Crystalline Silicon Nanowires Research Articles

Single-nanowire-morphed mechanical slingshot for directional shooting delivery of micro-payloads

Stretching elastomer bands to accumulate strain energy, for a sudden projectile launching, has been an old hunting skill that will continue to find new applications in miniaturized worlds. In this work, we explore the use of highly resilient and geometry-tailored ultrathin crystalline silicon nanowires (SiNWs) as elastic medium to fabricate the first, and the smallest, mechanical slingshot. These NW-morphed slingshots were first grown on a planar surface, with desired layout, and then mounted upon standing pillar frames, with a unique self-hooking structure that allows for a facile and reliable assembly, loading and shooting maneuver of microsphere payloads. Impressively, the elastic spring design can help to store 10 times more strain energy into the NW springs, compared with the straight ones under the same pulling force, which has been strong enough to overcome the sticky van der Waals (vdW) force at the touching interfaces that otherwise will hinder a reliable releasing onto soft surface with low-surface energy or adhesion force, and to achieve a directional shooting delivery of precise amount of tiny payload units onto delicate target with the least impact damage. This NW-morphing construction strategy also provides a generic protocol/platform to fast design, prototype, and deploy new nanoelectromechanical and biological applications at extremely low costs.

Precise morphology control of in-plane silicon nanowires via a simple plasma pre-treatment

Silicon nanowires (SiNWs) are advantageous building blocks to explore a wide range of high performance nanoelectronics and photonics devices. In-plane solid-liquid-solid (IPSLS) SiNWs, grown by metal catalyst droplets that absorb hydrogenated amorphous silicon (a-Si:H) thin film to produce crystalline SiNWs, are particularly suitable for planar device fabrication and integration. Here, we explore a new growth control dimension to tailor the geometry of the in-plane SiNWs from island-chain to zigzag and to straight morphologies by using a simple plasma modification of the a-Si:H thin film precursor. This unique capability is due to fact that the Gibbs energy and bonding status of the a-Si:H layer can be largely modified by the plasma treatments at various substrate temperatures, which in turn have a huge impact on the growth balance condition of the SiNWs and consequently on their morphologies. These results highlight a facile and yet highly effective strategy to tailor the morphologyof in-plane SiNWs that will find important applications in fabricating nanoelectronic, sensor and logic devices.

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.

Thermally induced evolution of the structure and optical properties of silicon nanowires

In the present paper, we report on the investigation of thermal annealing (TA) effect on structural and optical properties of crystalline silicon nanowires produced by metal-assisted chemical etching approach. In particular, the impact of TA on nanowire length, relative volume and size distribution of voids is described in terms of Lifshitz-Slyozov-Wagner theory considering the TA induced Oswald ripening in the SiNW arrays. It was also found that TA leads to a decrease of the SiNWs total reflection in the wide UV–VIS-IR spectral range. The reported effects can be used for tuning of crystalline SiNWs arrays in view of their further applications in photonics related fields.

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.

Single-Nanowire Fuse for Ionization Gas Detection.

Local electric field enhancement is crucial to detect gases for an ionization gas sensor. Nanowires grown collectively along the identical lattice orientation have been claimed to show a strong tip effect in many previous studies. Herein, we propose a novel ionization gas detector structure by using a single crystalline silicon nanowire as one electrode that is placed above the prepatterned nanotips. A significant improvement of the local electric field in its radical direction was obtained leading to an ultralow operation voltage for gas breakdown. Different from the tip of the nanowire in the reported ionization gas sensors, the gaseous discharge current in this device flows towards the sidewall in the case of a trace amount of gas environment change. Technically, this discharge current brings about a sudden temperature rise followed by a fusion of the silicon nanowire. Such unique fusibility of a single nanowire in this gas detection device suggests a novel architecture that is portable and in-site executable and can be used as an integrated gas environmental monitor.

Structural and electronic properties of lithiated Si nanowires: An ab initio study

Ab initio density-functional theory is used to study the structural changes of a crystalline silicon nanowire during lithiation and delithiation processes. Based on the obtained results quantum transport calculations are performed to shed light on the electrical current flowing at varying lithium concentrations. It is found that inserting lithium atoms into the crystalline silicon system leads to a steplike process, where each Si shell is successively lithiated from the outside. While increasing the Li concentration, the volume of the nanowire quadruples and the silicon network becomes fully amorphous. During delithiation the Li ions are initially homogeneously distributed throughout the nanowire cross section, eliminating the steplike behavior observed in the lithiation cycle. Hereby, the volume of the system decreases to 113% of its initial value, with a small amount of Li ions remaining in the Si network and a loss of storage capacity as a consequence. A simultaneous analysis of the current trajectories within the lithiated nanowires reveals that the main contribution shifts from the center to the surface of the nanostructure as the Li concentration increases.

