Abstract:Bulk TMDCs are diamagnetic materials; however, two-dimensional TMDCs exhibit spin polarized edge states, which results in a coupling between the unsaturated transition metal and chalcogenide atoms at the edges. The magnetism in two-dimensional TMDCs broadens their applications in spintronic and multi-functional devices. Herein, by combining macro/micro-magnetic experimental measurements and density functional theory (DFT) calculations, we demonstrate that among five possible edge-terminated WSe nanosheets only… Show more
“…Additionally, there was no W-4f peak at 32 eV and 34 eV, which represent the 1T phase; therefore, it can be confirmed at the WSe 2 of the 2H phase. These results are consistent with previous reports on WSe 2 [12,44].…”
The electrical and optical properties of semiconducting transition metal dichalcogenides (TMDs) can be tuned by controlling their composition and the number of layers they have. Among various TMDs, the monolayer WSe2 has a direct bandgap of 1.65 eV and exhibits p-type or bipolar behavior, depending on the type of contact metal. Despite these promising properties, a lack of efficient large-area production methods for high-quality, uniform WSe2 hinders its practical device applications. Various methods have been investigated for the synthesis of large-area monolayer WSe2, but the difficulty of precisely controlling solid-state TMD precursors (WO3, MoO3, Se, and S powders) is a major obstacle to the synthesis of uniform TMD layers. In this work, we outline our success in growing large-area, high-quality, monolayered WSe2 by utilizing methane (CH4) gas with precisely controlled pressure as a promoter. When compared to the catalytic growth of monolayered WSe2 without a gas-phase promoter, the catalytic growth of the monolayered WSe2 with a CH4 promoter reduced the nucleation density to 1/1000 and increased the grain size of monolayer WSe2 up to 100 μm. The significant improvement in the optical properties of the resulting WSe2 indicates that CH4 is a suitable candidate as a promoter for the synthesis of TMD materials, because it allows accurate gas control.
“…Additionally, there was no W-4f peak at 32 eV and 34 eV, which represent the 1T phase; therefore, it can be confirmed at the WSe 2 of the 2H phase. These results are consistent with previous reports on WSe 2 [12,44].…”
The electrical and optical properties of semiconducting transition metal dichalcogenides (TMDs) can be tuned by controlling their composition and the number of layers they have. Among various TMDs, the monolayer WSe2 has a direct bandgap of 1.65 eV and exhibits p-type or bipolar behavior, depending on the type of contact metal. Despite these promising properties, a lack of efficient large-area production methods for high-quality, uniform WSe2 hinders its practical device applications. Various methods have been investigated for the synthesis of large-area monolayer WSe2, but the difficulty of precisely controlling solid-state TMD precursors (WO3, MoO3, Se, and S powders) is a major obstacle to the synthesis of uniform TMD layers. In this work, we outline our success in growing large-area, high-quality, monolayered WSe2 by utilizing methane (CH4) gas with precisely controlled pressure as a promoter. When compared to the catalytic growth of monolayered WSe2 without a gas-phase promoter, the catalytic growth of the monolayered WSe2 with a CH4 promoter reduced the nucleation density to 1/1000 and increased the grain size of monolayer WSe2 up to 100 μm. The significant improvement in the optical properties of the resulting WSe2 indicates that CH4 is a suitable candidate as a promoter for the synthesis of TMD materials, because it allows accurate gas control.
