Single Crystalline Silicon Nanowires Research Articles

Contamination‐Assisted Rather Than Metal Catalyst‐Free Bottom‐Up Growth of Silicon Nanowires

AbstractWell‐established metal‐catalyzed vapor–liquid–solid (VLS) growth represents still undoubtedly the key technology for bottom‐up synthesis of single‐crystalline silicon nanowires (SiNWs). Although various SiNW applications are demonstrated, electrical and optical properties are exposed to the inherent risk of electronic deep trap state formation by metal impurities. Therefore, metal catalyst‐free growth strategies are intriguing. The oxid‐assisted SiNW synthesis is explored and it is shown that contamination control is absolutely crucial. Slightest metal impurities, such as iron, are sufficient to trigger SiNW growth, calling into question true metal catalyst‐free SiNW synthesis. Therefore, the term contamination‐assisted is rather introduced and it is shown that contamination‐assisted SiNW growth is determined by the chemical surface treatment (e.g., with KOH solution), but also by the crystal orientation of a silicon substrate. SiNWs are grown in this regards in a reproducible manner, but so far with a distinct tapering, using a conventional gas‐phase reactor system at temperatures of about 680 °C and monosilane (SiH4) as the precursor gas. The synthesized SiNWs show convincing electrical properties compared to Au‐catalyzed SiNWs. Nevertheless, contamination‐assisted growth of SiNWs appears to be an important step toward bottom‐up synthesis of high‐quality SiNWs with a lower risk of metal poisoning, such as those needed for CMOS and other technologies.

Exploring nanowire regrowth for the integration of bottom-up grown silicon nanowires into AFM scanning probes

Bottom-up grown single-crystalline silicon nanowires (SiNWs) are highly intriguing to build nanoscale probes, for instance for atomic force microscopy (AFM), due to their mechanical robustness and high aspect ratio geometry. Several strategies to build such nanowire-equipped probes were explored but their fabrication is still elaborate, time-consuming and relies partly on single-crystalline substrates. Here, we explore a new strategy to fabricate AFM probes that are equipped with single-SiNW scanning tips. The conceptual evaluation begins with a discussion on the overall design and softness of such probes based on finite-element-method simulations. For the experimental realization, SiNWs were grown by the well-established gold-catalyzed vapor–liquid–solid method employing gaseous monosilane. As-grown SiNWs were subsequently transferred onto flexible membranes and even freestanding AFM microcantilever beams via mechanical nanowire contact printing. Elongation of the deposited nanowires by so-called regrowth was triggered by reusing the original gold catalyst to yield the prospective AFM scanning tip. SiNW-equipped scanning probes were created in this manner and were successfully employed for topography imaging. Although a multitude of challenges remains, the created probes showed an overall convincing performance and a superior durability.

Towards the fabrication of third generation solar cells on amorphous, flexible and transparent substrates with well-ordered and disordered Si-nanowires/pillars

Abstract Si Nanowires (NWs) are typically synthesized on limited substrates that lack essential characteristics, such as mechanical flexibility and optical transparency. Most of the synthesis processes are also costly and inhibit widespread applications enabled by NWs. Throughout this study, therefore, we have shown that ordered and disordered single crystalline silicon nanowires can be grown and transferred from Si wafer to a broad range of foreign substrates while maintaining their original order on the mother substrates. Vertically-aligned Si NWs have been successfully transferred to Ag-pre-coated glasses, transparent-conductive-oxides and metal foils that ensure ohmic contacts between Si NWs and the transferred substrates, which are critical for scalable manufacturing of electronics and opto-electronic devices. This strategy presents an opportunity to develop low-cost device manufacturing with highly crystalline semiconductor materials, and is a critical leap towards the next generation high performance core-shell Si-NWs solar cells. In order to demonstrate devices based on the transferred NWs, the NWs are coated with a thin layer of CZTS to fabricate a third generation solar cell. The devices were characterized and exhibited the highest power conversion efficiency of 1.31% reported to date, for such transferred NWs and material combinations.

Effect of Silicon Wafer Resistivity on Morphology and Wettability of Silicon Nanowires Arrays

Herein, we prepare vertical and single crystalline silicon nanowires (SiNWs) via a one-step metal-assisted chemical etching method in aqueous NH4HF2/AgNO3 solution. The effects of silicon substrate resistivity and concentrations of NH4HF2 and AgNO3 on the etching process were examined. Two properties were studied, the morphology and the wettability of etched layers. The morphology of the silicon nanowire (SiNW) layers was investigated by scanning electron microscopy (SEM) while the wettability was studied by contact angle measurement system. It was established that the morphology is strongly influenced by etching parameters and their wettability changes from superhydrophilic to hydrophobic.

