arXiv Material Science

/arXiv Material Science

Correlated optical and electron microscopy reveal the role of multiple defect species and local strain on quantum emission. (arXiv:1901.05952v1 [cond-mat.mtrl-sci])

2019-01-18T02:29:30+00:00January 18th, 2019|Categories: Publications|Tags: |

Color-centers in solids have emerged as promising candidates for quantum photonic computing, communications, and sensing applications. Defects in hexagonal boron nitride(hBN) possess high-brightness, room-temperature quantum emission, but their large spectral variability and unknown local structure significantly challenge their technological utility. Here, we directly correlate hBN quantum emission with its local, atomic-scale crystalline structure using correlated photoluminescence (PL) and cathodoluminescence (CL) spectroscopy. Across 20 emitters, we observe zero phonon lines (ZPLs) in PL and CL ranging from 540-720 nm. CL mapping reveals that multiple defects and distinct defect species located within an optically-diffraction-limited region can each contribute to the observed PL spectra. Through high resolution transmission electron imaging, we find that emitters are located in regions with multiple fork-like dislocations. Additionally, local strain maps indicate that strain is not responsible for observed ZPL spectral range, though it can enable spectral tuning of particular emitters. While many emitters have identical ZPLs in CL and PL, others exhibit reversible but distinct CL and PL peaks; density functional calculations indicate that defect complexes and charge-state transitions influence such emission spectra. Our results highlight the sensitivity of defect-driven quantum emission to the surrounding crystallography, providing a foundation for atomic-scale optical characterization.

Published in: "arXiv Material Science".

Generation of sub-20-fs pulses from a graphene mode-locked laser. (arXiv:1901.05069v1 [cond-mat.mtrl-sci])

2019-01-17T02:29:55+00:00January 17th, 2019|Categories: Publications|Tags: , |

We demonstrate, what is to our knowledge, the shortest pulses directly generated to date from a solid-state laser, mode locked with a graphene saturable absorber (GSA). In the experiments, a low-threshold diode-pumped Cr3+:LiSAF laser was used near 850 nm. At a pump power of 275 mW provided by two pump diodes, the Cr3+:LiSAF laser produced nearly transform-limited, 19-fs pulses with an average output power of 8.5 mW. The repetition rate was around 107 MHz, corresponding to a pulse energy and peak power of 79 pJ and 4.2 kW, respectively. Once mode locking was initiated with the GSA, stable, uninterrupted femtosecond pulse generation could be obtained. In addition, the femtosecond output of the laser could be tuned from 836 nm to 897 nm with pulse durations in the range of 80-190 fs. We further performed detailed mode locking initiation tests across the full cavity stability range of the laser to verify that pulse generation was indeed started by the GSA and not by Kerr lens mode locking.

Published in: "arXiv Material Science".

An Element Replacement Approach by Reaction with Lewis acidic Molten Salts to Synthesize Nanolaminated MAX Phases and MXenes. (arXiv:1901.05120v1 [cond-mat.mtrl-sci])

2019-01-17T02:29:53+00:00January 17th, 2019|Categories: Publications|

Nanolaminated materials are important because of their exceptional properties and wide range of applications. Here, we demonstrate a general approach to synthesize a series of Zn-based MAX phases and Cl-terminated MXenes originating from the replacement reaction between the MAX phase and the late transition metal halides. The approach is a top-down route that enables the late transitional element atom (Zn in the present case) to occupy the A site in the pre-existing MAX phase structure. Using this element replacement reaction between Zn2+ cation in molten ZnCl2 and Al element in MAX phase precursors (Ti3AlC2, Ti2AlC, Ti2AlN, and V2AlC), the novel MAX phases Ti3ZnC2, Ti2ZnC, Ti2ZnN, and V2ZnC were synthesized. When employing excessive ZnCl2, Cl terminated MXenes (such as Ti3C2Cl2 and Ti2CCl2) were derived by a subsequent exfoliation of Ti3ZnC2 and Ti2ZnC due to the strong Lewis acidity of molten ZnCl2. These results indicate that A-site element replacement in traditional MAX phases by late transition metal halides opens the door to explore MAX phases that are not thermodynamically stable at high temperature and would be difficult to synthesize through the commonly employed powder metallurgy approach. Moreover, the etching effect of Lewis acid in molten salts provides a clean and safe chemistry for the delamination of MAX phase to obtain MXene when compared with the commonly used HF-etching process.

