Silicon Photonics IV: Innovative Frontiers

Part IAdvances in Fundamental Research

© The Author(s), under exclusive license to Springer Nature Switzerland AG 2021

D. J. Lockwood, L. Pavesi (eds.)Silicon Photonics IVTopics in Applied Physics139https://doi.org/10.1007/978-3-030-68222-4_1

1. Optical Properties of Si Nanocrystals Enhanced by Ligands

Kateřina Dohnalová¹   and Kateřina Kůsová²  

(1)

Institute of Physics, University of Amsterdam, Science Park 904, 1098XH Amsterdam, The Netherlands

(2)

Institute of Physics, Czech Academy of Sciences, Cukrovarnická 10, 162 00 Prague 6, Czech Republic

Kateřina Dohnalová (Corresponding author)

Email: [email protected]

Kateřina Kůsová

Email: [email protected]

Abstract

Compared to bulk silicon, silicon nanocrystals (Si-NCs) show modified properties, such as tunable emission and enhanced radiative rate, as a result of the quantum confinement, surface chemistry and environment. While the effect of quantum confinement is well understood and experimentally confirmed on the hydrogen-capped Si-NCs, the surface effects in Si-NC with other types of ligands can be very complex and hard to predict. In our work, we argue that the surface chemistry, be it ligands and/or shell, can be designed to further improve the radiative rate of the Si-NCs, beyond what is achievable by the quantum confinement alone. Our experimental work shows a number of effects that indicate that in many instances, the core and surface capping cannot be separated, and optical properties cannot be clearly interpreted as extrinsic (related to the surface capping agent) or intrinsic (related to the core only). To this end, we performed also a detailed theoretical analysis of a number of surface ligands, to identify the role of chemistry and how that improves the optical properties of Si-NCs. Based on these investigations and findings, we realized two main things. Firstly, we argue that one cannot derive a simple rule to predict which type of element or molecule will improve or deteriorate the optical properties, because every individual element added (covalently) to the surface of Si-NC contributes to the electronic density via several mutually dependent effects, such as (i) orbital displacement, (ii) direct contribution of surface species into the density of states close to the bandgap, (iii) charge transfer due to the relative polarity of the surface capping element and Si, or (iv) ligand/matrix induced strain. Secondly, we realized that the $$\textbf{k}$$ -space projections of the molecular orbitals, i.e., the band structure of the nanocrystal, are an essential and critical tool for investigations of the electronic and optical properties in materials with an originally indirect bandgap, since the surface chemistry in our simulations affects strongly the whole band structure.

1.1 Introduction

Silicon is a cornerstone of the modern civilization. Thanks to all its superlatives, such as abundant resources, non-toxicity, bio-compatibility and biodegradability, chemical robustness, low cost production, naturally forming oxide, and many more advantageous properties, it is in fact desirable material for any application. Silicon makes up about 28% of the Earth’s crust by mass. Bulk silicon dominates CMOS micro-electronics technologies and enabled digital technologies through transistors to the computing central processing unit (CPU). It also plays an important role in photovoltaics and the detector industry, despite its poor band-edge absorption, for which it compensates by a higher material thickness. Silicon is currently entering also battery applications, for its enormous capacity for Li ion intake [27]. Silicon was reported to be, even in its nanocrystalline form, non-toxic [3, 9, 56, 125] and bio-degradable [154], with superior photo- and pH-stability [82], which opens opportunities also in the traditionally high health risk areas such as cosmetics, agriculture or medicine, for example, as theranostic agents [170]. However, for optical applications in lighting, displays, lasers, and amplifiers, as well as thin film photovoltaics, bulk silicon is not best suited due to its indirect bandgap (Fig. 1.1a). Radiative recombinations of electrons and holes, as well as optical excitations of electrons across the bandgap, require the participation of phonons, which lowers the probability of such transitions. The resulting weak oscillator strength of the optical transitions leads to a low radiative rate and a slow absorption onset with a weak absorption at the band-edge.

Nevertheless, finding/designing a silicon form that can efficiently emit light is obviously highly desirable. A silicon light source would enable the realization of the long awaited on-chip-integrated silicon laser and hence also all-Si photonics (the main topic of this book series), and consequently also optical CPU architecture [1]. Moreover, such a light source could also be implemented in the on-chip integration of the light emitting diodes (LEDs) [146], desired for the light-weight and modular micro-LED displays and lighting. Efficient band-edge absorption would be beneficial for silicon-based thin solar cells, which would also enable a wide spread of solar energy for transportation and other areas where besides efficiency also portability or light-weightiness are essential. Also, because of its non-toxicity and bio-compatibility, silicon can play an essential role in the bio-imaging [56] and bio-integration (with human body) of optically driven micro-devices, sensors, and interfaces [101].

For the last two decades, we have been searching experimentally and theoretically for paths toward enhanced optical capabilities of silicon, especially the wavelength tunability and radiative rate, via combining the quantum confinement and surface engineering in ligand-capped silicon nanocrystals (Si-NCs) (We note that for simplicity, we use the Si-NCs term also for materials, where the crystallinity of the nanoparticle has not been proven or measured and should be better named silicon nanoparticles). Properties induced by the surface chemistry are measured experimentally on both single nanoparticle and ensemble levels. Such analysis is combined with theoretical simulations via semi-empirical tight binding and from first principles by the use of self-consistent ground state density functional theory (DFT).

Fig. 1.1

Quantum confinement effects in Si-NCs. a Bulk silicon band structure in the $$\Gamma $$ –X direction, critical for the optical properties, where the bandgap can be identified. Bottom panel shows a silicon unit cell with the diamond crystalline structure. b $$\textbf{k}$$ -space resolved density of states (DOS) for the H-capped Si-NCs of diameter from 1.3 to 3.2 nm, simulated by the DFT using cp2k code (full settings are discussed in [44]). Bottom panel shows the relative sizes and shapes of the simulated Si-NCs. c Fuzzy band structures of 2.3 nm Si-NC capped 50% by hydrogen and 50% by a butyl ligand –C $$_4$$ H $$_9$$ (left), partly capped with –H and partly oxidized with –OH and -O- bonds (middle) and fully –Br capped surface (right). Below are shown real-space 2D cross sections of the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) wavefunctions. The color scheme in (b, c) represents the DOS on a log-scale. Bandgap energy and phonon-less thermalized (T = 300 K) radiative rates are given in (b, c) in each respective band structure plot. d–g Comparison of photoluminescence (PL) tunability, experimentally reported by various sources: d Fully H-capped Si-NCs [45, 65, 211]. The gray data points [180] are possibly influenced by strain and are therefore not included in the fit. e Alkyl-capped Si-NCs [68, 79, 85, 96, 145, 162, 175, 207, 214]. f Oxidized Si-NCs [116]. Solid symbols in (f) denote Si-NCs prepared as free-standing (e.g., by wet etching or plasma synthesis followed by slow oxidation). Open symbols in (f) stand for matrix-embedded Si-NCs (prepared, e.g., as SiO $$_x$$ /Si superlattices or by ion implantation or samples involving thermal oxidation). The fits are meant only as a description of the data and are

for the free-standing and

for the matrix-embedded samples. Data from [116] are reused with permission from AIP Publishing. Panels (d–f) contain also fits of the data and the corresponding 95% confidence bands. g Comparison of fit curves from panels (d–f)

1.1.1 Quantum Confinement

The most reliable route toward an enhanced oscillator strength, and hence also the enhanced radiative rate and absorption cross section, has been so far via the utilization of the quantum confinement in Si-NCs. Bright size-tunable emission from the Si-NCs was first reported in 1990 in a pioneering work of Canham et al. [19] on H- and oxidized porous silicon, followed by many more reports from differently prepared and capped Si nanostructures in the following three decades [8, 26, 29, 33, 43, 67, 78, 107, 140, 143, 144, 150, 179, 185].

