Free carrier enhanced depletion in ZnO nanorods decorated with bimetallic AuPt nanoclusters.

The decoration of semiconductor nanostructures with small metallic clusters usually leads to an improvement of their properties in sensing or catalysis. Bimetallic cluster decoration typically is claimed to be even more effective. Here, we report a detailed investigation of the effects of Au, Pt or AuPt nanocluster decoration of ZnO nanorods on charge transport, photoluminescence and UV sensitivity. ZnO nanorods were synthesized by chemical bath deposition while decoration with small nanoclusters (2-3 nm in size) was achieved by a laser-ablation based cluster beam deposition technology. The structural properties were investigated by scanning electron microscopy, high resolution transmission electron microscopy, X-ray photoelectron spectroscopy and Rutherford backscattering spectrometry, and the optoelectronic properties by current-voltage and photoluminescence measurements. The extent of band bending at the cluster-ZnO interface was quantitatively modeled through numerical simulations. The decoration of ZnO nanorods with monometallic Au or Pt nanoclusters causes a significant depletion of free electrons below the surface, leading to a reduction of UV photoluminescence, an increase of ZnO nanorod dark resistance (up to 200 times) and, as a consequence, an improved sensitivity (up to 6 times) to UV light. These effects are strongly enhanced (up to 450 and 10 times, respectively) when ZnO nanorods are decorated with bimetallic AuPt nanoclusters that substantially augment the depletion of free carriers likely due to a more efficient absorption of the gas molecules on the surface of the bimetallic AuPt nanoclusters than on that of their monometallic counterparts. The depletion of free carriers in cluster decorated ZnO nanorods is quantitatively investigated and modelled, allowing the application of these composite materials in UV sensing and light induced catalysis.

Circulating fractures-related microRNAs distinguish primary hyperparathyroidism-related from estrogen withdrawal-related osteoporosis in postmenopausal osteoporotic women: A pilot study.

Primary hyperparathyroidism (PHPT) represents a common cause of secondary osteoporosis in postmenopausal women, where the negative effect of estrogen withdrawal and that of hyperparathyroidism on bone mineralization coexist. Circulating microRNAs (miRNAs) expression profile has been correlated to both osteoporosis and fragility fractures. The study aimed to profile a set of miRNAs associated with osteoporotic fractures, namely miR-21-5p, miR-23a-5p, miR-24-2-5p, miR-24-3p, miR-93-5p, miR-100-5p, miR-122-5p, miR-124-3p, miR-125b-5p and miR-148-3p, in the plasma of 20 postmenopausal PHPT women. PHPT miRNAs profiles were compared with those detected in 10 age-matched postmenopausal non-PHPT osteoporotic women (OP). All the 10 miRNAs were detected in the plasma samples of both PHPT and OP women. The miRNA profiles clearly distinguished PHPT from OP samples, and identified within the PHPT group, two clusters differing for the PHPT severity, in term of ionized calcium and bone mineralization. In particular, miR-93-5p was significantly downregulated in PHPT samples, while miR-24-3p negatively correlated with the T-score at lumbar, femur neck and total hip sites. PHPT women who experienced osteoporotic fractures had plasma miR-24-3p levels higher than those detected in unfractured PHPT women. In conclusion, PHPT may modulate circulating fractures-related miRNAs, in particular, miR-93-5p, which may distinguish estrogen-related from PHPT-related osteoporosis.

Radiative mechanism and surface modification of four visible deep level defect states in ZnO nanorods.

Visible luminescence from ZnO nanorods (NRs) is attracting large scientific interest for light emission and sensing applications. We study visible luminescent defects in ZnO NRs as a function of post growth thermal treatments, and find four distinct visible deep level defect states (VDLSs): blue (2.52 eV), green (2.23 eV), orange (2.03 eV), and red (1.92 eV). Photoluminescence (PL) studies reveal a distinct modification in the UV (3.25 eV) emission intensity and a shift in the visible spectra after annealing. Annealing at 600 °C in Ar (Ar600) and O2 (O600) causes a blue and red-shift in the visible emission band, respectively. All samples demonstrate orange emission from the core of the NR, with an additional surface related green, blue, and red emission in the As-Prep, Ar600, and O600 samples, respectively. From PL excitation (PLE) measurements we determine the onset energy for population of the various VDLSs, and relate it to the presence of an Urbach tail below the conduction band due to a presence of ionized Zni or Zni complexes. We measured an onset energy of 3.25 eV for the as prepared sample. The onset energy red-shifts in the annealed samples by about 0.05 to 0.1 eV indicating a change in the defect structure, which we relate to the shift in the visible emission. We then used X-ray photoemission spectroscopy (XPS), and elastic recoil detection analysis (ERDA) to understand changes in the surface structure, and H content, respectively. The results of the XPS and ERDA analysis explain how the chemical states are modified due to annealing. We summarize our results by correlating our VDLSs with specific intrinsic defect states to build a model for PL emission in ZnO NRs. These results are important for understanding how to control defect related visible emission for sensing and electroluminescence applications.

The role of the interface in germanium quantum dots: when not only size matters for quantum confinement effects.

Quantum confinement (QC) typically assumes a sharp interface between a nanostructure and its environment, leading to an abrupt change in the potential for confined electrons and holes. When the interface is not ideally sharp and clean, significant deviations from the QC rule appear and other parameters beyond the nanostructure size play a considerable role. In this work we elucidate the role of the interface on QC in Ge quantum dots (QDs) synthesized by rf-magnetron sputtering or plasma enhanced chemical vapor deposition (PECVD). Through a detailed electron energy loss spectroscopy (EELS) analysis we investigated the structural and chemical properties of QD interfaces. PECVD QDs exhibit a sharper interface compared to sputter ones, which also evidences a larger contribution of mixed Ge-oxide states. Such a difference strongly modifies the QC strength, as experimentally verified by light absorption spectroscopy. A large size-tuning of the optical bandgap and an increase in the oscillator strength occur when the interface is sharp. A spatially dependent effective mass (SPDEM) model is employed to account for the interface difference between Ge QDs, pointing out a larger reduction in the exciton effective mass in the sharper interface case. These results add new insights into the role of interfaces on confined systems, and open the route for reliable exploitation of QC effects.