Silicon Nanosheets Research Articles

Liquid-Phase Exfoliated Silicon Nanosheets: Saturable Absorber for Solid-State Lasers.

As a newly-developed two-dimensional (2D) material of group-IVA, few-layer silicon (Si) nanosheets were prepared by the liquid phase exfoliation (LPE) method. Its non-linear saturable adsorption property was investigated by 532 and 1064 nm nanosecond lasers. Using Si nanosheets as the saturable absorber (SA), passive Q-switched all-solid-state lasers were demonstrated for the first time. For different laser emissions of Nd3+ at 0.9, 1.06, and 1.34 µm, the narrowest Q-switched pulse widths were 200.2, 103.7, and 110.4 ns, corresponding to the highest peak powers of 2.76, 2.15, and 1.26 W. The results provide a promising SA for solid-state pulsed lasers and broaden the potential application range of Si nanosheets in ultrafast photonics and optoelectronics.

Two-dimensional porous silicon nanosheets as anode materials for high performance lithium-ion batteries.

In this paper, silicon nanosheets (Si-NSs) are chemically synthesized by using graphene oxide nanosheets as the template. The obtained Si-NSs, which are aggregations of silicon nanocrystals with a size of ∼10 nm, are applied directly as the anode material for lithium ion batteries, delivering a reversible capacity of 800 mA h g-1 after 900 cycles at a rate as high as 8400 mA g-1. Ex situ measurements and in situ observations show the positive effect of the mesoporous structure on the structural stability of Si-NSs. The evolution and survivability of the porous structures during lithiation and delithiation processes are investigated by molecular dynamics simulations, demonstrating that the porous structure can enhance the amount of "active" Li atoms during the stable stage of cycling and therefore promote mass capacity. The longer the survival of the porous structure, the longer the high mass capacity can be retained.

Catalytic CO2 reduction by palladium-decorated silicon–hydride nanosheets

Heterogeneous conversion of CO2 to fuels by Si surface hydrides has recently attracted broad research interest. Being earth-abundant, low-cost and non-toxic, elemental Si is a very attractive candidate for this process, which targets CO2 conversion to synthetic fuels on a gigatonne-per-year scale. It is well known, however, that silicon hydrides react stoichiometrically with CO2, and all attempts have failed to achieve catalytic conversion. The problem originates from the formation of inactive silanols and siloxanes with permanent loss of Si hydrides. Here, we deposit Pd on the surface of Si nanosheets, aiming to address the core of the problem. An operando infrared study shows Si hydrides successfully regenerating on such surfaces exposed to CO2 and H2. We demonstrate that silicon–hydride nanosheets decorated with Pd nanoparticles can enable the reverse water–gas shift reaction in a catalytic cycle. Silicon–hydride materials are attractive candidates for the photoreduction of carbon dioxide into fuels, although they have only worked stoichiometrically so far. Now, Ozin and co-workers show how decorating silicon nanosheets with palladium nanoparticles renders the process catalytic.

Silicon nanosheets as co-initiators for diaryliodonium induced radical and cationic polymerization

A system of silicon nanosheets and a diaryliodonium salt was found to initiate cationic and radical polymerizations. The polymerization relies on a syngergistic interaction between the silicon nanomaterial and the diaryliodonium salt, whereby the silicon nanomaterial acts as a co-initiator, inducing the decomposition of the diaryliodonium salt. The decomposition products, in turn, are able to initiate both cationic and radical polymerizations thereby enabling a mild and straightforward reaction procedure to obtain a variety of polymer/nanomaterial composites with cationically and radically polymerizable monomers. Most importantly, this work highlights the potential of using silicon nanomaterials’ unique properties not just for physical applications, but also in chemical applications.