Molecular dynamics simulation of the uniaxial tensile test of silicon nanowires using the MEAM potential

The silicon nanowire is a novel nanometer-scale structure which can be used in various electronic and optoelectronic devices. While the plastic deformation and failure mechanisms, including the brittle-to-ductile transition at room temperature, of nanowires have been studied both experimentally and theoretically, the fundamental atomic-scale mechanisms of these behaviors still remain incompletely understood. In this article we study the mechanical and failure behaviors of the [110]-oriented single-crystalline silicon nanowires by performing uniaxial tensile tests using molecular dynamics simulations with three modified embedded-atom-method (MEAM) potentials, referred to as Baskes, Lee, and Lee-modified. The effects of several key parameters such as size, temperature, and strain rate are investigated. A preliminary examination of the three MEAM potentials reveals that Lee and Lee-modified models outperform Baskes model in predicting the thermal expansion coefficient and surface energies. The uniaxial tensile simulations show that some mechanical properties such as Young’s modulus and tensile strength exhibit size-dependence while there is little size-effect on the failure strain. A novel parameter named the ductile failure probability is introduced to quantify the failure behavior of the nanowire during the tensile test, which scales from 0 for the pure brittle to 1 for the pure ductile mode failure. Overall, the ductile failure probability increases with decreasing size and increasing temperature in all of the three models. The Baskes nanowires exhibit the most ductile failures among the three models whereas most Lee-modified nanowires fail through fractures on the (111) plane. The Lee model shows some intermediate levels of ductile failure behavior. The difference due to the strain rate is very small, but overall nanowires become slightly more brittle as the strain rate decreases.

SiNWs Biophysically Regulate the Fates of Human Mesenchymal Stem Cells

While biophysical stimuli from polymeric matrices are known to significantly affect the fates of human mesenchymal stem cells (hMSCs), the stimulatory effects of nano-sized silicon-based matrices on hMSCs have not been thoroughly investigated. We previously demonstrated that vertically aligned, single-crystalline silicon nanowires (SiNWs) can control the osteogenicity of hMSCs via controllable spring constants from SiNWs matrix. However, other possible differentiation fates of hMSCs on SiNWs have not been explored. We hypothesize that tunable spring constant from artificial SiNWs matrices can direct different types of hMSC differentiations. The spring constants of tunable SiNW matrices can be consistently controlled by tuning the SiNW length. The results of gene expression and cell stiffness suggest that hMSCs differentiations are sensitive to our distinguishable spring constants from the SiNWs groups, and simultaneously conduct osteogenicity and adipogenicity. These findings suggest that SiNW matrices can regulate the fates of hMSCs when the SiNW characteristics are carefully tuned.

Probing Stress States in Silicon Nanowires During Electrochemical Lithiation Using In Situ Synchrotron X-Ray Microdiffraction

Silicon is considered as a promising anode material for the next generation lithium-ion battery due to its high capacity at nanoscale. However, silicon expands up to 300% during lithiation, which induces high stresses and leads to fractures. To design silicon nanostructures that could minimize fracture, it is important to understand and characterize stress states in the silicon nanostructures during lithiation. Synchrotron X-ray microdiffraction has proven to be effective in revealing insights of mechanical stress and other mechanics considerations in small-scale crystalline structures used in many important technological applications, such as microelectronics, nanotechnology and energy systems. In the present study, an in situ synchrotron X-ray microdiffraction experiment was conducted to elucidate the mechanical stress states during the first electrochemical cycle of lithiation in single-crystalline silicon nanowires (SiNWs) in a lithium-ion battery test cell. Morphological changes in the SiNWs at different levels of lithiation were also studied using scanning electron microscope (SEM). It was found from SEM observation, that lithiation commenced predominantly at the top-surface of SiNWs followed by further progression towards the bottom of the SiNWs gradually. The hydrostatic stress of the crystalline core of the SiNWs at different levels of electrochemical lithiation was determined using the in situ synchrotron X-ray microdiffraction technique. We found that the crystalline core of the SiNWs became highly compressive (up to -~325.5 MPa) once lithiation started. This finding helps unravel insights about mechanical stress states in the SiNWs during the electrochemical lithiation, which could potentially pave the path towards the fracture-free design of silicon nanostructure anode materials in the next generation lithium-ion battery.

