Highlights - Modeling Light-Driven Processes at the Surfaces and Interfaces of Nanomaterials

Computational modeling at the atomistic level has twofold significance. It provides insights important for understanding of fundamental processes in molecular systems and nanocomposites, which are challenging, if not impossible, to obtain experimentally. Plus, computational predictions can be used as guidelines in rational design of new molecular structures and nanomaterials for various technological applications, ranging from optoelectronics and solar cells to devices for disease treatment and diagnostics. One of the most challenging problems for computations is modeling light-driven processes at interfaces and surfaces of nano-sized materials. This challenge is addressed in Dr. Kilina’s group by developing and applying quantum chemistry tools, including originally designed methodologies for simulating phonon-mediated non-adiabatic dynamics in semiconductor nanosystems. 

Research of Dr. Kilina’s group is focused on three main topics, all related to nanoscale systems and their properties critical for energy applications: (i) modeling of charge transfer process in heterostructured nanocomposites; (ii) improving optical properties of carbon nanotubes by chemical functionalization; and (iii) rational design of metal-organic complexes with enhanced near infrared (NIR) emission. In each of these projects, atomistic calculations provide important interpretations and explanations of a range of experimentally detected phenomena and reveal structure-to-property relations useful for novel material design strategies. Most of scientific results obtained by Dr. Kilina’s group were calculated using CCAST's HPC clusters in collaboration with other NDSU research groups (Dr. Wenfang Sun, Dr. Dmitri Kilin, Dr. Andrei Kryjevski, and Dr. Bakhtiyor Rasulev), as well as scientists from Los Alamos National Laboratory (LANL; Dr. Segei Tretiak, Dr. Brendan Gifford, and Dr. Hang Htun).     

I. Modeling of charge transfer process in heterostructured nanocomposites (funded by NSF under grant no. 2004197)

Developing energy sources that are both sustainable and efficient is one of the greatest challenges facing mankind. One attractive solution is solar-based fuel cells that utilize solar energy to create hydrogen from water, which is then used as a fuel producing no harmful air pollutants. However, cost, efficiency, and durability are the main factors that hinder large-scale use of this technology. One way to meet these challenges is to utilize quantum dots, which are tiny pieces of crystalline solids of one billionth of a meter in size. Attaching a molecule to the quantum dot surface alters its properties. This modification can significantly enhance efficiency of solar energy conversion to chemical or electrical energy. However, precise control over the processes governing interactions between molecules and quantum dots is currently lacking. To close this gap in our knowledge, Professors Kilina, Kilin, and Kryjevski of NDSU are developing computational methods that model light-induced processes taking place in quantum dots with complicated surface structure. Computational predictions obtained from this research are expected to guide rational design of novel cost-effective materials for energy applications. This project provides educational and research experience for high school, undergraduate and graduate students in computational chemistry and materials modeling. Remote training/research activities are offered to increase participation of female and Native American students. The project helps prepare a diverse STEM workforce with the skills and knowledge critical for the real-world design of novel materials for energy applications. 

The research team proposes to create and implement novel quantum chemistry methods capable of accurate modeling of the photoexcited dynamics in extended nanostructures with complex surfaces and interfaces. Specifically, this methodology is suitable for the Janus-type quantum dots composed of two different semiconductors, such as PbS(e)/CdS(e) (Fig. 1), and covalently functionalized by organic dyes, to determine the conditions that govern dynamics of charge transfer in the presence of other competing processes, such as carrier multiplication, energy transfer, phonon-mediated carrier relaxation and carrier recombination. These computations are useful for interpreting data from time-resolved optical spectroscopy and guiding new experimental probes. Knowledge of dependence of charge and exciton transfer efficiency on the surface and interface effects is critical for controlling the photoexcited processes via chemical engineering of quantum dot/dye composites, thus improving their functionality for energy conversion applications (Fig. 2). 

