Rudy Schlaf

Effect of ultraviolet and x-ray radiation on the work function of TiO2 surfaces

The work functions of nanocrystalline anatase (TiO2) thin films and a rutile single crystal were measured using photoemission spectroscopy (PES). The nanocrystalline titanium dioxide films were deposited in-vacuum using electrospray thin film deposition. A comparison between ultraviolet photoemission spectroscopy (UPS) and low intensity x-ray photoemission spectroscopy (LIXPS) work function measurements on these samples revealed a strong, immediate, and permanent work function reduction (>0.5u2002eV) caused by the UPS measurements. Furthermore, it was found that regular XPS measurements also reduce the work function after exposure times ranging from seconds to minutes. These effects are similar in magnitude to artifacts seen previously on indium tin oxide (ITO) substrates characterized with XPS and UPS, and are likely related to the formation of a surface dipole through the photochemical hydroxylation of oxygen vacancies present on the TiO2 surface.

Characterization of indium tin oxide surfaces and interfaces using low intensity x-ray photoemission spectroscopy

Ultraviolet photoemission spectroscopic (UPS) and x-ray photoemission spectroscopic (XPS) characterizations of indium tin oxide (ITO) surfaces prepared in ambient environment significantly lower the work function of the ITO surface. This artifact complicates the investigation of ITO surfaces and interfaces using XPS and UPS. The presented results demonstrate that, while the exposure of the sample surface to standard UPS UV sources results in a reduction of the work function within a second or less, XPS measurements show a more gradual work function change over the course of hundreds of seconds. This allowed the design of a measurement protocol based on low intensity x-ray photoelectron spectroscopy work function measurements, which do not cause significant work function changes during the exposure time needed for characterization. Applying this technique, the orbital lineup between the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital of the semiconducting polymer poly(3-he...

Efficient Mercury Capture Using Functionalized Porous Organic Polymer

The primary challenge in materials design and synthesis is achieving the balance between performance and economy for real-world application. This issue is addressed by creating a thiol functionalized porous organic polymer (POP) using simple free radical polymerization techniques to prepare a cost-effective material with a high density of chelating sites designed for mercury capture and therefore environmental remediation. The resulting POP is able to remove aqueous and airborne mercury with uptake capacities of 1216 and 630 mg g-1 , respectively. The material demonstrates rapid kinetics, capable of dropping the mercury concentration from 5 ppm to 1 ppb, lower than the US Environmental Protection Agencys drinking water limit (2 ppb), within 10 min. Furthermore, the material has the added benefits of recyclability, stability in a broad pH range, and selectivity for toxic metals. These results are attributed to the materials physical properties, which include hierarchical porosity, a high density of chelating sites, and the materials robustness, which improve the thiol availability to bind with mercury as determined by X-ray photoelectron spectroscopy and X-ray absorption fine structure studies. The work provides promising results for POPs as an economical material for multiple environmental remediation applications.

Work function measurements on nano-crystalline zinc oxide surfaces

The work function of nano-crystalline zinc oxide (ZnO) thin films was examined using photoemission spectroscopy (PES). Colloidally dispersed ZnO nano-particles were electrospray-deposited in vacuum to form nano-crystalline thin films. The samples showed an immediate work function reduction by 0.35u2009eV during ultraviolet photoemission spectroscopy (UPS) measurements. This artifact was detected and quantified through low intensity x-ray photoemission spectroscopy (LIXPS) measurements, which use a very low photon flux. This prevented significant photochemical changes on the measured surface, i.e. the true work function unaffected by the UPS artifact can be measured. Annealing of an identical sample removed all ambient contamination from the ZnO surface with the effect to prevent the work function lowering artifact. This allowed the conclusion that ambient contamination is essential for the artifact to occur, similar to what was observed earlier on indium tin oxide and TiO2 surfaces. In an additional experimen...

Charge Transfer through Modified Peptide Nucleic Acids

We studied the charge transfer properties of bipyridine-modified peptide nucleic acid (PNA) in the absence and presence of Zn(II). Characterization of the PNA in solution showed that Zn(II) interacts with the bipyridine ligands, but the stability of the duplexes was not affected significantly by the binding of Zn(II). The charge transfer properties of these molecules were examined by electrochemistry for self-assembled monolayers of ferrocene-terminated PNAs and by conductive probe atomic force microscopy for cysteine-terminated PNAs. Both electrochemical and single molecular studies showed that the bipyridine modification and Zn(II) binding do not affect significantly the charge transfer of the PNA duplexes.