“…Therefore, these findings highlight a new and exciting perspective for simultaneously thermal and magnetic control of the magnetization state in atomically thin magnetic semiconductors at desirable ambient temperatures for wide‐ranging applications in spintronics, ultrafast sensing, and high‐speed information storage systems. [ 3–5 ] We note that TISF is unique to the V‐doped WSe 2 monolayer films, as it is absent in magnetically defect‐induced monolayer semiconductors such as WSe 2 and MoS 2 , [ 35,36 ] as well as in metallic monolayer magnets such as VSe 2 and MnSe 2 . [ 18,25 ] The TISF is also absent in V‐doped/alloyed WS 2 monolayers, [ 31 ] in which V and W1 magnetic moments are ferromagnetically coupled.…”
The outstanding optoelectronic and valleytronic properties of transition metal dichalcogenides (TMDs) have triggered intense research efforts by the scientific community. An alternative to induce long‐range ferromagnetism (FM) in TMDs is by introducing magnetic dopants to form a dilute magnetic semiconductor. Enhancing ferromagnetism in these semiconductors not only represents a key step toward modern TMD‐based spintronics, but also enables exploration of new and exciting dimensionality‐driven magnetic phenomena. To this end, tunable ferromagnetism at room temperature and a thermally induced spin flip (TISF) in monolayers of V‐doped WSe2 are shown. As vanadium concentration increases, the saturation magnetization increases, which is optimal at ≈4 at% vanadium; the highest doping level ever achieved for V‐doped WSe2 monolayers. The TISF occurs at ≈175 K and becomes more pronounced upon increasing the temperature toward room temperature. The TISF can be manipulated by changing the vanadium concentration. The TISF is attributed to the magnetic‐field‐ and temperature‐dependent flipping of the nearest W‐site magnetic moments that are antiferromagnetically coupled to the V magnetic moments in the ground state. This is fully supported by a recent spin‐polarized density functional theory study. The findings pave the way for the development of novel spintronic and valleytronic nanodevices and stimulate further research.
“…With this assumption, comparison between the two set of XPS responses reveals that the doublet W 4f 7/2 and 4f 5/2 responses are reduced when WSe 2 is in contact with Al. Actually, this is attributed to the formation of W 2 Se 3 , which originates from the covalent bonding between Al and WSe 2 layers 27–29 .Figure 5( a,b ) X-ray photoemission spectroscopy data around the W 4f peaks for a bare WSe 2 ( a ) and Al/WSe 2 ( b ). Four peaks can be fitted with contributions of W 6+ and W 4+ states, and two peaks of W 4+ states become weaker for Al/WSe 2 sample.…”
Section: Electronic and Crystal Structural Properties Of Surface And mentioning
confidence: 99%
“…Figure 5(a,b) displays W 4f peaks obtained for a bare WSe 2 and the Al(5 nm)/WSe 2 specimen. Two peaks around 32.2 eV and 34.4 eV correspond to the doublet W 4f 7/2 and W 4f 5/2 , respectively 27–29 , of the WSe 2 composition. The other two peaks around 35.8 eV and 38.0 eV correspond to the doublet W 6f 7/2 and W 6f 5/2 , which are attributed to the WO 3 precursor 27 .…”
Section: Electronic and Crystal Structural Properties Of Surface And mentioning
In nano-device applications using two-dimensional (2D) van der Waals materials, a heat dissipation through nano-scale interfaces can be a critical issue for optimizing device performances. By using a time-domain thermoreflectance measurement technique, we examine a cross-plane thermal transport through mono-layered (
n
= 1) and bi-layered (
n
= 2) WSe
2
flakes which are sandwiched by top metal layers of Al, Au, and Ti and the bottom Al
2
O
3
substrate. In these nanoscale structures with hetero- and homo-junctions, we observe that the thermal boundary resistance (TBR) is significantly enhanced as the number of WSe
2
layers increases. In particular, as the metal is changed from Al, to Au, and to Ti, we find an interesting trend of TBR depending on the WSe
2
thickness; when referenced to TBR for a system without WSe
2
, TBR for
n
= 1 decreases, but that for
n
= 2 increases. This result clearly demonstrates that the stronger bonding for Ti leads to a better thermal conduction between the metal and the WSe
2
layer, but in return gives rise to a large mismatch in the phonon density of states between the first and second WSe
2
layers so that the WSe
2
-WSe
2
interface becomes a major thermal resistance for
n
= 2. By using photoemission spectroscopy and optical second harmonic generation technique, we confirm that the metallization induces a change in the valence state of W-ions, and also recovers a non-centrosymmetry for the bi-layered WSe
2
.
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