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

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%

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

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

Twisted MoSe₂ Bilayers with Variable Local Stacking and Interlayer Coupling Revealed by Low-Frequency Raman Spectroscopy

Unique twisted bilayers of MoSe₂ with multiple stacking orientations and interlayer couplings in the narrow range of twist angles, 60 ± 3°, are revealed by low-frequency Raman spectroscopy and theoretical analysis. The slight deviation from 60° allows the concomitant presence of patches featuring all three high-symmetry stacking configurations (2H or AA′, AB′, and A′B) in one unique bilayer system. In this case, the periodic arrangement of the patches and their size strongly depend on the twist angle. <i>Ab initio</i> modeling predicts significant changes in frequencies and intensities of low-frequency modes <i>versus</i> stacking and twist angle. Experimentally, the variable stacking and coupling across the interface are revealed by the appearance of two breathing modes, corresponding to the mixture of the high-symmetry stacking configurations and unaligned regions of monolayers. Only one breathing mode is observed outside the narrow range of twist angles. This indicates a stacking transition to unaligned monolayers with mismatched atom registry without the in-plane restoring force required to generate a shear mode. The variable interlayer coupling and spacing in transition metal dichalcogenide bilayers revealed in this study may provide an interesting platform for optoelectronic applications of these materials

Molecular Selectivity of Graphene-Enhanced Raman Scattering

Graphene-enhanced Raman scattering (GERS) is a recently discovered Raman enhancement phenomenon that uses graphene as the substrate for Raman enhancement and can produce clean and reproducible Raman signals of molecules with increased signal intensity. Compared to conventional Raman enhancement techniques, such as surface-enhanced Raman scattering (SERS) and tip-enhanced Raman scattering (TERS), in which the Raman enhancement is essentially due to the electromagnetic mechanism, GERS mainly relies on a chemical mechanism and therefore shows unique molecular selectivity. In this paper, we report graphene-enhanced Raman scattering of a variety of different molecules with different molecular properties. We report a strong molecular selectivity for the GERS effect with enhancement factors varying by as much as 2 orders of magnitude for different molecules. Selection rules are discussed with reference to two main features of the molecule, namely its molecular energy levels and molecular structures. In particular, the enhancement factor involving molecular energy levels requires the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energies to be within a suitable range with respect to graphene’s Fermi level, and this enhancement effect can be explained by the time-dependent perturbation theory of Raman scattering. The enhancement factor involving the choice of molecular structures indicates that molecular symmetry and substituents similar to that of the graphene structure are found to be favorable for GERS enhancement. The effectiveness of these factors can be explained by group theory and the charge-transfer interaction between molecules and graphene. Both factors, involving the molecular energy levels and structural symmetry of the molecules, suggest that a remarkable GERS enhancement requires strong molecule–graphene coupling and thus effective charge transfer between the molecules and graphene. These conclusions are further experimentally supported by the change of the UV–visible absorption spectra of molecules when in contact with graphene and these conclusions are theoretically corroborated by first-principles calculations. These research findings are important for gaining fundamental insights into the graphene–molecule interaction and the chemical mechanism in Raman enhancement, as well as for advancing the role of such understanding both in guiding chemical and molecule detection applications and in medical and biological technology developments

Enhanced Raman Scattering on In-Plane Anisotropic Layered Materials

Surface-enhanced Raman scattering (SERS) on two-dimensional (2D) layered materials has provided a unique platform to study the chemical mechanism (CM) of the enhancement due to its natural separation from electromagnetic enhancement. The CM stems from the charge interactions between the substrate and molecules. Despite the extensive studies of the energy alignment between 2D materials and molecules, an understanding of how the electronic properties of the substrate are explicitly involved in the charge interaction is still unclear. Lately, a new group of 2D layered materials with anisotropic structures, including orthorhombic black phosphorus (BP) and triclinic rhenium disulfide (ReS₂), has attracted great interest due to their unique anisotropic electrical and optical properties. Herein, we report a unique anisotropic Raman enhancement on few-layered BP and ReS₂ using copper phthalocyanine (CuPc) molecules as a Raman probe, which is absent on isotropic graphene and h-BN. According to detailed Raman tensor analysis and density functional theory calculations, anisotropic charge interactions between the 2D materials and molecules are responsible for the angular dependence of the Raman enhancement. Our findings not only provide new insights into the CM process in SERS, but also open up new avenues for the exploration and application of the electronic properties of anisotropic 2D layered materials

