Observation of Nanosecond Hot Carrier Decay in Graphene - The

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Surfaces, Interfaces, and Catalysis; Physical Properties of Nanomaterials and Materials

Observation of Nanosecond Hot Carrier Decay in Graphene Lin Fan, Suk Kyoung Lee, Pai-Yen Chen, and Wen Li J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.8b00234 • Publication Date (Web): 06 Mar 2018 Downloaded from http://pubs.acs.org on March 8, 2018

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The Journal of Physical Chemistry Letters

Observation of Nanosecond Hot Carrier Decay in Graphene Lin Fan1, Suk Kyoung Lee1, Pai-Yen Chen2 and Wen Li1* Department of Chemistry, Wayne State University, Detroit, 48202 2 Department of Electrical and Computer Engineering, Wayne State University, Detroit, MI, 48202 [email protected] 1

Abstract: An extremely long decay time of hot carriers in graphene at room temperature was observed for the first time by monitoring the photo-induced thermionic emission using a highlysensitive time-of-flight (TOF) angle-resolved photoemission spectroscopy (ARPES) method. The emission persisted beyond one nanosecond, two orders of magnitude longer than previously reported carrier decay. The long lifetime was attributed to the excitation of image potential states (IPS) at very low laser fluencies. TOC graphic

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The lifetime of hot carriers in materials upon photoexcitation dictates the practicality of the materials in many applications such as solar energy conversion and surface chemistry. For example, the photocatalytic activity of doped titanium dioxide (TiO2) was found to correlate with the lifetime of the photo-induced carriers1. More recently, Regan et al. found that UV-irradiated tungsten oxide (WO3) nanoparticles have a higher electrode potential than granular WO3 due to long-lived carriers2. Because of many attractive properties arising from its unique band-structure, graphene has been considered as one of the most promising material for optoelectronic devices. Theoretically, the lifetime of hot carriers in graphene should be quite long (hundreds of picoseconds to a few nanoseconds) due to a high optical phonon energy (~200 meV) and restricted phase space for acoustic phonon emission3. However, numerous experimental measurements in the past decade employing various pump-probe methods such as differential transmission4-13/reflectivity spectroscopy14-16 and time-resolved ARPES (TR-ARPES)17-28 have always measured a few ps decay time. One of the main reasons is that the excitation pulse energies are often high in the NIR and mid-IR region and extremely efficient carrier-carrier scattering can thermalize electron temperature to a few thousand Kelvins so that optical phonon emission becomes the dominant decay pathway. Supercollision involving multiple collisions between carriers and defects facilitating momentum transfer to acoustic phonons has also been proposed to explain the fast decay process when thermalized electron temperature is below optical phonon energy.29-31 The fast decay of hot carriers (similar to that of metal) and a very small bandgap seem to suggest that graphene might not be ideal for real applications. Recent study proposed to use tri-layer graphene with high carrier mobility to circumvent this problem32. In this work, we show at very low irradiation levels, there exists a significant slow-decay process in graphene at room temperature, which are two orders of magnitude slower than previous results4-16. We attribute this to the excitation of image potential states (IPS) with a long lifetime. This result was achieved by measuring the time-resolved thermionic emission from graphene. Recent work has identified the pathways for thermionic emission from photo-excited graphite.33 We measured for the first time the decay of thermionic emission in graphene to the nanosecond range, which showed unexpected slow hot carrier dynamics. The employed laser fluence was very low at ~10 µJ/cm2. At such low fluencies, due to the extremely low signal level, many experimental tools cannot be applied anymore. We developed a new TOF-based ARPES apparatus (Fig. 1(a)) to achieve a high detection efficiency and thus enabled such measurements. Furthermore, the momenta and the energies of emitted electrons could be extracted with the new apparatus while the hot carrier lifetime was obtained directly through TOF measurements without employing a conventional pump-probe setup.


