Bulletin of the American Physical Society
APS March Meeting 2013
Volume 58, Number 1
Monday–Friday, March 18–22, 2013; Baltimore, Maryland
Session Z5: Focus Session: Graphene: Transport and Optical Phenomena: Hot Electrons and Photocurrents |
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Sponsoring Units: DCMP Chair: Alex Holleitner, Technische Universitaet Muenchen Room: 301 |
Friday, March 22, 2013 11:15AM - 11:27AM |
Z5.00001: Hot carriers, phonons and electron-phonon decoupling in graphite Tung-Wu Hsieh, Chih-Wei Lai Visible and near-IR radiation and hot phonons are observed in HOPG graphite following the excitation of picosecond laser pulses at 1.58 eV of fluences exceeding 1000 J/m$^2$. The optically generated electron-hole carriers lead to non-thermal radiation ranging from 1.2 to 2.8 eV, including black-body-like emissions above the excitation and a broad spectral peak near 1.4 eV. We determine an effective electronic temperature (Te) by fitting the high energy radiation to a Plank spectrum; Tg from G-mode Stokes/anti-Stokes Raman peaks; Tl from spectral line shifts of G-mode. With increasing incident fluence from 10$^3$ to 10$^4$ J/m$^2$, Te, Tg and Tl are decoupled and increase from 1000 to 5000, 1000 to 2500, and 300 to 500K, respectively. At a fluence below 10$^3$ J/m$^2$, Te approaches Tg near 2000K, which is $\sim$ 1000K above Tl. This is indicative of quasi-equilibrium, but decoupled, distributions of carriers and phonons. The transient radiation decays within 2ps, limited to instrument response. Similar effects are observed for excitations at 1.53 and 1.49 eV. Experiments are conducted in vacuum at ambient T$=$300K. [Preview Abstract] |
Friday, March 22, 2013 11:27AM - 11:39AM |
Z5.00002: Doping dependence of the ultrafast relaxation dynamics of hot electrons in graphene Liang Zhao, Jie Shan, Kin Fai Mak, Tony Heinz The ultrafast relaxation dynamics of highly excited electrons in graphene has attracted much attention due to both its fundamental interest and its practical importance in relation for optoelectronic devices. Several mechanisms including electron-optical phonon scattering and disorder assisted electron-phonon scattering have been proposed to be responsible for electron cooling on the picosecond time scale. In this work, we apply the technique of two-color femtosecond pump-probe spectroscopy to investigate the electron relaxation dynamics as a function of doping. The photo-induced absorption in graphene is seen to vary rapidly in the first a few 100's fs, followed by a slower decay of $\sim$ ps. The dynamics depend sensitively on the doping level. We will present our analysis of the results in terms of the transient electron chemical potential and temperature and discuss the role of different doping mechanisms, in particular, in the regime where the Fermi energy approaches half of the probe photon energy. [Preview Abstract] |
Friday, March 22, 2013 11:39AM - 11:51AM |
Z5.00003: Hot Carrier Transport at the Graphene-Metal Interface Induced by Strong Lateral Photo-Dember Effect Chang-Hua Liu, You-Chia Chang, Nanditha Dissanayake, Yaozhong Zhang, Zhaohui Zhong Ultrafast photo-excitation in a semiconductor can lead to transient spatial charge gradient if electrons and holes have different drift velocities. The charge gradient builds up the transient electric field and causes the subsequent terahertz pulse emission. This phenomenon, known as the photo-Dember effect, was typically considered insignificant in graphene due to its similar electron and hole mobilities. Here, we observe hot carrier transport at the graphene-metal interface driven by the photo-Dember electric field under femtosecond pulse laser excitation. The polarity of hot carrier transport is determined by the asymmetry of electron and hole mobilities of the graphene device and cannot be flipped sign by tuning graphene doping level. This indicates the formation of strong photo-Dember field, dominating over the graphene/metal built-in electric field or thermal electric field. We further analyze the spatial distribution and temporal evolution of the transient electric field near the contact edge by using the drift-diffusion model. The modeling results suggest that strong photo-Dember effect is caused by the low electronic specific heat of graphene and a huge charge gradient near the graphene-metal interface under pulse laser excitation. [Preview Abstract] |
Friday, March 22, 2013 11:51AM - 12:03PM |
