Browsing by Author "Lu, Li-Syuan"

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Atomic scale depletion region at one dimensional MoSe2-WSe2 heterointerface
(2018-12-10)
Chu, Yu-Hsun
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Wang, Li-Hong
;
Lee, Shin-Ye
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Chen, Hou-Ju
;
Yang, Po-Ya
Lateral heterojunctions based on two dimensional (2D) transition metal dichalcogenides (TMDCs) potentially realize monolayer devices exploiting 2D electronic structures and the functions introduced by the presence of 1D heterointerfaces. Electronic structures of a lateral MoSe2-WSe2 junction have been unveiled using scanning tunneling microscopy and spectroscopy. A smooth and narrow depletion region exists despite a defect-rich heterointerface deviating from the preferred zigzag orientations of the TMDC lattice. From the characteristics of the depletion region, a high carrier concentration and high internal electric fields are inferred, offering to benefit designs of lateral TMDC devices. Published by AIP Publishing.
Dielectric impact on exciton binding energy and quasiparticle bandgap in monolayer WS2 and WSe2
(2019-04-01)
Hsu, Wei-Ting
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Quan, Jiamin
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Wang, Chun-Yuan
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Lu, Li-Syuan
;
Campbell, Marshall
Excitons, bound electron-hole pairs in a 2D plane, dominate the optical properties of monolayer transition metal dichalcogenides (TMDs). A large exciton binding energy on the order of 0.5 eV was theoretically predicted and experimentally determined recently. These ultrastable excitons thus open an avenue to explore the exciton physics such as Bose-Einstein condensation and superfluidity at room temperature (Kasprzak et al 2006 Nature 443 409; Plumhof et al 2014 Nat. Mater. 13 247; Fogler et al 2014 Nat. Commun. 5 4555; Jiang and John 2014 Sci. Rep. 4 7432). Recent experiments further demonstrated the concept of Coulomb engineering via dielectric environments based on either solutions or few-layer graphene. However, the conducting nature of these dielectrics can lead to quenching of optical transitions. Thus, in order to utilize 'dielectric tuning' of the exciton binding energy and quasiparticle band gaps, one must use insulating dielectrics. Here, we investigate the impact of insulating dielectric environments on the exciton binding energy of monolayer WS2 and WSe2 by exciton Rydberg spectroscopy, in which the dielectric environment is systematically varied from kappa= 1.49 to 3.82. We found that, with increasing kappa value, the exciton binding energy and quasiparticle bandgap exhibit significant reductions. Quantitatively, our result follows the prediction of nonlocally-screened Keldysh potential very well. The fitted 2D polarizability, Chi(2D), agrees rather well with previous density function theory calculations. Their close agreement validates the nonlocally-screened Keldysh model which can be used to quantitatively predict the exciton binding energy for monolayer TMDs (and possibly other 2D materials) in different dielectric environments. Such a predictive model will play an important role for the design of van der Waals heterostructures and TMD-based optoelectronic devices.
Epitaxial growth of a monolayer WSe2-MoS2 lateral p-n junction with an atomically sharp interface
(2015-07-31)
Li, Ming-Yang
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Shi, Yumeng
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Cheng, Chia-Chin
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Lu, Li-Syuan
;
Lin, Yung-Chang
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.
Evidence of indirect gap in monolayer WSe2
(2017-10-13)
Hsu, Wei-Ting
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Lu, Li-Syuan
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Wang, Dean
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Huang, Jing-Kai
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Li, Ming-Yang
Monolayer transition metal dichalcogenides, such as MoS2 and WSe2, 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 MoS2 and WSe2. Instead of applying stress, we investigate the strain effects by imaging the direct exciton populations in monolayer WSe2-MoS2 and MoSe2-WSe2 lateral heterojunctions with inherent strain inhomogeneity. We find that unstrained monolayer WSe2 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 WSe2 exhibits an indirect quasiparticle gap, which has to be reconsidered in further studies for its fundamental properties and device applications.
