Controllable physicochemical properties of WOx thin films grown under glancing angle.

In this work, various physicochemical properties are investigated in nanostructured WOx thin films prepared by radio-frequency magnetron sputtering for optoelectronic applications. A glancing angle of 87° is employed to grow films of different thicknesses, which are then exposed to post-growth annealing. Detailed local probe analyses in terms of morphology and work function of WOx films are carried out to investigate thickness-dependent property modulations of the as-deposited and annealed films. The analyses show a reasonably good correlation with photoelectron spectroscopic measurements on the films and the bulk I-V characteristics acquired on a series of WOx/p-Si heterojunction diodes. The presence of a critical WOx thickness is identified to regulate the rectification ratio values at the WOx/p-Si heterostructures and increase in series resistance within the bulk of the films. The present study provides valuable insights to correlate optical, electrical, and structural properties of WOx thin films, which will be beneficial for fabricating WOx-based optoelectronic devices, including photovoltaic cells.

Site-Specific Emulation of Neuronal Synaptic Behavior in Au Nanoparticle-Decorated Self-Organized TiOx Surface.

Neuromorphic computing is a potential approach for imitating massive parallel processing capabilities of a bio-synapse. To date, memristors have emerged as the most appropriate device for designing artificial synapses for this purpose due to their excellent analog switching capacities with high endurance and retention. However, to build an operational neuromorphic platform capable of processing high-density information, memristive synapses with nanoscale footprint are important, albeit with device size scaled down, retaining analog plasticity and low power requirement often become a challenge. This paper demonstrates site-selective self-assembly of Au nanoparticles on a patterned TiOx layer formed as a result of ion-induced self-organization, resulting in site-specific resistive switching and emulation of bio-synaptic behavior (e.g., potentiation, depression, spike rate-dependent and spike timing-dependent plasticity, paired pulse facilitation, and post tetanic potentiation) at nanoscale. The use of local probe-based methods enables nanoscale probing on the anisotropic films. With the help of various microscopic and spectroscopic analytical tools, the observed results are attributed to defect migration and self-assembly of implanted Au atoms on self-organized TiOx surfaces. By leveraging the site-selective evolution of gold-nanostructures, the functionalized TiOx surface holds significant potential in a multitude of fields for developing cutting-edge neuromorphic computing platforms and Au-based biosensors with high-density integration.

Controlled creation and annihilation of isolated robust emergent magnetic monopole like charged vertices in square artificial spin ice.

Magnetic analogue of an isolated free electric charge, i.e., a magnet with a single north or south pole, is a long sought-after particle which remains elusive so far. In magnetically frustrated pyrochlore solids, a classical analogue of monopole was observed as a result of excitation of spin ice vertices. Direct visualization of such excitations were proposed and later confirmed in analogous artificial spin ice (ASI) systems of square as well as Kagome geometries. However, such magnetically charged vertices are randomly created as they are thermally driven and are always associated with corresponding equal and opposite emergent charges, often termed as monopole-antimonopole pairs, connected by observable strings. Here, we demonstrate a controlled stabilisation of a robust isolated emergent monopole-like magnetically charged vertices in individual square ASI systems by application of an external magnetic field. The excitation conserves the magnetic charge without the involvement of a corresponding excitation of opposite charge. Well supported by Monte Carlo simulations our experimental results enable, in absence of a true elemental magnetic monopole, creation of electron vortices and studying electrodynamics in presence of a monopole-like field in a solid state environment.

Tuning the Electrical and Thermoelectric Properties of N Ion Implanted SrTiO3 Thin Films and Their Conduction Mechanisms.