Cohesive zone model for crack propagation in crystalline silicon nanowires

A fracture of the active particle such as silicon anode results in a potential degradation in lithium-ion batteries with charge/discharge cycle. The massive volume expansion during the lithiation generates large stress which induces the cracking on its surface. We propose the finite element model to have a deep insight into the diffusion-induced stress with fracture process for crystalline silicon nanowires. This model is constructed based on the large deformation theory and cohesive zone model that can describe the failure of Si nanowires. At the beginning of lithiation, high compressive hoop stress is generated on its surface. However, further lithiation, the hoop stress near the surface become less compressive and tensile, which is associated with the “push out effect” as a result of the volume expansion of lithiated material near the phase boundary. When the diffusion induced tensile stress exceeds the fracture threshold, a crack is initiated on the surface and propagates continuously to the center of the silicon nanowire. This crack follows the phase boundary rapidly, but the crack growth speed decreases due to the neighboring compressive hoop stress generated near the phase boundary. From the observation of failure mechanism in Si, we characterize a critical size of Si nanowire, below which fracture can be averted, and we suggest a safe state of charge (safe SOC) depending on its radial size during lithiation.

Oxide reduced silicon nanowires

Core crystalline silicon nanowires with a heavily reduced amorphous shell have been successfully synthesised using palladium as a metal catalyst. We present two approaches to reduce the oxidation of the nanowires during the thermal annealing growth. The ratios of the amorphous shell to crystalline core of the nanowires produced, from the two methods, are compared and show a remarkable drop (hence thinner oxide) compared to wires fabricated using currently available techniques. In addition, a focused ion beam was utilised to contact the oxide-reduced nanowires for transport measurements, without first removing the thin oxide shell. The oxygen-reduced core-shell silicon nanowires showed a very low electrical resistivity (4 × 10−1 Ω cm). Our novel approach presents a new alternative to the production of low cost, high yield, highly conducting silicon nanowires offering a wide range of opportunities for semiconductor based technology.

Synthesis and Characterization of Silicon Nanowires by Electroless Etching

Silicon nanowires (SiNWs) were synthesized by two-step electroless etching of p-type Si (100) wafer and characterized by field emission scanning electron microscopy, UV–Vis spectroscopy, x-ray photoelectron spectroscopy, Fourier transform infrared spectroscopy and Raman spectroscopy. The vibrational signature at 1108 and 2087 cm−1 confirmed SiNWs were passivated by both oxygen and hydrogen atoms. Raman peak at 517 cm−1 indicated crystalline SiNWs with tailing toward redshift due to Fano effect. The Si(2p) and Si(2s) core orbital spectra of SiNWs were found at 99.8 and 150.5 eV, respectively. Moreover, the reflection of SiNWs is minimized to ~ 1 to 5% in the 650-nm wavelength.

Suppressing phonon transport in nanowires: A simple model for phonon–surface-roughness interaction

Suppressing phonon propagation in nanowires is an essential goal towards achieving efficient thermoelectric devices. Recent experiments have shown unambiguously that surface roughness is a key factor that can reduce the thermal conductivity well below the Casimir limit in thin crystalline silicon nanowires. We use insights gained from the experimental studies to construct a simple analytically tractable model of the phonon--surface-roughness interaction that provides a better theoretical understanding of the effects of surface roughness on the thermal conductivity, which could potentially help in designing better thermoelectric devices.

Engineering island-chain silicon nanowires via a droplet mediated Plateau-Rayleigh transformation.

The ability to program highly modulated morphology upon silicon nanowires (SiNWs) has been fundamental to explore new phononic and electronic functionalities. We here exploit a nanoscale locomotion of metal droplets to demonstrate a large and readily controllable morphology engineering of crystalline SiNWs, from straight ones into continuous or discrete island-chains, at temperature <350 °C. This has been accomplished via a tin (Sn) droplet mediated in-plane growth where amorphous Si thin film is consumed as precursor to produce crystalline SiNWs. Thanks to a significant interface-stretching effect, a periodic Plateau-Rayleigh instability oscillation can be stimulated in the liquid Sn droplet, and the temporal oscillation of the Sn droplets is translated faithfully, via the deformable liquid/solid deposition interface, into regular spatial modulation upon the SiNWs. Combined with a unique self-alignment and positioning capability, this new strategy could enable a rational design and single-run fabrication of a wide variety of nanowire-based optoelectronic devices.

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.

Smart integration of silicon nanowire arrays in all-silicon thermoelectric micro-nanogenerators

Micro and nanotechnologies are called to play a key role in the fabrication of small and low cost sensors with excellent performance enabling new continuous monitoring scenarios and distributed intelligence paradigms (Internet of Things, Trillion Sensors). Harvesting devices providing energy autonomy to those large numbers of microsensors will be essential. In those scenarios where waste heat sources are present, thermoelectricity will be the obvious choice. However, miniaturization of state of the art thermoelectric modules is not easy with the current technologies used for their fabrication. Micro and nanotechnologies offer an interesting alternative considering that silicon in nanowire form is a material with a promising thermoelectric figure of merit. This paper presents two approaches for the integration of large numbers of silicon nanowires in a cost-effective and practical way using only micromachining and thin-film processes compatible with silicon technologies. Both approaches lead to automated physical and electrical integration of medium-high density stacked arrays of crystalline or polycrystalline silicon nanowires with arbitrary length (tens to hundreds microns) and diameters below 100 nm.

