Si Nanolayers Research Articles

Architecting Sturdy Si/Graphite Composite with Lubricative Graphene Nanoplatelets for High-Density Electrodes.

Densification of the electrode by calendering is essential for achieving high-energy density in lithium-ion batteries. However, Si anode, which is regarded as the most promising high-energy substituent of graphite, is vulnerable to the crack during calendering process due to its intrinsic brittleness. Herein, a distinct strategy to prevent the crack and pulverization of Si nanolayer-embedded Graphite (Si/G) composite with graphene nanoplatelets (GNP) is proposed. The thickly coated GNP layer on Si/G by simple mechanofusion process imparts exceptional mechanical strength and lubricative characteristic to the Si/G composite, preventing the crack and pulverization of Si nanolayer against strong external force during calendering process. Accordingly, GNP coated Si/G (GNP-Si/G) composite demonstrates excellent electrochemical performances including superior cycling stability (15.6% higher capacity retention than P-Si/G after 300 cycles in the full-cell) and rate capability under the industrial testing condition including high electrode density (>1.6gcm-3) and high areal capacity (>3.5mAhcm-2). The material design provides a critical insight for practical approach to resolve the fragile properties of Si/G composite during calendering process.

Visualization-enhanced under-oil open microfluidic system for in situ characterization of multi-phase chemical reactions

Under-oil open microfluidic system, utilizing liquid-liquid boundaries for confinements, offers inherent advantages including clogging-free flow channels, flexible access to samples, and adjustable gas permeation, making it well-suited for studying multi-phase chemical reactions that are challenging for closed microfluidics. However, reports on the novel system have primarily focused on device fabrication and functionality demonstrations within biology, leaving their application in broader chemical analysis underexplored. Here, we present a visualization-enhanced under-oil open microfluidic system for in situ characterization of multi-phase chemical reactions with Raman spectroscopy. The enhanced system utilizes a semi-transparent silicon (Si) nanolayer over the substrate to enhance visualization in both inverted and upright microscope setups while reducing Raman noise from the substrate. We validated the system’s chemical stability and capability to monitor gas evolution and gas-liquid reactions in situ. The enhanced under-oil open microfluidic system, integrating Raman spectroscopy, offers a robust open-microfluidic platform for label-free molecular sensing and real-time chemical/biochemical process monitoring in multi-phase systems.

Design of phosphorus-doped porous hard carbon/Si anode with enhanced Li-ion kinetics for high-energy and high-power Li-ion batteries

With the rapid progress of portable electronics and electric vehicles, high-energy lithium-ion batteries (LIBs) with fast-charging technology are urgently required. Silicon (Si) is considered as the most promising anode candidate for high-energy LIBs but challenging to large-scale commercialization due to its huge volume change and poor conductivity. To develop high-capacity Si-based anode materials with enhanced lithium-ion diffusion and fast reaction kinetics, we design a novel high-Si-content Si/hard carbon composite via scalable methods. The combination between phosphorus-doped hard carbon with porous structure and uniformly distributed Si nanolayers effectively improve lithium-ion kinetics and mitigate the volume change of Si. As a result, the architecture delivers a reversible capacity of 1124 mAh/g at 0.1C and superior cycling stability with an 87.4% capacity retention after 200 cycles at 1C. The 1.5 A h pouch-type full-cell tests further demonstrate good cycling stability and high rate performance at 4C under an electrode density of 1.6 g cm−3 and areal capacity loading of 3.53 mAh cm−2. This work paves a new way for the rational design of Si-based anode materials for high-energy and high-power LIBs.

Au-Hyperdoped Si Nanolayer: Laser Processing Techniques and Corresponding Material Properties.

The absorption of light in the near-infrared region of the electromagnetic spectrum by Au-hyperdoped Si has been observed. While silicon photodetectors in this range are currently being produced, their efficiency is low. Here, using the nanosecond and picosecond laser hyperdoping of thin amorphous Si films, their compositional (energy-dispersion X-ray spectroscopy), chemical (X-ray photoelectron spectroscopy), structural (Raman spectroscopy) and IR spectroscopic characterization, we comparatively demonstrated a few promising regimes of laser-based silicon hyperdoping with gold. Our results indicate that the optimal efficiency of impurity-hyperdoped Si materials has yet to be achieved, and we discuss these opportunities in light of our results.

