Howard W. H. Lee

Photo‐oxidation of electroluminescent polymers studied by core‐level photoabsorption spectroscopy

The C 1s and O 1s core‐level photoabsorption spectra of poly[2‐methoxy,5‐(2′‐ethyl‐ hexoxy)‐1,4‐phenylene vinylene] (MEH‐PPV) before and after exposure O2 and broadband visible light were recorded to determine the degradation pathway for this polymer. The change in the O 1s spectra as a function of exposure demonstrates that the O adds to the polymer chain to form a carbonyl group. Exposure to only O2 or only light causes no change in the C 1s or O 1s spectra. In the C 1s spectra, the change in the dependence on the photon angle of incidence after exposure demonstrates that O attacks the polymer at the double bond in the vinyl group thereby altering the extended conjugation of the polymer.

ULTRAVIOLET-BLUE EMISSION AND ELECTRON-HOLE STATES IN ZNSE QUANTUM DOTS

We observed the quantum-confined band edge emission from ZnSe quantum dots and the size dependence of the energy states, spin-orbit interaction, and Stokes shift. The band edge emission occurs in the ultraviolet blue. The energy gap=Eg+C/dn where d is the diameter and n is 1.19±0.13 and 1.21±0.13 for the first and second electron-hole transitions, respectively. The separation between these transitions approaches the bulk spin-orbit splitting, while the Stokes shift decreases with particle size. Effective mass theories cannot explain these results. Trap emission is observed in some samples in the green and red, resulting from Se-related traps.

Blue luminescence from amorphous GaN nanoparticles synthesized in situ in a polymer

Amorphous GaN nanoparticles were synthesized by the in situ thermal decomposition of cyclotrigallazane incorporated into a polystyrene–poly(N,N-dimethyl-4-vinylaniline) copolymer. Transmission electron microscopy, energy dispersive x-ray spectrometry, and x-ray photoelectron spectroscopy show that the composite material consists of amorphous GaN nanoparticles (average diameter ∼40 nm) well dispersed in the copolymer. The photoluminescence spectra show blue light emission from the amorphous GaN nanoparticles, peaked at ∼426 nm.

SYNTHESIS OF GALLIUM NITRIDE QUANTUM DOTS THROUGH REACTIVE LASER ABLATION

Nanocrystalline GaN was synthesized through reactive laser ablation of gallium metal in a N2 atmosphere. X-ray diffraction, selected-area electron diffraction, and transmission electron microscopy measurements show that the GaN crystallites are as small as 2 nm in diameter, and follow a log-normal size distribution with a mean particle diameter of 12 nm. Size-selective photoluminescence and photoluminescence excitation spectroscopy reveal a continuous range of blueshifted band-edge emissions and absorptions starting from the bulk value for gallium nitride and continuing to below 300 nm. These results are consistent with a GaN particle size distribution that encompasses regions above and below the excitonic-Bohr radius of GaN, such that the GaN material shows combined bulk and quantum confined optical properties.

ZnO nanoparticles embedded in polymeric matrices

Highly stable, wurtzite quantum sized ZnO colloids encapsulated in polymers have been synthesized. The particles can be obtained in a powder form and are partially redissolvable in organic media. A shift in the optical absorption spectrum confirms quantum size effects. Stationary fluorescence measurements exhibit excitonic as well as trapped fluorescence. The intensity of trapped fluorescence changes with capping. The time resolved fluorescence measurements indicate considerably short decay times.

Investigation of optically generated transient electronic defects and protonic transport in hydrogen-bonded molecular solids. Isomorphs of potassium dihydrogen phosphate

Abstract Recent results are reported from our study of a new class of transient electronic defects generated with ultraviolet (UV) photons in hydrogen-bonded molecular solids represented by isomorphs of dihydrogen phosphate. At room temperature, the defect forms 1.6 ± 0.4 ps after UV two-photon excitation, decays nonexponentially in ≈ 20 s, and absorbs from the UV to the near-IR. The defect is identified as a HPO·−4 radical and supports proposed formation and decay mechanisms based on protonic transport. These findings also call into question interpretations of ionizing radiation results.

X-ray photoemission and photoabsorption of organic electroluminescent materials

Thin films of tris-(8, hydroxyquinoline) aluminum (Alq3) and N,N′-diphenyl-N,N′-bis(3-methylphenyl)-1,1′-biphenyl-4,4′-diamine (TPD) were measured using synchrotron radiation-based core and valence level photoemission and core level photoabsorption to elucidate the element-specific electronic structure of organic electroluminescent materials. The energy level alignment of an Alq3/TPD interface is given for both occupied and unoccupied states. A comparison of freshly evaporated films of Alq3 and TPD with films that have been exposed to intense radiation or oxidative conditions sheds light on possible damage mechanisms of the molecular solid.

Accurate thickness/density measurements of organic light-emitting diodes

We report on the use of Rutherford backscattering spectroscopy for thickness analysis of organic light-emitting diode structures (OLEDs) with subnanometer resolution and a spatial resolution <1 mm. A careful study of ion beam induced effects revealed some organic film degradation, but not so severe as to inhibit meaningful measurements. The method is independent of the substrate and is still applicable if the organic film is capped with a metal cathode. Common OLED materials have been the subject of this study: poly(2-methoxy,5-(2′-ethylhexoxy)-1,4-phenylene-vinylene (MEH-PPV), N′,N′-diphenyl-N, N′-bis(3-methylphenyl)-1,1′ biphenyl-4,4′-diamine (TPD), and tris-(8-hydroxyquinoline) aluminum (Alq3). The densities of thin films of evaporated TPD (ρ=1.22±0.05 g/cm3) and Alq3 (ρ=1.51±0.03 g/cm3) have been established.

Observation of quantum confined excited states of GaN nanocrystals

GaN nanocrystals with an average diameter of 4.5 nm±1.6 nm were synthesized by pulsed laser ablation of a gallium metal target in a nitrogen atmosphere. Transmission electron microscopy and selected area electron diffraction confirmed the hexagonal structure and size of the nanocrystals. Optical absorption and photoluminescence spectroscopy revealed quantum confined excited states in the nanocrystalline samples with features at 4.43 eV (280 nm) and 5.47 eV (227 nm).

Solution preparation of Ge nanoparticles with chemically tailored surfaces

Boyd R. Taylor *, Glenn A. Fox , Lousia J. Hope-Weeks , Robert S. Maxwell , Susan M. Kauzlarich , Howard W.H. Lee d a Chemistry and Chemical Engineering Division, Lawrence Livermore National Laboratory, Livermore, CA 94550, USA b Analytical and Nuclear Chemistry Division, Lawrence Livermore National Laboratory, Livermore, CA 94550, USA c Chemistry Department, University of California, One Shields Avenue, Davis, CA 95616, USA d UltraPhotonics Inc., 48611 Warm Springs Blvd. Fremont, CA 94536, USA