Engineering topics

Joly, Alan G

Publications and source records attributed to Joly, Alan G.

18 records

DOE OSTI2013

Structure And Radiation Damage Behavior Of Epitaxial CrxMo1-x Alloy Thin Films On MgO

Phenomena related to the interaction of point defects and dopants with grain boundaries and interfaces have been very well documented. However, a quantitative understanding of such an interaction is still missing. In this paper we explore the correlation between radiation damage and interface structure. In doing so, CrxMo1-x(001) films of thickness ~100 nm were epitaxially grown on MgO (001) using molecular beam epitaxy. The interface dislocation density can be systematically varied by controlling the composition of the film. This system allows us to probe the response of the defects generated during the irradiation to the interface dislocation density. The microstructural features of these films before and after irradiation are carefully studied using high resolution scanning/transmission electron microscopy and electron diffraction. It has been found that the film/substrate system is very resistant to ion-induced irradiation damage. No visible point defect clusters, dislocation network or amorphization has been identified, which is contrasted with other materials of either metallic or ionic bonding. We conclude that the defects in the present system appear to be either sub-microscopic and highly dispersed or are annihilated during the irradiation process. Further detailed work is needed to identify the spatial distribution of the defects.

Environmental Molecular Sciences Laboratory↗

DOE OSTI2012

Demonstrated Wavelength Portability of Raman Reference Data for Explosives and Chemical Detection

As Raman spectroscopy continues to evolve, questions arise as to the portability of Raman data: dispersive versus Fourier transform, wavelength calibration, intensity calibration, and in particular the frequency of the excitation laser. While concerns about fluorescence arise in the visible or ultraviolet, most modern (portable) systems use near-infrared excitation lasers, and many of these are relatively close in wavelength. We have investigated the possibility of porting reference data sets from one NIR wavelength system to another. We have constructed a reference library consisting of 145 spectra, including 20 explosives, as well as sundry other compounds and materials using a 1064 nm spectrometer. These data were used as a reference library to evaluate the same 145 compounds whose experimental spectra were recorded using a second 785 nm spectrometer. In 128 cases of 145 (or 88.3% including 20/20 for the explosives) the compounds were correctly identified with a mean 'hit score' of 954 of 1000. Adding in criteria for when to declare a correct match versus when to declare uncertainty, the approach was able to correctly categorize 134 out of 145 spectra, giving a 92.4% accuracy. For the few that were incorrectly identified, either the matched spectra were spectroscopically similar to the target or the 785 nm signal was degraded due to fluorescence. The results indicate that imported data recorded at a different NIR wavelength can be successfully used as reference libraries, but key issues must be addressed: The reference data must be of equal or higher resolution, the systems require rigorous wavelength calibration, and wavelength-dependent intensity response should be accounted for in the different systems.

47 OTHER INSTRUMENTATION↗

DOE OSTI2012

Structural characterization of epitaxial CrxMo1-x alloy thin films

To develop a model system containing regularly-spaced misfit dislocations for studies of the radiation resistance of nanoscale defects, epitaxial thin films of Cr, Mo, and Cr{sub x}Mo{sub 1-x} alloys were deposited on MgO(001) by molecular beam epitaxy. Film compositions were chosen to vary the lattice mismatch with MgO. The film structure was investigated with x-ray diffraction (XRD), Rutherford backscattering spectrometry (RBS), and scanning transmission electron microscopy (STEM). Epitaxial films with reasonably high crystalline quality and abrupt interfaces were achieved at a relatively low deposition temperature, as confirmed by STEM. However, it was found by XRD and RBS in the channeling geometry that increasing the Mo content of the CrMo alloy films degraded the crystalline quality, despite the improved lattice match with MgO. XRD rocking curve data indicated that regions of different crystalline order may be present within the films with higher Mo content. This is tentatively ascribed to spinodal decomposition into Cr-rich and Mo-rich regions, as predicted by the Cr{sub x}Mo{sub 1-x} phase diagram.