Ultrafast Infrared Laser Crystallization of Amorphous Si/Ge Multilayer Structures.

Silicon-germanium multilayer structures consisting of alternating Si and Ge amorphous nanolayers were annealed by ultrashort laser pulses at near-infrared (1030 nm) and mid-infrared (1500 nm) wavelengths. In this paper, we investigate the effects of the type of substrate (Si or glass), and the number of laser pulses (single-shot and multi-shot regimes) on the crystallization of the layers. Based on structural Raman spectroscopy analysis, several annealing regimes were revealed depending on laser fluence, including partial or complete crystallization of the components and formation of solid Si-Ge alloys. Conditions for selective crystallization of germanium when Si remains amorphous and there is no intermixing between the Si and Ge layers were found. Femtosecond mid-IR laser annealing appeared to be particularly favorable for such selective crystallization. Similar crystallization regimes were observed for both single-shot and multi-shot conditions, although at lower fluences and with a lower selectivity in the latter case. A theoretical analysis was carried out based on the laser energy absorption mechanisms, thermal stresses, and non-thermal effects.

Investigation of the interface between LiNbO3 and Si fabricated via room-temperature bonding method using activated Si nano layer

Wafer-level bonding of LiNbO3 and Si has been difficult to achieve owing to the large mismatch in their thermal expansion coefficients, which prevents the use of bonding methods involving annealing. As a solution, we have developed a room-temperature wafer-bonding method that uses an activated Si nanolayer as an adhesive. In this study, we analyzed the bond interface between LiNbO3 and Si that formed via this room-temperature bonding method. The atomic structures of the bonding interface of LiNbO3/Si and the debonded surfaces were investigated in detail. Furthermore, it was found that the bond strength between the activated Si nanolayers and Si was as strong as that of Si/Si bonded using the standard surface-activated bonding method. These findings provide evidence for a strong bond between LiNbO3 and Si at room temperature.

A scalable silicon/graphite anode with high silicon content for high-energy lithium-ion batteries

High-capacity Si-based anodes are urgently needed for high-energy lithium-ion batteries in forthcoming electric vehicles and energy storage systems. However, it is still a challenge to prepare Si anodes with high Si content materials as well as good electrochemical performance because the accumulation and growth of Si lead to large volume expansion, pulverization and cycling degradation. Herein, we demonstrate feasibility of a high-capacity Si-based anode via a scalable and simple route using high Si content (31.6 wt%) Si/graphite composite, consisting of thin Si nanolayer and edge-sites-exposed graphite. This architecture effectively alleviates the volume changes of Si and facilitates lithium-ion transport. As a result, the Si/graphite composite delivers a reversible capacity of 1,080 mAh/g with an initial Coulombic efficiency of 86.3% and achieves outstanding cycling stability (94.3% capacity retention after 100 cycles). When combined with commercial graphite-blended systems for a 1.5 Ah pouch-type full-cell test under a high electrode density (1.6 g/cm3) and areal capacity (3.46 mAh/cm2), the composite demonstrates excellent cycling stability at 25 and 45 °C, respectively, and good high-rate performance. This strategy offers a practical feasibility of next-generation high-capacity anode for high-energy LIBs.

Change of Bandgap Energy in Quantum System of Nanolayer on Silicon

In the quantum system of nanolayer (NL) on silicon, the bandgap energy obviously increases with the decrease of NL thickness, where the quantum confinement (QC) effect plays the main role as the thickness of Si NL changes along with (100), (110) and (111) directions, respectively. And the simulation result demonstrated that the direct bandgap can be obtained as the NL with (001) direction is thinner than 10 nm on Si surface. However, it is discovered in the simulated calculation that the QC effect disappears as the NL thickness arrives at the size of the monoatomic layer, in which its bandgap sharply decreases, where the abrupt change effect in bandgap energy occurs near-ideal 2D-layer. In the experiment, we fabricated the Si NL structure by using electron beam irradiation and laser deposition methods, in which a novel way was used to control the NL thickness by modulating irradiation time of the electron beam. The new effect should have a good application on a photonic-electronic chip of silicon.

