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NAP-XPS and NAP-UPS

X-ray Photoelectron Spectroscopy (XPS) and Ultraviolet Photoelectron Spectroscopy (UPS) are well-established and versatile techniques commonly used in surface chemical analysis.

XPS spectra are obtained by illuminating the sample surface with monochromatic X-rays and measuring the energy of the photo-emitted electrons with an information depth of up to 10 nm for standard soft X-ray excitation sources. Thus, it provides qualitative and quantitative information about the elemental composition and the chemical state of the surface.

The classical XPS analysis technique

The classical XPS analysis technique is used under UHV conditions and this strongly restricts the type of samples that can be investigated mainly to solid samples or liquids with a very low vapor pressure. Therefore, model systems rather than real samples in their generic environments can be investigated by using standard XPS techniques in UHV.

courtesy of J- J. Gallet, SOLEIL Synchrotron

Thus, in Near-Ambient-Pressure X-ray Photoelectron Spectroscopy (NAP-XPS) the sample is surrounded by a gas atmosphere and no ultra-high vacuum (UHV) conditions are required in the analysis area. Therefore, investigations of a large variety of different samples, including insulating samples, biological samples, gases, liquids and their interfaces are easily accessible. When measuring XPS in a gas atmosphere, the emitted photo-electrons from samples are scattered by collisions with surrounding gas molecules before entering the hemispherical electron analyzer. 

Limitations of standard XPS

courtesy of Miguel Salmeron, LBNL

UHV–XPS versus NAP-XPS

Requirements for XPS measurements

In order to be able to perform XPS measurements at elevated pressures, different requirements have to be fulfilled:

  • The electrons have to be “caught” before most of them are inelastically scattered by collisions with gas molecules. Thus, short electron and X-ray travel lengths as well as large angular acceptance of the analyzer are required. 
  • The pressure gap has to be crossed, which implies the development of analytical components (hemispherical electron analyzer, small spot X-ray and UV-sources) that can work in pressure ranges from UHV to tens of mbar regime or even higher.
  • The exact composition, temperature and pressure of the sample environment have to be controlled.
  • High signal intensity and fast data acquisition is required, since the chemical processes at surfaces are taking place on a fast time scale.

Many fundamental processes that occur at solid-gases or solid-liquids interfaces play a key role in different application fields like heterogeneous catalysis, energy generation and storage, or environmental science. By using NAP-XPS, in-situ and in-operando studies under realistic working conditions are possible.

Strengths of NAP-XPS:

  • Chemical state measurements on surfaces
  • Sensitive to all elements except for H and He
  • Huge variety of addressable sample systems including insulating samples, powders, liquids, gases, biological molecules
  • Investigation at elevated pressures under well controlled conditions (temperature, pressure, type of gas/liquid)
  • In-operando studies of real devices

Key Applications:

  • In-operando studies of electrochemical energy conversion and storage devices
  • In-situ investigations of medical and biological materials
  • Processes at interfaces under reactive conditions for e.g. corrosion and catalytic studies
  • Surface studies in contact with gaseous or liquid environments

A NAP-XPS system mainly consists of three building blocks, the analysis chamber that accommodates the sample under controlled pressure and temperature conditions, a special electron analyzer with a dedicated differential pumping scheme that collects as many photoelectrons as possible at a distance that is shorter than their mean free path and a high flux small spot X-ray source, with spot sizes smaller than the entrance nozzle of the electron analyzer (usually < 300 µm). There are different system layouts for NAP-XPS systems which mainly differ in the way the sample environment is realized. All designs have specific advantages for specific experimental tasks or conditions. 


Additional to NAP-XPS, small spot UV-sources have been developed that are able to work under different gas atmospheres by using a Al-window and differential pumping. With this, NAP-UPS investigations (classically with He I and He II radiation) can be carried out in order to investigate changes in the valence band structure in gas pressures of up to 1 mbar. 


NAP-XPS / NAP-UPS techniques are experimentally available for laboratories as well as synchrotrons. In the case of synchrotrons, a special beam entrance stage either with differential pumping or a Si3N4-window can be used as connection of the NAP-XPS system to beamline. 
 

