Structure-embedding network for predicting the transmission spectrum of a multilayer deep etched grating.

This Letter presents a structure-embedding network (SEmNet) to predict the transmission spectrum of a multilayer deep etched grating (MDEG). Spectral prediction is an important procedure in the MDEG design process. Existing approaches based on deep neural networks have been applied to spectral prediction to improve the design efficiency of similar devices, such as nanoparticles and metasurfaces. Due to a dimensionality mismatch between a structure parameter vector and the transmission spectrum vector, however, the prediction accuracy decreases. The proposed SEmNet can overcome the dimensionality mismatch problem of deep neural networks to increase the accuracy of predicting the transmission spectrum of an MDEG. SEmNet consists of a structure-embedding module and a deep neural network. The structure-embedding module increases the dimensionality of the structure parameter vector with a learnable matrix. The augmented structure parameter vector then becomes the input to the deep neural network to predict the transmission spectrum of the MDEG. Experiment results demonstrate that the proposed SEmNet improves the prediction accuracy of the transmission spectrum in comparison with the state-of-the-art approaches.

Layer-dependent anisotropic frictional behavior in two-dimensional monolayer hybrid perovskite/ITO layered heterojunctions.

Two-dimensional (2D) organic-inorganic hybrid perovskites, which possess outstanding optical and electrical properties, are promising semiconductor materials that have attracted significant interest in widespread applications. The frictional behavior of 2D perovskite materials with other transparent conductive materials, such as indium tin oxide (ITO), offers promising developments in optoelectronic devices. Therefore, the understanding of this frictional behavior is essential. Atomic force microscopy (AFM) is employed here to measure the frictional behavior between the (001) plane of the 2D organic-inorganic hybrid (C4H9NH3)2PbBr4 perovskite and the (111) plane of the ITO. The experimental analyses characterizing the nature of the friction in a single-crystalline heterojunction are reported. Based on the results of the analyses of interfaces between 2D monolayer perovskites and ITO, a strong anisotropy of friction is clearly demonstrated. The anisotropy of friction is observed as a four-fold symmetry with low a frictional coefficient, 0.035, in misaligned contacts, and, 0.015, in aligned contacts in the heterojunction configuration. In addition, atomistic simulations reveal underlying frictional mechanisms in the dynamical regimes. A new phenomenon discovered in the studies establishes that the measured frictional anisotropy surprisingly depends on the number of atomic layers in the 2D perovskite. The frictional anisotropy decreases significantly with the increase in the number of layers up to 16 layers, and then it becomes independent of the thickness. Our results are predicted to be of a general nature and should be applicable to other 2D hybrid perovskite heterojunction configurations, and thus, furthers the development of adaptive and stretchable optoelectronic nanodevices.

The smallest resonator arrays in atmosphere by chip-size-grown nanowires with tunable Q-factor and frequency for subnanometer thickness detection.

A chip-size vertically aligned nanowire (NW) resonator arrays (VNRs) device has been fabricated with simple one-step lithography process by using grown self-assembled zinc oxide (ZnO) NW arrays. VNR has cantilever diameter of 50 nm, which breakthroughs smallest resonator record (>100 nm) functioning in atmosphere. A new atomic displacement sensing method by using atomic force microscopy is developed to effectively identify the resonance of NW resonator with diameter 50 nm in atmosphere. Size-effect and half-dimensional properties of the NW resonator have been systematically studied. Additionally, VNR has been demonstrated with the ability of detecting nanofilm thickness with subnanometer (<10(-9)m) resolution.

Selective adsorption of bismuth telluride nanoplatelets through electrostatic attraction.

We demonstrate a facile technique to assemble solution phase-synthesized bismuth telluride (Bi2Te3) nanoplatelets into arrays of micropatterns. Aminosilane self-assembled monolayers (SAMs) are printed on silicon dioxide (SiO2) substrates using microcontact printing (µCP). The SAM printed surfaces are terminated with amine-groups allowing Bi2Te3 nanoplatelet selective adsorption by electrostatic attraction. Using Kelvin probe force microscopy, the electrical potential difference between aminosilane SAM and Bi2Te3 nanoplatelet surfaces is found to be ∼650 mV, which is larger than that (∼400 mV) between the SiO2 substrate and Bi2Te3 nanoplatelet surfaces. The selective adsorption provides an opportunity for integrating solution phase-grown topological insulators toward several device-level applications.

Nonlinear length dependent electrical resistance of a single crystal zinc oxide micro/nanobelt.