Z5.00004: Giant Nonlocal Photocurrent at the Charge Neutrality Point in Graphene Qiong Ma, Nathan Gabor, Nityan Nair, Wenjing Fang, Jing Kong, Pablo Jarillo-Herrero Graphene based photosensitive devices have attracted considerable attention due to monolayer graphene's broadband optical absorption and gate tunable capacities. As the quality of graphene increases, emergent phenomena are being observed in both transport and optical measurements. Here we report measurements of giant nonlocal photocurrent that emerges at the charge neutrality point in graphene transistor devices. Scanning photocurrent imaging of uniformly undoped monolayer graphene transistors reveals highly ordered spatial patterns with alternating photocurrent signs as a function of laser position. The charge density dependence of the photoresponse, combined with in-situ improvement of device mobility, reveals a strong correlation between the nonlocal photocurrent and the derivative of the thermopower as a function of charge density. Photocurrent enhancement is pronounced in high-mobility devices and at intermediate temperatures. Such behaviors are suggestive of phonon drag effects that emerge at the charge neutrality point under photoexcitation. [Preview Abstract] |
Friday, March 22, 2013 12:03PM - 12:15PM |
Z5.00005: Hot carrier response in gapped bilayer graphene Grant Aivazian, Jason Ross, K. Watanabe, T. Taniguchi, K. Kitamura, David Cobden, Xiaodong Xu Recently bilayer graphene has been shown to develop a bandgap upon breaking of inversion symmetry by a perpendicular electric field that is \textit{in situ }tunable between zero and several hundred meV (corresponding to wavelengths in the mid-IR). Such unique tunability offers bilayer graphene a niche in mid-IR optoelectronic devices where a lack of high performance photodetectors exists. In this work we have performed spatially and temporally resolved photocurrent measurements in a dual-gated bilayer graphene FET under continuous-wave and pulsed laser excitation. We find that photocurrent generation in native bilayer graphene is dominated by hot carriers, as is the case in monolayer graphene, but it behaves very differently from monolayer graphene once a bandgap has been opened. [Preview Abstract] |
Friday, March 22, 2013 12:15PM - 12:27PM |
Z5.00006: Supercollision cooling in undoped graphene Sung Ho Jhang, Andreas Betz, Emiliano Pallecchi, Robson Ferreira, Gwendal Feve, Jean-Marc Berroir, Bernard Placais We have investigated the electron-lattice cooling rate in graphene by means of GHz Johnson noise thermometry. For phonon temperatures ($T_{ph}$) larger than Bloch-Gr\"{u}neisen temperature ($T_{\mathrm{BG}}$), we find the energy relaxation rate $J$ obeys a cubic law as a function of electron temperature $T_e$. In this regime, the small Fermi surface of graphene drastically restricts the allowed phonon energy in ordinary electron-phonon scattering, and disorder-assisted supercollisions dominate over the conventional electron-phonon collisions. In the low-temperature regime, for $T_{ph} < T_{\mathrm{BG}}$, we regain $J \propto T_e^{4}$ dependence, which is the signature of standard electron-phonon interaction in the 2D graphene. Beside its implication for electron-phonon physics, our observations are of direct relevance for the performance of graphene bolometers and photo-detectors. [Preview Abstract] |
Friday, March 22, 2013 12:27PM - 12:39PM |
Z5.00007: Focused Laser Induced Spatially Controllable p-n junction in Graphene Field-Effect Transistor Young Duck Kim, Myung-Ho Bae, Jung-tak Shu, Young Seung Kim, Joung Real Ahn, Seung-Hyun Chun, Yun Daniel Park Tunable local doping on graphene is an important issue for future graphene-based electronics. Here we investigate a local doping effect by a focused laser irradiation and demonstrate a spatially controllable p-n junction in graphene field-effect transistor. Scanning photocurrent microscopy with varying back-gate voltages reveals the local charge trap in gate oxide near the laser-irradiated region. This is manifested by itself as double peaks in resistance as a function of gate voltage in graphene device, where the region between the double peaks corresponds to the p-n junction. Irradiation of a focused laser on graphene device suggests a new pave to spatially control the doping level, position and size of doped segment on graphene channel in a nondestructive way without high electrical bias, local gate electrode and chemical process. [Preview Abstract] |
