Papers by Aleksey Kocherzhenko
Personalization enables businesses to learn customer preferences from past interactions and thus ... more Personalization enables businesses to learn customer preferences from past interactions and thus to target individual customers with more relevant content. We consider the problem of predicting the optimal promotional offer for a given customer out of several options as a contextual bandit problem. Identifying information for the customer and/or the campaign can be used to deduce unknown customer/campaign features that improve optimal offer prediction. Using a generated synthetic email promo dataset, we demonstrate similar prediction accuracies for (a) a wide and deep network that takes identifying information (or other categorical features) as input to the wide part and (b) a deep-only neural network that includes embeddings of categorical features in the input. Improvements in accuracy from including categorical features depends on the variability of the unknown numerical features for each category. We also show that selecting options using upper confidence bound or Thompson sampl...
Organic and Hybrid Sensors and Bioelectronics XI, 2018
Standard models for evaluating the electro-optic (EO) response of organic materials typically ass... more Standard models for evaluating the electro-optic (EO) response of organic materials typically assume that the refractive index of the material in the absence of a RF modulation field is isotropic and homogeneous. Such assumptions work very well for low-concentration guest-host materials in bulk devices. However, current generation organic EO materials at high densities and under nanoscale confinement can show sufficient birefringence to affect device performance. We use computer simulations and spectroscopic experiments to characterize and predict changes in the index of refraction under poling. We also demonstrate that poling-induced birefringence can lead to a non-linear relationship between the apparent EO coefficient and poling field strength.
Journal of Visualized Experiments, 2020
Rational design of disordered molecular aggregates and solids for optoelectronic applications rel... more Rational design of disordered molecular aggregates and solids for optoelectronic applications relies on our ability to predict the properties of such materials using theoretical and computational methods. However, large molecular systems where disorder is too significant to be considered in the perturbative limit cannot be described using either first principles quantum chemistry or band theory. Multiscale modeling is a promising approach to understanding and optimizing the optoelectronic properties of such systems. It uses first-principles quantum chemical methods to calculate the properties of individual molecules, then constructs model Hamiltonians of molecular aggregates or bulk materials based on these calculations. In this paper, we present a protocol for constructing a tight-binding Hamiltonian that represents the excited states of a molecular material in the basis of Frenckel excitons: electron-hole pairs that are localized on individual molecules that make up the material. The Hamiltonian parametrization proposed here accounts for excitonic couplings between molecules, as well as for electrostatic polarization of the electron density on a molecule by the charge distribution on surrounding molecules. Such model Hamiltonians can be used to calculate optical absorption spectra and other optoelectronic properties of molecular aggregates and solids.
The Journal of Physical Chemistry C, 2019
Excitonic interactions often significantly affect the optoelectronic properties of molecular mate... more Excitonic interactions often significantly affect the optoelectronic properties of molecular materials. However, their role in determining the nonlinear optical response of organic electro-optic materials remains poorly understood. In this paper, we explore the effects of excitonic interactions on the first hyperpolarizability for aggregates of donor− acceptor chromophores. We show that calculations of the first hyperpolarizabilty of chromophore aggregates based on a two-state model agree well with the more rigorous coupled perturbed Hartree−Fock method. We then use both time-dependent density functional theory calculations and the molecular exciton approximation to parametrize the two-state model. Use of the molecular exciton approximation to the two-state model (i) is appropriate for disordered aggregates (unlike band theory), (ii) is computationally efficient enough for calculating the first hyperpolarizability of materials that consist of thousands of interacting chromophores, and (iii) allows the unraveling of the effects of both excitonic interactions and electrostatic polarization of the chromophore electron density by its environment on the first hyperpolarizability of molecular materials. We find that use of the molecular exciton approximation to the two-state model does not introduce significant additional errors compared to those introduced by applying the two-state model alone. We determine that the absolute change to the first hyperpolarizability of chromophore aggregates due to excitonic interactions increases with the size of the aggregate. For all sizes of disordered aggregates of chromophores considered in this paper, the inclusion of excitonic interactions on average decreases the magnitude of the first hyperpolarizability by 12−14% compared to the case of non-interacting chromophores. Finally, we present a method for analytically calculating the first hyperpolarizability of a one-dimensional periodic array of chromophores within the molecular exciton approximation to the two-state model. This technique can be used to include an approximate correction for excitonic effects when simulating the electro-optic response of disordered and ordered organic materials.
