Hot-Carrier Cooling in High-Quality Graphene is Intrinsically Limited by Optical Phonons
Eva A. A. Pogna, Xiaoyu Jia, Alessandro Principi, Alexander Block, Luca Banszerus, Jincan Zhang, Xiaoting Liu, Thibault Sohier, Stiven Forti, Karuppasamy Sounda… 展开作者
Eva A. A. Pogna, Xiaoyu Jia, Alessandro Principi, Alexander Block, Luca Banszerus, Jincan Zhang, Xiaoting Liu, Thibault Sohier, Stiven Forti, Karuppasamy Soundarapandian, Bernat Terrés, Jake D. Mehew, Chiara Trovatello, Camilla Coletti, Frank H. L. Koppens, Mischa Bonn, Niek van Hulst, Matthieu J. Verstraete, Hailin Peng, Zhongfan Liu, Christoph Stampfer, Giulio Cerullo, Klaas-Jan Tielrooij
Many promising optoelectronic devices, such as broadband photodetectors, nonlinear frequency converters, and building blocks for data communication systems, exploit photoexcited charge carriers in graphene. For these systems, it is essential to understand, and eventually control, the cooling dynamics of the photoinduced hot-carrier distribution. There is, however, still an active debate on the different mechanisms that contribute to hot-carrier cooling. In particular, the intrinsic cooling mechanism that ultimately limits the cooling dynamics remains an open question. Here, we address this question by studying two technologically relevant systems, consisting of high-quality graphene with a mobility >10,000 cm$^2$V$^{-1}$s$^{-1}$ and environments that do not efficiently take up electronic heat from graphene: WSe$_2$-encapsulated graphene and suspended graphene. We study the cooling dynamics of these two high-quality graphene systems using ultrafast pump-probe spectroscopy at room temperature. Cooling via disorder-assisted acoustic phonon scattering and out-of-plane heat transfer to the environment is relatively inefficient in these systems, predicting a cooling time of tens of picoseconds. However, we observe much faster cooling, on a timescale of a few picoseconds. We attribute this to an intrinsic cooling mechanism, where carriers in the hot-carrier distribution with enough kinetic energy emit optical phonons. During phonon emission, the electronic system continuously re-thermalizes, re-creating carriers with enough energy to emit optical phonons. We develop an analytical model that explains the observed dynamics, where cooling is eventually limited by optical-to-acoustic phonon coupling. These fundamental insights into the intrinsic cooling mechanism of hot carriers in graphene will play a key role in guiding the development of graphene-based optoelectronic devices.
We perform a femtosecond-THz optical pump-probe spectroscopy to investigate the cooling dynamics of hot carriers in quasi-free standing bilayer epitaxial graphene. We observed longer decay time constants, in the range of 2.6 to 6.4 ps, compared to previous studies on monolayer graphene, which increase nonlinearly with excitation intensity. The increased relaxation times are due to the decoupling of the graphene layer from the SiC substrate after hydrogen intercalation which increases the distance between graphene and substrate. Furthermore, our measurements do not show that the supercollision …
In the past 50 years, the high gain in quantum efficiency of photoconductors is often explained by a widely accepted theory in which the photogain is proportional to the minority carrier lifetime and inversely proportional to the carrier transit time across the photoconductor. It occasionally misleads scientists to believe that a high-speed and high-gain photodetector can be made simply by shortening the device length. The theory is derived on the assumption that the distribution of photogenerated excess carriers is spatially uniform. In this Letter, we find that this assumption is not valid f…
Being used in optoelectronic devices as ultra-thin conductor-insulator junctions, detailed investigations are needed about how exactly h-BN and graphene hybridize. Here, we present a comprehensive ab initio study of hot carrier dynamics governed by electron-phonon scattering at the h-BN/graphene interface, using graphite (bulk), monolayer and bilayer graphene as benchmark materials. In contrast to monolayer graphene, all multilayer structures possess low-energy optical phonon modes that facilitate carrier thermalization. We find that the h-BN/graphene interface represents an exception with com…
