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Doan, Manh-Ha,Jin, Youngjo,Adhikari, Subash,Lee, Sanghyub,Zhao, Jiong,Lim, Seong Chu,Lee, Young Hee American Chemical Society 2017 ACS NANO Vol.11 No.4
<P>Despite numerous studies on two-dimensional van der Waals heterostructures, a full understanding of the charge transport and photoinduced current mechanisms in these structures, in particular, associated with charge depletion/inversion layers at the interface remains elusive. Here, we investigate transport properties of a prototype multilayer MoS2/WSe2 heterojunction via a tunable charge inversion/depletion layer. A charge inversion layer was constructed at the surface of WSe2 due to its relatively low doping concentration compared to that of MoS2, which can be tuned by the back-gate bias. The depletion region was limited within a few nanometers in the MoS2 side, while charges are fully depleted on the whole WSe2 side, which are determined by Raman spectroscopy and transport measurements. Charge transport through the heterojunction was influenced by the presence of the inversion layer and involves two regimes of tunneling and recombination. Furthermore, photocurrent measurements clearly revealed recombination and space-charge-limited behaviors, similar to those of the heterostructures built from organic semiconductors. This contributes to research of various other types of heterostructures and can be further applied for electronic and optoelectronic devices.</P>
Enhanced Cathodoluminescence of KOH-treated InGaN/GaN LEDs with Deep Nano-Hole Arrays
Manh-Ha Doan,이재진 한국광학회 2014 Current Optics and Photonics Vol.18 No.3
Square lattice nano-hole arrays with diameters and periodicities of 200 and 500 nm, respectively, are fabricated on InGaN/GaN blue light emitting diodes (LEDs) using electron-beam lithography and inductively coupled plasma reactive ion etching processes. Cathodoluminescence (CL) investigations show that light emission intensity from the LEDs with the nano-hole arrays is enhanced compared to that from the planar sample. The CL intensity enhancement factor decreases when the nano-holes penetrate into the multiple quantum wells (MQWs) due to the plasma-induced damage and the residues. Wet chemical treatment using KOH solution is found to be an effective method for light extraction from the nano-patterned LEDs, especially, when the nano-holes penetrate into the MQWs. About 4-fold CL intensity enhancement factor is achieved by the KOH treatments after the dry etching for the sample with a 250-nm deep nano-hole array.
Quantifying the Effect of Economic Sanctions on Trade in Cultural Goods
Doan Ngoc Thang,Tran Manh Ha 한국국제경제학회 2023 International Economic Journal Vol.37 No.3
The question of whether the imposition of economic sanctions impairs or nurtures cultural affinity remains controversial. In effect, this paper investigates the effect of economic sanctions on the trade of cultural goods as a proxy for cultural proximity, using cross-country data with 5,304 country pairs ranging from the years 1996–2019. Through extensive robustness checks, our main findings reveal that economic sanctions drive up the trade of cultural goods. The effect on the trade of cultural goods is heterogeneous across various forms of economic sanctions. While military, arms, trade, and travel sanctions are trade facilitators, financial and other sanctions are trade barriers. These effects are conditional on the level of economic development in the sanctioned countries and change over time. Our findings suggest cultural implications of economic sanctions in foreign policy design.
Roy Shrawan,Doan Manh-Ha,Kim Jeongyong,Kang Seon Kyeong,Ahn Gwang Hwi,Lee Hyun Seok,Yun Seok Joon 한국물리학회 2021 THE JOURNAL OF THE KOREAN PHYSICAL SOCIETY Vol.78 No.8
Atomically thin monolayer transition-metal dochalcogenides (1L-TMDs) are optically active direct band gap semiconducting materials with interesting properties; they are appropriate platform to study and investigate the modulated optoelectronic properties due to locally induced charge transfer phenomenon using various approaches. Herein, 1L-TMDs ( MoS2, WS2 and WSe2) grown using chemical vapor deposition (CVD) were transferred above 10-nm-thick patterned platinum (Pt) stripes deposited on SiO2/ Si substrate to fabricate a local vertical heterostructure of 1L-TMDs with Pt. The optical characterization showed that the PL intensities of n (p)-type 1L-TMDs, namely MoS2 and WS2 ( WSe2), deposited above Pt were reduced with peak positions blue (red)-shifted by 40 (16) meV compared to the samples on SiO2/ Si substrates. This was attributed to the transfer of electrons from the 1L-TMDs to the Pt due to a charge transfer process at the interface. At the same time, an enhanced photocurrent, in comparison to 1L-MoS2 alone was observed under a negative gate voltage of − 40 V from the homojunctions of 1L-MoS2 and 1L-MoS2/Pt formed within the same grain structure due to a Pt-induced local p-doping effect. The charge modulation of the opto-electrical properties of 1L-TMDs due to charge transfer caused using patterned metal provides a simple lateral homojunction for enhanced photovoltaic applications.
