Direct biexciton generation in Si nanocrystal by a single photon

It has been shown theoretically that a strong quantum confinement regime in Si nanocrystals promotes highly efficient simultaneous excitation of two electron-hole pairs (biexciton) by a single photon. The rate (inverse lifetime) of biexciton generation has been calculated analytically as a function of the nanocrystal radius. The size-dependence of the rate in Si nanocrystal turns out to be sharp enough-in fact, it is inversely proportional to the sixth power of the radius. At radii values approaching a nanometer, the lifetime of biexciton generation falls into the nanosecond range. The threshold energy of this process in Si nanocrystals is exactly equal to twice the nanocrystal gap in contrast to the case of nanocrystals formed of direct-bandgap semiconductors, where the direct photon-induced creation of a biexciton with such an energy is, in fact, suppressed.

Energy transfer between Si nanocrystals and protoporphyrin molecules as a function of distance, orientation and size

The interaction between protoporphyrin molecules (donor) and Si nanocrystals (acceptor) up to 2.5 nm for varying distances and orientations is studied by DFT, semi-empirical and TDDFT methods. Simulations show an effect on electronic structure, indicative of electron charge transfer in parallel orientation and small distances and nonelectron energy transfer for different orientations and larger distances. An absorption-emission spectral overlap is observed. The calculations of coupling and energy transfer rates show a distance dependence typical of fluorescence resonance energy transfer (FRET) in the long range, while in the short and ultra-short range the distance dependence indicates electron transfer in addition to FRET. The Si NCs with the smallest size yield larger couplings than the larger nanocrystals. The PPIX/Si NC coupling was enhanced by adding a plasmon nanoparticle as a bridge in the donor-acceptor system. Results using Au nanoparticles show increased energy transfer rates up to 10⁴ and lower distance dependence.

Raman amplification for trapped radiation in crystalline single Si nanoparticle

In a single crystalline Si particle, we observed a huge amplification of the Raman peak at 521 cm^-1. With an AFM microscope, coupled with a Micro-Raman spectrometer, we investigate a single Si particle at wavelengths of 532 nm, 633 nm, and 785 nm. As observed by transmission electron microscopy, it has an octahedral shape of 150 nm in size. Thermal effects were detected on the Raman peak when the laser radiation, trapped inside, determines the heating of the particle up to its fusion. In these cases, the Raman peak splits into two components, the first at the crystal position and the other shifted at a lower value. The data permit the identification of the amplification mechanism of the Raman peak as trapped radiation moving forward and backwards into the particle. The thermal effects are attributed to phonon confinement and reduced thermal exchange with the surrounding. The present results are discussed in light of local order, the uncertainty principle, and phonon dispersion curves, and corroborated by shape-dependent simulation of absorption, scattering, and extinction behaviour.

Indirect to Direct Band Gap Transformation by Surface Engineering in Semiconductor Nanostructures

Indirect band gap semiconductor materials are routinely exploited in photonics, optoelectronics, and energy harvesting. However, their optical conversion efficiency is low, due to their poor optical properties, and a wide range of strategies, generally involving doping or alloying, has been explored to increase it, often, however, at the cost of changing their material properties and their band gap energy, which, in essence, amounts to changing them into different materials altogether. A key challenge is therefore to identify effective strategies to substantially enhance optical transitions at the band gap in these materials without sacrificing their intrinsic nature. Here, we show that this is indeed possible and that GaP can be transformed into a direct gap material by simple nanostructuring and surface engineering, while fully preserving its "identity". We then distill the main ingredients of this procedure into a general recipe applicable to any indirect material and test it on AlAs, obtaining an increase of over 4 orders of magnitude in both emission intensity and radiative rates.

Exciton-Photon Interactions in Semiconductor Nanocrystals: Radiative Transitions, Non-Radiative Processes and Environment Effects

In this review, we discuss several fundamental processes taking place in semiconductor nanocrystals (quantum dots (QDs)) when their electron subsystem interacts with electromagnetic (EM) radiation. The physical phenomena of light emission and EM energy transfer from a QD exciton to other electronic systems such as neighbouring nanocrystals and polarisable 3D (semi-infinite dielectric or metal) and 2D (graphene) materials are considered. In particular, emission decay and FRET rates near a plane interface between two dielectrics or a dielectric and a metal are discussed and their dependence upon relevant parameters is demonstrated. The cases of direct (II–VI) and indirect (silicon) band gap semiconductors are compared. We cover the relevant non-radiative mechanisms such as the Auger process, electron capture on dangling bonds and interaction with phonons. Some further effects, such as multiple exciton generation, are also discussed. The emphasis is on explaining the underlying physics and illustrating it with calculated and experimental results in a comprehensive, tutorial manner.