Materials Research Express, 2018
We review the principles of formation, physical properties, and current or envisaged applications... more We review the principles of formation, physical properties, and current or envisaged applications for a wide range of carbon allotropic forms. We discuss experimental and theoretical advances relating to staple zero-, one-, and two-dimensional carbon structures, such as fullerenes, carbon nanotubes, and graphene. In addition we emphasize research on emerging carbon allotropes (carbon nanoscrolls, funnels, etc) that result from combining or deforming allotropic forms with well-defined dimensionality. Such materials fall in-between clearly delineated dimensional categories and consequently exhibit unique characteristics that are promising for electronic, optical, and mechanical applications. We also consider other approaches to tuning properties of carbon-based materials, such as chemical functionalization, intentional introduction of structural disorder, and placement of guest atoms or molecules inside hollow structures. Finally, we discuss the properties of and experimental methods for studying zero-dimensional systems (paramagnetic nitrogen impurity atoms) in diamond matrix. The review emphasizes the interplay between the various material properties of carbon-based nanostructures and the designs for nanoscale devices that rely on synergistic combinations of these properties. For example, an electromechanical vibrator, a strain sensor, a nanodynamometer, and a nanoelectromechanical memory cell that we describe exploit both electronic and nanomechanical properties of low-dimensional carbon structures, a reed switch and a magnetic field sensor use magnetic and nanomechanical properties, a maser based on nitrogen-doped diamond uses thermal and optoelectronic properties, etc. All presented device concepts have been validated by calculations, and some have been implemented experimentally.
Journal of Lightwave Technology, 2018
Chipscale integration of electronics and photonics is a logical next step in the evolution of inf... more Chipscale integration of electronics and photonics is a logical next step in the evolution of information technology; however, given issues related to the footprint of photonic devices and circuits, bandwidth of current electro-optic (EO) modulators, energy efficiency of EO devices, and optical loss budget, electronic photonic integration represents a grand challenge requiring both improvement of electro-optic materials and implementation of novel device architectures. Progress in silicon photonics and plasmonics has brought chipscale integration closer to reality. This presentation focuses on the utilization of multiscale theoretical methods to significantly increase the electro-optic activity of organic π-electron materials. Such improvement is crucial to reducing EO device footprint, energy requirements, optical insertion loss, and improving operational bandwidth. Indeed, current improvement of in-device EO activity to values of several hundred picometers (pm) per volt (V) has permitted device voltage-length parameters to be improved to 40 V-μm, energy efficiency to approximately 1 femtojoule/bit, bandwidth to > 170 GHz, and optical insertion loss to < 6 dB.
Journal of Chemical Theory and Computation, 2017
Optimizing the optical properties of large chromophore aggregates and molecular solids for applic... more Optimizing the optical properties of large chromophore aggregates and molecular solids for applications in photovoltaics and nonlinear optics is an outstanding challenge. It requires efficient and reliable computational models that must be validated against accurate theoretical methods. We show that linear absorption spectra calculated using the molecular exciton model agree well with spectra calculated using time-dependent density functional theory and configuration interaction singles for aggregates of strongly polar chromophores. Similar agreement is obtained for a hybrid functional (B3LYP), a long-range corrected hybrid functional (ωB97X), and configuration interaction singles. Accounting for the electrostatic environment of individual chromophores in the parametrization of the exciton model with the inclusion of atomic point charges significantly improves the agreement of the resulting spectra with those calculated using all-electron methods; different charge definitions (Mulliken and ChelpG) yield similar results. We find that there is a size-dependent error in the exciton model compared with all-electron methods, but for aggregates with more than six chromophores, the errors change slowly with the number of chromophores in the aggregate. Our results validate the use of the molecular exciton model for predicting the absorption spectra of bulk molecular solids; its formalism also allows straightforward extension to calculations of nonlinear optical response.
The Journal of Physical Chemistry C, 2017
We simulate subpicosecond charge separation in two donor-acceptor molecular dyads. Charge separat... more We simulate subpicosecond charge separation in two donor-acceptor molecular dyads. Charge separation dynamics is described using a quantum master equation, with parameters of the dyad Hamiltonian obtained from density functional theory (DFT) and timedependent density functional theory (TDDFT) calculations and the rate of energy dissipation estimated from Ehrenfest-TDDFT molecular dynamics simulations. We find that higher-energy charge transfer states must be included in the dyad Hamiltonian in order to obtain agreement of charge separation rates with the experimental values. Our results show that efficient and irreversible charge separation involves both coherent electron transfer from the donor excited state to higher-energy unoccupied states on the acceptor and incoherent energy dissipation that relaxes the dyad to the lowest energy charge transfer state. The role of coherence depends on the initial excited state, with electron delocalization within Hamiltonian eigenstates found to be more important than coherence between eigenstates. We conclude that ultrafast charge separation is most likely to occur in donor-acceptor dyads possessing dense manifolds of charge transfer states at energies close to those of Frenkel excitons on the donor, with strong couplings to these states enabling partial delocalization of eigenstates over acceptor and donor.