Functionalized and oxidized silicon nanosheets: Customized design for enhanced sensitivity towards relative humidity

The use of completely oxidized two-dimensional (2D) silicon nanosheets (SiNSs) represents a novel approach for the application of 2D silicon-based materials in the nanoelectronics field. Densely stacked and highly porous oxidized SiNSs (OSiNSs) act as a sensitive layer for humidity detection. Due to the oxidation-caused porosity of the SiNSs and the possibility functionalize the 2D surface with hydrophilic groups, this hybrid material exhibits an extremely good sensitivity towards relative humidity (RH). In this work, precise tuning of the SiNSs’ sensing properties by their functionalization is demonstrated. In particular, the modification with methacrylic acid (MAA) groups, leading to SiNS-MAA, and the subsequent deposition on interdigitated electrodes double the capacitance value in the range of 20–85%RH. These values were achieved after the full oxidation of SiNS-MAA in ambient conditions. The mentioned changes in capacitance are extremely high compared to the response of the so far known common polymer humidity sensors. Contrary to that, this response is neutralized when the SiNSs are functionalized with tert-butyl acrylic acid (tBMA), a rather hydrophobic functional group. The fabricated devices show, how the specific functionalization of SiNSs serves as a reliable tool to provide sensitivity towards RH. Similar approach, based on tuning the functionality, can be applied to achieve e.g., sensor array selectivity. For this purpose, the functional groups on the surface of the nanomaterial can be further modified. Additional molecules with sensitivities towards various surrounding conditions could be attached. Furthermore, these functional molecules can be used for subsequent (bio)molecule immobilization, which can serve as sensitive molecular groups towards surrounding substrates and gases. However, one of the main challenges in sensor technology is to find a highly selective solution: a sensor system capable to differentiate among different vapor species. The described strategy can serve as an access towards new and promising solutions, which can help to face this issue in modern nanomaterials-based technology.

Pressure-driven phase transitions and reduction of dimensionality in 2D silicon nanosheets

In-situ high-pressure synchrotron X-ray powder diffraction studies up to 21 GPa of CVD-grown silicon 2D-nanosheets establish that the structural phase transitions depend on size and shape. For sizes between 9.3(7) nm and 15.2(8) nm we observe an irreversible phase transition sequence from I (cubic) → II (tetragonal) → V (hexagonal) during pressure increase and during decompression below 8 GPa the emergence of an X-ray amorphous phase. High-angle annular dark field scanning transmission electron microscopy (HAADF-STEM) and atomic force microscopy (AFM) images of this X-ray amorphous phase reveal the formation of significant numbers of 1D nanowires with aspect ratios > 10, which are twinned and grow along the <111> direction. We discovered a reduction of dimensionality under pressure from a 2D morphology to a 1D wire in a material with a diamond structure. MD simulations indicate the reduction of thermal conductivity in such nanowires.

Vermiculite derived porous silicon nanosheet as a scalable and low cost anode material for lithium-ion batteries

Silicon is considered to be a promising candidate as the anode material for next-generation lithium-ion batteries. However, Si-based material is still facing great challenge to be commercialized due to the unsatisfied cycle life and rate capability. Here, we demonstrate a facile and low-cost method to prepare two-dimension (2D) silicon nanosheets from natural vermiculite, which is an earth-abundant non-metallic mineral with stratified structure. The obtained porous 2D silicon material (Ver-Si) exhibits promising electrochemical performance with a reversible capacity of 900 mAh g−1 and high capacity retention. The promising electrochemical performance is originated from the unique 2D mesoporous structure, which can facilitate charge (Li-ion and electron) transportation and electrolyte penetration, as well as accommodate the volume changes during cycling.

Performance Comparison of &lt;inline-formula&gt; &lt;tex-math notation="LaTeX"&gt;${n}$ &lt;/tex-math&gt; &lt;/inline-formula&gt;–Type Si Nanowires, Nanosheets, and FinFETs by MC Device Simulation

The performance of n-type silicon nanosheets, nanowires, and FinFETs is benchmarked by Monte Carlo (MC) device simulation. Measurements of nanowire transfer characteristics are provided to validate the MC model supplemented by a corresponding comparison for nanosheets with literature data. At an OFF-current per effective gate width of 10 nA/ $\mu \text{m}$ , the ON-currents of nanowires, FinFETs, and nanosheets are 500, 545, and 570 $\mu \text{A}/\mu \text{m}$ , respectively. A major reason for this inferior nanowire performance is found to be the stronger impact of surface roughness scattering in nanowires due to a higher surface-to-volume ratio. However, the nanowires’ performance disadvantage is reduced for shorter gate lengths due to their better electrostatic control and reduced impact of surface roughness upon scaling.

