Heh-Nan Lin

Nanoscale charge transport in an electroluminescent polymer investigated by conducting atomic force microscopy

We report conducting atomic force microscopy study of nanoscale hole transport in an electroluminescent polymer. Imaging of current variation with a spatial resolution of around 20 nm is achieved. Local current–voltage measurements are performed and the mobilities obtained from space-charge-limited current analysis are two orders of magnitude higher than macroscopic results. A Poole–Frenkel plot shows a clear transition at a field of around 1.5×106 V/cm, which is consistent with reported values of polaron dissociation field.

Nanoscale surface electrical properties of indium–tin–oxide films for organic light emitting diodes investigated by conducting atomic force microscopy

Nanoscale surface electrical properties of indium–tin–oxide films prepared by different cleaning methods for use as anode materials in organic light emitting diodes are studied by conducting atomic force microscopy. It is found that most of the surface area possesses a nonconducting feature, and an ultraviolet-ozone treatment produces the most nonconductive sample. The conducting regions, which distribute randomly and range from 6 to 50 nm in size, are attributed to the existence of Sn-rich oxide by a comparison with reported scanning electron microscopy images. After scanning the tip with a bias of −8 V on the nonconducting regions, oxide decomposition occurs on as-received and wet-cleaning processed samples, whereas no structure change appears on the ozone treated sample. The results indicate that the generation of stable oxide after ozone treatment is one of the origins for improved device performance.

Creating Optical Near-Field Orbital Angular Momentum in a Gold Metasurface

Nanocavities inscribed in a gold thin film are optimized and designed to form a metasurface. We demonstrate both numerically and experimentally the creation of surface plasmon (SP) vortex carrying orbital angular momentum in the metasurface under linearly polarized optical excitation that carries no optical angular momentum. Moreover, depending on the orientation of the exciting linearly polarized light, we show that the metasurface is capable of providing dynamic switching between SP vortex formation or SP subwavelength focusing. The resulting SP intensities are experimentally measured using a near-field scanning optical microscope and are found in excellent quantitative agreements as compared to the numerical results.

Comparative measurements of the piezoelectric coefficient of a lead zirconate titanate film by piezoresponse force microscopy using electrically characterized tips

We report comparative measurements of the piezoelectric coefficient d33 of a lead zirconate titanate (PZT) film by piezoresponse force microscopy based on tip/PZT/electrode and tip/electrode/PZT/electrode configurations. With the use of electrically characterized Au-coated tips, the obtained values are in good agreement and indicate a negligible methodological effect. It is concluded that the d33 can be reasonably determined without the use of a top electrode as long as the electrical quality of the tip is qualified.

Plasmonic properties of a nanoporous gold film investigated by far-field and near-field optical techniques

We report a study of the plasmonic properties of a 20-nm-thick nanoporous Au film by far-field and near-field optical techniques. The film is prepared sequentially by deposition of gold and copper, thermal annealing, and chemical etching, and has randomly distributed nanopores with sizes ranging between 20 and 350 nm. The absorbance of the nanoporous Au film is much higher than that of a plain Au film and can be attributed to the conversion of incident light into surface plasmon polaritons (SPPs). In addition, a broad peak appears at around 630 nm in the scattering spectrum and serves as evidence of hole plasmon resonance. From transmission mode near-field scanning optical microscopy measurements, two types of local field enhancement are observed. One has a small spatial extent of around 200 nm and the other has a large spatial extent of around 1 μm. The two types of enhancement correspond to strong and weak SPP localizations, respectively.

Force modulation microscopy study of phase separation on blend polymer films

We report force modulation microscopy study of phase separation on blend polymer films of ether-type phenylene vinylene based copolymer and cyano-substituted poly(2,5-didecyloxy- p-phenylene vinylene). Three different tips with spring constants of 0.6, 3, and 60 N/m, and various modulation frequencies between 5 and 10 kHz have been employed. It is found that consistent and correct image contrast can be obtained only with the stiffest tip. Theoretical calculation based on Sneddon mechanics also predicts a tip-sample stiffness close to that of the preferred tip. These results suggest strongly that the tip spring constant used in force modulation microscopy should be comparable to (or greater than) the interaction stiffness.

Nanoscale optical imaging on an electroluminescent polymer by conducting atomic force microscopy

We demonstrate optical imaging with a resolution of around 50 nm on an electroluminescent polymer by conducting atomic force microscopy. The results indicate that brighter light emission occurs from asperities on the polymer surface. By comparing surface morphologies of the polymer and the indium tin oxide substrate, it is found that similar asperities exist on both surfaces and the polymer becomes thinner on these locations. Therefore, stronger luminescence intensity from asperities is caused by higher electric field due to reduced polymer thickness. The present method can also be extended to obtain simultaneous optical and electrical transport properties.

Direct observation of surface plasmon vortex and subwavelength focusing with arbitrarily-tailored intensity patterns

Surface plasmon (SP) vortices typically have annular intensity patterns. Here we show that fractional SP vortices with arbitrary, asymmetric intensity patterns can be synthesized via simple geometric arrangements. We provide the direct experimental observations over the existence of fractional SP vortices. Triangular SP vortex intensities are synthesized by circularly polarized plane waves. On the other hand, subwavelength focusing with user-desired patterns is also experimentally demonstrated. The synthesized SP vortex and focusing patterns are experimentally measured using a near-field scanning optical microscope with uncoated fiber probe and are found in good agreements as compared to numerical results.

Controllable fabrication of bent near-field optical fiber probes by electric arc heating

We describe the construction of a high voltage electric arc puller for controllable fabrication of bent near-field optical fiber probes. Various probes with bent angles ranging from 30° to 75° and bent lengths between 600 and 900 μm were successfully produced. The tip diameters achieved are between 100 and 200 nm. These bent type probes can be made into cantilevered probes that can be used for any dynamic mode atomic force microscope, and make the construction of a scanning near-field optical microscope easily attainable.

Conducting atomic force microscopy study of phase transformation in silicon nanoindentation

We report the study of phase transformation in the nanoindentation of Si by conducting atomic force microscopy. Distinctively high current features with a smallest size of around 20nm have been observed and correspond directly to the generated conductive Si-III and/or Si-XII phases under pressure release. Local current-voltage relationships on the high current sites have also been obtained and found to follow the Fowler-Nordheim tunneling equation.