Bulletin of the American Physical Society
APS March Meeting 2014
Volume 59, Number 1
Monday–Friday, March 3–7, 2014; Denver, Colorado
Session A51: Focus Session: Beyond Graphene: Synthesis, Defects, Structure, and Properties I |
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Sponsoring Units: DMP Chair: Linyou Cao, North Carolina State University Room: Mile High Ballroom 1E |
Monday, March 3, 2014 8:00AM - 8:36AM |
A51.00001: Exploring the Flatlands: Synthesis, Characterization and Engineering of Two-Dimensional Materials Invited Speaker: Jun Lou In this talk, we report the controlled vapor phase synthesis of MoS$_{2}$ atomic layers and elucidate a fundamental mechanism for the nucleation, growth, and grain boundary formation in its crystalline monolayers. The atomic structure and morphology of the grains and their boundaries in the polycrystalline molybdenum disulfide atomic layers are examined and first-principles calculations are applied to investigate their energy landscape. The electrical properties of the atomic layers are examined and the role of grain boundaries is evaluated. More importantly, if precise two-dimensional domains of graphene, h-BN and MoS$_{2}$ atomic layers can be seamlessly stitched together, in-plane heterostructures with interesting electronic applications could potentially be created. Here, we show that planar graphene/h-BN heterostructures can be formed by growing graphene in lithographically-patterned h-BN atomic layers. Our approach can create periodic arrangements of domains with size that ranging from tens of nanometers to millimeters. The resulting graphene/h-BN atomic layers can be peeled off from their growth substrate and transferred to various platforms including flexible substrate. Finally, we demonstrate how self-assembled monolayers with a variety of end termination chemistries can be utilized to tailor the physical properties of single-crystalline MoS$_{2}$ atomic-layers. Our data suggests that combined interface-related effects of charge transfer, built-in molecular polarities, varied densities of defects, and remote interfacial phonons strongly modify the electrical and optical properties of MoS$_{2}$, illustrating an engineering approach for local and universal property modulations in two-dimensional atomic-layers. [Preview Abstract] |
Monday, March 3, 2014 8:36AM - 8:48AM |
A51.00002: Molecular Beam Epitaxy of Layered Material Superlattices and Heterostructures Suresh Vishwanath, Xinyu Liu, Sergei Rouvimov, Jacek K. Furdyna, Debdeep Jena, Huili Grace Xing Stacking of various layered materials is being pursued widely to realize various devices and observe novel physics. Mostly, these have been limited to exfoliation and stacking either manually or in solution, where control on rotational alignment or order of stacking is lost. We have demonstrated molecular beam epitaxy (MBE) growth of Bi$_{\mathrm{2}}$Se$_{\mathrm{3}}$/MoSe$_{\mathrm{2}}$ superlattice and Bi$_{\mathrm{2}}$Se$_{\mathrm{3}}$/MoSe$_{\mathrm{2}}$/SnSe$_{\mathrm{2}}$ heterostructure on sapphire. We have achieved a better control on the order of stacking and number of layers as compared to the solution technique. We have characterized these structures using RHEED, Raman spectroscopy, XPS, AFM, X-ray reflectometry, cross-section (cs) and in-plane (ip) TEM. The rotational alignment is dictated by thermodynamics and is understood using ip-TEM diffraction patterns. Layered growth and long range order is evident from the streaky RHEED pattern. Abrupt change in RHEED pattern, clear demarcation of boundary between layers seen using cs-TEM and observation of Raman peaks corresponding to all the layers suggest van-der-waals epitaxy. In our knowledge this is a first demonstration of as grown superlattices and heterostuctures involving transition metal dichalcogenides and is an important step towards the goal of stacking of 2D crystals like lego blocks. [Preview Abstract] |
Monday, March 3, 2014 8:48AM - 9:00AM |
