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Optically initialized robust valley-polarized holes in monolayer WSe2
(2015-11-01)
Hsu, Wei-Ting
;
Chen, Yen-Lun
;
Chen, Chiang-Hsiao
;
Liu, Pang-Shiuan
;
Hou, Tuo-Hung
A robust valley polarization is a key prerequisite for exploiting valley pseudospin to carry information in next-generation electronics and optoelectronics. Although monolayer transition metal dichalcogenides with inherent spin-valley coupling offer a unique platform to develop such valleytronic devices, the anticipated long-lived valley pseudospin has not been observed yet. Here we demonstrate that robust valley-polarized holes in monolayer WSe2 can be initialized by optical pumping. Using time-resolved Kerr rotation spectroscopy, we observe a long-lived valley polarization for positive trion with a lifetime approaching 1 ns at low temperatures, which is much longer than the trion recombination lifetime (similar to 10-20 ps). The long-lived valley polarization arises from the transfer of valley pseudospin from photocarriers to resident holes in a specific valley. The optically initialized valley pseudospin of holes remains robust even at room temperature, which opens up the possibility to realize room-temperature valleytronics based on transition metal dichalcogenides.
Oxygen Passivation Mediated Tunability of Trion and Excitons in MoS2
(2017-08-16)
Gogoi, Pranjal Kumar
;
Hu, Zhenliang
;
Wang, Qixing
;
Carvalho, Alexandra
;
Schmidt, Daniel
Using wide spectral range in situ spectroscopic ellipsometry with systematic ultrahigh vacuum annealing and in situ exposure to oxygen, we report the complex dielectric function of MoS2 isolating the environmental effects and revealing the crucial role of unpassivated and passivated sulphur vacancies. The spectral weights of the A (1.92 eV) and B (2.02 eV) exciton peaks in the dielectric function reduce significantly upon annealing, accompanied by spectral weight transfer in a broad energy range. Interestingly, the original spectral weights are recovered upon controlled oxygen exposure. This tunability of the excitonic effects is likely due to passivation and reemergence of the gap states in the band structure during oxygen adsorption and desorption, respectively, as indicated by ab initio density functional theory calculation results. This Letter unravels and emphasizes the important role of adsorbed oxygen in the optical spectra and many-body interactions of MoS2.
Self-assembly of hierarchical MoSx/CNT nanocomposites (2 < x < 3): towards high performance anode materials for lithium ion batteries
(2013-07-09)
Shi, Yumeng
;
Wang, Ye
;
Wong, Jen It
;
Tan, Alex Yuan Sheng
;
Hsu, Chang-Lung
Two dimension (2D) layered molybdenum disulfide (MoS2) has emerged as a promising candidate for the anode material in lithium ion batteries (LIBs). Herein, 2D MoSx (2 <= x <= 3) nanosheet-coated 1D multiwall carbon nanotubes (MWNTs) nanocomposites with hierarchical architecture were synthesized via a high-throughput solvent thermal method under low temperature at 2006 degrees C. The unique hierarchical nanostructures with MWNTs backbone and nanosheets of MoSx have significantly promoted the electrode performance in LIBs. Every single MoSx nanosheet interconnect to MWNTs centers with maximized exposed electrochemical active sites, which significantly enhance ion diffusion efficiency and accommodate volume expansion during the electrochemical reaction. A remarkably high specific capacity (i.e., >1000 mAh/g) was achieved at the current density of 50 mA g(-1), which is much higher than theoretical numbers for either MWNTs or MoS2 along (similar to 372 and similar to 670 mAh/g, respectively). We anticipate 2D nanosheets/1D MWNTs nanocomposites will be promising materials in new generation practical LIBs.
