Heinz 1departments of physics and electrical engineering, columbia university, 538 west 120th. Silicon photonic crystal cavity enhanced secondharmonic generation from monolayer wse 2 view the table of contents for this issue, or go to the journal homepage for more. Strong photoluminescence emission is caused by the transition from an indirect band gap semiconductor of bulk. Chemical vapor deposition growth of monolayer mose2. Electronic structure of transition metal dichalcogenides.
Valley zeeman effect in elementary optical excitations of. In recent years, there have been tremendous advancements in the growth of monolayer transition metal dichalcogenides tmdcs by chemical vapor deposition cvd. Silicon photonic crystal cavity enhanced secondharmonic. A monolayer of a transition metal dichalcogenide such as wse2 is a twodimensional directbandgap valleysemiconductor1,2 having an effective honeycomb lattice structure with broken inversion. Straininduced directindirect bandgap transition and phonon. Electronic properties of ws2 and wse2 monolayers with biaxial. The calculations indicate that an increasing external stain tends to depress the ripple structure with a shorter ss interlayer spacing and to enlarge the length of the. This bandbending appears to be dictated by the edge. Specifically, the asgrown monolayers often exhibit lower pl qy than.
We show that only by treating the electronelectron and electronphonon interactions at the same time it is possible to obtain an unprecedented agreement of the zero and finitetemperature optical gaps and. Tungsten diselenide is an inorganic compound with the formula wse 2. The compound adopts a hexagonal crystalline structure similar to molybdenum disulfide. Conley hj1, wang b, ziegler ji, haglund rf jr, pantelides st, bolotin ki. Download a prospectus taught masters admissions process.
Layerdependent electronic structure of an atomically heavy two. The valley degree of freedom in twodimensional 2d crystals recently emerged as a novel information carrier in addition to spin and charge. Twodimensional materials such as graphene are attractive materials for making smaller transistors because they are inherently nanoscale and can carry high currents. Specifically, the pl intensity of bilayer wse2 is amplified by. The monolayer wse2 is interesting and important for future application in nanoelectronics.
We mechanically exfoliate mono and fewlayers of the transition metal dichalcogenides molybdenum disulfide, molybdenum diselenide, and tungsten diselenide. Bandgap engineering of strained monolayer and bilayer mos2. We developed a nonmechanical straining method to simultaneously modulate the bandgap and photoluminescence pl quantum efficiency of a synthesized molybdenum disulfide mos 2 monolayer on sio 2, by vacuum annealing and subsequent quick cooling in ethanol. Jul 31, 2015 s5 demonstrates that the mos 2 monolayer also grew out from the prepatterned wse 2 monolayer. By comparison, wse2, in its monolayer form ml, should.
However, graphene has no band gap and the transistors are leaky. The pet stamp with hbnmos 2 was then stamped onto a monolayer wse 2 flake to form a wse 2 mos 2 heterostructure. Emitting materials and technology northeast normal university, ministry of education, changchun, 024 china. Raman spectroscopic studies show a single prominent peak at 250 cm. Unveiling bandgap evolution and carrier redistribution in multilayer wse 2. Covalent nitrogen doping in molecular beam epitaxygrown. Monolayer mos2 bandgap modulation by dielectric environments. Electronic band structure and density of states calculations found that the states around the fermi energy are mainly due to metal d states.
S4b, consistent with the direct bandgap emission of monolayer wse 2. Strong photoluminescence emission is caused by the transition from an indirect. These data are calculated using the atomistix tool kit atk tool10. Controllable synthesis of bandgaptunable and monolayer. Molybdenum disulfide or moly is an inorganic compound composed of molybdenum and sulfur. It has been found that the monolayer tx 2 type t, transitionmetal atom. The sts measurements of ml and bl wse2 domains of the same sample reveal not only the bandgap narrowing upon increasing the film thickness from ml to bl, but also a bandbending effect across the boundary step between ml and bl domains. To examine the optical bandgap, pl spectroscopy was employed on the same exfoliated wse 2 flakes as were probed with raman spectroscopy.
Synthetic wse2 monolayers with high photoluminescence quantum. Direct observation of the transition from indirect to. Monolayer tmdcs such as mos2, ws2 and wse2, are direct band gap. Observation of longlived interlayer excitons in monolayer. Chemical vapor deposition growth of monolayer mose2 nanosheets.
Our monolayer wse2 crystals may constitute a valuable device platform. Specifically, monolayer mos 2 bandgap is shown to change from 2. The intrinsic valley lifetime in 2d transition metal dichalcogenides tmd is expected to be markedly long due to the unique spinvalley locking behavior, where the intervalley scattering of the electron simultaneously requires. Twodimensional largescale bandgaptunable monolayer mos2. Straininduced directindirect band gap transition and phonon. X, chalcogen atom have several distinctive electronic and optical properties including a direct band gap the band gap is in the range of 1. K points e represents units of energy on the zaxis, while k x and k y represent units of momentum in reciprocal space. Bandgap modulation of mos2 monolayer by thermal annealing. My research is about two kinds of popular 2d materialsgraphene and transition metal dichalcogenides tmdcs. Wse2 tungsten diselenide wse2 is a semiconductor with an indirect band gap of 1. We observe that a bilayer of wse2 is a direct bandgap semiconductor, when integrated in a blgbased heterostructure, thus shifting the directindirect band gap crossover to trilayer wse2. Electronic band structure and density of states calculations found that the states around the fermi energy are mainly due to metal d.
