Abstract
Two-dimensional atomic crystals can radically change their properties in response to external influences, such as substrate orientation or strain, forming materials with novel electronic structure. Applying strain to graphene has the same effect as introducing an effective pseudo-magnetic field. Doing so, one has been able to experimentally quantize the electron spectrum into...
Abstract
The future application of few-layer semiconductors, such as transition metal dichalcogenides (TMDCs), as building blocks for opto-electronic devices relies on a full understanding of the light-induced strongly bound electron-hole pairs (excitons) in these materials. Through the investigation of Zeeman and diamagnetic shifts in the excitonic peaks under high magnetic fields,...
Abstract
Twisted van der Waals (vdW) heterostructures exhibit periodic variations, leading to a new type of in-plane superlattice known as moiré superlattice/pattern which modifies considerably the optical properties of excitons in transition metal dichalcogenides (TMD) vdW heterostructures. The period of these moiré superlattices is determined by the lattice constant mismatch and the twist...
Abstract
We carried out first-principles density functional theory calculations of hydrogen and oxygen adsorption and diffusion on subnanometer MoS nanowires. The nanowires are robust against adsorption of hydrogen. On the other hand, interaction with oxygen shows that the nanowires can oxidize with a small barrier. Our results open the path for understanding the behavior of MoS nanowires...
Abstract
Fractals have been used since long as decorative art, but only in the last century they have been classified mathematically. In the 80’s and 90’s, the foundational work of Mandelbrot triggered an enormous activity in the field. The focus was on understanding classical fractals. This century, the task is to understand quantum fractals. In 2019, we realized a Sierpinski gasket...
Abstract
The discovery of two dimensional (2D) graphene has opened the doors to investigate a myriad of new 2D materials that have better characteristics. Out of these are the transition metal dichalcogenides (TMDs). In this talk, I will shine light on the electronic, optical and thermal properties of the 1T [1] Pd-based dichalcogenides, namely PdS2, PdSe2, PdSSe, PdSTe, and PdSeTe systems,...
While it is well known that the III-Nitrides are the materials for the highly efficient light-emitting diodes, among other optoelectronic devices, the transition metal dichalcogenides (TMDC) also offer a great potential use in the field of 2D materials with interesting electronic and optoelectronic properties. In the case of III-Nitrides, the interest on these materials was renewed, now as a...
We briefly review the rich aspects of the three-body physics in two dimensions with attractive short-range potentials and contrast it with the three-dimensional case. Then we address the interesting case of two attractive and one repulsive potential appropriate to describe trions in 2D materials. The emergent property is the frustration of the trion binding with respect to the exciton, which...
Abstract
Singlet fission in organic semiconductors is a promising concept that can be used to increase the efficiency of solar photovoltaic cells. Upon light absorption a singlet exciton (S = 0) is created, which splits into two triplet excitons (S = 1), via an intermediate triplet-pair state (S = 1 ⊗ S = 1). This carrier-multiplication process potentially reduces the thermalization losses...
Abstract
For the past 40 years density functional theory (DFT) has been the dominant method for the quantum mechanical simulation of periodic systems, predicting the ground state properties of metals, semiconductors, and insulators with great success. This success not only encompasses standard bulk materials but also complex materials such as proteins, polymers, solids, nanostructures and...
Abstract
For the past 40 years density functional theory (DFT) has been the dominant method for the quantum mechanical simulation of periodic systems, predicting the ground state properties of metals, semiconductors, and insulators with great success. This success not only encompasses standard bulk materials but also complex materials such as proteins, polymers, solids, nanostructures and...
Abstract
Most of 2D superconductors are of type II, i.e., they are penetrated by quantized vortices when exposed to out-of-plane magnetic fields. In a presence of a supercurrent, a Lorentz-like force acts on the vortices, leading to drift and dissipation. The current-induced vortex motion is impeded by pinning at defects. Usually, the pinning strength decreases upon any type of...
Abstract
Crystal nucleation and growth are fundamental natural processes and central phenomena in several technologies. Crystal nucleation relies on the emergence of a critical nucleus from an SCL that enables other atoms or molecules to join the system, starting crystal growth. According to classical theories, the crystal growth rate can be separated into thermodynamic and kinetic terms....
