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Electrophysiological recording and stimulation are the gold standard for functional mapping during surgical and therapeutic interventions as well as capturing cellular activity in the intact human brain. A critical component probing human brain activity is the interface material at the electrode contact that electrochemically transduces brain signals to and from free charge carriers in the measurement system. Here, we summarize state-of-the-art electrode array systems in the context of translation for use in recording and stimulating human brain activity. We leverage parametric studies with multiple electrode materials to shed light on the varied levels of suitability to enable high signal-to-noise electrophysiological recordings as well as safe electrophysiological stimulation delivery. We discuss the effects of electrode scaling for recording and stimulation in pursuit of high spatial resolution, channel count electrode interfaces, delineating the electrode–tissue circuit components that dictate the electrode performance. Finally, we summarize recent efforts in the connectorization and packaging for high channel count electrode arrays and provide a brief account of efforts toward wireless neuronal monitoring systems. Graphical Abstract

The Utah array powers cutting-edge projects for restoration of neurological function, such as BrainGate, but the underlying electrode technology has itself advanced little in the last three decades. Here, advanced dual-side lithographic microfabrication processes is exploited to demonstrate a 1024-channel penetrating silicon microneedle array (SiMNA) that is scalable in its recording capabilities and cortical coverage and is suitable for clinical translation. The SiMNA is the first penetrating microneedle array with a flexible backing that affords compliancy to brain movements. In addition, the SiMNA is optically transparent permitting simultaneous optical and electrophysiological interrogation of neuronal activity. The SiMNA is used to demonstrate reliable recordings of spontaneous and evoked field potentials and of single unit activity in chronically implanted mice for up to 196 days in response to optogenetic and to whisker air-puff stimuli. Significantly, the 1024-channel SiMNA establishes detailed spatiotemporal mapping of broadband brain activity in rats. This novel scalable and biocompatible SiMNA with its multimodal capability and sensitivity to broadband brain activity will accelerate the progress in fundamental neurophysiological investigations and establishes a new milestone for penetrating and large area coverage microelectrode arrays for brain–machine interfaces.

Over the past decade, stereotactically placed electrodes have become the gold standard for deep brain recording and stimulation for a wide variety of neurological and psychiatric diseases. Current electrodes, however, are limited in their spatial resolution and ability to record from small populations of neurons, let alone individual neurons. Here, we report on a novel, reconfigurable, monolithically integrated human-grade flexible depth electrode capable of recording from up to 128 channels and able to record at a depth of 10 cm in brain tissue. This thin, stylet-guided depth electrode is capable of recording local field potentials and single unit neuronal activity (action potentials), validated across species. This device represents a major new advance in manufacturing and design approaches which extends the capabilities of a mainstay technology in clinical neurology.One-Sentence SummaryA human-grade thin-film depth electrode offers new opportunities in spatial and temporal resolution for recording brain activity.

This paper describes the fabrication process and characteristics of dimension- and position-controlled gallium nitride (GaN) microstructure arrays grown on graphene films and their quantum structures for use in flexible light-emitting device applications. The characteristics of dimension- and position-controlled growth, which is crucial to fabricate high-performance electronic and optoelectronic devices, were investigated using scanning and transmission electron microscopes and power-dependent photoluminescence spectroscopy measurements. Among the GaN microstructures, GaN microrods exhibited excellent photoluminescence characteristics including room-temperature stimulated emission, which is especially useful for optoelectronic device applications. As one of the device applications of the position-controlled GaN microrod arrays, we fabricated light-emitting diodes (LEDs) by heteroepitaxially growing InxGa1−xN/GaN multiple quantum wells (MQWs) and a p-type GaN layer on the surfaces of GaN microrods and by depositing Ti/Au and Ni/Au metal layers to prepare n-type and p-type ohmic contacts, respectively. Furthermore, the GaN microrod LED arrays were transferred onto Cu foil by using the chemical lift-off method. Even after being transferred onto the flexible Cu foil substrate, the microrod LEDs exhibited strong emission of visible blue light. The proposed method to enable the dimension- and position-controlled growth of GaN microstructures on graphene films can likely be used to fabricate other high-quality flexible inorganic semiconductor devices such as micro-LED displays with an ultrahigh resolution.