Friday, March 22, 2013 12:39PM - 12:51PM |
Z5.00008: Multiple regimes of carrier cooling in photoexcited graphene probed by time-resolved terahertz spectroscopy A.J. Frenzel, N.M. Gabor, P.K. Herring, W. Fang, J. Kong, P. Jarillo-Herrero, N. Gedik Energy relaxation and cooling of photoexcited charge carriers in graphene has recently attracted significant attention due to possible hot carrier effects, large quantum efficiencies, and photovoltaic applications. However, the details of these processes remain poorly understood, with many conflicting interpretations reported. Here we use time-resolved terahertz spectroscopy to explore multiple relaxation and cooling regimes in graphene in order to elucidate the fundamental physical processes which occur upon photoexcitation of charge carriers. We observe a novel negative terahertz photoconductivity that results from the unique linear dispersion and allows us to measure the electron temperature with ultrafast time resolution. Additionally, we present measurements of the relaxation dynamics over a wide range of excitation fluence. By varying the pump photon energy, we demonstrate that cooling dynamics of photoexcited carriers depend on the amount of energy deposited in the graphene system by the pump pulse, not the number of absorbed photons. The data suggest that fundamentally different regimes are encountered for different excitation fluences. These results may provide a unifying framework for reconciling various measurements of energy relaxation and cooling in graphene. [Preview Abstract] |
Friday, March 22, 2013 12:51PM - 1:03PM |
Z5.00009: Terahertz generation and picosecond photo-thermoelectric currents in graphene Alexander Holleitner We demonstrate that THz radiation is generated in optically pumped bilayer graphene. The electro-magnetic radiation is detected via a time-domain THz spectroscopy utilizing coplanar metal stripline circuits in combination with an on-chip pump/probe scheme [1]. The striplines act as highly sensitive near-field antennae with a bandwidth of up to 1 THz. Our ultrafast experiments further clarify the optoelectronic mechanisms contributing to the photocurrent generation at graphene-metal interfaces. We verify that both built-in electric fields, similar to those in semiconductor-metal interfaces, and a photo-thermoelectric effect give rise to the photocurrent at graphene-metal interfaces at different time scales. We particularly discuss how the picosecond photocurrents in monolayer graphene depend on the geometry and the thermal coupling of the devices to the environment [2]. We acknowledge the very fruitful cooperation with L. Prechtel, S. Manus, D. Schuh, W. Wegscheider, L. Song, and P. Ajayan.\\[4pt] [1] L. Prechtel, L. Song, P. Ajayan, D. Schuh, W. Wegscheider, A.W. Holleitner, Nature Communications 3, 646 (2012).\\[0pt] [2] A. Brenneis et al. (2013). [Preview Abstract] |
Friday, March 22, 2013 1:03PM - 1:15PM |
Z5.00010: Optical pure spin current injection in graphene Julien Rioux, Guido Burkard Pure spin current injection by optical methods is investigated in single-layer and bilayer graphene within the tight-binding model, including bias and interlayer coupling effects. Interlayer coupling in bilayer graphene has a distinct qualitative effect on the polarization dependence of the spin current injection. In combination with interlayer coupling, which induces trigonal warping of the electronic bands, the bias voltage allows to control the warping at the Fermi surface. The resulting implications for the spin current injection are presented. Unlike the previously presented charge current injection [J. Rioux et al., PRB 83, 195406 (2011)], the effect presented here relies on a single monochromatic beam. [Preview Abstract] |
Friday, March 22, 2013 1:15PM - 1:27PM |
Z5.00011: Probing the Optoelectronic Response of a Monolayer MoS$_{2}$ Field-Effect Transistor Kathryn L. McGill, Kin Fai Mak, Joshua W. Kevek, Jiwoong Park, Paul L. McEuen Two-dimensional materials contain a wealth of interesting optoelectronic properties. Single-layer molybdenum disulfide (MoS$_{2})$, with its broken inversion symmetry, is of particular interest. This broken symmetry results in the formation of direct band gaps at the K and K' valleys in its band structure, allowing long-lived optical excitations. Furthermore, monolayer MoS$_{2}$ has valley-dependent electronic properties allowing confinement of charge carriers to a single valley by optical pumping [1]. We have fabricated two- and four-terminal devices based on single layers of MoS$_{2}$. We observe an efficient photocurrent response at the two-dimensional semiconductor-metal interface displaying Shottky diode behavior, in which an interfacial field splits excitons at the contacts to produce current. We also find that the photocurrent drastically increases under reverse biasing of the diode. Additionally we are exploring the dependence of this photocurrent response on the polarization state of incident light.\\[4pt] [1] Xiao, D., \textit{et al.}, \textit{Phys. Rev. Lett.} \textbf{108}, 196802 (2012). [Preview Abstract] |