Quantum confinement in Si-NCs has a considerable effect on their electronic structure, but only for radii comparable or smaller than the bulk silicon’s excitonic Bohr radius of $$\sim $$ 4.9 nm. In such a case, strong spatial confinement of carriers results in a shift of valence and conduction bands, leading to a bandgap opening, where the bandgap energy increases with the decreasing Si-NC size (Fig. 1.1b, d–g). According to the simplest effective mass approximation (EMA) model, the optical bandgap $$E_g$$ scales with the NC diameter d via an inverse parabolic dependence

as a result of the quantum localization. We also need to add the Coulomb term, which scales with $$\propto d^{-1}$$ and small polarization terms [14]. In reality, experimentally investigated Si-NC samples show slightly different values of the exponent, reported in the literature between 1.3 and 2.0 [122, 214]. The bandgap energy determines the photoluminescence (PL) peak from the Si-NCs, which can be tuned in a wide spectral range, from ultraviolet (UV) at $$\sim $$ 260 nm to near infrared (NIR) at 1100 nm [43, 179] (Fig. 1.1d–g). This broad spectral tunability has been experimentally and theoretically proven in the hydrogen-capped Si-NCs (Fig. 1.1b, d, g) [43]. Despite being an ideal model of an Si-NC for theoretical calculations, H-capped Si-NCs are only rarely studied experimentally due to their increased sensitivity to air and UV light, leading almost immediately to at least partial oxidation, as discussed in more detail below. This is why only a few reports are included in the collection of literature data plotting the experimentally reported PL maxima as a function of NC size, which yields an exponent of 1.33, but shows a low spread of data points around the fitted curve. In Si-NCs with other than H-capping, often a much narrower tunability range is observed [37, 46, 76, 124, 172, 207] due to either a presence of surface sites that act as a traps for the excited carriers [43, 211], or due to the presence of strain [116], or simply as a result of limitations of the preparation procedure. Moreover, chemically different surface ligands were themselves experimentally [22] and theoretically [215] shown to cause spectral shifts.

The effects of the quantum confinement on the bandgap energy is common to all types of semiconductor NCs. In an indirect bandgap semiconductor, such as silicon, additional effects occur due to the relaxation of the $$\textbf{k}$$ -selection rule as a consequence of the larger spatial confinement of carriers. In silicon NCs, this in turn enhances the radiative rate $$k_{\mathrm{rad}}$$ , based on the theoretical calculations, proportionally to the inverse cubic function

[8]. In particular, there is about a 3–4 orders of magnitude increased phonon-less radiative rate in the smallest Si-NCs, when compared to the bulk Si, reaching $$\sim $$ 10 $$^7$$  s $$^{-1}$$ for the simulated $$\sim $$ 1 nm H-capped Si-NCs [39, 44, 75, 159]. Despite such a considerable improvement, radiative rates for the Si-NCs emitting in the visible spectral range still remain much lower than those in the traditionally employed direct bandgap semiconductor NCs with the radiative rates exceeding 10 $$^8$$ –10 $$^9$$  s $$^{-1}$$ , suggesting a persistently indirect nature of the bandgap in the Si-NCs and the importance of the k-space information in general.

An interesting development with respect to the enhanced radiative rate in Si-NCs has been published only very recently [180]; The authors study a series of H-capped Si-NCs prepared by a standard bottom-up sol-gel approach and observe a gradual size-induced PL shift from 700 to 530 nm. Whereas the larger Si-NCs exhibit the typical slow PL decay rates, at the size range below 1.7 nm, the PL decay rate is enhanced by about five orders of magnitude. (Argumentation is provided as to whether this PL decay rate is most likely the radiative rate.) This observation could be interpreted as the switch between indirect bandgap-like and direct bandgap-like emission in Si-NCs; the authors interpret their experimental data in terms of a switch between a bulk-like and molecular size regime. It is unknown why these particular very small Si-NCs exhibit fast PL decay, but the authors admit to the possibility that the small Si-NCs are strained.

Despite the generally limited success in the enhancement of the radiative rate for Si-NCs with emission in a visible spectral range and often observed limited tunability range of the bandgap in various ligand-capped Si-NCs, some of the best Si-NC materials with bright emission have already been implemented in various optoelectronic prototypes of light emitting devices [6, 43, 51, 55, 139, 163]. Nevertheless, such devices have not yet been commercialized and remain in a research and development stage, suggestive of the fact that a much needed improvement of their optical properties is still required.

1.1.2 Complex Role of Surface Chemistry

The most frequently quoted property of a nanocrystalline semiconductor material is the above discussed size-effect. One of its manifestations is the optical bandgap tunability via quantum confinement in the NCs smaller than the Bohr’s excitonic radius. In most semiconductor NCs, such as the direct bandgap III–V or II–VI NCs, the size of the core offers a robust parametrization for the optical bandgap energy. However, in the case of the Si-NCs, the size of the NC core is not enough to determine neither the optical bandgap energy nor the radiative rate, both being significantly affected by the surface chemistry, e.g., by organic coatings (Fig. 1.1e), oxide capping and strain (Fig. 1.1f).

