Applications In Future Nanodevices Research Articles

Analysis of nonlinear bending behavior of nano-switches considering surface effects

Nano-switch structures are important control elements in nanoelectromechanical systems and have potential applications in future nanodevices. This paper analyzes the effects of surface effects, geometric nonlinearity, electrostatic forces, and intermolecular forces on the nonlinear bending behavior and adhesion stability of nano-switches. Based on the Von Karman geometric nonlinearity theory, four types of boundary conditions for the nano-switch structure were specifically calculated. The results show that surface effects have a significant impact on the nonlinear bending and adhesion stability of nano-switches. Surface effects increase the adhesion voltage of the nano-switch and decrease its adhesion displacement, and as the size of the nano-switch structure increases, the impact of surface effects decreases. A comparative analysis of the linear theory and the nonlinear theory results shows that the adhesion voltage predicted by the linear theory is smaller than that predicted by the nonlinear theory. The effect of geometric nonlinearity increases as the size of the nano-switch structure increases, as the distance between the electrodes increases, and as the aspect ratio of the nano-switch structure increases. These findings provide theoretical support and reference for the design and use of future nanodevices and nanoelectromechanical systems.

On-Surface Synthesis of Graphene Nanoribbons with Atomically Precise Structural Heterogeneities and On-Site Characterizations.

Graphene nanoribbons (GNRs) are strips of graphene, with widths of a few nanometers, that are promising candidates for future applications in nanodevices and quantum information processing due to their highly tunable structure-dependent electronic, spintronic, topological, and optical properties. Implantation of periodic structural heterogeneities, such as heteroatoms, nanopores, and non-hexagonal rings, has become a powerful manner for tailoring the designer properties of GNRs. The bottom-up synthesis approach, by combining on-surface chemical reactions based on rationally designed molecular precursors and in situ tip-based microscopic and spectroscopic techniques, promotes the construction of atomically precise GNRs with periodic structural modulations. However, there are still obstacles and challenges lying on the way toward the understanding of the intrinsic structure-property relations, such as the strong screening and Fermi level pinning effect of the normally used transition metal substrates and the lack of collective tip-based techniques that can cover multi-internal degrees of freedom of the GNRs. In this Perspective, we briefly review the recent progress in the on-surface synthesis of GNRs with diverse structural heterogeneities and highlight the structure-property relations as characterized by the noncontact atomic force microscopy and scanning tunneling microscopy/spectroscopy. We furthermore motivate to deliver the need for developing strategies to achieve quasi-freestanding GNRs and for exploiting multifunctional tip-based techniques to collectively probe the intrinsic properties.

Layer-dependent dielectric modulation in WS2/GaN heterostructures

Controlling over the electronic structure of two-dimensional (2D) semiconductors is crucial to unlock their full potential in the applications in future nanodevices. Here, we demonstrate a layer-dependent dielectric modulation of electronic structures in multilayer ${\mathrm{WS}}_{2}/\mathrm{GaN}$ heterostructures. The results investigated by means of in situ scanning tunneling microscopy/spectroscopy and photoluminescence spectroscopy show that the fluctuations of the dielectric environment induced by local corrugation of the GaN surface dramatically tune the band structure of monolayer ${\mathrm{WS}}_{2}$ with a large downshift of the valence band maximum by about 1.12 eV. Under the shielding effect of the first-layer ${\mathrm{WS}}_{2}$, the dielectric environment shows a weak modulation effect on the second-layer, and even has negligible influence on the third- and fourth-layer ${\mathrm{WS}}_{2}$. The relationship between dielectric environment and interfacial distance is further simulated through first-principles calculations. The electrostatic potential energy, differential charge densities, and charge transfer of the ${\mathrm{WS}}_{2}/\mathrm{GaN}$ heterostructure under different interfacial distances reveal that the differences of charge redistributions between the ${\mathrm{WS}}_{2}/\mathrm{GaN}$ interface and ${\mathrm{WS}}_{2}$ interlayer are responsible for the layer-dependent dielectric modulation. This work offers some references for the design and fabrication of novel 2D optoelectronic devices.

MOCVD Growth and Structural Properties of ZnS Nanowires: A Case Study of Polytypism.

