arXiv subjects

Qi Feng

Publications and source records attributed to Qi Feng.

103 records · Page 6Linked to original sources

arXiv2017

Fast multicolor photodetectors based on graphene-contacted p-GaSe/n-InSe van der Waals heterostructures

The integration of different two-dimensional materials within a multilayer van der Waals (vdW) heterostructure offers a promising technology for realizing high performance opto-electronic devices such as photodetectors and light sources1-3. Transition metal dichalcogenides, e.g. MoS2 and WSe2, have been employed as the optically-active layer in recently developed heterojunctions. However, MoS2 and WSe2 become direct band gap semiconductors only in mono- or bilayer form4,5. In contrast, the metal monochalcogenides InSe and GaSe retain a direct bandgap over a wide range of layer thicknesses from bulk crystals down to exfoliated flakes only a few atomic monolayers thick6,7. Here we report on vdW heterojunction diodes based on InSe and GaSe: the type II band alignment between the two materials and their distinctive spectral response, combined with the low electrical resistance of transparent graphene electrodes, enable effective separation and extraction of photoexcited carriers from the heterostructure even when no external voltage is applied. Our devices are fast (< 10 μs), self-driven photodetectors with multicolor photoresponse ranging from the ultraviolet to the near-infrared and have the potential to accelerate the exploitation of two-dimensional vdW crystals by creating new routes to miniaturized optoelectronics beyond present technologies.

physics.ins-det↗

arXiv2016

Towards Fast Multicolor Photodetectors based on Graphene Contacted Vertical p-GaSe/n-InSe van der Waals Heterostructure

We investigated the electronic and optoelectronic properties of vertical van der Waals heterostructure photodetectors using layered p type GaSe and n type InSe, with graphene as the transparent electrodes. Not only the photocurrent peaks from the layered GaSe and InSe themselves were observed, also the interlayer optical transition peak was observed, which is consistent with the first-principles calculation. The built-in electric field between p-n heterojunction and the advantage of the graphene electrodes can effectively separate the photo-induced electron-hole pairs, and thus lead to the response time down to 160 μs. Making use of the interlayer as well as intralayer optical transitions of the vertical layered p-n heterostructure and graphene electrodes, we could achieve high performance multi-color photodetectors based on two-dimensional layered materials, which will be important for future optoelectronics.

cond-mat.mes-hall↗

arXiv2016

The analysis of VERITAS muon images using convolutional neural networks

Imaging atmospheric Cherenkov telescopes (IACTs) are sensitive to rare gamma-ray photons, buried in the background of charged cosmic-ray (CR) particles, the flux of which is several orders of magnitude greater. The ability to separate gamma rays from CR particles is important, as it is directly related to the sensitivity of the instrument. This gamma-ray/CR-particle classification problem in IACT data analysis can be treated with the rapidly-advancing machine learning algorithms, which have the potential to outperform the traditional box-cut methods on image parameters. We present preliminary results of a precise classification of a small set of muon events using a convolutional neural networks model with the raw images as input features. We also show the possibility of using the convolutional neural networks model for regression problems, such as the radius and brightness measurement of muon events, which can be used to calibrate the throughput efficiency of IACTs.

astro-ph.IM↗

arXiv2015

Sparse Multipath Channel Estimation and Decoding for Broadband Vector OFDM Systems

Vector orthogonal frequency division multiplexing (V-OFDM) is a general system that builds a bridge between OFDM and single-carrier frequency domain equalization in terms of intersymbol interference and receiver complexity. In this paper, we investigate the sparse multipath channel estimation and decoding for broadband V-OFDM systems. Unlike the non-sparse channel estimation, sparse channel estimation only needs to recover the nonzero taps with reduced complexity. Consider the pilot signals are transmitted through a sparse channel that has only a few nonzero taps with and without additive white Gaussian noise, respectively. The exactly and approximately sparse inverse fast Fourier transform (SIFFT) can be employed for these two cases. The SIFFT-based algorithm recovers the nonzero channel coefficients and their corresponding coordinates directly, which is significant to the proposed partial intersection sphere (PIS) decoding approach. Unlike the maximum likelihood (ML) decoding that enumerates symbol constellation and estimates the transmitted symbols with the minimum distance, the PIS decoding first generates the set of possible transmitted symbols and then chooses the transmitted symbols only from this set with the minimum distance. The diversity order of the PIS decoding is determined by not only the number of nonzero taps, but also the coordinates of nonzero taps, and the bit error rate (BER) is also influenced by vector block size to some extent but roughly independent of the maximum time delay. Simulation results indicate that by choosing appropriate sphere radius, the BER performance of the PIS decoding outperforms the conventional zero-forcing decoding and minimum mean square error decoding, and approximates to the ML decoding with the increase of signal-to-noise ratio, but reduces the computational complexity significantly.

