Nanocrystalline Si Research Articles

Improvement of the Anode Properties of Lithium-Ion Batteries for SiO x with a Third Element.

Silicon oxide (SiOx) has been placed into practical use as an anode active material for next-generation Li-ion batteries because it has a higher theoretical capacity than graphite anodes. However, the synthesis method is typically vapor deposition, which is expensive, and the poor electron conductivity of SiOx restricts high performance. In this study, we prepared M/SiOx active materials consisting of SiOx and a third element (M = Al, B, Sn) using a low-cost mechanical milling (MM) method and investigated their electrode properties as Li-ion battery anodes. Also, the authors added a third element to improve the conductivity of the SiO2 matrix. Al, B, and Sn were selected as elements that do not form a compound with Si, exist as a simple substance, and can be dispersed in SiO2. As a result, we confirmed that SiOx has a nanostructure of nanocrystalline Si dispersed in an amorphous-like SiO2 matrix and that the third element M exists not in the nanocrystalline Si but in the SiO2 matrix. The electron conductivity of SiOx was improved by the addition of B and Sn. However, it was not improved by the addition of Al. This is because Al2O3 was formed in the insulator due to the oxidization of Al. The charge–discharge cycle tests revealed that the cycle life was improved from 170 cycles to 330 or 360 cycles with the addition of B or Sn, respectively. The improvement in electron conductivity is assumed to make it possible for SiO2 to react with Li ions more uniformly and form a structure that can avoid the concentration of stress due to the volume changes of Si, thereby suppressing the electrode disintegration.

Tuning Inactive Phases in Si-Ti-B Ternary Alloy Anodes to Achieve Stable Cycling for High-Energy-Density Lithium-Ion Batteries.

Cycle stability improvement of a high-capacity Si anode is a challenge for its wide application in high-energy-density lithium-ion batteries. Active amorphous/nanosized Si embedded in an inactive matrix is a strategy to improve the cycle stability of Si anodes. Ternary Si100-x-yTixBy (5 ≤ y ≤ x ≤ 20) alloys are designed and prepared by ball milling using elemental Si, Ti, and B as starting materials. The formation sequence of inactive phases during mechanical alloying is predicted by an effective heat-of-formation model and verified by microstructural characterization. The local-fine distribution of free amorphous and nanocrystalline Si in the Si100-x-yTixBy is analyzed by confocal μ-Raman spectroscopy. When used as lithium-ion anodes, the capacity and voltage affected by Si and inactive compounds in the Si100-x-yTixBy are concerned to assess their high energy density. Furthermore, the impact of free active Si, the inactive phase, and amorphous Si on the cyclability of Si100-x-yTixBy is studied. The results show that the Si100-x-yTixBy material is a potential anode for high-energy-density Li-ion batteries and could be used to guide the design of multi-component Si-alloy anodes.

Hybrid Au@Si microspheres produced via laser irradiation in liquid for nonlinear photonics

Hybrid nanomaterials with chemical composition integrating light-emitting low-loss semiconductors with plasmon-active metals are highly demanded for optoelectronics, nanophotonics and sensors. However, there is a still lack for high-performing and inexpensive methods allowing facile integration of plasmonic and all-dielectric concepts within unified practically relevant nanostructures and ensuring their production at gram-per-hour yield. Here, we report one-step synthesis of hybrid Au@Si nanomaterial with unique composition and morphology via scalable and high-performing pulsed laser irradiation of isopropanol solution containing commercial Si micro-powder and AuCl4- ions. The resulting hybrid nanomaterial represents sub-micron nanocrystalline Si grains embedded into Au surrounding forming micro-spheres (MSs) additionally decorated with Au nanoparticles. Such unique structure and chemical composition of the Au@Si MSs permits to efficiently absorb and enhance incident radiation within rather broad spectral range spanning from visible to near-IR making the nanomaterial promising for plasmon-mediated amplification of linear and nonlinear optical effects. Efficient generation of broadband hot-carrier-induced photoluminescence of nanocrystalline Si grains upon IR femtosecond-laser sub-nJ pump proves the produced nanomaterial as advanced nanophotonic platform for nano-spectroscopy and sensing.