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Figure 1. (a) The schematic of the experiment setup. (b) The XZ image of electron distributions (Z is TOF axis and X is one of the spatial coordinate) for graphene at 80 MHz, 10.4 µJ/cm2. (c) Same as (b) but at 2 kHz, 500 µJ/ cm2. (d) Same as (b)(c) but for copper film at 2 kHz, 173 µJ/ cm2. The vertical white line indicates the time zero (pz=0) as defined in the main text. Experimentally, the employed laser system was a mode-locked Ti: Sapphire laser (a repetition rate of 80 MHz) with and without amplified stages (the amplified beam has repetition rate of 2 kHz). The center wavelength was 790 nm and the pulse duration was ~35 fs. The input power was a few tens of mW when using the oscillator beam directly, and around 100 µW when using the amplified beam. Commercial chemical vapor deposition (CVD) graphene on fused silica surface (graphenesquare.com) was used without further modification or treatment. The sample was placed in a high vacuum chamber (~10-9 torr) at room temperature. Emitted electrons were detected by a 3D detection system34-35, which was consisted of a MCP (microchannel plate)/phosphor screen detector, a fast frame camera, and a high-speed digitizer. The camera records the positions of electrons on the MCP and the generated electric pulses on MCP were picked off by the digitizer to provide corresponding timing information. The obtained 3 coordinates (x, y, t) were used to calculate the 3D momentum of each electron, which could be then converted into energy and parallel momentum to produce a dispersion diagram. Our previous study showed this 3D momentum-imaging system was compatible with a laser repetition rate up to 10 kHz.36 A new method was developed here to remove this limit: the digitizer was triggered by the electric signals from MCP while the camera was operated in freerun mode; the synchronization between the camera frames and TOFs was achieved using built-in time-stamps of the camera and the digitizer. The electron TOF was measured against the laser pulses, which was picked-off using a photodiode. This new method removed the rate limit



imposed by a fast camera (usually a few kframes/s) so that a high repetition rate laser (such as a Ti:Sapphire oscillator) can be used. This was critical for investigating hot carrier dynamics at very low fluencies. Inside the experimental chamber, we adopted a transmission geometry to introduce a laser beam normal to the graphene sample (Fig. 1(a)). The sample was installed on the first electrode plate. This electrode, together with the rest three optics, was able to velocity map emitted electrons from the sample37. These electrodes accelerated electrons along the surface normal direction (TOF axis) toward the detector with inhomogeneous electric fields (13V/cm at the surface). The open-electrode design allowed a 2π collection efficiency, which helped achieve a high overall detection efficiency (>50%, limited by the quantum efficiency of MCPs) enabling single-particle detection at very low fluencies. The overall count rate was less than one count per laser shot and thus space charge effects could be ignored. For comparison, we also performed experiments on a 10 nm thick copper film. The setup is very similar to the velocity map imaging (VMI) method, which was conventionally used in gas phase measurements of photoionization and photodissociation dynamics37. We would like to note that there have been great efforts38-41 in transferring the conventional VMI method from gas phase to surface studies. However, the 3D capability has been lacking until this work. In Fig. 1(b-d) we show the time-space (XZ) images of 3D electron Newton spheres. We define the time zero of electrons as the TOF of electrons with a zero momentum along TOF axis (pz = 0). In this case, the actual TOF is solely determined by the TOF tube length and electric fields. Electrons with a momentum component toward the detector arrive at the detector earlier than this time zero, which we label as negative TOF in Fig. 1(b-d). Electrons with pz opposite to the detector cannot be ejected from the surface, so the electron cloud is a hemisphere, shown in Fig. 1(d), which are electrons emitted from the thin copper film at 173 µJ/ cm2. Electrons emitted from graphene sample, however, seem to deviate from the expected hemispherical distribution. Fig. 1(b) was obtained using a fluence of 10.4 µJ/cm2 while Fig. 1(c) displays electrons emitted at a fluence of 500 µJ/cm2. The image size (X axis) and the extent of the negative TOF of Fig. 1(b) are smaller than those of Fig. 1(c), which means that the photoelectrons have higher kinetic energies when the excitation laser beam has higher fluencies. Compared with the copper sample, graphene sample has a significant amount of electrons arriving after the time zero (positive time), especially at a low fluence. While positive time does not indicate momentum inward toward the surface, such delayed arrival time can only be interpreted as delayed electron emission from surface, i.e. an extended lifetime of hot carriers. At low fluencies, such a delay can be seen to go beyond one nanosecond. Such a slow decay of hot carriers in graphene is unprecedented at room temperature and indicates at low laser fluencies, a different decay or excitation mechanism is at play.


