Papers by Dmitri S. Kilin
RSC Advances
First principles modeling of excited state dynamics of charge carriers at the interface between t... more First principles modeling of excited state dynamics of charge carriers at the interface between the perovskite and electron transport layer in perovskite solar cells identifies an effect of the interface morphology onto efficiency of charge transfer.
<div><p>The surface of charged wet TiO<sub>2</sub> anatase (001) function... more <div><p>The surface of charged wet TiO<sub>2</sub> anatase (001) functionalised by ruthenium ion at ambient temperatures is studied by computational modelling. Response of this model to photoexcitations at ambient temperatures is explored with the Redfield density matrix equation of motion on the basis of Kohn–Sham orbitals. The parameters of the Redfield equation are on-the-fly non-adiabatic couplings for electronic degrees of freedom obtained along the <i>ab initio</i> molecular dynamics nuclear trajectories. The main results in this study are the following: (1) optical properties of the doped models such as light absorption intensity and transition energies can be tuned by modifying total charge; (2) electron and hole relaxation rates depend on the initial excitation; and (3) in the doped model, excitations of lower energy provide quicker relaxation. Results of computational modelling would benefit understanding of the mechanism of electron transfer processes on the surface of ruthenium-doped TiO<sub>2</sub>.</p></div
Molecular Physics, 2016
In this study, a computational examination of the electronic transitions and through-space energy... more In this study, a computational examination of the electronic transitions and through-space energy transfer processes lends insight into the experimental electronic spectra of a redox-sensitive rhodamine–anthraquinone dyad. Electronic transitions were calculated using density functional theory (DFT) and time-dependent DFT (TDDFT) based on models optimised from single-crystal X-ray diffraction (XRD) ion positions. DFT calculations were performed on gas-phase models using the Vienna Ab Initio Software Package (VASP) with the functional developed by Perdew, Burke, and Ernzerhof (PBE) on a basis set of plane waves. Using the DFT results, select transitions were evaluated based on a dipole–dipole coupling mechanism to find the Förster resonance energy transfer coupling, the square of which is approximately proportional to the rate of energy transfer between the donor and the acceptor. Electronic transitions during the relaxation of charge carriers are also investigated using nonadiabatic ...
Using a combination of density-gradient and analytical ultracentrifugation, we study the photophy... more Using a combination of density-gradient and analytical ultracentrifugation, we study the photophysical profile of CsPbBr 3 nanocrystal (NC) suspensions by separating them into size-resolved fractions. Ultracentrifugation drastically alters the ligand profile of the NCs, which necessitates post-processing 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
SiNC size purification was performed in a Beckman Coulter Optima L-80 XP ultracentrifuge using a ... more SiNC size purification was performed in a Beckman Coulter Optima L-80 XP ultracentrifuge using a five-layer step gradient layered with 50 %, 60 %, 70 %, 80 %, and 90 % chloroform in m-xylene (by volume). All solvents were processed to remove oxygen. Custom polyvinylidene fluoride (Kynar) ultracentrifuge tubes were obtained from Seton Scientific. The step gradient was layered with a pipette, with subsequent layers deposited in order of decreasing density. A 0.3 mL solution of SiNCs in mesitylene (around 1 mg/mL) was layered on top of the
The Journal of Physical Chemistry C, 2019
APbX 3 (A=Cs,Methylammonium{MA}; X=I,Br,Cl) lead halide perovskites are of interest for lightemit... more APbX 3 (A=Cs,Methylammonium{MA}; X=I,Br,Cl) lead halide perovskites are of interest for lightemitting applications due to the tuneability of their bandgap across the visible and near infrared spectrum (IR) coupled with efficient photoluminescence quantum yields (PLQYs). Is widely speculated that photo-excited electrons and holes spatially separate into large (Frölich) negative and positive polarons which are stabilized by the A cations. Polarons are expected to be optically active with recent IR transient absorption experiments showing spectral features consistent with photo-ionization of the polaron into the continuum band states. For large polarons in the intermediate coupling regime it would also be expected to observe spectral signatures of transitions within the polaronic potential well producing polaron excited-states. From the polaron excited-state we predict that large polarons should be capable of spontaneous emission (photoluminescence) in the mid-IR to far-IR regime based on the concept of inverse occupations within the polaron potential well. To test this hypothesis, we use density-functional theory (DFT) based calculations using a CsPbBr 3 nanocrystal atomistic model as a host material for either negative (electron) or positive (hole) polarons. We dynamically couple electronic and nuclear degrees of freedom by computing non-adiabatic couplings which allow to explore non-radiative relaxation of excited polaronic states. Radiative relaxation of excited polaronic states are found from Einstein coefficients for spontaneous emission. Efficiency of polaron emission is determined from rates of non-radiative recombination () and radiative recombination () as. It is found that both the positive and negative polaron show bright absorption features and photoluminescence from the relaxed-excited state (RES) to the polaron ground states (PGS) but is an inefficient process (PLQY ~ 10-4-10-7). Methodology considerations for improving the computed PLQY of polaron emission are discussed, such as Marcus rate corrections and coherence. This work provides computational support for observation of IR polaron absorption and a potential direction towards extending the emission capabilities of APbX 3 perovskites into the mid-IR to far-IR regime.
