DONGSEOK SUH | Sungkyunkwan University (original) (raw)

Papers by DONGSEOK SUH

ACS Applied Materials & Interfaces, 2019

The transport behaviors of MoS 2 field-effect transistors (FETs) with various channel thicknesses... more The transport behaviors of MoS 2 field-effect transistors (FETs) with various channel thicknesses are studied. In a 12 nm thick MoS 2 FET, a typical switching behavior is observed with an I on /I off ratio of 10 6. However, in 70 nm thick MoS 2 FETs, the gating effect weakens with a large off-current, resulting from the screening of the gate field by the carriers formed through the ionization of S vacancies at 300 K. Hence, when the latter is dual-gated, two independent conductions develop with different threshold voltage (V TH) and field-effect mobility (μ FE) values. When the temperature is lowered for the latter, both the ionization of S vacancies and the gate-field screening reduce, which revives the strong I on /I off ratio and merges the two separate channels into one. Thus, only one each of V TH and μ FE are seen from the thick MoS 2 FET when the temperature is less than 80 K. The change of the number of conduction channels is attributed to the ionization of S vacancies, which leads to a temperature-dependent intra-and interlayer conductance and the attenuation of the electrostatic gate field. The defect-related transport behavior of thick MoS 2 enables us to propose a new device structure that can be further developed to a vertical inverter inside a single MoS 2 flake.

ACS applied materials & interfaces, Jan 20, 2017

Stretchable conductors can be used in various applications depending on their own characteristics... more Stretchable conductors can be used in various applications depending on their own characteristics. Here, we demonstrate simple and robust elastomeric conductors that are optimized for stretchable electrical signal transmission line. They can withstand strains up to 600% without any substantial change in their resistance (≤ 10% as is and ≤ 1% with passivation), and exhibit suppressed charge fluctuations in the medium. The inherent elasticity of a polymeric rubber and the high conductivity of flexible, highly oriented carbon nanotube sheets were combined synergistically, without losing both properties. The nanoscopic strong adhesion between aligned carbon nanotube arrays and strained elastomeric polymers induces conductive wavy folds with microscopic bending of radii on the scale of a few micrometers. Such features enable practical applications such as in elastomeric length-changeable electrical digital and analog signal transmission lines at above MHz frequencies. In addition to repo...

Physical Phenomena At High Magnetic Fields IV, 2002

The effect of interfibrillar interaction on the charge transport of doped polyacetylene is invest... more The effect of interfibrillar interaction on the charge transport of doped polyacetylene is investigated by studying the high field magnetoconductivity of iodine doped helical polyacetylene. The zero-field resistivity ratio, ρ r = ρ(1.2 K )/ρ(300 K ), is comparable to that of stretch-oriented high-density polyacetylene, which indicates the partial alignment of chains inside a polymer fiber. At low magnetic fields, the small negative component of magnetoconductivity was observed and its magnitude increases as the ρ r value increases. In the high field region, the magnetoconductivity is positive and it clearly shows the linear dependence on the magnetic field up to H = 30 T. The linear field dependence of magnetoconductivity is different from what is expected in the three-dimensional weak localization picture. For the same ρ r value samples, the magnitude of negative magnetoconductivity of S-polyacetylene is much bigger than that of R-polyacetylene, which could be attributed to the difference in the degree of helicity determining the strength of interfibrillar interaction.

Nanotechnology, Jan 5, 2015

The combination of quantum Hall conductance and charge-trap memory operation was qualitatively ex... more The combination of quantum Hall conductance and charge-trap memory operation was qualitatively examined using a graphene field-effect transistor. The characteristics of two-terminal quantum Hall conductance appeared clearly on the background of a huge conductance hysteresis during a gate-voltage sweep for a device using monolayer graphene as a channel, hexagonal boron-nitride flakes as a tunneling dielectric and defective silicon oxide as the charge storage node. Even though there was a giant shift of the charge neutrality point, the deviation of quantized resistance value at the state of filling factor 2 was less than 1.6% from half of the von Klitzing constant. At high Landau level indices, the behaviors of quantum conductance oscillation between the increasing and the decreasing electron densities were identical in spite of a huge memory window exceeding 100 V. Our results indicate that the two physical phenomena, two-terminal quantum Hall conductance and charge-trap memory opera...