36 MATERIALS SCIENCE↗

DOE OSTI2011

Communication: Spectroscopic phase and lineshapes in high-resolution broadband sum frequency vibrational spectroscopy: Resolving interfacial inhomogeneities of "identical" molecular groups

The ability to achieve sub-wavenumber resolution (0.6 cm-1) and large signal-to-noise ratios in high-resolution broadband sum-frequency generation vibrational spectroscopy (HR-BB-SFG-VS) allows for the detailed SFG spectral lineshapes to be used in the unambiguous determination of fine spectral features. Changes in the structural spectroscopic phase in SFG-VS as a function of beam polarization and experimental geometry proved to be instrumental in the identification of an unexpected 2.78 +/- 0.07 cm-1 spectral splitting for two methyl groups at the vapor/dimethyl sulfoxide liquid interface, as well as in the determination of their orientational angles.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗

DOE OSTI2011

Raman Database Considerations for Near-Infrared Systems

For Raman spectroscopy the ability to detect is often limited by the existence and quality of the reference library to which field spectra are compared. Developing such databases is often labor- and resource-intensive; typically the generated data are not transferred to other instruments. Still other considerations may exist for comparing data at visible and ultraviolet excitation wavelengths such as resonance enhancement. However, for the common near-infrared wavelengths of 785, 830, 960, 1047 and 1064 nm where this is normally of a lesser concern, it is logical to consider whether data can be ported from one spectrometer to another so as to obviate the expensive and time-consuming process of generating reference data for each system. The present experiment generated a list of 125 chemical and common substances and formed a database from their corresponding 1064 nm spectra. The same molecules were then measured using a 785 nm system the new spectra were treated as “unknowns” and compared to the 1064 nm database using a commercial search algorithm. We found that at least 108 of the 125 spectra recorded at 785 nm were correctly identified using the search algorithm. For the few that were incorrectly identified, in most cases the spectra were extremely similar or the 785 nm signal was degraded due to fluorescence, as would occur regardless of reference data. Our results indicate that if the spectrometers are properly calibrated on both their wavelength and intensity axes, “foreign” data recorded at a different NIR wavelength can be successfully used as reference libraries

Kunkel, Brenda M↗

DOE OSTI2011

Photochemical Properties, Composition, and Structure in Molecular Beam Epitaxy Grown Fe “Doped” and (Fe,N) Codoped Rutile TiO2(110)

We have investigated the surface photochemical properties of Fe "doped" and (Fe,N) co-doped homoepitaxial rutile TiO2 (110) films grown by plasma assisted molecular beam epitaxy. Fe does not incorporate as an electronic dopant in the rutile lattice, but rather segregates to the film surface. However, co-deposition of Fe with N enhances the solubility of Fe, and DFT calculations suggest that co-dopant complex formation is the driving force behind the enhanced solubility. The co-doped films, in which a few atomic percent of Ti (O) are replaced with Fe (N), exhibit significant disorder compared to undoped films grown under the same conditions, presumably due to dopant-induced strain. Co-doping redshifts the rutile bandgap into the visible. However, the film surfaces are photochemically inert with respect to hole-mediated decomposition of adsorbed trimethyl acetate. The absence of photochemical activity may result from dopant-induced trap and/or recombination sites within the film. This study indicates that enhanced visible light absorptivity in TiO2 does not necessarily result in visible light initiated surface photochemistry.

36 MATERIALS SCIENCE↗

DOE OSTI2011

Plasmonic enhancement of thin-film solar cells using gold-black coatings

Coatings of conducting gold-black nano-structures on commercial thin-film amorphous-silicon solar cells enhance the short-circuit current by 20% over a broad spectrum from 400 to 800 nm wavelength. The efficiency, i.e. the ratio of the maximum electrical output power to the incident solar power, is found to increase 7% for initial un-optimized coatings. Metal blacks are produced cheaply and quickly in a low-vacuum process requiring no lithographic patterning. The inherently broad particle-size distribution is responsible for the broad spectrum enhancement in comparison to what has been reported for mono-disperse lithographically deposited or self-assembled metal nano-particles. Photoemission electron microscopy reveals the spatial-spectral distribution of hot-spots for plasmon resonances, where scattering of normally-incident solar flux into the plane increases the effective optical path in the thin film to enhance light harvesting. Efficiency enhancement is correlated with percent coverage and particle size distribution, which are determined from histogram and wavelet analysis of scanning electron microscopy images. Electrodynamic simulations reveal how the gold-black particles scatter the radiation and locally enhance the field strength.

