An anomalous thermalization effect induced by optical excitation of Er 3+ ions in nanocrystals of Y 2 O 2 S is investigated. Due to the absence of low-energy phonon modes in the 20−40 nm crystals we studied, phonon relaxation is restricted, and as a result, the intensity of hot bands that originate from the upper crystal field levels in the 4 I 15/2 ground state increases rapidly as temperature decreases below 8 K. This unusual increase in hot band intensity is interpreted satisfactorily by calculations of temperature dependent multiphonon relaxation rates in nanoparticles.Our theoretical analysis provides a fundamental understanding of confinement effects on the spectroscopic properties of rare earth ions in nanocrystals. This analysis applies also to the previously observed unusual hot bands in 4−6 nm particles of Eu 2 O 3 .

In this paper, a leaf-like porous CuO-graphene nanostructure is synthesized by a hydrothermal method.The as-prepared composite is characterized using XRD, Raman, SEM, TEM and nitrogen adsorptiondesorption. The growth mechanism is discussed by monitoring the early growth stages. It is shown that the CuO nanoleaves are formed through oriented attachment of tiny Cu(OH) 2 nanowires.Electrochemical characterization demonstrates that the leaf-like CuO-graphene are capable of delivering specific capacitances of 331.9 and 305 F g À1 at current densities of 0.6 and 2 A g À1 , respectively. A capacity retention of 95.1% can be maintained after 1000 continuous charge-discharge cycles, which may be attributed to the improvement of electrical contact by graphene and mechanical stability by the layer-by-layer structure. The method provides a facile and straightforward approach to synthesize CuO nanosheets on graphene and may be readily extended to the preparation of other classes of hybrids based on graphene sheets for technological applications. Recently, graphene nanosheets (GNS) based on transition metal oxides 2,3,22 have been studied and are expected to show improved capacitance owing their enhanced electronic conductivity, due to graphene materials possessing rapid electron transfer, high mechanical strength, high elasticity, and

The luminescence dynamics of optical centers in nanocrystals depends critically on the phonon density of states (PDOS), which is quite distinct from that of bulk materials. It is shown that energy transfer (ET) in nanocrystals is confined by discrete PDOS as well as direct size restriction. Temperature-, concentration-, and size-dependence of the fluorescence decay from the S3/24 state of Er3+ in Y2O2S nanocrystals have been investigated using laser spectroscopic experiments and computational simulations. A set of microscopic rate equations that govern the evolution of the excitation probability Pi(t) are solved iteratively using a Monte Carlo method. The simulations of ET based on a theoretical model with five parameters are in good agreement with the experimental results. It is shown that phonon-assisted ET processes in Er3+:Y2O2S nanocrystals contribute partly to the fluorescence decay at 295 K, and is negligible at 5 K. For applications, the nanoconfinement effects on ET may significantly reduce the efficiency of sensitized or upconversion luminescence due to the lack of low-frequency phonon modes and restricted excitation migration in nanophosphors.

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