Blog
The above images show the transition of our UHV system from its start as a standalone XPS to being coupled to a thermal evaporator and having XPS, UPS, and now IPES. The spectrum shown is a combination of UPS and IPES measurements taken with our system on a ZnPc film. Our IPES design is based on a publication by Hiroyuki Yoshida (Chemical Physics Letters, 2012, 539–540, 180–185.) that utilizes a photomultiplier tube with bandpass filters as the detector. The system operates in the Bremsstrahlung isochromat mode, where the electron energy is scanned while the photon energy probed is constant. Our IPES system measures photon energies of less than 5 eV, as selected with an interchangeable bandpass filter, which is less than half of the photon energy probed by typical IPES instruments. Through probing lower energy photons we are utilizing lower energy electrons, which greatly reduces sample damage. Combined with our H Lyman-α source for UPS, our system provides an unprecedented ability to investigate more damage prone materials, such as organics and perovskites. Congratulations to graduate students Alex and Ashkan for setting up this awesome system!
We report on a novel VUV photon source for UPS measurements that utilizes H Lyman-α emission with a narrow linewidth and a widely tunable intensity, and apply it to a number of organic materials of interest to show its ability to overcome the problem of sample degradation commonly observed with typical He I UPS sources. The H Lyman-α source displays no measureable higher energy emission lines, which significantly reduces the background over typical He I discharge sources and allows for the onset of the density of states to be clearly observed over several orders of magnitude.
The new design minimizes error associated with the measurement of Seebeck coefficients in thin films by utilizing a more appropriate geometry (Reenen, S. v.; Kemerink, M. Organic Electronics 2015, 15, 2250-2255) and a reference material with a known Seebeck coefficient deposited directly alongside the sample to be measured. The design also no longer requires a probe station, which minimizes the footprint (allowing measurements inside or outside of a glovebox) and the time required for measurement of each sample.
The above image shows our setup for measuring the Seebeck coefficients in thin-film thermoelectric materials. A temperature gradient is applied accross a thin film on a glass substrate, while thermocouples and thin gold wires are utilized to contact the sample for temperature and voltage measurements between the two electrodes. Depending on the material, a temperature difference of 10 degrees C can result in over a millivolt difference in electrical potential between the electrodes. This setup highlights the basics of measuring the Seebeck coefficient, and highlights how small amounts of otherwise wasted thermal energy can be utilized to generate electrical power!
We demonstrate that surface modifying groups can be utilized to influence both film morphology and electrical conductivity in silver nanowire-polymer composites. This work shows that an adsorbed polymer on the silver nanowire surface can be displaced by thiols with varying end groups, and through choice of thiol end group the electrical and morphological properties of the composite, as well as the solvents utilized to disperse the silver nanowires, can be manipulated. Using this approach the authors demonstrate a one-step process for fabricating homogeneous composite transparent electrodes. http://pubs.acs.org/doi/abs/10.1021/ acsami.5b06489
A big thanks to all the folks at Angstrom Engineering for building such a great system! Thanks to Sean for helping to train the group and get it all setup and running properly.
Thanks to Jochen Weisser at Excitech for building the ultraviolet source to our specifications! The above image shows the vacuum ultraviolet source and the HOMO onset of a poly(3-hexylthiophene) thin film.
Thanks to Randy at RBD instruments for providing us with a great system and training the group to use the system. The system is a used PHI 5600 instrument with a multichannel plate detector, 11 inch diameter hemisphere, and dual anode (Mg and Al) X-ray source. The system will soon be outfit with an ultraviolet photon source, an inverse photoelemission spectroscopy setup, and necessary components for a direct connection to a thermal evaporator and glovebox.
