FREITAG ET AL. VOL. 8 ’ NO. 8 ’ 8350–8356 ’ 2014

www.acsnano.org

8350

July 17, 2014

C 2014 American Chemical Society

Substrate-Sensitive Mid-infraredPhotoresponse in GrapheneMarcus Freitag,†,^,* Tony Low,†,^ Luis Martin-Moreno,‡,^ Wenjuan Zhu,† Francisco Guinea,§ and

Phaedon Avouris†

†IBM T.J. Watson Research Center, Yorktown Heights, New York 10598, United States, ‡Instituto de Ciencia de Materiales de Aragon and Departamento de Fisica dela Materia Condensada, CSIC-Universidad de Zaragoza, E-50009 Zaragoza, Spain, and §Instituto de Ciencia de Materiales de Madrid, CSIC, Sor Juana Ines de la Cruz 3,28049 Madrid, Spain. ^M.F., T.L., and L.M.-M. contributed equally to this work.

Monolayer 2D systems can interactwith the substrate supportingthem, and as a result, their proper-

ties become substrate-dependent. An inter-esting case involves the remote interactionbetween theplasmonmodesof graphene1�6

or graphene nanoribbons7�10 with the sur-facepolar phonons of a dielectric substrate.3,9

In particular, photocurrent generation11 canbe strongly affected because the absorptioncross section and the subsequent decay ofthe excitations are substrate-dependent.Previous measurements of plasmonic pho-tocurrents in graphene nanoribbon arrays11

involved the excitation of a specific mixedplasmon�phononmode with a CO2 laser at943 cm�1, and the photocurrent was shownto modulate as a function of electrostaticdoping. In this work, photocurrent spec-troscopy of graphene nanoribbon arrays isperformed using a quantum cascade laser,allowing the access of a wide range of mid-infrared frequencies from 850 to 1600 cm�1.Our study reveals a rich interplay betweenvarious polaritonic modes, due to strongcoupling between light and various di-pole-carrying excitations such as plasmons,phonons, and their hybrids. Clear signaturesof enhanced light�matter interaction are

revealed through the measured spectrallyresolved photocurrent.Plasmons are collective electronic oscilla-

tions, and in graphene, they follow a square-root dispersion relation E ∼

√q, where q

is the plasmon wavevector. Direct opticalexcitation of plasmons in graphene is pre-cluded due to the large momentum mis-match with photons. Near-field excita-tion is one way to circumvent this.3�5

In graphene metamaterials, such as arraysof graphene nanoribbons (GNRs), mo-mentum conservation is also relaxed, andstanding plasmon modes with momentumthat goes with q ≈ π/w, where w is thenanoribbon width, can be excited.7�9 Notonly does the energy of the plasmon de-pend on the GNR width, it also depends ondoping of the graphene. The former makesGNR arrays tunable by design, while thelatter makes them in situ tunable (over alimited range) by simply applying a back-gate voltage.For graphene in contact with a polar

substrate, interaction of the plasmon witha substrate surface polar phonon (SPP) leadsto their hybridization and splitting into twohybrid plasmon�phonon modes. In thecase of a SiO2 substrate, the surface polar

* Address correspondence tomfreitag@us.ibm.com.

Received for review May 23, 2014and accepted July 17, 2014.

Published online10.1021/nn502822z

ABSTRACT We report mid-infrared photocurrent spectra of graphene nanoribbon

arrays on SiO2 dielectrics showing dual signatures of the substrate interaction. First,

hybrid polaritonic modes of graphene plasmons and dielectric surface polar phonons

produce a thermal photocurrent in graphene with spectral features that are tunable by

gate voltage, nanoribbon width, and light polarization. Second, phonon polaritons

associated with the substrate are excited, which indirectly heat up the graphene, leading

to a graphene photocurrent with fixed spectral features. Models for other commonly

used substrates show that the responsivity of graphene infrared photodetectors can be

tailored to specific mid-IR frequency bands by the choice of the substrate.

