Size Of Silicon Nanocrystals Research Articles

Photoluminescence and Stability of Sputtered SiOx Layers

SiOx layers with thicknesses of about 300–1000 nm are produced by sputtering silicon onto glass and quartz substrates. Silicon is oxidized during deposition, and the refractive index of the sputtered layers vary from 1.63 to 1.86. Oxygen atomic concentration in SiOx is in the range of . Photoluminescence of SiOx layers is observed and assumed to originate from silicon nanocrystals embedded in a SiOx matrix that is formed after thermal treatment by a Xe flash‐lamp. A layer on quartz substrate exhibits the strongest emission and shows Raman spectral peak at 507 , which is smaller than the value of 521 of bulk silicon crystal. Based on this difference, the size of silicon nanocrystals is estimated to be equal to 1.2 nm. The photoluminescence spectrum consists of overlapping Gaussian functions indicating the size distribution of the silicon nanocrystals. Mean silicon nanocrystals size in the layer on a quartz substrate is estimated at 1.4 nm from the photoluminescence peak datasets reported in previous researches. Thus, sputtering of silicon in a low‐pressure air and subsequent flash‐heating seems to be a convenient method to produce silicon nanocrystals in SiOx layers. These could be useful to realize Si‐LEDs or even silicon lasers. In addition, the aging of SiOx layers during 6 months is evaluated.

Formation of Silicon Nanocrystals under Friction

It is found using triboluminescence (TL) that nanocrystals of silicon (ncSi) with linear dimensions of about 4 nm are formed from a single crystal of silicon (Si) under friction. The time dependence of the intensity of TL signals is investigated. It consists of many superimposed signals. An analysis of their dynamics shows that nanocrystals are located on the microcrack faces formed during fracture. At the same time, a powder flies out of the friction region. Using the photoluminescence (PL) and Raman spectroscopy methods, the size of silicon nanocrystals in the powder was determined. It is about 2.2 nm. Probably, microcracks grow further in the time interval between friction and recording the Raman and PL spectra (several hours), which leads to an approximately twofold decrease in the size of nanocrystals.

Spectroscopic and Microscopic Correlation of SRO-HFCVD Films on Quartz and Silicon

This work is focused on making a correlation between results obtained by using spectroscopy and microscopy techniques from single and twofold-layer Silicon-Rich Oxide (SRO) films. SRO films single-layer and twofold-layer characterizations were compared considering the conditions as-grown and with thermal treatment at 1100 °C for 60 min in a nitrogen atmosphere. The thickness of the single-layer film is 324.7 nm while for the twofold-layer film it is 613.2 nm; after heat-treated, both thicknesses decreased until 28.8 nm. X-ray Photoelectron Spectroscopy shows changes in the excess-silicon in single-layer SRO films, with 10% in as-grown films and decreases to 5% for the heat-treated films. Fourier Transform Infrared Spectroscopy (FTIR) exhibits three characteristic vibrational modes of SiO2, as well as, the vibrating modes associated with the Si-H bonds, which disappear after the heat treatment. With UV–Vis spectroscopy results we obtained the absorbance and the absorption coefficient for the SRO films in order to calculate the optical bandgap energy (Egopt), which increased with heat-treatment. The energy peaks of the photoluminescence spectra were used to calculate the silicon nanocrystal size, obtaining thus an average size of 1.89 ± 0.32 nm for the as-grown layer, decreasing the size to 1.64 ± 0.01 nm with the thermal treatment. On the other hand, scanning electron microscopy and high-resolution transmission electron microscopy images confirm the thickness of the twofold-layer SRO films as 628 nm for the as-grown layer and 540 nm for the layer with heat-treatment, and the silicon nanocrystal size of 2.3 ± 0.6 nm for the films with thermal treatment.

Образование нанокристаллов кремния при трении

Abstract It is found using triboluminescence (TL) that nanocrystals of silicon (ncSi) with linear dimensions of about 4 nm are formed from a single crystal of silicon (Si) under friction. The time dependence of the intensity of TL signals is investigated. It consists of many superimposed signals. An analysis of their dynamics shows that nanocrystals are located on the microcrack faces formed during fracture. At the same time, a powder flies out of the friction region. Using the photoluminescence (PL) and Raman spectroscopy methods, the size of silicon nanocrystals in the powder was determined. It is about 2.2 nm. Probably, microcracks grow further in the time interval between friction and recording the Raman and PL spectra (several hours), which leads to an approximately twofold decrease in the size of nanocrystals.

Radiofrequency Hyperthermia of Cancer Cells Enhanced by Silicic Acid Ions Released During the Biodegradation of Porous Silicon Nanowires.