Optical Conductivity of Two-Dimensional Silicon: Evidence of Dirac Electrodynamics.

The exotic electrodynamics properties of graphene come from the linearly dispersive electronic bands that host massless Dirac electrons. A similar behavior was predicted to manifest in freestanding silicene, the silicon counterpart of graphene, thereby envisaging a new route for silicon photonics. However, the access to silicene exploitation in photonics was hindered so far by the use of optically inappropriate substrates in experimentally realized silicene. Here we report on the optical conductivity of silicon nanosheets epitaxially grown on optically transparent Al2O3(0001) from a thickness of a few tens of nanometers down to the extreme two-dimensional (2D) limit. When a 2D regime is approached, a Dirac-like electrodynamics can be deduced from the observation of a low-energy optical conductivity feature owing to a silicene-based interfacing to the substrate.

Energy filtering in silicon nanowires and nanosheets using a geometric superlattice and its use for steep-slope transistors

This paper investigates energy filtering in silicon nanowires and nanosheets by resonant electron tunneling through a geometric superlattice. A geometric superlattice is any kind of periodic geometric feature along the transport direction of the nanowire or nanosheet. Multivalley quantum-transport simulations are used to demonstrate the manifestation of minibands and minibandgaps in the transmission spectra of such a superlattice. We find that the presence of different valleys in the conduction band of silicon favors a nanowire with a rectangular cross section for effective energy filtering. The obtained energy filter can consequently be used in the source extension of a field-effect transistor to prevent high-energy electrons from contributing to the leakage current. Self-consistent Schrödinger-Poisson simulations in the ballistic limit show minimum subthreshold swings of 6 mV/decade for geometric superlattices with indentations. The obtained theoretical performance metrics for the simulated devices are compared with conventional III-V superlatticeFETs and TunnelFETs. The adaptation of the quantum transmitting boundary method to the finite-element simulation of 3-D structures with anisotropic effective mass is presented in Appendixes A and B. Our results bare relevance in the search for steep-slope transistor alternatives which are compatible with the silicon industry and can overcome the power-consumption bottleneck inherent to standard CMOS technologies.

Surface Engineering of Two-Dimensional Hydrogenated Silicon Nanosheets for Tailored Applications

Approaching the predicted limits by Moor’s law, which come with the shrinking size of electronics, science needs to find alternatives for the fabrication of novel devices. Within the last decades, more and more hybrid materials have been developed based on 0D- 1D- and also 2D nanomaterials. This development is also driven by the search for fast, cheap, reproducible and environmental-friendly low-cost devices. Hydrogenated silicon nanosheets (SiNSs) consist of just a buckled monolayer of sp3-hybridized silicon atoms, which are decorated with hydrogen atoms. The sheets show green and consistent photoluminescence at ~510 nm and are predicted show semiconducting behaviour. SiNSs may be used as an alternative for not only bulk silicon, but also already known two dimensional materials. Well-studied and established nanosilicon surface chemistry offers solid ground for precise surface engineering, leading to control of the nanomaterial for reproducible device fabrication. New applications and processing techniques can thus be developed, which lead to facile fabrication of enhanced sensors and (opto)electronics. In this regard, research pursues additionally the combination of organic materials’ properties with those of the nanomaterials, building the basis for new devices. One example is the combination of the widely used organic semiconductor poly(3-hexylthiophene-2,5-diyl) with the precisely modified SiNSs. Again, surface engineering plays the key role in the hybrid materials’ synthesis and subsequent application. This gives the opportunity to tune the properties of the hybrid material in the desired way. In this context, we present different ways of surface engineering with the modification of SiNSs as an example for the subsequent application in various (opto)electronic devices (e.g., SGFETs and photonic sensors).

