Sungwoo Hwang

Wafer-Scale Growth of Single-Crystal Monolayer Graphene on Reusable Hydrogen-Terminated Germanium

Smoothing Graphene Several methods have been reported for the growth of monolayer graphene into areas large enough for integration into silicon electronics. However, the electronic properties of the graphene are often degraded by grain boundaries and wrinkles. Lee et al. (p. 286, published online 3 April) showed that flat, single crystals of monolayer graphene can be grown by chemical-vapor deposition on silicon wafers covered by a germanium layer that aligns the grains. The graphene can be dry-transferred to other substrates, and the germanium layer can be reused for further growth cycles. Wafer-scale single-crystal monolayer graphene can be repeatedly grown on a hydrogen-terminated germanium (110) surface. The uniform growth of single-crystal graphene over wafer-scale areas remains a challenge in the commercial-level manufacturability of various electronic, photonic, mechanical, and other devices based on graphene. Here, we describe wafer-scale growth of wrinkle-free single-crystal monolayer graphene on silicon wafer using a hydrogen-terminated germanium buffer layer. The anisotropic twofold symmetry of the germanium (110) surface allowed unidirectional alignment of multiple seeds, which were merged to uniform single-crystal graphene with predefined orientation. Furthermore, the weak interaction between graphene and underlying hydrogen-terminated germanium surface enabled the facile etch-free dry transfer of graphene and the recycling of the germanium substrate for continual graphene growth.

Surfactant‐Free Scalable Synthesis of Bi2Te3 and Bi2Se3 Nanoflakes and Enhanced Thermoelectric Properties of Their Nanocomposites

Surfactant-free nanoflakes of n-type Bi2 Te3 and Bi2 Se3 are synthesized in high yields. Their suspensions are mixed to create nanocomposites with heterostructured nanograins. A maximum ZT (0.7 at 400 K) is achieved with a broad content of 10-15% Bi2 Se3 in the nanocomposites.

Highly stretchable transistors using a microcracked organic semiconductor.

A. Chortos, J. Lim, J. W. F. To, Dr. M. Vosgueritchian, T. J. Dusseault, Prof. Z. Bao Department of Chemical Engineering Stanford University 381 North-South Mall , Stanford , CA , USA E-mail: zbao@stanford.edu T.-H. Kim, S. Hwang Nano Electronics Laboratory Samsung Advanced Institute of Technology Suwon 443–803 , Korea E-mail: swnano.hwang@samsung.com

Fabrication and room-temperature characterization of a silicon self-assembled quantum-dot transistor

A quantum-dot transistor based on silicon self-assembled quantum dots has been fabricated. The device shows staircases and oscillations in the drain current at room temperature. These data are interpreted as due to single electron tunneling through the dots located in the shortest current path between the source and the drain electrodes. The dot size calculated from the data is ∼7 nm, which is consistent with the size of the self-assembled dots incorporated in the transistor.

DNA hydrogel-based supercapacitors operating in physiological fluids

DNA nanostructures have been attractive due to their structural properties resulting in many important breakthroughs especially in controlled assemblies and many biological applications. Here, we report a unique energy storage device which is a supercapacitor that uses nanostructured DNA hydrogel (Dgel) as a template and layer-by-layer (LBL)-deposited polyelectrolyte multilayers (PEMs) as conductors. Our device, named as PEM-Dgel supercapacitor, showed excellent performance in direct contact with physiological fluids such as artificial urine and phosphate buffered saline without any need of additional electrolytes, and exhibited almost no cytotoxicity during cycling tests in cell culture medium. Moreover, we demonstrated that the PEM-Dgel supercapacitor has greater charge-discharge cycling stability in physiological fluids than highly concentrated acid electrolyte solution which is normally used for supercapacitor operation. These conceptually new supercapacitors have the potential to be a platform technology for the creation of implantable energy storage devices for packageless applications directly utilizing biofluids.

Electrical transport through 60 base pairs of poly(dG)-poly(dC) DNA molecules

We report electrical transport through 60 base pairs of poly~dG!-poly~dC! DNA molecules. The DNA solution is dropped on two metal electrodes with the gap of 20 nm. The current‐voltage characteristics measured between the electrodes exhibits clear staircases, which are reproducible over repeated measurements. The size of the observed staircases is consistent with the energy gap obtained from a tight binding calculation.

Polypyrrole/Agarose-based electronically conductive and reversibly restorable hydrogel.

Conductive hydrogels are a class of composite materials that consist of hydrated and conducting polymers. Due to the mechanical similarity to biointerfaces such as human skin, conductive hydrogels have been primarily utilized as bioelectrodes, specifically neuroprosthetic electrodes, in an attempt to replace metallic electrodes by enhancing the mechanical properties and long-term stability of the electrodes within living organisms. Here, we report a conductive, smart hydrogel, which is thermoplastic and self-healing owing to its unique properties of reversible liquefaction and gelation in response to thermal stimuli. In addition, we demonstrated that our conductive hydrogel could be utilized to fabricate bendable, stretchable, and patternable electrodes directly on human skin. The excellent mechanical and thermal properties of our hydrogel make it potentially useful in a variety of biomedical applications such as electronic skin.

Graphene and Thin-Film Semiconductor Heterojunction Transistors Integrated on Wafer Scale for Low-Power Electronics

Graphene heterostructures in which graphene is combined with semiconductors or other layered 2D materials are of considerable interest, as a new class of electronic devices has been realized. Here we propose a technology platform based on graphene-thin-film-semiconductor-metal (GSM) junctions, which can be applied to large-scale and power-efficient electronics compatible with a variety of substrates. We demonstrate wafer-scale integration of vertical field-effect transistors (VFETs) based on graphene-In-Ga-Zn-O (IGZO)-metal asymmetric junctions on a transparent 150 × 150 mm(2) glass. In this system, a triangular energy barrier between the graphene and metal is designed by selecting a metal with a proper work function. We obtain a maximum current on/off ratio (Ion/Ioff) up to 10(6) with an average of 3010 over 2000 devices under ambient conditions. For low-power logic applications, an inverter that combines complementary n-type (IGZO) and p-type (Ge) devices is demonstrated to operate at a bias of only 0.5 V.

RF characterization and modeling of various wire bond transitions

This paper presents radio-frequency (RF) characterization and modeling of various wire bond transitions between chips and packages. Test modules composed of Si chips and alumina packages are fabricated in conductor-backed (CB) structures, and they are characterized at frequencies up to 20 GHz. It is shown that the parallel plate resonance of the CB coplanar waveguide (CPW) persists in wire bonding transitions, and narrower-ground CPW-CPW transitions show better characteristics than wider-ground CPW-CPW transitions. The results of three-dimensional full-wave electromagnetic simulation on the test modules reproduce the measured results with a reasonable accuracy. Simple equivalent circuit modeling can reproduce the measured results of the narrower ground CPW transition with no resonance structures. Finally, the effects of bond wire length and impedance mismatch on RF performance are investigated.

Room temperature single electron effects in Si quantum dot memory with oxide-nitride tunneling dielectrics

We have developed a repeatable process of forming uniform, small-size and high-density Si quantum dots on oxide-nitride tunneling dielectrics and have fabricated an EEPROM which showed room temperature single electron effects. This proved the feasibility of practical Si quantum dot memory with ON film.