Quantitatively predicting modal thermal conductivity of nanocrystalline Si by full-band Monte Carlo simulations

Thermal transport of nanocrystalline Si is of great importance for the application of thermoelectrics. A better understanding of the modal thermal conductivity of nanocrystalline Si will be expected to benefit the efficiency of thermoelectrics. In this work, the variance reduced Monte Carlo simulation with full band of phonon dispersion is applied to study the modal thermal conductivity of nanocrystalline Si. Importantly, the phonon modal transmissions across the grain boundaries which are modeled by the amorphous Si interface are calculated by the mode-resolved atomistic Greens function method. The predicted ratios of thermal conductivity of nanocrystalline Si to that of bulk Si agree well with that of the experimental measurements in a wide range of grain size. The thermal conductivity of nanocrystalline Si is decreased from 54 percent to 3 percent and the contribution of phonons with mean free path larger than the grain size increases from 30 percent to 96 percnet as the grain size decreases from 550 nm to 10 nm. This work demonstrates that the full band Monte Carlo simulation using phonon modal transmission by the mode-resolved atomistic Greens function method can capture the phonon transport picture in complex nanostructures, and therefore can provide guidance for designing high performance Si based thermoelectrics.

Structures, Electronic Properties and Carrier Transport Mechanisms of Si Nano-Crystalline Embedded in the Amorphous SiC Films with Various Si/C Ratios.

Recent investigations of fundamental electronic properties (especially the carrier transport mechanisms) of Si nanocrystal embedded in the amorphous SiC films are highly desired in order to further develop their applications in nano-electronic and optoelectronic devices. Here, Boron-doped Si nanocrystals embedded in the amorphous SiC films were prepared by thermal annealing of Boron-doped amorphous Si-rich SiC films with various Si/C ratios. Carrier transport properties in combination with microstructural characteristics were investigated via temperature dependence Hall effect measurements. It should be pointed out that Hall mobilities, carrier concentrations as well as conductivities in films were increased with Si/C ratio, which could be reached to the maximum of 7.2 cm2/V∙s, 4.6 × 1019 cm−3 and 87.5 S∙cm−1, respectively. Notably, different kinds of carrier transport behaviors, such as Mott variable-range hopping, multiple phonon hopping, percolation hopping and thermally activation conduction that play an important role in the transport process, were identified within different temperature ranges (10 K~400 K) in the films of different Si/C ratio. The changes from Mott variable-range hopping process to thermally activation conduction process with temperature were observed and discussed in detail.

Determination of localized surface phonons in nanocrystalline silicon by inelastic neutron scattering spectroscopy and its application to deuterium isotope enrichment

The hydrogen (H) isotope deuterium (D) has attracted special interest for the manufacture of silicon (Si) semiconductors, Si microchips, and optical fibers, as well as for the synthesis of isotopically labeled compounds. However, the efficient production of D or H deuteride in a controlled manner is challenging, and rational H isotope enrichment protocols are still lacking. Here, we demonstrate a highly efficient exchange reaction from H to D on the surface of nanocrystalline Si (n-Si). Fourfold enrichment of D termination was successfully achieved by dipping n-Si into a dilute D solution. By determining the surface-localized vibrational modes for H- and D-terminated n-Si using inelastic neutron scattering spectroscopy, we found that the physical mechanism responsible for this enrichment originates from the difference in the zero-point oscillation energies and entropies of the surface-localized vibrations. Theoretically, the extent of enrichment could be greatly enhanced (\ensuremath{\sim}15 times) using a gas-phase reaction. This enrichment protocol, which avoids the use of precious metal catalysts, opens the way for sustainable H-to-heavy H exchange reactions.

Maintaining superior crystallinity and conductivity in boron-doped nc-Si ultra-thin films by hydrogen plasma treatment

In view of using doped nanocrystalline Si (nc-Si) as the emitter layer in nc-Si/crystalline-Si heterojunction solar cells, a very thin (≤ 50 nm) emitter layer with high conductivity and superior crystallinity is optimum. In the present work, p-nc-Si:H thin film prepared in inductively coupled plasma chemical vapor deposition, without using additional H2-dilution in [SiH4 + B2H6 (1% in H2)]-plasma, retains conductivity ~10−1 S cm−1 and crystallinity ~68% at a low thickness (t) ~50 nm. The charge carrier transport through the heavily doped nc-Si network demonstrates reverse Meyer–Neldel characteristics for t ≥ 50 nm. However, with reduction to t < 50 nm, the evolution of the ultra-nanocrystalline dominated amorphous network, accompanied by accumulation of Si–H–Si platelet-like components in the grain boundary, leads to a drastic reduction in conductivity to 10−4 S cm−1. In this context, the post-deposition and short-time hydrogen plasma treatment (PSHPT) on the as-deposited ultra-thin (t ~30 nm) film becomes instrumental in substantially enhancing the crystallinity from 40% to 69% and increasing conductivity by two orders of magnitude to 10−2 S cm−1. The network modification by PSHPT proceeds via transformation of the surface and sub-surface weak bonds and is significant for the ultra-thin film compared to the thick film, due to retaining a superior surface-to-bulk ratio.