(Invited) Epitaxy of (SiGe/Si) Superlattices for the Fabrication of Horizontal Gate-All-Around Nanosheet Transistors

Stacked horizontal Gate All Around Nanosheet transistors were recently proposed as a replacement of FinFET for the sub-7nm device nodes. In order to stack Nanosheet channels, a specific SiGe/Si superlattice epitaxy is employed, with SiGe being used as a sacrificial layer that will define the suspension thickness (Tsus) between the channels. In this paper, we will consider the optimization of SiGe/Si multilayer epitaxy for Nanosheet channel definition (Fig. 1). Stacks with different Ge compositions are physically characterized and studied. With strain characterization and x-ray diffraction measurements on (100) substrates, we will show that it is possible to form pseudomorphic defect-free SiGe/Si multilayers over a wide range of Ge concentrations in the sacrificial SiGe (Fig. 2 and 3). The uniformity obtained on blanket 300mm substrates will be presented (Fig. 4), with a discussion on the SiGe/Si interface abruptness. In the context of CMOS device integration, it is necessary to employ a low temperature STI process to mitigate the Ge inter-diffusion in the silicon channel and to avoid the possible generation of defects from plastic relaxation of SiGe. The SiGe and Ge-Si intermixed regions are removed with appropriate selective etch process in two Nanosheet specific modules: inner spacer formation in the extension regions (Fig. 5) and post channel release in the gate regions (Fig. 6), leaving only 5nm intrinsic silicon Nanosheets. We will report the etch rate and selectivity of our in-house vapor-phase channel release process performed at low temperature. This process enables us to remove SiGe versus Si with a great selectivity and a limited amount of silicon loss during channel release, before gate stack deposition around the stacked Nanosheets. From a device perspective, we will show that the optimization of the silicon thickness in the SiGe/Si superlattice plays an important role in the Nanosheet device performance and electrostatics (Fig. 7). Finally, with a good SiGe/Si multilayer uniformity control, a reduced thermal budget and after optimization of our in-house SiGe selective etch process, we will show the capability to form 5nm silicon Nanosheet channels with small within wafer variation, low device variability and good process control, compatible with the fabrication of very large scale integrated circuits for future CMOS device nodes. Acknowledgements: This work was performed by the Research Alliance Teams at various IBM Research Facilities. Figure 1

Indirect-to-Direct Band Gap Transition of Si Nanosheets: Effect of Biaxial Strain

The effect of biaxial strain on the band structure of two-dimensional silicon nanosheets (Si NSs) with (111), (110), and (001) exposed surfaces was investigated by means of density functional theory calculations. For all the considered Si NSs, an indirect-to-direct band gap transition occurs as the lateral dimensions of Si NSs increase; that is, increasing lateral biaxial strain from compressive to tensile always enhances the direct band gap characteristics. Further analysis revealed the mechanism of the transition which is caused by preferential shifts of the conduction band edge at a specific k-point because of their bond characteristics. Our results explain a photoluminescence result of the (111) Si NSs [U. Kim et al., ACS Nano 2011, 5, 2176–2181] in terms of the plausible tensile strain imposed in the unoxidized inner layer by surface oxidation.

Excited state dynamics study of the self-trapped exciton formation in silicon nanosheets

The exciton formation dynamics of several model silicon nanosheets (SiNSs) are investigated using a time-dependent density functional tight binding method. The first excited-state (S1) self-trapped exciton formation in the SiNSs is obtained by observing the frontier orbital localization related to the characteristic size of the electronic excitations. The frontier molecular orbitals are highly localized in the S1 state on the stretched Si-Si bond due to the photo-excited structural relaxation, leading to a significant Stokes shift. A time domain study of the photo-excited emission gap correlated with the frontier orbital localization properties for exciton formation. The stretched Si-Si softer bonds provide a favorable site for exciton localization, resulting in exciton trapping. The exciton formation time was found to be around ∼450-850 fs, showing the consistency of the initial exciton formation time with a recent measurement (∼500 to ∼900 fs). This study reveals that Si-Si bond breaking acts as an optical activity center and provides regulation of the self-trapped exciton formation time by the quantum confinement effect in SiNSs; significant to the Si nanomaterial properties.