The often observed strong influence of ligands on the optical properties of the Si-NCs is due to a complex interplay between the core and the covalent surface chemistry with other elements, and the relatively large percentage of atoms being located at the surface (a very useful tool giving insight into the basic possible arrangements of atoms and bonds can be found in  [105]). This interplay manifests via charge transfer, strain, orbital displacement, and the direct contribution of the surface elements to the density of states near the bandgap. Silicon is a light element, and its valence electron states are relatively close to the elements that are often considered as capping agents, such as oxygen, nitrogen, or carbon. Silicon can covalently bond with many elements with a various degree of electron sharing/transfer, stability, opto-chemical robustness or resistance against oxidation (which is a strongly preferred reaction under normal conditions). The most robust and studied types of cappings in the literature are H–, silica oxide (O–), hydrocarbon (organic) C–, O– or N-linked ligands, and halides (–Cl, –Br, –I or –F), together with the P- and B- surface co-doped Si-NCs [182]. For the sake of completeness, we note that it is important to realize that the photo-chemical stability of a certain bond cannot be easily deduced from the Si-X bond dissociation energy (DE), derived for the diatomic species [132], because the covalent bond strength changes in the presence of other bonded species, or under strains [202]. This is a result of the different orbital displacement caused by the additional bonded species, which can weaken the bond. Hence, the silane SiH $$_4$$ molecule is extremely unstable, violently reacting with oxygen, but Si-NC capped with H can resist oxidation for a very long time. This is because the dissociation of the Si-H bond is easier when in the –Si-H $$_2$$ or even –Si-H $$_3$$ form, and therefore, the –SiX $$_3$$ species on the Si-NC surface are less stable than the –SiX species. Also, various facets on the Si-NC surface have different Si coordination and are therefore subject to different strains in the presence of ligands due to steric hindrance effects. The highly curved surface of small Si-NCs also results in distorted/strained bonds, which further decreases their stability and makes them more prone to reactions or disintegration. For these reasons, stability of, e.g., H-capping on the surface of a Si-NC will differ for a crystalline and an amorphous Si-NC, as well as for large or small Si-NCs, and possibly also for Si-NCs prepared by different methods. This further complicates the main question of this chapter on how a certain element or molecule influences the optical properties of a Si-NC, because one type of ligand can lead to very different results, when placed in different positions on the Si-NC surface.

The H-capping is the simplest and best understood ligand, and as such is also a great clean starting point for the subsequent surface treatments [149]. Despite the extreme reactivity of the SiH $$_4$$ with air, as discussed above, the H-terminated Si surfaces are on their own chemically very stable, because the Si-H bond is strong and nonpolar; however, a direct photo-desorption occurs under a UV-blue irradiation [104]. As a result of the native indirect bandgap, persisting also in small Si-NCs [80, 159], Si-NCs absorb efficiently only from the blue-UV range, rendering the H-capped Si-NCs surfaces photo-chemically unstable for the light-related applications. Nevertheless, despite their opto-chemical instability, H-capped Si-NCs are often presented in the literature as a reference material and a starting point for theoretical simulations, because their optical properties are purely core-driven.

1.1.2.1 Oxide Capping

Silica oxide is naturally forming on the surface of silicon under exposure to air. Therefore, silica oxide-capped Si-NCs are one of the most accessible and analyzed types of Si-NCs. Capping with oxygen atoms can be done in several ways, as a double bond Si=O, as a bridging bond Si-O-Si, or via –OH groups [25]. The first two types have been reported to lead to limited bandgap tunability [74, 166, 167, 200, 211, 216] and slower radiative rates [40, 78]. The limiting effect of oxidation on spectral tunability has been first experimentally reported by Wolkin et al. [211] on porous silicon, prepared from bulk crystalline silicon by electro-chemical etching in solution composed of ethanol, water, and HF acid. This resulted in a H-passivated nano-sponge surface [83, 211], which contained small H-capped Si-NCs. As demonstrated by Wolkin et al. [211], the H-capped surface oxidized under exposure to air, when excited by UV light, which was accompanied by a change in color of the emitted PL.

For the larger oxide-capped Si-NCs, the PL peak was shown to be size tunable [10, 23, 52, 84, 122, 187, 211]; however, the tunability would often end in the red range close to 620–650 mm, indicating possible involvement of the surface oxide states [133]. This PL is always characterized by a slow $$\sim \upmu $$ s decay, for which it has been named in literature as an S-band [17]. For the smaller Si-NCs, the limit to which the emission can be tuned varies greatly through the literature [46]. A detailed overview can be found in [116] with the data sets re-plotted in Fig. 1.1f, clearly illustrating the existence of two distinct classes of oxide-capped Si-NCs, characterized by a different size dependence of the PL maximum. These two classes of Si-NCs correlate with the way the samples were prepared—either as free-standing (e.g., by a wet etching) or as matrix-embedded (e.g., by an ion implantation). We have hypothesized that in the matrix-embedded samples, the matrix exerts a compressive strain on the crystalline Si-NC cores, which leads to the corresponding shift in the electronic bands and consequently to the reported red-shifted emission, when compared to the free-standing oxidized Si-NCs. Interestingly, an analogical influence of the matrix was also reported in CdS $$_x$$ Se $$_{1-x}$$ NCs [176]. An opposite effect, i.e., the expansion of the crystalline lattice in oxidized Si-NCs, was demonstrated by doping Si-NCs with lithium [102], leading to a blue-shifted PL emission. Interestingly, the collection of the experimental data on the PL peak maximum as a function of the size of the NC d from the literature in Fig. 1.1f could not be satisfactorily fitted using the EMA-like model

. Therefore, a polynomial, meant simply as a quantitative description, was used. The different shape of the size dependence of the bandgap might imply that an additional effect of possibly size-dependent strain is present in such oxidized samples. These oxide-capped Si-NCs exhibit typical stretched-exponential PL decay of the order of tens or hundreds of microseconds [72], whose various properties [81, 197], including (the lack of) saturation of its power dependence [36, 73], were discussed in countless publications.

Interestingly, unlike other types of capping, a thin oxide shell was shown to lead to a very narrow luminescence linewidth [183], which could be very beneficial, for example, a high color definition required in application for displays. Oxidized Si-NCs can be also very interesting for application in the medical field. They are relatively photo-stable and were shown to be non-toxic [98, 155, 170, 177, 193]. In aqueous environments, oxide-capped Si-NCs slowly dissolve into the omnipresent, benign silicic acid [155], i.e., they are bio-degradable. The size of the lumminescent oxide-capped Si-NCs is always below $$\sim $$ 5 nm, which is a limit for a safe excretion from a living organism via urine [126], but can be an issue for its possible entry through the blood-brain barrier [71]. Also, upon excitation, oxide-capped Si-NCs are a source of oxygen radicals that are toxic to the neighboring tissue, an effect suggested for possible use in photo-induced local cancer treatment [108, 156, 191], or as a carrier of radioactive isotopes [158]. The red and slow decaying emission offers also a great contrast to the often fast decaying blue-green emission from organic tissue [144]. For the optical applications, however, the oxidation of the Si-NC surface might not be desired, when a full spectral tunability through the whole visible range is required, as well as high radiative rates.

1.1.2.2 Organic Capping

Another type of surface capping, well represented in the literature, but less understood theoretically, is via organic molecules, often long alkyl chains attached via hydrosilylation. Unlike in oxidized Si-NCs, the bandgap tunability curve for alkyl-capped SiNCs (Fig. 1.1e) can be fitted with an EMA-like model, albeit using a slightly different exponent than that in H-capped Si-NCs. Thus, this shape of the tunability curve suggests that in the alkyl-capped Si-NCs, the emitted PL resembles more that of the ideal H-capped Si-NCs system rather than the PL of the oxidized Si-NCs, in agreement with ab-initio studies [44, 171]. The large spread of the reported data around the fitted curve in Fig. 1.1e, also critically pointed out in [85] for this type of Si-NCs, is partly a result of the inclusion of many reports in this data set, where every measurement, especially the determination of size, is inevitably connected with an error [189, 190]. However, as some recent literature suggests, it can also signify some degree of influence of the alkyl chains on the emitted PL, be it their length, type of attachment or surface coverage [44]. Furthermore, synthesis of organically capped Si-NCs often involves heating of organic solvents, which has been reported to lead to the presence of other than Si-NC emitters, such as e.g. carbon dots, which could skew the reported PL properties if not carefully eliminated [17, 151, 209].