Abstract
Graphene has weak spin-orbit coupling and no magnetic order. But when placed in contact with a strong spin-orbit coupling material, such as a TMDC, or a ferromagnet, such as Cr2Ge2Te6, Dirac electrons acquire strong spin-orbit or exchange coupling, respectively. Such proximity effects render graphene suitable for spintronic applications that require spin manipulation [1]. In...
The wurtzite phase group III-Nitrides (AlN, GaN, InN) have attracted great interest due to their successful applications in the optoelectronics since the 90’s. In this paper we perform a comprehensive study of AlN, GaN and InN structural elastic and electronic properties using hybrid and conventional Density Functional Theory, presenting a comparison of the features of the three compounds. We...
Abstract
Bismuth telluride (Bi2Te3) is an archetype of a three-dimensional topological insulator, which presents topological surface sates (TSS) with a linear dispersion like in a Dirac cone positioned between the valence and conduction bands. The Dirac fermions on the surface are protected against scattering by the time inversion symmetry [1]. On the other hand, Pb1-xSnxTe is a topological...
Abstract
Molecular electronics has attracted attention due application in nanoscale electronic devices. Feynman was the first scientist to propose that a molecular machine could be built, in which atoms would work the same role as the bricks of a regular size structure, composing a nanometer appliance. These nanostructures can present features which are sometimes similar to well-known...
Abstract
Electrostatically defined quantum dots in bilayer graphene offer a promising platform for spin qubits with presumably long coherence times due to low spin-orbit coupling and low nuclear spin density. We employ a capacitively coupled charge sensor to study the time dynamics of the excited state using the Elzerman technique. We find that the relaxation time of the excited state is of...
The II-VI semiconductor compound CdMnTe have been studied for a long time due to its optoelectronic properties and application as solar cells, x-ray detectors and other devices. The great majority of these studies have used GaAs(001) as substrates since II-VI substrates with good quality are rare and very expensive. This work describes the characterization of CdTe/CdMnTe quantum wells grown...
Abstract
Light matter interaction is of utmost importance in a number of technological applications. In photovoltaics, the excitation of carriers is a key ingredient. It leads to the formation of excitons, which are strongly bound in low dimensional systems, and the dynamics of carriers upon excitations if energy harvesting is possible.
In this talk I will discuss some recent developments...
Abstract
Atomically-thin materials and systems have provided theorists with new perspectives to exploit the electronic structure under direct and indirect interactions. For example, the electron response to static electric field, bias voltage, or even by including the spin-orbit coupling may lead to the discovery of new phenomena, as well as interesting electronic properties at low...
Abstract
The development of spintronic devices demands the existence of materials with some kind of spin splitting (SS). With de the advance in the development of 2D materials in the last two decades, this family of compounds brought great opportunities for applications in spintronic devices. Finding the best materials for this assignment, however, is a challenging task. To advance the...
Abstract
Superconducting quantum computing is a burgeoning field that seeks to develop Josephson-junction-based qubits and superconducting circuitry as a scalable architecture for quantum information processing. In particular, advancements in qubits design and fabrication techniques have led to the development of the building blocks necessary for the development of one of the leading...
Abstract
Superconducting quantum computing is a burgeoning field that seeks to develop Josephson-junction-based qubits and superconducting circuitry as a scalable architecture for quantum information processing. In particular, advancements in qubits design and fabrication techniques have led to the development of the building blocks necessary for the development of one of the leading...
Abstract
The interplay of the spin and the orbital angular momenta of electrons in semiconductors governs the Zeeman splitting, often described by the g-factors. In this talk, I will cover the basic physics behind the Zeeman splitting and g-factors, with recent examples involving two-
dimensional materials and related van der Waals heterostructures. Particularly, I will show that in...
Abstract
The GW plus the Bethe-Salpeter (BSE) equation approach becomes a methodology commonly used for computing the quasiparticle and optical properties of condensed-matter systems. However, GW approach requires a fine k-point sampling of the Brillouin zone, and GW plus BSE (GW-BSE) demands an even finer k-point sampling. Hence it is rather easy to reach the limits of what can be...
