Wen-Di Li

Engineering nonlinearity into memristors for passive crossbar applications

Although TaOx memristors have demonstrated encouraging write/erase endurance and nanosecond switching speeds, the linear current-voltage (I-V) characteristic in the low resistance state limits their applications in large passive crossbar arrays. We demonstrate here that a TiO2-x/TaOx oxide heterostructure incorporated into a 50 nm× 50 nm memristor displays a very large nonlinearity such that I(V/2) ≈ I(V)/100 for V ≈ 1 volt, which is caused by current-controlled negative differential resistance in the device.

High-Performance Flexible Transparent Electrode with an Embedded Metal Mesh Fabricated by Cost-Effective Solution Process

A new structure of flexible transparent electrodes is reported, featuring a metal mesh fully embedded and mechanically anchored in a flexible substrate, and a cost-effective solution-based fabrication strategy for this new transparent electrode. The embedded nature of the metal-mesh electrodes provides a series of advantages, including surface smoothness that is crucial for device fabrication, mechanical stability under high bending stress, strong adhesion to the substrate with excellent flexibility, and favorable resistance against moisture, oxygen, and chemicals. The novel fabrication process replaces vacuum-based metal deposition with an electrodeposition process and is potentially suitable for high-throughput, large-volume, and low-cost production. In particular, this strategy enables fabrication of a high-aspect-ratio (thickness to linewidth) metal mesh, substantially improving conductivity without considerably sacrificing transparency. Various prototype flexible transparent electrodes are demonstrated with transmittance higher than 90% and sheet resistance below 1 ohm sq(-1) , as well as extremely high figures of merit up to 1.5 × 10(4) , which are among the highest reported values in recent studies. Finally using our embedded metal-mesh electrode, a flexible transparent thin-film heater is demonstrated with a low power density requirement, rapid response time, and a low operating voltage.

Combined helium ion beam and nanoimprint lithography attains 4 nm half-pitch dense patterns

The authors demonstrated a promising technique that yielded single-digit nanometer features for nanotechnology research and possible future electronic circuit fabrication by combining high resolution helium ion beam patterning and nanoimprint lithography. They fabricated a series of line patterns with single-digit nanometer half-pitches by exposing a layer of hydrogen silsesquioxane (HSQ) resist with a scanning focused helium ion beam. The smallest half-pitch of clearly resolved line patterns was 4 nm. Using the HSQ patterns as a nanoimprint template, nanoscale patterns down to 4 nm half-pitch were transferred into nanoimprint resist through a UV-curable nanoimprint process.

Diamond nitrogen-vacancy centers created by scanning focused helium ion beam and annealing

We demonstrate a method to create nitrogen-vacancy (NV) centers in diamond using focused helium ion microscopy. Near-surface NV centers can be created with spatial resolution below 0.6 μm. We studied the density, creation efficiency, and spectral linewidths at optical and microwave frequencies for NV centers produced using various helium ion implantation doses. The optical linewidths are narrower than those of similar nitrogen-vacancy centers produced using nitrogen ion implantation.

High density nitrogen-vacancy sensing surface created via He+ ion implantation of 12C diamond

We present a promising method for creating high-density ensembles of nitrogen-vacancy centers with narrow spin-resonances for high-sensitivity magnetic imaging. Practically, narrow spin-resonance linewidths substantially reduce the optical and RF power requirements for ensemble-based sensing. The method combines isotope purified diamond growth, in situ nitrogen doping, and helium ion implantation to realize a 100 nm-thick sensing surface. The obtained 1017 cm−3 nitrogen-vacancy density is only a factor of 10 less than the highest densities reported to date, with an observed 200 kHz spin resonance linewidth over 10 times narrower.

Solution-Processed Transparent Nickel-Mesh Counter Electrode with in-Situ Electrodeposited Platinum Nanoparticles for Full-Plastic Bifacial Dye-Sensitized Solar Cells

A new type of embedded metal-mesh transparent electrode (EMTE) with in-situ electrodeposited catalytic platinum nanoparticles (PtNPs) is developed as a high-performance counter electrode (CE) for lightweight flexible bifacial dye-sensitized solar cells (DSSCs). The thick but narrow nickel micromesh fully embedded in a plastic film provides superior electrical conductivity, optical transmittance, and mechanical stability to the novel electrode. PtNPs decorated selectively on the nickel micromesh surface provide catalytic function with minimum material cost and without interfering with optical transparency. Facile and fully solution-processed fabrication of the novel CE is demonstrated with potential for scalable and cost-effective production. Using this PtNP-decorated nickel EMTE as the CE and titanium foil as the photoanode, unifacial flexible DSSCs are fabricated with a power conversion efficiency (PCE) of 6.91%. By replacing the titanium foil with a transparent ITO-PEN photoanode, full-plastic bifacial DSSCs are fabricated and tested, demonstrating a remarkable PCE of 4.87% under rear-side illumination, which approaches 85% of the 5.67% PCE under front-side illumination, among the highest ratio in published results. These promising results reveal the enormous potential of this hybrid transparent CE in scalable production and commercialization of low-cost and efficient flexible DSSCs.

