Unraveling the Nature of Excitons and their Interactions through Time-Resolved Photoemission and Optical Spectroscopy

Okinawa Institute of Science and Technology Graduate UniversityDoctor of PhilosophyThe exciton – a coulomb-bound electron-hole pair, was first conceptualized by Frenkel and Wannier in the 1930s. Since then, it has been integral to understanding the optoelectronic response in semiconductors, particularly in low-dimensional semiconductors. Therein, excitons have large binding energies due to quantum confinement and reduced dielectric screening, thus dominating the optical response of the material even at room temperature. Despite their importance, a critical fundamental property of excitons remains inaccessible – the momentum of the constituent electrons and holes! Such a measurement would immediately reveal valuable information, such as their direct or indirect nature, their wave function, their size, and the nature of their interactions. Resolving the momentum coordinates of excitons requires the development of a new instrumentation platform that probes the excitons in time, energy, space, and momentum. This thesis describes the need for such an instrumentation platform and its development, namely the development of time-resolved momentum microscopy. It then describes three studies on the nature of excitons and their interactions. First, we study the interlayer excitons in a WSe2/MoS2 heterostructure. Using time-resolved Momentum Microscopy, we resolve the momentum coordinates of the constituent electrons and holes within the interlayer exciton, directly measuring its size and confinement within the moir unit cell. Next, we demonstrate Floquet effects in monolayer WS2, in the absence of optical fields, resulting from the time-periodic oscillations in the electron self-energy due to excitons. The strong amplitude of the time-periodic perturbation allows us to observe the hybridization of the original band structure with the exciton-dressed one. Finally, we use traditional µ-TAS to study the exciton-exciton annihilation process in bilayer black phosphorus. We show that it is possible to alter the dimensionality of the exciton-exciton annihilation process from one dimensional-like to two dimensional-like by tuning the exciton density and temperature. In conclusion, this thesis answers some fundamental questions about excitons and their interactions in two dimensional semiconductors and paves the way for uncovering novel non-equilibrium phenomena in two dimensional materials.doctoral thesi

Ultrafast Control of the Dimensionality of Exciton-Exciton Annihilation in Atomically Thin Black Phosphorus

Using microtransient absorption spectroscopy, we show that the dynamical form of exciton-exciton annihilation in atomically thin black phosphorous can be made to switch between time varying 1D scattering and time-independent 2D scattering. At low carrier densities, anisotropy drives the 1D behavior, but as the photoexcitation density approaches the exciton saturation limit, the 2D nature of exciton-exciton scattering takes over. Furthermore, lowering the temperature provides a handle on the ultrafast timescale at which the 1D to 2D transition occurs. We understand our results quantitatively using a diffusion based model of exciton-exciton scattering

Visualizing superconductivity in a doped Weyl semimetal with broken inversion symmetry

The Weyl semimetal MoTe₂ offers a rare opportunity to study the interplay between Weyl physics and superconductivity. Recent studies have found that Se substitution can boost the superconductivity up to 1.5 K, but suppresses the T-d structure phase that is essential for the emergence of the Weyl state. A microscopic understanding of the possible coexistence of enhanced superconductivity and the Td phase has not been established so far. Here, we use scanning tunneling microscopy to study an optimally doped superconductor MoTe₁.₈₅Se₀.₁₅ with bulk T-c similar to 1.5K. By means of quasiparticle interference imaging, we identify the existence of a low-temperature Td phase with broken inversion symmetry where superconductivity globally coexists. Furthermore, we find that the superconducting coherence length, extracted from both the upper critical field and the decay of density of states near a vortex, is much larger than the characteristic length scale of the existing chemical disorder. Our findings of robust superconductivity arising from a Weyl semimetal normal phase in MoTe₁.₈₅Se₀.₁₅ make it a promising candidate for realizing topological superconductivity

Pulling apart photoexcited electrons by photoinducing an in-plane surface electric field

