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Hipolito F.,National University of Singapore | Pedersen T.G.,University of Aalborg | Pedersen T.G.,Center for Nanostructured Graphene | Pereira V.M.,National University of Singapore
Physical Review B - Condensed Matter and Materials Physics | Year: 2016

The dc photoelectrical currents can be generated purely as a nonlinear effect in uniform media lacking inversion symmetry without the need for a material junction or bias voltages to drive it, in what is termed photogalvanic effect. These currents are strongly dependent on the polarization state of the radiation, as well as on topological properties of the underlying Fermi surface such as its Berry curvature. In order to study the intrinsic photogalvanic response of gapped graphene, biased bilayer graphene (BBG), and hexagonal boron nitride (hBN), we compute the nonlinear current using a perturbative expansion of the density matrix. This allows a microscopic description of the quadratic response to an electromagnetic field in these materials, which we analyze as a function of temperature and electron density. We find that the intrinsic response is robust across these systems and allows for currents in the range of pA cm/W to nA cm/W. At the independent-particle level, the response of hBN-based structures is significant only in the ultraviolet due to their sizable band gap. However, when Coulomb interactions are accounted for by explicit solution of the Bethe-Salpeter equation, we find that the photoconductivity is strongly modified by transitions involving exciton levels in the gap region, whose spectral weight dominates in the overall frequency range. Biased bilayers and gapped monolayers of graphene have a strong photoconductivity in the visible and infrared window, allowing for photocurrent densities of several nA cm/W. We further show that the richer electronic dispersion of BBG at low energies and the ability to change its band gap on demand allows a higher tunability of the photocurrent, including not only its magnitude but also, and significantly, its polarity. © 2016 American Physical Society.


Pedersen T.G.,University of Aalborg | Pedersen T.G.,Center for Nanostructured Graphene
Physical Review B - Condensed Matter and Materials Physics | Year: 2015

Dopants positioned near edges in nanostructured graphene behave differently from bulk dopants. Most notable, the amount of charge transferred to delocalized states (i.e., doping efficiency) depends on position as well as edge chirality. We apply a self-consistent tight-binding model to analyze this problem focusing on substitutional nitrogen and boron doping. Using a Green's-function technique, very large structures can be studied, and artificial interactions between dopants in periodically repeated simulations cells are avoided. We find pronounced signatures of edges in the local impurity density of states. Importantly, the doping efficiency is found to oscillate with sublattice position, in particular, for dopants near zigzag edges. Finally, to assess the effect of electron-electron interactions, we compute the self-energy corrected Green's function. © 2015 American Physical Society.


Thomsen M.R.,University of Aalborg | Thomsen M.R.,Center for Nanostructured Graphene | Brun S.J.,University of Aalborg | Brun S.J.,Center for Nanostructured Graphene | And 2 more authors.
Journal of Physics Condensed Matter | Year: 2014

In order to use graphene for semiconductor applications, such as transistors with high on/off ratios, a band gap must be introduced into this otherwise semimetallic material. A promising method of achieving a band gap is by introducing nanoscale perforations (antidots) in a periodic pattern, known as a graphene antidot lattice (GAL). A graphene antidot barrier (GAB) can be made by introducing a 1D GAL strip in an otherwise pristine sheet of graphene. In this paper, we will use the Dirac equation (DE) with a spatially varying mass term to calculate the electronic transport through such structures. Our approach is much more general than previous attempts to use the Dirac equation to calculate scattering of Dirac electrons on antidots. The advantage of using the DE is that the computational time is scale invariant and our method may therefore be used to calculate properties of arbitrarily large structures. We show that the results of our Dirac model are in quantitative agreement with tight-binding for hexagonal antidots with armchair edges. Furthermore, for a wide range of structures, we verify that a relatively narrow GAB, with only a few antidots in the unit cell, is sufficient to give rise to a transport gap. © 2014 IOP Publishing Ltd.


Brun S.J.,University of Aalborg | Brun S.J.,Center for Nanostructured Graphene | Thomsen M.R.,University of Aalborg | Thomsen M.R.,Center for Nanostructured Graphene | And 2 more authors.
Journal of Physics Condensed Matter | Year: 2014

The electronic properties of graphene may be changed from semimetallic to semiconducting by introducing perforations (antidots) in a periodic pattern. The properties of such graphene antidot lattices (GALs) have previously been studied using atomistic models, which are very time consuming for large structures. We present a continuum model that uses the Dirac equation (DE) to describe the electronic and optical properties of GALs. The advantages of the Dirac model are that the calculation time does not depend on the size of the structures and that the results are scalable. In addition, an approximation of the band gap using the DE is presented. The Dirac model is compared with nearest-neighbour tight-binding (TB) in order to assess its accuracy. Extended zigzag regions give rise to localized edge states, whereas armchair edges do not. We find that the Dirac model is in quantitative agreement with TB for GALs without edge states, but deviates for antidots with large zigzag regions. © 2014 IOP Publishing Ltd.


