, for example, reduction of Si halides [26], [36], [37] and oxidation of metal silicides [38]. However, the PL of Si NPs fabricated from solution chemistry methods usually lies in the blue to green region with much shorter PL lifetime (~nanoseconds); even the NPs are also stated as C/H-terminated [39], [40], [41], [42]. Figure 1 is a summary of the syntheses of Si NPs; other synthesis methods, as well as their advantages and disadvantages, have recently been reviewed elsewhere and thus will not be discussed here [5], [16].

Figure 1:

Scheme of the common synthesis methods for colloidal Si NPs. Reproduced and adapted with permission from John Wiley and Sons [15].

As Si is scaled down to dimensions less than ~10 nm, the quantum confinement and surface effects both become significant, which exert strong influences on the optical properties of Si NPs. The quantum-confinement effect expands the original bandgap of Si (E_g~1.1 eV) to the visible spectrum along with the shrinking of the dimensions of Si NPs; meanwhile, the indirect nature of the bandgap undergoes a transformation, which renders the relaxation of excited electrons through this band possible. On the other hand, researchers gradually realize the strong effects of surface on the Si NPs’ optical properties from two different phenomena. First, different research groups reported that the PL of Si NPs gradually changed or diminished during the storage, and this was later demonstrated to arise from slow oxidation by air [43], [44], [45]. Second, in early years of research, a major question was the controversy regarding the association of optical spectroscopic results (in particular PL) with the size of Si NPs, which is an important signature of quantum confinement [46], [47]. However, for those Si NPs made from solution chemistry methods, their PL peaks always lie in the blue to green region with much shorter PL lifetime (~nanoseconds). In recent years, researchers have noticed that the surface effects may be the origin of this discrepancy.

In this review, we focus on different kinds of PL from Si NPs. We first briefly summarize the PL of those Si NCs from high-temperature aerosol methods, which are well-recognized as arising from quantum-confinement effect. In the second part, we will discuss the recent surface chemical modification on these aerosol-made Si NPs, highlighting how the surface modification can alter or even totally change the PL properties of Si NPs. In the third part, the effect of surface oxidation will be discussed, and finally, the “fast” PL of Si NPs synthesized from solution chemistry methods will be reviewed. In addition, it should also be noted that besides controlling the size and surface, doping is another effective method to tailor the PL of Si NPs. Exemplary work on the doping of Si NPs has been reported by Fujii et al. [48], [49], Kortshagen et al. [50] and Pi et al. [51] groups. Such doping research is not covered in our review.

2 PL of Si NPs

2.1 Quantum-confinement effect

As aforementioned, bulk Si has an indirect band structure with a 1.1-eV bandgap, and the different positions of valence band maximum and conduction band minimum in k-space result in a low probability of bandgap radiative transition. As the dimension is scaled down to below 10 nm, quantum confinement relaxes the selection rules for momentum conservation, providing the band edge optical transition a further enhanced cross section, which is sufficient to render Si NPs strongly emissive. Based on the quantum-confinement model, the widening of the bandgap occurs, and the smaller the size, the larger bandgap the nanoparticle should have. Thus, the PL peak wavelength of Si NPs is expected to monotonically blue-shift from ~1000 nm to ~600 nm as the size of Si NPs decreases from ~10 nm to <2 nm (Figure 2A) [54], [55], which is recently demonstrated experimentally by different groups [31], [56]. Especially, Ozin’s group recently utilized density gradient ultracentrifugation [57], which successfully prepared monodisperse Si NCs with a very narrow size distribution, and the size-dependent PL bandgap law was further confirmed. The indirect nature of the band structure remains in the Si NPs; sharp exciton peaks will not appear in the absorption spectra (Figure 2B), and the recombination rate is lower than those direct-band quantum dots, which gives the PL lifetime of Si NPs in the range of hundreds of nanoseconds to microseconds (Figure 2C). Quantum yields of the PL based on quantum confinement effects were reported to reach as high as ~60% in the near-infrared regime for those larger sizes around 4–5 nm, but it undergoes a monotonic decrease along with the further reduction in size of Si NPs and the blue-shift of PL to the visible regime (Figure 2D). This is due to the increasing contribution of non-radiative recombination such as surface defects and ligand vibrations in the smaller size regime [52], [53], [56]. In Si QDs with PL arising from the core-based quantum-confinement effect, the surface of the Si QDs must be engineered to avoid the formation of defects such as dangling bonds at the QD surface and to build an energy barrier that can effectively prevent carriers in the Si QDs from tunneling out, which is critical to the optimization of the PL QY [58]. In addition, the PL excitation peak is around the T-T direct high-energy range, which results in a large Stokes shift to 800 nm (Figure 2A). It should be noted that the indirect nature of the bandgap retained in the quantum-confined form is also extremely important for the application of Si NPs to photonic technologies relying on suppressed reabsorption by the emitters. For example, Meinardi et al. [59] recently demonstrate that Si NPs could be the best candidates, so far, for efficient luminescent solar concentration due to the strong above-gap absorption and zero band-edge re-absorption.

