196 research outputs found
Asynchronous Distributed ADMM for Large-Scale Optimization- Part I: Algorithm and Convergence Analysis
Aiming at solving large-scale learning problems, this paper studies
distributed optimization methods based on the alternating direction method of
multipliers (ADMM). By formulating the learning problem as a consensus problem,
the ADMM can be used to solve the consensus problem in a fully parallel fashion
over a computer network with a star topology. However, traditional synchronized
computation does not scale well with the problem size, as the speed of the
algorithm is limited by the slowest workers. This is particularly true in a
heterogeneous network where the computing nodes experience different
computation and communication delays. In this paper, we propose an asynchronous
distributed ADMM (AD-AMM) which can effectively improve the time efficiency of
distributed optimization. Our main interest lies in analyzing the convergence
conditions of the AD-ADMM, under the popular partially asynchronous model,
which is defined based on a maximum tolerable delay of the network.
Specifically, by considering general and possibly non-convex cost functions, we
show that the AD-ADMM is guaranteed to converge to the set of
Karush-Kuhn-Tucker (KKT) points as long as the algorithm parameters are chosen
appropriately according to the network delay. We further illustrate that the
asynchrony of the ADMM has to be handled with care, as slightly modifying the
implementation of the AD-ADMM can jeopardize the algorithm convergence, even
under a standard convex setting.Comment: 37 page
Electric-field-induced strong enhancement of electroluminescence in multilayer molybdenum disulfide.
The layered transition metal dichalcogenides have attracted considerable interest for their unique electronic and optical properties. While the monolayer MoS2 exhibits a direct bandgap, the multilayer MoS2 is an indirect bandgap semiconductor and generally optically inactive. Here we report electric-field-induced strong electroluminescence in multilayer MoS2. We show that GaN-Al2O3-MoS2 and GaN-Al2O3-MoS2-Al2O3-graphene vertical heterojunctions can be created with excellent rectification behaviour. Electroluminescence studies demonstrate prominent direct bandgap excitonic emission in multilayer MoS2 over the entire vertical junction area. Importantly, the electroluminescence efficiency observed in multilayer MoS2 is comparable to or higher than that in monolayers. This strong electroluminescence can be attributed to electric-field-induced carrier redistribution from the lowest energy points (indirect bandgap) to higher energy points (direct bandgap) in k-space. The electric-field-induced electroluminescence is general for other layered materials including WSe2 and can open up a new pathway towards transition metal dichalcogenide-based optoelectronic devices
Large area growth and electrical properties of p-type WSe2 atomic layers.
Transition metal dichacogenides represent a unique class of two-dimensional layered materials that can be exfoliated into single or few atomic layers. Tungsten diselenide (WSe(2)) is one typical example with p-type semiconductor characteristics. Bulk WSe(2) has an indirect band gap (⌠1.2 eV), which transits into a direct band gap (⌠1.65 eV) in monolayers. Monolayer WSe(2), therefore, is of considerable interest as a new electronic material for functional electronics and optoelectronics. However, the controllable synthesis of large-area WSe(2) atomic layers remains a challenge. The studies on WSe(2) are largely limited by relatively small lateral size of exfoliated flakes and poor yield, which has significantly restricted the large-scale applications of the WSe(2) atomic layers. Here, we report a systematic study of chemical vapor deposition approach for large area growth of atomically thin WSe(2) film with the lateral dimensions up to ⌠1 cm(2). Microphotoluminescence mapping indicates distinct layer dependent efficiency. The monolayer area exhibits much stronger light emission than bilayer or multilayers, consistent with the expected transition to direct band gap in the monolayer limit. The transmission electron microscopy studies demonstrate excellent crystalline quality of the atomically thin WSe(2). Electrical transport studies further show that the p-type WSe(2) field-effect transistors exhibit excellent electronic characteristics with effective hole carrier mobility up to 100 cm(2) V(-1) s(-1) for monolayer and up to 350 cm(2) V(-1) s(-1) for few-layer materials at room temperature, comparable or well above that of previously reported mobility values for the synthetic WSe(2) and comparable to the best exfoliated materials
A Highly Active Star Decahedron Cu Nanocatalyst for Hydrocarbon Production at Low Overpotentials
