Transient Absorption and Photocurrent Microscopy Show That Hot Electron Supercollisions Describe the Rate-Limiting Relaxation Step in Graphene

Graham, Mary; Shi, Su‐Fei; Wang, Zenghui; Ralph, Daniel; Park, Jiwoong; McEuen, Paul L.

doi:10.1021/nl4030787

Cited by 66 publications

(103 citation statements)

References 35 publications

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“…Such hot carrier dynamics have been discussed extensively in the literature. We therefore estimated the transient temperature profile from previous publications [32,33,39] and simulated the temporal PC dynamics. In particular, we assumed a biexponential decay with time constants τ 1 ¼ 0.3 ps and τ 2 ¼ 3.1 ps and a 200 fs rise time [Refs.…”

Section: Prl 113 056602 (2014) P H Y S I C a L R E V I E W L E T T Ementioning

confidence: 99%

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Semiconducting-to-Metallic Photoconductivity Crossover and Temperature-Dependent Drude Weight in Graphene

et al. 2014

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We investigate the transient photoconductivity of graphene at various gate-tuned carrier densities by optical-pump terahertz-probe spectroscopy. We demonstrate that graphene exhibits semiconducting positive photoconductivity near zero carrier density, which crosses over to metallic negative photoconductivity at high carrier density. These observations can be accounted for by the interplay between photoinduced changes of both the Drude weight and carrier scattering rate. Our findings provide a complete picture to explain the opposite photoconductivity behavior reported in (undoped) graphene grown epitaxially and (doped) graphene grown by chemical vapor deposition. Notably, we observe nonmonotonic fluence dependence of the photoconductivity at low carrier density. This behavior reveals the nonmonotonic temperature dependence of the Drude weight in graphene, a unique property of two-dimensional massless Dirac fermions. DOI: 10.1103/PhysRevLett.113.056602 PACS numbers: 72.80.Vp, 72.40.+w, 73.40.Qv, 78.20.−e Charge carriers in graphene mimic two-dimensional (2D) massless Dirac fermions with linear energy dispersion, resulting in unique optical and electronic properties [1]. They exhibit high mobility and strong interaction with electromagnetic radiation over a broad frequency range [2]. Interband transitions in graphene give rise to pronounced optical absorption in the midinfrared to visible spectral range, where the optical conductivity is close to a universal value σ 0 ¼ πe 2 =2h [3]. Free-carrier intraband transitions, on the other hand, cause lowfrequency absorption, which varies significantly with charge density and results in strong light extinction at high carrier density [4]. In addition to this density dependence, the massless Dirac particles in graphene are predicted to exhibit a distinctive nonmonotonic temperature dependence of the intraband absorption strength, or Drude weight, due to their linear dispersion [5,6]. This behavior contrasts with the temperature-independent Drude weight expected in conventional systems of massive particles with parabolic dispersion [7,8]. Although the unique behavior of the Drude weight in graphene has been considered theoretically, experimental signatures are still lacking.The intrinsic properties of Drude absorption in graphene can be revealed by studying its dynamical response to photoexcitation. In particular, optical-pump terahertz-probe spectroscopy provides access to a wide transient temperature range via pulsed optical excitation, and allows measurement of the ac Drude conductivity by a timedelayed terahertz probe pulse [9]. This technique has been applied to study transient photoconductivity (PC) in graphene, but conflicting results have been reported [9][10][11][12][13][14][15]. Positive PC was observed in epitaxial graphene on SiC (Ref.[15]), while negative PC was seen in graphene grown by chemical vapor deposition (CVD) [11][12][13][14]. It has been argued that the opposite behavior in these samples arises from their different charge densities. Here we study...

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Section: Prl 113 056602 (2014) P H Y S I C a L R E V I E W L E T T Ementioning

confidence: 99%

“…In particular, we assumed a biexponential decay with time constants τ 1 ¼ 0.3 ps and τ 2 ¼ 3.1 ps and a 200 fs rise time [Refs. [33,39]; see inset of Fig. 3(b)].…”

Section: Prl 113 056602 (2014) P H Y S I C a L R E V I E W L E T T Ementioning

confidence: 99%

Semiconducting-to-Metallic Photoconductivity Crossover and Temperature-Dependent Drude Weight in Graphene

et al. 2014

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“…At low temperatures, the intrinsic process is expected to be dominant. However, the intrinsic NC process has never been experimentally observed before [11,14].Here, we report on temperature-dependent spatially resolved photocurrent measurements of high-quality monolayer graphene (MLG) p-n junction devices. At the p-n interface, the photothermoelectric (PTE) effect dominates the photocurrent generation [2-4] and exhibits a nonmonotonic temperature dependence.…”

mentioning

confidence: 95%

“…At low temperatures, the intrinsic process is expected to be dominant. However, the intrinsic NC process has never been experimentally observed before [11,14].…”

mentioning

confidence: 99%

“…The sensitivity of T Ã to the charge density and disorder concentration may account for the different (monotonic) temperature-dependent behavior observed in previous studies [1,4,11,14,[24][25][26][27][28]. In addition, all the above arguments are based on the T e ≳ T 0 condition, while otherwise we need to consider the full expression of the relative cooling weight between the SC and NC, which is derived in Ref.…”