The comparison of the PL tunability curves in Fig. 1.1g confirms the presence of a limit around the end of the red spectral range for most of the experimentally investigated Si-NCs (except the H-capped ones) [37, 76, 180, 207]. It is very likely that most of the experimentally investigated organically capped Si-NCs are partly oxidized under UV-blue illumination, since due to the steric hindrance between the organic ligands, the surface cannot be 100% capped by organic molecules [44]. A possible culprit of the observed difference in the tunability limit of alkyl- and oxide-capped Si-NCs with respect to the H-capped ones might also be a stability problem, where the surface of a too small Si-NC might be too reactive to be passivated after the core has been formed, since the attachment of ligands can simply lead to the disintegration of the whole Si-NC. Such a stability problem could in principle be circumvented by the various one-pot synthesis schemes of already-passivated Si-NCs [165].

The PL decay is for the larger, red-NIR emitting alkyl-capped Si-NCs, similarly slow as for the oxide-capped Si-NCs, albeit with much better quantum yields [85, 97, 175]. Moreover, there are certainly some new developments in this field of research: Ensemble-induced effects start to be discussed in these Si-NCs [92, 145], the discussion on the periodicity of the core and its influence on the PL has been opened [189, 190], and even a fast PL component has been reported within this red emission band [11]. Pressure-dependent PL studies of organically capped Si-NCs perfectly agree with bulk-like band shifts [79], pointing toward a possible core-related origin. Interestingly, very similar band shifts were reported also in oxide-capped Si-NCs [116]. Comparable PL changes in alkyl- and oxide-capped Si-NCs were also reported in temperature-resolved measurements [138]. On the contrary, alkyl- and oxide-capped SiNCs were observed to exhibit different phonon modes in Raman measurements [86], interpreted in terms of oxide-induced strain.

While origin of the slow decaying PL in organically capped Si-NCs is accepted as core-related, the vast majority of the bottom-up wet-chemically synthesized Si-NCs exhibit unusually fast, nanosecond decaying PL in the blue-green spectral range [41, 53, 85, 113, 173, 206, 210, 217], whose origin is still a subject of intense debate and will be discussed in the following sections.

1.1.2.3 Direct Bandgap-Like Emission from Si-NCs: The F-Band

Quite controversially, a bright emission with direct bandgap-like radiative rates of $$10^7$$ – $$10^9~\mathrm{s}^{-1}$$ , and often a blue-green spectrum, has been reported experimentally from various Si-NCs capped with oxide [5, 90, 109] (see the latest review in [17]), and also for organic ligands [41, 53, 85, 113, 173, 206, 210, 217]. Throughout the literature, to differentiate between the traditionally slow decaying Si emission and the often observed fast decaying emission bands, the two are often labeled as the S-band for the slow emission and the F-band for the fast one. The S-band PL has typically 1–100s microsecond lifetime and red-NIR tunable energy, as described above. The F-band is very often confined to the blue-green spectral region and has a fast 1–10 nanosecond PL decay. In the past, the F-band has been assigned mostly to an extrinsic origin, such as oxygen- [12, 58, 59, 66, 69, 77, 168, 192, 194, 212] or nitrogen-related surface sites [34]. Other literature sources gave evidence for its possible intrinsic origin [42, 99, 152, 164, 195, 205] connected with the core or a combination of intrinsic and surface-related emissive sites [30, 44, 64]. A large body of literature explores also the possibility that this emission is entirely unrelated to the Si-NCs and is caused by carbon dots or other organic impurities [18, 20, 21, 24, 35, 57, 121, 129, 130, 151, 201, 209]. With respect to the carbon impurities, in our recent critical study [209], we show that despite the confirmed presence of Si and NCs in the elemental and materials analysis of the sample, the emission still did not originate from the Si-NC, but from ill-defined organic impurities, possibly carbon dots. Hence, one must perform synthesis of a control sample, as well as a direct (preferably correlative) single-dot microscopy, to be able to confirm that the emissive nanoparticle is indeed the Si-NC, and not a carbon dot, which would also exhibit size-dependent properties.

At least some blue PL can be emitted by nearly any (mostly organic or oxide-related) compound, when detected with high enough sensitivity. Therefore, clearly, extreme care needs to be taken to rule out other species than Si-NCs as the source of the observed PL. However, if we exclude the possibly of an erroneous assignment, given the broad scope of the reported origins of the fast emission, it is highly improbable that a single, all-encompassing explanation exists. More probably, different physical phenomena are responsible for the fast emission in the different samples, and often, more than one of the proposed mechanisms might apply even in a single sample. Thus, we do not believe that the F-band label should apply to such a broad scope of phenomena and that several, possibly overlapping, sub-groups exist within this type of emission, as we have already suggested elsewhere [117]. Therefore, a detailed preparation history of any studied sample needs to be complemented by a thorough characterization to differentiate between the individual modes of emission. Additionally, taking into account the very small size of just a few crystallographic planes, sometimes even the concepts of, e.g., a surface-site might not be that easy to grasp. In most cases, the wavefunction of an electron or a hole inside a NC will be spread over a large part of the core [44], rather than just residing on a certain surface site or being homogeneously delocalized over the whole core volume. Such an effect is evident, e.g., from the real-space localization of the highest occupied molecular orbital (HOMO) and the lowest unoccupied MO (LUMO) wavefunctions in, e.g.,  Fig. 1.1c. Therefore, except for few extreme cases, a differentiation between a surface- or core-related origin can be blurred [110]. In fact, the reality of the light emission of the Si-NC is governed by a complex interplay of many intertwined phenomena, as we have recently discussed in [44], and parallel experimental and theoretical approaches are a must in order to achieve further progress.

Despite the described uncertainties, the silicon community is dedicated to uncover the origin of this fast decaying and often very efficient F-band emission, as it holds great promises for optical applications of silicon. In our experimental work, we researched the F-band emission using various methods and differently prepared Si-NCs, including reports of positive optical gain [49]. In our most recent work, we focus on the $$\textbf{k}$$ -space projected density of states of Si-NCs with various ligands [48, 80, 115, 159], described in the following section, to uncover the possible evolution of the direct bandgap-like optical transitions that could stand behind the F-band emission.