Luminescence of Nanocrystalline Si in (Si + CaF2)/CaF2 Structures in the Visible Range of the Spectrum

Luminescence of Nanocrystalline Si in (Si + CaF2)/CaF2 Structures in the Visible Range of the Spectrum

Laser-printed hemispherical silicon Mie resonators

Subwavelength nanostructures made of high-index low-loss materials have revolutionized the fields of linear and nonlinear nanophotonics, stimulating growing demands for efficient and inexpensive fabrication technologies. Here, we demonstrate high-precision and reproducible printing of hemispherical Si nanoparticles (NPs) via controllable dewetting of glass-supported $\alpha$-Si films driven by a single femtosecond laser pulse. The diameter of the formed nanocrystalline NPs can be fully controlled by initial $\alpha$-Si film thickness as well as lateral size of the laser spot and can be predicted by a simple empirical model based on conservation of energy and mass. A resonant optical response associated with Mie-type resonances supported by hemispherical NPs was confirmed by combining numerical modeling with optical microspectroscopy. Inexpensive and high-performing direct laser printing of nanocrystalline Si Mie resonators with a user-defined arrangement opens a pathway for various applications in optical sensing and nonlinear nanophotonics.

Fiber Bragg grating-electromagnetically induced transparent fast optical switch

Here, an all-optical switch is modeled in the form of fiber Bragg grating (FBG) based on electromagnetically induced transparency (EIT) phenomena dealing with a three-level EIT silicon (Si) nanocrystalline medium. The EIT consists of two fields, namely a strong control and weak probe fields that interact with the medium. Fast-optical switching of the probe is achieved by applying a pulsed control field, which relies on both steady-state and transient density matrices. The effects of four significant parameters of interest, i.e., Rabi frequency of the control field, EIT nanocrystal density, FBG length, and spontaneous decay rate, have been extensively investigated to optimize the probe transmission. Subsequently, the time-dependent density matrix is solved to obtain the response time of the FBG-EIT switch. Eventually, FBG is modeled such that low index layers contain EIT material and the high index layers do not. The FBG-EIT switch satisfies the criteria of maximum transmission and minimum rise time to give out the optimal operational condition. Despite the optical switching is in OFF mode at Ωc=0 THz demonstrating a high FBG reflectance, the reflectance suddenly vanishes at a certain Ω c s to satisfy the equivalence of low and high refractive indices (switching ON). Therefore, a fast-optical switch is envisaged operating as large as 100 GHz in theory. The future applications of the FBG- EIT include the fast Q-switching of fiber lasers, ultra-fast modulation, optical quantum communication, and high speed optical processing and computation.

Derivation of Luminescent Mesoporous Silicon Nanocrystals from Biomass Rice Husks by Facile Magnesiothermic Reduction.

High-quality silicon (Si) nanocrystals that simultaneously had superior mesoporous and luminescent characteristics were derived from sticky, red, and brown rice husks via the facile and cost-effective magnesiothermic reduction method. The Si nanocrystals were confirmed to comprise an aggregated morphology with spherical nanocrystals (e.g., average sizes of 15–50 nm). Due to the surface functional groups formed at the nanocrystalline Si surfaces, the Si nanocrystals clearly exhibited multiple luminescence peaks in visible-wavelength regions (i.e., blue, green, and yellow light). Among the synthesized Si nanocrystals, additionally, the brown rice husk (BRH)-derived Si nanocrystals showed to have a strong UV absorption and a high porosity (i.e., large specific surface area: 265.6 m2/g, small average pore diameter: 1.91 nm, and large total pore volume: 0.5389 cm3/g). These are indicative of the excellent optical and textural characteristics of the BRH-derived Si nanocrystals, compared to previously reported biomass-derived Si nanocrystals. The results suggest that the biomass BRH-derived Si nanocrystals hold great potential as an active source material for optoelectronic devices as well as a highly efficient catalyst or photocatalyst for energy conversion devices.

Fabricating Femtosecond Laser-Induced Periodic Surface Structures with Electrophysical Anisotropy on Amorphous Silicon.

