Si Microwire Arrays Research Articles

Enhancing positron production using front surface target structures

We report a target design which produced a substantial gain in relativistic electron-positron pair production using high-intensity lasers and targets with large-scale micro-structures on their surface. Comparing to an unstructured target, a selected Si microwire array target yielded a near 100% increase in the laser-to-positron conversion efficiency and produced a 10 MeV increase in the average emitted positron energy under nominally the same experimental conditions. We had established a multi-scale particle-in-cell simulation scheme to simulate both the laser absorption and the subsequent pair productions in a thick metal target. The experimental results are supported by the simulations demonstrating the performance increase is due to a higher conversion efficiency of laser energy into electrons with kinetic energies greater than 10 MeV due to enhanced direct laser acceleration of electrons enabled by the microwire array.

Primary Corrosion Processes for Polymer-Embedded Free-Standing or Substrate-Supported Silicon Microwire Arrays in Aqueous Alkaline Electrolytes

Solar fuel devices have shown promise as a sustainable source of chemical fuels. However, long-term stability of light absorbing materials remains a substantial barrier to practical devices. Herein, multiple corrosion pathways in 1 M KOH(aq) have been defined for TiO2-protected Si microwire arrays in a polymer membrane either attached to a substrate or free-standing. Top-down corrosion was observed in both morphologies through defects in the TiO2 coating. For the substrate-based samples, bottom-up corrosion was observed through the substrate and up the adjacent wires. In the free-standing samples, uniform bottom-up corrosion was observed through the membrane with all wire material corroded within 10 days of immersion in the dark in 1 M KOH(aq).

Si Microwire-Array Photocathodes Decorated with Cu Allow CO2 Reduction with Minimal Parasitic Absorption of Sunlight

High loadings of Cu were integrated on the light-facing side of Si microwire arrays used under simulated sunlight for the photoelectrochemical reduction of CO2(aq) to hydrocarbons in 0.10 M KHCO3(a...

Effects of bubbles on the electrochemical behavior of hydrogen-evolving Si microwire arrays oriented against gravity

Silicon microwire-arrays mitigate the effects of gas bubbles on the total overpotential of hydrogen-evolving cathodes oriented against gravity.

Enhanced Stability and Efficiency for Photoelectrochemical Iodide Oxidation by Methyl Termination and Electrochemical Pt Deposition on n-Type Si Microwire Arrays

Arrays of Si microwires doped n-type (n-Si) and surface-functionalized with methyl groups have been used, with or without deposition of Pt electrocatalysts, to photoelectrochemically oxidize I–(aq) to I3–(aq) in 7.6 M HI(aq). Under conditions of iodide oxidation, methyl-terminated n-Si microwire arrays exhibited stable short-circuit photocurrents over a time scale of days, albeit with low energy-conversion efficiencies. In contrast, electrochemical deposition of Pt onto methyl-terminated n-Si microwire arrays consistently yielded energy-conversion efficiencies of ∼2% for iodide oxidation, with an open-circuit photovoltage of ∼400 mV and a short-circuit photocurrent density of ∼10 mA cm–2 under 100 mW cm–2 of simulated air mass 1.5G solar illumination. Platinized electrodes were stable for >200 h of continuous operation, with no discernible loss of Si or Pt. Pt deposited using electron-beam evaporation also resulted in stable photoanodic operation of the methyl-terminated n-Si microwire arrays but yielded substantially lower photovoltages than when Pt was deposited electrochemically.

Hydrogen Evolution with Minimal Parasitic Light Absorption by Dense Co–P Catalyst Films on Structured p-Si Photocathodes

Planar and three-dimensionally structured p-Si devices, consisting of an electrodeposited Co–P catalyst on arrays of Si microwires or Si micropyramids, were used as photocathodes for solar-driven hydrogen evolution in 0.50 M H2SO4(aq) to assess the effects of electrode structuring on parasitic absorption by the catalyst. Without the use of an emitter layer, p-Si/Co–P microwire arrays produced a photocurrent density of −10 mA cm–2 at potentials that were 130 mV more positive than those of optimized planar p-Si/Co–P devices. Champion p-Si/Co–P microwire array devices exhibited ideal regenerative cell solar-to-hydrogen efficiencies of >2.5% and were primarily limited by the photovoltage of the p-Si/Co–P junction. The vertical sidewalls of the Si microwire photoelectrodes thus minimized effects due to parasitic absorption at high loadings of catalyst for device structures with or without emitters.

