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1. Inter-Oligomer Interaction Influence on Photoluminescence in Cis-Polyacetylene Semiconductor Materials

   https://doi.org/10.3390/polym16131896  

   Keya, Kamrun N; Han, Yulun; Xia, Wenjie; Kilin, Dmitri (July 2024, Polymers)

   Semiconducting conjugated polymers (CPs) are pivotal in advancing organic electronics, offering tunable properties for solar cells and field-effect transistors. Here, we carry out first-principle calculations to study individual cis-polyacetylene (cis-PA) oligomers and their ensembles. The ground electronic structures are obtained using density functional theory (DFT), and excited state dynamics are explored by computing nonadiabatic couplings (NACs) between electronic and nuclear degrees of freedom. We compute the nonradiative relaxation of charge carriers and photoluminescence (PL) using the Redfield theory. Our findings show that electrons relax faster than holes. The ensemble of oligomers shows faster relaxation compared to the single oligomer. The calculated PL spectra show features from both interband and intraband transitions. The ensemble shows broader line widths, redshift of transition energies, and lower intensities compared to the single oligomer. This comparative study suggests that the dispersion forces and orbital hybridizations between chains are the leading contributors to the variation in PL. It provides insights into the fundamental behaviors of CPs and the molecular-level understanding for the design of more efficient optoelectronic devices. 

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2. Role of perovskite thickness on optoelectronic properties in lead bromide and lead iodide thin film perovskites: A DFT study

   https://doi.org/10.1557/s43580-023-00641-y  

   Graupner, David R; Kilin, Dmitri (November 2023, MRS Advances)

   Two-dimensional organic–inorganic hybrid lead halide perovskites are of interest for photovoltaic and light emitting devices due to their favorable properties that can be tuned. Here we use density functional theory to model two-dimensional lead halide perovskites of different thicknesses and using two different hallogens. Excited-state optoelectronic properties of the perovskite models are examined using excited-state dynamics treated by reduced density matrix method. Nonadiabatic couplings were computed based on the on-the-fly approach along a molecular dynamics trajectory at ambient temperatures. The density matrix-based equation of motion for electronic degrees of freedom was used to determine the dynamics of electronic degrees of freedom. We observe that the thickness of the perovskite layer shows a redshift in the absorption spectra with increasing thickness but has minimal effect on the photoluminescence quantum yield of the material. 

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3. Simulating ultrafast transient absorption spectra from first principles using a time-dependent configuration interaction probe

   https://doi.org/10.1063/5.0215890  

   Mehmood, Arshad; Silfies, Myles C; Durden, Andrew S; Allison, Thomas K; Levine, Benjamin G (July 2024, The Journal of Chemical Physics)

   Transient absorption spectroscopy (TAS) is among the most common ultrafast photochemical experiments, but its interpretation remains challenging. In this work, we present an efficient and robust method for simulating TAS signals from first principles. Excited-state absorption and stimulated emission (SE) signals are computed using time-dependent complete active space configuration interaction (TD-CASCI) simulations, leveraging the robustness of time-domain simulation to minimize electronic structure failure. We demonstrate our approach by simulating the TAS signal of 1′-hydroxy-2′-acetonapthone (HAN) from ab initio multiple spawning nonadiabatic molecular dynamics simulations. Our results are compared to gas-phase TAS data recorded from both jet-cooled (T ∼ 40 K) and hot (∼403 K) molecules via cavity-enhanced TAS (CE-TAS). Decomposition of the computed spectrum allows us to assign a rise in the SE signal to excited-state proton transfer and the ultimate decay of the signal to relaxation through a twisted conical intersection. The total cost of computing the observable signal (∼1700 graphics processing unit hours for ∼4 ns of electron dynamics) was markedly less than that of performing the ab initio multiple spawning calculations used to compute the underlying nonadiabatic dynamics. 

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4. Two-dimensional electronic–vibrational sum frequency spectroscopy for interactions of electronic and nuclear motions at interfaces

   https://doi.org/10.1073/pnas.2100608118  

   Deng, Gang-Hua; Qian, Yuqin; Zhang, Tong; Han, Jian; Chen, Hanning; Rao, Yi (August 2021, Proceedings of the National Academy of Sciences)

   Interactions of electronic and vibrational degrees of freedom are essential for understanding excited-states relaxation pathways of molecular systems at interfaces and surfaces. Here, we present the development of interface-specific two-dimensional electronic–vibrational sum frequency generation (2D-EVSFG) spectroscopy for electronic–vibrational couplings for excited states at interfaces and surfaces. We demonstrate this 2D-EVSFG technique by investigating photoexcited interface-active ( E )-4-((4-(dihexylamino) phenyl)diazinyl)-1-methylpyridin-1- lum (AP3) molecules at the air–water interface as an example. Our 2D-EVSFG experiments show strong vibronic couplings of interfacial AP3 molecules upon photoexcitation and subsequent relaxation of a locally excited (LE) state. Time-dependent 2D-EVSFG experiments indicate that the relaxation of the LE state, S 2 , is strongly coupled with two high-frequency modes of 1,529.1 and 1,568.1 cm −1 . Quantum chemistry calculations further verify that the strong vibronic couplings of the two vibrations promote the transition from the S 2 state to the lower excited state S 1 . We believe that this development of 2D-EVSFG opens up an avenue of understanding excited-state dynamics related to interfaces and surfaces. 

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5. Relaxation of Photoexcited Electron–Hole Pairs at Si(111) Surfaces with Adsorbed Ag Monolayered Clusters of Increasing Size

   https://doi.org/10.1021/acs.jpclett.4c03735  

   Han, Yulun; Vazhappilly, Tijo; Micha, David A; Kilin, Dmitri S (March 2025, The Journal of Physical Chemistry Letters)

   The efficiency of silicon solar cells is affected by the light absorption and recombination losses of photoexcited charge carries. One possible way to improve the efficiency is through the deposition of transition metal nanoparticles on Si surfaces. Here, we first carry out density functional theory (DFT) calculations to obtain electronic structures for Agn (n = 1–7) monolayered clusters adsorbed on Si(111)/H surfaces. Results are presented in the form of the density of states, band gaps, and light absorption, which allow for the investigation of the interaction of Ag clusters with Si. Different behaviors can be expected depending on the size of the deposited Ag clusters. Overall, the deposition of Ag clusters leads to smaller band gaps, red-shifts, and large increases in light absorption compared to the pristine Si slab. We then study the relaxation dynamics of electron–hole pairs for slabs based on nonadiabatic couplings using the reduced density matrix approach within the Redfield formalism. Nonradiative relaxation rates are noticeably different for various structures and transitions. One observes higher relaxation rates for surfaces with adsorbates than for the pristine Si surface due to charge transfer events involving Ag orbitals. We also compute emission spectra from excited-state relaxation dynamics. The band gap emission is dark for the pristine Si due to the indirect nature of its band gap. The addition of larger Ag clusters breaks the symmetry of Si slabs, enabling indirect gap transitions. These slabs thus exhibit bright band gap emission. The introduction of adsorbates is advantageous for applications in photovoltaics and photocatalysis. 

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