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Figure 2. (a) TOF decay curves after time zero of different samples and irradiation conditions. Red curve is for graphene at 80 MHz, 12.3 µJ/cm2. Blue curve is for graphene at 2 kHz, 500 µJ/cm2. Black curve is for Cu at 2 kHz, 173 µJ/cm2. (b) Exponential decay fitting of graphene decay curves at different laser fluencies. Red dots are natural log of normalized counts at 12.3 µJ/cm2. Blue circles are natural log of normalized counts at 500 µJ/cm2. Black lines are exponential fittings. With the 3D electron momentum measurements, the apparatus can produce dispersion diagrams similar as a conventional ARPES. We demonstrate this in Supplemental Information for the electrons arriving before the time zero. In the following, we will focus on the decay dynamics of the delayed emission. Figure 2(a) shows the electron yield decay curves after time zero. Counts are normalized to the highest intensity at time zero. From Fig. 2(a) we see the copper film has the fastest nominal decay (mainly instrument response function and background). Because there is no real delayed emission for copper film after time zero (Fig. 1(d)), the baseline of the copper decay curve appears noisy. An exponential decay fitting of the copper transient yields a decay rate constant of 64 ps, which indicates the lower limit of decay time measurement of our current method. It can be seen that graphene at a low fluence has a much slower decay rate (red curve in Fig. 2(a)) than at a high fluence (blue curve). The hot carriers in graphene at ~10 µJ/cm2 have a decay constant of ~302 ps while at a high laser fluence (~500 µJ/cm2), it drops to ~100 ps. We note that the 100 ps is very close to the lower limit (64 ps) and suggests that this value might not be the real decay time of graphene hot carrier at high fluencies. Instead, this number could be an overall fitting result of background plus a very small contribution of slow decay process. Indeed, at low fluencies, the signal after time zero contributes ~20% of the overall signal and increases as the fluence decreases, while for at high fluencies, it is less than 4% of total signal. Such a small signal level means that the decay dynamics at high fluencies can be easily overwhelmed by the background electrons. Therefore, from our current measurement it is difficult to draw a conclusion on decay time of carrier at high fluencies (~500 µJ/cm2). At lower fluencies, the fitted decay constants are reliable and show very little variation when fluencies are varied about 200%. This suggests the slow decay dynamics do not change with fluence in the measured range. However, its contribution to the total emission does change. All previous studies have measured a lifetime of hot carriers in room temperature graphene to be a few ps, which is likely to be dominated by the fast decay processes due to employed higher fluencies. Our results indicate a more than two orders of magnitude slower decay process, which only becomes important when the fluence reaches down to ~10 µJ/cm2 (~ 0.3 GW/cm2) level.



Interestingly, in a previous study at lower temperature (10 K), two decay rates were observed when the excitation energy (tens of meV) was below optical phonon energy (200 meV), ~14-25 ps and ~300 ps, respectively.11 The second time constant is very close to our result and was attributed to acoustic phonon scattering that leads to a very slow decay rate. However, in our study the photon energy is high at 1.6 eV. Therefore, the acoustic phonon decay mechanism does not apply here.

Figure 3. (a) Electronic excitation and decay processes in graphene. The red arrows in the left panel are the optical excitation at the K point. The green and black wavy arrows in the middle panel indicate the electron-electron scattering leading to prompt thermionic emission and excitation of IPS states, respectively. The main hot carrier distribution reaches a quasiequilibrium through carrier-carrier scattering in the middle panel while quickly cools down through electron-phonon scattering in the right panel. (b) Fluence dependent photoemission yield at low fluencies. The prompt thermionic emission is for the electrons arriving before time zero while delayed emission is for electrons arriving after time zero. The slopes show the multiphoton orders of each process. (c) The XY distribution (spatial) of the prompt thermionic emission. (d) The XY distribution of delayed emission with a round laser excitation spatial profile. (d) Same with (c), but with an elliptical laser profile. No size-scaling has been applied to (c) (d) (e).