Published in: "arXiv Material Science".

Discovery of Hidden Classes of Layered Electrides by Extensive High-throughput Materials Screening. (arXiv:1901.05121v1 [cond-mat.mtrl-sci])

2019-01-17T02:29:49+00:00January 17th, 2019|Categories: Publications|Tags: |

Despite their extraordinary properties, electrides are still a relatively unexplored class of materials with only a few compounds grown experimentally. Especially for layered electrides, the current researches mainly focus on several isostructures of Ca2N with similar interlayer two-dimensional (2D) anionic electrons. An extensive screening for different layered electrides is still missing. Here, by screening materials with anionic electrons for the structures in Materials Project, we uncover 12 existing materials as new layered electrides. Remarkably, these layered electrides demonstrate completely different properties from Ca2N. For example, unusual fully spin-polarized zero-dimensional (0D) anionic electrons are shown in metal halides with MoS2-like structures; unique one-dimensional (1D) anionic electrons are confined within the tubes of the quasi-1D structures; a coexistence of magnetic and non-magnetic anionic electrons is found in ZrCl-like structures and a new ternary Ba2LiN with both 0D and 1D anionic electrons. These materials not only significantly increase the pool of experimentally synthesizable layered electrides but also are promising to be exfoliated into advanced 2D materials.

Published in: "arXiv Material Science".

Tin+1Cn MXene with fully saturated and thermally stable Cl terminations. (arXiv:1901.05212v1 [cond-mat.mtrl-sci])

2019-01-17T02:29:45+00:00January 17th, 2019|Categories: Publications|Tags: |

MXenes are a rapidly growing family of 2D materials that exhibit a highly versatile structure and composition, allowing for significant tuning of the material properties. These properties are, however, ultimately limited by the surface terminations, which are typically a mixture of species, including F and O that are inherent to the MXene processing. Other and robust terminations are lacking. Here, we apply high-resolution scanning transmission electron microscopy (STEM), corresponding image simulations and first-principles calculations to investigate the surface terminations on MXenes synthesized from MAX phases through Lewis acidic melts. The results show that atomic Cl terminates the synthesized MXenes, with mere residual presence of other termination species. Furthermore, in situ STEM-electron energy loss spectroscopy (EELS) heating experiments show that the Cl terminations are stable up to 750 {deg}C. Thus, we present an attractive new termination that widely expands the MXenes functionalization space and enable new applications.

Published in: "arXiv Material Science".

Phonon polaritonics in two-dimensional materials. (arXiv:1901.05308v1 [cond-mat.mes-hall])

2019-01-17T02:29:43+00:00January 17th, 2019|Categories: Publications|Tags: , |

Extreme confinement of electromagnetic energy by phonon polaritons holds the promise of strong and new forms of control over the dynamics of matter. To bring such control to the atomic-scale limit, it is important to consider phonon polaritons in two-dimensional (2D) systems. Recent studies have pointed out that in 2D, splitting between longitudinal and transverse optical (LO and TO) phonons is absent at the $Gamma$ point, even for polar materials. Does this lack of LO–TO splitting imply the absence of a phonon polariton in polar monolayers? Here, we derive a first-principles expression for the conductivity of a polar monolayer specified by the wavevector-dependent LO and TO phonon dispersions. In the long-wavelength (local) limit, we find a universal form for the conductivity in terms of the LO phonon frequency at the $Gamma$ point, its lifetime, and the group velocity of the LO phonon. Our analysis reveals that the phonon polariton of 2D is simply the LO phonon of the 2D system. For the specific example of hexagonal boron nitride (hBN), we estimate the confinement and propagation losses of the LO phonons, finding that high confinement and reasonable propagation quality factors coincide in regions which may be difficult to detect with current near-field optical microscopy techniques. Finally, we study the interaction of external emitters with two-dimensional hBN nanostructures, finding extreme enhancement of spontaneous emission due to coupling with localized 2D phonon polaritons, and the possibility of multi-mode strong and ultra-strong coupling between an external emitter and hBN phonons. This may lead to

Published in: "arXiv Material Science".

