•Single crystalline AlN films were obtained on sapphire with MoS2 as buffer layer.•The dislocation density of AlN films were decreased using the inserted MoS2 layer.•The growth temperature is reduced ...to 300 °C, a record low for epitaxial AlN growth.•The finding is important for GaN industry, as AlN often serves as a seed layer.•The finding is beneficial to the exploration for the application of 2-D materials.
AlN films are commonly deposited on sapphire substrates. However, the growth process usually requires high temperature and multi-steps to improve the film quality, due to the large lattice mismatch between the film and substrate. Here we demonstrate that using monolayer MoS2 grown on sapphire as a template, single crystalline AlN films with high crystallinity can be obtained at a much lower growth temperature using helicon sputtering system. The x-ray rocking curve of AlN films prepared on MoS2/sapphire template at 400 °C shows a full width at half maximum of 0.050°, showing a dramatic reduction compared to that of 0.803° for those grown directly on sapphire substrate. The estimated dislocation density of AlN on MoS2/sapphire is decreased to 7.7x107 cm−2, which is comparable to that prepared by metal-organic chemical vapor deposition at high temperatures. The small lattice mismatch between MoS2 and AlN, as well as the polar crystal of monolayer MoS2 apparently improve the quality of AlN films and facilitate AlN formed at lower temperature.
Growing a single-crystalline film on a substrate relies on the compatibility of crystal symmetry and lattice constant between the two materials. Such limitations can be circumvented by introducing ...van der Waals epitaxy of three-dimensional (3D) crystals on two-dimensional (2D) layered materials. Recently, buffer-assisted growth of III-nitride films on graphene has been demonstrated. However, the low chemical reactivity of graphene surface considerably limits the large-area and single-crystalline growth of planar 3D films on 2D layered materials. Here, we demonstrate that using highly oriented monolayer MoS2 as a buffer layer, single-crystalline AlN thin films can be grown on Si(100) substrates, which possess a different crystal symmetry with the films. The AlN films were grown by helicon sputtering system at low temperature (400 °C), showing a very flat surface with a root-mean-square roughness of 1.0 nm and an X-ray rocking curve with a full width at half-maximum of 0.336°, indicating a high-crystalline quality. Because the buffer layer as well as the AlN films were prepared at low temperatures, our results not only pave the way for integrating III-nitride with the Si wafer industry process but also open a new possibility for growing III-nitride thin film on various foreign substrates.
Direct magnetron sputtering of transition metal dichalcogenide targets is proposed as a new approach for depositing large-area two-dimensional layered materials. Bilayer to few-layer MoS2 deposited ...by magnetron sputtering followed by post-deposition annealing shows superior area scalability over 20 cm2 and layer-by-layer controllability. High crystallinity of layered MoS2 was confirmed by Raman, photo-luminescence, and transmission electron microscopy analysis. The sputtering temperature and annealing ambience were found to play an important role in the film quality. The top-gate field-effect transistor by using the layered MoS2 channel shows typical n-type characteristics with a current on/off ratio of approximately 104. The relatively low mobility is attributed to the small grain size of 0.1-1 m with a trap charge density in grain boundaries of the order of 1013 cm-2.
We investigated hybrid zero-dimensional core–shell CdSe/ZnS quantum dot (QD)/two-dimensional monolayer WSe2 semiconductors with an Ag nanodisk (ND) for manipulating plasmonic-enhanced ...photoluminescence (PL) and color conversion efficiency. The absorption spectrum of the local surface plasmon resonance (LSPR) effectively overlaps with that of QDs or monolayer WSe2 to considerably enhance PL. The broad absorption spectrum of the LSPR simultaneously overlapped with the emission spectrum of QDs and the absorption spectrum of excitons in WSe2 to enhance the color conversion efficiency. The highest efficiency of color conversion from QDs to WSe2 with Ag ND was 53%. In the future, hybrid QD/transition metal dichalcogenide light emitters could be further integrated with GaN-based white light-emitting diodes to manipulate the color temperature and expand the color gamut to develop a miniature white light-emitting diode.
