Physics
Permanent URI for this communityhttps://scholarworks.montana.edu/handle/1/52
The Physics department is committed to education and research in physics, the study of the fundamental universal laws that govern the behavior of matter and energy, and the exploration of the consequences and applications of those laws. Our department is widely known for its excellent teaching and student mentoring. Our department plays an important role in the university’s Core Curriculum. We have strong academic programs with several options for undergraduate physics majors, leading to the B.S. degree, as well as graduate curricula leading to the M.S. and Ph.D. degrees. Our research groups span a variety of fields within physics. Our principal concentrations are in Astrophysics, Relativity, Gravitation and Cosmology, Condensed Matter Physics, Lasers and Optics, Physics Education, Solar Physics, and the Space Science and Engineering Lab.
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Item Nanoscale Raman Characterization of a 2D Semiconductor Lateral Heterostructure Interface(American Chemical Society, 2022-01) Garg, Sourav; Fix, J. Pierce; Krayev, Andrey V.; Flanery, Connor; Colgrove, Michael; Sulkanen, Audrey R.; Wang, Minyuan; Liu, Gang-Yu; Borys, Nicholas J.; Kung, PatrickThe nature of the interface in lateral heterostructures of 2D monolayer semiconductors including its composition, size, and heterogeneity critically impacts the functionalities it engenders on the 2D system for next-generation optoelectronics. Here, we use tip-enhanced Raman scattering (TERS) to characterize the interface in a single-layer MoS2/WS2 lateral heterostructure with a spatial resolution of 50 nm. Resonant and nonresonant TERS spectroscopies reveal that the interface is alloyed with a size that varies over an order of magnitude─from 50 to 600 nm─within a single crystallite. Nanoscale imaging of the continuous interfacial evolution of the resonant and nonresonant Raman spectra enables the deconvolution of defect activation, resonant enhancement, and material composition for several vibrational modes in single-layer MoS2, MoxW1–xS2, and WS2. The results demonstrate the capabilities of nanoscale TERS spectroscopy to elucidate macroscopic structure–property relationships in 2D materials and to characterize lateral interfaces of 2D systems on length scales that are imperative for devices.Item Anisotropic 2D excitons unveiled in organic–inorganic quantum wells(Royal Society of Chemistry, 2020-11) Maserati, Lorenzo; Refaely-Abramson, Sivan; Kastl, Christoph; Chen, Christopher T.; Borys, Nicholas J.; Eisler, Carissa N.; Collins, Mary S.; Smidt, Tess E.; Barnard, Edward S.; Strasbourg, Matthew; Schriber, Elyse A.; Shevitski, Brian; Yao, Kaiyuan; Hohman, J. Nathan; Schuck, P. James; Aloni, Shaul; Neaton, Jeffrey B.; Schwartzberg, Adam M.Hybrid layered metal chalcogenide crystalline polymer hosts strongly anisotropic two-dimensional excitons with large binding energies.Item The ultrafast onset of exciton formation in 2D semiconductors(Springer Science and Business Media LLC, 2020-10) Trovatello, Chiara; Katsch, Florian; Borys, Nicholas J.; Selig, Malte; Yao, Kaiyuan; Borrego-Varillas, Rocio; Scotognella, Francesco; Kriegel, Ilka; Yan, Aiming; Zettl, Alex; Schuck, P. James; Knorr, Andreas; Cerullo, Giulio; Dal Conte, StefanoThe equilibrium and non-equilibrium optical properties of single-layer transition metal dichalcogenides (TMDs) are determined by strongly bound excitons. Exciton relaxation dynamics in TMDs have been extensively studied by time-domain optical spectroscopies. However, the formation dynamics of excitons following non-resonant photoexcitation of free electron-hole pairs have been challenging to directly probe because of their inherently fast timescales. Here, we use extremely short optical pulses to non-resonantly excite an electron-hole plasma and show the formation of two-dimensional excitons in single-layer MoS2 on the timescale of 30 fs via the induced changes to photo-absorption. These formation dynamics are significantly faster than in conventional 2D quantum wells and are attributed to the intense Coulombic interactions present in 2D TMDs. A theoretical model of a coherent