Defect Band Intensity Research Articles

Laser‐induced annealing of aged PuO2

AbstractPlutonium dioxide (PuO2) is an important compound used in nuclear fuel, irradiation targets, and heat sources. As such, improved understanding of its structural and spectroscopic properties has numerous applications. Alpha particle‐induced damage of a PuO2 crystal lattice modifies several properties of its Raman spectrum, including band intensities, positions, and widths. The decay also induces growth of new bands and creates electron‐trapped defects with luminescent properties. Herein, we show for the first time that laser‐induced heating can reverse damage to an aged and damaged PuO2 lattice. Using automated instrumentation to heat a single 6–10 μm spot to temperatures above 1,300°C, we show that laser‐induced annealing of aged PuO2 results in restoration of T2g band intensity with a concomitant decrease in defect band intensity, a result that indicates laser annealing can be used to reverse age‐damage in PuO2. This laser annealing approach permitted in situ observation of temperature‐dependent Raman spectral changes, thereby providing insight into the thermodynamics of structural alterations in a radiolytically damaged PuO2.

Oxygen ion conductivity studies of bismuth and bismuth-calcium co-doped ThO2

A series of trivalent (Bi3+) doped and divalent (Ca2+) co-doped ThO2 samples i.e., Th0.50-xCaxBi0.50O2-δ (x = 0.00, 0.05, 0.10, 0.15 and 0.20) have been synthesized by citrate-nitrate solution combustion route and investigated in the context of oxygen ion conductivity and dielectric relaxation phenomena. The Rietveld refinement of the Powder X-ray diffraction data confirmed monophasic fluorite structures (S.G. Fm3‾m) for calcium concentrations up to 20 mol %. The optical band gap decreased with the increase in Ca2+ concentration up to 10 mol %. In contrast, the defect band's intensity in the Raman spectra increased due to oxygen vacancies on divalent addition. The quantitative aspect of oxygen vacancy and defect concentration was derived from Raman spectra. The crystallographic index application was further employed to interpret the optimum doping concentration to maximize oxide ion conductivity. Remarkably high oxide ion conductivity (~10−3 S/cm) was observed for Bi3+ doped (50 mol %), and Bi3+ (50 mol %)-Ca2+ (10 mol %) co-doped ThO2 samples at 773 K. The Nyquist plot exhibited grain contribution for low dopant levels. Both grain and grain boundary contribution were present in the higher dopant concentrations. Conductivity, dielectric, and modulus properties of doped and co-doped samples have been compared, from which 10 mol % of Ca2+-doping was identified to be the optimum concentration.

Low temperature optical sensor based on non-thermally coupled level of Ho3+ and defect level of Zn2+ in Yb3+:Y2Ti2O7 phosphor

In this paper, non-contact optical low temperature sensing behavior is explored between non-thermally coupled level of Ho3+ and the defect level of Zn2+ ion on doping in Yb3+:Y2Ti2O7 phosphor for the first time. The Ho3+/Zn2+ co-doped phosphor prepared by solid state reaction method demonstrates a temperature dependent behavior of emission intensity below the room temperature (25 °C). Under 450 nm (diode laser) excitation, the fluorescence at 552 nm (Green) and 754 nm (NIR) from Ho3+ ions and at 713 nm (NIR) from defect level of Zn2+ ions has been observed and studied at different temperatures in between 300K-100K. An enhancement in the emission intensity of defect band along with Ho3+ ions emission bands is observed on lowering the temperature. The ratio of emission intensity of peaks from Ho3+ level and defect level is considered for temperature sensing by polynomial fitting in FIR technique. The emission intensity of defect band increases with a larger rate than Ho3+ ions emission bands which increase the temperature sensor sensitivity. This paper opens a new field to use defect emission as a low temperature sensor. Thus, the present phosphor may be used as a low temperature sensing device.