One-dimensional periodic surface structures were formed by femtosecond laser irradiation of amorphous hydrogenated silicon (a-Si:H) films. The a-Si:H laser processing conditions influence on the periodic relief formation as well as correlation of irradiated surfaces structural properties with their electrophysical properties were investigated. The surface structures with the period of 0.88 and 1.12 μm were fabricated at the laser wavelength of 1.25 μm and laser pulse number of 30 and 750, respectively. The orientation of the surface structure is defined by the laser polarization and depends on the concentration of nonequilibrium carriers excited by the femtosecond laser pulses in the near-surface region of the film, which affects a mode of the excited surface electromagnetic wave which is responsible for the periodic relief formation. Femtosecond laser irradiation increases the a-Si:H films conductivity by 3 to 4 orders of magnitude, up to 1.2 × 10−5 S∙cm, due to formation of Si nanocrystalline phase with the volume fraction from 17 to 28%. Dark conductivity and photoconductivity anisotropy, observed in the irradiated a-Si:H films is explained by a depolarizing effect inside periodic microscale relief, nonuniform crystalline Si phase distribution, as well as different carrier mobility and lifetime in plane of the studied samples along and perpendicular to the laser-induced periodic surface structures orientation, that was confirmed by the measured photoconductivity and absorption coefficient spectra.

Crystallite distribution analysis based on hydrogen content in thin-film nanocrystalline silicon solar cells by atom probe tomography

The three-dimensional (3D) distribution of nanosized silicon (Si) crystallites within a hydrogenated nanocrystalline Si (nc-Si:H) material is examined by laser-assisted atom probe tomography (APT). The amorphous and crystalline phases in nc-Si:H are distinguished by obtaining the 3D density distribution of H atoms, because the former contains a high H density. The H content in the amorphous phase is estimated to be approximately 15 at% by APT, which is consistent with that obtained by infrared spectroscopy. Thus, the 3D analysis of H distribution via APT is a powerful method to visualize the real shape of nanosized crystallites within nc-Si:H materials.

Fabrication of Anisotropic Structures on the Surface of Amorphous Silicon by Femtosecond Laser Pulses

Anisotropic periodic relief in form of ripples was formed on surface of amorphous hydrogenated silicon (a-Si:H) films by femtosecond laser pulses with the wavelength of 1.25 μm. The orientation of the surface structures relative to laser radiation polarization vector depended on the number of laser pulses N acting on the film surface. When N = 30, the structures with 0.88 μm period were formed orthogonal to the laser radiation polarization; at N = 750 the surface structures had period of 1.12 μm and direction parallel to the polarization. The conductivity of the laser-modified a-Si:H films increased by 3 to 4 orders of magnitude, up to 3.8·10–5 (Ω∙cm)–1, due to formation of nanocrystalline Si phase with a volume fraction from 17 to 30%. Anisotropy of the dark conductivity, as well as anisotropy of the photoconductivity spectral dependences was observed in the modified films due to depolarizing influence of periodic microscale relief and uneven distribution of nanocrystalline Si phase within such laser-induced structure.

Properties of nanocrystalline silicon probed by optomechanics

Abstract Nanocrystalline materials exhibit properties that can differ substantially from those of their single crystal counterparts. As such, they provide ways to enhance and optimize their functionality for devices and applications. Here, we report on the optical, mechanical and thermal properties of nanocrystalline silicon probed by means of optomechanical nanobeams to extract information of the dynamics of optical absorption, mechanical losses, heat generation and dissipation. The optomechanical nanobeams are fabricated using nanocrystalline films prepared by annealing amorphous silicon layers at different temperatures. The resulting crystallite sizes and the stress in the films can be controlled by the annealing temperature and time and, consequently, the properties of the films can be tuned relatively freely, as demonstrated here by means of electron microscopy and Raman scattering. We show that the nanocrystallite size and the volume fraction of the grain boundaries play a key role in the dissipation rates through nonlinear optical and thermal processes. Promising optical (13,000) and mechanical (1700) quality factors were found in the optomechanical cavity realized in the nanocrystalline Si resulting from annealing at 950°C. The enhanced absorption and recombination rates via the intragap states and the reduced thermal conductivity boost the potential to exploit these nonlinear effects in applications including Nanoelectromechanical systems (NEMS), phonon lasing and chaos-based devices.

Comparative study on P and B doped nano-crystalline Si multilayers

We fabricated P and B-doped Si nanocrystals (Si NCs) by high temperature annealing doped amorphous Si/SiO2 multilayers. We found that B impurity had the higher doping efficiency but the less activation efficiency than P atoms in Si NCs. The electrical properties of Si NCs films were systematically investigated by temperature-dependent conductivity and Hall measurements. The carrier transport properties exhibited the different characteristics after P and B-doping, especially the metal-like conductance performance at high doping levels. Our results revealed the different P and B doping behaviors in Si NCs, which is helpful for developing the Si NCs-based devices.