(Re-)crystallization mechanism of highly oriented Si-microwire arrays by TEM analysis

Novel anodes for Li-ion batteries have been characterized by transmission electron microscopy (TEM). The anodes consist of arrays of ordered silicon microwires obtained from conventional silicon wafers by controlled etching processes. Suitable preparation methods for wires in the micrometer range have been tested, optimized, and subsequently applied. Ex situ TEM measurements have been performed for pristine as well as for electrochemically cycled wires that have been charged and discharged for several times. The influences of cycling rate, an electrolyte additive, as well as elevated temperatures on the anodes’ stability have been investigated.

Si microwire array photoelectrochemical cells: Stabilized and improved performances with surface modification of Pt nanoparticles and TiO2 ultrathin film

Abstract To achieve the semiconductor photoelectrochemical (PEC) cells targeting the industry applications with commercial competitiveness, high efficiency and good stability are requisite properties. To improve the PEC response, the vertically-aligned silicon microwire arrays (SiMWAs) modified with isolated Pt nanoparticles (PtNPs) and conformal TiO 2 ultrathin film (TiO 2 /Pt@SiMWAs) are fabricated and examined in this study. The modified system shows the significantly enhanced responses and operation stability, that is, an enhancement of ∼30.0% in saturation photocurrent density ( J sat ), a cathodic shift of ∼0.85 V (relative to the bare SiMWAs) in applied potential for J sat , and an attenuation ratio of the photocurrent 2 ultrathin film protects the SiMWAs from photo-oxidation/corrosion as well as suppresses the surface recombination. Our results indicate a feasible route for the practical applications of oxidizable and corrodible semiconductor micro-/nanostructures in the fields of PEC solar cells, water splitting, photodegradation, and so on.

Silicon microwire arrays decorated with amorphous heterometal-doped molybdenum sulfide for water photoelectrolysis

Silicon is a promising photocathode material for solar hydrogen evolution because of its small band gap, negative conduction band position, and ideal theoretical current density. In this study, p-type Si microwire (p-Si MW) arrays were prepared as photocathodes because of the large surface area and high light-harvesting capability. However, Si MWs suffered from low photocatalytic activity because of slow photo-induced carriers during driving of water-splitting reaction. Therefore, molybdenum sulfide (MoS2) with appropriate band alignment with p-Si material was employed for surface modification to function as a co-catalyst for collecting photo-generated minority carriers and reducing recombination possibility. The onset potential and current density at 0V versus reversible hydrogen electrode (RHE) of Si@MoS2 MWs were +0.122V and −8.41mAcm−2. Heterometal atoms were employed to dope MoS2 co-catalyst and expose more sulfur-terminated active sites to further boost photoelectrochemical performance. Optimal activity of Si@MMoSx (M = Fe, Co, Ni) was achieved by doping Co heteroatoms, and its turn-on voltage and photocurrent density at 0V (vs. RHE) were respectively increased to +0.192V and −17.2mAcm−2. X-ray absorption spectroscopy was applied to demonstrate that Fe ions of FeMoSx were dichalcogenide materials, forming a composite with MoS2 and contributing better photoelectrolytic efficiency. By contrast, two-valent heteroatoms of CoMoSx and NiMoSx substituted the Mo4+ ions in MoS2. For charge compensation, more defects and edges were revealed as active sites of solar hydrogen production by adding Co or Ni dopants in MoS2 co-catalyst, which led to lower overpotential.

Enhanced Absorption and <1% Spectrum-and-Angle-Averaged Reflection in Tapered Microwire Arrays

We report ordered, high aspect ratio, tapered Si microwire arrays that exhibit an extremely low angular (0° to 50°) and spectrally averaged reflectivity of <1% of the incident 400–1100 nm illumination. After isolating the microwires from the substrate with a polymer infill and peel off process, the arrays were found to absorb 89.1% of angular averaged incident illumination (0° to 50°) in the equivalent volume of a 20 μm thick Si planar slab, reaching 99.5% of the classical light trapping limit between 400 and 1100 nm. We explain the broadband absorption by enhancement in coupling to waveguide modes due to the tapered microstructure of the arrays. Time-resolved microwave photoconductivity decay measurements yielded charge-carrier lifetimes of 0.75 μs (more than an order of magnitude higher than vapor–liquid–solid-grown Si microwires) in the tapered microwires, resulting in an implied Voc of 0.655 V. The high absorption and high aspect ratio in these ordered microwire arrays make them an attractive platform for high-efficiency thin-film crystalline Si solar cells and as well as for the photoelectrochemical production of fuels from sunlight.