Graphene-based Photodetector with Engineered Hot Carrier Cooling Dynamics
Imec；Ghent University–imec（根特大学-imec）；Catalan Institute of Nanoscience and Nanotechnology (ICN2), CSIC and BIST（加泰罗尼亚纳米科学和纳米技术研究所（ICN2），西班牙高等科学研究委员会和巴塞尔研究所）；TU Eindhoven（埃因霍温理工大学）；Utrecht University（乌得勒支大学）；The University of Manchester（曼彻斯特大学）
Silicon nanoparticles have the promise to surpass the theoretical efficiency limit of single-junction silicon photovoltaics by the creation of a "phonon bottleneck", a theorized slowing of the cooling rate of hot optical phonons that in turn reduces the cooling rate of hot carriers in the material. To verify the presence of a phonon bottleneck in silicon nanoparticles requires simultaneous resolution of electronic and structural changes at short timescales. Here, extreme ultraviolet transient absorption spectroscopy is used to observe the excited state electronic and lattice dynamics in polycr…
We monitor the dynamics of hot carriers in InSe by means of two photons photoelectron spectroscopy (2PPE). The electrons excited by photons of 3.12 eV experience a manifold relaxation. First, they thermalize to the electronic states degenerate with the $\bar M$ valley. Subsequently, the electronic cooling is dictated by Fröhlich coupling with phonons of small momentum transfer. Ab-initio calculations predict cooling rates that are in good agreement with the observed dynamics. We argue that electrons accumulating in states degenerate with the $\bar M$ valley could travel through a multilayer fl…
Controlling energy flows in solids through switchable electron-lattice cooling can grant access to a range of interesting and potentially useful energy transport phenomena. Here we discuss a unique switchable electron-lattice cooling mechanism arising in graphene due to phonon emission mediated by resonant scattering on defects in crystal lattice, which displays interesting analogy to the Purcell effect in optics. This mechanism strongly enhances the electron-phonon cooling rate, since non-equilibrium carriers in the presence of momentum recoil due to disorder can access a larger phonon phase …
High Intrinsic Mobility and Ultrafast Carrier Dynamics in Multilayer Metal Dichalcogenide MoS2
在金属二硫属化物中载流子迁移率的最终限制，以及与载流子弛豫相关的动力学，尚不清楚。我们提出了测量的多层dichalcogenide二硫化钼的频率依赖性的电导率的光泵太赫兹探测光谱。我们发现这种材料在低温下的迁移率接近4200 cm 2/Vs。散射的温度依赖性表明，迁移率，一个数量级大于以前报道的二硫化钼，是固有的限制在太赫兹频率的声学声子散射。我们的测量载流子弛豫揭示皮秒冷却时间，然后再复合持续数十纳秒，并占主导地位的俄歇散射到缺陷。我们的研究结果提供了一个有用的背景下，了解和评估的二硫化钼为基础的电子和光电子器件的性能。
Dielectric control of ultrafast carrier dynamics and transport in graphene
Max Planck Institute for Polymer Research（马克斯·普朗克聚合物研究所）；Debye Institute for Nanomaterials Science, Utrecht University（乌得勒支大学德拜纳米科学研究所）；Catalan Institute of Nanoscience and Nanotechnology (ICN2), CSIC and BIST（加泰罗尼亚纳米科学与纳米技术研究所（ICN2，西班牙高等科学研究委员会和巴塞罗那独立科学与技术基金会））；School of Physics and Astronomy, University of Manchester（曼彻斯特大学物理与天文学学院）；Department of Applied Physics, TU Eindhoven（埃因霍温理工大学应用物理系）
We show that the conductivity of a two-dimensional electron gas can be intrinsically anisotropic despite isotropic Fermi surface, energy dispersion, and disorder configuration. In the model we study, the anisotropy stems from the interplay between Dirac and Schrödinger features combined in a special two-band Hamiltonian describing the quasiparticles similar to the low-energy excitations in phosphorene. As a result, even scalar isotropic disorder scattering alters the nature of the carriers and results in anisotropic transport. Solving the Boltzmann equation exactly for such carriers with point…
Carrier relaxation measurements in moiré materials offer a unique probe of the microscopic interactions, in particular the ones that are not easily measured by transport. Umklapp scattering between phonons is a ubiquitous momentum-nonconserving process that governs the thermal conductivity of semiconductors and insulators. In contrast, Umklapp scattering between electrons and phonons has not been demonstrated experimentally. Here, we study the cooling of hot electrons in moiré graphene using time- and frequency-resolved photovoltage measurements as a direct probe of its complex energy pathways…
Low Carrier Density Epitaxial Graphene Devices On SiC
