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Observation of Ultrafast Carrier Dynamics and Phonon Relaxation of Graphene from the Deep-Ultraviolet to the Visible Region Kawon Oum,† Thomas Lenzer,*,† Mirko Scholz,† Dae Yool Jung,‡ Onejae Sul,*,‡ Byung Jin Cho,‡ Jens Lange,§ and Andreas Müller§ †

Physikalische Chemie, Universität Siegen, Adolf-Reichwein-Str. 2, 57076 Siegen, Germany Department of Electrical Engineering, KAIST, 291, Daehak-ro, Guseong-dong, Yuseong-gu, Daejeon 305-701, Republic of Korea § Institut für Physik, Martin-Luther-Universität Halle-Wittenberg, von-Danckelmann-Platz 3, 06120 Halle, Germany ‡

ABSTRACT: We investigated the ultrafast carrier dynamics and phonon relaxation of CVD-grown monolayer and 9-layer graphene on a quartz substrate. Excitation was performed at 400 and 800 nm. The normalized change in optical density ΔOD was probed over the range 260−640 nm (1.94−4.77 eV), reaching down into the region of graphene’s Fano resonance, previously not investigated in femtosecond broadband pump−probe experiments. Time constants of 160 fs and 4 ps were found and assigned to carrier−optical phonon scattering and slower phonon relaxation processes, respectively. The carrier distribution at early times was clearly hotter for 400 nm excitation than for 800 nm excitation. A pronounced spectral bleach feature was observed below 300 nm. It immediately formed after photoexcitation and recovered slowly, with a time constant of 35 ps for monolayer and time constants of 120 and 970 ps for 9-layer graphene. The same dynamics were found for weak transient absorption features above 300 nm, which emerged after ca. 0.5 ps. The slow dynamics were assigned to interfacial heat flow from graphene to the quartz substrate. The bleach and absorption features were well described by a simple model assuming a red-shift of the Fano resonance. This red-shift disappeared with progressive cooling of graphene. We therefore suggest that the red-shift is induced by shrinking of the band separation due to lattice heating.

1. INTRODUCTION A thorough characterization of the ultrafast charge carrier dynamics of graphene is an important prerequisite for tailoring its properties in optical and high-speed electronic applications.1−6 Laser-based femtosecond pump−probe spectroscopy is very helpful in this respect. Yet, information so far has been largely obtained by using single wavelength pump−probe techniques or probing over restricted spectral ranges. A comprehensive overview of the state-of-the-art of femtosecond pump−probe spectroscopy on graphene can be found e.g. in very recent papers by Winnerl and co-workers,7 Brida et al.,8 Carbone et al.,9 and Gurzadyan and co-workers.10 Briefly, there appear to be three main channels for carrier relaxation. (a) Intraband carrier−carrier scattering: energy and momentum are both conserved when the scattering of both carriers takes place on one line in k-space. Previous studies suggest a sub-100 fs time scale for thermalization by this mechanism.7,11,12 (b) Inverse Auger scattering (= impact excitation): this process is similar to channel a, but involves interband scattering. Energy and momentum transfer occurs between a conduction band electron and a valence band electron. The latter one is lifted into the conduction band, resulting in charge carrier multiplication.13,14 (c) Carrier−phonon scattering. Reported time constants for these processes, which at the early stage of relaxation mainly involve optical phonons, span the range from the 100 fs region (here competing with carrier−carrier scattering) up to 25 ps.7,15 The time constants are reported © 2014 American Chemical Society

to be strongly dependent on the initial excitation energy, with an “optical phonon bottleneck” around 0.1 eV photon energy.7 This bottleneck appears in experiments employing low photon energies because intraband optical−phonon emission is forbidden due to energy conservation reasons.7 On the other hand, so far there have been no ultrafast broadband optical studies probing the photoinduced dynamics of graphene into the deep-UV region. Here, however, key features of its optical spectrum are located. One approaches the M-point in the graphene band structure, which is characterized by the appearance of the so-called Fano resonance, peaking around 270 nm.16,17 The lowest wavelength covered so far by femtosecond broadband absorption studies is ca. 400 nm.10,12,18 In addition, the time-zero assignments in these previous experiments were not internally consistent, and consequently these authors applied different kinetic models to deduce their results, leading to different interpretations of the carrier scattering dynamics.10,12 Here, we therefore would like to provide a comprehensive picture of the ultrafast dynamics of monolayer and 9-layer graphene on quartz, probing the broad wavelength range 260−640 nm (15 600−38 500 cm−1, 1.94− 4.77 eV), with ca. 90 fs time resolution. This unique wide spectral coverage into the Fano resonance region allows us to Received: July 21, 2013 Revised: February 27, 2014 Published: March 12, 2014 6454