Low-Frequency Interlayer Raman Modes to Probe Interface of Twisted Bilayer MoS₂

van der Waals homo- and heterostructures assembled by stamping monolayers together present optoelectronic properties suitable for diverse applications. Understanding the details of the interlayer stacking and resulting coupling is crucial for tuning these properties. We investigated the low-frequency interlayer shear and breathing Raman modes (<50 cm^–1) in twisted bilayer MoS₂ by Raman spectroscopy and first-principles modeling. Twisting significantly alters the interlayer stacking and coupling, leading to notable frequency and intensity changes of low-frequency modes. The frequency variation can be up to 8 cm^–1 and the intensity can vary by a factor of ∼5 for twisting angles near 0° and 60°, where the stacking is a mixture of high-symmetry stacking patterns and is thus sensitive to twisting. For twisting angles between 20° and 40°, the interlayer coupling is nearly constant because the stacking results in mismatched lattices over the entire sample. It follows that the Raman signature is relatively uniform. Note that for some samples, multiple breathing mode peaks appear, indicating nonuniform coupling across the interface. In contrast to the low-frequency interlayer modes, high-frequency intralayer Raman modes are much less sensitive to interlayer stacking and coupling. This research demonstrates the effectiveness of low-frequency Raman modes for probing the interfacial coupling and environment of twisted bilayer MoS₂ and potentially other two-dimensional materials and heterostructures

Low-Frequency Raman Fingerprints of Two-Dimensional Metal Dichalcogenide Layer Stacking Configurations

The tunable optoelectronic properties of stacked two-dimensional (2D) crystal monolayers are determined by their stacking orientation, order, and atomic registry. Atomic-resolution Z-contrast scanning transmission electron microscopy (AR-Z-STEM) and electron energy loss spectroscopy (EELS) can be used to determine the exact atomic registration between different layers, in few-layer 2D stacks; however, fast optical characterization techniques are essential for rapid development of the field. Here, using two- and three-layer MoSe₂ and WSe₂ crystals synthesized by chemical vapor deposition, we show that the generally unexplored low frequency (LF) Raman modes (<50 cm^–1) that originate from interlayer vibrations can serve as fingerprints to characterize not only the number of layers, but also their stacking configurations. <i>Ab initio</i> calculations and group theory analysis corroborate the experimental assignments determined by AR-Z-STEM and show that the calculated LF mode fingerprints are related to the 2D crystal symmetries

Raman Shifts in Electron-Irradiated Monolayer MoS₂

We report how the presence of electron-beam-induced sulfur vacancies affects first-order Raman modes and correlate the effects with the evolution of the <i>in situ</i> transmission-electron microscopy two-terminal conductivity of monolayer MoS₂ under electron irradiation. We observe a red-shift in the E′ Raman peak and a less pronounced blue-shift in the A′₁ peak with increasing electron dose. Using energy-dispersive X-ray spectroscopy and selected-area electron diffraction, we show that irradiation causes partial removal of sulfur and correlate the dependence of the Raman peak shifts with S vacancy density (a few %). This allows us to quantitatively correlate the frequency shifts with vacancy concentration, as rationalized by first-principles density functional theory calculations. <i>In situ</i> device current measurements show an exponential decrease in channel current upon irradiation. Our analysis demonstrates that the observed frequency shifts are intrinsic properties of the defective systems and that Raman spectroscopy can be used as a quantitative diagnostic tool to characterize MoS₂-based transport channels