The Journal of Physical Chemistry C, 2019
The Journal of Physical Chemistry Letters, 2018
Photoinduced reactions of a pair of cyclohexasilane (CHS) monomers are explored by timedependent ... more Photoinduced reactions of a pair of cyclohexasilane (CHS) monomers are explored by timedependent 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 linewidth broadening in the computed photoluminescence spectra compared to a pair of CHS monomers.
The Journal of Physical Chemistry C, 2018
Fully inorganic lead halide perovskite nanocrystals (NCs) are of interest for optoelectronic and ... more Fully inorganic lead halide perovskite nanocrystals (NCs) are of interest for optoelectronic and light emitting devices due to their photoluminescence (PL) emission properties which can be tuned/optimized by (I) surface passivation and (II) doping. (I) Surface passivation of the NC affects PL capabilities, as an under-passivated surface can introduce trap states which reduces PL quantum yields (QY). (II) Doping NCs and quantum dots (QDs) with transition metal ions provides stable optical transitions. Doping perovskite NCs with Mn 2+ ions provides high intensity 4 T 1 6 A 1 optical transitions in addition to the bright, intrinsic NC emission. Here we use noncollinear Density Functional Theory (DFT) to investigate the roles of surface passivation and doping on PL emission stability of perovskite NCs. Two models are investigated: (i) a pristine NC and (ii) a NC doped with Mn 2+ ion. The noncollinear DFT includes spin-orbit coupling (SOC) between different spin-states and produces spin-adiabatic molecular orbitals. These orbitals are used to calculate the transition dipoles between electronic states, oscillator strengths, radiative transition rates, and emission spectra. It was found that non-collinear spin basis with spin-orbit coupling slows down hole relaxation in the doped NC by two orders of magnitude compared to spin-polarized basis. This is attributed to 'spin-flip' transition from the perovskite NC to the Mn 2+ dopant and low-probability non-radiative d-d transition.
Molecular Physics, 2017
ABSTRACT In this work, non-collinear spin DFT + U approaches with spin-orbit coupling (SOC) are a... more ABSTRACT In this work, non-collinear spin DFT + U approaches with spin-orbit coupling (SOC) are applied to Ln3+ doped β-NaYF4 (Ln = Ce, Pr) nanocrystals in Vienna ab initio Simulation Package taking into account unpaired spin configurations using the Perdew–Burke–Ernzerhof functional in a plane wave basis set. The calculated absorption spectra from non-collinear spin DFT + U approaches are compared with that from spin-polarised DFT + U approaches. The spectral difference indicates the importance of spin–flip transitions of Ln3+ ions. Suite of codes for nonadiabatic dynamics has been developed for 2-component spinor orbitals. On-the-fly nonadiabatic coupling calculations provide transition probabilities facilitated by nuclear motion. Relaxation rates of electrons and holes are calculated using Redfield theory in the reduced density matrix formalism cast in the basis of non-collinear spin DFT + U with SOC. The emission spectra are calculated using the time-integrated method along the excited state trajectories based on nonadiabatic couplings.
