Bulletin of the American Physical Society
APS March Meeting 2015
Volume 60, Number 1
Monday–Friday, March 2–6, 2015; San Antonio, Texas
Session G15: Focus Session: Exciton and Energy Transport in 2D Materials |
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Sponsoring Units: DMP Chair: Hayk Harutyunyan, Emory University Room: 008B |
Tuesday, March 3, 2015 11:15AM - 11:27AM |
G15.00001: Ultrafast mid-infrared intraexcitonic spectroscopy of monolayer MoS2 Soonyoung Cha, Ji Ho Sung, Moon-Ho Jo, Hyunyong Choi The optical properties of transition metal dichalcogenide (TMD) are currently active research topics for understanding the two-dimensional nature of carrier dynamics. For monolayer TMDs, reduced dielectric screening invokes strong Coulomb interactions, which lead to the large exciton binding energy. Recent theory predicts that the subset of excitons is much richer, such that internal transitions between excitons (ex. 1s, 2s, 2p) should prevail the photo-induced optical response. Here, we performed ultrafast optical pump and mid-infrared (IR) probe spectroscopy to investigate transient intraexcitonic dynamics in a monolayer MoS2. To obtain a complete excitonic dynamics, the probe photon energy is tuned over a broad range from mid-IR to IR, 0.24 eV to 0.66 eV. Our study reveals that the mid-IR responses exhibit photo-induced absorption after 3.1 eV pump excitation, which then relax within tens of picosecond and show multiple intraexcitonic and interexcitonic transitions. Our experiment shows that the fast decay component of the dynamics closely follows transient dynamics of so called A-exciton population, suggesting the photo-induced absorption indeed originates from the internal excitonic transitions. [Preview Abstract] |
Tuesday, March 3, 2015 11:27AM - 11:39AM |
G15.00002: Infrared nano-imaging of plasmonic hotspots on graphene nano-bubbles Zhe Fei, Jonathan Foley, Will Gannett, Alex Zettl, Mengkun Liu, Guangxin Ni, Siyuan Dai, Fritz Keilmann, Antonio Castro Neto, Stephen Gray, Gary Wiederrecht, Michael Fogler, Dimitri Basov One of the major goals of plasmonics is to achieve strong enhancement of electromagnetic energy by forming plasmonic hot spots for various applications including bio-sensing, single molecule fingerprinting, surface enhanced spectroscopy, and etc. Here, we demonstrate by infrared nano-imaging that nano-bubbles formed on graphene/hexagonal boron nitride heterostructures are ideal for trapping electromagnetic energy thus forming ultra-confined plasmonic hot spots. The distributions of these hot spots are sensitively dependent on the size and shape of these nano-bubbles as well as the ingredients inside. Further analysis indicates that the observed plasmonic hotspots are formed due to a significant enhancement of the plasmon wavelength and intensity above graphene nano-bubbles filled with air or other low-k dielectric materials. Our work presents a novel scheme for plasmonic hot spots formation and sheds light on future applications of graphene nano-bubbles for plasmon-enhanced single molecule characterization. [Preview Abstract] |
Tuesday, March 3, 2015 11:39AM - 11:51AM |
G15.00003: Temperature-induced plasma excitations in gapped graphene and silicene Andrii Iurov, Godfrey Gumbs, Danhong Huang Both closed-form analytic and numerical results are presented for the polarization function, as well as the plasmon excitations of gapped graphene and silicene. The calculations are carried out within the random phase approximation (RPA). We investigate the behavior of the plasmon dispersion as a function of sublattice potential difference in silicene. These results could be used to compare the effective mass model with the tight-binding approximation at finite temperature in which spin-orbit coupling is included. [Preview Abstract] |
Tuesday, March 3, 2015 11:51AM - 12:03PM |
G15.00004: ABSTRACT MOVED TO Z1.00007 |
Tuesday, March 3, 2015 12:03PM - 12:15PM |