Figure 2:

Optical properties of Si NPs based on quantum-confinement effect. (A) Room temperature PL (λ_exc=420 nm) and PLE (measured at emission maximum) spectroscopy of Si nanocrystals with different sizes. Reprinted with permission from American Chemical Society [31]. (B) Molar absorption coefficients (ε) of Si nanocrystals with the indicated diameters. Reprinted with permission from the American Chemical Society [31]. (C) Typical fits used to extract PL lifetime for a sample and PL lifetime as a function of peak emission and nanocrystal size. Reprinted with permission from the American Chemical Society [52]. (D) Size-dependent photoluminescence absolute quantum yields of Si NPs with decreasing sizes. Reprinted with permission from the American Chemical Society [53].

As we mentioned before, C/H-terminated Si NPs synthesized from pyrolysis of silanes and thermal decomposition methods of Si-rich oxides are well consistent with the quantum confinement model [31], [52]. The H-terminated Si NPs can be directly obtained from pyrolysis of silanes or liberated from the silica host by etching with HF [30]. To obtain the surface Si-C bonds, hydrosilylation is the most common method by addition of Si-H bonds across multiple C-C bonds. The hydrosilylation reactions can be initiated by exposure to heat [60], light [61], radical initiators [62], transition-metal catalysts [63], Lewis acids [64], plasmons [65] or borane catalysts [66]. In most cases, C-terminated surfaces show minor influences on the optical properties of these Si NPs, and the PLs are still coming from intrinsic bandgap emission. Very recently, the Rieger group reported that the PL of phenylacetylene-functionalized Si NPs red-shifts relative to hexyl- and phenyl-capped counterparts and an in-gap state near the conduction band edge, which can account for the PL shift via relaxation across this state [67].

2.2 Chemical surface modification

Generally speaking, the smaller the size of the Si NPs, the more profound the influence that the surface will play. In the case of PL, the influence of the surface declares itself by inducing surface states in the bandgap. How the surface states behave and how much it will influence the optical properties are determined by the chemical nature of the surface motifs. Different from the H- and C-termination, which brings minimal interference on the Si NPs’ optical relaxation process, surface passivation by oxygen and nitrogen will lead to strong surface states, which may affect or even be decisive in the recombination process of excitons. It should be noted that the influence of surface on the optical properties is complex, and a clear distinction between the core and surface (and possibly an interface between these two) sometimes cannot be easily made. In some instances, the NP core and its surface both contribute to the PL process. In this section, we focus on the state-of-art surface chemical modifications, which have been demonstrated with the ability to significantly alter the PL of Si NPs.