Terahertz optical properties of multilayer graphene: Experimental observation of strong dependence on stacking arrangements and misorientation angles
(2012-12-27)
Lin, I-Tan
;
Liu, Jia-Ming
;
Shi, Kai-Yao
;
Tseng, Pei-Shan
;
Wu, Kuang-Hsiung
The optical conductivity of monolayer and multilayer graphene in the terahertz spectral region is experimentally measured using terahertz time-domain spectroscopy. The stacking arrangement and the misorientation angle of each sample are determined by Raman spectroscopy. The chemical potential of each sample is measured using ultrafast midinfrared pump-probe spectroscopy to be 63 or 64 meV for all samples. The intraband scattering rate can be obtained by fitting the measured data with theoretical models. Other physical parameters, including carrier density, dc conductivity, and carrier mobility, of each sample can also be deduced from the theoretical fitting. The fitting results show the existence of misoriented or AA-stacked layers with an interaction energy of alpha(1) = 217 meV in our multilayer samples. Here we show that the scattering rate strongly depends on the stacking arrangement of the sample. High scattering rates and high optical conductivity are associated with AA-stacked samples, while lower ones are associated with misoriented multilayer graphene. This implies that the THz optoelectronic properties of multilayer graphene can be tuned by purposefully misorienting layers or employing different stacking schemes. DOI: 10.1103/PhysRevB.86.235446 PACS number(s): 78.67.Wj, 61.48.Gh, 72.80.Vp, 73.50.Mx
Giant photoluminescence enhancement in tungsten-diselenide-gold plasmonic hybrid structures
(2016-05-01)
Wang, Zhuo
;
Dong, Zhaogang
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Gu, Yinghong
;
Chang, Yung-Huang
;
Zhang, Lei
Impressive properties arise from the atomically thin nature of transition metal dichalcogenide two-dimensional materials. However, being atomically thin limits their optical absorption or emission. Hence, enhancing their photoluminescence by plasmonic nanostructures is critical for integrating these materials in optoelectronic and photonic devices. Typical photoluminescence enhancement from transition metal dichalcogenides is 100-fold, with recent enhancement of 1,000-fold achieved by simultaneously enhancing absorption, emission and directionality of the system. By suspending WSe2 flakes onto sub-20-nm-wide trenches in gold substrate, we report a giant photoluminescence enhancement of similar to 20,000-fold. It is attributed to an enhanced absorption of the pump laser due to the lateral gap plasmons confined in the trenches and the enhanced Purcell factor by the plasmonic nanostructure. This work demonstrates the feasibility of giant photoluminescence enhancement in WSe2 with judiciously designed plasmonic nanostructures and paves a way towards the implementation of plasmon-enhanced transition metal dichalcogenide photodetectors, sensors and emitters.
Evidence of indirect gap in monolayer WSe2
(2017-10-13)
Hsu, Wei-Ting
;
Lu, Li-Syuan
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Wang, Dean
;
Huang, Jing-Kai
;
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.
Ultrafast Multi-Level Logic Gates with Spin-Valley Coupled Polarization Anisotropy in Monolayer MoS2
(2015-02-06)
Wang, Yu-Ting
;
Luo, Chih-Wei
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Yabushita, Atsushi
;
Wu, Kaung-Hsiung
;
Kobayashi, Takayoshi
The inherent valley-contrasting optical selection rules for interband transitions at the K and K' valleys in monolayer MoS2 have attracted extensive interest. Carriers in these two valleys can be selectively excited by circularly polarized optical fields. The comprehensive dynamics of spin valley coupled polarization and polarized exciton are completely resolved in this work. Here, we present a systematic study of the ultrafast dynamics of monolayer MoS2 including spin randomization, exciton dissociation, free carrier relaxation, and electron-hole recombination by helicity- and photon energy-resolved transient spectroscopy. The time constants for these processes are 60 fs, 1 ps, 25 ps, and, similar to 300 ps, respectively. The ultrafast dynamics of spin polarization, valley population, and exciton dissociation provides the desired information about the mechanism of radiationless transitions in various applications of 2D transition metal dichalcogenides. For example, spin valley coupled polarization provides a promising way to build optically selective-driven ultrafast valleytronics at room temperature. Therefore, a full understanding of the ultrafast dynamics in MoS2 is expected to provide important fundamental and technological perspectives.