在六方Si端SiC衬底上生长的单层外延石墨烯（EG）本质上是电子掺杂的（载流子密度约为10^13 cm^（-2））。我们展示了一个干净的设备制造过程中使用的贵金属保护层，并表明，与王水蚀刻的结果在p型（空穴）分子掺杂我们的非门控，无污染的EG。通过这种简单的工艺制造的器件可以达到10^10 cm^（-2）到10^11 cm^（-2）的载流子密度，迁移率约为8000 cm ^2/V/s或更高。在载流子密度n = 2.4 x 10^11 cm^（-2）、迁移率= 5200 cm ^2/V/s的中度掺杂器件中，我们观察到高度发达的量子化霍尔电阻平台，在磁场强度小于4 T时填充因子为2。掺杂浓度可以通过在Ar中的热处理恢复到更高的水平，而具有p型和n型多数载流子的器件在环境空气中倾向于向较低的载流子浓度漂移。
Carrier dynamics in metallic nanostructures is strongly influenced by their confining dimensions. Gold nanoparticles of size $\sim 2$ nm lie at the boundary separating metallic and non-metallic behavior. In this work, we report carrier dynamics in high aspect ratio ultrathin gold nanowires (Au-UNWs) of average diameter $\sim$2 nm using pump (3.1 eV) and coherent white light continuum as probe in the spectral range of 1.15 eV to 2.75 eV. The transient carrier dynamics in Au-UNWs under extreme excitation regime is slower than predicted by the often used two temperature model. We identify Auger-a…
Measurement of intrinsic Dirac fermion cooling on the surface of a topological insulator Bi$_2$Se$_3$ using time- and angle-resolved photoemission spectroscopy
我们进行时间和角度分辨的光电子能谱的原型拓扑绝缘体Bi $_2$ Se $_3$的表面和体电子光激发后的超快动力学研究。通过分析表面态和体能带谱的演化，分别得到了它们的电子温度和化学势弛豫动力学。这些动力学揭示了强声子辅助的表面-体耦合在高晶格温度和总抑制在低晶格温度的表面和体之间的非弹性散射。在这个低温制度下，独特的冷却狄拉克费米子在TI的声学声子表现出通过幂律依赖的表面温度衰减速率载流子密度。
Anisotropic hot carrier relaxation mediated by electron phonon scattering in TiN thin films
National Taiwan University（国立台湾大学）；Research Center for Applied Sciences, Academia Sinica（中央研究院应用科学研究中心）；Nano Science and Technology Program, TIGP, Academia Sinica（中央研究院纳米科技计划，全球研究生学位学程）；Institute of Atomic and Molecular Sciences, Academia Sinica（中央研究院原子与分子科学研究所）
With the technology of artificial defects creating, we can tune the band structure and transport properties of many two-dimensional (2D) layered materials. One prototype materials system is the antidoted graphene sheet, where periodical pores are made using focuses ion or electron beams in the nanoscale. We here study the electrical conductivity, thermopower, and active rates of cooling and refrigeration of antidoted graphene samples with different pore-radii and interporous distances. We use a calculation method that takes into consider the sensitivity of transport to charge carrier energy, w…
Nanowires (NWs) with their quasi-one-dimensionality often present different structural and opto-electronic properties than their thin-film counterparts. The thinner they are the larger these differences are, in particular in the carrier-phonon scattering and thermal conductivity. In this work, we present femtosecond transient absorbance measurements on GaAs0.8P0.2 NWs of two different diameters, 36 and 51 nm. The results show that thinner NWs sustain the hot-carriers at a higher temperature for longer times than thicker NWs. We explain the observation suggesting that in thinner NWs, the build-…
The timescale of electronic cooling is an important parameter controlling the performance of devices based on quantum materials for optoelectronic, thermoelectric and thermal management applications. In most conventional materials, cooling proceeds via the emission of phonons, a relatively slow process that can bottleneck the carrier relaxation dynamics, thus degrading the device performance. Here we present the theory of near-field radiative heat transfer, that occurs when a two-dimensional electron system is coupled via the non-retarded Coulomb interaction to a three-dimensional bulk that ca…
In semiconductor materials, hot exciton cooling is the process by which highly excited carriers nonradiatively relax to form a band edge exciton. While cooling plays an important role in determining the thermal losses and quantum yield of a system, the timescales and mechanism of cooling are not well understood in confined semiconductor nanocrystals (NCs). A mismatch between electronic energy gaps and phonon frequencies in NCs has led to the hypothesis of a phonon bottleneck, in which cooling would be extremely slow, while enhanced electron-hole interactions in NCs have been used to explain co…
Charge carrier scattering is critical to the electrical properties of two-dimensional materials such as graphene, transition metal dichalcogenide monolayers, black phosphorene, and tellurene. Beyond pristine two-dimensional materials, further tailored properties can be achieved by nanoporous patterns such as nano- or atomic-scale pores (antidots) across the material. As one example, structure-dependent electrical/optical properties for graphene antidot lattices (GALs) have been studied in recent years. However, detailed charge carrier scattering mechanism is still not fully understood, which h…