Ultrawide-bandgap (6.14 eV) (AlGa)2O3/Ga2O3 heterostructure designed by lattice matching strategy for highly sensitive vacuum ultraviolet photodetection

One judiciously designed strategy of utilizing an ultrathin but conductive Ga2O3:Si nanolayer to prepare (AlGa)2O3 crystalline film is demonstrated. Benefiting from the existence of Ga2O3:Si nanolayer, a high-quality (Al0.68Ga0.32)2O3 sesquioxide film with 68 at.% aluminum was epitaxially grown on sapphire substrates, which was characterized by high-resolution transmission electron microscopy, X-ray photoelectron spectroscopy and X-ray diffraction. Its bandgap was broadened to 6.14 eV, and a vacuum ultraviolet (VUV) (AlGa)2O3/Ga2O3:Si photodetector was subsequently fabricated. The detector exhibits a pretty high on-off ratio of about 103, an open-circuit voltage of 1.0 V and a responsivity of 8.1 mA W−1 at 0 V bias voltage. The performances imply that the proposed strategy is valuable for improving the quality and also adjusting the bandgap of (AlGa)2O3 sesquioxides, which is expected to facilitate their application in VUV photodetection.

Bonding of LiNbO3 and Si wafers at room temperature using Si nanolayers

We report the room temperature bonding of LiNbO3 and Si wafers based on the use of Si nanolayers. The proposed method employs physical sputtering, which simultaneously activates the surface of an etched Si wafer and forms a Si nanolayer on the surface of a LiNbO3 wafer. Following sputtering, both wafers are immediately brought into contact and the newly formed Si nanolayer acts as a nanoadhesive. The data presented herein demonstrate that this technique is more effective at directly bonding LiNbO3 and Si than the conventional surface-activated bonding method. Following activation, the bonded surface energy, which reflects the bond strength, was estimated to be approximately 2.2 J m−2. This result indicates that the bonding was strong enough to withstand the processes associated with the fabrication of microelectronics devices, including wafer thinning.

X-ray photoelectron studies of near surface oxidation and plasmon excitation in spatially confined bi- and tri- layers periodic multilayer mirrors

The alternate bi- and tri- layers nanoscale periodic multilayers of Mo/Si, Mo/Be and Mo/Be/Si are useful for the high reflectivity mirrors, operating effectively around 13.5 nm of wavelength. In X-ray photoelectron spectroscopy (XPS), binding energy shift of BeO was observed in the Mo/Be structures. However, the chemical shifts of BeO and SiO2 was observed in the Mo/Be/Si multilayer structures. These evidences have indicated the near surface oxidation of multilayers. In the spatially confined layers, the plasmon was excited by the inelastic scattering of Mo4p photoelectrons, generated by the AlKα X-ray radiation of XPS. In the periodic multilayers, the plasmon was excited at higher energy as compared to the bulk counterpart, which was associated to quantum confinement of the condensed Si and Be nanolayers. In the bi- and tri- layers, the quantum confinement of plasmon in the Si nanolayer was fundamentally described by the density of valence electron and energy width of the band gap. However, in the bi- layer of Mo/Be, the plasmon shift of metallic Be layer was associated to confinement of the conduction electron density.

Realization of wafer-scale single-crystalline GaN film on CMOS-compatible Si(100) substrate by ion-cutting technique

Heterogeneous integration of gallium nitride (GaN) film on complementary metal-oxide-semiconductor (CMOS)-compatible Si(100) substrate provides a material platform for future high-performance chips with multiple functions. In this work, a 2 inch wafer-scale single-crystalline GaN film is transferred from commercialized bulk GaN wafer onto Si(100) substrate by combining ion-slicing and modified surface-activated bonding with a sputtering-deposited Si nanolayer. The H+ implantation fluence for the exfoliation of GaN film is as low as 2.5 × 1017 cm−2 and the full width at half maximum of the (0002) x-ray rocking curve of GaN film is 203 arcsec. The sliced bulk GaN wafer is recycled, which is beneficial to reduce the cost and to enhance the mass application of the ion-cutting technique to GaN. The exfoliation mechanism of H-implanted GaN is investigated. The activation energy for slicing GaN is only 2.08 eV owing to the high quality of the GaN wafer, while the wide residual damage band is still an obstacle to improving the quality of the GaN film. The successful demonstration of wafer-scale single-crystalline GaN film on Si(100) substrate will be of great benefit to the integration of high-performance GaN devices and Si CMOS integrated circuits with mature processing technology.

