Strain effects on phase transitions in transition metal dichalcogenides

강승훈,권영균 한국물리학회 2019 Current Applied Physics Vol.19 No.6

We perform density functional theory calculation to investigate the structural and electronic properties of various two-dimensional transition metal dichalcogenides, MX2 (M=Ti, V, Cr, Zr, Nb, Mo, Hf, Ta, or W, and X=S or Se), and their strain-induced phase transitions. We evaluate the relative stability and the activation barrier between the octahedral-T and the trigonal-H phases of each MX2. It is found that the equilibrium and phase transition characteristics of MX2 can be classified by the group to which its metal element M belongs in the periodic table. MX2 with M in the group 4 (Ti, Zr, or Hf), forms an octahedral-T phase, while that with an M in the group 6 (Cr, Mo, or W) does a trigonal-H phase. On the other hand, MX2 with M in the group 5 (V, Nb, or Ta), which is in-between the groups 4 and 6, may form either phase with a similar stability. It is also found that their electronic structures are strongly correlated to the structural configurations: mostly metallic in the T phase, while semiconducting in the H phase, although there are some exceptions. We also explore the effects of an applied stress and find for some MX2 materials that the resultant strain, either tensile or compressive, may induce a structural phase transition by reducing the transition energy barrier, which is, in some cases, accompanied by its metal-insulator transition.

Strong Thermopower Enhancement and Tunable Power Factor <i>via</i> Semimetal to Semiconductor Transition in a Transition-Metal Dichalcogenide

Moon, Hongjae,Bang, Joonho,Hong, Seokkyoon,Kim, Gwansik,Roh, Jong Wook,Kim, Jeongmin,Lee, Wooyoung American Chemical Society 2019 ACS NANO Vol.13 No.11

<P>Electronic band engineering is a promising approach to enhance the thermopower of thermoelectric materials. In transition-metal dichalcogenides (TMDCs), this has so far only been achieved using their inherent semiconducting nature. Here, we report the thickness-modulated band engineering of nanosheets based on semimetallic platinum diselenide (PtSe<SUB>2</SUB>) resulting in a thermopower enhancement of more than 50 times than that of the bulk. We obtained this by introducing a semimetal to semiconductor (SMSC) transition resulting in the formation of a bandgap. This approach based on semimetallic TMDCs provides potential advantages such as a large variation of transport properties, a decrease of the ambipolar transport effect, and a high carrier density dependence of the transport properties. Our observations suggest that the SMSC transition in TMDCs is a promising and straightforward strategy for the development of two-dimensional nanostructured thermoelectric materials.</P> [FIG OMISSION]</BR>

MoTe₂ Lateral Homojunction Field-Effect Transistors Fabricated using Flux-Controlled Phase Engineering

Ma, Rui,Zhang, Huairuo,Yoo, Youngdong,Degregorio, Zachary Patrick,Jin, Lun,Golani, Prafful,Ghasemi Azadani, Javad,Low, Tony,Johns, James E.,Bendersky, Leonid A.,Davydov, Albert V.,Koester, Steven J. American Chemical Society 2019 ACS NANO Vol.13 No.7

<P>The coexistence of metallic and semiconducting polymorphs in transition-metal dichalcogenides (TMDCs) can be utilized to solve the large contact resistance issue in TMDC-based field effect transistors (FETs). A semiconducting hexagonal (2H) molybdenum ditelluride (MoTe<SUB>2</SUB>) phase, metallic monoclinic (1T′) MoTe<SUB>2</SUB> phase, and their lateral homojunctions can be selectively synthesized <I><I>in situ</I></I> by chemical vapor deposition due to the small free energy difference between the two phases. Here, we have investigated, in detail, the structural and electrical properties of <I>in situ</I>-grown lateral 2H/1T′ MoTe<SUB>2</SUB> homojunctions grown using flux-controlled phase engineering. Using atomic-resolution plan-view and cross-sectional transmission electron microscopy analyses, we show that the round regions of near-single-crystalline 2H-MoTe<SUB>2</SUB> grow out of a polycrystalline 1T′-MoTe<SUB>2</SUB> matrix. We further demonstrate the operation of MoTe<SUB>2</SUB> FETs made on these <I>in situ</I>-grown lateral homojunctions with 1T′ contacts. The use of a 1T′ phase as electrodes in MoTe<SUB>2</SUB> FETs effectively improves the device performance by substantially decreasing the contact resistance. The contact resistance of 1T′ electrodes extracted from transfer length method measurements is 470 ± 30 Ω·μm. Temperature- and gate-voltage-dependent transport characteristics reveal a flat-band barrier height of ∼30 ± 10 meV at the lateral 2H/1T′ interface that is several times smaller and shows a stronger gate modulation, compared to the metal/2H Schottky barrier height. The information learned from this analysis will be critical to understanding the properties of MoTe<SUB>2</SUB> homojunction FETs for use in memory and logic circuity applications.</P> [FIG OMISSION]</BR>