Controlling the morphology, orientation, and crystal phase of semiconductor nanowires is crucial for their future applications in nanodevices. In this work, zinc sulfide (ZnS) nanowires have been grown by metalorganic chemical vapor deposition (MOCVD), using gold or gold–gallium alloys as catalyst. At first, basic studies on MOCVD growth regimes (mass-transport, zinc- or sulfur- rich conditions) have been carried out for ZnS thin films. Subsequently, the growth of ZnS nanowires was investigated, as a function of key parameters such as substrate temperature, S/Zn ratio, physical state and composition of the catalyst droplet, and supersaturation. A detailed analysis of the structural properties by transmission electron microscopy (TEM) is given. Depending on the growth conditions, a variety of polytypes is observed: zinc-blende (3C), wurtzite (2H) as well as an uncommon 15R crystal phase. It is demonstrated that twinning superlattices, i.e., 3C structures with periodic twin defects, can be achieved by increasing the Ga concentration of the catalyst. These experimental results are discussed in the light of growth mechanisms reported for semiconductor nanowires. Hence, in this work, the control of ZnS nanowire structural properties appears as a case study for the better understanding of polytypism in semiconductor 1D nanostructures.

Theoretical prediction of Janus PdXO (X = S, Se, Te) monolayers: structural, electronic, and transport properties.

Due to the broken vertical symmetry, the Janus material possesses many extraordinary physico-chemical and mechanical properties that cannot be found in original symmetric materials. In this paper, we study in detail the structural, electronic, and transport properties of 1T Janus PdXO monolayers (X = S, Se, Te) by means of density functional theory. PdXO monolayers are observed to be stable based on the analysis of the vibrational characteristics and molecular dynamics simulations. All three PdXO structures exhibit semiconducting characteristics with indirect bandgap based on evaluations with hybrid functional Heyd–Scuseria–Ernzerhof (HSE06). The influences of the spin–orbit coupling (SOC) on the band diagram of PdXO are strong. Particularly, when the SOC is included, PdTeO is calculated to be metallic by the HSE06+SOC approach. With high electron mobility, Janus PdXO structures have good potential for applications in future nanodevices.

Tuning the electronic properties of Stone-Wales defected graphene using uniaxial longitudinal and horizontal strain: A first-principles study

We investigated the influence of horizontal and longitudinal strain on the electronic proprieties of Stone-Wales defected honeycomb graphene structure using the density functional theory. Tensile and compressive uniaxial strains were independently applied on the armchair (horizontal) and the zigzag (longitudinal) directions of a defected honeycomb graphene monolayer. We found the responses of the band gap are dependent on the applied strain direction. Likewise, the induced band gap openings show evident correlations between Stone-Wales defect orientations and the applied strain directions. Our results show that by choosing the strain values and directions the band gap values can be precisely achieved. Additionally, the effects of the applied strain were calculated and analyzed for the Fermi level, work function and the charge distributions. For both the defect-free and the defected samples, Fermi level shifts up in response to the compressive strain while it shifts down in response to the tensile strain. Our study shows the electronic properties are significantly affected by the alignment between the Stone-Wales C-C middle bond orientation and the applied stain direction. Finally, we hope providing a precise tuning for the electronic properties of defected graphene will allow for future nanodevice applications.

A molecular dynamics study of the mechanical properties of h-BCN monolayer using a modified Tersoff interatomic potential

Using molecular dynamics (MD) simulations, we investigate the mechanical properties of hexagonal BCN monolayer, a newly synthesized two-dimensional material with an atom ratio of B/C/N = 1:1:1. The Tersoff potential is modified to get good agreement between predicted and measured fracture strengths of graphene. With this modified Tersoff potential, we perform extensive MD simulations to study the effect of temperature, strain rate and vacancy defect on the mechanical properties of h-BCN. It is found that h-BCN is a strong material with fracture strength of 81.4–93.5 GPa, albeit ∼35% lower than that of graphene. Similar to graphene, temperature has strong effect on the mechanical properties of h-BCN. As the temperature increases from 10 K to 1300 K, the fracture strength and strain of h-BCN drops by 55% and 62%, respectively. The strain rate is found to have a moderate effect. When the strain rate increases from 0.00002 to 0.0125 ps−1, the fracture strength and strain of h-BCN increases 6.1% and 12%, respectively. As for the atomic defect, a very small concentration (0.028%) of vacancy in h-BCN is able to cause a 28% reduction in fracture strength and a 35.5% reduction in fracture strain. These findings have significance for its future applications in nanodevices.

Calculations of defect states in various sizes of InN nanowires

InN has received considerable attentions due to its small band-gap and unique properties in the III-nitride family. Understanding the effects of defects on optoelectronic properties of InN nanowire is essential for exploring its applications in future nanodevices. In this work, we have systematically calculated defect states in InN nanowires based on density-functional theory. Hydrogen passivation and several potential intrinsic point defects are considered in various sizes of nanowires, as well as charged defect states. For small-sized hexagonal nanowires, VN at N-poor condition or NIn at N-rich condition is the most stable defect. Whereas for larger-sized nanowires, VN and InN defects are competing when the N chemical potential changes, showing obvious size effect of the defect stability on the nanowire surface. Those defect states change the electronic structure of the nanowires drastically by introducing empty bands or deep level and provide possibility to tailor the optical properties in terms of forming different stable defects.