dx.doi.org/10.1021/jp4072197 | J. Phys. Chem. C 2014, 118, 6454−6461

The Journal of Physical Chemistry C

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extract previously unattainable information regarding the thermally induced red-shift of this band and thermal cooling processes of graphene in contact with a quartz substrate.

2. EXPERIMENTAL SECTION Ultrafast transient spectra were recorded at a repetition rate of 920 Hz using a new setup, employing the pump−supercontinuum probe (PSCP) technique,19−24 which is dedicated to experiments in the deeper-UV range: A multifilament supercontinuum was generated in a translating 2 mm thick CaF2 plate from the second harmonic (400 nm, 12 mW) of the fundamental beam of a regeneratively amplified titanium:sapphire system (Coherent Libra USP-HE, 800 nm). This way, the energy range is further extended by roughly 10 000 cm−1 into the UV compared to our standard PSCP setup.20−24 The UV extension enables us to study the broadband response of graphene down to the region of its Fano resonance.16,25 In the PSCP experiment, graphene samples at T = 297 K were either constantly moved in a plane perpendicular to the probe beam direction using an x/y translation stage or measured at fixed position. In both cases, identical results were obtained. Pump and probe beams were polarized at magic angle (54.7°),26,27 and therefore the reported signals are free from orientational carrier relaxation dynamics of the initially prepared anisotropic carrier distribution. Note that isotropy should be reestablished within 50 fs.28,29 Excitation was performed either at 800 nm (= 12 500 cm−1 = 1.55 eV) or 400 nm (= 25 000 cm−1 = 3.10 eV). The pump fluence was varied in the range 0.1−0.4 mJ cm−2, and the cross-correlation was typically ca. 70 fs, depending on the probe wavelength. For such conditions, one is still clearly below the experimentally found and theoretically predicted saturation fluence.28,30 Diameters of 300 and 160 μm were typically used for the pump and supercontinuum probe beams, respectively. No differences were observed for signals under an inert atmosphere of nitrogen or in contact with air. Graphene samples were prepared at KAIST as follows: A thin layer of Cu film with thickness of 300 nm was deposited on a 4 in. wafer by a thermal evaporation process as a catalytic film. Then the wafer was loaded in an ICP-CVD (inductively coupled plasma enhanced chemical vapor deposition) system to grow the mono- or multilayer graphene. The growth condition was 950 °C in methane gas as a carbon atom feedstock for 5 min (for 9-layer graphene) or 3 min (for monolayer graphene).31 After the growth, the graphene samples were transferred to quartz substrates (Suprasil I, thickness 200 μm) by the wet transfer method.5 The graphene/SiO2 samples were characterized by micro-Raman spectroscopy with an excitation wavelength of 514.5 nm and a spot size of 5 μm. Transmission spectra were recorded using a Varian Cary 5000 spectrophotometer against a quartz reference substrate.

Figure 1. Steady-state transmission spectra of the monolayer graphene/SiO2 (1LG/SiO2) and 9-layer graphene/SiO2 (9LG/SiO2) samples measured against a SiO2 reference substrate. The dashed lines indicate the positions of the respective minima (268.3 and 272.3 nm) obtained from a numerical analysis of the smoothed first derivative of each spectrum.

previous experimental studies of graphene’s optical conductivity.25 In addition, steady-state micro-Raman spectra of the samples are shown in Figure 2. The G and 2D peaks are located

Figure 2. Steady-state Raman spectra of (A) the 1LG/SiO2 and (B) the 9LG/SiO2 CVD graphene samples on a quartz substrate.

at 1591 and 2694 cm−1 for 1LG/SiO2 and at 1583 and 2698 cm−1 for 9LG/SiO2, as obtained from a numerical analysis of the smoothed first derivative of each spectrum. The values for the Raman peak positions and the G/2D intensity ratio are in good agreement with results obtained previously.10,32 3.2. Ultrafast Broadband Transient Absorption Spectra of 9LG/SiO2. Figure 3 contains examples for time-resolved PSCP broadband spectra of 9LG/SiO2 recorded after excitation at 400 nm (A, B, and C) and 800 nm (A′, B′, and C′), respectively. The data are presented in terms of the normalized change in optical density ΔOD = log(I0/I) (with pump beam) − log(I0/I) (without pump beam), which was obtained over the wavelength range 260−640 nm.

