arXiv Material Science

/arXiv Material Science

Uniform vapor pressure based CVD growth of MoS2 using MoO3 thin film as a precursor for co-evaporation. (arXiv:1811.06119v1 [cond-mat.mtrl-sci])

2018-11-16T02:29:27+00:00November 16th, 2018|Categories: Publications|Tags: |

Chemical vapor deposition (CVD) is a powerful method employed for high quality monolayer crystal growth of 2D transition metal dichalcogenides with much effort invested toward improving the growth process. Here, we report a novel method for CVD based growth of monolayer molybdenum disulfide (MoS2) by using thermally evaporated thin films of molybdenum trioxide (MoO3) as the molybdenum (Mo) source for co-evaporation. Uniform evaporation rate of the MoO3thin films provides uniform Mo vapor which promotes highly reproducible single crystal growth of MoS2throughout the substrate. These high-quality crystals are as large as 95um and were characterized by scanning electron microscopy, Raman spectroscopy, photoluminescence spectroscopy,atomic force microscopy and transmission electron microscopy. The bottom gated field effect transistors fabricated on as grown single crystals shows n-type transistor behavior with a good on/off ratio of 106 under ambient conditions.Our results presented here addresses the precursor vapor control during the CVD processand is a major step forward toward reproduciblegrowth of MoS2 for future semiconductor device applications.

Published in: "arXiv Material Science".

Interlayer excitons in bilayer MoS2 with strong oscillator strength up to room temperature. (arXiv:1811.06469v1 [cond-mat.mtrl-sci])

2018-11-16T02:29:23+00:00November 16th, 2018|Categories: Publications|Tags: , |

Coulomb bound electron-hole pairs, excitons, govern the optical properties of semi-conducting transition metal dichalcogenides like MoS$_2$ and WSe$_2$. We study optical transitions at the K-point for 2H homobilayer MoS$_2$ in Density Functional Theory (DFT) including excitonic effects and compare with reflectivity measurements in high quality samples encapsulated in hexagonal BN. In both calculated and measured spectra we find a strong interlayer exciton transition in energy between A and B intralayer excitons, observable for T$=4 -300$ K, whereas no such transition is observed for the monolayer in the same structure in this energy range. The interlayer excitons consist of an electron localized in one layer and a hole state delocalized over the bilayer, which results in the unusual combination of high oscillator strength and a static dipole moment. We also find signatures of interlayer excitons involving the second highest valence band (B) and compare absorption calculations for different bilayer stackings. For homotrilayer MoS$_2$ we also observe interlayer excitons and an energy splitting between different intralayer A-excitons originating from the middle and outer layers, respectively.

Published in: "arXiv Material Science".

Computational insights and the observation of SiC nanograin assembly: towards 2D silicon carbide. (arXiv:1701.07387v2 [cond-mat.mtrl-sci] UPDATED)

2018-11-16T02:29:21+00:00November 16th, 2018|Categories: Publications|Tags: , , , , |

While an increasing number of two-dimensional (2D) materials, including graphene and silicene, have already been realized, others have only been predicted. An interesting example is the two-dimensional form of silicon carbide (2D-SiC). Here, we present an observation of atomically thin and hexagonally bonded nanosized grains of SiC assembling temporarily in graphene oxide pores during an atomic resolution scanning transmission electron microscopy experiment. Even though these small grains do not fully represent the bulk crystal, simulations indicate that their electronic structure already approaches that of 2D-SiC. This is predicted to be flat, but some doubts have remained regarding the preference of Si for sp$^{3}$ hybridization. Exploring a number of corrugated morphologies, we find completely flat 2D-SiC to have the lowest energy. We further compute its phonon dispersion, with a Raman-active transverse optical mode, and estimate the core level binding energies. Finally, we study the chemical reactivity of 2D-SiC, suggesting it is like silicene unstable against molecular absorption or interlayer linking. Nonetheless, it can form stable van der Waals-bonded bilayers with either graphene or hexagonal boron nitride, promising to further enrich the family of two-dimensional materials once bulk synthesis is achieved.

Published in: "arXiv Material Science".

