Bulletin of the American Physical Society
APS March Meeting 2014
Volume 59, Number 1
Monday–Friday, March 3–7, 2014; Denver, Colorado
Session L51: Focus Session: Beyond Graphene Devices: Function, Fabrication, and Characterization III |
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Sponsoring Units: DMP Chair: Jing Shi, University of California, Riverside Room: Mile High Ballroom 1E |
Wednesday, March 5, 2014 8:00AM - 8:12AM |
L51.00001: Electric-field tuning phonon in single layer WS2 Yiling Yu, Yifei Yu, Alper Gurarslan, Linyou Cao The physical properties of two-dimensional semiconductor materials play crucial role in realizing next generation electronic and opto-electronic devices. In this work, we observe a dramatic change in Raman spectrum for single layer WS2 under external electric field. The intensity of Raman peak will increase or decrease for different direction of bias voltages. This indicates we can tune the optical phonon behavior by external electric field and enable a strong electron-phonon coupling in the single layer WS2. Our results can provides new physical understanding to electron-phonon coupling to two dimensional material systems, and suggest a potential promising way to control thermal conductivity of layered materials through external electric field, which is very interesting to both basic physics and device applications. [Preview Abstract] |
Wednesday, March 5, 2014 8:12AM - 8:24AM |
L51.00002: Thermopower of graphene and the validity of Mott's formula Fereshte Ghahari, Takashi Taniguchi, Kenji Watanabe, Philip Kim Thermoelectric power (TEP) of graphene is previously measured in the disorder limited transport regime where the semiclassical Mott relation agrees with experimental data. In this presentation, we report the TEP measurement on graphene samples deposited on hexagonal boron nitride substrates where drastic suppression of disorder is achieved. Our results show that at high temperatures the measured thermopower deviates from Mott relation and this deviation is greater for higher mobility samples. We quantify this deviation in both degenerate and non-degenerate regime using Boltzmann transport theory considering different scattering mechanisms in the system. [Preview Abstract] |
Wednesday, March 5, 2014 8:24AM - 8:36AM |
L51.00003: Ballistic Thermal Conductance in Layered Two-Dimensional Materials Zuanyi Li, Yizhou Liu, Yong Xu, Wenhui Duan, Eric Pop The thermal properties of two-dimensional (2D) materials like graphene, h-BN, MoS$_{\mathrm{2}}$ and WS$_{\mathrm{2}}$ are uniquely anisotropic, including high in-plane but low out-of-plane thermal conductivity $\kappa $. Here we provide a comparative study of the ballistic limits of heat flow in these 2D layers and stacks. Based on full phonon dispersions from density functional theory, we calculate their in-plane and cross-plane ballistic thermal conductance per cross-sectional area, $G$. For a given material, monolayers and multilayers have similar in-plane $G$ above 100 K, but monolayers show higher $G$ at low temperature due to the contribution of flexural phonons. At 300 K, graphene has the highest $G$ $\sim$ 4.2 GWK$^{\mathrm{-1}}$m$^{\mathrm{-2}}$, about 20{\%} higher than h-BN and 5 times higher than MoS$_{\mathrm{2}}$ and WS$_{\mathrm{2}}$. Cross-plane values are about one order of magnitude lower than in-plane values due to weak van der Waals interactions. Based on the calculated $G$, we can obtain phonon mean free path, given diffusive $\kappa $. These results are important as they establish the length scales of the ballistic-diffusive transition of heat flow and the non-classical regime where $\kappa $ depends on the system size. [Preview Abstract] |
Wednesday, March 5, 2014 8:36AM - 9:12AM |
L51.00004: Thermal and Thermoelectric Transport in Two-Dimensional Materials and Devices beyond Graphene Invited Speaker: Li Shi Besides the switching speed and on-off ratio, the hot spot temperature is one important performance metric of novel electronic devices fabricated from two-dimensional (2D) materials beyond graphene. This performance metric depends sensitively on the largely unknown thermal transport properties of various 2D materials. In addition, it still remains a grand challenge to experimentally verify the theoretical predictions of enhanced thermoelectric figure of merit in 2D systems and by topologically protected surface states. Following our prior works on thermal transport measurements of graphene, we have recently studied thermal transport in few-layer h-BN, MoS2, and germanane, and the thermoelectric properties of bismuth telluride nanoplates. The results reveal that surface perturbation suppresses the in-plane lattice thermal conductivity of these 2D materials. The thickness needed for recovery to the bulk lattice thermal conductivity scales with the bulk phonon mean free path. In addition, we have observed decrease in both the electrical conductivity and thermal conductivity with decreasing thickness of bismuth telluride nanoplates. While the electrical conductivity is still within the bulk range, the thermal conductivity is reduced to below the bulk range for nanoplates thinner than 20 nm. These results are explained by the presence of surface band bending and diffuse surface scattering of electrons and phonons in the nanoplates, where pronounced n-type surface band bending can yield suppressed and even negative Seebeck coefficient in unintentionally p-type doped nanoplates. Sb doping and surface functionalization are employed in our works to tune the Fermi level and surface band bending and modify the thermoelectric properties. [Preview Abstract] |
