Quantum confinement and surface chemistry of 0.8–1.6 nm hydrosilylated silicon nanocrystals

In the framework of density functional theory （DFT）, we have studied the electronic properties of alkene/alkyne- hydrosilylated silicon nanocrystals （Si NCs） in the size range from 0.8 nm to 1.6 nm. Among the alkenes with all kinds of functional groups considered in this work, only those containing -NH2 and -C4H3S lead to significant hydrosilylation- induced changes in the gap between the highest occupied molecular orbital （HOMO） and the lowest unoccupied molecular orbital （LUMO） of an Si NC at the ground state. The quantum confinement effect is dominant for all of the alkene- hydrosilylated Si NCs at the ground state. At the excited state, the prevailing effect of surface chemistry only occurs at the smallest （0.8 nm） Si NCs hydrosilylated with alkenes containing -NH2 and -C4H3S. Although the alkyne hydrosilylation gives rise to a more significant surface chemistry effect than alkene hydrosilylation, the quantum confinement effect remains dominant for alkyne-hydrosilylated Si NCs at the ground state. However, at the excited state, the effect of surface chemistry induced by the hydrosilylation with conjugated alkynes is strong enough to prevail over that of quantum confinement.

Effect of Hydrogen Dilution on Growth of Silicon Nanocrystals Embedded in Silicon Nitride Thin Film by Plasma-Enhanced CVD

An investigation was conducted into the effect of hydrogen dilution on the microstructure and optical properties of silicon nanograins embedded in silicon nitride （Si/SiNx） thin film deposited by the helicon wave plasma-enhanced chemical vapour deposition technique. With Ar-diluted SiH4 and N2 as the reactant gas sources in the fabrication of thin film, the film was formed at a high deposition rate. There was a high density of defect at the amorphous silicon （a-Si）/SiNx interface and a relative low optical gap in the film. An addition of hydrogen into the reactant gas reduced the film deposition rate sharply. The silicon nanograins in the SiNx matrix were in a crystalline state, and the density of defects at the silicon nanocrystals （nc-Si）/SiNx interface decreased significantly and the optical gap of the films widened. These results suggested that hydrogen activated by the plasma could not only eliminate in the defects between the interface of silicon nanograins and SiNx matrix, but also helped the nanograins transform from the amorphous into crystalline state. By changing the hydrogen dilution ratio in the reactant gas sources, a tunable band gap from 1.87 eV to 3.32 eV was obtained in the Si/SiNx film.

Photoluminescence from Silicon Nanocrystals in Encapsulating Materials

Naturally oxidized freestanding silicon nanocrystals （Si NCs） are incorporated in commonly used encapsulating materials to explore the photoluminescent application of Si NCs in device structures such as solid-state lighting light-emitting diodes （LEDs） and solar cells. The quantum yield of Si NCs before the incorporation has reached about 45% at the excitation wavelength of 370 nm without any special surface modification. It is found that medium Ioadings, e.g., 5 wt% of Si NCs in encapsulating materials help to obtain high external quantum efficiency （EQE） of the mixtures of Si NCs and encapsulating materials. The curing of encapsulating materials significantly reduces EQE. Among all the encapsulating materials investigated in this work, silicone- OE6551 enables the highest EQE （21% at excitation wavelength λex = 370 nm） after curing. Based on current findings, we have discussed the continuous efforts to advance the photoluminescent application of Si NCs.

Density Functional Theory Study on the Oxidation of Hydrosilylated Silicon Nanocrystals

As a leading surface modification approach, hydrosilylation enables freestanding silicon nanocrystals （Si NCs） to be well dispersed in a desired medium. Although hydrosilylation-induced organic layers at the NC surface may somehow retard the oxidation of Si NCs, oxidation eventually occurs to Si NCs after relatively long time exposure to air. We now investigated the oxidation of hydrosilylated Si NCs in the frame work of density functional theory （DFT）. Three oxygen configurations that may be introduced by the oxidation of a Si NC are considered. It is found that a hydrosilylated Si NC is less prone to oxidation than a fully H-passivated Si NC in the point of view of thermodynamics. At the ground state, backbond oxygen （BBO） and hydroxyl （OH） hardly change the gap between the highest occupied molecular orbital （HOMO） and the lowest unoccupied molecular orbital （LUMO） of a hydrosilylated Si NC. At the excited state, the decrease in the HOMO-LUMO gap induced by the introduction of doubly bonded oxygen （DBO） is more significant than that induced by the introduction of BBO or OH. We have correlated the changes in the optical absorption （emission） of a hydrosilylated Si NC after oxidation to those of the HOMO-LUMO gap at the ground state （excited state）.

