2D ferroelectric material has emerged as an attractive building block for high‐density data storage nanodevices. Although monolayer van der Waals ferroelectrics have been theoretically predicted, a ...key experimental breakthrough for such calculations is still not realized. Here, hexagonally stacking α‐In2Se3 nanoflake, a rarely studied van der Waals polymorph, is reported to exhibit out‐of‐plane (OOP) and in‐plane (IP) ferroelectricity at room temperature. Ferroelectric multidomain states in a hexagonal α‐In2Se3 nanoflake with uniform thickness can survive to 6 nm. Most strikingly, the electric‐field‐induced polarization switching and hysteresis loop are, respectively, observed down to the bilayer and monolayer (≈1.2 nm) thicknesses, which designates it as the thinnest layered ferroelectric and verifies the corresponding theoretical calculation. In addition, two types of ferroelectric nanodevices employing the OOP and IP polarizations in 2H α‐In2Se3 are developed, which are applicable for nonvolatile memories and heterostructure‐based nanoelectronics/optoelectronics.
The thinnest layered ferroelectric is demonstrated for the first time at room temperature. The semiconducting hexagonal α‐In2Se3 nanoflakes exhibit out‐of‐plane and in‐plane ferroelectricity that are closely intercorrelated. The polarization switching and hysteresis loops can be realized in the thickness as thin as ≈2.3 nm (bilayer) and ≈1.2 nm (monolayer). Two types of ferroelectric switchable devices are proposed to show the potential application in nonvolatile memories.
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.
Two-dimensional materials such as graphene are attractive materials for making smaller transistors because they are inherently nanoscale and can carry high currents. However, graphene has no band gap ...and the transistors are "leaky"; that is, they are hard to turn off. Related transition metal dichalcogenides (TMDCs) such as molybdenum sulfide have band gaps. Transistors based on these materials can have high ratios of "on" to "off" currents. However, it is often difficult to make a good voltage-biased (p-n) junction between different TMDC materials. Li et al. succeeded in making p-n heterojunctions between two of these materials, molybdenum sulfide and tungsten selenide. They did this not by stacking the layers, which make a weak junction, but by growing molybdenum sulfide on the edge of a triangle of tungsten selenide with an atomically sharp boundary Science, this issue p. 524 Two-dimensional transition metal dichalcogenides (TMDCs) such as molybdenum sulfide MoS2 and tungsten sulfide WSe2 have potential applications in electronics because they exhibit high on-off current ratios and distinctive electro-optical properties. Spatially connected TMDC lateral heterojunctions are key components for constructing monolayer p-n rectifying diodes, light-emitting diodes, photovoltaic devices, and bipolar junction transistors. However, such structures are not readily prepared via the layer-stacking techniques, and direct growth favors the thermodynamically preferred TMDC alloys. We report the two-step epitaxial growth of lateral WSe2-MoS2 heterojunction, where the edge of WSe2 induces the epitaxial MoS2 growth despite a large lattice mismatch. The epitaxial growth process offers a controllable method to obtain lateral heterojunction with an atomically sharp interface.
Abstract
Van der Waals heterobilayers of transition metal dichalcogenides with spin–valley coupling of carriers in different layers have emerged as a new platform for exploring spin/valleytronic ...applications. The interlayer coupling was predicted to exhibit subtle changes with the interlayer atomic registry. Manually stacked heterobilayers, however, are incommensurate with the inevitable interlayer twist and/or lattice mismatch, where the properties associated with atomic registry are difficult to access by optical means. Here, we unveil the distinct polarization properties of valley-specific interlayer excitons using epitaxially grown, commensurate WSe
2
/MoSe
2
heterobilayers with well-defined (AA and AB) atomic registry. We observe circularly polarized photoluminescence from interlayer excitons, but with a helicity opposite to the optical excitation. The negative circular polarization arises from the quantum interference imposed by interlayer atomic registry, giving rise to distinct polarization selection rules for interlayer excitons. Using selective excitation schemes, we demonstrate the optical addressability for interlayer excitons with different valley configurations and polarization helicities.
Palladium diselenide (PdSe2), a peculiar noble metal dichalcogenide, has emerged as a new two-dimensional material with high predicted carrier mobility and a widely tunable band gap for device ...applications. The inherent in-plane anisotropy endowed by the pentagonal structure further renders PdSe2 promising for novel electronic, photonic, and thermoelectric applications. However, the direct synthesis of few-layer PdSe2 is still challenging and rarely reported. Here, we demonstrate that few-layer, single-crystal PdSe2 flakes can be synthesized at a relatively low growth temperature (300 °C) on sapphire substrates using low-pressure chemical vapor deposition (CVD). The well-defined rectangular domain shape and precisely determined layer number of the CVD-grown PdSe2 enable us to investigate their layer-dependent and in-plane anisotropic properties. The experimentally determined layer-dependent band gap shrinkage combined with first-principle calculations suggest that the interlayer interaction is weaker in few-layer PdSe2 in comparison with that in bulk crystals. Field-effect transistors based on the CVD-grown PdSe2 also show performances comparable to those based on exfoliated samples. The low-temperature synthesis method reported here provides a feasible approach to fabricate high-quality few-layer PdSe2 for device applications.
