Transition Metal Chalcogenides: Ultrathin Inorganic Materials with Tunable Electronic Properties

Heine, Thomas

doi:10.1021/ar500277z

Cited by 309 publications

(163 citation statements)

References 59 publications

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“…The next couple of TMDs are WX 2 (X = S and Se) compounds, which are quite intensively studied in recent years 14,19–21,23 , and are known as indirect gap semiconductors in their bulk form, but the value of the indirect gap is not well established for these compounds. The band gap determined from PA measurements for WS 2 and WSe 2 is 1.57 and 1.33 eV, respectively.…”

Section: Resultsmentioning

confidence: 99%

Photoacoustic and modulated reflectance studies of indirect and direct band gap in van der Waals crystals

Zelewski

Kudrawiec

2017

Sci Rep

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Photoacoustic (PA) and modulated reflectance (MR) spectroscopy have been applied to study the indirect and direct band gap for van der Waals (vdW) crystals: dichalcogenides (MoS 2 , MoSe 2 , MoTe 2 , HfS 2 , HfSe 2 , WS 2 , WSe 2 , ReS 2 , ReSe 2 , SnS 2 and SnSe 2 ) and monochalcogenides (GaS, GaSe, InSe, GeS, and GeSe). It is shown that the indirect band gap can be determined by PA technique while the direct band gap can be probed by MR spectroscopy which is not sensitive to indirect optical transitions. By measuring PA and MR spectra for a given compound and comparing them with each other it is easy to conclude about the band gap character in the investigated compound and the energy difference between indirect and direct band gap. In this work such measurements, comparisons, and analyses have been performed and chemical trends in variation of indirect and direct band gap with the change in atom sizes have been discussed for proper sets of vdW crystals. It is shown that both indirect and direct band gap in vdW crystals follow the well-known chemical trends in semiconductor compounds.Van der Waals (vdW) crystals are known for a long time 1-14 but in recent years some of them have gained significant interest because of unique mechanical and optical properties of samples obtained by exfoliation of bulk material to atomically thin layers [15][16][17][18][19][20][21][22][23][24][25] . Since pioneering reports on optical properties of MoS 2 layers 15,16 , it is well established that the electronic band structure of MoS 2 strongly varies with the number of layers and exhibits indirect-to-direct band gap transition with the size reduction to a single layer. Similar studies of optical properties and the electronic band structure have been performed on other van der Waals crystals such as MoSe 2 , MoTe 2 , WS 2 , and WSe 2 22-25 . Some of them were investigated quite recently as bulk materials 13,14 but interest in these materials was low due to no potential applications in semiconductor devices. At this moment the situation is different. VdW crystals are exciting because of interesting physics of two-dimensional (2D) layers and their potential applications in novel semiconductor devices [26][27][28] . Because of this even bulk van der Waals crystals are interesting to study since many of them are not explored experimentally. One method which has never been applied to study most of van der Waals crystals is photoacoustic (PA) spectroscopy. We show that this technique is an excellent tool to study the band gap in van der Waals crystals since it is sensitive to indirect gap transitions which cannot be observed in photoluminescence. In combination with modulated reflectance (MR), which is known as a state of the art absorption-like technique to study direct optical transitions 29 , it is possible to conclude about the band gap character in van der Waals crystals and the spectral separation between the indirect and direct gap. In this work we applied PA and MR to study indirect and direct gap in many van der Waals crystals ...

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Section: Resultsmentioning

confidence: 99%

Photoacoustic and modulated reflectance studies of indirect and direct band gap in van der Waals crystals

Zelewski

Kudrawiec

2017

Sci Rep

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“…30 A Se-vacancy introduces two acceptor states in the structure. These holes are charge carriers, which, on interaction with electrons, alter the energy levels and produce extended states in the band gap, 31 as seen in Fig.…”

Section: -8mentioning

confidence: 99%

A DFT study of intrinsic point defects in monolayer MoSe2

2017

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This study is a computational investigation of the electronic structure of the eight most-frequently observed intrinsic point-defect configurations in monolayer Molybdenum diselenide (m-MoSe2); analyzed using the Amsterdam Density Functional (ADF) BAND package. Pristine m-MoSe2 is an intrinsic semiconductor with a direct band gap of 1.44 eV. MoSe2 is defect-sensitive due to the similar orbital character of the Valence Band Maximum (VBM) and Conduction Band Minimum (CBM), with deep states induced in the structure by the defects. These states can be attributed solely to the metal d and chalcogen p states, which spring enhanced photoluminescence, making MoSe2 a potential candidate for optoelectronic applications. Band-gap narrowing is proportional to the number of chalcogen vacancies. All defect configurations cause shifting of the Fermi-level, with metal vacancies shifting the semiconducting character of pristine m-MoSe2 to metallic. Only the antisite defect configuration of MoSe2 and Mo-vacancies at a large distance could introduce spin in the structure, with spin attributed to the metal d and chalcogen p states. These findings suggest the possible application of m-MoSe2 for fabricating DMS by defect engineering.

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“…When reducing the number of layers from bulk to ML, the band gap of TMDs evolves from an indirect band gap to a direct band gap with an increased gap size due to quantum confinement [2,3]. The layer-dependent tunability of the electronic structure together with other distinct physical properties of ML TMDs make them promising candidates of applications in fields like electronics, optoelectronics, spintronics and valleytronics, sensing, and catalysis [4][5][6][7].…”

Section: Introductionmentioning

confidence: 99%

Strong spin-orbit splitting and magnetism of point defect states in monolayer WS2

Fang

Huis

2016

Phys. Rev. B

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The spin-orbit coupling (SOC) effect has been known to be profound in monolayer pristine transition metal dichalcogenides (TMDs). Here we show that point defects, which are omnipresent in the TMD membranes, exhibit even stronger SOC effects and change the physics of the host materials drastically. In this article we chose the representative monolayer WS 2 slabs from the TMD family together with seven typical types of point defects including monovacancies, interstitials, and antisites. We calculated the formation energies of these defects, and studied the effect of spin-orbit coupling (SOC) on the corresponding defect states. We found that the S monovacancy (V S ) and S interstitial (adatom) have the lowest formation energies. In the case of V S and both of the W S and W S2 antisites, the defect states exhibit strong splitting up to 296 meV when SOC is considered. Depending on the relative position of the defect state with respect to the conduction band minimum (CBM), the hybrid functional HSE will either increase the splitting by up to 60 meV (far from CBM), or decrease the splitting by up to 57 meV (close to CBM). Furthermore, we found that both the W S and W S2 antisites possess a magnetic moment of 2 μ B localized at the antisite W atom and the neighboring W atoms. The dependence of SOC on the orientation of the magnetic moment for the W S and W S2 antisites is discussed. All these findings provide insights in the defect behavior under SOC and point to possibilities for spintronics applications for TMDs.

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Transition Metal Chalcogenides: Ultrathin Inorganic Materials with Tunable Electronic Properties

Cited by 309 publications

References 59 publications

Photoacoustic and modulated reflectance studies of indirect and direct band gap in van der Waals crystals

Photoacoustic and modulated reflectance studies of indirect and direct band gap in van der Waals crystals

A DFT study of intrinsic point defects in monolayer MoSe2

Strong spin-orbit splitting and magnetism of point defect states in monolayer WS2

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