Xuan Nui Pham,Manh B. Nguyen,Ha Son Ngo,Huan V. Doan 한국공업화학회 2020 Journal of Industrial and Engineering Chemistry Vol.90 No.-
In recent decades, highly efficient deep desulfurization processes have become very necessary todecrease environmental pollution due to sulfur emissions from fuels. Herein, an enhanced photocatalyticdesulfurization of a model fuel was investigated under sunlight irradiation using H2O2 as the oxidant andAg@AgBr loaded mesoporous silica Al-SBA-15 as a catalyst. In this study, the photocatalyst (Ag@AgBr/Al-SBA-15) was synthesized via a chemical deposition using halloysite clay as the silica–aluminum sourceand characterized by X-ray diffraction, N2 adsorption, scanning electron microscopy, transmissionelectron microscopy, and UV–Vis diffuse reflectance spectroscopy (UV–Vis DRS). The UV–Vis DRS resultsrevealed that the light absorption expanded to the visible region (l > 400 nm) for the various Ag@AgBrnanoparticles doped in the mesoporous Al-SBA-15 material. The 30% Ag@AgBr/Al-SBA-15 sample with a30% Ag@AgBr doping exhibited enhanced photocatalytic activity and showed high stability even afterfour successive cycles. The results demonstrated that initial dibenzothiophene concentrations (500 ppm)reached 98.66% removal with 50 mg of the catalyst dosage, 1.0 mL of H2O2, for 360 min of sunlightirradiation at 70 C.
Optical detection of charge accumulation in an electrically anisotropic semimetal of WTe2
Choi Young-Gwan,Doan Manh-Ha,Choi Gyung-Min 한국물리학회 2022 THE JOURNAL OF THE KOREAN PHYSICAL SOCIETY Vol.81 No.3
Charge accumulation in a current-injected device provides important information about the electrical properties of materials. Conventionally, local charge accumulation is measured by potential drops at multiple positions in the device using a voltage probe. Here, we report on direct observations of the spatial distribution of charge accumulation in semimetal WTe2 via the polarization rotation microscopy technique. The mappings of the polarization rotation of the refected light show that the injected charge carriers accumulate near the transverse boundaries of the device. Our results demonstrate an optical method to visualize charge accumulation profles of the electrically anisotropic medium
Optical thermometry of anisotropic semimetal of WTe2 using polarization-sensitive microscopy
Choi Young-Gwan,Doan Manh-Ha,Choi Gyung-Min 한국물리학회 2022 THE JOURNAL OF THE KOREAN PHYSICAL SOCIETY Vol.81 No.2
We report an optical thermometry technique using a polarization-sensitive laser scanning microscopy. WTe2 multilayer is heated by Joule heating from an alternating charge current, and its local temperature is detected via polarization rotation of the refected light. We found a linear relationship between the temperature and the polarization rotation angle with a conversion coefcient of 25±4 µrad/K. Considering the uncertain level of the polarization rotation microscopy, our system has a temperature resolution of less than 10 mK. We also confrmed that the spatial mapping of the temperature rise well matches the simulated current density distribution, which confrms that local heating is driven by Joule heating.
Facile Doping in Two-Dimensional Transition-Metal Dichalcogenides by UV Light
Ly, Thuc Hue,Deng, Qingming,Doan, Manh Ha,Li, Lain-Jong,Zhao, Jiong American Chemical Society 2018 ACS APPLIED MATERIALS & INTERFACES Vol.10 No.35
<P>Two-dimensional (2D) materials have been emerging as potential candidates for the next-generation materials in various technology fields. The performance of the devices based on these 2D materials depends on their intrinsic band structures as well as the extrinsic (doping) effects such as surrounding chemicals and environmental oxygen/moisture, which strongly determines their Fermi energy level. Herein, we report the UV treatments on the 2D transition-metal dichalcogenides, to controllably dope the samples without damaging the crystal structures or quenching the luminescence properties. More surprisingly, both n-type and p-type doping can be achieved depending on the initial status of the sample and the UV treatment conditions. The doping mechanisms were elaborated on the atomic scale with transmission electron microscopy and ab initio calculations. The facile doping by UV light has potential to be integrated with photolithography processes, aiming for the large-scale integrated device/circuits design and fabrications.</P> [FIG OMISSION]</BR>