Reductive growth of nanosized ligated metal clusters on silicon nanowires

The reductive growth of metal clusters on silicon nanowires (SiNWs) is reported. The HF-etched SiNWs were found to reduce ligated Au-Ag clusters of single size, shape, composition, and structure. In the process, the surfaces of the SiNWs were reoxidized. The reductive cluster growth on the SiNW surface was followed by high-resolution transmission electron microscopy (HRTEM). The reduced metal clusters grew to different sizes in the nanometer regime (1-7 nm in diameter) on the SiNW surfaces. At sizes greater than approximately 7 nm, they tend to separate from the SiNW surfaces. Further growth and/or agglomeration of these colloidal particles to sizes greater than roughly 25 nm in diameter eventually causes the particles to precipitate from solution. Two interesting phenomena, the "sinking cluster" and the "cluster fusion" processes, were observed under TEM.     

Mechanisms and energetics of hydride dissociation reactions on surfaces of plasma-deposited silicon thin films

We report results from a detailed analysis of the fundamental silicon hydride dissociation processes on silicon surfaces and discuss their implications for the surface chemical composition of plasma-deposited hydrogenated amorphous silicon (a-Si:H) thin films. The analysis is based on a synergistic combination of first-principles density functional theory (DFT) calculations of hydride dissociation on the hydrogen-terminated Si(001)-(2x1) surface and molecular-dynamics (MD) simulations of adsorbed SiH(3) radical precursor dissociation on surfaces of MD-grown a-Si:H films. Our DFT calculations reveal that, in the presence of fivefold coordinated surface Si atoms, surface trihydride species dissociate sequentially to form surface dihydrides and surface monohydrides via thermally activated pathways with reaction barriers of 0.40-0.55 eV. The presence of dangling bonds (DBs) results in lowering the activation barrier for hydride dissociation to 0.15-0.20 eV, but such DB-mediated reactions are infrequent. Our MD simulations on a-Si:H film growth surfaces indicate that surface hydride dissociation reactions are predominantly mediated by fivefold coordinated surface Si atoms, with resulting activation barriers of 0.35-0.50 eV. The results are consistent with experimental measurements of a-Si:H film surface composition using in situ attenuated total reflection Fourier transform infrared spectroscopy, which indicate that the a-Si:H surface is predominantly covered with the higher hydrides at low temperatures, while the surface monohydride, SiH((s)), becomes increasingly more dominant as the temperature is increased.     

Surface chemistry of aerosolized silicon nanoparticles: evolution and desorption of hydrogen from 6-nm diameter particles

The surface chemistry of pristine, 6-nm silicon nanoparticles has been investigated. The particles were produced in an RF plasma and studied using a tandem differential mobility analysis apparatus, Fourier transform infrared spectroscopy (FTIR), time-of-flight secondary ion mass spectrometry (ToF-SIMS), and transmission electron microscopy. Particles were extracted from the plasma, which operates at approximately 20 Torr, into an atmospheric pressure aerosol flow tube, and then through a variable-temperature furnace that could be adjusted between room temperature and 1200 degrees C. DMA measurements show that freshly generated silicon particles shrink with heating, with particle diameters decreasing by approximately 0.25 nm between 350 and 400 degrees C. FTIR results indicate that freshly generated particles are primarily covered with SiH2 groups and smaller amounts of SiH and SiH3. Spectra recorded as a function of heating temperature indicate that the amount of surface hydrogen, as measured by the intensity of modes associated with SiH, SiH2, and SiH3, decreases with heating. ToF-SIMS measurements also suggest that hydrogen desorbs from the particles surfaces over the same temperature range that the particles shrink.     