Multi-chromatic silicon nanocrystals

Silicon nanocrystals(SiNCs)have great potential to become environmental friendly alternatives to heavy-metal containing nanocrystals for applications including medical imaging,lighting and displays.SiNCs exhibit excellent photostability,non-toxicity and abundant resources,but their often reported inefficient and spectrally limited light emission seriously impair their applications.Here we demonstrate a new method that converts SiNCs into an efficient and robust multi-chromatic phosphor.Using~15 keV electron-beam irradiation of oxide-capped SiNCs,we introduce several types of color centers into the nanocrystal’s oxide shell with efficient blue,green and red emission bands,together yielding warm-white photoluminescence,even for a single SiNC.Introduced centers are not native to the original system and we relate them to known defects in silica.Unlike in the silica host,however,here the centers are efficiently optically excitable.Provided further optimization and up-scaling of this method,e-beam irradiated SiNCs can be of great interest as white phosphors for applications such as LEDs.

Bright trions in direct-bandgap silicon nanocrystals revealed by low-temperature single-nanocrystal spectroscopy

Strain-engineered silicon nanocrystals(SiNCs)have recently been shown to possess direct bandgap.Here,we report the observation of a rich structure in the single-nanocrystal photoluminescence spectra of strain-engineered direct-bandgap SiNCs in the temperature range of 9–300 K.The relationship between individual types of spectra is discussed,and the numerical modeling of spectral diffusion of the experimentally acquired spectra reveals a common origin for most types.The intrinsic spectral shape is shown to be a structure that contains three peaks,approximately 150 meV apart,each of which possesses a Si phonon substructure.Narrow spectral lines,reaching ≤ meV at 20 K,are detected.The observed temperature dependence of the spectral structure can be assigned to the radiative recombination of positively charged trions,in contrast to several previous reports linking a very similar shape to phonons in the surface capping layers.Our result serves as strong additional support for the direct-bandgap nature of the investigated SiNCs.

Temperature-dependent Photoluminescence of Silicon Nanocrystals Embedded in SiO2 Matrix

Nanostructured silicon plays a key role in the fidds of microelectronics, optoelectronics and photonics. Since near-infrared photolummescence（PL） was first observed in nanoporous silicon at room temperature, silicon nanocrystals（Si-NCs） have attracted considerable attention due to their tunable structures and properties. Considerable efforts have recently been devoted to obtaining deeper insight in the PL emission of colloidal and solid Si-NCs and the applications of these materials.

Different charging behaviors between electrons and holes in Si nanocrystals embedded in SiN_x matrix by the influence of near-interface oxide traps

Si-rich silicon nitride films are prepared by plasma-enhanced chemical vapor deposition method, followed by thermal annealing to form the Si nanocrystals（Si-NCs） embedded in Si Nx floating gate MOS structures. The capacitance–voltage（C–V）, current–voltage（I–V）, and admittance–voltage（G–V） measurements are used to investigate the charging characteristics. It is found that the maximum flat band voltage shift（△VFB） due to full charged holes（～ 6.2 V） is much larger than that due to full charged electrons（～ 1 V）. The charging displacement current peaks of electrons and holes can be also observed by the I–V measurements, respectively. From the G–V measurements we find that the hole injection is influenced by the oxide hole traps which are located near the Si O2/Si-substrate interface. Combining the results of C–V and G–V measurements, we find that the hole charging of the Si-NCs occurs via a two-step tunneling mechanism. The evolution of G–V peak originated from oxide traps exhibits the process of hole injection into these defects and transferring to the Si-NCs.

Substrate-induced stress in silicon nanocrystal/SiO_2 multilayer structures

A Raman frequency upshift in the nc-Si phonon mode is observed at room temperature, which is attributed to a strong compressive stress in the Si nanocrystals. The 10-period amorphous-Si（3 nm）/amorphous-SiO2 （3 nm） layers are deposited by high-vacuum radio-frequency magnetron sputtering on quartz and sapphire substrates at different temperatures. The samples are then annealed in N2 atmosphere at 1100 ℃ for 1 h for Si crystallization. It is demonstrated that the presence of a supporting substrate at the high growth temperature can induce different types of stresses in the Si nanocrystal layers. The strain is attributed to the difference in the thermal expansion coefficient between the substrate and the Si/SiO2 SL film. Such a substrate-indueed stress indicates a new method for tuning the optical and electronic properties of Si nanocrystals for strained engineering.

Four-wave mixing in silicon-nanocrystal embedded high-index doped silica micro-ring resonator

A nonlinear integrated optical platform that allows the fabrication of waveguide circuits with different material composition,and at small dimensions,offers advantages in terms of field enhancement and increased interaction length,thereby facilitating the observation of nonlinear optics effects at a much lower power level.To enhance the nonlinearity of the conventional waveguide structure,in this work,we propose and demonstrate a microstructured waveguide where silicon rich layer is embedded in the core of the conventional waveguide in order to increase its nonlinearity.By embedding a 20 nm thin film of silicon nanocrystal(Si-nc),we achieve a twofold increase of the nonlinear parameter,γ.The linear relationship between the fourwave mixing conversion efficiency and pump power reveals the negligible nonlinear absorption and small dispersion in the micro-ring resonators.This simple approach of embedding an ultra-thin Si-nc layer into conventional high-index doped silica dramatically increases its nonlinear performance,and could potentially find applications in all-optical processing functions.

