Ji-Hoon Ahn

Deterministic Two-Dimensional Polymorphism Growth of Hexagonal n-Type SnS2 and Orthorhombic p-Type SnS Crystals

van der Waals layered materials have large crystal anisotropy and crystallize spontaneously into two-dimensional (2D) morphologies. Two-dimensional materials with hexagonal lattices are emerging 2D confined electronic systems at the limit of one or three atom thickness. Often these 2D lattices also form orthorhombic symmetries, but these materials have not been extensively investigated, mainly due to thermodynamic instability during crystal growth. Here, we show controlled polymorphic growth of 2D tin-sulfide crystals of either hexagonal SnS2 or orthorhombic SnS. Addition of H2 during the growth reaction enables selective determination of either n-type SnS2 or p-type SnS 2D crystal of dissimilar energy band gap of 2.77 eV (SnS2) or 1.26 eV (SnS) as a final product. Based on this synthetic 2D polymorphism of p-n crystals, we also demonstrate p-n heterojunctions for rectifiers and photovoltaic cells, and complementary inverters.

Rotation‐Misfit‐Free Heteroepitaxial Stacking and Stitching Growth of Hexagonal Transition‐Metal Dichalcogenide Monolayers by Nucleation Kinetics Controls

2D vertical stacking and lateral stitching growth of monolayer (ML) hexagonal transition-metal dichalcogenides are reported. The 2D heteroepitaxial manipulation of MoS2 and WS2 MLs is achieved by control of the 2D nucleation kinetics during the sequential vapor-phase growth. It enables the creation of hexagon-on-hexagon unit-cell stacking and hexagon-by-hexagon stitching without interlayer rotation misfits.

Interlayer orientation-dependent light absorption and emission in monolayer semiconductor stacks

Two-dimensional stacks of dissimilar hexagonal monolayers exhibit unusual electronic, photonic and photovoltaic responses that arise from substantial interlayer excitations. Interband excitation phenomena in individual hexagonal monolayer occur in states at band edges (valleys) in the hexagonal momentum space; therefore, low-energy interlayer excitation in the hexagonal monolayer stacks can be directed by the two-dimensional rotational degree of each monolayer crystal. However, this rotation-dependent excitation is largely unknown, due to lack in control over the relative monolayer rotations, thereby leading to momentum-mismatched interlayer excitations. Here, we report that light absorption and emission in MoS2/WS2 monolayer stacks can be tunable from indirect- to direct-gap transitions in both spectral and dynamic characteristics, when the constituent monolayer crystals are coherently stacked without in-plane rotation misfit. Our study suggests that the interlayer rotational attributes determine tunable interlayer excitation as a new set of basis for investigating optical phenomena in a two-dimensional hexagonal monolayer system.

Thermoelectric elastomer fabricated using carbon nanotubes and nonconducting polymer

The electrical and thermoelectric properties of an organic elastomer composite composed of carbon nanotubes (CNTs) and a nonconductive polymer were systemically investigated as a function of CNT content. As the CNT content of the poly(dimethylsiloxane) (PDMS) matrix increased, the electrical conductivity increased remarkably (by about 250 times) without a large increase in the thermal conductivity, which could lead to significant improvement in the ZT value. Moreover, the Seebeck coefficient was also enhanced by increasing the CNT content. Consequently, the ZT value was effectively increased by a small increase in the quantity of CNTs in the nonconductive polymer matrix.

Vapor phase synthesis of TaS2 nanocrystals with iodine as transport agent

TaS2 is a transition metal dichalcogenide material with a layered two-dimensional crystal structure and physically interesting behavior. 1T-TaS2 in particular exhibits a complex interaction between electron–electron and electron–lattice, as well as unique two-dimensional charge density wave characteristics with varying phase transitions depending on the temperature range. However, few reports exist on the synthesis of TaS2 crystals, and a relatively long time is required for the synthesis of TaS2 nanostructures. Here, we present an efficient method for 1T-TaS2 synthesis by chemical vapor deposition using iodine as a transport agent. We successfully synthesized 1T-TaS2 nanocrystals or thin films in a short processing time of a few hours, and their structural and chemical properties were characterized.

Wafer‐Scale Synthesis of Reliable High‐Mobility Molybdenum Disulfide Thin Films via Inhibitor‐Utilizing Atomic Layer Deposition

A reliable and rapid manufacturing process of molybdenum disulfide (MoS2 ) with atomic-scale thicknesses remains a fundamental challenge toward its successful incorporation into high-performance nanoelectronics. It is imperative to achieve rapid and scalable production of MoS2 exhibiting high carrier mobility and excellent on/off current ratios simultaneously. Herein, inhibitor-utilizing atomic layer deposition (iALD) is presented as a novel method to meet these requirements at the wafer scale. The kinetics of the chemisorption of Mo precursors in iALD is governed by the reaction energy and the steric hindrance of inhibitor molecules. By optimizing the inhibition of Mo precursor absorption, the nucleation on the substrate in the initial stage can be spontaneously tailored to produce iALD-MoS2 thin films with a significantly increased grain size and surface coverage (>620%). Moreover, highly crystalline iALD-MoS2 thin films, with thicknesses of only a few layers, excellent room temperature mobility (13.9 cm2 V-1 s-1 ), and on/off ratios (>108 ), employed as the channel material in field effect transistors on 6″ wafers, are successfully prepared.