Single-Crystal Si Microwire Anodes for Li Ion Batteries and Their Necessary Conditioning during the First Charge/Discharge Cycles: A Voltammetry Study

In the past years it has been proved that it s posible to make use of the high storage capacity of Si (4200 mAh/g) in real Li ion batteries. Micro- or nano-structuring the material has improved its mechanical properties, which a couple of decades ago were unthinkable for batteries (Si suffers of an expansion of up to 300 % when lithiated, causing its pulverization if it is in bulk). One of the most important concepts, due to its high areal capacity, is the one developed by us consisting of single-crystal Si microwire arrays with optimum spacing for allowing Si expansion. The fabrication method is unique and scalable, based on a series of electrochemical and chemical etching steps of Si wafers [1, 2]. It is known that Si could reach a very high capacity, but its performance is much dependent on the charging/discharging conditions. It has been seen that th first cycles must be done at slow rates of C/10 or slower, to assure the formation of a stable SEI and to minimize the mechanical stress in the wires [3]. Nevertheless, a systematic and fast analytic approach for confirming the descibed findings and particularizing to Si wire of specific dimensions. Cyclic voltammetry is a technique commonly used for identifying redox pairs and the electric potentials at which electrochemical events occur. However, during this kind of test on Li ion batteries, the electrodes get lithiated/delithiated up to certain degree regulated by the mesuring time. In this way, this technique could be used for identifying different phenomena occurring during the lithiation/delithiation of Si. Fort he tests of the present report, Si microwires of 1.8 µm in diameter and 60 µm in length have been prepared. 4.5 mg of the material have been mixed with an equal weight of Carbon black and 10 % of CMC, and have been casted on a Cu current collector to make anodes. The tests have been done in half cells, with Li metal as counter electrode. Cyclic voltammograms of the anodes can be observe don the left figure. The sweeping rate for the measurement has been 0.1 mV/s in the range of 20 mV to 1 V. Fort the analysis, special attention has been paid to the delithiation process, considering that the SEI is not delithiated (delithiation just related with Si is observed). The right figure presents the calculated values of capacity and C rate from the data. The C rate has been calculated from the amplitude of the first delithiation peak as (height [mA]/weight [g])/4200 mAh/g. The total delithiation capacity has been obtained from the area under the whole positive current curve as (Area [mA*V]/(rate [V/h] * weight [g]). As can be observed the values tend to stabilize after 5 cycles, indicating that after cycle 5 the lithiated/delithiated portion (about 3.7 %) reaches a stable amorphous state. The final C rate is 0.0344 (about C/29), very slow, but necessary for self-organization at the given potential of the delithiation peak. After cycle 5 the capacity does not increase since the measurement time, given by the sweep rate, is fast (2.7 h) in comparison to the ideal C rates (29 h). The implications of the right conditioning in the performance of the Si microwire anodes will be discussed in detail in the paper at the conference. Cyclic voltammetry is a simple and fast technique with high analytic potential for different unrevealed phenomena. [1] E. Quiroga-González, E. Ossei-Wusu, J. Carstensen, H. Föll, “How to Make Optimized Arrays of Si Wires Suitable as Superior Anode for Li-ion Batteries,” J. Electrochem. Soc. 158(11) (2011) E119-E123. [2] E. Quiroga-González, J. Carstensen, H. Föll, "Scalable processing and capacity of Si microwire array anodes for Li ion batteries", Nanoscale Res. Lett.9 (2014) 417. [3] E. Quiroga-González, J. Carstensen, H. Föll, “Good cycling performance of high-density arrays of Si microwires as anodes for Li ion batteries,” Electrochim. Acta 101 (2013) 93-98. Figure 1

Microengineering Laser Plasma Interactions at Relativistic Intensities.

We report on the first successful proof-of-principle experiment to manipulate laser-matter interactions on microscales using highly ordered Si microwire arrays. The interaction of a high-contrast short-pulse laser with a flat target via periodic Si microwires yields a substantial enhancement in both the total and cutoff energies of the produced electron beam. The self-generated electric and magnetic fields behave as an electromagnetic lens that confines and guides electrons between the microwires as they acquire relativistic energies via direct laser acceleration.

