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Ledge-directed epitaxy of continuously self-aligned single-crystalline nanoribbons of transition metal dichalcogenides
(1970-01-01)
Aljarb, Areej
;
Fu, Jui-Han
;
Hsu, Chih-Chan
;
Chuu, Chih-Piao
;
Wan, Yi
Two-dimensional transition metal dichalcogenide nanoribbons are touted as the future extreme device downscaling for advanced logic and memory devices but remain a formidable synthetic challenge. Here, we demonstrate a ledge-directed epitaxy (LDE) of dense arrays of continuous, self-aligned, monolayer and single-crystalline MoS(2)nanoribbons on beta-gallium (iii) oxide (beta-Ga2O3) (100) substrates. LDE MoS(2)nanoribbons have spatial uniformity over a long range and transport characteristics on par with those seen in exfoliated benchmarks. Prototype MoS2-nanoribbon-based field-effect transistors exhibit high on/off ratios of 10(8)and an averaged room temperature electron mobility of 65 cm(2) V-1 s(-1). The MoS(2)nanoribbons can be readily transferred to arbitrary substrates while the underlying beta-Ga(2)O(3)can be reused after mechanical exfoliation. We further demonstrate LDE as a versatile epitaxy platform for the growth of p-type WSe(2)nanoribbons and lateral heterostructures made of p-WSe(2)and n-MoS(2)nanoribbons for futuristic electronics applications. Aligned arrays of single-crystalline monolayer TMD nanoribbons with high aspect ratios, as well as their lateral heterostructures, are realized, with the growth directed by the ledges on the beta-Ga(2)O(3)substrate. This approach provides an epitaxy platform for advanced electronics applications of TMD nanoribbons.
Hybrid Composites of Quantum Dots, Monolayer WSe2, and Ag Nanodisks for White Light-Emitting Diodes
(2020-07-24)
Chang, Chiao-Yun
;
Yu, Cheng-Li
;
Lin, Chen-An
;
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.
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
;
Kuo, Po-Chun
;
Kao, Hui-Ling
;
Chen, Jyh-Shin
;
Chen, Meei-Ru
Purely in-plane ferroelectricity in monolayer SnS at room temperature
(2020-05-15)
Higashitarumizu, Naoki
;
Kawamoto, Hayami
;
Lee, Chien-Ju
;
Lin, Bo-Han
;
Chu, Fu-Hsien
2D van der Waals ferroelectrics have emerged as an attractive building block with immense potential to provide multifunctionality in nanoelectronics. Although several accomplishments have been reported in ferroelectric switching for out-of-plane ferroelectrics down to the monolayer, a purely in-plane ferroelectric has not been experimentally validated at the monolayer thickness. Herein, an in-plane ferroelectricity is demonstrated for micrometer-size monolayer SnS at room temperature. SnS has been commonly regarded to exhibit the odd-even effect, where the centrosymmetry breaks only in the odd-number layers to exhibit ferroelectricity. Remarkably, however, a robust room temperature ferroelectricity exists in SnS below a critical thickness of 15 layers with both an odd and even number of layers, suggesting the possibility of controlling the stacking sequence of multilayer SnS beyond the limit of ferroelectricity in the monolayer. This work will pave the way for nanoscale ferroelectric applications based on SnS as a platform for in-plane ferroelectrics. Out-of-plane ferroelectricity is usually unstable in the two dimensional limit due to the presence of the depolarization field. Here, the authors successfully circumvent this issue by growing mu m-sized SnS monolayers that exhibit in-plane ferroelectricity that is stable at room temperature.
Bidirectional All-Optical Synapses Based on a 2D Bi2O2Se/Graphene Hybrid Structure for Multifunctional Optoelectronics
(1970-01-01)
Yang, Chia-Ming
;
Chen, Tsung-Cheng
;
Verma, Dharmendra
;
Li, Lain-Jong
;
Liu, Bo
Neuromorphic computing has been extensively studied to mimic the brain functions of perception, learning, and memory because it may overcome the von Neumann bottleneck. Here, with the light-induced bidirectional photoresponse of the proposed Bi2O2Se/graphene hybrid structure, its potential use in next-generation neuromorphic hardware is examined with three distinct optoelectronic applications. First, a photodetector based on a Bi2O2Se/graphene hybrid structure presents positive and negative photoresponsibility of 88 and -110 A W-1 achieved by the excitation of visible wavelength and ultraviolet wavelength light at intensities of 1.2 and 0.3 mW cm(-2), respectively. Second, this unique photoresponse contributes to the realization of all optically stimulated long-term potentiation or long-term depression to mimic synaptic short-term plasticity and long-term plasticity, which are attributed to the combined effect of photoconductivity, bolometric, and photoinduced desorption. Third, the devices are applied to perform digital logic functions, such as "AND" and "OR," using full light modulation. The proposed Bi2O2Se/graphene-based optoelectronic device represents an innovative and efficient building block for the development of future multifunctional artificial neuromorphic systems.
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.
Nitride-stressor and quantum-size engineering in Ge quantum-dot photoluminescence wavelength and exciton lifetime
(2020-03-01)
Kuo, Yu-Hong
;
Chiu, Shih-Hsuan
;
Tien, Che-Wei
;
Lin, Sheng-Di
;
Chang, Wen-Hao
We report local nitride-stressor engineering in combination with quantum-size tunability in Ge quantum dots (QDs) for tailoring photoluminescence (PL) wavelength and exciton lifetime. Spherical-shaped Ge QDs with tunable diameters ranging from 25-90 nm embedded within layers of thermally-grown SiO2 and chemical-vapor-deposited Si3N4 were fabricated by thermal oxidation of lithographically-patterned poly-Si0.85Ge0.15 pillars on top of buffer layers of SiO2 and Si3N4, respectively, in a self-organization approach. The effects of local stressors and quantum confinement on the strain and optical properties of Ge QDs were systematically investigated using Raman and PL measurements. We observed that when Ge QD diameter gets smaller than 60 nm, quantum confinement sets in and has a predominant influence on PL wavelength and exciton lifetime. Our self-organized Ge QD/Si3N4 system provides a generic building block for the fabrication of active photonic devices on Si3N4 platform.
Monolayer MoS2 Enabled Single-Crystalline Growth of AlN on Si(100) Using Low-Temperature Helicon Sputtering
(2019-04-01)
Hsu, Wei-Fan
;
Lu, Li-Syuan
;
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.
Layer-Dependent and In-Plane Anisotropic Properties of Low-Temperature Synthesized Few-Layer PdSe2 Single Crystals
(2020-04-28)
Lu, Li-Syuan
;
Chen, Guan-Hao
;
Cheng, Hui-Yu
;
Chuu, Chih-Piao
;
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.
Femtosecond-Laser Ablation of Monolayer Tungsten Diselenide (WSe2) on Sapphire
(2018-01-01)
Chen, Yu-Ling
;
Tseng, Ya-Hsin
;
Chen, Yen-Chun
;
Chang, Wen-Hao
;
Her, Tsing-Hua
Near-IR femtosecond laser ablation of monolayer tungsten diselenide WSe2 on sapphire is demonstrated. The ablation threshold is found low (below 10 mJ/cm(2)), due to its low dimensionality and weak coupling to substrates. Substrate-dependent ablation threshold is predicted and will be discussed.

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