Destaques Estudantes

Charge layering in the close vicinity of charged interfaces is a well-known effect, extensively reported in both experiments and simulations of Room Temperature Ionic Liquids (RTILs) and concentrated electrolytes. The traditional Poisson–Fermi (PF) theory is able to successfully describe overcrowding effects but fails to reproduce charge ordering even in strong coupling regimes. Simple models, yet capable of investigating the interplay between these important interfacial phenomena, are still lacking. In order to bridge this gap, we herein present a modified PF approach that is able to capture layering effects in strong coupling regimes typical of RTIL. The modification is based on the introduction of charge cavities around test-particles, which simply extend the exclusion volume effects to also incorporate the accompanying depletion of charges due to particle insertion. The addition of this simple ingredient is shown to reproduce overscreening and charge ordering, thereby extending the predictive power of the PF approach to strong coupling regimes. Using a linear response theory, we were able to study the emergence of charge ordering based on two characteristic lengths: a wavelength responsible for charge layering, along with a damping length that screens charge oscillations. At large ionic strengths and strong couplings, the system undergoes a transition to undamped charge layering. The transition takes place when the poles of the Fourier components of the linear potential become real-valued. This criterion allows one to identify the transition line across the parameter space, thus delimiting the region of stability against unscreened charge ordering.

The concept of entropy is mostly recognized as one of the most difficult for the students to comprehend. This makes entropy also a subject hard to teach. In this paper, we discuss the employment of a simplified version of the solid Einstein model, largely employed to illustrate multiplicity, to construct the concept of entropy and to obtain the Boltzmann entropy equation. In this constructive process, STEM students can gain intuition on that entropy is a statistical quantity and on what it represents. The process is carried out in a close way to the Clausius definition, allowing the connection of the result of the model to other thermodynamic quantities.

In this study, 4-pyrrolidinopyridine (PPY) and 2,5-diphenyloxazole (DPO) were employed as N-heterocyclic derivatives to synthesize novel picrate salts. These salts were subsequently subjected to characterization using infrared spectroscopy, ultraviolet spectroscopy, and single-crystal X-ray diffraction. Comparison of these structures with entries in the Cambridge Structural Database (CSD) and the application of two multivariate statistical analyses, namely Principal Component Analysis (PCA) and Hierarchical Cluster Analysis (HCA), facilitated the differentiation of salts and co-crystals based on the structural behaviors of the picrate molecule. Additionally, Hirshfield Surface Analysis and 2D fingerprint plots were employed to investigate intermolecular contacts. The fluorescence emission spectra revealed that compound 2 has the potential to serve as a probe for turn-off detection of picric acid (PA).

Phyllosilicate minerals, which form a class of naturally occurring layered materials (LMs), have been recently considered as a low-cost source of two-dimensional (2D) materials. Clinochlore [Mg5Al(AlSi3)O10(OH)8] is one of the most abundant phyllosilicate minerals in nature, exhibiting the capability to be mechanically exfoliated down to a few layers. An important characteristic of clinochlore is the natural occurrence of defects and impurities which can strongly affect their optoelectronic properties, possibly in technologically interesting ways. In the present work, we carry out a thorough investigation of the clinochlore structure on both bulk and 2D exfoliated forms, discussing its optical features and the influence of the insertion of impurities on its macroscopic properties. Several experimental techniques are employed, followed by theoretical first-principles calculations considering several types of naturally-ocurring transition metal impurities in the mineral lattice and their effect on electronic and optical properties. We demonstrate the existence of requirements concerning surface quality and insulating properties of clinochlore that are mandatory for its suitable application in nanoelectronic devices. The results presented in this work provide important informations for clinochlore potential applications and establish a basis for further works that intend to optimize its properties to relevant 2D technological applications through defect engineering.

Co-, Ni-, and Mn-doped BiFeO3 (BFO) ceramics were synthesized herein through a solid-state reaction. All doped BFO samples exhibit visible-light response, and the Co- and Ni-doped BFO samples present enhanced ferromagnetic order at room temperature. All doped samples show secondary phases in minor quantities. Optical spectra reveal two absorptions bands, indicating multiple electron transitions for BFO and its secondary phases. M − H hysteresis loops suggest enhanced ferromagnetism in the Co- and Ni-doped BFO samples because of magnetic spinel CFP and NFO phases, respectively, whereas changes in oxygen vacancies and Fe–O–Fe bond angle play minor roles in the ferromagnetic behavior.