Antireflection subwavelength structures based on silicon nanowires arrays fabricated by metal-assisted chemical etching

In this paper, we have obtained a series of large-area and different diameters nanosphere lithography (NSL) to obtain the required silicon nanowires (SiNWs) arrays. The single-crystalline SiNWs have been presented by combining nanosphere lithography (NSL) and metal-assisted chemical etching (MACE). The period of SiNW arrays can be controlled by adjusting the original diameter of polystyrene nanosphere (PSs) and the etching time during the NSL process. The special SiNWs structure obtained can be demonstrated to be significant for improving the antireflection properties of silicon substrate. The results show that SiNW arrays with various parameters, such as diameter, distance and height can be obtained by controlling the key etching parameter during the MACE process, which are important to obtain the structures of different parameters to adapt an appropriate value to decrease the light scattering. For a wide wavelength range of 300–1200 nm, the reflectance is below 10% or less, which is due to an ultra-high surface area. Especially, the reflectance of antireflection structure (ARS) surface reduces below 1% over a wavelength range of 300–400 nm. Furthermore, the silicon nanowire (SiNW) arrays with highly efficient antireflection obtained by MACE exhibit different surface roughness from the bottom to the top part of SiNWs by high resolution images, which is benefit for further improving the ARS of SiNWs.

Enabling the study of stress states using in situ µSXRD in the silicon nanowire anode during electrochemical lithiation in a specially designed Li-ion battery test cell

Synchrotron X-ray microdiffraction has proven to be effective in revealing insights of mechanical stress and other mechanics considerations in small scale crystalline structures in many important technological applications, such as microelectronics, nanotechnology and energy systems. In the present study, a special Li-ion battery test cell was designed to enable an in situ synchrotron X-ray microdiffraction experiment to elucidate the mechanical stress states during the first electrochemical cycle of lithiation in single-crystalline silicon nanowires (SiNWs). Silicon is considered as a promising anode material for the next generation lithium-ion battery due to its high capacity at nanoscale. However, silicon expands up to 300% during lithiation, which induces high stresses and leads to fractures. To design silicon nanostructures that could minimize fracture, it is important to understand and characterize stress states in the silicon nanostructures during lithiation. The stress of the crystalline core of the SiNWs during electrochemical lithiation was determined (at different levels of the electrochemical lithiation) using the in situ synchrotron X-ray microdiffraction technique. We found that the crystalline core of the SiNWs became highly compressive (up to -~325.5 MPa) once lithiation started. This finding helps unravel insights about mechanical stress states in the SiNWs during the electrochemical lithiation, which could potentially pave the path to fracture-free design of silicon nanostructure anode materials in the next generation lithium-ion battery.

Approaching the ideal elastic strain limit in silicon nanowires.

Achieving high elasticity for silicon (Si) nanowires, one of the most important and versatile building blocks in nanoelectronics, would enable their application in flexible electronics and bio-nano interfaces. We show that vapor-liquid-solid-grown single-crystalline Si nanowires with diameters of ~100 nm can be repeatedly stretched above 10% elastic strain at room temperature, approaching the theoretical elastic limit of silicon (17 to 20%). A few samples even reached ~16% tensile strain, with estimated fracture stress up to ~20 GPa. The deformations were fully reversible and hysteresis-free under loading-unloading tests with varied strain rates, and the failures still occurred in brittle fracture, with no visible sign of plasticity. The ability to achieve this "deep ultra-strength" for Si nanowires can be attributed mainly to their pristine, defect-scarce, nanosized single-crystalline structure and atomically smooth surfaces. This result indicates that semiconductor nanowires could have ultra-large elasticity with tunable band structures for promising "elastic strain engineering" applications.

Synthesis of Highly Crystalline Needle-Like Silicon Nanowires for Enhanced Field Emission Applications

Needle-like silicon nanowires (SiNWs) were successfully synthesized on gold-coated silicon substrates using a very high frequency plasma enhanced chemical vapor deposition method (VHF-PECVD). The prepared samples were characterized by field emission scanning electron microscopy (FESEM) with energy dispersive X-ray spectroscopy (EDX), high resolution transmission electron microscopy (HRTEM), X-ray diffraction (XRD) and photoluminescence (PL). XRD analysis confirmed formation of single crystalline SiNWs along (111) crystalline planes and microscopic studies revealed formation of NWs with diameters ranging from 10 to 100 nm and lengths of a few micrometers. Furthermore, the presence of gold nanoparticles on the tip of the NWs verified the vapor–liquid–solid growth mechanism of SiNWs. It was also demonstrated that SiNWs are composed of well-crystallized silicon cores and an amorphous shell. The obtained results verified potential application of such structures in field emission displays.

