performance. Here we demonstrate that graphene/WSe2/graphene heterostructures ally the high photodetection efficiency of TMDs 5,6 with a picosecond photoresponse comparable to that of graphene 7-9 , thereby optimizing both speed and efficiency in a single photodetector. We follow in time the extraction of photoexcited carriers in these devices using timeresolved photocurrent measurements and demonstrate a photoresponse time as short as 5.5 ps, which we tune by applying a bias and by varying the TMD layer thickness. Our study provides direct insight into the physical processes governing the detection speed and quantum efficiency of these van der Waals (vdW) heterostuctures, such as out-of-plane carrier drift and recombination.The observation and understanding of ultrafast and efficient photodetection demonstrate the potential of hybrid TMD-based heterostructures as a platform for future optoelectronic devices. 2The optoelectronic response of 2D crystals is currently the subject of intense investigation [1][2][3][5][6][7][8][9][10][11][12][13][14][15][16] prompted by the need for next-generation photodetectors with superior performance in terms of efficiency, detection speed, as well as flexibility and transparency 17 . High photon absorption 5,18 and large photoconducting gain 11,12,14 have been observed in devices based on semiconducting 2D crystals. Yet, the observed response time typically ranges from nanoseconds 16 to seconds 11,14 , with faster devices often displaying lower responsivity 11 . Therefore, the main challenge is to develop and assess photodetectors based on 2D semiconductor crystals that simultaneously possess a large active area, high internal efficiency, and fast response time.A promising approach to create such versatile devices is to sandwich a TMD layer between two graphene sheets serving as charge extraction contacts. In contrast to lateral photodetectors, such vertical van der Waals (vdW) heterostructures 4 have the advantage of possessing a large, scalable active area and an atomically short charge extraction channel, potentially enabling both efficient and fast photodetection. Whereas the quantum efficiency of these vdW devices 5,6,13 and the dynamics of photocarrier creation and relaxation in TMDs [19][20][21][22][23][24][25][26][27] have been addressed, the response time of TMD-based photodetectors, as well as the dynamic processes governing their quantum efficiency remain elusive.Here, we report on the intrinsic processes that limit the performance of photodetectors based on high-quality G/WSe2/G (with G representing graphene) vdW heterostructures encapsulated in hexagonal boron nitride (hBN) 28 . We perform time-resolved photocurrent measurements 7,29 on devices consisting of WSe2 flakes with a range of thicknesses (monolayer and multilayers from 2.2 to 40 nm). This technique, which combines electronic detection with subpicosecond optical excitation, allows probing of the extraction ( Figure 1a) and loss dynamics of the photoexcited charge carriers in the photoactive TMD layer. We ...

This review examines the properties of graphene from an experimental perspective. The intent is to review the most important experimental results at a level of detail appropriate for new graduate students who are interested in a general overview of the fascinating properties of graphene. While some introductory theoretical concepts are provided, including a discussion of the electronic band structure and phonon dispersion, the main emphasis is on describing relevant experiments and important results as well as some of the novel applications of graphene. In particular, this review covers graphene synthesis and characterization, field-effect behavior, electronic transport properties, magnetotransport, integer and fractional quantum Hall effects, mechanical properties, transistors, optoelectronics, graphene-based sensors, and biosensors. This approach attempts to highlight both the means by which the current understanding of graphene has come about and some tools for future contributions.

