Seong Chan Jun

Enhanced Supercapacitive Performance of Chemically Grown Cobalt–Nickel Hydroxides on Three-Dimensional Graphene Foam Electrodes

Chemical growth of mixed cobalt-nickel hydroxides (CoxNi1-x(OH)2), decorated on graphene foam (GF) with desirable three-dimensional (3D) interconnected porous structure as electrode and its potential energy storage application is discussed. The nanostructured CoxNi1-x(OH)2 films with different Ni:Co (x) compositions on GF are prepared by using the chemical bath deposition (CBD) method. The structural studies (X-ray diffraction and X-ray photoelectron spectroscopy) of electrodes confirm crystalline nature of CoxNi1-x(OH)2/GF and crystal structure consists of Ni(OH)2 and Co(OH)2. The morphological properties reveal that nanorods of Co(OH)2 reduce in size with increases in nickel content and are converted into Ni(OH)2 nanoparticles. The electrochemical performance reveals that the Co0.66Ni0.33(OH)2/GF electrode has maximum specific capacitance of ∼1847 F g(-1) in 1 M KOH within a potential window 0 to 0.5 V vs Ag/AgCl at a discharge current density of 5 A g(-1). The superior pseudoelectrochemical properties of cobalt and nickel are combined and synergistically reinforced with high surface area offered by a conducting, porous 3D graphene framework, which stimulates effective utilization of redox characteristics and communally improves electrochemical performance with charge transport and storage.

Nanomechanical hydrogen sensing

A nanomechanical beam resonator is used as a sensitive, specific hydrogen sensor. The beam is fabricated from AuPd alloy and tested by magnetomotive transduction at room temperature. The fundamental resonance frequency decreases significantly and reversibly at hydrogen pressures above 10−5Torr, whereas the frequency shifts observed for other gases are significantly smaller. The large frequency shift is likely due to the formation of interstitial hydrogen in the metal alloy lattice, which relieves the built-in tensile stress in the resonator beam. The uptake of hydrogen as measured by frequency shift is consistent with previous studies.

Electrothermal tuning of Al–SiC nanomechanical resonators

A highly effective electrothermal tuning method is demonstrated for Al–SiC nanomechanical resonators. Doubly clamped beam devices are actuated and read out using a magnetomotive technique under a moderate vacuum at room temperature. Direct current applied to a beam heats the structure and shifts the resonance frequency downward. Frequency shifts of 10% are easily achievable, and the thermal time constant of these structures is in the submicrosecond range. The initial frequency and frequency tunability are studied for beams of varying Al thickness, and the device performance can be accurately modelled using simple mechanical and thermal models. Because of the different mechanical properties of SiC and Al, both the initial frequency and the frequency tunability can be modified by varying the Al layer thickness. This approach has the potential to become an important tool for effective frequency tuning in deployable SiC-based nanoelectromechanical system devices and systems for applications that would benefit from SiC as the structural material.

Hierarchical MnCo-layered double hydroxides@Ni(OH)2 core–shell heterostructures as advanced electrodes for supercapacitors

Rational assembly and hetero-growth of hybrid structures consisting of multiple components with distinctive features are a promising and challenging strategy to develop materials for energy storage applications. Herein, we propose a supercapacitor electrode comprising a three-dimensional self-supported hierarchical MnCo-layered double hydroxides@Ni(OH)2 [MnCo-LDH@Ni(OH)2] core–shell heterostructure on conductive nickel foam. The resultant MnCo-LDH@Ni(OH)2 structure exhibited a high specific capacitance of 2320 F g−1 at a current density of 3 A g−1, and a capacitance of 1308 F g−1 was maintained at a high current density of 30 A g−1 with a superior long cycle lifetime. Moreover, an asymmetric supercapacitor was successfully assembled using MnCo-LDH@Ni(OH)2 as the positive electrode and activated carbon (AC) as the negative electrode. The optimized MnCo-LDH@Ni(OH)2//AC device with a voltage of 1.5 V delivered a maximum energy density of 47.9 W h kg−1 at a power density of 750.7 W kg−1. The energy density remained at 9.8 W h kg−1 at a power density of 5020.5 W kg−1 with excellent cycle stability.

Controlled electrochemical growth of Co(OH)2 flakes on 3D multilayered graphene foam for high performance supercapacitors

The present research describes successful enchase of Co(OH)2 microflakes by the potentiodynamic mode of electro-deposition (PED) on porous, light weight, conducting 3D multilayered graphene foam (MGF) and their synergistic effect on improving the supercapacitive performance. Structural and morphological analyses reveal uniform growth of Co(OH)2 microflakes with an average flake width of ∼30 nm on the MGF surface. Moreover, electrochemical capacitive measurements of the Co(OH)2/MGF electrode exhibit a high specific capacitance of ∼1030 F g−1 with ∼37 W h kg−1 energy and ∼18 kW kg−1 power density at 9.09 A g−1 current density. The superior pseudoelectrochemical properties of cobalt hydroxide are synergistically decorated with high surface area offered by a conducting, porous 3D graphene framework, which stimulates the effective utilization of redox characteristics and mutually improves electrochemical capacitive performance with charge transport and storage. This work evokes scalable electrochemical synthesis with the enhanced supercapacitive performance of the Co(OH)2/MGF electrode in energy storage devices.

