텅스텐 셀레늄 화합물 박막의 두께 조절 합성과 두께에 따른 라만 분광 특성 연구

강건희,최병기,김지호,장영준 한국물리학회 2015 새물리 Vol.65 No.7

텅스텐 셀레늄 화합물 (WSe2) 박막을 화학기상증착법 (chemical vapor deposition, CVD)으로 센티미터 크기의 사파이어 기판 위에 두께를 증가시키면서 성장시켰다. 다양한 수소비율과 온도의 성장조건에 따른 라만분광측정 결과를 통해, 성장조건에 민감하게 변하는 박막의 결정성을 확인하고 최적화된 성장조건을 수소비율 15%와 증착온도 875 ℃ 로 추정할 수 있었다. 박막 두께 (1 - 8 nm)에 따라서 주요 라만 봉우리들 (E112g, A1g, 2LA)의 파수와 세기의 변화를 분석하였고, WSe2 벌크 물질의 1-3층에서 보고된 결과와의 비교를 통해서 대면적 WSe2 박막의 두께확인이 가능하였다. 이로써 WSe2 박막을 두께를 조절하여 성장하는 것이 가능했고, 라만 봉우리 분석을 이용해 비파괴적인 두께 분석법을 넓은 두께 범위의 박막에 적용하였다. Tungsten diselenide (WSe2) thin films with different thicknesses were fabricated on sapphire wafers. To fabricate centimeter-scale large-area films, we evaporated tungsten-oxide films followed by selenization under various hydrogen gas concentrations (0 - 20%) and at various substrate temperatures (700 - 1000 ℃) in a chemical-vapor-deposition chamber. We utilized the Raman spectral intensity to evaluate the optimum growth conditions (15% and 875 ℃) and observed a thicknessdependent change in the phonon peaks in the thickness range from 0.3 to 8 nm. By comparing the major peaks (E1 2g, A1g, and 2LA), we confirmed that the WSe2 films had the expected film thicknesses and the corresponding peak shapes. Our results demonstrate both a growth method for the large-area WSe2 films with thickness controllability and a nondestructive analysis method for determining the layer thickness over a wide thickness range.

Morphological characteristics in polycrystalline tungsten diselenide regulating transport properties lead to predominant thermoelectric performance

Kim, Cham,Baek, Ju Young,Kim, Dong Hwan,Kim, Jong Tae,Lopez, David Humberto,Kim, Taewook,Kim, Hoyoung Elsevier 2017 Journal of Alloys and Compounds Vol.722 No.-

<P><B>Abstract</B></P> <P>We studied a polycrystalline p-type WSe<SUB>2</SUB> semiconductor for thermoelectric applications. The polycrystalline WSe<SUB>2</SUB> nanocompound was prepared via a thermal reaction process of tungsten and selenium elements and it was sintered to produce a bulk structure using spark plasma sintering equipment. The resulting bulk specimen showed different morphological aspects, in which we observed irregularly-shaped grains along the direction perpendicular to the sintering pressing direction (i.e., along transversal direction) while finding thin layers along the parallel direction (i.e., along longitudinal direction). The specimen recorded a significantly low longitudinal thermal conductivity possibly because longitudinal phonon transport should be hindered due to the thin layers. Electron transport along the longitudinal direction might not be greatly interrupted by the morphological characteristics because the specimen recorded high carrier mobility along the direction resulting in lower electrical resistivity than that of a single crystalline equivalent. The specimen also showed moderate carrier concentration, which led to a plausible Seebeck coefficient. Since the specimen exhibited the significantly low thermal conductivity with the electrical properties, it recorded a higher figure of merit than the equivalent, which is the highest thermoelectric performance for p-type WSe<SUB>2</SUB> in bulk phase ever developed.</P> <P><B>Highlights</B></P> <P> <UL> <LI> A new thermal reaction process was devised for a polycrystalline WSe<SUB>2</SUB> nanocompound. </LI> <LI> The nanocompound showed morphological anisotropy led to decoupling of conductivities. </LI> <LI> The highest thermoelectric performance was done for bulk phase WSe<SUB>2</SUB> ever reported. </LI> </UL> </P> <P><B>Graphical abstract</B></P> <P>[DISPLAY OMISSION]</P>