A51.00003: Synthesis of large-scale MoS2 films using gas phase precursor Jinhwan Lee, Youngbin Lee, Jeong Ho Cho, Jong-Hyun Ahn, Changgu Lee We present layer-controlled synthesis of large-scale and uniform molybdenum disulfide films on insulating substrates. For the synthesis, we used gas phase sulfuric precursor and molybdenum metal source. By controlling the deposition thickness of the metal, we could vary the synthesized film thickness in the precision of number of layers. From the synthesis, 2, 4, 8, 12 layers were grown on 2-in scale Si/SiO2 and quartz substrates up to 8 cm with almost perfect uniformity over the entire area. Also on one substrate, films with different thicknesses were grown in separate areas with layer (or atomic)-level uniformity. AFM, TEM, XPS, and optical spectroscopy characterizations show that the films have high crystalline quality without a sigh of amorphous phase. The films synthesized on quartz substrate were transparent and the transparency depended linearly on the film thickness. We also fabricated arrays of field effect transistor device for electrical characterization. 90{\%} among the devices operated with functionality and the measured mobility was on the level of 0.1 cm$^{2}$/Vs and on/off ratio was 10$^{5}$. [Preview Abstract] |
Monday, March 3, 2014 9:00AM - 9:12AM |
A51.00004: Controlled physical vapor growth of WSe$_{2}$ and other MX$_{2}$ monolayers Paul Nguyen, Joe Finney, Chunming Huang, Pasqual Rivera, Sanfeng Wu, Genevieve Clark, Zaiyao Fei, Xiaodong Xu, David Cobden Although exfoliated monolayers of two-dimensional semiconductors such as WSe$_{2}$ show extraordinary and potentially useful optical properties, the ability to grow them in a controlled way will be critical for tuning their properties, incorporating dopants, and making devices on larger scales and with high yield. We are investigating their growth by physical vapor deposition on insulators such as silicon dioxide without catalyst, systematically varying the growth parameters (gas flow and type, sources, temperature, and substrate), with a focus on WSe$_{2}$ which has the smallest gap and strongest spin-orbit coupling of the MX$_{2}$s. While MoS$_{2}$ monolayers of high optical quality can easily be grown as triangular single crystals tens of microns in size using a simple MoS$_{2}$ source, WSe$_{2}$ proves to be much more sensitive to the growth parameters, as well as to air leaks and contamination of the furnace tube. Nevertheless we have reproducibly grown monolayer WSe$_{2}$ crystals up to 15 microns in size showing excellent optical properties using a WSe$_{2}$ source and pure hydrogen carrier gas. [Preview Abstract] |
Monday, March 3, 2014 9:12AM - 9:24AM |
A51.00005: Growth of Molybdenum disulfide by Mo foil and ammonium heptamolybdate Carl Naylor, Ganghee Han, Nicholas Kybert, Charlie Johnson Molybdenum disulfide (MoS$_{\mathrm{2}})$ is one of the latest semiconducting materials to show huge attention, due to its tunable band gap by controlling the number of layers and reasonable values of mobility. Indeed, its astonishing electrical properties combined with having a high on/off ratio for field effect transistors that is difficult to reach with graphene, make MoS$_{\mathrm{2}}$ a promising material for nanosensing and many other applications. Here we introduce two different growth techniques for MoS$_{\mathrm{2}}$. Molybdenum foil and ammonium heptamolybdate were used as molybdenum feedstock while we sublimated sulfur source from solid for both techniques. Crystallinity of MoS$_{\mathrm{2}}$ from both techniques was checked by optical microscope, atomic force microscope, Raman spectroscopy and Transmission electron microscopy. We believe that our techniques would be facile routes for MoS$_{\mathrm{2}}$ growth. [Preview Abstract] |
Monday, March 3, 2014 9:24AM - 9:36AM |
A51.00006: ABSTRACT WITHDRAWN |
Monday, March 3, 2014 9:36AM - 9:48AM |
