Sample Preparation Methods
Sample Preparation Variables
- Angle lapping
- Ball cratering
- Blisters – break open with tweezers
- Carbon is often deposited by SEM
- Chemical derivatization
- Chemical etch
- Clean sample mount
- Clean with gas jet
- Clean with solvents
- Damage can be caused by SEM
- Doping: n-, p- vs none
- Floating or grounded
- Fracture (in air, in LN2, under solvent)
- Glove bag (argon fill)
- Horizontal traps
- In air – oxidation
- In nitrogen – drying
- In vacuum – drying – cooling
- Indium foil – press onto
- LN2 fracturing in air
- Magnetic sample
- Need to cut to fit on sample mounts
- Never store in Plastic bags – contaminated inside
- Never touch Surface Contact
- Oil analysis
- Outgassing materials
- Plasma clean
- Powder tray
- Pressed pellet
- Sample conductivity
- Sample damage
- Sample degradation
- Sample fracturing
- Sample history
- Sample mounting
- Sample porosity
- Sample preparation
- Sample roughness
- Sample scraping
- Sample shape
- Sample treatments
- Sample storage
- Scrape (in air, under hexane or other solvent)
- Scribe Analysis Area for transparent sample
- Selective rinsing with solvents
- Sequence of analysis
- Super gentle ion etching
- Surface derivatization
- Surface roughness
- Tools for preparation
- Use Double-sided tape to hold sample
XPS Sample Preparation
by NESAC/BIO an advanced Polymer Analysis Lab at the University of Washington
Gloves: Use only polyethylene gloves. Other gloves may contain silicones that can contaminate the surface.Clean: Use only clean utensils (tweezers, etc) when handling samples. Make sure theses have been cleaned to remove hydrocarbon and silicone contaminants and dedicate their use to ESCA samples only. Keep your samples in a “clean” environment. A laminar flow hood or a clean laboratory environment is strongly suggested.
Common Contaminants: Common surface contaminants include:
|Hydrocarbon: Pump oil, greasy finger prints, dirty dessicators, dirty solvents.|
|Silicones: Non-approved gloves, glass-fitting-grease, dirty dessicators, hair, hand lotion.|
|Salts: Sodium, chlorine, potassium, can be introduced through improper rinsing or exposure to water that has not been properly purified.|
Controls: Always include control samples with the sample of interest. A control could include a sample of just the underlying substrate and/or a sample of the underlying substrate exposed to solvent used for the surface modification without the actual modifying agent.Duplicates: It is best, if possible, to send duplicate samples, even if only one is to be analyzed. Sometimes samples are damaged in transport or, on a very rare occasion, can be damaged while loading for analysis.
Include With Samples:
|Samples Summary: Include a sheet with a list of the specific samples that are included and what is on them.|
|Structure: Proper ESCA analysis requires the structure of the surface bound species and knowledge of the underlying substrate. Without this information accurate analysis of the ESCA data is difficult.|
Packaging: We suggest shipping samples in tissue culture polystyrene (TCPS) dishes sealed with parafilm. To prevent the samples from rattling around during transport, we use a small amount of double sided tape to secure the back of the samples to the bottom of the TCPS dish. It is very important to only stick a small corner of the sample to the tape. If the center of the samples is stuck to the tape or there is tape under the entire sample it is nearly impossible to remove the samples without damaging them. Please try sticking down a “test” sample and be sure it can easily be removed before packaging other samples. We have shipped many samples without incident and have discovered that only taping a small part of the corner is required to secure the sample (see Figure 2).
Know Your Contact: Be sure that you are shipping your samples to the correct address and contact person. Our shipping address is NOT the same as the usual mailing address. The shipping address will be given out once samples have been approved for analysis. Also, inform your contact that the samples are being shipped to ensure that they will look out for your samples, and inform you if they do not arrive. If you have questions, please contact us.
Sample Physical Requirements:
Sample Size: Our ESCA systems can accommodate a wide variety of sizes and shapes in samples. We have also been known to “cut” samples down to size if necessary. Please inform your NESAC/BIO contact of the shape and size of the sample and we will suggest options.
Outgassing: One of the main limitations for ESCA samples is their outgassing properties. ESCA is a technique which MUST be preformed in a vacuum chamber with pressures of ~5×10-9 Torr. (See ESCA Techniques).Samples such as polymerized tetraglyme, “wet” silicones, or any “spongy” type sample which soaks up water will have some trouble pumping down to the appropriate pressure. However, there are methods we can use to analyze these samples, such as reducing the size of the sample. If your sample may present a problem, please notify your contact person and arrangements will be made for special analysis.
Good sample preparation methods are vital in surface science as the signals emanating from surface contamination can overwhelm the signals from the sample. Gloves and clean tweezers must be used and any glassware must be thoroughly cleaned before use. Tweezers should be cleaned regularly by sonication in isopropyl alcohol (IPA).
Samples can be stored or transported in clean poly(styrene) petri dishes and well plates, or clean glass vials. Avoid ALL other plastic containers, including plastic sample bags. A good alternative to plastic or glass containers is new, clean aluminium foil. An argon etch is available to XPS users for in situ sample cleaning. This method is recommended for the removal of thin oxide layers however it will reduce your available analysis time so it should be avoided where possible.
Typical samples for XPS are 0.5 – 1 cm2 in size and up to 4 mm thick. Thicker samples may also be accommodated – please contact us for details.
The Axis Ultra and Thermo XL systems use magnetic immersion lenses to focus the photoelectrons emitted from the surface towards the detector. Magnetic samples can still be analysed, but the experimental set up for these samples is slightly different. If you have magnetic samples that you would like to submit for XPS analysis, then please contact us prior to booking the instrument to discuss the available options.