The radiofrequency (RF) mild hyperthermia effect sensitized by biodegradable nanoparticles is a promising approach for therapy and diagnostics of numerous human diseases including cancer. Herein, we report the significant enhancement of local destruction of cancer cells induced by RF hyperthermia in the presence of degraded low-toxic porous silicon (PSi) nanowires (NWs). Proper selection of RF irradiation time (10 min), intensity, concentration of PSi NWs, and incubation time (24 h) decreased cell viability to 10%, which can be potentially used for cancer treatment. The incubation for 24 h is critical for degradation of PSi NWs and the formation of silicic acid ions H+ and H3SiO4– in abundance. The ions drastically change the solution conductivity in the vicinity of PSi NWs, which enhances the absorption of RF radiation and increases the hyperthermia effect. The high biodegradability and efficient photoluminescence of PSi NWs were governed by their mesoporous structure. The average size of pores was 10 nm, and the sizes of silicon nanocrystals (quantum dots) were 3–5 nm. Degradation of PSi NWs was observed as a significant decrease of optical absorbance, photoluminescence, and Raman signals of PSi NW suspensions after 24 h of incubation. Localization of PSi NWs at cell membranes revealed by confocal microscopy suggested that thermal poration of membranes could cause cell death. Thus, efficient photoluminescence in combination with RF-induced cell membrane breakdown indicates promising opportunities for theranostic applications of PSi NWs.

Changes of the Thermodynamic Properties at Isochoric and Isobaric Decrease of the Silicon Nanocrystal Size

Equation of state P(ν/νo) and the baric dependences of the lattice and surface properties of silicon macro- and nanocrystals have been calculated using the method of calculation of crystal properties from the pair Mie–Lennard-Jones interatomic potential and the RP-model of nanocrystal. The isothermal dependences of P(ν/νo) for the macro- and the nanocrystal are shown to be intersected at a certain value of relative volume (ν/νo)0. The surface pressure becomes zero at the intersection point (at (ν/νo)0). The value of (ν/νo)0 decreases upon isomorphic–isomeric increase in temperature and also at isomorphic–isothermic decrease in the number of atoms N in the nanocrystal, or at isomeric–isothermic deviation of the nanocrystal shape from the most energetically optimal shape (in the RP-model, this shape is a cube). The obtained equation of state is used to study the changes of the silicon properties at isochoric (ν/νo = 1) and also isobaric (P = 0) decrease in N at temperatures 300 and 1000 K.

Size distribution and visible luminescence of silicon nanoparticles embedded in SiNx thin film: Role of RF power in PECVD

This paper presents, the studies of the influence of (radio frequency) RF power on the size distribution and visible photoluminescence (PL) of SiNx thin film deposited at 300[Formula: see text]C of substrate temperature by plasma enhanced chemical vapor deposition. RF power was varied (5–50[Formula: see text]W), and its aftereffect on the optical properties of thin films was investigated. By increasing the RF power between 5[Formula: see text]W and 25[Formula: see text]W, main PL peak showed a red shift with an increase in PL intensity, which is associated with an increase in the silicon nanocrystals size and density, respectively. Results obtained were confirmed with High-resolution transmission electron microscopy micrographs and from the statistical calculations. By attaining a precise RF power value, stable silicon nitride thin film with suitable optical properties can be achieved for the potential fabrication of optoelectronic devices.

Silicon Nanocrystals: It's Simply a Matter of Size

AbstractComposed of one of the most earth abundant, low cost and least toxic elements, silicon nanocrystals exhibit quantum and spatial confinement effects when their size is diminished to that of the exciton, around 5 nm. With the recent discovery of various means for synthesizing and separating silicon nanocrystals into narrow size distribution mono‐dispersions, it became possible for the first time to implement more deeply analytical studies of their size‐dependent chemical, physical and biological properties than was possible with poly‐dispersions. In this article we take a look at some recent studies of mono‐dispersions of silicon nanocrystals where it is apparent that size really matters. With this newfound knowledge we imagine the new directions that the field may take in the future. The topics covered include how silicon nanocrystal size is manifest on their (i) surface structure and reactivity, (ii) optical and electronic properties, (iii) chemical and photochemical stability, and (iv) biochemical and cytotoxicity behavior. The article concludes with a vision of what the future might hold for this important class of nanocrystals.

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.

Evaluation of optical and electronic properties of silicon nano-agglomerates embedded in SRO: applying density functional theory.

