Wafer-Scale Epitaxial Growth of an Atomically Thin Single-Crystal Insulator as a Substrate of Two-Dimensional Material Field-Effect Transistors.

As the electron mobility of two-dimensional (2D) materials is dependent on an insulating substrate, the nonuniform surface charge and morphology of silicon dioxide (SiO2) layers degrade the electron mobility of 2D materials. Here, we demonstrate that an atomically thin single-crystal insulating layer of silicon oxynitride (SiON) can be grown epitaxially on a SiC wafer at a wafer scale and find that the electron mobility of graphene field-effect transistors on the SiON layer is 1.5 times higher than that of graphene field-effect transistors on typical SiO2 films. Microscale and nanoscale void defects caused by heterostructure growth were eliminated for the wafer-scale growth of the single-crystal SiON layer. The single-crystal SiON layer can be grown on a SiC wafer with a single thermal process. This simple fabrication process, compatible with commercial semiconductor fabrication processes, makes the layer an excellent replacement for the SiO2/Si wafer.

Enhanced Performance of WS2 Field-Effect Transistor through Mono and Bilayer h-BN Tunneling Contacts.

Transition metal dichalcogenides (TMDs) are of great interest owing to their unique properties. However, TMD materials face two major challenges that limit their practical applications: contact resistance and surface contamination. Herein, a strategy to overcome these problems by inserting a monolayer of hexagonal boron nitride (h-BN) at the chromium (Cr) and tungsten disulfide (WS2 ) interface is introduced. Electrical behaviors of direct metal-semiconductor (MS) and metal-insulator-semiconductor (MIS) contacts with mono- and bilayer h-BN in a four-layer WS2 field-effect transistor (FET) are evaluated under vacuum from 77 to 300 K. The performance of the MIS contacts differs based on the metal work function when using Cr and indium (In). The contact resistance is significantly reduced by approximately ten times with MIS contacts compared with that for MS contacts. An electron mobility up to ≈115 cm2  V-1  s-1 at 300 K is achieved with the insertion of monolayer h-BN, which is approximately ten times higher than that with MS contacts. The mobility and contact resistance enhancement are attributed to Schottky barrier reduction when h-BN is introduced between Cr and WS2 . The dependence of the tunneling mechanisms on the h-BN thickness is investigated by extracting the tunneling barrier parameters.

Super-Nernstian pH Sensor Based on Anomalous Charge Transfer Doping of Defect-Engineered Graphene.

The conventional pH sensor based on the graphene ion-sensitive field-effect transistor (Gr-ISFET), which operates with an electrostatic gating at the solution-graphene interface, cannot have a pH sensitivity above the Nernst limit (∼59 mV/pH). However, for accurate detection of the pH levels of an aqueous solution, an ultrasensitive pH sensor that can exceed the theoretical limit is required. In this study, a novel Gr-ISFET-based pH sensor is fabricated using proton-permeable defect-engineered graphene. The nanocrystalline graphene (nc-Gr) with numerous grain boundaries allows protons to penetrate the graphene layer and interact with the underlying pH-dependent charge-transfer dopant layer. We analyze the pH sensitivity of nc-Gr ISFETs by adjusting the grain boundary density of graphene and the functional group (OH-, NH2-, CH3-) on the SiO2 surface, confirming an unusual negative shift of the charge-neutral point (CNP) as the pH of the solution increases and a super-Nernstian pH response (approximately -140 mV/pH) under optimized conditions.

Self-Catalytic Growth of Elementary Semiconductor Nanowires with Controlled Morphology and Crystallographic Orientation.

While the orientation-dependent properties of semiconductor nanowires have been theoretically predicted, their study has long been overlooked in many fields owing to the limits to controlling the crystallographic growth direction of nanowires (NWs). We present here the orientation-controlled growth of single-crystalline germanium (Ge) NWs using a self-catalytic low-pressure chemical vapor deposition process. By adjusting the growth temperature, the orientation of growth direction in GeNWs was selectively controlled to the ⟨110⟩, ⟨112⟩, or ⟨111⟩ directions on the same substrate. The NWs with different growth directions exhibit distinct morphological features, allowing control of the NW morphology from uniform NWs to nanoribbon structures. Significantly, the VLS-based self-catalytic growth of the ⟨111⟩ oriented GeNW suggests that NW growth is possible for single elementary materials even without an appropriate external catalyst. Furthermore, these findings could provide opportunities to investigate the orientation-dependent properties of semiconductor NWs.

