Federico A. Rabuffetti

Controlled Growth of Platinum Nanoparticles on Strontium Titanate Nanocubes by Atomic Layer Deposition

With an eye toward using surface morphology to enhance heterogeneous catalysis, Pt nanoparticles are grown by atomic layer deposition (ALD) on the surfaces of SrTiO(3) nanocubes. The size, dispersion, and chemical state of the Pt nanoparticles are controlled by the number of ALD growth cycles. The SrTiO(3) nanocubes average 60 nm on a side with {001} faces. The Pt loading increases linearly with Pt ALD cycles to a value of 1.1 x 10(-6) g cm(-2) after five cycles. Scanning electron microscopy images reveal discrete, well-dispersed Pt nanoparticles. Small- and wide-angle X-ray scattering show that the Pt nanoparticle spacing and size increase as the number of ALD cycles increases. X-ray absorption spectroscopy shows a progression from platinum(II) oxide to metallic platinum and a decrease in Pt--O bonding with an increase in Pt--Pt bonding as the number of ALD cycles increases.

Infrared to-visible upconversion luminescence in Er:Yb:SrFBr nanocrystals

The ability of SrFBr nanocrystals codoped with ∼1 mol% of Er3+ and Yb3+ to perform infrared-to-visible light upconversion is demonstrated. The average and local crystal structures conform to those expected on the basis of the P4/nmm tetragonal space-group. Excitation of Yb3+ at 980 nm results in two-photon upconversion and green-yellow luminescence from Er3+. The 2H11/2, 4S3/2, and 4F9/2 excited states of the Er3+ activator in the SrFBr host exhibit a biexponential decay with lifetimes in the 100–200 and 300–400 μs ranges.

NIR-to-NIR and NIR-to-blue light upconversion in stoichiometric NaYb(MO4)2 (M = Mo, W)

Polycrystalline NaYb(MoO4)2 and NaYb(WO4)2 exhibited NIR-to-NIR and NIR-to-blue light upconversion under 973 nm excitation. The emission spectra were dominated by a strong NIR (∼795 nm) band. Blue (∼475 nm), green (∼525 and 545 nm), and red (∼650 nm) bands were also observed. The origin of these bands was investigated using a combination of steady-state and time-dependent spectrofluorometry, elemental analysis, and Rietveld analysis of synchrotron X-ray diffraction data. The strong NIR emission at ∼795 nm was assigned to two-photon upconversion from Yb3+-sensitized Tm3+, which was found to be present at trace levels (∼1 ppm) in both NaYb(MoO4)2 and NaYb(WO4)2. Due to the high efficiency of the energy-transfer from Yb3+ to Tm3+, the intensity of the NIR emission exhibited a linear dependence on the excitation power. Green and red bands were assigned to two-photon upconversion from Yb3+-sensitized Er3+, which was also found to be present at trace levels in both hosts (∼1 ppm). In the case of the blue emission, power-dependence and time-dependent spectrofluorometric studies favored cooperative luminescence of Yb3+–Yb3+ dimers, rather than three-photon upconversion from Yb3+-sensitized Tm3+. Local clustering of Yb3+ ions yielding Yb3+–Yb3+ dimers that interact cooperatively under NIR excitation was feasible considering (i) the intrinsic disorder of Na+ and Yb3+ over the same crystallographic site, and (ii) the shortest distance between two adjacent Yb3+ ions (∼3.81 A). The significance of probing energy-transfer processes relevant to light absorption and emission in fully concentrated metalate hosts is highlighted.

n-Type PbSe Quantum Dots via Post-Synthetic Indium Doping

We developed a postsynthetic treatment to produce impurity n-type doped PbSe QDs with In3+ as the substitutional dopant. Increasing the incorporated In content is accompanied by a gradual bleaching of the interband first-exciton transition and concurrently the appearance of a size-dependent, intraband absorption, suggesting the controlled introduction of delocalized electrons into the QD band edge states under equilibrium conditions. We compare the optical properties of our In-doped PbSe QDs to cobaltocene treated QDs, where the n-type dopant arises from remote reduction of the PbSe QDs and observe similar behavior. Spectroelectrochemical measurements also demonstrate characteristic n-type signatures, including both an induced absorption within the electrochemical bandgap and a shift of the Fermi-level toward the conduction band. Finally, we demonstrate that the In3+ dopants can be reversibly removed from the PbSe QDs, whereupon the first exciton bleach is recovered. Our results demonstrate that PbSe QDs can be controllably n-type doped via impurity aliovalent substitutional doping.