Electronic and mechanical responses of two-dimensional HfS2, HfSe2, ZrS2, and ZrSe2 from first-principles

During the last decade, numerous high-quality two-dimensional (2D) materials with semiconducting electronic character have been synthesized. Recent experimental study (Sci. Adv. 2017;3: e1700481) nevertheless confirmed that 2D ZrSe2 and HfSe2 are among the best candidates to replace the silicon in nanoelectronics owing to their moderate band-gap. We accordingly conducted first-principles calculations to explore the mechanical and electronic responses of not only ZrSe2 and HfSe2, but also ZrS2 and HfS2 in their single-layer and free-standing form. We particularly studied the possibility of engineering of the electronic properties of these attractive 2D materials using the biaxial or uniaxial tensile loadings. The comprehensive insight provided concerning the intrinsic properties of HfS2, HfSe2, ZrS2, and ZrSe2 can be useful for their future applications in nanodevices.

A Novel Domain-Confined Growth Strategy for In Situ Controllable Fabrication of Individual Hollow Nanostructures.

The manipulation and tailoring of the structure and properties of semiconductor nanocrystals (NCs) is significant particularly for the design and fabrication of future nanodevices. Here, a novel domain‐confined growth strategy is reported for controllable fabrication of individual monocrystal hollow NCs (h‐NCs) in situ inside a transmission electron microscope, which enables the atomic scale monitoring of the entire reaction. During the process, the preformed carbon shells serve as nanoreaction cells for the formation of CdSeS h‐NCs. Electron beam (e‐beam) irradiation is demonstrated to be the key activation factor for the solid‐to‐hollow shape transformation. The formation of CdSeS hollow NCs is also found to be sensitive to the volume ratio of the CdSe/CdS NCs to the carbon shell and only those CdSe/CdS NCs with a volume ratio in the range 0.2–0.8 are successfully converted into hollow NCs. The method paves the way to potentially use an e‐beam for the in situ tailoring of individual semiconductor NCs targeted toward future nanodevice applications.

New group-V elemental bilayers: A tunable structure model with four-, six-, and eight-atom rings

Two-dimensional (2D) group V elemental materials have attracted widespread attention due to their nonzero band gap while displaying high electron mobility. Using first-principles calculations, we propose a series of new elemental bilayers with group V elements (Bi, Sb, As). Our study reveals the dynamical stability of 4, 6, and 8-atom ring structures, demonstrating their possible coexistence in such bilayer systems. The proposed structures for Sb and As are large-gap semiconductors that are potentially interesting for applications in future nanodevices. The Bi structures have nontrivial topological properties with a large and direct nontrivial band gap. The nontrivial gap is shown to arise from a band inversion at the Brillouin zone center due to the strong intrinsic spin-orbit coupling (SOC) in Bi atoms. Moreover, we demonstrate the possibility to tune the properties of these materials by enhancing the ratio of 6-atom rings to 4 and 8-atom rings, which results in wider nontrivial band gaps and lower formation energies.

Two-dimensional Penta-BP5 Sheets: High-stability, Strain-tunable Electronic Structure and Excellent Mechanical Properties

Two-dimensional (2D) crystals exhibit unique and exceptional properties and show promise for various applications. In this work, we systematically studied the structures of a 2D boronphosphide (BP) monolayer with different stoichiometric ratios (BPx, x = 1, 2, 3, 4, 5, 6 and 7) and observed that each compound had a stable 2D structure with metallic or semiconducting electronic properties. Surprisingly, for the BP5 compounds, we discovered a rare penta-graphene-like 2D structure with a tetragonal lattice. This monolayer was a semiconductor with a quasi-direct band gap of 2.68 eV. More importantly, investigation of the strain effect revealed that small uniaxial strain can trigger the band gap of the penta-BP5 monolayer to transition from a quasi-direct to direct band gap, whereas moderate biaxial strain can cause the penta-BP5 to transform from a semiconductor into a metal, indicating the great potential of this material for nanoelectronic device applications based on strain-engineering techniques. The wide and tuneable band gap of monolayer penta-BP5 makes it more advantageous for high-frequency-response optoelectronic materials than the currently popular 2D systems, such as transition metal dichalcogenides and black phosphorus. These unique structural and electronic properties of 2D BP sheets make them promising for many potential applications in future nanodevices.

