X-ray Photoelectron Spectroscopy (XPS)

• Description

X-ray Photoelectron Spectroscopy, XPS, or ESCA, Electron Spectroscopy for Chemical Analysis) is a non-destructive and quantitative technique for the chemical, physics and electronics properties analysis of elements (Z>2) present in the shallowest layers (20-30Å), making it a unique technique for the chemical analysis of surfaces. The sample is excited with an X-ray source (usually AlKα, 1486.7 eV). When the x-rays imping on the sample, photoelectrons with different kinetic energies (KE) are emitted. This kinetic energy is related to the binding energy (BE, eV) and the incident radiation energy (hv, eV) by equation 1. BE is characteristic of an electron from a given atomic orbital and element (tabulated values).

hv (eV) = KE (eV) + BE (eV)    (eq. 1)

Depending on the chemical environment of the atom, the binding energy (BE) of the emitted electrons changes slightly (called "chemical shift") so that both the element and its chemical environment can be identified (for example, in the carbon C1s region, C-C, C≡N, C-O-C, C-F₃ and O-C=O species can be differentiated).

XPS technique is therefore a powerful surface analysis technique that can identify (and quantify) the elements present in the sample and provide information about the oxidation/reduction state of an element and its chemical environment.

Conventional XPS machines usually work under UHV (Ultra-high Vacuum) conditions. However, thanks to scientific advances in recent decades, systems have been developed that are capable of operating in the analysis chamber under NAP (Near Ambient Pressure) conditions. Different gases can be introduced into the analysis chamber at a pressure of around 1-25 mbar, thanks to several differential pumping stages between the analysis chamber and the analyzer.

• Tecnology

Figure 1. Panoramic view of the XPS machine "ProvenX-NAP System".

SPECS Provenx NAP System

ProvenX-NAP System (Figure 1), designed by SPECS and acquired by the SRCiT thanks to the grant EQC2021-007785-P awarded by the Spanish Ministry of Science and Innovation), is divided in different parts: (1) sample introduction chamber "Load Lock", (2) preparation chamber "Pre-chamber", (3) coupled reactor "High Pressure Cell - HPC" and (4) analysis chamber which have been designed for a specific use, detailed below:

Figure 2. a) "Load Lock" chamber, b) sample preparation chamber, c) HPC coupled reactor, d) analysis chamber and e) different types of sample holders.

• The sample introduction chamber "Load Lock" chamber (Figure 2a) is designed for fast sample introduction, reaching Ultra-high Vacuum (UHV) conditions in approx. one hour. So, the sample can be transferred to the analysis chamber in ~1 hour. In addition, it has a parking (Figure 3b) that allows up to 5 different samples to be stored at the same time.

• The preparation chamber "Pre-chamber" (Figure 2b) is equipped with an evaporator "SPECS EBE-4 Multi-Pocket Electron Beam Evaporator". It is capable of evaporating small quantities of almost any material. The material, either in crucible or rod form, is heated by bombarding it with electrons from a circle-shaped filament covering the material. In combination with the EBE-M Power Supply, which can run every pocket individually or in any combination with the other pockets, EBE-4 allows an optimal evaporation by controlling filament current (A), regulating emission (mA) or flux current (nA).

• The coupled reactor "High-Pressure Cell - HPC" (Figure 2c) is a high-pressure cell designed to carry out reductions, oxidations and chemical reactions. It is designed so that the reaction volume is minimized and temperatures up to 800 °C can be reached in the presence of gases. Heating of the sample is achieved by the use of a halogen lamp. Different types of gases, O₂, CO_x, H₂ and inert gases can be introduced into the reactor. Currently, only one gas can be introduced, but in the future, it will probably be possible to introduce several different gases at the same time.

• The analysis chamber "NAP Backfilling" (Figure 2d), coated with μmetal to protect it magnetically, has different devices attached: (i) FlexMan NAP manipulator with 4 axes (X, Y, Z and inclination) with temperature control (-150º to 600ºC), which allows the movement of the sample and ARPES measurements (changing the angle of inclination of the sample); (ii) Scanning Ion Source which allows the sample to be bombarded with Ar ions in order to clean the sample; (iii) Flood Gun source with automatic adjustment for charge neutralization of positively charged insulators or semiconductors; (iv) digital camera for sample observation; (v) monochromatic X-ray source μFocus 600 (AlK, 1486.7 eV); (vi) UV 300 ultraviolet source to study the secondary electrons and the Fermi level of different samples (Ultraviolet Photoelectron Spectroscopy, UPS); (vii) PHOIBOS 150 NAP analyzer with 1D-DLD detector with different differential pumping stages, allowing the analyzer to work in the analysis chamber at pressures of the order of millibars and the analyzer and detector to be under UHV conditions; (viii) gas panel for the dosing of up to three different gases (inert gases as well as reactive gases H₂, O₂ and CO_x, among others) at the same time with a constant flow rate.. In addition, the evolution of the gases can be followed by an RGA "Residual Gas Analyzer" mass spectrometer located in the second differential pumping stage of the analyzer.