First Principles Study of Intrinsic and Extrinsic Point Defects in Monolayer WSe2. (arXiv:1901.05238v1 [cond-mat.mtrl-sci])

2019-01-17T02:29:36+00:00January 17th, 2019|Categories: Publications|Tags: , |

We present a detailed first principles density functional theory study of intrinsic and extrinsic point defects in monolayer (ML) WSe2. Among the intrinsic point defects, Se vacancies (Sevac) have the lowest formation energy (disregarding Se adatoms that can be removed with annealing). The defects with the next smallest formation energies (at least 1 eV larger) are SeW (Se substituting W atoms in an antisite defect), Wvac (W vacancies) and 2Sevac (Se divacancies). All these intrinsic defects have gap states that are not spin-polarized. The presence of a graphite substrate does not change the formation energies of these defects significantly. For the extrinsic point defects, we focus on O, O2, H, H2 and C interacting with perfect WSe2 and its intrinsic point defects. The preferred binding site in perfect WSe2 is the interstitial site for atomic O, H and C. These interstitial defects have no gap states. The gap states of the intrinsic defects are modified by interaction with O, O2, H, H2 and C. In particular, the gap states of Sevac and 2Sevac are completely removed by interaction with O and O2. This is consistent with the significantly larger stability of O-related defects compared to H- and C-related defects. The preferred binding site for O is Sevac, while that for H is SeW. H bonded to SeW results in spin-polarized gap states, which may be useful in defect engineering for spintronics applications. The charge transition levels and ionization energies of these defects are also computed. H in the interstitial site

Published in: "arXiv Material Science".

Signatures of Gate-Tunable Superconductivity in Trilayer Graphene/Boron Nitride Moir’e Superlattice. (arXiv:1901.04621v1 [cond-mat.supr-con])

2019-01-16T02:29:34+00:00January 16th, 2019|Categories: Publications|Tags: , , |

Understanding the mechanism of high temperature (high Tc) superconductivity is a central problem in condensed matter physics. It is often speculated that high Tc superconductivity arises from a doped Mott insulator as described by the Hubbard model. An exact solution of the Hubbard model, however, is extremely challenging due to the strong electron-electron correlation. Therefore, it is highly desirable to experimentally study a model Hubbard system in which the unconventional superconductivity can be continuously tuned by varying the Hubbard parameters. Here we report signatures of tunable superconductivity in ABC-trilayer graphene (TLG) / boron nitride (hBN) moir’e superlattice. Unlike “magic angle” twisted bilayer graphene, theoretical calculations show that under a vertical displacement field the ABC-TLG/hBN heterostructure features an isolated flat valence miniband associated with a Hubbard model on a triangular superlattice. Upon applying such a displacement field we find experimentally that the ABC-TLG/hBN superlattice displays Mott insulating states below 20 Kelvin at 1/4 and 1/2 fillings, corresponding to 1 and 2 holes per unit cell, respectively. Upon further cooling, signatures of superconducting domes emerge below 1 kelvin for the electron- and hole-doped sides of the 1/4 filling Mott state. The electronic behavior in the TLG/hBN superlattice is expected to depend sensitively on the interplay between the electron-electron interaction and the miniband bandwidth, which can be tuned continuously with the displacement field D. By simply varying the D field, we demonstrate transitions from the candidate superconductor to Mott insulator and metallic phases. Our study shows that TLG/hBN heterostructures offer an attractive model

Published in: "arXiv Material Science".

Kondo Impurities in Two Dimensional MoS2 for Achieving Ultrahigh Thermoelectric Powerfactor. (arXiv:1901.04661v1 [cond-mat.mes-hall])

2019-01-16T02:29:31+00:00January 16th, 2019|Categories: Publications|Tags: , |

Local magnetic impurities arising from atomic vacancies in two-dimensional (2D) nanosheets are predicted to have a profound effect on charge transport due to resonant scattering, and provide a handle for enhancing thermoelectric properties through the Kondo effect. However, the effects of these impurities are often masked by external fluctuations and turbostratic interfaces, therefore, it is highly challenging to probe the correlation between magnetic impurities and thermoelectric parameters experimentally. In this work, we demonstrate that by placing Molybdenum Disulfide on a hexagonal Boron Nitride substrate, a colossal spin splitting of the conduction sub-band up to ~50.0 meV is observed at the sulfur vacancies, suggesting that these are local magnetic states. Transport measurements reveal a large anomalous positive Seebeck coefficient in highly conducting n type MoS2, originating from quasiparticle resonance near the Fermi level described by the Kondo effect. Furthermore, by tuning the chemical potential, a record power factor of 50mW/mK2 in low-dimensional materials was achieved. Our work shows that defect engineering of 2D materials affords a strategy for controlling Kondo impurities and tuning thermoelectric transport.