Electronic structure and charge-density wave transition in monolayer VS2

김혁진,최병기,이인학,김민재,천승현,Jozwiak Chris,Bostwick Aaron,Rotenberg Eli,Chang Young Jun 한국물리학회 2021 Current Applied Physics Vol.30 No.-

Vanadium disulfide (VS2) attracts elevated interests for its charge-density wave (CDW) phase transition, ferromagnetism, and catalytic reactivity, but the electronic structure of monolayer has not been well understood yet. Here we report synthesis of epitaxial 1T VS2 monolayer on bilayer graphene grown by molecular-beam epitaxy (MBE). Angle-resolved photoemission spectroscopy (ARPES) measurements reveal that Fermi surface with six elliptical pockets centered at the M points shows gap opening at low temperature. Temperature-dependence of the gap size suggests existence of CDW phase transition above room temperature. Our observations provide important evidence to understand the strongly correlated electron physics and the related surface catalytic properties in two-dimensional transition-metal dichalcogenides (TMDCs).

핫픽업 전사기술을 이용한 고성능 WSe₂ 기반 전계효과 트랜지스터의 제작

김현호 ( Hyun Ho Kim ) 한국접착및계면학회 2020 접착 및 계면 Vol.21 No.3

원자층 두께의 전이금속 칼코겐화합물(transition-metal dichalcogenide, TMD) 기반 반도체 소재는 그래핀과 비슷한 구조의 이차원구조를 지니는 소재로서 조절 가능한 밴드갭 뿐만 아니라 우수한 유연성, 투명성 등 다양한 장점으로 인해 다양한 미래사회의 전자소자에 활용될 수 있는 소재로서 각광받고 있다. 하지만 이러한 TMD 소재들은 수분과 산소에 매우 취약하다는 단점 때문에 대기안정성을 해결할 수 있는 다양한 시도가 이루어지고 있다. 본 연구에서는 핫픽업 전사기술을 이용하여 TMD 반도체 소재 중 하나인 WSe₂ 와 이차원 절연체 h-BN와의 수직 헤테로 구조를 제작하여 WSe₂의 대기안정성을 향상시키기 위한 연구를 수행하였으며, h-BN/WSe₂ 구조를 활용하여 WSe₂ 기반 고성능 전계효과 트랜지스터 제작에 대한 연구를 수행하였다. 제작된 소자의 전기적 특성을 분석한 결과, h-BN에 의해 표면이 안정화된 WSe₂ 기반 소자는 대기안정성 뿐만 아니라 150 cm²/Vs의 상온 정공 이동도, 3×10⁶의 온/오프 전류비, 192 mV/decade의 서브문턱스윙 등 우수한 전기적 특성을 갖는다는 것 또한 확인할 수 있었다. Recently, the atomically thin transition-metal dichalcogenide (TMD) semiconductors have attracted much attention owing to their remarkable properties such as tunable bandgap with high carrier mobility, flexibility, transparency, etc. However, because these TMD materials have a significant drawback that they are easily degraded in an ambient environment, various attempts have been made to improve chemical stability. In this research article, I report a method to improve the air stability of WSe₂ one of the TMD materials via surface passivation with an h-BN insulator, and its application to field-effect transistors (FETs). With a modified hot pick-up transfer technique, a vertical heterostructure of h-BN/WSe₂ was successfully made, and then the structure was used to fabricate the top-gate bottom-contact FETs. The fabricated WSe₂-based FET exhibited not only excellent air stability, but also high hole mobility of 150 cm²/Vs at room temperature, on/off current ratios up to 3×10⁶, and 192 mV/decade of subthreshold swing.

Epitaxial Synthesis of Molybdenum Carbide and Formation of a Mo₂C/MoS₂ Hybrid Structure <i>via</i> Chemical Conversion of Molybdenum Disulfide

Jeon, Jaeho,Park, Yereum,Choi, Seunghyuk,Lee, Jinhee,Lim, Sung Soo,Lee, Byoung Hun,Song, Young Jae,Cho, Jeong Ho,Jang, Yun Hee,Lee, Sungjoo American Chemical Society 2018 ACS NANO Vol.12 No.1