Performance Improvement of Residue-Free Graphene Field-Effect Transistor Using Au-Assisted Transfer Method.

We report a novel graphene transfer technique for fabricating graphene field-effect transistors (FETs) that avoids detrimental organic contamination on a graphene surface. Instead of using an organic supporting film like poly(methyl methacrylate) (PMMA) for graphene transfer, Au film is directly deposited on the as-grown graphene substrate. Graphene FETs fabricated using the established organic film transfer method are easily contaminated by organic residues, while Au film protects graphene channels from these contaminants. In addition, this method can also simplify the device fabrication process, as the Au film acts as an electrode. We successfully fabricated graphene FETs with a clean surface and improved electrical properties using this Au-assisted transfer method.

Wafer-Scale and Low-Temperature Growth of 1T-WS2 Film for Efficient and Stable Hydrogen Evolution Reaction.

The metallic 1T phase of WS2 (1T-WS2 ), which boosts the charge transfer between the electron source and active edge sites, can be used as an efficient electrocatalyst for the hydrogen evolution reaction (HER). As the semiconductor 2H phase of WS2 (2H-WS2 ) is inherently stable, methods for synthesizing 1T-WS2 are limited and complicated. Herein, a uniform wafer-scale 1T-WS2 film is prepared using a plasma-enhanced chemical vapor deposition (PE-CVD) system. The growth temperature is maintained at 150 °C enabling the direct synthesis of 1T-WS2 films on both rigid dielectric and flexible polymer substrates. Both the crystallinity and number of layers of the as-grown 1T-WS2 are verified by various spectroscopic and microscopic analyses. A distorted 1T structure with a 2a0 × a0 superlattice is observed using scanning transmission electron microscopy. An electrochemical analysis of the 1T-WS2 film demonstrates its similar catalytic activity and high durability as compared to those of previously reported untreated and planar 1T-WS2 films synthesized with CVD and hydrothermal methods. The 1T-WS2 does not transform to stable 2H-WS2 , even after a 700 h exposure to harsh catalytic conditions and 1000 cycles of HERs. This synthetic strategy can provide a facile method to synthesize uniform 1T-phase 2D materials for electrocatalysis applications.

Rational Design of Ultrathin Gas Barrier Layer via Reconstruction of Hexagonal Boron Nitride Nanoflakes to Enhance the Chemical Stability of Proton Exchange Membrane Fuel Cells.

Hexagonal boron nitride (hBN) has great potential as a promising gas barrier layer in proton exchange membrane fuel cells (PEMFCs) as it shows high proton conductivity as well as excellent gas-blocking capability. However, structural defects and mechanical damage during the transfer of the hBN layer and membrane swelling have limited the application of hBN sheets to PEMFCs. Here, an ultrathin gas barrier layer is successfully fabricated on a proton exchange membrane via reconstruction of mechanically exfoliated hBN nanoflakes using a direct spin-coating process. The hBN-coated layer effectively suppresses the gas crossover and inhibits the formation of reactive oxygen radicals in the electrodes without reducing the proton conductivity of the membrane. It is also demonstrated that the structural advantages of hBN-coated gas barrier layers promise high performance of a unit cell even after a open-circuit voltage (OCV) hold test for 100 h. Furthermore, through in-depth postmortem analyses, a time-dependent degradation mechanism of membrane electrode assembly under the OCV condition is rationally proposed.

High performance self-gating graphene/MoS2 diode enabled by asymmetric contacts.

