David R. Tallant

Mechanisms behind green photoluminescence in ZnO phosphor powders

We explore the interrelationships between the green 510 nm emission, the free‐carrier concentration, and the paramagnetic oxygen‐vacancy density in commercial ZnO phosphors by combining photoluminescence, optical‐absorption, and electron‐paramagnetic‐resonance spectroscopies. We find that the green emission intensity is strongly influenced by free‐carrier depletion at the particle surface, particularly for small particles and/or low doping. Our data suggest that the singly ionized oxygen vacancy is responsible for the green emission in ZnO; this emission results from the recombination of a photogenerated hole with the singly ionized charge state of this defect.

CORRELATION BETWEEN PHOTOLUMINESCENCE AND OXYGEN VACANCIES IN ZNO PHOSPHORS

By combining electron paramagnetic resonance (EPR), optical absorption, and photoluminescence (PL) spectroscopy, a strong correlation is observed between the green 510 nm emission, the free‐carrier concentration, and the density of singly ionized oxygen vacancies in commercial ZnO phosphor powders. From these results, we demonstrate that free‐carrier depletion at the particle surface, and its effect on the ionization state of the oxygen vacancy, can strongly impact the green emission intensity. The relevance of these observations with respect to low‐voltage field emission displays is discussed.

The short-range structure of zinc polyphosphate glass

Abstract 31 P magic angle spinning nuclear magnetic resonance (MAS-NMR) spectroscopy and Raman spectroscopy have been used to examine the polyhedral arrangements in x ZnO · (1 − x )P 2 O 5 (0.50 ≤ x ≤ 0.71) glasses. The depolymerization of P metaphosphate chains by the addition of ZnO is quantitatively described by the increase in the concentration of Q 1 -phosphate sites, determined from the 31 P MAS-NMR spectra. When x > 0.60, the NMR and Raman spectra exhibit peaks due to Q 0 and Q 2 tetrahedra, indicating that structures disproportionate in glass melts near the pyrophosphate composition. The splitting of the Raman peak due to the Q 1 terminal oxygen stretching mode indicates that a variety of P-O-Zn bonds participate in the polyphosphate glass structure. The complex mixture of P and Zn polyhedra contributes to the glass-forming tendency of the high ZnO (> 60 mol%) compositions.

Thick stress-free amorphous-tetrahedral carbon films with hardness near that of diamond

We have developed a process for making thick, stress-free, amorphous-tetrahedrally bonded carbon (a-tC) films with hardness and stiffness near that of diamond. Using pulsed-laser deposition, thin a-tC films (0.1–0.2 μm) were deposited at room temperature. The intrinsic stress in these films (6–8 GPa) was relieved by a short (2 min) anneal at 600 °C. Raman and electron energy-loss spectra from single-layer annealed specimens show only subtle changes from as-grown films. Subsequent deposition and annealing steps were used to build up thick layers. Films up to 1.2 μm thick have been grown that are adherent to the substrate and have low residual compressive stress (<0.2 GPa). The values of hardness and modulus determined directly from an Oliver–Pharr analysis of nanoindentation experimental data were 80.2 and 552 GPa, respectively. We used finite-element modeling of the experimental nanoindentation curves to separate the “intrinsic” film response from the measured substrate/film response. We found a hardness ...

Raman Spectroscopy Study of the Structure of Lithium and Sodium Ultraphosphate Glasses

Anhydrous binary phosphate glasses containing from 0 to 50 mol% Li2O or Na2O have been prepared and examined by Raman scattering spectroscopy. The unpolarized Raman spectrum of vitreous P2O5 has intense bands near 640 cm−1, attributed to the symmetric stretching mode of POP bridging oxygens, (POP)sym, between Q3 phosphate tetrahedra, and at 1390 cm−1 due to the symmetric stretch of the PO terminal oxygens, (PO)sym. With the addition of alkali oxide to P2O5, a new feature appears in the Raman spectra near 1160 cm−1 indicating the formation of Q2 phosphate tetrahedra with two bridging and two non-bridging oxygens. The increase in relative amplitude of this new (PO2)sym band with increasing modifier content is consistent with a simple depolymerization of the phosphate network. From 20 to 50 mol% alkali oxide, the position of the (PO)sym Raman band decreases by ∼ 130 cm−1 whereas the frequency of the (POP)sym band increases by ∼ 60 cm−1. These frequency shifts are the result of π-bond delocalization on Q3 species that effectively lengthens the PO terminal oxygen bond and strengthens the POP linkages with increasing alkali oxide content. The compositional dependence of the π-bond delocalization on Q3 tetrahedra is described by considering the interconnections between neighboring Q3 and Q2 tetrahedra. The onset of π-bond delocalization on Q3 species corresponds with the anomalous Tg minimum at 20 mol% alkali oxide in alkali ultraphosphate glasses. The increase in Tg between 20 and 50 mol% alkali oxide is attributed to the increased ionic interconnection of what becomes a chain-like phosphate network at higher alkali contents. Finally, the Raman spectra of several alkali ultraphosphate glasses show high frequency shoulders on the Raman bands attributed to the (PO2)sym and (PO2)asym vibrational modes. These shoulders represent the presence of strained structural units, possibly three- or four-membered rings.