Utilizing the excess energy of photoexcitation that is otherwise lost as thermal effects can improve the efficiency of next-generation light-harvesting devices. Multiple exciton generation (MEG) in ...semiconducting materials yields two or more excitons by absorbing a single high-energy photon, which can break the Shockley-Queisser limit for the conversion efficiency of photovoltaic devices. Recently, monolayer transition metal dichalcogenides (TMDs) have emerged as promising light-harvesting materials because of their high absorption coefficient. Here, we report efficient MEG with low threshold energy and high (86%) efficiency in a van der Waals (vdW) layered material, MoS2. Through different experimental approaches, we demonstrate the signature of exciton multiplication and discuss the possible origin of decisive MEG in monolayer MoS2. Our results reveal that vdW-layered materials could be a potential candidate for developing mechanically flexible and highly efficient next generation solar cells and photodetectors.
We demonstrate that the photoluminescence emission intensity of the CVD-grown MoSe 2 monolayers can be effectively enhanced more than 30 times after a simple hydrohalic acid treatment, providing the ...cost-effect manufacturing of atomically-thin two-dimensional semiconductor materials.
We report an ultrafast increase of the quasi-particle bandgap and effective mass in photoexcited monolayer MoS 2 on HOPG, utilizing extreme-ultraviolet time- and angle-resolved photoemission ...spectroscopy (XUV-trARPES). Combined with theoretical models, we attribute these compelling band renormalizations to the excitonic effects from bound electron-hole pairs.
Utilization of the excess energy of photoexcitation that is otherwise lost as thermal effects can improve the efficiency of next-generation light-harvesting devices. Multiple exciton generation (MEG) ...in semiconducting materials yields two or more excitons by absorbing a single high-energy photon, which can break the Shockley-Queisser limit for the conversion efficiency of photovoltaic devices. Recently, monolayer transition metal dichalcogenides (TMDs) have emerged as promising light-harvesting materials because of their high absorption coefficient. Here, we report efficient MEGs with low threshold energy and high (86%) efficiency in a van der Waals (vdW) layered material, MoS
. Through different experimental approaches, we demonstrate the signature of exciton multiplication and discuss the possible origin of decisive MEG in monolayer MoS
. Our results reveal that vdW-layered materials could be a potential candidate for developing mechanically flexible and highly efficient next-generation solar cells and photodetectors.
Optical excitation serves as a powerful approach to control the electronic structure of layered Van der Waals materials via many-body screening effects, induced by photoexcited free carriers, or via ...light-driven coherence, such as optical Stark and Bloch-Siegert effects. Although theoretical work has also pointed to an exotic mechanism of renormalizing band structure via excitonic correlations in bound electron-hole pairs (excitons), experimental observation of such exciton-driven band renormalization and the full extent of their implications is still lacking, largely due to the limitations of optical probes and the impact of screening effects. Here, by using extreme-ultraviolet time-resolved angle-resolved photoemission spectroscopy together with excitonic many-body theoretical calculations, we directly unmask the band renormalization effects driven by excitonic correlations in a monolayer semiconductor. We revealed a surprising bandgap opening, increased by 40 meV, and a simultaneous enhancement of band effective mass. Our findings unmask the novel exciton-driven mechanism towards the band engineering in photoexcited semiconducting materials, opening a new playground to manipulate the transient energy states in layered quantum materials via optical controls of excitonic many-body correlations.
By creating a large-scale and highly oriented monolayer-MoS2/multilayer-MoSe2 heterostructure with an atomically clean interface, we use angle-resolved photoemission spectroscopy (ARPES) to probe the ...electronic structure of the created heterostructure and its respective constituents. This comparative study allows us to determine the band offset at the K-point and to reveal k-resolved interlayer hybridization in the Brillouin zone. We further discuss the effect of moiré potential on the state at the K-valley for MoS2. We expect that our approach of creating heterostructures and the k-resolved study of the electronic band structure will elucidate how interlayer coupling manifests in determining the electronic structure formation of transition metal dichalcogenide (TMD) vertical heterostructures.