In systems in atomic scale and nanoscale such as clusters or agglomerates constituted by particles from a few to less than 100 atoms, quantum confinement effects are very important. Their optical and electronic properties are often dependent on the size of the systems and the way in which the atoms in these clusters are bonded. Generally, these nanostructures display optical and electronic properties significantly different to those found in corresponding bulk materials. Silicon agglomerates embedded in silicon rich oxide (SRO) films have optical properties, which have been reported to be directly dependent on silicon nanocrystal size. Furthermore, the room temperature photoluminescence (PL) of SRO has repeatedly generated a huge interest due to its possible applications in optoelectronic devices. However, a plausible emission mechanism has not been widely accepted in the scientific community. In this work, we present a short review about the experimental results on silicon nanoclusters in SRO considering different techniques of growth. We focus mainly on their size, Raman spectra, and photoluminescence spectra. With this as background, we employed the density functional theory with a functional B3LYP and a basis set 6-31G* to calculate the optical and electronic properties of clusters of silicon (constituted by 15 to 20 silicon atoms). With the theoretical calculation of the structural and optical properties of silicon clusters, it is possible to evaluate the contribution of silicon agglomerates in the luminescent emission mechanism, experimentally found in thin SRO films.

Synthesis and properties of nanosilicon prepared by homogeneous and heterogeneous reduction of tetraethyl orthosilicate

Nanocrystalline silicon was synthesized by reduction of tetraethyl orthosilicate both in homogeneous and heterogeneous processes with surface stabilization by nitrogen heterocyclic carbene (NHC) ligands. The presence of NHC ligands on the surface of silicon nanocrystals was demonstrated by IR spectroscopy. According to transmission electron microscopy data, the size of silicon nanocrystals is in the range of 2.2–8.4 nm. Photoluminescence of the samples is clearly recorded in the blue spectral region.

Breakdown point of quantum confinement photoluminescence and space-separation dependent energy transfer from silicon nanocrystals

A detailed photoluminescence (PL) analysis and transmission electron microscopy studies on silicon nanocrystals (SNCs) are presented to probe the carrier recombination mechanism. There exist obvious distinctive phenomena in different sizes of SNCs. Clear evidence for the breakdown point of quantum confinement PL is confirmed in small SNCs; it is regarded as inflection point for SNCs' corresponding electronic states. Moreover, the energy-transfer based carrier multiplication and SNCs nanotwin are also observed between neighboring nanocrystals in large ones. Finally, a systematic model has been proposed to explain the evolution of SNCs' PL mechanism along with the size and space-separation of SNCs.

Vplyv vysokotemperaturnoho vidpalu na strukturu ta kray vlasnoho pohlynannya tonkoplivkovoho kremniyu, lehovanoho olovom

Influence of isochronal annealing in the range of 350–1100 oC on the structural properties and the intrinsic absorption edge in thin silicon films doped with tin (a-SiSn) has been studied. It is found that as-deposited a-SiSn films with a tin content of about 4 at.%, unlike undoped a-Si ones, contain silicon nanocrystals with a crystallite size of about 4 nm and a crystalline fraction of about 65%. It is shown that, in the course of isochronal annealing of a-SiSn specimens in the interval of 350–1100 oC, the size of silicon nanocrystals in the amorphous matrix gradually increases to about 7 nm, and the fraction of crystalline phase to about 100%. Crystallization in undoped a-Si is observed only after the annealing at temperatures above 700 oC. The influence of tin on the optical band gap in a-Si as a function of the isochronal annealing temperature is analyzed.

Improved Performance of Silicon Nanocrystal Memories for Application Working Over a Wide Range of Temperature

The microelectronic industry requires more and more low consumption and high reliability solutions. In this scenario the silicon nanocrystal memories (Si-nc) are one of the most mature technologies able to replace the Flash floating gate in NOR embedded applications. The main advantages of Si-nc memories are: the full compatibility with the CMOS process using a reduced number of masks (1) and the robustness against SILC (2). In this paper we present an experimental work on the optimized silicon nanocrystal cell industrially manufactured, where the reliability is improved (3). In particular the impact of silicon nanocrystals size and temperature on cell programming window is shown. Moreover we demonstrated for the first time at our knowledge that it is possible to achieve 1Mcycles endurance characteristic in the temperature range of [-40°C; 150°C], programming by channel hot electron (CHE) and erasing by Fowler-Nordheim (FN).

The Effect of the MeV Si-Ion Irradiation on the Photoluminescence of Silicon Nanocrystals

Silicon nanocrystals embedded in silicon nitride films were irradiated with Si-ions at 8 MeV in order to modify their optical response. The samples were characterized by means of Rutherford Backscattering Spectrometry, Elastic Recoil Detection Analysis, High-Resolution Transmission Electronic Microscopy and Photoluminescence analysis. It was found a blue-shift in the photoluminescence emission from the as-grown films after they were irradiated with high energetic silicon ions. According to the quantum confinement theory, this fact is related to a decrease in size of the silicon nanocrystals, which means that a higher silicon fluence irradiation is related with a diminishing in silicon nanocrystal size.

