Bulletin of the American Physical Society
APS March Meeting 2011
Volume 56, Number 1
Monday–Friday, March 21–25, 2011; Dallas, Texas
Session V36: Graphene: Optical Properties I |
Hide Abstracts |
Sponsoring Units: DCMP Chair: Farhan Rana, Cornell University Room: C142 |
Thursday, March 24, 2011 8:00AM - 8:12AM |
V36.00001: Imaging stacking order in few-layer graphene Chun Hung Lui, Zhiqiang Li, Zheyuan Chen, Paul V. Klimov, Louis E. Brus, Tony F. Heinz Few-layer graphene (FLG) has been predicted to exist in various crystallographic stacking sequences, which can strongly influence the material's electronic properties We demonstrate an accurate and efficient method of characterizing stacking order in FLG using the distinctive features of the Raman 2D-mode. Raman mapping allows us to visualize directly the spatial distribution of Bernal (ABA) and rhombohedral (ABC) stacking in tri- and tetra-layer graphene. We find that $\sim $15{\%} of exfoliated graphene tri- and tetra-layers is comprised of micron-sized domains with rhombohedral stacking, rather than the Bernal stacking. These domains are stable and remain unchanged for annealing to temperatures exceeding 800 $^{\circ}$C. [Preview Abstract] |
Thursday, March 24, 2011 8:12AM - 8:24AM |
V36.00002: Electric-field induced changes in the band structure of trilayer graphene: The effect of crystallographic stacking order Zhiqiang Li, Chun Hung Lui, Kin Fai Mak, Emmanuele Cappelluti, Tony F. Heinz We have studied by means of infrared spectroscopy the influence of a strong perpendicular electric-field on the band structure of graphene trilayers with two different types of crystallographic stacking: ABA (Bernal) and ABC (rhombohedral) stacking. The symmetries of the two crystallographic structures are different, the former having mirror symmetry and the latter inversion symmetry. Distinct infrared response was observed when breaking their respective symmetries by the application of the electric field. We observed an electrically tunable band gap of over 100 meV in ABC trilayers, while no band gap was found for ABA trilayers. Our results will be compared to the induction of a band gap in AB bilayer graphene [K. F. Mak \textit{et al}, PRL \textbf{102}, 256405 (2009); Y. Zhang \textit{et al}, Nature \textbf{459}, 820 (2009)] [Preview Abstract] |
Thursday, March 24, 2011 8:24AM - 8:36AM |
V36.00003: Photoluminescence in highly doped graphene Baisong Geng, Chi-Fan Chen, Liang Zheng Tan, Bryan W. Boudouris, Jason Horng, Caglar Girit, Alex Zettl, Michael F. Crommie, Rachel Segalman, Steve G. Louie, Feng Wang Pristine graphene is a zero-bandgap semiconductor. Usually no photoluminescence can be observed from such zero-bandgap material upon laser excitation. In highly doped graphene, however, we observed a strong broadband photoluminescence. We will discuss the mechanism of this photoluminescence in graphene, which arises from new recombination pathways enabled by strong electrical doping. We will also describe the polarization dependence of this newly observed photoluminescence. [Preview Abstract] |
Thursday, March 24, 2011 8:36AM - 8:48AM |
V36.00004: Response of graphene to intense optical irradiation Adam Roberts, Collin Reynolds, Daniel Hemmer, Brian Leroy, Arvinder Sandhu We investigate the modification of graphene under intense ultrashort laser irradiation. Our observations indicate that the graphene structure is very resilient and exhibits a high damage threshold, which is promising for high order non-linear applications. In the case of epitaxially grown samples, we find that single-shot damage threshold is 5x10$^{10 }$Wcm$^{-2}$ for 50 fs pulse duration. Raman and optical microscopy measurements of irradiated samples show that the carbon lattice completely disappears from the region where the laser intensity exceeds the threshold without leaving any visual or spectroscopic signature. Below the threshold, single-shot irradiation does not exhibit a significant defect formation. However, repeated laser irradiation below the threshold leads to formation of defects. The mechanisms underlying the defect formation and lattice reduction will be discussed. [Preview Abstract] |
Thursday, March 24, 2011 8:48AM - 9:00AM |
