23 research outputs found
Theory of photoluminescence spectral line shapes of semiconductor nanocrystals
Single-molecule photoluminescence (PL) spectroscopy of semiconductor
nanocrystals (NCs) reveals the nature of exciton-phonon interactions in NCs.
Understanding the narrow line shapes at low temperatures and the significant
broadening as temperature increases remains an open problem. Here, we develop
an atomistic model to describe the PL spectrum of NCs, accounting for excitonic
effects, phonon dispersion relations, and exciton-phonon couplings. We use
single-molecule PL measurements on CdSe/CdS core-shell NCs from T=4 to T=290K
to validate our model and find that the slightly-asymmetric main peak at low
temperatures is comprised of a narrow zero-phonon line (ZPL) and several
acoustic phonon sidebands. Furthermore, we identify the distinct CdSe optical
modes that give rise to the optical phonon sidebands. As the temperature
increases, the spectral width shows a stronger dependence on temperature, which
we demonstrated to be correlated with frequency shifts and mode-mixing,
reflected as higher-order exciton-phonon couplings (Duschinsky rotations). We
also model the PL dependence on core size and shell thickness and provide
strategies for the design of NCs with narrow linewidths at elevated
temperatures
Simulating noise on a quantum processor: interactions between a qubit and resonant two-level system bath
Material defects fundamentally limit the coherence times of superconducting
qubits, and manufacturing completely defect-free devices is not yet possible.
Therefore, understanding the interactions between defects and a qubit in a real
quantum processor design is essential. We build a model that incorporates the
standard tunneling model, the electric field distributions in the qubit, and
open quantum system dynamics, and draws from the current understanding of
two-level system (TLS) theory. Specifically, we start with one million TLSs
distributed on the surface of a qubit and pick the 200 systems that are most
strongly coupled to the qubit. We then perform a full Lindbladian simulation
that explicitly includes the coherent coupling between the qubit and the TLS
bath to model the time dependent density matrix of resonant TLS defects and the
qubit. We find that the 200 most strongly coupled TLSs can accurately describe
the qubit energy relaxation time. This work confirms that resonant TLSs located
in areas where the electric field is strong can significantly affect the qubit
relaxation time, even if they are located far from the Josephson junction.
Similarly, a strongly-coupled resonant TLS located in the Josephson junction
does not guarantee a reduced qubit relaxation time if a more strongly coupled
TLS is far from the Josephson junction. In addition to the coupling strengths
between TLSs and the qubit, the model predicts that the geometry of the device
and the TLS relaxation time play a significant role in qubit dynamics. Our work
can provide guidance for future quantum processor designs with improved qubit
coherence times.Comment: 8 pages, 5 figure
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.Comment: 22 pages, 4 figures, SI included as ancilliary fil
Dynamic lattice distortions driven by surface trapping in semiconductor nanocrystals
Nonradiative processes limit optoelectronic functionality of nanocrystals and
curb their device performance. Nevertheless, the dynamic structural origins of
nonradiative relaxations in nanocrystals are not understood. Here, femtosecond
electron diffraction measurements corroborated by atomistic simulations uncover
transient lattice deformations accompanying radiationless electronic processes
in semiconductor nanocrystals. Investigation of the excitation energy
dependence shows that hot carriers created by a photon energy considerably
larger than the bandgap induce structural distortions at nanocrystal surfaces
on few picosecond timescales associated with the localization of trapped holes.