1.1.3 The $$\text{K}$$ -Space Projections of the Density of States

The physics of emission from the Si-NCs is in general challenging and not yet fully controlled due to a complex, almost organic-like covalent chemistry on the silicon surfaces [16], which can, moreover, depend also on the preparation technique of the Si-NC core [208]. This leads to a complex electronic interplay between the silicon and ligand states, where the surface bonded elements can strongly affect the whole electronic structure of the Si-NCs [44] (Fig. 1.1c), especially the real- and $$\textbf{k}$$ -space density of states (DOS) profiles, and consequently also the bandgaps and radiative rates. In fact, the $$\textbf{k}$$ -space resolved DOS in NCs is under-represented in most of the available theoretical literature and is not a standard option in the DFT simulations packages (unlike for bulk crystals). However, this might be slowly changing, as more recent work is adopting this approach as well [11, 162].

Band structure, or a $$\textbf{k}$$ -space projected DOS, is a very useful formalism for the description of the electronic and optical properties in bulk crystalline materials. This often used approximation uses a simplified description of a quantum state of a solid based on single electron states. It assumes that the electrons travel in a static potential without dynamically interacting with the lattice vibrations, other electrons, etc., (or in other words the adiabatic and Hartree–Fock approximations are applied to the corresponding Schrödinger equation). This approach has proven very successful even beyond the scope of the initial approximation, as, for example, an exciton or a dopant/ impurity/ defect can be viewed as a small correction to the states within the band structure. One important assumption from which the band structure concept is derived is the requirement for a long-range translational symmetry in an ideally infinite crystalline material. In a macroscopic crystal, the lattice can be considered infinite without too much simplification, since 1 cm $$^3$$ of a crystalline material can contain some $$10^{23}$$  atoms. Similar simplification, however, would be far too crude for a small nanocrystal, whose crystalline core can contain as few as 500 atoms. Despite clearly breaking the requirement for being infinite and having a long-range translational symmetry, the band structure description has been used by the experimentalists to qualitatively and sometimes quantitatively describe some of the aspects of the behavior of semiconductor nanocrystals [60, 89].

Fig. 1.2

a Real-space (gray) and $$\textbf{k}$$ -space projection in $$\Gamma -X$$ (black) and $$\Gamma -L$$ (red) directions of the HOMO and LUMO states. Data are simulated using the tight binding approach described in more detail in [159]. Data are reused from [48] with permission from Nature Springer. b1–b4 Fuzzy band structures of three sizes of H-capped (b1–b3) and one size of OH-capped (b4) Si-NCs. Data are simulated using DFT code Fireball [95]. On the right side of panels, (b3) and (b4) are shown the real-space cross sections of the wavefunctions of the three states closest to the bandgap.

Modified after [80], reused with permission from APS Physics

In our tight binding simulations [48], we adopted this approach for the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) (see Supplementary information in [48]; Fig. 1.2a). To uncover the $$\textbf{k}$$ -space profile of the density of states $$|\Psi _i(\textbf{k})|^2$$ , we performed a Fourier transform of the real-space molecular orbital $$|\Psi _i(\textbf{r})|$$ . The density of states

can be then plotted along a specific direction, e.g., $$\Gamma -$$ X and $$\Gamma -$$ L. This allowed us to identify an enhancement in the DOS around the $$\Gamma $$ point for the LUMO, indicating direct bandgap-like transitions. This finding was accompanied by a 1000 $$\times $$ enhancement in the phonon-less radiative rates and enhanced absorption cross section (Supplementary information in [48]).

The HOMO–LUMO $$\textbf{k}$$ -space profile is very important for the emission from the Si-NC at $$T=0$$  K, but for a more general understanding of the ligand effect and a better comparison with experiments, performed at $$T > 0$$  K, many more states around the Fermi energy need to be resolved in the $$\textbf{k}$$ -space. To achieve this goal, a more suitable approach was demonstrated independently at that time by Hapala et al. [80]. In this approach, hundreds of molecular orbitals (MOs) in real space are Fourier transformed and projected along the specific line (direction) in the momentum space. This is done for a broad range of energy levels $$E_i$$ , symmetrically around the Fermi energy. The most important message in  [80], with respect to the earlier works that utilized Fourier transform for similar purposes, is the detailed discussion concerning the folding of the bands. This is not a trivial step, since the first Brillouin zone is almost empty and most of the momentum space density is located in the higher reciprocal cells. Also, not all the symmetry bulk crystal directions (such as $$\Gamma -$$ X) in the small NCs are equivalent, as the symmetry is broken by the involvement of the surface and ligands. After folding all the contributions of the higher reciprocal cells into the first Brillouin zone, all the densities from the all the $$\Gamma $$ -X directions are summed up to form the final fuzzy band structure, named after its blurred character in the $$\textbf{k}$$ -direction.

Examples of such a fuzzy band structure for H- and OH-capped Si-NCs, simulated using DFT code Fireball [95], are shown in Fig. 1.2b1–b4. Panels (b1–b3) demonstrate the evolution of this $$\textbf{k}$$ -space projection and the related radiative rates of an H-capped Si-NCs, depending on the size of the crystalline core. For the smallest size (1.5 nm) in Fig. 1.2b1, the $$\textbf{k}$$ -space blur is very high, which makes it difficult for any trend to be discernible. Despite the blurring, the highest DOS seems to be close to the $$\Gamma $$ point for the HOMO, and close to the X point for the LUMO states, suggesting a persistently indirect bandgap in the HOMO–LUMO transitions. For the two larger H-capped Si-NCs in Fig. 1.2b2, b3, despite the extensive blurring in the $$\textbf{k}$$ -direction, we can clearly recognize the shape of the original bulk Si bands in the $$\Gamma -$$ X direction, which makes the adoption of the band structure concept in this size range even more tenable. The original shape of the bulk silicon bands remains present even when the H-capping is replaced with various ligands: e.g., an –OH-capping in Fig. 1.2b4. More examples of the same type of simulations are shown also in the introductory Figs. 1.1b, c and 1.4b and later in Sect. 1.4, this time with a logarithmic DOS scale and simulated using DFT code cp2k [31, 44]. Nevertheless, despite this clear similarity in the overall band structure for all the different simulated Si-NC-ligand systems, there are also great differences, especially close to the bandgap. The comparison of the band structures for Si-NCs with various ligands presented here clearly indicates that the surface capping plays a very complex role and influences the whole band structure.