3. RESULTS AND DISCUSSION 3.1. Steady-State Spectroscopy of the Graphene/SiO2 Samples. Steady-state transmission spectra are shown in Figure 1. On the basis of the transmission values at longer wavelength, we can identify the graphene/SiO2 samples as 1layer and 9-layer systems. In the following, they will be termed 1LG/SiO2 and 9LG/SiO2. In the UV we observe the characteristic Fano resonance of graphene. It peaks at 268.3 and 272.3 nm for 1LG/SiO2 and 9LG/SiO2, respectively. The weak red-shift with increasing layer number is in accord with 6455



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Figure 3B contains the spectral development up to 2 ps. At 0.2 ps (black line), the transient bleach amplitude at 600 nm is about 50% of that at 273 nm. The bleach signal quickly recovers up to ca. 0.5 ps (green line). This process is assigned to predominantly carrier−optical phonon scattering. The recovery then further proceeds on time scales up to 2 ps (red line). By 2 ps pump−probe delay, the bleach signal above ca. 305 nm has turned into an absorption feature, whereas below this wavelength (in the region of the Fano resonance), there is still a substantial bleach remaining. We believe that the absorption feature is already superimposed at much earlier times (but is canceled out by Pauli blocking) because the spectral shape in this region does not vary appreciably in this range of delay times. In Figure 3C, the dynamics at longer times up to 800 ps are depicted. We observe that both the bleach recovery in the Fano resonance region and the decay of the absorption feature happen on the same time scale, which points at a common physical origin of this relaxation process. The crossing point (ΔOD = 0) slightly shifts to the blue, as does the bleach minimum in the deep-UV. These spectral features must be due to a red-shift of the Fano resonance after photoexcitation. This shift is reduced on longer time scales. We demonstrate this process qualitatively in Figure 4. Here, the steady-state

Figure 3. Transient PSCP spectra of the 9-layer CVD graphene sample on quartz: Pump wavelength for (A)−(C) = 400 nm and for (A′)−(C′) = 800 nm; fluence ca. 0.38 mJ cm−2. Time scales of the transient spectra in one row are identical. The dashed magenta lines in panels C and C′ correspond to the scaled and inverted steady-state optical density of 9-layer graphene at 297 K. Each transient spectrum corresponds to an average of 5000 laser shots.

In Figure 3A, we show the spectral development around t = 0. Excitation of graphene by the 400 nm pump pulse promotes electrons into the conduction band by π−π* excitation in the vicinity of the K and K′ points of the graphene band structure. At this early time, a broadband bleach (= increase in optical transmission) occurs. This bleach originates from partial blocking of the interband transitions by the generated charge carrier population which spreads out extremely quickly, predominantly due to carrier−carrier scattering, which occurs within the time resolution of our experiment, presumably on the 10−30 fs scale.8,11,33 A corresponding relaxation process occurs for the holes remaining in the valence band. Broadband transient absorption studies of graphene so far have been restricted to probe wavelengths above 400 nm.10−12,18 The deep-UV probing employed in the current experiments enables us to follow the response down to 260 nm, approaching the Mpoint in the graphene band structure. Here, we observe a pronounced instantaneous bleach, with a peak at ca. 273 nm which coincides with the peak of the Fano resonance16,25 (compare the inverted steady-state absorption spectrum which is included for comparison in panel C as a dashed magenta line).