14 SOLAR ENERGY↗

DOE OSTI2011

Luminescence of Lanthanide-Dimethyl Sulfoxide Compound Solutions

Dimethyl sulfoxide (DMSO) has the ability to penetrate living tissues without causing significant damage. Of foremost importance to our understanding of the possible functions of DMSO in biological systems is its ability to replace some of the water molecules associated with the cellular constituents, or to affect the structure of the omnipresent water. Luminescence probes have been widely used for biological studies such as labeling, imaging and detection. Luminescence probes formed in DMSO may find new applications. Here, luminescence compounds formed by refluxing lanthanide nitrates of Ce, La, Tb, Yb, Nd, Gd and Eu in DMSO are reported and their luminescence properties investigated. Based on their luminescence spectral properties, the compounds can be classified into four classes. For compounds-I with Yb, Ce, and La, the excitation and emission spectra are very broad and their excitation or emission peaks are shifted to longer wavelengths when the monitored emission or excitation wavelength is longer . For compounds-II with Gd and Nd, both the excitation and emission spectra are very broad but their emission wavelengths change little at different excitation wavelengths. For Tb-DMSO as compound-III, both the typical emissions from the f - f transitions of Tb3+ and a broad emission at 445 nm are observed. At low temperatures of reaction, the f - f emissions are dominant, while at high temperatures such as 180 oC of reaction, the broad emission at 445 nm is dominant. For compound-IV with Eu-DMSO compounds, the dominant emissions are from the f - f transitions of Eu3+ and only a weak broad emission is observed, which is likely from the d - f transition of Eu2+ rather than from the metal to ligand charge transfer states.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗

DOE OSTI2011

In Situ Infrared Spectroscopic Study of Forsterite Carbonation in Wet Supercritical CO2

Carbonation reactions are central to the prospect of CO2 trapping by mineralization in geologic reservoirs. In contrast to the relevant aqueous-mediated reactions, little is known about the propensity for carbonation in the long-term partner fluid: water-containing supercritical carbon dioxide (‘wet’ scCO2). We employed in situ mid-infrared spectroscopy to follow the reaction of a model silicate mineral (forsterite, Mg2SiO4) for 24 hr with wet scCO2 at 50°C and 180 atm, using water concentrations corresponding to 0%, 55%, 95%, and 136% saturation. Results show a dramatic dependence of reactivity on water concentration and the presence of liquid water on the forsterite particles. Exposure to neat scCO2 showed no detectable carbonation reaction. At 55% and 95% water saturation, a liquid-like thin water film was detected on the forsterite particles; less than 1% of the forsterite transformed, mostly within the first 3 hours of exposure to the fluid. At 136% saturation, where an (excess) liquid water film approximately several nanometers thick was intentionally condensed on the forsterite, the carbonation reaction proceeded continuously for 24 hr with 10% to 15% transformation. Our collective results suggest constitutive links between water concentration, water film formation, reaction rate and extent, and reaction products in wet scCO2.

54 ENVIRONMENTAL SCIENCES↗

DOE OSTI2011

Structure of Cr Film Epitaxially Grown on MgO (001)

Cr(001) films of thickness ~100 nm were epitaxially grown on MgO (001) using molecular beam epitaxy at both room temperature and 550 °C. The structure of the films was characterized using high resolution transmission electron microscopy, electron diffraction, and electron energy-loss spectroscopy. Room temperature growth leads to highly oriented columnar structured grains in the film, while growth at 550 °C yields high quality single crystal Cr film, possessing a 45 degree rotational epitaxial relationship between the film and the substrate. For the single crystal film, the interface exhibited coherent regions separated by equally-spaced misfit dislocations. When imaged from the MgO[100]//Cr[110] direction, the dislocation spacing is 5.38 nm and 4.64 nm for the films grown at room temperature and 550 °C, respectively, which is slightly smaller than the expected value of 6.25 nm calculated from the differences in lattice spacing based on known lattice constants. The results presented here indicate that the lattice mismatch between the film and the substrate is accommodated mainly by interface misfit dislocations above some critical thickness. The electronic structure of the interface between the film and the substrate is also explored using electron energy-loss spectroscopy.

