Session Z48: Excited State VIII: Hybrid Materials and Molecules

Excited states of solids and nanomaterials using quantum Monte Carlo methods

Quantum Monte Carlo (QMC) methods promise a highly accurate fully many-body ab initio treatment of the electronic structure problem, applicable not only to ground states but also excited states. While it has been standard practice to perform these calculations with geometries and input trial wavefunctions obtained with other electronic structure methods, this approach precludes application to systems where the parent methodology does not reproduce the geometry or band structure with sufficient accuracy. Here I will introduce modern QMC methodology and the challenges that need to be solved to develop an approach were all uncertainties and errors can be assessed and the true uncertainties in the predictions obtained. I will then describe (1) development of a surrogate line-search based approach to find geometries, applicable to any statistical approach, and (2) progress in systematically improving trial wavefunctions for QMC. Using these methods we find the structure and optical properties of  2D GeSe to be strongly coupled, and we are now able to apply QMC to transition metal oxides in a convergent manner.   

A Novel Pathway from DFT to CI

Noble gases are of great importance in the development of new theoretical tools before studying more complex and interesting systems, such as molecules and clusters, because their properties are well established experimentally. Noble gas atoms are also small enough so when testing a new theory, basis set dependencies can be ruled out. Here we describe a method for approaching configuration interaction accuracy by analyzing past results obtained by density functional theory. We present core level excitations for Neon and other systems.   

Quasiparticle Electronic Structure and Optical Properties of CdS Quantum Dots and their Assemblies in Different Dimensions

Quantum dots have received considerable attention due to the exceptional optoelectronic properties which stem from their confinement. Covalently linking these quantum dots forms gel-like assemblies, whose properties differ from the isolated quantum dots. The understanding of the electronic structure of these materials at the atomistic level is crucial for optimizing their performances in devices. Prior computational investigations on the structure-property relationship have been limited to empirical, tight-binding models, and local/semi-local density functional studies on very small quantum dots. The fundamental differences in the electronic structure and optical properties between quantum dot assemblies and isolated quantum dots are largely unknown. In this work, we employ the first-principles GW-BSE approach to study the structural-property relationships of CdS quantum dots and their assemblies in various dimensions (i.e., 0D, 1D, 2D, and 3D). Our results are in quantitative agreement with experiments and unveil the effect of dimensionality on the modulation of the quantum dot properties.   

Molecular dielectric embedding GW for ionically bonded hybrid perovskites

Ruddelsden-Popper (RP) perovskites are a class of hybrid organic-inorganic lead halide perovskites with unique optoelectronic properties. First-principles GW and GW-BSE calculations of RP perovskites' excited state properties are expensive due to the O(N^4) scaling of the RPA dielectric screening with system size. We introduce an approximation for the dielectric screening, treating RP perovskites as charged organic molecules embedded in an anionic lead halide structure. DFT and RPA polarizability calculations for organic and inorganic sublattices are carried out separately, then the RPA polarizabilities are combined into one dielectric matrix. This method assigns formal charges to estimate total inorganic/organic charge transfer, and it is successful despite the strong ionic bonds between organic ions and lead halide layers. We validate our approximation by closely reproducing the GW quasiparticle band structures for several RP perovskites, and show applicability to GW-BSE calculations on disordered cells, as calculations for the inorganic sublattice can be reused as molecules are repositioned.   

Excited-state forces in organic metal halide perovskites from GW/BSE calculations

Organic metal halide perovskites are promising materials for solar cells and optoelectronic devices, but one of the main barriers to their use is light-induced degradation. Light-induced structural changes also play an important role in Stokes shifts and exciton transport. They have been attributed to mechanisms such as the creation of polaronic trap states from free carriers, halide ion migration, and changes in stress.

 

To understand those structural changes, we calculate forces induced by absorption of light, with the GW approximation and Bethe-Salpeter equation (GW/BSE). Our excited-state forces method, an improved version of Ismail-Beigi and Louie’s [Phys. Rev. Lett. 90, 076401 (2003)], combines quasiparticle energies and exciton wavefunctions from GW/BSE calculations in BerkeleyGW with electron-phonon matrix elements from Density Functional Perturbation Theory (DFPT)

calculations.

We first benchmark excited-state forces on simple test systems, such as the CO molecule, and then we apply this approach to methylammonium lead iodide (CH₃NH₃PbI₃), exploring different excitations and crystal phases. Our calculated excited-state forces are the starting point for structural changes induced by light absorption in hybrid perovskites   

Quantum Simulations of Thermally Activated Delayed Fluorescence in an All-organic Emitter

We investigate the prototypical thermally activated delayed fluorescence (TADF) emitter NAI-DMAC^[1] [C₃₇H₃₂N₂O₂], in the dilute- and high-packing fraction limits at finite temperature (T= 300 K), by combining first principles molecular dynamics^[2]with a quantum thermostat^[3,4] to account for nuclear quantum effects (NQE). We find a weak dependence of the singlet-triplet energy gap (ΔE_ST) on temperature in both the solid and the molecule, and a substantial effect of packing. While the ΔE_STvanishes in the perfect crystal, it is of the order of ~ 0.3 eV in the molecule, with fluctuations ranging from 0.1 to 0.4 eV at 300 K. The transition probability between HOMO and LUMO molecular orbitals has a stronger dependence on temperature than the singlet-triplet gap, with a desirable effect for thermally activated fluorescence; such temperature effect is weaker in the condensed phase than in the molecule. Our results ^[5] show that optimization of packing and geometrical conformation is critical to increase the efficiency of TADF compounds and highlight the importance of considering thermal fluctuations and NQE to obtain robust predictions of the electronic properties of NAI-DMAC. 