Singlet oxygen generation by nanoporous silicon: photoluminescence dynamics in magnetic field

Singlet oxygen generation in porous silicon (PSi) was investigated by a magneto-optical experiment. Photoluminescence (PL) quenching due to an energy transfer (ET) process mediated by an exchange interaction was monitored in the spectral range 1.4-2.5 eV and in a magnetic field of 0-6 Tesla at different levels of oxygen concentration and excitation pump power. When a magnetic field was applied, both PL recovery and, for magnetic fields below 2 Tesla and high concentrations of oxygen, an unusual additional pump power dependent quenching of the PL was observed. A rate equation model describing the behavior of PL from PSi with oxygen adsorbed at cryogenic temperatures in magnetic field was developed. The model has been expanded to cover the ET process as a function of the nanoparticle size.

First synthesis of silicon nanocrystals in amorphous silicon nitride from a preceramic polymer

We report the first synthesis of silicon nanocrystals embedded in a silicon nitride matrix through a direct pyrolysis of a preceramic polymer (perhydropolysilazane). Structural analysis carried out by XRD, XPS, Raman and TEM reveals the formation of silicon quantum dots and correlates the microstructures with the annealing temperature. The photoluminescence of the nanocomposites was investigated by both linear and nonlinear measurements. Furthermore we demonstrate an enhanced chemical resistance of the nitride matrix, compared to the typical oxide one, in both strongly acidic and basic environments. The proposed synthesis via polymer pyrolysis is a striking innovation potentially allowing a mass-scale production nitride embedded Si nanocrystals.

Reappraising the Luminescence Lifetime Distributions in Silicon Nanocrystals

The luminescence dynamics in ensembles of nanocrystals are complicated by a variety of processes, including the size-dependence of the radiative and non-radiative rates in inhomogeneous broadened samples and interparticle interactions. This results in a non-exponential decay, which for the specific case of silicon nanocrystals (SiNCs) has been widely modeled with a Kohlrausch or "stretched exponential" (SE) function. We first derive the population decay function for a luminescence decay following exp[- (t/τ)^β]. We then compare the distributions and mean times calculated by assuming that either the luminescence decay or the population decay follows this function and show that the results are significantly different for β much below 1. We then apply these two types of SE functions as well as other models to the luminescence decay data from two thermally grown SiNC samples with different mean sizes. The mean lifetimes are strongly dependent on the experimental setup and the chosen fitting model, none of which appears to adequately describe the ensemble decay dynamics. Frequency-resolved spectroscopy (FRS) techniques are then applied to SiNCs in order to extract the lifetime distribution directly. The rate distribution has a half width of ~ 0.5 decades and mainly resembles a somewhat high-frequency-skewed lognormal function. The combination of TRS and FRS methods appear best suited to uncovering the luminescence dynamics of NC materials having a broad emission spectrum.

Curved surface effect and manipulation of electronic states in nanosilicon

It is interesting in low-dimensional nanostructures of silicon that the two quantum effects play different roles in nanosilicon emission, in which the quantum confinement (QC) effect opens band gap and makes emission shift into shorter wavelengths (blue-shift) as the size of the nanocrystals is reduced; however the breaking symmetry originating from impurities on nanosilicon produces the localized electronic states in band gap and makes emission shift into longer wavelengths (red-shift). The results of experiment and calculation demonstrated that the energy levels of nanosilicon can be manipulated through these quantum effects, where the curved surface (CS) effect of impurity atoms bonding on nanosilicon is important in breaking symmetry of nanosilicon system. Here, the CS effect plays an important role on impuritied nanosilicon in smaller scale with larger surface curvature, in which a few characteristic parameters have been found to describe the breaking symmetry of nanosilicon system, such as bonding angle and projecting length of bonds on curved surface. More interesting, the coupling ways between the QC effect and the CS effect determinate the levels position of localized states in band gap and manipulate emission wavelength, where a few new phenomena were explored.

