Monocrystalline Nanowires Research Articles

Nanotwinning and tensile behavior in cold-welded high-entropy-alloy nanowires

Since the fabrication technique for high-entropy alloy (HEA) nanowires/nanopillars is still in its infancy, neither experimental nor modeling analyses of their cold-welding performance have been reported. Based on insights accumulated in our previous experiments and simulations regarding cold-welded metallic nanowires, in this study, the cold-welding performance of HEA nanowires is probed by atomistic simulations. Among different materials, our simulations reveal that extensively twinned structures are formed in CoCrMnFeNi samples, but not in CoCrCuFeNi or Ni samples. The larger fracture strain in certain HEAs is due to the improved ductility around the fracturing area as well as multiple twinning. Unlike in Ni samples, the fracture strains in HEA samples, regardless of being cuboid or cylindrical, are improved by shrinking the sample size. Among different orientations, the [010]-direction monocrystalline nanowires fail at a strain over 0.6, which is almost double that of the [111] direction. The fracture strains in polycrystalline HEA samples are, on average, larger than those in polycrystalline Ni samples. Furthermore, fracture strains in randomly generated polycrystalline HEA samples are more predictable than those in polycrystalline Ni samples with identical grain configurations. As previously reported, dislocation emission is still a prerequisite to fracture in all cold-welded samples.

Laser-induced crystallization of monoclinic nanowires in glassy selenium films

The authors investigate influence of 632.8 nm laser irradiation on structural properties of thin glassy (amorphous) Se films synthesized on quartz substrates using vacuum thermal evaporation. They first propose the fact that low power laser irradiation of Se films (power density PS=80 W cm−2) initiates phase transformations, accompanied by growth of monocrystalline nanowires of metastable phase of monoclinic beta-Se8 in the amorphous matrix at room temperature. New structural properties of Se films are proven to be long term stable at room temperature. It is shown that laser irradiation at PS ≥ 800 W cm−2 induces synthesis of a mixture of nanocrystallites of trigonal Se and monoclinic alpha-Se8 in the amorphous films and their mass crystallization with growth of spherulites at room temperature.

Flexible PbTiO3-nanowires/ P(VDF-TrFE) composite films and their dielectric, ferroelectric and pyroelectric properties

Flexible composite films with PbTiO3 monocrystalline nanowires as fillers and poly(vinylidene fluoride-trifluoroethylene) (P(VDF-TrFE)) as matrix were prepared by solution-casting and subsequent hot-pressing process. It was found that hot-pressing step is necessary to eliminate the small portion of γ-phase in the solution-cast P(VDF-TrFE) film which could strongly suppress the switchability of the P(VDF-TrFE) matrix. Dielectric, ferroelectric and pyroelectric properties of the nanocomposites have been systematically studied by varying mass fraction of the nanowire fillers from 0 wt% to 50 wt%. Enhanced dielectric permittivities were obtained by incorporating PbTiO3 nanowires without sacrificing the dielectric loss properties. The relative permittivity and dielectric loss of the composites were greatly decreased after poling. P-E loops characteristic of ferroelectrics were observed in all the two-step processed films, showing a highest Pr of about 6.21 μC/cm2 at 30 wt% loading content. Pyroelectric coefficient p of the composites increases with the mass fraction of PbTiO3 nanowires as well as with the temperature, with a value of 52.7 μC/(m2 K) for pure P(VDF-TrFE) film at 45 °C and 72.8 μC/(m2 K) for 50 wt% PT/P(VDF-TrFE) composite film at 45 °C. The voltage FOM (FV) decreases as the loading of PbTiO3 nanowires increases (more prominently after 10 wt%), while detectivity FOM (FD) keeps a relatively high value on the same level (between 18 × 10−6 Pa−1/2–20 × 10−6 Pa−1/2) for 0–30 wt% loading content.