Figure 4. Modeled differential absorption spectra for the Fano resonance red-shift of 9LG/SiO2. Red, blue, and black spectra result from shifting the original steady-state absorption spectrum by −1000, −500, and −100 cm−1, respectively, and subtracting the steady-state absorption without shift in each case.

transmission spectrum of 9LG/SiO2 from Figure 1 (red line) was converted into an absorption spectrum OD = log(1/T). Then different red-shifts were applied to this spectrum (−1000, −500, and −100 cm−1), and the original spectrum (no redshift) was subtracted. This results in modeled ΔOD spectra, shown as red, blue, and black lines, respectively. As can be clearly seen, these modeled spectra nicely reproduce key features of the transient PSCP spectra in Figure 3C and also for the results at the pump wavelength 800 nm, shown in Figure 3C′: (a) the bleach in the deeper-UV and the absorption in the UV−vis region, which also both have the correct order of magnitude, and (b) a uniform reduction of the spectral amplitudes of bleach and absorption combined with a blue-shift of the zero crossing point (ΔOD = 0) with decreasing red-shift (therefore the PSCP experiments obviously evidence a 6456



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excitation. The electron (hole) distributions are located at lower (higher) energies, and therefore the bleach is more dominant at longer wavelengths and weaker at shorter wavelengths compared to excitation with 400 nm. We note that pump-wavelength-dependent differences in the carrier distribution due to inverse Auger scattering (= impact excitation) are likely not visible in the wavelength range covered by our spectra. Such differences should be easier to observe at longer wavelengths, where the dynamics deeper in the Dirac cone are probed. The PSCP spectra for the two different excitation wavelengths were subjected to a global kinetic analysis. Figure 6

reduction of the Fano red-shift with time). We note that there are also some deviations remaining: The simulated bleach is less and the absorption is more pronounced, indicating that the picture of a simple shift of the spectrum is somewhat oversimplified. The slow kinetics and possible explanations for the physical process behind the transient shift of the Fano resonance will be discussed in more detail in section 3.4. Additional PSCP data for 9LG/SiO2 were recorded for the pump wavelength 800 nm. The corresponding spectra are shown in panels A′, B′, and C′ for identical pump−probe delays as in panels A, B, and C. Also, data are shown as contour plots in Figure 5A,C for a comprehensive comparison (the

Figure 6. Selected kinetic traces extracted from 9LG/SiO2 PSCP spectra recorded after excitation at 400 nm. Probe wavelengths: (A) 266, (B) 292, (C) 420, and (D) 620 nm. Black open circles are experimental data (compare Figure 3A−C). The insets show the corresponding dynamics at early times. The red lines represent the fits from a global kinetic analysis of all 448 transients (time constants: 160 fs, 4 ps, 120 ps, and 970 ps).

Figure 5. Comparison of transient PSCP spectra of the 9-layer CVD graphene sample on quartz at different pump wavelengths and fluences: (A) pump wavelength at 400 nm, fluence = 0.38 mJ cm−2; (B) pump wavelength at 400 nm, fluence = 0.18 mJ cm−2; (C) pump wavelength at 800 nm, fluence = 0.38 mJ cm−2.

highlights selected kinetic traces at four representative probe wavelengths and the corresponding simulations (in red) from a global analysis procedure, described in more detail in our previous publications.23,34,35 The fast recovery of the bleach in the whole spectral range is well described by the two time constants 160 fs and 4 ps. The first time constant is assigned to predominantly carrier−optical phonon scattering.7,8,11,29,36 The second time constant must be due to slower carrier−optical phonon scattering processes and subsequent decay of optical phonons into other phonon modes.37 At long times, relaxation should be dominated by relaxation via acoustic phonons. The latter process must be in fact entirely responsible for the slowest portion of the dynamics where also the coupling to phonons of the SiO2 substrate should play a key role. This part of the dynamics is most clearly visible as a slowly disappearing bleach component in the deep-UV transient at λprobe = 266 nm

pump fluence was the same in both cases). We note that no substantial differences in dynamics are found, when the fluence of the 400 nm pump beam is reduced (see the contour plots in Figure 5A,B). The spectra for the different pump wavelengths in Figure 5A,C look at first sight qualitatively similar. However, there are distinct differences for delay times up to 2 ps, and these are a direct consequence of the lower excitation energy. For instance, in Figure 3A,A′ the bleach amplitude for probe wavelengths above 550 nm for excitation at 800 nm is larger than for 400 nm excitation, and it is smaller at short wavelengths especially below 300 nm. This suggests that the carrier distribution on these time scales is cooler after 800 nm 6457