Self-Formed Channel Devices Based on Vertically Grown 2D Materials with Large-Surface-Area and Their Potential for Chemical Sensor Applications

2D layered materials with sensitive surfaces are promising materials for use in chemical sensing devices, owing to their extremely large surface-to-volume ratios. However, most chemical sensors based on 2D materials are used in the form of laterally defined active channels, in which the active area is limited to the actual device dimensions. Therefore, a novel approach for fabricating self-formed active-channel devices is proposed based on 2D semiconductor materials with very large surface areas, and their potential gas sensing ability is examined. First, the vertical growth phenomenon of SnS2 nanocrystals is investigated with large surface area via metal-assisted growth using prepatterned metal electrodes, and then self-formed active-channel devices are suggested without additional pattering through the selective synthesis of SnS2 nanosheets on prepatterned metal electrodes. The self-formed active-channel device exhibits extremely high response values (>2000% at 10 ppm) for NO2 along with excellent NO2 selectivity. Moreover, the NO2 gas response of the gas sensing device with vertically self-formed SnS2 nanosheets is more than two orders of magnitude higher than that of a similar exfoliated SnS2 -based device. These results indicate that the facile device fabrication method would be applicable to various systems in which surface area plays an important role.

Photo-thermoelectric properties of SnS nanocrystals with orthorhombic layered structure

The photo-thermoelectric properties of SnS nanocrystals, two-dimensional materials with an orthorhombic symmetry, were investigated using a focused laser scanning method. The SnS nanocrystals were synthesized by a vapor transport method, and their fundamental material and electrical properties were investigated. Upon shining a laser onto the SnS channel region under a positive source-drain bias, a positive photocurrent was observed due to photo-excited electron-hole pairs. On the other hand, when this external electric field was not applied, a strong photocurrent was observed within the metal electrode region rather than at the metal-semiconductor interface, which indicated that the major mechanism for the photocurrent under zero external bias was a photo-induced thermoelectric effect rather than a photovoltaic effect. Moreover, the Seebeck coefficient of the SnS nanocrystal device was approximately 1735 μV/K, which is 3.5 times larger than that of its bulk counterpart.

Low Power Switching Characteristics of CNT Field Effect Transistor Device with Al-Doped ZrHfO2 Gate Dielectric

In this report, we demonstrated a reliable switching effect of carbon nanotube (CNT) field-effect transistor (FET) devices integrated with 99% semiconducting CNT as a channel and high-k oxide as the dielectric. CNT FET devices with high-k oxides of Al-ZrHfO2 and Al2O3 were electrically characterized and compared. There was no considerable hysteresis in the Al2O3-based CNT FET device. The Al-ZrHfO2 with a tetragonal phase-based high dielectric constant (~47), designed by an atomic layer deposition process, showed a reliable switching effect as well as low operation voltage (<±3 V). Charge trapping/detrapping process via oxygen vacancy-related defects of Al-ZrHfO2 was proposed as a primary mechanism to explain a current change of a counterclockwise direction and threshold voltage (Vth) shift for transfer properties. The suggested charge trapping model within bulk oxide was experimentally proven since the hysteresis from the adsorption/desorption of gas molecules to CNT surface was negligible. Endurance characteristics of the CNT switching devices remained stable without any serious current fluctuation during a repetitive cycling test. The memory device with reliable switching properties as well as low operation power would pave a road toward next-generation memory components of portable electronic gadgets.

A Hybrid Gate Dielectrics of Ion Gel with Ultra-Thin Passivation Layer for High-Performance Transistors Based on Two-Dimensional Semiconductor Channels

We propose a hybrid gate structure for ion gel dielectrics using an ultra-thin Al2O3 passivation layer for realizing high-performance devices based on electric-double-layer capacitors. Electric-double-layer transistors can be applied to practical devices with flexibility and transparency as well as research on the fundamental physical properties of channel materials; however, they suffer from inherent unwanted leakage currents between electrodes, especially for channel materials with low off-currents. Therefore, the Al2O3 passivation layer was introduced between the metal electrodes and ion gel film as a leakage current barrier; this simple approach effectively reduced the leakage current without capacitance degradation. In addition, we confirmed that a monolayer MoS2 transistor fabricated with the proposed hybrid gate dielectric exhibited remarkably enhanced device properties compared to a transistor using a normal ion gel gate dielectric. Our findings on a simple method to improve the leakage current properties of ion gels could be applied extensively to realize high-performance electric-double-layer transistors utilizing various channel materials.