Formation and Luminescence of Single Oxygen Impurities on the Surface of SiC Nanocrystals

Impurities that hinder luminescence are a common problem in the synthesis of nanocrystals, and controlling the synthesis reaction could provide a way to avoid or use impurities beneficially. Excited state molecular dynamics is used to determine how oxygen impurities appear in the plasma synthesis of silicon carbide nanocrystals (SiC NCs). Formation of impurities is studied by considering the intermediate structures in the simulated photoreaction. The results show the most probable bonding patterns of silicon, carbon, and oxygen. These intermediates are used as a basis for studying the luminescence of expected oxygen impurities in SiC NCs, where luminescence is studied by first-principles modeling and density matrix dissipative dynamics based on on-the-fly non-adiabatic couplings and the Redfield tensor. Modeling the dissipation of energy from electronic to nuclear degrees of freedom reveals multiple impurities with significant photoluminescence quantum yields.

Photoabsorbance of supported metal clusters: ab initio density matrix and model studies of large Ag clusters on Si surfaces

Metal clusters with 10 to 100 atoms supported by a solid surface show electronic structure typical of molecules and require ab initio treatments starting from their atomic structure, and they also can display collective electronic phenomena similar to plasmons in metal solids. We have employed ab initio electronic structure results from two different density functionals (PBE and the hybrid HSE06) and a reduced density matrix treatment of the dissipative photodynamics to calculate light absorbance by the large Ag clusters Ag_N, N = 33, 37(open shell) and N = 32, 34 (closed shell), adsorbed at the Si(111) surface of a slab, and forming nanostructured surfaces. Results on light absorption are quite different for the two functionals, and are presented here for light absorbances using orbitals and energies from the hybrid functional giving correct energy band gaps. Absorption of Ag clusters on Si increases light absorbance versus photon energy by large percentages, with peak increases found in regions of photon energies corresponding to localized plasmons. The present metal clusters are large enough to allow for modelling with continuum dielectric treatments of their medium. A mesoscopic Drude-Lorentz model is presented in a version suitable for the present structures, and provides an interpretation of our results. The calculated range of plasmon energies overlaps with the range of solar photon energies, making the present structures and properties relevant to applications to solar photoabsorption and photocatalysis.

First-Principles Study on Optoelectronic Properties of Fe-Doped Montmorillonite Clay

A theoretical investigation is conducted to describe optoelectronic properties of Fe-doped montmorillonite nanoclay under spin states of low spin (LS), intermediate spin (IS), and high spin (HS). Ground state electronic properties are studied using spin-polarized density functional theory calculations. The nonradiative and radiative relaxation channels of charge carriers are studied by computing nonadiabatic couplings (NACs) using an "on-the-fly" approach from adiabatic molecular dynamics trajectories. The NACs are further processed using a reduced density matrix approach with the Redfield formalism. The computational results are presented for electronic density of states, absorption spectra, charge carrier dynamics, and photoluminescence (PL) by comparing various spin multiplicities. Results on spin α and spin β components are independent and quite different because of the partial occupation of Fe 3d states. Overall, HS is the most stable with the largest Fe-O distances. One finds different nonradiative relaxation pathways in space and on the time scale for electrons and holes. The Redfield PL reveals obvious Fe 3d-3d transitions for LS and IS.

Hot Carrier Dynamics at Ligated Silicon(111) Surfaces: A Computational Study

We provide a case-study for thermal grafting of benzenediazonium bromide onto a hydrogenated Si(111) surface using ab initio molecular dynamics (AIMD) calculations. A sequence of reaction steps is identified in the AIMD trajectory, including the loss of N₂ from the diazonium salt, proton transfer from the surface to the bromide ion that eliminates HBr, and deposition of the phenyl group onto the surface. We next assess the influence of the phenyl groups on photophysics of hydrogen-terminated Si(111) slabs. The nonadiabatic couplings necessary for a description of the excited-state dynamics are calculated by combining ab initio electronic structures and reduced density matrix formalism with Redfield theory. The phenyl-terminated slab shows reduced nonradiative relaxation and recombination rates of hot charge carriers in comparison with the hydrogen-terminated slab. Altogether, our results provide atomistic insights revealing that (i) the diazonium salt thermally decomposes at the surface allowing the formation of covalently bonded phenyl group, and (ii) the coverage of phenyl groups on the surface slows down charge carrier cooling driven by electron-phonon interactions, which increases photoluminescence efficiency at the near-infrared spectral region.