Bandgap tunability at single-layer molybdenum disulphide grain boundaries
(2015-02-01)
Huang, Yu Li
;
Chen, Yifeng
;
Zhang, Wenjing
;
Quek, Su Ying
;
Chen, Chang-Hsiao
Two-dimensional transition metal dichalcogenides have emerged as a new class of semiconductor materials with novel electronic and optical properties of interest to future nanoelectronics technology. Single-layer molybdenum disulphide, which represents a prototype two-dimensional transition metal dichalcogenide, has an electronic bandgap that increases with decreasing layer thickness. Using high-resolution scanning tunnelling microscopy and spectroscopy, we measure the apparent quasiparticle energy gap to be 2.40 +/- 0.05 eV for single-layer, 2.10 +/- 0.05 eV for bilayer and 1.75 +/- 0.05 eV for trilayer molybdenum disulphide, which were directly grown on a graphite substrate by chemical vapour deposition method. More interestingly, we report an unexpected bandgap tunability (as large as 0.85 +/- 0.05 eV) with distance from the grain boundary in single-layer molybdenum disulphide, which also depends on the grain misorientation angle. This work opens up new possibilities for flexible electronic and optoelectronic devices with tunable bandgaps that utilize both the control of two-dimensional layer thickness and the grain boundary engineering.
Plasmonic Gold Nanorods Coverage Influence on Enhancement of the Photoluminescence of Two-Dimensional MoS2 Monolayer
(2015-11-17)
Lee, Kevin C. J.
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Chen, Yi-Huan
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Lin, Hsiang-Yu
;
Cheng, Chia-Chin
;
Chen, Pei-Ying
The 2-D transition metal dichalcogenide (TMD) semiconductors, has received great attention due to its excellent optical and electronic properties and potential applications in field-effect transistors, light emitting and sensing devices. Recently surface plasmon enhanced photoluminescence (PL) of the weak 2-D TMD atomic layers was developed to realize the potential optoelectronic devices. However, we noticed that the enhancement would not increase monotonically with increasing of metal plasmonic objects and the emission drop after the certain coverage. This study presents the optimized PL enhancement of a monolayer MoS2 in the presence of gold (Au) nanorods. A localized surface plasmon wave of Au nanorods that generated around the monolayer MoS2 can provide resonance wavelength overlapping with that of the MoS2 gain spectrum. These spatial and spectral overlapping between the localized surface plasmon polariton waves and that from MoS2 emission drastically enhanced the light emission from the MoS2 monolayer. We gave a simple model and physical interpretations to explain the phenomena. The plasmonic Au nanostructures approach provides a valuable avenue to enhancing the emitting efficiency of the 2-D nano-materials and their devices for the future optoelectronic devices and systems.
Spectroscopic Signatures for Interlayer Coupling in MoS2-WSe2 van der Waals Stacking
(2014-09-01)
Chiu, Ming-Hui
;
Li, Ming-Yang
;
Zhang, Wengjing
;
Hsu, Wei-Ting
;
Chang, Wen-Hao
Stacking of MoS2 and WSe2 monolayers is conducted by transferring triangular MoS2 monolayers on top of WSe2 monolayers, all grown by chemical vapor deposition (CVD). Raman spectroscopy and photoluminescence (PL) studies reveal that these mechanically stacked monolayers are not closely coupled, but after a thermal treatment at 300 degrees C, it is possible to produce van der Waals solids consisting of two interacting transition metal dichalcogenide (TMD) monolayers. The layer-number sensitive Raman out-of-plane mode A(1g)(2) for WSe2 (309 cm(-1)) is found sensitive to the coupling between two TMD monolayers. The presence of interlayer excitonic emissions and the changes in other intrinsic Raman modes such as E \'\' for MoS2 at 286 cm(-1) and A(1g)(2) for MoS2 at around 463 cm(-1) confirm the enhancement of the interlayer coupling.

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