The optical and electronic properties of 2D semiconductors are intrinsically linked via the strong interactions between optically excited bound species and free carriers. Here we use near-field scanning microwave microscopy (SMM) to image spatial variations in photoconductivity in MoS$_2$--WS$_2$ lateral multijunction heterostructures using photon energy-resolved narrowband illumination. We find that the onset of photoconductivity in individual domains corresponds to the optical absorption onset, confirming that the tightly bound excitons in transition metal dichalcogenides can nonetheless dis…
Pauli blockade occurs when the excited electrons fill up the states near the conduction bands and block subsequent absorption in semiconductors, and has been widely applied in mode-locking for passively-pulsed-laser systems. In this letter, we report the first direct observation that the Pauli blockade is broken by ultrafast cooling of hot electrons in optically-pumped graphene. With femtosecond spectroscopy, we demonstrate that the time scale to excite an electron (~100 fs) is of the same order as that of the electron decay via electron-electron scattering, which allows the electron excitatio…
In this work, we report on hot carrier diffusion in graphene across large enough length scales that the carriers are not thermalized across the crystal. The carriers are injected into graphene at one site and their thermal transport is studied as a function of applied power and distance from the heating source, up to tens of micrometers away. Superconducting contacts prevent out-diffusion of hot carriers to isolate the electron-phonon coupling as the sole channel for thermal relaxation. As local thermometers, we use the amplitude of the Universal Conductance Fluctuations, which varies monotoni…
For high-speed optoelectronic applications relying on fast relaxation or energy transfer mechanisms, understanding of carrier relaxation and recombination dynamics is critical. Here, we compare the differences in photoexcited carrier dynamics in 2D and quasi-3D colloidal methylammonium lead iodide perovskite nanoplatelets via differential transmission spectroscopy. We find that the cooling of excited electron-hole pairs by phonon emission progresses much faster and is intensity-independent in the 2D-case. This is due to the low dielectric surrounding of the thin perovskite layers, for which th…
Revealing the ultrafast dynamics of three-dimensional (3D) Dirac fermions upon photoexcitation is critical for both fundamental science and device applications. So far, how the cooling of 3D Dirac fermions differs from that of two-dimensional (2D) Dirac fermions and whether there is population inversion are fundamental questions that remain to be answered. Here we reveal the ultrafast dynamics of Dirac fermions in a model 3D Dirac semimetal Cd$_3$As$_2$ by ultrafast time- and angle-resolved photoemission spectroscopy (TrARPES) with a tunable probe photon energy from 5.3 - 6.9 eV. The energy- a…
The acoustic, optic, and surface polar optic phonons are the three important intrinsic and extrinsic phononic modes that increasingly populate graphene on a substrate with rising temperatures; the coupling of which with photoexcited hot carriers in the equipartition regime provides significant pathways for electron-phonon relaxation. In this paper, we theoretically investigate the relative significance of the three phononic modes in electron scattering and cooling phenomena in single layer graphene, including their comparison with supercollision driven power loss, and obtain analytical formula…
We present a numerical study on the intraband optical conductivity of hot carriers at quasi-equilibria in photoexcited graphene based on the semiclassical Boltzmann transport equations (BTE) with the aim of understanding the effects of intrinsic optical phonon and extrinsic coulomb scattering caused by charged impurities at the graphene--substrate interface. Instead of using full-BTE solutions, we employ iterative solutions of the BTE and the comprehensive model for the temporal evolutions of hot-carrier temperature and hot-optical-phonon occupations to reduce computational costs. Undoped grap…