Magnetic-Field-Driven Electron Dynamics in Graphene

Graphene exhibits unique optoelectronic properties originating from the band structure at the Dirac points. It is an ideal model structure to study the electronic and optical properties under the influence of the applied magnetic field. In graphene, electric field, laser pulse, and voltage can create electron dynamics which is influenced by momentum dispersion. However, computational modeling of momentum-influenced electron dynamics under the applied magnetic field remains challenging. Here, we perform computational modeling of the photoexcited electron dynamics achieved in graphene under an applied magnetic field. Our results show that magnetic field leads to local deviation from momentum conservation for charge carriers. With the increasing magnetic field, the delocalization of electron probability distribution increases and forms a cyclotron-like trajectory. Our work facilitates understanding of momentum resolved magnetic field effect on non-equilibrium properties of graphene, which is critical for optoelectronic and photovoltaic applications.

Nonradiative Relaxation Dynamics of a Cesium Lead Halide Perovskite Photovoltaic Architecture: Effect of External Electric Fields

Lead halide perovskites have attracted much attention as an active material in solar cells. In this first-principles study, we consider a cesium lead halide perovskite slab interfacing with electron transport and hole transport layers, relevant to the practical photovoltaic architecture. We apply external electric fields normal to the surface of the perovskite slab and explore the induced changes onto optoelectronic properties. It is found that the bandgap increases linearly and the conductivity diminishes exponentially with decreasing electric field strengths. Furthermore, we study the influence of electric fields onto nonradiative relaxation of photoexcited electrons and holes using the reduced density matrix in the formalism of Redfield theory. Our calculations provide relaxation rates and relaxation pathways, illustrating the mechanisms of modulations of electric field strengths onto charge carrier dynamics. Our results show that holes have longer lifetimes than electrons at various external electric fields. It is also found that the patterns of charge carrier dynamics depend on the direction of external electric fields. Specifically, in comparison with the system under zero field, our findings show that (i) the positive electric field facilitates the relaxation of electrons and holes and (ii) the negative electric field facilitates the relaxation of electrons but inhibits the relaxation of holes.

Brightly Luminescent CsPbBr3 Nanocrystals through Ultracentrifugation

Using a combination of density-gradient and analytical ultracentrifugation, we studied the photophysical profile of CsPbBr₃ nanocrystal (NC) suspensions by separating them into size-resolved fractions. Ultracentrifugation drastically alters the ligand profile of the NCs, which necessitates postprocessing to restore colloidal stability and enhance quantum yield (QY). Rejuvenated fractions show a 50% increase in QY compared to no treatment and a 30% increase with respect to the parent. Our results demonstrate how the NC environment can be manipulated to improve photophysical performance, even after there has been a measurable decline in the response. Size separation reveals blue-emitting fractions, a narrowing of photoluminescence spectra in comparison to the parent, and a crossover from single- to stretched-exponential relaxation dynamics with decreasing NC size. As a function of edge length, L, our results confirm that the photoluminescence peak energy scales a L^-2, in agreement with the simplest picture of quantum confinement.

Synthesis of Holey Graphene Nanoparticle Compounds

Bulk-scale syntheses of sp² nanocarbon have typically been generated by extensive chemical oxidation to yield graphite oxide from graphite, followed by a reductive step. Materials generated via harsh random processes lose desirable physical characteristics. Loss of sp² conjugation inhibits long-range electronic transport and the potential for electronic band manipulation. Here, we present a nanopatterned holey graphene material electronically hybridized with metal-containing nanoparticles. Oxidative plasma etching of highly ordered pyrolytic graphite via previously developed covalent organic framework (COF)-5-templated patterning yields bulk-scale materials for electrocatalytic applications and fundamental investigations into band structure engineering of nanocomposites. We establish a broad ability (Ag, Au, Cu, and Ni) to grow metal-containing nanoparticles in patterned holes in a metal precursor-dependent manner without a reducing agent. Graphene nanoparticle compounds (GNCs) show metal-contingent changes in the valence band structure. Density functional theory investigations reveal preferences for uncharged metal states, metal contributions to the valence band, and embedding of nanoparticles over surface incorporation. Ni-GNCs show activity for oxygen evolution reaction in alkaline media (1 M KOH). Electrocatalytic activity exceeds 10,000 mA/mg of Ni, shows stability for 2 h of continuous operation, and is kinetically consistent via a Tafel slope with Ni(OH)₂-based catalysis.

