What is x-ray photoelectron spectroscopy?
X-ray photoelectron spectroscopy (XPS)
, a scientific analytical instrument, is a surface analysis technique used to characterize the elements and their chemical state on the surface of a material.
X-ray photoelectron spectroscopy applications
performs qualitative, quantitative or semi-quantitative and valence state analysis of the elemental composition of solid samples. Composition and chemical state analysis of solid sample surface, widely used in elemental analysis, multiphase studies, compound structure identification, trace element analysis by enrichment method, elemental valence identification. In addition, it has applications in micromechanical studies of oxidation, corrosion, friction, lubrication, combustion, bonding, catalysis, coating, etc.; environmental determination of pollution chemistry, dust particle studies, etc.; molecular biochemistry and three-dimensional profiling such as the study of interfaces and transition layers.
The specific scope of use of x-ray photoelectron spectrometer
Phase analysis, composition analysis and identification of inclusions in metallic materials.
Analysis of surface coating and plating of solid materials, such as: detection of metallized film surface coating.
Qualitative and quantitative analysis of micro-area components on the surface of materials, and analysis of surface, line and point distribution of elements on the surface of materials.
X-ray photoelectron spectroscopy principle
The principle of x-ray photoelectron spectrometer is that the detected rays enter the sensitive area and produce ionization, generating a large number of electron-hole pairs. Under the action of the applied electric field, the electrons and holes drift rapidly to the positive and negative poles, respectively, and are collected, forming a pulsed electrical signal in the output circuit The electron-hole pairs in the semiconductor detector are called the information carriers of the detector. The multi-channel pulse amplitude analyzer (MCA) is an instrument that processes the amplitude distribution of a pulse signal. It classifies pulsed signals according to their amplitude and records the number of signals in each category. It mainly consists of an analog-to-digital converter (ADC), an address encoder and a memory. The detector converts gamma rays of different energies into a pulse signal with amplitude proportional to energy, which is input to the ADC, converted into a digital representation, and enters the memory coded with an address Each address memory is a channel with a counter. After the measurement is completed, the two-dimensional spectral line displayed according to the count of different channels is the energy spectrum map.
Advantages of using x-ray photoelectron spectroscopy
Fast analysis speed
XPS can simultaneously accept and detect all the different energy X-ray photon signals, so it can analyze and determine all the elements contained in the sample within a few minutes, the detector with beryllium window can detect the elements in the range of 11Na ~ 92U, the new window material introduced to the market in the 1980s can enable the spectrometer to analyze the light elements above Be, detecting the elements in the range of 4Be ~ 92U.
Photo electron spectroscopy has a large stereo angle for x-ray collection. Since the Si(Li) probe in the energy spectrometer can be placed very close to the emission source (about 10㎝) without going through crystal diffraction, there is almost no loss of signal intensity, so the sensitivity is high (up to 104cps/nA, the X-ray count rate produced per unit intensity of the incident electron beam).
In addition, the energy spectrometer can operate at low incident electron beam current (10-11A), which facilitates the improvement of spatial resolution of the analysis.
Good repeatability of spectral lines
Because x-ray electron spectrometer There are no moving parts, good stability, and no focusing requirements, so the repeatability of the peak position of the spectral line is good and there is no defocusing problem, which is suitable for the analysis of relatively rough surfaces.
X-ray photoelectron spectroscopy construction
X-ray photoelectron spectroscopy is composed of a sampling chamber, ultra-high vacuum system, X-ray excitation source, ion source, energy analysis system and computer data acquisition and processing system, etc.
X-ray photoelectron spectrometer is equipped with a fast injection chamber, which is designed for fast injection without breaking the ultra-high vacuum of the analysis chamber. The volume of the rapid injection chamber is small enough to achieve a high vacuum of 10-3 Pa within 5-10 min.
Ultra-high vacuum systemsUltra-high vacuum
systems are necessary for X-ray photoelectron spectrometry for two main reasons. First, XPS is a surface analysis technique, and if the vacuum in the analysis chamber is poor, the clean surface of the specimen may be covered by residual gas molecules in the vacuum in a very short time. Secondly, since the signal and energy of photoelectrons are very weak, if the vacuum level is poor, the photoelectrons can easily collide with the residual gas molecules in the vacuum and lose energy, and finally cannot reach the detector. In an X-ray photoelectron spectrometer, to make the vacuum of the analysis chamber can reach 10-8 Pa, the three-stage vacuum pump system is generally used.
