Crystalline Silicon Nanoparticles Research Articles

A Two-dimensional Assembly of Luminescent Silicon Nanocrystals at Air-Water Interface

We report the formation, through self-organization, of a two-dimensional assembly of crystalline silicon nanoparticles (SiNPs) made amphiphilic by attaching the long-chain fatty acid — Stearic Acid, at air-water interface. The bulk film of amphiphilized SiNP (ASiNP), floating on water surface, exhibits intense luminescence, under ultraviolet (UV) excitation, that can be viewed with the naked eye. A monolayer of ASiNP on water surface can be transferred horizontally onto any arbitrary substrate through a modified inverse Langmuir-Schaefer (MILS) deposition technique and it retains its luminescence. Brewster Angle Microscopy (BAM) of the film on water and Imaging Ellipsometry (IE) of transferred film show circular domains of diameter ∼ 100 μm, whereas Atomic Force Microscopy (AFM) of the latter show fractal patterns of height ∼ 1 nm and lateral size ∼ 10 nm, within these domains. Transmission electron microscopy (TEM) and the corresponding selected area electron diffraction (SAED) patterns reveal crystalline Si with very small sizes in the range of 2-10 nm having a lateral spread of several hundreds of nanometers. Spectroscopic investigations by UV-visible spectrometer of functionalized Si films reveals features that are very different from that of bulk Si and clearly bear signature of nanostructures. These thin films hold immense potential for applications in opto-electronic devices.

Operando Raman Spectroscopy and Synchrotron X-ray Diffraction of Lithiation/Delithiation in Silicon Nanoparticle Anodes.

Operando Raman spectroscopy and synchrotron X-ray diffraction were combined to probe the evolution of strain in Li-ion battery anodes made of crystalline silicon nanoparticles. The internal structure of the nanoparticles during two discharge/charge cycles was evaluated by analyzing the intensity and position of Si diffraction peaks and Raman TO-LO phonons. Lithiation/delithiation of the silicon under limited capacity conditions triggers the formation of "crystalline core-amorphous shell" particles, which we evidenced as a stepwise decrease in core size, as well as sequences of compressive/tensile strain due to the stress applied by the shell. In particular, we showed that different sequences occur in the first and the second cycle, due to different lithiation processes. We further evidenced critical experimental conditions for accurate operando Raman spectroscopy measurements due to the different heat conductivity of lithiated and delithiated Si. Values of the stress extracted from both operando XRD and Raman are in excellent agreement. Long-term ex situ measurements confirmed the continuous increase of the internal compressive strain, unfavorable to the Si lithiation and contributing to the capacity fading. Finally, a simple mechanical model was used to estimate the sub-nanometer thickness of the interfacial shell applying the stress on the crystalline core. Our complete operando diagnosis of the strain and stress in SiNPs provides both a detailed scenario of the mechanical consequences of lithiation/delithiation in SiNP and also experimental values that are much needed for the benchmarking of theoretical models and for the further rational design of SiNP-based electrodes.

Control of nanoparticle size and amount by using the mesh grid and applying DC-bias to the substrate in silane ICP-CVD process

The effect of applying a bias to the substrate on the size and amount of charged crystalline silicon nanoparticles deposited on the substrate was investigated in the inductively coupled plasma chemical vapor deposition process. By inserting the grounded grid with meshes above the substrate, the region just above the substrate was separated from the plasma. Thereby, crystalline Si nanoparticles formed by the gas-phase reaction in the plasma could be deposited directly on the substrate, successfully avoiding the formation of a film. Moreover, the size and the amount of deposited nanoparticles could be changed by applying direct current bias to the substrate. When the grid of 1 × 1-mm-sized mesh was used, the nanoparticle flux was increased as the negative substrate bias increased from 0 to – 50 V. On the other hand, when a positive bias was applied to the substrate, Si nanoparticles were not deposited at all. Regardless of substrate bias voltages, the most frequently observed nanoparticles synthesized with the grid of 1 × 1-mm-sized mesh had the size range of 10–12 nm in common. When the square mesh grid of 2-mm size was used, as the substrate bias was increased from – 50 to 50 V, the size of the nanoparticles observed most frequently increased from the range of 8–10 to 40–45 nm but the amount that was deposited on the substrate decreased.