Etching behavior of silicon nanowires with HF and NH4F and surface characterization by attenuated total reflection Fourier transform infrared spectroscopy: similarities and differences between one-dimensional and two-dimensional silicon surfaces

A systematic study of the etching behavior of one-dimensional (1-D) Si nanowires (SiNWs) in various HF and NH4F etching solutions is reported. The concentration and pH dependences of the etching time (which is inverse to the "stability") of the SiNWs in these solutions were investigated. A V-shaped bimodal etching curve was observed for HF solutions with concentrations of 0.5-40%. Specifically, SiNWs exhibit high stability in both low (0.5%) and high (40%) concentrations of HF solution, with the lowest stability (i.e., fastest etching rate) occurring at 2% (1 M) HF solution. With NH4F, the time needed to totally etch away the SiNWs sample decreases with increasing concentration (from 1-40%). The opposite is true when the pH of the NH4F solution was maintained at 14. These surprising results were rationalized in terms of "passivation" of the SiNW surfaces by HF or related molecules via hydrogen bonding for Si-H-terminated surfaces in HF solutions (with low pH values) and by NH4(+) ions via ionic bonding for Si-O(-)-terminated surfaces in NH4F solutions (with high pH values), respectively. Furthermore, it was found that SiNWs are stable only in relatively narrow pH ranges in these solutions. When SiNWs are etched with HF, the stability range is pH = 1-2 where the surface moieties are Si-H(x) species (x = 1-3). When SiNWs are etched with NH4F, the stability range is pH = 12-14 where the surface moieties are mainly Si-(O-)x species (x = 1-3). These rationales were confirmed by attenuated total reflection Fourier transform infrared spectroscopy measurements, which showed that, while etching SiNWs with HF gave rise to Si-H(x) surface species, no Si-H(x) species were observed when SiNWs were etched with NH4F. The latter finding is at odds with the corresponding results reported for the two-dimensional (2-D) Si wafers where etching with either HF or NH4F produces Si-H(x) species on the surface. This difference suggests either that the etching mechanisms for NH4F versus HF are different for SiNWs or, more likely, that the Si-H(x) surface species produced in NH4F solutions are so unstable that they are hydrolyzed readily at pH > 4. The similarities and differences of the etching behaviors and the resulting surface speciations between the 1-D SiNWs and the 2-D Si wafers suggest that the nanoscale structures as well as the low dimensionality of SiNWs may have contributed to the rapid hydrolysis of the surface Si-H(x) species in NH4F solutions, especially at high pH values.     

Silver Coated Platinum Core–Shell Nanostructures on Etched Si Nanowires: Atomic Layer Deposition (ALD) Processing and Application in SERS

A new method to prepare plasmonically active noble metal nanostructures on large surface area silicon nanowires (SiNWs) mediated by atomic layer deposition (ALD) technology has successfully been demonstrated for applications of surface‐enhanced Raman spectroscopy (SERS)‐based sensing. As host material for the plasmonically active nanostructures we use dense single‐crystalline SiNWs with diameters of less than 100 nm as obtained by a wet chemical etching method based on silver nitrate and hydrofluoric acid solutions. The SERS active metal nanoparticles/islands are made from silver (Ag) shells as deposited by autometallography on the core nanoislands made from platinum (Pt) that can easily be deposited by ALD in the form of nanoislands covering the SiNW surfaces in a controlled way. The density of the plasmonically inactive Pt islands as well as the thickness of noble metal Ag shell are two key factors determining the magnitude of the SERS signal enhancement and sensitivity of detection. The optimized Ag coated Pt islands on SiNWs exhibit great potential for ultrasensitive molecular sensing in terms of high SERS signal enhancement ability, good stability and reproducibility. The plasmonic activity of the core‐shell Pt//Ag system that will be experimentally realized in this paper as an example was demonstrated in numerical finite element simulations as well as experimentally in Raman measurements of SERS activity of a highly diluted model dye molecule. The morphology and structure of the core‐shell Pt//Ag nanoparticles on SiNW surfaces were investigated by scanning‐ and transmission electron microscopy. Optimized core–shell nanoparticle geometries for maximum Raman signal enhancement is discussed essentially based on the finite element modeling.     

Quantitative real-time measurements of DNA hybridization with alkylated nonoxidized silicon nanowires in electrolyte solution

The quantitative, real-time detection of single-stranded oligonucleotides with silicon nanowires (SiNWs) in physiologically relevant electrolyte solution is demonstrated. Debye screening of the hybridization event is circumvented by utilizing electrostatically adsorbed primary DNA on an amine-terminated NW surface. Two surface functionalization chemistries are compared: an amine-terminated siloxane monolayer on the native SiO2 surface of the SiNW, and an amine-terminated alkyl monolayer grown directly on a hydrogen-terminated SiNW surface. The SiNWs without the native oxide exhibit improved solution-gated field-effect transistor characteristics and a significantly enhanced sensitivity to single-stranded DNA detection, with an accompanying 2 orders of magnitude improvement in the dynamic range of sensing. A model for the detection of analyte by SiNW sensors is developed and utilized to extract DNA-binding kinetic parameters. Those values are directly compared with values obtained by the standard method of surface plasmon resonance (SPR) and demonstrated to be similar. The nanowires, however, are characterized by higher detection sensitivity. The implication is that SiNWs can be utilized to quantitate the solution-phase concentration of biomolecules at low concentrations. This work also demonstrates the importance of surface chemistry for optimizing biomolecular sensing with silicon nanowires.     

