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Titanate nanotubes were synthesized and subjected to an ion exchange reaction with erbium salt aqueous solution to obtain titanate nanotubes exchanged with erbium (3+) ions. In order to evaluate the effects of the thermal treatment atmosphere on the structural and optical properties of erbium titanate nanotubes, we subjected them to heat treatment in air and argon atmospheres. For comparison, titanate nanotubes were also treated in the same conditions. A complete structural and optical characterizations of the samples was performed. The characterizations evidenced the preservation of the morphology with the presence of phases of erbium oxides decorating the surface of the nanotubes. Variations in the dimensions of the samples (diameter and interlamellar space) were promoted by the replacement of Na+ by Er3+ and the thermal treatment in different atmospheres. In addition, the optical properties were investigated by UV-Vis absorption spectroscopy and photoluminescence spectroscopy. The results revealed that the band gap of the samples depends on the variation of diameter and sodium content caused by ion exchange and thermal treatment. Furthermore, the luminescence strongly depended on vacancies, evidenced mainly by the calcined erbium titanate nanotubes in argon atmosphere. The presence of these vacancies was confirmed by the determination of Urbach energy. The results suggest the use of thermal treated erbium titanate nanotubes in argon atmosphere in optoelectronics and photonics applications, such as photoluminescent devices, displays, and lasers.
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Understanding the impact of doping variations on the physical properties of two-dimensional materials is important for their application in electronic and optoelectronic devices. Here we report a nano-optical study on graphene and MoS2 homojunctions by placing these two materials partly on top of a layered talc substrate, partly on top of an SiO2 substrate. By analyzing the nano-Raman scattering from graphene and the nanophotoluminescense emission from MoS2, two different doping zones are evident with sub-100 nm wide charge oscillations. The oscillations occur abruptly at the homojuction and extend over longer distances away from the interface, indicating imperfect deposition of the two-dimensional layer on the substrate. These results evidence fine and unexpected details of the homojuctions, important to build better electronic and optoelectronic devices.
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Twisted bilayer graphene is created by slightly rotating the two crystal networks in bilayer graphene with respect to each other. For small twist angles, the material undergoes a self-organized lattice reconstruction, leading to the formation of a periodically repeated domain1-3. The resulting superlattice modulates the vibrational3,4 and electronic5,6 structures within the material, leading to changes in the behaviour of electron-phonon coupling7,8 and to the observation of strong correlations and superconductivity9. However, accessing these modulations and understanding the related effects are challenging, because the modulations are too small for experimental techniques to accurately resolve the relevant energy levels and too large for theoretical models to properly describe the localized effects. Here we report hyperspectral optical images, generated by a nano-Raman spectroscope10, of the crystal superlattice in reconstructed (low-angle) twisted bilayer graphene. Observations of the crystallographic structure with visible light are made possible by the nano-Raman technique, which reveals the localization of lattice dynamics, with the presence of strain solitons and topological points1 causing detectable spectral variations. The results are rationalized by an atomistic model that enables evaluation of the local density of the electronic and vibrational states of the superlattice. This evaluation highlights the relevance of solitons and topological points for the vibrational and electronic properties of the structures, particularly for small twist angles. Our results are an important step towards understanding phonon-related effects at atomic and nanometric scales, such as Jahn-Teller effects11 and electronic Cooper pairing12-14, and may help to improve device characterization15 in the context of the rapidly developing field of twistronics16.
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Plasmon-tunable tip pyramids (PTTPs) are reproducible and efficient nanoantennas for tip-enhanced Raman spectroscopy (TERS). Their fabrication method is based on template stripping of a segmented gold pyramid with a size-adjustable nanopyramid end, which is capable of supporting monopole localized surface plasmon resonance (LSPR) modes leading to high spectral enhancement when its resonance energy is matched with the excitation laser energy. Here, we describe in detail the PTTP fabrication method and report a statistical analysis based on 530 PTTPs' and 185 ordinary gold micropyramids' templates. Our results indicate that the PTTP method generates probes with an apex diameter smaller than 30 nm on 92.4% of the batch, which is a parameter directly related to the achievable TERS spatial resolution. Moreover, the PTTPs' nanopyramid edge size L, a critical parameter for LSPR spectral tuning, shows variability typically smaller than 12.5%. The PTTP's performance was tested in TERS experiments performed on graphene, and the results show a spectral enhancement of up to 72-fold, which is at least one order of magnitude higher than that typically achieved with gold micropyramids. Imaging resolution is in the order of 20 nm.
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Nano-heterostructures of titanate nanotubes were synthesized and they revealed a complex structure with the formation of TiO2 (anatase), CeO2, Ag2O and metallic silver nanoparticles on the outer walls and intercalation of Ce4+ and Ag+ into the interlayer spaces of the nanotubes by microwave-assisted hydrothermal process and subjected to ion exchange reactions. To the best of our knowledge, this is the first reported silver and cerium co-exchanged titanate nanotubes for bio-applications. The co-ion exchange processes preserved the original tubular structure of titanate nanotubes with significant changes of the superficial as well as interlamellar environment. This study opens up possibility of synthesizing complex, functional nano-heterostructure with the scope of modification of the final structure, especially the amount and oxidation state of the intercalated cation (Ce4+, Ce3+ and Ag+) as well as the quantity and variety of the decorating nanoparticles (CeO2, Ag2O or metallic Ag). The interplay of which, in turn, can lead to important biological properties and applications, owing to their ion-liberation capacity. The samples were tested in antibacterial activity with two different kind of bacteria (gram positive and negative), cell cytotoxicity and adhesion, and it was found that the nano-heterostructure formed shows high antibacterial activity with low cytotoxicity and high cell adhesion, which makes it a promising material for further health applications.
