The photoresponse of semiconductors, which determines the performance of optoelectronic devices, is governed by energy relaxation pathways of photoexcited electron-hole (e-h) pairs: energy transferred to the lattice is lost as heat while energy transported through charge carriers may be used to drive an optoelectronic circuit ^[1] . In graphene, energy relaxation pathways are strongly altered by the vanishing electronic density of states. After initial relaxation of photo-excited carriers caused by electron-electron scattering and optical phonon emission, electron-lattice energy relaxation can be quenched ^[2] , resulting in a novel transport regime in which thermal energy is redistributed solely among electronic charge carriers.

While graphene is considered an excellent candidate for photodetection and energy-harvesting applications due to its broadband optical response and high internal quantum efficiency, measurements have not clearly determined the photocurrent generation mechanism. Here, we report on the intrinsic photoresponse of dual-gated monolayer and bilayer graphene p-n junction devices (Figure 1). Local laser excitation of wavelength 850 nm at the p-n interface leads to striking six-fold photovoltage patterns as a function of bottom- and top-gate voltages (Figure 2). These patterns, together with the measured spatial and density dependence of the photoresponse, provide strong evidence that non-local hot carrier transport, rather than the photovoltaic effect, dominates the intrinsic photoresponse in graphene ^{[3] [4]} . The hot carrier regime manifests as a strong photo-thermoelectric effect in which the photogenerated carrier population remains hot while the lattice stays cool.

1. S. M. Sze, Physics of Semiconductor Devices, 2^nd ed., London: Wiley, Dec. 1981.
2. R. Bistritzer and A. H. MacDonald, “Electronic cooling in graphene,” Physical Review Letters, vol. 102, p. 206410, May 2009.
3. N. M. Gabor, J. C. W. Song, Q. Ma, N. Nair, T. Taychatanapat, K. Watanabe, T. Taniguchi, L. S. Levitov, and P. Jarillo-Herrero, “Hot carrier-assisted intrinsic photoresponse in graphene,” Science, vol. 334, pp. 648-652, Oct. 2011.
4. J. C. W. Song, M. S. Rudner, C. M. Marcus, and L. S. Levitov, “Hot carrier transport and photoresponse in graphene,” Nano Letters, vol. 11, pp. 4688-4692, Sep. 2011.

The bilayer 2-dimensional electron gas (2DEG) consists of two closely spaced 2DEGs, between which Coulomb interactions and tunneling effects can lead to new behaviors which are absent in the individual layers ^{[1] [2] [3]} .  In these bilayers, an insulating spacer is necessary to separate the 2DEG layers.  In twisted bilayer graphene, layers can be stacked directly on each other yet still retain a degree of independence.  This independence is possible because of the carbon honeycomb lattice of graphene, which results in weak coupling between layers ^[4] and a circular Fermi surface centered at nonzero K vectors ^[5] .  The latter is key, because a relative twist angle between the graphene bilayer lattices can cause the Fermi surfaces of the layers to not overlap at low densities (Figure 1).  This fact preserves the linear Dirac dispersion in the twisted bilayer graphene ^{[1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11]} but with twice the number of Dirac cones due to the two layers ^{[1] [3] [6]} .

We measure the magnetoresistance of dual-gated twisted bilayer graphene devices, which exhibit the quantum Hall effect and magnetoresistance oscillations of two monolayer graphene sheets conducting in parallel. As we vary the gate voltages, we observe inter-layer Landau level crossings, allowing us to quantify the layer charge transfer and finite screening effects between the layers. This incomplete screening of the applied field, due to graphene’s small density of states and the close spacing between the layers, lets us extract the inter-layer capacitance of the atomically-spaced graphene sheets. At high magnetic fields, we observe a pattern of insulating states centered at zero density originating from layer-polarized edge modes.

Figure 1: Magnetoresistance studies of twisted bilayer graphene. (a) Lattice structure. (b) Twist angle separates the Fermi surface of each layer in K-space. (c) Device schematic (d) Magnetoresistance as a function of filling factor ν_tot and displacement field at B=4Tesla. Peaks in magnetoresistance are due to Landau levels. Landau levels of each layer move in different directions with displacement field. (e) Insulating states due to electron-electron interactions develop in twisted bilayers at very high magnetic field, which varies with filling factor and displacement field.