The Journal of Physical Chemistry C, 2016
Photo-induced generation of excitons and their non-radiative relaxation dynamics are 7 simulated ... more Photo-induced generation of excitons and their non-radiative relaxation dynamics are 7 simulated at the interface of (10,0) carbon nanotubes (CNT) and a PbSe nanowire (NW). 8 Possible pathways of photoinduced excitations are explored by combining reduced density 9 matrix approach in the basis of Kohn Sham orbitals and on-the-fly nonadiabatic couplings. A 10 range of neutral photoexcitations localized on the CNT is followed by formation of charge 11 transfer (CT) states involving PbSe NW. Depending on the wavelength of the incident light, the 12 initial photoexcitation can be followed by two directions of charge transfer: either 13 () ' () + or () + () '. Excitation of a hot electron results in the CT state with an 14 electron located at the NW and the hole at the CNT with shorter lifetime, while excitation of a 15 hot hole leads to the CT state with an electron at the CNT and the hole at the PbSe having much 16 longer lifetime. Observed ability to control the direction and the lifetime of the CT state makes 17 the CNT/ PbSe NW composites promising for photovoltaic applications.
Organometallics, 2016
Photofragmentation mechanisms of gas-phase lanthanide tris-(isopropylcyclopentadienyl) complexes,... more Photofragmentation mechanisms of gas-phase lanthanide tris-(isopropylcyclopentadienyl) complexes, Ln(iCp) 3 , were studied through experimental photoionization time-of-flight mass spectrometry (PI-TOF-MS). A DFTbased time-dependent excited-state molecular dynamics (TDESMD) algorithm, under standard approximations, was used to simulate the photofragmentation process. Two competing reaction pathways, intact ligand stripping and ligand cracking within the metal−ligand complex, were hypothesized based on experimental data. It was evident that intramolecular hydrogen, methyl, and isopropyl abstraction play an important role in the ligand-cracking reaction pathway, leading to metal carbide and metal hydrocarbide products. The TDESMD simulations also produced branching reaction pathways for ligand ejection and ligand cracking and further suggested that both pathways are initiated by ligand-to-metal charge transfer. Although the simulations reproduced several of the proposed reactions and several of the products of cracking observed in the PI-TOF mass spectra, differences between the simulation and experimental results suggest specific directions for improvement in the computational model.
Accounts of chemical research, Jan 18, 2016
Colloidal quantum dots (QDs) are near-ideal nanomaterials for energy conversion and lighting tech... more 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 s...
The Journal of chemical physics, Jan 21, 2016
Efficient multiple exciton generation (MEG) in chiral single-wall carbon nanotubes (SWCNTs) is pr... more Efficient multiple exciton generation (MEG) in chiral single-wall carbon nanotubes (SWCNTs) is present within the solar spectrum range as shown by the many-body perturbation theory calculations combined with the density functional theory simulations. To describe the impact ionization process, we calculate exciton-to-biexciton decay rates R1→2 and biexciton-to-exciton rates R2→1 in the (6,2) and (10,5) SWCNTs. Within the solar energy range, we predict R1→2 ∼ 10(14) s(-1), while biexciton-to-exciton recombination is weak with R2→1/R1→2 ≤ 10(-2). Also we calculate quantum efficiency (QE), the average number of excitons created by a single absorbed photon, for which we find QE ≃ 1.2-1.6, that is 20%-60%. However, MEG strength in these SWCNTs varies strongly with the excitation energy due to highly non-uniform density of states at the low energy. We hypothesize that MEG efficiency in the chiral SWCNTs can be enhanced by altering the low-energy electronic spectrum via surface functionaliz...
The time-development of photoexcitations in molecular aggregates exhibits specific dynamics of el... more The time-development of photoexcitations in molecular aggregates exhibits specific dynamics of electronic states and vibrational wavefunction. We discuss the dynamical formation of entanglement between electronic and vibrational degrees of freedom in molecular aggregates with theory of electronic energy transfer and the method of vibronic 2D wavepackets [Cina, Kilin, Humble, J. Chem. Phys. 118, 46 (2003)]. The vibronic dynamics is also described by applying Jaynes-Cummings model to the electronic energy transfer [Kilin, Pereverzev, Prezhdo, J. Chem. Phys. 120, 11209 (2004);math-ph/0403023]. Following the ultrafast excitation of donor[chem-ph/9411004] the population of acceptor rises by small portions per each vibrational period, oscillates force and back between donor and acceptor with later damping and partial revivals of this oscillation. The transfer rate gets larger as donor wavepacket approaches the acceptor equilibrium configuration, which is possible at specific energy differ...