Published in: "arXiv Material Science".

Nonmonotonic band gap evolution in bent phosphorene nanosheets. (arXiv:1901.04665v1 [cond-mat.mtrl-sci])

2019-01-16T02:29:29+00:00January 16th, 2019|Categories: Publications|Tags: |

Nonmonotonic bending-induced changes of fundamental band gaps and quasiparticle energies are observed for realistic nanoscale phosphorene nanosheets. Calculations using stochastic many-body perturbation theory (sGW) show that even slight curvature causes significant changes in the electronic properties. For small bending radii (< 4 nm) the band-gap changes from direct to indirect. The response of phosphorene to deformation is strongly anisotropic (different for zig-zag vs. armchair bending) due to an interplay of exchange and correlation effects. Overall, our results show that fundamental band gaps of phosphorene sheets can be manipulated by as much as 0.7 eV depending on the bending direction.

Published in: "arXiv Material Science".

Moir’e quantum well states in tiny angle two dimensional semi-conductors. (arXiv:1901.04679v1 [cond-mat.mes-hall])

2019-01-16T02:29:27+00:00January 16th, 2019|Categories: Publications|Tags: , |

The valence band edge in tiny angle twist bilayers of MoS$_2$ and phosphorene is shown to consist of highly localized energy levels created by a `moir’e quantum well’, i.e. trapped by the interlayer moir’e potential. These approximately uniformly spaced energy levels exhibit a richly modulated charge density, becoming ultra-localized at the valence band maximum. The number and spacing of such levels is controllable by the twist angle and interlayer interaction strength, suggesting the possibility of `moir’e engineering’ ordered arrays of quantum dots in 2d twist semi-conductors.

Published in: "arXiv Material Science".

Doping optimization for the power factor of bipolar thermoelectric materials. (arXiv:1901.04718v1 [cond-mat.mtrl-sci])

2019-01-16T02:29:25+00:00January 16th, 2019|Categories: Publications|

Bipolar carrier transport is often a limiting factor in the thermoelectric efficiency of narrow bandgap materials at high temperatures due to the reduction in the Seebeck coefficient and the introduction of an additional term to the thermal conductivity. Using the Boltzmann transport formalism and a two-band model, we simulate transport through bipolar systems and calculate their thermoelectric transport properties: the electrical conductivity, the Seebeck coefficient and the thermoelectric power factor. We present an investigation into the doping optimisation of such materials, showing the detrimental impact that rising temperatures have if the doping (and the Fermi level) is not optimised for each operating temperature. We also show that the doping levels for optimized power factors at a given operating temperature differ in bipolar systems compared to unipolar ones. We show finally that at 600 K, in a bipolar material with bandgap approximately that of Bi2Te3, the optimal doping required can reside between 10% – 30% larger than that required for an optimal unipolar material depending on the electronic scattering details of the material.

Published in: "arXiv Material Science".

Magnetic dielectric- graphene- ferroelectric system as a promising non-volatile device for modern spintronics. (arXiv:1901.04550v1 [cond-mat.mtrl-sci])

2019-01-16T02:29:23+00:00January 16th, 2019|Categories: Publications|Tags: , |

The conductivity of the system magnetic dielectric (EuO) – graphene channel – ferroelectric substrate was considered. The magnetic dielectric locally transforms the band spectrum of graphene by inducing an energy gap in it and making it spin-asymmetric with respect to the free electrons. The range of spontaneous polarization (2- 5)mC/m2 that can be easily realized in thin films of proper and incipient ferroelectrics, was under examination. It was demonstrated, that if the Fermi level in the graphene channel belongs to energy intervals where the graphene band spectrum, modified by EuO, becomes sharply spin-asymmetric, such a device can be an ideal non-volatile spin filter. Controlling of the Fermi level (e.g. by temperature that changes ferroelectric polarization) can convert a spin filter to a spin valve.