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Because the measured parallel momentum is very small, the emission should proceed through thermionic emission at the Γ points while the excitation takes place at the K points, which possess significant parallel momenta. Strong electron-electron scattering has been proposed to facilitate the electron energy thermalizing and transferring the excitation energy from the K points to the Γ points. This is shown as the green wavy arrow in Fig. 3(a), middle panel. This is the excitation process of the prompt thermionic emission electrons (before time zero) in Fig. 1(b) and (c) that decays within the first few picoseconds. Meanwhile, carrier-carrier scattering can also excite high-lying states, from which electrons can escape to the vacuum level through delayed thermionic emission. To identify these states, we use the Richardson-Dunshman equation42 to estimate their work functions. For thermionic emission, it is appropriate to use this ି

∅

equation, ‫ ܶܣ = ܬ‬ଶ ݁ ೖಳ೅ , in which J is the current density, A is the Richardson constant (1.2 ×106 A⋅m-2⋅K-2), T is the temperature, ∅ is the work function and kB is the Boltzmann constant. If assuming a room temperature and using the measured electron count rate, a work function of ~1 eV is obtained (see Supplemental Material for computation details). A prominent candidate for such excited states are graphene image potential states (IPS) at the Γ points, whose work functions have been previously measured in the range of 0.7-0.9 eV43. The IPS states of graphene have been studied previously both theoretically and experimentally43-44. However, the lifetime of IPS states of graphene on fused silica has not been measured directly. Some previous work measured the lifetime of IPS states of graphene on metal surfaces, which has been shown to be extremely short (~100 fs) due to the coupling between the IPS and surface states of metals. Current study employs fused silica as substrate, which appears to be effectively decoupled from the graphene and thus allows to observe inherent lifetime of IPS of graphene. Interestingly, delayed ionization was also observed in fullerene molecules, C60 and C70, and was attributed to strong coupling between electronic and nuclear degrees of freedom.45-46 Similarly in graphene, the IPS states can couple to the lattice motion in the long-time scale and thus IPS electrons can either absorb energy to be thermionically emitted or lose energy to decay. The observed decay of thermionic emission is due to the decay of the population of IPS states and thus the lifetime can be measured. We provide two additional experimental evidences to further support our interpretation: 1. the multiphoton orders of the prompt and delayed emission are compared in Fig. 3(b), in which the fluence-dependent electron yields are plotted in double-log axes and the slopes are extracted. We see both are between 5 and 6, which suggest 6-photon processes. The slightly lower multiphoton order of the delayed emission is likely due to the fact that the energy of the IPS is lying ~1 eV below the vacuum level, which prompt thermionic emission will have to reach. 2. Because the bound IPS states are located near the Γ points, the parallel momentum should be small. In Fig. 3(c-e), we compare the spatial extension of prompt (Fig. 3(c)) and delayed emission (Fig. 3(d)(e)) on the detector. Indeed, the spatial distribution of the delayed emission is very small with a K|| less than 0.08 Å-1. Interestingly, due to this small momentum, we can directly control and monitor the spatial profile of IPS excitations on the graphene surface. Fig. 3(d) is the case when the laser profile is made to be round with an aperture while Fig. 3(e) is when we send in unclipped beam which is elliptically-shaped. Besides thermionic emission through IPS, other potential mechanisms were also considered. For example, because our samples were polycrystalline, defect states at the domain boundary might be important47. However, it is not known that the defects can produce excited states with a low work function. Previous study showed such defect states were located at the Fermi energy level48. Finally, it is still possible that adsorbate molecules such as water could



play a role in the observed carrier dynamics because the sample was not pristine. However, the absence of delayed emission on copper surface suggested this is an unlikely scenario. To conclude, we have employed a novel TOF-ARPES to investigate carrier decay dynamics. The TOF capability enables us to directly measure the lifetime of hot carrier in graphene at long time scale without a pump-probe setup. We propose that hot carriers in graphene can be excited to image potential states and have a long lifetime at low laser fluencies. It should be interesting to investigate how important are these IPS electrons at even lower laser fluencies and whether these carriers can be effectively extracted to implement optoelectric devices. Acknowledgement: Research supported by the U.S. Department of Energy (DOE), Office of Science, Basic Energy Sciences (BES), under Award # DE-SC0012628 (graphene work), and by U. S. Army Research Office, under Award # W911NF-12-1-0598 (the development of the 3D VMI system). We would like to thank Drs. Z. S. Tao, J. Huang and J. T. Thomson for helpful discussions. Supporting Information. 1. Dispersion diagrams for thermionic emission in graphene 2. The procedure for estimating the work function of the IPS states.

