Oriented Si Nanowires Research Articles

Optimization of GOPS-Based Functionalization Process and Impact of Aptamer Grafting on the Si Nanonet FET Electrical Properties as First Steps towards Thrombin Electrical Detection.

Field effect transistors (FETs) based on networks of randomly oriented Si nanowires (Si nanonets or Si NNs) were biomodified using Thrombin Binding Aptamer (TBA–15) probe with the final objective to sense thrombin by electrical detection. In this work, the impact of the biomodification on the electrical properties of the Si NN–FETs was studied. First, the results that were obtained for the optimization of the (3-Glycidyloxypropyl)trimethoxysilane (GOPS)-based biofunctionalization process by using UV radiation are reported. The biofunctionalized devices were analyzed by atomic force microscopy (AFM) and scanning transmission electron microscopy (STEM), proving that TBA–15 probes were properly grafted on the surface of the devices, and by means of epifluorescence microscopy it was possible to demonstrate that the UV-assisted GOPS-based functionalization notably improves the homogeneity of the surface DNA distribution. Later, the electrical characteristics of 80 devices were analyzed before and after the biofunctionalization process, indicating that the results are highly dependent on the experimental protocol. We found that the TBA–15 hybridization capacity with its complementary strand is time dependent and that the transfer characteristics of the Si NN–FETs obtained after the TBA–15 probe grafting are also time dependent. These results help to elucidate and define the experimental precautions that must be taken into account to fabricate reproducible devices.

DNA grafting on silicon nanonets using an eco-friendly functionalization process based on epoxy silane

A new glycidoxypropyltrimethoxysilane (GOPS) deposition process using chemical vapor phase at ambient pressure in anhydrous atmosphere has been successfully implemented on Si nanonets which are randomly oriented Si nanowire (NW) networks. As a result, a covalent grafting of DNA probes followed by DNA hybridization could be obtained. Compared to the usually used deposition of GOPS in liquid phase, this process is shorter in time, toxic solvent-free, with the same or better efficiency. Moreover, this simple, fast and eco-friendly functionalization process can advantageously replace the previously studied amino-propyltriethoxysilane (APTES) deposition process and is therefore well adapted for the fabrication of Si nanonet based DNA biosensors.

First evidence of superiority of Si nanonet field effect transistors over multi-parallel Si nanowire ones in view of electrical DNA hybridization detection

Si nanonets (SiNN, networks of randomly oriented Si nanowires) and multi-parallel silicon nanowires (MP-SiNW) were integrated into field effect transistor using standard and low cost microelectronic technologies. The SiNN field effect transistors exhibit high initial ON-state current (in the range of 10–7 A), ION/IOFF ratio up to 104 and rather homogeneous transfer characteristics. In contrast, the MP-SiNW ones present smaller modulation between ON and OFF currents, higher IOFF and more scattered electrical characteristics. In view of DNA hybridization detection, a simple and eco-friendly functionalization process with glycidyloxypropyltrimethoxysilane (GOPS) was used to covalently graft single strand DNA probes on both SiNN and MP-SiNW devices. Validated by fluorescence measurement, DNA hybridization leads to a systematic decrease of ON-state current of SiNN devices. In addition, SiNN-based sensors exhibit more homogeneous and reproducible current variation in response to DNA hybridization step as compared to MP-SiNW configuration. This result highlights the better sensing performances of SiNN FETs as compared to MP-SiNW ones and emphasizes the SiNN potential for label-free detection of DNA.