Environmental Charge Compensation

In conventional XPS systems where the analysis region needs to be kept under UHV an effect occurs that is named surface charging. A nonconductive sample that has its electrons continuously removed from the analysis region by the impinging X-ray photons will slowly be charged up positively. The escaping electrons will be influenced in their path and energy by the positive charges at the surface. To prevent the surface from charging up, conventional XPS systems are equipped with charge compensation system (electron and ion sources) that can be tuned in a way that additional charges will be brought to the surface region compensating the lost charges due to the photoionization process. This is a really difficult and time consuming task. 

(1)
(2)

(1) NAP-XPS systems or EnviroESCA are not conventional UHV-XPS systems but Ambient Pressure XPS (AP-XPS) systems, which means that the sample is kept under vapor or gas pressures up to 50 mbar. This implies that there are neutral gas atoms and molecules surrounding the sample, whereas it is not important if the gas atoms or molecules evaporate from the sample surface or if they are introduced inside the analysis chamber by using a built-in gas delivery system.

The impinging X-ray photons will also interact with the neutral gas atoms and ionize them, hence generating free positive charged ions and electrons. These free charges act as a charge cloud above the surface layer of the sample, allowing every escaping electron from the surface to be exchanged against an electron of the charge cloud. We call this intrinsic charge neutralization effect Environmental Charge Compensation.


(2) The Environmental Charge Compensation Effect is an intrinsic effect that takes place in any NAP-XPS system with sufficiently broad X-ray beam. It allows the user to easily acquire high resolution XPS spectra from nearly any surface, independent if it is liquid, solid, conductive or nonconductive, without using any additional charge compensation system.
 

EnviroESCA: Polymers and Plastics

EnviroESCA’s special system design

EnviroESCA’s special system design and many NAP-XPS system designs allow for investigations of a large variety of different samples, including insulating samples, gases, liquids and their interfaces that are not accessible with standard XPS systems. The sample to be investigated is the central part of every X-ray Photoelectron (XPS) system.

In a NAP-XPS system or EnviroESCA the sample needs to be positioned underneath the analyzer nozzle which is the entrance to the analysis section of the machine. Samples can be flat surfaces or really rough three dimensions structures. They can be very tiny with a diameter of just few hundreds of microns or as large as 10 mm in NAP-XPS and 120 mm in EnviroESCA. 

EnviroESCA: Zeolites

The effectivity of the Environmental Charge Compensation

The effectivity of the Environmental Charge Compensation that is intrinsic to all NAP-XPS systems is shown in the NAP-XPS spectra of zeolites. The NAP-XPS measurements are carried out by using the EnviroESCA system, in 1 mbar air without using an electron source for charge compensation. The sharp peaks in the XPS spectrum are an evidence of the efficient charge compensation process that takes place in gas atmospheres.

NAP-XPS systems as well as EnviroESCA allow for performing XPS measurements on samples kept under gas or liquid vapor environments up to 50 mbar and thus, open up the possibility of investigating fundamental processes in the field of biology, biochemistry, astrobiology, medicine, chemistry, etc. that are not accessible by using conventional XPS systems.
 