High resolution photolithography using arrays of polystyrene and SiO2 micro- and nano-sized spherical lenses

Photolithography mask made of close-packed array of micro- and nano-sized spherical lenses allows to obtain the ordered structures and provides highest “optical resolution/cost” ratio between all existing photolithography and laser direct writing methods. In this letter, we present results of modeling the propagation of a plane wave falling on the array of quartz (SiO2) microspherical lenses and focusing in the image reverse photoresist layer. We present here experimental results on fabrication of ordered arrays of submicron wells and columns and substrate preparation for growth of monocrystalline nanowires on metal surface using photolithography with mask of SiO2 microspheres. Such ordered nano-sized arrays of wells and columns can be used in fabrication of further growth of monocrystalline nanowires, quantum dots and production of plasmon structures.

Metal to insulator transition in Sb doped SnO2 monocrystalline nanowires thin films

We report on the growth and transport properties of single crystalline Sb doped SnO2 wires grown from chemical vapour deposition. While undoped samples presented semiconducting behaviour, doped ones clearly undergo a transition from an insulating state (dR/dT<0) to a metallic one (dR/dT>0) around 130−150 K depending on the doping level. Data analysis in the framework of the metal-to-insulator transition theories allowed us to investigate the underlying physics: electron-electron and electron-phonon interactions were identified as the scattering mechanisms present in the metallic phase, while the conduction mechanism of the semiconducting phase (undoped sample) was characterized by thermal activation and variable range hopping mechanisms.

Low temperature humidity sensor based on Ge nanowires selectively grown on suspended microhotplates

The gas sensing properties of germanium (Ge) monocrystalline nanowires (NWs) at temperatures up to 100°C have been demonstrated for the first time. The devices have been fabricated based on an energy efficient and site-specific vapor-liquid-solid growth of NW meshes on top of microhotplates, which contain a buried heater and top electrodes. The devices have been investigated for the detection of oxygen, nitrogen dioxide and carbon monoxide gases, showing the important effect played by pre-adsorbed surface oxygen in the response to the different gases. The Ge NW-based devices exhibit p-type conductivity and show high selectivity in their response towards water vapor. Water vapor interaction is not dependent on the presence of oxygen and the adsorption leads to electron donation in the Ge nanowires. TEM analysis of the NWs proves that they are covered by a thin, outer germanium oxide layer, which is stable and does not grow upon exposure to these gases and operation temperatures up to 100°C. The presence of this oxide layer plays a key role in the sensing mechanisms.

Lower limits of line resistance in nanocrystalline back end of line Cu interconnects

The strong non-linear increase in the Cu interconnect line resistance with decreasing linewidth presents a significant obstacle to their continued downscaling. In this letter we use the first principles density functional theory based electronic structure of Cu interconnects to find the lower limits of their line resistance for metal linewidths corresponding to future technology nodes. We find that even in the absence of scattering due to grain boundaries, edge roughness or interfaces, quantum confinement causes a severe increase in the line resistance of Cu. We also find that when the simplest scattering mechanism in the grain boundary scattering dominated limit is added to otherwise coherent electronic transmission in monocrystalline nanowires, the lower limit of line resistance is significantly higher than projected roadmap requirements in the International Technology Roadmap for Semiconductors.

Electrochemical properties of Li2MnO3 nanowires with polycrystalline and monocrystalline states

In this article, monocrystalline bundles and polycrystalline nanowires were synthesized using α-MnO2 and MnOOH as precursors via an in-situ reaction. The electrochemical properties of the resulting nanocrystals were investigated. Results reveal that the polycrystalline nanowires are more activated than the monocrystalline nanowires. The polycrystalline nanowires also deliver a maximum discharge capacity of 226 mA h/g at 10 mA/g. Compared with the linear charging plateau at >4.3 V in the monocrystalline bundles, the charging platues of (100) and (001) domains in the polycrystalline nanowires are 4.3–4.55 and 4.55–4.6 V, respectively. The high capacity and rate performance of the polycrystalline nanowires are attributed to their good electroconductivity and rapid Li+ diffusion, which originate from defects and small domains.