A graphene-MoS2 (GM) heterostructure based diode is fabricated using asymmetric contacts to MoS2, as well as an asymmetric top gate (ATG). The GM diode exhibits a rectification ratio of 5 from asymmetric contacts, which is improved to 105 after the incorporation of an ATG. This improvement is attributed to the asymmetric modulation of carrier concentration and effective Schottky barrier height (SBH) by the ATG during forward and reverse bias. This is further confirmed from the temperature dependent measurement, where a difference of 0.22 eV is observed between the effective SBH for forward and reverse bias. Moreover, the rectification ratio also depends on carrier concentration in MoS2 and can be varied with the change in temperature as well as back gate voltage. Under laser light illumination, the device demonstrates strong opto-electric response with 100 times improvement in the relative photo current, as well as a responsivity of 1.9 A W-1 and a specific detectivity of 2.4 × 1010 Jones. These devices can also be implemented using other two dimensional (2D) materials and suggest a promising approach to incorporate diverse 2D materials for future nano-electronics and optoelectronics applications.

Direct growth of graphene on rigid and flexible substrates: progress, applications, and challenges.

Graphene has recently been attracting considerable interest because of its exceptional conductivity, mechanical strength, thermal stability, etc. Graphene-based devices exhibit high potential for applications in electronics, optoelectronics, and energy harvesting. In this paper, we review various growth strategies including metal-catalyzed transfer-free growth and direct-growth of graphene on flexible and rigid insulating substrates which are "major issues" for avoiding the complicated transfer processes that cause graphene defects, residues, tears and performance degradation in graphene-based functional devices. Recent advances in practical applications based on "direct-grown graphene" are discussed. Finally, several important directions, challenges and perspectives in the commercialization of 'direct growth of graphene' are also discussed and addressed.

Large reduction in thermal conductivity for SiGe alloy nanowire wrapped with a Ge nanoparticle-embedded SiO2 shell.

We demonstrate silicon germanium (SiGe) alloy nanowires (NWs) with Ge nanoparticles (GeNPs) embedded in a SiO2 shell as a material for decreasing thermal conductivity. During thermal oxidation of SiGe NWs to form SiGe-SiO2 core-shell structures, Ge atoms were diffused into the SiO2 shell to relax the strain in the SiGe core, and agglomerated as a few nanometer-sized particles. This structure leads to a large reduction in thermal conductivity due to the GeNP-phonon interaction, while electrical conductivity is sustained because the core of the SiGe alloy NW provides a current path for the charged carriers. The thermal conductivity of the SiGe alloy NWs wrapped with a GeNP-embedded SiO2 shell is 0.41 W m(-1) K(-1) at 300 K.

Catalytic etching of monolayer graphene at low temperature via carbon oxidation.

In this work, an easy method to etch monolayer graphene is shown by catalytic oxidation in the presence of ZnO nanoparticles (NPs). The catalytic etching of monolayer graphene, which was transferred to the channel of field-effect transistors (FETs), was performed at low temperature by heating the FETs several times under an inert gas atmosphere (ZnO + C → Zn + CO or CO2). As the etching process proceeded, diverse etched structures in the shape of nano-channels and pits were observed under microscopic observation. To confirm the evolution of etching, current-voltage characteristics of monolayer graphene were measured after every step of etching by catalytic oxidation. As a result, the conductance of monolayer graphene decreased with the development of etched structures. This decrease in conductance was analyzed by percolation theory in a honeycomb structure. Finally, well-patterned graphene was obtained by oxidizing graphene under air in the presence of NPs, where Al was deposited on graphene as a mask for designed patterns. This method can substitute graphene etching via carbon hydrogenation using H2 at high temperature.

Epitaxial growth of a single-crystal hybridized boron nitride and graphene layer on a wide-band gap semiconductor.