KEYWORDS: graphene photodetector . graphene�substrate interaction . plasmon�phonon polariton . phonon polariton .mid-infrared detector

ARTIC

LE

FREITAG ET AL. VOL. 8 ’ NO. 8 ’ 8350–8356 ’ 2014

www.acsnano.org

8351

phonon modes at 460, 800, and 1170 cm�1 hybridizewith the graphene plasmon to form the plasmon�phonon resonances (Figure 1a).3,9 The characteristicsof the hybrid modes are inherited partly from plasmo-ns and partly from phonons, and as such, they showdispersion and lifetimes somewhere between thehighly dispersive plasmon and the nondispersivephonon.9

In order to detect the optical energy deposited in theGNR array via the excitation of a hybrid plasmon�phonon mode, the excitation has to be converted intoan electrical signal. A photocurrent in graphene canarise by a number of different mechanisms.12 Photo-voltaic effects,13�17 thermoelectric effects,18�21 bolo-metric effects,12,22,23 and phototransistor action24,25 allhave been reported. In the photovoltaic effect, built-inelectric fields separate photogenerated electrons andholes, which leads to a photocurrent, for example,upon selective illumination at a contact or illuminationat a p�n junction. The thermoelectric effect is causedby the selective light-induced heating of part of thedevice in combination with spatial variations in theFermi-level-dependent Seebeck coefficient. Photo-transistor action requires another material in closeproximity where either holes or electrons can gettrapped while capacitively coupled to the channeland affecting the transport current.24,25 The photocur-rent spectrum then reflects the absorption spectrum ofthe added particles. Finally, in the bolometric effect,biased but otherwise homogeneous graphene is

illuminated, which heats it up and changes the trans-port current.To explore the plasmonic photocurrents in gra-

phene nanoribbon arrays, wework with homogeneousnanoribbons since any varying doping profiles such asin p�n junctions would lead to inhomogeneousbroadening of the plasmon modes. The photocurrentmechanism under these circumstances is of the bolo-metric nature, and the sign of the photocurrent de-pends on the electrostatic doping.12 Close to the Diracpoint, photogenerated carriers dominate and increasethe transport current. Away from the Dirac point, thetemperature dependence of the carrier mobility dom-inates, which leads to an overall current decrease. Forour p-doped samples, the transport current is reducedupon photoexcitation unless we apply a gate voltageon the order of 40 V or more. Unless otherwise noted,we focus on the gate voltage range of �40 V to20 V, where the reduction in current due to theheating strongly dominates. Please see Figure 1 andthe Methods section for the details of the sample andexperiment.

RESULTS AND DISCUSSION

Our tunable quantum cascade laser (QCL) coversmid-infrared frequencies ranging from 850 to1600 cm�1 and therefore allows the excitation of twoof the plasmon�phononmodes illustrated in Figure 1a.Photocurrent spectroscopy (Figure 2) shows severalpeaks in this mid-IR window. We identify the most

Figure 1. Mid-IR excitation of GNR array photodetector on Si/SiO2. (a) Dispersion relation of hybrid plasmon�phononmodesin grapheneon SiO2with chemical potential μ=0.43 eV. The sp0 to sp2 are polar SiO2 phonons that interactwith the grapheneplasmon; ωop is the optical phonon energy in graphene. The dashed curve shows the graphene plasmon dispersion beforehybridization with the SiO2 phonon. The shaded triangle indicates the Landau damping region, where plasmons woulddecay rapidly into e�hpairs. Cutting the graphene into nanoribbonswithwidthwmeans selecting awavevector that satisfiesthe usual half-wavelength condition q ≈ π/w and therefore choosing specific energies for the resonant plasmon�phononmodes. (b) Contacted array of graphene nanoribbons, 130 nm in width and 190 nm spacing, fabricated on silicon with 90 nmsilicon oxide. The entire array is 30 μm long and 10 μm wide. (c) Mid-IR radiation from a tunable quantum cascade laser,pulsed at 100 kHz, is focused by a ZnSe objective into a 20 μm spot centered on the GNR array detector. A DC bias on theorder of VD =�8 V is applied at the drain contact. DC and AC electrical signals are separated on the source side by a bias teeand sent to either a preamplifier (DC) or a lock-in amplifier (AC) to measure the DC transport current or AC photocurrent,respectively.