II. Improving optical properties of carbon nanotubes by chemical functionalization (funded by DOE EPSCoR: Building EPSCoR-State/National Laboratory Partnerships grant no. DE-SC0021287)

Progress in quantum computing and quantum cryptography requires efficient and tunable single-photon emission (SPE) sources at room temperature in the telecom wavelengths. This demand has refocused the materials research interest toward functionalization of single-walled carbon nanotubes (SWCNTs) by small organic molecules. Such covalent functionalization creates the sp³-defect at the tube’s surface, which results in the exciton localization at the defect leading to its energy redshift and emission enhancement; Fig. 3. Among a few materials showing SPE at room temperature, SWCNTs offer a key advantage in their emission that can be altered from near IR to telecom wavelengths via tube diameters, chiralities, and modifications of functionalizing groups. However, to practically realize this exciting potential of functionalized SWCNTs enabling quantum behaviors, an advanced level of understanding of the impact of the sp³-defects on emission of SWCNTs is critically needed. 

The project aims to generate important insights into defect-state relaxation pathways to determine how the electron-dynamics and optically active states in SWCNTs can be tuned via chemical modifications of molecular adducts at the SWCNT surface. This knowledge is critical for design strategies of novel materials suitable for quantum lighting and quantum information applications. Targeting this goal, the team from NDSU will develop a new fully integrated methodology of the photophysical characterization and tunable design of SWCNTs functionalized by molecular adducts through combining quantum chemistry computations with data-driven methods of cheminformatics and machine learning.

Collaboration with scientists from LANL will be initiated to allow for direct comparison between computational and experimental data that provides a tool for method validation and error analysis/correction, while also assist in rational design of novel SWCNT-based quantum materials; Fig. 4. The NDSU team will use the Center for Integrated Nanotechnologies (CINT) at LANL accessible through the user facility program. In addition, the NDSU-LANL collaboration will be maintained via LANL summer internships for several NDSU graduate students involved in this project. Thus, this project offer a unique research experience for NDSU graduate students preparing them for future scientific careers related to materials design and big data processing for information technologies.

III. Rational design of metal-organic complexes with enhanced near infrared (NIR) emission(funded by NSF under grant no. CHE-1800476)

The transition-metal complexes of interest display a rich array of photophysical properties where substituted ligands can be used both for the tuning of excited states and as conduits for electron- and energy-transfer. These complexes consist of two or more components – a metal and several ligands, each allowing for a wide range of variations in their chemical composition, structure and size to be designed to tune their photophysics, including the ground- and excited-state absorption, and the triplet excited-state lifetime and quantum yield, for optimizing the NIR emission effect; Fig. 5. 

In this project, the state-of-the-art combination of (1) experimental, (2) computational, (3) cheminformatics and data mining methods will help to reveal the factors responsible for linear and non-linear optical properties to be able to boost a rational design of these chemical systems offering great promises in the field of dye-sensitized solar cells, catalysis, sensing, displays, optical limiting, and biotechnology. This is performed by experimental (Dr. Sun) and computational study (Dr. Kilina and Dr. Kilin) of a broad library of organometallic complexes that possess specific optical properties, by application in-house data, as well as a large set of data collected from various public sources using computer science methods (Dr. Rasulev). 

The project explores and identifies several structural factors responsible for nonlinear absorption and/or near-IR emission via a high-performance screening using a generated virtual library of thousands hypothetical organometallic complexes; then the best selected virtual organometallic complexes will be synthesized and tested, to confirm data-driven findings. With a prudent choice of ligands and metals, new complexes with specifically designed excited state properties will be explored: (i) weak and broad absorption at visible to (NIR) region, (ii) intense excited state absorption to guaranty a large ratio of the excited-state absorption cross section relative to that of the ground-state absorption, (iii) long-lived triplet excited states, and (iv) relatively high triplet quantum yield. The project lies at the interface of organic, inorganic, materials chemistry, theoretical quantum chemistry, cheminformatics and data mining approaches, and is therefore well suited to the education of scientists at all levels.