Epitaxial growth of silicon nanowire arrays at wafer-scale using high-speed rotating-disk CVD for improved light-trapping

Silicon nanowires (Si-NWs) have been fabricated at wafer-scale with Au nanoparticles as catalysts at 500 °C using a chemical-vapor-deposition (CVD) method combined with a high-speed rotation wafer-disk. High resolution TEM and SAED patterns were used to characterize the microstructures of the grown Si-NWs. Single crystalline Si-NWs appear self-oriented on Si (100) substrates along the growth axis. The nanowires have a uniform diameter of 50 nm and varying lengths and densities depending on the rotation speed of the wafer substrate. The effects of wafer-rotation speed on the growth of Si-NWs are discussed. Due to a reduced boundary layer thickness, fast mass transport leads to both an enhanced growth rate and density of the Si-NWs. Moreover, the density of the Si-NW array is optimized to achieve a low reflectance loss for their further application in tandem solar cells as the light-trapping layer.

Surface morphology and strain coupling effects on phonon transport in silicon nanowires

We perform non-equilibrium molecular dynamics calculations to study the coupling effects of surface, defects, and strain on heat transport in silicon nanowires (SiNWs). It is found that, for single-crystalline SiNWs, rippled surface and vacancy defects can significantly reduce the thermal conductivity κ of SiNWs. In addition, through surface amorphization, the thermal conductivity can be further remarkably reduced, due to strong phonon scattering at the interface as well as the non-propagating diffusion of phonons in the amorphous region. However, the κ value of amorphous-surface SiNWs is not sensitive to the roughness and vacancy in amorphous surface. Besides surface and defects, it has been observed that stress-induced defects/deforms can introduce additional mechanism of phonon scattering, which decrease the thermal conductivity. The present work demonstrates that, through phonoic engineering to tune the thermal conductivity, SiNWs can be promising candidates for thermoelectric applications.

Thickness effect of catalyst layer on silicon nanowires morphology and features

Silicon nanowires (SiNWs) have been synthesized on gold layer-coated silicon substrates via plasma enhanced chemical vapor deposition method (PECVD). Various thicknesses of Au layers were coated on Si (111) substrates using radio frequency magnetron sputtering. Field emission scanning electron microscopy (FESEM), X-ray diffraction (XRD), energy dispersive X-ray spectroscopy (EDX), and transmission electron microscopy (TEM) were used to characterize the morphology, compositions, and structures of the samples. The results show that the sample consisted of single-crystalline SiNWs with the diameters ranging from 40 to 160nm and length up to 3μm. It was observed that the diameter of SiNWs increases with increasing of Au catalyst layers thickness. Raman spectra display peaks with narrow and asymmetric shape at 518cm−1 for the SiNWs, indicating the high crystalline nature of the samples. A possible growth mechanism is proposed for the formation of nanowires (NWs). It has been found that the features of SiNWs depend on the thickness of Au layers.

The mechanisms of platinum-catalyzed silicon nanowire growth

Platinum (Pt) has been known as a catalyst material for vapor-liquid-solid (VLS) synthesis since the mid 1960s with the potential to grow electronic grade silicon nanowires (SiNWs). In contrast to gold-based growth, Pt-catalyzed SiNW synthesis has rarely been studied, most likely due to higher synthesis temperatures and the formation of multiple Pt silicide phases. Here we present the growth of SiNWs from a Pt catalyst deposited by a focused ion or electron beam, which opens new strategies for the assembly of Pt-catalyzed SiNW-based devices, as well as SiNW growth from Pt nanoparticles and thin films. We show that single-crystalline SiNWs exhibit either the well-known catalyst tip or a polycrystalline silicon tip so far not reported. The local Pt concentration was found to be one key parameter triggering the growth mode. The proposed growth model for both types of SiNWs is based on a solid-state silicide-mediated crystallization rather than VLS. The discussion of the growth modes is supported by a variation of several growth parameters and SiNW synthesis using the substrate materials silicon nitride, single-crystalline silicon, fused silica, and sapphire.

Synthesis and Characterization of Microwave-Exfoliated Graphene Oxide-Wrapped Silicon Nanowire via Hydrosilylation.

Single-crystalline silicon nanowires (SiNWs) were fabricated by using an electroless metal-assisted etching of bulk silicon wafers with silver nanoparticles obtained by wet electroless deposition. The etching of SiNWs is based on sequential treatment in aqueous solutions of silver nitrate followed by hydrofluoric acid and hydrogen peroxide. Free-standing SiNWs were then obtained using ultra-sono method in toluene. Graphene oxide was prepared using the modified Hummers' process. Activated microwave-exfoliated graphite oxide (MEGO) was prepared and used for composition of silicon nanowires and graphene oxide via hydrosilylation. The silicon nanowire-graphene composite materials were characterized using XPS and FE-SEM.