<P>The epitaxial synthesis of molybdenum carbide (Mo<SUB>2</SUB>C, a 2D MXene material) <I>via</I> chemical conversion of molybdenum disulfide (MoS<SUB>2</SUB>) with thermal annealing under CH<SUB>4</SUB> and H<SUB>2</SUB> is reported. The experimental results show that adjusting the thermal annealing period provides a fully converted metallic Mo<SUB>2</SUB>C from MoS<SUB>2</SUB> and an atomically sharp metallic/semiconducting hybrid structure <I>via</I> partial conversion of the semiconducting 2D material. Mo<SUB>2</SUB>C/MoS<SUB>2</SUB> hybrid junctions display a low contact resistance (1.2 kΩ·μm) and low Schottky barrier height (26 meV), indicating the material’s potential utility as a critical hybrid structural building block in future device applications. Density functional theory calculations are used to model the mechanisms by which Mo<SUB>2</SUB>C grows and forms a Mo<SUB>2</SUB>C/MoS<SUB>2</SUB> hybrid structure. The results show that Mo<SUB>2</SUB>C conversion is initiated at the MoS<SUB>2</SUB> edge and undergoes sequential hydrodesulfurization and carbide conversion steps, and an atomically sharp interface with MoS<SUB>2</SUB> forms through epitaxial growth of Mo<SUB>2</SUB>C. This work provides the area-controllable synthesis of a manufacturable MXene from a transition metal dichalcogenide material and the formation of a metal/semiconductor junction structure. The present results will be of critical importance for future 2D heterojunction structures and functional device applications.</P><P><B>Graphic Abstract</B> <IMG SRC='http://pubs.acs.org/appl/literatum/publisher/achs/journals/content/ancac3/2018/ancac3.2018.12.issue-1/acsnano.7b06417/production/images/medium/nn-2017-064177_0005.gif'></P><P><A href='http://pubs.acs.org/doi/suppl/10.1021/nn7b06417'>ACS Electronic Supporting Info</A></P>

Quantification of Schottky barrier height and contact resistance of a Au electrode on multilayer WSe2

Jung Dae Hyun,Kim TaeWan 한국물리학회 2022 THE JOURNAL OF THE KOREAN PHYSICAL SOCIETY Vol.80 No.4

Two-dimensional transition-metal dichalcogenide (TMD) device performance is significantly affected by the contact resistance of Schottky contacts at the p-type TMD–metal–electrode interface. The contact resistance and the Schottky barrier height (SBH) of a chemical-vapor-deposition-grown multilayer WSe2 film-based field-effect transistor with Au electrodes were investigated. The experimentally measured and calculated SBH was determined from temperature-dependent current– voltage measurements and thermionic emission model. The transfer length method was employed to investigate the contact resistance between the WSe2 and the Au electrode. SBH values of 56 meV for negative Vds and 55.7 meV for positive Vds and a contact resistance of 12 kΩ·μm were found. An ion-gel-gated field-effect transistor based on multilayer WSe2 exhibited ambipolar behavior with a mobility and an on/off ratio of 1.09 × 10−2 cm2/ V s and 103, respectively.

nType Doping Technique using triphenylphosphine (PPh₃) based on Transition-Metal Dichalcogenides (TMDs)

구지완,박진홍 한국진공학회 2020 한국진공학회 학술발표회초록집 Vol.2020 No.8

Optoelectric Properties of Gate-Tunable MoS₂/WSe₂ Heterojunction

Sum-Gyun Yi,Joo Hyoung Kim,Jung Ki Min,Min Ji Park,Young Wook Chang,Kyung-Hwa Yoo IEEE 2016 IEEE TRANSACTIONS ON NANOTECHNOLOGY Vol. No.

<P>Two-dimensional transition-metal dichalcogenides semiconductors are attractive materials for optoelectric devices because of their direct energy bandgap and transparency. To investigate the feasibility of transparent p-n junctions, we have fabricated heterojunctions consisting of WSe2 and MoS2 since WSe2 and MoS2 with proper electrode metals exhibit p-type and n-type behaviors, respectively. These heterojunctions showed rectifying behaviors, indicating that p-n junctions were formed. In addition, photocurrent and photovoltaic effects were observed under light illumination, which were dependent on the gate voltage. Possible origins of gate-tunability are discussed.</P>

First-principles study on the Poisson's ratio of transition-metal dichalcogenides

유용민,양진훈,이주형 한국물리학회 2018 Current Applied Physics Vol.18 No.7

In this study, we investigate the Poisson's ratio of transition-metal dichalcogenides (TMDCs) with a chemical formula of MX2, where M=Mo, W and X=S, Se, respectively, from first-principles. Through density functional theory calculations, it is demonstrated that the Poisson's ratio of MX2 exhibits not only a substantial difference between the planar and vertical values but also a systematic dependence on the chalcogen species. Among the TMDCs, MoS2 displays the strongest anisotropy, which entails a distinctive contracting response under a planar strain. We find that such pronounced anisotropy in the Poisson's ratio of the TMDCs originates from the different filling of the in- (px, py, dxy, and dx2−y2) and out-of-plane (pz, dyz, dzx, and dz2) electronic orbitals depending on the transition-metal elements. These findings shed a new light on the elastic properties of TMDCs which continue to be interesting and show intriguing phenomena.