Characterization of Lattice Thermal Transport in Two-Dimensional BAs, BP, and BSb: A First-Principles Study. (arXiv:1811.05597v1 [cond-mat.mtrl-sci])

2018-11-15T02:29:22+00:00November 15th, 2018|Categories: Publications|Tags: |

Ever since the high thermal conductivity in cubic boron arsenide (c-BAs) was predicted theoretically by Lindsay et. al in 2013, countless studies have zeroed in on this particular material. Most recently, c-BAs has been confirmed experimentally to have a thermal conductivity of around 1,100 W/m-K. In this study, we investigate the seldom studied two dimensional hexagonal form of boron arsenide (h-BAs) using a first-principles approach and by solving the Boltzmann Transport Equation for phonons. Traditionally, a good indicator of a high thermal conductivity material is its high Debye temperature and high phonon group velocity. However, we determine h-BAs to have a much lower Debye temperature and average phonon group velocity compared to the other monolayer boron-V compounds of boron nitride (h-BN) and boron phosphide (h-BP), yet curiously it possesses a higher thermal conductivity. Further investigation reveals that this is due to the phonon frequency gap caused by large mass imbalances, which results in a restricted Umklapp phonon-phonon scattering channel and consequently a higher thermal conductivity. We determine the intrinsic lattice thermal conductivity of monolayer h-BAs to be 362.62 W/m-K at room temperature, which is considerably higher compared to the other monolayer boron-V compounds of h-BN (231.96 W/m-K), h-BP (187.11 W/m-K), and h-BSb (87.15 W/m-K). This study opens the door for investigation into a new class of monolayer structures and the properties they possess.

Published in: "arXiv Material Science".

Unique gap structure and symmetry of the charge density wave in single-layer VSe$_2$. (arXiv:1811.05690v1 [cond-mat.mes-hall])

2018-11-15T02:29:20+00:00November 15th, 2018|Categories: Publications|Tags: |

Single layers of transition metal dichalcogenides (TMDCs) are excellent candidates for electronic applications beyond the graphene platform; many of them exhibit novel properties including charge density waves (CDWs) and magnetic ordering. CDWs in these single layers are generally a planar projection of the corresponding bulk CDWs because of the quasi-two-dimensional nature of TMDCs; a different CDW symmetry is unexpected. We report herein the successful creation of pristine single-layer VSe$_2$, which shows a ($sqrt7 times sqrt3$) CDW in contrast to the (4 $times$ 4) CDW for the layers in bulk VSe$_2$. Angle-resolved photoemission spectroscopy (ARPES) from the single layer shows a sizable ($sqrt7 times sqrt3$) CDW gap of $sim$100 meV at the zone boundary, a 220 K CDW transition temperature twice the bulk value, and no ferromagnetic exchange splitting as predicted by theory. This robust CDW with an exotic broken symmetry as the ground state is explained via a first-principles analysis. The results illustrate a unique CDW phenomenon in the two-dimensional limit.

Published in: "arXiv Material Science".

Quasi-free-standing single-layer WS2 achieved by intercalation. (arXiv:1811.05748v1 [cond-mat.mtrl-sci])

2018-11-15T02:29:18+00:00November 15th, 2018|Categories: Publications|Tags: |

Large-area and high-quality single-layer transition metal dichalcogenides can be synthesized by epitaxial growth on single-crystal substrates. An important advantage of this approach is that the interaction between the single-layer and the substrate can be strong enough to enforce a single crystalline orientation of the layer. On the other hand, the same interaction can lead to hybridization effects, resulting in the deterioration of the single-layer’s native properties. This dilemma can potentially be solved by decoupling the single-layer from the substrate surface after the growth via intercalation of atoms or molecules. Here we show that such a decoupling can indeed be achieved for single-layer WS2 epitaxially grown on Ag(111) by intercalation of Bi atoms. This process leads to a suppression of the single-layer WS2-Ag substrate interaction, yielding an electronic band structure reminiscent of free-standing single-layer WS2.

Published in: "arXiv Material Science".