Synthesis and Structural Characterization of Monocrystalline <i>α</i>-V<sub>2</sub>O<sub>5</sub> Nanowires

A stable one-dimensional system in an orthorhombic α-V2O5 nanowires monocrystalline structure was obtained by a solvothermal method from a polymorphic V2O5 structure. The starting material was firstly submitted to acid hydrolysis in H2O2 followed by a solvothermal treatment. The outcome of this procedure, a metastable phase of the one-dimensional system V10O24·12H2O/V3O7·H2O, was subsequently reoxidized by controlled heating in an open air system. The final product was an orange crystalline solid mainly formed by monocrystalline nanowires of α-V2O5 having lengths of tens of micrometers and widths of about 75 nm with a preferential [200] growth direction. It was found that the pH value of the initial solution plays an important role in the formation of the crystalline phase in the final products. Characterization was performed by X-ray diffraction (XRD), scanning electron microscopy (SEM) and high-resolution transmission electron microscopy (HRTEM). This study offers an alternate route for the synthesis of vanadium oxides and related compounds.

Chlorine based focused electron beam induced etching: A novel way to pattern germanium

Focused electron beam induced etching (FEBIE) with chlorine as etching agent has been used to geometrically shape and to electrically modify semiconductor nanodevices. Selected sections of monocrystalline nanowires were modified directly without the requirement for a photomask or a resist layer. FEBIE as a subtractive nanofabrication technology allows to locally etch active semiconductor devices made of Si or Ge. In this work, chlorine is used as the etchant gas to thin germanium channel structures fabricated by standard photolithography. For effective material removal a sufficiently high electron influence is essential to avoid the pitfalls of this method. Topography and conductivity of FEBIE-modified structures prior and after the etching process was studied by AFM and by electrical I–V characteristics. The presented work demonstrates the potential of Cl-based FEBIE for device prototyping and electrical trimming of future Ge-based nanodevices.

Facile synthesis of porous ZnO nanowires consisting of ordered nanocrystallites and their enhanced gas-sensing property

Porous ZnO nanowires were obtained by a facile chemical solution route combined with subsequent calcination. Calcination of the precursors produced porous ZnO nanowires, which consist of numerous ordered nanocrystallites with high porosity resulting from the thermal decomposition of the as-prepared precursors, i.e., wire-like zinc hydroxide carbonate. Moreover, the nanostructures were characterized through X-ray diffraction, field emission scanning electron microscopy, thermogravimetric-differential thermal analysis, and Brunauer–Emmett–Teller N2 adsorption–desorption analyses, among others. The BET surface area of the porous ZnO nanowires can be calculated to be 39.1m2g−1. Furthermore, the gas sensing properties of the as-prepared porous ZnO nanowires were investigated using volatile organic compounds. Compared with the ZnO monocrystalline nanowires, the porous ZnO nanowires exhibited higher response with certain organic vapors, such as toluene, 2-propanol, and acetone. The responses to 100ppm toluene, 2-propanol and acetone were 29.1, 53.7, and 57.2, respectively, at a working temperature of 300°C. These results showed that the porous ZnO nanowires consisting of nanocrystallites are highly promising candidates for gas sensing applications.

Orbital Overlap Effects in Electron Transfer Reactions across a Metal Nanowire/Electrolyte Solution Interface

In this paper, we report on calculations of the orbital overlap between Fe(III) and Cr(III) aquacomplexes and different electrode surfaces: Cu(111), Ag (111), Au(111), Pt(111), and corresponding monatomic wires. The electronic structure of the monocrystalline surfaces and nanowires are described in terms of the electronic spillover and density of electronic states at the Fermi level obtained from periodic density functional theory (DFT) calculations. The transmission coefficients (κ) characterizing the first stage of outer-sphere electron transfer for the reduction of aquacomplexes are calculated on the basis of Landau–Zener theory as a function of electrode–reactant separation; the electronic transmission coefficients for the [Cr(H2O)6]3+/2+ redox couple were found to be smaller than those for [Fe(H2O)6]3+/2+. Two different intervals can be clearly distinguished for Cu, Au and Pt: “a catalytic region”, where κ(wire) > κ(Me slab) and “an inhibition region”, where κ(wire) < κ(Me slab). A similar behavior exhibits the coupling constant estimated for a hydrogen atom adsorbed at the Au(111) surface and the Au monatomic wire. These effects originate from some specific features of electronic density profile for metal nanowires: at short distances the electronic density of nanowires is higher compared with the (111) metal surfaces, while at larger separations it decreases more sharply.

