Browsing by Author "Medina, Henry"

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Enhanced Photocarrier Generation with Selectable Wavelengths by M-Decorated-CuInS2 Nanocrystals (M = Au and Pt) Synthesized in a Single Surfactant Process on MoS2 Bilayers
(2019-02-22)
Tang, Shin-Yi
;
Medina, Henry
;
Yen, Yu-Ting
;
Chen, Chia-Wei
;
Yang, Tzu-Yi
A facile approach for the synthesis of Au- and Pt-decorated CuInS2 nanocrystals (CIS NCs) as sensitizer materials on the top of MoS2 bilayers is demonstrated. A single surfactant (oleylamine) is used to prepare such heterostructured noble metal decorated CIS NCs from the pristine CIS. Such a feasible way to synthesize heterostructured noble metal decorated CIS NCs from the single surfactant can stimulate the development of the functionalized heterostructured NCs in large scale for practical applications such as solar cells and photodetectors. Photodetectors based on MoS2 bilayers with the synthesized nanocrystals display enhanced photocurrent, almost 20-40 times higher responsivity and the On/Off ratio is enlarged one order of magnitude compared with the pristine MoS2 bilayers-based photodetectors. Remarkably, by using Pt- or Au-decorated CIS NCs, the photocurrent enhancement of MoS2 photodetectors can be tuned between blue (405 nm) to green (532 nm). The strategy described here acts as a perspective to significantly improve the performance of MoS2-based photodetectors with the controllable absorption wavelengths in the visible light range, showing the feasibility of the possible color detection.
Low-Temperature and Ultrafast Synthesis of Patternable Few-Layer Transition Metal Dichacogenides with Controllable Stacking Alignment by a Microwave-Assisted Selenization Process
(2016-02-23)
Chen, Yu-Ze
;
Medina, Henry
;
Wang, Sheng-Wen
;
Su, Teng-Yu
;
Li, Jian-Guang
To date, the chemical vapor deposition (CVD) process is the most popular approach because of its high yield and quality. Nevertheless, the need for a high temperature and the relatively long process time within each cycle hinder the commercial development in terms of production cost. In this work, we demonstrate a fast (<3 min) and feasible approach to synthesizing a few-layers of WSe2 and MoSe2 on arbitrary substrates by a microwave-assisted selenization process. The transition metal dichalcogenides (TMDs) can be patterned by standard photolithography. Furthermore, controllable layered growth from horizontal to vertical alignment can be achieved, leading to an enhanced chemical catalytic activity caused by large edge sites (exposed areas). As a proof, a highly sensitive NO gas sensor based on vertical WSe2 was fabricated. Moreover, our microwave assisted selenization process can be further extended to achieve other two-dimensional TMD materials because of the simplicity of the process.
Phase-Engineered PtSe2-Layered Films by a Plasma-Assisted Selenization Process toward All PtSe2-Based Field Effect Transistor to Highly Sensitive, Flexible, and Wide-Spectrum Photoresponse Photodetectors
(2018-05-09)
Su, Teng-Yu
;
Medina, Henry
;
Chen, Yu-Ze
;
Wang, Sheng-Wen
;
Lee, Shao-Shin
The formation of PtSe2-layered films is reported in a large area by the direct plasma-assisted selenization of Pt films at a low temperature, where temperatures, as low as 100 degrees C at the applied plasma power of 400 W can be achieved. As the thickness of the Pt film exceeds 5 nm, the PtSe2-layered film (five monolayers) exhibits a metallic behavior. A clear p-type semiconducting behavior of the PtSe2-layered film (approximate to trilayers) is observed with the average field effective mobility of 0.7 cm(2) V-1 s(-1) from back-gated transistor measurements as the thickness of the Pt film reaches below 2.5 nm. A full PtSe2 field effect transistor is demonstrated where the thinner PtSe2, exhibiting a semiconducting behavior, is used as the channel material, and the thicker PtSe2, exhibiting a metallic behavior, is used as an electrode, yielding an ohmic contact. Furthermore, photodetectors using a few PtSe2-layered films as an adsorption layer synthesized at the low temperature on a flexible substrate exhibit a wide range of absorption and photoresponse with the highest photocurrent of 9 mu A under the laser wavelength of 408 nm. In addition, the device can maintain a high photoresponse under a large bending stress and 1000 bending cycles.
Realizing thermal strain of patterned sapphire substrates dominate the bandgap-shifted of bilayer MoS2
(2017-01-01)
Wang, Sheng-Wen
;
Medina, Henry
;
Hong, Kuo-Bin
;
Wu, Chun-Chia
;
Arumugam, Manikandan
Using thermal strain concept, we can tune the bandgap of bilayer MoS2 through the two different thermal expansion coefficients of sapphire. Also, we propose a simple model to explain and precisely predict the bandgap-shifted behavior.
Thermally Strained Band Gap Engineering of Transition-Metal Dichalcogenide Bilayers with Enhanced Light Matter Interaction toward Excellent Photodetectors
(2017-09-01)
Wang, Sheng-Wen
;
Medina, Henry
;
Hong, Kuo-Bin
;
Wu, Chun-Chia
;
Qu, Yindong
Integration of strain engineering of two-dimensional (2D) materials in order to enhance device performance is still a challenge. Here, we successfully demonstrated the thermally strained band gap engineering of transition-metal dichalcogenide bilayers by different thermal expansion coefficients between 2D materials and patterned sapphire structures, where MoS2 bilayers were chosen as the demonstrated materials. In particular, a blue shift in the band gap of the MoS2 bilayers can be tunable, displaying an extraordinary capability to drive electrons toward the electrode under the smaller driven bias, and the results were confirmed by simulation. A model to explain the thermal strain in the MoS2 bilayers during the synthesis was proposed, which enables us to precisely predict the band gap-shifted behaviors on patterned sapphire structures with different angles. Furthermore, photodetectors with enhancement of 286% and 897% based on the strained MoS2 on cone and pyramid-patterned sapphire substrates were demonstrated, respectively.
Wafer Scale Phase-Engineered 1T-and 2H-MoSe2/Mo Core-Shell 3D-Hierarchical Nanostructures toward Efficient Electrocatalytic Hydrogen Evolution Reaction
(2016-11-23)
Qu, Yindong
;
Medina, Henry
;
Wang, Sheng-Wen
;
Wang, Yi-Chung
;
Chen, Chia-Wei
The necessity for new sources for greener and cleaner energy production to replace the existing ones has been increasingly growing in recent years. Of those new sources, the hydrogen evolution reaction has a large potential. In this work, for the first time, MoSe2/Mo core-shell 3D-hierarchical nanostructures are created, which are derived from the Mo 3D-hierarchical nanostructures through a low-temperature plasma-assisted selenization process with controlled shapes grown by a glancing angle deposition system.