V36.00005: Mid Infrared Near Field Study of Monolayer Graphene Z. Fei, G.O. Andreev, W. Bao, L.M. Zhang, Z. Zhao, G. Dominguez, M. Thiemens, M.M. Fogler, C.N. Lau, F. Keilmann, D.N. Basov We have performed near-field spectroscopic studies of both monolayer suspended graphene (SG) and graphene on SiO$_{2}$/Si substrate (GOS) using scattering-type scanning near-field optical microscope (s-SNOM). Our data show that SG produces reliable near-field signal in mid-infrared frequencies. Images taken with high spatial resolution ($\sim $20nm) show nanoscopic features such as ripples and electronic inhomogeneities. The SiO$_{2}$/Si substrate contributes a phonon resonance in the near-field signal around 1130 cm$^{-1}$. This resonance is remarkably strengthened and broadened by just a single layer of graphene in the case of GOS. By probing the resonance spectrum we find over 400{\%} contrast in near field signal between GOS and the bare substrate. The detailed analysis of the contrast suggests that GOS is slightly doped. This study therefore provides much needed insight into the thickness resolution of the s-SNOM technique, proving it can be sensitive to just a single layer of atoms, and advances the fundamental understanding of graphene-light interactions by probing in the near-field regime. [Preview Abstract] |
Thursday, March 24, 2011 9:00AM - 9:12AM |
V36.00006: Terahertz and Infrared Conductivity of Large-Area Graphene Lei Ren, Qi Zhang, Takashi Arikawa, Layla G. Booshehri, Junichiro Kono, Zhong Jin, Zhengzong Sun, Zheng Yan, James M. Tour Graphene is predicted to offer new opportunities for terahertz (THz) science and technology. Its zero-gap linear band dispersion is expected to lead to exotic nonlinear electromagnetic properties, which can be probed through frequency-dependent conductivity measurements. Here, we use THz time-domain spectroscopy and Fourier-transform infrared spectroscopy to investigate carrier dynamics in large-area graphene grown by chemical vapor deposition. We studied both nitrogen-doped and nominally-undoped graphene; the latter had accidental doping presumably through air and acid exposure. Absorption increased with the number of graphene layers and was larger in the nominally-undoped samples especially in the 0.2-2.2 THz range. For the highest-mobility samples, we observed Drude-like frequency dependence in the THz range. Further measurements in a wider spectral range are in progress to understand the differences between these samples and the interplay between intra-band and inter-band dynamics. [Preview Abstract] |
Thursday, March 24, 2011 9:12AM - 9:24AM |
V36.00007: Carrier Cooling in Graphene Measured by THz Time-Domain Spectroscopy Jared Strait, Haining Wang, Shriram Shivaraman, Virgil Shields, Carlos Ruiz-Vargas, Jiwoong Park, Michael Spencer, Farhan Rana We present results on the ultrafast relaxation dynamics of photoexcited electrons and holes in graphene using optical-pump terahertz-probe spectroscopy. Measurements done at different temperatures show that the measured differential transmission as a function of the probe delay decays on time scales that become very long at low temperatures with decay times exceeding $\sim$150 ps at temperatures lower than $\sim$50K. We interpret these transients as carrier cooling due to a combination of electron-optical phonon and electron-acoustic phonon scattering. When the carrier temperature goes below $\sim$250 K, optical-phonon scattering ceases to effectively cool the carriers given the large optical phonon energies in graphene. Since acoustic phonon scattering is not efficient in removing the heat from the carriers, the carrier distribution cools very slowly. Our data is in agreement with the theoretical predictions [1].\\[4pt] [1] Phys. Rev. B 79, 235406 (2009) and Phys. Rev. Lett., 102, 206410 (2009). [Preview Abstract] |
Thursday, March 24, 2011 9:24AM - 9:36AM |