Fabrication and Field-Emission Properties of Vertically-Aligned Tapered [110]Si Nanowire Arrays Prepared by Nanosphere Lithography and Electroless Ag-Catalyzed Etching

In this study, the controllable fabrication of a variety of vertically aligned, single-crystalline [110]-oriented Si nanowire arrays with sharp tips on (110)Si substrates is achieved using a combined self-assembled nanosphere lithography and multiple electroless Ag-catalyzed Si etching processes. All of the experiments were performed at room temperature. The morphological evolution and formation mechanism of long tapered [110]Si nanowire arrays during the multiple tip-sharpening cycle processes have been investigated by scanning electron microscopy, transmission electron microscopy and water contact angle measurements. Field emission measurements demonstrate that the field-emission behaviors of all nanowire samples produced in this study agree well with the Fowler–Nordheim theory, and the produced long tapered [110]Si nanowire array possesses superior electron emission characteristics, with a very low turn-on field of 1.4[Formula: see text]V/[Formula: see text]m and a high field enhancement factor of 3816. The simple and room temperature fabrication of the well-ordered long tapered [110]Si nanowire array and its excellent electron field emission performance suggest that it can serve as a good candidate for applications in high-performance Si-based vacuum electronic nanodevices.

“Deep Ultra-Strength”-Induced Band Structure Evolution in Silicon Nanowires

Recently we experimentally demonstrated that vapor–liquid–solid (VLS) grown silicon (Si) nanowires can be stretched to above 10% elastic strain through in situ nanomechanical strategies (Zhang et al. Sci. Adv. 2016, 2 (8), e1501382). Here, based on first-principles calculation, the geometric and electric properties of <110>-oriented Si nanowires with diameter ∼1.5 nm under ultralarge strain are comparatively investigated. The anisotropic Poisson effect was observed in the silicon nanowire that the change of diameter along the (100) orientation decreases dramatically while that along the (110) orientation shows only minor influence. For the purpose of obtaining a thorough understanding of the influence of applied strain on the electronic properties, the electronic band structure, effective mass of electron, and work function are systematically studied. Direct-to-indirect band gap transformation occurs for hydrogen terminated nanowires when the applied strain is larger than 4%. When the applied strain is more than 19% (may vary depending on different chemical modifications), the Si nanowires could even transform from semiconductor to metallic behavior, with no band gap! Our results show the possibility of tuning Si nanowires’ electronic properties through mechanical straining, which may offer a new route in Si-based optoelectronics applications.

Nanotransplantation Printing of Crystallographic-Orientation-Controlled Single-Crystalline Nanowire Arrays on Diverse Surfaces.

The fabrication of a highly ordered array of single-crystalline nanostructures prepared from solution-phase or vapor-phase synthesis methods is extremely challenging due to multiple difficulties of spatial arrangement and control of crystallographic orientation. Herein, we introduce a nanotransplantation printing (NTPP) technique for the reliable fabrication, transfer, and arrangement of single-crystalline Si nanowires (NWs) on diverse substrates. NTPP entails (1) formation of nanoscale etch mask patterns on conventional low-cost Si via nanotransfer printing, (2) two-step combinatorial plasma etching for defining Si NWs, and (3) detachment and transfer of the NWs onto various receiver substrates using an infiltration-type polymeric transfer medium and a solvent-assisted adhesion switching mechanism. Using this approach, high-quality, highly ordered Si NWs can be formed on almost any type of surface including flexible plastic substrates, biological surfaces, and deep-trench structures. Moreover, NTPP provides controllability of the crystallographic orientation of NWs, which is confirmed by the successful generation of (100)- and (110)-oriented Si NWs with different properties. The outstanding electrical properties of the NWs were confirmed by fabricating and characterizing Schottky junction field-effect transistors. Furthermore, exploiting the highly flexible nature of the NWs, a high-performance piezoresistive strain sensor, with a high gauge factor over 200 was realized.

Piezoresistive effect of n-type 〈111〉-oriented Si nanowires under large tension/compression

Small-scale samples enable us to understand changes in physical properties under larger strain due to their higher tolerance to deformation. In this study, the piezoresistive character of n-type 〈111〉-oriented Si nanowires under large strain was measured during tensile and compressive deformations. The Si nanowires were directly cut from the wafer using top-down technology and deformed while capturing their electrical properties inside a transmission electron microscope. The experimental results show that both tensile and compressive deformation enhanced their electrical transport properties. The piezoresistance coefficient is of the same order of magnitude as its bulk counterpart, but half as large, which may be attributed to a larger strain magnitude. We also studied the circulatory characteristics and influence of electron beam radiation. This study provided new physical insights into piezoresistive effects under large strain.