Stacking orders induced direct band gap in bilayer mose2wse2. Despite these promising properties, a lack of efficient largearea. Twodimensional transition metal dichalcogenides tmdcs such as molybdenum sulfide mos 2 and tungsten sulfide wse 2 have potential applications in electronics because they exhibit high onoff current ratios and distinctive electrooptical properties. Dielectric impact on exciton binding energy and quasiparticle bandgap in monolayer ws 2 and wse 2 weiting hsu 1, jiamin quan 1,2, chunyuan wang 1,3, lisyuan lu 4, marshall campbell 1, wenhao chang 4, lainjong li 5, xiaoqin li 1 and chihkang shih 1,6. Vaportransport growth of high optical quality wse2. Jun 19, 2015 in this paper, density functional theory calculations are used to investigate the monolayer mos2 in terms of the strain by analyzing the structure parameters. Mos2 and wse2 monolayers are direct bandgap and show bright photoluminescence pl, whereas multilayers exhibit much weaker pl due to their indirect optical bandgap. Evidence of indirect gap in monolayer wse 2 nature. Differences in the mechanical properties of monolayer and multilayer wse2mose2. Covalent nitrogen doping in molecular beam epitaxygrown and. Photoluminescence emission and raman response of monolayer.
Hanbicki a t, currie m, kioseoglou g, friedman a l and jonker b t 2015 measurement of high exciton binding energy in the monolayer transitionmetal dichalcogenides ws2 and wse2 arxiv. This drastic pl enhancement is attributed to an indirect to direct bandgap transition for strained bilayer wse2, as confirmed by density functional theory dft calculations. Single layer wse2 actually has indirect band gap published by admin on september 29, 2015 september 29, 2015 writing in nano letters a team led by professor chihkang shih from the university of texas at austin studied the electronic structures of single layer transition metal dichalcogenides wse2 and ws2, by scanning tunneling spectroscopy. Observation of ultralong valley lifetime in wse2mos2. Bandgap modulation of mos2 monolayer by thermal annealing and. However, the largescale synthesis of tmds 2017 journal of materials chemistry c hot papers. Its chemical formula is mos 2 the compound is classified as a transition metal dichalcogenide. Manipulation of electronic structure in wse2 monolayer by strain.
Experimental study of nanomaterials graphene, mos2, and wse2 zhang, fan. Multilayer samples were cleaved from the single crystals. Highperformance fieldeffecttransistors on monolayer wse. Here, the authors locate the midgap states originating from single chalcogenatom vacancies in four representative semiconducting monolayer films, and analyse their implications for the. Stability of direct band gap under mechanical strains for. Monolayer transition metal dichalcogenides have so far been thought to be direct bandgap semiconductors. The cvdgrown mos2, mose2, ws2, and wse2 monolayers were. Our calculations indicate that, at equilibrium, the ws 2 and wse 2 monolayers are semiconductors with a direct band gap of respectively 1. In this paper, density functional theory calculations are used to investigate the monolayer mos2 in terms of the strain by analyzing the structure parameters. Straininduced directindirect band gap transition and. Synthetic wse2 monolayers with high photoluminescence. Electronic band gaps and exciton binding energies in monolayer mo xw 1xs 2 transition metal dichalcogenide alloys probed by scanning tunneling and optical spectroscopy albert f. This bandbending appears to be dictated by the edge states at steps of the bl islands.
Oct, 2017 monolayer transition metal dichalcogenides have so far been thought to be direct bandgap semiconductors. Tuning the bandgap of thinlayered twodimensional transition metal dichalcogenide 2d tmdcs nanocrystals by controlling their composition or structure is considered to be an important method of tailoring light absorption, electron transition and carrier mobility. Since the successful isolation of graphene in 2004, twodimensional 2d materials have become one of the hottest research fields in material science. Using a fully abinitio methodology, we demonstrate how the lattice vibrations couple with neutral excitons in monolayer wse2 and contribute to the nonradiative excitonic lifetime. Tungsten diselenide belongs to the groupvi transition metal dichalcogenides tmdc. The calculations indicate that an increasing external stain tends to depress the ripple structure with a shorter ss interlayer spacing and to. Electronic band gaps and exciton binding energies in. Spectroscopic studies of atomic defects and bandgap. Methanemediated vapor transport growth of monolayer wse2.