(1) Dozzi, M. V.; D’Andrea, C.; Ohtani, B.; Valentini, G.; Selli, E. Fluorine-Doped TiO2 Materials: Photocatalytic Activity vs Time-Resolved Photoluminescence. J. Phys. Chem. C 2013, 117, 25586-25595. (2) Regan, K. P.; Koenigsmann, C.; Sheehan, S. W.; Konezny, S. J.; Schmuttenmaer, C. A. Size-Dependent Ultrafast Charge Carrier Dynamics of WO3 for Photoelectrochemical Cells. J. Phys. Chem. C 2016, 120, 14926-14933. (3) Bistritzer, R.; MacDonald, A. H. Electronic Cooling in Graphene. Phys. Rev. Lett. 2009, 102, 206410. (4) Dawlaty, J. M.; Shivaraman, S.; Chandrashekhar, M.; Rana, F.; Spencer, M. G. Measurement of ultrafast carrier dynamics in epitaxial graphene. Appl. Phys. Lett. 2008, 92, 042116. (5) George, P. A.; Strait, J.; Dawlaty, J.; Shivaraman, S.; Chandrashekhar, M.; Rana, F.; Spencer, M. G. Ultrafast Optical-Pump Terahertz-Probe Spectroscopy of the Carrier Relaxation and Recombination Dynamics in Epitaxial Graphene. Nano Lett. 2008, 8, 4248-4251. (6) Sun, D.; Wu, Z.-K.; Divin, C.; Li, X.; Berger, C.; de Heer, W. A.; First, P. N.; Norris, T. B. Ultrafast Relaxation of Excited Dirac Fermions in Epitaxial Graphene Using Optical Differential Transmission Spectroscopy. Phys. Rev. Lett. 2008, 101, 157402. (7) Breusing, M.; Ropers, C.; Elsaesser, T. Ultrafast Carrier Dynamics in Graphite. Phys. Rev. Lett. 2009, 102, 086809. (8) Wang, H.; Strait, J. H.; George, P. A.; Shivaraman, S.; Shields, V. B.; Chandrashekhar, M.; Hwang, J.; Rana, F.; Spencer, M. G.; Ruiz-Vargas, C. S.; Park, J. Ultrafast relaxation dynamics of hot optical phonons in graphene. Appl. Phys. Lett. 2010, 96, 081917. (9) Breusing, M.; Kuehn, S.; Winzer, T.; Malić, 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.