The study and control of spatiotemporal dynamics of photocarriers at the interfaces of materials have led to transformative modern technologies, such as light-harvesting devices and photodetectors. At the heart of these technologies is the ability to separate oppositely charged electrons and holes. Going further, the ability to separate like charges and manipulate their distribution could provide a powerful new paradigm in opto-electronic control, more so when done on ultrafast time scales. However, this requires one to selectively address subpopulations of the photoexcited electrons within the distribution—a challenging task, particularly on ultrafast time scales. By exploiting the spatial intensity variations in an ultrafast light pulse, we generate local surface fields within the optical spot of a doped semiconductor and thereby pull apart the electrons into two separate distributions. Using time-resolved photoemission microscopy, we directly record a movie of this redistribution process lasting a few hundred picoseconds, which we control via the spatial profile and intensity of the photoexciting pulse. Our quantitative model explains the underlying charge transport phenomena, thus providing a roadmap to the more generalized ability to manipulate photocarrier distributions with high spatiotemporal resolution

Directly visualizing the momentum forbidden dark excitons and their dynamics in atomically thin semiconductors

Resolving the momentum degree of freedom of excitons - electron-hole pairs bound by the Coulomb attraction in a photoexcited semiconductor, has remained a largely elusive goal for decades. In atomically thin semiconductors, such a capability could probe the momentum forbidden dark excitons, which critically impact proposed opto-electronic technologies, but are not directly accessible via optical techniques. Here, we probe the momentum-state of excitons in a WSe2 monolayer by photoemitting their constituent electrons, and resolving them in time, momentum and energy. We obtain a direct visual of the momentum forbidden dark excitons, and study their properties, including their near-degeneracy with bright excitons and their formation pathways in the energy-momentum landscape. These dark excitons dominate the excited state distribution - a surprising finding that highlights their importance in atomically thin semiconductors.Comment: 34 page

Performance-limiting nanoscale trap clusters at grain junctions in halide perovskites.

Halide perovskite materials have promising performance characteristics for low-cost optoelectronic applications. Photovoltaic devices fabricated from perovskite absorbers have reached power conversion efficiencies above 25 per cent in single-junction devices and 28 per cent in tandem devices1,2. This strong performance (albeit below the practical limits of about 30 per cent and 35 per cent, respectively3) is surprising in thin films processed from solution at low-temperature, a method that generally produces abundant crystalline defects4. Although point defects often induce only shallow electronic states in the perovskite bandgap that do not affect performance5, perovskite devices still have many states deep within the bandgap that trap charge carriers and cause them to recombine non-radiatively. These deep trap states thus induce local variations in photoluminescence and limit the device performance6. The origin and distribution of these trap states are unknown, but they have been associated with light-induced halide segregation in mixed-halide perovskite compositions7 and with local strain8, both of which make devices less stable9. Here we use photoemission electron microscopy to image the trap distribution in state-of-the-art halide perovskite films. Instead of a relatively uniform distribution within regions of poor photoluminescence efficiency, we observe discrete, nanoscale trap clusters. By correlating microscopy measurements with scanning electron analytical techniques, we find that these trap clusters appear at the interfaces between crystallographically and compositionally distinct entities. Finally, by generating time-resolved photoemission sequences of the photo-excited carrier trapping process10,11, we reveal a hole-trapping character with the kinetics limited by diffusion of holes to the local trap clusters. Our approach shows that managing structure and composition on the nanoscale will be essential for optimal performance of halide perovskite devices

Synergistic impact of nanomaterials and plant probiotics in agriculture: A tale of two-way strategy for long-term sustainability