1. G. Boebinger, H. Jiang, L. Pfeiffer, and K. West, “Magnetic-field-driven destruction of quantum Hall states in a double quantum well,” Physical Review Letters, vol. 64, no. 15, pp. 1793-1796, Apr. 1990.
2. T. J. Gramila, J. P. Eisenstein, A. H. MacDonald, L. N. Pfeiffer, and K. W. West, “Mutual friction between parallel two-dimensional electron systems,” Phys. Rev. Lett., vol. 66, no. 9, pp. 1216-1219, Mar. 1991.
3. J. P. Eisenstein and A. H. Macdonald, “Bose-Einstein condensation of excitons in bilayer electron systems,” Nature, vol. 432, no. 7018, pp. 691-4, Dec. 2004.
4. M. S. Dresselhaus and G. Dresselhaus, “Intercalation compounds of graphite,” Advances in Physics, vol. 51, no. 1, pp. 1-186, 2002.
5. A. H. Castro Neto, F. Guinea, N. M. R. Peres, K. S. Novoselov, and A. K. Geim, “The electronic properties of graphene,” Reviews of Modern Physics, vol. 81, no. 1, pp. 109-162, 2009.
6. J. M. B. Lopes dos Santos, N. M. R. Peres, and A. H. Castro Neto, “Graphene Bilayer with a Twist: Electronic Structure,” Physical Review Letters, vol. 99, no. 25, p. 256802, Dec. 2007.
7. J. Hass, F. Varchon, J. E. Millán-Otoya, M. Sprinkle, N. Sharma, W. A. de Heer, C. Berger, P. N. First, L. Magaud, and E. H. Conrad, “Why Multilayer Graphene on 4H-SiC(0001¯) Behaves Like a Single Sheet of Graphene,” Physical Review Letters, vol. 100, no. 12, p. 125504, Mar. 2008.
8. H. Schmidt, T. Lüdtke, P. Barthold, E. McCann, V. I. Fal’ko, and R. J. Haug, “Tunable graphene system with two decoupled monolayers,” Applied Physics Letters, vol. 93, no. 17, p. 172108, Oct. 2008.
9. G. Li, A. Luican, J. M. B. Lopes dos Santos, A. H. Castro Neto, A. Reina, J. Kong, and E. Y. Andrei, “Observation of Van Hove singularities in twisted graphene layers,” Nature Physics, vol. 6, no. 2, pp. 109-113, Nov. 2009.
10. A. Luican, G. Li, A. Reina, J. Kong, R. R. Nair, K. S. Novoselov, A. K. Geim, and E. Y. Andrei, “Single-Layer Behavior and Its Breakdown in Twisted Graphene Layers,” Physical Review Letters, vol. 106, no. 12, p. 126802, Mar. 2011.
11. H. Schmidt, T. Lüdtke, P. Barthold, and R. J. Haug, “Mobilities and scattering times in decoupled graphene monolayers,” Physical Review B, vol. 81, no. 12, Mar. 2010.

Postdoctoral Associates

• Leonardo Campos
• Nathaniel Gabor
• Tchefor Ndukum
• Hadar Steinberg

Collaborators

• Nuh Gedik (MIT)
• Brian LeRoy (U of Arizona)
• Tomás Palacios (MIT)
• Amir Yacoby (Harvard)
• Michael Strano (MIT)
• Jing Kong (MIT)
• Karl Berggren (MIT)
• Charles Marcus (Harvard)
• Silvano De Franceschi (CEA)
• Cees Dekker (TU Delft)
• Philip Kim (Columbia)
• Leo Kouwenhoven (TU Delft)
• Leonid Levitov (MIT)
• Alberto Morpurgo (TU Delft)
• Barbaros Oezyilmaz (NUS)
• Lieven Vandersypen (TU Delft)

Graduate Students

• Britt Baugher – MIT Physics
• Valla Fatemi – MIT Physics
• Qiong Ma – MIT Physics
• Javier Sanchez-Yamagishi – MIT Physics
• Thiti Taychatanapat – Harvard Physics
• Joel I-Jan Wang – Harvard SEAS

Support Staff

• Monica Wolf, Administrative Assistant

Publications

(View full list of publications at:

http://jarilloherrero.mit.edu/)

Yankowitz, M., Xue, J., Cormode, D., Sanchez-Yamagishi, J. D., Watanabe, K., Taniguchi, T., Jarillo-Herrero, P., Jacquod, P., LeRoy. B.J., “Emergence of Superlattice Dirac Points in Graphene on Hexagonal Boron Nitride”, Nature Physics 2012 (advance online publication).

Sanchez-Yamagishi, J. D., Taychatanapat, T., Watanabe, K., Taniguchi, T., Yacoby, A., Jarillo-Herrero, P., “Quantum Hall Effect, Screening and Layer-Polarized Insulating States in Twisted Bilayer Graphene“ Physical Review Letters 108, 076601 (2012).

Xue, J., Sanchez-Yamagishi, J. D., Watanabe, K., Taniguchi, T., Jarillo-Herrero, P., LeRoy, B. J., “Long wavelength local density of states oscillations near graphene step edges”, Physical Review Letters 108, 016801 (2012).

McIver, J. W., Hsieh, D., Steinberg, H., Jarillo-Herrero, P., Gedik, N., “Control Over Topological Insulator Photocurrents with Light Polarization”, Nature Nanotechnology 7, 96–100 (2012).

Steinberg, H., Laloë, J.-B., Fatemi, V., Moodera, J. S., Jarillo-Herrero, P., “Electrically tunable surface-to-bulk coherent coupling in topological insulator thin films”, Physical Review B, 84, 233101 (2011), highlighted at condmatjournalclub.org with a commentary by Leonid Glazman.

Gabor, N. M., Song, J. C. W., Ma, Q., Nair, N. L., Taychatanapat, T., Watanabe, K., Taniguchi, T., Levitov, L. S., Jarillo-Herrero, P., “Hot Carrier Assisted Intrinsic Photoresponse in Graphene”, Science 334, 648-652 (2011).

Wang, H., Taychatanapat, T., Hsu, A., Watanabe, K., Taniguchi, T., Jarillo-Herrero, P., Palacios, T., “BN/Graphene/BN Transistors for RF Applications”, IEEE Elec. Dev. Lett. 32, 1209-1211 (2011).

Taychatanapat, T., Watanabe, K., Taniguchi, T., Jarillo-Herrero, P., “Quantum Hall effect and Landau level crossing of Dirac fermions in trilayer graphene”, Nature Physics 7, 621-625 (2011) (also see News & Views from Nature Physics).

Xue, J., Sanchez-Yamagishi, J., Bulmash, D., Jacquod, P., Deshpande, A., Watanabe, K., Taniguchi, T., Jarillo-Herrero, P., LeRoy, B. J., “Scanning tunnelling microscopy and spectroscopy of ultra-flat graphene on hexagonal boron nitride”, Nature Materials 10, 282-285 (2011).