Photocatalytic activity of ZnO nanoparticles with optimization of defects

Tuning of defect levels is the major concern for the application of ZnO nanoparticles. These nanoparticles with different defect concentration were prepared by simple combustion method at 700°C and subsequent quenching in air. The catalyst was characterized by X-Ray Diffraction (XRD), Field Emission Scanning Electron Microscope (FESEM), UV–vis spectroscopy measurements. XRD reveals that the cell volume increased with large temperature difference between 700°C and quenched media. FTIR confirmed that the vibrational frequencies shifted from 519cm−1 to 535cm−1 point out the role of quenching defects. FESEM shows the size is of the order of 20–45nm and found irregular distribution of grain boundaries in the quenched induced defects. Different morphology plays a major role in the enhancement of photocatalytic efficiency of synthesized nanoparticles. PL intensity indicates more oxygen vacancies for air quenched ZnO whereas; DTA study demonstrates that the synthesized ZnO is thermally stable. Kinetics study confirmed the photodegradation of methyl orange (MO) dye using this photocatalyst follows first order rate constant. It exhibits the complete degradation of dye only in 150min under UV irradiation. Kinetic study revealed the rate constant of nanoparticles is 0.0165min−1. Air quenched sample showed the higher intensity of defect band as compared to pristine ZnO.

A comparison of Raman and photoluminescence spectra for the assessment of single-wall carbon nanotube sample quality

We compare mass-yields, Raman band intensity ratios and photoluminescence quantum yields (PLQY) of (6,5) single-wall carbon nanotubes (SWNT), produced by alcohol catalytic chemical vapor deposition at reaction temperatures ranging from 600°C to 850°C. The highest PLQYs for suspensions of unprocessed raw material and for density gradient ultracentrifuged (DGU) samples are obtained if SWNTs were grown at 850°C. Accordingly, the Raman defect band intensity ratios of DGU samples decrease toward the highest CVD growth temperatures. In contrast, PLQYs of non-DGU processed, polydisperse raw material suspensions, do not correlate with commonly used Raman D/G defect band intensity ratios.

Evolution and defect analysis of vertical graphene nanosheets

We report catalyst‐free direct synthesis of vertical graphene nanosheets (VGNs) on SiO2/Si and quartz substrates using microwave electron cyclotron resonance – plasma enhanced chemical vapor deposition. The evolution of VGNs is studied systematically at different growth stages. Raman analysis as a function of growth time reveals that two different disorder‐induced competing mechanisms contributing to the defect band intensity. The VGNs grown on SiO2/Si substrates predominantly consists of both vacancy‐like and hopping defects. On the other hand, the VGNs grown on quartz substrates contain mainly boundary‐like defects. X‐ray photoemission spectroscopy studies also corroborate Raman analysis in terms of defect density and vacancy‐like defects for the VGNs grown on SiO2/Si substrates. Moreover, the grown VGNs exhibit a high optical transmittance from 95% to 78% at 550 nm and the sheet resistance varies from 30 to 2.17 kΩ/sq. depending on growth time. Copyright © 2014 John Wiley & Sons, Ltd.

Microwave-assisted synthesis of layered basic zinc acetate nanosheets and their thermal decomposition into nanocrystalline ZnO

We have developed a low-cost technique using a conventional microwave oven to grow layered basic zinc acetate (LBZA) nanosheets (NSs) from a zinc acetate, zinc nitrate and HMTA solution in only 2 min. The as-grown crystals and their pyrolytic decomposition into ZnO nanocrystalline NSs are characterized by scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), atomic force microscopy (AFM), X-ray diffraction (XRD) and photoluminescence (PL). SEM and AFM measurements show that the LBZA NSs have typical lateral dimensions of 1 to 5 μm and thickness of 20 to 100 nm. Annealing in air from 200°C to 1,000°C results in the formation of ZnO nanocrystalline NSs, with a nanocrystallite size ranging from 16 nm at 200°C to 104 nm at 1,000°C, as determined by SEM. SEM shows evidence of sintering at 600°C. PL shows that the shape of the visible band is greatly affected by the annealing temperature and that the exciton band to defect band intensity ratio is maximum at 400°C and decreases by a factor of 15 after annealing at 600°C. The shape and thickness of the ZnO nanocrystalline NSs are the same as LBZA NSs. This structure provides a high surface-to-volume ratio of interconnected nanoparticles that is favorable for applications requiring high specific area and low resistivity such as gas sensing and dye-sensitized solar cells (DSCs). We show that resistive gas sensors fabricated with the ZnO NSs showed a response of 1.12 and 1.65 to 12.5 ppm and 200 ppm of CO at 350°C in dry air, respectively, and that DSCs also fabricated from the material had an overall efficiency of 1.3%.PACS81.07.-b; 62.23.Kn; 61.82.Fk

