Single-layer metal dichalcogenides are two-dimensional semiconductors that present strong potential for electronic and sensing applications complementary to that of graphene.

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Electronics and optoelectronics of two-dimensional transition metal dichalcogenides.

Probability Distributions.- Linear Models for Regression.- Linear Models for Classification.- Neural Networks.- Kernel Methods.- Sparse Kernel Machines.- Graphical Models.- Mixture Models and EM.- Approximate Inference.- Sampling Methods.- Continuous Latent Variables.- Sequential Data.- Combining Models.

Pattern Recognition and Machine Learning

Graphene’s success has shown that it is possible to create stable, single and few-atom-thick layers of van der Waals materials, and also that these materials can exhibit fascinating and technologically useful properties. Here we review the state-of-the-art of 2D materials beyond graphene. Initially, we will outline the different chemical classes of 2D materials and discuss the various strategies to prepare single-layer, few-layer, and multilayer assembly materials in solution, on substrates, and on the wafer scale. Additionally, we present an experimental guide for identifying and characterizing single-layer-thick materials, as well as outlining emerging techniques that yield both local and global information. We describe the differences that occur in the electronic structure between the bulk and the single layer and discuss various methods of tuning their electronic properties by manipulating the surface. Finally, we highlight the properties and advantages of single-, few-, and many-layer 2D materials in...

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Progress, Challenges, and Opportunities in Two-Dimensional Materials Beyond Graphene

Preceding the current interest in layered materials for electronic applications, research in the 1960's found that black phosphorus combines high carrier mobility with a fundamental band gap. We introduce its counterpart, dubbed few-layer phosphorene, as a new 2D p-type material. Same as graphene and MoS2, phosphorene is flexible and can be mechanically exfoliated. We find phosphorene to be stable and, unlike graphene, to have an inherent, direct and appreciable band-gap that depends on the number of layers. Our transport studies indicate a carrier mobility that reflects its structural anisotropy and is superior to MoS2. At room temperature, our phosphorene field-effect transistors with 1.0 um channel length display a high on-current of 194 mA/mm, a high hole field-effect mobility of 286 cm2/Vs, and an on/off ratio up to 1E4. We demonstrate the possibility of phosphorene integration by constructing the first 2D CMOS inverter of phosphorene PMOS and MoS2 NMOS transistors.

Phosphorene: A New 2D Material with High Carrier Mobility

The lithium metal battery is strongly considered to be one of the most promising candidates for high-energy-density energy storage devices in our modern and technology-based society. However, uncontrollable lithium dendrite growth induces poor cycling efficiency and severe safety concerns, dragging lithium metal batteries out of practical applications. This review presents a comprehensive overview of the lithium metal anode and its dendritic lithium growth. First, the working principles and technical challenges of a lithium metal anode are underscored. Specific attention is paid to the mechanistic understandings and quantitative models for solid electrolyte interphase (SEI) formation, lithium dendrite nucleation, and growth. On the basis of previous theoretical understanding and analysis, recently proposed strategies to suppress dendrite growth of lithium metal anode and some other metal anodes are reviewed. A section dedicated to the potential of full-cell lithium metal batteries for practical applicatio...

Toward Safe Lithium Metal Anode in Rechargeable Batteries: A Review.

Improving the rate performance of LiFePO4 by Fe-site doping

The atomic and electronic properties of black phosphorus (BP), which has been recently shown to have potential application as anode material for lithium ion batteries, are studied via ab initio calculations. The calculations reveal that the interlayer interaction in BP is Van der Waals Keesom force, which is critical to the formation of the layered structure. Interestingly, we also found that the small band gap of bulk BP (0.19 eV) when compared with that of single layer BP (0.75 eV) is partly because of the interlayer Van der Waals interaction in BP. The change in a materials band structure because of Van der Waals interaction is rarely reported in literature.

Ab initio studies on atomic and electronic structures of black phosphorus

The diffusion mechanism of Li ions in the olivine ${\mathrm{LiFePO}}_{4}$ is investigated from first-principles calculations. The energy barriers for possible spatial hopping pathways are calculated with the adiabatic trajectory method. The calculations show that the energy barriers running along the $c$ axis are about 0.6, 1.2, and 1.5 eV for ${\mathrm{LiFePO}}_{4},$ ${\mathrm{FePO}}_{4},$ and ${\mathrm{Li}}_{0.5}{\mathrm{FePO}}_{4},$ respectively. However, the other migration pathways have much higher energy barriers resulting in very low probability of Li-ion migration. This means that the diffusion in ${\mathrm{LiFePO}}_{4}$ is one dimensional. The one-dimensional diffusion behavior has also been shown with full ab initio molecular dynamics simulation, through which the diffusion behavior is directly observed.

First-principles study of Li ion diffusion in LiFePO 4

We present a first-principles electronic band structure for pure ${\mathrm{LiFePO}}_{4},$ delithiated ${\mathrm{FePO}}_{4},$ and Cr-doped ${\mathrm{LiFePO}}_{4}.$ It indicates that not only Fe but also O atoms are oxidized in the delithiation process, while P is little affected. This is in contrast to the usual view of the intercalation reaction that the removal of Li only transforms Fe from ${\mathrm{Fe}}^{2+}$ to ${\mathrm{Fe}}^{3+},$ but in agreement with the present x-ray photoemission spectroscopy experiment. Calculation also assumes a significant enhancement of electronic conductivity when lithium ions are replaced by cations with higher valence, ${\mathrm{Cr}}^{3+}.$ We also confirm experimentally, for ${\mathrm{Li}}_{1\ensuremath{-}3x}{\mathrm{Cr}}_{x}{\mathrm{FePO}}_{4}$ with $x=0.01$ and 0.03, an enhancement of the electronic conductivity up to eight orders of magnitude comparing with pure ${\mathrm{LiFePO}}_{4}.$ Besides the conventional p-type doping conductivity, another mechanism has been suggested, which involves the electron hopping within a cluster surrounding the doping atom and related vacancies, and electron tunneling between these conducting clusters.

Enhancement of electronic conductivity of LiFePO 4 by Cr doping and its identification by first-principles calculations

Understanding and improving Li transport through crystalline Li2CO3, a stable component of the solid electrolyte interphase (SEI) films in Li-ion batteries, is critical to battery rate performance, capacity drop, and power loss. Identification of the dominant diffusion carriers and their diffusion pathways in SEI films coated on anode and cathode surfaces is a necessary step toward the development of methods to increase Li conductivity. In this paper, we identify the dominant Li diffusion carriers in Li2CO3 over a voltage range (0–4.4 V) that includes typical anode and cathode materials by computing and comparing thermodynamics of all seven Li-associated point defects with density functional theory (DFT). The main diffusion carriers in Li2CO3 below 0.98 V are excess Li-ion interstitials; however, above 3.98 V, Li-ion vacancies become the dominant diffusion carrier type, and they have the same concentration in the voltage range of 0.98–3.98 V. Diffusion mechanisms of the main diffusion carriers were comput...

Siqi Shi

Papers

Improving the rate performance of LiFePO4 by Fe-site doping

Ab initio studies on atomic and electronic structures of black phosphorus

First-principles study of Li ion diffusion in LiFePO 4

Enhancement of electronic conductivity of LiFePO 4 by Cr doping and its identification by first-principles calculations

Defect Thermodynamics and Diffusion Mechanisms in Li2CO3 and Implications for the Solid Electrolyte Interphase in Li-Ion Batteries