The Journal of Physical Chemistry C, 2014
We study the dynamics of charge separation in bulk heterojunction organic photovoltaic systems in... more We study the dynamics of charge separation in bulk heterojunction organic photovoltaic systems in light of recent experimental observations that this process is characterized by multiple time scales in the range of 10 fs to 100 ps. Coherent evolution of the excitonic state has been suggested to dominate the early stages of the charge separation process and diffusion of localized excitons to be dominant at longer times. Both of these processes obviously depend on the system morphology, in particular on the grain sizes of the donor and acceptor phases. Here we analyze these mechanisms and their characteristic time scales, aiming to verify the consistency of the proposed mechanisms with the experimentally observed time scales of charge separation. We suggest that the coherent mechanism that dominates the early stage of charge separation involves delocalized excitons. These excitons are formed by optical excitation of clusters of strongly interacting donor sites, and the charge separation rate is determined by the probability that such sites lie at the donor-acceptor interface. The (relatively) slow diffusive rate is estimated from the mean first passage time for a diffusing exciton to reach the donor grain surface. Our estimates, based on available exciton diffusion rates and morphology data, are consistent with experimental observations.
How does classical reality emerge from quantum mechanics? When does an electron in a molecule sto... more How does classical reality emerge from quantum mechanics? When does an electron in a molecule stop behaving as a wave and start behaving as a particle? What is the best way to describe the intermediate regime? Can quantum mechanical effects be put to practical use in molecular electronics? These are the questions that this thesis asks and attempts to answer. The topics covered include quantum interference and decoherence in molecular systems, band-like and incoherent hopping charge transport in polymers, the effects of the molecular environment on single-molecule charge transport and supramolecular control of charge transport.
A consistent analysis of linear spectroscopy for arrays of dipole-coupled two-level molecules rev... more A consistent analysis of linear spectroscopy for arrays of dipole-coupled two-level molecules reveals distinct signatures of weak and strong coupling regimes separated by a critical point. Multiple molecular excitations (odd(even) in weak(strong) coupling) are accessed from the ground state. As the coupling increases, the single excitation oscillator strength rapidly exceeds the Heitler-London value and diverges at the critical point, returning to a quadratic size scaling in strong coupling, where also the photon frequency decreases with size. The lowest accessible excitation is found to show a one-photon hyperradiance.
The Journal of Physical Chemistry C, 2015
In organic materials, exciton dissociation into free charges requires overcoming an electron-hole... more In organic materials, exciton dissociation into free charges requires overcoming an electron-hole Coulomb interaction that exceeds the thermal energy and may still be large after charge transfer at a donor/acceptor interface. We analyze the factors affecting efficiency of charge separation and subsequent removal of electrons and holes from such a donor-acceptor interface and suggest strategies for optimizing these processes. Energy transfer, charge separation and charge transfer in the vicinity of the donor-acceptor interface are studied within a common theoretical framework based on a quantum master equation, for a model system with realistic excitation energies and electronic couplings. We find that enhancing the efficiency of both charge transfer from the donor to the acceptor and of charge removal from the donor-acceptor interface requires an intricate balance between the extent of electronic delocalization throughout the material and rates of energy dissipation. For very large exciton binding energies, cascade charge separation in systems with more than one donor and one acceptor species, such as molecular polyads, is found to greatly facilitate the dissociation of geminate pairs. Our calculations predict charge separation on sub-picosecond timescales for several parameter combinations, leading to design principles for enhancing charge separation in multi-chromophore systems.