Controlled Pore Formation on Mesoporous Single Crystalline Silicon Nanowires: Threshold and Mechanisms

Silicon nanowires are prepared by the method of the two-step metal-assisted wet chemical etching. We analyzed the structure of solid, rough, and porous nanowire surfaces of boron-doped silicon substrates with resistivities ofρ> 1000 Ωcm,ρ= 14–23 Ωcm, andρ< 0.01 Ωcm by scanning electron microscopy and nitrogen gas adsorption. Silicon nanowires prepared from highly doped silicon reveal mesopores on their surface. However, we found a limit for pore formation. Pores were only formed by etching below a critical H2O2concentration (cH2O2<0.3 M). Furthermore, we determined the pore size distribution dependent on the etching parameters and characterized the morphology of the pores on the nanowire surface. The pores are in the regime of small mesopores with a mean diameter of 9–13 nm. Crystal and surface structure of individual mesoporous nanowires were investigated by transmission electron microscopy. The vibrational properties of nanowire ensembles were investigated by Raman spectroscopy. Heavily boron-doped silicon nanowires are highly porous and the remaining single crystalline silicon nanoscale mesh leads to a redshift and a strong asymmetric line broadening for Raman scattering by optical phonons at 520 cm−1. This redshift,λSi bulk=520 cm−1 →λSi nanowire=512 cm−1, hints to a phonon confinement in mesoporous single crystalline silicon nanowires.

Ultrafast chemical lithiation of single crystalline silicon nanowires: in situ characterization and first principles modeling

Through a combined density functional theory andin situscanning electron microscopy study, we demonstrate the ultrafast chemical lithiation of a singlec-Si nanowire brought into direct contact with Li metal in the absence of an electric field.

Well-aligned NiSi/Si heterostructured nanowire arrays as field emitters

Nickel silicides are promising candidates in modern semiconductor devices as contact metals, metal gate electrodes, and source/drains in field effect transistors. A multistep template replication process was utilized to fabricate highly periodic and well-aligned silicon nanowires (SiNWs), and the size could be further reduced via dry oxidation and post chemical treatment. NiSi/Si heterostructured nanowire arrays were fabricated by nickel film deposition on SiNWs and then rapid thermal annealing at 550 °C. Field emission (FE) measurements illustrated that the turn-on field and field enhancement factor β of NiSi/Si heterostructures were 1.3 V/μm and ∼9000, better than that of the SiNWs, which were about 1.9 V/μm and ∼3900. These results are superior to the FE properties of other silicon-based nanomaterials. The excellent FE characteristics are attributed to the uniformly distributed nanowires with high aspect ratio and large proximity, high conductivity of uniform NiSi coating, and favorable morphological features with sharp and rough tips as hot emission spots. The single-crystalline silicon nanowires shelled with polycrystalline NiSi provide substantial opportunities for integration with Si devices to form active field emitters, and for exploration of the nickel silicides on other electrical and optical properties.

The thermal transport properties of single-crystalline nanowires covered with amorphous shell: A molecular dynamics study

The thermal transport properties of single-crystalline nanowires (carbon nanowires (CNWs) and silicon nanowires (SiNWs)) covered with amorphous silicon shell are investigated by non-equilibrium molecular dynamics (NEMD). The thermal conductivity of SiNWs covered with amorphous silicon shell (Si/a-Si core–shell NWs) is obviously lower than that of single-crystalline SiNWs and it keeps approximately constant when the temperature exceeds a definite value for the diffusion properties of amorphous shell. It is quite different from that of single-crystalline SiNWs whose thermal conductivity decreases significantly for relatively higher temperature. Compared to single-crystalline SiNWs, Si/a-Si core–shell NWs express stronger non-propagation, diffusion thermal transport properties. The thermal transport properties of CNWs covered with amorphous silicon shell (C/a-Si core–shell NWs) are essentially similar to that of Si/a-Si core–shell NWs. It expresses stronger non-propagation, diffusion thermal transport properties and its thermal conductivity increases very little with the growth of length. However, its diffusion characteristics are weaker than that of Si/a-Si core–shell NWs. Generally speaking, the amorphous shell and amorphous/crystalline interface play a negative role to the thermal conductivities and an enhance role to the diffusion characteristics of single-crystalline nanowires, however, their degree of effect on different single-crystalline nanomaterials is different.