X-ray photoelectron spectroscopy (XPS) is a surface-sensitive quantitative spectroscopic technique, which measures the elemental composition, empirical formula, chemical state, and electronic state of the top approximately 10 nm of a material. XPS spectra are obtained by irradiating the material with a beam of X-rays while simultaneously measuring the number of electrons emitted by the material in dependence on their kinetic energy.
Running the X-ray source and the electron analyzer requires high vacuum (p ~ 10−8 mbar) conditions in the analysis chamber and, therefore, conventional XPS is possible only under UHV conditions, making it difficult to conduct investigations of surfaces under real-world conditions (i.e. in the presence of gases and possibly liquids), such as is the case of interfacial chemical reactions in catalysis, chemical vapor deposition, electrochemistry, and environmental chemistry. Near-ambient-pressure XPS (NAP-XPS) is an XPS system capable of operating at pressures of a few tens of millibars. We can now probe chemical interactions on the atomic level for vapor–solid interfaces. NAP-XPS also allows to investigate electronic and structural properties of small organics.
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The main chamber serves for the XPS and UPS measurements performed in either UHV or NAP conditions. The chamber is connected to the preparation chamber and to the NAP load lock chamber through ports fitted with gate valves. UHV measurements are performed on a standard UHV manipulator, transferable from the preparation chamber, and NAP measurements are performed in a dedicated inner NAP cell, transferable from the NAP load lock chamber.
The main chamber is equipped with an X-ray radiation source, a monochromator, a UV lamp, a differentially pumped hemispherical electron energy analyzer, a quadrupole mass spectrometer, a flood gun, a high resolution CCD camera, and a wobble stick. The X-ray source emits Al K? radiation (h? = 1486.6 eV), which is monochromatized and its spot size diameter can be focused down to 0.3 mm. The hemispherical analyzer is fitted with a pre-lens system and with differential pumping in order to facilitate the measurements up to NAP conditions. The quadrupole mass spectrometer is fitted in the 1st pumping stage of the analyzer pre-lens and is used for monitoring of the composition of the (residual) gas atmosphere inside the NAP cell (when used) or in the main chamber. The QMS has a Faraday detector and so has only moderate sensitivity (detectable partial pressures starting from 10?9 mbar). The flood gun emits low-energy electrons, which can be used to compensate for charging effects of samples with low electric conductivity. The flood gun can be used only under the UHV working conditions. The CCD camera is used to monitor the sample approach and positioning in the NAP cell. The wobble stick serves for the transfer of samples between the UHV manipulator and the NAP cell.
The NAP cell is fitted on a 3-axis manipulator in the NAP load lock chamber separated from the main chamber by a gate valve. The manipulator enables movement of the NAP cell through the gate valve to the main chamber, docking of the NAP cell to the entrance aperture of the analyzer, approach of the sample to the working position, and its precise positioning within the NAP cell. The NAP cell is made of stainless steel, and it is fitted with a sample holder stage, a loading door, a nozzle with a 0.3 mm opening, three gas inlets, a gas outlet, a pressure gauge, an external filament for e-beam heating, an air/LN2 cooling circuit, two thermocouples, and windows for X-rays, light, and a camera.
Samples are transferred between the sample holder stage in the NAP cell and the UHV manipulator in the main chamber (under UHV conditions) through a Viton-sealed door by the wobble stick. The sample is placed in vertical position in the sample holder stage, and it is grounded through the sample holder and the sample holder stage. Sample holders are optionally fitted with a K-type thermocouple contact to measure the sample temperature. A second K-type thermocouple is mounted permanently on the sample holder stage.
Sample heating is realized by an external filament placed at the back side of the sample holder stage, outside of the NAP cell. The external filament heats the back side of the sample holder stage by electron bombardment; the heat from the stage is then conductively transferred to the sample holder and to the sample. The highest reachable temperature of a sample in the NAP cell is 850 K (heating in UHV, presence of a gas atmosphere additionally cools the sample, presence of an oxidizing atmosphere – oxygen or a stronger agent – limits the maximal temperature to approx. 750 K because of possible damage to inner parts of the stage and possible Mo contamination of the sample surface); the heating rate is approx. 1 K / 5 s; the cooling rate is slower and depends on the actual temperature; there is no automatic control over the temperature ramp. The sample holder stage has an inner cooling circuit for the sample cooling by air or LN2. The lowest sample temperature achievable by LN2 cooling is 200 K (after approx. 1 hour of cooling). The heating/cooling of the sample results in thermally induced drifts, which should always be compensated for, because they significantly affect the sample working position (uncompensated drift during extensive heating can even result in a sample-nozzle collision).
