Hydrogen-terminated Silicon Research Articles

Flexible transparent and hydrophobic SiNCs/PDMS coatings for anti-counterfeiting applications.

Silicon nanocrystals (SiNCs) have attracted considerable attention in many advanced applications due to silicon's high natural abundance, low toxicity, and impressive optical properties. However, little attention has been paid to fluorescence anti-counterfeiting applications based on lipophilic silicon nanocrystals. Moreover, it is also a challenge to fabricate aging-resistant anti-counterfeiting coatings based on silicon nanocrystals. Herein, this paper presents a demonstration of aging-resistant fluorescent anti-counterfeiting coatings based on red fluorescent silicon nanocrystals. In this work, lipophilic silicon nanocrystals (De-SiNCs) with red fluorescence were prepared first by thermal hydrosilylation between hydrogen-terminated silicon nanocrystals (H-SiNCs) and 1-decene. Subsequently, a new SiNCs/PDMS coating (De-SiNCs/DV) was fabricated by dispersing De-SiNCs into reinforcing PDMS composites with vinyl-capped silicone resin. Interestingly, the De-SiNCs/DV composites exhibit superior transparency (up to 85%) in the visible light range, outstanding fluorescence stabilities with an average lifetime of 20.59 μs under various conditions including acidic/alkaline environments, different organic solvents, high-humidity environments and UV irradiation. Meanwhile, the encapsulation of De-SiNCs is beneficial to enhancing the mechanical properties and thermal stability of De-SiNCs/DV composites. Additionally, the De-SiNCs/DV coating exhibits an excellent anti-counterfeiting effect on cotton fabrics when used as an ink in screen-printing. These findings pave the way for developing innovative flexible multifunctional anti-counterfeiting coatings in the future.

Grafting of Organophosphonic Acid Monolayers on Hydrogen-Terminated Silicon Surface and Secondary Functionalization in Supercritical Carbon Dioxide Media.

The monolayer grafting on the oxide-free Si surface is challenging due to vulnerability of the surface against oxide formation in an ambient atmosphere. Most of the conventional studies focused on organic solvent-based chemistry and solvent and substrate interfaces, and residual solvents after the monolayer grafting play a key role in producing the highly stable monolayers. CO2 in its supercritical state (SCCO2) provides an elegant engineering solution for the problem faced as it can be used as inert processing environment and as carrier fluid for monolayer grafting taking up the role of organic solvents. In this work, monolayers of alkyl organophosphonic acids (OPAs) and functional OPAs were grafted on hydrogen-terminated oxide-free Si surfaces using the SCCO2 process. Grafted monolayers were physically and chemically characterized to verify the successful monolayer formation and determine the nature of the covalent binding configuration on the surface. To broaden the prospects of practical utility of the process and the OPA monolayer, the (3-bromopropyl)phosphonic acid (BPPA) monolayer was demonstrated to undergo secondary functionalization by terminal group substitution to convert the Br terminal group to the OH terminal group and secondary monolayer grafting to assemble 4-fluorothiophenol on top of the BPPA monolayer. The ability of monolayers to sustain secondary functionalization processing qualitatively hints toward ordered and stable monolayers of OPAs. The developed SCCO2 process in this work presents a single-step, green, and scalable method to graft the OPA monolayer on oxide-free Si which can employed in the future for monolayer doping, highly selective biochemical sensors, and targeted biological interactions.

Pt-Coated Silicon Nanoparticles: An Investigation into the Hydrosilylation on Hydrogen-Terminated Silicon Surfaces Using Pt(dvs).

The interaction of hydrogen-terminated silicon nanoparticles (H-SiNPs) with Karstedt's catalyst at various temperatures was investigated. The results indicate that at room temperature, the oxidative addition of Pt(0) onto H-SiNPs is irreversible, and the catalyst is not eliminated from the surface of H-SiNPs, enabling a facile synthesis of Pt-loaded SiNPs that can undergo ligand exchange. The nature of the Pt-on-Si ensemble is characterized by Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy, transmission electron microscopy, and energy-dispersive X-ray spectroscopy. Reaction conditions that enable effective hydrosilylation are discussed. It is found that higher temperatures favor reductive elimination of the catalyst and hydrosilylation of 1-octene onto the surface of the H-SiNPs.