In 2013, Dasog et al. [47] demonstrated that surface treatment with nitrogen-containing molecules would switch the Si NPs’ PL from the “slow red” quantum-confinement state to the “fast blue” charge-transfer surface state. The surface passivation resulting from this reaction was incomplete, and significant oxidation was also observed [47]. In 2014, the same group reported that H-terminated Si-NPs can react with CO₂, producing an acetal surface, and the Si NPs after modification show blue-green PLs with excitation wavelength dependence, which is regarded as arising from multiple surface states [68]. Besides, phosphine oxide-stabilized Si NPs in organic solvents can also be obtained after H-terminated Si NPs reacting with phosphine oxides in air. The optical properties of these phosphine-oxide-stabilized, oxide-coated Si NCs show the dual effects of bandgap and defect states, both of which play pronounced roles in determining the emission maxima [68]. In addition, halogen-terminated Si NPs was also obtained from H-terminated Si NPs. For example, In 2015, Veinot’s group reported that for the Si NPs, chlorination by PCl₅ can proceed at room temperature even without a radical initiator, and similarly, bromination and iodination can also be achieved upon the reaction of H-terminated Si NPs with elemental bromine and iodine [69]. Halide-terminated Si NPs did not exhibit detectable PL in the visible region under UV illumination. However, partial oxidation by air induces blue PL from chloride-terminated Si NPs as well as yellow-orange PL for both the bromide- and iodide-terminated systems [69]. The authors [69] attribute the blue emission to be from the oxychloride defects, while the yellow-orange PL likely originates from the oxide defects. Alkylation was then performed using alkylmagnesium halide reagents [69]. They further demonstrated that the defects play an important role in the optical properties of Si NPs [69]. The oxychloride impurities resulted in blue PL in Si NPs irrespective of NC size, whereas bromide-derived alkyl Si NPs yielded size-dependent PL properties. The PL from alkyl Si NPs obtained from the NPs with an iodide surface was dominated by the oxide defects and exhibited orange-yellow color under UV illumination [69]. The aforementioned surface modification pathways and their resulted PLs are summarized in Figure 3. In addition, several theoretical studies have also been conducted to explain the influence of the different kinds of surface modification on the PL of Si NPs [70], [71], [72].

Figure 3:

Tailoring the PL of Si NPs by chemical surface modification. (A) Switching Si NPs’ PL from the “slow red” quantum-confinement state to the “fast blue” charge-transfer surface state by treatment with nitrogen-containing molecules and oxygen. Reproduced and adapted with permission from the American Chemical Society [47]. (B) Reactivity of H-terminated Si-NPs with different organic moieties and the resulted colorful PL. Reproduced and adapted with permission from the American Chemical Society [68]. (C) Schematic representation of halogenation and alkylation of hydride terminated Si-NPs and PL spectra of alkylated Si-NPs derived from halogenated surfaces. Reproduced and adapted with permission from the American Chemical Society [69].

Recently, Si NPs with surfaces modified by N-containing molecules have been experimentally obtained and theoretically studied by different groups [47], [68], [73], [74], [75], [76], [77], [78]. The PL energies of surface nitrogen-capped Si NPs from our own group’s work and other recent studies are shown in Figure 4. It is very interesting that PL peak energies of surface nitrogen-capped Si NPs from different fabrication methods, though of different sizes, different degrees of crystallinity, different amounts of ligands on the surface, and even different degrees of oxidation, can still be fit into a general ligand structure-based law, that is, arylamine shows a significant red-shift effect on PL, and more delocalized/conjugated groups on the ligands lead to further bandgap narrowing, while the PLs of allylamine-capped Si NPs remain in the blue range [75]. In addition, all the surface nitrogen-capped Si NPs show a short lifetime in the nanosecond range, except the work of He’s group, in which the PL lifetime was not reported [76]. Thus, it can now be reasonably deduced that this kind of PL with nanosecond lifetime should arise from the surface nitrogen sites, and the PL energy of Si NPs is determined by the structure of surface ligands (Figure 4B). This type of surface PL is totally different from the quantum-confinement state in conventional Si QDs, which arises from the indirect X−L transition and shows a size-dependent bandgap and microsecond lifetime. It should be noted that Si NPs with the aminated surfaces are able to exhibit very high quantum yields (up to 90%) with a narrow PL bandwidth [full width at half maximum (FWHM)=40 nm] [75].

Figure 4:

PL mechanism of surface nitrogen-capped Si NPs. (A) Spectral tunability of surface nitrogen-capped Si NPs from different fabrication methods that follow the ligand structure law. (B) Scheme of the surface PL from surface nitrogen-capped Si NPs. (C) Scheme of the quantum-confinement PL from conventional Si QDs, as a contrast. Reprinted and adjusted with permission from the American Chemical Society [75].