Universal Size-Dependent Stokes Shifts in Lead Halide Perovskite Nanocrystals

Size-dependent photoluminescence Stokes shifts (ΔE_s) universally exist in CsPbX₃ (X = Cl^-, Br^-, or I^-) perovskite nanocrystals (NCs). ΔE_s values, which range from ∼15 to 100 meV for NCs with average edge lengths (l) from approximately 13 to 3 nm, are halide-dependent such that ΔE_s(CsPbI₃) > ΔE_s(CsPbBr₃) ≳ ΔE_s(CsPbCl₃). Observed size-dependent Stokes shifts are not artifacts of ensemble size distributions as demonstrated through measurements of single CsPbBr₃ NC Stokes shifts (⟨ΔE_s⟩ = 42 ± 5 meV), which are in near quantitative agreement with associated ensemble (l = 6.8 ± 0.8 nm) ΔE_s values (ΔE_s ≈ 50 meV). Transient differential absorption measurements additionally illustrate no significant spectral dynamics on the picosecond time scale that would contribute to ΔE_s. This excludes polaron formation as being responsible for ΔE_s. Altogether, the results point to an origin for ΔE_s, intrinsic to the size-dependent electronic properties of individual perovskite NCs.

Bright Silicon Nanocrystals from a Liquid Precursor: Quasi-Direct Recombination with High Quantum Yield

Silicon nanocrystals (SiNCs) with bright bandgap photoluminescence (PL) are of current interest for a range of potential applications, from solar windows to biomedical contrast agents. Here, we use the liquid precursor cyclohexasilane (Si₆H₁₂) for the plasma synthesis of colloidal SiNCs with exemplary core emission. Through size separation executed in an oxygen-shielded environment, we achieve PL quantum yields (QYs) approaching 70% while exposing intrinsic constraints on efficient core emission from smaller SiNCs. Time-resolved PL spectra of these fractions in response to femtosecond pulsed excitation reveal a zero-phonon radiative channel that anticorrelates with QY, which we model using advanced computational methods applied to a 2 nm SiNC. Our results offer additional insight into the photophysical interplay of the nanocrystal surface, quasi-direct recombination, and efficient SiNC core PL.

First-Principles Molecular Dynamics of Monomethylhydrazine and Nitrogen Dioxide

The exploration of chemical reactions preceding ignition is essential for the development of ideal hypergolic propellants. Unexpected reaction pathways of a hypergolic mixture composed of monomethylhydrazine and nitrogen dioxide are predicted through a cooperative combination of (i) spin-unrestricted ab initio molecular dynamics (AIMD) and (ii) wave packet dynamics of protons. Ensembles of AIMD trajectories reveal a sequence of reaction steps for proton transfer and rupture of the C-N bond. The details of proton transfer are explored by wave packet dynamics on the basis of ab initio potential energy surfaces from AIMD trajectories. The possibility of spontaneous ignition of this hypergolic mixture at room temperature is predicted as a quantized feature of proton-transfer dynamics.

Unraveling Photodimerization of Cyclohexasilane from Molecular Dynamics Studies

Photoinduced reactions of a pair of cyclohexasilane (CHS) monomers are explored by time-dependent excited-state molecular dynamics (TDESMD) calculations. In TDESMD trajectories, one observes vivid reaction events including dimerization and fragmentation. A general reaction pathway is identified as (i) ring-opening formation of a dimer, (ii) rearrangement induced by bond breaking, and (iii) decomposition through the elimination of small fragments. The identified pathway supports the chemistry proposed for the fabrication of silicon-based materials using CHS as a precursor. In addition, we find dimers have smaller HOMO-LUMO gaps and exhibit a red shift and line-width broadening in the computed photoluminescence spectra compared with a pair of CHS monomers.

Unexpected high binding energy of CO2 on CH3NH3PbI3 lead-halide organic–inorganic perovskites via bicarbonate formation

The adsorption kinetics of CO₂ was experimentally characterized in ultra-high vacuum (UHV). In addition, density functional theory (DFT) calculations were included.