Resistless EUV lithography: Photon-induced oxide patterning on silicon

In this work, we show the feasibility of extreme ultraviolet (EUV) patterning on an HF-treated silicon (100) surface in the absence of a photoresist. EUV lithography is the leading lithography technique in semiconductor manufacturing due to its high resolution and throughput, but future progress in resolution can be hampered because of the inherent limitations of the resists. We show that EUV photons can induce surface reactions on a partially hydrogen-terminated silicon surface and assist the growth of an oxide layer, which serves as an etch mask. This mechanism is different from the hydrogen desorption in scanning tunneling microscopy-based lithography. We achieve silicon dioxide/silicon gratings with 75-nanometer half-pitch and 31-nanometer height, demonstrating the efficacy of the method and the feasibility of patterning with EUV lithography without the use of a photoresist. Further development of the resistless EUV lithography method can be a viable approach to nanometer-scale lithography by overcoming the inherent resolution and roughness limitations of photoresist materials.

Solar light-driven selective photoelectrochemical CO2 reduction to CO in aqueous media using Si nanowire arrays decorated with Au and Au-based metal nanoparticles

To address recent energy and environmental issues, such as global warming and resource depletion, significant interest has been shown in carbon dioxide (CO2) fixation based on photoelectrochemical processes under solar light irradiation. The present paper describes the applicability of gold nanoparticles-decorated silicon nanowire arrays (Au/SiNW) as photoelectrodes to promote CO2 reduction. The decoration with Au nanoparticles of SiNW was performed by an electroless plating utilizing surface hydrogen-terminated silicon groups, generated during the nanowire formation process. Au/SiNW exhibits efficient photoelectrochemical performance for CO2 reduction to produce CO selectively in an aqueous medium under simulated solar light irradiation owing to its vertically aligned nanowire structure and Au nanoparticles as cocatalysts. The former provides high specific surface area and light trapping effect, and the latter induces selective interaction with CO2. Moreover, a unique two-steps method for Au decoration that consists of photo-assisted deposition of copper nanoparticles and the following electroless plating to replace Cu atoms to Au ones achieves more uniform decoration of SiNW with highly dispersed core–shell structured Cu@Au nanoparticles. The resulting photoelectrode, termed Cu@Au/SiNW, shows improved selectivity toward CO production and gives a good Faradic efficiency of 72% in an aqueous medium.

Synthesis and Surface Attachment of Molecular Re(I) Complexes Supported by Functionalized Bipyridyl Ligands.

Eleven 2,2'-bipyridine (bpy) ligands functionalized with attachment groups for covalent immobilization on silicon surfaces were prepared. Five of the ligands feature silatrane functional groups for attachment to metal oxide coatings on the silicon surfaces, while six contain either alkene or alkyne functional groups for attachment to hydrogen-terminated silicon surfaces. The bpy ligands were coordinated to Re(CO)5Cl to form complexes of the type Re(bpy)(CO)3Cl, which are related to known catalysts for CO2 reduction. Six of the new complexes were characterized using X-ray crystallography. As proof of principle, four molecular Re complexes were immobilized on either a thin layer of TiO2 on silicon or hydrogen-terminated silicon. The surface-immobilized complexes were characterized using X-ray photoelectron spectroscopy, IR spectroscopy, and cyclic voltammetry (CV) in the dark and for one representative example in the light. The CO stretching frequencies of the attached complexes were similar to those of the pure molecular complexes, but the CVs were less analogous. For two of the complexes, comparison of the electrocatalytic CO2 reduction performance showed lower CO Faradaic efficiencies for the immobilized complexes than the same complex in solution under similar conditions. In particular, a complex containing a silatrane linked to bpy with an amide linker showed poor catalytic performance and control experiments suggest that amide linkers in conjugation with a redox-active ligand are not stable under highly reducing conditions and alkyl linkers are more stable. A conclusion of this work is that understanding the behavior of molecular Re catalysts attached to semiconducting silicon is more complicated than related complexes, which have previously been immobilized on metallic electrodes.