2D materials have been studied in basic research and used in technological applications in many areas of physics and related fields because they have extraordinary properties and are relatively easy to obtain and friendly to work with. In laser physics and technology, 2D materials simplify the way to obtain femtosecond pulses and have become a powerful tool in the ultra-fast field. In this...
Abstract
Polaritons, which are quasiparticles composed of a photon coupled to an electric or magnetic dipole, are a major focus in nanophotonic research of low-dimensional materials. Polaritons can be active in a broad range of the electromagnetic spectrum (meVs to eVs) and exhibit momenta much higher than the corresponding free-space radiation. Hence, the use of high momentum broadband...
Abstract
Hybrid graphene-hexagonal boron nitride (hBN) monolayers have already been synthesized, but most investigations on their properties have only considered relaxed structures. In this talk, I will discuss the mechanical and electronic properties of two types of monolayers: in (i), we have a graphene sheet with hBN domains; in (ii), we have an hBN sheet with graphene domains. The...
Abstract
The 2D materials, such as monolayer transition metal dichalcogenides (TMDs) 1-4, can form van der Waals (vdWs) heterostructures held together by weak van der Waals forces, providing an unprecedented platform to engineer quantum materials with exotic physical properties. Among the different vdWs heterostructures, the most interesting ones for optical applications are those...
Abstract
Recently, the existence of an allotrope phase of bismuthene called pentaoctite, in which all hexagonal rings are replaced by either pentagons or octagons, has been proposed.[1,2] These structures show a sizeable bandgap, can be stable under strain, and have topological insulator behavior with protected surface nontrivial Dirac states. From this information, we extend our...
Abstract
In this seminar, some applications of optical spectroscopy in materials and food science will be presented. The first subject consists of quantum cutting luminescence for solar cells application. In the quantum-cut optical phenomenon, two lower-energy photons are obtained by the energy partition of a high-energy photon. As a consequence, this process opens the possibility of its...
Abstract
Nowadays, the so-called van der Waals heterostructures represent a prominent research area within optoelectronics in semiconductor 2D materials. The layered structures are related to the fact that they support the formation of excitons – bound electron-hole pairs – and excitonic complexes with binding energies more than an order of magnitude greater than conventional...
Atomically thin membranes are ideal building blocks for nanoelectromechanical systems (NEMS) because of their unique mechanical properties and their low mass. We make membranes by transferring atomically thin layers on top of silicon oxide substrates that are pre-patterned with (circular) holes. The suspended membranes are characterized by a laser interferometer set-up that gives access to...
Abstract
Naturally occurring van der Waals crystals have brought unprecedented interest to nanomaterial researchers in recent years. So far, more than 1800 layered materials (LMs) have been identified but only a few insulating and naturally occurring LMs were deeply investigated [1,2]. Thus, as soon as a new LM is identified, the investigation of its optical, mechanical, and electrical...
Abstract
The Stark effect is one of the most efficient mechanisms to manipulate many-body states in semiconductor nanostructures. In mono- and few-layer transition metal dichalcogenides, it
is usually induced by optical and electric field means. In this contribution [1], we address the tunability of the optical emission energies of excitonic states in MoSe2 monolayers employing
the 220...
Abstract
First-principles calculations reported here illuminate the effects of the interface properties of Al2O3 and graphene, with emphasis on the structural and electrical properties. Various contact interfaces and with different alpha-Al 2O3 surface terminations are considered with on and slightly-off stoichiometric aluminum oxide. We show that depending on whether aluminum or oxygen is...
Renewable energy production is a key component in the drive towards a safe, secure energy supply for future low-carbon economies. Using energy from the sun to generate electricity provides a sustainable source of free, abundant, safe, clean energy, without use of any fossil fuels and without waste or pollution.
Solar cells (photovoltaic cells) are made of semiconductor materials that...
Abstract
The atomic layer deposition (ALD) of metallic oxides, mainly alumina (Al2O3), when performed in thermal mode uses deionized water (DI) as oxidant source and trimethylaluminum (TMA) as a metal reactant. However, growth per cycle (GPC) of Al2O3 thin films for the reactant and co-reactant mentioned above is limited to 0.1 nm/cycle [1]. This barrier in the GPC is overcome by using...