Wednesday, March 5, 2014 9:12AM - 9:24AM |
L51.00005: Thermoelectric performance in ultra-thin transition metal dichalcogenides Darshana Wickramaratne, Ferdows Zahid, Roger Lake The thermoelectric figure of merit, ZT, is calculated for one to four monolayers of MoS$_{2}$, MoSe$_{2}$, WS$_{2}$ and WSe$_{2}$. The maximum ZT in this family of materials occurs in bilayer MoSe$_{2}$. Its ZT value of 2.39 is a factor of 8 increase compared to that of the bulk at room temperature. The values for the power factors and ZT change non-monotonically as the film thicknesses are increased from a single monolayer up to four layers. In contrast to Bi$_{2}$Te$_{3}$, the peak value of ZT occurs at a thickness greater than a single monolayer for all 4 materials. The shape of the distribution of the valence band and the conduction band density of modes explains the enhanced thermoelectric performance that occurs for film thicknesses above a single monolayer. Ab-initio electronic structure calculations are used in a Landauer approach to calculate the thermoelectric transport coefficients. [Preview Abstract] |
Wednesday, March 5, 2014 9:24AM - 9:36AM |
L51.00006: First-principles Raman spectra of MoS$_2$, WS$_2$ and their heterostructures Liangbo Liang, Vincent Meunier MoS$_2$ and WS$_2$ are graphene-like layered structures that are considered as alternative and complement to graphene. Raman spectroscopy is a very powerful tool to study them. Despite the extensive experimental Raman study on MoS$_2$ and WS$_2$, it remains unclear how Raman intensities and especially intensity ratio of Raman modes E$_{2g}$ and A$_{1g}$ depend on the thickness. To clarify such issues, we carried out density functional theory calculations for both MoS$_2$ and WS$_2$ to simulate their Raman spectra and reveal the intrinsic thickness dependence of Raman intensities and intensity ratio. More importantly, we quantitatively analyzed the laser polarization effect on the intensity ratio and revealed its high sensitivity to laser polarization, which could explain the large discrepancy between measured intensity ratios by different groups. We also studied $\textit{ab initio}$ Raman spectra of MoS$_2$/WS$_2$ heterostructures up to four layers in every possible combinations and stacking orders. Each configuration is found to possess a unique Raman spectrum in both frequency and intensity that can be explained by changes in dielectric screening and interlayer interactions. Our findings serve as guidelines for the experimental identification of heterostructure configurations. [Preview Abstract] |
Wednesday, March 5, 2014 9:36AM - 9:48AM |
L51.00007: Nano-imaging and nano-spectroscopy of tunable surface phonon polaritons in hexagonal boron nitride Siyuan Dai, Zhe Fei, Qiong Ma, Aleksandr Rodin, Martin Wagner, Alexander McLeod, Mengkun Liu, Will Gannett, William Regan, Mark Thiemens, Gerardo Dominguez, Antonio Castro Neto, Alex Zettl, Fritz Keilmann, Pablo Jarillo-Herrero, Michael Fogler, Dimitri Basov Van der Waals crystals such as graphene, topological insulators, cuprate high-temperature superconductors, and many other layered structures reveal a rich variety of enigmatic electronic, photonic and magnetic properties. We report infrared (IR) nano-imaging of surface phonon polaritons in a prototypical van-der-Waals crystal: hexagonal boron nitride (hBN). In the setting of an antenna-based IR spectroscopic nanoscope, we accomplished launching, detecting, and real space imaging of the polaritonic waves. We were able to alter both the wavelength and the amplitude of such waves by varying the number of crystal layers in our specimens. We demonstrated a new nano-photonics method for mapping the polariton dispersion. The dispersion is shown to be governed by the crystal thickness according to a scaling law that persists down to a few monolayers. Our results point to novel functionalities of van-der-Waals crystals as reconfigurable nano-photonic materials. [Preview Abstract] |
Wednesday, March 5, 2014 9:48AM - 10:00AM |
L51.00008: Generation and Detection of Coherent THz Acoustic Phonons in Few Atomic Layer MoS2 Haining Wang, Changjian Zhang, Wei Min Chan, Sandip Tiwari, Rana Farhan We present, for the first time, results on the generation and detection of coherent THz acoustic phonon oscillations in few-monolayer MoS2 by ultrafast pump-probe technique. In $\sim$ 1nm thick Dichalcogenides, the lowest confined LA phonon modes in the out-of-plane direction can have frequencies approaching one THz. In our experiments, a pump pulse is used to excite these phonon modes through the Raman process. The refractive index of few-layer MoS2 is sensitive to the layer separation, and, therefore, the transmission of the probe pulse changes with the layer separation allowing us to observe coherent phonon oscillations in real time. The measured phonon frequencies, for different number of monolayers (from $\sim$3 to $\sim$100), agree well with analytical model based on the quantization of the bulk LA phonon dispersion in the out-of-plane direction and the interlayer force constant was extracted. Our data also allow us to extract phonon lifetimes and quality factors. The observed ultrafast dynamics of the photoexcited carriers also evolve with the number of monolayers as the electronic bandstructure evolves. [Preview Abstract] |