Published in: "arXiv Material Science".

A non-perturbative theory of effective Hamiltonians: example of moir’e materials. (arXiv:1901.04535v1 [cond-mat.mtrl-sci])

2019-01-16T02:29:20+00:00January 16th, 2019|Categories: Publications|Tags: , |

We demonstrate that there exists a continuum Hamiltonian $H(bf{r},bf{p})$ that is formally the operator equivalent of the general tight-binding method, inheriting the associativity and Hermiticity of the latter operator. This provides a powerful and controlled method of obtaining effective Hamiltonians via Taylor expansion with respect to momentum and, optionally, deformation fields. In particular, for fundamentally non-perturbative defects, such as twist faults and partial dislocations, the method allows the deformation field to be retained to all orders, providing an efficient scheme for the generation of transparent and compact Hamiltonians for such defects. We apply the method to a survey of incommensurate physics in twist bilayers of graphene, graphdiyne, MoS$_2$, and phosphorene. For graphene we are able to reproduce the `reflected Dirac cones’ of the $30^circ$ quasi-crystalline bilayer found in a recent ARPES experiment, and show it is an example of a more general phenomena of coupling by the moir’e momentum. We show that incommensurate physics is governed by the decay of the interlayer interaction on the scale of the single layer reciprocal lattices, and demonstrate that if this is slow incommensurate scattering effects lead to very rapid broadening of band manifolds as the twist angle is tuned through commensurate values.

Published in: "arXiv Material Science".

A Facile Method for Precise Layer Number Identification of Two-Dimensional Materials through Optical Images. (arXiv:1901.04102v1 [cond-mat.mtrl-sci])

2019-01-15T02:29:14+00:00January 15th, 2019|Categories: Publications|Tags: , , |

Optical microscopy is believed to be an efficient method for identifying layer number of two-dimensional 2D materials. However, since illuminants, cameras and their parameters are different from lab to lab, it is impossible to identify layer numbers just by comparing a given optical image with standard or calculated images under standard conditions. Here we reported an image reconstruction method, converting raw optical images acquired by arbitrary illuminants and cameras into reconstructed images at specified illuminant and specified camera. After image reconstruction, the color differences of each layer number roughly equaled those calculated under specified condition. By comparing the color differences in reconstructed image with those calculated under specified condition, the layer numbers of 2D materials in our lab and published papers, including MoS2, WS2 and WSe2, were ambiguously identified. This study makes optical microscopy a precise method for identifying layer numbers of 2D materials on known substrate.

Published in: "arXiv Material Science".

Tunability of Magnetic Anisotropy of Co on Two-Dimensional Materials by Tetrahedral Bonding. (arXiv:1901.03490v1 [cond-mat.mtrl-sci])

2019-01-14T02:29:16+00:00January 14th, 2019|Categories: Publications|Tags: |

Pairing of $pi$ electronic state structures with functional or metallic atoms makes them possible to engineer physical and chemical properties. Herein, we predict the reorientation of magnetization of Co on hexagonal BN (h-BN) and graphene multilayers. The driving mechanism is the formation of the tetrahedral bonding between sp$^3$ and d orbitals at the interface. More specifically, the intrinsic $pi$-bonding of h-BN and graphene is transformed to sp$^3$ as a result of strong hybridization with metallic $d_{z^2}$ orbital. The different features of these two tetrahedral bondings, sp$^2$ and sp$^3$, are well manifested in charge density and density of states in the vicinity of the interface, along with associated band structure near the $bar{K}$ valley. Our findings provide a novel approach to tailoring magnetism by means of degree of the interlayer hybrid bonds in 2D layered materials.

Published in: "arXiv Material Science".

