Julia Zanoni

A high-temperature furnace was constructed to perform in situ grazing-incidence small-angle x-ray scattering (GISAXS) experiments at high temperatures for isothermal studies. The furnace consists of two interconnected chambers: (i) a main chamber housing the heating elements, where the sample is inserted during GISAXS measurements, and (ii) a pre-chamber where the sample is maintained near room temperature until the temperature in the main chamber reaches the pre-selected value. The two-chamber design allows a rapid increase of the sample temperature and avoids the sample overheating, conditions desirable for studies of the formation and growth of nanoparticles under isothermal annealing. In a practical application, the furnace was used to investigate the kinetics of the formation of NiSi₂ silicide nanocrystals endotaxially grown in Si(001) wafers. This experiment allowed us to gain insights into the growth process and evaluate the performance of the furnace for in situ GISAXS studies. The high-temperature furnace provides a robust tool for studying the effect of annealing temperature on the kinetics of the growth of nanoparticles in thermally activated processes with special advantages in studies in which isothermal conditions are required.

Cr-doped Al₂O₃ spheres with strong red emission were produced using a simple laser processing approach with a 50 W continuous CO₂ laser. Structural characterization revealed that the produced spheres were monophasic, comprising the α-Al₂O₃ phase. Photoluminescence (PL) studies indicated that the observed red emission originates from multiple Cr³⁺ optical centers being dominated by the ²E → ⁴A₂ transition (R-lines), with a further contribution from the parity and spin forbidden ²T₁ → ⁴A₂ transition (R′-lines). The identification of additional radiative recombination from chromium ion pairs (N-lines) evidences that the produced samples are heavily doped. As such, energy transfer processes between the different chromium optical centers are seen to take place, as suggested by the lifetime decay analysis. PL excitation revealed that the room temperature luminescence is preferentially populated via the spin-allowed ⁴A₂ → ⁴T₂,⁴T₁ transitions and by the parity and spin-forbidden ⁴A₂ → ²T₂ (B-lines), ²T₁ (R′-lines)_, ²E (R-lines). Such results demonstrate that the present synthesis method is able to deliver high-optical-quality Al₂O₃:Cr crystals in a fast and simple way, with potential interest for optical, sensing, or lasing applications.

In situ ion-beam-induced luminescence measurements reveal a strong enhancement of the Cr³⁺ emission yield in electrically conductive chromium doped β-Ga₂O₃ single crystals upon proton irradiation. The observed effect can be explained based on the Fermi-level pinning caused by radiation defects. This pinning of the Fermi level activates deep carrier traps that can act as sensitizers of the Cr³⁺ emission. In agreement with this model, in semi-insulating samples, where the Fermi level lies deep in the bandgap, the Cr³⁺ emission is present already in as-grown samples, and no enhancement of its intensity is observed upon proton irradiation. The boost of the Cr³⁺ emission yield by irradiation, observed in conductive samples, is reversed by thermal annealing in argon at temperatures above 550 °C for 30 s. The results reveal a high potential of Cr-doped β-Ga₂O₃ for in situ and ex situ optical radiation detection and dosimetry.

Zn_1+xGa_2−2xGe_xO₄ (ZGGO:Cr)-persistent phosphor, with a molar fraction, x, of x = 0.1, doped with a 0.5% molar of chromium, was synthesised via solid-state reaction at 1350 °C for 36 h. X-ray diffraction measurements and Raman spectroscopy evidence a single crystalline phase corresponding to the cubic spinel structure. Room temperature (RT) photoluminescence (PL) and afterglow decay profiles were investigated using above and below bandgap excitation. In both cases, persistent PL was observed for almost 8 h, mainly originating from a Cr³⁺ defect, the so-called N2 optical centre. RT PL excitation and diffuse reflectance allow identification of the best pathways of Cr³⁺ red/NIR emission, as well as estimation of the ZGGO bandgap energy at 4.82 eV. An in-depth investigation of the observed luminescence at 15 K and temperature-dependent PL under site-selective excitation reveals the spectral complexity of the presence of several optically active Cr³⁺ centres in the ZGGO host that emit in almost the same spectral region. Furthermore, the temperature dependence of the R-lines’ intensity indicates the existence of thermal populating processes between the different optical centres. Such observations well account for a wide distribution of defect trap levels available for carrier capture/release, as measured by the persistent luminescence decay, from which the carriers are released preferentially to the N2 Cr³⁺-related optical centre.