G15.00005: Dynamics of Ion-Gating 2D Crystals Using a Solid Polymer Electrolyte Hua-Min Li, Buchanan Bourdon, Yu-Chuan Lin, Joshua Robinson, Alan Seabaugh, Susan Fullerton Ion-gating can significantly increase the static carrier density of graphene due to the formation of an electric double layer (EDL); however, the dynamics of ion-gating have not been extensively reported. A comprehensive understanding of ion dynamics is important because it establishes the timescales required to achieve EDL equilibrium, and directly affects the operating speed of devices and circuits employing electrolytic gates. Here, ion dynamics are measured on epitaxial graphene Hall-bar devices that are electrolytically gated with polyethylene oxide and lithium perchlorate. The time constants for EDL formation and dissipation are measured as a function of temperature. The measured formation time is slower than the dissipation time, because ion diffusion resulting from a concentration gradient must be opposed during EDL formation. These results quantitatively agree with COMSOL multiphysics simulations. EDL dissipation follows a stretched exponential decay described by the Kohlrausch-Williams-Watts (KWW) equation. The temperature-dependent relaxation times extracted from the KWW fit follow the Vogel-Fulcher-Tammann (VFT) temperature dependence. At temperatures approaching the glass transition temperature of the electrolyte, the relaxation times exceed several hours, demonstrating the long timescales over which the EDL can persist in the absence of a gate bias. [Preview Abstract] |
Tuesday, March 3, 2015 12:15PM - 12:27PM |
G15.00006: Electronic transport in graphene/CdSe nanoparticle monolayer/graphene tunneling devices Datong Zhang, Chenguang Lu, Arend van der Zande, Philip Kim, Irving P. Herman We fabricated graphene/CdSe nanoparticle monolayer/graphene sandwich device structures. The CdSe nanoparticle monolayer is formed on a liquid-air surface before transferring it onto the bottom graphene layer that had been micro-exfoliated onto a 285 nm SiO2/Si substrate. The top graphene layer is transferred to the targeted area on the CdSe nanoparticle monolayer via a dry transfer technique. Tunneling-type vertical transport is observed, which is fitted by tunneling models that suggest that ligand shell instead of nanoparticle core is the major barrier of tunneling. Photoconductivity is enhanced but with low exciton separation efficiency when the laser is on the junction area, also suggesting that ligand shell is the major barrier of electronic transport in the sandwich structure. [Preview Abstract] |
Tuesday, March 3, 2015 12:27PM - 12:39PM |
G15.00007: PVA:LiClO$_{4}$: a robust, high T$_{g}$ polymer electrolyte for adjustable ion gating of 2D materials Erich Kinder, Susan Fullerton Polymer electrolytes are an effective way to gate organic semiconductors and nanomaterials, such as nanotubes and 2D materials, by establishing an electrostatic double layer with large capacitance. Widely used solid electrolytes, such as those based on polyethylene oxide, have a glass transition temperature below room temperature.~This permits relatively fast ion mobility at T $=$ 23 $^{\circ}$C, but requires a constant applied field to maintain a doping profile. Moreover, PEO-based electrolytes cannot withstand a variety of solvents, limiting its use. Here, we demonstrate a polymer electrolyte using polyvinyl alcohol (PVA) with T$_{g}$~\textgreater 23 $^{\circ}$C, through which a doping profile can be defined by a potential applied when the polymer is heated above T$_{g}$, then ``locked-in'' by cooling the electrolyte to room temperature (\textless T$_{g})$ to limit ion mobility. Current-voltage measurements of a graphene field effect transistor verify the ``lock-in'' process, showing constant drain current regardless of the applied electrolyte gate bias. Hall bar measurements are used to quantify the charge carrier density. Owing to PVA's chemical stability, photolithography can be performed directly on the polymer electrolyte, which allows for the deposition of a patterned, metal gate directly on the electrolyte, as well as the ability to pattern the electrolyte itself. [Preview Abstract] |
Tuesday, March 3, 2015 12:39PM - 12:51PM |
G15.00008: Near-field optical second-harmonic technique for detection and characterization of semiconductor thin film electron-scattering domain boundaries Farbod Shafiei, Tommaso Orzali, Gennadi Bersuker, Downer Michael Understanding electron transport in epitaxial semiconductor thin films and low dimension systems is crucial for new electro-optic devices. III-V films grown on Si integrate high carrier mobility into the established Si platform, but are susceptible to formation of sub-micron anti-phase domains that possess unwanted Ga-Ga or As-As electron-scattering defects at their boundaries. Optical second-harmonic generation provides sensitive, specific and noninvasive but so far only spatially-integrated characterization for these defects [1]. We introduce a fiber based nearfield scanning optical second harmonic microscopy for the first time to fully resolve the electron scattering boundaries on III-V/Si films. This technique reveal variations in electron scattering boundaries structure as growth conditions, epitaxial film composition, and substrate vary, and are compared with surface topography, darkfield transmission electron microscopy and electron back scatter diffraction. Suppression of the electron-scattering boundaries has been explored. \\[4pt] [1] Lei \textit{et al}., Appl. Phys. Lett. 102, 152103 (2013). [Preview Abstract] |