2.3 Surface oxidation

Oxygen forms a variety of types of bonds on the Si NP surface, for example, double-bonded oxygen in the silanone bond (Si=O), bridging oxygen (Si-O-Si), and single-bonded oxygen in the silanol (Si-OH). Surface oxidation plays a profound, but very tricky and complex role on the PLs of Si NPs. For example, red-orange PL, frequently identified as the “S-band”, has been reported for surface-oxidized porous Si NCs embedded in matrix and has been attributed to quasi-direct transitions or surface states [79], [80], [81]. Under some circumstances, partially oxidized nanocrystalline Si shows yellow emission, which was suggested to arise from either surface Si=O species [29] (for which there is currently no known molecular equivalent) or affected by Si-O-C bonds [82]. Blue to cyan PLs are also reported to emerge from Si NPs after oxidation treatment [13], [44], [45], [47], [83], [84], [85]. It should be noted that as Si NPs are under oxidation and Si-O species are formed on the surface, the Si core will also be simultaneously etched to smaller size. Thus, the blue shift of PL in the process of oxidation can also be explained by shrinking of the core size [86]. On the other hand, in some instances, surface effects will not behave so predominantly that the dual quantum confinement and surface bands can be observed in the PL spectra at the same time [87]. Also, in some circumstances, the surface-oxidized Si NPs can still show size-dependent PL from red to blue with a broader spectral tunability [86], [88]. These samples were usually chemically treated in peroxide and were finally capped to a great degree by silanol. In addition, it has also been reported that the surface oxidation and other surface motifs (e.g. nitrogen-containing ligands) can play a synergistic role, forming new surface states on Si NPs [47], [73]. The aforementioned different PLs from surface-oxidized Si NPs are shown in Figure 5.

Figure 5:

Effect of surface oxidation on the PL of Si NPs. (A) Comparison of the optical properties of the Si-dioxide-embedded and organic-treated Si NPs. Reproduced and adapted with permission from the American Chemical Society [82]. (B) Photographs and spectra of PL from styrene-grafted Si NPs after different periods of time under 254 nm UV illumination. Reproduced and adapted with permission from the American Chemical Society [45]. (C) After ~3-nm H-Si QDs are oxidized in EtOH/H₂O₂, and the emission color of the oxidized Si QDs ranges from salmon pink to blue. Reproduced and adapted with permission from John Wiley and Sons [88].

Based on the above results, the influence of surface oxides on the Si NPs’ PL is very complex, which is related to several factors. For example, what is exactly the atomic structure of the oxides formed on the NP surface (Si=O, Si-O-Si, Si-OH or other unknown ones)? Is there any synergetic or completive effect with other surface motifs (such as ligands)? What about the effect of crystallinity of Si NPs? It should also be noted that a spectrally similar blue emission band can also be found in silica-based nanostructures, and in these cases [89], [90], [91], [92], the origin of PLs is typically assigned to the formation of oxygen-related defects [93], [94]. These results give an indication that for the surface-oxidized Si NPs, the photo excitation and emission processes do not necessarily require the participation of the Si(0) core, and the surface can fulfill the photoluminescence process by itself.

2.4 The “fast” PL of Si NPs synthesized from solution chemistry

As mentioned before, Si NPs from wet-chemical synthesis usually show a “fast” PL (i.e. with a nanosecond lifetime) in the blue to green region [38], [42], [82], [95], [96], though they are also stated to have a C- or H-terminated surface (Figure 6). In addition, the He group recently reported a kind of one-dimensional, biocompatible fluorescent Si nanorods (Si NRs) with tunable lengths ranging from ~100 to ~250 nm, which can be facilely prepared through one-pot microwave synthesis. The resultant Si NRs exhibit excitation wavelength-dependent blue to green maximum emission wavelength ranges from ~450 to ~600 nm under serial excitation wavelengths from 390 to 560 nm [4]. There are two explanations for this kind of “fast” PL. The Gregorkiewicz and Pelant groups recently performed theoretical modeling [95], [97] to explain this unordinary emission, and in their views, it is the carbon surface termination that gives rise to drastic modification in electron and hole wavefunctions. In their study, the theoretical analysis using both the TB model [95] and DFT [97] revealed the formation of a direct bandgap transition at the band edge, with the radiative recombination rate enhanced up to 10⁷⁻⁸ s⁻¹ [98]. The excitonic origin of the fast-decaying emission has also been confirmed by a detailed analysis of single quantum dot spectroscopy data [96] and further corroborated by oxidation experiments, where the blue-green emission with nanosecond decay from C-terminated Si QDs has been converted into the red/NIR emission with microsecond decay upon oxidation [95]. However, this theory cannot explain why more research reported the “slow red” PLs of C-terminated Si NPs from aerosol methods. On the contrary, Kauzlarich and other groups hold the view that this fast PL is from the surface or other traps, which are formed during the process of chemical synthesis [31], [47], [99].