Flexible FETs using ultrathin Si microwires embedded in solution processed dielectric and metal layers

This work presents a novel manufacturing route for obtaining high performance bendable field effect transistors (FET) by embedding silicon (Si) microwires (2.5 μm thick) in layers of solution-processed dielectric and metallic layers. The objective of this study is to explore heterogeneous integration of Si with polymers and to exploit the benefits of both microelectronics and printing technologies. Arrays of Si microwires are developed on silicon on insulator (SOI) wafers and transfer printed to polyimide (PI) substrate through a polydimethylsiloxane (PDMS) carrier stamp. Following the transfer printing of Si microwires, two different processing steps were developed to obtain top gate top contact and back gate top contact FETs. Electrical characterizations indicate devices having mobility as high as 117.5 cm2 V−1 s−1. The fabricated devices were also modeled using SILVACO Atlas. Simulation results show a trend in the electrical response similar to that of experimental results. In addition, a cyclic test was performed to demonstrate the reliability and mechanical robustness of the Si μ-wires on flexible substrates.

Amorphous oxygen-rich molybdenum oxysulfide Decorated p-type silicon microwire Arrays for efficient photoelectrochemical water reduction

We report the fabrication of p-type silicon (Si) photocathodes consisting of well-ordered Si microwire (Si-MW) arrays coupled with non-precious and earth-abundant amorphous oxygen-rich molybdenum oxysulfide (MoOxSy) as both a hydrogen evolution catalyst and a passivation layer. The MoOxSy is conformally grown on the Si-MW surface through photo-assisted cyclic voltammetric (CV) deposition. By adjusting the cycle numbers of the CV deposition, Si-MW array electrodes with different MoOxSy catalyst loadings (Si-MWs@MoOxSy) have been obtained and comprehensively characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), and Raman and Fourier-transform infrared spectroscopies. The photoelectrochemical performance of the Si-MWs@MoOxSy cathodes toward water reduction is investigated and compared with that of platinum nanoparticle decorated Si-MW array electrodes (Si-MWs@PtNPs). An optimized Si-MWs@MoOxSy photocathode is found to exhibit activity comparable to that of the Si-MWs@PtNPs one, with a much better stability in acidic medium. In neutral electrolyte, Si-MWs@MoOxSy outperforms Si-MWs@PtNPs in terms of both activity and stability. Given the low materials cost, easy and well-established electrode fabrication procedure, and high demonstrated photoelectrochemical performance, the Si-MWs@MoOxSy arrays reported here hold substantial promise for use as low-cost and efficient photocathodes for water reduction.

Versatile control of metal-assisted chemical etching for vertical silicon microwire arrays and their photovoltaic applications.

A systematic study was conducted into the use of metal-assisted chemical etching (MacEtch) to fabricate vertical Si microwire arrays, with several models being studied for the efficient redox reaction of reactants with silicon through a metal catalyst by varying such parameters as the thickness and morphology of the metal film. By optimizing the MacEtch conditions, high-quality vertical Si microwires were successfully fabricated with lengths of up to 23.2 μm, which, when applied in a solar cell, achieved a conversion efficiency of up to 13.0%. These solar cells also exhibited an open-circuit voltage of 547.7 mV, a short-circuit current density of 33.2 mA/cm2, and a fill factor of 71.3% by virtue of the enhanced light absorption and effective carrier collection provided by the Si microwires. The use of MacEtch to fabricate high-quality Si microwires therefore presents a unique opportunity to develop cost-effective and highly efficient solar cells.

Ordered silicon microwire arrays grown from substrates patterned using imprint lithography and electrodeposition.

Silicon microwires grown by the vapor-liquid-solid process have attracted a great deal of interest as potential light absorbers for solar energy conversion. However, the research-scale techniques that have been demonstrated to produce ordered arrays of micro and nanowires may not be optimal for use as high-throughput processes needed for large-scale manufacturing. Herein we demonstrate the use of microimprint lithography to fabricate patterned templates for the confinement of an electrodeposited Cu catalyst for the vapor-liquid-solid (VLS) growth of Si microwires. A reusable polydimethylsiloxane stamp was used to pattern holes in silica sol-gels on silicon substrates, and the Cu catalyst was electrodeposited into the holes. Ordered arrays of crystalline p-type Si microwires were grown across the sol-gel-patterned substrates with materials quality and performance comparable to microwires fabricated with high-purity metal catalysts and cleanroom processing.