ARTIC

LE

FREITAG ET AL. VOL. 8 ’ NO. 8 ’ 8350–8356 ’ 2014

www.acsnano.org

8352

prominent resonances, labeled A and B, as two hybridplasmon�phonon modes9,11 due to their clear polar-ization dependence, as shown in Figure 2a. Electro-magnetic simulations of the absorption spectra ofarrays of 130 nm GNRs, in keeping with the sameplasmon oscillator strength (EF/εeff)

1/2 as in the experi-ment, are displayed in the top panel of Figure 2b. Thered curve gives the absorption under perpendicularpolarization (E-vector perpendicular to the GNR axis),while the blue curve describes the parallel polarizationcase. The model takes into account the polar opticalsurface phonons of the SiO2 substrate, which hy-bridize with graphene plasmons.3,8,9 For the details ofthe dielectric parameters of SiO2, please see theSupporting Information. The energy of the hybridplasmon�phonon modes, which are only observedunder perpendicular polarization, matches the energyof the experimental photocurrent peaks A and B(Figure 2a) quite well. Furthermore, the computedabsorption in mode A is 3 times larger than theabsorption in mode B, and this is also reflected in thephotocurrent spectrum.Notably absent from the calculated absorption of

the GNR array is anymode in the vicinity of 1070 cm�1.This resonance appears in all experimental spectraindependently of infrared polarization (Figure 2a),

graphene nanoribbon width, or electrostatic dop-ing (Figure 3a). In Figure 4, we show an analogousmeasurement using simple graphene photodetectors(not cut into GNRs), which lacks the plasmon�phonon resonances but also exhibits this mode at1070 cm�1. Themode has not been reported before ingraphene photocurrent measurements. It is also miss-ing in the absorption spectrum of graphene26 orgraphene nanoribbons.9 The feature at 1070 cm�1

therefore has to have an origin beyond the grapheneplasmons or their hybridization with surface polarphonons.The phonon spectrum of SiO2 includes an infrared-

active polar phonon near 1100 cm�1. When resonantlyexcited with light, the resultant substrate phononsproduce enhanced optical fields at the substrate sur-face (i.e., phonon polaritons). For a simple semi-infiniteSiO2 substrate, normal incidence optical fields decayexponentially according to exp(�Im(kz)z), where kz =(εSiO2

)1/2k0 and k0 is the free space wavevector. Thelight absorption coefficient therefore is proportionalto exp(�z/lz), where lz t 1/2Im(kz) is the absorptiondepth. The bottom panel of Figure 2b plots thecumulative absorption from the surface to the depthof 90 nm (the SiO2 thickness), showing enhancedsurface absorption around 1100 cm�1. Figure 2c

Figure 2. Photocurrent spectroscopy in the mid-IR. (a) Normalized photoconductance vs excitation energy for an array of130 nmGNRs for two different light polarizations. The photoconductanceΔG is normalized by the incident laser power P andthe dark conductance G. Hybrid plasmon�phonon modes A and B are excited and produce a photocurrent for perpendi-cularly polarized light. The substrate phonon-labeled SiO2 on the other hand is not polarization-dependent. A residualphotocurrent at parallel polarization belowpeakA is likely due to another infrared-active SiO2 phononnear 800 cm

�1. (b) Toppanel: Absorption of an array of 130 nm GNRs on SiO2 calculated for perpendicular (red) and parallel (blue) polarization.Bottompanel: Calculated absorption due to the infrared-active polar phonon of 90 nm SiO2. (c) Transversemagnetic field |Hy|contour plots in a plane perpendicular to the GNR array for excitationwith energy 1256 cm�1 in the hybrid plasmon�phononmode B (left) and with energy 1112 cm�1 at the SiO2 phonon (right). Scale bars are 100 nm. (d) Calculated temperatureincrease at the graphene position upon photoexcitation of the graphene hybrid plasmon�phononmodes (red) and the SiO2

phonon (blue). A SiO2 thermal conductivity of κSiO2= 1.5 W/mK and interface thermal conductivity between graphene and

SiO2 of κC = 10 MW/m2K are used for the calculations.

ARTIC

LE

FREITAG ET AL. VOL. 8 ’ NO. 8 ’ 8350–8356 ’ 2014

www.acsnano.org

8353

plots the intensity of the transverse magnetic fieldcomponent |Hy| in the device cross section at fre-quency coinciding with the hybrid plasmon phononresonance B (1256 cm�1) and the infrared-activesurface phonon (1112 cm�1). It reveals the enhancedfield intensity at the interface. The former has sur-face light confinement that goes with w/π, whilethe latter decays with lz, which are both ∼100 nmat their respective resonance conditions in ourexperiment.