Photoluminescence through in-gap states in phenylacetylene functionalized silicon nanocrystals

Optoelectronic properties of Si nanocrystals (SiNCs) were studied by combining scanning tunneling spectroscopy (STS) and optical measurements. The photoluminescence (PL) of phenylacetylene functionalized SiNCs red shifts relative to hexyl- and phenyl-capped counterparts, whereas the absorption spectra and the band gaps extracted from STS are similar for all surface groups. However, an in-gap state near the conduction band edge was detected by STS only for the phenylacetylene terminated SiNCs, which can account for the PL shift via relaxation across this state.

Quasi-Direct Optical Transitions in Silicon Nanocrystals with Intensity Exceeding the Bulk

Comparison of the measured absolute absorption cross section on a per Si atom basis of plasma-synthesized Si nanocrystals (NCs) with the absorption of bulk crystalline Si shows that while near the band edge the NC absorption is weaker than the bulk, yet above ∼ 2.2 eV the NC absorbs up to 5 times more than the bulk. Using atomistic screened pseudopotential calculations we show that this enhancement arises from interface-induced scattering that enhances the quasi-direct, zero-phonon transitions by mixing direct Γ-like wave function character into the indirect X-like conduction band states, as well as from space confinement that broadens the distribution of wave functions in k-space. The absorption enhancement factor increases exponentially with decreasing NC size and is correlated with the exponentially increasing direct Γ-like wave function character mixed into the NC conduction states. This observation and its theoretical understanding could lead to engineering of Si and other indirect band gap NC materials for optical and optoelectronic applications.

Observation of oxygen dimers via energy transfer from silicon nanoparticles

Energy transfer from photo-excited excitons confined in silicon nanoparticles to oxygen dimers adsorbed on the nanoparticle surfaces is studied as a function of temperature and magnetic field. Quenching features in the nanoparticle photoluminescence spectrum arise from energy transfer to the oxygen dimers with and without the emission of Si TO(Δ) phonons and, also, with and without the vibrational excitation of the dimers. The dependence of the quenching on magnetic field shows that energy transfer is fast when a dimer is present, allowing an estimate of the proportion of the nanoparticles with adsorbed dimers.

A transfer hamiltonian model for devices based on quantum dot arrays

We present a model of electron transport through a random distribution of interacting quantum dots embedded in a dielectric matrix to simulate realistic devices. The method underlying the model depends only on fundamental parameters of the system and it is based on the Transfer Hamiltonian approach. A set of noncoherent rate equations can be written and the interaction between the quantum dots and between the quantum dots and the electrodes is introduced by transition rates and capacitive couplings. A realistic modelization of the capacitive couplings, the transmission coefficients, the electron/hole tunneling currents, and the density of states of each quantum dot have been taken into account. The effects of the local potential are computed within the self-consistent field regime. While the description of the theoretical framework is kept as general as possible, two specific prototypical devices, an arbitrary array of quantum dots embedded in a matrix insulator and a transistor device based on quantum dots, are used to illustrate the kind of unique insight that numerical simulations based on the theory are able to provide.

Octahedral faceted Si nanoparticles as optical traps with enormous yield amplification

We describe a method for the creation of an efficient optical scatter trap by using fully crystalline octahedral Silicon nanoparticles (Si-NPs) of approximately 100 nanometres in size. The light trapping, even when probing an isolated nanoparticle, is revealed by an enormous amplification of the Raman yield of up to 10⁸ times that of a similar Si bulk volume. The mechanism conceived and optimised for obtaining such a result was related to the capability of a Si octahedron to trap the light because of its geometrical parameters. Furthermore, Si-NPs act as very efficient light scatterers not only for the direct light beam but also for the trapped light after it escapes the nanoparticle. These two effects are observed, either superimposed or separated, by means of the Raman yield and by photoluminescence enhancements. The inductively coupled plasma synthesis process performed at a temperature of only 50°C allows for the ubiquitous use of these particles on several substrates for optical and photovoltaic applications.