polarization that dephases and relaxes to an incoherent exciton population reproduces the experimental dynamics on the sub-100-fs timescale and sheds light into the underlying mechanism of how the lowest-energy excitons, which are the most important for optoelectronic applications, form from higher-energy excitations. Importantly, a phonon-mediated exciton cascade from higher energy states to the ground excitonic state is found to be the rate-limiting process. These results set an ultimate timescale of the exciton formation in TMDs and elucidate the exceptionally fast physical mechanism behind this process.Item Electrically driven photon emission from individual atomic defects in monolayer WS2(2020-09) Schuler, Bruno; Cochrane, Katherine A.; Kastl, Christoph; Barnard, Edward S.; Wong, Edward; Borys, Nicholas J.; Schwartzberg, Adam M.; Ogletree, D. Frank; Garcia de Abajo, F. Javier; Weber-Bargioni, AlexanderQuantum dot–like single-photon sources in transition metal dichalcogenides (TMDs) exhibit appealing quantum optical properties but lack a well-defined atomic structure and are subject to large spectral variability. Here, we demonstrate electrically stimulated photon emission from individual atomic defects in monolayer WS2 and directly correlate the emission with the local atomic and electronic structure. Radiative transitions are locally excited by sequential inelastic electron tunneling from a metallic tip into selected discrete defect states in the WS2 bandgap. Coupling to the optical far field is mediated by tip plasmons, which transduce the excess energy into a single photon. The applied tip-sample voltage determines the transition energy. Atomically resolved emission maps of individual point defects closely resemble electronic defect orbitals, the final states of the optical transitions. Inelastic charge carrier injection into localized defect states of two-dimensional materials provides a powerful platform for electrically driven, broadly tunable, atomic-scale single-photon sources.Item Imaging strain-localized excitons in nanoscale bubbles of monolayer WSe2 at room(2020-11) Darlington, Thomas P.; Carmesin, Christian; Florian, Matthias; Yanev, Emanuil; Ajayi, Obafunso; Ardelean, Jenny; Rhodes, Daniel A.; Ghiotto, Augusto; Krayev, Andrey; Watanabe, K.; Taniguchi, T.; Kysar, Jeffrey W.; Pasupathy, Abhay N.; Hone, James C.; Jahnke, Frank; Borys, Nicholas J.; Schuck, P. JamesIn monolayer transition-metal dichalcogenides, localized strain can be used to design nanoarrays of single photon sources. Despite strong empirical correlation, the nanoscale interplay between excitons and local crystalline structure that gives rise to these quantum emitters is poorly understood. Here, we combine room-temperature nano-optical imaging and spectroscopic analysis of excitons in nanobubbles of monolayer WSe2 with atomistic models to study how strain induces nanoscale confinement potentials and localized exciton states. The imaging of nanobubbles in monolayers with low defect concentrations reveals localized excitons on length scales of around 10 nm at multiple sites around the periphery of individual nanobubbles, in stark contrast to predictions of continuum models of strain. These results agree with theoretical confinement potentials atomistically derived from the measured topographies of nanobubbles. Our results provide experimental and theoretical insights into strain-induced exciton localization on length scales commensurate with exciton size, realizing key nanoscale structure–property information on quantum emitters in monolayer WSe2.Item Light-Driven Permanent Charge Separation across a Hybrid Zero-Dimensional/Two-Dimensional Interface(2020-04) Kriegel, Ilka; Ghini, Michele; Bellani, Sepastiano; Zhang, Kehao; Jansons, Adam W.; Crockett, Brandon M.; Koskela, Kristopher M.; Barnard, Edward S.; Penzo, Erika; Hutchison, James E.; Robinson, Joshua A.; Manna, Liberato; Borys, Nicholas J.; Schuck, P. JamesWe report the first demonstration of light-driven permanent charge separation across an ultrathin solid-state zero-dimensional (0D)/2D hybrid interface by coupling photoactive Sn-doped In2O3 nanocrystals with monolayer MoS2, the latter serving as a hole collector. We