Grace M. Credo

EXTERNAL QUANTUM EFFICIENCY OF SINGLE POROUS SILICON NANOPARTICLES

We use a combination of single nanoparticle luminescence and scanning force microscopy to determine the quantum efficiency (QE) of single porous Si nanoparticles and to determine the ratio of luminescent nanoparticles deposited on a silica surface to the total nanoparticles. An estimate of the QE of bulk porous Si based on these data compares favorably to the QE measured experimentally. From this we conclude that the 1% QE of bulk porous Si measured experimentally results primarily from a statistical distribution of high QE quantum-confined Si chromophores.

Probing nanoscale photo-oxidation in organic films using spatial hole burning near-field scanning optical microscopy

Spatial hole burning near-field scanning optical microscopy (SHB–NSOM) is used to locally photopattern three species of organic thin films, poly(2-methoxy, 5-(2′-ethyl hexyloxy)–p-phenylene vinylene) (MEH–PPV), tris-8-hydroxyquinoline aluminum (Alq3) and dye-functionalized polyelectrolyte self-assembled layers, on a 100 nm length scale. In SHB–NSOM the film is illuminated with light from a stationary NSOM tip to induce photo-oxidation. The reduction in the fluorescence yield resulting from this exposure is then mapped using fluorescence NSOM (FL–NSOM). We have examined the localized photo-oxidation as a function of time, position, and environment free from the limits of far-field spatial averaging. In all of the thin film materials studied we find that the long-time diameter of the dark spot is much larger than the tip diameter and is a signature of energy migration. Characteristic lengths of the energy migration are extracted from this data by a simple diffusion model and are found to be of the order of ...

Nanoscale photophysics of Alq3 films

We use near-field scanning optical microscopy (NSOM) and spectroscopy to probe the photophysics of thin films of molecular semiconductor tris--hydroxyquinoline aluminum (Alq 3 ) on a 10-100 nm scale. Photoluminescence NSOM is used to simultaneously map variations in emission and morphology of both solution-cast and vacuum-deposited films. Vacuum-deposited films annealed at 200°C (T > T g ) exhibit greater fluorescence and topographical contrast than films annealed at 100°C (T<T g ), with the most dramatic changes occurring in films less than 50 nm thick. These results indicate the formation of microcrystalline domains in annealed Alq3 films, which vary with annealing temperature and film thickness. We have also used NSOM to locally photo-pattern films on a 100 nm scale and to directly measure exciton diffusion. Our results indicate very long diffusion lengths in comparison to indirect methods.

Near‐Field Fluorescence Microscopy of Tris‐8‐hydroxyquinoline Aluminum Films

We use near-field scanning optical microscopy (NSOM) to probe the local optical and morphological properties in drop-cast, spin-cast, and vacuum-deposited tris-8-hydroxyquinoline aluminum (Alq3) films with 10±100 nm resolution, the length scale of many interesting structural domains. We use concurrent shear force microscopy (an analog to atomic force microscopy, AFM) to correlate morphology (different regions) to intensity variations in our fluorescence images as well as variations in the localized fluorescence spectra. Our studies show that drop-cast films have the largest surface roughness and most spatial variation in film fluorescence while vacuum-deposited films have the smallest surface roughness and the least spatial variation in film fluorescence. Thin films of the luminescent organic semiconductor Alq3 have been widely studied due to their tremendous potential as the active layer in organic light-emitting devices (LEDs). In contrast to the majority of conventional inorganic materials (III±V semiconductors) used in current generation LEDs, a large number of organic molecular semiconductors, such as Alq3, have extremely high fluorescence efficiencies in the blue and green visible region. Large-scale production and purification of such materials is likely to be less costly than for current inorganic-based LEDs. The use of a thin film of Alq3 lmax = 550 nm, green) as the emissive layer in a practical electroluminescent device constructed from layered organic materials was first demonstrated in 1987. Despite the numerous spectroscopy techniques applied to Alq3 films, the dependence of its nanoscale optical properties on film morphology, particularly on a submicrometer level, remains poorly understood. Previous studies rely on far-field spectroscopy techniques that average over many morphological domains or on topographical images taken without the benefits of concurrent optical probing. In this work, we apply NSOM to directly probe the nanoscale (10±100 nm) optical properties of Alq3. NSOM allows optical microscopy and spectroscopy with greater than 100 nm spatial resolution, a length scale where many of the optical and transport properties of Alq3 are defined. [10] In NSOM, a sub-wavelength (10±200 nm) aperture is placed in close proximity to the surface of interest (near-field region »10 nm) and the interaction between laser light passing through the aperture and the sample is limited to the aperture diameter. If the aperture is maintained in the near-field and scanned over a sample surface, an image can be reconstructed point by point with spatial resolution limited by the aperture diameter rather than by the wavelength of light (the diffraction limit, l/2). The combination of the proximity of the tip to the sample and small tip aperture in NSOM has made it possible to achieve ultrahigh spatial resolution. In our NSOM apparatus, depicted in Figure 1, a tapered fiber optic tip with laser light coupled to it is fixed while the sample, mounted film-side down on a piezo tube