(001)↗

DOE OSTI2011

Luminescence enhancement of CaZnGe2O6:Tb3+ afterglow phosphors synthesized using ZnO nanopowders

CaZnGe2O6:Tb3+ afterglow phosphors were prepared by solid state reaction using organic coated ZnO 30 nanopowders and their photoluminescence, X-ray luminescence and afterglow properties were investigated. The CaZnGe2O6:Tb3+ samples emit a green luminescence at 548 nm attributed to the 5D4-7F5 transition of Tb3+. It was observed that the replacement of bulk ZnO by ZnO nanopowder in the sample synthesis increases the luminescence intensity. By adjusting the mass ratio of bulk ZnO to nanopowder ZnO, the photoluminescence intensity, X-ray luminescence intensity, and afterglow efficiency are improved. The optimized sample made with a 0.71 ratio of nano ZnO to bulk ZnO has a factor of four enhancement in X-ray luminescence, photoluminescence and afterglow intensities in comparison with the sample made with 100% bulk ZnO.

36 MATERIALS SCIENCE↗

DOE OSTI2011

Exciton-driven highly hyperthermal O-atom desorption from nanostructured CaO

We report qualitatively new highly-hyperthermal (HHT) oxygen atom emission from nanostructured CaO excited by 6.4 eV nanosecond laser pulses. The kinetic energy distribution of emitted O-atoms peaks at 0.7 eV, which is over four times greater than previously observed. Excitation of MgO and CaO nanostructures with UV laser pulses is known to result in thermal and hyperthermal emission of oxygen atoms when photons with energies below and above the band gap, respectively, are used. The highly energetic atomic desorption we observe, following bulk excitation, challenges the conventional view that bulk excitation can only induce thermal desorption. Using density functional theory and an embedded cluster method, we propose a mechanism for this HHT feature based on the interaction of surface holes with bulk excitons. These experimental and theoretical results suggest that specific atomic desorption mechanisms in wide-bandgap materials can be controlled by selective electronic excitation of not only the surface but also the bulk of these materials

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗

DOE OSTI2011

Plasmonic Field Enhancement of Individual Nanoparticles by Correlated Scanning and Photoemission Electron Microscopy

We present results of a combined two-photon photoemission and scanning electron microscopy investigation to determine the electromagnetic enhancement factors of silver-coated spherical nanoparticles deposited on an atomically flat mica substrate. Femtosecond laser excitation, of the nanoparticles, produces intense photoemission, attributed to near-resonant excitation of localized surface plasmons. Enhancement factors are determined by comparing the respective two-photon photoemission yield measured for equal areas between single nanoparticles to that of the surrounding flat surface. For s-polarized, 400 nm (~ 3.1 eV) femtosecond radiation a distribution of enhancement factors are found with a large percentage (77%) of the nanoparticles falling within a median range. A correlated scanning electron microscopy analysis demonstrated that the nanoparticles typifying the median of the distribution were characterized by ideal spherical shapes and defect-free morphologies. The single largest enhancement factors were in contrast produced by a very small percentage (8%) of the total, for which evidence of silver defect anomalies were found that contributed to the overall structure of the nanoparticle. Comparisons are made between the experimentally measured enhancement factors and previously reported theoretical predictions of the localized surface plasmon near-field intensities for isolated nanometer-sized silver spheres.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗

DOE OSTI2011

Observation of green emission from Ce3+ doped gadolinium oxide nanoparticles

Green emission at around 500 nm is observed in Gd2O3:Ce3+ nanoparticles and the intensity is highly dependent on the concentration of Ce3+ in the nanoparticles. The luminescence of this emission displays both picosecond and millisecond lifetimes. The msec lifetime is over four orders of magnitude longer than typical luminescence lifetimes (10-40 ns) of Ce3+ in traditional Ce3+ doped phosphors and therefore likely originates from defect states. The picosecond lifetime is shorter than the typical Ce3+ value and is also likely due to defect or surface states. When the samples are annealed at 700 oC, this emission disappears possibly due to changes in the defect moieties or concentration. In addition, a blue emission at around 430 nm is observed in freshly-prepared Gd2O3 undoped nanoparticles which is attributed to the stabilizer, polyethylene glycol biscarboxymethyl ether. Upon aging, the undoped particles show similar emission to the doped particles with similar luminescence lifetimes. When Eu3+ ions are co-doped in Gd2O3:Ce nanoparticles, both the green emission and the emission at 612 nm from Eu3+ are observed.