Stone-Wales graphene: A Two Dimensional Carbon Semi-Metal with Magic Stability. (arXiv:1901.02966v1 [cond-mat.mtrl-sci])

2019-01-11T02:29:47+00:00January 11th, 2019|Categories: Publications|Tags: , |

A two-dimensional carbon allotrope, Stone-Wales graphene, is identified in stochastic group and graph constrained searches and systematically investigated by first-principles calculations. Stone-Wales graphene consists of well-arranged Stone-Wales defects, and it can be constructed through a 90$^circ$ bond-rotation in a $sqrt{8}$$times$$sqrt{8}$ super-cell of graphene. Its calculated energy relative to graphene, +149 meV/atom, makes it more stable than the most competitive previously suggested graphene allotropes. We find that Stone-Wales graphene based on a $sqrt{8}$ super-cell is more stable than those based on $sqrt{9} times sqrt{9}$, $sqrt{12} times sqrt{12}$ and $sqrt{13} times sqrt{13}$ super-cells, and is a “magic size” that can be further understood through a simple “energy splitting and inversion” model. The calculated vibrational properties and molecular dynamics of SW-graphene confirm that it is dynamically stable. The electronic structure shows SW-graphene is a semimetal with distorted, strongly anisotropic Dirac cones.

Published in: "arXiv Material Science".

Edge mode based graphene nanomechanical resonators for high-sensitivity mass sensor. (arXiv:1901.03043v1 [cond-mat.mes-hall])

2019-01-11T02:29:45+00:00January 11th, 2019|Categories: Publications|Tags: |

We perform both molecular dynamics simulations and theoretical analysis to study the sensitivity of the graphene nanomechanical resonator based mass sensors, which are actuated following the global extended mode or the localized edge mode. We find that the mass detection sensitivity corresponding to the edge mode is about three times higher than that corresponding to the extended mode. Our analytic derivations reveal that the enhancement of the sensitivity originates in the reduction of the effective mass for the edge mode due to its localizing feature.

Published in: "arXiv Material Science".

Revealing the Physico-Chemical Basis of Organic Solid-Solid Wetting Deposition: Casimir-Like Forces, Hydrophobic Collapse, and the Role of the Zeta Potential. (arXiv:1901.02917v1 [physics.chem-ph])

2019-01-11T02:29:42+00:00January 11th, 2019|Categories: Publications|Tags: |

Supramolecular self-assembly at the solid-solid interface enables the deposition and monolayer formation of insoluble organic semiconductors under ambient conditions. The underlying process, termed as the Organic Solid-Solid Wetting Deposition (OSWD), generates two-dimensional adsorbates directly from dispersed three-dimensional organic crystals. This straightforward process has important implications in various fields of research and technology, such as in the domains of low-dimensional crystal engineering, the chemical doping and band-gap engineering of graphene, and in the area of field-effect transistor fabrication. However, till date, lack of an in-depth understanding of the physico-chemical basis of the OSWD prevented the identification of important parameters, essential to achieve a better control of the growth of monolayers and supramolecular assemblies with defined structures, sizes, and coverage areas. Here we propose a detailed model for the OSWD, derived from experimental and theoretical results that have been acquired by using the organic semiconductor quinacridone as an example system. The model reveals the vital role of the zeta potential and includes Casimir-like fluctuation-induced forces and the effect of dewetting in hydrophobic nano-confinements. Based on our results, the OSWD of insoluble organic molecules can hence be applied to environmental friendly and low-cost dispersing agents, such as water. In addition, the model substantially enhances the ability to control the OSWD in terms of adsorbate structure and substrate coverage.

Published in: "arXiv Material Science".

Electrically Tunable Wafer-Sized Three-Dimensional Topological Insulator Thin Films Grown by Magnetron Sputtering. (arXiv:1901.02611v1 [physics.app-ph])

2019-01-10T02:29:24+00:00January 10th, 2019|Categories: Publications|

Three-dimensional (3D) topological insulators (TIs) are candidate materials for various electronic and spintronic devices due to their strong spin-orbit coupling and unique surface electronic structure. Rapid, low-cost preparation of large-area TI thin films compatible with conventional semiconductor technology is key to the practical applications of TIs. Here, we show that wafer-sized Bi2Te3 family TI and magnetic TI films with decent quality and well-controlled composition and properties can be prepared on amorphous SiO2/Si substrates by magnetron cosputtering. The SiO2/Si substrates enable us to electrically tune (Bi1-xSbx)2Te3 and Cr-doped (Bi1-xSbx)2Te3 TI films between p-type and n-type behavior and thus study the phenomena associated with topological surface states, such as the quantum anomalous Hall effect (QAHE). This work significantly facilitates the fabrication of TI-based devices for electronic and spintronic applications.

Published in: "arXiv Material Science".

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