Surface Immobilization of a Re(I) Tricarbonyl Phenanthroline Complex to Si(111) through Sonochemical Hydrosilylation

A sonochemical-based hydrosilylation method was employed to covalently attach a rhenium tricarbonyl phenanthroline complex to silicon(111). fac-Re(5-(p-Styrene)-phen)(CO)3Cl (5-(p-styrene)-phen = 5-(4-vinylphenyl)-1,10-phenanthroline) was reacted with hydrogen-terminated silicon(111) in an ultrasonic bath to generate a hybrid photoelectrode. Subsequent reaction with 1-hexene enabled functionalization of remaining atop Si sites. Attenuated total reflectance-Fourier transform infrared spectroscopy confirms attachment of the organometallic complex to silicon without degradation of the organometallic core, supporting hydrosilylation as a strategy for installing coordination complexes that retain their molecular integrity. Detection of Re(I) and nitrogen by X-ray photoelectron spectroscopy (XPS) further support immobilization of fac-Re(5-(p-styrene)-phen)(CO)3Cl. Cyclic voltammetry and electrochemical impedance spectroscopy under white light illumination indicate that fac-Re(5-(p-styrene)-phen)(CO)3Cl undergoes two electron reductions. Mott-Schottky analysis indicates that the flat band potential is 239 mV more positive for p-Si(111) co-functionalized with both fac-Re(5-(p-styrene)-phen)(CO)3Cl and 1-hexene than when functionalized with 1-hexene alone. XPS, ultraviolet photoelectron spectroscopy, and Mott-Schottky analysis show that functionalization with fac-Re(5-(p-styrene)-phen)(CO)3Cl and 1-hexene introduces a negative interfacial dipole, facilitating reductive photoelectrochemistry.

Effects of dopant type and concentration on surface recombination velocity in hydrogen-terminated silicon

Isolating the effects of the type and concentration of the dopant in lightly doped regions in the reaction between hydrogen-terminated silicon surface and atmospheric impurities in air is very difficult. However, changes in the surface recombination sites can be analyzed accurately through recombination lifetime measurements performed using the microwave photoconductive decay method. Thus, we investigated variations in the effective recombination lifetime in hydrogen-terminated silicon surfaces over time in air for different dopant types and concentrations. For both p-type and n-type silicon wafers, surface recombination velocity, S, increased with decreasing resistivity, namely, increasing dopant concentration. The time-dependent variations of the S for the p-type wafers decreased, and those for the n-type wafers increased with decreasing resistivity. Thus, it was shown that the time-dependent variation of the S depends on the type and concentration of the dopant used.

Energetic disorder impacts energy-level alignment of alpha-sexithiophene on hydrogen-terminated silicon and silicon oxide

The energy-level alignment at hybrid organic-inorganic interfaces is decisive for the performance of (opto-)electronic devices. We use ultraviolet and x-ray photoelectron spectroscopy (UPS and XPS) to measure the energy-level alignment of vacuum-sublimed α-sexithiophene (6 T) thin films with HF-etched n-type Si(100) and with Si with a native oxide layer (SiOx). The 6 T thin films induce a small (<0.1 eV) downwards band bending into both substrates as shown by XPS. The well-ordered growth of 6 T on Si leads to a relatively narrow density of states (DOS) distribution of the highest occupied molecular orbital (HOMO) as shown by UPS. Furthermore, the Fermi-level comes to lie at rather mid-gap position and, consequently, no energy-level bending occurs in the 6 T layer. Structural disorder in the 6 T thin film on SiOx leads to a broad HOMO DOS distribution and to tailing states into the energy gap. Consequently, downwards energy-level bending (by around 0.20 eV) takes place in the 6 T layer.

Water-Enabled H2 Generation from Hydrogenated Silicon Nanosheets for Efficient Anti-Inflammation.