Three-dimensional etching profiles and surface speciations (via attenuated total reflection-fourier transform infrared spectroscopy) of silicon nanowires in NH4F-buffered HF solutions: a double passivation model

A systematic study of the etching behavior, in terms of three-dimensional profiles, of one-dimensional (1-D) silicon nanowires (SiNWs) in NH(4)F-buffered hydrofluoric acid (BHF) solutions of varying concentrations and pH values and the surface speciations of the resulting etched SiNW surfaces, as characterized by attenuated total reflection-Fourier transform infrared (ATR-FTIR) spectroscopy, is reported. It was found that SiNWs are stable only in relatively narrow pH ranges of the BHF solutions. The results are rationalized in terms of a "double passivation" model. When SiNWs are etched in BHF solutions with pH values of 1-3, the surfaces are passivated with hydrogen (inner layer) giving rise to surface moieties such as Si-H(x) species (x = 1-3); at high HF concentrations, the H-terminated Si surfaces are covered with a hydrogen bonding network of HF and related molecules (oligomers, etc.), providing an outer-layer passivation. When SiNWs are etched in BHF solutions with pH values of 11-14 (by adding a strong base such as NaOH), the surfaces are oxygen-terminated with surface moieties such as Si-(O(-))(x)() species (x = 1-3); at high NH(4)F concentrations, the negatively charged Si surfaces are stabilized by NH(4)(+) ions via ionic bonding, again providing outer-layer passivation. In BHF solutions with pH values of 3-11, the surface speciation, consisting of Si-(OH)(x)(O(-))(y) (x + y = 1-3) species, is unstable and etched away rapidly. The surface speciations of SiNWs etched in various BHF solutions were explored via ATR-FTIR spectroscopy. It was found that, while etching SiNWs with HF-rich BHF solutions with pH < 4 gave rise to Si-H(x)() surface species, no surface Si-H(x) species were observed with SiNWs etched in BHF solutions with pH >/= 4 (HF/NH(4)F /= 4 on the other. These two factors, among others, contribute to the rapid hydrolysis of the surface Si-H(x)() species (and the etching of the SiNWs), particularly in BHF solutions with low HF/NH(4)F ratios and high pH values (pH >/= 4).     

Au掺杂硅纳米线的稳定性和电子结构

采用基于密度泛函理论的第一性原理的方法, 对Au掺杂[100]方向氢钝化硅纳米线(SiNWs)不同位置的形成能、能带结构、态密度及磁性进行了计算, 考虑了Au占据硅纳米线的替代、四面体间隙和六角形间隙的不同位置. 结果表明: Au偏爱硅纳米线中心的替代位置. Au掺杂后的硅纳米线引入了杂质能级, 禁带宽度变窄. 对于Au替代掺杂, 杂质能级主要来源于Au的d、p态和Si的p态, 由于Au的d态和Si的p态的耦合, Au掺杂硅纳米线具有铁磁性. 对于间隙掺杂, 杂质能级主要来源于Au的s态, 是非磁性的. 另外, 根据原子轨道和电子填充模型分析了其电子结构和磁性.     

Tripeptide-modified silicon nanowire based field-effect transistors as real-time copper ion sensors

A highly sensitive and specific copper ion sensor using single crystalline silicon nanowires (SiNWs) configured as field-effect transistors (FETs) is reported. The surface of SiNWs is functionalized with a His-containing tripeptide which serves as a copper sensitive layer. It is found that the complexation between copper ions and His-containing tripeptides leads to an increase in the SiNW conductance, and the position of His residue in the tripeptide plays an important role for the binding of copper ions. Only Gly-Gly-His-modified SiNWs exhibit selectivity for copper ions in the presence of zinc ions whereas Gly-His-Gly-modified SiNWs do not. Our experimental results demonstrate the potential utility of oligopeptide-modified SiNWs as metal ion sensors.     