Hydrogen-assisted spark discharge generation of highly crystalline and surface-passivated silicon nanoparticles

The ability to produce highly pure and crystalline nanoparticles with prescribed surface chemistry is of vital importance as these properties play crucial roles in enhancing the performance of nanoparticle-embedded opto- and nano-electronic devices. In this study, we sought to improve the purity, crystallinity, and surface passivation of silicon nanoparticles as they are produced via spark discharge generation by introducing a hydrogen atmosphere. When pure argon is used as the carrier gas, we find that some particles are oxidized (silicon oxide rather than silicon) due to the trace amounts of oxygen present in the system, and those that are not oxides are often amorphous, as the spark energy is insufficient to produce highly crystalline silicon nanoparticles. When hydrogen is introduced into the chamber, it creates a reducing environment within the spark discharge zone, which effectively reduces the formation of oxide particles. The portion of generated silicon oxide nanoparticles decreases as more hydrogen is introduced into the system. In addition, hydrogen plasma generated from the spark discharge process improved the crystallinity of the produced silicon nanoparticles, and aids passivation of the silicon nanoparticle surface by forming Si-Hx bonds, enhancing their stability during handling in oxygen containing atmosphere. Thus, the presented technique can be used to produce hydrogen-passivated silicon nanoparticles with high purity and crystallinity, and it is expected that these particles can be utilized as quantum dots after size-selection or be incorporated into luminescence devices.

Resistive switching behavior of SiOx layers with Si nanoparticles

First results on resistive switching in SiOx film containing crystalline silicon nanoparticles are reported. SiOx layers (x = 1.15) with thickness of 50 nm were deposited on n-Si crystalline substrates and annealed for 60 min at 1000oC to grow crystalline nanoparticles. Part of the samples were annealed in an inert atmosphere, while the rest were subjected to a two-step (O2+N2/N2) annealing process. Current-voltage (I-V) characteristics were by applying positive or negative voltage to the top contact. For both types of samples the I-V characteristics were asymmetric with lower currents measured at negative voltage, especially in the case of two-step annealed samples. In most of the N2 annealed structures switching behavior high-low/low-high resistance state was observed in both polarities at voltages with amplitudes in the range (2 - 4) V. Uncontrolled switching low/high resistance was also seen, more frequently at positive voltages. In contrast, the two-step annealed samples showed stable behavior. The transition high-low resistance state was achieved by negative voltages in the (-2, -5) V range leading to an increase of the current by more than three orders of magnitude. The structures were reset to the high resistive state, by positive voltage in the range (3 - 4) V. Uncontrolled switching was not observed in the two-step annealed samples for both polarities and they showed higher reliability regarding the number of switching cycles.

Photoluminescence from 20 MeV electron beam irradiated homogeneous SiOx and composite Si-SiOx films

The effect of 20 MeV electron irradiation on the room temperature photoluminescence from homogeneous SiOx and composite Si-SiOx films, containing amorphous or crystalline Si nanoparticles, is studied. Layers with x = 1.5 and 1.7 and thickness of 200 nm were deposited on crystalline silicon substrates by thermal evaporation of SiO in vacuum. Film annealing in an inert atmosphere at 700 oC or 1000 oC for 60 min was applied to grow amorphous or crystalline silicon nanoparticles, respectively, in a SiOx matrix. Samples from all types of films were irradiated with 20 MeV electrons at close to room temperature and a fluence of 2.4x1014 el.cm-2. Photoluminescence was measured under excitation with the 488 nm line of an Ar+ laser. The electron irradiation causes a decrease of the integrated photoluminescence intensity in composite samples with initial x = 1.7 containing amorphous or crystalline nanoparticles and x = 1.5 samples with Si nanocrystals. The electron irradiation of x = 1.5 samples with amorphous nanoparticles slightly increases the photoluminescence intensity. The obtained results are discussed in terms of electron beam induced phase separation and Si nanoparticle size increase.

Synthesis of Silicon Nanoparticles in Nonthermal Capacitively-Coupled Flowing Plasmas: Processes and Transport

Control of the size and material properties of silicon nanoparticles plays a critical role in optimizing applications using those nanoparticles, such as photovoltaics and biomedical devices. While synthesis of silicon nanoparticles in low temperature plasmas has many attractive features, the basic mechanisms leading to formation of nanoparticles in these plasmas are poorly understood. A two-dimensional numerical model for synthesis of silicon nanoparticles (<5 nm in diameter) in radio frequency (RF) discharges was developed and used to investigate mechanisms for particle growth for Ar/He/SiH4 gas mixtures. Algorithms for the kinetics of nanoparticle formation were self-consistently embedded into a plasma hydrodynamics simulation to account for nucleation, growth, charging, and transport of nanoparticles. We found that with RF excitation in narrow tubes at pressures of a few Torr, the electric field does not fully confine charged nanoparticles in the axial direction, which then results in a finite residence time of particles in the plasma. We found that because of the high neutral nanoparticle density, coagulation plays a significant role in growth. The model predicts the possibility of synthesizing crystalline silicon nanoparticles under these conditions. Trends in the growth of nanoparticles as a function of power are discussed.