The measured bolometric photocurrent is directlyproportional to the increase in graphene lattice tem-perature upon photoexcitation. A simple linear heat-flow model can be applied to estimate the steady-state temperature in graphene. Heat flow into the airis several orders smaller than heat flow into the gatestack, and we can therefore assume all heat flow intothe gate stack. Lateral heat flow along the grapheneto the contacts can be neglected since the devicesare very long (30 μm) compared to the dielectricthickness (90 nm). The silicon is assumed to be theheat sink at room temperature, and the temperaturedrops across the SiO2 with thermal conductivityκSiO2

= 1.5 W m�1 K�1. The thermal contact resistancebetween graphene and SiO2 is κC = 10 MW/Km2.27

Laser power is P = 1 MW/m2. For absorption in thegraphene plasmon�phonon mode, heat generationis a delta function centered at the graphene position,while in the case of the SiO2 phonon, heat is gener-ated continuously along the 90 nm dielectric. Theformer (latter) leads to direct (indirect) heating ofgraphene. The temperature increase of the graphenegiven by

ΔTph ¼ agPL

KSiO2

þ 1Kc

� �

and

ΔTph ¼ a¥l zPKSiO2

1 � 1þ L

l z

� �exp � L

l z

� �" #

for the direct and indirect heating of the graphene,respectively. Here ag and a¥ are the simulated lightabsorption in graphene and semi-infinite dielectric,respectively, while in the experiment, a finite dielec-tric thickness L = 90 nm is used. The calculatedtemperatures at the position of the graphene as afunction of excitation energy are shown in Figure 2d.The relative intensity of the peaks A, SiO2, and B inFigure 2a are captured well.Since the photocurrent measurements with tunable

QCL allow us to acquire entire photocurrent spectra,we are now ready to tune the photocurrent maxima byvarying GNRwidths and electrostatic doping. Figure 3ashows normalized photoconductance spectra for a90 nm GNR array under two different gate voltages.In the zero gate voltage case (red squares), which cor-responds to a Fermi level of EF =�0.33 eV, peaks A andB are slightly up-shifted compared to their counter-parts in Figure 2a, which was taken on a 130 nm GNRarray. Furthermore, at a reduced gate voltage of�40 V,which corresponds to EF =�0.44 eV, a strong blue shiftin peaks A and B is observed (blue circles). In additionto the blue shift, plasmon�phonon mode B broadenssubstantially with increased electrostatic doping. Thisbroadening is associated with the opening of addi-tional decay channels for the hybrid plasmon�phonon

Figure 3. Doping dependence of the photocurrent spectra.(a) Normalized photoconductance vs excitation energyfor an array of 90 nm GNRs and two different backgatevoltages. The hybrid plasmon�phonon modes A and B arehighly tunable,while the SiO2phonon isfixed. (b) Plasmon�phonon mode B in 90 nm GNRs: The 3D false-color plotshows the experimental photocurrent vs gate voltage andexcitation energy. (c) Calculated electron loss function,11

defined as inverse of the imaginary part of the dielectricfunction for 90 nm GNRs. See the Supporting Informationfor details of modeling. (d,e) Same as (b,c), but for 130 nmGNRs.

Figure 4. Photocurrent spectroscopy in the mid-IR forgraphene. The SiO2-related resonance is present even inthe absence of plasmonic resonances.

ARTIC

LE

FREITAG ET AL. VOL. 8 ’ NO. 8 ’ 8350–8356 ’ 2014

www.acsnano.org

8354

mode due to optical phonon scattering, which dam-pens the plasmon.9 On the other hand, the elec-trostatic doping does not alter the SiO2 phononpeak, which stays fixed. Finally, the color plots inFigures 3b,d show the normalized photoconduc-tance in the vicinity of the hybrid plasmon�phononmode B color-coded as a function of gate voltageand laser energy for both 90 nm GNRs and 130 nmGNRs. The broadening of peak B, which is very strongfor 90 nm GNRs, is not observed in the 130 nm GNRcase. In 130 nm GNRs, the energy of the hybridplasmon�phonon mode even with doping at EF =�0.44 eV is not high enough to reach the energies ofthe optical phonons that are responsible for thedecay channels.In previous work,9 we have studied the disper-

sion and damping of these hybrid plasmon�phononmodes within the standard theory based on randomphase approximation (RPA); see Supporting Informa-tion. Both the substrate phonons and graphene'sintrinsic optical phonon are included in this theory.Figure 3c,e plots the RPA plasmon loss function L as afunction of frequency and gate voltage for q corre-sponding to 90 and 130 nm ribbons. Qualitative fea-tures of the experiment in Figure 3b,d are captured,including the broadening.Most dielectrics host vibrational mid-infrared-active