Charge transfer state emission dynamics in blue-emitting functionalized silicon nanocrystals

Time-resolved photoluminescence spectroscopy reveals a charge transfer state dynamics responsible for the size-independent blue emission observed from dodecylamine and ammonia functionalized silicon nanocrystals.

Size vs surface: tuning the photoluminescence of freestanding silicon nanocrystals across the visible spectrum via surface groups

The syntheses of colloidal silicon nanocrystals (Si-NCs) with dimensions in the 3-4 nm size regime as well as effective methodologies for their functionalization with alkyl, amine, phosphine, and acetal functional groups are reported. Through rational variation in the surface moieties we demonstrate that the photoluminescence of Si-NCs can be effectively tuned across the entire visible spectral region without changing particle size. The surface-state dependent emission exhibited short-lived excited-states and higher relative photoluminescence quantum yields compared to Si-NCs of equivalent size exhibiting emission originating from the band gap transition. The Si-NCs were exhaustively characterized using transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), and Fourier transformed infrared spectroscopy (FTIR), and their optical properties were thoroughly investigated using fluorescence spectroscopy, excited-state lifetime measurements, photobleaching experiments, and solvatochromism studies.

Enhancing Hydrogen Diffusion in Silica Matrix by Using Metal Ion Implantation to Improve the Emission Properties of Silicon Nanocrystals

Efficient silicon-based light emitters continue to be a challenge. A great effort has been made in photonics to modify silicon in order to enhance its light emission properties. In this aspect silicon nanocrystals (Si-NCs) have become the main building block of silicon photonic (modulators, waveguide, source, and detectors). In this work, we present an approach based on implantation of Ag (or Au) ions and a proper thermal annealing in order to improve the photoluminescence (PL) emission of Si-NCs embedded in SiO2. The Si-NCs are obtained by ion implantation at MeV energy and nucleated at high depth into the silica matrix (1-2 μm under surface). Once Si-NCs are formed inside the SiO2we implant metal ions at energies that do not damage the Si-NCs. We have observed by, PL and time-resolved PL, that ion metal implantation and a subsequent thermal annealing in a hydrogen-containing atmosphere could significantly increase the emission properties of Si-NCs. Elastic Recoil Detection measurements show that the samples with an enhanced luminescence emission present a higher hydrogen concentration. This suggests that ion metal implantation enhances the hydrogen diffusion into silica matrix allowing a better passivation of surface defects on Si NCs.

Magnetic field dependence of singlet oxygen generation by nanoporous silicon

Energy transfer from photoexcited excitons localized in silicon nanoparticles to adsorbed oxygen molecules excites them to the reactive singlet spin state. This process has been studied experimentally as a function of nanoparticle size and applied external magnetic field as a test of the accepted understanding of this process in terms of the exchange coupling between the nano-Si exciton and the adsorbed O2 molecules.

Electronic States and Optical Gap of Phosphorus-Doped Silicon Nanocrystals Embedded in a Silica Host Matrix

Using the envelope-function approximation the electronic states and the optical gap of silicon nanocrystals heavily doped with phosphorus have been calculated. Assuming the uniform impurity distribution over the crystallite volume we have found the fine structure of the electron ground state (induced by the valley-orbit interaction) and the optical gap as a function of the crystallite size and donor concentration. It is shown that the energy of the ground singlet state decreases almost linearly as the concentration increases, while the valley-orbit splitting increases nonlinearly. Phosphorus doping also results in the decrease of the nanocrystal gap with increasing the impurity concentration.

Giant photoluminescence emission in crystalline faceted Si grains

Empowering an indirect band-gap material like Si with optical functionalities, firstly light emission, represents a huge advancement constantly pursued in the realization of any integrated photonic device. We report the demonstration of giant photoluminescence (PL) emission by a newly synthesized material consisting of crystalline faceted Si grains (fg-Si), a hundred nanometer in size, assembled in a porous and columnar configuration, without any post processing. A laser beam with wavelength 632.8 nm locally produce such a high temperature, determined on layers of a given thickness by Raman spectra, to induce giant PL radiation emission. The optical gain reaches the highest value ever, 0.14 cm/W, representing an increase of 3 orders of magnitude with respect to comparable data recently obtained in nanocrystals. Giant emission has been obtained from fg-Si deposited either on glass or on flexible, low cost, polymeric substrate opening the possibility to fabricate new devices.