Composition and Stacking Dependent Topology in Bilayers from the Graphene Family. (arXiv:1811.05525v1 [cond-mat.mtrl-sci])

2018-11-15T02:29:17+00:00November 15th, 2018|Categories: Publications|Tags: , , |

We present a compositional and structural investigation of silicene, germanene, and stanene bilayers from first-principles. Due to the staggering of the individual layers, several stacking patterns are possible, most of which are not available to the bilayer graphene. This structural variety, in conjunction with the presence of the spin-orbit coupling, unveil a diversity of the electronic properties, with the appearance of distinct band features, including orbital hybridization and band inversion. We show that for particular cases, the intrinsic spin Hall response exhibits signatures of non-trivial electronic band topology, making these structures promising candidates to probe Dirac-like physics.

Published in: "arXiv Material Science".

Toward the Identification of Atomic Defects in Hexagonal Boron Nitride: X-Ray Photoelectron Spectroscopy and First-Principles Calculations. (arXiv:1811.05924v1 [cond-mat.mtrl-sci])

2018-11-15T02:29:15+00:00November 15th, 2018|Categories: Publications|Tags: , |

Defects in hexagonal boron nitride (hBN) exhibit single-photon emission (SPE) and are thus attracting broad interest as platforms for quantum information and spintronic applications. However, the atomic structure and the specific impact of the local environment on the defect physical properties remain elusive. Here we articulate X-ray photoelectron spectroscopy (XPS) and first-principles calculations to discern the experimentally-observed point defects responsible for the quantum emission observed in hBN. XPS measurements show a broad band, which was deconvolved and then assigned to $N_{B}V_{N}$, $V_{N}$, $C_{B}$, $C_{B}V_{N}$, and $O_{2B}V_{N}$ defect structures using Density Functional Theory (DFT) core-level binding energy (BE) calculations.

Published in: "arXiv Material Science".

High-quality, large-grain MoS2 films grown on 100 mm sapphire substrates using a novel molybdenum precursor. (arXiv:1811.05044v1 [cond-mat.mtrl-sci])

2018-11-14T05:29:26+00:00November 14th, 2018|Categories: Publications|Tags: |

Two-dimensional MoS2 is a crystalline semiconductor with high potential for numerous technologies. Research in recent years has sought to exploit the direct band gap and high carrier mobility properties of monolayer MoS2 for functional applications. To date, the production of MoS2 has remained at the research level and samples are usually synthesized in small quantities using small yield, expensive techniques. In order to realize scalable MoS2-based technology, large-area, high-quality and affordable MoS2 wafers must become available. Here we report on MoS2 films grown on 100 mm sapphire wafers by a chemical vapor deposition process utilizing hydrogen sulfide and molybdenum precursor mixtures consisting of Na2MoO4 and NaCl. The grains of these films are faceted, large-area, on the order of 1-75 microns in length on edge. Growth conditions are identified that yield monolayer MoS2 films. Raman spectroscopy shows an E2g and A1g peak separation of 17.9-19.1 cm-1. Photoluminescence spectroscopy shows that annealing the films post-growth suppresses trion emission. X-ray photoelectron spectroscopy of the annealed films shows an in increase in the S:Mo concentration ratio from 1.90:1.00 to 2.09:1.00.

Published in: "arXiv Material Science".

Two dimensional crystals in three dimensions: electronic decoupling of single-layered platelets in colloidal nanoparticles. (arXiv:1811.05238v1 [cond-mat.mtrl-sci])

2018-11-14T05:29:24+00:00November 14th, 2018|Categories: Publications|Tags: , |

Two-dimensional crystals, single sheets of layered materials, often show distinct properties desired for optoelectronic applications, such as larger and direct band gaps, valley- and spinorbit effects. Being atomically thin, the low amount of material is a bottleneck in photophysical and photochemical applications. Here, we propose the formation of stacks of two-dimensional crystals intercalated with small surfactant molecules. We show, using first principles calculations, that already the very short surfactant methyl amine electronically decouples the layers. We demonstrate the indirect-direct band gap transition characteristic for Group 6 transition metal dichalcogenides experimentally by observing the emergence of a strong photoluminescence signal for ethoxide-intercalated WSe2 and MoSe2 multilayered nanoparticles with lateral size of about 10 nm and beyond. The proposed hybrid materials offer the highest possible density of the two-dimensional crystals with electronic properties typical for monolayers. Variation of the surfactant’s chemical potential allows fine-tuning of electronic properties and potentially elimination of trap states caused by defects.