V36.00008: Ultrafast electron dynamics in freely suspended graphene Leandro Malard, Kin Fai Mak, Tony F. Heinz The optical conductivity of free-standing graphene under the non-equilibrium conditions was investigated by femtosecond pump-probe spectroscopy. The conductivity transient exhibited a strong dependence on pump fluence, with a crossover from enhanced to reduced absorbance occurring with increasing pump fluence. The observed phenomena can be understood by taking into account both the induced intra- and inter-band optical response. Intra-band transitions dominate the transient at low pump fluence (and electronic temperature) and inter-band transitions dominate at high pump fluence (and electronic temperature). Analysis within a model incorporating these two responses allows us to infer the variation of carrier scattering rate with electronic temperature. The temporal evolution of the conductivity transient is controlled by the anharmonic decay of the optical phonons; a lifetime of $\sim$1.4 ps was inferred for intrinsic, suspended graphene. [Preview Abstract] |
Thursday, March 24, 2011 9:36AM - 9:48AM |
V36.00009: Ultrafast dynamics of highly-excited Dirac fermions in monolayer graphene Junhua Zhang, J\"org Schmalian, Tianqi Li, Jigang Wang One of the striking optical properties of single-layer graphene is the universal absorbance in the near-infrared-to-visible spectral range due to the Dirac spectrum of the low-energy electronic structure. High-fluence laser pump can produce superdense Dirac-fermionic excitations at the order of 10 femtoseconds so to reach the non-linear saturation of absorption. We construct a simple model for the transient state of the photo-excited graphene to explore the non-linear saturation of photoexcitations and the transport property of carries. The comparison of our model calculations with the experimental results shows good agreements. [Preview Abstract] |
Thursday, March 24, 2011 9:48AM - 10:00AM |
V36.00010: Ultrafast relativistic response of Photo-excited Carriers in Graphene J. Lee, K.M. Dani, R. Sharma, A.D. Mohite, A.M. Dattelbaum, H. Htoon, A.J. Taylor, R.P. Prasankumar, C.M. Galande, P.M. Ajayan Understanding the ultrafast non-equilibrium dynamics of photocarriers in graphene's unique relativistic band structure is important for the development of such high-speed, graphene-based photonic devices and also from a fundamental point of view. Here, we directly demonstrate the relativistic nature of a non-equilibrium gas of electrons and holes photogenerated in a graphene monolayer as early as 100 femtoseconds (fs) after photoexcitation. We photoexcited carriers in graphene and then measured the time-resolved, pump-induced change in reflection at various visible probe photon energies. We observe a nonlinear scaling in the Drude-like optical conductivity of the photocarriers with respect to their density, in striking contrast to the linear scaling expected from conventional materials with parabolic dispersion relations. [Preview Abstract] |
Thursday, March 24, 2011 10:00AM - 10:12AM |
V36.00011: Carrier Dynamics in Colloidal Graphene Quantum Dots Cheng Sun, Xin Yan, Liang-shi Li, John A. McGuire We describe carrier dynamics for single and multiple excitons in colloidal graphene quantum dots (GQDs). Strong confinement and corresponding size-tunable electronic structure make GQDs potentially useful sensitizers in photovoltaic devices. We have studied the optical response of GQDs consisting of 132 and 168 sp$^{2}$ hybridized carbon atoms dissolved in toluene with HOMO-LUMO transitions of 1.4-1.6 eV. From measurements of ultrafast ($\sim$100 fs) transient absorption over nanosecond timescales, we extract the single-photon absorption cross-section and observe carrier-induced Stark shifts of the order of 0.1 eV indicating strong carrier-carrier interactions, as expected for the relatively weak screening of a two-dimensional nanostructure. Multiexcitons are observed to decay nonradiatively on $\sim$1 to 20 ps timescales, while single excitons display dynamics on multiple timescales due to carrier cooling, singlet-to-triplet intersystem crossing, and, on nanosecond to microsecond timescales, radiative recombination. [Preview Abstract] |
Follow Us |
Engage
Become an APS Member |
My APS
Renew Membership |
Information for |
About APSThe American Physical Society (APS) is a non-profit membership organization working to advance the knowledge of physics. |
© 2024 American Physical Society
| All rights reserved | Terms of Use
| Contact Us
Headquarters
1 Physics Ellipse, College Park, MD 20740-3844
(301) 209-3200
Editorial Office
100 Motor Pkwy, Suite 110, Hauppauge, NY 11788
(631) 591-4000
Office of Public Affairs
529 14th St NW, Suite 1050, Washington, D.C. 20045-2001
(202) 662-8700