phonon modes which can also interact with light andplasmons in the same fashion as described above.Here, we consider the cases of hexagonal boron nitride(hBN) and silicon carbide (SiC), common substratesfor graphene devices. Their bulk optical phononfrequencies and related dielectric parameters, aswell as thermal conductivities are summarized in theSupporting Information. Figure 5a shows the hybrid

plasmon�phonon modes for an array of 130 nm widegraphene nanoribbons on SiO2, SiC, and hBN, assum-ing doping of 0.5 eV. These various polariton modesdistribute across the mid-infrared to the far-infrared,clearly demonstrating that graphene photodetectorscan be spectrally tailored by hybridization of theplasmons with substrate phonons. In Figure 5b, weplot the light absorption depth for the variousdielectrics as a function of frequency, which is re-sponsible for the indirect heating and resulting pho-tocurrent in graphene. Here, we see that siliconcarbide and boron nitride both accommodate surfacephonon polaritons, which are more strongly localizedthan the SiO2 counterpart. However, thermal conduc-tivities of SiC and hBN are 360 and 30 W/mK (out-of-plane), respectively, higher than the SiO2 thermalconductivity of 1.5 W/mK, which reduces peak tem-peratures achievable in those materials. Ideally, onewould engineer the gate stack in a way that a thinlayer of strongly absorbing material such as SiC or BNis deposited onto a thicker dielectric like SiO2 with lowthermal conductivity.

CONCLUSIONS

We presented a spectroscopic study of the photo-current in graphene nanoribbon arrays over a widerange of mid-IR wavelengths from about 6 to 12 μm(850 to 1600 cm�1). Our experimental observationprovides direct proof of the importance of the sub-strate's phonons in the photocurrent generation pro-cess in graphene. Thesemeasurements and associatedmodeling show that graphene photodetection inthe mid-infrared can be spectrally tailored in manyways, such as by substrate engineering, designing ofthe nanoribbon width, and electrostatic doping. In

Figure 5. Effects of various substrate dielectrics. (a) Absorption in graphene due to plasmon�phonon modes of GNR arraysfabricated on different polar substrates, calculated by solving the Maxwell equation of semi-infinite substrates. A GNR widthof 130 nm is assumed. (b) Light absorption depth for different substrates as indicated. The dielectric parameters for thevarious substrates are tabulated in the Supporting Information.

ARTIC

LE

FREITAG ET AL. VOL. 8 ’ NO. 8 ’ 8350–8356 ’ 2014

www.acsnano.org

8355

contrast, graphene photocurrent spectroscopy canreveal signatures of the phononic modes, allowing

the vibrational characterization of thin dielectric filmsor even molecular layers.

METHODSOur photosensitive graphene structures consist of arrays

of graphene nanoribbons 90 or 130 nm in width on a Si/SiO2

substrate (Figure 1b). The graphene is grown by chemical vapordeposition on copper foil using methane.28 This process is self-limiting due to the low solubility of carbon in copper and yieldsin excess of 95% single-layer graphene with only small patchesof few-layer graphene. After PMMA is deposited, the copper isdissolved by wet-etching with etchant CE200, and the gra-phene, now attached to the PMMA, is transferred onto silicon/SiO2 chips with 90 nm oxide thickness. With the gate stack inplace, source and drain electrodes consisting of 1/20/40 nmTi/Pd/Au are fabricated by e-beam lithography on the top of thegraphene. Finally, the graphene is etched into nanoribbonsusing e-beam lithography, lift-off, and oxygen plasma. In thisstep, the array dimensions are also established (30 μm long and10 μm wide).The as-produced graphene nanoribbon devices are chemi-

cally p-doped to a level of 0.33 eV as determined from transfercharacteristics. Electrostatic doping through the global back-gate lets us vary the Fermi level from EF =�0.44 eV for VG =�40Vto EF = �0.16 eV for VG = 40 V. The nanoribbon arrays aredesigned with GNR width equal to the spacing between them.However, by AFM, we measure a GNR width 30 nm smaller thanthe designwidth, and it is this AFMwidth thatwe cite throughoutthis paper. The width as measured by AFM closely matches theelectronic width we used to model the plasmons in a previouspaper,9 and therefore, there is no need for a distinction betweenelectronic and geometric width anymore. The edge roughness ofthe ribbons is on the order of 10 nm, again as measured by AFM.Transport and photocurrents are measured in an AC photo-

