Epitaxial growth of atomically thin two-dimensional crystals such as transition metal dichalcogenides remains challenging, especially for producing large-size transition metal dichalcogenides bilayer ...crystals featuring high density of states, carrier mobility and stability at room temperature. Here we achieve in epitaxial growth of the second monolayer from the first monolayer by reverse-flow chemical vapor epitaxy and produce high-quality, large-size transition metal dichalcogenides bilayer crystals with high yield, control, and reliability. Customized temperature profiles and reverse gas flow help activate the first layer without introducing new nucleation centers leading to near-defect-free epitaxial growth of the second layer from the existing nucleation centers. A series of bilayer crystals including MoS
and WS
, ternary Mo
W
S
and quaternary Mo
W
S
Se
are synthesized with variable structural configurations and tunable electronic and optical properties. The robust, potentially universal approach for the synthesis of large-size transition metal dichalcogenides bilayer single crystals is highly-promising for fundamental studies and technological applications.
There have been continuous efforts to seek novel functional two-dimensional semiconductors with high performance for future applications in nanoelectronics and optoelectronics. In this work, we ...introduce a successful experimental approach to fabricate monolayer phosphorene by mechanical cleavage and a subsequent Ar＊ plasma thinning process. The thickness of phosphorene is unambiguously determined by optical contrast spectra combined with atomic force microscopy （AFM）. Raman spectroscopy is used to characterize the pristine and plasma-treated samples. The Raman frequency of the A2g mode stiffens, and the intensity ratio of A2g to Alg modes shows a monotonic discrete increase with the decrease of phosphorene thickness down to a monolayer. All those phenomena can be used to identify the thickness of this novel two-dimensional semiconductor. This work on monolayer phosphorene fabrication and thickness determination will facilitate future research on phosphorene.
Two-dimensional (2D) transition metal dichalcogenide (TMDC) monolayers, a class of ultrathin materials with a direct bandgap and high exciton binding energies, provide an ideal platform to study the ...photoluminescence (PL) of light-emitting devices. Atomically thin TMDCs usually contain various defects, which enrich the lattice structure and give rise to many intriguing properties. As the influences of defects can be either detrimental or beneficial, a comprehensive understanding of the internal mechanisms underlying defect behaviour is required for PL tailoring. Herein, recent advances in the defect influences on PL emission are summarized and discussed. Fundamental mechanisms are the focus of this review, such as radiative/nonradiative recombination kinetics and band structure modification. Both challenges and opportunities are present in the field of defect manipulation, and the exploration of mechanisms is expected to facilitate the applications of 2D TMDCs in the future.
Owing to its excellent electrical, mechanical, thermal and optical properties, graphene has attracted great interests since it was successfully exfoliated in 2004. Its two dimensional nature and ...superior properties meet the need of surface plasmons and greatly enrich the field of plasmonics. Recent progress and applications of graphene plasmonics will be reviewed, including the theoretical mechanisms, experimental observations, and meaningful applications. With relatively low loss, high confinement, flexible feature, and good tunability, graphene can be a promising plasmonic material alternative to the noble metals. Optics transformation, plasmonic metamaterials, light harvesting etc. are realized in graphene based devices, which are useful for applications in electronics, optics, energy storage, THz technology and so on. Moreover, the fine biocompatibility of graphene makes it a very well candidate for applications in biotechnology and medical science.
Two-dimensional (2D) materials have attracted great attention in optoelectronics because of their unique structure, optical and electrical properties. Designing high-performance photodetectors and ...implementing their applications are eager to promote the development of 2D materials. Position-sensitive detector (PSD) is an optical inspection device for the precise measurements of position, distance, angle, and other relevant physical variables. It is a widely used component in the fields of tracking, aerospace, nanorobotics, and so forth. Essentially, PSD is also a photodetector based on the lateral photovoltaic effect (LPE). This article reviews recent progress in high-performance PSD based on 2D materials. The high-sensitive photodetectors and LPE involved in 2D photodetectors are firstly discussed. Then, we introduce the research progress of PSD based on 2D materials and analyze the carrier dynamics in different device structures. Finally, we summarize the functionalities and applications of PSD based on 2D materials, and highlight the challenges and opportunities in this research area.
Two-dimensional (2D) materials have been extensively studied in recent years due to their unique properties and great potential for applications. Different types of structural defects could present ...in 2D materials and have strong influence on their properties. Optical spectroscopic techniques, e.g. Raman and photoluminescence (PL) spectroscopy, have been widely used for defect characterization in 2D materials. In this review, we briefly introduce different types of defects and discuss their effects on the mechanical, electrical, optical, thermal, and magnetic properties of 2D materials. Then, we review the recent progress on Raman and PL spectroscopic investigation of defects in 2D materials, i.e. identifying of the nature of defects and also quantifying the numbers of defects. Finally, we highlight perspectives on defect characterization and engineering in 2D materials.