P-doping Effects of Tungsten Diselenide Transistors by Fluoropolymer Encapsulation

Hyeonji Lee,Seongin Hong,Hocheon Yoo 한국진공학회 2021 한국진공학회 학술발표회초록집 Vol.2021 No.2

We show that the ambipolar charge transport of the WSe₂ transistor can be made into a unipolar p- type through Cytop fluoropolymer and thermal annealing. After doping, the charge transport can be controlled by thermal annealing temperature. The C-F bond is rearranged through thermal annealing, and the well-aligned dipole moment causes the Fermi level to be lowered, enabling the channel transport to be p-doped.

Highly Enhanced Photoresponsivity of a Monolayer WSe₂ Photodetector with Nitrogen-Doped Graphene Quantum Dots

Nguyen, Duc Anh,Oh, Hye Min,Duong, Ngoc Thanh,Bang, Seungho,Yoon, Seok Jun,Jeong, Mun Seok American Chemical Society 2018 ACS APPLIED MATERIALS & INTERFACES Vol.10 No.12

<P>Hybrid structures of two-dimensional (2D) materials and quantum dots (QDs) are particularly interesting in the field of nanoscale optoelectronic devices because QDs are efficient light absorbers and can inject photocarriers into thin layers of 2D transition-metal dichalcogenides, which have high carrier mobility. In this study, we present a heterostructure that consists of a monolayer of tungsten diselenide (ML WSe<SUB>2</SUB>) covered by nitrogen-doped graphene QDs (N-GQDs). The improved photoluminescence of ML WSe<SUB>2</SUB> is attributed to the dominant neutral exciton emission caused by the n-doping effect. Owing to strong light absorption and charge transfer from N-GQDs to ML WSe<SUB>2</SUB>, N-GQD-covered ML WSe<SUB>2</SUB> showed up to 480% higher photoresponsivity than that of a pristine ML WSe<SUB>2</SUB> photodetector. The hybrid photodetector exhibits good environmental stability, with 46% performance retention after 30 days under ambient conditions. The photogating effect also plays a key role in the improvement of hybrid photodetector performance. On applying the back-gate voltage modulation, the hybrid photodetector shows a responsivity of 2578 A W<SUP>-1</SUP>, which is much higher than that of the ML WSe<SUB>2</SUB>-based device.</P> [FIG OMISSION]</BR>

A systematic study of the synthesis of monolayer tungsten diselenide films on gold foil

윤석준,김수민,김기강,이영희 한국물리학회 2016 Current Applied Physics Vol.16 No.9

We report a systematic study of the synthesis of large-area monolayer WSe2 on gold foil by controlling the growth temperature and the partial pressure of hydrogen during chemical vapor deposition. The gold surface causes surface-mediated growth to form monolayer WSe2 films. The amount of the tungsten source is controlled by adjusting the partial pressure of hydrogen, which plays a role in the reduction of WO3 (solid phase) into WO3x (vapor phase). The coverage of monolayer WSe2 can be effectively controlled by changing either the partial pressure of hydrogen or the growth temperature at a fixed growth time under an Se-rich atmosphere, resulting in 100% coverage of the WSe2 film. The crystallinity and thickness uniformity are characterized by Raman spectroscopy, photoluminescence, and transmission electron microscopy. This characterization reveals that the quality of the WSe2 film is comparable to mechanically-exfoliated monolayer WSe2 and possesses good thickness uniformity.

