Síntese de resinas ligno-fenol-formaldeído para aplicação em painéis de média densidade.

bitstream/item/219766/1/TS2020-010-dis-MEPA.pdfDissertação (Mestrado em Química) - Universidade Federal do Ceará, Centro de Ciências, Fortaleza. Coorientador: Renato Carrhá Leitã

Width and Crystal Orientation Dependent Band Gap Renormalization in Substrate-Supported Graphene Nanoribbons

The excitation energy levels of two-dimensional (2D) materials and their one-dimensional (1D) nanostructures, such as graphene nanoribbons (GNRs), are strongly affected by the presence of a substrate due to the long-range screening effects. We develop a first-principles approach combining density functional theory (DFT), the GW approximation, and a semiclassical image-charge model to compute the electronic band gaps in planar 1D systems in weak interaction with the surrounding environment. Application of our method to the specific case of GNRs yields good agreement with the range of available experimental data and shows that the band gap of substrate-supported GNRs are reduced by several tenths of an electronvolt compared to their isolated counterparts, with a width and orientation-dependent renormalization. Our results indicate that the band gaps in GNRs can be tuned by controlling screening at the interface by changing the surrounding dielectric materials

Atomically Precise Graphene Nanoribbon Heterojunctions for Excitonic Solar Cells

By mixing pure precursor monomers and nitrogen-doped equivalents, atomically sharp wiggle-edged heterojunctions can be obtained via the combined action of Ullmann coupling followed by cyclodehydrogenation [Cai et al., <i>Nat. Nanotechnol.</i> <b>2014</b>, <i>9</i>, 896]. We used first-principles density functional theory and the many-body <i>GW</i> approach to establish the role of doping (boron and nitrogen) in a variety of graphene nanowiggles displaying a range of band gaps. The substitution of C atoms located at the edges of the structures does not significantly affect the magnitude of the band gaps, but leads to their relative upshift or downshift depending on the dopant. This shift is found to vary quasi-linearly as the concentration of dopant increases. Consequently, tunable type-II staggered band alignments are formed in graphene nanowiggle heterojunctions. We predict that these type-II heterojunctions can provide ultrathin solar cells with power conversion efficiencies up to 22.0%

Investigating Orientational Defects in Energetic Material RDX Using First-Principles Calculations

Orientational defects are molecular-scale point defects consisting of misaligned sterically trapped molecules. Such defects have been predicted in α-RDX using empirical force fields. These calculations indicate that their concentration should be higher than that of vacancies. In this study we confirm the stability of a family of four orientational defects in α-RDX using first-principles calculations and evaluate their formation energies and annealing barrier heights. The charge density distribution in the defective molecules is evaluated and it is shown that all four orientational defects exhibit some level of charge reduction at the midpoint of the N–N bond, which has been previously related to the sensitivity to initiation of the material. We also evaluate the vibrational spectrum of the crystal containing orientational defects and observe band splitting relative to the perfect crystal case. This may assist the experimental identification of such defects by Raman spectroscopy

Electrolyte Diffusion in Gyroidal Nanoporous Carbon

The structural properties of gyroidal nanoporous carbon (GNC) materials and their diffusion properties are investigated using a combination of molecular dynamics methods. We consider nine different GNC materials with variable pore geometry and pore size to establish that the local curvature induced by the presence of specific carbon ring size imposes highly specific behavior on electrolyte diffusion inside the GNC channels. We also find that GNC materials containing carbon square and heptagon motifs are globally more rigid and locally more flexible than GNC materials containing octagonal rings. The most rigid GNC’s present a faster water diffusion, indicating that the diffusion properties can be controlled by a proper choice of gyroid size and density. The analysis emphasizes that a fine balance between water permeation and ionic conduction can lead to GNC materials with attractive properties for nanofluidic applications. The impact of these findings are discussed in terms of their ionic transport, water filtration, and energy storage properties

Phonon-Enabled Carrier Transport of Localized States at Non-Polar Semiconductor Surfaces: A First-Principles-Based Prediction