Zinc oxide (ZnO) is a wide bandgap semiconductor material that has been widely explored for countless applications, including in biosensing. Among its interesting properties, its remarkable photoluminescence (PL), which typically exhibits an intense signal at room temperature (RT), arises as an extremely appealing alternative transduction approach due to the high sensitivity of its surface properties, providing high sensitivity and selectivity to the sensors relying on luminescence output. Therefore, even though not widely explored, in recent years some studies have been devoted to the use of the PL features of ZnO as an optical transducer for detection and quantification of specific analytes. Hence, in the present paper, we revised the works that have been published in the last few years concerning the use of ZnO nanostructures as the transducer element in different types of PL-based biosensors, namely enzymatic and immunosensors, towards the detection of analytes relevant for health and environment, like antibiotics, glucose, bacteria, virus or even tumor biomarkers. A comprehensive discussion on the possible physical mechanisms that rule the optical sensing response is also provided, as well as a warning regarding the effect that the buffer solution may play on the sensing experiments, as it was seen that the use of phosphate-containing solutions significantly affects the stability of the ZnO nanostructures, which may conduct to misleading interpretations of the sensing results and unreliable conclusions.

Zinc oxide (ZnO)/laser-induced graphene (LIG) composites were prepared by mixing ZnO, grown by laser-assisted flow deposition, with LIG produced by laser irradiation of a polyimide, both in ambient conditions. Different ZnO:LIG ratios were used to infer the effect of this combination on the overall composite behavior. The optical properties, assessed by photoluminescence (PL), showed an intensity increase of the excitonic-related recombination with increasing LIG amounts, along with a reduction in the visible emission band. Charge-transfer processes between the two materials are proposed to justify these variations. Cyclic voltammetry (CV) and electrochemical impedance spectroscopy evidenced increased electron transfer kinetics and an electrochemically active area with the amount of LIG incorporated in the composites. As the composites were designed to be used as transducer platforms in biosensing devices, their ability to detect and quantify hydrogen peroxide (H₂O₂) was assessed by both PL and CV analysis. The results demonstrated that both methods can be employed for sensing, displaying slightly distinct operation ranges that allow extending the detection range by combining both transduction approaches. Moreover, limits of detection as low as 0.11 mM were calculated in a tested concentration range from 0.8 to 32.7 mM, in line with the values required for their potential application in biosensors.

A scalable laser scribing approach to produce zinc oxide (ZnO) decorated laser-induced graphene (LIG) in a unique laser-processing step was developed by irradiating a polyimide sheet covered with a Zn/ZnO precursor with a CO₂ laser (10.6 μm) under ambient conditions. The laser scribing parameters revealed a strong impact on the surface morphology of the formed LIG, on ZnO microparticles' formation and distribution, as well as on the physical properties of the fashioned composites. The ZnO microparticles were seen to be randomly distributed along the LIG surface, with the amount and dimensions depending on the used laser processing conditions. Besides the synthesis conditions, the use of different precursors also resulted in distinct ZnO growth's yields and morphologies. Raman spectroscopy revealed the existence of both wurtzite-ZnO and sp² carbon in the majority of the produced samples. Broad emission bands in the visible range and the typical ZnO near band edge (NBE) emission were detected by photoluminescence studies. The spectral shape of the luminescence signal was seen to be extremely sensitive to the employed processing parameters and precursors, highlighting their influence on the composites' optical defect distribution. The sample produced from the ZnO-based precursor evidenced the highest luminescence signal, with a dominant NBE recombination. Electrochemical measurements pointed to the existence of charge transfer processes between LIG and the ZnO particles.