current setup described in Figure 1c. Mid-IR radiation from atunable quantum cascade laser is focused into a spot about 20μm in diameter and centered in the middle of the GNR array.The tuning range of the QCL (Block LaserScope) covers themid-IR region between 850 and 1600 cm�1. The peak laser powervaries with wavelength between 5 and 50 mW. Pulses from theQCL have a duty factor of 2.5% at 100 kHz repetition rate, so thateffective AC laser power amplitudes are between 250 μW and2.5 mW. A bias on the order of �8 V is applied at the draincontact of the GNR array, and the source contact is connected tothe ACþDC port of a bias tee. The DC port of the bias tee isgrounded through a preamplifier to measure the transportcurrent, and the AC port is connected to a lock-in amplifier,referenced to the laser pulses at 100 kHz. This allows us to utilizea higher sensitivity preamp for the AC photocurrent measure-ment on top of the larger DC transport current.

Conflict of Interest: The authors declare no competingfinancial interest.

Supporting Information Available: Note concerning the cal-culation of the RPA loss function and table specifying thesubstrate parameters used for the calculations. This material isavailable free of charge via the Internet at http://pubs.acs.org.

REFERENCES AND NOTES1. Hwang, E. H.; Das Sarma, S. Dielectric Function, Screening,

and Plasmons in Two-Dimensional Graphene. Phys. Rev. B2007, 75, 205418.

2. Liu, Y.; Willis, R. F.; Emtsev, K. V.; Seyller, T. PlasmonDispersion and Damping in Electrically Isolated Two-Di-mensional Charge Sheets. Phys. Rev. B 2008, 78, 201403.

3. Fei, Z.; Andreev, G. O.; Bao, W.; Zhang, L. M.; McLeod, A. S.;Wang, C.; Stewart, M. K.; Zhao, Z.; Dominguez, G.;Thiemens, M.; et al. Infrared Nanoscopy of Dirac Plasmonsat the Graphene�SiO2 Interface. Nano Lett. 2011, 11,4701–4705.

4. Chen, J.; Badioli, M.; Alonso-Gonzalez, P.; Thongrattanasiri,S.; Huth, F.; Osmond, J.; Spasenovic, M.; Centeno, A.;Pesquera, A.; Godignon, P.; et al. Optical Nano-imagingof Gate-Tunable Graphene Plasmons. Nature 2012, 487,77–81.

5. Fei, Z.; Rodin, A. S.; Andreev, G. O.; Bao, W.; McLeod, A. S.;Wagner,M.; Zhang, L.M.; Zhao,Z.; Thiemens,M.;Dominguez,G.; et al. Gate-Tuning of Graphene Plasmons Revealed byInfrared Nano-imaging. Nature 2012, 487, 82–85.

6. Grigorenko, A. N.; Polini, M.; Novoselov, K. S. GraphenePlasmonics. Nat. Photonics 2012, 6, 749–758.

7. Ju, L.; Geng, B.; Horng, J.; Girit, C.; Martin, M.; Hao, Z.;Bechtel, H. A.; Liang, X.; Zettl, A.; Shen, Y. R.; et al.GraphenePlasmonics for Tunable Terahertz Metamaterials. Nat.Nanotechnol. 2011, 6, 630–634.

8. Yan, H.; Li, X.; Chandra, B.; Tulevski, G.; Wu, Y.; Freitag, M.;Zhu, W.; Avouris, P.; Xia, F. Tunable Infrared PlasmonicDevices Using Graphene/Insulator Stacks. Nat. Nanotech-nol. 2012, 7, 330–334.

9. Yan, H.; Low, T.; Zhu, W.; Wu, Y.; Freitag, M.; Li, X.; Guinea, F.;Avouris, P.; Xia, F. Damping Pathways of Mid-infraredPlasmons in Graphene Nanostructures. Nat. Photonics2013, 7, 394–399.

10. Nikitin, A. Y.; Guinea, F.; Garcia-Vidal, F. J.; Martin-Moreno,L. Surface Plasmon Enhanced Absorption and SuppressedTransmission in Periodic Arrays of Graphene Ribbons.Phys. Rev. B 2012, 85, 081405.

11. Freitag, M.; Low, T.; Zhu, W.; Yan, H.; Xia, F.; Avouris, P.Photocurrent in Graphene Harnessed by Tunable IntrinsicPlasmons. Nat. Commun. 2013, 4, 1951.