Femtosecond-laser ablation of monolayer molybdenum disulfide (MoS2) on sapphire
(2019-01-01)
Solomon, Joel M.
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Yao, Hsin-Yu
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Lu, Li-Syuan
;
Chang, Wen-Hao
;
Her, Tsing-Hua
Single-shot femtosecond laser ablation of monolayer molybdenum disulfide is demonstrated. An ablation threshold was found 0.9 nJ/mu m(2), which is too low for two-photon photoionization alone. We show that surface defects and avalanche ionization are important. (C) 2019 The Author(s)
Hybrid Composites of Quantum Dots, Monolayer WSe2, and Ag Nanodisks for White Light-Emitting Diodes
(2020-07-24)
Chang, Chiao-Yun
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Yu, Cheng-Li
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Lin, Chen-An
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Lin, Hsiang-Ting
;
Lee, Andrew Boyi
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.
Large-area few-layer MoS2 deposited by sputtering
(2016-06)
Huang, Jyun-Hong
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Chen, Hsing-Hung
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Liu, Pang-Shiuan
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Lu, Li-Syuan
;
Wu, Chien-Ting
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 cm(2) 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 10(4). The relatively low mobility is attributed to the small grain size of 0.1-1 mu m with a trap charge density in grain boundaries of the order of 10(13) cm(-2).
Layer-Dependent and In-Plane Anisotropic Properties of Low-Temperature Synthesized Few-Layer PdSe2 Single Crystals
(2020-04-28)
Lu, Li-Syuan
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Chen, Guan-Hao
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Cheng, Hui-Yu
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Chuu, Chih-Piao
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Lu, Kuan-Cheng
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 degrees 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.
Low temperature deposition of high quality single crystalline AlN thin films on sapphire using highly oriented monolayer MoS 2 as a buffer layer
(2020-08-15)
Chien, Hung-Yi
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Kuo, Po-Chun
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Kao, Hui-Ling
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Chen, Jyh-Shin
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Chen, Meei-Ru
Monolayer MoS2 Enabled Single-Crystalline Growth of AlN on Si(100) Using Low-Temperature Helicon Sputtering
(2019-04-01)
Hsu, Wei-Fan
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Lu, Li-Syuan
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Kuo, Po-Chun
;
Chen, Jie-He
;
Chueh, Wei-Chen
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 degrees 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 degrees, 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.
Negative circular polarization emissions from WSe2/MoSe2 commensurate heterobilayers
(2018-04-10)
Hsu, Wei-Ting
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Lu, Li-Syuan
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Wu, Po-Hsun
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Lee, Ming-Hao
;
Chen, Peng-Jen
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 WSe2/MoSe2 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.
Photoluminescence Enhancement and Structure Repairing of Monolayer MoSe2 by Hydrohalic Acid Treatment
(2016-01-01)
Han, Hau-Vei
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Lu, Ang-Yu
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Lu, Li-Syuan
;
Huang, Jing-Kai
;
Li, Henan
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.
Photoluminescence Enhancement and Structure Repairing of Monolayer MoSe2 by Hydrohalic Acid Treatment
(2016-01)
Han, Hau-Vei
;
Lu, Ang-Yu
;
Lu, Li-Syuan
;
Huang, Jing-Kai
;
Li, Henan
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.
Room-Temperature Ferroelectricity in Hexagonally Layered alpha-In2Se3 Nanoflakes down to the Monolayer Limit
(2018-12-12)
Xue, Fei
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Hu, Weijin
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Lee, Ko-Chun
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Lu, Li-Syuan
;
Zhang, Junwei
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 alpha-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 alpha-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 (approximate to 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 alpha-In2Se3 are developed, which are applicable for nonvolatile memories and heterostructure-based nanoelectronics/optoelectronics.
Tailoring excitonic states of van der Waals bilayers through stacking configuration, band alignment, and valley spin
(2019-12-01)
Hsu, Wei-Ting
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Lin, Bo-Han
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Lu, Li-Syuan
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Lee, Ming-Hao
;
Chu, Ming-Wen
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.