A51.00007: Single-Crystal Growth and Optical Characterization of Large-Area Monolayer WS$_{2}$ Ting Yu Nowadays, two-dimensional (2D) transition metal dichalcogenides (TMDs) have caused a great deal of interest in view of their unique properties and potential novel applications. Beyond graphene, monolayers of WS$_{2}$ with a direct band gap are attracting for developing 2D field-effect transistor and visible light-emitting devices. Till now, controllable synthesis of high-quality single-crystal WS$_{2}$ monolayer is still very challenging but highly demanded. In this study, we have successfully grown large-area single-crystal monolayer WS$_{2}$ by use of a modified chemical vapor deposition system. Particularly, the photoluminescence (PL) of WS$_{2}$ has been investigated. Valley-selective circular dichroism and intense red emission demonstrate high quality of as-grown WS$_{2}$ monolayers. Besides, uniform and non-uniform PL distributions over various samples were analyzed to identify the intrinsic emission characters. The PL weakening and its blue shift are attributed to the as-grown structural defects and the defect-induced n-doping. The present work paves the pathway to prepare large-scale single-crystal 2D TMDs and highlights the promising optical performance of WS$_{2}$ for future optoelectronics. [Preview Abstract] |
Monday, March 3, 2014 9:48AM - 10:00AM |
A51.00008: Chemically-exfoliated single-layer MoS$_2$: stability, lattice dynamics and catalytic adsorption from first principles Matteo Calandra Chemically and mechanically exfoliated MoS$_2$ single-layer samples have substantially different properties. While mechanically exfoliated single-layers are mono-phase ( 1H polytype with Mo in trigonal prismatic coordination), the chemically exfoliated samples show coexistence of three different phases, 1H, 1T (Mo in octahedral coordination) and 1T$^{'}$ (a distorted $2\times 1$ 1T-superstructure). By using first-principles calculations, we investigate the energetics and the dynamical stability of the three phases. We show that the 1H phase is the most stable one, while the metallic 1T phase, strongly unstable, undergoes a phase transition towards a metastable and insulating 1T$^{'}$ structure composed of separated zig-zag chains. We calculate electronic structure, phonon dispersion, Raman frequencies and intensities for the 1T$^{'}$ structure. We provide a microscopical description of the J$_1$, J$_2$ and J$_3$ Raman features first detected more then $20$ years ago, but unexplained up to now. Finally, we show that H adsorbates, that are naturally present at the end of the chemical exfoliation process, stabilize the 1T$^{\prime}$ over the 1H one. [Preview Abstract] |
Monday, March 3, 2014 10:00AM - 10:12AM |
A51.00009: Fabrication of Atomically Layered Material Heterostructures of WSe2 and hBN Yafang Yang, Hugh Churchill, Britt Baugher, Javier Sanchez-Yamagishi, Pablo Jarillo-Herrero We discuss fabrication methods for hBN-WSe2-hBN heterostructures designed to create high quality and high mobility monolayer WSe2 devices by encapsulating the WSe2 in a relatively clean and impurity-free environment.~ We use a release polymer to pick up hBN and WSe2 from a SiO2 substrate, and transfer the stack onto another pre-cleaned hBN flake. In this way the WSe2 channel is protected from resist residue by hBN above and below, and thus stays pristine and clean.~ Various fabrication strategies will be discussed, including a comparison of MMA and PPC as release polymers.~ We characterize the performance of these devices with electrical transport measurements.~ [Preview Abstract] |
Monday, March 3, 2014 10:12AM - 10:24AM |