Published in: "arXiv Material Science".

A semi-empirical integrated microring cavity approach for 2D material optical index identification at 1.55 {mu}m. (arXiv:1811.05323v1 [physics.app-ph])

2018-11-14T05:29:22+00:00November 14th, 2018|Categories: Publications|Tags: |

Atomically thin two-dimensional (2D) materials provide a wide range of basic building blocks with unique properties, making them ideal for heterogeneous integration with a mature chip platform for advances in optical communication technology. Control and understanding of the precise value of the optical index of these materials, however, is challenging, due to the small lateral flake dimension. Here we demonstrate a semi-empirical method to determine the index of a 2D material (nMoTe2 of 4.36+0.011i) near telecommunication-relevant wavelength by integrating few layers of MoTe2 onto a micro-ring resonator. The placement, control, and optical-property understanding of 2D materials with integrated photonics paves a way for further studies of active 2D material-based optoelectronics and circuits.

Published in: "arXiv Material Science".

Modeling and Thermal Metrology of Thermally Isolated MEMS Electrothermal Actuators for Strain Engineering of 2D Materials. (arXiv:1811.05450v1 [physics.app-ph])

2018-11-14T05:29:19+00:00November 14th, 2018|Categories: Publications|

We present electrothermal microelectromechanical (MEMS) actuators as a practical platform for straining 2D materials. The advantages of the electrothermal actuator is its high output force and displacement for low input voltage, but its drawback is that it is actuated by generating high amounts of heat. It is crucial to mitigate the high temperatures generated during actuation for reliable 2D material strain device implementation. Here, we implement a chevron actuator design that incorporates a thermal isolation stage in order to avoid heating the 2D material from the high temperatures generated during the actuation. By comparing experiment and simulation, we ensure our design does not compromise output displacement, while keeping the 2D material strain device stage cool. We also provide a simple analytical model useful for quickly evaluating different thermal isolation stage designs.

Published in: "arXiv Material Science".

Microfocus laser-ARPES on encapsulated mono-, bi-, and few-layer 1T’-WTe$_2$. (arXiv:1811.04629v1 [cond-mat.mtrl-sci])

2018-11-13T02:29:27+00:00November 13th, 2018|Categories: Publications|Tags: |

Two-dimensional crystals of semimetallic van der Waals materials hold much potential for the realization of novel phases, as exemplified by the recent discoveries of a polar metal in few layer 1T’-WTe$_2$ and of a quantum spin Hall state in monolayers of the same material. Understanding these phases is particularly challenging because little is known from experiment about the momentum space electronic structure of ultrathin crystals. Here, we report direct electronic structure measurements of exfoliated mono-, bi-, and few-layer 1T’-WTe$_2$ by laser-based micro-focus angle resolved photoemission. This is achieved by encapsulating with monolayer graphene a flake of WTe$_2$ comprising regions of different thickness. Our data support the recent identification of a quantum spin Hall state in monolayer 1T’-WTe$_2$ and reveal strong signatures of the broken inversion symmetry in the bilayer. We finally discuss the sensitivity of encapsulated samples to contaminants following exposure to ambient atmosphere.

Published in: "arXiv Material Science".

Low Lattice Thermal Conductivity of a Two-Dimensional Phosphorene Oxide. (arXiv:1811.04612v1 [cond-mat.mtrl-sci])

2018-11-13T02:29:25+00:00November 13th, 2018|Categories: Publications|Tags: |

A fundamental understanding of the phonon transport mechanism is important for optimizing the efficiency of thermoelectric devices. In this study, we investigate the thermal transport properties of the oxidized form of phosphorene called phosphorene oxide (PO) by solving phonon Boltzmann transport equation based on first-principles density functional theory. We reveal that PO exhibits a much lower thermal conductivity (2.42-7.08 W/mK, at 300 K) than its pristine counterpart as well as other two-dimensional materials. To comprehend the physical origin of such low thermal conductivity, we scrutinize the contribution of each phonon branch to the thermal conductivity by evaluating various mode-dependent quantities including Gruneisen parameters, anharmonic threep-honon scattering rate, phase space of three-phonon scattering processes. Our results show that its exible puckered structure of PO leads to smaller sound velocities; its broken-mirror symmetry allows more ZA phonon scattering; and the relatively-free vibration of dangling oxygen atoms in PO gives rise to additional scattering resulting in further reduction in the phonon lifetime. These results can be verified by the fact that PO has larger phase space for three-phonon processes than phosphorene. Furthermore we show that the thermal conductivity of PO can be optimized by controlling its size or its phonon mean free path, indicating that PO can be a promising candidate for low-dimensional thermoelectric devices.

