Out-of-plane heat transfer in van der Waals stacks through electron–hyperbolic phonon coupling

Abstract

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...

Ultrafast relaxation of hot phonons in graphene-hBN heterostructures

“…Faster relaxation of carriers in graphene-hBN heterostructures has also recently been observed through photoconductivity measurements and attributed to hyperbolic phonon coupling. 39 See supplementary material for the following sections: S1-Lateral heat transport, S2-hBN thickness dependence and baseline of ∆R(t)/R 0 curves, S3-Details of the two temperature model and reflectivity calculation, and S4-Evolution of the electronic and phonon temperature. …”

Hyperbolic Phonon Polariton Electroluminescence as an Electronic Cooling Pathway

“…In contrast to this well‐known configuration, graphene has no bandgap, however, the threshold energy is now the RS band energy μ c − μ v ≲ ℏΩ II . For unbiased graphene, out‐of‐equilibrium electronic distribution can be transiently created by optical pumping and have been observed to relax on sub‐ps timescales by optical pump–probe techniques . In the next section, we show that electrical pumping of charges occurs similarly in high mobility graphene under large bias and leads to a favorable situation for HPhP electroluminescence cooling.…”

“…From a microscopic point of view, this mismatch is related to the small density of electromagnetic modes within the light cone as compared to that of the electron gas at high temperature. This issue has been overcame in recent studies, by using a so‐called hyperbolic substrate. Within the Reststrahlen (RS) bands (which roughly corresponds to the optical phonon modes) hyperbolic materials sustain long‐distance propagating hyperbolic phonon‐polariton (HPhP) modes that can carry a large momentum .…”

Surprisingly fast cooling in graphene-based van der Waals stacks

“…Thus, the hBN/graphene heterostructure provides a perfect research platform for nanoscale heat transfer of van der Waals heterostructures. Recently, Koppens et al [6] and Placais et al [7] independently verified highly surprisingly fast heat flow within van der Waals heterojunctions through different solid experiments, which made great progress in the study of nanoscale thermal transport.…”

“…Koppens et al [6] identified the picosecond relaxation times and dynamics of the hyperbolic cooling process in graphene heterostructures. In order to measure the ultrafast carrier dynamics of hBN-encapsulated graphene (illustrated in Fig.…”