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Interface confined hydrogen evolution reaction in zero valent metal nanoparticles-intercalated molybdenum disulfide

February 23rd, 2017|Publications|

Interface confined hydrogen evolution reaction in zero valent metal nanoparticles-intercalated molybdenum disulfideNature Communications, Published online: 23 February 2017; doi:10.1038/ncomms14548Interface confined reactions are a viable strategy for achieving stable and selective catalysts. Here, the authors demonstrate that 1T’-enriched lithiated MoS2 can reduce metal ions in situ, forming zero valent platinum nanoparticle-intercalated MoS2, with enhanced hydrogen evolution activity.

Published in: "Nature Communications".

Characterization and electrolytic cleaning of poly(methyl methacrylate) residues on transferred chemical vapor deposited graphene

February 23rd, 2017|Publications|

By Jianbo Sun, Harry O Finklea and Yuxin Liu

Poly(methyl methacrylate) (PMMA) residue has long been a critical challenge for practical applications of the transferred chemical vapor deposited (CVD) graphene. Thermal annealing is empirically used for the removal of the PMMA residue; however experiments imply that there are still small amounts of residues left after thermal annealing which are hard to remove with conventional methods. In this paper, the thermal degradation of the PMMA residue upon annealing was studied by Raman spectroscopy. The study reveals that post-annealing residues are generated by the elimination of methoxycarbonyl side chains in PMMA and are believed to be absorbed on graphene via the π – π interaction between the conjugated unsaturated carbon segments and graphene. The post-annealing residues are difficult to remove by further annealing in a non-oxidative atmosphere due to their thermal and chemical stability. An electrolytic cleaning method was shown to be effective in removing these po…

Published in: "Nanotechnology".

Temperature-driven single-valley Dirac fermions in HgTe quantum wells. (arXiv:1702.06869v1 [cond-mat.mtrl-sci])

February 23rd, 2017|Publications|

By M. Marcinkiewicz, S. Ruffenach, S. S. Krishtopenko, A. M. Kadykov, C. Consejo, D. B. But, W. Desrat, W. Knap, J. Torres, A. V. Ikonnikov, K. E. Spirin, S. V. Morozov, V. I. Gavrilenko, N. N. Mikhailov, S. A. Dvoretskii, F. Teppe

We report on temperature-dependent magnetospectroscopy of two HgTe/CdHgTe quantum wells below and above the critical well thickness $d_c$. Our results, obtained in magnetic fields up to 16 T and temperature range from 2 K to 150 K, clearly indicate a change of the band-gap energy with temperature. The quantum well wider than $d_c$ evidences a temperature-driven transition from topological insulator to semiconductor phases. At the critical temperature of 90~K, the merging of inter- and intra-band transitions in weak magnetic fields clearly specifies the formation of gapless state, revealing the appearance of single-valley massless Dirac fermions with velocity of $5.6times10^5$ m$times$s$^{-1}$. For both quantum wells, the energies extracted from experimental data are in good agreement with calculations on the basis of the 8-band Kane Hamiltonian with temperature-dependent parameters.

Published in: "arXiv Material Science".

Probing Single Vacancies in Black Phosphorus at the Atomic Level. (arXiv:1702.06753v1 [cond-mat.mtrl-sci])

February 23rd, 2017|Publications|

By Brian Kiraly, Nadine Hauptmann, Alexander N. Rudenko, Mikhail I. Katsnelson, Alexander A. Khajetoorians

Utilizing a combination of low-temperature scanning tunneling microscopy/spectroscopy (STM/STS) and electronic structure calculations, we characterize the structural and electronic properties of single atomic vacancies within several monolayers of the surface of black phosphorus. With a combination of experimental analysis and tight-binding calculations, we illustrate that we can depth profile these vacancies and assign them to specific sublattices within the unit cell. Measurements reveal that the single vacancies exhibit strongly anisotropic and highly delocalized charge density, laterally extended up to 20 atomic unit cells. The vacancies are then studied with STS, which reveals in-gap resonance states near the valence band edge and a strong p-doping of the bulk black phosphorus crystal. Finally, quasiparticle interference generated near these vacancies enables the direct visualization of the anisotropic band structure of black phosphorus.

Published in: "arXiv Material Science".

Ultrabroadband MoS2 Photodetector with Spectral Response from 445 to 2717 nm

February 23rd, 2017|Publications|

By Ying Xie, Bo Zhang, Shuxian Wang, Dong Wang, Aizhu Wang, Zeyan Wang, Haohai Yu, Huaijin Zhang, Yanxue Chen, Mingwen Zhao, Baibiao Huang, Liangmo Mei, Jiyang Wang

Photodetectors with excellent detecting properties over a broad spectral range have advantages for the application in many optoelectronic devices. Introducing imperfections to the atomic lattices in semiconductors is a significant way for tuning the bandgap and achieving broadband response, but the imperfection may renovate their intrinsic properties far from the desire. Here, by controlling the deviation from the perfection of the atomic lattice, ultrabroadband multilayer MoS2 photodetectors are originally designed and realized with the detection range over 2000 nm from 445 nm (blue) to 2717 nm (mid-infrared). Associated with the narrow but nonzero bandgap and large photoresponsivity, the optimized deviation from the perfection of MoS2 samples is theoretically found and experimentally achieved aiming at the ultrabroadband photoresponse. By the photodetection characterization, the responsivity and detectivity of the present photodetectors are investigated in the wavelength range from 445 to 2717 nm with the maximum values of 50.7 mA W−1 and 1.55 × 109 Jones, respectively, which represent the most broadband MoS2 photodetectors. Based on the easy manipulation, low cost, large scale, and broadband photoresponse, this present detector has significant potential for the applications in optoelectronics and electronics in the future. Ultrabroadband multilayer MoS2 photodetectors with the optical response up to 4.7 µm are designed and realized. Their detection properties, ranging from 445 nm (blue) to 2717 nm (mid-infrared), are investigated at room temperature by controlling the S defects, which show the broadest detecting range with low-cost fabrication process and have potential applications in many optoelectronic devices.