Electron–phonon coupling can hamper carrier transport either by scattering or by the formation of mass-enhanced polarons. Here, we use time-dependent density functional theory-molecular dynamics simulations to show that phonons can also promote the transport of excited carriers. Using nonpolar InAs (110) surface as an example, we identify phonon-mediated coupling between electronic states close in energy as the origin for the enhanced transport. In particular, the coupling causes localized excitons in the resonant surface states to propagate into bulk with velocities as high as 10⁶ cm/s. The theory also predicts temperature enhanced carrier transport, which may be observable in ultrathin nanostructures

Voltage Dependent Charge Storage Modes and Capacity in Subnanometer Pores

Using molecular dynamics simulations, we show that charge storage in subnanometer pores follows a distinct voltage-dependent behavior. Specifically, at lower voltages, charge storage is achieved by swapping co-ions in the pore with counterions in the bulk electrolyte. As voltage increases, further charge storage is due mainly to the removal of co-ions from the pore, leading to a capacitance increase. The capacitance eventually reaches a maximum when all co-ions are expelled from the pore. At even higher electrode voltages, additional charge storage is realized by counterion insertion into the pore, accompanied by a reduction of capacitance. The molecular mechanisms of these observations are elucidated and provide useful insight for optimizing energy storage based on supercapacitors

Electronic Bandgap and Edge Reconstruction in Phosphorene Materials

Single-layer black phosphorus (BP), or phosphorene, is a highly anisotropic two-dimensional elemental material possessing promising semiconductor properties for flexible electronics. However, the direct bandgap of single-layer black phosphorus predicted theoretically has not been directly measured, and the properties of its edges have not been considered in detail. Here we report atomic scale electronic variation related to strain-induced anisotropic deformation of the puckered honeycomb structure of freshly cleaved black phosphorus using a high-resolution scanning tunneling spectroscopy (STS) survey along the light (<i>x</i>) and heavy (<i>y</i>) effective mass directions. Through a combination of STS measurements and first-principles calculations, a model for edge reconstruction is also determined. The reconstruction is shown to self-passivate most dangling bonds by switching the coordination number of phosphorus from 3 to 5 or 3 to 4

Quantum-Confined Stark Effect of Individual Defects in a van der Waals Heterostructure

The optical properties of atomically thin semiconductor materials have been widely studied because of the isolation of monolayer transition metal dichalcogenides (TMDCs). They have rich optoelectronic properties owing to their large direct bandgap, the interplay between the spin and the valley degree of freedom of charge carriers, and the recently discovered localized excitonic states giving rise to single photon emission. In this Letter, we study the quantum-confined Stark effect of these localized emitters present near the edges of monolayer tungsten diselenide (WSe₂). By carefully designing sequences of metallic (graphene), insulating (hexagonal boron nitride), and semiconducting (WSe₂) two-dimensional materials, we fabricate a van der Waals heterostructure field effect device with WSe₂ hosting quantum emitters that is responsive to external static electric field applied to the device. A very efficient spectral tunability up to 21 meV is demonstrated. Further, evaluation of the spectral shift in the photoluminescence signal as a function of the applied voltage enables us to extract the polarizability volume (up to 2000 ų) as well as information on the dipole moment of an individual emitter. The Stark shift can be further modulated on application of an external magnetic field, where we observe a flip in the sign of dipole moment possibly due to rearrangement of the position of electron and hole wave functions within the emitter

Probing the Interlayer Coupling of Twisted Bilayer MoS₂ Using Photoluminescence Spectroscopy

Two-dimensional molybdenum disulfide (MoS₂) is a promising material for optoelectronic devices due to its strong photoluminescence emission. In this work, the photoluminescence of twisted bilayer MoS₂ is investigated, revealing a tunability of the interlayer coupling of bilayer MoS₂. It is found that the photoluminescence intensity ratio of the trion and exciton reaches its maximum value for the twisted angle 0° or 60°, while for the twisted angle 30° or 90° the situation is the opposite. This is mainly attributed to the change of the trion binding energy. The first-principles density functional theory analysis further confirms the change of the interlayer coupling with the twisted angle, which interprets our experimental results