Self-referenced luminescence thermometry with Sm(3+) doped TiO2 nanoparticles.

The performance of Sm(3+) doped TiO2 nanoparticles for luminescence temperature sensing was tested over a temperature range from room to 110 °C. The Sm(3+) ions were incorporated into TiO2 nanocrystals using hydrolytic sol-gel route. Microstructural characterization of the obtained material was performed using transmission electron microscopy and x-ray diffraction measurements. Luminescence emission spectra of Sm(3+) doped TiO2 nanoparticles consists of two distinct spectral regions: the high energy region associated with the trap emission of the TiO2 host, and the low energy region with well-resolved emission peaks of the Sm(3+) ions. The ratio between Sm(3+) emission and TiO2 trap emission shows strong temperature dependence, and is tested for temperature sensing. The relative sensor sensitivity was found to be higher than 1% °C(-1) over given temperature range with the maximum value of 10.54% °C(-1) at 57.5 °C. Lifetime data derived from the Sm(3+) emission decay revealed that time-resolved measurements provide comparable quality of temperature sensing as corresponding ratiometric measurements, with a maximum relative sensitivity of 10.14% °C(-1) at 66.5 °C.

Mechanism of the enhancement of electrical conductivity of nanocrystalline silicon due to hydrogen plasma treatment.

The mechanism of electrical charge transport in hydrogenated nanocrystalline silicon (nc-Si:H) and the enhancement in electrical conductivity by hydrogen plasma exposure has been studied. Nanoscale electrical conduction measurements (laterally on the surface) suggested that the dominant charge transport in nc-Si:H occurs through the crystalline grain interiors while grain boundaries are highly resistive. Room temperature low-power/short-duration (10 W, 10 s) surface hydrogen plasma treatment enhanced the local surface and bulk electrical conductivity of nc-Si:H films which was attributed to improved passivation of surface and bulk dangling bonds, increase in crystalline fraction and decrease in grain boundary (GB) fraction. However, the improvement in electrical conductivity due to high-power/long-duration (50 W, 10 min) hydrogen plasma exposure was not as pronounced as low-power/short-duration exposure. Temperature-dependent dark conductivity measurements showed dual activation-energy behavior; increase in activation energy in the high-temperature regime (400-585 K) was attributed to the temperature dependence of tunneling probability of carriers and explained using a heteroquantum dots model. A decrease in activation energy with plasma exposure was observed which was explained using the framework of a three-phase model of nc-Si:H where GB width and barrier potential played a critical role in determining the relative contribution of tunneling and thermally activated carrier transport.

A standards-based method for compositional analysis by energy dispersive X-ray spectrometry using multivariate statistical analysis: application to multicomponent alloys.

Given an unknown multicomponent alloy, and a set of standard compounds or alloys of known composition, can one improve upon popular standards-based methods for energy dispersive X-ray (EDX) spectrometry to quantify the elemental composition of the unknown specimen? A method is presented here for determining elemental composition of alloys using transmission electron microscopy-based EDX with appropriate standards. The method begins with a discrete set of related reference standards of known composition, applies multivariate statistical analysis to those spectra, and evaluates the compositions with a linear matrix algebra method to relate the spectra to elemental composition. By using associated standards, only limited assumptions about the physical origins of the EDX spectra are needed. Spectral absorption corrections can be performed by providing an estimate of the foil thickness of one or more reference standards. The technique was applied to III-V multicomponent alloy thin films: composition and foil thickness were determined for various III-V alloys. The results were then validated by comparing with X-ray diffraction and photoluminescence analysis, demonstrating accuracy of approximately 1% in atomic fraction.

Nanoparticle shape and configuration analysis by transmission electron tomography.

Tomographic reconstruction by transmission electron microscopy is used to reveal three-dimensional nanoparticle shapes and the stacking configurations of nanoparticle ensembles. Reconstructions are generated from bright-field image tilt series, with a sample tilt range up to +/- 70 degrees, using single or dual tilt axes. We demonstrate the feasibility of this technique for the analysis of nanomaterials, using appropriate acquisition conditions. Tomography reveals both cubic and hexagonal close-packing configurations in multi-layered arrays of size-selected In nanospheres. By tomography and phase-contrast lattice imaging, we relate the three-dimensional shape of PbSe octahedral nanoparticles to the underlying crystal structure. We also confirm simple-cubic packing in multi-layers of PbSe nanocubes and see evidence that the particle shapes have cubic symmetry. The shapes of TiO(2) nanorod bundles are shown by tomographic reconstruction to resemble flattened ellipsoids.

Zero thermal expansion in a nanostructured inorganic-organic hybrid crystal.

There are very few materials that exhibit zero thermal expansion (ZTE), and of these even fewer are appropriate for electronic and optoelectronic applications. We find that a multifunctional crystalline hybrid inorganic-organic semiconductor, beta-ZnTe(en)(0.5) (en denotes ethylenediamine), shows uniaxial ZTE in a very broad temperature range of 4-400 K, and concurrently possesses superior electronic and optical properties. The ZTE behavior is a result of compensation of contraction and expansion of different segments along the inorganic-organic stacking axis. This work suggests an alternative route to designing materials in a nanoscopic scale with ZTE or any desired positive or negative thermal expansion (PTE or NTE), which is supported by preliminary data for ZnTe(pda)(0.5) (pda denotes 1,3-propanediamine) with a larger molecule.