Ultrafast Transient Absorption MicroscopyStudies of Carrier Dynamics in EpitaxialGrapheneLibai Huang,*,† Gregory V. Hartland,‡ Li-Qiang Chu,§ Luxmi,⊥ Randall M. Feenstra,⊥Chuanxin Lian,| Kristof Tahy,| and Huili Xing|

†Notre Dame Radiation Laboratory, ‡Department of Chemistry and Biochemistry, §Department of Chemical andBiomolecular Engineering, and |Department of Electrical Engineering, University of Notre Dame, Notre Dame,Indiana 46556-5670 and ⊥Department of Physics, Carnegie Mellon University, Pittsburgh, Pennsylvania 15213

ABSTRACT Transient absorption microscopy was employed to image charge carrier dynamics in epitaxial multilayer graphene. Thecarrier cooling exhibited a biexponential decay that showed a significant dependence on carrier density. The fast and slow relaxationtimes were assigned to coupling between electrons and optical phonon modes and the hot phonon effect, respectively. The limitingvalue of the slow relaxation time at high pump intensity reflects the lifetime of the optical phonons. Significant spatial heterogeneityin the dynamics was observed due to differences in coupling between graphene layers and the substrate.

KEYWORDS Transient absorption imaging, epitaxial graphene, carrier dynamics, electron-phonon coupling

Graphene is a two-dimensional material with a singleatomic layer of carbon atoms arranged in a hexago-nal lattice. Due to its unique structure, graphene

exhibits unusual optical and electronic properties.1,2 Inparticular, the ballistic nature of carrier transport in graphenemakes it highly desirable for applications such as nanoscalefield effect transistors and single-electron transistors.3 Re-cent research efforts have successfully produced grapheneby mechanical exfoliation,2 epitaxial growth,4 and chemicalsynthesis.5-7 Epitaxial growth is a promising approach forapplications, as it has the ability to prepare graphene on alarge scale and supported on a substrate. In this process,vacuum graphitization of SiC at high temperatures resultsin 1-40 layers of graphene.4 Despite the multilayer struc-ture, evidence suggests that these samples have many of thecharacteristics of single-layer graphene.8

Energy exchange between the electrons and phonons isparticularly important to electron transport, and understand-ing this process will be vital for the realization of futuregraphene-based electronics. Transient absorption spectros-copy is a powerful tool to probe energy relaxation of pho-toexcited carriers and has been applied extensively to closelyrelated carbon nanotubes.9-11 There have been severalreports on the ultrafast dynamics of charge carriers ingraphene, using optical pump-probe12-14 and terahertztechniques.15,16 However, understanding of the carrier re-laxation pathways is far from complete. For example, mostexperiments see a fast decay of several hundred femtosec-

onds, followed by a slower picosecond decay. The slowerdecay has been attributed to minority carrier recombi-nation16 or coupling between electrons and acousticphonons.17 There is also debate about whether the dynamicsare carrier concentration dependent.15 Furthermore, almostall measurements reported thus far have been carried outwith low spatial resolution. However, epitaxially growngraphene is highly inhomogeneous, with variations in thesample thickness occurring over length scale of a fewmicrometers.4 This means that these measurements inte-grated over a distribution of numbers of layers, making itdifficult to interpret how the number of layers affects thedynamics.14 In addition to variation in thickness, recentRaman measurement also revealed Raman peak shiftsresulting from inhomogeneity in the graphene/substrateinteraction.18,19 It is not clear how substrate interactions anddoping17 affect the dynamics. To pave the road for electronicdevices based on epitaxial graphene, characterization meth-ods with high spatial resolution are needed to understandthese effects.

In this Letter, we report transient absorption microscopyas a novel tool to characterize graphene and to interrogatethe charge carrier dynamics. This technique has the abilityto directly image carrier dynamics with a diffraction-limitedspatial resolution. The intensity of the transient absorptionsignal is shown to correlate with the number of graphenelayers. The carrier cooling exhibits a biexponential decay,consisting of an instrument-response-limited fast decay timeτ1 (<0.2 ps) and a slower decay time τ2.12,14,17 The value ofτ2 was found to increase with increasing pump fluence.15

The fast decay is assigned to coupling between the electronsand optical phonons in graphene,20,21 and the slower decay

* To whom correspondence should be addressed: e-mail, lhuang2@nd.edu;telephone, 1-574-631-2657; fax, 1-574-631-8068.Received for review: 12/11/2009Published on Web: 03/08/2010

pubs.acs.org/NanoLett

© 2010 American Chemical Society 1308 DOI: 10.1021/nl904106t | Nano Lett. 2010, 10, 1308–1313

is attributed to the hot phonon effect. At high pump intensi-ties the slow decay reaches a limiting value, which isassigned to the relaxation time of the optical phonons. Thecontribution of the slow component to the overall decay wasfound to vary with spatial position in the sample. This isattributed to differences in coupling between the graphenelayers and the substrate.