Decoupling of thermal and electrical conductivities by adjusting the anisotropic nature in tungsten diselenide causing significant enhancement in thermoelectric performance

Kim, Cham,Baek, Ju Young,Kim, Dong Hwan,Kim, Jong Tae,Lopez, David Humberto,Kim, Taewook,Kim, Hoyoung THE KOREAN SOCIETY OF INDUSTRIAL AND ENGINEERING 2018 JOURNAL OF INDUSTRIAL AND ENGINEERING CHEMISTRY -S Vol.60 No.-

<P><B>Abstract</B></P> <P>A polycrystalline WSe<SUB>2</SUB> nanocompound was produced via a brief thermal reaction between the atomic elements. It should grow along the in-plane direction with covalent bonds rather than along the through-plane direction with van der Waals forces, leading to both crystallographic and morphological anisotropies. Not only the anisotropies should structurally induce strong phonon scattering but they alleviate possible electron scattering at the van der Waals forces; thus, we greatly reduced thermal conductivity while minimizing electrical conductivity loss. The decoupled conductivities resulted in enhancement in figure of merit, by approximately 70% at 350°C, thus affording a promising material for mid-temperature thermoelectric operations.</P> <P><B>Graphical abstract</B></P> <P>[DISPLAY OMISSION]</P>

폭발물 감지 시스템 개발을 위한 TNT 분자 흡착에 대한 WSe2 소자의 전기적 반응 특성 평가

김찬휘,조수연,김형태,이원주,박준홍 한국결정성장학회 2023 한국결정성장학회지 Vol.33 No.6

분자 단위의 폭발물질을 탐지하기 위하여, 고감도 응답성 센서의 개발이 요구되고있다. 2차원 반도체는얇은 적층형 구조를 가져 전하 캐리어가 축적될 수 있어, 전하 캐리어의 급격한 신호 변조 특성을 기대할 수 있다. WSe2 반도체 소재의 TNT(Trinitrotoluene) 폭발물질에 대한 탐지 효용성을 연구하기 위해, CVD(Chemical Vapor Deposition) 공정을 이용해 WSe2 박막을 합성하여 FET(Field Effect Transistors)을 제작하였다. 라만 분석과 FT-IR(Fourier-transform infrared) 분광 결과는 TNT 분자의 흡착과 WSe2 결정질의 구조적 전이 분석 정보를 나타내었다. 또한, WSe2 표면의 TNT 분자 흡착 전후의 전기적 특성을 비교하였다 . TNT 도포 전, WSe2 FET에 백 게이트 바이어스로 50 V를 인가함에 따라 0.02 A의 최대 전류값이 관측되었고, 0.6%(w/v) TNT 용액을 도포하였을 때 Drain 전류는 p-type 거동을 보이면서 0.41 A의 최대 전류 값을 기록하였다. 이후 On/Off Ratio 및 캐리어 이동도, 히스테리시스를 추가적으로 평가하였다. 본 연구에서는 WSe2의 TNT 분자에 대한 고감도와 신속한 응답성을 통해 폭발물질 탐지 센서 소재로서의 가능성을 제시하였다. As demanding the detection of explosive molecules, it is required to develop rapidly and precisely responsivesensors with ultra-high sensitivity. Since two-dimensional semiconductors have an atomically thin body nature where mobilecarriers accumulate, the abrupt modulation carrier in the thin body channel can be expected. To investigate the effectivenessof WSe2 semiconductor materials as a detection material for TNT (Trinitrotoluene) explosives, WSe2 was synthesized using thermal chemical vapor deposition, and afterward, WSe2 FETs (Field Effect Transistors) were fabricated using standard photo-lithograph processes. Raman Spectrum and FT-IR (Fourier-transform infrared) spectroscopy reveal that the adsorption of TNT molecules induces the structural transition of WSe2 crystalline. The electrical properties before and after adsorption of TNT molecules on the WSe2 surface were compared; as 50 V was applied as the back gate bias, 0.02 A was recorded in the bare state, and the drain current increased to 0.41 A with a dropping 0.6% (w/v) TNT while maintaining the p-type behavior. Afterward, the electrical characteristics were additionally evaluated by comparing the carrier mobility, hysteresis, and on/off ratio. Consequently, the present report provides the milestone for developing ultra-sensitive sensors with rapid response and high precision.