cs.IT↗

arXiv2015

Charge Trap Memory Based on Few-Layered Black Phosphorus

Atomically thin layered two-dimensional materials, including transition-metal dichacolgenide (TMDC) and black phosphorus (BP), (1) have been receiving much attention, because of their promising physical properties and potential applications in flexible and transparent electronic devices . Here, for the first time we show non-volatile chargetrap memory devices, based on field-effect transistors with large hysteresis, consisting of a few-layer black phosphorus channel and a three dimensional (3D) Al2O3 /HfO2 /Al2O3 charge-trap gate stack. An unprecedented memory window exceeding 12 V is observed, due to the extraordinary trapping ability of HfO2. The device shows a high endurance and a stable retention of ?25% charge loss after 10 years, even drastically lower than reported MoS2 flash memory. The high program/erase current ratio, large memory window, stable retention and high on/off current ratio, provide a promising route towards the flexible and transparent memory devices utilising atomically thin two-dimensional materials. The combination of 2D materials with traditional high-k charge-trap gate stacks opens up an exciting field of nonvolatile memory devices.

cond-mat.mtrl-sci↗

arXiv2015

Gate Tuning of High-Performance InSe-Based Photodetectors Using Graphene Electrodes

In order to increase the response speed of the InSe-based photodetector with high photoresponsivity, graphene is used as the transparent electrodes to modify the difference of the work function between the electrodes and the InSe. As expected, the response speed of InSe/graphene photodetectors is down to 120 μs, which is about 40 times faster than that of our InSe/metal device. And it can also be tuned by the back-gate voltage from 310 μs down to 100 μs. With high response speed, the photoresponsivity can reach as high as 60 AW-1 simultaneously. Meanwhile the InSe/graphene photodetectors possess a broad spectral range at 400-1000 nm. The design of 2D crystal/graphene electrical contacts could be important for high performance optoelectronic devices.

physics.optics↗

arXiv2015

Log-Sobolev inequalities on the horizontal path space of a totally geodesic foliation

We develop a Malliavin calculus on the horizontal path space of a totally geodesic Riemannian foliation. As a first application, under suitable assumptions, we prove a log-Sobolev inequality for a natural one-parameter family of infinite-dimensional Ornstein-Uhlenbeck type operators. As a second application, we obtain concentration and tail estimates for the horizontal Brownian motion of the foliation.

math.PR↗

arXiv2013

Grain boundary ferromagnetism in vanadium-doped In$_2$O$_3$ thin films

Room temperature ferromagnetism was observed in In$_2$O$_3 thin films doped with 5 at.% vanadium, prepared by pulsed laser deposition at substrate temperatures ranging from 300 to 600 $\,^{\circ}{\rm C}$. X-ray absorption fine structure measurement indicated that vanadium was substitutionally dissolved in the In$_2$O$_3$ host lattice, thus excluding the existence of secondary phases of vanadium compounds. Magnetic measurements based on SQUID magnetometry and magnetic circular dichroism confirm that the magnetism is at grain boundaries and also in the grains. The overall magnetization originates from the competing effects between grains and grain boundaries.

cond-mat.mtrl-sci↗

arXiv2013

Contrasting behavior of the structural and magnetic properties in Mn- and Fe-doped In$_2$O$_3$ films

We have observed room temperature ferromagnetism (FM) in In$_2$O$_3$ thin films doped with either 5 at.% Mn or Fe, prepared by pulsed laser deposition (PLD) at substrate temperatures ranging from 300 to 600$\,^{\circ}{\rm C}$. The dependence of saturation magnetization on grain size was investigated for both types of In$_2$O$_3$ films. It is revealed that, for the Mn-doped films, the magnetization was largest with small grains, indicating the importance of grain boundaries. In contrast, for Fe-doped films, the largest magnetization was observed with large grains.

cond-mat.mtrl-sci↗

arXiv2010

On Using Magnetic and optical methods to determine the size and characteristics of nanoparticles embedded in oxide semiconductors

Films of oxides doped with transition metals are frequently believed to have magnetic inclusions. Magnetic methods to determine the amount of nanophases and their magnetic characteristics are described. The amount of the sample that is paramagnetic may also be measured. Optical methods are described and shown to be very powerful to determine which defects are also magnetic.

cond-mat.mtrl-sci↗

arXiv2009

Magneto-optical properties of Co/ZnO multilayer films

Multilayer films of ZnO with Co were deposited on glass substrates then annealed in a vacuum. The magnetisation of the films increased with annealing but not the magnitude of the magneto-optical signals. The dielectric functions for the films were calculated using the MCD spectra. A Maxwell Garnett theory of a metallic Co/ZnO mixture is presented. The extent to which this explains the MCD spectra taken on the films is discussed.

cond-mat.mtrl-sci↗

arXiv2007

Empirical study on clique-degree distribution of networks

The community structure and motif-modular-network hierarchy are of great importance for understanding the relationship between structures and functions. In this paper, we investigate the distribution of clique-degree, which is an extension of degree and can be used to measure the density of cliques in networks. The empirical studies indicate the extensive existence of power-law clique-degree distributions in various real networks, and the power-law exponent decreases with the increasing of clique size.

physics.soc-ph↗

arXiv1994

Projective Stationary Sets and Strong Reflection Principle

We study projective stationary sets. The Projective Stationary Reflection principle is the statement that every projective stationary set contains an increasing continuous $\in$--chain of length $ω_1$. We show that if Martin's Maximum holds, then the Projective Stationary Reflection Principle holds. Also it is equivalent to the Strong Reflection Principle. We show that the saturation of the nonstationary ideal on $ω_1$ is equivalent to a certain kind of reflection.

math.LO↗