12. Freitag, M.; Low, T.; Xia, F.; Avouris, P. Photoconductivity ofBiased Graphene. Nat. Photonics 2013, 7, 53–59.

13. Lee, E. J. H.; Balasubramanian, K.; Weitz, R. T.; Burghard, M.;Kern, K. Contact and Edge Effects in Graphene Devices.Nat. Nanotechnol. 2008, 3, 486–490.

14. Mueller, T.; Xia, F.; Freitag, M.; Tsang, J.; Avouris, P. Role ofContacts in Graphene Transistors: A Scanning Photocur-rent Study. Phys. Rev. B 2009, 79, 245430.

15. Xia, F.; Mueller, T.; Golizadeh-Mojarad, R.; Freitag, M.; Lin,Y.-m.; Tsang, J.; Perebeinos, V.; Avouris, P. PhotocurrentImaging and Efficient Photon Detection in a GrapheneTransistor. Nano Lett. 2009, 9, 1039–1044.

16. Peters, E. C.; Lee, E. J.; Burghard, M.; Kern, K. Gate Depen-dent Photocurrents at a Graphene p�n Junction. Appl.Phys. Lett. 2010, 97, 193102.

17. Rao, G.; Freitag, M.; Chiu, H.-Y.; Sundaram, R. S.; Avouris, P.Raman and Photocurrent Imaging of Electrical Stress-Induced p�n Junctions in Graphene. ACS Nano 2011, 5,5848–5854.

18. Xu, X.; Gabor, N. M.; Alden, J. S.; van der Zande, A. M.;McEuen, P. L. Photo-thermoelectric Effect at a GrapheneInterface Junction. Nano Lett. 2010, 10, 562–566.

19. Lemme, M. C.; Koppens, F. H. L.; Falk, A. L.; Rudner, M. S.;Park, H.; Levitov, L. S.; Marcus, C. M. Gate-Activated Photo-response in aGraphene p�n Junction.Nano Lett.2011, 11,4134–4137.

20. Song, J. C. W.; Rudner, M. S.; Marcus, C. M.; Levitov, L. S. HotCarrier Transport and Photocurrent Response in Gra-phene. Nano Lett. 2011, 11, 4688–4692.

21. 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 inGraphene.Science 2011, 334, 648–652.

22. Vora, H.; Kumaravadivel, P.; Nielsen, B.; Du, X. BolometricResponse in Graphene Based Superconducting TunnelJunctions. Appl. Phys. Lett. 2012, 100, 153507–5.

23. Yan, J.; Kim, M. H.; Elle, J. A.; Sushkov, A. B.; Jenkins, G. S.;Milchberg, H. M.; Fuhrer, M. S.; Drew, H. D. Dual-Gated

ARTIC

LE

FREITAG ET AL. VOL. 8 ’ NO. 8 ’ 8350–8356 ’ 2014

www.acsnano.org

8356

Bilayer Graphene Hot-Electron Bolometer. Nat. Nanotech-nol. 2012, 7, 472–478.

24. Konstantatos, G.; Badioli, M.; Gaudreau, L.; Osmond, J.;Bernechea, M.; de Arquer, F. P. G.; Gatti, F.; Koppens, F. H. L.Hybrid Graphene�Quantum Dot Phototransistors withUltrahigh Gain. Nat. Nanotechnol. 2012, 7, 363–368.

25. Liu, Y.; Cheng, R.; Liao, L.; Zhou, H.; Bai, J.; Liu, G.; Liu, L.;Huang, Y.; Duan, X. Plasmon Resonance Enhanced Multi-colour Photodetection by Graphene. Nat. Commun. 2011,2, 579.

26. Kuzmenko, A. B.; van Heumen, E.; Carbone, F.; van derMarel, D. Universal Optical Conductance of Graphite. Phys.Rev. Lett. 2008, 100, 117401.

27. Low, T.; Perebeinos, V.; Kim, R.; Freitag, M.; Avouris, P.Cooling of Photoexcited Carriers in Graphene by Internaland Substrate Phonons. Phys. Rev. B 2012, 86, 045413.

28. Li, X.; Cai, W.; An, J.; Kim, S.; Nah, J.; Yang, D.; Piner, R.;Velamakanni, A.; Jung, I.; Tutuc, E.; et al. Large-AreaSynthesis of High-Quality and Uniform Graphene Filmson Copper Foils. Science 2009, 324, 1312–1314.

ARTIC

LE