The photophysical properties of two difluoroboron flavanone β-diketonates (DK1 and DK2) and their interaction with fullerene (C60) in toluene solution and spin-coated films were investigated using time-correlated single-photon counting, absorption spectroscopy, and steady-state fluorescence spectroscopy. In molecular crystal, the complexes exhibited red-shifted absorption and emission relative to their spectra in solution. Additionally, both complexes displayed bi-exponential decay behavior in time-resolved fluorescence measurements, indicating their capability to form both H and J types of aggregates in the solid state. The introduction of C60 resulted in significant fluorescence quenching and reduced excited-state lifetimes for both complexes. This quenching, observed in both solution and spin-coated films, was primarily driven by photo-induced electron transfer (PET) processes, underscoring the potential of these complexes as donors in fullerene-based heterojunction organic solar cells. To elucidate the process of aggregate formation and the impacts of different dimerization types within the crystalline structure of the complexes, first-principles calculations using Density Functional Theory (DFT) and time-dependent density functional theory (TD-DFT) were performed. We also employed DFT to explore various DK configurations on the fullerene surface, evaluating intermolecular distances and formation energies. These calculations highlighted the energetically favorable gap between the low-lying LUMO levels of the complexes and C60, confirming their suitability for such applications.

The advancement of nanobiocomposites reinforced with 2D nano-materials plays a pivotal role in enhancing bone tissue engineering. In this study, we introduce a nanobiocomposite that reinforces bovine collagen with 2D nano-talc, a recently exfoliated nano-mineral. These nanobiocomposites were prepared by blending collagen with varying concentrations of 2D nano-talc, encompassing mono- and few-layers talc from soapstone nanomaterial. Extensive characterization techniques including AFM, XPS, nano-FTIR, s-SNOM nanoimaging, Force Spectroscopy, and PeakForce QNM® were employed. The incorporation of 2D nano-talc significantly enhanced the mechanical properties of the nanobiocomposites, resulting in increased stiffness compared to pristine collagen. In vitro studies supported the growth and proliferation of osteoblasts onto 2D nano-talc-reinforced nanobiocomposites, as well as showed the highest mineralization potential. These findings highlight the substantial potential of the developed nanobiocomposite as a scaffold material for bone tissue engineering applications.

The role of defects in two-dimensional semiconductors and how they affect the intrinsic properties of these materials have been a widely researched topic over the past few decades. Optical characterization techniques such as photoluminescence and Raman spectroscopies are important tools to probe the physical properties of semiconductors and the impact of defects. However, confocal optical techniques present a spatial resolution limitation lying in a μm-scale, which can be overcome by the use of near-field optical measurements. Here, we use tip-enhanced photoluminescence and Raman spectroscopies to unveil the nanoscale optical properties of grown MoS2 monolayers, revealing that the impact of doping and strain can be disentangled by the combination of both techniques. A noticeable enhancement of the exciton peak intensity corresponding to trion emission quenching is observed at narrow regions down to a width of 47 nm at grain boundaries related to doping effects. Besides, localized strain fields inside the sample lead to non-uniformities in the intensity and energy position of photoluminescence peaks. Finally, two distinct MoS2 samples present different nano-optical responses at their edges associated with opposite strains. The edge of the first sample shows a photoluminescence intensity enhancement and energy blueshift corresponding to a frequency blueshift for E2g and 2LA Raman modes. In contrast, the other sample displays a photoluminescence energy redshift and frequency red shifts for E2g and 2LA Raman modes at their edges. Our work highlights the potential of combining tip-enhanced photoluminescence and Raman spectroscopies to probe localized strain fields and doping effects related to defects in two-dimensional materials.

In this work, we employed Density Functional Theory calculations combined with search techniques based on evolutionary algorithms to predict and characterize crystalline structures composed of nitrogen (N6) cage-like molecules. We found stable molecular crystals and a rich phenomenology associated with their behavior under pressure, including atomic rebonding and semiconductor-metal transitions. This investigation resides in the context of high-energy-density materials, since molecular species containing only nitrogen atoms tend to dissociate into N2 molecules, releasing large amounts of energy.

Despite several theoretically proposed two-dimensional (2D) diamond structures, experimental efforts to obtain such structures are in initial stage. Recent high-pressure experiments provided significant advancements in the field, however, expected properties of a 2D-like diamond such as sp3 content, transparency and hardness, have not been observed together in a compressed graphene system. Here, we compress few-layer graphene samples on SiO2/Si substrate in water and provide experimental evidence for the formation of a quenchable hard, transparent, sp3-containing 2D phase. Our Raman spectroscopy data indicates phase transition and a surprisingly similar critical pressure for two-, five-layer graphene and graphite in the 4-6 GPa range, as evidenced by changes in several Raman features, combined with a lack of evidence of significant pressure gradients or local non-hydrostatic stress components of the pressure medium up to ≈ 8 GPa. The new phase is transparent and hard, as evidenced from indentation marks on the SiO2 substrate, a material considerably harder than graphene systems. Furthermore, we report the lowest critical pressure (≈ 4 GPa) in graphite, which we attribute to the role of water in facilitating the phase transition. Theoretical calculations and experimental data indicate a novel, surface-to-bulk phase transition mechanism that gives hint of diamondene formation.