Core-Shell Structural Evolution of Crystalline Silicon Nanoparticles upon Lithiation/Delithiation By Ex Situ Raman Spectroscopy and Operando Synchrotron X-Ray Diffraction

Core-shell structural evolution of crystalline silicon nanoparticles upon lithiation/delithiation by ex situ Raman spectroscopy and operando synchrotron X-ray diffraction Silicon has attracted substantial attention as an alternative anode material for next-generation Li-ion batteries (LiBs) as it has a high theoretical capacity of 3580 mA h g-1 that is about ten times that of the commonly used graphite. (1) Upon charging, Si and Li react via an alloying process with a sharp interphase separating the growing LixSi amorphous phase from the pristine crystalline Si. In case of spherical particles this leads to a core-shell structure captured in Figure1b and c. An enormous volume expansion of Si associated with alloying leads to cracking of the electrode material and rapid fading of its performance. This hinders Si from being widely used in LiBs. It has been reported that for Si nano-particles (diameter<150nm) volume expansion does not lead to cracking (2). To benefit from silicon unique properties and enable realization of smart electrode design, an enhanced understanding of structural evolution along charging/discharging is required. In the present work we were able to evidence the gradual amorphisation of crystalline Si nanoparticles upon cycling using ex situ Raman spectroscopy. Furthermore we probed the resulting stresses found in cores of the core-shell structures of Si nanoparticles and their evolution along lithiation/delithiation. Operando synchrotron X-ray diffraction (XRD) has also been performed to monitor the changes of the crystalline lattice during the initial cycles and to further quantify the constrained/unconstrained state of the silicon core. By means of combined ex situ Raman spectroscopy and in situ XRD we were able to provide in-depth understanding of the nanoparticles evolution. Figure 1 . Schematic representation of silicon electrodes: a) pristine b) after 1 cycle lithiation c) after 100 cycles lithiation d) corresponding Raman spectra. Black, blue and green, respectively. (All spectra were normalized to the maximum of the c-Si peak) 1. Obrovac, M. N., and Leif Christensen. "Structural changes in silicon anodes during lithium insertion/extraction." Electrochemical and Solid-State Letters 7.5 (2004): A93-A96. 2. Liu, Xiao Hua, et al. "Size-dependent fracture of silicon nanoparticles during lithiation." Acs Nano 6.2 (2012): 1522-1531. Figure 1

Plant-derived fluorescent silicon nanoparticles featuring excitation wavelength-dependent fluorescence spectra for anti-counterfeiting applications.

By using gramineae plants as natural and accessible reaction precursors, we herein introduce a green synthetic strategy, which is efficacious for the facile production of crystalline, excitation-wavelength-dependent fluorescent and small-sized silicon nanoparticles (SiNPs). We further explore the prepared SiNPs as a novel kind of fluorescent label for anti-counterfeiting applications.

Structural, compositional and electrical characterization of Si-rich SiOx layers suitable for application in light sensors

Metal-Oxide-Silicon (MOS) structures containing silicon nanoparticles (SiNPs) in three different gate dielectrics, single SiOx layer (c-Si/SiNPs-SiOx), two-region (c-Si/thermal SiOx/SiNPs-SiOx) or three-region (c-Si/thermal SiO2/SiNPs-SiOx/SiO2) oxides, were prepared on n-type (100) c-Si wafers. The silicon nanoparticles were grown by a high temperature furnace annealing of sub-stoichiometric SiOx films (x=1.15) prepared by thermal vacuum evaporation technique. Annealing in N2 at 700 or 1000°C leads to formation of amorphous or crystalline SiNPs in a SiOx amorphous matrix with x=1.8 or 2.0, respectively. The three-region gate dielectric (thermal SiO2/SiNPs-SiO2/SiO2) was prepared by a two-step annealing of c-Si/thermal SiO2/SiOx structures at 1000°C . The first annealing step was carried out in an oxidizing atmosphere while the second one was performed in N2. Cross-sectional Transmission Electron Microscopy and X-ray Photoelectron Spectroscopy have proven both the nanoparticle growth and the formation of a three region gate dielectric. Annealed MOS structures with semitransparent aluminum top electrodes were characterized electrically by current/capacitance–voltage measurements in dark and under light illumination. A strong variation of the current at negative gate voltages on the light intensity has been observed in the control and annealed at 700°C c-Si/SiNPs-SiOx/Al structures. The obtained results indicate that MOS structures with SiO1.15 gate dielectric have potential for application in light sensors in the NIR–Visible Light–UV range.