(Research Award of the Energy and Technology Division) (Photo)Electrochemistry in the Service of Solar Energy Conversion

The paper is a short summary of the lecture delivered for the 2011 Research Award of the Energy Technology Division. The main expertise field of my group is materials science and (photo)electrochemistry for energy conversion and storage. The motivations that oriented my research are described, spanning from photoelectrochemistry of single crystal semiconductors, p-n photoelectroelectrochemical cells for solar hydrogen generation, to the photoetching of semiconductors, electrochemical processing of silicon surface including porous silicon, electrodeposition of polycrystalline semiconductor thin films and monocrystalline nanowires and development of Eta (extremely thin absorber) solar cells.

Reversible nanoparticle gels with colour switching

Assembly of semiconductor nanoparticles into gel structures and their subsequent behaviour is one of the less-developed areas in nanochemistry. We demonstrate here a simple luminescent gel from CdTe nanoparticles in aqueous solution. Its structure can be described as an infinite network from chainlike branched structures. The recrystallization into the solid monocrystalline nanowires is prevented by increasing content of sulphur in the nanoparticles, which drastically increases the recrystallization energy. Brief sonication returns the system into the sol state. This switching behaviour is reversible and is accompanied by equally reversible emission colour switching. Such properties are much needed in a variety of media-responsive (i.e. “smart”) optoelectronic materials. This system will also be useful as a convenient research tool for the observation of dynamics of aqueous nanoscale colloids.

The electrochemistry of transparent quantum size rutile nanowire thin films prepared by one-step low temperature chemical bath deposition

We performed a spectro- and photoelectrochemical study of electrodes consisting of oriented rutile TiO 2 nanowires with a diameter of ∼2 nm prepared directly by chemical bath deposition on conducting glass. A significant increase (around 0.25 eV with respect to bulk rutile) of the band gap energy for the nanowire film is observed and attributed to quantum size effects, pointing to the presence of individual monocrystalline nanowires with no significant electronic communication among them. This allows for the investigation of morphologically well defined electrodes in the two dimensional quantum confinement regime, which are characterized by particularly good photoelectrocatalytic and electrochromic properties.

MBE growth of ultra small coherent Ge quantum dots in silicon for applications in nanoelectronics

Nanostructure based on IV–IV semiconductor has attracted great attention and large interest in academic community in recent year due to its quantum effect. Among these structures, quantum dots and wires have been considered for its potential application in Si-based optoelectronic and microwave devices. This work aimed at the research of monocrystalline Ge nano-islands, nanoclusters and nanowires formation at very high density and extra small size in Si matrix by molecular beam epitaxy (MBE). Initial nucleation stages, structural and morphological transitions depending on thermodynamic and kinetic parameters of deposition will be thoroughly studied. Experimental studies are focused on atomic and molecular processes on the growth surface in the course of formation of quantum dots and wires using MBE. New data on atomic structure and evolution of Si surface with Ge nanoclusters are obtained during MBE by reflection high-energy electron diffractometry (RHEED) and scanning-tunneling microscopy (STM). Superstructure and surface morphology is found to effect on ordering of nanoclusters lateral arrangement. The possibility of reproducible technology for manufacturing small-sized ordered Ge nanocrystals ensembles in the Si epitaxial films for device application is discussed.