The SrTiO3 thin films were fabricated by pulsed laser deposition. Subsequently ion implantation with 60 keV N ions at two different fluences 1 × 1016 and 5 × 1016 ions/cm2 and followed by annealing was carried out. Thin films were then characterized for electronic structure, morphology and transport properties. X-ray absorption spectroscopy reveals the local distortion of TiO6 octahedra and introduction of oxygen vacancies due to N implantation. The electrical and thermoelectric properties of these films were measured as a function of temperature to understand the conduction and scattering mechanisms. It is observed that the electrical conductivity and Seebeck coefficient (S) of these films are significantly enhanced for higher N ion fluence. The temperature dependent electrical resistivity has been analysed in the temperature range of 80-400 K, using various conduction mechanisms and fitted with band conduction, near neighbour hopping (NNH) and variable range hopping (VRH) models. It is revealed that the band conduction mechanism dominates at high temperature regime and in low temperature regime, there is a crossover between NNH and VRH. The S has been analysed using the relaxation time approximation model and dispersive transport mechanism in the temperature range of 300-400 K. Due to improvement in electrical conductivity and thermopower, the power factor is enhanced to 15 µWm-1 K-2 at 400 K at the higher ion fluence which is in the order of ten times higher as compared to the pristine films. This study suggests that ion beam can be used as an effective technique to selectively alter the electrical transport properties of oxide thermoelectric materials.

Growth of Wafer-Scale ReS2 with "Tunable" Geometry toward Electron Field-Emission Application.

Despite high potential, the promise of 2D materials has not been realized practically because of limits of tiny grown size and difficult manipulation of the active spot. The utilization of 2D layers is the ultimate approach, which should be supported by large-scale production. In this very first report, we demonstrate the wafer-scale production of ReS2 using the conventional sputtering method. The controllability of ReS2 geometry has been investigated to form typical thin films or vertically aligned layers that are further applied for field emission. The vertically aligned ReS2 layers exhibit ultralow turn-on electric field (0.6 V µm-1) with the current density (0.6 mA cm-2) and significantly low threshold electric field (0.8 V µm-1), respectively, along with outstanding emission stability. The results are attributed to weakly coupled ReS2 layers and the high geometrical field enhancement factor (∼1.08 × 105). Further, Kelvin probe force microscopy measurements confirm that lowering the work function is not solely responsible to achieve the ultralow operative field. Moreover, finite element simulation suggests that not only the length, width, and separation of the nanostructures but also the local slope plays an important role in suppressing screening effects.

A Transparent Photonic Artificial Visual Cortex.

Mimicking brain-like functionality with an electronic device is an essential step toward the design of future technologies including artificial visual and memory applications. Here, a proof-of-concept all-oxide-based (NiO/TiO2 ) highly transparent (54%) heterostructure is proposed and demonstrated, which mimics the primitive functions of the visual cortex. Specifically, orientation selectivity and spatiotemporal processing similar to that of the visual cortex are demonstrated using direct optical stimuli under the self-biased condition due to photovoltaic effect, illustrating an energy-efficient approach for neuromorphic computing. The photocurrent of the device can be modulated from zero to 80 µA by simply rotating the slit by 90°. The device shows fast rise and fall times of 3 and 6 ms, respectively. Based on Kelvin probe force measurements, the observed results are attributed to a lateral photovoltaic effect. This highly transparent, self-biased, photonic triggered device paves the way for the advancement of energy-efficient neuromorphic computation.

Temporal evolution on SiO2 surface under low energy Ar+-ion bombardment: roles of sputtering, mass redistribution, and shadowing.

Self-organized pattern evolution on SiO2 surface under low energy Ar-ion irradiation has been investigated extensively at varied ion energies, angles of ion incidence, and ion flux. Our investigations reveal an instability on SiO2 surface in an angular window of 40° ̶ 70° and for a comprehensive range of Ar-ion energies (200-1000 eV). Different topographical features, viz. ripples, mounds, and elongated nanostructures evolve on the surface, depending upon the angle of incidence and ion fluence. The results are compiled in the form of a parametric phase diagram (ion energy versus angle of incidence) which summarizes the pattern formation on SiO2 surface. To understand the evolution of observed patterns, we have carried out theoretical estimation, taking into account the synergetic roles of ion induced curvature-dependent sputter erosion and prompt atomic redistribution. It is shown that irradiation-induced mass redistribution of target atoms plays a crucial role in determining the critical angle of ion incidence for pattern formation on SiO2 under the present experimental conditions, whereas the contribution of curvature-dependent sputtering needs to be considered to understand the existence of the angular window of pattern formation. In addition, ion-beam shadowing by surface features are shown to play a dominant role in the formation of mounds and elongated structures at higher ion fluences.