We have systematically investigated the intrinsic electrical property of a single crystal zinc oxide (ZnO) micro/nanobelt (MB/NB) using a conductive atomic force microscopy (AFM) technique. By mounting one end of the MB/NB on a flat nonconductive silicon substrate, a platform for performing electrical property characterization using conductive current AFM is established. The quantitative characterization of the electrical resistance of the MB/NBs was performed by acquiring I-V curves for the MB/NB in between the electrode and the conductive AFM tip. The resistance of the single crystalline ZnO MB/NB was measured to be exponentially dependent on the length of the MB/NB. A systematic model based on the anisotropic velocity of the carriers in the crystal planes has been proposed and fits the experimental measurement well. This research reveals that the electrical resistance shows a nonlinear length dependence in the semiconducting single crystal MB/NB, which is significantly different from the bulk counterpart. Understanding such a property could definitely improve the design and the performance of next generation electrical nanodevices.

Controllable growth of laterally aligned zinc oxide nanorod arrays on a selected surface of the silicon substrate by a catalyst-free vapor solid process--a technique for growing nanocircuits.

We report a simple and effective vapor deposition method for directly growing ultra-long, laterally aligned, zinc oxide (ZnO) nanorod arrays only on the side edges of a bare silicon (Si) substrate without using any catalysts and precursors. The growth on the top surface of the substrate is restrained by controlling the flow of source vapor in a tube furnace through the chemical vapor solid process. The optimized growth parameters have been thoroughly investigated and identified. Direct growth of laterally aligned ZnO nanowire arrays on the desired surface of the substrate is successfully achieved. A vapor solid mechanism with source vapor flow rate control has been proposed to explain the synthesis: ZnO nanodots first form on the bare Si substrate side edges due to the local large binding energy and high zinc (Zn) vapor concentration, and then nanorods epitaxially grow from the nanodots. In addition, the lateral, ultra-long ZnO nanorods grown on orthogonal silicon microelectrodes are achieved and could be expected to find important applications in a bottom-up way of fabricating the next generation nanoelectronics.

Light-Driven Charge Transport and Optical Sensing in Molecular Junctions.

Probing charge and energy transport in molecular junctions (MJs) has not only enabled a fundamental understanding of quantum transport at the atomic and molecular scale, but it also holds significant promise for the development of molecular-scale electronic devices. Recent years have witnessed a rapidly growing interest in understanding light-matter interactions in illuminated MJs. These studies have profoundly deepened our knowledge of the structure-property relations of various molecular materials and paved critical pathways towards utilizing single molecules in future optoelectronics applications. In this article, we survey recent progress in investigating light-driven charge transport in MJs, including junctions composed of a single molecule and self-assembled monolayers (SAMs) of molecules, and new opportunities in optical sensing at the single-molecule level. We focus our attention on describing the experimental design, key phenomena, and the underlying mechanisms. Specifically, topics presented include light-assisted charge transport, photoswitch, and photoemission in MJs. Emerging Raman sensing in MJs is also discussed. Finally, outstanding challenges are explored, and future perspectives in the field are provided.

A potential fluorescence detection approach to trace hexachlorobenzene via disaggregating with ethanol.

A potential ultra-sensitive detection approach for hexachlorobenzene (HCB), based on the measurement of the intrinsic fluorescence of well-separated HCB molecules in ethanol, has been proposed. Owing to the strong intermolecular π-π stacking interaction of the planar aromatic rings, self-aggregated HCB shows almost no fluorescence. However, the intrinsic emission of HCB can readily be detected in ethanol due to the enhanced emission from the disaggregated HCB, which is related to the hydrogen bond formation between ethanol and HCB. By simply measuring the HCB intrinsic fluorescence, a HCB concentration a little bit higher than 10(-14) M (~0.001 ppt) in ethanol can be detected; moreover, the fluorescence intensity of the HCB increases linearly with the HCB concentration ranging from 10(-10) to 10(-7) M. The approach might provide a simple, fast and efficient method for HCB quantification.

Fluorescence detection of trace PCB101 based on PITC immobilized on porous AAO membrane.