Assuntos
Antibacterianos/farmacologia , Cério/química , Prata/química , Titânio/química , Animais , Antibacterianos/química , Linhagem Celular , Escherichia coli , Nanopartículas Metálicas/química , Camundongos , Testes de Sensibilidade Microbiana , Viabilidade Microbiana/efeitos dos fármacos , Nanotubos/química , Tamanho da Partícula , Staphylococcus aureus/efeitos dos fármacos , Propriedades de SuperfícieRESUMO
Nanoconfinement is one of the most intriguing nanoscale effects and affects several physical and chemical properties of molecules and materials, including viscosity, reaction kinetics, and glass transition temperature. In this work, liquid nuclear magnetic resonance (NMR) was used to analyze the behavior of 2,4-pentadienone in ordered mesoporous materials with a pore diameter of between 3 and 10 nm. The liquid NMR results showed meaningful changes in the hydrogen chemical shift and the keto-enol chemical equilibrium, which were associated with the pore diameter, allowing the authors to observe the effects of nanoconfinement. An interesting phenomenon was observed where the chemical equilibria of 2,4-pentadienone confined in a mesoporous material with a pore diameter of 3.5 nm was similar to that obtained with free (bulk) 2,4-pentadienone in larger pore materials. Another interesting result was observed for the enthalpy and entropy of the tautomeric equilibria of 2,4-pentadienone confined in mesoporous materials with a 5.5 nm pore diameter being -7.9 kJ mol-1 and -15.9 J mol-1.K. These values are similar to those obtained by dimethyl sulfoxide. This phenomenon indicates the possible use of ordered mesoporous materials as a reaction substitute in organic solvents. It was further observed that while the values of enthalpy (ΔH) and entropy (ΔS) had been modified by confinement, the Gibbs free energy (ΔG) value remained closer to that observed in free (bulk) 2,4-pentadienone. It is expected that this study will help in understanding the effects of nanoconfinement and provide a simple method to employ NMR techniques to analyze these phenomena.
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The time scale for interfacial photoinduced electron transfer (PeT) in plasmonic nanoparticles is not well established and the details are still under debate. This has renewed the interest in studying the electron transfer effect from both experimental and theoretical points of view. We present a quantitative analysis of PeT in single spherical gold (Au) and gold@palladium core@shell (Au@Pd) nanoparticles supported on reduced graphene oxide (RGO) using dark-field hyperspectral microscopy (DFHM) and electrochemical impedance spectroscopy (EIS). By studying the plasmon bandwidth in the scattering spectra of single particles and by correlating it to the plasmon damping processes we showed that PeT occurs from the AuNPs to RGO in a 10 fs time scale with a quantum efficiency of 35%. The introduction of a Pd shell on the AuNPs decreases the PeT time, with transfer occurring in as little as 1.7 fs with quantum yield higher than 74%. Furthermore, EIS showed a smaller resistance for PeT on RGO/Au@PdNPs under green light illumination. Our results can improve the understanding of the chemical interface damping process due to PeT in plasmonic nanomaterials and can enable the design of more efficient plasmon enhanced photocatalysts.
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The knowledge of the phonon coherence length is of great importance for two-dimensional-based materials since phonons can limit the lifetime of charge carriers and heat dissipation. Here we use tip-enhanced Raman spectroscopy (TERS) to measure the spatial correlation length Lc of the A1g1 and A1g2 phonons of monolayer and few-layer gallium sulfide (GaS). The differences in Lc values are responsible for different enhancements of the A1g modes, with A1g1 always enhancing more than the A1g2, independently of the number of GaS layers. For five layers, the results show an Lc of 64 and 47 nm for A1g1 and A1g2, respectively, and the coherence lengths decrease when decreasing the number of layers, indicating that scattering with the surface roughness plays an important role.
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A reproducible route for tuning localized surface plasmon resonance in scattering type near-field optical microscopy probes is presented. The method is based on the production of a focused-ion-beam milled single groove near the apex of electrochemically etched gold tips. Electron energy-loss spectroscopy and scanning transmission electron microscopy are employed to obtain highly spatially and spectroscopically resolved maps of the milled probes, revealing localized surface plasmon resonance at visible and near-infrared wavelengths. By changing the distance L between the groove and the probe apex, the localized surface plasmon resonance energy can be fine-tuned at a desired absorption channel. Tip-enhanced Raman spectroscopy is applied as a test platform, and the results prove the reliability of the method to produce efficient scattering type near-field optical microscopy probes.
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Combined microscopy techniques are used to establish the usability of phosphonic acid layers as promoters of hydroxyapatite (HAp) growth. Using spread coating, octadecylphosphonic acid (OPA) self-assembled bilayers are delivered to the thin natural oxide layer of a titanium film surface with no prior treatment. These bilayers aggregate two major advantages of phosphonic moieties to titanium surfaces: nucleation of hydroxyapatite crystals from ionic solution and affinity for both titanium oxide surface and HAp crystals. The functionalized substrates and bare titanium (control) samples are immersed in an aqueous solution containing calcium and phosphorus ions. Over a 4-week immersion time, OPA-functionalized substrates present numerous large agglomerates of inorganic crystals, in contrast to control samples, with no significant amount of deposits. Initial sample characterization was performed with atomic force microscopy (AFM). Compositional and structural characterization of these agglomerates (using TEM, EDS, and electron diffraction), revealed that they are indeed HAp, the main component of the inorganic bone matrix.