Vertical and lateral heterogeneous structures of two-dimensional (2D) materials have paved the way for pioneering studies on the physics and applications of 2D materials. A hybridized hexagonal boron nitride (h-BN) and graphene lateral structure, a heterogeneous 2D structure, has been fabricated on single-crystal metals or metal foils by chemical vapor deposition (CVD). However, once fabricated on metals, the h-BN/graphene lateral structures require an additional transfer process for device applications, as reported for CVD graphene grown on metal foils. Here, we demonstrate that a single-crystal h-BN/graphene lateral structure can be epitaxially grown on a wide-gap semiconductor, SiC(0001). First, a single-crystal h-BN layer with the same orientation as bulk SiC was grown on a Si-terminated SiC substrate at 850 °C using borazine molecules. Second, when heated above 1150 °C in vacuum, the h-BN layer was partially removed and, subsequently, replaced with graphene domains. Interestingly, these graphene domains possess the same orientation as the h-BN layer, resulting in a single-crystal h-BN/graphene lateral structure on a whole sample area. For temperatures above 1600 °C, the single-crystal h-BN layer was completely replaced by the single-crystal graphene layer. The crystalline structure, electronic band structure, and atomic structure of the h-BN/graphene lateral structure were studied by using low energy electron diffraction, angle-resolved photoemission spectroscopy, and scanning tunneling microscopy, respectively. The h-BN/graphene lateral structure fabricated on a wide-gap semiconductor substrate can be directly applied to devices without a further transfer process, as reported for epitaxial graphene on a SiC substrate.

Ultrastable-Stealth Large Gold Nanoparticles with DNA Directed Biological Functionality.

The stability of gold nanoparticles (AuNPs) in biological samples is very important for their biomedical applications. Although various molecules such as polystyrenesulfonate (PSS), phosphine, DNA, and polyethylene glycol (PEG) have been used to stabilize AuNPs, it is still very difficult to stabilize large AuNPs. As a result, biomedical applications of large (30-100 nm) AuNPs are limited, even though they possess more favorable optical properties and are easier to be taken up by cells than smaller AuNPs. To overcome this limitation, we herein report a novel method of preparing large (30-100 nm) AuNPs with a high colloidal stability and facile chemical or biological functionality, via surface passivation with an amphiphilic polymer polyvinylpyrrolidone (PVP). This PVP passivation results in an extraordinary colloidal stability for 13, 30, 50, 70, and 100 nm AuNPs to be stabilized in PBS for at least 3 months. More importantly, the PVP capped AuNPs (AuNP-PVP) were also resistant to protein adsorption in the presence of serum containing media and exhibit a negligible cytotoxicity. The AuNP-PVPs functionalized with a DNA aptamer AS1411 remain biologically active, resulting in significant increase in the uptake of the AuNPs (∼12,200 AuNPs per cell) in comparison with AuNPs capped by a control DNA of the same length. The novel method developed in this study to stabilize large AuNPs with high colloidal stability and biological activity will allow much wider applications of these large AuNPs for biomedical applications, such as cellular imaging, molecular diagnosis, and targeted therapy.

Selective exfoliation of single-layer graphene from non-uniform graphene grown on Cu.

Graphene growth on a copper surface via metal-catalyzed chemical vapor deposition has several advantages in terms of providing high-quality graphene with the potential for scale-up, but the product is usually inhomogeneous due to the inability to control the graphene layer growth. The non-uniform regions strongly affect the reliability of the graphene in practical electronic applications. Herein, we report a novel graphene transfer method that allows for the selective exfoliation of single-layer graphene from non-uniform graphene grown on a Cu foil. Differences in the interlayer bonding energy are exploited to mechanically separate only the top single-layer graphene and transfer this to an arbitrary substrate. The dry-transferred single-layer grapheme showed electrical characteristics that were more uniform than those of graphene transferred using conventional wet-etching transfer steps.

Tunable threshold voltage of an n-type Si nanowire ferroelectric-gate field effect transistor for high-performance nonvolatile memory applications.

We successfully fabricated ferroelectric-gate field effect transistor (FEFET)-based nonvolatile memory devices using an n-type Si nanowire coated with omega-shaped-gate organic ferroelectric poly(vinylidene fluoride-trifluoroethylene) via a low-temperature fabrication process. Our FEFET memory devices with controllable threshold voltage via adjustment of the doping concentration exhibit excellent memory characteristics with ultra-low ON state power dissipation (≤3 nW), a large modulation in channel conductance between the ON and OFF states exceeding 10(5), a long retention time of over 3 × 10(4) s and a high endurance of over 10(5) programming cycles whilst maintaining an I ON/I OFF ratio higher than 10(3). This result may be promising for next-generation nonvolatile memory on flexible substrate applications.