demonstrate that the nanocrystals in this device-ready architecture act as local light-controlled charge sources by quasi-permanently donating ∼5 holes per nanocrystal to the monolayer MoS2. The amount of photoinduced contactless charge transfer to the monolayer MoS2 competes with what is reached in electrostatically gated devices. Thus, we have constructed a hybrid bilayer structure in which the electrons and holes are separated into two different solid-state materials. The temporal evolution of the local doping levels of the monolayer MoS2 follows a capacitive charging model with effective total capacitances in the femtofarad regime and areal capacitances in the μF cm–2 range. This analysis indicates that the 0D/2D hybrid system may be able to store light energy at densities of at least μJ cm–2, presenting new potential foundational building blocks for next-generation nanodevices that can remotely control local charge density, power miniaturized circuitry, and harvest and store optical energy.Item Long-Range Exciton Diffusion in Two-Dimensional Assemblies of Cesium Lead Bromide Perovskite Nanocrystals(2020-05) Penzo, Erika; Loiudice, Anna; Barnard, Edward S.; Borys, Nicholas J.; Jurow, Matthew J.; Lorenzon, Monica; Rajzbaum, Igor; Wong, Edward K.; Liu, Yi; Schwartzberg, Adam M.; Cabrini, Stefano; Whitelam, Stephen; Buonsanti, Raffaella; Weber-Bargioni, AlexanderFörster resonant energy transfer (FRET)-mediated exciton diffusion through artificial nanoscale building block assemblies could be used as an optoelectronic design element to transport energy. However, so far, nanocrystal (NC) systems supported only diffusion lengths of 30 nm, which are too small to be useful in devices. Here, we demonstrate a FRET-mediated exciton diffusion length of 200 nm with 0.5 cm2/s diffusivity through an ordered, two-dimensional assembly of cesium lead bromide perovskite nanocrystals (CsPbBr3 PNCs). Exciton diffusion was directly measured via steady-state and time-resolved photoluminescence (PL) microscopy, with physical modeling providing deeper insight into the transport process. This exceptionally efficient exciton transport is facilitated by PNCs’ high PL quantum yield, large absorption cross section, and high polarizability, together with minimal energetic and geometric disorder of the assembly. This FRET-mediated exciton diffusion length matches perovskites’ optical absorption depth, thus enabling the design of device architectures with improved performances and providing insight into the high conversion efficiencies of PNC-based optoelectronic devices.Item Light-Driven Permanent Charge Separation across a Hybrid Zero-Dimensional/Two-Dimensional Interface(2020-03) Kriegel, Ilka; Ghini, Michele; Bellani, Sebastiano; Zhang, Kehao; Jansons, Adam W.; Crockett, Brandon M.; Koskela, Kristopher M.; Barnard, Edward S.; Penzo, Erika; Hutchison, James E.; Robinson, Joshua A.; Manna, Liberato; Borys, Nicholas J.; Schuck, P. JamesWe report the first demonstration of light-driven permanent charge separation across an ultrathin solid-state zero-dimensional (0D)/2D hybrid interface by coupling photoactive Sn-doped In2O3 nanocrystals with monolayer MoS2, the latter serving as a hole collector. We demonstrate that the nanocrystals in this device-ready architecture act as local light-controlled charge sources by quasi-permanently donating ∼5 holes per nanocrystal to the monolayer MoS2. The amount of photoinduced contactless charge transfer to the monolayer MoS2 competes with what is reached in electrostatically gated devices. Thus, we have constructed a hybrid bilayer structure in which the electrons and holes are separated into two different solid-state materials. The temporal evolution of the local doping levels of the monolayer MoS2 follows a capacitive charging model with effective total capacitances in the femtofarad regime and areal capacitances in the μF cm–2 range. This analysis indicates that the 0D/2D hybrid system may be able to store light energy at densities of at least μJ cm–2, presenting new potential foundational building blocks for next-generation nanodevices that can remotely control local charge density, power miniaturized circuitry, and harvest and store optical energy.