Session N68: Probing Charge, Heat and Energy Transport at the Nanoscale

Having it all: Discerning charge and heat in energy transduction and nanoscale transport

Very few materials are able to absorb visible light without dissipating some of the resulting energy into phonon modes, and these excited modes have the capability to act back on the electronic excitation that is generated. By the same token, very few probes of photophysical processes in materials are able to directly probe the coexistence of both electronic and thermal departures from equilibrium or directly visualize the impact of the spatiotemporal interaction of electronic and thermal excitations. I will nevertheless, describe such a capability that leverages not only the ps time resolution associated with electronic to thermal energy transduction but also direct spatial maps of localized photoinduced electronic and temperature profiles and their coupled evolution. This approach shows thermoelectric effects in few-layer MoS₂ and can be more broadly applied to other emerging semiconductors.   

Terahertz emission spectroscopy for monitoring hot-carrier transport in Plasmonics

Hot carriers –electrons and holes with energies notably larger than k­_BT_room­– play a key role in many photon-driven processes ranging from solar energy harvesting to photochemistry. In majority these applications, the transport of hot carriers across metal/dielectric interfaces is necessary. Understanding characteristic time and length scales for the interfacial transport of hot carriers is a challenging but highly rewarding task, as optimizing the performance of hot carrier devices heavily depends on such knowledge. Our experimental knowledge of the transport dynamic is limited to gathered timescales from pump-probe experiments. However, considering that generation of hot carriers is only one of many transient processes with similar timescales, often such recorded data provide an indirect picture of charge transport and require theoretically heavy modelings to unravel the contribution of transport among other competing transient effects. Here, we discuss the measurement of hot-carrier transport dynamics via the recording of time variation of emitted terahertz waveforms upon the interfacial transport of hot carriers. The role of geometrical symmetry, propagation direction of charge transport, and the external DC electrical bias on the terahertz response will be discussed.   

Hot electron relaxation in periodic type-II quantum well

In hot carrier photovoltaic devices, photoexcited electrons and holes are spatially separated in "Type II" quantum wells to reduce their recombination rate. The excess energy of the hot electrons can be lost to optical phonons through electron-phonon interactions. These optical phonons decay into acoustic phonons and eventually leave the system as heat. Previously, we have calculated the phonon modes of the repeated quantum well/barrier supercell and shown that the energy escaping rate is significantly reduced in this structure compared to a pure semiconductor. We build on this to develop a model to predict the steady-state of the hot electrons. We describe the electron dynamics with a fermionic Boltzmann rate equation, where an external light source pumps the hot electron population, and the electron-phonon interaction changes the population distribution of electrons in energy space. The distribution for the relevant optical phonon is modeled with a bosonic rate equation, including their decay into acoustic modes and leaking to a normal mode of the supercell and leaving the system. Solving these coupled equations with realistic parameters can predict and possibly optimize the steady-state, non-equilibrium, hot electron distribution.    

Directing exciton propagation in monolayer TMDCs through patterned dielectric substrates

The local control of exciton transport in monolayer TMDCs has manifested as a particular challenge so far. Although these excitons are known to be quite mobile  [1,2], their lack of charge precludes common ways to create directed transport via electrical potentials. An alternative route to this may be found in their inherent 2D nature since these excitons are particularly susceptible to their local dielectric environment [3,4]. This opens a novel way to tailor the excitonic energy landscape, envisioning directed exciton propagation along a defined energy pathway created from dielectric patterns through nanostructured substrates.

Here, we couple monolayers of WSe₂ to nano-patterned substrates with abrupt changes in their dielectric constants from ultra-sharp interfaces of alternating materials. The structures are created via block copolymer lithography and atomic layer deposition on large areas, scalable to 100’s of µm in either direction, with pattern dimensions down to 20nm. Our substrates of choice are 50nm interdigitated oxide/air line-grids. This transfers into narrow potential trenches in the excitonic energy landscape through distinct, local dielectric screening, creating a preferred direction for exciton propagation. We use time and spatially resolved micro-spectroscopies to monitor exciton transport across this engineered energy landscape via the phonon-assisted PL emission from long-lived states at cryogenic temperatures.

[1]   Annual Review of Physical Chemistry 71, 1 (2020).

[2]   Physical Review Letters 127, 76801 (2021)

[3]   Nature Materials 18, 541 (2019)

[4]   Nature Electronics 2, 60 (2019)   

Room temperature 1D excitonic guides

The emerging field of excitonics, made possible by the remarkable advancement in material science and nanofabrication, presents the possibility of improving on the usual on-chip interconnect paradigm of using electrons. In this work, we report a room temperature 1D excitonic guide in tungsten diselenide (WSe₂) monolayer, where photo generated excitons drift under a gradient in the exciton potential. The excitonic potential energy gradient is created by engineering the band structure along the guide using underlying nanostructures. [1] We achieve drift length in excess of 2.5 μm with 60.8 % efficiency and exciton drift velocity reaching ~8 × 10⁵ cm²/s. [2] In addition, we observe fluence dependent excitonic drift suggesting contributions from nonequilibrium many-body effects. [3] These results are critical for realizing optoexcitonic circuits: a next generation ultrafast and efficient data communication and processing platform.     