Crystal Structure and Band Gap of Nanoscale Phases of Si Formed at Various Depths of the Near-Surface Region of SiO2

Si nanophases and nanolayers were obtained by bombardment with Ar+ ions followed by annealing at various depths of silicon oxide. As ion energy E0 varies from 10 to 25 keV, the average depth of Si nanophase formation varies from 15 to 25 nm. It is shown that, as the sizes of Si nanophases vary from ~10 to 25 nm, band gap Eg decreases from 1.9 to 1.5 eV. For Si nanolayers, Eg is ~1.1–1.2 eV.

Calendering-Compatible Macroporous Architecture for Silicon-Graphite Composite toward High-Energy Lithium-Ion Batteries.

Porous strategies based on nanoengineering successfully mitigate several problems related to volume expansion of alloying anodes. However, practical application of porous alloying anodes is challenging because of limitations such as calendering incompatibility, low mass loading, and excessive usage of nonactive materials, all of which cause a lower volumetric energy density in comparison with conventional graphite anodes. In particular, during calendering, porous structures in alloying-based composites easily collapse under high pressure, attenuating the porous characteristics. Herein, this work proposes a calendering-compatible macroporous architecture for a Si-graphite anode to maximize the volumetric energy density. The anode is composed of an elastic outermost carbon covering, a nonfilling porous structure, and a graphite core. Owing to the lubricative properties of the elastic carbon covering, the macroporous structure coated by the brittle Si nanolayer can withstand high pressure and maintain its porous architecture during electrode calendering. Scalable methods using mechanical agitation and chemical vapor deposition are adopted. The as-prepared composite exhibits excellent electrochemical stability of >3.6 mAh cm-2 , with mitigated electrode expansion. Furthermore, full-cell evaluation shows that the composite achieves higher energy density (932 Wh L-1 ) and higher specific energy (333 Wh kg-1 ) with stable cycling than has been reported in previous studies.

Wafer Bonding of SiC-AlN at Room Temperature for All-SiC Capacitive Pressure Sensor.

Wafer bonding of a silicon carbide (SiC) diaphragm to a patterned SiC substrate coated with aluminum nitride (AlN) film as an insulating layer is a promising choice to fabricate an all-SiC capacitive pressure sensor. To demonstrate the bonding feasibility, a crystalline AlN film with a root-mean-square (RMS) surface roughness less than ~0.70 nm was deposited on a SiC wafer by a pulsed direct current magnetron sputtering method. Room temperature wafer bonding of SiC-AlN by two surface activated bonding (SAB) methods (standard SAB and modified SAB with Si nano-layer sputtering deposition) was studied. Standard SAB failed in the bonding, while the modified SAB achieved the bonding with a bonding energy of ~1.6 J/m2. Both the microstructure and composition of the interface were investigated to understand the bonding mechanisms. Additionally, the surface analyses were employed to confirm the interface investigation. Clear oxidation of the AlN film was found, which is assumed to be the failure reason of direct bonding by standard SAB.

The influence of Kanthal wire surface defects on the formation of Si nanolayer deposited by PVD method

Abstract The subject of this research is the structure of a Si nanolayer deposited on a FeCrAl wire surface by means of magnetron sputtering method. Si layer was selected as one of possible protections of the wire surface against excessive corrosive-erosive wear. In order to increase the power necessary for the DC discharge of the magnetron with Si cathode, a second magnetron with an aluminum disc as a cathode was used. The wire was attached to a carousel holder to ensure its rotation around the magnetron. The thickness of the deposited layers was about 150 nm. A wire surface examination indicated the presence of defects such as gaps between grains, cavities as well as severely deformed grains of surface layer. The research was conducted on the sample sections which had been prepared by focused ion beam method (FIB). The technique of transmission microscopy, which was used for observation, allowed us to obtain images in bright field (BF), dark field (DF), as well as in high resolution (HREM). The studies were also performed on the wire surface after the cutting process of the expanded polystyrene blocks. A metallographic optical microscope Nikon MA200 with a large depth of field was used for the examination which showed the presence of carbon deposit products. Additionally, a composition microanalysis was carried out along the line within selected areas of samples, with the use of energy dispersive spectroscopy (EDS). A large impact of wire surface defects on Si layer forming was found as well as a high direct homogeneous growth. The examination of the sections indicated the existence of a mechanism of defects sealed by Si layer, where directionality of grains growth in these areas revealed the tendency for vertical location relative to defects surface. Consequently, closed nanopores, i.e. spaces not covered with Si layer, were created. It is a characteristic feature of areas with defects covered with an oxide film created in a natural way.