Applied Physics Letters, 2014

Synthetic Metals, 2001

Starting with a review of general principles of transport processes in polyacetylene (PA) which c... more Starting with a review of general principles of transport processes in polyacetylene (PA) which can be studied by electron spin resonance (ESR) new results on highly doped PA are presented. The AuCl3 doped PA samples stored under vacuum at room temperature in ESR sample tubes reach a steady state in the temperature dependent intensity and linewidth behavior after three to four month storage. The ESR spectra show two lines with an equal g-factor. In particular, the linewidth of the small line demonstrates a remarkable property. The temperature dependence of the linewidth shows a maximum of ΔB=0.30mT at 30K, and it is constant at ΔB=0.17mT above this temperature in the whole temperature region between 70 and 300K. The ESR data are discussed in a two spin model of delocalized states at the PA chain and fixed spins near the dopants. Broadening and motional narrowing influence the ESR linewidth. The interpretation points to the metallic behavior of the highly doped PA at low temperatures.

Physical Review B, 2006

A detailed theoretical investigation on the global and local structures of the Ge-Sb-Te͑GST͒ tern... more A detailed theoretical investigation on the global and local structures of the Ge-Sb-Te͑GST͒ ternary alloy system for the phase-change memory is presented. We examine the cohesive energy of the ͑GeTe͒ n ͑Sb 2 Te 3 ͒ m homologous series as well as the dependence of the energy on the atomic distribution. We show that the cohesive energy decreases with increasing vacancy concentration and the vacancies repel each other to minimize the number of dangling bonds. In Ge 2 Sb 2 Te 5 , Sb and Ge atoms favor two-dimensional ͑layered͒ and three-dimensional ͑agglomerated͒ arrangements, respectively. In Ge 1 Sb 2 Te 4 , on the other hand, Ge atoms tend to form a two-dimensional layered structure. Possible structural building blocks of the GST system are proposed based on the density-functional theory total energy calculations.

Applied Physics Letters, 2008

We introduce single-phase In-Ge-Sb-Te (IGST) quaternary thin film (fcc structure when crystallize... more We introduce single-phase In-Ge-Sb-Te (IGST) quaternary thin film (fcc structure when crystallized) deposited by cosputtering from Ge2Sb2Te5(GST) and In3Sb1Te2 targets. This film, compared with the GST ternary system, provides a significant increase of amorphous-to-crystalline transformation temperature. High-resolution x-ray photoelectron spectroscopy (HRXPS) revealed that, with increasing In amounts, the Sb 4d and Ge 3d core peaks shift toward lower binding energies (BEs), with negligible changes in spectral linewidths, whereas the In 4d and Te 4d core peaks show insignificant changes in BEs. HRXPS interpretation suggests that the Na site in IGST can be occupied by Te, Sb, In, and vacancy, whereas in GST it is occupied only by Te.

Advanced Functional Materials, 2017

Flexible superconducting yarns consisting of sputter-deposited NbN nanowires on highly aligned ca... more Flexible superconducting yarns consisting of sputter-deposited NbN nanowires on highly aligned carbon nanotube (CNT) array sheets are reported. In the microscopic view, the NbN nanowires are formed on top of individual CNT fibrils, and the superconductivity property of the twist-spun NbN-CNT yarn system is comparable to that of a typical NbN thin film on a normal solid substrate. Because of its intrinsic porosity, the system exhibits superior mechanical flexibility with a small bending radius. It also remains a superconducting state even when subjected to severe mechanical deformations, primarily due to the proximity superconductivity through carbon nanotube bundles. The results demonstrate the possibility of fabricating flexible superconducting yarns in a conventional thin-film deposition process, using ultraflexible free-standing CNT sheets as a template. In addition, preliminary tests on reducing the normal-state resistance toward superconducting cable applications are presented.

ACS applied materials & interfaces, Jan 15, 2016

For transition metal dichalcogenides, the fluctuation of the channel current due to charged impur... more For transition metal dichalcogenides, the fluctuation of the channel current due to charged impurities is attributed to a large surface area and a thickness of a few nanometers. To investigate current variance at the interface of transistors, we obtain the low-frequency (LF) noise features of MoTe2 multilayer field-effect transistors with different dielectric environments. The LF noise properties are analyzed using the combined carrier mobility and carrier number fluctuation model which is additionally parametrized with an interfacial Coulomb-scattering parameter (α) that varies as a function of the accumulated carrier density (Nacc) and the location of the active channel layer of MoTe2. Our model shows good agreement with the current power spectral density (PSD) of MoTe2 devices from a low to high current range and indicates that the parameter α exhibits a stronger dependence on Nacc with an exponent -γ of -1.18 to approximately -1.64 for MoTe2 devices, compared with -0.5 for Si de...