A sensitive and selective fluorescent membrane for rapid detection of trace 2,2',4,5,5'-pentachlorinated biphenyl (PCB101) has been achieved by immobilizing the fluorophore phenyl isothiocyanate (PITC) onto porous anodic aluminium oxide (AAO) membrane (denoted as PITC@AAO). The fluorescence of the PITC@AAO membrane is obviously enhanced after titrating the analyte PCB101 into the membrane, being ascribed to the halogen-bonding interaction between the fluorophore PITC and the analyte PCB101. The fluorescence intensity increases with the PCB101 concentration in the low range below 1 ppm, and there exists an approximate linear relationship between the relative fluorescence intensity and the PCB101 concentration in the low range of 1-6 ppb. Moreover, the PITC@AAO membrane shows good selectivity; for example, it is insensitive to common structural analogs (polychlorinated aromatics). The mechanisms of the fluorescence enhancement and the better sensitivity and selectivity of the PITC@AAO membrane to PCB101 than that of PITC/n-hexane solution are also discussed. This work demonstrates that trace (in ppb range) PCBs can be detected by simple fluorescence measurement.

Third-order nonlinear Hall effect induced by the Berry-connection polarizability tensor.

Nonlinear responses in transport measurements are linked to material properties not accessible at linear order1 because they follow distinct symmetry requirements2-5. While the linear Hall effect indicates time-reversal symmetry breaking, the second-order nonlinear Hall effect typically requires broken inversion symmetry1. Recent experiments on ultrathin WTe2 demonstrated this connection between crystal structure and nonlinear response6,7. The observed second-order nonlinear Hall effect can probe the Berry curvature dipole, a band geometric property, in non-magnetic materials, just like the anomalous Hall effect probes the Berry curvature in magnetic materials8,9. Theory predicts that another intrinsic band geometric property, the Berry-connection polarizability tensor10, gives rise to higher-order signals, but it has not been probed experimentally. Here, we report a third-order nonlinear Hall effect in thick Td-MoTe2 samples. The third-order signal is found to be the dominant response over both the linear- and second-order ones. Angle-resolved measurements reveal that this feature results from crystal symmetry constraints. Temperature-dependent measurement shows that the third-order Hall response agrees with the Berry-connection polarizability contribution evaluated by first-principles calculations. The third-order nonlinear Hall effect provides a valuable probe for intriguing material properties that are not accessible at lower orders and may be employed for high-order-response electronic devices.

Magnetic Proximity Effect in Graphene/CrBr3 van der Waals Heterostructures.

2D van der Waals heterostructures serve as a promising platform to exploit various physical phenomena in a diverse range of novel spintronic device applications. Efficient spin injection is the prerequisite for these devices. The recent discovery of magnetic 2D materials leads to the possibility of fully 2D van der Waals spintronics devices by implementing spin injection through the magnetic proximity effect (MPE). Here, the investigation of MPE in 2D graphene/CrBr3 van der Waals heterostructures is reported, which is probed by the Zeeman spin Hall effect through non-local measurements. Quantitative estimation of the Zeeman splitting field demonstrates a significant MPE field even in a low magnetic field. Furthermore, the observed anomalous longitudinal resistance changes at the Dirac point RXX,D with increasing magnetic field near ν = 0 may be attributed to the MPE-induced new ground state phases. This MPE revealed in the graphene/CrBr3 van der Waals heterostructures therefore provides a solid physics basis and key functionality for next-generation 2D spin logic and memory devices.

Photoelectric Property Modulation by Nanoconfinement in the Longitude Direction of Short Semiconducting Nanorods.

Photoelectric property change in half-dimensional (0.5D) semiconducting nanomaterials as a function of illumination light intensity and materials geometry has been systematically studied. Through two independent methods, conductive atomic force microscopy (C-AFM) direct current-voltage acquisition and scanning kelvin probe microscopy (SKPM) surface potential mapping, photoelectric property of 0.5D ZnO nanomaterial has been characterized with exceptional behaviors compared with bulk/micro/one-dimensional (1D) nanomaterial. A new model by considering surface effect, quantum effect, and illumination effect has been successfully built, which could more accurately predict the photoelectric characteristics of 0.5D semiconducting nanomaterials. The findings reported in this study could potentially impact three-dimensional (3D) photoelectronics.

Improved SERS performance from Au nanopillar arrays by abridging the pillar tip spacing by Ag sputtering.

Ag-capped Au nanopillar arrays on a resin supporter (see left upper figure), with a typical adjacent pillar tip gap of 10 nm, show obviously higher surface-enhanced Raman scattering (SERS) sensitivity (right column in red) than that of the bare Au nanopillar array while using 10 nM R6G as probe molecules. The large-area Ag-capped Au nanopillar array has potential in trace detection of special chemicals.