[1] W. Zeng, et al., Adv. Mat., 2018, 30(5), 1704961

[2] F. Gygi, IBM J. Res. Dev., 2008, 52, p.137; code Qbox: http://qboxcode.org.

[3] M. Ceriotti et al., PRL 2009, 103 (3), 030603

[4] A. Kundu et al., PRM 2021, 5 (7).

[5] T.Francese et al. 2021 (submitted)

 

We acknowledge support by MURI Project N00014-19-1-2453   

Quasiparticle and optical properties of phthalocyanine-TMD interfaces from first-principles GW-BSE

Heterogeneous interfaces formed between (metallo)phthalocyanine molecules and transition-metal dichalcogenides (TMDs) are both promising energy materials and excellent platforms for studying mixed-dimensional molecule-semiconductor heterostructures. First-principles calculations of the quasiparticle electronic structure and the optical properties at these interfaces are desired for a fundamental understanding of the energy and charge transfer. In this work, we employ the first-principles GW-BSE formalism to accurately characterize the quasiparticle and optical properties of a series of phthalocyanine-TMD interfaces. To reduce the computational cost, we leverage the substrate screening approximation and carry out dielectric embedding GW-BSE calculations. We discuss the dielectric screening effects of typical substrates used in experiments and the structure-property relationships derived from our calculations. Our results provide a microscopic description of the energy and charge transfer at these interfaces and benchmarks for future experimental and theoretical studies.   

Energy and Charge Transfer Excitations in a Bacterial Reaction Center from First Principles

In purple bacteria, the fundamental charge separation step that drives the conversion of radiation energy into chemical energy, proceeds along one branch of a heterodimeric pigment-protein complex, the reaction center. An atomistic explanation of the directionality of this charge transfer process has not been provided yet. Here, we use first principles (time-dependent) density functional theory and Green's function-based many-body perturbation theory to investigate the electronic and excited state structure of the primary four bacteriochlorophyll pigments in the reaction center. We systematically study the effect of the protein environment and molecular vibrations on energy and charge transfer excitations. We show a pronounced effect of a subset of amino acids in the vicinity of the primary photoactive pigments on both types of excitations. However, analysis of transition and difference densities reveals transfer of charges along both reaction center branches, and both forward and backward charge transfer. A vibrational mode decomposition allows us to unravel the coupling between molecular vibrations and excited states and provides an intuitive understanding of vibronic coupling in these complex multichromophoric systems.   

Optimal polymorphs of tetracene for singlet fission.

The efficiency of solar cells may be improved using singlet fission (SF), where one singlet exciton splits into two triplet excitons. A molecule may crystallize in more than one form. This phenomenon, known as polymorphism, may affect SF. Molecular crystal structure prediction (CSP) can identify potential crystal forms of a molecule. Here we search for polymorphs of tetracene using Genarris, a random structure generator, and GAtor, a genetic algorithm-based CSP code. Potential polymorphs of tetracene, in addition to the two experimentally known polymorphs, were identified. Using many-body perturbation theory in the GW approximation and Bethe-Salpeter equation, the SF performance of tetracene polymorphs was assessed. The excitation energies and the charge transfer character of the exciton wave function were computed. We observe that crystal packing can significantly affect excitonic properties in tetracene.   

First-principles Studies of Tl activated Scintillator Phosphor Materials: Towards an understanding of the Scintillation mechanism

Tl doped halide scintillator phosphors are amongst the most commonly used gamma ray detector materials for medical imaging, high energy physics and nuclear materials detection applications (e.g. CsI:Tl, NaI:Tl). Even so the complete scintillation process in these materials is poorly understood. Recently there has been interest in co-doping these materials to try and improve their detection performance. We have performed first-principles studies based on GGA, hybrid functionals and the GW/BSE method in tandem with experiments to understand the scintillation mechanism in these materials and how it could be improved by co-doping. In particular we have looked at the Tl exciton optical emission states and energy transfer mechanisms from the gamma ray to the Tl. Recently there has also been interest in new Tl bulk scintillators such as TLYC (Tl₂LiYCl₆) which we have also studied.   

Exploration of ab-initio calculations for 3d transition metal Kα X-ray Emission Spectra (XES)

Core-to-core spectroscopy is an intensely many-body problem, which makes it difficult to treat highly correlated materials such as transition metals, lanthanides, and actinides in which many body effects are prominent. I have expanded upon the work of Haverkort et al [1] to apply a DFT + MLFT approach to core-to-core XES, which is ideal for treating these highly correlated systems in a computationally efficient and accurate manner. In this approach we project the DFT calculated Bloch states onto a more localized Wannier basis, which is used to form the Tight Binding (TB) Hamiltonian that describes the system. This TB-Hamiltonian is then projected onto a symmetry reduced basis within Quanty, which is used to perform an ab-initio multiplet ligand field calculation which includes scaling of the Slater-Condon parameter, Crystal Field terms, and Charge Transfer effects. We have found excellent agreement between theory and experiment for the 3d transition metal crystalline systems that we have studied. Most notably we reproduce the spectral shift between the K-alpha XES of Cr6+ and Cr3+ materials, which has important applications to environmental safety standards.