In this work we studied the shape anisotropy and its relation with the band alignment in the InAsP/GaAs quantum dots by means of three technics: polarized photoluminescence, time resolved photoluminescence and magneto photoluminescence. For comparison, InAs/GaAs and InP/GaAs quantum dots were also analyzed, as not only their recombination energy sets a lower and an upper limit to the...
Abstract
Infrared sensors have many important applications both in civilian and military sectors. Due to the military application, the commercialization of such devices are controlled by the governors of the countries that fabricate such devices, specifying the types and the performance of the devices that can be sold to each country, even to civilian applications. This situation asks for...
The intrinsic ferromagnetism in two-dimensional (2D) materials has been a long-term concern and pursuit. Only few years ago it has been realized, after thinning CrGeTe 3 and CrX3 (X = Cl, Br, I) from bulk down to a monolayer. These materials were not only ferromagnetic, but also semiconducting - which stimulated intensive research on novel emergent phenomena and creative concepts. In this talk...
Abstract
Model hamiltonians are a useful tool to approach the many-body problem in materials. They often provide valuable physical insight and enable larger length-scale studies. However, since model parameters are invariably unknown, this approach often lacks predictive power, an issue that turns more significant in materials with strong electronic correlations. Being able to controllably...
Abstract
In this talk we will present a new code, called QE2KP, which calculates the effective kp Hamiltonian using the ab initio wave-functions as basis functions to calculate the matrix elements of the kp theory within the Löwdin perturbation approach. The kp method is widely used to obtain effective Hamiltonians to describe a chosen set of bands of crystalline materials. The derivation...
Using a combination of in situ high-resolution transmission electron microscopy (Fig. 1) and density functional theory (Fig. 2), we report the formation and rupture of ZrO2 atomic ionic wires. Near rupture, under tensile stress, the system favors the spontaneous formation of oxygen vacancies, a critical step in the formation of the monatomic bridge. In this length scale, vacancies provide...
The improving ability to synthesize new materials has intensified the interest in describing properties of systems modeled by more complex lattices. The 2D super-honeycomb lattices, including the Kagomé-graphene lattice, have been explored recently in metallic organic frameworks. They have been revealed as an essential route to achieving localized electronic responses, manifested as flat bands...
Abstract
Beyond graphene, most of the attempts in finding interesting layered materials (LMs) that are capable of being reduced to mono and few-layers have been made in synthesized materials such as hexagonal boron nitride and transition metal dichalcogenides. In an effort to increase the list of naturally occurring LMs that are abundant in nature and could become an alternative low-cost...
Organic eletronics based on thin films as electrodes or active layers offer some processing advantages and new possibilities in the manufacture of these devices, such as flexibility and large areas. Interest in this area of research has grown significantly in last decade, presenting many innovations, whether in the synthesis of new materials, in the understanding of optoelectronic properties...
Abstract
Biosensors based on graphene field-effect transistors (GFETs) are highly attractive technology, as they allow real-time label-free electrical detection, scalability, inexpensive mass production, miniaturization, the use of a low volume of sample, and the possibility of on-chip integration of both sensor and measurement systems. Besides that, graphene possesses unique properties...
Abstract
This work presents a study about the structure of thin films, deposited to be a protection layer against oxidation of microlamps, as well as chemical and structural modifications induced by their heating under operation. The studied microlamps (fig. 1) were produced by Plasma Enhanced Chemical Vapor Deposition (PECVD) and sputtering, over silicon substrates, and have applications...
Abstract
The n-acenes are a class of polyaromatic hydrocarbons (PAHs) composed of linearly condensed benzene rings, resembling a quasi-1D graphene strip with zigzag boundaries. The two-dimensional structures are known as periacenes. The acenes with more than five linear benzene rings, are characterized by having a singlet open shell wave function in the ground state, showing that these...
Abstract
The possibility of controlling the optical and electronic structure in semiconductor by quantum confinement has turn semiconductor colloidal nanocrystals (NCs) one of the most investigates class of materials for optoelectronics applications. In the last two decades, much effort has been devoted to gain further control over those properties by electronic wavefunction engineering via...