*Graphene is a promising material for ultrafast and broadband photodetection. Earlier studies have addressed the general operation of graphene-based photothermoelectric devices and the switching speed, which is limited by the charge carrier cooling time, on the order of picoseconds. However, the generation of the photovoltage could occur at a much faster timescale, as it is associated with the carrier heating time. Here, we measure the photovoltage generation time and find it to be faster than 50 fs. As a proof-of-principle application of this ultrafast photodetector, we use graphene to directly measure, electrically, the pulse duration of a sub-50 fs laser pulse. The observation that carrier heating is ultrafast suggests that energy from absorbed photons can be efficiently transferred to carrier heat. To study this, we examine the spectral response and find a constant spectral responsivity of between 500 and 1,500 nm. This is consistent with efficient electron heating. These results are promising for ultrafast femtosecond and broadband photodetector applications. Photovoltage generation through the photothermoelectric (PTE) effect occurs when light is focused at the interface of monolayer and bilayer graphene, or at the interface between regions of graphene with different Fermi energies E F (refs 1-6). In such graphene PTE devices-which operate over a large spectral range 7,8 that extends even into the far-infrared 9 -local heating of electrons by absorbed light, in combination with a difference in Seebeck coefficients between the two regions, gives rise to a PTE voltage V PTE = (S 2 − S 1 )(T el − T 0 ). Here, S 1 and S 2 are the Seebeck coefficients of regions 1 and 2, respectively, T el is the hot electron temperature after photoexcitation and electron heating, and T 0 is the temperature of the electrode heat sinks. The performance of PTE graphene devices is intimately connected to the dynamics of the photoexcited electrons and holes, which have mainly been studied in graphene samples through ultrafast optical pumpprobe measurements [10][11][12][13][14][15][16][17] . As shown in Fig. 1a, the dynamics start with (i) photoexcitation and electron-hole pair generation, followed by (ii) electron heating through carrier-carrier scattering, in competition with lattice heating, both of which take place on a sub-100 fs timescale, and finally (iii) electron cooling by thermal equilibration with the lattice, which takes place on a picosecond timescale. The effect of the picosecond cooling step (iii) on the switching speed of graphene devices has been studied using timeresolved photovoltage scanning experiments with ∼200 fs time resolution [18][19][20] . These studies showed that the picosecond electron cooling time limits the intrinsic photo-switching rate of these devices to a few hundred gigahertz, because faster switching would reduce the switching contrast, as the system does not have time to return to the ground state. Indeed, gigahertz switching speeds have been demonstrated in graphene-based devices [21][22][23]...

Finding alternative optoelectronic mechanisms that overcome the limitations of conventional semiconductor devices is paramount for detecting and harvesting low-energy photons. A highly promising approach is to drive a current from the thermal energy added to the free-electron bath as a result of light absorption. Successful implementation of this strategy requires a broadband absorber where carriers interact among themselves more strongly than with phonons, as well as energy-selective contacts to extract the excess electronic heat. Here we show that graphene-WSe2-graphene heterostructure devices offer this possibility through the photo-thermionic effect: the absorbed photon energy in graphene is efficiently transferred to the electron bath leading to a thermalized hot carrier distribution. Carriers with energy higher than the Schottky barrier between graphene and WSe2 can be emitted over the barrier, thus creating photocurrent. We experimentally demonstrate that the photo-thermionic effect enables detection of sub-bandgap photons, while being size-scalable, electrically tunable, broadband and ultrafast.

Two-dimensional (2D) semiconducting materials are promising building blocks for optoelectronic applications, many of which require efficient dissociation of excitons into free electrons and holes. However, the strongly bound excitons arising from the enhanced Coulomb interaction in these monolayers suppresses the creation of free carriers. Here, we identify the main exciton dissociation mechanism through time and spectrally resolved photocurrent measurements in a monolayer WSe2 p–n junction. We find that under static in-plane electric field, excitons dissociate at a rate corresponding to the one predicted for tunnel ionization of 2D Wannier–Mott excitons. This study is essential for understanding the photoresponse of 2D semiconductors and offers design rules for the realization of efficient photodetectors, valley dependent optoelectronics, and novel quantum coherent phases.

