ASU Scholarship Showcase

Permanent Link Feedback

Date Range
2015 2017

The valley degree of freedom in two-dimensional (2D) crystals recently emerged as a novel information carrier in addition to spin and charge. The intrinsic valley lifetime in 2D transition metal dichalcogenides (TMD) is expected to be markedly long due to the unique spin-valley locking behavior, where the intervalley scattering of the electron simultaneously requires a large momentum transfer to the opposite valley and a flip of the electron spin. However, the experimentally observed valley lifetime in 2D TMDs has been limited to tens of nanoseconds thus far. We report efficient generation of microsecond-long-lived valley polarization in WSe[subscript 2]/MoS[subscript 2] heterostructures ...

Contributors
Kim, Jonghwan, Jin, Chenhao, Chen, Bin, et al.
Created Date
2017-07-26

Modulated reflectance (contactless electroreflectance (CER), photoreflectance (PR), and piezoreflectance (PzR)) has been applied to study direct optical transitions in bulk MoS[subscript 2], MoSe[subscript 2], WS[subscript 2], and WSe[subscript 2]. In order to interpret optical transitions observed in CER, PR, and PzR spectra, the electronic band structure for the four crystals has been calculated from the first principles within the density functional theory for various points of Brillouin zone including K and H points. It is clearly shown that the electronic band structure at H point of Brillouin zone is very symmetric and similar to the electronic band structure at K ...

Contributors
Kopaczek, J., Polak, M. P., Scharoch, P., et al.
Created Date
2016-06-21

The strong light-matter interaction and the valley selective optical selection rules make monolayer (ML) MoS[subscript 2] an exciting 2D material for fundamental physics and optoelectronics applications. But, so far, optical transition linewidths even at low temperature are typically as large as a few tens of meV and contain homogeneous and inhomogeneous contributions. This prevented in-depth studies, in contrast to the better-characterized ML materials MoSe[subscript 2] and WSe[subscript 2]. In this work, we show that encapsulation of ML MoS[subscript 2] in hexagonal boron nitride can efficiently suppress the inhomogeneous contribution to the exciton linewidth, as we measure in photoluminescence and reflectivity ...

Contributors
Cadiz, F., Courtade, E., Robert, C., et al.
Created Date
2017-05-18

Transition metal trichalcogenides form a class of layered materials with strong in-plane anisotropy. For example, titanium trisulfide (TiS[subscript 3]) whiskers are made out of weakly interacting TiS[subscript 3] layers, where each layer is made of weakly interacting quasi-one-dimensional chains extending along the b axis. Here we establish the unusual vibrational properties of TiS[subscript 3] both experimentally and theoretically. Unlike other two-dimensional systems, the Raman active peaks of TiS[subscript 3] have only out-of-plane vibrational modes, and interestingly some of these vibrations involve unique rigid-chain vibrations and S–S molecular oscillations. High-pressure Raman studies further reveal that the A[subscript g][superscript S–S] S-S molecular ...

Contributors
Wu, Kedi, Torun, Engin, Sahin, Hasan, et al.
Created Date
2016-09-22

We present two-dimensional Mg(OH)[subscript 2] sheets and their vertical heterojunctions with CVD-MoS[subscript 2] for the first time as flexible 2D insulators with anomalous lattice vibration and chemical and physical properties. New hydrothermal crystal growth technique enabled isolation of environmentally stable monolayer Mg(OH)[subscript 2] sheets. Raman spectroscopy and vibrational calculations reveal that the lattice vibrations of Mg(OH)[subscript 2] have fundamentally different signature peaks and dimensionality effects compared to other 2D material systems known to date. Sub-wavelength electron energy-loss spectroscopy measurements and theoretical calculations show that Mg(OH)[subscript 2] is a 6 eV direct-gap insulator in 2D, and its optical band gap displays ...

Contributors
Tuna, Aslihan, Wu, Kedi, Sahin, Hasan, et al.
Created Date
2016-02-05

Binary transition metal dichalcogenide monolayers share common properties such as a direct optical bandgap, spin-orbit splittings of hundreds of meV, light–matter interaction dominated by robust excitons and coupled spin-valley states. Here we demonstrate spin-orbit-engineering in Mo[subscript (1−x)]W[subscript x]Se[subscript 2] alloy monolayers for optoelectronics and applications based on spin- and valley-control. We probe the impact of the tuning of the conduction band spin-orbit spin-splitting on the bright versus dark exciton population. For MoSe[subscript 2] monolayers, the photoluminescence intensity decreases as a function of temperature by an order of magnitude (4–300 K), whereas for WSe[subscript 2] we measure surprisingly an order of ...

Contributors
Wang, Gang, Robert, Cedric, Tuna, Aslihan, et al.
Created Date
2015-12-14

The electronic band structure of MoS[subscript 2], MoSe[subscript 2], WS[subscript 2], and WSe[subscript 2], crystals has been studied at various hydrostatic pressures experimentally by photoreflectance (PR) spectroscopy and theoretically within the density functional theory (DFT). In the PR spectra direct optical transitions (A and B) have been clearly observed and pressure coefficients have been determined for these transitions to be: α[subscript A] = 2.0 ± 0.1 and α[subscript B] = 3.6 ± 0.1 meV/kbar for MoS[subscript 2], α[subscript A] = 2.3 ± 0.1 and α[subscript B] = 4.0 ± 0.1 meV/kbar for MoSe[subscript 2], α[subscript A] = 2.6 ± 0.1 ...

Contributors
Dybala, F., Polak, M. P., Kopaczek, J., et al.
Created Date
2016-05-24

Black phosphorus attracts enormous attention as a promising layered material for electronic, optoelectronic and thermoelectric applications. Here we report large anisotropy in in-plane thermal conductivity of single-crystal black phosphorus nanoribbons along the zigzag and armchair lattice directions at variable temperatures. Thermal conductivity measurements were carried out under the condition of steady-state longitudinal heat flow using suspended-pad micro-devices. We discovered increasing thermal conductivity anisotropy, up to a factor of two, with temperatures above 100 K. A size effect in thermal conductivity was also observed in which thinner nanoribbons show lower thermal conductivity. Analysed with the relaxation time approximation model using phonon ...

Contributors
Lee, Sangwook, Yang, Fan, Suh, Joonki, et al.
Created Date
2015-10-16

Plasmonic and metamaterial based nano/micro-structured materials enable spectrally selective resonant absorption, where the resonant bandwidth and absorption intensity can be engineered by controlling the size and geometry of nanostructures. Here, we demonstrate a simple, lithography-free approach for obtaining a resonant and dynamically tunable broadband absorber based on vanadium dioxide (VO2) phase transition. Using planar layered thin film structures, where top layer is chosen to be an ultrathin (20 nm) VO2 film, we demonstrate broadband IR light absorption tuning (from similar to 90% to similar to 30% in measured absorption) over the entire mid-wavelength infrared spectrum. Our numerical and experimental results ...

Contributors
Kocer, Hasan, Butun, Serkan, Palacios, Edgar, et al.
Created Date
2015-08-21