Nonlinear Metasurface for Simultaneous Control of Spin and Orbital Angular Momentum in Second Harmonic Generation

The spin and orbital angular momentum (SAM and OAM) of light is providing a new gateway toward high capacity and robust optical communications. While the generation of light with angular momentum is well studied in linear optics, its further integration into nonlinear optical devices will open new avenues for increasing the capacity of optical communications through additional information channels at new frequencies. However, it has been challenging to manipulate the both SAM and OAM of nonlinear signals in harmonic generation processes with conventional nonlinear materials. Here, we report the generation of spin-controlled OAM of light in harmonic generations by using ultrathin photonic metasurfaces. The spin manipulation of OAM mode of harmonic waves is experimentally verified by using second harmonic generation (SHG) from gold meta-atom with 3-fold rotational symmetry. By introducing nonlinear phase singularity into the metasurface devices, we successfully generate and measure the topological charges of spin-controlled OAM mode of SHG through an on-chip metasurface interferometer. The nonlinear photonic metasurface proposed in this work not only opens new avenues for manipulating the OAM of nonlinear optical signals but also benefits the understanding of the nonlinear spin-orbit interaction of light in nanoscale devices.

Nanostructure transfer using cyclic olefin copolymer templates fabricated by thermal nanoimprint lithography

The authors demonstrate the application of cyclic olefin copolymer (COC) films as secondary nanoimprint templates for transferring sub-100 nm nanostructures. Featureless COC films were first patterned by a thermal nanoimprint process using silicon molds with gratings of various periods from 140 to 420 nm. Morphology of COC gratings imprinted at different processing parameters was characterized by scanning electron microscopy and atomic force microscopy and the grating transfer fidelity was systematically investigated. The nanoimprinted COC substrates were then used as secondary templates in an ultraviolet (UV)-cured nanoimprint lithography process to transfer the grating patterns onto UV-curable epoxy. The authors also demonstrate the application of using these nanoimprinted COC templates to transfer metallic nanostructures onto fiber facets. With good mechanical strength, high transparency to UV light, easy fabrication, and excellent chemical compatibility, COC is a promising material that can be used in...

Light-stimulated actuators based on nickel hydroxide-oxyhydroxide

An actuating material can be powered wirelessly by water desorption induced by low-intensity visible light. Light-induced actuators that are self-contained and compact can be used as artificial muscles for microrobotics because their actuation can be induced wirelessly, which reduces the complexity of the device or system. Here, we report a material system, nickel hydroxide-oxyhydroxide, that could actuate because of a volume change stimulated by illumination of visible light of low intensities. The actuating material here exhibited a turbostratic crystal structure capable of intercalating water, and we show that the intercalated water can be rapidly and reversibly desorbed into the environment under visible light of low intensities, resulting in fast actuation driven wirelessly by light. By electroplating the actuating material on passive substrates, we have fabricated film actuators capable of undergoing reversible bending and curling with an intrinsic actuating stress of 5 to 65 megapascals at response rates in the order of tens to hundreds of degrees per second depending on the light intensity, which are comparable to mammalian skeletal muscles. By intentionally electroplating the nickel hydroxide-oxyhydroxide on selected areas of the substrate, a hinged actuator that can lift objects ~100 times the weight of the actuating material is achieved. Other demonstrations show the potential uses in robotic devices, including sunlight-induced actuation, a biomimicked “sensitive plant” with rapid leaf movement, and a light-powered walking bot.

Dynamic nuclear polarization enhanced magnetic field sensitivity and decoherence spectroscopy of an ensemble of near-surface nitrogen-vacancy centers in diamond

We perform pulsed optically detected electron spin resonance to measure the DC magnetic field sensitivity and electronic spin coherence time T2 of an ensemble of near-surface, high-density nitrogen-vacancy centers engineered to have a narrow magnetic resonance linewidth. Combining pulsed spectroscopy with dynamic nuclear polarization, we obtain the photon-shot-noise-limited DC magnetic sensitivity of 35 nT Hz−0.5. We find that T2 is controlled by instantaneous diffusion, enabling decoherence spectroscopy on residual nitrogen impurity spins in the diamond lattice and a quantitative determination of their density. The demonstrated high DC magnetic sensitivity and decoherence spectroscopy are expected to broaden the application range for two-dimensional magnetic imaging.X iv :1 61 2. 00 08 8v 1 [ co nd -m at .m es -h al l] 1 D ec 2 01 6 Magnetic field sensitivity and decoherence spectroscopy of an ensemble of narrow-linewidth nitrogen-vacancy centers close to a diamond surface Kento Sasaki, Ed E. Kleinsasser, Zhouyang Zhu, 4 Wen-Di Li, 4 Hideyuki Watanabe, Kai-Mei C. Fu, 6 Kohei M. Itoh, 7, a) and Eisuke Abe b) School of Fundamental Science and Technology, Keio University, 3-14-1 Hiyoshi, Kohoku-ku, Yokohama 223-8522, Japan Department of Electrical Engineering, University of Washington, Seattle, Washington 98195-2500, USA HKU-Shenzhen Institute of Research and Innovation (HKU-SIRI), Shenzhen 518000, China Department of Mechanical Engineering, The University of Hong Kong, Pokfulam, Hong Kong, China Correlated Electronics Group, Electronics and Photonics Research Institute, National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba Central 5, 1-1-1 Higashi, Tsukuba, Ibaraki 305-8565, Japan Department of Physics, University of Washington, Seattle, Washington 98195-1560, USA Spintronics Research Center, Keio University, 3-14-1 Hiyoshi, Kohoku-ku, Yokohama 223-8522, Japan