Influence of Graphite Source on Chemical Oxidative Reactivity

Although all graphites share the same idealized chemical structure, marked differences in fact exist between their reactivities, such as the propensity for oxidation, that need to be taken into consideration for the development of applications. Here we show that five different commercially sourced natural and synthetic graphites differ significantly in their response to a modified Staudenmaier oxidation that produces substoichiometric graphene oxides (sub-GOx). The dominant oxidation product is hydroxyl groups, which can be dehydrate to epoxy groups under mild heating even below 120 °C. The extent of oxidation correlates broadly with the defect band intensity in the starting graphites as measured by Raman spectroscopy. FTIR shows there is a significant concentration of H defects at the % atom level. The results suggest that defects in the graphite plane are more prevalent than previously thought. Finally, the properties of the thermally reduced sub-GOx are also different. The product from the least defective starting graphite ultimately exhibits the lowest activation energies for both electron and hole transport, of the order of 10 μeV below 25 K, that is characteristic of band-like transport. These results are important because they show that the quality of the starting graphite significantly affects the properties of the derived products.

Controlled Hydrogenation of Graphene Sheets and Nanoribbons

The electronic properties of graphene sheets and nanoribbons with different degrees of hydrogenation have been investigated using a combination of charge transport and Raman spectroscopy experiments. The field-effect transistor mobility of graphene is shown to be highly sensitive to the treatment time during atomic hydrogen dose and follows an exponential decrease with time. Raman spectroscopy demonstrates linearly increasing defect-band intensity, and when considered together with transport data, the relationship between graphene mobility and the crystalline size of intact sp(2) carbon regions can be derived. Further, the increase in width of the voltage plateau for monolayer and bilayer graphene points to the formation of midgap states. For partially hydrogenated graphene, the temperature-dependent transport in these states shows a weak insulating behavior. A comparison of Raman spectrum and conductivity data of partially hydrogenated monolayer and bilayer graphene suggests that the latter is also quite susceptible to adsorption of hydrogen atoms, despite a stiffer lattice structure.

Raman spectra of Taunit carbon nanomaterial

Raman spectra of samples of the Taunit carbon nanomaterial (CNM) annealed at different temperatures are studied. The effect that 3–6-nm Pt-catalyst nanoparticles have on the Raman spectra of the Taunit CNM is studied as well. It is found that Taunit is a poorly organized CNM, but crystalline structure ordering has been observed with an increase in the annealing temperature. It is shown that Pt-catalyst precipitation on the surface of the Taunit CNM leads to an increase in the defect band intensity due to the appearance of some crystal lattice defects.

Effects of plasma conditions on properties of ZnO films grown by plasma-assisted molecular beam epitaxy

The influence of rf power and aperture plate configuration on the growth of ZnO thin films using plasma-assisted molecular beam epitaxy was investigated. It was found that by changing the aperture plate geometry from 276 to 25 holes, an increase of growth rate was observed, suggesting that the latter is more efficient for ZnO growth. The structural, electrical, and optical properties were also improved as measured by in situ electron diffraction, single field Hall effect, and photoluminescence. A background carrier concentration of 1×1018cm−3 and a mobility of 52cm2∕Vs were measured, and a room-temperature band edge peak 200 times the defect band intensity was observed. Optical emission spectroscopy shows significantly different behavior in plasma generated using the two plates and suggests that more than increased atomic oxygen production is occurring.

Effect of in situ CdCl2 treatment on spray deposited CdTe∕CdS heterostructure

CdTe thin films have been deposited without and with in situ CdCl2 treatment using spray pyrolysis technique. Scanning electron microscopy studies show enhanced grain growth in the presence of CdCl2. Glancing incidence angle x-ray diffraction is used to observe the microstructural changes of CdTe∕CdS heterostructure at different depths by changing the incident angle. Spraying of CdCl2 on CdS prior to CdTe deposition promotes S diffusion throughout CdTe film and also Te diffusion into CdS. There is an associated change in the microstress of the CdTe film at different layers. The films without CdCl2 treatment show compressive microstress varying from −98to−158MPa with increasing incident angle. CdCl2 spray during CdTe deposition leads to compressive microstress varying from −98MPa at the interface to −19MPa near the surface and CdCl2 spray prior to CdTe deposition leads to a mildly tensile stress (+20to+40MPa). Photoluminescence spectra for CdTe films with the in situ treatment show a reduction in the band gap due to S diffusion as well as the reduction in the defect band intensity. An in situ CdCl2 treatment results in less surface oxidation compared to ex situ process, as seen from x-ray photoelectron spectroscopy study. A shift of ∼200meV in the Fermi energy towards the valence band is also observed in valence band spectra after the in situ CdCl2 treatment.

