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.
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
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.
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.
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.
Excitons in monolayer semiconductors have a large optical transition dipole for strong coupling with light. Interlayer excitons in heterobilayers feature a large electric dipole that enables strong ...coupling with an electric field and exciton-exciton interaction at the cost of a small optical dipole. We demonstrate the ability to create a new class of excitons in hetero- and homobilayers that combines advantages of monolayer and interlayer excitons, i.e., featuring both large optical and electric dipoles. These excitons consist of an electron confined in an individual layer, and a hole extended in both layers, where the carrier-species-dependent layer hybridization can be controlled through rotational, translational, band offset, and valley-spin degrees of freedom. We observe different species of layer-hybridized valley excitons, which can be used for realizing strongly interacting polaritonic gases and optical quantum controls of bidirectional interlayer carrier transfer.
Monolayer (ML) transition metal dichalcogenides (TMDCs) are suitable for use in low‐threshold, room‐temperature lasers. 2D semiconductor lasers are coherent, compact, and are suitable for ...multispectral light sources. 0D quantum dot technology has been applied in broadband‐emitting devices and high‐quality display systems. Herein, dual‐color continuous‐wave microcavity lasers are investigated by integrating a tungsten diselenide (WSe2) monolayer and cadmium selenide (CdSe) quantum dots (QDs) into a single microdisk cavity. The hybrid TMDC/QD microcavity device not only achieves lasing in two distinct wavelength regions but also boosts the lasing performances of the WSe2 monolayer due to the energy conversion between the two gain materials. Variations in the spectra obtained under power‐dependent lasing are examined, and the temporal coherence properties of the lasing signals are also characterized to verify the lasing actions. The results indicate that the lasing threshold of the 2D WSe2 monolayer cavity with the CdSe QDs is reduced by more than 2.5 times compared with the WSe2 cavity without the QDs. These findings both expand the wavelength range of TMDC‐based compact lasers at room temperature and facilitate their implementation in applications such as photonic integrated circuits, broadband light‐emitting diodes, and quantum display systems.
A dual‐color continuous‐wave microdisk laser is achieved by integrating a tungsten diselenide (WSe2) monolayer and cadmium selenide quantum dots (QDs). The hybrid WSe2/QDs device provides lasing actions in two wavelength regions, and boosts WSe2 lasing performances because of the energy conversion. These findings expand the 2D semiconductor laser wavelength range and support its implementation in integrated circuits.
Two-dimensional (2D) materials are popular for fundamental physics study and technological applications in next-generation electronics, spintronics, and optoelectronic devices due to a wide range of ...intriguing physical and chemical properties. Recently, the family of 2D metals and 2D semiconductors has been expanding rapidly because they offer properties once unknown to us. One of the challenges to fully access their properties is poor stability in ambient conditions. In the first half of this Review, we briefly summarize common methods of preparing 2D metals and highlight some recent approaches for making air-stable 2D metals. Additionally, we introduce the physicochemical properties of some air-stable 2D metals recently explored. The second half discusses the air stability and oxidation mechanisms of 2D transition metal dichalcogenides and some elemental 2D semiconductors. Their air stability can be enhanced by optimizing growth temperature, substrates, and precursors during 2D material growth to improve material quality, which will be discussed. Other methods, including doping, postgrowth annealing, and encapsulation of insulators that can suppress defects and isolate the encapsulated samples from the ambient environment, will be reviewed.
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