Bulletin of the American Physical Society, Mar 3, 2015

tum Hall effect (QHE) is one of the unique properties of two-dimensional electronic systems provi... more tum Hall effect (QHE) is one of the unique properties of two-dimensional electronic systems providing the universal standard of electrical resistance. Due to edge-state transport features in quantum Hall regime, the two-terminal graphene field-effect transistor (FET) is frequently examined for the study of the integer as well as the fractional QHEs of graphene. In this work, we present a simple method to identify the existence of quantum Hall state in the graphene FET especially at high temperatures. Using the monolayer graphene FET sample with fully broken degeneracy, we modified the equipotential line inside graphene FET by the addition of extra electrode for the clear identification of the quantum Hall state formation at given temperature and magnetic field. We suggest a simple model to explain the difference and similarity between two-terminal and multi-terminal configurations, including the discussion about the QHE devices connected in series.

Sensors and Actuators B: Chemical, 2006

Thin, strongly adhering films of single-walled carbon nanotube bundles (SWNT) on flexible substra... more Thin, strongly adhering films of single-walled carbon nanotube bundles (SWNT) on flexible substrates such as poly(ethyleneterephthalate) (PET) were used for vapour sensing (hexane, toluene, acetone, chloroform, acetonitrile, methanol, water, etc.). These sensors are extremely easy to fabricate using the line patterning method. For example, '4-probe' sensor patterns are drawn on a computer and then printed on overhead transparency (PET) sheets. These PET patterns were coated with films of electronically conductive SWNT bundles (1-2 m thick) by dipcoating in aqueous surfactant-supported dispersions and mounted in glass chambers equipped for vapour sensing. Experiments conducted under saturated vapour conditions in air showed sensor responses that correlated well with solvent polarity [E T (30) scale]. Similar results were obtained under controlled vapour conditions (no air) at 10,000 ppm. Control experiments using films of carbon black on PET (Aquadag-E ®), also prepared by the line patterning method, showed very little response to vapours under identical experimental conditions. The sensors are very flexible, e.g., they can be bent to diameters as small as 10 mm without significantly compromising sensor function.

Science, 2012

Nanotube Yarn Actuators Actuators are used to convert heat, light, or electricity into a twisting... more Nanotube Yarn Actuators Actuators are used to convert heat, light, or electricity into a twisting or tensile motion, and are often described as artificial muscles. Most materials that show actuation either provide larger forces with small-amplitude motions, such as the alloy NiTi, or provide larger motions with much less force, such as polymeric materials. Other problems with such actuators can include slow response times and short lifetimes. Lima et al. (p. 928 , see the Perspective by Schulz ) show that a range of guest-filled, twist-spun carbon nanotube yarns can be used for linear or torsional actuation, can solve the problems of speed and lifetime, and do not require electrolytes for operation.

Nano Letters, 2014

We report electrochemically powered, all-solid-state torsional and tensile artificial yarn muscle... more We report electrochemically powered, all-solid-state torsional and tensile artificial yarn muscles using a spinnable carbon nanotube (CNT) sheet that provides attractive performance. Large torsional muscle stroke (53°/mm) with minor hysteresis loop was obtained for a low applied voltage (5 V) without the use of a relatively complex three-electrode electromechanical setup, liquid electrolyte, or packaging. Useful tensile muscle strokes were obtained (1.3% at 2.5 V and 0.52% at 1 V) when lifting loads that are ∼25 times heavier than can be lifted by the same diameter human skeletal muscle. Also, the tensile actuator maintained its contraction following charging and subsequent disconnection from the power supply because of its own supercapacitor property at the same time. Possible eventual applications for the individual tensile and torsional muscles are in micromechanical devices, such as for controlling valves and stirring liquids in microfluidic circuits, and in medical catheters.

IEEE Electron Device Letters, 2010

Nano letters, Jan 8, 2016

Electrical transport in monolayer graphene on SrTiO3 (STO) thin film is examined in order to prom... more Electrical transport in monolayer graphene on SrTiO3 (STO) thin film is examined in order to promote gate-voltage scaling using a high-k dielectric material. The atomically flat surface of thin STO layer epitaxially grown on Nb-doped STO single-crystal substrate offers good adhesion between the high-k film and graphene, resulting in non-hysteretic conductance as a function of gate voltage at all temperatures down to 2 K. The two-terminal conductance quantization under magnetic fields corresponding to quantum Hall states survives up to 200 K at a magnetic field of 14 T. In addition, the substantial shift of charge neutrality point in graphene seems to correlate with the temperature-dependent dielectric constant of the STO thin film, and its effective dielectric properties could be deduced from the universality of quantum phenomena in graphene. Our experimental data prove that the operating voltage reduction can be successfully realized due to the underlying high-k STO thin film, with...