Made from stacks of two-dimensional materials, van der Waals heterostructures exhibit unique light-matter interactions and are promising for novel optoelectronic devices. The performance of such devices
is governed by near-field coupling through, e.g., interlayer charge and/or energy transfer. New concepts
and experimental methodologies are needed to properly describe two-dimensional heterointerfaces. We propose to study interlayer charge transfer (ICT) and energy transfer (IET) in transition-metal dichalcogenides based heterostructures. After proper characterization with atomic force microscopy, the samples will be studied with an optical scheme, namely pump&probe, in which a pump pulse excites electrons in TMD and the probe pulse is used for a time delayed Raman measurement. The dependence of the Raman measurements on the electronic (via the resonance enhancement) and phonon properties will allow to discriminate among different exchange effects.
From the proposed project, we expect to overcome the main limitations of the current spectroscopic approaches employed to study charge and energy transfer in vdWH:
- Lack of temporal resolution: The time-resolved scheme, which we propose in this project, aims at disclosing the dynamics and associated timescales of the transfer processes observed in [1]. We expect to temporally resolve the effect of the donor photoexcitation by time-delayed probing of electronic and vibrational properties of graphene acceptor. Specifically, we expect to track, in the time domain, the peak position and linewidth of the graphene Raman modes. The latter provide a determination of the Fermi level (or, equivalently, of the charge density), and change according to the transfer dynamics.
- Lack of donor/acceptor selectivity: Taking advantage of the wavelength tunablity of our setup we will probe the acceptor (graphene) by simultaneously enabling/disabling the donor photoexcitation using pump wavelengths below/above the TMD gap, respectively.
In order to address these open questions, this study will undertake a fundamental research effort, in the wake of our recent achievements on two-dimensional materials. Benefitting from the complementary expertise, the project joints the preliminary results performed at IPCMS using optical spectroscopy in the continuous wave (cw) regime (Fig 1 and [1]) with the acknowledged ultrafast (femtosecond) optical studies performed in Rome, able to provide the necessary time resolution to unveil the details of IET and ICT. Indeed, our group in Rome has recently developed a Coherent Vibrational spectro-microscopy setup, optimized to perform Impulsive Raman Spectroscopy and Coherent Vibrational spectroscopy in low dimensional materials, e.g. graphene [2].
For this project we will perform original combinations of Raman scattering, photoluminescence (PL) and non-linear ¿pump and probe¿ spectroscopies. In particular, we will photoexcite (pump) the vdWH above the MX2 (donor) energy gap, probing at different time delays upon photoexcitation the response on graphene, which represents the acceptor component in the ICT and IET process. Further, the electronic and vibrational response of graphene will be detected as function of pump photon flux and pump photon energy, tuned below and above the MX2 energy gap, as control knobs to facilitate or suppress the transfer process.
[1] G. Froehlicher et al., Phys. Rev. X, 8, 011007 (2018)
[2] C. Ferrante et al, Nat. Commun., 9, 308, (2018)