Investigation of crystallinity and planar defects in the Si nanowires grown by vapor–liquid–solid mode using indium catalyst for solar cell applications

Stacking-fault-free and planar defect (twinning plane)-free In-catalyzed Si nanowires (NWs) are essential for carrier transport and nanoscale device applications. In this article, In-catalyzed, vertically aligned, and cone-shaped Si NWs on Si(111) were grown successfully, in the vapor–liquid–solid (VLS) mode. In particular, the influences of substrate temperature (TS) and cooling rate (ΔTS/Δt) on the formation of planar defects, twinning planes along the [112] direction, and stacking faults in Si NWs were investigated. When TS was decreased from 600 °C to room temperature at a rate of 100 °C/240 s after Si NW growth, twinning plane defects perpendicular to the substrate and along different segments of (111)-oriented Si NWs were observed. Finally, one simple model was proposed to explain the stacking fault formation as well as Si NW length limitation due to the In-nanoparticle (In-NP) migration, and root causes of the twinning plane defects in the Si-NWs.

Enhanced diode performance in cadmium telluride–silicon nanowire heterostructures

We report on the structural and optoelectronic characteristics and photodetection properties of cadmium telluride (CdTe) thin film/silicon (Si) nanowire heterojunction diodes. A simple and cost-effective metal-assisted etching (MAE) method is applied to fabricate vertically oriented Si nanowires on n-type single crystalline Si wafer. Following the nanowire synthesis, CdTe thin films are directly deposited onto the Si nanowire arrays through RF magnetron sputtering. A comparative study of X-ray diffraction (XRD) and Raman spectroscopy shows the improved crystallinity of the CdTe thin films deposited onto the Si nanowires. The fabricated nanowire based heterojunction devices exhibit remarkable diode characteristics, enhanced optoelectronic properties and photosensitivity in comparison to the planar reference device. The electrical measurements revealed that the diodes have a well-defined rectifying behavior with a superior rectification ratio of 105 at ±5V and a relatively small ideality factor of n=1.9 with lower reverse leakage current and series resistance at room temperature in dark condition. Moreover, an open circuit voltage of 120mV is also observed under illumination. Based on spectral photoresponsivity measurements, the nanowire based device exhibits a distinct responsivity (0.35–0.5AW−1) and high detectivity (6×1012−9×1012cmHz1/2W−1) in near-infrared wavelength region. The enhanced device performance and photosensitivity is believed to be due to three-dimensional nature of the interface between the CdTe thin film and the Si nanowires. The device characteristics observed here reveals that fabricated CdTe thin film/Si nanowire heterojunctions are promising for high-performance and low-cost optoelectronic device applications, near-infrared photodetectors in particular.

Improved diode properties in zinc telluride thin film-silicon nanowire heterojunctions

In this study, structural and optoelectronic properties and photodedection characteristics of diodes constructed from p-zinc telluride (ZnTe) thin film/n-silicon (Si) nanowire heterojunctions are reported. Dense arrays of vertically aligned Si nanowires were successfully synthesized on (1 1 0)-oriented n-type single crystalline Si wafer using simple and inexpensive metal-assisted etching (MAE) process. Following the nanowire synthesis, p-type ZnTe thin films were deposited onto vertically oriented Si nanowires via radio frequency magnetron sputtering to form three-dimensional heterojunctions. A comparative study of the structural results obtained from X-ray diffraction and Raman spectroscopy measurements showed the improved crystallinity of the ZnTe thin films deposited onto the Si nanowire arrays. The fabricated nanowire-based heterojunction devices exhibited remarkable diode characteristics and enhanced optoelectronic properties and photosensitivity in comparison to the planar reference. The electrical measurements revealed that the diodes with nanowires had a well-defined rectifying behaviour with a rectification ratio of 104 at ±2 V and a relatively small ideality factor of n = 1.8 with lower reverse leakage current and series resistance at room temperature in dark condition. Moreover, an open-circuit voltage of 100 mV was also observed under illumination. Based on spectral photoresponsivity measurements, the nanowire-based device exhibited a distinct responsivity and high detectivity in visible and near-infrared (NIR) wavelength regions. The device characteristics observed here offer that the fabricated ZnTe thin film/Si nanowire-based p–n heterojunction structures will find important applications in future and will be a promising candidate for high-performance and low-cost optoelectronic device applications, NIR photodedectors in particular.