X-ray excitation source
The binding energy of electrons in the inner shell layer of atoms is high, and higher energy photons are required to beat them out. The photon energies obtained from X-ray sources with magnesium or aluminum as anode materials are 1253.6eV and 1486.6eV, respectively, and the photon energies in this range are sufficient to beat out 1s electrons from atoms with small atomic masses. The X-ray source of this instrument is Al Ka, and after monochromatization, the linewidth can be reduced from 0.8eV to 0.2eV, and the stray lines and tough radiation in the X-ray can be eliminated.Ion source
The purpose of having an ion source in XPS is to clean the sample surface or to quantitatively strip the sample surface. In XPS spectrometers, Ar ion sources are often used, which can be divided into fixed and scanning sources. Fixed Ar ion sources are used only for surface cleaning because they cannot perform scanning stripping and have poor uniformity of etching on the sample surface. For deep analysis, a scanning Ar ion source should be used.Energy analyzers
There are two types of energy analyzers for X-ray photoelectrons, hemispherical energy analyzers and barrel mirror energy analyzers. The hemispherical energy analyzer is mostly used in XPS spectrometers because of its high efficiency of transmission of photoelectrons and good energy resolution. The barrel mirror energy analyzer is mainly used in the X-ray spectrometer because of the high efficiency of the transmission of oscillating electrons. For some multi-functional electron spectrometers, the choice of energy analyzer is mainly based on the analysis method because of the commonality and focus of XPS and AES. A hemispherical energy analyzer is used mainly for XPS, while a cartridge-type energy analyzer is used mainly for OSCE.Computer system
Due to the complexity of data acquisition and control of X-ray photoelectron spectrometer, computer processing of spectra is also an important part, such as automatic identification of elements, semi-quantitative calculation, fitting of spectral peaks, etc.
X-ray photoelectron spectroscopy specifications
|X-ray source||Mgkα 12kV, 30mA (Mg/Al target optional)
|Vacuum level||5× 10-7Pa or less
|Resolution||FWHM Ag3d5/2 0.8eV
|Gas reaction cell ||1000℃@1bar, 400℃@6bar
1. XPS allows high sensitivity analysis from small to large areas.
2. Excellent depth profiling performance.
3. Advanced automatic charge double beam neutralization technology.
4. Automation and remote control for a flexible working environment.
5. Versatile configurations.
X-ray photoelectron spectroscopy test sample preparation requirements
The following are the different requirements for the use of XPS spectrometer for different nature of samples
a. The amount of powder required for testing is generally not less than 0.2ml or 10mg, and the sample should be dry.
b. Solid sample size block 5*5mm, thickness as much as possible below 4mm, the surface must be flat, and the sample needs to be dried.
c. Liquid samples must be dried on aluminum foil or silicon wafers and other carriers to form a liquid film, generally repeated 3-5 times drying drops almost can not be measured on the substrate.
d. Materials must be non-radioactive, non-toxic, and non-volatile substances (such as monomers Na, K, S, P, Zn, Se, As, I, Te, Hg, etc.).
e. If there is no special requirement, the strongest peak will be tested by default, and if the strongest peak overlaps with the peaks of other elements, the second strongest peak will be tested by default. Please note if you need to sweep special peaks.
f. The signal may not be obvious for elements smaller than 1%.
g. General sample testing generally includes a full spectrum and a single element fine spectrum containing up to five elements of carbon; more than five elements are charged at an additional cost per element; samples are not provided for data analysis and processing, if analysis and processing are required, additional fees are charged.
h. Other questions and special testing requirements can be communicated online.
How to maintain x-ray photoelectron spectroscopy?
As an important tool for material surface analysis, X-ray photoelectron spectrometer is widely used, not only to analyze the elemental chemical information of material surface, but also to analyze micro-areas and depth distribution, etc. The testing volume of X-ray photoelectron spectrometer is usually large, so the routine maintenance of X-ray photoelectron spectrometer can be triggered by vacuum system, circulating water system, gas circuit system, etc.
Maintenance of vacuum system
Preparation of samples needs to ensure that the samples do not decompose or release gas under ultra-high vacuum and X-ray irradiation conditions.
The sample should be fully pumped into the sample inlet chamber.
Keep the inlet chamber door clean.
Run the titanium sublimation pump regularly to adsorb the reaction gases that affect the vacuum.
Cooling and dehumidification measures are required in case of high ambient temperature and humidity.
Maintenance of circulating water system
If the leakage current of cooling water is higher than 10 mA needs to replace with the circulating water.
Clean the water cooler screen and metal filter ball frequently, and regularly clean the radiator screen on the front panel of the instrument cabinet capable.
It is recommended to replace the deionized or distilled water every six months. About 500 mL of ethanol can be added to prevent the growth of algae plants and microorganisms.
Maintenance of air circuit system
Compressed air pressure needs to be maintained between 0.3~0.7 MPa.
When the compressed air pressure is low, the air cylinder needs to be replaced in time. When replacing the gas cylinder with high-purity nitrogen, you need to check the gas tightness of the gas line interface with soapy water.
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