Browsing by Author "Chou, Fang-Cheng"

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Femtosecond time-evolution of mid-infrared spectral line shapes of Dirac fermions in topological insulators
(2020-06-17)
Yeh, Tien-Tien
;
Tu, Chien-Ming
;
Lin, Wen-Hao
;
Cheng, Cheng-Maw
;
Tzeng, Wen-Yen
Mid-infrared (MIR) light sources have much potential in the study of Dirac-fermions (DFs) in graphene and topological insulators (TIs) because they have a low photon energy. However, the topological surface state transitions (SSTs) in Dirac cones are veiled by the free carrier absorption (FCA) with same spectral line shape that is always seen in static MIR spectra. Therefore, it is difficult to distinguish the SST from the FCA, especially in TIs. Here, we disclose the abnormal MIR spectrum feature of transient reflectivity changes (Delta R/R) for the non-equilibrium states in TIs, and further distinguish FCA and spin-momentum locked SST using time-resolved and linearly polarized ultra-broadband MIR spectroscopy with no environmental perturbation. Although both effects produce similar features in the reflection spectra, they produce completely different variations in the Delta R/R to show their intrinsic ultrafast dynamics.
Manifestation of a Second Dirac Surface State and Bulk Bands in THz Radiation from Topological Insulators
(2015-09-15)
Tu, Chien-Ming
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Yeh, Tien-Tien
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Tzeng, Wen-Yen
;
Chen, Yi-Ru
;
Chen, Hsueh-Ju
Topological insulators (TIs) are interesting quantum matters that have a narrow bandgap for bulk and a Dirac-cone-like conducting surface state (SS). The recent discovered second Dirac surface state (SS) and bulk bands (BBs) located similar to 1.5 eV above the first SS are important for optical coupling in TIs. Here, we report on the time-domain measurements of THz radiation generated from TIs n-type Cu0.02Bi2Se3 and p-type Bi2Te3 single crystals by ultrafast optical pulse excitation. The observed polarity-reversal of the THz pulse originated from transient current is unusual, and cannot be reconciled with the photo-Dember effect. The second SS and BBs are found to be indispensable for the explanation of the unusual phenomenon. Thanks to the existence of the second SS and BBs, TIs manifest an effective wide band gap in THz generation. The present study demonstrates that time-domain THz spectroscopy provide rich information of the optical coupling and the electronic structure of TIs.
Selenium nanoparticle prepared by femtosecond laser-induced plasma shock wave
(2020-01-06)
Tzeng, Wen-Yen
;
Tseng, Ya-Hsin
;
Yeh, Tien-Tien
;
Tu, Chien-Ming
;
Sankar, Raman
A novel approach for the production of both amorphous and crystalline selenium nanoparticles (SeNPs) using femtosecond laser-induced plasma shock wave on the surface of Bi2Se3 topological insulators at room temperature and ambient pressure is demonstrated. The shape and size of SeNPs can be reliably controlled via the kinetic energy obtained from laser pulses, so these are applicable as active components in nanoscale applications. Importantly, the rapid, low-cost and eco-friendly synthesis strategy developed in this study could also be extendable to other systems. (C) 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
Solution-processable bismuth iodide nanosheets as hole transport layers for organic solar cells
(2014-02-01)
Boopathi, Karunakara Moorthy
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Raman, Sankar
;
Mohanraman, Rajeshkumar
;
Chou, Fang-Cheng
;
Chen, Yang-Yuang
In this paper we demonstrate the use of low-temperature-solution-processable bismuth iodide (BiI3) nanosheets as hole transport layers in organic photovoltaics with an active layer comprising poly(3-hexylthiophene) (P3HT) mixed with a fullerene derivative. The performance of the resulting devices was comparable with that of corresponding conventionally used systems incorporating polyethylenedioxythiophene:polystyrenesulfonate (PEDOT:PSS). UV-vis spectroscopy revealed that the transparency of a BiI3 layer in the visible ( > 620 nm) and near-infrared range is greater than that of a PEDOT:PSS layer. X-ray photoemission spectroscopy of a BiI3 film revealed signals at 158.8, 164, 618.6, and 630 eV-characteristic of Bi 4f(7/2), Bi 4f(5/2), I 3d(5/2), and I 3d(3/2), respectively-that indicated a stoichiometric BiI3 film. Wet milling of BiI3 crystals resulted in the formation of nanosheets, the presence of which we confirmed using scanning electron microscopy. The resultant power conversion efficiency of the device was approximately 3.5%, with an open-circuit voltage of 0.56 V, a short-circuit current density of 10.4 mA cm(-2), and a fill factor of 60.1% under AM1.5G irradiation (100 mW cm(-2)). (C) 2013 Elsevier B.V. All rights reserved.
Ultralow Schottky Barriers in Hexagonal Boron Nitride-Encapsulated Monolayer WSe2 Tunnel Field-Effect Transistors
(2020-04-22)
Pande, Gaurav
;
Siao, Jyun-Yan
;
Chen, Wei-Liang
;
Lee, Chien-Ju
;
Sankar, Raman
To explore the potential of field-effect transistors (FETs) based on monolayers (MLs) of the two-dimensional semiconducting channel (SC) for spintronics, the two most important issues are to ensure the formation of variable low-resistive tunnel ferromagnetic contacts (FCs) and to preserve intrinsic properties of the SC during fabrication. Large Schottky barriers lead to the formation of high resistive contacts, and methods adopted to control the barriers often alter the intrinsic properties of the SC. This work aims at addressing both issues in fully encapsulated ML WSe2 FETs using bilayer hexagonal boron nitride (h-BN) as a tunnel barrier at the FC/SC interface. We investigate the electrical transport in ML WSe2 FETs with the current-in-plane geometry that yields hole mobilities of similar to 38.3 cm(2) V-1 s(-1) at 240 K and on/off ratios of the order of 10(7), limited by the contact regions. We have achieved an ultralow effective Schottky barrier (similar to 5.34 meV) with an encapsulated tunneling device as opposed to a nonencapsulated device in which the barrier heights are considerably higher. These observations provide an insight into the electrical behavior of the FC/h-BN/SC/h-BN heterostructures, and such control over the barrier heights opens up the possibilities for WSe2-based spintronic devices.