Subeutectic growth of single-crystal silicon nanowires grown on and wrapped with graphene nanosheets: high-performance anode material for lithium-ion battery.

A novel one-pot synthesis for the subeutectic growth of (111) oriented Si nanowires on an in situ formed nickel nanoparticle catalyst prepared from an inexpensive nickel nitrate precursor is developed. Additionally, anchoring the nickel nanoparticles to a simultaneously reduced graphene oxide support created synergy between the individual components of the c-SiNW-G composite, which greatly improved the reversible charge capacity and it is retention at high current density when applied as an anode for a Li-ion battery. The c-SiNW-G electrodes for Li-ion battery achieved excellent high-rate performance, producing a stable reversible capacity of 550 mAh g(-1) after 100 cycles at 6.8 A g(-1) (78% of that at 0.1 A g(-1)). Thus, with further development this process creates an important building block for a new wave of low-cost silicon nanowire materials and a promising avenue for high rate Li-ion batteries.

Laterally assembled nanowires for ultrathin broadband solar absorbers

We studied optical resonances in laterally oriented Si nanowire arrays by conducting finite-difference time-domain simulations. Localized Fabry-Perot and whispering-gallery modes are supported within the cross section of each nanowire in the array and result in broadband light absorption. Comparison of a nanowire array with a single nanowire shows that the current density (J(SC)) is preserved for a range of nanowire morphologies. The J(SC) of a nanowire array depends on the spacing of its constituent nanowires, which indicates that both diffraction and optical antenna effects contribute to light absorption. Furthermore, a vertically stacked nanowire array exhibits significantly enhanced light absorption because of the emergence of coupled cavity-waveguide modes and the mitigation of a screening effect. With the assumption of unity internal quantum efficiency, the J(SC) of an 800-nm-thick cross-stacked nanowire array is 14.0 mA/cm², which yields a ~60% enhancement compared with an equivalent bulk film absorber. These numerical results underpin a rational design strategy for ultrathin solar absorbers based on assembled nanowire cavities.

Study on electrical transport properties of strained Si nanowires by in situ transmission electron microscope

Strain engineering in semiconductor nanostructure has been received great attention because their ultra-large elastic limit can induce a broad tuning range of the physical properties. Here, we report how the electrical transport properties of the p-type -oriented Si nanowires may be tuned by bending strain and affected by the plastic deformation in a transmission electron microscope. These freestanding nanowires were prepared from commercial silicon-on-insulator materials using the focusing ion beam technique. Results show that the conductivity of these Si nanowires is improved remarkably by bending strain when the strain is lower than 2%, while the improvement is nearly saturated when the strain approaches to 2%. The electric current will reduce a little sometimes when strain exceeds 3%, which may result from plastic events. Our experimental results may be helpful to Si strain engineering.

Rational Defect Introduction in Silicon Nanowires

The controlled introduction of planar defects, particularly twin boundaries and stacking faults, in group IV nanowires remains challenging despite the prevalence of these structural features in other nanowire systems (e.g., II-VI and III-V). Here we demonstrate how user-programmable changes to precursor pressure and growth temperature can rationally generate both transverse twin boundaries and angled stacking faults during the growth of <111> oriented Si nanowires. We leverage this new capability to demonstrate prototype defect superstructures. These findings yield important insight into the mechanism of defect generation in semiconductor nanowires and suggest new routes to engineer the properties of this ubiquitous semiconductor.

