Role of the carrier density in the transport mechanisms of polycrystalline ZnO films.

The transport processes occurring in polycrystalline ZnO have been investigated by measuring the resistivity as a function of temperature in ZnO films with different n-doping levels, obtained by varying the oxygen pressure during the deposition process. These films show an electrical resistivity spanning about two orders of magnitude, from 4 to 8 × 10-2Ω cm at room temperature, corresponding to low and high levels of n-type doping, respectively. The present results indicate a relevant role of the carrier density in determining the dominant transport mechanisms in these samples by showing that the picture characterizing a highly n-doped ZnO sample, where an intra-grain mechanism and a grain-boundary mechanism dominate the high temperature and low temperature transport processes, respectively, is thoroughly overturned in lightly n-doped samples, where a grain-boundary mechanism and an intra-grain mechanism govern the charge transport in the same temperature regimes, respectively. Moreover, the present results indicate a critical role of the conditions limiting the occurrence of the Mott variable range hopping regime. They show indeed that an incomplete check of such conditions can result in erroneous conclusions about the prevalent transport mechanisms.

Transport mechanisms in Co-doped ZnO (ZCO) and H-irradiated ZCO polycrystalline thin films.

In the present study, the electrical resistivity (ρ) as a function of the temperature (T) has been measured in polycrystalline ZnO, Co-doped ZnO (ZCO) and H irradiated ZCO (HZCO) samples, in the 300-20 K range. The achieved results show impressive effects of Co doping and H irradiation on the ZnO transport properties. The Co dopant increases the ZnO resistivity at high T (HT), whereas it has an opposite effect at low T (LT). H balances the Co effects by neutralizing the ρ increase at HT and strengthening its decrease at LT. A careful analysis of the ρ data permits to identify two different thermally activated processes as those governing the charge transport in the three materials at HT and LT, respectively. The occurrence of such processes has been fully explained in terms of a previously proposed model based on an acceptor impurity band, induced by the formation of Co-oxygen vacancy complexes, as well as known effects produced by H on the ZnO properties. The same analysis shows that both Co and H reduce the effects of grain boundaries on the transport processes. The high conductivity of HZCO in the whole T-range and its low noise level resulting from electric noise spectroscopy make this material a very interesting one for technological applications.

Ferromagnetism and Conductivity in Hydrogen Irradiated Co-Doped ZnO Thin Films.

Impressive changes in the transport and ferromagnetic properties of Co-doped ZnO thin films have been obtained by postgrowth hydrogen irradiation at temperatures of 400 °C. Hydrogen incorporation increases the saturation magnetization by one order of magnitude (up to ∼1.50 µB/Co) and increases the carrier density and mobility by about a factor of two. In addition to the magnetic characterization, the transport and structural properties of hydrogenated ZnO:Co have been investigated by Hall effect, local probe conductivity measurements, micro-Raman, and X-ray absorption spectroscopy. Particular care has been given to the detection of Co oxides and metal Co nanophases, whose influence on the increase in the transport and ferromagnetic properties can be excluded on the ground of the achieved results. The enhancement in ferromagnetism is directly related to the dose of H introduced in the samples. On the contrary, despite the shallow donor character of H atoms, the increase in carrier density n is not related to the H dose. These apparently contradictory effects of H are fully accounted for by a mechanism based on a theoretical model involving Co-VO (Co-O vacancy) pairs.

Clinical utility of preoperative screening with STOP-Bang questionnaire in elective surgery.

BACKGROUND: Obstructive sleep apnea (OSA) is a common disease which increases the risk of perioperative complications. The aim of this study is to assess the clinical utility of preoperative screening for OSA in determining the prevalence of patients at high risk of OSA in a surgical population, the incidence of difficult airway management and the incidence of perioperative complications. METHODS: We conducted a multisite, prospective observational study on adult patients scheduled for elective surgery. All patients completed a STOP-Bang questionnaire as a part of their preoperative evaluation. Collected data included: demographic data, type of surgery, ASA class, postoperative course, complications within 48 hours, difficult intubation (DI) and difficult mask ventilation (DMV) rates. RESULTS: A total of 3452 consecutive patients were recruited; 2997 (87%) were identified as low OSA risk patients and 455 (13%) were identified as high OSA risk patients; 113 (3%) postoperative complications, 315 (9%) cases of DMV and 375 (11%) of DI were observed. The percentage of postoperative complications in patients with HR-OSA was 9%, while the percentage of DI was 20% and the percentage of DMV was 23%. High risk for OSA and higher BMI (≥30 Kg m-2) were independently associated with risk for perioperative complications. CONCLUSION: In conclusion, this study demonstrates that the prevalence of high OSA risk patients in the surgical population is high. The increase in the rates of perioperative complications justifies the implementation of perioperative strategies that use the STOP-Bang as a tool for triage.

Evidence of cobalt-vacancy complexes in Zn(1-x)Co(x)O dilute magnetic semiconductors.

We investigate the local structure of ferromagnetic Zn(1-x)Co(x)O epilayers by coupling polarization-dependent x-ray absorption spectroscopy and ab initio calculations of selected defect structures. We give clear evidence of the presence of oxygen vacancies, located close to the Co atoms in a specific complex configuration. We also establish the upper concentration limit of metallic parasitic nanophases and their contribution to magnetism. Our results lead to the conclusion that oxygen vacancies play an important role in originating the high temperature ferromagnetism of Zn(1-x)Co(x)O.