Tuning ultrafast electron thermalization pathways in a van der Waals heterostructure

Ultrafast electron thermalization—the process leading to carrier multiplication via impact ionization1,2, and hot-carrier luminescence3,4—occurs when optically excited electrons in a material undergo rapid electron–electron scattering3,5–7 to redistribute excess energy and reach electronic thermal equilibrium. Owing 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 techniques8–14. Here, we overcome this challenge by leveraging the atomic thinness of two-dimensional van der Waals (vdW) materials to introduce a highly tunable electron transfer pathway that directly competeswith electron thermalization.We realize this scheme in a graphene–boron nitride–graphene (G–BN–G) vdW heterostructure15–17, through which optically excited carriers are transported from one graphene layer to the other. By applying an interlayer bias voltage or varying the excitation photon energy, interlayer carrier transport can be controlled to occur faster or slower than the intralayer scattering events, thus e ectively tuning the electron thermalization pathways in graphene. Our findings, which demonstrate a means to probe and directly modulate electron energy transport in nanoscale materials, represent a step towards designing and implementing optoelectronic and energy-harvesting devices with tailored microscopic properties. Immediately after photoexcitation of an optoelectronic device, energetic electrons scatter with other high-energy and ambient charge carriers to form a thermalized hot electron gas, which further cools by dissipating excess energy to the lattice. Owing to the short distance travelled by charge carriers between electron–electron scattering events in solids18, equilibration among the electrons occurs on timescales from tens of femtoseconds to picoseconds19,20. In graphene, a low-dimensional material with much enhanced Coulomb interaction21, electron thermalization is known to occur on extremely fast timescales (<30 fs; refs 22–25), reflecting the extremely short transit length between scattering events. Most analyses of graphene have, therefore, treated its electrons as being instantaneously thermalized26–29, and slightly non-thermal electronic behaviour has thus far only been reported in pumpprobe experiments with ultrashort (∼10 fs) laser pulses and low excitation density8,9. Owing to such short time (femtosecond) and length (nanometre) scales, it is challenging to detect and control the thermalization process in graphene or, more generally, in any solid-state systems. In this letter, we report an approach to probe and manipulate the electron thermalization in graphene by introducing a new energy transport channel that competes with the thermalization process. Such an additional dynamical pathway is realized in a vdW heterostructure30 that consists of a G–BN–G stack (Fig. 1a–c). In this layered structure, the photoexcited electrons in one graphene layer can travel vertically to the other graphene layer through the very thin middle BN layer (blue dashed arrow in Fig. 1b). Given the close proximity of these layers, interlayer charge transport can occur on extremely fast timescales31, and thus compete directly with the intralayer thermalization process (red arrows in Fig. 1b). In our experiment, we have observed such competing processes by measuring the interlayer photocurrent under different bias and excitation conditions. Remarkably, by adjusting the interlayer bias voltage or varying the excitation photon energy, we can control the interlayer charge transport to occur slower or faster than the intralayer thermalization, thus tuning the thermalization process. Our experiments not only provide valuable insight into the electron dynamics of graphene, but also demonstrate a means to manipulate electron thermalization in low-dimensional materials. We fabricated the G–BN–G heterostructure devices on Si/SiO2 substrates by mechanically co-laminating graphene sheets and hexagonal boron nitride (BN) flakes32 with 5–30 nm thickness (Fig. 1a,d; see Methods). In our experiment, we applied a bias voltage Vb between the top and bottom graphene and measured the corresponding interlayer current I under optical excitation. The main light source is a broadband supercontinuum laser that provides bright semi-continuous radiation fromwavelength λ=450 to 2,000 nm (see Methods). To probe the interlayer current in the time domain, we also used femtosecond laser pulses from an 80MHz Ti:sapphire oscillator. We have measured four G–BN–G devices with monolayer graphene and two devices with fewlayer (≤ four layers) graphene, and found similar results. The device characteristics are therefore insensitive to a slight change of graphene layer thickness. Figure 1d–f shows the optical image of a G–BN–G device and the corresponding device characterization using scanning photocurrent microscopy. Photocurrent images at interlayer bias voltages Vb=−0.5 and 0.5V under laser excitation at a wavelength of 600 nm show that the photocurrent I appears only in the