Thermal Transport in Nanostructured Solid-State Cooling Devices

Low-dimensional nanostructured materials are promising candidates for high efficiency solid-state cooling devices based on the Peltier effect. Thermal transport in these low-dimensional materials is a key factor for device performance since the thermoelectric figure of merit is inversely proportional to thermal conductivity. Therefore, understanding thermal transport in nanostructured materials is crucial for engineering high performance devices. Thermal transport in semiconductors is dominated by lattice vibrations called phonons, and phonon transport is often markedly different in nanostructures than it is in bulk materials for a number of reasons. First, as the size of a structure decreases, its surface area to volume ratio increases, thereby increasing the importance of boundaries and interfaces. Additionally, at the nanoscale the characteristic length of the structure approaches the phonon wavelength, and other interesting phenomena such as dispersion relation modification and quantum confinement may arise and further alter the thermal transport. In this paper we discuss phonon transport in semiconductor superlattices and nanowires with regards to applications in solid-state cooling devices. Systematic studies on periodic multilayers called superlattices disclose the relative importance of acoustic impedance mismatch, alloy scattering, and crystalline imperfections at the interfaces. Thermal conductivity measurements of mono-crystalline silicon nanowires of different diameters reveal the strong effects of phonon-boundary scattering. Experimental results for Si/SiGe superlattice nanowires indicate that different phonon scattering mechanisms may disrupt phonon transport at different frequencies. These experimental studies provide insight regarding the dominant mechanisms for phonon transport in nanostructures. Finally, we also briefly discuss Peltier coolers made from nanostructured materials that have shown promising cooling performance.

Controlled synthesis and characterization of monazite type monocrystalline nanowires of mixed lanthanide orthophosphates

Monazite type monocrystalline nanowires of mixed lanthanide orthophosphates LnTb(PO4)2 (Ln=Ce, Nd, Sm) were synthesized via a simple hydrothermal process under optimal heat-treatment temperature (240 °C) and the starting acidity of the stock solution (pH 0.8). The as-synthesized nanowires were characterized by X-ray diffraction, transmission electron microscopy (TEM) and high-resolution TEM coupled with energy dispersive X-ray analysis, and infrared spectroscopy. The length and width of the nanowires were in the range of 2–10 μm and 20–350 nm, respectively.

From Complex Chains to 1D Metal Oxides: A Novel Strategy to Cu2O Nanowires

A new strategy has been put forward that small complexes with several linear-aligned metal cations can provide precursors for the growth of metal oxide nanowires, if they can be linearly connected with each other by bridging anions with rodlike micelles confined. As an example, Cu2O monocrystalline nanowires were prepared via a novel complex-precursor surfactant-assisted (CPSA) route, in which linear alignment of copper cations in Cu3(dmg)2Cl4 as precursors provided orientation for the growth of Cu2O nanowires while rodlike SDS micelles drove the linear units of Cu3(dmg)2Cl4 to connect with each other by Cl anions to form [Cu3(dmg)2Cl2]n2n+ and confined the diameter of nanowires. The infrared absorption spectra and laser light scattering show our strategy is reasonable and successful. The band gap of Cu2O nanowires was determined to 2.34 eV by UV−vis absorption spectrum, showing its promising application for the reversible conversion between solar energy and electrical or chemical energy. This method can ...

Single gallium nitride nanowire lasers.

There is much current interest in the optical properties of semiconductor nanowires, because the cylindrical geometry and strong two-dimensional confinement of electrons, holes and photons make them particularly attractive as potential building blocks for nanoscale electronics and optoelectronic devices, including lasersand nonlinear optical frequency converters. Gallium nitride (GaN) is a wide-bandgap semiconductor of much practical interest, because it is widely used in electrically pumped ultraviolet-blue light-emitting diodes, lasers and photodetectors. Recent progress in microfabrication techniques has allowed stimulated emission to be observed from a variety of GaN microstructures and films. Here we report the observation of ultraviolet-blue laser action in single monocrystalline GaN nanowires, using both near-field and far-field optical microscopy to characterize the waveguide mode structure and spectral properties of the radiation at room temperature. The optical microscope images reveal radiation patterns that correlate with axial Fabry-Perot modes (Q approximately 10(3)) observed in the laser spectrum, which result from the cylindrical cavity geometry of the monocrystalline nanowires. A redshift that is strongly dependent on pump power (45 meV microJ x cm(-2)) supports the idea that the electron-hole plasma mechanism is primarily responsible for the gain at room temperature. This study is a considerable advance towards the realization of electron-injected, nanowire-based ultraviolet-blue coherent light sources.