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Page 9 of 15 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(10) Strait, J. H.; Wang, H.; Shivaraman, S.; Shields, V.; Spencer, M.; Rana, F. Very Slow Cooling Dynamics of Photoexcited Carriers in Graphene Observed by Optical-Pump TerahertzProbe Spectroscopy. Nano Lett. 2011, 11, 4902-4906. (11) Winnerl, S.; Orlita, M.; Plochocka, P.; Kossacki, P.; Potemski, M.; Winzer, T.; Malic, E.; Knorr, A.; Sprinkle, M.; Berger, C.; de Heer, W. A.; Schneider, H.; Helm, M. Carrier Relaxation in Epitaxial Graphene Photoexcited Near the Dirac Point. Phys. Rev. Lett. 2011, 107, 237401. (12) Brida, D.; Tomadin, A.; Manzoni, C.; Kim, Y. J.; Lombardo, A.; Milana, S.; Nair, R. R.; Novoselov, K. S.; Ferrari, A. C.; Cerullo, G.; Polini, M. Ultrafast collinear scattering and carrier multiplication in graphene. Nat. Commun. 2013, 4, 1987. (13) Trushin, M.; Grupp, A.; Soavi, G.; Budweg, A.; De Fazio, D.; Sassi, U.; Lombardo, A.; Ferrari, A. C.; Belzig, W.; Leitenstorfer, A.; Brida, D. Ultrafast pseudospin dynamics in graphene. Phys. Rev. B 2015, 92, 165429. (14) Ishioka, K.; Hase, M.; Kitajima, M.; Wirtz, L.; Rubio, A.; Petek, H. Ultrafast electronphonon decoupling in graphite. Phys. Rev. B 2008, 77, 121402. (15) Li, T.; Luo, L.; Hupalo, M.; Zhang, J.; Tringides, M. C.; Schmalian, J.; Wang, J. Femtosecond Population Inversion and Stimulated Emission of Dense Dirac Fermions in Graphene. Phys. Rev. Lett. 2012, 108, 167401. (16) Yan, X.-Q.; Yao, J.; Liu, Z.-B.; Zhao, X.; Chen, X.-D.; Gao, C.; Xin, W.; Chen, Y.; Tian, J.-G. Evolution of anisotropic-to-isotropic photoexcited carrier distribution in graphene. Phys. Rev. B 2014, 90, 134308. (17) Bostwick, A.; Ohta, T.; Seyller, T.; Horn, K.; Rotenberg, E. Quasiparticle dynamics in graphene. Nat. Phys. 2007, 3, 36-40. (18) 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. (19) Gilbertson, S.; Dakovski, G. L.; Durakiewicz, T.; Zhu, J.-X.; Dani, K. M.; Mohite, A. D.; Dattelbaum, A.; Rodriguez, G. Tracing Ultrafast Separation and Coalescence of Carrier Distributions in Graphene with Time-Resolved Photoemission. J. Phys. Chem. Lett. 2012, 3, 6468. (20) Gierz, I.; Petersen, J. C.; Mitrano, M.; Cacho, C.; Turcu, I. C. E.; Springate, E.; Stöhr, A.; Köhler, A.; Starke, U.; Cavalleri, A. Snapshots of non-equilibrium Dirac carrier distributions in graphene. Nat. Mater. 2013, 12, 1119-1124. (21) Johannsen, J. C.; Ulstrup, S.; Cilento, F.; Crepaldi, A.; Zacchigna, M.; Cacho, C.; Turcu, I. C. E.; Springate, E.; Fromm, F.; Raidel, C.; Seyller, T.; Parmigiani, F.; Grioni, M.; Hofmann, P. Direct View of Hot Carrier Dynamics in Graphene. Phys. Rev. Lett. 2013, 111, 027403. (22) Gierz, I.; Link, S.; Starke, U.; Cavalleri, A. Non-equilibrium Dirac carrier dynamics in graphene investigated with time- and angle-resolved photoemission spectroscopy. Faraday Discussions 2014, 171, 311-321. (23) Ulstrup, S.; Johannsen, J. C.; Cilento, F.; Miwa, J. A.; Crepaldi, A.; Zacchigna, M.; Cacho, C.; Chapman, R.; Springate, E.; Mammadov, S.; Fromm, F.; Raidel, C.; Seyller, T.; Parmigiani, F.; Grioni, M.; King, P. D. C.; Hofmann, P. Ultrafast Dynamics of Massive Dirac Fermions in Bilayer Graphene. Phys. Rev. Lett. 2014, 112, 257401. (24) Gierz, I.; Calegari, F.; Aeschlimann, S.; Chávez Cervantes, M.; Cacho, C.; Chapman, R. T.; Springate, E.; Link, S.; Starke, U.; Ast, C. R.; Cavalleri, A. Tracking Primary Thermalization Events in Graphene with Photoemission at Extreme Time Scales. Phys. Rev. Lett. 2015, 115, 086803.