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Summary of Technical Characteristics ProvenX-NAP System

• PHOIBOS 150 NAP 1D-DLD analyzer, XPS and UPS compatible
• Monochromatic X-ray source μ-FOCUS 600 NAP (AlKα, 1486.7 eV). It is expected to be replaced by a μFocus 450 dual source (AlKα/AgLα, 1486.7 and 2984.3 eV, respectively) in the next 6-9 months.
• 4-axis (X, Y, Z and tilt) FlexMan NAP sample manipulator with control temperature (-192º to 600ºC)
• NAP Backfilling analysis chamber (μ-metal)
• "Load Lock" chamber with with storage of up to 5 samples.
• SpecsLab Prodigy and Prodigy ISQAR software
• EV2-220-000 mass spectrometer (<200 amu)
• Gas control panel coupled to the analysis chamber for gas dosing. It allows working at the same time with three different gases (O₂, H₂, CO_x, inert) up to 25 mbar.
• Digital camera for observing the sample with a laser pointer
• Charge neutralizer Flood Gun FG 22/35
• UVS 300 ultraviolet source
• Scanning Ion Source IQE 12/38 for sputtering Ar ions to erode the sample
• "EBE-4 Multi-Pocket Electron Beam" Evaporator to evaporate for up to 4 different materials
• "HPC 20" High Pressure Cell, just with one gas line installed (O₂, H₂, CO_x, inert), working up to 20 bar and 800ºC.

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• Samples to be analyzed and sample holders

• Organic and inorganic solid samples (powder or bulk)
• Membranes and filters
• Capillaries and fibers
• Thin layers
• Electric boards
• Samples sensitive to the environment
• Single crystals
• Paints and coatings
• Catalysts
• The samples can be mounted in different sample holders (Figure 2e):

• Heating sample holders (stainless steel) with thermocouple: Sample holders (1) will be used to heat the samples (by resistive heater) in the analysis chamber and sample holders (2) to heat the samples in the high-pressure cells (by using halogen lamp).

• No heating and no thermocouples sample holders (3): stainless steel or molybdenum.

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Applications

Some books d ivide the applications of the XPS technique into three main topics, in materials science, organic materials and corrosion science and electrochemistry.

Within the first block, there are different fields in which the use of XPS is very useful, as information can be obtained about the composition and chemical state of surfaces and interfaces (surface distribution, thickness and structure of surface layers, molecules adsorbed to the surface, coating composition, among others). XPS, in materials science, is used to: (i) characterizing semiconductors; (ii) for the understanding of complex surface processes that affect various minerals during metal extraction and ore processing, e.g. met al readsorption; (iii) the study in nanoscience of compounds that have combined properties of both the organic and inorganic part, due to their potential application in medicine and biotechnology; (iv) for the understanding of different phenomena that take place at the surface level in metallurgical materials, e.g. alloy formation; (v) coatings, (vi) to understand the behavior of inorganic materials, e.g. catalysts, during a chemical reaction, by obtaining information on the species adsorbed on the catalyst surface, the chemical environment and the oxidation/reduction state of the active centers, whether or not catalyst restructuring occurs under reaction conditions; (vii) to understand the friction and wear mechanisms leading to structural failure in tribology and to analyze the chemical composition of worn surfaces, adsorption and the reaction of lubricating oil additives and wear resistant coatings.

XPS provides a similar kind of information in the field of biology and organic materials as in the case of material science. However, due to the complex nature of biological systems, XPS is commonly used in combination with other relevant techniques for obtaining reliable information. Further, there are challenges related to sample preparation, sample damage and interpretation of the data. XPS is used to study the surface chemistry of microbes, formation of biofilms, biocompatible materials and also pharmaceutical materials. The surface chemistry of microbes is important because they can form biofilms on material surfaces. Similarly in medicine, the interaction of biomaterials and pharmaceutical drugs within the host body is also dependent on their surface properties.

In the field of corrosion studies, the XPS provides quantitative insights into the chemical composition, the nature of valence states, elemental distribution within the surface films (including multi-layer structure), the thickness of the films and the composition of alloy surface under the films. Surface films are usually very complex with multilayered structures and reflect the electrochemical properties of metals and components of alloys. XPS can detect the changes in the composition and chemical structure of surface layers with potential, time and electrochemical conditions. The XPS analysis is helpful in understanding the general and localized corrosion mechanism and is usually used in the study of the passivity of metals and alloys, surface treatments and coatings.

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