Physical Review A, 2014
We present a consistent analysis of linear spectroscopy for arrays of nearest neighbor dipolecoup... more We present a consistent analysis of linear spectroscopy for arrays of nearest neighbor dipolecoupled two-level molecules that reveals distinct signatures of weak and strong coupling regimes separated for infinite size arrays by a quantum critical point. In the weak coupling regime, the ground state of the molecular array is disordered, but in the strong coupling regime it has (anti)ferroelectric ordering. We show that multiple molecular excitations (odd/even in weak/strong coupling regime) can be accessed directly from the ground state. We analyze the scaling of absorption and emission with system size and find that the oscillator strengths show enhanced superradiant behavior in both ordered and disordered phases. As the coupling increases, the single excitation oscillator strength rapidly exceeds the well known Heitler-London value. In the strong coupling regime we show the existence of a unique spectral transition with excitation energy that can be tuned by varying the system size and that asymptotically approaches zero for large systems. The oscillator strength for this transition scales quadratically with system size, showing an anomalous one-photon superradiance. For systems of infinite size, we find a novel, singular spectroscopic signature of the quantum phase transition between disordered and ordered ground states. We outline how arrays of ultra cold dipolar molecules trapped in an optical lattice can be used to access the strong coupling regime and observe the anomalous superradiant effects associated with this regime.
Journal of Physical Chemistry C, 2010
ABSTRACT Quantum interference effects occurring in molecules through which a charge can travel vi... more ABSTRACT Quantum interference effects occurring in molecules through which a charge can travel via multiple pathways can be the basis for new unconventional design principles in molecular scale electronics. However, these quantum interference effects can be reduced by interaction between the charge and molecular vibrations. In this work dephasing (decoherence) effects have been studied using a model that combines a (classical) molecular mechanics description of molecular vibrations with a quantum mechanical propagation of the charge. It is found that despite the clear effect of dephasing on the charge propagation, interference effects are largely retained at room temperature if vibrations are accounted for. Additionally, it is shown that taking electronic interactions between non-nearest neighbor atoms into account also diminishes interference effects but not sufficiently to destroy them completely. It is concluded that interference effects are strong enough to use them in a functional manner in molecular electronics. This opens up new ways to design molecular electronic components that exploit quantum interference.
Technical Physics, 2004
ABSTRACT The current induced by the passage of an external point charge through a plane vacuum ca... more ABSTRACT The current induced by the passage of an external point charge through a plane vacuum capacitor in an RCL circuit free of current (voltage) sources is calculated. The case is also analyzed when an internal point charge is emitted by one of the capacitor plates, moves to the other plate, and is absorbed by it. A technique is proposed to measure the internal charge and its velocity component perpendicular to the capacitor plates in a passive RCL circuit.
The Journal of Physical Chemistry C, 2012
Potential barrier for the relative rotation of Zn-Porphyrin fragments (adapted from Ref. [22]). 2... more Potential barrier for the relative rotation of Zn-Porphyrin fragments (adapted from Ref. [22]). 2) Potential barriers for the relative rotation of phenyl fragments in PFBP:-red: if neither phenyl is part of a fluorene unit;-blue: if one of the phenyls is part of a fluorene unit. When both phenyls are part of a fluorene unit, they are constrained to a planar geometry.
The Journal of Physical Chemistry C, 2011
Charge carrier transport in organic semiconductors is at the heart of many revolutionary technolo... more Charge carrier transport in organic semiconductors is at the heart of many revolutionary technologies ranging from organic transistors, light-emitting diodes, flexible displays and photovoltaic cells. Yet, the nature of charge carriers and their transport mechanism in these materials is still unclear. Here we show that by solving the time-dependent electronic Schrödinger equation coupled to nuclear motion for eight organic molecular crystals, the excess charge carrier forms a polaron delocalized over up to 10-20 molecules in the most conductive crystals. The polaron propagates through the crystal by diffusive jumps over several lattice spacings at a time during which it expands more than twice its size. Computed values for polaron size and charge mobility are in excellent agreement with experimental estimates and correlate very well with the recently proposed transient localization theory.
The Journal of Physical Chemistry C, 2010
ABSTRACT Incoherent hopping models are commonly used to describe charge transport in many classes... more ABSTRACT Incoherent hopping models are commonly used to describe charge transport in many classes of electrically conducting materials. An important parameter in these models is the charge carrier hopping rate or its inverse, the charge carrier lifetime at a localization site. Among the most common approaches to calculating the charge hopping rate are those based on the hydrogen molecular ion (H2+) theory and on the Marcus theory of electron transfer reactions. The former is typically used when describing doped inorganic crystalline semiconductors, while the latter is commonly employed for organic systems. In the present paper the limitations of these approaches are examined and a generalized expression for the charge carrier lifetime at a localization site is proposed, which includes the expressions found from H2+ theory and Marcus theory as limiting cases. Charge transport simulations based on all three expressions are compared.
Physical Chemistry Chemical Physics, 2011
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Papers by Aleksey Kocherzhenko