ABSTRACT
This paper investigates the electronic structure of wurtzite (W) and rock-salt (RS) Zn1-xCoxO nanoparticles (NPs) by means of optical measurements under pressure (up to 25 GPa), X-ray absorption, and transmission electron microscopy. W-NPs were chemically synthesized at ambient conditions and RS-NPs were obtained by pressure-induced transformation of W-NPs. In contrast to the abrupt phase transition in W-Zn1-xCoxO as thin film or single crystal, occurring sharply at about 9 GPa, spectroscopic signatures of tetrahedral Co(2+) are observed in NPs from ambient pressure to about 17 GPa. Above this pressure, several changes in the absorption spectrum reveal a gradual and irreversible W-to-RS phase transition: (i) the fundamental band-to-band edge shifts to higher photon energies; (ii) the charge-transfer absorption band virtually disappears (or overlaps the fundamental edge); and (iii) the intensity of the crystal-field absorption peaks of Co(2+) around 2 eV decreases by an order of magnitude and shifts to 2.5 eV. After incomplete phase transition pressure cycles, the absorption edge of nontransformed W-NPs at ambient pressure exhibits a blue shift of 0.22 eV. This extra shift is interpreted in terms of quantum confinement effects. The observed gradual phase transition and metastability are related to the NP size distribution: the larger the NP, the lower the W-to-RS transition pressure.
ABSTRACT
The intrahepactic lithisis is unusual in the Occidental hemisphere. The prevalence in Venezuela in unknown. We study all the edoscipic retrograde cholangiography carried for lithiasic pathology, in the Servicio de Gastroenterología, Hospital Universitario de Caracas, since January 1993 until April 1995. Over 342 patients, 9 were intrahepatic lithisis (1.16%). Seven cases of Caracas and two refered of the rest of the country. The sex was female in seven patients and males in two. The age range was 15 to 67 years old. The clinic feature much frequent was abdominal pain. The diagnosis was ultrasound and endoscopic retrograde cholangiography. The treatment was endoscopic with sphincterotomy and removal stones and surgical with resection in two cases, and choledocotomy in the other four cases.
