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Semiconductor nanostructures or thin films are vital components of modern optoelectronic devices, such as light-emitting diodes, sensors, or transistors. While single crystalline wafers are used as heteroepitaxial templates for them, increasing demands on flexibility or transferability require separation of the grown semiconductor structures on such substrates, which is technically challenging and expensive. Recent research suggests that large-scale 2D nanomaterials can serve as heteroepitaxial templates and provide additional functionalities such as transferability to foreign substrates or mechanical flexibility. In this paper, growth, structural properties, and optoelectronic device applications of semiconductor nanostructures or thin films which are heteroepitaxially grown on large-scale 2D nanomaterials are reviewed.

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.

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.

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.

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 flexible and monolithically integrated multicolor light-emitting diode (LED) arrays using morphology-controlled growth of GaN microstructures on chemical-vapor-deposited (CVD) graphene films. As the morphology-controlled growth template of GaN microstructures, we used position-controlled ZnO nanostructure arrays with different spacings grown on graphene substrates. In particular, we investigated the effect of the growth parameters, including micropattern spacings and growth time and temperature, on the morphology of the GaN microstructures when they were coated on ZnO nanostructures on graphene substrates. By optimizing the growth parameters, both GaN microrods and micropyramids formed simultaneously on the graphene substrates. Subsequent depositions of InGaN/GaN quantum well and p-GaN layers and n- and p-type metallization yielded monolithic integration of microstructural LED arrays on the same substrate, which enabled multicolor emission depending on the shape of the microstructures. Furthermore, the CVD graphene substrates beneath the microstructure LEDs facilitated transfer of the LED arrays onto any foreign substrate. In this study, Cu foil was used for flexible LEDs. The flexible devices exhibited stable electroluminescence, even under severe bending conditions. Cyclic bending tests demonstrated the excellent mechanical stability and reliability of the 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.

We report the experimental realization of piezoelectric ZnO dual-gate thin film transistors (TFTs) as highly sensitive force sensors and discuss the physical origins of its electrically tunable piezoelectric response using a simple analytical model. A dual gate TFT is fabricated on a polyimide substrate using radio-frequency (RF) magnetron sputtering of piezoelectric ZnO thin film as a channel. The ZnO TFTs exhibited a field effect mobility of ~ 5 cm(2)/Vs, I(max) to I(min) ratio of 10(7), and a subthreshold slope of 700 mV/dec. Notably, the TFT exhibited static and transient current changes under external force stimuli, with varying amplitude and polarity for different gate bias regimes. To understand the current modulation of the dual-gate TFT with independently biased top and bottom gates, an analytical model is developed. The model includes accumulation channels at both surfaces and a bulk channel within the film and accounts for the force-induced piezoelectric charge density. The microscopic piezoelectric response that modulates the energy-band edges and correspondent current-voltage characteristics are accurately portrayed by our model. Finally, the field-tunable force response in single TFT is demonstrated as a function of independent bias for the top and bottom gates with a force response range from −0.29 nA/mN to 22.7 nA/mN. This work utilizes intuitive analytical models to shed light on the correlation between the material properties with the force response in piezoelectric TFTs.

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.

In this paper, we describe a new method for the selective extraction and quantification of glutathione (GSH) using matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS) and maleimide-presenting gold nanoparticles (Mal-AuNPs). Our strategy utilizes the Michael addition to selectively extract GSH, from chosen samples, onto the maleimide of Mal-AuNPs. After the extraction step, the GSH bound to the AuNPs was analyzed by MALDI-TOF MS in the presence of an internal standard which was prepared by reacting Mal-AuNPs with isotope-labeled GSH (GSH*). The GSH* has the same structure as GSH but a higher molecular weight, and therefore, enables absolute quantification of GSH by comparing the mass signal intensities of the GSH- and GSH*-conjugated alkanethiols. Our strategy was verified by analyzing GSHspiked fetal bovine serum and NIH 3T3 cells.

E-mail: wsyeo@konkuk.ac.krReceived May 4, 2012, Accepted June 11, 2012Key Words : Gold shells, MALDI-TOF MS, Matrix-free, ThiolsThiol-containing (bio)molecules, such as cysteine (Cys),homocysteine (Hcy), and glutathione (GSH), are recognizedto be physiologically important and known to be involved inmany diseases, such as leucocyte loss, psoriasis, liver damage,and cancer, depending on changes in the cellular level.