A51.00010: Large-area growth of molybdenum disulphide monolayers for integrated photonics Y. Jia, T. Stanev, E. Lenferink, N.P. Stern Electronic devices based on single/few-layer transition-metal dichacogenide semiconductors heavily rely on mechanically exfoliated micro-flakes. Uncontrollable position and dimension are significant obstacles to integration of electronics and photonics using these layered two-dimensional materials. In this report, we grow continuous few-layer MoS$_2$ film on SiO$_2$/Si wafers using a cost-effective solution process and thermal decomposition. The number of the layers can be controlled by the spin-coating rate of the solution. Multi-layers can be controllably reduced layer-by-layer using an Ar-plasma etch. Compared with chemical vapor depositions which usually require temperature of 600-900 C, the low temperature of 450 C used here offers more flexibility in MoS$_2$ direct growth on other materials such as flexible plastic substrates. The good crystalline quality over area of $50 \times 50$ $\mu$m$^2$ and the controlled layer thickness enable broad applications of 2D semiconductor films to realizing integrated photonic devices. [Preview Abstract] |
Monday, March 3, 2014 10:24AM - 10:36AM |
A51.00011: Synthesis and Characterization of Liquid Phase Exfoliated Tungsten Disulphide (WS$_{2}$) Flakes Milinda Wasala, Sujoy Ghosh, Andrew Winchester, Logan Moore, Barbara Nichols, Madan Dubey, Saikat Talapatra We report on the synthesis of 2D thin flakes of WS$_{2}$ obtained from liquid phase exfoliation of their bulk powder. Temperature dependent conductivity measurement as well as photo response of thin films prepared from these flakes will be presented. Our preliminary data, studied with in the temperature range 320K \textless\ T \textless\ 25, indicates that under a constant laser powers of wavelength $=$ 658 nm, photocurrent (I$_{\mathrm{ph}}$) decrease with decreasing temperature and becomes temperature independent at low temperatures. Further, it was found that I$_{\mathrm{ph}}$ $\sim$ (laser intensity)$^{\gamma}$ with 0.5 \textless\ $\gamma $ \textless\ 1.0. These findings will be discussed under various available models related to photoconductivity in semiconductors. [Preview Abstract] |
Monday, March 3, 2014 10:36AM - 10:48AM |
A51.00012: Stability of MoS$_{\mathrm{2(1-x)}}$Se$_{\mathrm{2x}}$ alloy: insight from first-principles calculations Duy Le, Talat S. Rahman Two-dimensional transition metal dichalcogenide (TMD) alloy is an interesting class of TMD because the ability to tune continuously its bandgap opens many new possibilities for basic studies, device concepts, and fabrication of novel heterostructures. We will present phonon dispersions of MoS$_{\mathrm{2(1-x)}}$Se$_{\mathrm{2x}}$ alloy calculated using Density Functional Perturbation Theory (DFPT). The dispersions do not show any existence of instability modes for various Se concentrations ($x)$, attesting the stability of this TMD alloy. We will, in addition, show electronic and optical properties, including Raman spectra, of MoS$_{\mathrm{2(1-x)}}$Se$_{\mathrm{2x}}$ alloy as a function of Se concentrations and their comparison with available experimental data. [Preview Abstract] |
Monday, March 3, 2014 10:48AM - 11:00AM |
A51.00013: Phonons and related spectra in V$_{2}$O$_{5}$ bulk and monolayer(001) Churna Bhandari, Walter R.L. Lambrecht We study the phonons at the zone center for the layered material V$_2$O$_5$ using density functional perturbation theory. The mode frequencies and their calculated infra-red and Raman spectra are shown to be in good agreement with results from literature. We find better agreement with the experiment, using a pseudopotential that treats vanadium semicore states 3s and 3p as bands. We also study the changes between bulk and monolayer using the same method. We find significant changes in some phonon frequencies. In particular, we see the high frequency modes related to bond-stretching between vanadium and vanadyl-oxygen exhibit a blue shift while a few low-frequency modes show a red-shift. The interatomic force constants, separated in their long-range and short range components are used to analyze the origin of these shifts. We find that the blue shifts arise predominantly from a change in the long-range force constants which is due both to the change in dielectric screening and the change in the Born effective charges. [Preview Abstract] |
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