Ensembles Of Silicon Nanocrystals Research Articles

Determination of the size dispersion of amorphous silicon quantum dots from a silicon nitride film containing silicon nanocrystals

In this work we will investigate the size distribution of the amorphous silicon quantum dots (Si-AQD) inside a silicon nitride film SiNx (x = 0.12). The studied film contains an ensemble of silicon nanocrystals (Si-nc) inside an amorphous silicon nitride matrix SiNy (y = 0.66). Since the nitrogen atoms number are less than the silicon atoms number in the amorphous matrix, we can suggest that silicon atoms can aggregate under amorphous quantum dots. In this context, we will first estimate the number of crystallized silicon atoms (c-Si). The number of amorphous silicon atoms (a-Si) can be deduced from the fraction of crystallized silicon atoms and leads to the determination of the number of nitrogen atoms.Results show that all the nitrogen atoms are localized in the Si-ncs cap shell. Thus, the amorphous quantum dots can be easily revealed from the scattering electron microscopy (SEM) image. The low yield of luminescence from amorphous quantum dots can explain the difference between both experimental and simulated photoluminescence (PL) from silicon nitride film containing Si-ncs.

Photoluminescence properties of silicon nanocrystals grown by nanosecond laser ablation of solid-state targets in an inert gas atmosphere

It was found that the photoluminescence intensity decay kinetics of nanocrystalline silicon layers formed by nanosecond laser ablation of crystalline silicon targets in a helium atmosphere exhibit a power-law behavior with an exponent from 0.9 to 1.5, depending on the temperature and luminescence photon energy in the range of 1.4–1.8 eV, which indicates photoexcited carrier recombination controlled by dissipative tunneling processes in silicon nanocrystal ensembles in a suboxide matrix.

Thermally stimulated exciton emission in Si nanocrystals

Increasing temperature is known to quench the excitonic emission of bulk silicon, which is due to thermally induced dissociation of excitons. Here, we demonstrate that the effect of temperature on the excitonic emission is reversed for quantum-confined silicon nanocrystals. Using laser-induced heating of silicon nanocrystals embedded in SiO2, we achieved a more than threefold (>300%) increase in the radiative (photon) emission rate. We theoretically modeled the observed enhancement in terms of the thermally stimulated effect, taking into account the massive phonon production under intense illumination. These results elucidate one more important advantage of silicon nanostructures, illustrating that their optical properties can be influenced by temperature. They also provide an important insight into the mechanisms of energy conversion and dissipation in ensembles of silicon nanocrystals in solid matrices. In practice, the radiative rate enhancement under strong continuous wave optical pumping is relevant for the possible application of silicon nanocrystals for spectral conversion layers in concentrator photovoltaics.

Observing the morphology of single-layered embedded silicon nanocrystals by using temperature-stable TEM membranes.

We use high-temperature-stable silicon nitride membranes to investigate single layers of silicon nanocrystal ensembles by energy filtered transmission electron microscopy. The silicon nanocrystals are prepared from the precipitation of a silicon-rich oxynitride layer sandwiched between two SiO2 diffusion barriers and subjected to a high-temperature annealing. We find that such single layers are very sensitive to the annealing parameters and may lead to a significant loss of excess silicon. In addition, these ultrathin layers suffer from significant electron beam damage that needs to be minimized in order to image the pristine sample morphology. Finally we demonstrate how the silicon nanocrystal size distribution develops from a broad to a narrow log-normal distribution, when the initial precipitation layer thickness and stoichiometry are below a critical value.

Phenomenological model of photoluminescence degradation and photoinduced defect formation in silicon nanocrystal ensembles under singlet oxygen generation

We propose a phenomenological model to explain photoluminescence degradation of silicon nanocrystals under singlet oxygen generation in gaseous and liquid systems. The model considers coupled rate equations, which take into account the exciton radiative recombination in silicon nanocrystals, photosensitization of singlet oxygen generation, defect formation on the surface of silicon nanocrystals as well as quenching processes for both excitons and singlet oxygen molecules. The model describes well the experimentally observed power law dependences of the photoluminescence intensity, singlet oxygen concentration, and lifetime versus photoexcitation time. The defect concentration in silicon nanocrystals increases by power law with a fractional exponent, which depends on the singlet oxygen concentration and ambient conditions. The obtained results are discussed in a view of optimization of the photosensitized singlet oxygen generation for biomedical applications.