Wednesday, March 5, 2014 10:00AM - 10:12AM |
L51.00009: Layer-dependent electronic and vibrational properties of SnSe$_{2}$ and SnS$_{2}$ 2D materials Joseph Gonzalez, Rudy Schalf, Ivan Oleynik Layered metal chalcogenides possess a wide range of unique electronic properties, which are currently explored for applications as novel two-dimensional electronic materials. SnS$_{2}$ and SnSe$_{2}$ layered materials consist of covalently bonded S-Se-S (Sn-Se-Sn) sheets bonded together by weak van der Waals interactions. The atomic, electronic and vibrational properties of SnS$_{2}$ and SnSe$_{2}$ thin films are investigated using first-principles density functional theory. The evolution of the thickness-dependent band structure and Raman spectra are discussed, as well as the effects of strain and the influence of the substrate. The first-principles results are compared with available experimental data. [Preview Abstract] |
Wednesday, March 5, 2014 10:12AM - 10:24AM |
L51.00010: Light Generation and Harvesting in a Van der Waals Heterostructure Oriol L\'opez S\'anchez, Esther Alarcon Llado, Volodymyr Koman, Anna Fontcuberta i Morral, Aleksandra Radenovic, Andras Kis We report on the realization of light-emitting diodes based on heterojunctions with monolayer MoS$_{2}$. Careful interface engineering allows us to realize diodes showing rectification and light emission from the entire surface of the heterojunction. Electroluminescence spectra show clear signs of the A and B excitons and the A- trion resonance related to the optical transitions between the conduction and valence bands. Our pn diodes can also operate as solar cells with an external quantum efficiency higher than 2{\%}. Our work opens up the way to more sophisticated optoelectronic devices such as 2D LEDs and solar cells based on monolayer MoS$_{2}$. [Preview Abstract] |
Wednesday, March 5, 2014 10:24AM - 10:36AM |
L51.00011: Electronic and optical properties of monolayer and few-layer of distorted transition-metal dichalcogenides Pierre Darancet, Andrew J. Millis, Chris A. Marianetti Groups IV, V, and VI- transition-metal dichalcogenides (TMDC) are layered compounds exhibiting a wealth of competing phenomena, ranging from charge density waves (CDW) to Mott transitions. We present investigations using density functional theory (DFT) and DFT+U regarding the electronic structure and electronic correlations arising in distorted tantalum disulfide (TaS2). We show that the monolayer material is a Mott insulator while the bulk is a metal, in contradiction with much of the existing literature, which argues that the bulk material is a Mott insulator. Properties of the few layer system will also be presented.Finally, we will discuss the influence of these competing energy scales on the transport and optical properties of these materials. [Preview Abstract] |
Wednesday, March 5, 2014 10:36AM - 10:48AM |
L51.00012: Persistent Photoconductivity in Monolayer MoS$_{2}$ on Organic-molecule-functionalized Substrates Wei-Hua Wang, Yueh-Chun Wu, Shao-Yu Chen, Cheng-Hua Liu, Po-Hsun Ho, Chun-Wei Chen, Chi-Te Liang We demonstrate a giant persistent photoconductivity (PPC) effect in monolayer MoS$_{2}$ in which the photocurrent robustly persists after illumination has ceased. This PPC effect in monolayer MoS$_{2}$ on organic-molecule-functionalized substrates sustains up to room temperature and can be highly suppressed by applying source--drain/back-gate voltages to the transistors. Based on this persistency and controllability of the PPC effect, we achieve a room-temperature conductance bistability by utilizing optical and electrical pulses. The observed giant PPC effect in MoS$_{2}$ can be attributed to a large electron-capture barrier of trap states, which is estimated to be as high as 390 meV. [Preview Abstract] |
Wednesday, March 5, 2014 10:48AM - 11:00AM |
L51.00013: Enhanced Photoluminescence and Photocurrent Spectra in MoS2 under Ionic Liquid Gating Zhen Li, Shun Wen Chang, Stephen Cronin We report substantial improvements in the photoluminescence (PL) and photocurrent (PC) spectra of monolayer MoS2 field effect transistors taken under electrostatic and ionic liquid gating conditions. The photocurrent and photoluminescence spectra show good agreement with a dominant peak at 1.9eV. The magnitude of the photoluminescence and photocurrent can be increased significantly by Si back gating and ionic liquid gating due to the passivation of surface states and trapped charges that act as recombination centers and cause non-radiative recombination of photoinduced electron-hole pairs. Under ionic liquid gating, we observe an increase in the photoluminescence intensity by 300{\%}, while the linewidth decreases by 37{\%}. The photocurrent also doubles when passivated by the ionic liquid. The acute sensitivity of monolayer MoS2 to ionic liquid gating and passivation arises because of its high surface-to-volume ratio, which makes it especially sensitive to trapped charge and surface states. Under high gating conditions, we observe a slight decrease in the photoluminescence intensity, most likely due to Auger recombination. These results reveal that, in order for efficient optoelectronic devices to be made from monolayer MoS2, some passivation strategies must be employed to mitigate the issues associated with surface states. [Preview Abstract] |
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