Step-by-Step Laser Crystallization of Amorphous Si：H/SiNx：H Multilayer for Active Layer in Microcavities

We report the crystallization and photoluminescence （PL） properties of amorphous Si：H/SiNx ：H multilayer （ML） films treated by step-by-step laser annealing. The results of Raman measurements show that the nanocrystalline Si （nc-Si） grains are formed in the a-Si：H layers under the constrained growth mechanism. The blue shift of PL peak with grain size is observed and can be attributed to the quantum confinement effect, For comparison, we also report the crystallization and PL of a-Si：H/SiNx：H ML samples by normal one-step treatment, This method of step-by-step laser treatment will be a candidate to make nc-Si quantum dots in amorphous Si：H/SiNx：H ML as an active layer in microcavities.

Electroluminescence Afterglow from Indium Tin Oxide/Si-Rich SiO2/p-Si Structure

Indium tin oxide/Si-rich SiO2/p-Si structured devices are fabricated to study the electroluminescence （EL） of the Si-rich SiO2 （SRO） material. The obvious peaks at～1050nm and ～1260nm in the EL are ascribed to localized state transitions of amorphous Si （a-Si） clusters. The EL afterglow associated with a-Si clusters is observed from this structure at room temperature, while the afterglow is absent in the case of optical pumping. It is believed that carrier-induced defects act as trap centres in the a-Si clusters, resulting in the EL afterglow. The phenomenon of the EL afterglow indicates the limits of EL performance and electrical modulation of the SRO material with a larger fraction of a-Si clusters.

Scaling dependence of memory windows and different carrier charging behaviors in Si nanocrystal nonvolatile memory devices

Based on the charge storage mode,it is important to investigate the scaling dependence of memory performance in silicon nanocrystal（Si-NC） nonvolatile memory（NVM） devices for its scaling down limit.In this work,we made eight kinds of test key cells with different gate widths and lengths by 0.13-μm node complementary metal oxide semiconductor（CMOS） technology.It is found that the memory windows of eight kinds of test key cells are almost the same of about1.64 V @ ±7 V/1 ms,which are independent of the gate area,but mainly determined by the average size（12 nm） and areal density（1.8×10^（11）/cm^2） of Si-NCs.The program/erase（P/E） speed characteristics are almost independent of gate widths and lengths.However,the erase speed is faster than the program speed of test key cells,which is due to the different charging behaviors between electrons and holes during the operation processes.Furthermore,the data retention characteristic is also independent of the gate area.Our findings are useful for further scaling down of Si-NC NVM devices to improve the performance and on-chip integration.

Multipeak-structured photoluminescence mechanisms of as-prepared and oxidized Si nanoporous pillar arrays

Silicon dominates the electronic industry, but its poor optical properties mean that it is not preferred for photonic applications. Visible photoluminescence （PL） was observed from porous Si at room temperature in 1990, but the origin of these light emissions is still not fully understood. This paper reports that an Si nanocrystal, silicon nanoporous pillar array （Si-NPA） with strong visible PL has been prepared on a Si wafer substrate by the hydrothermal etching method. After annealing in 02 atmosphere, the hydride coverage of the Si pillar internal surface is replaced by an oxide layer, which comprises of a great quantity of Si nanocrystal （nc-Si） particles and each of them axe encapsulated by an Si oxide layer. Meanwhile a transition from efficient triple-peak PL bands from blue to red before annealing to strong double-peak blue PL bands after annealing is observed. Comparison of the structural, absorption and luminescence characteristics of the as-prepared and oxidized samples provides evidence for two competitive transition processes, the band-to-band recombination of the quantum confinement effect of nc-Si and the radiative recombination of excitons from the luminescent centres located at the surface of nc-Si units or in the Si oxide layers that cover the nc-Si units because of the different oxidation degrees. The sizes of nc-Si and the quality of the Si oxide surface are two major factors affecting two competitive processes. The smaller the size of nc-Si is and the stronger the oxidation degree of Si oxide layer is, the more beneficial for the luminescent centre recombination process to surpass the quantum confinement process is. The clarification on the origin of the photons may be important for the Si nanoporous pillar array to control both the PL band positions and the relative intensities according to future device requirements and further fabrication of optoelectronic nanodevices.

Coloidal Silocon Nnaocrystals and Their Functionalization with Carbon Nanotubes

1 Results The silicon nanocrystals (Si-ncs) are quasi-0D materials with rather unique physical and chemical properties that raise hope for optoelectronic applications. The strategy to improve the Si-ncs performances, as ensemble, is to increase their concentration in the device active layer. In particularly, substantial interest has arisen in the development of numerous synthesis techniques to produce luminescent Si-ncs with high concentrations. In recent years, we have concentrated our activities on pr...