Intraoperative neuromonitoring (IONM) is a widely used practice in spine surgery for early detection and minimization of neurological injury. IONM is most commonly conducted by indirectly recording motor and somatosensory evoked potentials from either muscles or the scalp, which requires large-amplitude electrical stimulation and provides limited spatiotemporal information. IONM may inform of inadvertent events during neurosurgery after they occur, but it does not guide safe surgical procedures when the anatomy of the diseased spinal cord is distorted. To overcome these limitations and to increase our understanding of human spinal cord neurophysiology, we applied a microelectrode array with hundreds of channels to the exposed spinal cord during surgery and resolved spatiotemporal dynamics with high definition. We used this method to construct two-dimensional maps of responsive channels and define with submillimeter precision the electrophysiological midline of the spinal cord. The high sensitivity of our microelectrode array allowed us to record both epidural and subdural responses at stimulation currents that are well below those used clinically and to resolve postoperative evoked potentials when IONM could not. Together, these advances highlight the potential of our microelectrode arrays to capture previously unexplored spinal cord neural activity and its spatiotemporal dynamics at high resolution, offering better electrophysiological markers that can transform IONM.

We report the fabrication of individually addressable, high-density, vertical zinc oxide (ZnO) nanotube pressure sensor arrays. High-sensitivity and flexible piezoelectric sensors were fabricated using dimension- and position-controlled, vertical, and free-standing ZnO nanotubes on a graphene substrate. Significant pressure/force responses were achieved from small devices composed of only single, 3 × 3, 5 × 5, and 250 × 250 ZnO nanotube arrays on graphene. An individually addressable pixel matrix was fabricated by arranging the top and bottom electrodes of the sensors in a crossbar configuration. We investigated the uniformity and robustness of pressure/force spatial mapping by considering the pixel size, the number of ZnO nanotubes in each pixel, and the lateral dimensions of individual ZnO nanotubes. A spatial resolution as high as 1058 dpi was achieved for a Schottky diode-based force/pressure sensor composed of ZnO nanotubes on a flexible substrate. Additionally, we confirmed the excellent flexibility and electrical robustness of the free-standing sensor arrays for high-resolution tactile imaging. We believe that this work opens important opportunities for 1D piezoelectric pressure/force sensor arrays with enormous applications in human-electronics interfaces, smart skin, and micro- and nanoelectromechanical systems.

Electrophysiological devices are critical for mapping eloquent and diseased brain regions and for therapeutic neuromodulation in clinical settings and are extensively used for research in brain-machine interfaces. However, the existing clinical and experimental devices are often limited in either spatial resolution or cortical coverage. Here, we developed scalable manufacturing processes with a dense electrical connection scheme to achieve reconfigurable thin-film, multithousand-channel neurophysiological recording grids using platinum nanorods (PtNRGrids). With PtNRGrids, we have achieved a multithousand-channel array of small (30 μm) contacts with low impedance, providing high spatial and temporal resolution over a large cortical area. We demonstrated that PtNRGrids can resolve submillimeter functional organization of the barrel cortex in anesthetized rats that captured the tissue structure. In the clinical setting, PtNRGrids resolved fine, complex temporal dynamics from the cortical surface in an awake human patient performing grasping tasks. In addition, the PtNRGrids identified the spatial spread and dynamics of epileptic discharges in a patient undergoing epilepsy surgery at 1-mm spatial resolution, including activity induced by direct electrical stimulation. Collectively, these findings demonstrated the power of the PtNRGrids to transform clinical mapping and research with brain-machine interfaces.

Nanoscale interfaces with biological tissue, principally made with nanowires (NWs), are envisioned as minimally destructive to the tissue and as scalable tools to directly transduce the electrochemical activity of a neuron at its finest resolution. This review lays the foundations for understanding the material and device considerations required to interrogate neuronal activity at the nanoscale. We first discuss the electrochemical nanoelectrode-neuron interfaces and then present new results concerning the electrochemical impedance and charge injection capacities of millimeter, micrometer, and nanometer scale wires with Pt, PEDOT:PSS, Si, Ti, ITO, IrO(x), Ag, and AgCl materials. Using established circuit models for NW-neuron interfaces, we discuss the impact of having multiple NWs interfacing with a single neuron on the amplitude and temporal characteristics of the recorded potentials. We review state of the art advances in nanoelectrode-neuron interfaces, the standard control experiments to investigate their electrophysiological behavior, and present recent high fidelity recordings of intracellular potentials obtained with ultrasharp NWs developed in our laboratory that naturally permeate neuronal cell bodies. Recordings from arrays and individually addressable electrically shorted NWs are presented, and the long-term stability of intracellular recording is discussed and put in the context of established techniques. Finally, a perspective on future research directions and applications is presented.