Published in: "arXiv Material Science".

Planar penta-transition metal phosphide and arsenide as narrow-bandgap semiconductors from first principle calculations. (arXiv:1811.04648v1 [cond-mat.mtrl-sci])

2018-11-13T02:29:23+00:00November 13th, 2018|Categories: Publications|Tags: |

Searching for materials with single atom-thin as well as planar structure, like graphene and borophene, is one of the most attractive themes in two dimensional materials. Herein, using density functional theory calculations, we have proposed a series of single layer planar penta-transition metal phosphide and arsenide, i.e. TM$mathrm{_2}$X$mathrm{_4}$ (TM= Ni, Pd and Pt; X=P, As). According to the calculated phonon dispersion relation and elastic constants, as well as ab initio molecular dynamics simulation results, monolayers of planar penta-TM$mathrm{_2}$X$mathrm{_4}$ are dynamically, mechanically, and thermally stable. In addition, the band structures calculated with the screened HSE06 hybrid functional including spin-orbit coupling show that these monolayers are direct-gap semiconductors with sizeable band gaps ranging from 0.14 eV to 0.69 eV. Besides, the optical properties in these monolayers are further investigated, where strong in-plane optical absorption with wide spectral range has been revealed. Our results indicate that planar penta-TM$mathrm{_2}$X$mathrm{_4}$ monolayers are interesting narrow gap semiconductors with excellent optical properties, and may find potential applications in photoelectronics.

Published in: "arXiv Material Science".

Effect of Sr doping on structure, morphology and transport properties of Bi2Se3 epitaxial thin films. (arXiv:1811.04442v1 [cond-mat.mtrl-sci])

2018-11-13T02:29:21+00:00November 13th, 2018|Categories: Publications|Tags: |

We report molecular beam epitaxy growth of Sr-doped Bi2Se3 films on (111) BaF2 substrate, aimed to realize unusual superconducting properties inherent to SrBi2Se3 single crystals. Despite wide range of the compositions, we do not achieve superconductivity. To explore the reason for that we study structural, morphological and electronic properties of the films and compare them to the corresponding properties of the single crystals. The dependence of the c-lattice constant in the films on Sr content appears to be more than an order of magnitude stronger than in the crystals. Correspondingly, all other properties also differ substantially, indicating that Sr atoms get different positions in lattices. We argue that these structural discrepancies come from essential differences in growth conditions. Our research calls for more detailed structural studies and novel growth approaches for design of superconducting SrxBi2Se3 thin films.

Published in: "arXiv Material Science".

Atomic process of oxidative etching in monolayer molybdenum disulfide. (arXiv:1811.04242v1 [cond-mat.mtrl-sci])

2018-11-13T02:29:19+00:00November 13th, 2018|Categories: Publications|Tags: , , , |

The microscopic process of oxidative etching of two-dimensional molybdenum disulfide (2D MoS2) at an atomic scale is investigated using a correlative TEM-etching study. MoS2 flakes on graphene TEM grids are precisely tracked and characterized by TEM before and after the oxidative etching. This allows us to determine the structural change with an atomic resolution on the edges of the domains, of well-oriented triangular pits and along the grain boundaries. We observe that the etching mostly starts from the open edges, grain boundaries and pre-existing atomic defects. A zigzag Mo edge is assigned as the dominant termination of the triangular pits, and profound terraces and grooves are observed on the etched edges. Based on the statistical TEM analysis, we reveal possible routes for the kinetics of the oxidative etching in 2D MoS2, which should also be applicable for other 2D transition metal dichalcogenide materials like MoSe2 and WS2.

Published in: "arXiv Material Science".

