Measuring the Ultrafast Spectral Diffusion and Vibronic Coupling Dynamics in CdSe Colloidal Quantum Wells using Two-Dimensional Electronic Spectroscopy

Exciton-phonon coupling and phonon-assisted exciton relaxation dynamics in In1-xGaxP quantum dots

Quantum dots leverage quantum confinement to modify the electronic structure of materials, separating electronic transitions from the composition of the corresponding bulk material. With ternary quantum dots, the composition may be varied continuously so that both composition and size may be used to tune the bandgap. As composition influences electron-phonon coupling which in turn governs relaxation dynamics, the composition of ternary quantum dots may be adjusted to change dynamics. Here, we show that exciton-phonon coupling and phonon-assisted exciton relaxation dynamics remain strongly correlated to material composition in ternary In0.62Ga0.38P/ZnS and In0.35Ga0.65P/ZnS quantum dots using both experimental two-dimensional electronic spectroscopy measurements and quantum dynamical simulations. Theoretical calculations show that alloyed In1-xGaxP quantum dots have more complex exciton level structure than parent InP quantum dots. We identify a slower hot exciton cooling rate in In0.62Ga0.38P/ZnS, attributed to the presence of 'energy-retaining' valley exciton states with strong exciton-phonon coupling. Experimental quantum beating maps reveal a more localized quantum beat pattern for In0.35Ga0.65P/ZnS quantum dots, which may relate to the increased number of 'dim' exciton levels with reduced spacings. These findings highlight that exciton relaxation dynamics and exciton-phonon coupling in an alloyed In1-xGaxP quantum dot system are composition-dependent.

Preparation of Colloidal Cadmium Sulfoselenide Nanoplatelets from Alloyed Quantum Dots

We report a novel approach for synthesizing cadmium sulfoselenide nanoplatelets from alloyed quantum dots. The resulting nanoplatelets, with a zinc-blende crystal structure, display uniform morphology, sharp photoluminescence, and spatially homogeneous elemental distributions. Correlated analyses corroborate the evolution of cadmium sulfoselenide nanoplatelets derived from alloyed quantum dots prepared with selenium sulfide.

Quantum Dynamics of Charge Carriers in Fullerenes Encapsulated by Covalent Organic Polyhedra: Choice of Fullerene Matters

Charge separation is at the heart of solar energy applications, and efficient materials require fast photoinduced electron transfer (ET) and slow charge recombination (CR). Using time-dependent self-consistent charge density functional tight-binding theory combined with nonadiabatic (NA) molecular dynamics, we report a detailed analysis of ET and CR in hybrids composed of photoactive covalent organic polyhedra (COP) and encapsulated fullerenes. The ET occurs on a subpicosecond time scale and accelerates with increasing fullerene diameter, C60 to C70 to C84. As the fullerene size increases, the π-electron system available for interaction with the COP grows, the fullerene-COP separation decreases, and the number of fullerene states available to accept the photoexcited electron increases, accelerating the ET. In comparison, the CR occurs on a nanosecond time scale and correlates with the length of the fullerene shortest axis because the relevant fullerene state is polarized in that direction. The largest and least symmetrical C84 exhibits the fastest ET and the slowest CR, making COP@C84 the most promising hybrid. Both high-frequency bond stretching and bending vibrations and low-frequency breathing modes are involved in the ET and CR processes, with more modes present in the C84 system due its lower symmetry. The 10-20 fs vibrationally induced coherence loss in the electronic subsystem contributes to long lifetimes of the charge-separated states. The comprehensive investigation of the structure-property relationship of the charge carrier dynamics in the COP@fullerene hybrids provides a detailed atomistic understanding of interfacial ET processes and generates guidelines for rational design of high-performance materials for solar energy and related applications.

Prolonged Exciton Lifetime Is Achieved in Porphyrin Nanoring by Template Engineering: A Nonadiabatic Tight Binding Approach

Porphyrin nanoring has been attracting immense attention due to its light harvesting capacity and potential applications in optical, catalysis, sensor, and electronic devices. We demonstrate by nonadiabatic quantum dynamics simulations that the photovoltaic efficiency can be enhanced by template engineering. Altering the hexadentate template (T6) with two tridentate templates (2T3) within the porphyrin ring (P6) cavity accelerated the electron transfer twice and suppressed the electron-hole recombination by nearly three times. The atomistic tight-binding simulation rationalized the dynamics by different localizations of charge of the band edge states, changes in nonadiabatic coupling, alteration in quantum coherence, and involvement of diverse electron-phonon vibrational modes. Further 2T3 templates more strongly hold the P6 ring than T6, reducing the structural fluctuation. As a result, the nonadiabatic coupling becomes weaker and suppresses the carrier recombination. Current atomistic simulation presents a template engineering strategy to enhance the exciton lifetime along with ultrafast charge separation, crucial factors for photovoltaic applications.

Controlling Charge Carrier Dynamics in Porphyrin Nanorings by Optically Active Templates

Understanding the dynamics of photogenerated charge carriers is essential for enhancing the performance of solar and optoelectronic devices. Using atomistic quantum dynamics simulations, we demonstrate that a short π-conjugated optically active template can be used to control hot carrier relaxation, charge carrier separation, and carrier recombination in light-harvesting porphyrin nanorings. Relaxation of hot holes is slowed by 60% with an optically active template compared to that with an analogous optically inactive template. Both systems exhibit subpicosecond electron transfer from the photoactive core to the templates. Notably, charge recombination is suppressed 6-fold by the optically active template. The atomistic time-domain simulations rationalize these effects by the extent of electron and hole localization, modification of the density of states, participation of distinct vibrational motions, and changes in quantum coherence. Extension of the hot carrier lifetime and reduction of charge carrier recombination, without hampering charge separation, demonstrate a strategy for enhancing efficiencies of energy materials with optically active templates.

Measuring the Ultrafast Correlation Dynamics of a Multilevel System Using the Center Line Slope Analysis in Two-Dimensional Electronic Spectroscopy

In a two-dimensional (2D) optical spectrum of a multilevel system, there are diagonal peaks and off-diagonal cross-peaks that correlate the different levels. The time-dependent properties of these diagonal peaks and cross-peaks contain much information about the dynamics of the multilevel system. The time-dependent diagonal peakshape that depends on the spectral diffusion dynamics of the associated transition and characterized by the frequency-fluctuation correlation function (FFCF) is well studied. However, the time-dependent peakshape of a cross-peak that provides the correlation dynamics between different transitions is much less studied or understood. We derived the third-order nonlinear response functions that describe the cross-peaks in a 2D electronic spectrum of a multilevel system that arise from processes sharing a common ground state and/or from internal conversion and population transfer. We can use the center line slope (CLS) analysis to characterize the cross-peaks in conjunction with the diagonal peaks. This allows us to recover the frequency-fluctuation cross-correlation functions (FXCFs) between two transitions. The FXCF and its subsidiary quantities such as the initial correlation and the initial covariance between different transitions are important for studying the correlation effects between states in complex systems, such as energy-transfer processes. Furthermore, knowledge of how various molecular processes over different timescales affect simultaneously different transitions can also be obtained from the measured FXCF. We validated and tested our derived equations and analysis process by studying, as an example, the 2D electronic spectra of metal-free phthalocyanine in solution. We measured and analyzed the diagonal peaks of the Qx and Qy transitions and the cross-peaks between these two transitions of this multilevel electronic system and obtained the associated FFCFs and FXCFs. In this model system, we measured negative components of FXCF over the tens of picosecond timescale. This suggests that in phthalocyanine, the Qx and Qy transitions coupling with the solvent molecule motion are anticorrelated to each other.