Chemical insight into the origin of red and blue photoluminescence arising from freestanding silicon nanocrystals

Silicon nanocrystals (Si NCs) are attractive functional materials. They are compatible with standard electronics and communications platforms and are biocompatible. Numerous methods have been developed to realize size-controlled Si NC synthesis. While these procedures produce Si NCs that appear identical, their optical responses can differ dramatically. Si NCs prepared using high-temperature methods routinely exhibit photoluminescence agreeing with the effective mass approximation (EMA), while those prepared via solution methods exhibit blue emission that is somewhat independent of particle size. Despite many proposals, a definitive explanation for this difference has been elusive for no less than a decade. This apparent dichotomy brings into question our understanding of Si NC properties and potentially limits the scope of their application. The present contribution takes a substantial step forward toward identifying the origin of the blue emission that is not expected based upon EMA predictions. It describes a detailed comparison of Si NCs obtained from three of the most widely cited procedures as well as the conversion of red-emitting Si NCs to blue emitters upon exposure to nitrogen-containing reagents. Analysis of the evidence is consistent with the hypothesis that the presence of trace nitrogen and oxygen even at the parts per million level in Si NCs gives rise to the blue emission.

Theory of nonlinear pulse propagation in silicon-nanocrystal waveguides

We develop a comprehensive theory of the nonlinear propagation of optical pulses through silica waveguides doped with highly nonlinear silicon nanocrystals. Our theory describes the dynamics of arbitrarily polarized pump and Stokes fields by a system of four generalized nonlinear Schrödinger equations for the slowly varying field amplitudes, coupled to the rate equation for the number density of free carriers. In deriving these equations, we use an analytic expression for the third-order effective susceptibility of the waveguide with randomly oriented nanocrystals, which takes into account both the weakening of the nonlinear optical response of silicon nanocrystals due to their embedment in fused silica and the change in the tensor properties of the response due to the modification of light interaction with electrons and phonons inside the silicon-nanocrystal waveguide. In order to facilitate the use of our theory by experimentalists, and for reasons of methodology, we provide a great deal of detail on the mathematical treatment throughout the paper, even though the derivation of the coupled-amplitude equations is quite straightforward. The developed theory can be applied for the solving of a wide variety of specific problems that require modeling of nonlinear optical phenomena in silicon-nanocrystal waveguides.

Microscopic origin of the fast blue-green luminescence of chemically synthesized non-oxidized silicon quantum dots

The microscopic origin of the bright nanosecond blue-green photoluminescence (PL), frequently reported for synthesized organically terminated Si quantum dots (Si-QDs), has not been fully resolved, hampering potential applications of this interesting material. Here a comprehensive study of the PL from alkyl-terminated Si-QDs of 2-3 nm size, prepared by wet chemical synthesis is reported. Results obtained on the ensemble and those from the single nano-object level are compared, and they provide conclusive evidence that efficient and tunable emission arises due to radiative recombination of electron-hole pairs confined in the Si-QDs. This understanding paves the way towards applications of chemical synthesis for the development of Si-QDs with tunable sizes and bandgaps.

Power-dependent spectral shift of photoluminescence from ensembles of silicon nanocrystals

: Nanocrystals are widely studied for their tunable optical properties, most importantly increased luminescence efficiency and emission energy. Quantum confinement effects are found for many different types of nanocrystals, and these introduce a relation between the emission wavelength and the size of nanocrystals. When ensembles of nanocrystals with a distribution of sizes are studied, this can have profound effects on their luminescence spectra. Here, we show how photoluminescence spectra of ensembles of silicon nanocrystals can shift under different excitation conditions, resulting from differences in absorption cross-section of the individual nanocrystal sizes. This effect, together with the fact that after a pulsed excitation a silicon nanocrystal can only emit a single photon, determines how the distribution of excited nanocrystals changes and leads to the spectral shift for different excitation powers. Next to this effect, the influence of different radiative rates in such ensembles is also addressed. These notions are important for the interpretation of photoluminescence data for silicon nanocrystals but can be extended to any nanoparticle system comprising size-distributed ensembles.