Influence of temperature on the displacement threshold energy in graphene. (arXiv:1811.04011v1 [cond-mat.mtrl-sci])

2018-11-12T02:29:26+00:00November 12th, 2018|Categories: Publications|Tags: , |

The atomic structure of nanomaterials is often studied using transmission electron microscopy. In addition to image formation, the energetic electrons may also cause damage while impinging on the sample. In a good conductor such as graphene the damage is limited to the knock-on process caused by elastic electron-nucleus collisions. This process is determined by the kinetic energy an atom needs to be sputtered, ie, its displacement threshold energy. This is typically assumed to have a fixed value for all electron impacts on equivalent atoms within a crystal. Here we show using density functional tight-binding simulations that the displacement threshold energy is affected by the thermal perturbation of the atoms from their equilibrium positions. We show that this can be accounted for in the estimation of the displacement cross section by replacing the constant threshold value with a distribution. The improved model better describes previous precision measurements of graphene knock-on damage, and should be considered also for other low-dimensional materials.

Published in: "arXiv Material Science".

Mg$_{3+delta}$Sb$_x$Bi$_{2-x}$ family: A promising substitute for the start-of-art n-type thermoelectric materials near room temperature. (arXiv:1811.03198v1 [cond-mat.mtrl-sci])

2018-11-09T02:29:36+00:00November 9th, 2018|Categories: Publications|

Bi2Te3-xSex family has been the n-type start-of-the-art thermoelectric materials near room temperatures (RT) for more than half-century, which dominates the active cooling and novel waves harvesting application near RT. However, the drawbacks of brittle nature and Te-containing restrict the further applications exploring. Here, we show that a Mg3+{delta}SbxBi2-x family ((ZT)avg =1.05) could be a promising substitute for the Bi2Te3-xSex family ((ZT)avg =0.9-1.0) in the temperature range of 50-250 {deg}C based on the comparable thermoelectric performance through a synergistic effect from the tunable band gap using the alloy effect and the suppressible Mg-vacancy formation using interstitial Mn dopant. The former is to shift the optimal thermoelectric performance to near RT, and latter is helpful to partially decouple the electrical transport and thermal transport in order to get an optimal RT power factor. A positive temperature-dependence of band gap suggested this family is also a superior medium-temperature thermoelectric material for the significantly suppressed bipolar effect. Furthermore, a two times higher mechanical toughness, compared with Bi2Te3-xSex family, consolidates the promising substitute for the start-of-art n-type thermoelectric materials near RT.

Published in: "arXiv Material Science".

Single-Layer Ferromagnetic and Piezoelectric CoAsS with Pentagonal Structure. (arXiv:1811.03469v1 [cond-mat.mtrl-sci])

2018-11-09T02:29:34+00:00November 9th, 2018|Categories: Publications|Tags: |

Single-layer pentagonal materials are an emerging family of two-dimensional (2D) materials that could exhibit novel properties due to the building blocks being pentagons instead of hexagons as in numerous 2D materials. Based on our recently predicted single-layer pentagonal CoS$_2$ that is an antiferromagnetic (AFM) semiconductor, we replace two S atoms by As atoms in a unit cell to form single-layer pentagonal CoAsS. The resulting single-layer material is dynamically stable ac-cording to the phonon calculations. We find two drastic changes in the properties of single-layer pentagonal CoAsS in comparison with those of CoS$_2$. First, we find a magnetic transition from the AFM to FM ordering. We understand that the transition is caused by the lower electronegativity of As atoms, leading to the weakened bridging roles on the superexchange interactions be-tween Co ions. Single-layer pentagonal CoAsS also shows significantly stronger magnetocrystalline anisotropy energy due to stronger spin-orbit coupling. We additionally perform Monte Carlo simulations to calculate the Curie temperature of single-layer pentagonal CoAsS and the predicted Curie temperature is 103 K. Second, we find that single-layer pentagonal CoAsS exhibits piezoelectricity, which is absent in single-layer pentagonal CoS$_2$ due to its center of symmetry. The computed piezoelectric coefficients are also sizable. The rare coexistence of FM ordering and piezoelectric properties makes single-layer pentagonal CoAsS a promising multifunctional 2D material.

Published in: "arXiv Material Science".

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