Improved model of optical phonon confinement in silicon nanocrystals

We develop a model for calculating the Raman scattering spectra from phonons confined in for silicon nanocrystals, which is based on the familiar approach taking into account the uncertainty in the quasi-momentum of phonons localized in the nanocrystals. The model is considerably improved by taking into account dispersion of phonons not only in the magnitude of the quasi-momentum, but also in its direction. A significant refinement of the model is also due to the fact that phonon dispersion is calculated using the widely approved Keating model instead of being approximated by empirical expressions as was done in earlier approaches. The calculations based on this model make it possible to determine the sizes of silicon nanocrystals more precisely from analysis of the experimental Raman spectra.

Structure and Optical Properties of Silicon Nanocrystals Embedded in Amorphous Silicon Thin Films Obtained by PECVD

Silicon nanocrystals embedded in amorphous silicon matrix were obtained by plasma enhanced chemical vapor deposition using dichlorosilane as silicon precursor. The RF power and dichlorosilane to hydrogen flow rate ratio were varied to obtain different crystalline fractions and average sizes of silicon nanocrystals. High-resolution transmission electron microscopy images and RAMAN measurements confirmed the existence of nanocrystals embedded in the amorphous matrix with average sizes between 2 and 6 nm. Different crystalline fractions (from 12% to 54%) can be achieved in these films by regulating the selected growth parameters. The global optical constants of the films were obtained by UV-visible transmittance measurements. Effective band gap variations from 1.78 to 2.3 eV were confirmed by Tauc plot method. Absorption coefficients higher than standard amorphous silicon were obtained in these thin films for specific growth parameters. The relationship between the optical properties is discussed in terms of the different internal nanostructures of the samples.

Variation in the structure and optical properties of polymorphous silicon thin films using dichlorosilane as silicon precursor

AbstractPolymorphous silicon thin films were obtained by plasma enhanced chemical vapor deposition using dichlorosilane as silicon precursor. The RF power and the dichlorosilane to hydrogen flow rate ratio were varied to obtain different crystalline fractions and average sizes of silicon nanocrystals embedded in the amorphous silicon matrix. Microscopy images confirmed the existence of nanocrystallites with averages sizes between 2 and 6 nm. Broader size distributions were obtained with increasing RF power. Raman results confirmed that different nanocrystalline fractions (from 12% to 54%) can be achieved in these films by regulating the selected growth parameters. The optical band gap calculated by the Tauc model from UV‐visible transmittance measurements varies between 1.8 to 2.3 eV. The relationship between the optical properties is discussed in terms of the different nanostructures of the samples. Depending on their absorption properties and effective band gap, these materials can be suitable for application in thin film solar cell devices (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Anharmonicity of Phonons in Silicon: Raman Spectroscopy Study

Anharmonicity of phonons in silicon manifesting in dependence of peak frequency in Raman spectrum versus temperature in range from 80 to 900 K was studied. The obtained data allow to improve method of definition of average sizes of silicon nanocrystals using Raman scattering data analysis, in the case of the not room temperature of nanocrystals during spectroscopic measurements

GISAXS and GIWAXS analysis of amorphous–nanocrystalline silicon thin films

Amorphous–nanocrystalline silicon thin films were deposited by Plasma Enhanced Chemical Vapor Deposition (PECVD) on glass substrate with various silicon nano-crystal size distributions and volume fractions. The samples were examined by Grazing Incidence Small Angle X-ray Scattering (GISAXS) and Grazing Incidence Wide Angle X-ray Scattering (GIWAXS) at the Austrian SAXS beamline (Synchrotron Elettra, Trieste) using an X-ray beam energy of 8 keV. The grazing incidence angle varied from the critical angle to 0.2° above the critical angle. This allowed the examination of the samples at different depths, and the distinction of the surface scattering contribution from the particles scattering in the bulk. The sizes of the “particles” obtained from the horizontal and vertical sections of 2D GISAXS patterns were between 2 nm and 6 nm. Since GISAXS is sensitive to electron density differences (contrast) between the scattering bodies and the surrounding matrix, it is not evident whether the particles are nano-crystals or just voids embedded in amorphous matrix. However, the size of the crystals calculated from the line-shape analysis of peaks in GIWAXS spectra and the crystal size distribution obtained from High-Resolution Transmission Electron Microscopy (HRTEM) images agree well with the size of “particles” estimated from GISAXS, strongly indicating that the observed particles are silicon nano-crystals.