Observation of ultrafast interfacial Meitner-Auger energy transfer in a Van der Waals heterostructure

Tracking and controlling ultrafast charge and energy flow in graphene-semiconductor heterostructures

Low-dimensional materials have left a mark on modern materials science, creating new opportunities for next-generation optoelectronic applications. Integrating disparate nanoscale building blocks into heterostructures offers the possibility of combining the advantageous features of individual components and exploring the properties arising from their interactions and atomic-scale proximity. The sensitization of graphene using semiconductors provides a highly promising platform for advancing optoelectronic applications through various hybrid systems. A critical aspect of achieving superior performance lies in understanding and controlling the fate of photogenerated charge carriers, including generation, transfer, separation, and recombination. Here, we review recent advances in understanding charge carrier dynamics in graphene-semiconductor heterostructures by ultrafast laser spectroscopies. First, we present a comprehensive overview of graphene-based heterostructures and their state-of-the-art optoelectronic applications. This is succeeded by an introduction to the theoretical frameworks that elucidate the fundamental principles and determinants influencing charge transfer and energy transfer-two critical interfacial processes that are vital for both fundamental research and device performance. We then outline recent efforts aimed at investigating ultrafast charge/energy flow in graphene-semiconductor heterostructures, focusing on illustrating the trajectories, directions, and mechanisms of transfer and recombination processes. Subsequently, we discuss effective control knobs that allow fine-tuning of these processes. Finally, we address the challenges and prospects for further investigation in this field.

k-Resolved Ultrafast Light-Induced Band Renormalization in Monolayer WS2 on Graphene

Understanding and controlling the electronic properties of two-dimensional materials are crucial for their potential applications in nano- and optoelectronics. Monolayer transition metal dichalcogenides have garnered significant interest due to their strong light-matter interaction and extreme sensitivity of the band structure to the presence of photogenerated electron-hole pairs. In this study, we investigate the transient electronic structure of monolayer WS2 on a graphene substrate after resonant excitation of the A-exciton using time- and angle-resolved photoemission spectroscopy. We observe a pronounced band structure renormalization, including a substantial reduction of the transient band gap in good quantitative agreement with our ab initio theory, revealing the importance of both intrinsic WS2 and extrinsic substrate contributions. Our findings deepen the fundamental understanding of band structure dynamics in two-dimensional materials and offer valuable insights for the development of novel electronic and optoelectronic devices based on monolayer TMDs and their heterostructures with graphene.

Free Carrier Auger-Meitner Recombination in Monolayer Transition Metal Dichalcogenides

Microscopic many-body models based on inputs from first-principles density functional theory are used to calculate the carrier losses due to free carrier Auger-Meitner recombination (AMR) processes in Mo- and W-based monolayer transition metal dichalcogenides as a function of the carrier density, temperature, and dielectric environment. Despite the exceptional strength of Coulomb interaction in the two-dimensional materials, the AMR losses are found to be similar in magnitude to those in conventional III-V-based quantum wells for the same wavelengths. Unlike the case in III-V materials, the losses show nontrivial density dependencies due to the fact that bandgap renormalizations on the order of hundreds of millielectronvolts can bring higher bands into or out of resonance with the optimal energy level for the AMR transition, approximately one bandgap from the lowest band. Similar nontrivial behaviors are found for the dependencies of AMR on the temperature and dielectric screening.

Nonlinear Nano-Imaging of Interlayer Coupling in 2D Graphene-Semiconductor Heterostructures

The emergent electronic, spin, and other quantum properties of 2D heterostructures of graphene and transition metal dichalcogenides are controlled by the underlying interlayer coupling and associated charge and energy transfer dynamics. However, these processes are sensitive to interlayer distance and crystallographic orientation, which are in turn affected by defects, grain boundaries, or other nanoscale heterogeneities. This obfuscates the distinction between interlayer charge and energy transfer. Here, nanoscale imaging in coherent four-wave mixing (FWM) and incoherent two-photon photoluminescence (2PPL) is combined with a tip distance-dependent coupled rate equation model to resolve the underlying intra- and inter-layer dynamics while avoiding the influence of structural heterogeneities in mono- to multi-layer graphene/WSe2 heterostructures. With selective insertion of hBN spacer layers, it is shown that energy, as opposed to charge transfer, dominates the interlayer-coupled optical response. From the distinct nano-FWM and -2PPL tip-sample distance-dependent modification of interlayer and intralayer relaxation by tip-induced enhancement and quenching, an interlayer energy transfer time ofτ ET ≈ ( 0 . 35 - 0.15 + 0.65 ) $\tau _{\rm ET} \approx (0.35^{+0.65}_{-0.15})$  ps consistent with recent reports is derived. As a local probe technique, this approach highlights the ability to determine intrinsic sample properties even in the presence of large sample heterogeneity.

Photoinduced dynamics during electronic transfer from narrow to wide bandgap layers in one-dimensional heterostructured materials

Electron transfer is a fundamental energy conversion process widely present in synthetic, industrial, and natural systems. Understanding the electron transfer process is important to exploit the uniqueness of the low-dimensional van der Waals (vdW) heterostructures because interlayer electron transfer produces the function of this class of material. Here, we show the occurrence of an electron transfer process in one-dimensional layer-stacking of carbon nanotubes (CNTs) and boron nitride nanotubes (BNNTs). This observation makes use of femtosecond broadband optical spectroscopy, ultrafast time-resolved electron diffraction, and first-principles theoretical calculations. These results reveal that near-ultraviolet photoexcitation induces an electron transfer from the conduction bands of CNT to BNNT layers via electronic decay channels. This physical process subsequently generates radial phonons in the one-dimensional vdW heterostructure material. The gathered insights unveil the fundamentals physics of interfacial interactions in low dimensional vdW heterostructures and their photoinduced dynamics, pushing their limits for photoactive multifunctional applications.

Distance Dependence of the Energy Transfer Mechanism in WS_{2}-Graphene Heterostructures

We report on the mechanism of energy transfer in Van der Waals heterostructures of the two-dimensional semiconductor WS_{2} and graphene with varying interlayer distances, achieved through spacer layers of hexagonal boron nitride (h-BN). We record photoluminescence and reflection spectra at interlayer distances between 0.5 and 5.8 nm (0-16  h-BN layers). We find that the energy transfer is dominated by states outside the light cone, indicative of a Förster transfer process, with an additional contribution from a Dexter process at 0.5 nm interlayer distance. We find that the measured dependence of the luminescence intensity on interlayer distances above 1 nm can be quantitatively described using recently reported values of the Förster transfer rates of thermalized charge carriers. At smaller interlayer distances, the experimentally observed transfer rates exceed the predictions and, furthermore, depend on excess energy as well as on excitation density. Since the transfer probability of the Förster mechanism depends on the momentum of electron-hole pairs, we conclude that, at these distances, the transfer is driven by nonrelaxed charge carrier distributions.