As an emerging therapeutic gas, hydrogen (H2) is gifted with excellent biosafety, high tissue permeability, and radical-trapping capacity and is extensively considered as a highly promising antioxidant in clinics. However, a facile and effective strategy of H2 production for major inflammatory disease treatments is still lacking. In this study, by a facile wet-chemical exfoliation synthesis, a hydrogen-terminated silicon nanosheet (H-silicene) has been synthesized, which can favorably react with environmental water to generate H2 rapidly and continuously without any external energy input. Furthermore, theoretical calculations were employed to reveal the mechanism of enhanced H2 generation efficacy of H-silicene nanosheets. The as-synthesized H-silicene has been explored as a flexible hydrogen gas generator for efficient antioxidative stress application for the first time, which highlights a promising prospect of this two-dimensional H-silicene nanomaterial for acute inflammatory treatments by on-demand H2 production-enabled reactive oxygen species scavenging. This study provides a novel and efficient modality for nanomaterial-mediated H2 therapy.

Si-H Surface Groups Inhibit Methacrylic Polymerization: Thermal Hydrosilylation of Allyl Methacrylate with Silicon Nanoparticles.

Hydrogen-terminated silicon nanoparticles (H-SiNPs) inhibit anerobic thermal autopolymerization of methacrylates. When heated to 100 °C under an inert atmosphere, allyl methacrylate (AMA) was stable for at least 95 h in the presence of 1.2 wt % H-SiNPs, exhibiting less than 0.15% conversion, whereas the neat monomer solidified within 24 h (over 10% conversion after 34 h). A mechanism is proposed that is based on H-transfer from SiNPs to the thermally activated methacrylic dimer biradical, quenching autopolymerization. An analysis of SiNPs isolated after heating in AMA reveals the grafting of ester groups. Thermal hydrosilylation offers a facile way to attach an allyl group to the surface of SiNPs.

Fabrication of Multiscale 1-Octadecene Monolayer Patterned Arrays Based on a Chemomechanical Method

A controlled and self-assembled micromachining system was built to fabricate a mico/nanoscale monolayer patterned array on a silicon surface using a diamond tip. The process was as follows: (1) we preprocessed a silicon wafer to obtain a hydrogen-terminated silicon surface; (2) we scratched three rectangular arrays of 10 μm × 3 μm with a spacing of 2 μm on the silicon surface with a diamond tip in 1-octadecene solution; the Si-H bonds were broken, and silicon free radicals were formed; (3) the 1-octadecene molecules were connected with silicon atoms based on Si-C covalent bonds, and the 1-octadecene nano monolayer was self-assembled on the patterned arrays of the silicon surface. Atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS), and Sessile water contact angles were used to detect and characterize the self-assembled monolayers (SAMs). The XPS results showed that the Si2p peak and the O1s peak were significantly decreased after self-assembly; however, the C1s peak was successively significantly increased. Sessile water contact angles showed that the hydrophilicity was weakened after the formation of 1-octenecene SAMs on the silicon substrate. The nanofriction of the sample was measured with AFM. The change in nanofriction also demonstrated that the SAMs were formed in accordance with the patterned array. We demonstrated that, by using this method, self-assembled multiscale structures on silicon substrate can be formed quickly and conveniently.

High Energy Limit of the Size-Tunable Photoluminescence of Hydrogen-Terminated Porous Silicon Nanostructures in HF

The photoluminescence (PL) of various porous silicon (PSi) layers was studied during chemical dissolution in HF. The relative PL quantum efficiency of some layers was also monitored. Typically, the PL increased, reached a maximum and then dropped down to complete extinction, accompanied with a PL blueshift. During PL fall, both the PL intensity and layer quantum efficiency fell sharply, accompanied by a decrease in full width at half maximum and a slowing blueshift. In the final stage, the PL intensity decreased without any further blueshift, the saturated PL peak wavelength being ∼515 nm (∼2.4 eV) for most layers, identifying a high energy limit for the achievable PL of hydrogen-terminated Si nanostructures. Our results show that sudden catastrophic mechanical failure of nanostructure cannot explain the sharp PL drop and saturation of PL blueshift. Rather, they support the idea of a critical size (∼1.5–2 nm) below which the PL quantum efficiency vanishes. The possible reasons were discussed, privileging the emergence of structural non-radiative defects below a certain size, though the decreasing intrinsic quantum efficiency of Si nanocrystals with decreasing size could also play an important role. Maximum PL intensity was generally obtained for a peak wavelength of ∼565 nm (∼2.2 eV).