Friday, March 22, 2013 1:27PM - 1:39PM |
Z5.00012: Photoresponse of Quasi-One Dimensional Graphene Nanostructures Tu Hong, Zeynab Jarrahi, Yunhao Cao, Alex Huffstutter, Yaqiong Xu Here, we perform simultaneous photocurrent and photoluminescence measurements of free-standing graphene nanostructures. Their photocurrent intensities show a linear relationship with the incident laser power, whereas their photoluminescence intensities increase non-linearly when the incident power rises. The photoluminescence may result from the thermal radiation generated during hot carrier relaxation. The power dependences of their photoluminescence reveal that these graphene nanostructures are quasi-one dimensional materials. [Preview Abstract] |
Friday, March 22, 2013 1:39PM - 1:51PM |
Z5.00013: Hybrid graphene-organic molecule transistors with large photoresponse Shao-Yu Chen, Yi-Ying Lu, Fu-Yu Shih, Po-Hsun Ho, Chun-Wei Chen, Yang-Fang Chen, Yit-Tsong Chen, Wei-Hua Wang We present large photoresponse in hybrid graphene-organic molecule transistors, which exhibit high gain and large responsitivity. High-quality graphene phototransistors are achieved via resist-free fabrication and noncovalent bonding of the organic molecules. The photocurrent of the devices is tunable with back gate which enables high controllability by electrical means. The strong photoresponse can be attributed to charge transfer and photogating effect in the layer of organic molecules. High photo-sensitivity in the hybrid graphene-organic molecule transistors is promising for the future development of graphene-based optoelectronic applications. [Preview Abstract] |
Friday, March 22, 2013 1:51PM - 2:03PM |
Z5.00014: Photocurrent Response of Graphene Heterostructures Joaquin Rodriguez-Nieva, Mildred S. Dresselhaus One of the obstacles to the use of graphene as an alternative to silicon electronics has been the absence of a band gap. One solution to some of the limitations that this obstacle introduces is to integrate graphene into a heterostructure such as a field-effect tunneling transistor that uses an atomically thin dielectric [1]. We explore theoretically some of the interesting properties of optically excited graphene heterostructures, where novel behaviors can appear due to the tunability of the Fermi level and thus, of the charge carrier densities and intrinsic electronic cooling mechanisms. We also discuss possible applications of such types of optically activated heterostructures in different areas of science and engineering. References: [1] L. Britnell, R. V. Gorbachev, R. Jalil, et al., Science, 335, 947-950 (2012) [Preview Abstract] |
Friday, March 22, 2013 2:03PM - 2:15PM |
Z5.00015: Near-field spectroscopy of graphene during ultrafast photoexcitation Martin Wagner, Zhe Fei, Alexander McLeod, Aleksandr Rodin, Wenzhong Bao, Lingfeng Zhang, Zeng Zhao, Eric Iwinski, Mark Thiemens, Michael Fogler, Antonio Castro-Neto, Chunning Lau, Fritz Keilmann, Dimitri Basov Recently, impressive progress in nanoplasmonics of graphene using near-field spectroscopy and imaging has been reported [Z. Fei et al., Nano Lett. 11, 4701 (2011); Z. Fei et al., Nature 487, 82 (2012)]. However, these studies of the interaction of the graphene plasmon with the SiO2 substrate surface phonon were time-independent. Here we combine imaging and material characterization on the nano scale with ultrafast sub-picosecond time resolution and present optical pump broadband mid-infrared probe spectroscopy of graphene. We discuss the optical pump induced changes of the coupled plasmon-phonon modes with respect to carrier density and time-dependence. The difference between ultrafast photoexcitation and conventional electrostatic doping via the field effect is analyzed and compared with modeling. [Preview Abstract] |
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