On the other hand, carriers created by a photon energy close to the bandgap
result in transient lattice heating that occurs on a much longer 200 ps
timescale, governed by an Auger heating mechanism. Elucidation of the
structural deformations associated with the surface trapping of hot holes
provides atomic-scale insights into the mechanisms deteriorating optoelectronic
performance and a pathway towards minimizing these losses in nanocrystal
devices.Comment: 17 pages, 4 figure
Circumventing the phonon bottleneck by multiphonon-mediated hot exciton cooling at the nanoscale
An atomistic understanding of exciton-phonon coupling and nonradiative processes in semiconductor nanocrystals
Semiconductor nanocrsytals (NCs) have been of much interest over the past several decades due to their highly tunable optoelectronic properties, which make thempromising materials for applications ranging from solar energy conversion to quantum information. These optoelectronic properties depend significantly on electron-hole and exciton-phonon interactions, which are enhanced in NCs due to quantum confinement. A fundamental understanding of these interactions as well as processes, such as exciton decay and dephasing, is key to developing rational design principles for NCs with decreased thermal losses and increased quantum yields. Yet the description of excited carriers in NCs remains a great challenge for modern computational science. In this dissertation, we address this gap by developing a unified, atomistic model to accurately describe excitonic properties and dynamics in experimentally relevant NC systems with thousands of atoms and tens of thousands of electrons.We develop an approach for calculating exciton-phonon couplings (EXPCs) and validate them by computing the reorganization energy, which is a measure of EXPC and is relevant for optical Stokes shifts, charge transfer processes, and NC-based device efficiencies. This microscopic theory allows us to delineate the dependence of the reorganization energy on NC size and structure as well as on phonon frequency and localization, resolving questions regarding the role of quantum confinement on EXPC. Additionally, we use this EXPC framework to perform quantum dynamics simulations of hot exciton cooling to address the longstanding controversy of the phonon bottleneck, which hypothesized slow nonradiative relaxation of hot carriers to the band edge in confined semiconductor nanostructures. Contrary to the phonon bottleneck but in agreement with recent experimental measurements, we find that cooling in CdSe NCs occurs in tens of femtoseconds. We show that this ultrafast timescale is governed by both electron-hole correlations and efficient multiphonon emission processes. Finally, we employ these tools in collaboration with experimentalists to understand lattice heating and polaron formation, photoluminescence, and charge trapping and transfer in various NC systems. The atomistic theories and calculations presented in this dissertation bridge our understanding of molecular and bulk systems to provide fundamental insight into exciton-phonon interactions and nonradiative processes at the nanoscale
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Interplay of Surface and Interior Modes in Exciton–Phonon Coupling at the Nanoscale
Exciton-phonon coupling (EXPC) plays a key role in the optoelectronic properties of semiconductor nanocrystals (NCs), but a microscopic picture of EXPC is still lacking, particularly regarding the magnitude and scaling with NC size, the dependence on phonon frequency, and the role of the NC surface. The computational complexity associated with accurately describing excitons and phonons has limited previous theoretical studies of EXPC to small NCs, noninteracting electron-hole models, and/or a small number of phonon modes. Here, we develop an atomistic approach for describing EXPC in NCs of experimentally relevant sizes. We validate our approach by calculating the reorganization energies, a measure of EXPC, for CdSe and CdSe-CdS core-shell NCs, finding good agreement with experimental measurements. We demonstrate that exciton formation distorts the NC lattice primarily along the coordinates of low-frequency acoustic modes. Modes at the NC surface play a significant role in smaller NCs while interior modes dominate for larger systems
Circumventing the phonon bottleneck by multiphonon-mediated hot exciton cooling at the nanoscale
Abstract Nonradiative processes govern efficiencies of semiconductor nanocrystal (NC)-based devices. A central process is hot exciton cooling, or the nonradiative relaxation of a highly excited electron/hole pair to form a band-edge exciton. Due to quantum confinement effects, the timescale and mechanism of cooling are not well understood. A mismatch between electronic energy gaps and phonon frequencies has led to the hypothesis of a phonon bottleneck and extremely slow cooling, while enhanced electron-hole interactions have suggested ultrafast cooling. Experimental measurements of the cooling timescale range six orders of magnitude. Here, we develop an atomistic approach to describe phonon-mediated exciton dynamics and simulate cooling in NCs of experimentally relevant sizes. We find that cooling occurs on ~30 fs timescales in CdSe NCs, in agreement with the most recent measurements, and that the phonon bottleneck is circumvented through a cascade of multiphonon-mediated relaxation events. Furthermore, we identify NC handles for tuning the cooling timescale