Photofragmentation of Tetranitromethane: Spin-Unrestricted Time-Dependent Excited-State Molecular Dynamics

In this study, the photofragmentation dynamics of tetranitromethane (TNM) is explored by a spin-unrestricted time-dependent excited-state molecular dynamics (u-TDESMD) algorithm based on Rabi oscillations and principles similar to trajectory surface hopping, with a midintensity field approximation. The leading order process is represented by the molecule undergoing cyclic excitations and de-excitations. During excitation cycles, the nuclear kinetic energy is accumulated to overcome the dissociation barriers in the reactant and a sequence of intermediates. The dissociation pathway includes the ejection of NO₂ groups followed by the formation of NO and CO. The simulated mass spectra at the ab initio level, based on the bond length in possible fragments, are extracted from simulation trajectories. The recently developed methodology has the potential to model and monitor photoreactions with open-shell intermediates and radicals.

Photoinduced Single- and Multiple-Electron Dynamics Processes Enhanced by Quantum Confinement in Lead Halide Perovskite Quantum Dots

Methylammonium lead iodide perovskite (MAPbI₃) is a promising material for photovoltaic devices. A modification of MAPbI₃ into confined nanostructures is expected to further increase efficiency of solar energy conversion. Photoexcited dynamic processes in a MAPbI₃ quantum dot (QD) have been modeled by many-body perturbation theory and nonadiabatic dynamics. A photoexcitation is followed by either exciton cooling (EC), its radiative (RR) or nonradiative recombination (NRR), or multiexciton generation (MEG) processes. Computed times of these processes fall in the order of MEG < EC < RR < NRR, where MEG is on the order of a few femtoseconds, EC is in the picosecond range, while RR and NRR are on the order of nanoseconds. Computed time scales indicate which electronic transition pathways can contribute to increase in charge collection efficiency. Simulated mechanisms of relaxation and their rates show that quantum confinement promotes MEG in MAPbI₃ QDs.

Mathematical modeling of gas desorption from a metal–organic supercontainer cavity filled with stored N2gas at critical limits

Gas escape rates from within the cavity of a MOSC were predicted by molecular dynamics and analytical equations.

Surface Chemistry of Semiconducting Quantum Dots: Theoretical Perspectives

Colloidal quantum dots (QDs) are near-ideal nanomaterials for energy conversion and lighting technologies. However, their photophysics exhibits supreme sensitivity to surface passivation and defects, of which control is problematic. The role of passivating ligands in photodynamics remains questionable and is a focus of ongoing research. The optically forbidden nature of surface-associated states makes direct measurements on them challenging. Therefore, computational modeling is imperative for insights into surface passivation and its impact on light-driven processes in QDs. This Account discusses challenges and recent progress in understanding surface effects on the photophysics of QDs addressed via quantum-chemical calculations. We overview different methods, including the effective mass approximation (EMA), time-dependent density functional theory (TDDFT), and multiconfiguration approaches, considering their strengths and weaknesses relevant to modeling of QDs with a complicated surface. We focus on CdSe, PbSe, and Si QDs, where calculations successfully explain experimental trends sensitive to surface defects, doping, and ligands. We show that the EMA accurately describes both linear and nonlinear optical properties of large-sized CdSe QDs (>2.5 nm), while TDDFT is required for smaller QDs where surface effects dominate. Both approaches confirm efficient two-photon absorption enabling applications of QDs as nonlinear optical materials. TDDFT also describes the effects of morphology on the optical response of QDs: the photophysics of stoichiometric, magic-sized X_nY_n (X = Cd, Pb; Y = S, Se) QDs is less sensitive to their passivation compared with nonstoichiometric X_n≠mY_m QDs. In the latter, surface-driven optically inactive midgap states can be eliminated by anionic ligands, explaining the better emission of metal-enriched QDs compared with nonmetal-enriched QDs. Ideal passivation of magic-sized QDs by amines and phosphine oxides leaves lower-energy transitions intact, while thiol derivatives add ligand-localized trap states to the band gap. Depending on its position, any loss of ligand from the QD's surface also introduces electron or hole traps, decreasing the QD's luminescence. TDDFT investigations of QD-ligand and QD-QD interactions provide an explanation of experimentally detected enhancement of blinking on-times in closely packed Si QDs and establish favorable conditions for hole transfer from the photoexcited CdSe QD to metal-organic dyes. While TDDFT well describes qualitative trends in optical response to stoichiometry and ligand modifications of QDs, it is unable to calculate highly correlated electronic states like biexcitons and magnetic-dopant-derived states. In these cases, multiconfiguration methods are applied to small nanoclusters and the results are extrapolated to larger-sized QDs, providing reasonable explanations of experimental observables. For light-driven dynamics, the electron-phonon couplings are important, and nonadiabatic dynamics (NAD) is applied. NAD based on first-principles calculations is a current grand challenge for the theory. However, it can be accomplished through sets of semiclassical approximations such as surface hopping (SH). We discuss validations of approximations used in photodynamics of ligated and doped QDs. Time-domain DFT-based SH-NAD reveals the ligand's role in ultrafast energy relaxation and the connection between the phonon bottleneck and the Zeno effect in CdSe QDs. The calculated results are helpful in controlling both dissipation and radiative processes in QDs via surface engineering and in explanations of experimental data.

