Kenneth D. Weston

Room-temperature fluorescence characteristics of single dye molecules adsorbed on a glass surface

Measurements of the total fluorescence and fluorescence spectra from single carbocyanine dye molecules (DiIC12) as a function of time reveal a wide range of phenomena. Discrete jumps in the fluorescence intensity from single molecules on a glass surface have been observed with correlation times spanning several orders of magnitude (1 ms–10 s). We propose a model for these fluctuations in which two or more ground state potential minima are accessed by twists of the chromophore backbone that alter the quantum efficiency of emission as well as the emission spectrum. Monte Carlo simulations based on this model are shown which qualitatively match experimental data. In addition, we observed emission spectra which range in shape from narrow, well-separated vibronic bands to a broad, featureless band. The distribution of emission parameters from different molecules, not obtained from ensemble measurements, indicates an abundance of distinct nanoenvironments of the glass surface sampled by the adsorbed molecules.

Single-molecule detection fluorescence of surface-bound species in vacuum

Abstract We have performed single-molecule fluorescence detection of DiIC 12 molecules adsorbed on a solid substrate where the surrounding pressure is controlled. The triplet lifetimes are significantly longer (4–100 ms) and far more easily observed at moderate vacuum pressures than in air (typically

Mesoscale optical properties of conjugated polymers probed by near-field scanning optical microscopy

Abstract Near-field scanning optical microscopy (NSOM) has been used to study the mesoscale optical properties of oriented and non-oriented free-standing thin films of poly( p -phenylene vinylene) (PPV). We report a strong correlation between film morphology and the optical properties on a 100 nm scale. The non-oriented PPV thin film exhibits mesoscale domains of local molecular orientation as well as areas which are characterized by a low photoluminescence efficiency. Upon stretch orientation, PPV thin films show average polymer chain orientation parallel to the stretch axis as well as distinct areas of perpendicular molecular orientation correlated to film topography.

Near‐field scanning optical microscopy in reflection: A study of far‐field collection geometry effects

We have designed and demonstrated two simple and versatile reflection mode near‐field scanning optical microscopes (NSOMs). In one scanner far‐field collection is coaxial with the NSOM tip, and in the other scanner, the far‐field collection is at a 45° angle to the NSOM tip. We quantitatively compare images obtained with the two scanners. While off‐axis collection offers a significantly higher signal‐to‐noise ratio, it also introduces tip shadowing in samples with topographic features larger than approximately 40 nm. The additional contrast from the shadowing further complicates image interpretation and must be considered when performing NSOM in reflection with off‐axis collection. In addition, we discuss some general issues that should be considered when designing a reflection NSOM.

Near-field scanning optical microscopy of cleaved vertical-cavity surface-emitting lasers

Near-field scanning optical microscopy (NSOM) and near-field optical spectroscopy (NFOS) techniques have been applied to cleaved vertical-cavity surface-emitting lasers (VCSELs). Collection mode NSOM of the cleaved VCSEL operating above the threshold current indicates emission outside the active layer. Using the spatially resolved spectroscopy afforded by NFOS, it was possible to map changes in the electroluminescence spectrum with spatial resolution of the order of 100 nm . The emission spectra acquired with the tip positioned over the p-DBR layers show evidence for p-DBR luminescence. The p-DBR luminescence indicates the presence of minority carriers in this region of the device which have been postulated as a cause of dark line defects (DLDs) which play an important role in device degradation.

Near-field microscopy of collapsed Langmuir-Blodgett films

Fluorescence near-field scanning optical microscopy (FL-NSOM) is used to probe the nanoscale structure in stained phospholipid monolayers deposited on glass substrates at moderate surface pressures. The FL-NSOM images reveal new liquid-expanded (LE) and liquid-condensed (LC) domains, including one-to-one correlation between fluorescence contrast and film topography. In particular, films of the phospholipid DPPC stained with DiIC12 exhibit multilayer structures that are observed within the solid phase domains and have LE-like fluorescence signals. These features are attributed to clusters of dye molecules resulting from the localized collapse of the film upon compression. Such collapsed features are also observed in supported films of 100% DiIC12 deposited at high surface pressure. In these films, spatially-resolved spectroscopy shows that the collapsed structures are amorphous based on the fluorescence spectrum while the molecules within the solid phase of the film have a fluorescence spectrum indicative of molecular aggregates.

Near-field scanning optical microscopy of nanostructures

Abstract The applications of near-field scanning optical microscopy (NSOM) to various nano-structured materials are reviewed. The review begins with a description of experimental aspects of NSOM and the various contrast mechanisms available with the technique. Applications of NSOM to a variety of different materials are then addressed including: NSOM of semiconductor heterostructures, NSOM of polymers and molecular crystals, and NSOM of Langmuir-Blodgett films and layered organic self-assemblies.