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.

Oxidation Induced Giant Modulation in the Luminescence of Colloidal Amorphous Porous Silicon Nanoparticles

ABSTRACTThe emission of crystalline silicon nanoparticles as well as nanowires can be tuned by varying their diameters. The diameter selection is achieved via a difficult chemical procedure that necessitates filtration which cannot be easily scaled up. Herein, we report a novel approach for producing and tuning the emission of freestanding colloidal of amorphous porous silicon nanoparticles (which should not be confused with bulk amorphous silicon nor with porous silicon) via a controlled oxidation without relying on size of nanoparticles. This oxidation increases local strain in the disordered network that causes orbital interactions which modifies the band-gap but a new hybridization.

Spectroscopic studies of SiOx films irradiated with high energy electrons

The effect of 20 MeV electron irradiation on the vibrational properties of homogeneous SiOx films and SiOx films containing amorphous or crystalline Si nanoparticles is studied. Layers with x = 1.15 and 1.3 and two different thicknesses, 200 nm and 1000 nm, were deposited on crystalline silicon substrates and annealed at 250 °C for stabilization. No nanoparticle growth occurs at this temperature. A part of the films were annealed in an inert atmosphere at 700 °C or 1000 °C for 60 min in order to grow amorphous (a-Si) or crystalline (nc-Si) silicon nanoparticles, respectively. Samples from all types of the films were irradiated with 20 MeV electrons at close to room temperature and a fluence of 2.4×1014 el.cm−2. The effect of the irradiation was investigated by means of Fourier Transform Infrared Absorption (FTIR) and Raman scattering spectroscopy. The FTIR results show no appreciable changes in the composition of the homogeneous SiOx films and the oxygen content in the matrix of a-Si- SiOx composite films. The electron irradiation causes an increase of the intensity of the light scattered from the pure silicon phase. The results obtained are discussed in terms of disorder decrease in the Si nanoparticles stimulated by MeV electron beam irradiation.

Generation of Silicon Nanostructures by Atmospheric Microplasma Jet: The Role of Hydrogen Admixture

Silicon nanostructures are synthesized with a DC atmospheric pressure microplasma jet using an Ar/SiH4/H2 gas mixture. The plasma is characterized by OES and imaged using an EMCCD camera. The effect of hydrogen admixture to the formed structures is studied by transmission electron microscopy. Under specific conditions, crystalline silicon nanoparticles grow in an amorphous matrix investigated by electron energy loss spectroscopy. As-grown silicon nanoparticles are collected in ethanol for dynamic light scattering and photoluminescence measurements. The size distribution peaks at 4 nm. The silicon nanocrystals exhibit roomtemperature photoluminescence that peaks at ∼415 and ∼465 nm.

Modification of silicon nanoparticle surface with gold or silver attenuates its biocompatibility in vitro

The effects of crystalline silicon (Si) nanoparticles covered with gold and silver on the viability and state of cellular organelles of cultured human peripheral blood lymphocytes were investigated. After interaction with Si nanoparticles covered with gold (Si/Au) or with silver (Si/Ag), the reactive oxygen species in cells increased, but their viability was not decreased. The Si/Au nanoparticles decreased functional activity of lysosomes and mitochondria, while Si/Ag decreased the functional activity only of mitochondria. It is concluded that modification of a surface with gold or silver results in a reduction of biocompatibility of crystalline silicon nanoparticles.

Size dependent melting of Silicon nanoparticles

Melting of crystalline silicon nanoparticles is studied by molecular dynamics (MD) simulations using Stillinger-Weber potential. Models are heated up from a crystalline to a normal liquid state. Temperature dependence of total energy and the Lindemann ratio exhibit a first-order-like behavior of the transition at the melting point. Heat capacity of the system presents a single peak at around the melting point. The size dependent melting is presented. As the size of the nanoparticles increases, the variation of the melting point becomes more monotonic and the temperature range of bistability shifts to higher temperatures. In large nanoparticles, the proportion of interior atoms increases and the average potential energy per atom converges to the bulk or thin films.