Modern agriculture is primarily focused on the massive production of cereals and other food-based crops in a sustainable manner in order to fulfill the food demands of an ever-increasing global population. However, intensive agricultural practices, rampant use of agrochemicals, and other environmental factors result in soil fertility degradation, environmental pollution, disruption of soil biodiversity, pest resistance, and a decline in crop yields. Thus, experts are shifting their focus to other eco-friendly and safer methods of fertilization in order to ensure agricultural sustainability. Indeed, the importance of plant growth-promoting microorganisms, also determined as “plant probiotics (PPs),” has gained widespread recognition, and their usage as biofertilizers is being actively promoted as a means of mitigating the harmful effects of agrochemicals. As bio-elicitors, PPs promote plant growth and colonize soil or plant tissues when administered in soil, seeds, or plant surface and are used as an alternative means to avoid heavy use of agrochemicals. In the past few years, the use of nanotechnology has also brought a revolution in agriculture due to the application of various nanomaterials (NMs) or nano-based fertilizers to increase crop productivity. Given the beneficial properties of PPs and NMs, these two can be used in tandem to maximize benefits. However, the use of combinations of NMs and PPs, or their synergistic use, is in its infancy but has exhibited better crop-modulating effects in terms of improvement in crop productivity, mitigation of environmental stress (drought, salinity, etc.), restoration of soil fertility, and strengthening of the bioeconomy. In addition, a proper assessment of nanomaterials is necessary before their application, and a safer dose of NMs should be applicable without showing any toxic impact on the environment and soil microbial communities. The combo of NMs and PPs can also be encapsulated within a suitable carrier, and this method aids in the controlled and targeted delivery of entrapped components and also increases the shelf life of PPs. However, this review highlights the functional annotation of the combined impact of NMs and PPs on sustainable agricultural production in an eco-friendly manner

Experimental measurement of the intrinsic excitonic wave function

An exciton, a two-body composite quasiparticle formed of an electron and hole, is a fundamental optical excitation in condensed matter systems. Since its discovery nearly a century ago, a measurement of the excitonic wave function has remained beyond experimental reach. Here, we directly image the excitonic wave function in reciprocal space by measuring the momentum distribution of electrons photoemitted from excitons in monolayer tungsten diselenide. By transforming to real space, we obtain a visual of the distribution of the electron around the hole in an exciton. Further, by also resolving the energy coordinate, we confirm the elusive theoretical prediction that the photoemitted electron exhibits an inverted energy-momentum dispersion relationship reflecting the valence band where the partner hole remains, rather than that of conduction band states of the electron

時間分解光電子放出と光学分光法による励起子とその相互作用の性質の解明

The exciton – a coulomb-bound electron-hole pair, was first conceptualized by Frenkel and Wannier in the 1930s. Since then, it has been integral to understanding the optoelectronic response in semiconductors, particularly in low-dimensional semiconductors. Therein, excitons have large binding energies due to quantum confinement and reduced dielectric screening, thus dominating the optical response of the material even at room temperature. Despite their importance, a critical fundamental property of excitons remains inaccessible – the momentum of the constituent electrons and holes! Such a measurement would immediately reveal valuable information, such as their direct or indirect nature, their wave function, their size, and the nature of their interactions. Resolving the momentum coordinates of excitons requires the development of a new instrumentation platform that probes the excitons in time, energy, space, and momentum. This thesis describes the need for such an instrumentation platform and its development, namely the development of time-resolved momentum microscopy. It then describes three studies on the nature of excitons and their interactions. First, we study the interlayer excitons in a WSe2/MoS2 heterostructure. Using time-resolved Momentum Microscopy, we resolve the momentum coordinates of the constituent electrons and holes within the interlayer exciton, directly measuring its size and confinement within the moir unit cell. Next, we demonstrate Floquet effects in monolayer WS2, in the absence of optical fields, resulting from the time-periodic oscillations in the electron self-energy due to excitons. The strong amplitude of the time-periodic perturbation allows us to observe the hybridization of the original band structure with the exciton-dressed one. Finally, we use traditional µ-TAS to study the exciton-exciton annihilation process in bilayer black phosphorus. We show that it is possible to alter the dimensionality of the exciton-exciton annihilation process from one dimensional-like to two dimensional-like by tuning the exciton density and temperature. In conclusion, this thesis answers some fundamental questions about excitons and their interactions in two dimensional semiconductors and paves the way for uncovering novel non-equilibrium phenomena in two dimensional materials