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Competing Channels for Hot-Electron Cooling in Graphene

et al. 2014

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We report on temperature-dependent photocurrent measurements of high-quality dual-gated monolayer graphene p-n junction devices. A photothermoelectric effect governs the photocurrent response in our devices, allowing us to track the hot-electron temperature and probe hot-electron cooling channels over a wide temperature range (4 to 300 K). At high temperatures (T > T Ã ), we found that both the peak photocurrent and the hot spot size decreased with temperature, while at low temperatures (T < T Ã ), we found the opposite, namely that the peak photocurrent and the hot spot size increased with temperature. This nonmonotonic temperature dependence can be understood as resulting from the competition between two hot-electron cooling pathways: (a) (intrinsic) momentum-conserving normal collisions that dominates at low temperatures and (b) (extrinsic) disorder-assisted supercollisions that dominates at high temperatures. Gate control in our high-quality samples allows us to resolve the two processes in the same device for the first time. The peak temperature T Ã depends on carrier density and disorder concentration, thus allowing for an unprecedented way of controlling graphene's photoresponse. Slow electron-lattice thermal equilibration is responsible for a plethora of new optoelectronic [1][2][3][4], transport [5,6], and thermoelectronic [7,8] phenomena in graphene. The wide temperature ranges (lattice temperature, 4 to 300 K) and long spatial scales in which hot carriers proliferate make graphene an ideal candidate for electronic energy transduction and numerous applications. Central to these are the unusual electron-phonon scattering pathways that dominate the cooling channels of graphene [7,[9][10][11][12][13].Unlike other materials, electron-lattice cooling at room temperature in graphene is dominated by an extrinsic threebody process [10]. This occurs when acoustic phonon emission is assisted by disorder scattering, called "supercollisions" (SC). SC dominates over the intrinsic momentumconserving emission of acoustic phonons (normal collisions [NC]) for high temperatures. At low temperatures, the intrinsic process is expected to be dominant. However, the intrinsic NC process has never been experimentally observed before [11,14].Here, we report on temperature-dependent spatially resolved photocurrent measurements of high-quality monolayer graphene (MLG) p-n junction devices. At the p-n interface, the photothermoelectric (PTE) effect dominates the photocurrent generation [2-4] and exhibits a nonmonotonic temperature dependence. We demonstrate that both the magnitude and spatial extent of the photoresponse are highly enhanced at an intermediate temperature T Ã , indicating the coexistence of momentum-conserving NC cooling and disorder-assisted SC mechanisms. NC (SC) cooling dominates below (above) T Ã , which can be tuned by varying the charge and impurity densities. In addition, we observed that the photoresponse at the graphene-metal (G-M) interface is also dominated by the PTE effect with a similar temperature-de...

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Ultrafast Laser Processing of 2D Materials: Novel Routes to Advanced Devices

Emelianov,

Pettersson,

Bobrinetskiy

2024

Advanced Materials

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Ultrafast laser processing has emerged as a versatile technique for modifying materials and introducing novel functionalities. Over the past decade, this method has demonstrated remarkable advantages in the manipulation of 2D layered materials, including synthesis, structuring, functionalization, and local patterning. Unlike continuous‐wave and long‐pulsed optical methods, ultrafast lasers offer a solution for thermal heating issues. Nonlinear interactions between ultrafast laser pulses and the atomic lattice of 2D materials substantially influence their chemical and physical properties. This paper highlights the transformative role of ultrafast laser pulses in maskless green technology, enabling subtractive, and additive processes that unveil ways for advanced devices. Utilizing the synergetic effect between the energy states within the atomic layers and ultrafast laser irradiation, it is feasible to achieve unprecedented resolutions down to several nanometers. Recent advancements are discussed in functionalization, doping, atomic reconstruction, phase transformation, and 2D and 3D micro‐ and nanopatterning. A forward‐looking perspective on a wide array of applications of 2D materials, along with device fabrication featuring novel physical and chemical properties through direct ultrafast laser writing, is also provided.

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Transient Absorption and Photocurrent Microscopy Show That Hot Electron Supercollisions Describe the Rate-Limiting Relaxation Step in Graphene

Cited by 66 publications

References 35 publications

Semiconducting-to-Metallic Photoconductivity Crossover and Temperature-Dependent Drude Weight in Graphene

Semiconducting-to-Metallic Photoconductivity Crossover and Temperature-Dependent Drude Weight in Graphene

Competing Channels for Hot-Electron Cooling in Graphene

Ultrafast Laser Processing of 2D Materials: Novel Routes to Advanced Devices

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