The enhanced electrochemical activity of nanostructured materials is readily exploited in energy devices, but their utility in scalable and human-compatible implantable neural interfaces can significantly advance the performance of clinical and research electrodes. We utilize low-temperature selective dealloying to develop scalable and biocompatible one-dimensional platinum nanorod (PtNR) arrays that exhibit superb electrochemical properties at various length scales, stability, and biocompatibility for high performance neurotechnologies. PtNR arrays record brain activity with cellular resolution from the cortical surfaces in birds and nonhuman primates. Significantly, strong modulation of surface recorded single unit activity by auditory stimuli is demonstrated in European Starling birds as well as the modulation of local field potentials in the visual cortex by light stimuli in a nonhuman primate and responses to electrical stimulation in mice. PtNRs record behaviorally and physiologically relevant neuronal dynamics from the surface of the brain with high spatiotemporal resolution, which paves the way for less invasive brain−machine interfaces.

We report on the selective-area growth of GaN microstructures on tungsten (W)-metal electrodes using a micro-patterned graphene intermediate layer between GaN and W, and demonstrate their use as light-emitting diodes (LEDs). Prior to the GaN growths, the cm-scale graphene layer was transferred on W and then further patterned into a regular array of few-μm-sized graphene microdots using conventional lithography. The graphene microdots served as a seed layer for selectively growing crack-free GaN microstructures with regular diameter and spacing. Each microstructure displayed a microdisk morphology, exhibiting a single crystalline phase from epitaxial lateral overgrowth (ELOG). We observed ohmic behavior between the as-grown GaN microdisks and underlying W film, facilitating the fabrication of LED microarrays. Using the underlying W layer as an ohmic contact, we fabricated p-n junction GaN microdisk LEDs, consisting of three periods of InxGa1–xN/GaN multiple quantum wells. Uniform electroluminescence was observed across the microdisks. These results open up novel strategies for streamlined fabrication of high-performance and high-resolution LEDs.

A facile and novel technique for the fabrication of pressure sensors is reported based on the hybridization of one-dimensional nanomaterials and two-dimensional graphene film. In particular, piezoelectric pressure sensors are fabricated by using vertically aligned and position- and dimension-controlled ZnO nanotube arrays grown on graphene layers. Graphene layers act not only as substrates for catalyst-free growth of high-quality ZnO nanotubes but also as flexible conduction channels connecting ZnO nanotubes and metal electrodes. Freestanding and flexible sensors have been efficiently obtained via mechanical lift-off of hybrid ZnO nanotube/graphene film structures and by exploiting the weak van der Waals forces existing between the graphene film and the original substrates. A prototype of such devices shows a high pressure sensitivity (−4.4 kPa−1), which would enable the detection of weak flows of inert gas. The relatively low wall thickness and large length of the ZnO nanotubes suggest a relatively high sensitivity to external pressures. The obtained nanotube sensors are attached to the philtrum and wrist of a volunteer and used to monitor his breath and heart rate. Overall, the prototype hybrid sensing device has great potential as wearable technology, especially in the sector of advanced healthcare devices. A novel technique is demonstrated for the fabrication of flexible and highly sensitive 1D piezoelectric pressure sensors containing ZnO nanotube arrays grown on 2D graphene layers. Due to the morphology-controlled tunable sensitivity, ultra-small size, and capability of detecting extremely low pressures, the sensors are able to efficiently detect human breath and pulse.

We report monolithic integration of indium arsenide (InAs) nanorods and zinc oxide (ZnO) nanotubes using a multilayer graphene film as a suspended substrate, and the fabrication of dual-wavelength photodetectors with the hybrid configuration of these materials. For the hybrid nanostructures, ZnO nanotubes and InAs nanorods were grown vertically on the top and bottom surfaces of the graphene films by metal-organic vapor-phase epitaxy and molecular beam epitaxy, respectively. The structural, optical, and electrical characteristics of the hybrid nanostructures were investigated using transmission electron microscopy, spectral photoresponse analysis, and current–voltage measurements. Furthermore, the hybrid nanostructures were used to fabricate dual-wavelength photodetectors sensitive to both ultraviolet and mid-infrared wavelengths. Wide- and narrow-bandgap semiconductor nanostructures were monolithically integrated on graphene layers by direct heteroepitaxial growth. The structural, optical, and electrical characteristics of the hybrid nanostructures were investigated. Furthermore, dual-wavelength photodetectors sensitive to both ultraviolet and mid-infrared wavelengths were fabricated using the hybrid nanostructures.

