Lead-Free Perovskite Narrow-Bandgap Oxide Semiconductors of Rare-Earth Manganates.

Tremendous success has been achieved in photovoltaic (PV) applications, but PV-generated electricity still cannot compete with traditional power in terms of price. Chemically stable and nontoxic all-oxide solar cells made from earth-abundant resources fulfill the requirements for low-cost manufacturing under ambient conditions and thus are promising as the next-generation approach to solar cells. However, the main obstacles to developing all-oxide solar cells are the spectral absorbers. Besides photovoltaics, novel chemically stable, nontoxic, and earth-abundant narrow-bandgap semiconductors are desired for photochemical applications in photodetectors, photoelectrodes, or photocatalysts. Herein, were report novel lead-free perovskite narrow-bandgap rare-earth semiconductors, YMnO3, HoMnO3, ErMnO3, and YbMnO3, which were identified by screening a family of perovskite rare-earth manganates, RMnO3 (R = Y, La, Ce, Pr, Nd, Sm, Gd, Tb, Dy, Ho, Er, and Yb). The sharp edge observed in their absorption spectra indicates the existence of band gaps, further confirmed with laser Raman fluorescence spectra. Good periodic on-off photoelectronic response was observed in 8 of the 12 members (i.e., R = La, Pr, Nd, Sm, Gd, Tb, Dy, and Yb). Among them, YbMnO3 is approved as an n-type semiconductor with a direct band gap near 1.35 eV, whose theoretical Shockley-Queisser efficiency is approximately 33.7% for single-p-n-junction solar cells. This work sheds light on exploring stable oxide semiconductors with a narrow band gap for future applications.

[Evaluation of the cavity cleaning of ultrasonic instruments and slow-speed handpiece in posterior teeth root-end preparation].

PURPOSE: To compare the cleanliness of root end preparations by using ultrasonic instrumentation and slow-speed handpiece. METHODS: Thirty-two mesial roots of the first mandibular molars with two canals and mature root apices were assigned randomly to 2 groups, each group had 16 teeth. The root-end preparations were made respectively using ultrasonic diamond tip Berutti and NiTi tip RE2 and slow-speed handpiece with No.2 round bur. Root end cavities were examined under scanning electron microscope for further evaluation of the superficial debris and smear layer of the root end preparations. SPSS 13.0 software package was used for Kruskal Wallis test. RESULTS: Ultrasonic preparation had significantly less superficial debris and smear layer than slow-speed handpiece preparation (P<0.05). CONCLUSIONS: Ultrasonic instrument creates cleaner surfaces for root end cavities than slow-speed handpiece preparation in posterior teeth root end preparation.

[Effects of different instruments in root-end preparation: scanning electron microscopic evaluation].

PURPOSE: To evaluate the effects of retrograde preparation between ultrasonic retrotips and traditional slow-speed handpiece. METHODS: 27 maxillary second bicuspid teeth with two canals and one isthmus were divided randomly into 3 groups with 9 in each. The root-end preparations were made respectively using ultrasonic diamond tip Berutti and NiTi tip RE2, ultrasonic #40K file and slow-speed handpiece with NO.2 round bur. The postoperative epoxy resin replicas were examined under scanning electron microscope for further evaluating the number and type of the micro-cracks, the quality of cavity margin. SPSS13.0 software package was used for Kruskal-Wallis test. RESULTS: The number of micro-crack of ultrasonic instruments were significantly less than that of slow-speed handpiece(P=0.002).The type of micro-cracks has no significant difference between the two techniques(P=0.657). On the quality of cavity margin, the slow-speed handpiece group had significant inferior to the ultrasonic #40K file group(P=0.039); there was no significantly difference between the ultrasonic Berutti and RE2 group and the slow-speed handpiece group (P=0.136); there was no significant difference between the two ultrasonic groups (P=0.637). CONCLUSION: Ultrasonic instrument is feasible in the root-end preparation.