Abstract
Monolayer transition metal dichalcogenides, such as MoS
2
and WSe
2
, have been known as direct gap semiconductors and emerged as new optically active materials for novel device ...applications. Here we reexamine their direct gap properties by investigating the strain effects on the photoluminescence of monolayer MoS
2
and WSe
2
. Instead of applying stress, we investigate the strain effects by imaging the direct exciton populations in monolayer WSe
2
–MoS
2
and MoSe
2
–WSe
2
lateral heterojunctions with inherent strain inhomogeneity. We find that unstrained monolayer WSe
2
is actually an indirect gap material, as manifested in the observed photoluminescence intensity–energy correlation, from which the difference between the direct and indirect optical gaps can be extracted by analyzing the exciton thermal populations. Our findings combined with the estimated exciton binding energy further indicate that monolayer WSe
2
exhibits an indirect quasiparticle gap, which has to be reconsidered in further studies for its fundamental properties and device applications.
Tuning band energies of semiconductors through strain engineering can significantly enhance their electronic, photonic, and spintronic performances. Although low-dimensional nanostructures are ...relatively flexible, the reported tunability of the band gap is within 100 meV per 1% strain. It is also challenging to control strains in atomically thin semiconductors precisely and monitor the optical and phonon properties simultaneously. Here, we developed an electromechanical device that can apply biaxial compressive strain to trilayer MoS2 supported by a piezoelectric substrate and covered by a transparent graphene electrode. Photoluminescence and Raman characterizations show that the direct band gap can be blue-shifted for ∼300 meV per 1% strain. First-principles investigations confirm the blue-shift of the direct band gap and reveal a higher tunability of the indirect band gap than the direct one. The exceptionally high strain tunability of the electronic structure in MoS2 promising a wide range of applications in functional nanodevices and the developed methodology should be generally applicable for two-dimensional semiconductors.
Stacking of MoS2 and WSe2 monolayers is conducted by transferring triangular MoS2 monolayers on top of WSe2 monolayers, all grown by chemical vapor deposition (CVD). Raman spectroscopy and ...photoluminescence (PL) studies reveal that these mechanically stacked monolayers are not closely coupled, but after a thermal treatment at 300 °C, it is possible to produce van der Waals solids consisting of two interacting transition metal dichalcogenide (TMD) monolayers. The layer-number sensitive Raman out-of-plane mode A2 1g for WSe2 (309 cm–1) is found sensitive to the coupling between two TMD monolayers. The presence of interlayer excitonic emissions and the changes in other intrinsic Raman modes such as E″ for MoS2 at 286 cm–1 and A2 1g for MoS2 at around 463 cm–1 confirm the enhancement of the interlayer coupling.
There exists a strong demand to replace expensive noble metal catalysts with cheap metal sulfides or phosphides for hydrogen evolution reaction (HER). Recently metal phosphides such as NixP, FeP and ...CoP have been considered as promising candidates to replace Pt cathodes. Here we report that the nanocrystalline CoP nanosheet assembly on carbon cloth can be formed by a two-step process: electrochemical deposition of Co species followed by gas phase phosphidation. The CoP catalyst in this report exhibits a Tafel slope of 30.1mV/dec in 0.5M H2SO4 and 42.6mV/dec in 1M KOH. The high HER performance of our CoP catalysts is attributed to the rugae-like morphology which results in a high double-layer capacitance and high density of active sites, estimated as 7.77×1017sites/cm2.
Nanocrystalline CoP nanosheet assembly on carbon cloth formed by electrochemical deposition of Co species followed by gas phase phosphidation is an efficient cathode for water electrolysis. Display omitted
•Nanocrystalline CoP is obtained by gas phosphidation of electroplated Co species.•The CoP catalyst exhibits a Tafel slope (30.1 mV/dec) comparable to Pt in acids.•The CoP catalyst shows a Tafel slope of 42.6 mV/dec in basic solutions.•The high HER activity is owing to its large surface area and active site density.