This paper reports on the controlled fabrication of a highly sensitive piezoresistive sensor by using Si nanorod (NR) arrays. An efficient, large-area, scalable strategy was adopted to fabricate the pressure sensors by incorporating chemically etched, high-aspect-ratio, vertical Si NR arrays between two thin Au layers. The piezoresistive properties corresponding to dimension- and position-controlled and randomly etched, closely packed, and thin Si NR arrays were exploited to fabricate the small, portable, and device-compatible pressure sensors. The Si-NR-based piezoresistive sensors exhibited a high sensitivity of 0.49 MPa−1, thereby demonstrating its superiority over other unconventional piezoresistive nanomaterials such as Si with different configurations of nanostructures. Furthermore, the sensors exhibited a large variation (~45%) in the current at a constant bias voltage of 2 V under a weak applied pressure corresponding to an inert gas flow of 5 sccm. The excellent pressure sensing performance of the piezoresistive Si NRs enabled the efficient detection of changes corresponding to the human breathing pattern. In particular, the key advantages of such pressure sensors is the simple, inexpensive, and scalable fabrication process; high sensitivity with ultra-low-pressure detection; and excellent ambient stability (>several months) with a high durability pertaining to more than 1,000 cycles of pressure loading/unloading. Furthermore, we demonstrated the ability of the pressure sensor to act as a portable human breath sensor to monitor respiratory parameters in a noninvasive and personalized manner. The results can provide direction for the realization of next-generation breath-sensing gadgets and other leading-edge applications in the domain of electronic and healthcare devices.

Despite ongoing advances in our understanding of local single-cellular and network-level activity of neuronal populations in the human brain, extraordinarily little is known about their “intermediate” microscale local circuit dynamics. Here, we utilized ultra-high-density microelectrode arrays and a rare opportunity to perform intracranial recordings across multiple cortical areas in human participants to discover three distinct classes of cortical activity that are not locked to ongoing natural brain rhythmic activity. The first included fast waveforms similar to extracellular single-unit activity. The other two types were discrete events with slower waveform dynamics and were found preferentially in upper cortical layers. These second and third types were also observed in rodents, nonhuman primates, and semi-chronic recordings from humans via laminar and Utah array microelectrodes. The rates of all three events were selectively modulated by auditory and electrical stimuli, pharmacological manipulation, and cold saline application and had small causal co-occurrences. These results suggest that the proper combination of high-resolution microelectrodes and analytic techniques can capture neuronal dynamics that lay between somatic action potentials and aggregate population activity. Understanding intermediate microscale dynamics in relation to single-cell and network dynamics may reveal important details about activity in the full cortical circuit.

The mechanism of surfaces/interfaces and precise control of growth morphology is a key parameter for any specific device application. Herein, we report on a qualitative growth study of molecular beam epitaxy-grown polycrystalline InAs thin films on a lattice-mismatched Si(1 0 0) substrate using atomic force microscopy. The height-height correlation function (HHCF) and power spectral density function (PSDF) were employed to analyze the surface structures. Clear oscillatory behavior in the HHCF for sufficiently larger lateral distances suggests a mound-like morphology, which was confirmed by the existence of a characteristic frequency peak in the PSDF. The growth mechanism is described qualitatively by the Schwoebel barrier (roughening) effect coupled with the Mullins diffusion model (smoothing effect).

Intracellular access with high spatiotemporal resolution can enhance our understanding of how neurons or cardiomyocytes regulate and orchestrate network activity, and how this activity can be affected with pharmacology or other interventional modalities. Nanoscale devices often employ electroporation to transiently permeate the cell membrane and record intracellular potentials, which tend to decrease rapidly to extracellular potential amplitudes with time. Here, we report innovative scalable, vertical, ultra-sharp nanowire arrays that are individually addressable to enable long-term, native recordings of intracellular potentials. We report large action potential amplitudes that are indicative of intracellular access from 3D tissue-like networks of neurons and cardiomyocytes across recording days and that do not decrease to extracellular amplitudes for the duration of the recording of several minutes. Our findings are validated with cross-sectional microscopy, pharmacology, and electrical interventions. Our experiments and simulations demonstrate that individual electrical addressability of nanowires is necessary for high-fidelity intracellular electrophysiological recordings. This study advances our understanding of and control over high-quality multi-channel intracellular recordings, and paves the way toward predictive, high-throughput, and low-cost electrophysiological drug screening platforms., Comment: Main manuscript: 33 pages, 4 figures, Supporting information: 43 pages, 27 figures, Submitted to Advanced Materials