Voltage-controlled moiré potentials and long-range propagation of indirect excitons in a van der Waals MoSe2/WSe2 heterostructure

Excitonic devices based on controlled propagation of indirect excitons (IXs) are demonstrated in GaAs structures but limited to low temperatures due to low IX binding energies [1]. IXs in transition-metal dichalcogenide (TMD) heterostructures are characterized by high binding energies offering the opportunity for room-temperature operation. However, strong moiré superlattice potentials in TMD heterostructures [2,3] localize IXs, making IX propagation different in TMD and GaAs heterostructures. In earlier excitonic devices IX transport was controlled by an energy barrier to IX propagation created by the gate electrode [1]. We present a new mechanism for exciton transport control. It is based on tuning the moiré potentials by voltage to enable IX delocalization. We present the long-range IX propagation due to this mechanism. We also establish a spatial correlation between IX linewidth, energy, and luminescence intensity.

[1] A. A. High et al. Science 321, 229 (2008).

[2] F. Wu et al. PRL 118, 147401 (2017).

[3] H. Yu et al. Sci. Adv. 3, e1701696 (2017).   

Exciton-phonon dephasing and linewidth from first-principles in monolayer MoS2

Exciton-phonon interactions dominate the temperature dependence of the absorption and luminescence spectrum and determines exciton transfer rates (on fs to ps time scales) in many materials. However, the direct experimental measurement of exciton-phonon interaction is challenging and often subjects to interpretation based on parameterized model Hamiltonians. We apply here a first-principles approach to study exciton-phonon coupling in monolayer MoS₂ and shows the highly selective nature of exciton-phonon coupling due to the internal spin structure of excitons, which leads to a surprisingly long lifetime of the lowest energy bright A exciton. Moreover, we show that interference terms due to off-diagonal exciton-phonon matrix elements, which have thus far been neglected in first-principles studies, are critical for the description of dephasing mechanisms, and once accounted for, yield exciton linewidths in excellent agreement with experiment.   

Signatures of Dimensionality and Symmetry in Exciton Bandstructure: Consequences for Exciton Dynamics and Transport

Exciton dynamics, lifetimes, and scattering are directly related to the exciton dispersion, or bandstructure. Here, we present a general theory for exciton bandstructure within both ab initio and model Hamiltonian approaches. We show that contrary to common assumption, the exciton bandstructure contains non-analytical discontinuities---a feature which is impossible to obtain from the electronic bandstructure alone. These discontinuities are purely quantum phenomena, arising from the exchange scattering of electron-hole pairs. We show that the degree of these discontinuities depends on materials' symmetry and dimensionality, with jump discontinuities occurring in 3D and different orders of removable discontinuities in 2D and 1D. We connect these features to the early stages of exciton dynamics, radiative lifetimes, and diffusion constants, in good correspondence with recent experimental observations, revealing that the discontinuities in the bandstrucutre lead to ultrafast ballistic transport and suggesting that measured exciton diffusion and dynamics are influenced by the underlying exciton bandstructure.   

Excitons in a hyperbolic medium

We study the interaction between excitons in a two-dimensional semiconductor and phonons in an anisotropic polar insulator that behaves as a hyperbolic medium. We show that the energy levels of the excitons are renormalized due to polaron-like effects. The excited states of excitons can either form quasi-bound complexes with phonons or exhibit an enhanced decay rate in the hyperbolic frequency region. Coupling to phonons also modifies van der Waals interaction, Förster resonant energy transfer, and collective mode dispersion of excitons. We discuss how these effects may be observed in optical experiments and their possible nanophotonic applications.   

Surface hopping modeling of coherent charge and energy transfer in molecular materials and beyond

Surface hopping is probably the most popular approach to model energy transfer processes and other non-adiabatic phenomena, but its original formulation does not specify coherent elements of the involved density matrices. This complicates the application of surface hopping to ultrafast processes as well as to nonlinear spectroscopy, where these elements are critical. In this talk, I will describe our efforts to develop surface hopping into a fully coherent method. Notably, I will show that previous coherent implementations based on an inconsistent treatment of coherences and populations lead to unphysical density matrices, which may manifest as a violation of the Cauchy–Schwarz inequality and negative populations in the site basis. We recently developed a fully-consistent coherent implementation that does not suffer from such unphysical behavior. I will present a comparison of the method against numerically-exact results for a multichromophoric system representative of a light-harvesting complex. A high accuracy is found throughout most of parameter space, demonstrating its promise as a robust high-accuracy/low-cost method to study charge and energy dynamics in a broad variety of nanostructured materials.   

Exciton Transport in the Su-Schrieffer-Heeger (SSH) Model and Molecular Aggregate Chains

The Su-Schrieffer-Heeger (SSH) model has provided the basis for studying topological effects in dimerized systems. Recently, there has been increased interest in uncovering the effects of topology in realistic systems. The Radiative Hamiltonian, a physically-motivated effective Hamiltonian that describes the interaction of a molecular aggregate with a radiation field, and includes long-range interactions as well as non-Hermitian contributions, allows for in-depth study into such systems. Using the average transfer time and steady-state current as metrics of exciton transport, we have investigated transport in a simple disordered molecular aggregate chain that displays topological properties, and made comparisons with an analytic approximation in which the system is mapped onto a pair of disordered SSH chains.   

Exciton mediated interaction between moiré trapped carriers

Moiré pattern formed by stacking two-dimensional materials can realize superlattice confinement of electrons or holes, which  is a promising platform to study their many-body interactions. Moire superlattices of semiconducting transition metal dichalcogenides also host excitons, the tightly bound electron-hole pairs that can be interconverted with photons. We investigate the Coulomb exchange interaction between excitons and moiré potential trapped carriers, and show that the optically excited excitons can mediate a ferromagnetic spin-spin interaction between the moire trapped carriers. This interaction can be of longer range than that of pure electronic origin in the absence of excitons, and allows optical tuning of the spin-spin interaction and magnetic orders in the moire superlattice.