April 2, 2012

TIM ECHTERMEYER ‘Photodetection in Graphene’

The richness of optical and electronic properties of graphene attracts enormous interest. So far, the main focus has been on fundamental physics and electronic devices. However, it has also great potential in photonics and optoelectronics, where the combination of its unique optical and electronic properties can be fully exploited, the absence of a bandgap can be beneficial, and the linear dispersion of the Dirac electrons enables ultra-wide-band tunability[1]. The rise of graphene in photonics and optoelectronics is shown by several recent results, ranging from solar cells and light emitting devices, to touch screens, photodetectors and ultrafast lasers. Despite being a single atom thick, graphene can be optically visualized. Its transmittance can be expressed in terms of the fine structure constant. The linear dispersion of the Dirac electrons enables broadband applications. Saturable absorption is observed as a consequence of Pauli blocking. Chemical and physical treatments enable luminescence. Graphene-polymer composites prepared using wet chemistry can be integrated in a fiber laser cavity, to generate ultrafast pulses, down to 200fs, and enable broadband tunability .

In this talk I will review and present results on the application of graphene for photodetection. Graphene’s suitability for high-speed photodetection was demonstrated in an optical communication link operating at 10 Gbit s-1. However, the underlying physical mechanism is still under debate as both photo-thermoelectric and photoelectric effects are being suggested. We carry out wavelength and polarization dependent photovoltage mapping of metal-graphene-metal junctions, demonstrating that both effects simultaneously contribute to the photoresponse. These measurements allow us to quantify the wavelength dependent ratio of thermo- vs. photoelectric effects from the visible to the near-infrared.

Overall, the electrical signal produced when shining light on graphene-based photodetectors is small compared to traditional semiconductor based detectors, due to the small 2.3% absorption of graphene. In order to increase the effective light absorption, we integrate plasmonic nanostructures into the devices, so to collect light over a larger area and concentrate the energy into the near-field region where the pn-junction is located. Metal nanogratings and nanodots are fabricated on graphene by e-beam lithography. Photovoltage mapping is then carried out at different gate voltages, laser powers, polarizations and wavelengths. Raman spectroscopy is used to confirm monolayer thickness, probe doping levels and absorption enhancement. We detect up to 20 times photovoltage enhancement in the device with metal nanostructures. Also, we find a wavelength dependent response, tuneable by the geometry of the applied metal nanostructures. Further, the polarization dependence of the incident light with respect to the nanostructures orientation strongly influences the magnitude of the generated photovoltage, being maximum for polarization perpendicular to the axis of our nanogratings.

Seminar, April 2, 2012, 12:00. Seminar Room

Hosted by Prof. Frank Koppens

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January 30, 2012

New Advances in Graphene

The authors demonstrate 100% light absorption can take place in a single patterned sheet of doped graphene. The results are relevant for infrared light detectors and sources, and have been highlighted in Physics Focus and Physics World.

The paper Complete optical absorption in periodically patterned graphene has been published in Physical Review Letters by Dr. Sukosin Thongrattanasiri, from FQRI-CSIC, Madrid; Prof. F. Javier García de Abajo, from FQRI-CSIC and Univ. Southampton; and Prof. Frank Koppens, leader at ICFO of the Nano-optoelectronics group.

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December 1, 2010

FRANCISCO GUINEA ‘Graphene and its Unique Properties’

Graphene, a two dimensional membrane one atom thick, is a novel material which shows features not found previously in other systems. Some of these properties, along with the research effort which is being carried out in order to elucidate their origin …

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October 14, 2010

NICOLAS CAMARA ‘High Quantity and High Quality Graphene Synthesis’

Graphene has already demonstrated its advantage over Silicon in many different aspects: thermal conductivity, mechanical robustness, mobility, sensibility to bio material, biocompatibility, mass less electrons behaving as photons….We can start dreaming about much faster computers without any heat issue consuming almost no energy with very high frequency wireless communications for ultra fast data transfers. Since graphene is a transparent flexible conductor, much cheaper and sustainable than ITO, it will be possible to fabricate flexible screens or touch screens combined with flexible ultra fast electronics.