Strong uniaxial magnetic anisotropy in Co films on highly ordered grating-like nanopatterned Ge surfaces.

We present a systematic investigation on uniaxial magnetic anisotropy (UMA) in Co thin films induced by high aspect ratio nanopatterned anisotropic substrates. Self-organized long grating-like nanostructures, with extreme regularities, are fabricated on Ge surfaces using Au-ion implantation at room temperature. Subsequently deposition of Co films are carried out on the same at two different angles. Magneto-optical Kerr effect measurements show strong UMA in Co films grown on ion-patterned Ge substrates, fabricated under different ion fluences, along and perpendicular to the direction of the patterns (long grating-like nanostructures). Magnetic force microscopy measurements under different externally applied magnetic fields reveal an easy domain wall motion when the field is applied along the grating-like nanostructures. On the other hand, high amplitude grating-like nanostructures hinder the spin rotation when the field is applied along the hard axis. The present study will be useful for magnetic recording media and ultra-small magnetic field sensors.

Surfing Silicon Nanofacets for Cold Cathode Electron Emission Sites.

Point sources exhibit low threshold electron emission due to local field enhancement at the tip. In the case of silicon, however, the realization of tip emitters has been hampered by unwanted oxidation, limiting the number of emission sites and the overall current. In contrast to this, here, we report the fascinating low threshold (∼0.67 V µm-1) cold cathode electron emission from silicon nanofacets (Si-NFs). The ensembles of nanofacets fabricated at different time scales, under low energy ion impacts, yield tunable field emission with a Fowler-Nordheim tunneling field in the range of 0.67-4.75 V µm-1. The local probe surface microscopy-based tunneling current mapping in conjunction with Kelvin probe force microscopy measurements revealed that the valleys and a part of the sidewalls of the nanofacets contribute more to the field emission process. The observed lowest turn-on field is attributed to the absence of native oxide on the sidewalls of the smallest facets as well as their lowest work function. In addition, first-principle density functional theory-based simulation revealed a crystal orientation-dependent work function of Si, which corroborates well with our experimental observations. The present study demonstrates a novel way to address the origin of the cold cathode electron emission sites from Si-NFs fabricated at room temperature. In principle, the present methodology can be extended to probe the cold cathode electron emission sites from any nanostructured material.

Gold-decorated highly ordered self-organized grating-like nanostructures on Ge surface: Kelvin probe force microscopy and conductive atomic force microscopy studies.

Nanoarchitecture by atomic manipulation is considered to be one of the emerging trends in advanced functional materials. It has a gamut of applications to offer in nanoelectronics, chemical sensing, and nanobiological science. In particular, highly ordered one-dimensional semiconductor nanostructures fabricated by self-organization methods are in high demand for their high aspect ratios and large number of applications. An efficient way of fabricating semiconductor nanostructures is by molecular beam epitaxy, where atoms are added to a crystalline surface at an elevated temperature during growth, yielding the desired structures in a self-assembled manner. In this article, we offer a room temperature process, in which atoms are sputtered away by ion impacts. Using gold ion implantation, the present study reports on the formation of highly ordered self-organized long grating-like nanostructures, with grooves between them, on a germanium surface. The ridges of the patterns are shown to have flower-like protruding nanostructures, which are mostly decorated by gold atoms. By employing local probe microscopic techniques like Kelvin probe force microscopy and conductive atomic force microscopy, we observe a spatial variation in the work function and different nanoscale electrical conductivity on the ridges of the patterns and the grooves between them, which can be attributed to gold atom decorated ridges. Thus, the architecture  presented offers the advantage of using the patterned germanium substrates as periodic arrays of conducting ridges and poorly conducting grooves between them.

Field-induced doping-mediated tunability in work function of Al-doped ZnO: Kelvin probe force microscopy and first-principle theory.

We demonstrate that the work function of Al-doped ZnO (AZO) can be tuned externally by applying an electric field. Our experimental investigations using Kelvin probe force microscopy show that by applying a positive or negative tip bias, the work function of AZO film can be enhanced or reduced, which corroborates well with the observed charge transport using conductive atomic force microscopy. These findings are further confirmed by calculations based on first-principles theory. Tuning the work function of AZO by applying an external electric field is not only important to control the charge transport across it, but also to design an Ohmic contact for advanced functional devices.