Figure 6:

Typical PLs of Si NPs from wet-chemical synthesis. Reproduced and adapted with permission from the American Chemical Society [47].

In recent years, several groups reported that the solution-made Si NPs can show “fast” PLs extended into the red region. For example, Kusová et al. [82] reported that bright yellow PL at ~570 nm with a lifetime of ~2 ns can be obtained after the initial surface-oxidized Si NC [100] was modified by capping involving methyl groups in a xylene-based suspension via a photochemical reaction. In 2015, the He group reported that fluorescent small-sized (~3.8 nm) Si NPs can be produced through biomimetic synthesis, and the as-prepared Si NPs feature bright red fluorescence at ~620 nm with lifetimes in the range of 1–11 ns [101].

It should be noted that the current characterization techniques are limited and not precise enough for the surface of Si NPs due to the following major difficulties: (i) polydispersity and heterogeneity of nanoparticles, and (ii) excess ligands or surfactants, residual reactants, and side products in the colloids [102]. Therefore, Si NPs from different fabrication methods, though “similar looking” in the IR, NMR and XPS spectra, may actually have some minor differences on the surface, which are hard to “map out” but might dramatically alter the optical properties of Si NPs. Especially for the Si NPs fabricated through solution chemistry, a lot of chemical substances (for example, air, solvents, stabilizing molecules) can attach to the highly reactive surface of Si NPs during the synthesis [47].

3 Summary and the future research

There is no doubt the surface plays a more significant role on the optical properties of Si NPs compared with other typical direct-bandgap semiconductor nanocrystals. Although being very complex and still somewhat mysterious, the surface effect can offer more chances to enhance the PL property and help Si NPs overcome the intrinsic drawback of an indirect bandgap, for example, achieving ultrahigh PL quantum yields up to 90%. The surface effect offers Si NPs the spectrally broadest emission tunability accessible with semiconductor QDs and allows for control of the radiative rates over many orders of magnitude [98].

On the other hand, current study and understanding on the surface are still not precise enough and, until now, there is no technique that provides atomic-level construction of the Si NP capping layer [103]. Some differences in the surfaces of Si NPs synthesized from different methods are very hard to discern, resulting in the controversy that “similar looking” Si NPs may show totally different PL properties. To address this issue in future research, the homogeneity of the produced samples needs to be improved; ideally, the future goal is to make every Si NP with the same size, surface, and structure. After achieving Si NPs with atomic precision and molecular purity, growing single crystals of Si NPs may be realized, and their atomic structure may be achieved by X-ray crystallographic analysis. With that, it will be very exciting to pursue fundamental understanding of the properties of Si NPs at the atomic level and establish definitive structure-property relationships [102], [104]. It should be noted that the atomic precision and X-ray total structures have been achieved for those organosilicon clusters [105], [106], but in these works, the PL was not discussed. Encouraged by the recent success in atomically precise gold nanoparticles and the determinations of their atomic structures (core plus surface ligands) by X-ray crystallography [107], [108], we believe that the day will come soon.

Acknowledgment

R. J. thanks the financial support from the U.S. Air Force Office of Scientific Research under AFOSR Award No. FA9550-15-1-9999 (FA9550-15-1-0154).

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Received: 2017-5-18

Accepted: 2017-7-31

Published Online: 2017-9-5

Published in Print: 2017-11-27

This article is distributed under the terms of the Creative Commons Attribution Non-Commercial License, which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.

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https://doi.org/10.1515/ntrev-2017-0145

Keywords for this article

quantum confinement; quantum dots; silicon nanoparticles; surface chemistry

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