Bang-Ying Yu

Depth Profiling of Organic Films with X-ray Photoelectron Spectroscopy Using C60+ and Ar+ Co-Sputtering

By sputtering organic films with 10 kV, 10 nA C60+ and 0.2 kV, 300 nA Ar+ ion beams concurrently and analyzing the newly exposed surface with X-ray photoelectron spectroscopy, organic thin-film devices including an organic light-emitting diode and a polymer solar cell with an inverted structure are profiled. The chemical composition and the structure of each layer are preserved and clearly observable. Although C60+ sputtering is proven to be useful for analyzing organic thin-films, thick organic-devices cannot be profiled without the low-energy Ar+ beam co-sputtering due to the nonconstant sputtering rate of the C60+ beam. Various combinations of ion-beam doses are studied in this research. It is found that a high dosage of the Ar+ beam interferes with the C60+ ion beam, and the sputtering rate decreases with increasing the total ion current. The results suggest that the low-energy single-atom projectile can disrupt the atom deposition from the cluster ion beams and greatly extend the application of the cluster ion-sputtering. By achievement of a steady sputtering rate while minimizing the damage accumulation, this research paves the way to profiling soft matter and organic electronics.

Effect of fabrication parameters on three-dimensional nanostructures of bulk heterojunctions imaged by high-resolution scanning ToF-SIMS.

Solution processable fullerene and copolymer bulk heterojunctions are widely used as the active layers of solar cells. In this work, scanning time-of-flight secondary ion mass spectrometry (ToF-SIMS) is used to examine the distribution of [6,6]phenyl-C61-butyric acid methyl ester (PCBM) and regio-regular poly(3-hexylthiophene) (rrP3HT) that forms the bulk heterojunction. The planar phase separation of P3HT:PCBM is observed by ToF-SIMS imaging. The depth profile of the fragment distribution that reflects the molecular distribution is achieved by low energy Cs(+) ion sputtering. The depth profile clearly shows a vertical phase separation of P3HT:PCBM before annealing, and hence, the inverted device architecture is beneficial. After annealing, the phase segregation is suppressed, and the device efficiency is dramatically enhanced with a normal device structure. The 3D image is obtained by stacking the 2D ToF-SIMS images acquired at different sputtering times, and 50 nm features are clearly differentiated. The whole imaging process requires less than 2 h, making it both rapid and versatile.

Tuning the surface potential of gold substrates arbitrarily with self-assembled monolayers with mixed functional groups.

Alkanethiol anchored self-assembled monolayers (SAMs) on gold are widely used to immobilize and detect molecules including DNA and proteins. Most of these molecules are covalently bonded with the SAM on the Au surface and cannot be released easily. By using different functional groups, the interfacial charge of SAMs can be selected, and thus, they can be considered as adaptors for immobilizing and releasing materials selectively through electrostatic interaction under given conditions. In this work, as an additional factor to control the surface charge, SAMs with mixed functional groups are presented, and it is demonstrated that the isoelectric point (IEP) can be tailored by the ratio of functional groups. Using carboxylic acid- and amine-SAM on gold substrates as an example, isoelectric points (IEPs) from 3.5 to 6.5 can be obtained arbitrarily. The ratio between the functional groups on the surface was quantified by X-ray photoelectron spectrometry (XPS) and was found to be slightly different from the deposition solution. The homogeneous spatial distribution of the functional groups was determined with scanning electrical potential microscopy (SEPM). The interfacial charge of SAMs with mixed functional groups on gold was investigated by electrokinetic analysis in aqueous electrolyte solutions.

Tailoring the surface potential of gold nanoparticles with self-assembled monolayers with mixed functional groups

Self-assembled monolayer (SAM)-modified gold nanoparticles can be used to immobilize and transport molecules including DNA and proteins. However, these molecules are usually covalently bound to the surface and chemical reactions are required to cleave and release them. Therefore, immobilizing molecules using electrostatic interactions might be beneficial. In this work, Au nanoparticles modified by SAMs with mixed carboxylic acid and amine functional groups are presented. The surface potential and the iso-electric point (IEP) of the nanoparticles can be tailored by the ratio of these functional groups and arbitrary IEPs between 3.2 and 7.3 can be achieved. As a result, based on electrostatic interactions, molecules could be triggered to adsorb/desorb by changing the environmental pH around this tunable IEP. These engineered nanoparticles were synthesized in a single-phase system based on the reduction of HAuCl4 by NaBH4 in ethanol with a mixture of 16-mercaptohexadecanoic acid and 8-amino-1-octanethiol that forms the SAM on the synthesized nanoparticles. Transmission electron microscopy, X-ray photoelectron spectroscopy, and electrophoresis light scattering revealed the particle size, ratio of the functional groups, and zeta-potential of the particles as a function of pH, respectively.