Published in: "Advanced Materials".

Electrical Detection of Individual Skyrmions in Graphene Devices. (arXiv:1702.06889v1 [cond-mat.mes-hall])

February 23rd, 2017|Publications|

By Francesca Finocchiaro, Jose Luis Lado, Joaquin Fernandez-Rossier

We study a graphene Hall probe located on top of a magnetic surface as a detector of skyrmions, using as working principle the anomalous Hall effect produced by the exchange interaction of the graphene electrons with the non-coplanar magnetization of the skyrmion. We study the magnitude of the effect as a function of the exchange interaction, skyrmion size and device dimensions. Our calculations for multiterminal graphene nanodevices, working in the ballistic regime, indicate that for realistic exchange interactions a single skyrmion would give Hall voltages well within reach of the experimental state of the art. The proposed device could act as an electrical transducer that marks the presence of a single skyrmion in a nanoscale region, paving the way towards the integration of skyrmion-based spintronics and graphene electronics.

Published : "arXiv Mesoscale and Nanoscale Physics".

Optomechanics for thermal characterization of suspended graphene. (arXiv:1702.06730v1 [cond-mat.mes-hall])

February 23rd, 2017|Publications|

By Robin J. Dolleman, Samer Houri, Dejan Davidovikj, Santiago J. Cartamil-Bueno, Yaroslav M. Blanter, Herre S. J. van der Zant, Peter G. Steeneken

Thermal properties of suspended single-layer graphene membranes are investigated by characterization of their mechanical motion in response to a high-frequency modulated laser. A characteristic delay time $tau$ between the optical intensity and mechanical motion is observed, which is attributed to the time required to raise the temperature of the membrane. We find, however, that the measured time constants are significantly larger than the predicted ones based on values of the specific heat and thermal conductivity. In order to explain the discrepancy between measured and modeled tau, a model is proposed that takes a thermal boundary resistance at the edge of the graphene drum into account. The measurements provide a noninvasive way to characterize thermal properties of suspended atomically thin membranes, providing information that can be hard to obtain by other means.

Published : "arXiv Mesoscale and Nanoscale Physics".

Layer-Pressure Topological Phase Diagram of Few-Layer Phosphorene. (arXiv:1702.06708v1 [cond-mat.mes-hall])

February 23rd, 2017|Publications|

By Peng-Lai Gong, Bei Deng, Liang Hu, Wei-Chao Wang, Da-Yong Liu, Xing-Qiang Shi, Zhi Zeng, Liang-Jian Zou

A layer-pressure topological phase diagram is obtained for few-layer phosphorene under increasing hydrostatic pressures by first-principles electronic structure calculations. We show that pressure can effectively manipulates the band structures of few-layer phosphorene — a pressure of less than 4.2 GPa can drive the quasi-two-dimensional (2D) phosphorene (of 4 layers or thicker) from normal insulators to nontrivial topological Dirac semimetals (TDSMs). Such 2D TDSMs phase possesses a single pair of Dirac cones exactly at the Fermi level, in startling contrast to the pressured bulk black phosphorous (BP) which has hole- and electron-types Dirac cones mixed with normal fermions. The band gap Eg and its change rate with pressure dEg/dP show a clear size-dependence on the thickness along the interlayer vertical direction, which are related to the quantum confinement effect (QCE). Although the QCE in 2D phosphorene makes the band inversion occur at a higher pressure with respect to the bulk, it becomes less important in thicker layers. Our results show that pressured few-layer phospherene is more suitable for electronic applications than bulk BP.

Published : "arXiv Mesoscale and Nanoscale Physics".

Voltage-induced suppression of weak localization in graphene. (arXiv:1702.06706v1 [cond-mat.mes-hall])

February 23rd, 2017|Publications|

By J. Fransson, R. Somphonsane, H. Ramamoorthy, G. He, J. P. Bird

In this theoretical study, we explore the manner in which the quantum correction due to weak localization is suppressed in weakly-disordered graphene, when it is subjected to the application of a non-zero voltage. Using a nonequilibrium Green function approach, we address the scattering generated by the disorder up to the level of the maximally crossed diagrams, hereby capturing the interference among different, impurity-defined, Feynman paths. Our calculations of the charge current, and of the resulting differential conductance, reveal the logarithmic divergence typical of weak localization in linear transport. The main finding of our work is that the applied voltage suppresses the weak localization contribution in graphene, by introducing a dephasing time that decreases inversely with increasing voltage.

Published : "arXiv Mesoscale and Nanoscale Physics".