From amorphous to crystalline silicon nanoclusters: structural effects on exciton properties

Synchrotron x-ray absorption spectroscopy (XAS) and electron spin resonance (ESR) experiments were performed to determine, in combination with Raman spectroscopy and x-ray diffraction (XRD) data from previous reports, the structure and paramagnetic defect status of Si-nanoclusters (ncls) at various intermediate formation stages in Si-rich Si oxide films having different Si concentrations (y = 0.36-0.42 in Si(y)O(1-y)), fabricated by electron cyclotron resonance plasma-enhanced chemical vapor deposition and isochronally (2 h) annealed at various temperatures (T(a) = 900-1100 °C) under either Ar or (Ar + 5%H(2)) atmospheres. The corresponding emission properties were studied by stationary and time dependent photoluminescence (PL) spectroscopy in correlation with the structural and defect properties. To explain the experimental data, we propose crystallization by nucleation within already existing amorphous Si-ncls as the mechanism for the formation of the Si nanocrystals in the oxide matrix. The cluster-size dependent partial crystallization of Si-ncls at intermediate T(a) can be qualitatively understood in terms of a 'crystalline core-amorphous shell' Si-ncl model. The amorphous shell, which is invisible in most diffraction and electron microscopy experiments, is found to have an important impact on light emission. As the crystalline core grows at the expense of a thinning amorphous shell with increasing T(a), the PL undergoes a transition from a regime dominated by disorder-induced effects to a situation where quantum confinement of excitons prevails.

Tailored nanoheterojunctions for optimized light emission

We present coupled classical and quantum simulations of 1 to 2 nm Si nanocrystals (NCs) embedded in amorphous SiO(2) and we show that by tuning the density of the oxide matrix one may change the relative alignment of Si NC and SiO(2) electronic states at the interface. We find that interfacial strain plays a key role in determining the variation of the nanaoparticle gap as a function of size, as well as of conduction band offsets with the oxide. In particular, our results show that it is the variation of the valence band offset with size that is responsible for the gap change. Our findings suggest that the elastic properties of the embedding matrix may be tuned to tailor the energy levels of small Si NCs so as to optimize their performance in optoelectronic devices and solar cells.

Colloidally stable silicon nanocrystals with near-infrared photoluminescence for biological fluorescence imaging

Luminescent silicon nanocrystals (ncSi) are showing great promise as photoluminescent tags for biological fluorescence imaging, with size-dependent emission that can be tuned into the near-infrared biological window and reported lack of toxicity. Here, colloidally stable ncSi with NIR photoluminescence are synthesized from (HSiO1.5)n sol-gel glasses and are used in biological fluorescence imaging. Modifications to the thermal processing conditions of (HSiO1.5)n sol-gel glasses, the development of new ncSi oxide liberation chemistry, and an appropriate alkyl surface passivation scheme lead to the formation of colloidally stable ncSi with photoluminescence centered at 955 nm. Water solubility and biocompatibility are achieved through encapsulation of the hydrophobic alkyl-capped ncSi within PEG-terminated solid lipid nanoparticles. Their applicability to biological imaging is demonstrated with the in-vitro fluorescence labelling of human breast tumor cells.

Red spectral shift and enhanced quantum efficiency in phonon-free photoluminescence from silicon nanocrystals

Crystalline silicon is the most important semiconductor material in the electronics industry. However, silicon has poor optical properties because of its indirect bandgap, which prevents the efficient emission and absorption of light. The energy structure of silicon can be manipulated through quantum confinement effects, and the excitonic emission from silicon nanocrystals increases in intensity and shifts to shorter wavelengths (a blueshift) as the size of the nanocrystals is reduced. Here we report experimental evidence for a short-lived visible band in the photoluminescence spectrum of silicon nanocrystals that increases in intensity and shifts to longer wavelengths (a redshift) with smaller nanocrystal sizes. This higher intensity indicates an increased quantum efficiency, which for 2.5-nm-diameter nanocrystals is enhanced by three orders of magnitude compared to bulk silicon. We assign this band to the radiative recombination of non-equilibrium electron-hole pairs in a process that does not involve phonons.