Intrinsic ferromagnetism and quantum transport transition in individual Fe-doped Bi2Se3 topological insulator nanowires.

Time-reversal symmetry is broken by magnetic doping in topological insulators (TIs). An energy gap at the Dirac point opens and thus, generates numerous surface carriers. TI nanostructures are an ideal platform to investigate exotic surface transport behavior due to their large surface-to-volume ratio, which enhances the contribution of the TI surface states. However, magnetic doping into TI nanostructures has been challenging, and induced magnetic behavior has remained elusive. Herein, we have synthesized Fe-doped Bi2Se3 nanowires using a facile chemical vapor deposition with a doping concentration of ∼1 at%. The combined structural characterizations illustrate the homogeneous distribution of the Fe dopants. Cryogenic magnetic force microscopy gives direct evidence of the spontaneous magnetization with a Curie temperature of ∼40 K in a single nanowire. The transport measurements show a quantum transition from weak anti-localization to weak localization behavior. All the evidence indicates the existence of intrinsic ferromagnetism and gapped topological surface states in the TI nanowires, paving a way for future memory and magnetoelectric nanodevice applications.

Mechanical and electronic properties of graphitic carbon nitride sheet: First-principles calculations

In this work, mechanical properties, elastic constants and the strain responses on the electronic properties of graphitic heptazine are investigated using density functional theory. The computed lattice constant and bulk modulus are in good agreement with the available literatures. The in-plane stiffness compared well with a similar two-dimensional structure, whereas the Poisson's ratio value is close to that of graphene. The calculated critical points (elastic and yielding points) for both the uni- and bi-axial strains indicate that the heptazine material can withstand longer tensions in the plastic region. This shows that the heptazine sheet is mechanically stable. Our calculations also predict enhanced band gap induced by small amount of bi-axial tensile strain within the elastic region. The increase in band gap is a result of symmetric deformations which predominantly affect the structural features of the sheet, leading to the eventual reorientation in the atomic orbitals of the sheet. We find no change in the electronic properties of the sheet under electric field up to a peak value of 10 V/nm. Such properties may serve as a guide for future nanodevice applications.

Metallization of DNA Origami to Form Electrically Conductive ~10 Nm Diameter Nanowires

DNA based bottom-up nanotechnology is seen as a promising technology for the development of future generations of nanodevices. The bottom-up approach takes advantage of molecular self-assembly to create useful architectures of nanometer dimensions. DNA, with its small size, functional groups and complementary base pairs, is a powerful template for nanodevice fabrication via bottom-up assembly. Recent advances in the field of “DNA origami” have enabled fabrication of nanostructures by folding single-stranded DNA into a variety of 2- and 3-D shapes. However, electrically conductive structures are required for many nanodevice applications. In this study, we report the fabrication of electrically conductive DNA origami-templated nanowires with the use of gold nanorods (Au NR’s) as a step towards DNA nanodevices. Our process begins with a seeding step where Au NR’s bind to the DNA origami backbone. The seeds are then connected by electroless deposition to make continuous wires. The use of nanorods as seeds provides two key advantages over previous work with particulate seeds. First, the surfactant-enabled anisotropic growth used to initially fabricate the nanorods appears to persist through the electroless plating process on the DNA template. This permits improved control of the width of the final metallized structure since growth in the axial direction is preferred. The average final width of our Au-metallized DNA wires was ~10nm. Continuous 400 nm long DNA wires were obtained by filling gaps between seed particles that ranged from 10.6 nm on average to gaps as wide as 25 nm. The second advantage of nanorods is the reduction in the number of connection points between seed particles by a factor of three relative to that observed for 5 nm seed particles. The reduced number of connection points is expected to increase the conductivity of the wires. Initial electrical characterization of the DNA wires shows that they are conductive, and more detailed measurements are in progress. Site-specific placement of Au NR's on a DNA origami template, demonstrated in this study, is capable of providing the controlled placement needed to enable future nanodevice applications. In summary, we have demonstrated a metallization process to fabricate thin, high aspect ratio, conductive wires on DNA-origami templates with the use of Au NR’s and electroless plating. Continuing efforts seek to interface these thin nanowires with appropriate semiconducting elements to enable fabrication of self-assembled nanoscale devices.

Morphology controlling method for amorphous silica nanoparticles and jellyfish-like nanowires and their luminescence properties.