Swift heavy ion track formation in nanoporous Si: Wave packet molecular dynamics study

In this article we study the process of silicon relaxation after swift heavy ion (SHI) irradiation using first-principles-based electron force field eFF. We show that early stages of so-called nonthermal melting take place in the nanometric viscinity of SHI trajectory in both bulk Si and Si nanolayer, but result in structural modifications only for the last one. Our results shed light on recent experimental evidence of significant reduction of threashold for SHI-induced damage creation in nanoporous Si.

The energy band structure of Si and Ge nanolayers

First-principles calculation based on density functional theory (DFT) with the generalized gradient approximation (GGA) were carried out to investigate the energy band gap structure of Si and Ge nanofilms. Calculation results show that the band gaps of Si(111) and Ge(110) nanofilms are indirect structures and independent of film thickness, the band gaps of Si(110) and Ge(100) nanofilms could be transfered into the direct structure for nanofilm thickness of less than a certain value, and the band gaps of Si(100) and Ge(111) nanofilms are the direct structures in the present model thickness range (about 7 nm). Moreover, the changes of the band gaps on the Si and Ge nanofilms follow the quantum confinement effects. It will be a good way to obtain direct band gap emission in Si and Ge materials, and to develop Si and Ge laser on Si chip.

NiMnGa/Si Shape Memory Bimorph Nanoactuation

The size dependences of thermal bimorph and shape memory effect of nanoscale shape memory alloy (SMA)/Si bimorph actuators are investigated in situ in a scanning electron microscope and by finite element simulations. By combining silicon nanomachining and magnetron sputtering, freestanding NiMnGa/Si bimorph cantilever structures with film/substrate thickness of 200/250 nm and decreasing lateral dimensions are fabricated. Electrical resistance and mechanical beam bending tests upon direct Joule heating demonstrate martensitic phase transformation and reversible thermal bimorph effect, respectively. Corresponding characteristics are strongly affected by the large temperature gradient in the order of 50 K/µm forming along the nano bimorph cantilever upon electro-thermal actuation, which, in addition, depends on the size-dependent heat conductivity in the Si nano layer. Furthermore, the martensitic transformation temperatures show a size-dependent decrease by about 40 K for decreasing lateral dimensions down to 200 nm. The effects of heating temperature and stress distribution on the nanoactuation performance are analyzed by finite element simulations revealing thickness ratio of SMA/Si of 90/250 nm to achieve an optimum SME. Differential thermal expansion and thermo-elastic effects are discriminated by comparative measurements and simulations on Ni/Si bimorph reference actuators.

Enhanced emission in the three-level system of Si and Ge nanostructures

Enhanced mission peak near 700nm is observed on the silicon quantum dots (QDs) embedded in Si amorphous film and the peak near 1100nm occurs on the silicon nanolayer, which have the emission characteristics of direct band-gap, such as the thresholds effect and the supper-line increasing effect in intensity with pumping in our experiment. It is interesting that the Si QDs embedded in nanosilicon layer are prepared by using pulsed laser deposition (PLD) method after annealing. In the same way, the peak near 900nm on the Ge QDs and the peak near 1500nm on the Ge nanolayer are measured in the PL spectra. It is very interesting that the sharper peaks with multi-longitudinal-mode occur in the Si and Ge nanolayers with the super-lattice on SOI in which the QDs are embedded. An emission model for Si and Ge laser on silicon chip with QDs pumping has been provided to explain the experimental results.