Metastable Metal-Monolayer-Semiconductor Junctions: Diverse Chain-Length-Dependent and Ultraslow Electrical Progression

Molecularly tunable metal-semiconductor (MS) junctions have been fabricated by modifying mercury drop electrodes with n-alkanethiols, 1-CH3(CH2)n–1SH (n = 10, 11, 12, 14, 16, and 18) prior to the formation of intimate contact with hydrogen-terminated silicon (H-Si≡) and characterized by both solid-state electrical and electrochemical measurements. We have demonstrated that the current-voltage properties of these molecular junctions change with time and that diverse time-dependent progression “patterns” were observed between the systems of long-chain alkanethiols (C14SH, C16SH, and C18SH) and short-chain alkanethiols (C10SH, C11SH, and C12SH). It is remarkable that for mercury contact electrodes modified with long-chain alkanethiolate self-assembled monolayers (SAMs), the junctions became more rectifying and stabilized over a prolonged period (∼2 h). For short-chain counterparts, surprisingly, they initially became more rectifying, then changed to ohmic over a similar period. It was proposed that at the MS interface, short-chain alkanethiolate SAMs first reorganize to be more ordered before eventually collapse. Long-chain alkanethiolate SAMs, on the other hand, achieve a more uniform, oriented, and stable packing as time goes by. These novel findings shed light on “long-term” intermolecular interactions that drive molecular systems to undergo ultraslow reorganizations, which can be readily modulated by simply varying the precursor structures (e.g., chain length of n-alkanethiols).

Characterization of Mechanochemical Modification of Porous Silicon with Arginine

Mechanochemistry initiated the reaction of hydrogen-terminated porous silicon (H/por-Si) powder with arginine. Samples were analyzed using Fourier-transform infrared spectroscopy (FTIR), dynamic light scattering (DLS), zeta potential, scanning electron microscopy (SEM), and photoluminescence (PL) spectroscopy. Arginine, which was physisorbed onto the surface of por-Si, blue-shifted the peak PL intensity from ~630 nm for the H/por-Si to ~565 nm for arginine-coated por-Si. Grinding for 4 h reduced &gt;80% of the initially 2–45 µm particles to &lt;500 nm, but was observed to quench the PL. With appropriate rinsing and centrifugation, particles in the 100 nm range were isolated. Rinsing ground powder with water was required to remove the unreacted arginine. Without rinsing, excess arginine induced the aggregation of passivated particles. However, water reacted with the freshly ground por-Si powder producing H2. A zeta potential of +42 mV was measured for arginine-terminated por-Si particles dispersed in deionized water. This positive value was consistent with termination such that NH2 groups extended away from the surface. Furthermore, this result was confirmed by FTIR spectra, which suggested that arginine was bound to silicon through the formation of a covalent Si–O bond.

Photoactive silicon surfaces functionalized with high-quality and redox-active platinum diimine complex monolayers

Platinum diimine complexes can covalently be grafted onto oxide-free, hydrogen-terminated silicon(111) surfaces into clean and high-quality monolayers. The so modified surfaces offer great prospects as photocathodes for solar-driven electrocatalysis.

Hydrosilylation of High Porosity Porous Silicon with 1-Hexene in Supercritical CO2 Fluid

Hydrosilylation of organic groups on hydrogen-terminated porous silicon (PSi) enhances its luminescence stability. Increasing PSi porosity improves its luminescence efficiency. However, for as-formed PSi layers with higher porosity than 75%, it is very difficult to preserve its fine structure during air-drying. Here, a process of both drying and hydrosilylation of highly porous silicon in ambient supercritical fluid was developed. 1-hexene was used for the hydrosilylation. Supercritical fluid hydrosilylation of PSi with porosity of 80% was achieved. The effects of the process were evaluated in terms of Fourier transform infrared spectroscopy. Stable photoluminescence of high porosity PSi was also obtained.