Tuesday, March 3, 2015 12:51PM - 1:03PM |
G15.00009: Semiclassical Boltzmann Theory Studies of the Electronic Resistances of Multilayered Silicene Junctions Yunpeng Wang, X.-G. Zhang, James Fry, Hai-Ping Cheng A layer-by-layer investigation is carried out to understand electron transport across metal-semiconductor-metal junctions. Structures of junctions are optimized using first-principles density functional theory with the generalized gradient approximation. The semiclassical Boltzmann theory of the electronic transport is revisited and applied to multilayer silicene and hexagonal BN based junctions. The calculated resistance is smaller than, but converges to that calculated by the Landauer formula as the thickness of the barrier increases. Our calculation results provide an upper limit on the transmission coefficient per channel, $ \sim 0.05 $, below which the Landauer formula is applicable for calculating the resistance. In addition, we find that the resistance of a junction is not determined entirely by the average transmission, but also by the distribution of the transmission over the first Brillouin zone. [Preview Abstract] |
Tuesday, March 3, 2015 1:03PM - 1:15PM |
G15.00010: Localized surface plasmon effects of two dimensional lattice of metal nanoislands Yukari Oda, Ryoko Shimada Localized surface plasmon (LSP) of metal nanoparticles results from non-propagating excitation of their conduction electrons coupled to the electromagnetic field. LSP localizes the electric field and enhances light emission from fluorescent materials. In this study, a two dimensional (2D) lattice of silver (Ag) nanoislands was fabricated by nanosphere lithography (NSL) method utilizing self-assembled, close-packed hexagonal structures of polystyrene spheres as the etching mask.~This 2D lattice was subjected to the electric field for investigating a role of the periodicity of metal islands in the LSP effect. 9,10-di(2-naphthyl) anthracene (ADN), a well-known blue-emitting material in the field of electroluminescence, was used for the study of the enhancement of emission due to the LSP effect. Hybrid thin films of poly(methyl methacrylate) containing ADN were prepared with spin-casting onto the 2D lattice of Ag nanoislands. Transmission and photoluminescence measurements were conducted for these hybrid thin films at room temperature. Detailed results will be presented on site. [Preview Abstract] |
Tuesday, March 3, 2015 1:15PM - 1:27PM |
G15.00011: Spatially indirect exciton condensate in bilayer systems Fei Xue, Fengcheng Wu, Allan MacDonald Bilayer equilibrium exciton condensates have attracted attention in recent years because of their interesting and anomalous transport properties. Here we report on the microscopic derivation of bosonic effective Hamiltonians which include exciton-exciton interaction terms and account realistically for band-structure and dielectric environment effects. We describe in detail the microscopic origin of different contributions to exciton-exciton interaction. We apply our theory to the case of transition metal dichalcogenides, addressing specifically the role of the excitonic flavor multiplicity in that system. [Preview Abstract] |
Tuesday, March 3, 2015 1:27PM - 1:39PM |
G15.00012: Coulomb drag in coupled electronic systems of different dimensionalities Ben Yu-Kuang Hu Coulomb drag occurs when two electrically independent electronic systems are situated in close enough proximity to each other that the Coulomb coupling between the two systems causes an electric current in one system to drag along a carriers in the other system. The magnitude of this effect is quantified by the drag rate. We derive the formalism to determine the drag rate in systems of different dimensionalities (for example, one-dimensional quantum wires coupled to two-dimensional quantum wells), based on coupled transport equations. We discuss how the hybrid coupled plasmons of the systems of different dimensionalities can affect the drag, and we investigate the effect that plasmons in the higher-dimensional system can have on the drag rate of the carriers in the lower-dimensional system. [Preview Abstract] |