Proton implantation effects on electrical and optical properties of undoped AlGaN with high Al mole fraction

Electrical and optical properties of undoped n-AlGaN films with Al composition close to 40% were studied before and after implantation of various doses of 100 keV protons. In the virgin samples, the electrical properties were determined by deep donor defects with an energy level near 0.25 eV from the conduction band edge and a concentration of ∼1018 cm−3. Other deep centers present had energy levels of 0.12, 0.3, and 0.45 eV. The luminescence spectra were dominated by two defect bands near 2.3 and 3.6 eV. Proton implantation significantly decreased the concentration of major donors even at the lowest doses of 1012 cm−2. For higher doses the Fermi level became progressively deeper and the data indicated complexing of defects present in the sample with either primary radiation defects or/and hydrogen introduced by implantation. The effect of the proton implantation on the intensity of luminescence bands was complicated but overall the intensity of defect bands was increased with implantation.

Effect of in-situ CdCl2 treatmenton spray deposited CdTe films: Photoluminescence study

In-situ ${CdCl}_2$ treatment for spray deposited CdTe films has been done by spraying a 0.01 M ${CdCl}_2$ solution on CdS substrates. Scanning electron micrographs show the improvement in grain growth with the ${CdCl}_2$ spray. Photoluminescence spectra for films with the in-situ treatment show a shift in the bandgap by 0.06 eV and a reduction in the defect band intensity as compared to those for the films without the treatment. The shift is due to conversion of CdTe into CdTeS by S diffusion from the CdS substrate. Our studies show that an in-situ ${CdCl}_2$ treatment is effective in improving the CdTe thin film properties.

Defect mapping of a synthetic diamond single crystal by cathodoluminescence spectroscopy

Cathodoluminescence (CL) spectroscopy in a scanning electron microscope was used to identify and to map the spatial distribution of luminescent defects in a synthetic diamond single crystal. Several defect CL bands were observed in the 1.5-3.5 eV region: (i) a band with a zero-phonon line at 2.156 eV, attributed to a center containing nitrogen and atomic vacancies; (ii) a broadband centered at ∼2.2 eV, tentatively attributed to a boron-containing center; (iii) a doublet line at 2.33 eV, attributed to a nitrogen-containing center; (iv) a zero-phonon line at 2.555 eV, attributed to a nickel-containing center; (v) a broadband centered at ∼2.85 eV, attributed to a dislocation-related center; and (vi) a zero-phonon line at 3.188 eV, attributed to a center containing nitrogen and a carbon interstitial. Lines due to free and acceptor-bound excitons were observed in the 5.0-5.4 eV region. The spatial variation of the CL was examined in the vicinity of regions of relatively high dislocation density (∼106 dislocations cm−2), which had been found in a previous x-ray diffraction imaging experiment. A quantitative analysis was made of the spatial variation of the band intensities. Upon moving from a relatively defect-free region to the center of a high dislocation density region, the intensities of defect bands (i) and (v) increased by very large factors (these bands were observed only within the high dislocation density regions); the intensity of defect band (vi) increased by a factor of ∼2; the acceptor-bound exciton intensity increased by a factor of 1.3; the intensities of defect bands (ii)-(iv) decreased by a factor of ∼2; and the free exciton intensity decreased by a factor of ∼7.5.

Photoluminescence and microstructural properties of high-temperature annealed buried oxide silicon-on-insulator

The defect properties of buried oxide silicon-on-insulator (SOI) formed by high dose O+ ion implantation and annealed in the temperature range of 1150–1300 °C were examined using photoluminescence (PL) spectroscopy and transmission electron microscopy. The intensity of radiative defect levels at 0.814 and 0.862 eV measured by PL at 4.2 K was observed to decrease with increasing post-implantation annealing temperature. A direct correlation between the radiative defect band intensities and etch defect density in the top silicon layer was observed. The correlation between defect density and PL defect band intensity was further verified by cross-sectional transmission electron microscopy of the top silicon. This letter demonstrates for the first time the correlation between PL defect properties and microstructure of buried oxide SOI. From comparisons with radiative defect centers in oxygen precipitated and plastically deformed silicon, the radiative defect states in SOI silicon are shown to result from residual dislocations in the top silicon layer of the SOI structure.