These results demonstrate that the band structure concept is still applicable even to a small NC with a relatively short-range translational symmetry. There are some limitations, though. First of all, the blurring in the $$\textbf{k}$$ -space is a consequence of the Heisenberg’s uncertainty relations: A higher spatial localization of carriers implies higher uncertainty in their $$\textbf{k}$$ -vector; this effect had already been proposed a long time ago [89]. Secondly, bands become very sparse close to the bandgap, where the DOS is naturally the lowest, and become separated by so-called mini-gaps, indicated by arrows in Fig. 1.2b1–b4. At a first glance, such splitted states could be mistaken for a surface state, but this is most of the time not the case. In order to find out whether a state is introduced by the surface capping, one should analyze the so-called projected DOS (PDOS), where element (and/or orbital) contributions to the DOS is given. An example is given in Fig. 1.4d1–d4 and e1–e4 for the various oxide cappings. From the PDOS elemental fraction analysis, one can see that for all the levels, including the split states inside the bandgap, more than 90% of the DOS is contributed by the Si core. Yet, there are clearly large differences between the, e.g., HOMO–LUMO energies and radiative rates, as well as the overall shape and the DOS distribution. Surface chemistry might or might not contribute a surface-localized state close to the band-edge, and there is no simple way of predicting it. Thus, such states separated by a mini-gap can, but do not have to be connected with the surface of the NC. Rather than applying such a distinction, these states should be treated as a result of a much more complex interplay between the electronic states of the surface ligands and the Si core. This close cooperation between silicon and the ligands makes the distinction between a core and a surface state more complicated than traditionally thought, as we pointed out also in [110] and our latest DFT study [44]. Importantly, the band structure approach is not only applicable, but also highly relevant, since it provides an explanation for the persistently low radiative rates in such small Si-NCs. Thus, instead of the traditional, atomistic approach, assuming that the NC behaves solely as a sum of its parts (core, surface, interface), we propose that a more holistic approach needs too be adopted, as backed up by in-depth simulations of the band structure and projected density of states.

In this chapter, we describe parallel and often joint efforts of our two independent research teams toward the understanding the Si-NCs with such enhanced optical properties, specifically with focus on (i) the bright fast decaying F-band emission from Si-NCs, as well as (ii) the $$\textbf{k}$$ -space representation of the effects of the surface chemistry on the directness of the radiative transitions in the Si-NCs. Our research of optical properties of Si-NCs covers parallel efforts in all three main directions of the research of nanomaterials, namely the technology of the preparation of samples, their experimental characterization as well as theoretical calculations. In the following text, we focus mainly on the promising yet still somewhat controversial topic of the fast radiative transitions detected in the various types of Si-NCs, and we discuss these transitions from both the experimental and theoretical point of view. We believe that experiment and theory need to go in this case hand in hand, especially given the number of issues with the precise material analysis and controversies surrounding the interpretations of the PL origin. To this date, we have employed the $$\textbf{k}$$ -space DOS projection theoretical approach to uncover the separate effects of ligand-induced tensile strain by DFT code Fireball in [115], effects of electronegative ligand and environment by tight binding in [159], and the joint effects of ligand-induced charge transfer and strain by DFT code cp2k [31] in [44]. These topics will be further discussed in Sect. 1.4. The chapter is organized as follows. In the two following sections, Sects. 1.2 and 1.3, experimental results related to fast radiative rates in Si-NCs from our two teams are discussed in detail. In the next Sect. 1.4, we present theoretical calculations aiming at the explanation of fast radiative rates experimentally found in a particular class of organically capped Si-NCs and discuss the implications of these theoretical results in detail. The presented work covers a long time-span of more than a decade, which lets us gain perspective into this problem.

1.2 Fast Radiative Rate in Hydrogen- and Oxide-Capped Silicon Nanocrystals

1.2.1 Oxidation of Hydrogen-Terminated Silicon Nanocrystals

To understand the underpinning mechanism of the emission in the oxide-capped Si-NCs, we resolved the effect of oxidation on PL in time [45], using a porous silicon, prepared by a similar process as described in [19, 211], but with the addition of hydrogen peroxide to achieve smaller Si-NCs, reported to yield blue emitting Si-NCs with fast emission rates [148]. The freshly etched porous silicon, with mostly H-capped Si-NCs, emitted in the green spectral range at around 525 nm with a fast, $$\sim $$ ns decay (Fig. 1.3) [45]. Such an emission wavelength could be attributed to a band-to-band radiative recombination from a Si-NC with core of diameter of $$\sim $$ 2 nm (Fig. 1.1d), which agrees with our experimental size estimation from TEM and Raman spectroscopy for this sample [47]. We need to note here that in the absence of a PL quantum yield measurement, we cannot a priori link the experimentally detected fast rates $$k_{{m\mathrm meas}}$$ to fast radiative rates $$k_{\mathrm{rad}}$$ , because the measured rates are determined by both the radiative and non-radiative processes [114]

and the effect of non-radiative rates needs to be determined separately. However, the green PL is easily observable with the naked eye, see Fig. 1.3b, which suggests that the radiative rates play an important role in the observed emission and the corresponding radiative rates might actually be relatively high.

To oxidize such a porous silicon thin layer, we immersed the sample in an ethanol bath (containing air, as it has not been degassed) and exposed it for few minutes to laser UV irradiation at 355 nm, while simultaneously registering the PL spectra. During this time, we observed a small gradual shift in the fast green component, until it eventually completely disappeared Fig. [45]. This fast component was gradually replaced by a much slower, microsecond decaying component in a deep red range at $$\sim $$ 650 nm (Fig. 1.3b). At the time, we have interpreted this emergent band as a surface oxide-induced emission, due to its gradual appearance during the oxidation under the UV illumination. The gradual shift of the PL emission was also found to be in a good agreement with the available theoretical simulations [133]. In retrospect, however, we would probably use the wording we observed emission characteristic of an oxidized Si-NC instead of a surface state (see Sect. 1.2.2 later on. Hence, exposure of H-terminated freshly etched Si-NCs to oxygen and UV irradiation leads to oxidation, accompanied by an emergence of a typical S-band emission, very much like in [211].

Fig. 1.3

a Schematic of the electro-chemical etching procedure to obtain porous silicon with small H-capped Si-NCs. b Sequence of real-color photographs of the PL from freshly etched porous silicon surface (the circle is about  1 in. in diameter, as defined by the size of the etching teflon container). Freshly etched H-capped Si-NCs exhibit a green emission ( $$t=0$$  s), which after about 1 min of continuous UV irradiation turns red. c Real-color photo of PL of the standard porous silicon before and after oxidation, emitting green and consequently red; yellow, white, and blue samples are made by prolonged post-etching in a bath of hydrogen peroxide. d Steady-state PL spectrum of freshly etched and oxidized standard porous silicon. e PL spectra of standard porous silicon after prolonged post-etching in hydrogen peroxide.

Reprinted from [45] and [46] with permission from AIP Publishing

1.2.2 Emergence of the F-Band in Oxidized Si-NC

As discussed above in Sect. 1.1.2.1, oxygen can bond on the surface of silicon in several different ways, causing different spectral limitations. Using the Firebal DFT code, we have simulated the effect of the –OH-capping on a 2.5 nm Si-NC (Fig. 1.2b4) and found quite a profound effect of the –OH ligand onto the whole band structure, including the levels close to the band-edge. Nevertheless, a closer look suggests that the band-edge states are not related to a surface localization, but are general Si-NC core states. This can be clearly demonstrated on the spatial cross section of the three HOMO and LUMO states, depicted for the largest H- and OH-capped Si-NC in Fig. 1.2b4, showing only a weak localization effect in the –OH-capped Si-NC.