The electrochemical carbon dioxide reduction reaction (CO_2RR) presents a viable approach to recycle CO_2 gas into low carbon fuels. Thus, the development of highly active catalysts at low overpotential is desired for this reaction. Herein, a highâyield synthesis of unique star decahedron Cu nanoparticles (SDâCu NPs) electrocatalysts, displaying twin boundaries (TBs) and multiple stacking faults, which lead to low overpotentials for methane (CH_4) and high efficiency for ethylene (C_2H_4) production, is reported. Particularly, SDâCu NPs show an onset potential for CH_4 production lower by 0.149 V than commercial Cu NPs. More impressively, SDâCu NPs demonstrate a faradaic efficiency of 52.43% ± 2.72% for C_2H_4 production at â0.993 ± 0.0129 V. The results demonstrate that the surface stacking faults and twin defects increase CO binding energy, leading to the enhanced CO_2RR performance on SDâCu NPs
Gate-induced insulator to band-like transport transition in organolead halide perovskite
Understanding the intrinsic charge transport in organolead halide perovskites
is essential for the development of high-efficiency photovoltaics and other
optoelectronic devices. Despite the rapid advancement of the organolead halide
perovskite in photovoltaic and optoelectronic applications, the intrinsic
charge carrier transport in these materials remains elusive partly due to the
difficulty of fabricating electrical devices and obtaining good electrical
contact. Here, we report the fabrication of organolead halide perovskite
microplates with monolayer graphene as low barrier electrical contact. A
systematic charge transport studies reveal an insulator to band-like transport
transition. Our studies indicate that the insulator to band-like transport
transition depends on the orthorhombic-to-tetragonal phase transition
temperature and defect densities of the organolead halide perovskite
microplates. Our findings are not only important for the fundamental
understanding of charge transport behavior but also offer valuable practical
implications for photovoltaics and optoelectronic applications based on the
organolead halide perovskite.Comment: 18 pages, 5 figure
Enhanced interlayer neutral excitons and trions in trilayer van der Waals heterostructures
Vertically stacked van der Waals heterostructures constitute a promising
platform for providing tailored band alignment with enhanced excitonic systems.
Here we report observations of neutral and charged interlayer excitons in
trilayer WSe2-MoSe2-WSe2 van der Waals heterostructures and their dynamics. The
addition of a WSe2 layer in the trilayer leads to significantly higher
photoluminescence quantum yields and tunable spectral resonance compared to its
bilayer heterostructures at cryogenic temperatures. The observed enhancement in
the photoluminescence quantum yield is due to significantly larger
electron-hole overlap and higher light absorbance in the trilayer
heterostructure, supported via first-principle pseudopotential calculations
based on spin-polarized density functional theory. We further uncover the
temperature- and power-dependence, as well as time-resolved photoluminescence
of the trilayer heterostructure interlayer neutral excitons and trions. Our
study elucidates the prospects of manipulating light emission from interlayer
excitons and designing atomic heterostructures from first-principles for
optoelectronics.Comment: 25 pages, 5 figures(Maintext). 9 pages, 7 figures(Supplementary
Information). - Accepted for publication in npg: 2D materials and
applications and reformatted to its standard. - Updated co-authors and
references. - Title and abstract are modified for clarity. - Errors have been
corrected, npg: 2D materials and applications (2018
Electroluminescence and photocurrent generation from atomically sharp WSe2/MoS2 heterojunction p-n diodes
The p-n diodes represent the most fundamental device building block for
diverse optoelectronic functions, but are difficult to achieve in atomically
thin transition metal dichalcogenides (TMDs) due to the inability to
selectively dope them into p- or n-type semiconductors. Here we report the
first demonstration of an atomically thin and atomically sharp heterojunction
p-n diode by vertically stacking p-type monolayer tungsten diselenide (WSe2)
and n-type few-layer molybdenum disulfide (MoS2). Electrical measurement
demonstrates excellent diode characteristics with well-defined current
rectification behaviour and an ideality factor of 1.2. Photocurrent mapping
shows fast photoresponse over the entire overlapping region with a highest
external quantum efficiency up to 12 %. Electroluminescence studies show
prominent band edge excitonic emission and strikingly enhanced hot electron
luminescence. A systematic investigation shows distinct layer-number dependent
emission characteristics and reveals important insight about the origin of
hot-electron luminescence and the nature of electron-orbital interaction in
TMDs. We believe that these atomically thin heterojunction p-n diodes represent
an interesting system for probing the fundamental electro-optical properties in
TMDs, and can open up a new pathway to novel optoelectronic devices such as
atomically thin photodetectors, photovoltaics, as well as
spin-/valley-polarized light emitting diodes and on-chip lasers.Comment: 27 pages, 7 figure
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