Two-dimensional (2D) transition-metal dichalcogenides (TMDCs) have shown promise for a variety of optoelectronic applications due to a wide range of optical, electrical, and mechanical properties. ...Large-area chemical vapor deposition (CVD)-grown TMDC flakes could be useful in such devices. However, the defects present in large-area TMDC flakes can significantly influence carrier dynamics and transport properties. Here, the ultrafast carrier dynamics of monolayer tungsten disulfide (WS2) covering a large area of the substrate was explored using transient absorption spectroscopy. By monitoring the transient optical response, exciton trapping by oxygen-induced defects has been identified in monolayer WS2. We observe excitation-density-dependent exciton decay dynamics for both band-edge and above band-edge excitations due to exciton–exciton annihilation. Our results demonstrate the impact of defect states on carrier recombination in CVD-grown TMDCs, which could pave the way for utilizing such materials in optoelectronic device applications.
2D materials have great potential for not only device scaling but also various applications. To prompt the development of 2D electronics and optoelectronics, a better understanding of the limitation ...of materials is essential. Material failure caused by bias can lead to variations in device behavior and even electrical breakdown. In this study, the structural evolution of monolayer MoS2 with high bias is revealed via in situ transmission electron microscopy at the atomic scale. The biasing process is recorded and studied with the aid of aberration‐corrected scanning transmission electron microscopy. The effects of electron beam irradiation and biasing are also discussed through the combination of experiments and theory. It is found that the Mo nanoclusters result from disintegration of MoS2 and sulfur depletion, which are induced by Joule heating. The thermal stress can also damage the MoS2 layer and form long cracks in both in situ and ex situ biasing cases. Investigation of the results obtained with different applied voltages helps to further verify the mechanism of evolution and provide a comprehensive study of the function of biasing.
The high‐voltage biasing of monolayer MoS2 devices is demonstrated through in situ TEM and aberration‐corrected STEM to explore the mechanism of the material failure. During in situ TEM biasing, the MoS2 device is damaged by knock‐on damage, and the atomic migration induced by Joule heating. Also, long cracks formed by thermal stress are discussed in this research.
Laser direct writing is an attractive method for patterning 2D materials without contamination. Literature shows that the ultrafast ablation threshold of graphene across substrates varies by an order ...of magnitude. Some attribute it to the thermal coupling to the substrates, but it remains by and large an open question. For the first time the effect of substrates on the femtosecond ablation of 2D materials is studied using MoS
as an example. We show unambiguously that femtosecond ablation of MoS
is an adiabatic process with negligible heat transfer to the substrates. The observed threshold variation is due to the etalon effect which was not identified before for the laser ablation of 2D materials. Subsequently, an intrinsic ablation threshold is proposed as a true threshold parameter for 2D materials. Additionally, we demonstrate for the first time femtosecond laser patterning of monolayer MoS
with sub-micron resolution and mm/s speed. Moreover, engineered substrates are shown to enhance the ablation efficiency, enabling patterning with low-power ultrafast oscillators. Finally, a zero-thickness approximation is introduced to predict the field enhancement with simple analytical expressions. Our work clarifies the role of substrates on ablation and firmly establishes ultrafast laser ablation as a viable route to pattern 2D materials.
Atomically thin two-dimensional transition-metal dichalcogenides (TMDCs) have attracted much attention recently due to their unique electronic and optical properties for future optoelectronic ...devices. The chemical vapor deposition (CVD) method is able to generate TMDCs layers with a scalable size and a controllable thickness. However, the TMDC monolayers grown by CVD may incorporate structural defects, and it is fundamentally important to understand the relation between photoluminescence and structural defects. In this report, point defects (Se vacancies) and oxidized Se defects in CVD-grown MoSe2 monolayers are identified by transmission electron microscopy and X-ray photoelectron spectroscopy. These defects can significantly trap free charge carriers and localize excitons, leading to the smearing of free band-to-band exciton emission. Here, we report that the simple hydrohalic acid treatment (such as HBr) is able to efficiently suppress the trap-state emission and promote the neutral exciton and trion emission in defective MoSe2 monolayers through the p-doping process, where the overall photoluminescence intensity at room temperature can be enhanced by a factor of 30. We show that HBr treatment is able to activate distinctive trion and free exciton emissions even from highly defective MoSe2 layers. Our results suggest that the HBr treatment not only reduces the n-doping in MoSe2 but also reduces the structural defects. The results provide further insights of the control and tailoring the exciton emission from CVD-grown monolayer TMDCs.
The electronic structure and dynamics of 2D transition metal dichalcogenide (TMD) monolayers provide important underpinnings both for understanding the many-body physics of electronic quasi-particles ...and for applications in advanced optoelectronic devices. However, extensive experimental investigations of semiconducting monolayer TMDs have yielded inconsistent results for a key parameter, the quasi-particle band gap (QBG), even for measurements carried out on the same layer and substrate combination. Here, we employ sensitive time- and angle-resolved photoelectron spectroscopy (trARPES) for a high-quality large-area MoS2 monolayer to capture its momentum-resolved equilibrium and excited-state electronic structure in the weak-excitation limit. For monolayer MoS2 on graphite, we obtain QBG values of ≈2.10 eV at 80 K and of ≈2.03 eV at 300 K, results well-corroborated by the scanning tunneling spectroscopy (STS) measurements on the same material.
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