The Substituent Effect in Chemistry and Machine Learning: Applications to Organic Photovoltaics and Energetic Materials

Itamar Borges Jr.
(IME – Dept. de Química)

Resumo:

The substituent effect – the modification of the electronic properties of molecules by changing groups of atoms – is ubiquitous in Chemistry and Materials Science. Machine learning (ML) techniques, a set of statistical methods that take advantage of recent developments in hardware and larger volumes of more accurate data, have become a handy tool for basic research. Combining these two approaches can reveal unknown phenomena and new physical insights. In this talk, we present Hammett’s theory, which quantifies via numerical constants, the electron-donor or -withdrawal power of substituents. We discuss a new comprehensive and systematic set of Hammett’s constants for general use determined with ML [1]. Afterward, we examine diketopyrrolopyrrole (DPP) molecules with one or two substituents, which can be used as a fine-tuned donor material in organic electronic devices. Several photovoltaic properties, including power conversion efficiencies in bulk heterojunctions, exciton binding energies and intramolecular charge transfer effects, are discussed in the framework of Hammett’s theory [2]. Finally, we examine the molecular origin of the impact sensitivity of an energetic material, which was rationalized with ML. In particular, we found that a positive or negative contribution of the molecular properties to the sensitivity depends on the value of this property [3]. Finally, we discuss some implications of combining quantum chemistry with ML and mention other research carried out by our group.

References:
[1] – MONTEIRO-DE-CASTRO, GABRIEL ; DUARTE, JULIO CESAR ; BORGES, ITAMAR . Machine Learning Determination of New Hammett’s Constants for meta- and para- Substituted Benzoic Acids Derivatives Employing Quantum Chemical Atomic Charge Methods. JOURNAL OF ORGANIC CHEMISTRY (JOC), 2023, 88, 9802.

Also featured in Chemistry World (Royal Society of Chemistry), July 31, 2023 “86-old Hammett equation gets a machine learning upgrade”, https://www.chemistryworld.com/news/86-year-old-hammett-equation-gets-a-machine-learning-update/4017798.article

86-year old Hammett equation gets a machine learning update
Algorithm opens the door to improved understanding of aromatic substituent effects
www.chemistryworld.com

Departamento de Física dos Sólidos
Instituto de Física UFRJ

Synthesis and optical properties of two-dimensional materials

Victor Carozo
(PUC-Rio)

Resumo:

Two-dimensional materials have recently emerged as a captivating class of new crystals, offering exceptional properties and immense potential for diverse technological applications. This study delves into the synthesis and optical analysis of these materials, with a specific emphasis on the application of Raman spectroscopy and second harmonic generation (SHG) to elucidate their unique optical attributes.

The synthesis of two-dimensional materials, encompassing transition metal dichalcogenides (TMDs), and other layered materials, constitutes a central aspect of this investigation. The controlled fabrication of high-quality two-dimensional layers is pivotal for harnessing their inherent optical characteristics, employing methods such as chemical vapor deposition (CVD) and vapor phase deposition (VPD) to tailor their composition and structure.

Raman spectroscopy emerges as an invaluable tool for the comprehensive characterization of two-dimensional materials, enabling insights into layer thickness, defects, strain, and interlayer interactions. Additionally, it facilitates non-destructive examinations of their electronic and lattice properties, facilitating precise material identification and quality assessment. Furthermore, the study explores the second harmonic generation (SHG) properties of two-dimensional materials, shedding light on their structural asymmetry, electronic band structures, and potential applications in nonlinear optics and optoelectronics.

This seminar highlights the increasing importance of two-dimensional materials in the field of physics and materials science. It underscores the roles played by Raman spectroscopy and SHG in advancing our comprehension of the synthesis and optical characteristics of these materials. These insights provide a solid foundation for the development of devices and applications centered on two-dimensional materials, paving the way for new opportunities in technological advancement.

Departamento de Física dos Sólidos
Instituto de Física UFRJ

Spin-lattice dynamics:
fundamentals and method

Ramon Cardias
(IF-UFF)

Resumo:

Spin-lattice dynamics represents a critical frontier in the study of magnetic materials, merging the intricate interactions of atomic vibrations (phonons) with electron spin waves (magnons). This computational approach delves into the mutual interplay between these two fundamental entities, elucidating phenomena such as magnon-phonon coupling. As magnetics evolve in realms like spintronics and high-density storage, a comprehensive understanding of spin-lattice interactions becomes imperative. Through spin-lattice dynamics, we gain insights into how thermal effects influence magnetic properties and unveil the underpinnings of spin-lattice relaxation processes. This presentation will shed light on the foundational concepts of spin-lattice dynamics, its computational developments, and its pivotal role in the next generation of magnetic materials research.

Departamento de Física dos Sólidos
Instituto de Física UFRJ

Bound states in the continuum:
the (un)known state with innovative applications

Prof. Pedro Pereira
(IF-UERJ)

Resumo:

Usually, in introductory quantum mechanics courses, we use the finite quantum well to study the possible quantum states, namely, bound states within the quantum well, and extended and resonant states in the continuum. In addition to these states, the bound state in the continuum is one less explored in introductory courses, even though it was proposed at the beginning of quantum mechanics. In 1929, von Neumann and Wigner noted that certain infinite periodic potentials gave rise to states with localized wavefunction embedded in the continuum. These periodic potentials are not found in nature, making the bound state in the continuum just a “toy model” of the Schrödinger equation at that time.