Template-based fabrication of SrTiO3 and BaTiO3 nanotubes.

In response to the growing need for metal oxide nanotubes and nanowires for nanoelectronic applications, polycrystalline titanate nanotubes are synthesized in this work at near-ambient conditions without the application of an external electric field or pre-existing solids. Nanotubes of complicated metal oxides including strontium titanate and barium titanate are fabricated inside anodic aluminum oxide (AAO) templates from aqueous solutions using a simple, inexpensive, reproducible, and environmentally friendly procedure. The deposition solution is prepared by dissolving ammonium hexafluorotitanate and strontium nitrate in a boric acid solution at a pH of 2.5. The typical lengths of SrTiO(3) nanotubes are 5-30 microm, with an average diameter of approximately 250 nm, which is defined by the pore diameter of the AAO template. After annealing at 800 degrees C in air, the resulting nanotubes are polycrystalline cubic SrTiO(3). The Sr:Ti ratio in the nanotube is controlled by the hydrolysis of TiF(6)(2-) ions, and the concentration of Sr(2+) and stoichiometric SrTiO(3) nanotubes can be obtained. As an additional controlling factor, the surface properties of the AAO can be modified by (octadecyl)trichlorosilane. Barium titanate is also prepared in a similar manner with barium nitrate and ammonium hexafluorotitanate as precursors. The polycrystalline cubic BaTiO(3) nanotubes are 12-30 microm long and approximately 250 nm in diameter.

Effect of Fabrication Parameters on Three-Dimensional Nanostructures and Device Efficiency of Polymer Light-Emitting Diodes

By using 10 kV C(60)(+) and 200 V Ar(+) ion co-sputtering, a crater was created on the light-emitting layer of phosphorescent polymer light-emitting diodes, which consisted of a poly(9-vinyl carbazole) (PVK) host doped with a 24 wt % iridium(III)bis[(4,6-difluorophenyl)pyridinato-N,C(2)] (FIrpic) guest. A force modulation microscope (FMM) was used to analyze the nanostructure at the flat slope near the edge of the crater. The three-dimensional distribution of PVK and FIrpic was determined based on the difference in their mechanical properties from FMM. It was found that significant phase separation occurred when the luminance layer was spin coated at 30 degrees C, and the phase-separated nanostructure provides a route for electron transportation using the guest-enriched phase. This does not generate excitons on the host, which would produce photons less effectively. On the other hand, a more homogeneous distribution of molecules was observed when the layer was spin coated at 60 degrees C. As a result, a 30% enhancement in device performance was observed.

Three-Dimensional Nanoscale Imaging of Polymer Bulk-Heterojunction by Scanning Electrical Potential Microscopy and C60+ Cluster Ion Slicing

Solution-processable fullerene and copolymer bulk-heterojunctions are widely used as the active layer of solar cells. It is known that the controlled phase-separation in the film provides a pathway for carrier transportation and is crucial to efficiency. In this work, scanning electrical potential microscopy (SEPM) is used to examine the surface distribution of [6,6]phenyl-C61-butyric acid methyl ester and poly(3-hexylthiophene), which form the bulk-heterojunction. Because the two components have different energies in the highest occupied molecular orbital (HOMO), the differences in contact potential yield strong contrast in SEPM. A cluster ion beam (C(60)(+)) is used to remove the surface in order to determine the structure below, and SEPM is used to analyze the newly exposed surface. With the SEPM images acquired from different depth through the material stacked, a 3D volume image is obtained. It is demonstrated that using SEPM with cluster ion slicing is an effective tool for studying the 3D nanostructures of soft materials.

Influence of as on the Morphologies and Optical Characteristics of GaSb/GaAs Quantum Dots

The influence of As atoms on the morphologies of GaSb quantum dots (QDs) is investigated. Without any special treatment, GaSb quantum rings (QRs) are observed in the embedded GaSb layer even when the uncapped layer reveals QD like morphologies. With intentional As supply after the uncapped GaSb QD deposition, a QD to QR transition is observed. The phenomenon suggests that insufficient Sb atoms on the GaSb QDs would lead to the QD to QR transition as in the case of embedded GaSb layers. With extended Sb soaking time following GaSb deposition, QD structures could be well maintained for the embedded GaSb layers. A light-emitting diode operated at room temperature is fabricated based on the GaSb/GaAs QD structure. Identical peak positions in photoluminescence and electroluminescence (EL) spectra of the device show that type-II GaSb QDs are responsible for the observed EL.