Charge transfer doping in functionalized silicon nanosheets/P3HT hybrid material for applications in electrolyte-gated field-effect transistors

This work introduces microwave-assisted thermal hydrosilylation reaction for the functionalization of silicon nanosheets, leading to the successful electronic interaction within SiNS–substrate/P3HT-based hybrid blends.

Enhancing the stability of silicon nanosheets electrodes by fluoroethylene carbonate

Stabilization of the SEI layer that is formed by consuming active lithium in electrolytes is a critical issue for improving the cyclability of lithium ion batteries. We used SiNSs as anodes and investigated the effect of fluoroethylene carbonate (FEC) additives on the SEI layer. Our XPS, EIS and TEM analysis of CO and CO bonds, interfacial resistance and thickness of SEl layer, respectively, shows that the FEC produces electrochemically and mechanically stable SEI layer. The SiNSs with FEC showed the capacity of over 1500mAhg−1 with 95% efficiency after 200 cycles.

IBM breakthrough leads to first 5-nm chip

An IBM-led research team has demonstrated a fabrication process that makes semiconductor chips featuring 5-nm transistors. Current state-of-the-art chips on the market have 10-nm transistors. An IBM team demonstrated 7-nm chips less than two years ago. The research team constructed the chips with stacks of silicon nanosheets. The method is an alternative to the current FinFET three-dimensional transistor architecture for cramming circuitry onto chips. Semiconductor makers will continue to use FinFET when they begin manufacturing 7-nm chips next year, but they have been looking for a new method to push into even smaller sizes. To make the 5-nm chips, the researchers also used extreme ultraviolet (EUV) lithography, which allows for the continuous adjustment of the nanosheet width and the fine tuning of circuit performance, IBM says. IBM is also using EUV lithography on 7-nm chips. “For business and society to meet the demand of cognitive and cloud computing in the

Silicon Nanosheets: Lewis Acid Induced Functionalization of Photoluminescent 2D Silicon Nanosheets for the Fabrication of Functional Hybrid Films (Adv. Funct. Mater. 21/2017)

Silicon Nanosheets: Lewis Acid Induced Functionalization of Photoluminescent 2D Silicon Nanosheets for the Fabrication of Functional Hybrid Films (Adv. Funct. Mater. 21/2017)

First-principles simulation on Seebeck coefficient in silicon nanowires

The Seebeck coefficients of silicon nanowires (SiNWs) were simulated on the basis of first-principles calculation using various atomistic structure models. The electronic band structures of fully hydrogen-terminated SiNW models give the correct image of quantum mechanical confinement from bulk silicon to SiNW for each axial direction, and the change in the density of states by dimensional reduction to SiNW enhances the thermoelectric performance in terms of the Seebeck coefficient, compared with those of bulk silicon and silicon nanosheets. The uniaxial tensile strain for the SiNW models does not strongly affect the Seebeck coefficient even for the SiNW system with giant piezoresistivity. In contrast, dangling bonds on a wire wall sharply reduce the Seebeck coefficient of SiNW and totally degrade thermoelectric performance from the viewpoint of the power factor. The exclusion of dangling bonds is a key element for the design and application of high-performance thermoelectric nanowires of semiconducting materials.

Lewis Acid Induced Functionalization of Photoluminescent 2D Silicon Nanosheets for the Fabrication of Functional Hybrid Films

Various Lewis acids are found to efficiently catalyze hydrosilylation reactions of hydride‐terminated 2D silicon nanosheets at room temperature. The hydride‐terminated nanosheets can be functionalized with a variety of unsaturated functional substrates and still possess their unique characteristic (opto)electronic properties (e.g., photoluminescence). This is demonstrated by successfully implementing the readily functionalized materials into new silicon/semiconducting polymer‐based field‐effect transistors (FETs). Surface modification of the freestanding silicon nanosheets opens new possibilities to form highly homogeneous blends with the already broadly used conventional polymers poly(3‐hexylthiophene‐2,5‐diyl). The consequential combination of the different properties of the materials enables the enhancement of the sensitivity of the solution‐gated FETs and increases the transconductance of the operating device.