Epitaxial graphene studied in this work was formed bythe sublimation of Si from the Si-face 4H-SiC (0001).13 Theaverage number of graphene layers was determined to be1.9 monolayers by Auger electron spectroscopy.13 Thesample was patterned by e-beam lithography with Cr/Aumetal markers in order to correlate the transient absorptionimages with Raman images of the same area. Micro-Ramanspectra and images were acquired in air at room tempera-ture using a confocal Raman microscope (Alpha300R, WITecGmbH, Germany), excited with Nd:Yag laser (λ ) 532 nm)and a 40× Nikon objective (NA) 0.6). A KMLabs Ti:sapphirelaser (89 MHz repetition rate, central wavelength 780 nm)pumped by a Coherent Verdi V-5 DPSS laser was employedfor the transient absorption measurements. The output fromthe oscillator was split into two beams, one of which isdoubled by a 0.4 mm thick �-barium borate crystal to serveas the pump pulse. The pump and probe beams werefocused at the sample by a 60×, 0.8 NA objective. After thesample the beams were recollimated by a second objective(50×, 0.65 NA), the pump was blocked by an interferencefilter, and the probe was detected with an avalanche pho-todiode (APD, Hamamatsu C5331-11). Lower NA objectiveswere used in this present study compared to our previouswork,22,23 because of the thickness of the sample (there is atrade-off between working distance and magnification).

Pump-induced changes in the probe intensity were mea-sured by chopping the pump with an acousto-optic modula-tor (AOM, Crystal Technologies, model no. 5100-35), andmonitoring the output of the APD with a lock-in amplifier(Stanford Research Systems, SR830). The AOM was drivenat 75 kHz by the internal reference of the lock-in. Transientabsorption traces were obtained by delaying the probe withrespect to the pump with a mechanical translation stage(Newport, UTM150PP.1). The powers used in the experi-ments were approximately 0.2-10 pJ/pulse for the pump,and 0.05 pJ/pulse for the probe. In all the experimentsdescribed below, the pump and probe beams had parallelpolarizations. The time resolution at the sample was ca. 230fs. Transient absorption images were obtained by rasterscanning the sample relative to the laser spot with a closedloop piezo-stage (Physik Instrumente, P-527.3Cl) at a fixeddelay between the pump and probe beams. The spatialresolution of our experiments was determined to be ap-proximately 0.3 µm by analyzing the profile of the smallestresolvable feature in the images. Since this is close to thediffraction limited spot size of the pump (0.4 µm/2 × NA )0.3 µm), we conclude that the spatial resolution in these

experiments is controlled by the smaller of the pump andprobe beam spot sizes.

Figure 1 shows a Raman image of the sample (Figure 1a)and a transient absorption image (Figure 1c) from the samearea. Graphene exhibits two major Raman bands: the Gband centered around 1584 cm-1 corresponding to in planeE2 g optical phonon, and the 2D band centered around 2704cm-1, which is the overtone of the disorder D band (∼1350cm-1) and is related to stacking order.19,24 The Raman imagewas obtained by integrating the intensity of the G band from1560 to 1620 cm-1 and plotting this versus position. Thisgives an approximate map of the relative thickness of thegraphene.24,25 The Raman image shows that the sample iscovered with few-layer graphene (dark areas in Figure 1a)and that there are scattered micrometer-sized thicker re-gions of multiple layers of graphene (bright areas in Figure1a) on top of the few-layer graphene. Pits are known to formduring the initial stage of the graphitization process, and theycoarsen into larger sizes as the graphitization proceeds.26

The size and number of the thick regions agree well withwhat we observe for the size and number of pits on the SiCsurface.13 Thus, we interpret the thick regions as occurringwithin the pits. Figure 1b presents Raman spectra fromvarious positions in the sample as marked in Figure 1a. Inthe thin regions of the sample (position 1), the Ramanspectrum resembles that of few-layer epitaxial graphene on

FIGURE 1. (a) Raman G band image from the epitaxial graphenesample, scale bar ) 1 µm. (b) Raman spectra taken at positions asmarked in (a). Spectra are displaced vertically for clarity. (c) Animage of differential transmission ∆T/T at zero pump-probe delayof the same sample area as in (a). Red and blue correspond to highto low magnitude of ∆T/T, respectively. Scale bar ) 1 µm. (d) Themagnitude of the transient absorption signal at zero time delayversus pump fluence from a thick region of the sample (filled circles)and a thin region of the sample (filled squares, ×10). Red solid linesare linear fits.

