Cation vacancies on both sublattices (V(Ti), V(Sr)) have been identified in homoepitaxial pulsed laser deposited SrTiO3 films using high intensity variable energy positron annihilation lifetime spectroscopy (PALS) measurements. Film nonstoichiometry was varied by varying laser fluence. PALS showed that on increasing the fluence above the Ti/Sr∼1 value, the concentration ratio [V(Sr)]/[V(Ti)] systematically increased. Reducing the fluence into the Ti-poor region below resulted in additional vacancy cluster defect formation. Vacancy concentrations greater than ∼50  ppm were observed in all films.

Metal-organic frameworks (MOFs) enable the design of host-guest systems with specific properties.I nt his work, we showh ow the confinement of anthracene in aw ellchosen MOF host leads to reversible yellow-to-purple photoswitching of the fluorescence emission. This behavior has not been observed before for anthracene,e ither in pure form or adsorbed in other porous hosts.The photoresponse of the hostguest system is caused by the photodimerization of anthracene, which is greatly facilitated by the pore geometry,c onnectivity, and volume as well as the structural flexibility of the MOF host. The photoswitching behavior was used to fabricate photopatternable and erasable surfaces that, in combination with data encryption and decryption, hold promise in product authentication and secure communication applications.

Vacancy‐type defects in Mg‐implanted GaN are probed using monoenergetic positron beams. Mg+ ions are implanted to provide a 500‐nm‐deep box profile with Mg concentrations, [Mg], of 1 × 1017–1 × 1019 cm−3 at room temperature. In the as‐implanted samples, the major defect species is a complex of a Ga vacancy (VGa) and a nitrogen vacancy (VN). After annealing above 1000 °C, the major defect species is changed to vacancy clusters due to vacancy agglomeration. This agglomeration is suppressed, and the agglomeration onset temperature is decreased with a decreasing [Mg]. For samples with [Mg] ≥ 1 × 1018 cm−3, the trapping rate of positrons by vacancy‐type defects decrease after annealing above 1100–1200 °C. This decreases is attributed to the change in the defect charge states from neutral to positive due to a downward shift of the Fermi level. The carrier trapping/detrapping properties of the vacancy‐type defects and their time dependences are also revealed.

Most applications of positron beams require knowledge of the implantation characteristics for an appropriate interpretation of the experimental data. In this work, the median implantation depth as a function of implantation energy, z 1/2 (E), of 3-18 keV positrons and their implantation profile P(z,E) in a total of 13 thin films of atactic polystyrene, poly͑styrene-co-acrylonitrile͒, and polymethylmethacrylate spin coated onto a silicon substrate were determined from positron lifetime measurements using a pulsed, low-energy positron beam. z 1/2 (E) and P(z,E) were determined from the measurement of the ortho-positronium yield obtained from the intensity I 3 of the long lifetime. z 1/2 (E) was parametrized with the commonly used power-law fit z 1/2 (E) ϭ(A/)E n , with and E in units of g cm Ϫ3 and keV, respectively, yielding Aϭ2.81(Ϯ0.2) g cm Ϫ2 and n ϭ1.71(Ϯ0.05). Excellent agreement between amorphous polymer and literature data on Al and Cu suggests that the median implantation depth of positrons for low-to medium-Z materials in the studied energy range is independent of structure and only a function of mass density. Fitting of the Makhovian implantation profile to the experimental data suggested that the value of the parameter m varies between 1.7 and 2.3, systematically increasing with z at constant implantation energy, but is independent of the implantation energy. Using an equation proposed by Baker et al., the experimental data of 12 of the 13 studied polymer films could be described with a slightly better agreement than the Makhovian equation.

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