Invited: Atmospheric-Pressure Plasmas Processes for Third Generation Photovoltaics

Non-thermal atmospheric pressure plasmas (APPs) have attracted great interest due the potential cost reductions in manufacturing and processing of materials for a wide range of applications [1]. Although APPs present several challenges, considerable progress has been achieved in recent years with an increasing number of industrial and commercial applications utilizing different forms of APPs. One of the most attractive application of APPs is for the synthesis and processing of nanomaterials with specific focus on materials for energy applications.In this contribution we will present recent advances in both the synthesis and surface engineering of quantum confined silicon-based nanoparticles [2-5]. In particular we will discuss a range of synthesis methods that rely on atmospheric pressure plasmas for producing crystalline silicon nanoparticles as well as alloyed silicon-tin nanoparticles; these will include laser-produced plasmas, rf-APPs and plasmas in liquids. The analysis will include, in some of the cases, theoeretical models that highlight key mechanisms of nanoparticle heating [6].We will then discuss the integration of silicon-based nanoparticles in photovoltaic devices, demonstrating their contribution as down-converters and/or as part of the active layer. In this context we will show how APP-based surface engineering can offer great opportunities for tailoring surface properties of the nanoparticles. Device parameters will be presented and analysed for different device architectures.Finally, tha applicability of APPs in different aspect of photovoltaic device fabrication will be discussed presenting opportunities and challenges for large scale nanomanufacturing. Acknowledgement This work was partially supported by a NEDO project, the Royal Society International Exchange Scheme (IE120884), the Leverhulme International Network (IN-2012-136) and EPSRC (EP/K022237/1). DM also acknowledges the support of the JSPS Bridge Fellowship. SM and SA thank the financial support of the University of Ulster Vice-Chancellor Studentship.

Crystalline Si nanoparticles below crystallization threshold: Effects of collisional heating in non-thermal atmospheric-pressure microplasmas

Nucleation and growth of highly crystalline silicon nanoparticles in atmospheric-pressure low-temperature microplasmas at gas temperatures well below the Si crystallization threshold and within a short (100 μs) period of time are demonstrated and explained. The modeling reveals that collision-enhanced ion fluxes can effectively increase the heat flux on the nanoparticle surface and this heating is controlled by the ion density. It is shown that nanoparticles can be heated to temperatures above the crystallization threshold. These combined experimental and theoretical results confirm the effective heating and structure control of Si nanoparticles at atmospheric pressure and low gas temperatures.

Depassivation kinetics in crystalline silicon nanoparticles

The formation of silicon dangling bond (Si-db) defects in crystalline silicon nanoparticles (Si-NPs) is studied by electron paramagnetic resonance combined with vacuum-annealing experiments. The kinetics of Si-db formation due to H desorption is found to be reliably described by a first-order-rate thermal model with a mean activation energy ${E}_{d}=2.25$ eV and a spread ${\ensuremath{\sigma}}_{Ed}=0.28$ eV in the activation energy distribution. These values deviate from those reported in previous studies of other Si-based materials, which is attributed to the presence of different interfacial hydrides Si${}_{4\ensuremath{-}n}\ensuremath{-}$Si$\ensuremath{-}$H${}_{n}$. Hence, the generation Si-db defects in Si-NPs initiates at a much lower temperature than one would expect based on the previously reported kinetics parameters. Unlike the case of planar Si/SiO${}_{2}$ interfaces, no permanent interface degradation is observed upon annealing of Si-NPs at temperatures $\ensuremath{\gtrsim}$600 ${}^{\ensuremath{\circ}}$C. This, together with the observation of an interfacial Si-db density similar to that typically incorporated in high quality thermally-grown SiO${}_{2}$ on bulk silicon, indicates the formation of a rather relaxed and thermally stable surface oxide shell during natural oxidation of Si-NPs.

Silanization effect on the photoluminescence characteristics of crystalline and amorphous silicon nanoparticles

Silicon nanoparticles synthesized by two different methods were surface modified with 3-mercaptopropyltrimethoxysilane. The particles of ~2 nm size exhibit photoluminescence (PL) in the UV-Vis range of the spectrum. The most intense PL band at 430 nm with an emission lifetime of 1-2 ns is attributed to the presence of the surface defects Si-O-Si, generated after anchoring the organic molecule onto the interface. The excitation-emission matrix of this band is essentially independent of the technique of synthesis, crystalline structure, and size of the silicon nanoparticles.