Atomistic modeling of anharmonic phonon-phonon scattering in nanowires

Phonon transport is simulated in ultrascaled nanowires in the presence of anharmonic phonon-phonon scattering. A modified valence-force-field model containing four types of bond deformation is employed to describe the phonon band structure. The inclusion of five additional bond deformation potentials allows us to account for anharmonic effects. Phonon-phonon interactions are introduced through inelastic scattering self-energies solved in the self-consistent Born approximation in the nonequilibrium Green's function formalism. After calibrating the model with experimental data, the thermal current, resistance, and conductivity of $\ensuremath{\langle}100\ensuremath{\rangle}$-, $\ensuremath{\langle}110\ensuremath{\rangle}$-, and $\ensuremath{\langle}111\ensuremath{\rangle}$-oriented Si nanowires with different lengths and temperatures are investigated in the presence of anharmonic phonon-phonon scattering and compared to their ballistic limit. It is found that all the simulated thermal currents exhibit a peak at temperatures around 200 K if phonon scattering is turned on while they monotonically increase when this effect is neglected. Finally, phonon transport through Si-Ge-Si nanowires is considered.

Atomistic modeling of electron-phonon interaction and electron mobility in Si nanowires

We investigate the electron mobility of Si nanowires with 〈100〉, 〈110〉, and 〈111〉 crystalline orientations by considering atomistic electron-phonon interactions. We calculate the electron band structures based on a semiempirical sp3d5s* tight-binding approach and the phonon band structures based on the Keating potential model. Then, by combining the electron and phonon eigenstates based on Fermi’s golden rule and solving the linearized Boltzmann transport equation while considering Pauli’s exclusion principle, we evaluate the electron mobility of Si nanowires. As expected, phonons in Si nanowires are found to behave quite differently from phonons in bulk Si because of phonon confinement. However, electron mobility in Si nanowires is primarily governed by the variation in the electron effective mass rather than that of the phonon eigenstates. As a result, the 〈110〉-oriented Si nanowires showed the highest electron mobility, because they have the smallest electron effective mass among the three orientations.

Surface and Intrinsic Conduction Properties of Au-Catalyzed Si Nanowires

An original atomic force microscopy (AFM) method is used to probe surface conduction properties of Au-catalyzed (111) oriented Si nanowires (NWs) attached on the substrate on which they were grown. Drastically different transport regimes are observed upon tuning the electronic junction between the AFM tip and NW (AFM tip work function and NW surface states) and temperature, which reveal the interplay between Schottky interface junctions, Au-mediated surface conduction along the NW sidewalls, and conduction through NWs. The method is applied to extract the intrinsic resistance of nominally undoped NWs when removed from surface Au catalyst residues, and provides evidence for transport through Si NWs with effective residual doping as low as ≈1014–1015 cm–3.

Phonon-limited and effective low-field mobility in n- and p-type [100]-, [110]-, and [111]-oriented Si nanowire transistors

Ultrascaled n- and p-type Si nanowire field-effect transistors (NW FETs) with [100], [110], and [111] as channel orientations are simulated in the presence of electron-phonon scattering using an atomistic quantum transport solver based on the sp3d5s∗ tight-binding model for electrons and holes, a modified Keating model for phonons, and the nonequilibrium Green’s function formalism. The channel resistances of devices with different gate lengths and carrier concentrations are computed at room temperature and used to extract phonon-limited, ballistic, and effective low-field mobilities. It is found that a [110] channel represents the best choice for high n- and p-type NW FET performances.