Modification of ensemble emission rates and luminescence spectra for inhomogeneously broadened distributions of quantum dots coupled to optical microcavities

We investigate the spontaneous emission modifications when ensembles of quantum dots (QDs) with differing emission frequencies and finite Lorentzian linewidths are coupled to a microcavity. Using contour integrals we develop a general expression for the rate enhancement when neither the emitter nor the cavity resonance can be treated as a delta function. We show that the ensemble cavity-coupled luminescence lifetimes are generally suppressed in the case of spherical cavities and that the spontaneous emission dynamics of the cavity coupled component becomes increasingly stretched as the coupling factor increases. The Q-factor measured from the luminescence spectrum can be much lower than the intrinsic cavity Q-factor, and is in many practical situations limited by the QD spectral width. The mode spectrum observed in the photoluminescence (PL) spectrum can be largely determined by the QD emission linewidth, permitting this parameter to be extracted without requiring single-particle spectroscopy. In the case of Si-QDs, the linewidth cannot be significantly greater than 10 meV in order to observe spherical cavity resonances in the PL spectrum.

Si-nanocrystal-based LEDs fabricated by ion implantation and plasma-enhanced chemical vapour deposition

An in-depth study of the physical and electrical properties of Si-nanocrystal-based MOSLEDs is presented. The active layers were fabricated with different concentrations of Si by both ion implantation and plasma-enhanced chemical vapour deposition. Devices fabricated by ion implantation exhibit a combination of direct current and field-effect luminescence under a bipolar pulsed excitation. The onset of the emission decreases with the Si excess from 6 to 3 V. The direct current emission is attributed to impact ionization and is associated with the reasonably high current levels observed in current-voltage measurements. This behaviour is in good agreement with transmission electron microscopy images that revealed a continuous and uniform Si nanocrystal distribution. The emission power efficiency is relatively low, approximately 10(-3)%, and the emission intensity exhibits fast degradation rates, as revealed from accelerated ageing experiments. Devices fabricated by chemical deposition only exhibit field-effect luminescence, whose onset decreases with the Si excess from 20 to 6 V. The absence of the continuous emission is explained by the observation of a 5 nm region free of nanocrystals, which strongly reduces the direct current through the gate. The main benefit of having this nanocrystal-free region is that tunnelling current flow assisted by nanocrystals is blocked by the SiO2 stack so that power consumption is strongly reduced, which in return increases the device power efficiency up to 0.1%. In addition, the accelerated ageing studies reveal a 50% degradation rate reduction as compared to implanted structures.

A study of laser-induced blue emission with nanosecond decay of silicon nanoparticles synthesized by a chemical etching method

Silicon nanoparticles (Si NPs), exhibiting a strong visible photoluminescence (PL), have found many applications in optoelectronics devices, biomedical tags and flash memories. Chemical etching is a well-known method for synthesizing orange-luminescent, hydride-capped silicon nanoparticles (H/Si NPs). However, a blueshift in emission wavelength occurs when reducing the particle size to exciton Bohr radius or less. In this paper, we attempted to synthesize and characterize H/Si NPs that emit lower wavelengths at room temperature. We proved that our method succeeded in synthesizing H/Si NPs with emission in the blue region. The wavelength-resolved and time-resolved studies of the PL were executed for H/Si NPs in methanol (MeOH), pyridine (py) and furan, using the 355 nm pulsed radiation from a Nd:YAG laser. In addition, excitation wavelength-dependent and PL studies were executed using the spectrofluorometer with a xenon (Xe) broad band light source. We noticed solvent-dependent PL spectra with sharp peaks near 420 nm and a short lifetime less than 100 ns. The morphology and particle size were investigated by high resolution transmission electron microscope (HRTEM). Particles as small as one nanometer were observed in MeOH and py suspensions while two-nanometer particles were observed in the furan suspension.

Improvement of the photon generation efficiency in phosphorus-doped silicon nanocrystals: Γ-X mixing of the confined electron states

It has been shown that the central-cell potential of a phosphorus ion embedded in a silicon nanocrystal effectively mixes the electronic states of X- and Γ-bands. Quantum confinement strengthens the Γ-X mixing which, in turn, straightens the nanocrystal's band structure, and substantially intensifies interband radiative recombination.