The sts measurements of ml and bl wse 2 domains of the same sample reveal not only the bandgap narrowing upon increasing the film thickness from ml to bl, but also a bandbending effect across the boundary step between ml and bl domains. Sep 29, 2015 single layer wse2 actually has indirect band gap published by admin on september 29, 2015 september 29, 2015 writing in nano letters a team led by professor chihkang shih from the university of texas at austin studied the electronic structures of single layer transition metal dichalcogenides wse2 and ws2, by scanning tunneling spectroscopy. Jan 26, 2015 a monolayer of a transition metal dichalcogenide such as wse2 is a twodimensional direct bandgap valleysemiconductor1,2 having an effective honeycomb lattice structure with broken inversion. Electronic properties of ws2 and wse2 monolayers with.
Wse2 monolayer turns to indirect gap semiconductor with compressive strain. Strain engineering in monolayer ws2, mos2, and the ws2. Ultrafast growth of large single crystals of monolayer ws2. Dielectric impact on exciton binding energy and quasiparticle. Electronic structure of exfoliated millimetersized monolayer wse 2. We show that only by treating the electronelectron and electronphonon interactions at the same time it is possible to obtain an unprecedented agreement of the zero and finitetemperature optical gaps. Here, the authors revisit this assumption and find that unstrained monolayer wse2 is an. Single layer wse2 actually has indirect band gap manchester.
Utilizing the bandgap modulation property, a tunable bandgap transistor, which can be in general made of a twodimensional semiconductor, is proposed. We used pet stamp to pick up the hbn flake and monolayer mos 2 in sequence. Molecularbeam epitaxy of monolayer and bilayer wse2. Direct observation of the transition from indirect to direct. Although these tmds share the same crystalline structure, their physical properties, such as bandgap, exciton resonance, and spinorbit coupling strengths, can vary significantly. Wse2 monolayer retains direct band gap when tensile strain is less than %. Electronic structure of exfoliated millimetersized monolayer wse2.
Pdf differences in the mechanical properties of monolayer. Subbandgap voltage electroluminescence and magneto. Aug 14, 20 bandgap engineering of strained monolayer and bilayer mos2. 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. However, obtaining high photoluminescence quantum yield pl qy, which is the key figure of merit for optoelectronics, is still challenging in the grown monolayers. Twodimensional largescale bandgaptunable monolayer mos21. Enhanced photon emission via heat engineering yuanzheng li centre for advanced optoelectronic functional materials research and key laboratory of uv. Synthetic twodimensional transition metal dichalcogenides such as tungsten disulphide ws2, tungsten diselenide wse2, molybdenum disulphide mos2 as well as mixed molybdenum tungsten disulphide mo0. Every tungsten atom is covalently bonded to six selenium ligands in a trigonal prismatic coordination sphere while each selenium is bonded to three tungsten atoms in a pyramidal geometry. Experimental study of nanomaterials graphene, mos2, and wse2. Experimental study of nanomaterials graphene, mos2, and.
The direct band gap occurs at the inequivalent k and. It is a silvery black solid that occurs as the mineral molybdenite, the principal ore for molybdenum. Transition metal dichalcogenides, such as mos2 and wse2, have recently gained tremendous interest for electronic and optoelectronic applications. In the monolayer limit, wse2 shows a spinsplitting of 475 mev in the valence band at the k point, the largest value observed among all the mx2 m mo, w. We observe a threshold voltage for electroluminescence significantly lower than the corresponding single particle band gap of monolayer wse2. The exact number of layers is unambiguously determined by atomic force microscopy and highresolution raman spectroscopy. Download scientific diagram a band structure of bulk hexagonal ws2 the blue arrow shows the indirect gap. We report first principles calculations of the electronic structure of monolayer 1hmx2 m mo, w. Unveiling bandgap evolution and carrier redistribution in. Electronic structure, surface doping, and optical response. This presents an obstacle for a number of device applications involving light. Spatially connected tmdc lateral heterojunctions are key components for constructing monolayer pn. However, the influences of strain on the direct bandgap feature, e. Straininduced directindirect bandgap transition and.
We developed a nonmechanical straining method to simultaneously modulate the bandgap and photoluminescence pl quantum efficiency of a synthesized molybdenum disulfide mos2 monolayer on sio2, by vacuum annealing and subsequent quick cooling in ethanol. Bandgap engineering of monolayer mos 2 under strain. We noticed that the mos 2 in wse 2 mos 2 heterostructures normally exhibits considerable pl energy differences at different locations fig. Related transition metal dichalcogenides tmdcs such as molybdenum sulfide have band gaps. This observation can be interpreted by considering the coulomb interaction and a tunneling process involving excitons, well beyond the picture of independent charge carriers. Because of its remarkable electronic structure, monolayer ws 21. Strain engineering in monolayer ws 2, mos 2, and the ws 2mos 2 heterostructure xin he,1,a hai li,1,2,a zhiyong zhu,3 zhenyu dai,1 yang yang,4 peng yang,1,2 qiang zhang,1 peng li,1 udo schwingenschlogl,1 and xixiang zhang1,b 1king abdullah university of science and technology kaus t, physical science and engineering division pse, thuwal 239556900, saudi arabia. S4a, and the pl spectrum shows a single peak at 760 nm supplementary fig.
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