In situ TEM electrochemistry of anode materials in lithium ion batteries

We created the first nanobattery inside a transmission electron microscope (TEM), allowing for real time and atomic scale observations of battery charging and discharging processes. Two types of nanobattery cells, one based on room temperature ionic liquid electrolytes (ILEs) and the other based on all solid components, were created. The former consists of a single nanowire anode, an ILE and a bulk LiCoO2 cathode; the latter uses Li2O as a solid electrolyte and metal Li as the anode. Some of the important latest results obtained by using the nanobattery setup are summarized here: (1) upon charging SnO2 nanowires in an ILE cell with one end of the nanowire contacting the electrolyte, a reaction front propagates progressively along the nanowire, causing the nanowire to swell, elongate, and spiral. The reaction front is a “Medusa zone” containing a high density of mobile dislocations, which continuously nucleate at the moving front and absorbed from behind. This dislocation cloud indicates large in-plane misfit stresses and is a structural precursor to electrochemically driven solid-state amorphization. When the nanowire is immersed in the electrolyte (in a flooding geometry), a multiple-strip-multiple-reaction-front lithiation mechanism operates. (2) Upon charging -oriented Si nanowires, the nanowires swell rather than elongate. We found unexpectedly the highly anisotropic volume expansion in lithiated Si nanowires, resulting in a surprising dumbbell-shaped cross-section, which developed due to plastic flow and necking instability. Driven by progressive charging, the stress concentration at the neck region led to cracking and eventually fracture of the single nanowire into sub-wires. Moreover, the fully lithiated phase was found to be crystalline Li15Si4, rather than the widely believed Li22Si5 phase, indicating the maximum capacity of Si being 3579 mA h g−1 at room temperature. (3) Carbon coating not only increases rate performance but also alters the lithiation induced strain of SnO2 nanowires. The SnO2 nanowires coated with carbon can be charged about 10 times faster than the non-coated ones. Intriguingly, the radial expansion of the coated nanowires was completely suppressed, resulting in enormously reduced tensile stress at the reaction front, as evidenced by the lack of formation of dislocations. (4) The lithiation process of individual Si nanoparticles was observed in real time in a TEM. A strong size dependent fracture behaviour was discovered, i.e., there exists a critical particle size with a diameter of ∼150 nm, below which the particles neither cracked nor fractured upon lithiation, above which the particles first formed cracks and then fractured due to lithiation induced huge volume expansion. For very large particles with size over 900 nm, electrochemical lithiation induced explosion of Si particles was observed. This strong size-dependent fracture behaviour is attributed to the competition between the stored mechanical energy and the crack propagation energy of the nanoparticles: smaller nanoparticles cannot store enough mechanical energy to drive crack propagation. These results indicate the strong material, size and crystallographic orientation dependent electrochemical behaviour of anode materials, highlighting the powerfulness of in situTEM electrochemistry, which provides not only deep understanding of the fundamental sciences of lithium ion batteries but also critical guidance in developing advanced lithium ion battery for electrical vehicle and backup power for fluctuation energy sources such as wind and solar energy.

Charged impurity scattering and mobility in gated silicon nanowires

We study the effects of charged impurity scattering on the electronic-transport properties of $⟨110⟩$-oriented Si nanowires in a gate-all-around geometry, where the impurity potential is screened by the gate, gate oxide and conduction-band electrons. The electronic structure of the doped nanowires is calculated with a tight-binding method and the transport properties with a Landauer-B\"uttiker Green's functions approach and the linearized Boltzmann transport equation (LBTE) in the first Born approximation. Based on our numerical results we argue that: (1) there are large differences between phosphorous- (P-) and boron- (B-) doped systems, acceptors behaving as tunnel barriers for the electrons, while donors give rise to Fano resonances in the transmission. (2) As a consequence, the mobility is much larger in P- than in B-doped nanowires at low carrier density but can be larger in B-doped nanowires at high carrier density. (3) The resistance of a single impurity is strongly dependent on its radial position in the nanowire, especially for acceptors. (4) As a result of subband structure and screening effects, the impurity-limited mobility can be larger in thin nanowires embedded in ${\text{HfO}}_{2}$ than in bulk Si. Acceptors might, however, strongly hinder the flow of electrons in thin nanowires embedded in ${\text{SiO}}_{2}$. (5) The perturbative LBTE largely fails to predict the correct mobilities in quantum-confined nanowires.