Structural design of sealing glasses

Requirements for enhanced component performance and reliability have led to the development of novel glass compositions for a variety of hermetic sealing applications. The development of technologically useful glass compositions was based on an understanding of the relationships between the molecular-level glass structure and important physical properties. The properties of the alkaline earth aluminoborate glasses for lithium batteries are sensitive to changes in B- and Al-coordination number, characterized by solid state nuclear magnetic resonance (NMR) spectroscopy. In general, the most useful compositions have structures that are dominated by tetrahedral Band Al-sites. Mixed alkali aluminophosphate glasses were developed for aluminum electrical connectors. The properties of sodium aluminophosphate glasses depend on the O/P ratio and significant property changes (e.g. maxima in T g and refractive index) occur when O/P exceeds the pyrophosphate limit at 3.5. Associated with these property changes is a decrease in the average Al-coordination number, from six to four, at O/P > 3.5. Raman spectroscopy provides additional information about the aluminophosphate network. Finally, zinc borophosphate glasses are developed for seals in flat panel displays. Boron-11 NMR shows that tetrahedral borons are preferred in xB 2 O 3 (1-x)(PO 3 ) 2 and in yB 2 O 3 (1 - y)Zn 2 P 2 O 7 glasses for x < 0.4 and y < 0.2. Raman spectroscopy reveals the concomitant evolution from a phosphate to a borophosphate network with increasing x and y.

Green photoluminescence efficiency and free-carrier density in ZnO phosphor powders prepared by spray pyrolysis

Abstract Electron paramagnetic resonance, optical absorption, and photoluminescence spectroscopy have been combined to characterize ZnO powders that were prepared by spray pyrolysis. We generally observe a good correlation between the 510 nm green emission intensity and the density of paramagnetic isolated oxygen vacancies. In addition, both quantities increase with free-carrier concentration n e , as long as n e 18 cm −3 . At higher free-carrier concentrations, both quantities decrease. A model is proposed involving the isolated oxygen vacancy as the luminescence center.

The thermal stability of diamond-like carbon

Abstract Diamond-like carbon (DLC) is a potential, low-cost substitute for diamond in certain applications, but little is known of the temperature range over which its desirable properties are retained. We have investigated the stability of DLC films at elevated temperature and high humidity using Raman spectroscopy, Auger electron spectroscopy and thermal desorption analysis. Exposure to boiling water and a hot (225 °C), humid environment does not appear to affect the DLC structure. Thermal desorption analysis detected the onset of hydrogen evolution from DLC in vacuum at 260 °C. Raman spectra show the conversion from DLC to nano-crystalline graphite (“glassy” carbon) beginning at 300 °C in ambient air. Auger spectroscopy confirms the conversion of sp 3 -bonded carbon to sp 2 -bonded carbon above 300 °C. Conversion to nano-crystalline graphite is complete by 450–600 °C in air. The structure and properties of DLC films are expected to be retained up to temperatures of at least 260 °C.

NMR confirmation of strained “defects” in amorphous silica☆

Abstract Solid state 29 Si magic angle sample spinning nuclear magnetic resonance spectroscopy and Raman spectroscopy were used to investigate the local silicon environment and siloxane ring vibrations in amorphous silica gels. Our results unambiguously relate the 608 cm −1 Raman “defect” in a-SiO 2 with reduced SiOSi bond angles indicative of strained 3-membered rings of silicate tetrahedra.

The short-range structure of sodium ultraphosphate glasses

Anhydrous sodium ultraphosphate glasses were prepared with Na2O contents between 0 and 50 mol% and were characterized by several structurally sensitive spectroscopic probes to determine the nature of the phosphate tetrahedra that constitute the short-range glass structure. Solid state 31P magic angle spinning nuclear magnetic resonance (MAS-NMR) spectroscopy reveals that Na2O depolymerizes the branched (Q3) P-O network of P2O5 to form metaphosphate (Q2) sites, in quantitative agreement with Van Wazers ‘chemically simple’ model. X-ray photoelectron spectroscopy reveals that the concomitant increase in non-bridging oxygen with increasing Na2O content is also in quantitative agreement with this structural model. Raman spectroscopic analyses of glasses with approximately 40 mol% Na2O suggest that some intermediate-range order, perhaps associated with strained rings, also exists within the glass network. Strained sites are eliminated when the solid glass is heated to melt temperatures.