THz photoconductivity in light-emitting surface-oxidized Si nanocrystals: the role of large particles

We propose an analytical description of the role of local depolarization fields in the terahertz conductivity of nanostructured samples and demonstrate this approach in a sample composed of silicon nanocrystals. This helps to uncover the nature of charge carrier transport at nanoscale. Time-resolved terahertz conductivity is investigated in an ensemble of silicon nanocrystals fabricated by electrochemical etching of silicon wafer followed by an H2O2 oxidizing treatment. The post-etching treatment leads to a decrease in the average nanocrystalline Si core size which enhances luminescence in the visible range. We show that the dominating microscopic photoconductive response of photocarriers is essentially Drude-like owing to the presence of a very small amount of large nanocrystals; the macroscopic character of the response is, however, deeply modified by the depolarization fields. Smaller nanocrystals appreciably contribute to the terahertz conductivity only at high photoexcitation densities where the screening due to depolarization fields suppresses the response of the large particles.

Influence of Size Purification and Self-Assembly on the Photoluminescence of Silicon Nanocrystal Ensembles

Silicon nanocrystals (SiNCs) synthesized through a nonthermal plasma reaction are purified through density-gradient ultracentrifugation in mixed organic solvents to achieve highly monodisperse fractions. These fractions show a reduction in photoluminescent (PL) linewidth, increased band alignment, and the emergence of short-range crystalline order in dried films. The films display a significant PL enhancement that we suggest arises from nanocrystal interactions in densely packed arrays and we exploit this to achieve photostable emission. We also explore how this PL enhancement emerges, from the scale of individual nanocrystals up to clusters of more than a thousand, and we examine the influence of different polymers on the self-assembly and PL stability of ensemble structures.

Influence of Size Purification and Self-Assembly on the Photoluminescence of Silicon Nanocrystal Ensembles

Silicon nanocrystals (SiNCs) synthesized through a nonthermal plasma reaction are purified through density-gradient ultracentrifugation in mixed organic solvents to achieve highly monodisperse fractions. These fractions show a reduction in photoluminescent (PL) linewidth, increased band alignment, and the emergence of short-range crystalline order in dried films. The films display a significant PL enhancement that we suggest arises from nanocrystal interactions in densely packed arrays and we exploit this to achieve photostable emission. We also explore how this PL enhancement emerges, from the scale of individual nanocrystals up to clusters of more than a thousand, and we examine the influence of different polymers on the self-assembly and PL stability of ensemble structures.

Electroluminescent devices based on nanosilicon multilayer structures

In this contribution we give an overview of our development of size-controlled multilayered ensembles of silicon nanocrystals (Si-NCs) for efficient light emitting devices (LEDs). The ensembles of Si-NCs embedded in silicon dioxide are grown by chemical vapour deposition methods and by using standard microelectronic processes. The formation of nanocrystals is monitored by several analytical techniques. Novel LED architectures of nanocrystal ensembles which improve electrical injection and light emission are described. Charge tunnelling in nanosilicon superlattices, light emission efficiency under direct and alternating current injection, and device stability are discussed. Visible light emission from an electroluminescence (EL) device with Si-NCs.

Temperature Dependent Photoluminescence of Size-Purified Silicon Nanocrystals

The photoluminescence (PL) of size-purified silicon nanocrystals is measured as a function of temperature and nanoparticle size for pure nanocrystal films and polydimethylsiloxane (PDMS) nanocomposites. The temperature dependence of the bandgap is the same for both sample types, being measurably different from that of bulk silicon because of quantum confinement. Our results also suggest weaker interparticle and environmental coupling in the nanocomposites, with enhanced PL and an unexpected dependence of lifetime on size for the pure nanocrystal films at low temperatures. We interpret these results through differences in the low-temperature size dependence of the ensemble nonradiative equilibrium constants. The response of the PDMS nanocomposites provides a consistent measure of local temperature through intensity, lifetime, and wavelength in a polymer-dispersed morphology suitable for biomedical applications, and we exploit this to fabricate a small-footprint fiber-optic cryothermometer. A comparison of the two sample types offers fundamental insight into the photoluminescent behavior of silicon nanocrystal ensembles.

Optical absorption and emission of silicon nanocrystals: From single to collective response

We report on the possibility of describing the absorption and emission characteristics of an ensemble of silicon nanocrystals (NCs) with realistic distributions in the NC size, by the sum of the responses of the single NCs. The individual NC responses are evaluated by means of ab initio theoretical calculations and the summation is performed by taking into account the trend of the optical properties as a function of NC size and oxidation degree. The comparison with experimental results shows a nice matching of the spectra, also without any tuning of the parameters. Finally, the possibility of adapting the model in order to reproduce the experimental data is explored and discussed.

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.