Tuesday, March 3, 2015 1:39PM - 1:51PM |
G15.00013: Three-dimensional plane-wave full-band quantum transport using empirical pseudopotentials Jingtian Fang, William Vandenberghe, Massimo Fischetti We study theoretically the ballistic performance of future sub-5 nm Field-Effect Transistors (FETs) using an atomistic quantum transport formalism based on empirical pseudopotentials, with armchair Graphene NanoRibbons (aGNRs), Silicon NanoWires (SiNWs) and zigzag Carbon NanoTubes (zCNTs) as channel structures. Due to the heavy computational burden from the plane-wave basis set, we restrict our study to ultrasmall devices, characterized by 5 nm channel lengths and 0.7 nm $\times$ 0.7 nm cross-sectional areas. Band structure calculations show that aGNRs have an oscillating chirality-dependent band gap. AGNRs with dimer lines N=3p+1 have large band gaps and aGNRFETs show promising device performance in terms of high $I_{on}$/$I_{off}$, small drain-induced barrier lowering and limited short channel effects due to their very thin body and associated excellent electrostatics control. N=3p+2 aGNRs have small band gaps and band-to-band tunneling generates a large current at high bias. We also discuss spurious solutions introduced by the envelope function approximation. Device characteristics of SiNWFETs and zCNTFETs are compared to aGNRFETs as well. [Preview Abstract] |
Tuesday, March 3, 2015 1:51PM - 2:03PM |
G15.00014: Dynamic Control of Thermal Emission with Plasmonically Active Graphene Metasurfaces Victor Brar, Michelle Sherrott, Min Jang, Seyoon Kim, Laura Kim, Josue Lopez, Mansoo Choi, Luke Sweatlock, Harry Atwater Thermal emission is typically viewed to be broadband, unpolarized and isotropic, with a spectral profile and intensity that depend on the emissivity of the material, and that vary only with changes in temperature. In this talk we demonstrate that the intensity, polarization and spectrum of thermal emission at constant temperature can be dynamically controlled through electrostatic gating of plasmonic graphene resonators on a heated SiNx substrate. We show that the plasmonic resonances in graphene act as antenna that to out-couple the thermal energy of substrate phonons and graphene electrons to create narrow, mid-infrared spectral features in the thermal emission profile. By varying the gate voltage and resonator width, we show that these features can be effectively turned on and off at kHz rates, and tuned across a broad frequency range. Our measurements show that at 7um the emissivity of the surface can be varied by 0.02, and that the emitted radiation is polarized, with a modulated power density of 0.02W/m2 over 100cm-1 of bandwidth. [Preview Abstract] |
Tuesday, March 3, 2015 2:03PM - 2:15PM |
G15.00015: Graphene-enabled electrically switchable radar absorbing surfaces Osman Balci, Emre Ozan Polat, Nurbek Kakenov, Coskun Kocabas Radar absorbing materials are used in stealth technologies for concealment of an object from radar detection. Resistive and/or magnetic composite materials are used to reduce the backscattered microwave signals. Inability to control electrical properties of these materials however, hinders the realization of active camouflage systems which require adaptive surfaces operating in microwave frequencies. Here, using large-area graphene electrodes, we demonstrate a new class of active surfaces which enables unprecedented ability to control reflection, transmission and absorption of microwaves by electrical means. Instead of tuning bulk material property, our strategy relies on electrostatic tuning of the charge density on an atomically thin electrode which operates as a tunable metal in microwave frequencies. Notably, we fabricated large area adaptive radar absorbing surfaces with tunable reflection suppression ratio up to 50 dB with operation voltages less than 5 V. These electrically switchable radar absorbing surfaces provide a significant step in realization of active camouflage systems and adaptive cloaking in microwave frequencies, which cannot be realized by conventional materials. [Preview Abstract] |
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