Transport phenomena like diffusion, convection, and drift play key roles in the sciences and engineering disciplines. They belong to the most omnipresent and important phenomena in nature that describe the motion of entities such as mass, charge or heat. Understanding and controlling these transport phenomena is crucial for a host of industrial technologies and applications, from cooling nuclear reactors to nanoscale heat‐management in the semiconductor industry. For decades, macroscopic transport techniques have been used to access important parameters such as charge mobilities or thermal conductivities. While being powerful, they often require physical contacts, which can lead to unwanted effects. Over the past years, an exciting solution has emerged: a technique called spatiotemporal microscopy (SPTM) that accesses crucial transport phenomena in a contactless, all‐optical, fashion. This technique offers powerful advantages in terms of accessible timescales, down to femtoseconds, and length scales, down to nanometres, and, further, selectively observes different species of interest. This tutorial review discusses common experimental configurations of SPTM and explains how they can be implemented by those entering the field. This review highlights the broad applicability of SPTM by presenting several exciting examples of transport phenomena that were unravelled thanks to this technique.

Spatiotemporal Microscopy: Shining Light on Transport Phenomena

Liquid-phase exfoliation enables tiled nanosheet films of 2D semiconductors with exceptionally long carrier lifetime for light harvesting and sensing.

Long lived photogenerated charge carriers in few-layer transition metal dichalcogenides obtained from liquid phase exfoliation

Since dissipative processes are ubiquitous in semiconductors, characterizing how electronic and thermal energy transduce and transport at the nanoscale is vital for understanding and leveraging their fundamental properties. For example, in low-dimensional transition metal dichalcogenides (TMDCs), excess heat generation upon photoexcitation is difficult to avoid since even with modest injected exciton densities exciton-exciton annihilation still occurs. Both heat and photoexcited electronic species imprint transient changes in the optical response of a semiconductor, yet the distinct signatures of each are difficult to disentangle in typical spectra due to overlapping resonances. In response, we employ stroboscopic optical scattering microscopy (stroboSCAT) to simultaneously map both heat and exciton populations in few-layer MoS2 on relevant nanometer and picosecond length- and time scales and with 100-mK temperature sensitivity. We discern excitonic contributions to the signal from heat by combining observations close to and far from exciton resonances, characterizing the photoinduced dynamics for each. Our approach is general and can be applied to any electronic material, including thermoelectrics, where heat and electronic observables spatially interplay, and it will enable direct and quantitative discernment of different types of coexisting energy without recourse to complex models or underlying assumptions. 

Detecting, Distinguishing, and Spatiotemporally Tracking Photogenerated Charge and Heat at the Nanoscale

Since dissipative processes are ubiquitous in semiconductors, characterizing how electronic and thermal energy transduce and transport at the nanoscale is vital for understanding and leveraging their fundamental properties. For example, in low-dimensional transition metal dichalcogenides (TMDCs), excess heat generation upon photoexcitation is difficult to avoid since even with modest injected exciton densities, exciton-exciton annihilation still occurs. Both heat and photoexcited electronic species imprint transient changes in the optical response of a semiconductor, yet the unique signatures of each are difficult to disentangle in typical spectra due to overlapping resonances. In response, we employ stroboscopic optical scattering microscopy (stroboSCAT) to simultaneously map both heat and exciton populations in few-layer \ch{MoS2} on relevant nanometer and picosecond length- and time scales and with 100-mK temperature sensitivity. We discern excitonic contributions to the signal from heat by combining observations close to and far from exciton resonances, characterizing photoinduced dynamics for each. Our approach is general and can be applied to any electronic material, including thermoelectrics, where heat and electronic observables spatially interplay, and lays the groundwork for direct and quantitative discernment of different types of coexisting energy without recourse to complex models or underlying assumptions.

pdf/detecting-distinguishing-and-spatiotemporally-tracking-36e7nw36.pdf

Detecting, distinguishing, and spatiotemporally tracking photogenerated charge and heat at the nanoscale