Electronic structure of indium tin oxide/nanocrystalline TiO2 interfaces as used in dye-sensitized solar cell devices

Dye-sensitized solar cells are typically prepared under ambient conditions and contamination is inevitably introduced during the fabrication process. Hence, the electronic structure and charge injection properties of the indium tin oxide (ITO)/nanocrystalline titanium dioxide (TiO2) interface was studied by photoemission spectroscopy (PES) in the presence of environmental contaminants. The interface was formed by in situ multi-step electrospray thin film deposition of TiO2 nanoparticles onto ITO substrates cleaned prior in solvent under ambient conditions. In between deposition steps, the samples were characterized with PES yielding the band line-up at the ITO/TiO2 interface. In addition, the band line-up before and after annealing of the TiO2 layer was determined. The results of these measurements have in common that there are only small charge injection barriers between the valence bands of the oxides (∼0–0.2 eV), but more significant barriers for electron injection from TiO2 to ITO (∼0.3–0.5 eV), which...

Determination of the charge neutrality level of poly(3-hexylthiophene)

The Al/poly(3-hexylthiophene) (P3HT) and Ag/P3HT interfaces were investigated using photoemission spectroscopy in combination with in situ thin-film deposition. The P3HT thin films were deposited directly into high vacuum from solution on the two metal substrates using an electrospray system and characterized via photoemission spectroscopy. The electronic structure and charge injection barriers at these interfaces were determined from the evaluation of the resulting spectra sequences. A linear correlation between barrier heights and substrate work functions was observed from the collected data in combination with previously published results, suggesting that the Induced Density of Interfaces States model for small molecular materials is also valid for conjugated polymer interfaces. The corresponding P3HT screening factor as well as its charge neutrality level was determined to be 0.48 and 3.44 eV, respectively.

A ZnO nanowire bio-hybrid solar cell

Harvesting solar energy as a carbon free source can be a promising solution to the energy crisis and environmental pollution. Biophotovoltaics seek to mimic photosynthesis to harvest solar energy and to take advantage of the low material costs, negative carbon footprint, and material abundance. In the current study, we report on a combination of zinc oxide (ZnO) nanowires with monolayers of photosynthetic reaction centers which are self-assembled, via a cytochrome c linker, as photoactive electrode. In a three-probe biophotovoltaics cell, a photocurrent density of 5.5 μA cm-2 and photovoltage of 36 mV was achieved, using methyl viologen as a redox mediator in the electrolyte. Using ferrocene as a redox mediator a transient photocurrent density of 8.0 μA cm-2 was obtained, which stabilized at 6.4 μA cm-2 after 20 s. In-depth electronic structure characterization using photoemission spectroscopy in conjunction with electrochemical analysis suggests that the fabricated photoactive electrode can provide a proper electronic path for electron transport all the way from the conduction band of the ZnO nanowires, through the protein linker to the RC, and ultimately via redox mediator to the counter electrode.

The electronic structure of co-sputtered zinc indium tin oxide thin films

Zinc indium tin oxide (ZITO) transparent conductive oxide layers were deposited via radio frequency (RF) magnetron co-sputtering at room temperature. A series of samples with gradually varying zinc content was investigated. The samples were characterized with x-ray and ultraviolet photoemission spectroscopy (XPS, UPS) to determine the electronic structure of the surface. Valence and conduction bands maxima (VBM, CBM), and work function were determined. The experiments indicate that increasing Zn content results in films with a higher defect rate at the surface leading to the formation of a degenerately doped surface layer if the Zn content surpasses ∼50%. Furthermore, the experiments demonstrate that ZITO is susceptible to ultraviolet light induced work function reduction, similar to what was earlier observed on ITO and TiO2 films.

Enhanced simulation of an RF ion funnel including gas turbulence.

Electrodynamic ion funnels are used to enhance the transmission of ions in electrospray-based ion injection systems in 0.1 to 30u2009Torr pressure range. Jet disrupters are commonly employed to prevent droplets and high pressure jets from entering subsequent vacuum regions. This study presents the simulation and testing of an ion funnel containing a jet disrupter using computational fluid dynamics (CFD) and SIMION ion trajectory simulations. Traditional modeling approaches have utilized approximations for the bulk fluid flow fields without including the time-varying nature of the turbulent flow present in the system, thus yielding idealized results. In this study, the fluid flow fields are calculated using CFD. In an effort to include time dependence, a random velocity vector, whose magnitude is proportional to the square root of the turbulence kinetic energy, was calculated at each time step and added to the velocity of the background gas. These simulations predicted that the transmitted ion current is effectively modulated by the variation of the jet disrupter voltage. The addition of the random velocity vector produced results that closely matched the experiments. The simulations yielded the dependence of the transmission on the jet disrupter voltage, and the voltage necessary for maximum ion throughput was accurately predicted. In addition, the magnitude of the predicted transmission closely matched that of the experimental results. This modeling approach could be extended to similar ion transport and filtering systems in which the effects of turbulent fluid flow cannot be ignored.