The gas is fed into the NAP cell through three independent gas lines. Two of them are fitted with manual variable leak valves. Manually operated gas lines are suitable for dosing of vapors or low-pressure gases. The third gas line is fitted with a system of three computer-controlled mass flow controllers. The mass flow controllers operate within the flow range of 0.5 – 50 sccm and can premix up to three gases before these enter the NAP cell. The minimum pressure in the NAP cell adjustable during operation of the MFCs is 0.3 mbar. The pressure in the NAP cell is monitored by a capacitance gauge with a working range of 0.03 – 133 mbar. In order to separate the high pressure region of the NAP cell from the low-pressure region of the analyzer pre-lens, the NAP cell is equipped with a nozzle of 0.3 mm opening diameter, which serves as a passage for the photo-electrons. The pressure drop across the nozzle is roughly five orders of magnitude. The maximum pressure in the NAP cell depends on the gas type; for nitrogen or a similar gas it is 20 mbar. However, the recommended maximum pressure for a photoemission experiment with respect to the photo-electron attenuation by the gas is 5 mbar (N2 or similar). The NAP cell is pumped by the nozzle itself continuously and additionally by the outlet (roughly of the same conductivity as the nozzle), fitted with a back pressure controller pumped by a turbomolecular pump. The back pressure controller can control the pressure within roughly a half of the particular pressure range given by the actual flow rate of gases fed into the NAP cell. Gas feeding while heating the sample usually unbalances the thermal equilibrium by additional heat transfer caused by the gas molecules.
The sample has to be brought close to the nozzle for NAP-XPS/UPS measurements. The sample approach and positioning are controlled using a high resolution camera through viewports installed on the NAP cell. The sample-nozzle working distance is approx. 0.3 mm. The sample can be laterally positioned by roughly +/? 2 mm. X-rays enter the NAP cell through an Al-coated Si3N4 window (of 100-nm thickness).
The preparation chamber serves for sample preparation in UHV. It is equipped with a four-axis (x, y, z, and rotation) manipulator, an argon sputter gun, an electron gun for low-energy electron diffraction (LEED), two e-beam evaporators of metals, a wobble stick, and a gas inlet through a variable leak valve.
The UHV manipulator allows sample transfer to and from the main chamber and its positioning in either chamber for the available experimental techniques. The manipulator is fitted with a sample holder stage for sample accommodation in vertical position, with a filament for e-beam heating, with an inner cooling circuit for LN2, and with two thermocouple connections for temperature measurements on the sample and on the sample holder stage. The sample is conductively coupled with the manipulator, which is isolated from the ground and can be biased or grounded through a dedicated connection.
The highest sample temperature achievable by the e-beam heating in the UHV manipulator is 1000 K. Incident electrons hit and heat up the back side of the sample holder, and the heat is conductively transferred to the sample. For the best thermal contact the back side of the sample should be perfectly flat. Specific sample holders with a drilled hole allow direct e-beam heating of the sample, resulting in the maximum sample temperature of around 1300 K. However, drilled sample holders have lower heating performance in the NAP cell due to a smaller contact area. The lowest temperature achievable by LN2 cooling in the UHV manipulator is 100 K (within about 1 hour).
The transfer chamber is maintained under UHV conditions and is equipped with linear transfer rods for the transfer of samples among the load lock chamber, the buffer chamber, and the preparation chamber. The transfer chamber is also fitted with a parking station for storing up to eight samples in UHV.
The load lock serves as the transition chamber for sample loading into (and removing from) the apparatus (the transfer chamber). It can be easily filled with N2 and is equipped with a fast-entry door.
The buffer chamber serves as the transition chamber for sample transfer between the electrochemical cell and the (UHV) transfer chamber through an inert atmosphere (Ar). It is equipped with a light-bulb-like heater used for sample drying.
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