Transition metal dichalcogenides (TMDs) are a class of layered materials that hold great promise for a wide range of applications. Their practical use can be limited by their thermal transport properties, which have proven challenging to determine accurately, both from a theoretical and experimental perspective. We have conducted a thorough theoretical investigation of the thermal conductivity of four common TMDs, ${\mathrm{MoSe}}_{2}, {\mathrm{WSe}}_{2}, {\mathrm{MoS}}_{2}$, and ${\mathrm{WS}}_{2}$, at room temperature, to determine the key factors that influence their thermal behavior. We analyze these materials using ab initio calculations performed with the siesta program, anharmonic lattice dynamics and the Boltzmann transport equation formalism, as implemented in the temperature-dependent effective potentials method. Within this framework, we analyze the microscopic parameters influencing the thermal conductivity, such as the phonon dispersion and the phonon lifetimes. The aim is to precisely identify the origin of differences in thermal conductivity among these canonical TMD materials. We compare their in-plane thermal properties in monolayer and bulk form, and we analyze how the thickness and the chemical composition affect the thermal transport behavior. We showcase how bonding and the crystal structure influence the thermal properties by comparing the TMDs with silicon, reporting the cases of bulk silicon and monolayer silicene. We find that the interlayer bond type (covalent vs. van der Waals) involved in the structure is crucial in the heat transport. In two-dimensional silicene, we observe a reduction by a factor $\ensuremath{\sim}15$ compared to the Si bulk thermal conductivity due to the smaller group velocities and shorter phonon lifetimes. In the TMDs, where the group velocities and the phonon bands do not vary significantly passing from the bulk to the monolayer limit, we do not see as strong a decrease in the thermal conductivity: only a factor 2--3. Moreover, our analysis reveals that differences in the thermal conductivity arise from variations in atomic species, bond strengths, and phonon lifetimes. These factors are closely interconnected and collectively impact the overall thermal conductivity. We inspect each of them separately and explain how they influence the heat transport. We also study artificial TMDs with modified masses, in order to assess how the chemistry of the compounds modifies the microscopic quantities and thus the thermal conductivity. 

Microscopic understanding of the in-plane thermal transport properties of 
2H
 transition metal dichalcogenides

Diffusion is one of the most ubiquitous transport phenomena in nature. Experimentally, it can be tracked by following point spreading in space and time. Here, we introduce a spatiotemporal pump-probe microscopy technique that exploits the residual spatial temperature profile obtained through the transient reflectivity when probe pulses arrive before pump pulses. This corresponds to an effective pump-probe time delay of 13 ns, determined by the repetition rate of our laser system (76 MHz). This pre-time-zero technique enables probing the diffusion of long-lived excitations created by previous pump pulses with nanometer accuracy and is particularly powerful for following in-plane heat diffusion in thin films. The particular advantage of this technique is that it enables quantifying thermal transport without requiring any material input parameters or strong heating. We demonstrate the direct determination of the thermal diffusivities of films with a thickness of around 15 nm, consisting of the layered materials MoSe2 (0.18 cm2/s), WSe2 (0.20 cm2/s), MoS2 (0.35 cm2/s), and WS2 (0.59 cm2/s). This technique paves the way for observing nanoscale thermal transport phenomena and tracking diffusion of a broad range of species.

A pre-time-zero spatiotemporal microscopy technique for the ultrasensitive determination of the thermal diffusivity of thin films.

https://export.arxiv.org/pdf/2501.16162v1.pdf

Quantum Transport with Spin-Orbit Coupling: New Developments in TranSIESTA

We evidence the possibility for coherent electrical manipulation of the spin orientation of topologically protected edge states in a low-symmetry quantum spin Hall insulator. By using a combination of ab-initio simulations, symmetry-based modeling, and large-scale calculations of the spin Hall conductivity, it is shown that small electric ﬁelds can eﬃciently vary the spin textures of edge currents in monolayer 1T’-WTe 2 by up to a 90-degree spin rotation, without jeopardizing their topological character. These ﬁndings suggest a new kind of gate-controllable spin-based device, topologically protected against disorder and of relevance for the development of topological spintronics. QSH phase via electric ﬁelds remains unexplored to date, albeit it could enable the design of all-electrically controlled spin devices, such as spin-dependent topological switches, that would enrich the prospects for dissipationaless spintronics metrology. This Letter reports on the possibility of a fully controllable variation of up to 90 degrees rotation of the spin polarization of chiral edge-states, dictating the canted QSH eﬀect, while preserving spin conservation. By combining density functional theory (DFT) with tight-binding methods and quantum transport simulations, we show that the emerging PST can be continuously varied from in-plane to out-of-plane under electric ﬁelds below 0.1 V/nm, making this eﬀect experimentally accessible. The experimental conﬁrmation of such fully electrically tunable spin-polarized topological currents would establish a new milestone towards replacing magnetic components in spintronic devices and all-electric spin circuit architectures, as well as optimized resistance quantum standards.

pdf/electrical-control-of-spin-polarized-topological-currents-in-11pnciwf.pdf

Electrical Control of Spin-polarized Topological Currents in Monolayer WTe$_2$

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A pre-time-zero spatiotemporal microscopy technique for the ultrasensitive determination of the thermal diffusivity of thin films.

Quantum Transport with Spin-Orbit Coupling: New Developments in TranSIESTA

Electrical Control of Spin-polarized Topological Currents in Monolayer WTe$_2$