Controllable sulfuration engineered NiO nanosheets with enhanced capacitance for high rate supercapacitors

NiO has been intensively studied as a promising electrode material for supercapacitors because of its high theoretical specific capacitance, well-defined redox behavior, and good chemical compatibility with nickel foam. However, it still suffers from inferior rate capability and cycling stability because of the simple component and random structural integration. Herein, we report a tunable sulfuration process of NiO nanosheets constructed on porous nickel foam for supercapacitor applications. The resulting NiO/Ni3S2 with distinct structural features exhibits an ultra-high specific capacitance of 2153 F g−1 at a current density of 1 A g−1, and the capacitance is retained at 1169 F g−1 even at a current density as high as 30 A g−1. An asymmetric supercapacitor device fabricated with NiO/Ni3S2 as the positive electrode and activated carbon as the negative electrode delivers high energy and power densities (52.9 W h kg−1 at 1.6 kW kg−1; 26.3 W h kg−1 at 6.4 kW kg−1), and good cycling stability (a capacitance retention of 92.9% over 5000 cycles).

Clean transfer of graphene and its effect on contact resistance

We demonstrate herein an effective method of forming a high-quality contact between metal and graphene on a wafer as large as 6 in. This gold-assisted transfer method producing no polymer residue on the graphene surface is introduced, and then the gold film is used directly as an electrode to form the transfer length method pattern for calculating the contact resistance. The graphene surface obtained using the gold-assisted transfer method is clean and uniform without residue or contamination, and its contact resistance is at least 60% lower than that obtained using the conventional poly(methyl methacrylate) assisted transfer method.

Radio-frequency transmission characteristics of a multi-walled carbon nanotube

Carbon nanotubes (CNTs) are considered as promising candidates for transmission lines as well as microcircuit interconnects in future nanoscale electronic systems. Owing to the growing interest in the use of microwave signals, understanding the transmission properties at high frequencies is essential to assess the applicability of multi-walled carbon nanotubes (MWNTs). In this work, we measured two-port properties of individual MWNTs using a network analyser from a frequency of 0.5 to 50 GHz. The radio-frequency transmission parameters were obtained from the measured S-parameter data. Our results show the frequency dependence of the equivalent resistance of MWNTs, which decreases with increasing frequency. This confirms that metallic CNTs will be useful for transmitting GHz signals in nanoelectric devices.

Scalable fabrication of micron-scale graphene nanomeshes for high-performance supercapacitor applications

Graphene nanomeshes (GNMs) with nanoscale periodic or quasi-periodic nanoholes have attracted considerable interest because of unique features such as their open energy band gap, enlarged specific surface area, and high optical transmittance. These features are useful for applications in semiconducting devices, photocatalysis, sensors, and energy-related systems. Here, we report on the facile and scalable preparation of multifunctional micron-scale GNMs with high-density of nanoperforations by catalytic carbon gasification. The catalytic carbon gasification process induces selective decomposition on the graphene adjacent to the metal catalyst, thus forming nanoperforations. The pore size, pore density distribution, and neck size of the GNMs can be controlled by adjusting the size and fraction of the metal oxide on graphene. The fabricated GNM electrodes exhibit superior electrochemical properties for supercapacitor (ultracapacitor) applications, including exceptionally high capacitance (253 F g−1 at 1 A g−1) and high rate capability (212 F g−1 at 100 A g−1) with excellent cycle stability (91% of the initial capacitance after 50 000 charge/discharge cycles). Further, the edge-enriched structure of GNMs plays an important role in achieving edge-selected and high-level nitrogen doping.

Static and Dynamic Performance of Complementary Inverters Based on Nanosheet α-MoTe2 p-Channel and MoS2 n-Channel Transistors

Molybdenum ditelluride (α-MoTe2) is an emerging transition-metal dichalcogenide (TMD) semiconductor that has been attracting attention due to its favorable optical and electronic properties. Field-effect transistors (FETs) based on few-layer α-MoTe2 nanosheets have previously shown ambipolar behavior with strong p-type and weak n-type conduction. We have employed a direct imprinting technique following mechanical nanosheet exfoliation to fabricate high-performance complementary inverters using α-MoTe2 as the semiconductor for the p-channel FETs and MoS2 as the semiconductor for the n-channel FETs. To avoid ambipolar behavior and produce α-MoTe2 FETs with clean p-channel characteristics, we have employed the high-workfunction metal platinum for the source and drain contacts. As a result, our α-MoTe2 nanosheet p-channel FETs show hole mobilities up to 20 cm(2)/(V s), on/off ratios up to 10(5), and a subthreshold slope of 255 mV/decade. For our complementary inverters composed of few-layer α-MoTe2 p-channel FETs and MoS2 n-channel FETs we have obtained voltage gains as high as 33, noise margins as high as 0.38 VDD, a switching delay of 25 μs, and a static power consumption of a few nanowatts.