Abstract
Two-dimensional (2D) semiconducting materials as active layers in photovoltaic devices is a subject that has attracted a lot of attention in the last years [1]. Nowadays, the most employed materials for this kind of application are transition metal dichalcogenides (TMDCs), which are semiconductors with chemical configuration MX2 [2], where M is a transition metal such as Mo, W,...
Abstract
Topological insulators (TIs) are materials that are insulating in their bulk but present metallic states on their surface. This is the simplest definition for a complex quantum effect that results from strong spin-orbit coupling that changes the topological order of the material. The metallic states host spin-polarized currents composed of Dirac fermions flowing on the topological...
Abstract
Stark many-body localization (SMBL) is a phenomenon observed in interacting systems with a nearly uniform spatial gradient applied field. Contrasting to the traditional many-body localization phenomenon, SMBL does not require disorder [1]. We have investigated SMBL in a spin-1/2 described by a Heisenberg model including a next-nearest-neighbor exchange coupling [2]. By employing an...
Abstract
The so-called “search for Majoranas” has mobilized several groups over the last decade with the goal of achieving the “holy grail” of topological quantum computation in condensed matter systems [1-2]. In spite of the advances, particularly in devices of semiconductor nanowires with proximity induced superconductivity, many unanswered questions and challenges remain, as highlighted...
Abstract
Vacancies in materials structure, lowering its atomic density, take the system closer to the atomic limit, to which all systems are topologically trivial. Here we show a mechanism of mediated interaction between vacancies inducing a topologically nontrivial phase. We explore topological transition dependence with the vacancy density in transition metal dichalcogenides. As a case of...
Professor Luiz Guimarães Ferreira, Guima as he was called by his friends, passed away last year. He was born in Rio de Janeiro in January 1937, he attended Santo Inácio College, graduating at the top of his class. He Joined ITA in 1955 in first place in the entrance exam and was most brilliant undergraduate student at ITA. He majored in Electronic Engineering in 1959, and soon joined MIT where...
Abstract
Techniques for studying short-range interactions have been successfully used to evaluate local parameters of crystals with high precision. In particular, experiments using perturbed angular correlation (PAC) spectroscopy were able to track the high-temperature phase transitions of hybrid improper ferroelectrics. Here, we continued such works by combining PAC spectroscopy with...
Abstract
In this talk we will present a new code, called QE2KP, which calculates the effective kp Hamiltonian using the ab initio wave-functions as basis functions to calculate the matrix elements of the kp theory within the Löwdin perturbation approach. The kp method is widely used to obtain effective Hamiltonians to describe a chosen set of bands of crystalline materials. The derivation...
Abstract
The so-called “search for Majoranas” has mobilized several groups over the last decade with the goal of achieving the “holy grail” of topological quantum computation in condensed matter systems [1-2]. In spite of the advances, particularly in devices of semiconductor nanowires with proximity induced superconductivity, many unanswered questions and challenges remain, as highlighted...
Abstract
Topological insulators (TIs) are materials that are insulating in their bulk but present metallic states on their surface. This is the simplest definition for a complex quantum effect that results from strong spin-orbit coupling that changes the topological order of the material. The metallic states host spin-polarized currents composed of Dirac fermions flowing on the topological...
Abstract
The discovery of two dimensional (2D) graphene has opened the doors to investigate a myriad of new 2D materials that have better characteristics. Out of these are the transition metal dichalcogenides (TMDs). In this talk, I will shine light on the electronic, optical and thermal properties of the 1T [1] Pd-based dichalcogenides, namely PdS2, PdSe2, PdSSe, PdSTe, and PdSeTe systems,...
While it is well known that the III-Nitrides are the materials for the highly efficient light-emitting diodes, among other optoelectronic devices, the transition metal dichalcogenides (TMDC) also offer a great potential use in the field of 2D materials with interesting electronic and optoelectronic properties. In the case of III-Nitrides, the interest on these materials was renewed, now as a...
Abstract
We carried out first-principles density functional theory calculations of hydrogen and oxygen adsorption and diffusion on subnanometer MoS nanowires. The nanowires are robust against adsorption of hydrogen. On the other hand, interaction with oxygen shows that the nanowires can oxidize with a small barrier. Our results open the path for understanding the behavior of MoS nanowires...