Single-crystal gallium nitride (GaN) layers were directly grown on centimeter-scale hexagonal boron nitride (h-BN). Using chemical vapor deposition (CVD), centimeter-scale h-BN films were synthesized on a single-crystal Ni(111) and readily transferred onto amorphous fused silica supporting substrates that had no epitaxial relationship with GaN. For growing fully coalescent GaN layers on h-BN, the achievement of high-density crystal growths was a critical growth step because the sp2-bonded h-BN layers are known to be free of dangling bonds. Unlike GaN layers grown on a typical heterogeneous sapphire substrate, the morphological and microstructural results strongly suggest a high-density growth feature that is driven by the atomic cliffs inherent in the CVD-grown h-BN layers. More importantly, the GaN layers grown on CVD-grown h-BN exhibited a flat and continuous surface morphology with well-aligned crystal orientations both along the c-axis and in-plane, indicating the characteristics of GaN heteroepitaxy on h-BN. A method for depositing high-quality semiconductor thin films on a two-dimensional material has been developed by researchers in Korea. Growing one defect-free material on another is difficult because of their different atomic structures, particularly when very thin samples are required. Now, Gyu-Chul Yi and co-workers from Seoul National University in Korea have grown large areas of single crystals of the semiconductor gallium nitride directly on boron nitride, which is useful for flexible electronics. They used chemical vapor deposition, which works by exposing a hot volatile gas incorporating atoms of the top material to a cooler substrate. The gallium nitride films could be readily transferred to silica substrates. Since the technique can produce large areas of the material it is a promising for making flexible devices on a commercial scale. Here, we utilize large-size scalable single-crystal 2D films to grow single crystalline inorganic semiconductors. Centimeter-scale hexagonal boron nitride (h-BN) films were synthesized on a single-crystal Ni(111) using chemical vapor deposition (CVD). Single-crystal GaN layers were directly grown on h-BN using metal–organic vapor phase epitaxy. The CVD-grown h-BN exhibited many atomic cliffs that enabled us to grow high-density GaN islands to be merged as homogeneous and flat GaN films. We also investigated the crystallinity and growth mechanism of the GaN films grown on CVD-grown h-BN using transmission electron microscopy and X-ray diffraction.

Heteroepitaxy of semiconductors on two-dimensional (2-d) atomic layered materials enables the use of flexible and transferable inorganic electronic and optoelectronic devices in various applications. Herein, we report the shape- and morphology-controlled van der Waals (vdW) epitaxy of ZnO nanostructures on hexagonal boron nitride (hBN) insulating layers for an architectured semiconductor integration on the 2-d layered materials. The vdW surface feature of the 2-d nanomaterials, because of the surface free of dangling bonds, typically results in low-density random nucleation–growth in the vdW epitaxy. The difficulty in controlling the nucleation sites was resolved by artificially formed atomic ledges prepared on hBN substrates, which promoted the preferential vdW nucleation–growth of ZnO specifically along the designed ledges. Electron microscopy revealed crystallographically domain-aligned incommensurate vdW heteroepitaxial relationships, even though ZnO/hBN is highly lattice-mismatched. First-principles theoretical calculations confirmed the weakly bound, noncovalent binding feature of the ZnO/hBN heterostructure. Electrical characterizations of the ZnO nanowall networks grown on hBN revealed the excellent electrical insulation properties of hBN substrates. An ultraviolet photoconductor device using the vdW epitaxial ZnO nanowall networks/hBN heterostructure was further demonstrated as an example of hBN substrate-based device applications. The architectured heteroepitaxy of semiconductors on hBN is thus expected to create many other device arrays that can be integrated on a piece of substrate with good electrical insulation for use in individual device operation. Scientists have demonstrated a new approach for fabricating nanoarchitectured semiconductors on dielectric hexagonal boron nitride (BN). The Korean team's approach involves forming atomic ledges on hexagonal BN substrate to realize shape- and morphology-controlled van der Waals (vdW) epitaxy of ZnO nanostructures on hexagonal BN insulating layers. The combination of hexagonal BN and vdW epitaxy seems like a perfect match – hexagonal BN has highly suitable properties for use as insulating substrates in electronic and optoelectronic devices, while vdW epitaxy is well suited for fabricating high-quality epitaxial semiconductor/hexagonal BN heterostructures. However, it has been difficult to control nucleation sites in vdW epitaxy. The researchers' method overcomes this problem through promoted vdW nucleation growth of ZnO along the artificially formed ledges. The team anticipates that the approach could be extended to many other combinations of semiconductors and layered insulating (or dielectric) nanomaterials. Architectured ZnO nanostructures were grown by van der Waals (vdW) heteroepitaxy on hexagonal BN (hBN) layers with artificially patterned atomic ledges. Electron microscopic and theoretical computational analyses presented non-covalent epitaxial features of domain-aligned incommensurate ZnO/hBN heterostructure. The vdW epitaxial ZnO/hBN heterostructures exhibited excellent electrical insulation of hBN, and were applied to fabricate the ultraviolet photodetector devices, as an example of functional optoelectronic device applications.

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.