High Pressure X-ray Phototelectron Spectroscopy (HP-XPS)
Dr James O'Shea
HP-XPS enables us to measure the chemical composition of a surface at pressures in the mbar range which allows it to be applied to more realistic environments than traditional ultra-high vacuum XPS.
The high-pressure (near-ambient pressure) XPS system comprises an ultra-high vacuum (UHV) analysis chamber equipped with a differentially pumped high-pressure hemispherical electron analyzer, a 4-axis sample manipulator and sources for monochromatic micro-focused x-rays, sputtering and charge neutralization. The chamber is also equipped with sources for evaporation and electrospray deposition of molecules. Samples can be introduced through a load-lock onto the UHV manipulator and prepared at pressures down into the 10-10 mbar range. Samples can then be transferred into the in-situ near-ambient pressure (NAP) cell that docks onto the front of the analyzer to perform XPS measurements at pressures up to 20 mbar in a range of gases.
The NAP cell is mounted on a UHV manipulator and can be accessed by its own load-lock. This allows non-UHV samples to be introduced into the system without ever encountering UHV pressures. This allows wet samples to be studied by maintaining an atmosphere of the relevant liquid vapour over the sample to prevent evaporation.
The NAP cell can be removed and exchanged through the load-lock for customized experiments requiring specialized sample mounting and electrical contacting. Standard samples can be transferred into and out of the NAP cell in the analysis chamber in-vacuo for further surface preparation as needed.
Our NAP cells incorporate fully motorized sample position inside the cell over 25 mm in the z-axis and ±4.5 mm in the x-axis and y-axis. Samples are mounted on transferable sample plates and can be up to 12mm x 12mm with a typical thickness of 0.5 mm.
We have two NAP cells. The first incorporates an e-beam heater with the possibility to heat the sample up to 600 °C at pressures up to 20 mbar N2 pressure. Sample cooling with liquid nitrogen allows the sample to be cooled down to -73 °C, again at pressures up to 20 mbar N2. This NAP cell also incorporates an imaging lens module for spatially resolved NAP-XPS mapping of the sample surface with resolutions better than 20 μm. The second NAP cell incorporates a Peltier cooling element for temperatures between -5 °C to 50 °C. This allows the sample to be the coldest thing in the NAP cell so that gases condense primarily on the sample surface, and enables us to study very thin films of liquids such as water on different surfaces. Both cells have 3 independent gas inlets with mass flow controllers pointing directly onto the sample surface for gas dosing.
We also have two quadrupole mass-spectrometers fitted to the gas inlet system and the first stage of the high-pressure electron analyzer, respectively. This allows us to measure the composition of the gases up to 100 amu entering and leaving the cell at the same time as studying what is going on at the surface itself with XPS.
The NAP-XPS spectrometer features a PHOIBOS 150 differentially pumped hemispherical analyzer with operating pressures from UHV up to 25 mbar. It has a wide angle pre-lens with magnification factor 7 and ±22° acceptance angle. It also features an integrated micro-valve for separation from the vacuum system of the analysis chamber.
The detector is a state-of-the-art 2D delay-line detector (DLD), with extremely low dark count and a linear response up to 1 million counts per second. This detector allows absolute quantification of the XPS peak intensities and facilitates the imaging mode of the system.
The X-ray source is a microfocus Al-kα monochromated source with a high-precision manipulator for accurate positioning of the spot on the sample. The spot size is variable from 250 μm to 1 mm with maximum powers of 20W and 180W, respectively. The system also incorporates a flood gun designed for sample charge neutralization of positively charged insulators or semiconductors. It provides a stable electron current of 0 – 50 μA and homogeneous current densities to achieve an effective charge neutralization on a large sample area. The beam energy can be controlled from 0 – 100 eV.
Catalysis, solid-liquid interfaces, photocatalysis, hydrated samples, electrochemical interfaces, water splitting surfaces, gas sensors, battery electrodes. Full service with academic or postgraduate researchers conducting the experiment alongside external researchers. Data analysis services can also be provided. £500 per day or specific cost per sample.
University of Nottingham
Research Acceleration and Demonstration Building (RAD)