We report on the fabrication and characteristics of an individually addressable gallium nitride (GaN) microdisk light-emitting diode (LED) array in free-standing and ultrathin form. A high-quality GaN microdisk array with n-GaN, InGaN/GaN quantum wells and p-GaN layers was epitaxially grown on graphene microdots patterned on SiO2/Si substrates. Due to the weak attachment of the graphene microdots to the growth substrate, a microdisk array coated with a polyimide layer was easily separated from the substrate using mechanical or chemical methods to form an ultrathin free-standing film. Individually addressable microdisk LEDs were created by forming thin metal contacts on the p-GaN and n-GaN surfaces in a crossbar configuration. Each microdisk LED that comprised an ultrahigh resolution array of 2500 pixels per inch was found to be uniquely addressable. The devices in free-standing form exhibited stable electrical and optoelectronic characteristics under extreme bending conditions and continuous operation mode despite the absence of a heat dissipating substrate. These results present promising approaches for the fabrication of high-quality inorganic semiconductor devices for ultrahigh resolution and high-performance flexible applications. A free-standing, ultrathin and flexible film of micrometer-scale semiconductor light sources has been created by scientists in South Korea. Light-emitting diodes are rapidly replacing less efficient alternatives in both lighting and displays but the traditional inorganic semiconductor materials they are made of are rigid, which means it is difficult to adapt them to applications requiring curved or flexible devices. Gyu-Chul Yi from the Seoul National University and co-workers developed uniquely addressable gallium nitride microdisk light-emitting diode arrays embedded in free-standing and ultrathin polyimide layers. Gallium nitride microdisk arrays they grew on graphene microdots were easily released from the substrate due to the weak bond between the graphene and the underlying substrate. This approach could be used to create ultraflexible and high-resolution light-sources for wearable, medical or implantable device applications. We report on the fabrication and characteristics of an individually addressable GaN microdisk LED array in free-standing and ultrathin form. GaN microdisk array was grown on graphene microdots, and the microstructures coated with a polyimide layer were easily separated from the substrate to form an ultrathin free-standing film. Crossbar configuration of metal leads enabled each microdisk LED in an array to be uniquely addressable. The devices in free-standing form exhibited stable electrical and optoelectronic characteristics under extreme bending conditions and continuous operation mode despite the absence of a heat dissipating substrate.

We report on the fabrication of individually addressable, high-density vertical zinc oxide (ZnO) nanotube Schottky diode arrays. The individually addressable nanotube Schottky diode arrays were fabricated by arranging the top and bottom electrodes in a crossbar configuration on a free-standing layer consisting of position-controlled ZnO nanotubes on graphene films. The electrical characteristics of each Schottky diode in the arrays were investigated by measuring current–voltage characteristics. We also investigated the variation in device characteristics within an array by spatially mapping the barrier height of individual devices. Additionally, we further confirmed the excellent flexibility and electrical robustness of the free-standing and thin Schottky diode arrays under extreme bending conditions and over multiple cycles. Moreover, the photoresponses of the nanotube Schottky diode arrays were investigated by measuring their spectral responses and current–voltage characteristics under light illuminations, yielding a maximum photocurrent to dark current ratio of 1400 and responsivity of 106 A/W. We believe that this work provides a general and rational route for developing many other two-terminal one-dimensional nanostructure device arrays for ultra-high density electronic and optoelectronic devices.

Scaling study of polycrystalline InAs thin films grown by molecular beam epitaxy on c-plane sapphire (001) substrate has been presented using atomic force microscopy, corresponding height-height correlation function and two-dimensional fast Fourier transform profiles. Height-height correlation function profiles exhibit oscillations at larger lateral distances along with ring-like features in the respective two-dimensional fast Fourier transform scans and clearly indicate mound-like surface morphology. Scaling parameters, interface width and lateral correlation length were extracted and their dependence on the growth temperatures were investigated. The growth mechanism has been well explained in terms of diffusion effects and the obtained parameters qualitatively agrees with the parameters obtained for the growth of InAs films grown on Si(100) substrate under identical growth conditions. Our study suggests that molecular beam epitaxy grown InAs thin films on lattice mismatched substrates (c-plane sapphire(001) and Si(100) substrate) follows the same growth behavior under identical growth conditions.

A novel characterization technique using both in situ reflection high-energy electron diffraction (RHEED) transmission mode and transmission electron microscopy (TEM) has been developed to investigate the growth behaviour of semiconductor nanostructures. RHEED employed in transmission mode enables the acquisition of structural information during the growth of nanostructures such as nanorods. Such real-time observation allows the investigation of growth mechanisms of various nanomaterials that is not possible with conventional ex situ analytical methods. Additionally, real-time monitoring by RHEED transmission mode offers a complete range of information when coupled with TEM, providing structural and chemical information with excellent spatial resolution, leading to a better understanding of the growth behaviour of nanomaterials. Here, as a representative study using the combined technique, the nucleation and crystallization of InAs nanorods and the epitaxial growth of InxGa1−xAs(GaAs) shell layers on InAs nanorods are explored. The structural changes in the InAs nanorods at the early growth stage caused by the transition of the local growth conditions and the strain relaxation processes that occur during epitaxial coating of the shell layers are shown. This technique advances our understanding of the growth behaviour of various nanomaterials, which allows the realization of nanostructures with novel properties and their application in future electronics and optoelectronics.