Unlike the unstable black phosphorous, another two-dimensional group-VA material, antimonene, was recently predicted to exhibit good stability and remarkable physical properties. However, the ...synthesis of high-quality monolayer or few-layer antimonenes, sparsely reported, has greatly hindered the development of this new field. Here, we report the van der Waals epitaxy growth of few-layer antimonene monocrystalline polygons, their atomical microstructure and stability in ambient condition. The high-quality, few-layer antimonene monocrystalline polygons can be synthesized on various substrates, including flexible ones, via van der Waals epitaxy growth. Raman spectroscopy and transmission electron microscopy reveal that the obtained antimonene polygons have buckled rhombohedral atomic structure, consistent with the theoretically predicted most stable β-phase allotrope. The very high stability of antimonenes was observed after aging in air for 30 days. First-principle and molecular dynamics simulation results confirmed that compared with phosphorene, antimonene is less likely to be oxidized and possesses higher thermodynamic stability in oxygen atmosphere at room temperature. Moreover, antimonene polygons show high electrical conductivity up to 10
S m
and good optical transparency in the visible light range, promising in transparent conductive electrode applications.
The electrical performance of two-dimensional transition metal dichalcogenides （TMDs） is strongly affected by the number of structural defects. In this work, we provide an optical spectroscopic ...characterization approach to correlate the number of structural defects and the electrical performance of WSe2 devices. Low-temperature photoluminescence （PL） spectra of electron-beam-lithography- processed WSe2 exhibit a clear defect-induced PL emission due to excitons bound to defects, which would strongly degrade the electrical performance. By adopting an electron-beam-free transfer-electrode technique, we successfully prepared a backgated WSe2 device containing a limited amount of defects. A maximum hole mobility of approximately 200 cm2.V -1.s-1 was achieved because of the reduced scattering sources, which is the highest reported value for this type of device. This work provides not only a versatile and nondestructive method to monitor the defects in TMDs but also a new route to approach the room-temperature phonon-limited mobility in high-performance TMD devices.
Abstract
Two-dimensional (2D) atomic crystal superlattices integrate diverse 2D layered materials enabling adjustable electronic and optical properties. However, tunability of the interlayer gap and ...interactions remain challenging. Here we report a solution based on soft oxygen plasma intercalation. 2D atomic crystal molecular superlattices (ACMSs) are produced by intercalating O
2
+
ions into the interlayer space using the plasma electric field. Stable molecular oxygen layer is formed by van der Waals interactions with adjacent transition metal dichalcogenide (TMD) monolayers. The resulting interlayer gap expansion can effectively isolate TMD monolayers and impart exotic properties to homo-(MoS
2
O
2
x
) and hetero-(MoS
2
O
2
x
/WS
2
O
2
x
) stacked ACMSs beyond typical capacities of monolayer TMDs, such as 100 times stronger photoluminescence and 100 times higher photocurrent. Our potentially universal approach to tune interlayer stacking and interactions in 2D ACMSs may lead to exotic superlattice properties intrinsic to monolayer materials such as direct bandgap pursued for future optoelectronics.
We report on a strong photoluminescence (PL) enhancement of monolayer MoS2 through defect engineering and oxygen bonding. Micro-PL and Raman images clearly reveal that the PL enhancement occurs at ...cracks/defects formed during high-temperature annealing. The PL enhancement at crack/defect sites could be as high as thousands of times after considering the laser spot size. The main reasons of such huge PL enhancement include the following: (1) the oxygen chemical adsorption induced heavy p doping and the conversion from trion to exciton; (2) the suppression of nonradiative recombination of excitons at defect sites, which was verified by low-temperature PL measurements. First-principle calculations reveal a strong binding energy of ∼2.395 eV for an oxygen molecule adsorbed on a S vacancy of MoS2. The chemically adsorbed oxygen also provides a much more effective charge transfer (0.997 electrons per O2) compared to physically adsorbed oxygen on an ideal MoS2 surface. We also demonstrate that the defect engineering and oxygen bonding could be easily realized by mild oxygen plasma irradiation. X-ray photoelectron spectroscopy further confirms the formation of Mo–O bonding. Our results provide a new route for modulating the optical properties of two-dimensional semiconductors. The strong and stable PL from defects sites of MoS2 may have promising applications in optoelectronic devices.
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