Electronic properties of nickel-doped TiO₂ anatase

Atomistic details of electron transfer in semiconductor materials are characterized for TiO2 thin film surfaces doped with nickel. A periodic slab model of eight atomic layers exposes the (1 0 0) crystallographic surface and is covered with a monolayer of water. The density of states, absorption spectra, partial charge densities, molecular dynamics, and non-adiabatic couplings are compared between doped and undoped models. Our results show that Ni doping improves several electronic properties including lowering the band gap, increasing visible light absorption, and shortening the relaxation time of holes rather than electrons, which maximizes charge separation. The different mechanisms of electron and hole dynamics are discussed. The computed characteristics of a doped semiconductor material have practical potential for increasing efficiency of a photo-electrochemical cells.

Modeling the surface photovoltage of silicon slabs with varying thickness

The variation with thickness of the energy band gap and photovoltage at the surface of a thin semiconductor film are of great interest in connection with their surface electronic structure and optical properties. In this work, the change of a surface photovoltage (SPV) with the number of layers of a crystalline silicon slab is extracted from models based on their atomic structure. Electronic properties of photoexcited slabs are investigated using generalized gradient and hybrid density functionals, and plane wave basis sets. Si(1 1 1) surfaces have been terminated by hydrogen atoms to compensate for dangling bonds and have been described by large supercells with periodic boundary conditions. Calculations of the SPV of the Si slabs have been done in terms of the reduced density matrix of the photoactive electrons including dissipative effects due to their interaction with medium phonons and excitons. Surface photovoltages have been calculated for model Si slabs with 4-12, and 16 layers, to determine convergence trends versus slab thickness. Band gaps and the inverse of the SPVs have been found to scale nearly linearly with the inverse thickness of the slab, while the electronic density of states increases quadratically with thickness. Our calculations show the same trends as experimental values indicating band gap reduction and absorption enhancement for Si films of increasing thickness. Simple arguments on confined electronic structures have been used to explain the main effects of changes with slab thickness. A procedure involving shifted electron excitation energies is described to improve results from generalized gradient functionals so they can be in better agreement with the more accurate but also more computer intensive values from screened exchange hybrid functionals.

Excited State Dynamics of Ru10 Cluster Interfacing Anatase TiO2(101) Surface and Liquid Water

Charge transfer dynamics at the interface of supported metal nanocluster and liquid water by GGA+U calculations combined with density matrix formalism is considered. The Ru10 cluster introduces new states into the band gap of TiO2 surface, narrows the band gap of TiO2, and enhances the absorption strength. The H2O adsorption significantly enhances the intensity of photon absorption, which is due to the formation of Ti-O(water) and Ru-O(water) bonds at the interfaces. The Ru10 cluster promotes the dissociation of water, facilitates charge transfer, and increases the relaxation rates of holes and electrons. We expect that our results are helpful in understanding basic processes contributing to photoelectrochemical water splitting.