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.

High thermosensitivity of silicon nanowires induced by amorphization

In this work, we demonstrate highly thermosensitive silicon nanowires (SiNWs) for thermal-sensing applications. Crystalline Si was amorphized by Focused Ion Beam in the fabrication process of the SiNWs, and subsequently recrystallized by a thermal annealing process to improve their electrical conductivity. A temperature coefficient of resistance (TCR) from −8000ppm/K to −12,000ppm/K was measured for the SiNWs. This large negative TCR is attributed to the boundary potential barrier of 110meV between silicon crystallites in the poly crystalline SiNWs.

Direct growth of Si nanowires on flexible organic substrates

A key characteristic of semiconductor nanowires (NWs) is that they grow on any substrate that can withstand the growth conditions, paving the way for their use in flexible electronics. We report on the direct growth of crystalline silicon nanowires on polyimide substrates. The Si NWs are grown by plasma-enhanced chemical vapor deposition, which allows the growth to proceed at temperatures low enough to be compatible with plastic substrates (350 °C), where gold or indium are used as growth seeds. In is particularly interesting as the seed not only because it leads to a better NW crystal quality but also because it overcomes a core problem induced by the use of Au in silicon processing, i.e. Au creates deep carrier traps when incorporated in the nanowires.

Role of a‐Si:H in lateral growth of crystalline silicon nanowires using Pb and In catalysts

Growth of crystalline silicon nanowires at low temperatures by chemical vapour deposition is an important technological challenge for electronic and sensor nanodevices. Here we present a comparative study of crystalline silicon nanowire growth by plasma‐enhanced chemical vapour deposition (PECVD) using lead and indium. We compare two different growth methods that produce either in‐plane or out‐of‐plane crystalline Si nanowires (SiNWs) at temperatures below 400 °C depending on the growth conditions and the catalyst used. The first growth method is based on the deposition of a thin film of amorphous silicon on a substrate at 150 °C and subsequent thermal annealing at 400 °C. The second method uses direct SiNWs growth at 400 °C in PECVD. While the first method produces in‐plane Si nanowires with In and no nanowire growth for Pb, the second method gives rise to out‐of‐plane Si nanowires for In and in‐plane nanowires for Pb. We show that one of the reasons for the difference between these two methods is the properties of amorphous hydrogenated silicon (a‐Si:H), which acts as the source material for in‐plane growth of Si nanowires.

Direct electrochemical deposition of crystalline silicon nanowires at T ≥ 60 °C

Direct synthesis of crystalline silicon (Si) nanowires at low temperatures has been achieved through an electrochemical liquid–liquid–solid (ec-LLS) process.

Electrical and Ultraviolet-A Detection Properties of E-Beam Evaporated n-TiO2 Capped p-Si Nanowires Heterojunction Photodiodes

Fabrication and Ultraviolet (UV) detection properties of p-Si Nanowires (NWs)/n-TiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> thin film-based heterojunction photodiodes have been reported in this paper. The highly oriented, uniform and vertically aligned p-type single crystalline silicon nanowire (SiNW) arrays have been synthesized by the two-step process of electroless metal deposition and etching technique. A thin layer (~120 nm) of anatase phase titanium dioxide (TiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> ) is then deposited on the top of p-SiNWs using electron-beam evaporation technique. The surface morphology and crystallinity of p-SiNWs and n-TiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> capped SiNWs have been characterized by scanning electron microscopy, energy dispersive X-ray spectroscopy and X-ray diffraction techniques. The optical properties of the as-deposited n-TiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> thin film grown on p-SiNWs were analyzed using the photoluminescence measurements. The UV detection properties of the p-SiNWs/n-TiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> heterojunction was studied by measuring the room temperature current-voltage (I - V ) characteristics under dark and UV illumination conditions with incident optical power(P <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">opt</sub> ) ≈ 650 μW at wavelength (λ) ≈ 365 nm. The as-fabricated p-SiNWs/n-TiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> thin film heterojunction photodiode showed an excellent value of Detectivity ~ 8.66 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">11</sup> mHz <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1/2</sup> W <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> with the external quantum efficiency (EQE) ~ 79.33%, responsivity~ 0.234 A/W, and contrast ratio ~ 113.82 at -11 V bias. Other parameters such as rectification ratio (~ 519.82), Turn-on voltage (~0.732 V) and effective barrier height (~0.7924 eV) under dark condition were also calculated. The proposed p-SiNWs/n-TiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> ) thin film hetrojunction can thus be explored for UV-A detection.detection.