© 2010 American Chemical Society 1309 DOI: 10.1021/nl904106t | Nano Lett. 2010, 10, 1308-–1313

SiC, and Raman signatures from SiC substrate are alsovisible.19 For positions 2, 3, 4, and 5 that are associated withhigh G band intensities (i.e., large number of graphenelayers), the spectra show predominately two peaks due tothe G and 2D bands of graphene.19 The transient absorptionimage shown in Figure 1c was recorded with temporallyoverlapped pump and probe beams, and the differentialtransmission of the probe beam ∆T/T is plotted as a functionof sample position. The features observed in the transientabsorption and micro-Raman images are very similar, whichshows that the magnitude of the transient absorption signalis proportional to the number of graphene layers.14

Figure 1d gives a plot of the magnitude of the transientabsorption signal at zero time delay versus pump power fora representative thick region of multilayer graphene and arepresentative thin region of few-layer graphene taken fromanother area of the sample. In both regions, a linear rela-tionship is observed, which shows that we are not saturatingthe optical transitions in these experiments. The absorptioncoefficient of few-layer graphene in the visible and near-IRspectral range has been found to be largely independent ofphoton energy and to have a simple dependence on thenumber of graphene layers, given by

where n is the number of layers.27 Given that we are in thelinear absorption regime, we can use this relationship tocalculate the carrier density. On the basis of the maximumpump power of 10 pJ/pulse and a spot size of ∼0.3 µm(corresponding to a pump laser fluence of 14 mJ/cm2), weestimate a maximum carrier density of ∼6.5 × 1014 cm-2

per graphene layer in these experiments.In principle the transient absorption image in Figure 1c

can be employed to determine the number of graphenelayers. In the linear absorption range, the differential probetransmission ∆T/T at zero time delay is proportional to theproduct of the absorbance R0(n), the pump laser fluence P,and a parameter C that depends on the interaction of theprobe with the excited sample and the experimental geom-

etry: (∆T/T) ≈ C × R0(n) × P. Thus, for a given P, ∆T/Tmapping of the sample provides a map of the relative valuesof a0(n), which can be related to the relative number ofgraphene layers. For example, the ratio of the slopes of thetwo lines illustrated in Figure 1d is 22.4 ( 1.2. This is equalto the ratio of the absorbance of the two regions. Assumingthe absorption of multilayer graphene can be estimated byeq 1 and that the thinner region is a single layer of graphene,the thicker region in Figure 1d corresponds to 18 ( 1graphene layers. If the thinner region is two graphene layers,then the thicker region would be 28 layers.

To determine the absolute number of graphene layersfrom the transient absorption images, we need to determinethe value of C. This could be done by performing experi-ments on samples with a known thickness. There may besignificant advantages for transient absorption microscopycompared to other optical techniques for measuring thethickness of graphene. For example, Figure 1 shows that thecontrast is much better in the transient absorption imagecompared to the Raman image. The low contrast in Ramanimages results in large errors in determining the thicknessof few-layer graphene samples (less than five layers).25

Rayleigh scattering has also been used to determine thenumber of graphene layers, but this technique also suffersfrom low contrast and is very sensitive to the opticalconstants of the substrate.28 Plans are underway to developthe transient absorption imaging technique as a character-ization tool for graphene.

Figure 2 presents transient absorption traces recorded atthree different positions in the sample. Based on the mag-nitude of the ∆T/T signal, the number of layers interrogatedincreases from A to C. All the traces show an instrumentalresponse limited rise. At position A the signal decays fasterthan our instrument response time (0.2 ps). At positions Band C the traces show both a fast instrument responselimited decay (τ1 < 0.2 ps) and a slower picosecond timescale decay τ2 that accounts for approximately 10-20% ofthe overall signal. The slow decay component is only ob-served in regions with a large ∆T/T signal, that is, in the thickregions of the sample where we have multilayers of graphene.In addition, the value of τ2 increases as ∆T/T increases. This

FIGURE 2. Transient absorption traces recorded at three different positions as marked in the right image. The traces were fitted with abiexponential decay convoluted with a Gaussian response function. Experimental data are black filled circles, and fits are red solid lines.Scale bar in the right image is 1 µm. These data were recorded with a pump fluence of 14 m J/cm2.