We report flexible resistive random access memory (ReRAM) arrays fabricated by using NiO

Microdonut-shaped GaN/Inx Ga1-x N light-emitting diode (LED) microarrays are fabricated for variable-color emitters. The figure shows clearly donut-shaped light emission from all the individual microdonut LEDs. Furthermore, microdonut LEDs exhibit spatially-resolved blue and green EL colors, which can be tuned by either controlling the external bias voltage or changing the size of the microdonut LED.

We report the growth of InAs and ZnO nanorods vertically on each surface of graphene layers and our investigation of the electrical characteristics.

ReRAM is a compelling candidate for next-generation non-volatile memory owing to its various advantages. However, fluctuation of operation parameters are critical weakness occurring failures in ‘reading’ and ‘writing’ operations. To enhance the stability, it is important to understand the mechanism of the devices. Although numerous studies have been conducted using AFM or TEM, the understanding of the device operation is still limited due to the destructive nature and/or limited imaging range of the previous methods. Here, we propose a new hybrid device composed of ReRAM and LED enabling us to monitor the conducting filament (CF) configuration on the device scale during resistive switching. We directly observe the change in CF configuration across the whole device area through light emission from our hybrid device. In contrast to former studies, we found that minor CFs were formed earlier than major CF contributing to the resistive switching. Moreover, we investigated the substitution of a stressed major CF with a fresh minor CF when large fluctuation of operation voltage appeared after more than 50 times of resistive switching in atmospheric condition. Our results present an advancement in the understanding of ReRAM operation mechanism, and a step toward stabilizing the fluctuations in ReRAM switching parameters.

The epitaxial lateral overgrowth (ELOG) of GaN microdisks on graphene microdots and the fabrication of flexible light-emitting diodes (LEDs) using these microdisks is reported. An ELOG technique with only patterned graphene microdots is used, without any growth mask. The discrete micro-LED arrays are transferred onto Cu foil by a simple lift-off technique, which works reliably under various bending conditions.

We report the fabrication and characteristics of vertical microtube light-emitting diode (LED) arrays with a metal core inside the devices. To make the LEDs, gallium nitride (GaN)/indium gallium nitride (In(x)Ga(1-x)N)/zinc oxide (ZnO) coaxial microtube LED arrays were grown on an n-GaN/c-aluminum oxide (Al2O3) substrate. The microtube LED arrays were then lifted-off the substrate by wet chemical etching of the sacrificial ZnO microtubes and the silicon dioxide (SiO2) layer. The chemically lifted-off LED layer was then transferred upside-down on other supporting substrates. To create the metal cores, titanium/gold and indium tin oxide were deposited on the inner shells of the microtubes, forming n-type electrodes inside the metal-cored LEDs. The characteristics of the resulting devices were determined by measuring electroluminescence and current-voltage characteristic curves. To gain insights into the current-spreading characteristics of the devices and understand how to make them more efficient, we modeled them computationally.

The bottom-up integration of a 1D-2D hybrid semiconductor nanostructure into a vertical field-effect transistor (VFET) for use in flexible inorganic electronics is reported. Zinc oxide (ZnO) nanotubes on graphene film is used as an example. The VFET is fabricated by growing position- and dimension-controlled single crystal ZnO nanotubes vertically on a large graphene film. The graphene film, which acts as the substrate, provides a bottom electrical contact to the nanotubes. Due to the high quality of the single crystal ZnO nanotubes and the unique 1D device structure, the fabricated VFET exhibits excellent electrical characteristics. For example, it has a small subthreshold swing of 110 mV dec-1 , a high Imax /Imin ratio of 106 , and a transconductance of 170 nS µm-1 . The electrical characteristics of the nanotube VFETs are validated using 3D transport simulations. Furthermore, the nanotube VFETs fabricated on graphene films can be easily transferred onto flexible plastic substrates. The resulting components are reliable, exhibit high performance, and do not degrade significantly during testing.

We report flexible resistive random access memory (ReRAM) arrays fabricated by using NiO x /GaN microdisk arrays on graphene films. The ReRAM device was created from discrete GaN microdisk arrays grown on graphene films produced by chemical vapor deposition, followed by deposition of NiO x thin layers and Au metal contacts. The microdisk ReRAM arrays were transferred to flexible plastic substrates by a simple lift-off technique. The electrical and memory characteristics of the ReRAM devices were investigated under bending conditions. Resistive switching characteristics, including cumulative probability, endurance, and retention, were measured. After 1000 bending repetitions, no significant change in the device characteristics was observed. The flexible ReRAM devices, constructed by using only inorganic materials, operated reliably at temperatures as high as 180 °C.