Exceptional electrochemical performance of Si-nanowires in 1,3-dioxolane solutions: a surface chemical investigation

The effect of 1,3-dioxolane (DOL) based electrolyte solutions (DOL/LiTFSI and DOL/LiTFSI-LiNO(3)) on the electrochemical performance and surface chemistry of silicon nanowire (SiNW) anodes was systematically investigated. SiNWs exhibited an exceptional electrochemical performance in DOL solutions in contrast to standard alkyl carbonate solutions (EC-DMC/LiPF(6)). Reduced irreversible capacity losses, enhanced and stable reversible capacities over prolonged cycling, and lower impedance were identified with DOL solutions. After 1000 charge-discharge cycles (at 60 °C and a 6 C rate), SiNWs in DOL/LiTFSI-LiNO(3) solution exhibited a reversible capacity of 1275 mAh/g, whereas only 575 and 20 mAh/g were identified in DOL/LiTFSI and EC-DMC solutions, respectively. Transmission electron microscopy (TEM) studies demonstrated the complete and uniform lithiation of SiNWs in DOL-based electrolyte solutions and incomplete, nonuniform lithiation in EC-DMC solutions. In addition, the formation of compact and uniform surface films on SiNWs cycled in DOL-based electrolyte solutions was identified by scanning electron microscopic (SEM) imaging, while the surface films formed in EC-DMC based solutions were thick and nonuniform. X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared (FTIR) spectroscopy were employed to analyze the surface chemistry of SiNWs cycled in EC-DMC and DOL based electrolyte solutions. The distinctive surface chemistry of SiNWs cycled in DOL based electrolyte solutions was found to be responsible for their enhanced electrochemical performances.     

Zero-Dimensional Nanodots on One-Dimensional Nanowires: Reductive Deposition of Metal Nanoparticles on Silicon Nanowires

We review herein the surface chemical properties of silicon nanowires (SiNWs) and show how SiNWs can be used as platforms in doing chemistry in the nanorealm. In particular, the surfaces of HF-treated SiNWs (which are H-terminated) exhibit interesting chemical reactivities towards reductive deposition of metal ions such as silver, copper, palladium, etc., giving rise to metal particles or aggregates on the SiNW surfaces. By varying the concentration of the metal ions in solution, nanostructures of these metals of different shapes, sizes, and morphologies can be fabricated. The reductive growth of ligated Au–Ag clusters of single size, shape, composition, and structure, on the SiNWs was also investigated. Two interesting phenomena, the sinking cluster and the cluster fusion processes, were observed by TEM. These assemblies of metal nanoparticles on silicon nanowires may be considered as zero-dimensional nanodots, on one-dimensional nanowires. It is hoped that fabrication of these metallic nanodots on silicon nanowires will lead to new and novel composite materials of importance in nanotechnology.     

硅纳米线的化学气相沉积法合成

硅纳米线是近十几年来在纳米科学与技术领域快速发展的一种重要材料．通过精细的结构设计与材料合成,硅纳米线在生物传感、锂离子电池、太阳能电池和光电化学等领域展示出良好的应用前景．化学气相沉积（CVD）法是一大类重要的自下而上合成硅纳米线方法．本文简介了CVD法合成硅纳米线的主要进展,包括具有单一结构和复合结构的硅纳米线的合成．其中,单一结构的硅纳米包括本征（无掺杂）、掺杂和超长的硅纳米线;复合结构的硅纳米线包括轴向异质结、径向异质结、转折结构和树枝状结构的硅纳米线．     

