Ultrafast Tracking of Exciton and Charge Carrier Transport in Optoelectronic Materials on the Nanometer Scale.

We present a novel optical transient absorption and reflection microscope based on a diffraction-limited pump pulse in combination with a wide-field probe pulse, for the spatiotemporal investigation of ultrafast population transport in thin films. The microscope achieves a temporal resolution down to 12 fs and simultaneously provides sub-10 nm spatial accuracy. We demonstrate the capabilities of the microscope by revealing an ultrafast excited-state exciton population transport of up to 32 nm in a thin film of pentacene and by tracking the carrier motion in p-doped silicon. The use of few-cycle optical excitation pulses enables impulsive stimulated Raman microspectroscopy, which is used for in situ verification of the chemical identity in the 100-2000 cm-1 spectral window. Our methodology bridges the gap between optical microscopy and spectroscopy, allowing for the study of ultrafast transport properties down to the nanometer length scale.We acknowledge financial support from the EPSRC and the Winton Program for the Physics of Sustainability. This project has received funding from the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation program (grant agreement No 758826). C.S. acknowledges financial support by the Royal Commission of the Exhibition of 1851

Correlating activity and defects in (photo)electrocatalysts using in-situ transient optical microscopy

(Photo)electrocatalysts capture sunlight and use it to drive chemical reactions such as water splitting to produce H2. A major factor limiting photocatalyst development is their large heterogeneity which spatially modulates reactivity and precludes establishing robust structure-function relationships. To make such links requires simultaneously probing of the electrochemical environment at microscopic length scales (nm to um) and broad timescales (ns to s). Here, we address this challenge by developing and applying in-situ steady-state and transient optical microscopies to directly map and correlate local electrochemical activity with hole lifetimes, oxygen vacancy concentration and the photoelectrodes crystal structure. Using this combined approach alongside spatially resolved X-Ray absorption measurements, we study microstructural and point defects in prototypical hematite (Fe2O3) photoanodes. We demonstrate that regions of Fe2O3, adjacent to microstructural cracks have a better photoelectrochemical response and reduced back electron recombination due to an optimal oxide vacancy concentration, with the film thickness and carbon impurities also dramatically influencing activity in a complex manner. Our work highlights the importance of microscopic mapping to understand activity and the impact of defects in even, seemingly, homogeneous solid-state metal oxide photoelectrodes

Enhancing Photoluminescence and Mobilities in WS2 Monolayers with Oleic Acid Ligands.

Many potential applications of monolayer transition metal dichalcogenides (TMDs) require both high photoluminescence (PL) yield and high electrical mobilities. However, the PL yield of as prepared TMD monolayers is low and believed to be limited by defect sites and uncontrolled doping. This has led to a large effort to develop chemical passivation methods to improve PL and mobilities. The most successful of these treatments is based on the nonoxidizing organic "superacid" bis(trifluoromethane)sulfonimide (TFSI) which has been shown to yield bright monolayers of molybdenum disulfide (MoS2) and tungsten disulfide (WS2) but with trap-limited PL dynamics and no significant improvements in field effect mobilities. Here, using steady-state and time-resolved PL microscopy we demonstrate that treatment of WS2 monolayers with oleic acid (OA) can greatly enhance the PL yield, resulting in bright neutral exciton emission comparable to TFSI treated monolayers. At high excitation densities, the OA treatment allows for bright trion emission, which has not been demonstrated with previous chemical treatments. We show that unlike the TFSI treatment, the OA yields PL dynamics that are largely trap free. In addition, field effect transistors show an increase in mobilities with the OA treatment. These results suggest that OA serves to passivate defect sites in the WS2 monolayers in a manner akin to the passivation of colloidal quantum dots with OA ligands. Our results open up a new pathway to passivate and tune defects in monolayer TMDs using simple "wet" chemistry techniques, allowing for trap-free electronic properties and bright neutral exciton and trion emission

