Philip D. Rack

The structure, device physics, and material properties of thin film electroluminescent displays

Abstract Thin film electroluminescent (TFEL) displays are complex optoelectronic devices with challenging material requirements. The multilayer device structure includes two electrodes (one metallic and one transparent), two insulators, and a semiconducting ‘phosphor’ layer. Each layer has unique electronic and optical properties that must be satisfied for device operation. In this article, we review the device structure, the electrical and optical device physics, and the material properties of TFEL displays. Particular attention is given to the phosphor layer properties and the radiative recombination phenomenon that is responsible for luminescence. The current status of the red, green, blue and white TFEL phosphors is also reviewed, and the current and future applications are discussed.

Self-Assembly versus Directed Assembly of Nanoparticles via Pulsed Laser Induced Dewetting of Patterned Metal Films

A nanoscale, synthetic perturbation was all that was required to nudge a natural, self-assembly process toward significantly higher order. Metallic thin film strips were transformed into nanoparticle arrays by nanosecond, liquid-phase dewetting. Arrays formed according to an evolving Rayleigh-Plateau instability, yet nanoparticle diameter and pitch were poorly controlled. However, by patterning a nanoscale sinusoid onto the original strip edge, a precise nanoparticle diameter and pitch emerged superseding the naturally evolving Rayleigh-Plateau instability.

Effects of atmospheric nonthermal plasma on invasion of colorectal cancer cells

The effect that the gas content and plasma power of atmospheric, nonthermal plasma has on the invasion activity in colorectal cancer cells has been studied. Helium and helium plus oxygen plasmas were induced through a nozzle and operated with an ac power of less than 10 kV which exhibited a length of 2.5 cm and a diameter of 3-4 mm in ambient air. Treatment of cancer cells with the plasma jet resulted in a decrease in cell migrationinvasion with higher plasma intensity and the addition of oxygen to the He flow gas.

Surface characterization and functionalization of carbon nanofibers

Carbon nanofibers are high-aspect ratio graphitic materials that have been investigated for numerous applications due to their unique physical properties such as high strength, low density, metallic conductivity, tunable morphology, chemical and environmental stabilities, as well as compatibility with organochemical modification. Surface studies are extremely important for nanomaterials because not only is the surface structurally and chemically quite different from the bulk, but its properties tend to dominate at the nanoscale due to the drastically increased surface-to-volume ratio. This review surveys recent developments in surface analysis techniques used to characterize the surface structure and chemistry of carbon nanofibers and related carbon materials. These techniques include scanning probe microscopy, infrared and electron spectroscopies, electron microscopy, ion spectrometry, temperature-programed desorption, and atom probe analysis. In addition, this article evaluates the methods used to modif...

On the breakup of patterned nanoscale copper rings into droplets via pulsed-laser-induced dewetting: competing liquid-phase instability and transport mechanisms.

Nanolithographically patterned copper rings were synthesized, and the self-assembly of the rings into ordered nanoparticle/nanodrop arrays was accomplished via nanosecond pulsed laser heating above the melt threshold. The resultant length scale was correlated to the transport and instability growths that occur during the liquid lifetime of the melted copper rings. For 13-nm-thick rings, a change in the nanoparticle spacing with the ring width is attributed to a transition from a Raleigh-Plateau instability to a thin film instability because of competition between the cumulative transport and instability timescales. To explore the competition between instability mechanisms further, we carried out experiments with 7-nm-thick rings. In agreement with the theoretical predictions, these rings break up in both the azimuthal and radial directions, confirming that a simple hydrodynamic model captures the main features of the processes leading to the breakup.

Pulsed laser dewetting of patterned thin metal films: A means of directed assembly

Thin nickel films were patterned into various shapes and treated with a series of laser pulses. The edges and vertices of the patterned shapes act as programable instabilities, which enable directed assembly via dewetting when the laser energy density is above the melting threshold. The pattern formations were monitored as a function of laser pulse and the retraction process was attributed liquid dewetting and a subsequent resolidification. The calculated retraction velocity (83m∕s) and liquid lifetime (12.3ns) were consistent with the measured nickel retraction distances. The vertices of the shapes had an initially larger retraction velocity which was attributed to an additional in-plane curvature.

Laser direct writing of phosphor screens for high-definition displays

A laser-based forward transfer direct writing technique was used to deposit phosphor powder screens for high-resolution display applications. With this technique, called matrix-assisted pulsed-laser evaporation direct write, dense oxide phosphor powders of Y2O3:Eu (red) and Zn2SiO4:Mn (green) were deposited on alumina and polymer substrates. All processing was performed in air at room temperature. Cathodoluminescent measurements showed that the luminous efficiency of the phosphor powders was not degraded by the deposition process. A 6×6 red and green matrix with pixel sizes of 100 μm (250 lines per inch) with a 100 μm spot size is demonstrated; however, with smaller spot sizes this technique is easily scalable to pixel sizes 2500 lines per inch).

Growth and simulation of high-aspect ratio nanopillars by primary and secondary electron-induced deposition

While several studies have suggested that secondary electrons dominate electron beam induced deposition (EBID), we demonstrate that primary electrons (PE’s) contribute significantly to the deposition for nanoscale EBID over the electron beam energy range (500–20keV). High-aspect ratio pillar growth is a signature of EBID; W nanopillar growth on SiO2 substrate yielded a growth rate of 6nms−1 and a nanopillar aspect ratio of ∼50. A simple integration of the primary, secondary, and backscattered electron distributions versus a dissociation cross section for WF6 suggests that all three electron species should contribute to the total volume of the deposited nanopillar, contrary to reports that suggest that secondary electrons dominate the process. A three-dimensional, Monte Carlo simulation including time correlated gas dynamics and species specific deposition was developed to help elucidate which of the relevant electron species, primary (PE’s), secondary (SE’s), and/or backscattered electrons (BSE’s), induce...

Effects of Calcium Codoping on Charge Traps in LSO:Ce Crystals

Experimental studies of Lu2SiO5:Ce (LSO:Ce) crystals codoped with various concentrations of Ca (0, 0.1, 0.2, 0.3, and 0.4 at% in the melt) are presented. The scintillation and optical properties including photoluminescence decay, emission, excitation, absorption, afterglow and thermoluminescence properties are investigated as a function of Ca concentration. Experimental data show Ca codoping significantly reduces the trapped charge population in the crystal matrix. Hence, the scintillation decay time of LSO:Ce is shortened and the afterglow is suppressed. Thermoluminescence studies show a strong correlation between the integrated thermoluminescence intensity and the Ca concentration.

Eu +3 and Cr +3 doping for red cathodoluminescence in ZnGa 2 O 4

Cathodoluminescence emission spectra and photoluminescence (PL) excitation spectra were used to evaluate Eu +3 and Cr +3 as activators for red luminescence in ZnGa 2 O 4 . In the ZnGa 2 O 4 :Eu materials red emission from Eu +3 and blue intrinsic emission were observed. The blue intrinsic emission increased relative the Eu +3 emission with increasing current density and is attributed to preferential current saturation of the Eu +3 activators. In addition, PL excitation measurements revealed that the inefficient energy transfer from the ZnGa 2 O 4 host to the Eu +3 is due to poor spectral overlap between the ZnGa 2 O 4 emission and the Eu +3 absorption. Cr-doping resulted in a saturated red-color, and no host emission was observed over the entire current density regime investigated. The PL excitation of the ZnGa 2 O 4 :Cr revealed good overlap between the ZnGa 2 O 4 host and the Cr +3 absorption. Efficient energy transfer to the Cr +3 activators occurs via multipolar interactions.