Unveiling the Effect of Surface and Bulk Structure on Electrochemical Properties of Disproportionated SiOx Anodes

AbstractAs a promising lithium‐ion battery anode materials, silicon suboxides (SiOx) exhibit elusive microstructure with atomic‐scale disproportionation which reveals a strong relation to performance. Generally, the structure of SiOx could be tuned via high‐temperature treatment. In this work, disproportionated SiOx are prepared to systematically discuss the internal relationship among microstructure, physicochemical properties and electrochemical performance. After annealing, amorphous SiO2 and nanocrystalline Si appear, and the SiO2 accumulating on the surface results in large resistance and low Li‐ion diffusivity, which leads to large overpotential and poor performance. If mechanical milling is employed followed by annealing, the SiO2 layer can be crushed, SiOx and Si with electrochemical activity are uncovered. Further research on the SiOx with diverse degrees of disproportionation indicates that proper amount of Si and SiO2 determines the optimal electrochemical performance. This result gives an in‐depth understanding and probable guidance to the modification investigation of SiOx anode for Li‐ion batteries.

Electron energy loss spectroscopy and energy filtered transmission electron microscopy of hydrogenated nano-crystalline Si thin films deposited via hydrogen profiling during hot-wire CVD for application in photovoltaics

Energy filtered transmission electron microscope imaging gives direct evidence of the nano-void network within hydrogenated nano-crystalline silicon thin films, deposited by reducing the hydrogen dilution as a function of deposition time (so called hydrogen profiling) during hot-wire chemical vapor deposition. Electron thickness maps show a t/λ value of approximately 0.38 in the void-rich areas compared to 0.47 in the continuous film, where t is the depth through which the electron beam is scattered and λ the electron mean free path in the Si thin film. Three-window elemental mapping shows no oxygen diffusion throughout the film architecture, whereas spectral imaging in combination with scanning transmission electron microscopy-coupled energy loss near edge fine-structure analyses along the growth direction of the film, across the tip and along the grain boundaries of the conically shaped crystals reveal changes in the Si-L3,2 and L1 energy-loss edges. In particular, the amorphous tissue between cone tips in the incubation layer typically shows a poorly defined L2 ionization edge, which progressively becomes more pronounced in the crystalline areas. Subtle variations in the L1 lineshape along the length of the film, characteristic of hydrogen passivation, confirm the profiling during synthesis.

Room temperature oxidation of Si nanocrystals at dry and wet air

Oxidation of HF vapor-etched nanocrystalline silicon films, prepared by drop coating from nanocrystalline Si sol in acetonitrile, was studied. Oxidation of nanocrystalline silicon at room temperature in air with 5% and 86% relative humidity was observed by means of infrared spectroscopy for 2 days. The change in film mass after 15 h of oxidation was determined using quartz crystal microbalance. In dry air, film mass and integral intensity of bands attributed to vibrations in Si3 − x–Si–Hx and Si–O–Si groups changed linearly with time. In humid air, intensity of in Si3 − x–Si–Hx band decays exponentially and intensity of Si–O–Si band increases as a square root of oxidation time. Film mass gain after 15 h of oxidation corresponds to an average oxide layer thickness of 0.02 nm in dry air and 0.51 nm in wet air.

Beneficial effects of Cr addition on the nanocrystalline Si and B modified Co-Zr permanent magnetic alloys

Co-Zr alloys with Si and B modifications show potential for rare earth-free permanent magnets. In this work, Cr was employed for improving the hard magnetic properties of melt-spun Co78-xZr16CrxSi3B3 (x = 0, 1, 2, 3, and 4) alloys. Their phase constitution, microstructure, and properties have been investigated. The results show that Cr addition can suppress the formation of the soft magnetic FCC-Co phase. The coercivity increased almost linearly with increasing Cr addition, accompanied by decreasing remanence. A high coercivity of 7.7 kOe was obtained in the alloy with 4 at.% Cr substitution for Co, manifesting a 67% coercivity enhancement from 4.6 kOe of the Cr-free alloy. Cr atoms were found to partially substitute Co atoms in the Co-Zr-Cr-Si-B alloys. The size of nanograin decreased with Cr addition alloys, partly due to the precipitation of the secondary Cr phase. After Cr addition, the enhanced positive peak in Henkel plots and the closed recoil loop indicate the improved exchange coupling and consistent magnetic reversal. The alloys with x = 0 and 4 exhibited much lower absolute values of coercivity temperature coefficient, 0.183%/℃ and 0.251%/℃, respectively, and one or two orders of magnitude smaller corrosion current density than that of melt-spun Nd-Fe-B alloys, demonstrating that Co-Zr based alloys have not only excellent thermal stability but also good corrosion resistance. In addition, Cr substitution can further reduce the corrosion rate of Co-Zr alloys in the 3.5 wt% NaCl solution.