Air-stable bismuth sulfobromide (BiSBr) visible-light absorbers : optoelectronic properties and potential for energy harvesting

ns2 compounds have recently attracted considerable interest due to their potential to replicate the defect tolerance of lead-halide perovskites and overcome their toxicity and stability limitations. However, only a handful of compounds beyond the perovskite family have been explored thus far. Herein, we investigate bismuth sulfobromide (BiSBr), which is a quasi-one-dimensional semiconductor, but very little is known about its optoelectronic properties or how it can be processed as thin films. We develop a solution processing route to achieve phase-pure, stoichiometric BiSBr films (ca. 240 nm thick), which we show to be stable in ambient air for over two weeks without encapsulation. The bandgap (1.91 ± 0.06 eV) is ideal for harvesting visible light from common indoor light sources, and we calculate the optical limit in efficiency (i.e., spectroscopic limited maximum efficiency, SLME) to be 43.6% under 1000 lux white light emitting diode illumination. The photoluminescence lifetime is also found to exceed the 1 ns threshold for photovoltaic absorber materials worth further development. Through X-ray photoemission spectroscopy and Kelvin probe measurements, we find the BiSBr films grown to be n-type, with an electron affinity of 4.1 ± 0.1 eV and ionization potential of 6.0 ± 0.1 eV, which are compatible with a wide range of established charge transport layer materials. This work shows BiSBr to hold promise for indoor photovoltaics, as well as other visible-light harvesting applications, such as photoelectrochemical cells, or top-cells for tandem photovoltaics

Ultrafast spatiotemporal dynamics of photoexcitations in two-dimensional semiconductors

This thesis focuses on two families of Van der Waals materials: the transition metal dichalcogenides and the group IV monochalcogenides. First, we explore the spatio-temporal dynamics of photo-excited species in monolayer transition metal dichalcogenides. Using pump-probe spectroscopy, and a range of steady state characterization techniques (ellipsometry, reflectance, photoluminescence), we study the formation and decay of excitons in these materials at low excitation densities. We find that the photodynamics are largely dominated by the screening of the Coulomb interaction by excited states. At high excitation densities, when the screening of the Coulomb interaction is such that the excitons are no longer stable, we observe their complete ionization into a dense plasma of unbound electrons and holes. This interaction driven transition from an insulator a metallic state is a typical Mott transition. Combining pump-probe techniques with microscopic imaging allows us to observe these processes in space with 10 fs temporal resolution and 10 nm spatial localisation. Above the Mott transition, we observe an ultrafast spatial propagation of the excitations in the first few hundred femtoseconds after excitation. We hypothesise that this new regime of ultrafast transport is driven by the Fermi pressure of the dense electron-hole gas. Because atomically thin materials are very sensitive to their environment, it is also possible to tune the energies of electronic states through changes in the surroundings. Here, we engineer a potential landscape for excitons in the monolayer by introducing spatial variations in the dielectric environment. We can then watch excitations evolve in this landscape on an ultrafast time scale. Specifically, we observe the funnelling of excitons in a lateral homojunction. Finally we demonstrate the exfoliation down to the monolayer of two representative member of the group-IV monochalcogenides family

Long-range corrected exchange-correlation kernels to describe excitons in second-harmonic generation

International audienceWe investigate the role of excitons in second-harmonic generation (SHG) through the long-range corrected (LRC) exchange-correlation kernels: empirical LRC, Bootstrap, and jellium-with-a-gap model. We calculate the macroscopic second-order frequency-dependent susceptibility χ(2). We also present the frequency-dependent macroscopic dielectric function ϵM which is a fundamental quantity in the theoretical derivation of χ(2). We assess the role of the long-range kernels in describing excitons in materials with different symmetry types: cubic zincblende, hexagonal wurtzite, and tetragonal symmetry. Our studies indicate that excitons play an important role in χ(2) bringing a strong enhancement of the SHG signal. Moreover, we found that the SHG enhancement follows a simple trend determined by the magnitude of the long-range corrected α-parameter. This trend is material dependent