RELATED PRODUCTS

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APPLICATION NOTES

Near ambient pressure photoelectron spectroscopy of fruit and vegetables
Near ambient pressure photoelectron spectroscopy of fruit and vegetables
In this note we present first NAP-XPS results from a fresh tomato and apple using the EnviroESCA. Portions of tomato and apple were introduced into the system and the pressure was stabilized at 10 mbar. Different regions on the surface were studied and the photoelectron spectra show significant chemical differences between these regions. This study demonstrates the unique NAP XPS capabilities of the EnviroESCA and extends the field of applications to (processed) food samples and other natural or biological samples that could not be studied by XPS up to now.
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AP-XPS investigations of a commercial lithium-based electrolyte
AP-XPS investigations of a commercial lithium-based electrolyte
In this note we present first AP-XPS results from a commercial liquid electrolyte for lithium ion battery production recorded at 10 mbar in the EnviroESCA. The solution is LiPF6 in EC/DMC as a common battery electrolyte. First we analyzed the LiPF6 solution in its vapor and liquid phase. In a second step we immersed V2O5, a typical cathode material, in this solution. Photoelectron spectra show significant chemical differences on the studied surfaces. This proof of principle study shows the enormous potential of the EnviroESCA for AP-XPS investigations of solid-liquid interfaces and the simplicity of the experimental setup.
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XPS of Ag and N2 at 25 mbar
XPS of Ag and N2 at 25 mbar
This application note shows near ambient pressure (NAP) measurements performed with the NAP XPS System for the University of Norte Dame, Prof. Franklin Tao.
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Investigation of Polymers and Plastics with EnviroESCA
Investigation of Polymers and Plastics with EnviroESCA
This application note describes how EnviroESCA can be used to analyze the surface of polymers and bulk insulating material. EnviroESCAs ability to perform X-Ray Photoelectron Spectroscopy (XPS) at non vacuum conditions shows its main advantage when being applied to insulating material. Specimens that tend to charge up under vacuum conditions and need to be extensive charge compensated with expensive low energy electron and low energy ion sources can be measured with ease and without the problem of overcompensation.
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Real World Device Inspection Application with EnviroESCA
Real World Device Inspection Application with EnviroESCA
This application note introduces EnviroESCA as a tool for real world device inspection for microelectronics. A printed circuit board was taken directly from the sales packaging and was investigated with XPS. The measurements concentrate on the crucial parts like contact surfaces and soldering joints. EnviroESCA and near ambient pressure XPS (NAP-XPS) in quality control applications aims to optimize manufacturing processes and device quality in general.
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Investigation of Hydrogel Contact Lenses with EnviroESCA
Investigation of Hydrogel Contact Lenses with EnviroESCA
In this application note we analyze the surface of water filled hydrogels with EnviroESCA, Here, contact lenses serve as an example for medical and biomaterials. The advantage of such investigations under non ultrahigh vacuum conditions is demonstrated and results from wet and dry samples are compared.
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XPS surface analysis of Zeolites with EnviroESCA
XPS surface analysis of Zeolites with EnviroESCA
In this note we demonstrate how the surface of Clinoptilolite, a natural zeolite, can be analysed with EnviroESCA. It describes how the Environmental Charge Compensation can be used to compensate for charging of the sample surface to be able to measure X-ray Photoelectron Spectra with ease.
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XPS surface analysis of a leaf with EnviroESCA
XPS surface analysis of a leaf with EnviroESCA
This application note presents the application of EnviroESCA to the field of biology and surface spectroscopy of biological samples. X-ray Photoelectron Spectroscopy (XPS) measurements on different sites of a leaf will be discussed. It will be shown that XPS is able to detect residues of calcium chloride, being used as a fertilizer, on the leafs surface.
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EnviroESCA and NAP-XPS in the field of Cosmetic Science
EnviroESCA and NAP-XPS in the field of Cosmetic Science
This application note presents the application of EnviroESCA to the field of Cosmetic Sciences and Forensic Sciences. Near Ambient Pressure X-ray Photoelectron Spectroscopy (NAP-XPS) measurements on human hair treated with different personal hair care products.
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EnviroESCA applications in Scientific Archaeology
EnviroESCA applications in Scientific Archaeology
In the present application note EnviroESCA is used for chemical Analysis in the field of Archaeometry. Besides the ability of Near Ambient Pressure X-ray Photoelectron Spectroscopy (NAP-XPS) to analyze the surface composition of metallic and nonmetallic specimens, EnviroESCA allows for a nondestructive analysis of artifacts without special pretreatment procedures.
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XPS surface analysis of a coffee bean with EnviroESCA
XPS surface analysis of a coffee bean with EnviroESCA
The Analysis of food and natural products under environmental conditions is of great importance due to their daily use and direct interaction with humans during consumption. In this study we investigated a roated coffee bean. Charge compensation of this insulating natural product is accomplished by Environmental Charge Compensation enabling X-ray photoelectron spectroscopy on such biological material with ease.
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XPS surface analysis of an ionic liquid with EnviroESCA
XPS surface analysis of an ionic liquid with EnviroESCA
This application note presents how EnviroESCA can be used to analyze the surface of ionic liquids in contact with gases under application relevant conditions in the near ambient pressure region.