Pedersen T.G.,University of Aalborg | Pedersen T.G.,Center for Nanostructured Graphene | Pedersen J.G.,University of Aalborg
Journal of Applied Physics | Year: 2012

Periodic arrays of antidots, i.e., nanoscale perforations, in graphene enable tight confinement of carriers and efficient transport barriers. Such barriers evade the Klein tunneling mechanism by being of the mass rather than electrostatic type. While all graphene antidot lattices (GALs) may support directional barriers, we show, however, that a full transport gap exists only for certain orientations of the GAL. Moreover, we assess the applicability of gapped graphene and the Dirac continuum approach as simplified models of various antidot structures showing that, in particular, the former is an excellent approximation for transport in GALs supporting a bulk band gap. Finally, the transport properties of a GAL based resonant tunneling diode are analyzed indicating that such advanced graphene based devices may, indeed, be realized using GAL structures. © 2012 American Institute of Physics.


Pedersen T.G.,University of Aalborg | Pedersen T.G.,Center for Nanostructured Graphene | Pedersen J.G.,Technical University of Denmark
Physical Review B - Condensed Matter and Materials Physics | Year: 2013

Boron and nitrogen substitutional impurities in graphene are analyzed using a self-consistent tight-binding approach. An analytical result for the impurity Green's function is derived taking broken electron-hole symmetry into account and validated by comparison to numerical diagonalization. The impurity potential depends sensitively on the impurity occupancy, leading to a self-consistency requirement. We solve this problem using the impurity Green's function and determine the self-consistent local density of states at the impurity site and, thereby, identify acceptor and donor energy resonances. © 2013 American Physical Society.


Christensen T.,Center for Nanostructured Graphene | Yan W.,Center for Nanostructured Graphene | Raza S.,Center for Electronic Nanoscopy | Jauho A.-P.,Technical University of Denmark | And 2 more authors.
ACS Nano | Year: 2014

Inspired by recent measurements on individual metallic nanospheres that cannot be explained with traditional classical electrodynamics, we theoretically investigate the effects of nonlocal response by metallic nanospheres in three distinct settings: atomic spontaneous emission, electron energy loss spectroscopy, and light scattering. These constitute two near-field and one far-field measurements, with zero-, one-, and two-dimensional excitation sources, respectively. We search for the clearest signatures of hydrodynamic pressure waves in nanospheres. We employ a linearized hydrodynamic model, and Mie-Lorenz theory is applied for each case. Nonlocal response shows its mark in all three configurations, but for the two near-field measurements, we predict especially pronounced nonlocal effects that are not exhibited in far-field measurements. Associated with every multipole order is not only a single blueshifted surface plasmon but also an infinite series of bulk plasmons that have no counterpart in a local-response approximation. We show that these increasingly blueshifted multipole plasmons become spectrally more prominent at shorter probe-to-surface separations and for decreasing nanosphere radii. For selected metals, we predict hydrodynamic multipolar plasmons to be measurable on single nanospheres. © 2014 American Chemical Society.


Pedersen T.G.,University of Aalborg | Pedersen T.G.,Center for Nanostructured Graphene
Physical Review B - Condensed Matter and Materials Physics | Year: 2015

A theory for the nonlinear excitonic optical response of semiconductors is developed. By adopting the length gauge, intraband effects are rigorously taken into account. We show that the second-order nonlinear response mixing intra- and interband transitions can be expressed in terms of generalized derivatives of the exciton Green's function. The theory is applied to hexagonal boron-nitride monolayers. For both the linear and nonlinear response, a dramatic influence of excitons is found. Hence, new discrete resonances appear as well as pronounced changes in the continuum spectrum. © 2015 American Physical Society.


Pedersen T.G.,University of Aalborg | Pedersen T.G.,Center for Nanostructured Graphene
Physical Review B - Condensed Matter and Materials Physics | Year: 2016

Excitons in transition-metal dichalcogenides can be dynamically manipulated using electrostatic fields. Analyzing both in-plane and out-of-plane fields, I compute the exciton Stark shift and electroabsorption spectrum of monolayer MoS2,MoSe2,WS2, and WSe2. The effect of in-plane fields is found to greatly surpass that of out-of-plane fields. In particular, if exciton binding is reduced through screening by surrounding dielectrics, such as in MoS2 encapsulated by hexagonal boron-nitride, the in-plane exciton polarizability exceeds the measured out-of-plane value by nearly two orders of magnitude. Accordingly, pronounced electroabsorption features are expected for fields as low as 10Vμm-1. © 2016 American Physical Society.