Direct Observation of Ultrafast Singlet Exciton Fission in Three Dimensions

We present quantitative ultrafast interferometric pump-probe microscopy capable of tracking of photoexcitations with sub 10 nm spatial precision in three dimensions with 15 fs temporal reso- lution, through retrieval of the full transient photoinduced complex refractive index. We use this methodology to study the spatiotemporal dynamics of the quantum coherent photophysical process of ultrafast singlet exciton fission. Measurements on microcrystalline pentacene films grown on glass (SiO2) and boron nitride (hBN) reveal a 25 nm, 70 fs expansion of the joint-density-of-states along the crystal a,c-axes accompanied by a 6 nm, 115 fs change in the exciton density along the crystal b-axis. We propose that photogenerated singlet excitons expand along the direction of max- imal orbital π-overlap in the crystal a,c-plane to form correlated triplet pairs, which subsequently electronically decouples into free triplets along the crystal b-axis due to molecular sliding motion of neighbouring pentacene molecules. Our methodology lays the foundation for the study of three dimensional transport on ultrafast timescales

Direct Imaging of Carrier Funneling in a Dielectric Engineered 2D Semiconductor.

Publication status: PublishedIn atomically thin transition-metal dichalcogenides (TMDCs), the environmental sensitivity of the strong Coulomb interaction offers promising approaches to create spatially varying potential landscapes in the same continuous material by tuning its dielectric environment. Thus, allowing for control of transport. However, a scalable and CMOS-compatible method for achieving this is required to harness these effects in practical applications. In addition, because of their ultrashort lifetime, observing the spatiotemporal dynamics of carriers in monolayer TMDCs, on the relevant time scale, is challenging. Here, we pattern and deposit a thin film of hafnium oxide (HfO2) via atomic layer deposition (ALD) on top of a monolayer of WSe2. This allows for the engineering of the dielectric environment of the monolayer and design of heterostructures with nanoscale spatial resolution via a highly scalable postsynthesis methodology. We then directly image the transport of photoexcitations in the monolayer with 50 fs time resolution and few-nanometer spatial precision, using a pump probe microscopy technique. We observe the unidirectional funneling of charge carriers, from the unpatterned to the patterned areas, over more than 50 nm in the first 20 ps with velocities of over 2 × 103 m/s at room temperature. These results demonstrate the possibilities offered by dielectric engineering via ALD patterning, allowing for arbitrary spatial patterns that define the potential landscape and allow for control of the transport of excitations in atomically thin materials. This work also shows the power of the transient absorption methodology to image the motion of photoexcited states in complex potential landscapes on ultrafast time scales

Research data supporting: "Direct imaging of carriers funnelling in a dielectric engineered 2D semiconductor."

This data set contains all data underlying the figures in the main text and supporting information. The information on how the data was acquired and processed is detailed in the open access manuscript + SI which has been deposited in this repository and is also available open-access via the publisher ACS Nano under the title: Direct imaging of carriers funnelling in a dielectric engineered 2D semiconductor. The data is arranged in several folders: AFM: contains the AFM profile of fig1 and AFM map of SI note 1. PL: contains the PL map and PL spectra of fig 1. Also contains the PL map of SI note 2 (map-PL_before_anneal_SI ) Simulations: contains the results of the simulations for Figure 4.c and 4.e. DY.npy is a 2-dimensional array containing the data for Fig 4.c. The first axis corresponds to the time delay and the second axis the positions. Amplitude.npy is a similar array containing the data of Figure 4.c. position.npy and time.npy are 1 dimensional array containing the time and positions values. Tam_fig2c. Corresponds to the data of Figure 2.c. time.npy and position.npy are 1 dimensional arrays containing the positions (in um) and time delay (in fs). Map_tam.npy is a 4-dimensional array containing the TAM images acquired at different time delay and positions. The first axis corresponds to positions on the sample, the second axis corresponds to the time delay. The last two axis correspond to image pixels so that Map_tam[p,t,:,:] is an image at position[p] and time[t]. TAM_fig4. Corresponds to the data of figure 4b and d. time.npy and position.npy are 1 dimensional arrays containing the positions (in um) and time delay (in fs). Map_tam.npy is a 4-dimensional array containing the TAM images acquired at different time delay and positions. The first axis corresponds to positions on the sample, the second axis corresponds to the time delay. The last two axis correspond to image pixels so that Map_tam[p,t,:,:] is an image at position[p] and time[t]