There are a few universally accepted methods of preparing powdered samples for XPS. Of these the favoured method is to press the powder into clean, high purity indium foil. Alternatively, the powder may be dissolved in a suitable solvent and then drop cast onto the surface of a clean silicon wafer. Finally, powders that can not be prepared by either of the above methods can be either sprinkled onto the surface of sticky carbon tape or pressed into a tablet for analysis. Please discuss these latter two options with the experimental officer prior to booking the instrument.
From PHI Handbook
Preparing and Mounting Samples
For the majority of XPS applications, sample preparation and mounting are not critical. Typically, the sample is mechanically attached to the specimen mount, and analysis is begun with the
sample in the as-received condition. Vital information is often hiding in the as-received surface so it can be vital to analyze the surface as-received. Additional sample preparation is discouraged because any preparation might modify the surface composition. For those samples where special preparation or mounting cannot be avoided, the following techniques are recommended.
1. Removing Volatile Material
Ordinarily, materials known to retain solvents or gases are dried in a separate small vacuum chamber before analysis. In exceptional cases, when the volatile layer is of interest, lhe sample may be cooled for analysis. The cooling must be to a sufficiently low temperature to guarantee that the volatile element is not warmed to evaporation by any heat load that the analysis conditions may impart. Removal of unwanted volatile materials is usually accomplished by long-term pumping in a separate vacuum system or by washing with a suitable solvent. If you rinse a surface with a volatile solvent, then use freshly distilled solvent to avoid contaminating the surface with high boiling point impurities hiding in the solvent. Choice of the solvent can be critical. Hexane or other light hydrocarbon solvents are probably least likely to alter the surface, providing the solvent properties are satisfactory. Samples may also be washed efficiently in a Soxhlet extractor using a suitable solvent.
2. Removing Nonvolatile Organic Contaminants
When the nature of an organic contaminant is not of interest or when a contaminant obscures underlying material that is of interest, the contaminant may be removed with appropriate organic solvents. As with volatile materials, the choice of solvent can be critical.
3. Ion Sputtering – Ion Etching
Ion sputter-etching or other erosion techniques, such as the use of an oxygen plasma on organic materials, may be used to remove surface contaminants. This technique is particularly useful when removing adventitious hydrocarbons from the sample or when the native oxides, formed by exposure to the atmosphere, are not of interest. Argon ion etching is commonly used to obtain information on composition as a function of the exposure time to ion etching. Calibration of the sputter rates can be used to convert sputter time to information on depth into the specimen. Because sputtering may cause changes in the surface chemistry, identification of the changes in chemical stales with depth may not reflect the true composition.
Abrasion of a surface can be done without significant contamination by using a laboratory wipe or a cork. To remove more material, use fine sandpaper, a file or a single edged razor blade. This may cause local heating, and reaction with laboratory air may occur (e.g., oxidation in air or formation of nitrides in nitrogen). To prevent oxidation of more active materials, perform abrasion in an inert atmosphere such as a glove bag or glove box or while immersing the sample in an appropriate volatile organic solvent. The abraded material should then be transferred to the load-lock with immediate pump-down, or if possible load the sample into a high vacuum transfer box in a sealed vessel to preserve the clean surface.
5. Fracturing and Scraping inside High Vacuum
With proper equipment many materials can be fractured or scraped within the test chamber under UHV conditions. While this minimizes contamination by reaction with atmospheric gases, attention must be given to unexpected results which might occur. Fracturing might occur along the grain boundaries which may not be representative of the bulk material. Scraping can cover hard material with soft material whcn the sample is multiphase.
6. Grinding Grains into Powder
If spectra that are more characteristic of the bulk composition are desired, then samples may be ground to a powder in a mortar or if possible fractured in air (e.g. single crystal of NaCl). Protection of the fresh surfaces from the atmosphere is required so use a gently flow or Argon or Nitrogen gas to minimize reaction with air. When grinding samples, localized high temperatures can be produced, so grinding should be done slowly to minimize heat-induced chemical changes at the newly created surfaces. The mortar and pestle should be well cleaned before reuse.
7. Mounting Powders for Analysis
There are a number of methods which can be used to mount powders for analysis. Perhaps the most widely used method is dusting the powder onto a polymer-based adhesive tape with a camel-hair brush. The powder can be dusted across the surface carefully and lightly, with no wiping strokes.
Some researchers shun double-sided adhesive tape for UHV work, but many have successfully used certain types of tape in the 9e(-10) Torr range.
Alternative methods for mounting powders include pressing (he powder into indium or other soft foils, supporting the powder on a metallic mesh, pressing the powder into pellets or simply depositing the powder by gravity.
With the foil method, the powder is pressed between two pieces of pure foil. The pieces arc then separated, and one of them is mounted for analysis. Success with this technique has been varied. Sometimes regions of bare foil remain exposed and, if the sample is an insulator, regions of the powder might charge differently.
Differential charging can also be a problem when a metallic mesh is used to support the powder. If a press is used to form the powder into a pellet of workable dimensions, a press with hard and extremely clean working surfaces should be used. Protecting the surface of the pellet by inserting either fresh Aluminum foil, weighing paper or wax paper.
Gravity can effectively hold some materials in place, particularly if a shallow well or depression is cut in the surface of a 1-2 mm thick of Aluminum metal.
Allowing a liquid suspension of the powder to dry on the specimen holder is also an effective way of mounting a powdered sample on a sample mount.
8. Sample Heights and Instrument Geometries must be Considered
The incident angle of the X-ray beam can be as low as 30 degrees relative to the plane of the sample so be sure that other samples on the sample mount will not block the X-ray beam or the flood gun beam or the Ar+ ion beam if needed. If you can use a rotating sample stage, then it can help avoid blockage of one of the beams.