The advancement of epitaxial growth techniques of semiconductor heterostructures allowed such periodic potentials to be fabricated in the laboratory. The bound state in the continuum was observed in 1992 in semiconductor superlattices, and was fundamental for the progress of the quantum cascade laser. Although the bound state in the continuum has been proposed in quantum systems, it is a universal feature of wave systems (acoustic, aquatic, optical, and photonic). In particular, they are of great interest for creating optical cavities with infinite quality factor.

The laboratory of semiconductors and optoelectronic devices (LSDopto) has been exploring these states for the development of infrared photodetectors with characteristics not obtained with traditional quantum well-infrared photodetectors. In this talk, I will present the fundamental concepts of bound states in the continuum in semiconductor superlattices, and, in particular, how they are being used in the development of innovative photodetectors by our group.

Departamento de Física dos Sólidos
Instituto de Física UFRJ

Magnetic fields as a tool to control superconducting devices

Dr. Davi Chaves
(DF/UFSCar)

Resumo:

Superconductors are at the forefront of an ongoing technological revolution. The emergence of novel computing schemes working at cryogenic temperatures has urged the development of new superconducting devices. Our research aims to explore the interaction between applied magnetic fields and superconducting thin films to provide avenues to control important features of such devices. A common thread in our works is to employ magneto-optical imaging (MOI) to visualize the local distribution of magnetic flux in the studied materials in real-time. In this presentation, I will discuss some of our recent results: (i) I will demonstrate how cooling under inhomogeneous magnetic fields may increase superconducting critical currents without requiring any nanostructuring process; (ii) We studied the effects of a single nanometric-deep weak-link on the superconducting and normal states’ properties of a prototypical niobium film; (iii) A quantitative MOI study allowed us to reveal how magnetic fields can be used to modify the expected weak-link behavior to that of a strong-link, drastically increasing the critical current density at particular fields. This is a tunable phenomenon that may have an immediate impact on superconducting devices; (iv) We have investigated the interaction of an applied magnetic field with a superconducting film presenting a nanoscale thickness gradient, revealing how the induced critical current modulation impacts the device’s response; (v) Lastly, I will introduce a magnetic field-assisted protocol that enables access to several memory states of nanobridge SQUIDs and demonstrate how these devices may work as superconducting multilevel memory elements.

Departamento de Física dos Sólidos
Instituto de Física UFRJ

Classical Density Functional Theory

Elvis Soares
(EQ-UFRJ)

Resumo:

Classical Density Functional Theory (DFT) has emerged as an indispensable theoretical tool, providing a robust framework to elucidate the microscopic structure of classical many-body systems. In this presentation, I will discuss my recent research in cDFT applied to confined fluids and electrolyte solutions. The initial part of the presentation will spotlight cDFT as a potent computational methodology capable of determining the properties of a many-body system under the influence of external potentials and crowded conditions. This approach has profound implications for our understanding of the behavior of confined fluids and electrolyte solutions. We will explore how the microscopic structure calculated from cDFT can be used to predict macroscopic properties, even within complex 3D geometries. Subsequently, we will explore the application of cDFT in characterizing the dynamical behavior of many-body particle systems under the influence of external potentials. Thereby, it will provide insights into a broad spectrum of physical phenomena. We will conclude with a brief discussion on the challenges we face in cDFT and the future directions we intend to pursue.

Departamento de Física dos Sólidos
Instituto de Física UFRJ

Vibrational Properties of Infinite and Finite Linear Carbon Chains

Graziâni Candiotto
(IF-UFRJ)

Resumo:

Departamento de Física dos Sólidos
Instituto de Física UFRJ

Anderson localization and spin-orbit induced emergent physics in elemental tellurium

Luis Martin-Moreno
(CSIC-Universidad de Zaragoza, Espanha)

Resumo:

Multidimensional data is notoriously troublesome to visualise; thus, it may contain symmetries that are difficult to detect. It is thus desirable to have methods that can detect them. Recently, machine learning methods have started to be applied to these problems. For instance, a machine learning technique was developed for finding continuous symmetries in fields that are solutions of partial differential equations [1]. In this talk, I will present a method for finding discrete symmetry in a general dataset (not necessarily representing a field) based on multi-evaluated neural networks. I will show how this technique works using some examples extracted from physics. This method may have applications not only to symmetry detection but also to optimal inverse design.

Departamento de Física dos Sólidos
Instituto de Física UFRJ

Anderson localization and spin-orbit induced emergent physics in elemental tellurium

Carsten Enderlein
(UFRJ-Xerém) 

Resumo: Elemental tellurium is a semiconductor with a small band gap of 330meV and a strong spin-orbit coupling. It is a van-der-Waals material with covalently bound helical atomic chains, which are arranged in a hexagonal lattice with van-der-Waals interhelical bonds. In recent years, tellurium has attracted a lot of attention as it has unique thermoelectric and magnetoelectric properties. For example, when an external current is applied along the screw axis of the helices, tellurium is magnetized parallel or antiparallel to the electrical current direction (depending on the chirality of the helices). In this talk, I will present theoretical and experimental results which demonstrate that tellurium is an Anderson insulator at low temperatures due to the existence of vacancies naturally present in elemental tellurium crystals. Furthermore, I will present data demonstrating the topological nature of the quasi-particles in tellurium. This topological nature leads to an emergent self-inductance in tellurium, which can demonstrate oscillative beahvior under magnetic fields. We have created nanostructures of tellurium, of which we believe that the inductant nature of the samples is even stronger than in the macrosocpic ones.

Departamento de Física dos Sólidos
Instituto de Física UFRJ