Ultrafast electron thermalization -the process leading to Auger recombination 1 , carrier multiplication via impact ionization 2, 3 , and hot carrier luminescence 4, 5 -occurs when optically excited electrons in a material undergo rapid electron-electron scattering 4, 6, 7, 8 to redistribute excess energy and reach electronic thermal equilibrium. Due to extremely short time and length scales, the measurement and manipulation of electron thermalization in nanoscale devices remains challenging even with the most advanced ultrafast laser techniques 9, 10, 11, 12 . Here, we overcome this challenge by leveraging the atomic thinness of two-dimensional van der Waals (vdW) materials in order to introduce a highly tunable electron transfer pathway that directly competes with electron thermalization. We realize this scheme in a graphene-boron nitride-graphene (G-BN-G) vdW heterostructure 13, 14, 15 ,

Van der Waals heterostructures have emerged as promising building blocks that offer access to new physics, novel device functionalities, and superior electrical and optoelectronic properties [1][2][3][4][5][6][7]. Applications such as thermal management, photodetection, light emission, data communication, high-speed electronics and light harvesting [8][9][10][11][12][13][14][15][16] require a thorough understanding of (nanoscale) heat flow. Here, using time-resolved photocurrent measurements we identify an efficient out-of-plane energy transfer channel, where charge carriers in graphene couple to hyperbolic phonon polaritons [17][18][19] in the encapsulating layered material. This hyperbolic cooling is particularly efficient, giving picosecond cooling times, for hexagonal BN, where the high-momentum hyperbolic phonon polaritons enable efficient near-field energy transfer. We study this heat transfer mechanism through distinct control knobs to vary carrier density and lattice temperature, and find excellent agreement with theory without any adjustable parameters. These insights may lead to the ability to control heat flow in van der Waals heterostructures.Owing to its large in-plane thermal conductivity, graphene has been suggested as material for the thermal management of nanoscale devices [8]. At the same time, graphene is well-known for its ability to convert incident light into electrical heat, i.e. hot electrons that can be used to generate photocurrent, with applications in photodetection, data communication and light harvesting [10,20,21]. Understanding, and ultimately controlling, heat flow in graphene-van der Waals heterostructures is therefore of paramount importance. For example, a short cooling time of graphene hot carriers is advantageous for thermal management and for high switching rates of photodetectors (PDs) for data communication, whereas a long cooling time is favorable for photodetection sensitivity [10,20,21]. Of particular relevance are heterostructure devices that contain high-quality graphene encapsulated by layered materials, such as hexagonal BN (hBN) and MoS 2 , which have the potential to crucially improve the performance of electronic and optoelectronic devices [1,2]. * Electronic address: Correspondence: klaas-jan.tielrooij@icfo.eu, frank.koppens@icfo.eu ‡ Equal contribution.A number of cooling pathways for graphene carriers have been proposed, involving among others strongly coupled optical phonons [22][23][24], acoustic phonons [25][26][27][28], substrate phonons [29] and plasmons [30] (see also Appendix 1). Here, using several experimental approaches, we show that cooling in graphene encapsulated by hBN is governed by out-of-plane coupling of graphene electrons to special polar phonon modes that occur in layered materials (LMs): hyperbolic phonon polaritons, where xx zz < 0, with xx and zz the permittivity parallel and perpendicular to the LM plane. Owing to this property, these materials carry deep sub-wavelength, raylike optical phonon polaritons. For hBN, within the two Rest...

The speed of solid-state electronic devices, determined by the temporal dynamics of charge carriers, could potentially reach unprecedented petahertz frequencies through direct manipulation by optical fields, consisting in a million-fold increase from state-of-the-art technology. In graphene, charge carrier manipulation is facilitated by exceptionally strong coupling to optical fields, from which stems an important back-action of photoexcited carriers. Here we investigate the instantaneous response of graphene to ultrafast optical fields, elucidating the role of hot carriers on sub-100 fs timescales. The measured nonlinear response and its dependence on interaction time and field polarization reveal the back-action of hot carriers over timescales commensurate with the optical field. An intuitive picture is given for the carrier trajectories in response to the optical-field polarization state. We note that the peculiar interplay between optical fields and charge carriers in graphene may also apply to surface states in topological insulators with similar Dirac cone dispersion relations.