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(Figure 6A), and the same time constants are indeed found in the corresponding absorption features at longer wavelengths (see e.g. the transients at 420 and 620 nm in panels C and D of Figure 6). Time constants of 120 ps (66% amplitude) and 970 ps (34% amplitude) were obtained from the global analysis. The underlying physical processes will be discussed below. 3.3. Ultrafast Broadband Transient Absorption Spectra of 1LG/SiO2. Figure 7 contains time-resolved PSCP

Figure 8. Comparison of transient PSCP spectra of the 1-layer CVD graphene sample on quartz at different pump wavelength and energy density per pulse: (A) pump wavelength at 400 nm, fluence = 0.38 mJ cm−2; (B) pump wavelength at 400 nm, fluence = 0.18 mJ cm−2; (C) pump wavelength at 800 nm, fluence = 0.38 mJ cm−2.

Stokes and Stokes features of quartz.38,39 The oscillations below 400 nm exhibit a characteristic blue-shift with time; see e.g. the red and green lines in panel A for delays of 20 and 50 fs, respectively, and also the “tilted” red features around t = 0 in the contour plot of Figure 8A. The structure has disappeared by 250 fs. Extensive separate PSCP measurements show that this structure is due to the coherent electronic contribution of the underlying quartz substrate,38,40 which is superimposed on the graphene relaxation dynamics. Fourier transformation of the signals in the UV region identifies an superimposed oscillatory component with a frequency of ca. 450 cm−1, in excellent agreement with literature values for a strong lower-frequency Raman band of quartz.39,41 Note also that the oscillatory structure becomes much weaker when the energy of the pump beam is further reduced (compare panels A and B in Figure 8). We note that these spectral oscillations are practically absent in the case of 9LG/SiO2 (Figure 3). This arises probably from the much larger amplitude of the superimposed graphene signal and the considerably reduced laser intensity arriving at the quartz sample in the UV region because of the substantial absorption of the multilayer graphene stack in that range. After complete disappearance of the oscillatory structure by ca. 0.25 ps in Figure 7B, the transient spectra look qualitatively similar to those of 9LG/SiO2. One observes a clear bleach feature over the whole spectral range, with a pronounced peak centered in the region of the Fano resonance at ca. 270 nm and residual bleach amplitude also remaining around 600 nm. Note, however, that the amplitude ratio of these two features (600 nm:270 nm) is only 25% and is thus smaller than in the case of 9LG/SiO2 (50%, compare Figure 3). The bleach then recovers,

Figure 7. Transient PSCP spectra of the monolayer CVD graphene sample on quartz: Pump wavelength for (A)−(C) = 400 nm (dashed vertical line) and for (A′)−(C′) = 800 nm. Time scales of the transient spectra in one row are identical. The dashed magenta lines in panels C and C′ correspond to the scaled and inverted steady-state optical density of monolayer graphene at 297 K. Transient spectra correspond to an average of 5000 laser shots, except for panels C and C′, where in each case 11 spectra in a time interval of ±2.5 ps around the given delay times have been averaged, representing a total of 55 000 laser shots.

broadband spectra of 1LG/SiO2 recorded after excitation at 400 nm (panels A−C) and 800 nm (panels A′−C′). In addition, contour plots of the dynamics are contained in Figure 8. The pump laser fluence employed was very similar to the fluence in the 9LG/SiO2 experiments (Figure 3). Therefore, the amplitude of the spectral response of the monolayer sample is considerably smaller because it scales with the number of layers. In Figure 7A, we observe the appearance of a highly regular oscillatory structure around t = 0, especially in the UV range. Centered around the pump wavelength 400 nm there is a distinct up- and down-going feature which is assigned to anti6458