Influence of the dielectric matrix on photoluminescence and energy exchange in ensembles of silicon nanocrystals

The results of a numerical simulation of photoluminescence in ensembles of Si nanocrystals incorporated into SiO2 and ZrO2 matrices are presented. It is shown that, in the ZrO2 matrix, which produces a lower potential barrier for electrons and holes in nanocrystals, the photoluminescence intensity decreases significantly and the spectral peak shifts towards lower energies.

Investigation of the dependence of the photoluminescence properties of silicon nanoclusters on their volume fraction in a silicon oxide matrix

The photoluminescence (PL) spectra and kinetics of amorphous and crystalline silicon nanoclusters are investigated. The given nanoclusters are formed by thermal annealing of thin suboxide silicon films with different volume fractions of silicon. It is demonstrated that the PL intensity and lifetime of the ensembles of silicon nanocrystals have a steplike dependence on the silicon volume fraction in the film. The influence of the percolation effect on the photoluminescence properties of the structures under study is discussed.

Formation of gold nanoparticles in single-layer and multi-layer ensembles of light-emitting silicon nanocrystals using ion implantation

The possibility of ion beam formation of gold nanoparticles in SiO2 and Al2O3 matrices with presynthesized single-layer and multi-layer ensembles of silicon nanocrystals is revealed by analyzing the spectra of optical transmission. The parameters of gold nanoparticles are estimated. It is shown that luminescent properties of Si nanocrystals are preserved under irradiation with gold ions.

Light Emission from Nanocrystalline Si Inverse Opals and Controlled Passivation by Atomic Layer Deposited Al2O3

3D-nanostructured ensembles of silicon nanocrystals forming inverse opals, directly prepared by magnesioreduction, exhibited intense photoluminescence in a broadband range (visible and NIR). Light emission, which strongly diminished under photoexcitation, was preserved by engineering the nanocrystals interface with atomic layer deposition of alumina. The high accessibility of the nanocrystals throughout the self-supported structure was a key factor for efficient photoluminescence stabilization.

Probing energy transfer in an ensemble of silicon nanocrystals

Time-resolved photoluminescence measurements of silicon nanocrystals formed by ion implantation of silicon into silicon dioxide reveal multi-exponential luminescence decays. Three discrete time components are apparent in the rise and decay data, which we associate with different classes of nanocrystals. The values of decay time are remarkably constant with emission energy, but the relative contributions of the three components vary strongly across the luminescence band. In keeping with the quantum confinement model for luminescence, we assign emission at high energies to small nanocrystals and that at low energies to large nanocrystals. By deconvolving the decay data over the full emission band, it is possible to study the migration of excitation from smaller (luminescence donor) to larger (luminescence acceptor) nanocrystals. We propose a model of diffusion of excitation between neighboring nanocrystals, with long lifetime emission being from the largest nanocrystal in the local neighborhood. Our data also allow us to study the saturation of acceptor nanocrystals, effectively switching off excitation transfer, and Auger recombination in non-interacting nanocrystals.

Luminescence and photosensitization properties of ensembles of silicon nanocrystals in terms of an exciton migration model

The relaxation processes that occur in ensembles of coupled silicon nanocrystals are described by a quantitative model that takes into account the energy transfer between them during exciton migration. This model is used to study the formation of singlet oxygen during the photoexcitation of silicon nanocrystals in porous silicon layers under various external conditions. It is experimentally found that, upon fine milling of as-deposited porous silicon films, the photoluminescence decay time increases despite an increase in the concentration of point defects. The photosensitized activity of ensembles of silicon nanocrystals degrades monotonically during their photostimulated oxidation. These experimental results agree completely with the conclusions of the model and are explained by a decrease in the number of exciton migration ways between nanocrystals when the granule size of a porous silicon powder decreases and by an increase in the efficiency of nonradiative recombination during the photostimulated oxidation of the nanocrystals. Our data indicate that nanocrystalline silicon is a promising material for the methods of nontoxic photodynamic therapy of oncological diseases.

Tunnel migration in ensembles of silicon nanocrystals doped with phosphorus

A comparative study of radiative recombination and tunnel migration in ensembles of P-doped Si nanocrystals embedded in wide-band dielectric matrix is performed. We have found the migration process to be strongly suppressed at donor concentration exceeding some critical value. At the same time, the radiative recombination rate gradually rises as the concentration increases. This essentially enhances the radiative-channel efficiency in the photoluminescence process.

Nonradiative relaxation in ensembles of silicon nanocrystals

The results of calculating the lifetimes of direct electron and hole tunneling and of nonradiative exciton transport between Si nanocrystals are presented. It is shown that tunnel transitions dominate in the process of the energy exchange between quantum dots and can considerably affect radiation in the ensemble of Si nanocrystals.