We report on scalable and position-controlled optical nanoprobe arrays using ZnO nanotube waveguides on graphene films for use in local optical excitation. For the waveguide fabrication, position-controlled and well-ordered ZnO nanotube arrays were grown on chemical vapor deposited graphene films with a submicron patterned mask layer and Au prepared between the interspace of nanotubes. Mammalian cells were cultured on the nanotube waveguide arrays and were locally excited by light illuminated through the nanotubes. Fluorescence and optogenetic signals could be excited through the optical nanoprobes. This method offers the ability to investigate cellular behavior with a high spatial resolution that surpasses the current limitation.

We report the growth and transfer of centimeter-sized, epitaxial hexagonal boron nitride (h-BN) few-layer films using Ni(111) single-crystal substrates. The h-BN films were heteroepitaxially grown on 10 × 10 mm2 or 20 × 20 mm2 Ni(111) substrates using atmospheric pressure chemical vapor deposition with a single ammonia-borane precursor. The grown films were transferred to arbitrary substrates via an electrochemical delamination technique, and the remaining Ni(111) substrates were repeatedly re-used. A careful analysis of the growth parameters revealed that the crystallinity and area coverage of the h-BN films were mostly sensitive to the sublimation temperature of the ammonia-borane source. Moreover, various physical characterizations confirmed that the grown films exhibited the typical characteristics of hexagonal boron nitride layers over the entire area. Furthermore, the heteroepitaxial relationship between h-BN and Ni(111) and the overall crystallinity of the film were thoroughly investigated using a synchrotron radiation X-ray diffraction analysis including θ–2θ scans, grazing incident diffraction, and reciprocal space mapping. The crystallinity at the microscopic scale was further investigated using transmission electron microscopy (TEM)-based techniques, including selective area electron diffraction pattern mapping, electron back-scattered diffraction, and high-resolution TEM. Transparent and flexible ceramic films for electronic devices can now be made at centimetre scales by a low-cost technique. Hexagonal boron nitride (h-BN) crystals are attractive platforms for building next-generation circuits because their flat, graphite-like surfaces resist mechanical and chemical damage while offering excellent signal isolation. Gyu-Chul Yi from Seoul National University and colleagues have developed a way to improve large-scale manufacturing of these films using a reusable nickel template. Nickel has a surface structure that helps h-BN films grow with high crystallinity following chemical vapor deposition, but detaching the resulting ceramic layer has always proved troublesome. The Korean team solved this problem through an electrochemical delamination process that seamlessly separates h-BN films with dimensions up to 20 × 20 square millimetres using a stream of hydrogen bubbles. Centimeter-sized, epitaxial hexagonal boron nitride (h-BN) few-layer films were heteroepitaxially grown on Ni(111) single-crystal substrates using atmospheric pressure chemical vapor deposition with ammonia-borane single precursor. The grown films were transferred to arbitrary substrates via an electrochemical delamination technique, and the remaining Ni(111) substrates were repeatedly re-used. Over repeated growth and transfer after the initial annealing, no significant degradation of Ni(111) substrates was observed and the crystallinity of h-BN layers was reproduced reliably. The grown h-BN films showed typical physical characteristics of h-BN, with a high uniformity over a wide area. The large-area synthesis and transfer of atomically thin uniform epitaxial h-BN layers can be applied in various fields where high quality two-dimensional insulating layers are required.

Skyrmions, once a hypothesized field-theoretical object believed to describe the nature of elementary particles, became common sightings in recent years among several non-centrosymmetric metallic ferromagnets. For more practical applications of Skyrmionic matter as carriers of information, thus realizing the prospect of "Skyrmionics", it is necessary to have the means to create and manipulate Skyrmions individually. We show through extensive simulation of the Landau-Lifshitz-Gilbert equation that a circulating current imparted to the metallic chiral ferromagnetic system can create isolated Skyrmionic spin texture without the aid of external magnetic field., 8 pages, 5 figures, submitted to PRB

One dimensional (1-D) semiconductor nanostructures are emerging as new candidates for the future electronic and optoelectronic devices due to their excellent physical properties, e.g. long lifetime and high efficiency, and various functionalities, e.g. flexibility and transparency [1,2]. Molecular beam epitaxy (MBE) provide accurate control over the various growth parameters, thereby allowing the growth of high-quality 1-D nanostructures with very clean and sharp interfaces [3]. Here, we grow InxGa1-xAs/InAs coaxial nanorod on graphene layers using catalyst-free MBE and investigate the chemical composition and crystal structure of nanorods using transmission electron microscope. Cross-sectional analysis shows that InxGa1-xAs coating layers have uniform thickness and cover the entire surface of the InAs core nanorods without any interfacial layers.