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XPS surface analysis of sesame seeds with EnviroESCA
XPS surface analysis of sesame seeds with EnviroESCA
Results of the surface analysis of untreated food samples, e.g., sesame seeds, measured with EnviroESCA are presented. High resolution and high quality spectra are recorded using Environmental Charge Compensation even for samples in bulk quantities.
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XPS surface analysis of printed paper samples with EnviroESCA
XPS surface analysis of printed paper samples with EnviroESCA
Results of the surface analysis of four paper samples obtained in EnviroESCA are presented. Neutralization of this insulating biopolymer is accomplished by Environmental Charge Compensation enabling X-ray Pho-toelectron Spectroscopy (XPS) on such important natural material with ease.
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XPS surface analysis of Italian hard cheese with EnviroESCA
XPS surface analysis of Italian hard cheese with EnviroESCA
This application note presents how EnviroESCA can be used to analyze the surface of food samples, e.g., Italian hard cheese. Due to the advanced vacuum system of EnviroESCA the surface analysis of Food samples even in the presence of fat and oils or volatiles is performed without problems.
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Investigation of a Superabsorbent Polymer (SAP) with EnviroESCA
Investigation of a Superabsorbent Polymer (SAP) with EnviroESCA
In this study we show how EnviroESCA can be used to analyze the surface of superabsorbent polymers under different environmental conditions. EnviroESCA’s ability to perform X-ray Photoelectron Spectroscopy (XPS) under ambient conditions shows its main advantage when being applied to insulating material that change their surface (chemistry) when they come in contact with different liquids and gases - in this case water. Specimens that tend to charge up under vacuum conditions can be measured with ease.
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XPS surface chemical analysis of aqueous solutions with EnviroESCA
XPS surface chemical analysis of aqueous solutions with EnviroESCA
In this study we present the capabilites of EnviroESCA to analyze the surface of various aqueous solutions under near ambient pressure conditions. Such investigations of aqueous solutions are of paramount importance due to the vast number of essential water based processes in nature and industry.
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XPS surface analysis of a human urolith with EnviroESCA
XPS surface analysis of a human urolith with EnviroESCA
A single human urinary stone (urolith) was characterized using EnviroESCA. The result of surface chemical analysis of the as received samples is presented. Charge neutralization on this insulating material is accomplished by Environmental Charge Compensation.
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XPS surface analysis of human tooth samples with EnviroESCA
XPS surface analysis of human tooth samples with EnviroESCA
Human teeth from an adult and a baby were studied using EnviroESCA. The results of surface chemical analysis of the as-received human tissues samples are presented. Neutralization of the insulating bio-material is accomplished by Environmental Charge Compensation enabling X-ray Photoelectron Spectroscopy (XPS) on tissues as tooth or bone.
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XPS surface analysis of cuttlefish bone samples with EnviroESCA
XPS surface analysis of cuttlefish bone samples with EnviroESCA
Biological mineral samples from a cuttlefish (sepia) were studied using EnviroESCA. The results of surface chemical analysis of the native and ion implantation treated samples are presented. Neutralization of the insulating biomaterial is accomplished by Environmental Charge Compensation enabling X-ray Photoelectron Spectroscopy (XPS) on tissue samples.
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XPS surface analysis of V2O5 upon heating with EnviroESCA
XPS surface analysis of V2O5 upon heating with EnviroESCA
This application note presents how EnviroESCA can be used to analyze samples during heating under near ambient pressure conditions using various gas atmospheres. Such investigations of surfaces at elevated temperatures and pressures are of paramount importance for catalysis in nature and industry.
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XPS surface chemical analysis of bacterial samples with EnviroESCA
XPS surface chemical analysis of bacterial samples with EnviroESCA
This application note presents how EnviroESCA can be used to analyze bacterial samples under near ambient pressure conditions in various states of hydration using different levels of humidity. Such investigations of bacterial cell wall surfaces in their hydrated state are essential for studying biological interfaces at work.
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Near-ambient pressure XPS of hydrated Escherichia coli samples with EnviroESCA
Near-ambient pressure XPS of hydrated Escherichia coli samples with EnviroESCA
This application note presents how EnviroESCA can be used to analyze E. coli biofilms on silicon under near ambient pressure conditions in various states of hydration. Such investigations of the outer bacterial cell surface in their hydrated state are essential for studying biological interfaces at work.
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Gas Cluster Ion Beam (GCIB) sputtering with EnviroESCA
Gas Cluster Ion Beam (GCIB) sputtering with EnviroESCA
This application note presents how the optional GCIB source at the EnviroESCA can be used to clean samples prior to XPS analysis to get reproducible analytical data and reliable quantification results. Moreover, Argon cluster Arn⁺ (n=500-5000) sputtering and depth profiling of (bio)organic samples is possible which cannot be done when using monoatomic Arn+ sputtering.
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Corrosion study of a paper clip in vinegar with EnviroESCA
Corrosion study of a paper clip in vinegar with EnviroESCA
In this note we present (N)AP XPS results from the first comparative ex-situ and operando corrosion study on the reaction of commercial paper clips in concentrated vinegar solution containing 25% acetic acid.
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