36 MATERIALS SCIENCE↗

DOE OSTI2010

Materials Applications of Photoelectron Emission Microscopy

Photoelectron emission microscopy (PEEM) is a versatile technique that can image a variety of materials including metals, semiconductors and even insulators. Under favorable conditions the most advanced aberration corrected instruments have a spatial resolution approaching 2 nm. Although PEEM cannot compete with transmission or scanning electron microscopies for ultimate resolution, the technique is much more gentle and has the unique advantage of imaging structure as well as electronic and magnetic states on the nanoscale. Since the image contrast is derived from spatial variations in electron photoemission intensity, PEEM is ideal for interrogating both static and dynamic electronic properties of complex nanostructured materials. PEEM can be performed using a variety of photoexcitation sources including synchrotron emission, femtosecond laser pulses and conventional UV lamp emission. Each source has advantages, for example, fs laser excitation enables time-resolved imaging for study of ultrafast dynamics of surface intermediate states while tunable synchrotron sources allow chemically specific excitation. Even more detail can be extracted from energy resolved PEEM. Here, we review the key principles and contrast mechanisms of PEEM and briefly summarize materials applications of PEEM with examples of a thermally-induced structural phase transformation in barium titanate, inter-diffusion between thin metal copper and ruthenium layers, and multiphoton imaging of polystyrene nanoparticles on a silver coated substrate.

43 PARTICLE ACCELERATORS↗

DOE OSTI2010

Luminescence Enhancement of CdTe Nanostructures in LaF3:Ce/CdTe Nanocomposites

Radiation detection demands new scintillators with high quantum efficiency, high energy resolution and short luminescence lifetimes. Nanocomposites consisting of quantum dots and Ce3+ doped nanophosphors may be able to meet these requirements. Here we report the luminescence of LaF3:Ce/CdTe nanocomposites which were synthesized by a wet chemistry method. In LaF3:Ce/CdTe nanocomposites the CdTe quantum dots are converted into nanowires, while in LaF3/CdTe nanocomposites no such conversion is observed. The CdTe luminescence in LaF3:Ce/CdTe nanocomposites is enhanced about 5 times, while in LaF3/CdTe nanocomposites no enhancement was observed. Energy transfer, light-re-absorption and surface passivation are likely the reasons for the luminescence enhancement.

36 MATERIALS SCIENCE↗

DOE OSTI2010

Surface-Specific Laser Matter Interactions and Dynamics

Neutral magnesium atom desorption is induced using infrared and visible femtosecond laser pulses at energies well below the excitation threshold of the bulk material. We find that infrared femtosecond laser excitation of MgO nanocrystalline samples desorbs neutral Mg-atoms with hyperthermal kinetic energies ranging from 0.1 to 0.6 eV, while visible femtosecond excitation desorbs neutral Mg-atoms with thermal kinetic energies ranging from 0.01 to 0.1 eV. The hyperthermal kinetic energy distribution is similar to the distribution observed previously under nanosecond laser excitation in the ultraviolet. The hyperthermal kinetic energy distribution supports the contention that emission is induced predominantly by electron trapping at Mg corner surface sites. The results demonstrate that femtosecond photon pulses can serve as a powerful tool for probing desorption dynamics at surfaces.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗

DOE OSTI2010

Effect of Surface Charge on Laser-induced Neutral Atom Desorption

When an ionic metal oxide crystal is cleaved, inhomogeneous electrical charging of the surface can be a result. Such an effect has been well-documented in magnesium oxide (100). For example, recent rigorous AFM studies indicate that nanoscale charged clusters of MgO are created during cleavage, with high concentrations often located at terrace step edges.[1] In addition, ablation processes of freshly cleaved magnesium oxide crystals may be effected by remnant surface charging and microstructures.[2] We report here that such surface charging strongly impacts even neutral atom desorption, even under conditions of extremely mild excitation of surface terrace features. In our experiments, single crystal MgO (100) is cleaved in air and placed in an ultra-high vacuum chamber (UHV). We irradiate the crystal at 6.4 eV, photon energy resonant with five-coordinated (5-C) terrace sites and probe desorbing neutral oxygen atoms. It is found that a significant fraction of desorbed neutral oxygen atoms from the charged surface possess kinetic energies in excess of 0.7 eV. This is in contrast to uncharged samples (discharged in vacuo over 24 hours) that display a near-thermal oxygen atom distribution.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