Structural defect-dependent resistive switching in Cu-O/Si studied by Kelvin probe force microscopy and conductive atomic force microscopy.

In this study, we show structural defect-dependent presence or absence of resistive switching in Cu-O films. We use Kelvin probe force microscopy and conductive atomic force microscopy to show the presence of resistive switching. In addition, local current mapping provides direct evidence on the formation of nanoscale filament. These findings match well with the existing theoretical model on resistive switching. In particular, understanding the role of structural defects in resistive switching can be considered as critically important to take a step forward for designing advanced nanoscale memory devices.

Semi-insulating behaviour of self-assembled tin(IV)corrole nanospheres.

Three novel tin(iv)corrole complexes have been prepared and characterized by various spectroscopic techniques including single crystal X-ray structural analysis. Packing diagrams of the tin(iv)corroles revealed that corrolato-tin(iv)-chloride molecules are interconnected by intermolecular C-HCl hydrogen bonding interactions. HCl distances are 2.848 Å, 3.051 Å, and 2.915 Å, respectively, for the complexes. In addition, the C-HCl angles are 119.72°, 144.70°, and 147.08°, respectively, for the complexes. It was also observed that in one of the three synthesized complexes dimers were formed, while in the other two cases 1D infinite polymer chains were formed. Well-defined and nicely organized three-dimensional hollow nanospheres (SEM images on silicon wafers) with diameters of ca. 676 nm and 661 nm are obtained in the complexes, forming 1D polymer chains. By applying a thin layer of tin(iv)corrole nanospheres to an ITO surface (AFM height images of ITO films; ∼200 nm in height), a device was fabricated with the following composition: Ag/ITO-coated glass/tin(iv)corrole nanospheres/ITO-coated glass/Ag. The resistivity (ρ) of the nanostructured film was calculated to be ∼2.4 × 10(8) Ω cm, which falls in the range of semi-insulating semiconductors. CAFM current maps at 10 V bias show bright spots with a 10-20 pA intensity and indicate that the nanospheres (∼250 nm in diameter) are the electron-conducting pathway in the device. The semi-insulating behavior arises from the non-facile electron transfer in the HOMOs of the tin(iv)corrole nanospheres.

Tunable antireflection from conformal Al-doped ZnO films on nanofaceted Si templates.

Photon harvesting by reducing reflection loss is the basis of photovoltaic devices. Here, we show the efficacy of Al-doped ZnO (AZO) overlayer on ion beam-synthesized nanofaceted silicon for suppressing reflection loss. In particular, we demonstrate thickness-dependent tunable antireflection (AR) from conformally grown AZO layer, showing a systematic shift in the reflection minima from ultraviolet to visible to near-infrared ranges with increasing thickness. Tunable AR property is understood in light of depth-dependent refractive index of nanofaceted silicon and AZO overlayer. This improved AR property significantly increases the fill factor of such textured heterostructures, which reaches its maximum for 60-nm AZO compared to the ones based on planar silicon. This thickness matches with the one that shows the maximum reduction in surface reflectance. PACS: 81.07.-b; 42.79.Wc; 81.16.Rf; 81.15.Cd.

Transition from ripples to faceted structures under low-energy argon ion bombardment of silicon: understanding the role of shadowing and sputtering.

In this study, we have investigated temporal evolution of silicon surface topography under 500-eV argon ion bombardment for two angles of incidence, namely 70° and 72.5°. For both angles, parallel-mode ripples are observed at low fluences (up to 2 × 1017 ions cm-2) which undergo a transition to faceted structures at a higher fluence of 5 × 1017 ions cm-2. Facet coarsening takes place at further higher fluences. This transition from ripples to faceted structures is attributed to the shadowing effect due to a height difference between peaks and valleys of the ripples. The observed facet coarsening is attributed to a mechanism based on reflection of primary ions from the facets. In addition, the role of sputtering is investigated (for both the angles) by computing the fractional change in sputtering yield and the evolution of surface roughness. PACS: 81.05.Cy, 81.16.Rf, 61.80.Jh, 87.64.Dz.