Young Ki Hong

Giant Photoamplification in Indirect‐Bandgap Multilayer MoS2 Phototransistors with Local Bottom‐Gate Structures

Local-gate multilayer MoS2 phototransistors exhibit a photoresponsivity of up to 342.6 A W(-1) , which is higher by 3 orders of magnitude than that of global-gate multilayer MoS2 phototransistors. These simulations indicate that the gate underlap is critical for the enhancement of the photoresponsivity. These results suggest that high photoresponsivity can be achieved in indirect-bandgap multilayer MoS2 phototransistors by optimizing the optoelectronic design.

Highly Crystalline CVD-grown Multilayer MoSe2 Thin Film Transistor for Fast Photodetector

Hexagonal molybdenum diselenide (MoSe2) multilayers were grown by chemical vapor deposition (CVD). A relatively high pressure (>760u2009Torr) was used during the CVD growth to achieve multilayers by creating multiple nuclei based on the two-dimensional crystal growth model. Our CVD-grown multilayer MoSe2 thin-film transistors (TFTs) show p-type-dominant ambipolar behaviors, which are attributed to the formation of Se vacancies generated at the decomposition temperature (650u2009°C) after the CVD growth for 10u2009min. Our MoSe2 TFT with a reasonably high field-effect mobility (10u2009cm2/Vu2009·u2009s) exhibits a high photoresponsivity (93.7u2009A/W) and a fast photoresponse time (τriseu2009~u20090.4u2009s) under the illumination of light, which demonstrates the practical feasibility of multilayer MoSe2 TFTs for photodetector applications.

High‐Mobility Transistors Based on Large‐Area and Highly Crystalline CVD‐Grown MoSe2 Films on Insulating Substrates

Large-area and highly crystalline CVD-grown multilayer MoSe2 films exhibit a well-defined crystal structure (2H phase) and large grains reaching several hundred micrometers. Multilayer MoSe2 transistors exhibit high mobility up to 121 cm(2) V(-1) s(-1) and excellent mechanical stability. These results suggest that high mobility materials will be indispensable for various future applications such as high-resolution displays and human-centric soft electronics.

Optically transparent thin-film transistors based on 2D multilayer MoS2 and indium zinc oxide electrodes

We report on optically transparent thin film transistors (TFTs) fabricated using multilayered molybdenum disulfide (MoS2) as the active channel, indium tin oxide (ITO) for the back-gated electrode and indium zinc oxide (IZO) for the source/drain electrodes, respectively, which showed more than 81% transmittance in the visible wavelength. In spite of a relatively large Schottky barrier between MoS2 and IZO, the n-type behavior with a field-effect mobility (μ(eff)) of 1.4 cm(2) V(-1) s(-1) was observed in as-fabricated transparent MoS2 TFT. In order to enhance the performances of transparent MoS2 TFTs, a picosecond pulsed laser was selectively irradiated onto the contact region of the IZO electrodes. Following laser annealing, μ(eff) increased to 4.5 cm(2) V(-1) s(-1), and the on-off current ratio (I(on)/I(off)) increased to 10(4), which were attributed to the reduction of the contact resistance between MoS2 and IZO.

Erratum to: A highly sensitive chemical gas detecting transistor based on highly crystalline CVD-grown MoSe2 films

Layered semiconductors with atomic thicknesses are becoming increasingly important as active elements in high-performance electronic devices owing to their high carrier mobilities, large surface-to-volume ratios, and rapid electrical responses to their surrounding environments. Here, we report the first implementation of a highly sensitive chemical-vapor-deposition-grown multilayer MoSe2 field-effect transistor (FET) in a NO2 gas sensor. This sensor exhibited ultra-high sensitivity (S = ca. 1,907 for NO2 at 300 ppm), real-time response, and rapid on–off switching. The high sensitivity of our MoSe2 gas sensor is attributed to changes in the gap states near the valence band induced by the NO2 gas absorbed in the MoSe2, which leads to a significant increase in hole current in the off-state regime. Device modeling and quantum transport simulations revealed that the variation of gap states with NO2 concentration is the key mechanism in a MoSe2 FET-based NO2 gas sensor. This comprehensive study, which addresses material growth, device fabrication, characterization, and device simulations, not only indicates the utility of MoSe2 FETs for high-performance chemical sensors, but also establishes a fundamental understanding of how surface chemistry influences carrier transport in layered semiconductor devices.

Real-time electrical detection of epidermal skin MoS2 biosensor for point-of-care diagnostics

Various approaches have been proposed for point-of-care diagnostics, and in particular, optical detection is preferred because it is relatively simple and fast. At the same time, field-effect transistor (FET)-based biosensors have attracted great attention because they can provide highly sensitive and label-free detection. In this work, we present highly sensitive, epidermal skin-type point-of-care devices with system-level integration of flexible MoS2 FET biosensors, read-out circuits, and light-emitting diode (LEDs) that enable real-time detection of prostate cancer antigens (PSA). Regardless of the physical forms or mechanical stress conditions, our proposed high-performance MoS2 biosensors can detect a PSA concentration of 1 pg·mL–1 without specific surface treatment for anti-PSA immobilization on the MoS2 surface on which we characterize and confirm physisorption of anti-PSA using Kelvin probe force microscopy (KPFM) and tapping-mode atomic force microscopy (tm-AFM). Furthermore, current modulation induced by the binding process was stably maintained for longer than 2–3 min. The results indicate that flexible MoS2-based FET biosensors have great potential for point-of-care diagnostics for prostate cancer as well as other biomarkers.