Desorption of nitric acid from boehmite and gibbsite

Solid-state Fourier transform infrared spectroscopy (FTIR), evolved gas analysis-FTIR (EGA-FTIR), thermal gravimetric analysis (TGA), and differential scanning calorimetry (DSC) have been used to investigate the desorption of nitric acid from boehmite and from gibbsite. Samples containing between 3 and 36% of adsorbed nitric acid by mass were prepared by placing the mineral in a 70% nitric acid solution or by the adsorption of nitric acid vapors in humid air. FTIR established that water-solvated nitrate was the main species adsorbed on the surface of either mineral under these conditions. The water-solvated nitrate vaporized as nitric acid at approximately 400 K with an enthalpy of desorption of approximately 50 kJ/mol for both surfaces. A second nitric acid desorption occurred at approximately 450 K and had an enthalpy of desorption of 85 kJ/mol (95 kJ/mol) for boehmite (gibbsite). This was assigned as desorption of partially solvated aluminum hydroxylated nitrate. Monodentate and bridging nitrate were also observed on the boehmite. These species desorbed at approximately 725 K as NO2 and O2 with an enthalpy of reaction of approximately 55 kJ/mol of NO2 desorbed.     

Electrochemical Detection of Acetaminophen with Silicon Nanowires

Acetaminophen (APAP) is an antipyretic, analgesic agent, the overdose of which during medical treatment poses a risk for liver failure. Hence, it is important to develop methods to monitor physiological APAP levels to avoid poisoning. Here, we report an efficient, selective electrochemical APAP sensor made from depositing silicon nanowires (SiNWs) onto glassy carbon electrodes (GCEs). Electrocatalytic activity of the SiNW/GCE sensors was monitored under varying pH and concentrations of APAP using cyclic voltammetry (CV) and chronoamperometry (CA). CV of the SiNWs at 0.5 to 13 mmol dm⁻³ APAP concentrations was used to determine the oxidation and reduction potentials of APAP. The selective detection of APAP was then demonstrated using CA at +0.568 V vs Ag/AgCl, where APAP is fully oxidized, in the 0.01 to 3 mmol dm⁻³ concentration range with potentially‐interfering analytes. The SiNW sensor has the ability to detect APAP well within the detection limits for APAP toxicity, showing promise as a practical biosensor.     

Effects of argon dilution on the translational and rotational temperatures of SiH in silane and disilane plasmas

The effects of argon dilution on the translational and rotational temperatures of SiH in both silane and disilane plasmas have been investigated using the imaging of radicals interacting with surfaces (IRIS) technique. The average rotational temperature of SiH determined from the SiH excitation spectra is approximately 500 K in both SiH(4)/Ar and Si(2)H(6)/Ar plasmas, with no obvious dependence on the fraction of argon dilution. Modeling of kinetic data yields average SiH translational temperatures of approximately 1000 K, with no dependence on the fraction of argon in the SiH(4)/Ar plasmas within the studied range. In the Si(2)H(6)/Ar plasmas, however, the translational temperature decreases from approximately 1000 to approximately 550 K as the Ar fraction in the plasma increases. Thus, at the highest Ar fractions, the translational and rotational temperatures are nearly identical, indicating that the SiH radicals are thermally equilibrated. The underlying chemistry and mechanisms of SiH energy equilibration in Ar-diluted plasmas are discussed.     

Location‐Controlled Growth of Vertically Aligned Si Nanowires using Au Nanodisks Patterned by KrF Stepper Lithography

The location‐controlled epitaxial growth of vertically aligned Si nanowire (v‐SiNW) arrays over large surface area was investigated with Au nanodisks (AuNDs) patterned by KrF stepper lithography. There are two steps for synthesizing v‐SiNWs from an AuND pattern: annealing and growth. The annealing process induces the formation of a single Au nanoparticle (AuNP) from an AuND pattern, which consists of several cracked AuNPs. Here, the oxide layer between the AuNDs and Si substrate is necessary for impeding the diffusion of Si atoms into the AuNPs. However, the oxide layer must be removed for properly aligned epitaxial SiNW growth. These SiNW arrays in large area can contribute highly to improve a nanowire‐based engineering by controlling the location of SiNWs with consistent pitch.     