Photoinduced Charge Transfer from Titania to Surface Doping Site

We evaluate a theoretical model in which Ru is substituting for Ti at the (100) surface of anatase TiO2. Charge transfer from the photo-excited TiO2 substrate to the catalytic site triggers the photo-catalytic event (such as water oxidation or reduction half-reaction). We perform ab-initio computational modeling of the charge transfer dynamics on the interface of TiO2 nanorod and catalytic site. A slab of TiO2 represents a fragment of TiO2 nanorod in the anatase phase. Titanium to ruthenium replacement is performed in a way to match the symmetry of TiO2 substrate. One molecular layer of adsorbed water is taken into consideration to mimic the experimental conditions. It is found that these adsorbed water molecules saturate dangling surface bonds and drastically affect the electronic properties of systems investigated. The modeling is performed by reduced density matrix method in the basis of Kohn-Sham orbitals. A nano-catalyst modeled through replacement defect contributes energy levels near the bottom of the conduction band of TiO2 nano-structure. An exciton in the nano-rod is dissipating due to interaction with lattice vibrations, treated through non-adiabatic coupling. The electron relaxes to conduction band edge and then to the Ru cite with faster rate than hole relaxes to the Ru cite. These results are of the importance for an optimal design of nano-materials for photo-catalytic water splitting and solar energy harvesting.

Quantum Zeno effect rationalizes the phonon bottleneck in semiconductor quantum dots

Quantum confinement can dramatically slow down electron-phonon relaxation in nanoclusters. Known as the phonon bottleneck, the effect remains elusive. Using a state-of-the-art time-domain ab initio approach, we model the observed bottleneck in CdSe quantum dots and show that it occurs under quantum Zeno conditions. Decoherence in the electronic subsystem, induced by elastic electron-phonon scattering, should be significantly faster than inelastic scattering. Achieved with multiphonon relaxation, the phonon bottleneck is broken by Auger processes and structural defects, rationalizing experimental difficulties.

Breaking the phonon bottleneck in PbSe and CdSe quantum dots: time-domain density functional theory of charge carrier relaxation

Spatial confinement can create relaxation bottlenecks by mismatch between electronic and vibrational frequencies. This hypothesis motivated discovery of multiple excitons, which could greatly enhance the efficiency of quantum dot (QD) solar cells. Surprisingly, recent experiments showed no bottleneck. Our time-domain ab initio study of the electron-phonon dynamics rationalizes the fast relaxation in PbSe and CdSe QDs, which have substantially different electronic properties. Atom fluctuations and surface effects lift degeneracies and create dense distributions of electronic levels at all but the lowest energies, while confinement enhances the electron-phonon coupling. The analysis applies to nanomaterials in general, modifying the fundamental view on the electron-phonon interaction.

Photoinduced Vibrational Coherence Transfer in Molecular Dimers

At short times that are faster than dephasing, photoinduced evolution of the vibrational subsystem in an electron-phonon molecular structure depends strongly on the electronic evolution. As the electronic population shifts between the donor and acceptor states, in the diabatic description the state with the largest population determines the equilibrium positions and frequencies of the vibrational modes, which oscillate continuously and without loss of coherence. The vibrational coherence transfer between the electronic states detected recently in a number of systems is described theoretically by application of the quantized Hamiltonian dynamics (QHD) formalism [J. Chem. Phys. 2000, 113, 6557] to the coupled electronic and vibrational degrees of freedom of a model heterodimer. The observed coherent modulation of the frequency of the probe signal is represented with simple analytic and numeric QHD models.

Electron-nuclear correlations for photo-induced dynamics in molecular dimers

Ultrafast photoinduced dynamics of electronic excitation in molecular dimers is drastically affected by the dynamic reorganization of inter- and intra- molecular nuclear configuration modeled by a quantized nuclear degree of freedom. The dynamics of the electronic population and nuclear coherence is analyzed by solving the chain of coupled differential equations for population inversion, electron-vibrational correlation, etc. Intriguing results are obtained in the approximation of a small change of the nuclear equilibrium upon photoexcitation. In the limiting case of resonance between the electronic energy gap and the frequency of the nuclear mode these results are justified by comparison to the exactly solvable Jaynes-Cummings model. It is found that the photoinduced processes in the model dimer are arranged according to their time scales: (i) Fast scale of nuclear motion, (ii) intermediate scale of dynamical redistribution of electronic population between excited states as well as growth and dynamics of electron-nuclear correlation, (iii) slow scale of electronic population approach to the quasi-equilibrium distribution, decay of electron-nuclear correlation, and decrease of the amplitude of mean coordinate oscillation. The latter processes are accompanied by a noticeable growth of the nuclear coordinate dispersion associated with the overall nuclear wave packet width. The demonstrated quantum relaxation features of the photoinduced vibronic dynamics in molecular dimers are obtained by a simple method, applicable to systems with many degrees of freedom.