(25) Gierz, I.; Mitrano, M.; Bromberger, H.; Cacho, C.; Chapman, R.; Springate, E.; Link, S.; Starke, U.; Sachs, B.; Eckstein, M.; Wehling, T. O.; Katsnelson, M. I.; Lichtenstein, A.; Cavalleri, A. Phonon-Pump Extreme-Ultraviolet-Photoemission Probe in Graphene: Anomalous Heating of Dirac Carriers by Lattice Deformation. Phys. Rev. Lett. 2015, 114, 125503. (26) Isabella, G.; Matteo, M.; Jesse, C. P.; Cephise, C.; Turcu, I. C. E.; Emma, S.; Alexander, S.; Axel, K.; Ulrich, S.; Andrea, C. Population inversion in monolayer and bilayer graphene. J. Phy. Condens. Matter 2015, 27, 164204. (27) Johannsen, J. C.; Ulstrup, S.; Crepaldi, A.; Cilento, F.; Zacchigna, M.; Miwa, J. A.; Cacho, C.; Chapman, R. T.; Springate, E.; Fromm, F.; Raidel, C.; Seyller, T.; King, P. D. C.; Parmigiani, F.; Grioni, M.; Hofmann, P. Tunable Carrier Multiplication and Cooling in Graphene. Nano Lett. 2015, 15, 326-331. (28) Søren, U.; Jens Christian, J.; Alberto, C.; Federico, C.; Michele, Z.; Cephise, C.; Richard, T. C.; Emma, S.; Felix, F.; Christian, R.; Thomas, S.; Fulvio, P.; Marco, G.; Philip, H. Ultrafast electron dynamics in epitaxial graphene investigated with time- and angle-resolved photoemission spectroscopy. J. Phy. Condens. Matter 2015, 27, 164206. (29) Song, J. C. W.; Reizer, M. Y.; Levitov, L. S. Disorder-Assisted Electron-Phonon Scattering and Cooling Pathways in Graphene. Phys. Rev. Lett. 2012, 109, 106602. (30) Betz, A. C.; Jhang, S. H.; Pallecchi, E.; Ferreira, R.; Feve, G.; Berroir, J. M.; Placais, B. Supercollision cooling in undoped graphene. Nat. Phys. 2013, 9, 109-112. (31) Graham, M. W.; Shi, S.-F.; Ralph, D. C.; Park, J.; McEuen, P. L. Photocurrent measurements of supercollision cooling in graphene. Nat. Phys. 2013, 9, 103-108. (32) Someya, T.; Fukidome, H.; Watanabe, H.; Yamamoto, T.; Okada, M.; Suzuki, H.; Ogawa, Y.; Iimori, T.; Ishii, N.; Kanai, T.; Tashima, K.; Feng, B.; Yamamoto, S.; Itatani, J.; Komori, F.; Okazaki, K.; Shin, S.; Matsuda, I. Suppression of supercollision carrier cooling in high mobility graphene on SiC. Phys. Rev. B 2017, 95, 165303. (33) Tan, S.; Argondizzo, A.; Wang, C.; Cui, X.; Petek, H. Ultrafast Multiphoton Thermionic Photoemission from Graphite. Phys. Rev. X 2017, 7, 011004. (34) Lee, S. K.; Cudry, F.; Lin, Y. F.; Lingenfelter, S.; Winney, A. H.; Fan, L.; Li, W. Coincidence ion imaging with a fast frame camera. Rev. Sci. Instrum. 2014, 85, 123303. (35) Lee, S. K.; Lin, Y. F.; Lingenfelter, S.; Fan, L.; Winney, A. H.; Li, W. Communication: Time- and space-sliced velocity map electron imaging. J. Chem. Phys. 2014, 141, 221101. (36) Fan, L.; Lee, S. K.; Tu, Y.-J.; Mignolet, B.; Couch, D.; Dorney, K.; Nguyen, Q.; Wooldridge, L.; Murnane, M.; Remacle, F.; Schlegel, H. B.; Li, W. A new electron-ion coincidence 3D momentum-imaging method and its application in probing strong field dynamics of 2-phenylethyl-N, N-dimethylamine. J. Chem. Phys. 2017, 147, 013920. (37) Eppink, A.; Parker, D. Velocity Map Imaging of Ions and Electrons Using Electrostatic Lenses: Application in Photoelectron and Photofragment Ion Imaging of Molecular Oxygen. Rev. Sci. Instrum. 1997, 68, 3477. (38) Monti, O. L. A.; Baker, T. A.; Nesbitt, D. J. Imaging nanostructures with scanning photoionization microscopy. J. Chem. Phys. 2006, 125, 154709. (39) Bainbridge, A. R.; Bryan, W. A. Velocity map imaging of femtosecond laser induced photoelectron emission from metal nanotips. New J. Phys. 2014, 16, 103031. (40) Ye, H.; Kienitz, J. M.; Fang, S.; Trippel, S.; Swanwick, M.; Keathley, P. D.; VelásquezGarcía, L. F.; Cirmi, G.; Rossi, G.; Fallahi, A.; Mücke, O. D.; Küpper, J.; Kärtner, F. X. In Velocity Map Imaging of Electrons Strong-Field Photoemitted from Si-Nanotip, 19th