α0(n) ) -ln(1 - 0.023n) (1)

© 2010 American Chemical Society 1310 DOI: 10.1021/nl904106t | Nano Lett. 2010, 10, 1308-–1313

will be discussed in more detail below. Recent ultrafast (10fs time resolution) measurements on graphite have shownthat the hot carriers lose a majority of their energy within0.2 ps by coupling to optical phonons.29 Thus, the fast decaypresent in our transient absorption traces is assigned tocoupling between the excited carriers and optical phononsof graphene. This assignment is consistent with theoreticalcalculations, which show that optical phonon emission is theprimary means of electron cooling in graphene.20 Note thatthe pump wavelength of 390 nm in our study is shorter thanthat used in previous works. However, initial electron scat-tering and thermalization processes are extremely fast witha characteristic time of ∼30 fs,29 so the higher energy of thepump photons should not have a significant effect on carrierdynamics for the time resolution of our measurements.

The assignment of the slow decay is more complicated.To investigate the origin of the slow decay component, westudied the pump intensity dependence of the carrier dy-namics. Figure 3a shows traces recorded at different intensi-ties at the same position as the thick position in Figure 1d.A plot of the value of τ2 from this position versus intensity ispresented in Figure 3b. The slow time constant initiallyincreases as the pump intensity is increased, levels off at apump fluence of 8 mJ/cm2, and reaches a constant value of1.8 ( 0.1 ps at higher powers. A plot of τ2 versus pumpfluence from a thinner region with fewer layers of grapheneis shown in Figure 3b. A similar saturation behavior isobserved at the thinner position; however, the limiting valueof τ2 at high pump fluence is faster (1.5 ( 0.1 ps). Note thatthese experiments were conducted in the linear absorptionrange, thus, this effect is not due to saturation of the opticaltransitions. On the basis of these measurements, the slowdecay τ2 is assigned to the hot phonon effect.30 At highexcitation densities electron relaxation creates a significantnumber of optical phonons. These phonons can coupleenergy back into the electron distribution, slowing the rateof cooling. This accounts for the appearance and increasein the slow relaxation time as the pump laser power in-creases. At very high powers the hot electrons and opticalphonon modes reach a quasi-equilibrium. In this case the

decay of the electronic temperature is controlled by therelaxation of the optical phonons, which occurs throughcoupling to low-energy acoustic phonon modes.30 Therefore,the limiting value of the slow relaxation time at high pumpintensity reflects the relaxation time for the optical phononmodes. Our results indicate that the lifetime of opticalphonons in epitaxial graphene is ∼2 ps. This value is similarto the lifetime of optical phonons in single-walled carbonnanotubes measured by time-resolved anti-Stokes Ramanspectroscopy.31 Note that the value of the slow decay isconsistent with previous optical pump-probe exper-iments,12,14,16,17 but our assignment of this time constantto optical phonon-acoustic phonon coupling differs fromsome previous reports.

The data in Figures 2 and 3 imply that the time constantfor relaxation of the optical phonons is different in differentregions of the film. Specifically, Figure 3b shows that thelimiting value of τ2 varies from a thick region to a thinnerregion of the same sample. Since the slow relaxation timeτ2 is limited by acoustic-optical phonon coupling, thisobservation implies that this coupling depends on graphenethickness. This is further illustrated in Figure 4, wheretransient absorption images of the same area at two delaytimes, 0 ps and 1 ps, are presented. If all the different regionsdisplayed the same dynamics, then these two images shouldlook the same. However, they do not; the contrast is differentin the two images: features are observed in the 1 ps delayimage that do not appear in the 0 ps delay image. This canbe clearly seen in the line profiles presented in Figure 4c.

We interpret these results as follows: the 0 ps delay imagemaps the thickness of the graphene sample. In general thethick regions with multilayers of graphene show the slowdecay, thus, features with high intensity in the 0 ps imagealso show up in the 1 ps image. Some of the thinner regionsof the sample that have few layer graphene display the slowdecay, and some do not. The regions with the slow decaycomponent appear in the 1 ps image, the regions with fastdynamics remain dark. The presence or absence of the slowdecay is due to differences in the coupling strength betweenthe optical and acoustic phonons. Thus, these results show

FIGURE 3. (a) Transient absorption traces at a thick position (black filled circles) recorded at three different pump fluences fitted with abiexponential decay convoluted with a Gaussian response (red solid lines). (b) Values of τ2 plotted as a function of pump fluence for the sameposition as Figure 3a (black filled circles) and another position with fewer layers of graphene (red triangle).

