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.

나트륨 이온 배터리의 성능 개선을 위한 전이 금속 기반 음극 소재 개발 동향

김민기,박정호 대한전기학회 2023 전기학회논문지 Vol.72 No.3

While lithium-ion battery (LIB) is limited in its large-scale energy storage usage due to the scarcity of lithium resource, sodium-ion battery (SIB) is a promising alternative to LIB in the field of large-scale energy storage since sodium is abundant resource. When the chemical reaction of SIB is converted into electrical energy, sodium ions are inserted more slowly because the radius of the sodium ion is larger than lithium ion, which causes a decrease in capacitance, deforms the structure in the electrode, and deteriorates the electrochemical performance. To solve this problem, the development of SIB anode material with high capacity and structural stability is investigated. Transition metal oxide (TMO) and transition metal dichalcogenide (TMD) are considered to be promising anode materials for SIB since they have high capacity and high energy density. The review of the edvelopment trend of TMO-based anode material for SID performance improvement will lead us to realize better SIB.

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>

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).

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>

Theoretical Study of Auger Recombination of Excitons in Monolayer Transition-metal Dichalcogenides

Hyun Cheol Lee 한국물리학회 2018 THE JOURNAL OF THE KOREAN PHYSICAL SOCIETY Vol.73 No.11

Excitons are the most prominent features of the optical properties of monolayer transition-metal dichalcogenides(TMDC). In view of optoelectronics it is very important to understand the decay mechanisms of the excitons of these materials. Auger recombination of excitons are regarded as one of the dominant decay processes. In this paper the Auger constant of recombination is com- puted based on the approach proposed by Kavoulakis and Baym. We obtain both temperature dependent (from type A, A' processes) and temperature independent (from type B, B' processes) contributions, and a numerical estimate of theoretical result yields the value of constant in the order of 102 cm2s1, being consistent with existing experimental data. This implies that Auger decay processes severely limit the photoluminescence yield of TMDC-based optoelectronic devices.

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>

Band Structure Engineering of Layered WSe₂<i>via</i> One-Step Chemical Functionalization

Park, Jun Hong,Rai, Amritesh,Hwang, Jeongwoon,Zhang, Chenxi,Kwak, Iljo,Wolf, Steven F.,Vishwanath, Suresh,Liu, Xinyu,Dobrowolska, Malgorzata,Furdyna, Jacek,Xing, Huili Grace,Cho, Kyeongjae,Banerjee, S American Chemical Society 2019 ACS NANO Vol.13 No.7

<P>Chemical functionalization is demonstrated to enhance the p-type electrical performance of two-dimensional (2D) layered tungsten diselenide (WSe<SUB>2</SUB>) field-effect transistors (FETs) using a one-step dipping process in an aqueous solution of ammonium sulfide [(NH<SUB>4</SUB>)<SUB>2</SUB>S(aq)]. Molecularly resolved scanning tunneling microscopy and spectroscopy reveal that molecular adsorption on a monolayer WSe<SUB>2</SUB> surface induces a reduction of the electronic band gap from 2.1 to 1.1 eV and a Fermi level shift toward the WSe<SUB>2</SUB> valence band edge (VBE), consistent with an increase in the density of positive charge carriers. The mechanism of electronic transformation of WSe<SUB>2</SUB> by (NH<SUB>4</SUB>)<SUB>2</SUB>S(aq) chemical treatment is elucidated using density functional theory calculations which reveal that molecular “SH” adsorption on the WSe<SUB>2</SUB> surface introduces additional in-gap states near the VBE, thereby, inducing a Fermi level shift toward the VBE along with a reduction in the electronic band gap. As a result of the (NH<SUB>4</SUB>)<SUB>2</SUB>S(aq) chemical treatment, the p-branch ON-currents (<I>I</I><SUB>ON</SUB>) of back-gated few-layer ambipolar WSe<SUB>2</SUB> FETs are enhanced by about 2 orders of magnitude, and a ∼6× increase in the hole field-effect mobility is observed, the latter primarily resulting from the p-doping-induced narrowing of the Schottky barrier width leading to an enhanced hole injection at the WSe<SUB>2</SUB>/contact metal interface. This (NH<SUB>4</SUB>)<SUB>2</SUB>S(aq) chemical functionalization technique can serve as a model method to control the electronic band structure and enhance the performance of devices based on 2D layered transition-metal dichalcogenides.</P> [FIG OMISSION]</BR>

Simple Chemical Treatment to n-Dope Transition-Metal Dichalcogenides and Enhance the Optical and Electrical Characteristics

Neupane, Guru P.,Tran, Minh Dao,Yun, Seok Joon,Kim, Hyun,Seo, Changwon,Lee, Jubok,Han, Gang Hee,Sood, A. K.,Kim, Jeongyong American Chemical Society 2017 ACS APPLIED MATERIALS & INTERFACES Vol.9 No.13

<P>The optical and electrical properties of monolayer transition-metal dichalcogenides (1L-TMDs) are critically influenced by two dimensionally confined exciton complexes. Although, extensive studies on controlling the optical properties of 1L-TIVIDs through external doping or defect engineering have been carried out, the effects of excess charges, defects, and the populations of exciton complexes on the light emission of 1L-TMDs are not yet fully understood. Here, we present a simple chemical treatment method for n dope 1L-TMDs, which also enhances their optical and electrical properties. We show that dipping lLs of MoS2 WS2, and WSe2, whether exfoliated or grown'y chemical vapor deposition, into methanol for several hours can increase the electron density and also can reduce the defects, resulting in the enhancement of their photoluminescence, light absorption, and the carrier mobility:. This methanol treatment was effective for both n- and p-type 1L-TMDs, suggesting that the surface restructuring around structural defects by methanol is responsible for the enhancement of optical and electrical characteristics. Our results have revealed a simple process for external doping.that can enhance both the optical and electrical properties of 1L-TMDs and help us understand how the exciton emission in 1L-TMDs can be modulated by chemical treatments.</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.