Hot phonon decay in supported and suspended exfoliated graphene

Hale, Peter John; Hornett, Samuel M.; Moger, Julian; Horsell, D. W.; Hendry, Euan

doi:10.1103/physrevb.83.121404

Cited by 76 publications

(134 citation statements)

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“…Ultrafast laser measurements on graphene have afforded great insight into quasiparticle relaxation mechanisms and lifetimes in this material [1][2][3][4]. For example, it is now known that, after ultrafast photoexcitation of the electrons, energy relaxation occurs through mechanisms with markedly different time scales: first through energy redistribution by electron-electron scattering (∼10 fs), then thermalization with optical phonons (∼100 fs), and finally, anharmonic decay of optical phonons and/or coupling to substrate phonons (∼1 ps) [2][3][4]. More recently, ultrafast laser measurements have been used to probe the time scales of charge transfer to graphene from photoexcited adsorbed molecules [5].…”

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confidence: 99%

“…We show the time scale of the relaxation associated with oxygen desorption to be ∼100 fs, suggesting the desorption proceeds through hot electron generation in the graphene rather than heating of the lattice through hot phonon generation. Ultrafast laser measurements on graphene have afforded great insight into quasiparticle relaxation mechanisms and lifetimes in this material [1][2][3][4]. For example, it is now known that, after ultrafast photoexcitation of the electrons, energy relaxation occurs through mechanisms with markedly different time scales: first through energy redistribution by electron-electron scattering (∼10 fs), then thermalization with optical phonons (∼100 fs), and finally, anharmonic decay of optical phonons and/or coupling to substrate phonons (∼1 ps) [2][3][4].…”

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confidence: 99%

“…It is known from optical pump-probe measurements that energy relaxation in graphene occurs via a three-stage process [2,3,23,24]: Electron-electron interactions lead to a hot Boltzmann distribution of carriers within a few femtoseconds following photoexcitation [25]. Within ∼100 fs following photoexcitation, electron-phonon scattering leads to cooling of the electron bath and heating of optical phonons.…”

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Optically induced oxygen desorption from graphene measured using femtosecond two-pulse correlation

et al. 2014

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Recently, there has been a great deal of interest in the effect of atmospheric gases on the properties of graphene. We investigate the electrical and optical response of graphene-based field effect transistors that have been exposed to high purity oxygen gas using a combination of ultrafast two-pulse correlation (to give high temporal resolution) and low-frequency transport measurements (to monitor the photoinduced changes in the Fermi level). By measuring the Fermi level shifts, we are only sensitive to the oxygen atoms that interact directly with the surface. We compare our results to predictions of the empirical friction model for molecular desorption. We show the time scale of the relaxation associated with oxygen desorption to be ∼100 fs, suggesting the desorption proceeds through hot electron generation in the graphene rather than heating of the lattice through hot phonon generation. Ultrafast laser measurements on graphene have afforded great insight into quasiparticle relaxation mechanisms and lifetimes in this material [1][2][3][4]. For example, it is now known that, after ultrafast photoexcitation of the electrons, energy relaxation occurs through mechanisms with markedly different time scales: first through energy redistribution by electron-electron scattering (∼10 fs), then thermalization with optical phonons (∼100 fs), and finally, anharmonic decay of optical phonons and/or coupling to substrate phonons (∼1 ps) [2][3][4]. More recently, ultrafast laser measurements have been used to probe the time scales of charge transfer to graphene from photoexcited adsorbed molecules [5]. However, experimentally the exact time scales and mechanisms associated with the adsorption and desorption of molecular species are not well known [6].Molecules that interact with graphene can significantly influence its electrical, optical, and mechanical properties [7][8][9]. This has led to graphene being used as the active element in a range of sensing applications [10]. Studies of changes to these properties have led to an understanding of the orientation of adsorbed molecular species [11], chemical activity mediated by the surface [12], and the type of electron scattering caused [13]. Adsorbed gases have also been shown to affect the THz conductivity of graphene [14]. Of particular interest is the interaction of graphene with molecular oxygen. Oxygen molecules can adsorb onto the surface of a pristine graphene sheet resulting in fractional charge transfer from the graphene to the oxygen. Recent studies have shown the importance of the substrate and environmental conditions to this charge transfer [15,16]. However, the nature of the interaction with oxygen has yet to be fully explored, and it is unclear exactly how and where the oxygen molecules bind to graphene and how efficiently molecules can dissociate at these sites.Here, we investigate the mechanisms, energies, and time scales relevant for the molecular adsorption of oxygen molecules on graphene. To do this, we have designed an experiment which permits high electrica...