We report the catalyst-free growth of InAs/InxGa1−xAs coaxial nanorod heterostructures on large-area graphene layers using molecular beam epitaxy and our investigation of the chemical composition and crystal structure of these heterostructures using electron microscopy. The graphene layers used as the substrate were prepared by chemical vapor deposition and transferred onto SiO2/Si substrates. InAs nanorods and their heterostructures were grown vertically on the graphene layers; electron microscopy images revealed uniform distributions for their diameter, length and density. Cross-sectional electron microscopy images showed that InxGa1−xAs layers, having uniform composition, coated heteroepitaxially the entire surface of the InAs nanorods, without interfacial layers or structural defects. The catalyst-free growth mechanism of InAs nanorods on graphene was investigated using in situ reflection high-energy electron diffraction. Nanorods with InAs cores and InxGa1−xAs shells have been grown catalyst-free on graphene using molecular beam epitaxy by scientists in Korea. Inorganic semiconductor nanorods on graphene layers are promising for producing electronic and optoelectronic devices that are simultaneously flexible and have long lifetimes and high efficiencies. Now, researchers at Seoul National University led by Gyu-Chul Yi have realized catalyst-free molecular beam epitaxy growth of InAs/InxGa1−xAs coaxial nanorods on graphene. This is an important step as catalyst-free molecular beam epitaxy can produce ultrapure single crystals and multilayered heterostructures with precisely-controlled thickness and chemical compositions. Analysis showed that the cores and shells were clearly defined with clean interfaces and had uniform compositions and thicknesses. This demonstration opens up the path for monolithic integration of semiconductor coaxial nanorod heterostructures on two-dimensional materials. We report the catalyst-free growth of InAs/InxGa1−xAs coaxial nanorod heterostructures on large-area graphene layers using molecular beam epitaxy and our investigation of the chemical composition and crystal structure of these heterostructures using electron microscopy. Cross-sectional electron microscopy images showed that InxGa1−xAs layers, having uniform composition, coated heteroepitaxially the entire surface of the InAs nanorods, without interfacial layers or structural defects. The catalyst-free growth mechanism of InAs nanorods on graphene was investigated using in situ reflection high-energy electron diffraction.

We report the growth of high-quality, smooth, and flat ZnO thin films on graphene layers and their photoluminescence (PL) characteristics. For the growth of high-quality ZnO thin films on graphene layers, ZnO nanowalls were grown using metal-organic vapor-phase epitaxy on oxygen-plasma treated graphene layers as an intermediate layer. PL measurements were conducted at low temperatures to examine strong near-band-edge emission peaks. The full-width-at-half-maximum value of the dominant PL emission peak was as narrow as 4 meV at T = 11 K, comparable to that of the best-quality films reported previously. Furthermore, the stimulated emission of ZnO thin films on the graphene layers was observed at the low excitation energy of 180 kW/cm2 at room temperature. Their structural and optical characteristics were investigated using X-ray diffraction, transmission electron microscopy, and PL spectroscopy.

We report the growth of GaN micro-rods and coaxial quantum-well heterostructures on graphene films, together with structural and optical characterization, for applications in flexible optical devices. Graphene films were grown on Cu foil by means of chemical vapor deposition, and used as the substrates for the growth of the GaN micro-rods, which were subsequently transferred onto SiO2/Si substrates. Highly Si-doped, n-type GaN micro-rods were grown on the graphene films using metal–organic chemical vapor deposition. The growth and vertical alignment of the GaN micro-rods, which is a critical factor for the fabrication of high-performance light-emitting diodes (LEDs), were characterized using electron microscopy and X-ray diffraction. The GaN micro-rods exhibited promising photoluminescence characteristics for optoelectronic device applications, including room-temperature stimulated emission. To fabricate flexible LEDs, InxGa1–xN/GaN multiple quantum wells and a p-type GaN layer were deposited coaxially on the GaN micro-rods, and transferred onto Ag-coated polymer substrates using lift-off. Ti/Au and Ni/Au metal layers were formed to provide electrical contacts to the n-type and p-type GaN regions, respectively. The micro-rod LEDs exhibited intense emission of visible light, even after transfer onto the flexible polymer substrate, and reliable operation was achieved following numerous cycles of mechanical deformation.