Uniform silica nanoparticles and jellyfish-like nanowires were synthesized by a chemical vapour deposition method on Si substrates treated without and with Ni(NO3)2, using silicon powder as the source material. Composition and structural characterization using field emission scanning electron microscopy, transmission electron microscopy, energy dispersive X-ray spectroscopy and fourier-transform infrared spectroscopy showed that the as-prepared products were silica nanoparticles and nanowires which have amorphous structures. The form of nanoparticles should be related to gas-phase nucleation procedure. The growth of the nanowires was in accordance with vapour-liquid-solid mechanism, followed by Ostwald ripening to form the jellyfish-like morphology. Photoluminescence and cathodoluminescence measurements showed that the silica products excited by different light sources show different luminescence properties. The emission spectra of both silica nanoparticles and nanowires are due to the neutral oxygen vacancies (≡Si-Si≡). The as-synthesized silica with controlled morphology can find potential applications in future nanodevices with tailorable photoelectric properties.

Correction: β-Sialon nanowires, nanobelts and hierarchical nanostructures: morphology control, growth mechanism and cathodoluminescence properties.

Correction for 'β-Sialon nanowires, nanobelts and hierarchical nanostructures: morphology control, growth mechanism and cathodoluminescence properties' by Juntong Huang, et al., Nanoscale, 2014, 6, 424-432.

Optical and Electrical Studies of Vertically Oriented Tellurium Nanowire Arrays Produced by Template Electrodeposition

We report the fabrication of highly ordered arrays of tellurium nanowires and investigate their electrical, optical and structural properties. The tellurium nanowire arrays were synthesized by electrochemical deposition via a template method and characterized by x-ray diffraction (XRD), field emission scanning electron microscopy (FESEM) and UV–Visible spectroscopy. The XRD and FESEM characterizations confirm the formation of a dense crop of tellurium nanowire arrays with hexagonal phase. The intense crystalline peak corresponding to the (100) plane observed in the XRD spectra suggests a preferential growth of wires along the [001] direction. The crystallite size and micro-strain effect were estimated by Williamson–Hall (WH) analysis. The average crystallite size and lattice strain extracted from WH plot were found to be ∼34 nm and 0.0044, respectively. Further, electrical properties of the arrays of nanowires were examined using a two-probe method. The current–voltage curve of the tellurium wires exhibits a non-linear behavior and a double diode-like characteristic, which signifies their novel applications in future nanodevices.

β-Sialon nanowires, nanobelts and hierarchical nanostructures: morphology control, growth mechanism and cathodoluminescence properties

Morphology control of one dimension (1D) nanomaterials is a pivotal issue in the field of nanoscience research to exploit their novel properties. Herein, we report the morphology controlled synthesis of 1D β-Sialon nanowires, nanobelts and hierarchical nanostructures via a thermal-chemical vapour deposition process using an appropriately selected catalyst and optimized temperature schedule. Vapour-solid (VS), a combination of vapour-liquid-solid (VLS)-based and VS-tip, and a combination of VS for one-generation nanowires with nucleation, growth and coalescence of two-generation nanobranches (NGCB) are used to explain the growth of β-Sialon nanowires, nanobelts and hierarchical nanostructures, respectively. Cathodoluminescence measurements show that the individual β-Sialon 1D nanostructures with different morphologies have different luminescent properties. All nanostructures exhibit two distinct emission peaks, the violet/blue emission centered at ~390 nm (3.18 eV), attributable to the near band edge (NBE) emission, and the red emission centered at ~728 nm (1.70 eV), assigned to the deep level (DL) emission. However, the DL emission is the ruling emission in the case of an individual β-Sialon nanowire, whereas the NBE emission becomes dominant in the case of an individual nanobelt as well as a hierarchical nanostructure due to the size and surface effects. The as-synthesized β-Sialon with controlled nanostructures and various morphologies can find potential applications in future nanodevices with tailorable or tunable photoelectric properties.

Structural and electrical properties of laser-crystallized nanocrystalline Ge films and nanocrystalline Ge/SiNx multilayers

Nanocrystalline Ge (nc-Ge) single layers and nc-Ge/SiNx multilayers are prepared by laser annealing amorphous Ge (a-Ge) films and a-Ge/SiNx multilayers. The microstructures as well as the electrical properties of laser-crystallized samples are systematically studied by using various techniques. It is found that the optical band gap of nc-Ge film is reduced compared with its amorphous counterpart. The formed nc-Ge film is of p-type, and the dark conductivity is enhanced by 6 orders for an nc-Ge single layer and 4 orders for a multilayer. It is suggested that the carrier transport mechanism is dominant by the thermally activation process via the nanocrystal, which is different from the thermally annealed nc-Ge sample at an intermediate temperature. The carrier mobility of nc-Ge film can reach as high as about 39.4 cm2·V−1·s−1, which indicates their potential applications in future nano-devices.