A similar simulation for a slightly smaller 2.2 nm Si-NC by the DFT code cp2k, using settings described in [44], is shown Fig. 1.4, where we also for comparison simulate bridging Si-O-Si bonds, located on the edges of the Si-NC (Fig. 1.4a), and a combination of the two. Again we see, with respect to the H-capped Si-NC of the same size, a profound effect on the whole electronic structure, including the HOMO–LUMO energy (assumed to be an approximation to a bandgap energy), radiative rates and spatial localization of the carriers (Fig. 1.4c). Panels (d1–d4) and (e1–e4) show element-resolved PDOS, confirming that over 90% of the DOS for all the systems originate from the Si core, despite the fact that the real-space cross section of the HOMO and LUMO wavefunctions, shown in (Fig. 1.4c), is clearly localized close to the surface. This might come across as a surprise, considering that the two mid-gap states found for the Si-O-Si capping are typically ascribed to trapping on Si-O-related sites [140, 167, 211].

Fig. 1.4

a From top down: 2.2 nmm Si-NC (Si $$_{235}$$ ) fully capped by –OH, –OH with Si-O-Si bridges on the edges and only the Si-O-Si bridges. b Fuzzy band structure of H-, –OH, –OH with Si-O-Si and Si-O-Si capping. Thermalized radiative rates at $$T=$$ 300 K and HOMO–LUMO energies are indicated in the graphs. The color scheme depicts the DOS on a log-scale. c Real-space projection of the HOMO and LUMO wavefunctions. d1–d4, e1–e4 Element-color-coded PDOS fraction (color legend is in (e1–e4)) for (d1, e1) H-capped, (d2, e2) OH-capped, (d3, e3) OH- and O-bridge-, and (d4, e4) O-bridge-capped Si-NCs

In order to introduce more –OH bonds onto the surface, which should have a good impact on the HOMO-LUMO energy and surface localization of the electron and hole (Fig. 1.4b,c), we introduced into our Si-NC preparation routine a hydrogen peroxide H $$_2$$ O $$_2$$ post-etching procedure, assuming the following chemical reaction [46]:

The idea that including hydrogen peroxide in the production of porous silicon was introduced by Nayfeh et al. [148], who demonstrated production of ultra small, $$\sim $$ 1 nm sized Si-NCs. In our case, when including the hydrogen peroxide in the etching bath, we were able to achieve only the $$\sim $$ 2.2 nm green emitting H-capped Si-NC, as described before (Fig. 1.3b, c; standard sample). Adding the post-etching procedure led to a gradual shift of the S-band of the oxidized sample slightly beyond 650 nm, resulting in a porous silicon layer we labeled as yellow (Fig. 1.3c). Nevertheless, prolonged post-etching introduced a strong F-band in the blue spectral range at $$\sim $$ 450 nm (Fig. 1.3e) with a nanosecond decay time [46]. At a certain point, with further prolonging the post-etching time, the F-band became in the steady-state PL equally strong as the S-band, leading to the white emitting porous silicon (Fig. 1.3c, e), with CIE chart coordinates of (0.35, 0.34), close to the white daylight standard D65 with coordinates (0.313, 0.329) [12] (Fig. 1.5e). Eventually, the prolonged post-etching led to a drastic decrease in the S-band and further increase in the F-band, resulting in a blue emitting porous silicon (Fig. 1.3c) with emissive properties similar to those observed by Nayfeh et al. [148]. In this way, we have demonstrated that by continuous tuning of the post-etching time, we can continuously shift the PL from the slow red (S-band) to a fast blue (F-band) spectral range, but the tunability obviously skipped the green spectral range, where we observed the original PL for the non-oxidized freshly etched H-capped porous silicon (Fig. 1.3b–e) [46].

Despite the fact that we observed the F-band in clearly oxidized Si-NCs, the origin of this emission cannot be straightforwardly assigned to an oxide surface state emission. In fact, in a collaborative research with Valenta et al., we found that the origin of the F-band in our material is likely of an intrinsic origin, from small and distorted Si-NCs [195], as inferred from the behavior of the excitation-dependence of the F-band. A similar conclusion was offered by the temporally resolved low-temperature PL measurements [152]. Nevertheless, a possible link to surface (silica-related) defects has not been completely ruled out. Interestingly, such an accompanying blue F-band is usually not reported in Si-NCs with long alkyl capping, but exceptions of the rule can be found also here. Fast blue emission has been recently reported in Si-NCs synthesized in plasma from liquid precursors and was interpreted as the intrinsic direct $$\Gamma -\Gamma $$ hot-electron emission [162].

Fig. 1.5

a Schematics of the Si-NCs samples used for the e-beam irradiation experiment. b Cathodoluminescence (CL) upon prolonged e-beam irradiation from the porous silicon sample. c CL from the lithography-made Si-NCs—(top) Scanning electron microscopy (SEM) image of the structure and schematics of the cross section (bottom) PL spectrum of the strained areas (spot 3 and 4) contain also green CL band, while the unstrained areas (spots 1 and 2) do not. d PL before and after e-beam irradiation for all the oxide-capped samples. e PL in the chromaticity diagram CIE for all the oxide-capped Si-NCs, before and after e-beam irradiation. f Possible origins of the red and blue bands in the PL and CL of the radiation damaged silica.

Figures after [12] are reprinted with permission from Springer Nature

1.2.3 Silica Defects

Another possible emission channel in the oxidized Si-NCs is the silica oxide defects. Numerous literature sources confirm that radiation damaged silica and its emission bands that can occur almost anywhere in between the UV and NIR spectral ranges (see for example [12, 17, 153] and references therein), coinciding with the spectral tunability range of the Si-NCs core. It is possibly inevitable that a strained, curved silica shell on the surface of oxidized Si-NCs contains numerous defects, some of them emissive.

To this end, we assembled a series of oxidized Si-NCs prepared by various methods by different research groups with different type of oxide—naturally grown and thermally induced, and an organically capped Si-NC as a control sample (Fig. 1.5a) and analyzed their cathodoluminescence (CL) and PL before and after irradiation by the e-beam during the CL measurement [12] (Fig. 1.5b–d).

While virtually no effect of the electron beam irradiation was observed for the organically capped Si-NCs, all the oxide-capped Si-NCs have exhibited drastic changes to their PL after the CL measurement. In particular, during the electron beam irradiation in vacuum, all samples have shown a decreasing original PL peak (with different spectral position for the different samples) and a rise of several new emission bands. The most prominent new peaks were a narrow, strong red double peak at 650 nm, and a broad featureless blue band at 450–480 nm (Fig. 1.5b–d). For the strained parts of the large, lithography prepared Si-NCs, we have also observed emergence of a green emission band at $$\sim $$ 560 nm (Fig. 1.5c), which did not show in the unstrained parts of the same sample (Fig. 1.5c), or the other oxide-capped Si-NCs.