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contribution from quartz,38 which was included in the fitting procedure. The transient for 260−270 nm shows a striking asymmetry around t = 0, arising from the bleach signal of graphene. A biexponential ultrafast decay is obtained from the fitting procedure, as in the results for 9LG/SiO2 (Figure 6), and it is compatible with the time constants of 160 fs and 4 ps determined for the multilayer sample. However, as already mentioned before, the most striking difference is that the bleach recovery and absorption decay for 1LG/SiO2 are substantially faster (35 ps). Therefore, these dynamics are essentially finished by 100 ps; see the transient for λprobe = 260−270 nm in Figure 9A on the long time scale. 3.4. Behavior on Longer Time Scales. The slowest time component, which is most obvious in the deep-UV transients (e.g., panel A in Figures 6 and 9), deserves a more detailed discussion. The global kinetic analysis of the transient spectra provided a time constant for the monoexponential fit of 35 ps for 1LG/SiO2. In the case of 9LG/SiO2, the slowest dynamics were well described by a biexponential fit with time constants of τ1 = 120 ps (A1 = 0.66) and τ2 = 970 ps (A2 = 0.34). For 9LG/ SiO2, the resulting average relaxation time is τ = A1τ1 + A2τ2 = 410 ps. For both types of samples, the time constants are independent of excitation wavelength (λpump = 400 or 800 nm). Following Heinz and co-workers,42 the thermal excess energy should be already deposited mostly in acoustic phonons on the 10 ps time scale. This interpretation is also supported by results from ultrafast electron diffraction experiments and femtosecond electron energy loss spectroscopy on graphite. These experiments suggest that carrier relaxation is largely completed within 1 ps and that the further dynamics are dominated by thermal relaxation, including structural changes, such as c-axis compression and subsequent expansion in the case of multilayer graphite-type structures.43−46 The pump beam diameter in our experiments is ca. 300 μm, so lateral heat flow is estimated to be on the order of 360 μs, and can therefore be safely neglected on the time scale of the PSCP experiments.42 Thus, we assign the observed final relaxation step to interfacial heat flow from graphene to SiO2. In such a case, the time constant should vary approximately linearly with the number of graphene layers.42 This is indeed in line with our data, where the average time constant for 9LG/SiO2 is roughly 10 times larger than the time constant for 1LG/SiO2. Based on the calculations of Heinz and co-workers, the observed time constants are consistent with a Kapitza thermal conductance of roughly 2000−2500 W cm−2 K−1, a reasonable value for graphene. They are also in line with values obtained from transient reflectivity experiments in the picosecond range.42 We therefore believe that the biexponential dependence observed for 9LG/SiO2 likely reflects more complex thermalization dynamics, which might be also influenced by smaller defects in the stacked layers of the CVD-grown graphene and possibly also imperfections of the 9LG/SiO2 interface structure. Both effects should lead to a heterogeneous distribution of time constants. As discussed in section 3.2, the slowest dynamics are directly linked to a reduction of the red-shift of the Fano resonance back to its value at thermal equilibrium. The final important question is therefore, what is the actual physical reason for the red-shift of the Fano resonance? Basically, there are two explanations possible: Breusing et al. suggested that such an effect can be caused by a renormalization of electron and hole states by the transient carrier populations.11 In contrast, Seibert et al. assign such a behavior to thermal diffusion and shrinkage

and very weak absorption is forming around 350 nm by 2 ps delay time (red line). In panel C, this absorption feature becomes more pronounced at 5 and 20 ps, as in panel C of Figure 3. The most striking difference with respect to 9LG/ SiO2 is the time scale of the final recovery of the bleach and the decay of absorption, with a much smaller time constant in the case of 1LG/SiO2. This behavior will be discussed in more detail in section 3.4. The signal shape in Figure 7C is similar to that in Figure 3C and must be therefore also due to a transient red-shift of the Fano resonance, the only difference being that the crossing point (ΔOD = 0) is shifted further to the blue, which is consistent with the larger blue-shift of the 1LG/SiO2 steady-state transmission/absorption spectrum (Figure 1). Additional PSCP broadband spectra of 1LG/SiO2 were recorded upon excitation at 800 nm, and the results are shown in panels A′−C′. The spectra resemble those for λpump = 400 nm. However, because of the different pump wavelength, in panel A′ there are no Raman features centered around 400 nm anymore. Very similar oscillatory structure around zero delay time is still observed (see also the contour plot of Figure 8C), and it is therefore also assigned to the response of the quartz substrate. A global kinetic analysis of the monolayer graphene results was performed, and kinetic traces for 1LG/SiO2 excited at λpump = 400 nm are shown in Figure 9. Here, averaged results from transients in a 10 nm probe window are shown in each case to arrive at a better signal-to-noise ratio. Clearly visible in the behavior at early times is the characteristic coherent electronic

Figure 9. Selected averaged kinetic traces extracted from 1LG/SiO2 PSCP spectra recorded after excitation at 400 nm. Probe wavelengths: (A) 260−270 nm, (B) 285−295 nm, (C) 415−425 nm, and (D) 615− 625 nm. Black open circles are experimental data (compare Figure 7). The insets show the corresponding dynamics at early times. The red lines represent the fits from a kinetic analysis (time constants: 160 fs, 4 ps, and 35 ps). 6459



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of the band separation due to heating of the lattice.47 Because our slow time constants for 1-LG/SiO2 and 9-LG/SiO2 correlate very well with a thermal diffusion process perpendicular to the graphene layers toward the SiO2 substrate, we believe that the red-shift is caused by shrinkage of the band separation due to lattice heating.