But first, perfect uniform graphene layers covering a full substrate have to be synthesized for a real microelectronics top down processing. In the talk I will give an overview of our graphene synthesis based on SiC sublimation and explain how we discriminate the number of layers, orientation, defects, uniformity, stress, doping….

Seminar, October 14, 2010, 12:00. Seminar Room

Hosted by Prof. Frank Koppens and Prof. Valerio Pruneri

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August 12, 2010

SUSHANT SONDE ‘Local Transport Properties in Graphene for Electronic Applications’

In view of possible applications in electrostatically tunable two-dimensional field-effect devices, this seminar is aimed at discussing electronic properties in substrate-supported graphene. Original methods based on various variants of Scanning Probe Microscopy techniques will be presented to analyze graphene exfoliated-and-deposited (DG) on SiO2/Si, SiC(0001) and high-κ dielectric substrate (Strontium Titanate) as well as graphene grown epitaxially (EG) on SiC(0001).

Scanning Capacitance Spectroscopy will be discussed as a probe to evaluate the electrostatic properties (quantum capacitance, local density of states) and transport properties (local electron mean free path) in graphene. Furthermore, based on this method two important issues adversely affecting room temperature charge transport in graphene will be addressed to elucidate the role of:

  1. Lattice defects in graphene introduced by ion irradiation and

  2. Charged impurities and Surface Polar Phonon scattering at the graphene/substrate interface.

Moreover, a comparative investigation of current transport across EG/SiC(0001) and DG/SiC(0001) interface by Scanning Current Spectroscopy will be discussed to explain electrical properties of the so-called ‘buffer layer’ commonly observed at the interface of EG/SiC(0001).

Seminar, August 12, 2010, 12:00. Seminar Room

Hosted by Prof. Frank Koppens

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May 27, 2009

Graphene @ ICFO 2009-05-27 21:00:00

The first part of this seminar will be dedicated to graphene. Since its observation in 2005, this truly bi-dimensional material has stimulated numerous studies at the interface between physics, chemistry and materials science. My research at Columbia deals with the optoelectronic properties of graphene layers. In particular, I’ll show that Raman scattering spectroscopy is a relatively simple, but extremely sensitive approach, that makes it possible to harvest useful and quantitative information (number of layers, doping level, influence or disorder, temperature or mechanical stress), that is complementary to electronic transport data. By performing a spatially resolved micro-Raman study on graphene layers partly suspended over micrometer-sized trenches, we have managed to demonstrate that a free standing layer of graphene is virtually undoped and insensitive to its local environment [1]. Without the deleterious perturbations induced by a solid substrate, such free-standing samples make it possible to probe the intrinsic electronic and vibrational properties of graphene. I will introduce some ongoing experiments performed in suspended graphene layers incorporated in field effect transistors.

The second part will review some recent experiments performed on individual carbon nanotubes. These quasi-1D systems can be seen as rolled up graphene sheets. In addition to several properties that derive directly from graphene, the quantum confinement gives rise to a wealth of remarkable (including enhanced excitonic effects) phenomena that can be probed optically. After a brief survey of the optical properties of carbon nanotubes, we will focus on the determination of the structural parameters (diameter, chiral angle) at the individual nanotube level, using various optical spectroscopies (absorption [2], luminescence [2, 3], Rayleigh [4] and Raman Scattering). If time allows, a study of the luminescence dynamics at the single nanotube level will be presented [3]. This work allowed us to probe the fine structure of the band edge exciton and to deduce the resonant absorption cross section of an individual nanotube of known structure.

    References :

  • [1] S. Berciaud, S. Ryu, L. E. Brus, T. F. Heinz, Nano Letters 9 346 (2009)
  • [2] S. Berciaud, L. Cognet, P. Poulin, R. B. Weisman & B. Lounis, Nano Letters 7, 1203 (2007).
  • [3] S. Berciaud, L. Cognet & B. Lounis, Phys. Rev. Lett. 101, 077402 (2008)
  • [4] M.Y. Sfeir, F. Wang, L. Huang, C. Chuang, J. Hone, S. O’Brien, T.F. Heinz, L.E. Brus, Science

    306, 1540 (2004).

Seminar, May 27, 2009, 10:00. Seminar Room

Hosted by ICFO

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