Phototransistors based on monolayer transition metal dichalcogenides (TMD) have high photosensitivity due to their direct band gap transition. However, there is a lack of understanding of the effect ...of metal contacts on the performance of atomically thin TMD phototransistors. Here, we fabricate phototransistors based on large-area chemical vapor deposition (CVD) tungsten diselenide (WSe2) monolayers contacted with the metals of different work function values. We found that the low Schottky-contact WSe2 phototransistors exhibit a very high photo gain (105) and specific detectivity (1014Jones), values higher than commercial Si- and InGaAs-based photodetectors; however, the response speed is longer than 5 s in ambient air. In contrast, the high Schottky-contact phototransistors display a fast response time shorter than 23 ms, but the photo gain and specific detectivity decrease by several orders of magnitude. Moreover, the fast response speed of the high Schottky-contact devices is maintained for a few months in ambient air. This study demonstrates that the contact plays an important role in TMD phototransistors, and barrier height tuning is critical for optimizing the photoresponse and photoresponsivity.
By using a comprehensive form of scanning tunneling spectroscopy, we have revealed detailed quasi-particle electronic structures in transition metal dichalcogenides, including the quasi-particle ...gaps, critical point energy locations, and their origins in the Brillouin zones. We show that single layer WSe2 surprisingly has an indirect quasi-particle gap with the conduction band minimum located at the Q-point (instead of K), albeit the two states are nearly degenerate. We have further observed rich quasi-particle electronic structures of transition metal dichalcogenides as a function of atomic structures and spin–orbit couplings. Such a local probe for detailed electronic structures in conduction and valence bands will be ideal to investigate how electronic structures of transition metal dichalcogenides are influenced by variations of local environment.
The valley pseudospin is a degree of freedom that emerges in atomically thin two-dimensional transition metal dichalcogenides (MX2). The capability to manipulate it, in analogy to the control of spin ...in spintronics, can open up exciting opportunities. Here, we demonstrate that an ultrafast and ultrahigh valley pseudo-magnetic field can be generated by using circularly polarized femtosecond pulses to selectively control the valley degree of freedom in monolayer MX2. Using ultrafast pump-probe spectroscopy, we observed a pure and valley-selective optical Stark effect in WSe2 monolayers from the nonresonant pump, resulting in an energy splitting of more than 10 milli-electron volts between the K and K' valley exciton transitions. Our study opens up the possibility to coherently manipulate the valley polarization for quantum information applications.
The emergence of two-dimensional electronic materials has stimulated proposals of novel electronic and photonic devices based on the heterostructures of transition metal dichalcogenides. Here we ...report the determination of band offsets in the heterostructures of transition metal dichalcogenides by using microbeam X-ray photoelectron spectroscopy and scanning tunnelling microscopy/spectroscopy. We determine a type-II alignment between MoS2 and WSe2 with a valence band offset value of 0.83 eV and a conduction band offset of 0.76 eV. First-principles calculations show that in this heterostructure with dissimilar chalcogen atoms, the electronic structures of WSe2 and MoS2 are well retained in their respective layers due to a weak interlayer coupling. Moreover, a valence band offset of 0.94 eV is obtained from density functional theory, consistent with the experimental determination.
2D layered heterostructures have attracted intensive interests due to their unique optical, transport, and interfacial properties. The laterally stitched heterojunction based on dissimilar 2D ...transition metal dichalcogenides forms an intrinsic p–n junction without the necessity of applying an external voltage. However, no scalable processes are reported to construct the devices with such lateral heterostructures. Here, a scalable strategy, two‐step and location‐selective chemical vapor deposition, is reported to synthesize self‐aligned WSe2–MoS2 monolayer lateral heterojunction arrays and demonstrates their light‐emitting devices. The proposed fabrication process enables the growth of high‐quality interfaces and the first successful observation of electroluminescence at the WSe2–MoS2 lateral heterojunction. The electroluminescence study has confirmed the type‐I alignment at the interface rather than commonly believed type‐II alignment. This self‐aligned growth process paves the way for constructing various 2D lateral heterostructures in a scalable manner, practically important for integrated 2D circuit applications.
A spatially controlled process for synthesizing self‐aligned monolayer lateral transition metal dichalcogenide heterojunction arrays in centimeter scale is demonstrated. The simplified fabrication process enables the growth of high‐quality junction interfaces and allows for successful demonstration of electroluminescence at the WSe2–MoS2 lateral heterojunction. The behavior of electroluminescence reveals the band structure nature of junction interfaces.
PDF