Chemical and electrical passivation of single-crystal silicon(100) surfaces through a two-step chlorination/alkylation process

Single-crystal Si(100) surfaces have been functionalized by using a two-step radical chlorination-Grignard (R = MgCl, R = CH3, C2H5, C4H9, C6H5, or CH2C6H5) alkylation method. After alkylation, no chlorine was detectable on the surface by X-ray photoelectron spectroscopy (XPS), and the C 1s region showed a silicon-induced peak shift indicative of a Si-C bond. The relative intensity of this peak decreased, as expected, as the steric bulk of the alkyl increased. Despite the lack of full alkyl termination of the atop sites of the Si(100) surface, functionalization significantly reduced the rate of surface oxidation in air compared to that of the H-terminated Si(100) surface, with alkylated surfaces forming less than half a monolayer of oxide after over one month of exposure to air. Studies of the charge-carrier lifetime with rf photoconductivity decay methods indicated a surface recombination velocity of <30 cm s(-1) for methylated surfaces, and <60 cm s(-1) for Si surfaces functionalized with the other alkyl groups evaluated. Soft X-ray photoelectron spectroscopic data indicated that the H-Si(100) surfaces were terminated by SiH, SiH2, and SiH3 species, whereas Cl-Si(100) surfaces were predominantly terminated by monochloro (SiCl and SiHCl) and dichloro (SiCl2 and SiHCl2) Si species. Methylation produced signals consistent with termination by Si-alkyl bonding arising from SiH(CH3)-, SiH2(CH3)-, and Si(CH3)2-type species.     

Tribological durability of silane monolayers on silicon

We report the frictional performance and long-term tribological stability of various alkyl silane monolayer films on silicon by using pin-on-disk tribometry at ambient conditions. We show that the durability of monolayers derived from n-alkyltrichlorosilanes on silicon increases exponentially with the chain length of the silane precursor, which we relate to the cohesive energy of these monolayers through molecular dynamics simulations. X-ray photoelectron spectroscopy (XPS) was used to show that tribological damage consisted of the loss of molecular components that could be partially replaced upon exposure to a solution containing perfluorinated silane precursors. For monolayers derived from n-octadecyltrichlorosilane, a critical load was identified to be approximately 250 mN (200 MPa), above which failure of films occurred within 100 cycles of testing. Monolayers with hydroxyl surfaces exhibited reduced stabilities due to stronger tip-surface interactions. Monolayers with the capability for cross-linking exhibited much greater stabilities than monolayers where cross-linking was limited or prevented. Collectively, these results demonstrate that the mechanical durability of monolayers when subjected to a tribological load is greatly enhanced by maximizing dispersional interactions and cross-linking and minimizing tip-surface interactions.     

The role of multifunctional kinetics during early-stage silicon hydride pyrolysis: reactivity of Si2H2 isomers with SiH4 and Si2H6

Kinetic parameters for the dominant pathways during the addition of the four Si(2)H(2) isomers, i.e., trans-HSiSiH, SiSiH(2), Si(H)SiH, and Si(H(2))Si, to monosilane, SiH(4), and disilane, Si(2)H(6), have been calculated using G3//B3LYP, statistical thermodynamics, conventional and variational transition state theory, and internal rotation corrections. The direct addition products of the multifunctional Si(2)H(2) isomers were monofunctional substituted silylenes, hydrogen-bridged species, and silenes. During addition to monosilane and disilane, the SiSiH(2) isomer was found to be most reactive over the temperature range of 800 to 1200 K. Revised parameters for the Evans-Polanyi correlation and a representative pre-exponential factor for multifunctional silicon hydride addition and elimination reaction families under pyrolysis conditions were regressed from the reactions in this study. This revised kinetic correlation was found to capture the activation energies and rate coefficients better than the current literature methods.     

Control of superhydrophilicity/superhydrophobicity using silicon nanowires via electroless etching method and fluorine carbon coatings

Surface roughness is promotive of increasing their hydrophilicity or hydrophobicity to the extreme according to the intrinsic wettability determined by the surface free energy characteristics of a base substrate. Top-down etched silicon nanowires are used to create superhydrophilic surfaces based on the hemiwicking phenomenon. Using fluorine carbon coatings, surfaces are converted from superhydrophilic to superhydrophobic to maintain the Cassie-Baxter state stability by reducing the surface free energy to a quarter compared with intrinsic silicon. We present the robust criteria by controlling the height of the nanoscale structures as a design parameter and design guidelines for superhydrophilic and superhydrophobic conditions. The morphology of the silicon nanowires is used to demonstrate their critical height exceeds several hundred nanometers for superhydrophilicity, and surpasses a micrometer for superhydrophobicity. Especially, SiNWs fabricated with a height of more than a micrometer provide an effective means of maintaining superhydrophilic (<10°) long-term stability.