© 2010 American Chemical Society 1311 DOI: 10.1021/nl904106t | Nano Lett. 2010, 10, 1308-–1313

that there is a significant amount of heterogeneity in theoptical phonon-acoustic phonon coupling in the thin re-gions of the sample.

Recent Raman measurement in epitaxial graphene re-ported an unexpected large variation in the 2D Raman peakposition that was not related to graphene thickness butrather due to mechanical decoupling from the substrate.18

We believe that a similar effect is responsible for the data inFigure 4. Regions that are strongly coupled to the substratehave strong optical phonon-acoustic phonon coupling anduniformly fast dynamics. The hot-phonon effect does notoccur in these strongly coupled regions because the opticalphonons have short lifetimes. These regions are dark in thetime-delayed image. In contrast, regions that are weaklycoupled have long optical phonon lifetimes and, thus, displaythe hot phonon effect. These regions show up in the timedelayed image. The change in contrast with delay time isonly observed in thin regions of the sample, as coupling tothe substrate becomes less important in thick regions (thesignal is dominated by layers that are not in contact withthe substrate). An alternative explanation is that the differ-ence in dynamics arises from differences in doping in thethin regions.17 However, the intensity-dependent dynamicsin Figure 3 show that the slow time constant arises from thehot phonon effect.30 Thus, doping would have to affect thelifetime of the optical phonons. While this is possible, itseems more likely to us that coupling to acoustic phononsis the explanation for the dynamical heterogeneity.

The results in this paper have important implications fordevice applications of graphene. Recent studies of theconductivity of single carbon nanotubes have shown thatnanotubes in contact with the substrate show enhanced highfield transport compared to suspended nanotubes.32 Thisarises because the suspended nanotubes have longer opticalphonon lifetimes. Thus, at high fields a significant populationof hot phonons is established, which reduces the conductiv-ity of the nanotube, and even produces negative differentialconductance.32 In contrast, hot phonons do not accumulatefor nanotubes in contact with the substrate because ofefficient coupling between the optical and acoustic phonons.Our lifetime measurements essentially show the same effectfor graphene (Figure 3b). This points to transient absorptionmicroscopy as a potentially important tool for characterizing

the electrical conductivity of graphene through measure-ment of the lifetime of optical phonon modes produced bycharge carrier relaxation. Research efforts are currentlyunderway to investigate the optical phonon lifetimes inchemical vapor deposition grown graphene samples thatoffer uniform few-layer graphene on a variety of substrates.This will help elucidate the interaction between grapheneand the substrate, as well as its role in electron transport.

In conclusion, we have examined the charge carrierdynamics of epitaxial graphene with high spatial andtemporal resolution. In general the transient absorptiontraces show a fast instrument response limited decay(<0.2 ps) due to coupling between the excited chargecarriers and optical phonon modes. A slower, carrierdensity dependent decay is also observed that is at-tributed to the hot-phonon effect. The time constant forthe slow decay increases with pump intensity up to ca.1.8 ps. The limiting value is determined by the lifetimeof the optical phonons, which depends on the couplingbetween the optical phonons and the acoustic phonons.Transient absorption images collected at different timedelays between the pump and probe pulses show thatthere is significant spatial heterogeneity in this couplingfor regions of the sample where there are only a few layersof graphene. This is attributed to differences in themechanical coupling between graphene and the substrate.

Acknowledgment. Huang acknowledges the support fromthe Office of Basic Energy Science of the U.S. Departmentof the Energy. Hartland acknowledges the support fromNational Science Foundation under Grant CHE-0647444.Xing thanks the Notre Dame Faculty Research Program, theNational Science Foundation for the CAREER award; Xing,Lian, and Tahy acknowledge the support from the NSF GrantECCS-0802125 and the Midwest Institute NanoelectronicDiscovery (MIND) Center sponsored by the NanoelectronicsResearch Initiative. Luxmi and Feenstra acknowledge NSFGrant DMR-0856240. The authors also thank ProfessorDebdeep Jena for helpful discussion on hot phonon effectin graphene. This is contribution No. NDRL 4842 from theNotre Dame Radiation Laboratory.

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FIGURE 4. Transient absorption images of the same sample area at two different pump-probe delay times, 0 ps (a) and 1 ps (b). Scale bars) 0.5 µm. (c) Line profiles taken along the dashed lines as indicated in the figures.

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