Improving the Stability of High-Performance Multilayer MoS2 Field-Effect Transistors

In this study, we propose a method for improving the stability of multilayer MoS2 field-effect transistors (FETs) by O2 plasma treatment and Al2O3 passivation while sustaining the high performance of bulk MoS2 FET. The MoS2 FETs were exposed to O2 plasma for 30 s before Al2O3 encapsulation to achieve a relatively small hysteresis and high electrical performance. A MoOx layer formed during the plasma treatment was found between MoS2 and the top passivation layer. The MoOx interlayer prevents the generation of excess electron carriers in the channel, owing to Al2O3 passivation, thereby minimizing the shift in the threshold voltage (Vth) and increase of the off-current leakage. However, prolonged exposure of the MoS2 surface to O2 plasma (90 and 120 s) was found to introduce excess oxygen into the MoOx interlayer, leading to more pronounced hysteresis and a high off-current. The stable MoS2 FETs were also subjected to gate-bias stress tests under different conditions. The MoS2 transistors exhibited negligible decline in performance under positive bias stress, positive bias illumination stress, and negative bias stress, but large negative shifts in Vth were observed under negative bias illumination stress, which is attributed to the presence of sulfur vacancies. This simple approach can be applied to other transition metal dichalcogenide materials to understand their FET properties and reliability, and the resulting high-performance hysteresis-free MoS2 transistors are expected to open up new opportunities for the development of sophisticated electronic applications.

Interstitial Mo‐Assisted Photovoltaic Effect in Multilayer MoSe2 Phototransistors

Thin-film transistors (TFTs) based on multilayer molybdenum diselenide (MoSe2 ) synthesized by modified atmospheric pressure chemical vapor deposition (APCVD) exhibit outstanding photoresponsivity (103.1 A W-1 ), while it is generally believed that optical response of multilayer transition metal dichalcogenides (TMDs) is significantly limited due to their indirect bandgap and inefficient photoexcitation process. Here, the fundamental origin of such a high photoresponsivity in the synthesized multilayer MoSe2 TFTs is sought. A unique structural characteristic of the APCVD-grown MoSe2 is observed, in which interstitial Mo atoms exist between basal planes, unlike usual 2H phase TMDs. Density functional theory calculations and photoinduced transfer characteristics reveal that such interstitial Mo atoms form photoreactive electronic states in the bandgap. Models indicate that huge photoamplification is attributed to trapped holes in subgap states, resulting in a significant photovoltaic effect. In this study, the fundamental origin of high responsivity with synthetic MoSe2 phototransistors is identified, suggesting a novel route to high-performance, multifunctional 2D material devices for future wearable sensor applications.

Massive, eco-friendly, and facile fabrication of multi-functional anodic aluminum oxides: application to nanoporous templates and sensing platforms

In this paper, we report on an efficient and eco-friendly approach to fabricate AAOs in oxalic acid electrolyte, which exhibits a relatively wide range of electrolyte temperatures for stable anodization. Our strategy consists of simultaneous multi-surfaces anodization (SMSA) for fabricating plural AAOs and direct detachment of those AAOs from an aluminum (Al) substrate by applying stair-like reverse biases (SRBs) inxa0the same electrolyte used for the SMSAs. A unit sequence including SMSA sequentially combined with SRBs-based detachment can be applied repeatedly to the same Al substrate for mass production of AAOs. Dimensional characteristics of AAOs were quantitatively controlled with respect to the electrolyte temperature as well as the number of applied sequences, and SRBs-based direct detaching characteristics depending on the AAO thickness were investigated as a function of the number of stairs in SRBs. The AAOs fabricated here were used as nanoporous templates for synthesizing π-conjugated polymer nanomaterials with various diameters, and their structural and optical characteristics were studied with respect to their physical dimensions. We also fabricated capacitive humidity sensors designed on interdigitated electrode structures with nanoporous AAOs, and discussed their superior device performances.

High performance and transparent multilayer MoS2 transistors: Tuning Schottky barrier characteristics

Various strategies and mechanisms have been suggested for investigating a Schottky contact behavior in molybdenum disulfide (MoS2) thin-film transistor (TFT), which are still in much debate and controversy. As one of promising breakthrough for transparent electronics with a high device performance, we have realized MoS2 TFTs with source/drain electrodes consisting of transparent bi-layers of a conducting oxide over a thin film of low work function metal. Intercalation of a low work function metal layer, such as aluminum, between MoS2 and transparent source/drain electrodes makes it possible to optimize the Schottky contact characteristics, resulting in about 24-fold and 3 orders of magnitude enhancement of the field-effect mobility and on-off current ratio, respectively, as well as transmittance of 87.4 % in the visible wavelength range.