After exposure to air, PL has stabilized for all the Si-NCs, independently of their origin, size, and original PL, into a broad, white PL composed of a mixture of the original PL band and the new blue, green, and red emission bands introduced by the electron beam irradiation (Fig. 1.5d). In fact, we realized that by the use of e-beam irradiation, we can modify the PL spectrum of any oxide-capped Si-NCs in such a way that it becomes very similar and white, independently of the particular Si-NCs synthesis method, size, shape, strain, or original PL spectrum (U.S. patent application number 16/331704) (Fig. 1.5d).

Due to the striking similarities between the emissive bands arising in the oxide-capped Si-NCs under electron beam irradiation and the usually observed blue F-band and red S-band in oxide-capped Si-NCs, we put forward a hypothesis that these bands are closely related to the well-known oxygen-deficiency centers in the Si-rich silica shell [12] (Fig. 1.5f). In radiation damaged silica, emission from such bands can be efficiently excited only over the wide silica bandgap ( $$\sim $$ 9 eV). However, in the case of oxide-capped Si-NCs, the very same defects are excitable under blue/UV light through the Si-NC core. This is a great advantage that could help such silica defects to be utilized in white light emitting phosphors for applications in lighting (U.S. patent application number 16/331704). Thus, we have realized that silica defects could be an additional emissive channel and offer an additional degree of freedom to tune the PL in the Si-NCs. Their role can be very important in the F-band emission, where the electron beam irradiation induced a spectrally broad blue band, possibly related to the double dangling bond =Si: site (Fig. 1.5f). In poorly emitting samples, they can contribute even to the S-band emission, which can be inferred from the observation that the oxidized Si-NCs had kept (at least part) of their original emission after the electron beam irradiation. Generally speaking, the S-band, as observed in various samples, does not completely copy the behavior of a silica defect, because the position of the PL peak can be changed by preparation conditions and tuned throughout the red spectral region (see e.g. [187]). Nevertheless, the S-band spectral tunability usually ends close to the range 620–650 nm, where this characteristic silica defect band related to oxygen dangling bond =Si-O $$\cdot $$ resides (Fig. 1.5f) [12]. Consequently, this work again emphasizes the possibility of different emissive channels and even the coexistence of the individual channels in certain samples.

1.2.4 Role of Nitrogen

The discussion over the origin of the blue F-band emission has been eventually stirred also toward a possible presence of nitrogen-based surface sites, inducing a fast blue emission even in the originally red-emitting Si-NCs [34]. To introduce nitrogen in our materials, we have treated our porous silicon with a non-thermal plasma in water [63, 64]. This treatment was shown to lead to the incorporation of nitrate-water complexes –O-NO $$_2$$ into the shell of the Si-NCs, which alleviates some of the strain in the surface oxide shell. We have investigated the time-resolved PL of both samples under different excitation wavelengths and applied a dedicated analysis [120]. We found that several components are present within the blue PL band Fig. 1.6a–d) The original oxide-capped Si-NCs exhibited a fast, single-exponential ( $$\tau _{PL}^{(2)}=1.2$$  ns) component, identified as related to a silica defect. Additionally, we also identified a slightly longer stretched-exponential ( $$\tau _{PL}^{(3)}\approx 10$$  ns) emission-wavelength dependent component, assigned to non-thermalized, direct core-related PL. Interestingly, in the nitrate-containing Si-NCs, the silica-defect-related PL is eliminated, and only a very weak signal from the non-thermalized carriers remains. These results prove that whereas most of the blue PL emitted by our oxide-capped Si-NCs is connected with the silica-related defects, a small portion of this PL still originates in the core states. This finding is in agreement with and an extension of our previous results [152, 195], where we were not able to identify the silica-defect-related contribution to the PL due to a less complex analysis of data [152].

Fig. 1.6

a, c Examples of the PL decays from a an O-capped Si-NCs and c a nitrogen-containing O-capped Si-NCs, containing a 3 and c 2 decay components. b, d The PL spectra of the PL decay components obtained by fitting, the corresponding symbols are also included in panels (a) and (d). The ultrafast ( $$\tau ^{(1)}_{PL}=40$$  ps) component corresponds to the Raman signal of the solvent and therefore is not of interest for the study of Si-NCs. e The simulated (elementally resolved) PDOS in a Si-NC capped with –OH (simulating oxide surface, top) and a combination of –OH and –O-NO $$_2$$ (bottom). The bottom panel also shows the projection of the real-space wavefunction for the HOMO and LUMO states, highlighted by the gray dashed lines. The values of the corresponding computed radiative rates are also included.

Reproduced from [64] with permission from The Royal Society of Chemistry

In parallel to the optical investigations, DFT calculations were also conducted for the nitrate-containing Si-NCs, shown in Fig. 1.6e [64]. Replacement of some of the –OH ligand groups with –O-NO $$_2$$ leads to the emergence of a few in-gap states with high surface real-space localization [the blue dashed line at the bottom of panel (e)]. However, these states, which are deep in the bandgap, would therefore act rather as non-radiative traps and would be inaccessible through the traditional PL measurements as a result of a phonon bottleneck. Therefore, the theoretical description relevant to the performed experiment compares the band-edge transitions in the two systems as indicated by the gray dashed lines in Fig. 1.6e. Clearly, for our O-linked nitrogen surface atoms, the change in the surface passivation leads to a small red-shift in PL, accompanied by a several-time enhancement in the radiative rate, in agreement with our experiment [63].

The presented results are not contradictory to the results obtained by the Veinot group [34], who reported the emergence of a large blue shift and a fast PL emission after the Si-NCs were exposed to nitrogen (and oxygen), because the type of incorporation of the nitrogen atoms into the Si-NC system can very well be different in the two cases (e.g., direct Si–N vs. our –O-NO $$_2$$ surface). However, they do illustrate the wealth of processes which can be observed in the PL of the Si-NCs and prove that the incorporation of nitrogen does not necessarily have to lead to a blue emission. A similar conclusion has been recently put forward also by an independent group [147].

Fig. 1.7

a Dramatic change in the PL decay and PL spectra of the oxide-capped Si-NCs, re-capped by organic methyl-based ligands upon UV irradiation in xylene-based solvent. b)Real-color photo of PL from the organically re-capped Si-NCs. c Schematic of the surface changes induced by the UV irradiation in xylene containing solvent. d Measured NMR spectra of the Si-NCs with methyl-based capping.

Reprinted with permission from [113] Copyright (2010) American Chemical Society

1.3 Organically Coated Si-NCs with a Fast Emission Rate

The fast decaying blue emission, very similar to that observed in our oxide caped Si-NCs, has been routinely reported from number of organically capped Si-NCs. Therefore, we decided to steer our investigations also toward this promising and rapidly developing field. During our investigation of suspensions of oxide-capped Si-NCs, we developed a photo-chemical capping procedure, which resulted in the re-passivation of the oxide-capped Si-NCs by organic ligands. In particular, the oxide-capped Si-NCs (made from from porous silicon) were kept in a mixture of hydrocarbons containing xylene and were periodically (for a long time)