(2) Geim, A. K. Graphene: Status and Prospects. Science 2009, 324, 1530−1534. (3) Bonaccorso, F.; Sun, Z.; Hasan, T.; Ferrari, A. C. Graphene Photonics and Optoelectronics. Nat. Photonics 2010, 4, 611−622. (4) Shin, W. C.; Kim, T. Y.; Sul, O.; Cho, B. J. Seeding Atomic Layer Deposition of High-k Dielectric on Graphene With Ultrathin Poly(4Vinylphenol) Layer for Enhanced Device Performance and Reliability. Appl. Phys. Lett. 2012, 101, 033507. (5) Hong, S. K.; Song, S. M.; Sul, O.; Cho, B. J. Carboxylic Group as the Origin of Electrical Performance Degradation during the Transfer Process of CVD Growth Graphene. J. Electrochem. Soc. 2012, 159, K107−K109. (6) Oh, J. G.; Shin, Y.; Shin, W. C.; Sul, O.; Cho, B. J. Dirac Voltage Tunability by Hf1‑xLaxO Gate Dielectric Composition Modulation for Graphene Field Effect Devices. Appl. Phys. Lett. 2011, 99, 193503. (7) Winnerl, S.; Göttfert, F.; Mittendorff, M.; Schneider, H.; Helm, M.; Winzer, T.; Malic, E.; Knorr, A.; Orlita, M.; Potemski, M.; et al. Time-Resolved Spectroscopy on Epitaxial Graphene in the Infrared Spectral Range: Relaxation Dynamics and Saturation Behavior. J. Phys.: Condens. Matter 2013, 25, 054202. (8) Brida, D.; Tomadin, A.; Manzoni, C.; Kim, Y. J.; Lombardo, A.; Milana, S.; Nair, R. R.; Novoselov, K. S.; Ferrari, A. C.; Cerullo, G.; et al. Ultrafast Collinear Scattering and Carrier Multiplication in Graphene. Nat. Commun. 2013, 4, 1987. (9) Carbone, F.; Aubock, G.; Cannizzo, A.; Van Mourik, F.; Nair, R. R.; Geim, A. K.; Novoselov, K. S.; Chergui, M. Femtosecond Carrier Dynamics in Bulk Graphite and Graphene Paper. Chem. Phys. Lett. 2011, 504, 37−40. (10) Shang, J.; Yu, T.; Gurzadyan, G. G. Femtosecond Energy Relaxation in Suspended Graphene: Phonon-Assisted Spreading of Quasiparticle Distribution. Appl. Phys. B: Laser Opt. 2012, 107, 131− 136. (11) Breusing, M.; Kuehn, S.; Winzer, T.; Malic, E.; Milde, F.; Severin, N.; Rabe, J. P.; Ropers, C.; Knorr, A.; Elsaesser, T. Ultrafast Nonequilibrium Carrier Dynamics in a Single Graphene Layer. Phys. Rev. B 2011, 83, 153410. (12) Shang, J.; Luo, Z.; Cong, C.; Lin, J.; Yu, T.; Gurzadyan, G. G. Femtosecond UV-Pump/Visible-Probe Measurements of Carrier Dynamics in Stacked Graphene Films. Appl. Phys. Lett. 2010, 97, 163103. (13) Winzer, T.; Knorr, A.; Malic, E. Carrier Multiplication in Graphene. Nano Lett. 2010, 10, 4839−4843. (14) Winzer, T.; Malic, E. Impact of Auger Processes on Carrier Dynamics in Graphene. Phys. Rev. B 2012, 85, 241404(R). (15) Kang, K.; Abdula, D.; Cahill, D. G.; Shim, M. Lifetimes of Optical Phonons in Graphene and Graphite by Time-Resolved Incoherent Anti-Stokes Raman Scattering. Phys. Rev. B 2010, 81, 165405. (16) Chae, D.-H.; Utikal, T.; Weisenburger, S.; Giessen, H.; v. Klitzing, K.; Lippitz, M.; Smet, J. Excitonic Fano Resonance in FreeStanding Graphene. Nano Lett. 2011, 11, 1379−1382. (17) Mak, K. F.; Ju, L.; Wang, F.; Heinz, T. F. Optical Spectroscopy of Graphene: From the Far Infrared to the Ultraviolet. Solid State Commun. 2012, 152, 1341−1349. (18) Shang, J.; Yu, T.; Lin, J.; Gurzadyan, G. G. Ultrafast ElectronOptical Phonon Scattering and Quasiparticle Lifetime in CVD-Grown Graphene. ACS Nano 2011, 5, 3278−3283. (19) Dobryakov, A. L.; Kovalenko, S. A.; Weigel, A.; Pérez Lustres, J. L.; Lange, J.; Müller, A.; Ernsting, N. P. Femtosecond Pump/ Supercontinuum-Probe Spectroscopy: Optimized Setup and Signal Analysis for Single-Shot Spectral Referencing. Rev. Sci. Instrum. 2010, 81, 113106. (20) Oum, K.; Lohse, P. W.; Klein, J. R.; Flender, O.; Scholz, M.; Hagfeldt, A.; Boschloo, G.; Lenzer, T. Photoinduced Ultrafast Dynamics of the Triphenylamine-Based Organic Sensitizer D35 on TiO2, ZrO2 and in Acetonitrile. Phys. Chem. Chem. Phys. 2013, 15, 3906−3916. (21) Oum, K.; Lohse, P. W.; Flender, O.; Klein, J. R.; Scholz, M.; Lenzer, T.; Du, J.; Oekermann, T. Ultrafast Dynamics of the Indoline