Decoupling of thermal and electrical conductivities by adjusting the anisotropic nature in tungsten diselenide causing significant enhancement in thermoelectric performance

김참,백주영,김동환,김종태,David Humberto Lopez,김태욱,김호영 한국공업화학회 2018 Journal of Industrial and Engineering Chemistry Vol.60 No.-

A polycrystalline WSe2 nanocompound was produced via a brief thermal reaction between the atomic elements. It should grow along the in-plane direction with covalent bonds rather than along the through-plane direction with van der Waals forces, leading to both crystallographic and morphological anisotropies. Not only the anisotropies should structurally induce strong phonon scattering but they alleviate possible electron scattering at the van der Waals forces; thus, we greatly reduced thermal conductivity while minimizing electrical conductivity loss. The decoupled conductivities resulted in enhancement in figure of merit, by approximately 70% at 350 °C, thus affording a promising material for mid-temperature thermoelectric operations.

Encapsulation of a Monolayer WSe₂ Phototransistor with Hydrothermally Grown ZnO Nanorods

Lee, Kang-Nyeoung,Bang, Seungho,Duong, Ngoc Thanh,Yun, Seok Joon,Park, Dae Young,Lee, Juchan,Choi, Young Chul,Jeong, Mun Seok American Chemical Society 2019 ACS APPLIED MATERIALS & INTERFACES Vol.11 No.22

<P>Transition metal dichalcogenides (TMDCs) are promising two-dimensional (2D) materials for realizing next-generation electronics and optoelectronics with attractive physical properties. However, monolayer TMDCs (<SUP>1L</SUP>TMDCs) have various serious issues, such as instability under ambient conditions and low optical quantum yield from their extremely thin thickness of ∼0.7 nm. To overcome these issues, we constructed a hybrid structure (HS) by growing zinc oxide nanorods (ZnO NRs) on a monolayer tungsten diselenide (<SUP>1L</SUP>WSe<SUB>2</SUB>) using the hydrothermal method. Consequently, we confirmed not only enhanced photoluminescence of <SUP>1L</SUP>WSe<SUB>2</SUB> but also improved optoelectronic properties by fabricating the HS phototransistor. Through various investigations, we found that these phenomena were due to the antenna and p-type doping effects attributed to the ZnO NRs. In addition, we verified that the optoelectronic properties of <SUP>1L</SUP>TMDCs are maintained for 2 weeks in ambient condition through the sustainable encapsulation effect induced by our HS. This encapsulation method with inorganic materials is expected to be applied to improve the stability and performance of various emerging 2D material-based devices.</P> [FIG OMISSION]</BR>

Recovery Improvement for Large-Area Tungsten Diselenide Gas Sensors

Ko, Kyung Yong,Park, Kyunam,Lee, Sangyoon,Kim, Youngjun,Woo, Whang Je,Kim, Donghyun,Song, Jeong-Gyu,Park, Jusang,Kim, Hyungjun American Chemical Society 2018 ACS APPLIED MATERIALS & INTERFACES Vol.10 No.28

<P>Semiconducting two-dimensional transition-metal dichalcogenides are considered promising gas-sensing materials because of their large surface-to-volume ratio, excellent electrical conductivity, and susceptible surfaces. However, enhancement of the recovery performance has not yet been intensively explored. In this study, a large-area uniform WSe<SUB>2</SUB> is synthesized for use in a high-performance semiconductor gas sensor. At room temperature, the WSe<SUB>2</SUB> gas sensor shows a significantly high response (4140%) to NO<SUB>2</SUB> compared to the use of NH<SUB>3</SUB>, CO<SUB>2</SUB>, and acetone. This paper demonstrates improved recovery of the WSe<SUB>2</SUB> gas sensor’s NO<SUB>2</SUB>-sensing performance by utilizing external thermal energy. In addition, a novel strategy for improving the recovery of the WSe<SUB>2</SUB> gas sensor is realized by reacting NH<SUB>3</SUB> and adsorbed NO<SUB>2</SUB> on the surface of WSe<SUB>2</SUB>: the NO<SUB>2</SUB> molecules are spontaneously desorbed, and the recovery time is dramatically decreased (85 min → 43 s). It is expected that the fast recovery of the WSe<SUB>2</SUB> gas sensor achieved here will be used to develop an environmental monitoring system platform.</P> [FIG OMISSION]</BR>