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confidence: 99%

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confidence: 99%

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confidence: 99%

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Optically induced oxygen desorption from graphene measured using femtosecond two-pulse correlation

et al. 2014

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“…In addition, the third order nonlinear optical responses associated with saturable absorption of graphene [42][43][44], few-layer graphite films [10,[43][44][45][46][47][48][49][50][51][52][53][54][55][56], and suspensions of graphene flakes [57] have been measured and timeresolved using a variety of pump-probe configurations. Typically, in these experiments, the interband absorption of the pump pulse produces nonequilibrium electron and hole populations, which subsequently relax by carrier-carrier scattering, phonon emission, diffusion and recombination.…”

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confidence: 99%

Third harmonic generation in graphene and few-layer graphite films

et al. 2013

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We observe optical third harmonic generation from graphene and few-layer graphite flakes produced by exfoliation. The emission scales with the cube of the intensity of the incident near-infrared femtosecond pulses and has a wavelength that is one-third of the incident wavelength, both consistent with third harmonic generation. We extract an effective third-order susceptibility for graphene that is on the order of 10 −16 m 2 /V 2 , which is comparable to that for materials that are resonantly excited, but larger than for materials that are transparent at the fundamental and third harmonic wavelengths. By measuring a set of flakes with different numbers of atomic layers, we find that the emission scales with the square of the number of atomic layers, which suggests that the susceptibility of graphene is independent of layer number, at least for a few layers.Graphene is a monolayer of carbon atoms arranged in a hexagonal two-dimensional lattice. It has a linear dispersion relationship between energy, E, and wavenumber, k, E(k) = ±hv F k, where the Fermi velocity v F ≈ 10 6 m/s [see Fig. 1 -3]. Since the material became readily available less than a decade ago [4], it has been the subject of extensive investigations [e.g., see Refs. 5 and 6 and references therein]. Graphene exhibits a number of unusual and remarkable transport properties that make it attractive for nano-electronic applications [7][8][9], including high mobilities [4,5,9, 10] and nearly-ballistic transport at room temperature [5,6]; however, it is the optical properties of graphene that are of primary interest here.The linear absorbance of graphene is flat and approximately 2.3% across the entire visible spectrum [11,12]. Thus, graphene can be considered to be both highly absorbing and/or highly transmitting, depending upon one's point of view or application. Since a negligible fraction (< 0.1%) is reflected [12], 97.7% of the incident light is transmitted. In this sense the sample is certainly transparent. On the other hand, if one were to assign an effective absorption coefficient, α, to a monolayer of thickness 0.33 nm (even though it may be questionable to use macroscopic parameters, such as α, for such thin samples), it would be very large (α ≈ 7 × 10 5 /cm). As suggested by this large effective absorption coefficient, graphene interacts strongly with light. The strong broadband nature of the interaction of light with graphene is consistent with its linear bandstructure, where interband transitions of roughly equal strength are available throughout the visible. The combination of broadband transparency and the high mobilities mentioned earlier makes graphene a promising candidate for use as a transparent conductor [13][14][15][16] in photovoltaic devices [17,18] and touch screens [14]. The high broadband absorption (i.e., optical conductivity) and high carrier mobility suggest that graphene may find applications in a range of optically-controlled transport devices, such as broadband and ultrafast photodetectors [19][20][21][22][23][24][25] The s...

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“…Pump-probe spectroscopy has been extensively employed to investigate relaxation processes in carbon-based materials: a variety of different samples have been studied, including thin graphite 13 , few- [14][15][16] and multi-layer 17 graphene, but very few did experiments on single-layer graphene (SLG) [18][19][20][21] . Furthermore, the temporal resolution reported in earlier literature, either in degenerate (i.e., pump and probe with same photon energy) or two-colour pump-probe, was in most cases Z100 fs [14][15][16][17][18]21 . This prevented the direct observation of the intrinsically fast e-e scattering processes.…”

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Ultrafast collinear scattering and carrier multiplication in graphene

et al. 2013

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Graphene is emerging as a viable alternative to conventional optoelectronic, plasmonic and nanophotonic materials. The interaction of light with charge carriers creates an out-ofequilibrium distribution, which relaxes on an ultrafast timescale to a hot Fermi-Dirac distribution, that subsequently cools emitting phonons. Although the slower relaxation mechanisms have been extensively investigated, the initial stages still pose a challenge. Experimentally, they defy the resolution of most pump-probe setups, due to the extremely fast sub-100 fs carrier dynamics. Theoretically, massless Dirac fermions represent a novel many-body problem, fundamentally different from Schrödinger fermions. Here we combine pump-probe spectroscopy with a microscopic theory to investigate electron-electron interactions during the early stages of relaxation. We identify the mechanisms controlling the ultrafast dynamics, in particular the role of collinear scattering. This gives rise to Auger processes, including charge multiplication, which is key in photovoltage generation and photodetectors.

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Hot phonon decay in supported and suspended exfoliated graphene

Cited by 76 publications

References 26 publications

Optically induced oxygen desorption from graphene measured using femtosecond two-pulse correlation

Optically induced oxygen desorption from graphene measured using femtosecond two-pulse correlation

Third harmonic generation in graphene and few-layer graphite films

Ultrafast collinear scattering and carrier multiplication in graphene

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