Nondestructive Single-Atom-Thick Crystallographic Scanner via Sticky-Note-Like van der Waals Assembling-Disassembling.

Crystallographic characteristics, including grain boundaries and crystallographic orientation of each grain, are crucial in defining the properties of two-dimensional materials (2DMs). To date, local microstructure analysis of 2DMs, which requires destructive and complex processes, is primarily used to identify unknown 2DM specimens, hindering the subsequent use of characterized samples. Here, a nondestructive large-area 2D crystallographic analytical method through sticky-note-like van der Waals (vdW) assembling-disassembling is presented. By the vdW assembling of veiled polycrystalline graphene (PCG) with a single-atom-thick single-crystalline graphene filter (SCG-filter), detailed crystallographic information of each grain in PCGs is visualized through a 2D Raman signal scan, which relies on the interlayer twist angle. The scanned PCGs are seamlessly separated from the SCG-filter using vdW disassembling, preserving their original condition. The remaining SCG-filter is then reused for additional crystallographic scans of other PCGs. It is believed that the methods can pave the way for advances in the crystallographic analysis of single-atom-thick materials, offering huge implications for the applications of 2DMs.

Fabrication and RF characterization of a single nickel silicide nanowire for an interconnect.

We fabricated a nickel silicide nanowire (NiSi NW) device with a low thermal budget and characterized it by measuring the S-parameters in the radio-frequency (RF) regime. A single silicon nanowire (Si NW) was assembled on a substrate with a two-port coplanar waveguide structure using the dielectrophoresis method. Then, the Si NW on the device was perfectly transformed into a NiSi NW. The NiSi NW device was characterized by performing measurements in the DC and RF regimes. The transformation into the NiSi NW resulted in reducing about three-order more the resistance than before the transformation. Hence, the transmission of the NiSi NW device was 25 dB higher than that of the Si NW device up to gigahertz. We also discussed extracting the intrinsic properties of the NiSi NW by using de-embedding, circuit modeling, and simulation.

Large-scale fabrication of 2-D nanoporous graphene using a thin anodic aluminum oxide etching mask.

A large-scale nanoporous graphene (NPG) fabrication method via a thin anodic aluminum oxide (AAO) etching mask is presented in this paper. A thin AAO film is successfully transferred onto a hydrophobic graphene surface under no external force. The AAO film is completely stacked on the graphene due to the van der Waals force. The neck width of the NPG can be controlled ranging from 10 nm to 30 nm with different AAO pore widening times. Extension of the NPG structure is demonstrated on a centimeter scale up to 2 cm2. AAO and NPG structures are characterized using optical microscopy (OM), Raman spectroscopy and field-emission scanning electron microscopy (FE-SEM). A field effect transistor (FET) is realized by using NPG. Its electrical characteristics turn out to be different from that of pristine graphene, which is due to the periodic nanostructures. The proposed fabrication method could be adapted to a future graphene-based nano device.

Real-time pulse measurement of nano-scale field effect transistors: cancellation of displacement current and extraction of coupling capacitance.

Real-time pulse measurements of nano-scale field effect transistors (FETs) are reported. We demonstrate the direct monitoring of the real-time current of bottom-up assembled silicon nanowire FET and top-down fabricated gate-all-around silicon nanowire FET, both with the diameter of approximately 50 nm. We demonstrate that the displacement current can be cancelled out from the measured pulse responses. On the other hand, the displacement current also can be utilized to obtain the coupling capacitance between the gate and source of the FETs.

Control of lateral dimension in metal-catalyzed germanium nanowire growth: usage of carbon sheath.

We report on the catalytic growth of thin carbon sheathed single crystal germanium nanowires (GeNWs), which can solve the obstacles that have disturbed a wide range of applications of GeNWs. Single crystal Ge NW core and amorphous carbon sheath are simultaneously grown via vapor-liquid-solid (VLS) process. The carbon sheath completely blocks unintentional vapor deposition on NW surface, thus ensuring highly uniform diameter, dopant distribution, and electrical conductivity along the entire NW length. Furthermore, the sheath not only inhibits metal diffusion but also improves the chemical stability of GeNWs at even high temperatures.