ABSTRACT
MIL-53(Al) is a metal-organic framework (MOF) with unique properties, including structural flexibility, thermal stability, and luminescence. Its ability to adsorb volatile organic compounds (VOCs) and water vapor makes it a promising platform for sensing applications. This study investigated the adsorption mechanism of MIL-53(Al) with different VOCs, including ketones, alcohols, aromatics, and water molecules, focusing on structural transformations due to pore size variation and photoluminescence properties. The reported results assess MIL-53(Al) selectivity towards different VOCs and provide insights into their fundamental properties and potential applications in sensing.
ABSTRACT
Graphene (Gr)-a single layer of two-dimensional sp2 carbon atoms-and Carbon Dots (CDs)-a novel class of carbon nanoparticles-are two outstanding nanomaterials, renowned for their peculiar properties: Gr for its excellent charge-transport, and CDs for their impressive emission properties. Such features, coupled with a strong sensitivity to the environment, originate the interest in bringing together these two nanomaterials in order to combine their complementary properties. In this work, the investigation of a solid-phase composite of CDs deposited on Gr is reported. The CD emission efficiency is reduced by the contact of Gr. At the same time, the Raman analysis of Gr demonstrates the increase of Fermi energy when it is in contact with CDs under certain conditions. The interaction between CDs and Gr is modeled in terms of an electron-transfer from photoexcited CDs to Gr, wherein an electron is first transferred from the carbon core to the surface states of CDs, and from there to Gr. There, the accumulated electrons determine a dynamical n-doping effect modulated by photoexcitation. The CD-graphene interaction unveiled herein is a step forward in the understanding of the mutual influence between carbon-based nanomaterials, with potential prospects in light conversion applications.
ABSTRACT
The effects of temperature and atmosphere (air and O2) on the doping of monolayers of graphene (Gr) on SiO2 and Si substrates, and on the doping of MoS2 multilayer flakes transferred on the same substrates have been investigated. The investigations were carried out by in situ micro-Raman spectroscopy during thermal treatments up to 430 °C, and by atomic force microscopy (AFM). The spectral positions of the G and 2D Raman bands of Gr undergo only minor changes during treatment, while their amplitude and full width at half maximum (FWHM) vary as a function of the temperature and the used atmosphere. The thermal treatments in oxygen atmosphere show, in addition to a thermal effect, an effect attributable to a p-type doping through oxygen. The thermal broadening of the line shape, found during thermal treatments by in situ Raman measurements, can be related to thermal phonon effects. The absence of a band shift results from the balance between a red shift due to thermal effects and a blue shift induced by doping. This shows the potential of in situ measurements to follow the doping kinetics. The treatment of MoS2 in O2 has evidenced a progressive erosion of the flakes without relevant spectral changes in their central zone during in situ measurements. The formation of MoO3 on the edges of the flakes is observed indicative of the oxygen-activated transformation.
ABSTRACT
ZnO nanoparticles (NPs) synthesized by pulsed laser ablation (PLAL) of a zinc plate in deionized water were investigated by time-resolved photoluminescence (PL) and complementary techniques (TEM, AFM, µRaman). HRTEM images show that PLAL produces crystalline ZnO NPs in wurtzite structure with a slightly distorted lattice parameter a. Consistently, optical spectra show the typical absorption edge of wurtzite ZnO (Eg = 3.38 eV) and the related excitonic PL peaked at 3.32 eV with a subnanosecond lifetime. ZnO NPs display a further PL peaking at 2.2 eV related to defects, which shows a power law decay kinetics. Thermal annealing in O2 and in a He atmosphere produces a reduction of the A1(LO) Raman mode at 565 cm(-1) associated with oxygen vacancies, accompanied by a decrease of defect-related emission at 2.2 eV. Based on our experimental results the emission at 2.2 eV is proposed to originate from a photo-generated hole in the valence band recombining with an electron deeply trapped in a singly ionized oxygen vacancy. This investigation clarifies important aspects of the photophysics of ZnO NPs and indicates that ZnO emission can be controlled by thermal annealing, which is important in view of optoelectronic applications.
ABSTRACT
We report the study of the visible-ultraviolet emission properties and the structural features of silica nanoparticles prepared through a laboratory sol-gel technique. Atomic force microscopy, Raman and Infrared investigations highlighted the 10 nm size, purity and porosity of the obtained nanoparticles. By using time resolved photoluminescence techniques in air and in a vacuum we were able to single out two contributions in the visible emission: the first, stable in both atmospheres, is a typical fast blue band centered around 2.8 eV; the second, only observed in a vacuum around the 3.0-3.5 eV range, is a vibrational progression with two phonon modes at 1370 cm(-1) and 360 cm(-1). By fully characterizing the spectroscopic features of this structured emission, we determine its vibronic properties and clarify the different origins with respect to the blue luminescent defect.