One-step conversion of Mg2Si into hydrogen-terminated porous silicon nanostructures

A simple, fast, and low-cost acid-treatment method is proposed for the conversion of magnesium silicide (Mg2Si) into hydrogen-terminated mesoporous silicon nanostructured material (M-pSi). The formation of porosity in M-pSi is attributed to the generation of gas species during the acid treatment, providing a large number of active sites. The adsorption and the photocatalytic performances of M-pSi toward the degradation of methyl orange are investigated. It is found that the mesoporous Si material exhibits an unusual and high photocatalytic ability toward the dye degradation in acidic conditions, outperforming the performance of commercially available Si nanoparticles. This unusual photocatalytic performance is related to the presence of hydrogen-terminated silicon nanostructures with a high surface area of 203 m2/g. The article proposes an alternative way of producing porous silicon materials with enhanced photocatalytic activity.

Impact of Interface Layer on Device Characteristics of Si:HfO2-Based FeFET’s

The impact of the interface layer on charge trapping and polarization switching at multiple temperatures is investigated. At high temperatures, a ferroelectric field effect transistor with a metal-ferroelectric-insulator-semiconductor (MFIS) gate-stack shows conventional electron-trapping dominated PBTI (i.e., ΔtV <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> t</sub> > 0), whereas at room temperature, negative threshold voltage shifts (ΔV <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> t</sub> ) are observed. We demonstrate the relationship between the interface layer and the subsequently deposited ferroelectric layers' effective permittivity: higher permittivity, consistent with a higher tetragonal phase/lower orthorhombic phase content, is noted when the ferroelectric layer is deposited on hydrogen-terminated silicon. In contrast, an anomalous BTI, consistent with a higher orthorhombic phase content, is observed at lower temperatures when the ferroelectric layer is deposited on oxygen-rich surfaces (SiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> , SiON).

The Role of the Molecular Hydrogen Formation in the Process of Metal-Ion Reduction on Multicrystalline Silicon in a Hydrofluoric Acid Matrix.

Metal deposition on silicon in hydrofluoric acid (HF) solutions is a well-established process for the surface patterning of silicon. The reactions behind this process, especially the formation or the absence of molecular hydrogen (H2), are controversially discussed in the literature. In this study, several batch experiments with Ag+, Cu2+, AuCl4− and PtCl62− in HF matrix and multicrystalline silicon were performed. The stoichiometric amounts of the metal depositions, the silicon dissolution and the molecular hydrogen formation were determined analytically. Based on these data and theoretical considerations of the valence transfer, four reasons for the formation of H2 could be identified. First, H2 is generated in a consecutive reaction after a monovalent hole transfer (h+) to a Si–Si bond. Second, H2 is produced due to a monovalent hole transfer to the Si–H bonds. Third, H2 occurs if Si–Si back bonds of the hydrogen-terminated silicon are attacked by Cu2+ reduction resulting in the intermediate species HSiF3, which is further degraded to H2 and SiF62−. The fourth H2-forming reaction reduces oxonium ions (H3O+) on the silver/, copper/ and gold/silicon contacts via monovalent hole transfer to silicon. In the case of (cumulative) even-numbered valence transfers to silicon, no H2 is produced. The formation of H2 also fails to appear if the equilibrium potential of the 2H3O+/H2 half-cell does not reach the energetic level of the valence bands of the bulk or hydrogen-terminated silicon. Non-hydrogen-forming reactions in silver, copper and gold deposition always occur with at least one H2-forming process. The PtCl62− reduction to Pt proceeds exclusively via even-numbered valence transfers to silicon. This also applies to the reaction of H3O+ at the platinum/silicon contact. Consequently, no H2 is formed during platinum deposition.