4. CONCLUSIONS Ultrafast broadband pump−probe spectroscopy probing deep into the UV range has enabled us to obtain a comprehensive overview of the photoinduced relaxation dynamics of monolayer and multilayer graphene on a quartz substrate. The data clearly show the transition from the ultrafast regime dominated by ultrafast carrier−carrier scattering (10−30 fs, within our time resolution) and carrier−optical phonon scattering (160 fs) to slower phonon relaxation processes (4 ps) and heat transfer to the underlying SiO2 substrate (35 ps for the monolayer and 120/970 ps for the 9-layer sample). At early times, the carrier distribution obtained after excitation at 400 nm is hotter than after 800 nm excitation. The average time constant for interfacial heat transfer appears to vary approximately linearly with layer thickness. In both monolayer and multilayer graphene we were able to clearly identify a redshift of the Fano resonance after photoexcitation which was identified by a strong bleach signal in the deep-UV and absorption in the UV to visible range. The disappearance of the red-shift is closely linked to the thermal diffusion time constant. We therefore believe that the red-shift of the Fano resonance is induced by thermal heating of the lattice. In follow-up experiments we plan to extend these studies to free-standing monolayer graphene samples, where only lateral heat flow within the graphene layer is possible and also to other types of substrates, which provide a different phonon spectrum and thus different coupling to the phonon modes of graphene. In all these cases, one would expect different time constants for the heat flow.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail [email protected]; Ph 00492717402803 (T.L.). *E-mail [email protected]; Ph 00428695485 (O.S.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS K.O. and T.L. thank N.P. Ernsting (Humboldt University, Berlin) for continuous support, B. Meyer, D. Gaumann, and M. Rabe (University of Siegen) for excellent technical assistance, and D. Neumaier (AMO GmbH, Aachen, Germany), M. C. Lemme, P. Haring Bolı ́var, and R. Bornemann (University of Siegen) for graphene test samples and helpful discussions. D.Y.J., O.S., and B.J.C. acknowledge financial support by a National Research Foundation of Korea (NRF) research grant (2008-2002744, 2010-0029132, and 2011-0031638). Finally, we thank the reviewers for their very helpful comments, especially regarding the red-shift of the Fano resonance.

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REFERENCES

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Observation of Ultrafast Carrier Dynamics and ... - ACS Publications

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