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International Conference on Ultrafast Phenomena, Okinawa, 2014/07/07; Optical Society of America: Okinawa, 2014; p 09.Wed.P03.37. (41) Pettine, J.; Grubisic, A.; Nesbitt, D. J. Angle- and Momentum-Resolved Photoelectron Velocity Map Imaging Studies of Thin Au Film and Single Supported Au Nanoshells. J. Phys. Chem. C 2018, 122, 3970-3984. (42) Richardson, O. W. Thermionic Emission from Hot Bodies. Wexford College Press: London, UK, 2003. (43) Daniel, N.; Thomas, F. Image-potential states and work function of graphene. J. Phy. Condens. Matter 2014, 26, 393001. (44) Silkin, V. M.; Zhao, J.; Guinea, F.; Chulkov, E. V.; Echenique, P. M.; Petek, H. Image potential states in graphene. Phys. Rev. B 2009, 80, 121408. (45) von Helden, G.; Holleman, I.; van Roij, A. J. A.; Knippels, G. M. H.; van der Meer, A. F. G.; Meijer, G. Shedding New Light on Thermionic Electron Emission of Fullerenes. Phys. Rev. Lett. 1998, 81, 1825-1828. (46) Campbell, E. E. B.; Ulmer, G.; Hertel, I. V. Delayed ionization of C60 and C70. Phys. Rev. Lett. 1991, 67, 1986-1988. (47) Banhart, F.; Kotakoski, J.; Krasheninnikov, A. V. Structural Defects in Graphene. ACS Nano 2011, 5, 26-41. (48) Ugeda, M. M.; Brihuega, I.; Guinea, F.; Gómez-Rodríguez, J. M. Missing Atom as a Source of Carbon Magnetism. Phys. Rev. Lett. 2010, 104, 096804.



(a) Computer 1 2 3 4 e5 6 MCP/ CMOS Camera 7 Phosphor 8 Signal decoupler 9 10 Digitizer 11 12 13 14 Time 15 0 0 0 2 kHz Graphene 80 MHz Graphene (d) (c) (b) 16 2 2 10.4 μJ/cm 500 μJ/cm 6.8 6.8 17 6.8 13.6 13.6 18 13.6 19 20.4 20.4 20.4 20 27.2 27.2 21 27.2 22 34.0 34.0 34.0 23 40.8 40.8 ACS Paragon Plus Environment × 1.75 40.8 24 -2.50 -2.00 -1.50 -1.00 -0.50 0.00 0.50 1.00 1.50 -2.50 -2.00 -1.50 -1.00 -0.50 0.00 0.50 1.00 1.50 -2.50 -2.00 -1.50 25 Time (ns) Time (ns) 26

Graphene/Quartz

TOF

Excitation laser (790 nm)

Amp.

Image

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X (mm)

2 kHz Cu 173 μJ/cm2

70

35

18 7 3 0

-1.00 -0.50 0.00 0.50 1.00 1.50

Time (ns)

Normalized Counts

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16


(b)

1.0

Cu 2 kHz

0.8

Graphene 80 MHz Graphene 2 kHz

0.6 0.4 0.2 0.0

0

0.25

0.5

0.75

1

Relative TOF (ns)

Ln (Normalized Counts)

(a)

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0 -1 τ = 302 ps

-2 -3 -4

τ = 103 ps

-5

-6 1.25 1.5 ACS Paragon Plus Environment 0

0.25

0.5

0.75

1

Relative TOF (ns)

1.25

1.5


(a)

EV

Prompt thermionic emission

IPS

EV

IPS

Excitation

EF

hv

EF

Γ

Κ 10-5

Γ

Κ 0

(b) Prompt emission slope 5.8 ± 0.3

10

-6

Delayed emission slope 5.2 ± 0.4

8

9

10

Fluence (µJ/cm2)

Κ (d) (d)

(c) (c)

50

6.8

Y (mm)

Counts per laser shot

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54

Delayed thermionic emission

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11

12

13.6

(e)

25

20.4 12

27.2 34.0

6 3 0

0


6.8

13.6

20.4

27.2

34.0

X (mm)

40.8

47.6

51.0

Page 15 of 15 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25


EV IPS e-e Scattering

Γ


K

Observation of Nanosecond Hot Carrier Decay in Graphene - The

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