News Article | November 14, 2016
Site: www.eurekalert.org

New research demonstrates the highest plasmon energy ever observed in graphene plasmons and brings graphene into the regime of telecommunication applications WASHINGTON - Graphene's unique properties can be both a blessing and a curse to researchers, especially to those at the intersection of optical and electronic applications. These single-atom thick sheets feature highly mobile electrons on their flexible profiles, making them excellent conductors, but in general graphene sheets do not interact with light efficiently. Problematic for shorter wavelength light, photons in the near infrared region of the spectrum, where telecommunication applications become realizable. In a paper published this week in the journal Optics Letters, from The Optical Society (OSA), researchers at the Technical University of Denmark have demonstrated, for the first time, efficient absorption enhancement at a wavelength of 2 micrometers by graphene, specifically by the plasmons of nanoscale graphene disks. Much like water ripples arising from the energy of a dropped pebble, electronic oscillations can arise in freely moving conduction electrons by absorbing light energy. The resulting collective, coherent motions of these electrons are called plasmons, which also serve to amplify the strength of the absorbed light's electric field at close proximity. Plasmons are becoming increasingly commonplace in various optoelectronic applications where highly conductive metals can be easily integrated. Graphene plasmons, however, face an extra set of challenges unfamiliar to the plasmons of bulk metals. One of these challenges is the relatively long wavelength needed to excite them. Many efforts taking advantage of the enhancing effects of plasmons on graphene have demonstrated promise, but for low energy light. "The motivation of our work is to push graphene plasmons to shorter wavelengths in order to integrate graphene plasmon concepts with existing mature technologies," said Sanshui Xiao, associate professor from the Technical University of Denmark. To do so, Xiao, Wang and their collaborators took inspiration from recent developments at the university's Center of Nanostructured Graphene (CNG), where they demonstrated a self-assembly method resulting in large arrays of graphene nanostructures. Their method primarily uses geometry to bolster the graphene plasmon effects at shorter wavelengths by decreasing the size of the graphene structures. Using lithographic masks prepared by a block copolymer based self-assembly method, the researchers made arrays of graphene nanodisks. They controlled the final size of the disks by exposing the array to oxygen plasma which etched away at the disks, bringing the average diameter down to approximately 18 nm. This is approximately 1000 times smaller than the width of a human hair. The array of approximately 18 nm disks, resulting from 10 seconds of etching with oxygen plasma, showed a clear resonance with 2 micrometer wavelength light, the shortest wavelength resonance ever observed in graphene plasmons. An assumption might be that longer etching times or finer lithographic masks, and therefore smaller disks, would result in even shorter wavelengths. Generally speaking this is true, but at 18 nm the disks already start requiring consideration of atomic details and quantum effects. Instead, the team plans to tune graphene plasmon resonances at smaller scales in the future using electrical gating methods, where the local concentration of electrons and electric field profile alter resonances. Xiao said, "To further push graphene plasmons to shorter wavelengths, we plan to use electrical gating. Instead of graphene disks, graphene antidots (i.e. graphene sheets with regular holes) will be chosen because it is easy to implement a back-gating technique." There are also fundamental limits to the physics that prevent shortening the graphene plasmon resonance wavelength with more etching. "When the wavelength becomes shorter, the interband transition will soon play a key role, leading to broadening of the resonance. Due to weak coupling of light with graphene plasmons and this broadening effect, it will become hard to observe the resonance feature," Xiao explained. This project is supported by Danish National Research Foundation Center for Nanostructured Graphene (DNRF103). Paper: Z. Wang, T. Li, K. Almdal, N. Mortensen, S. Xiau and S. Ndoni, "Experimental demonstration of graphene plasmons working close to the near-infrared window," Opt. Lett. 41, 5345-5348. DOI: 10.1364/OL.41.005345 Optics Letters offers rapid dissemination of new results in all areas of optics with short, original, peer-reviewed communications. Optics Letters covers the latest research in optical science, including optical measurements, optical components and devices, atmospheric optics, biomedical optics, Fourier optics, integrated optics, optical processing, optoelectronics, lasers, nonlinear optics, optical storage and holography, optical coherence, polarization, quantum electronics, ultrafast optical phenomena, photonic crystals and fiber optics. Founded in 1916, The Optical Society (OSA) is the leading professional organization for scientists, engineers, students and business leaders who fuel discoveries, shape real-life applications and accelerate achievements in the science of light. Through world-renowned publications, meetings and membership initiatives, OSA provides quality research, inspired interactions and dedicated resources for its extensive global network of optics and photonics experts. For more information, visit osa.org/100.

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