Functional integration of Ni–Mo electrocatalysts with Si microwire array photocathodes to simultaneously achieve high fill factors and light-limited photocurrent densities for solar-driven hydrogen evolution

An n+p-Si microwire array with a Ni–Mo/TiO2 catalyst/light-scattering bi-layer structure that simultaneously achieves high fill factors and light-limited photocurrent densities for direct solar H2(g) production.

Unassisted solar-driven photoelectrosynthetic HI splitting using membrane-embedded Si microwire arrays

Demonstration of unassisted solar-driven photoelectrosynthetic hydriodic acid splitting using freestanding membrane-embedded Si microwire arrays.

Scalable processing and capacity of Si microwire array anodes for Li ion batteries.

Si microwire array anodes have been prepared by an economical, microelectronics compatible method based on macropore etching. In the present report, evidence of the scalability of the process and the areal capacity of the anodes is presented. The anodes exhibit record areal capacities for Si-based anodes. The gravimetric capacity of longer anodes is comparable to the one of shorter anodes at moderate lithiation/delithiation rates. The diffusion limitation of the lithium ions through the electrolyte in depth among the wires is the limiting factor for cycling longer wires at high rates.PACS82.47.Aa; 82.45.Vp; 81.16.-c

Chemical Modification of Semiconductor Interfaces for Mechanically Robust Solar Fuel Generating Devices

Introduction The increasing demand for lightweight, low cost, and mechanically robust solar-to-fuel electrochemical energy conversion devices motivates development of integrated semiconductor-polymer systems. [1, 2] One promising design for flexible photovoltaic [3, 4] or solar fuel generating [5, 6] devices employs arrays of Si microwires embedded in a polymer matrix that serves as a mechanical support, an electronic contact, and/or an ion conducting and fuel-separating membrane. Chemical and mechanical stability of the Si-polymer interface is critical for maintaining high junction quality and electronic/mechanical contact during sustained device operation. However, quantitative performance metrics and design methodologies for tailoring robust interfaces are needed. Previously, we demonstrated that single-wire in-situ mechanical pull-out tests can be used to measure interfacial shear strength between Si microwires with systematically varied surface functionalities and a polydimethylsiloxane (PDMS) matrix. The interfacial shear stress was found to correlate with the chemical bond strength resulting from covalent (-H, mixed –CH3/butenyl) or van der Waals (-CH3, octadecyl) bonds with PDMS. This revealed that macroscopic mechanical properties of the Si/polymer composite can be tailored by tuning Si surface chemistry. Results and Discussion Si microwire arrays (Fig. 1a) were functionalized with electrostatically charged end groups (-COO- and NH3 +) to target directed bonding with ionic sites in conducting polymers such as Nafion, which have relevance as membranes in solar water-splitting devices. Negatively-charged surfaces were introduced via functionalization with decanoic acid (CH3(CH2)8COOH, Fig. 1b) by a UV-initiated hydrosilylation reaction on a Si-H terminated surface in 10% decanoic / 90% toluene. FTIR spectroscopy on a planar Si(111) wafer indicated the disappearance of Si-H stretches after hydrosilyation and the appearance of characteristic C-H, C=O and C-O-H vibrational modes (Fig. 2a) indicative of successful attachment of surface groups in the desired orientation with –COOH ends exposed, as further confirmed by XPS in the C 1s region (Fig. 2b). Amine-functionalized Si microwire surfaces were also developed to introduce positive charges onto Si surfaces when protonated in acid. Functionalized microwire arrays were embedded into a Nafion membrane by spin casting of a Nafion-DMF solution. Wire-polymer arrays were then removed from the substrate and mounted on an SEM stub for nanomechanical testing. Specimens for single-wire pull-out tests were prepared using a focused ion beam (FIB) to carve a “dog-bone” shape into the tops of wires (Fig. 1a inset), and were then manipulated by a diamond grip attached to a nanomechanical testing arm within an in-situ SEM setup (Fig. 1c-d). Wire pull-out (tensile) tests were used to measure the maximum interfacial shear strength at the Si microwire-polymer interface prior to debonding of the wire and release from the matrix, and the strength of electrostatic (attractive and repulsive) interactions assembled under a range of pH conditions was measured. Si-H and Si-CH3 surface interaction strengths with Nafion were also studied and compared with prior results in PDMS. We also discuss the role of these molecular surfaces as junctions in electrochemical devices, including their surface chemical and electrochemical stability during operation.