MRS Proceedings, 2014
ABSTRACTWe analyze and compare optoelectronic properties and hot carrier relaxation dynamics in d... more ABSTRACTWe analyze and compare optoelectronic properties and hot carrier relaxation dynamics in different forms of TiO2 anatase materials: nanowires and thin films. The models are chosen in such way that the same crystallographic surfaces are exposed and any difference in properties is attributed to the change of the dimensionality of the nanostructure. Specifically, we give a brief review of the electronic properties and non-adiabatic excited state dynamics of <001> anatase TiO2 nanowire as well as (100) and (001) anatase TiO2 surfaces. The calculated band gap of nanowire is larger than the ones of surfaces. The hole relaxation rate is higher than the electron relaxation rate for both the surfaces and nanowire, and the electron and hole relaxation rates of surfaces are larger than the ones of nanowire.
Molecular Physics, 2015
ABSTRACT Density functional theory and density matrix theory are employed to investigate the time... more ABSTRACT Density functional theory and density matrix theory are employed to investigate the time-dependent optical and electronic properties of an Au14 nanocluster protected by six cyclic thiolate ligands, Au4(SCH3)4. The Au14[Au4(SCH3)4]6 nanocluster, i.e. Au38(SCH3)24, is equivalent to a truncated-octahedral face-centred cubic Au38 core coated by a monolayer of 24 methylthiol molecules. The electronic and optical properties, such as density of states, linear absorption spectra, nonradiative nonadiabatic dissipative electronic dynamics and radiative emission spectra were calculated and compared for the core Au14 and thiolate-protected Au38(SCH3)24 nanocluster. The main observation from computed photoluminescence for both models is a mechanism of radiative emission. Specifically, a strong contribution to light emission intensity originates from intraband transitions inside the conduction band (CB) in addition to interband LUMO → HOMO transition (HOMO: highest occupied molecular orbital and LUMO: lowest unoccupied molecular orbital). Such comparison clarifies the contributions from Au core and methylthiol ligands to the electronic and optical properties of the Au38(SCH3)24 nanocluster.
The Journal of Physical Chemistry Letters, 2014
Charge transfer dynamics at the interface of supported metal nanocluster and liquid water by GGA+... more 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 Ru 10 cluster introduces new states into the band gap of TiO 2 surface, narrows the band gap of TiO 2 , and enhances the absorption strength. The H 2 O 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 Ru 10 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.
Molecular Physics, 2013
ABSTRACT We present an electronic structure and non-adiabatic excited state dynamics study of 001... more ABSTRACT We present an electronic structure and non-adiabatic excited state dynamics study of 001 anatase TiO2 nanowire (NW) by combining density matrix formalism and ab initio electronic structure calculations. Our results show that quantum confinement increases the energy gap as the dimension of TiO2 is reduced from the bulk to a NW with a diameter of several nanometres and that the probability of electronic transitions induced by lattice vibrations for the NW follows band gap law. The electron non-radiative relaxation to the bottom of the conduction band is involving Ti 3d orbitals, while the hole non-radiative relaxation of holes to the top of the valence band occurs by subsequent occupation of O 2p orbitals.
Molecular Physics, 2013
ABSTRACT We use the density functional theory (DFT) combined with the many-body perturbation theo... more ABSTRACT We use the density functional theory (DFT) combined with the many-body perturbation theory to derive expressions for the rates of the optical photon→exciton and photon→bi-exciton processes in nanoparticles, and for quantum efficiency, all to the leading order in the screened Coulomb interaction between Kohn–Sham quasiparticles. Also, we calculate exciton→bi-exciton rates due to the impact ionisation (II) mechanism in Si29H36 quantum dots (QDs) with both crystalline and amorphous core structures, and in quasi-one dimensional (1-D) arrays constructed from these QDs. We observe significant dependence of the carrier multiplication rates on the structure’s morphology and structural disorder. Amorphous silicon QD arrays are predicted to have more efficient bi-exciton generation rates as a function of exciton energy compared to their crystalline counterparts, and the isolated QDs of both kinds.
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Papers by Dmitri S. Kilin