Sample preparation

All NanoESCA measurements are extremely surface sensitive. As an approximate guide, XPS measurements probe up to 10 nm of the sample surface, ARPES and UPS measurements probe between 0.5 and 1 nm, and LEED measurements typically probe the first few monolayers. Preparing your sample to be free of surface contamination and representative of the material under study is crucial to obtaining good results.

General considerations

All samples must be delivered to the NanoESCA at least 24 hours prior to the scheduled experiment. This ensures that the sample can be introduced to ultra-high vacuum (UHV) and pumped down in time. If your sample outgasses, it may take longer, or in worst cases it may not be possible to continue with your experiment.

Materials

A variety of material forms may be analysed using the NanoESCA. All materials must be suitable for UHV, i.e. low outgassing.

  • Single crystals
  • Thin films on a substrate
  • Flakes on a substrate
  • Foils
  • Powders (XPS only)
  • Drop cast from solution (XPS only)

PEEM and/or ARPES

Materials for PEEM and/or ARPES must also be metallic or have good conductivity. In particular, they must have an electrical path from the surface analysis region to ground (via the sample holder). Samples must also be as smooth and flat as possible to avoid large electric fields during analysis. Rough samples are usually not suitable.

Typically, samples are grounded by the sample holder at the top of the sample (same side as the analysis region), so metallic films on an insulating substrate are usually suitable, as long as the metallic film is continuous across the surface. Similar restrictions apply for LEED measurements.

XPS

For XPS, insulating samples may be measured under favourable circumstances by the use of a flood electron source, which compensates for the charge built up during photoemission. Note that this typically requires optimisation for each individual sample, and therefore extends the measurement time.

Ex situ preparation

Before your sample is introduced to UHV, take extra care to keep it clean and free from contaminants. For example:

  • Do not handle the sample or any material intended for UHV with your hands. Always use gloves.
  • Do not allow your sample to contact water or moisture (unless this is part of your experiment, and only by arrangement). Wash/rinse samples with IPA if necessary.
  • Avoid contamination with any oily substances or residues. An ultrasonic bath in IPA may help if you suspect contamination.
  • Do not use acetone to clean samples (acetone reacts with filaments within the NanoESCA chamber). Rinse any samples that have been exposed to acetone with IPA.
  • For powder samples, try to store the powders in an inert and dry (moisture-free) environment.
  • For air sensitive samples, please discuss with NanoESCA staff prior to the measurement. In most cases, it should be possible to keep air exposure to a maximum of < 5 mins, and in some cases (e.g. exfoliation) even a matter of tens of seconds.

Sample mounting

The NanoESCA uses Omicron flag-style sample holders, including flat plates and square/circular PEEM holders. The maximum dimensions of samples for PEEM and/or ARPES are 10.4 x 10.4 mm (square holder) or 10.5 mm diameter (circular holder); the minimum dimensions for both holders is approx. 3 mm width or diameter. 

In situ sample cleaning

Unless you are cleaving or exfoliating your sample (see below), most samples require cleaning under UHV before analysis to remove contaminant (mainly carbon-containing and water) species from the atmosphere. The amount of time required and degree of temperatures needed depend on how clean the sample was before it was introduced to UHV, what the contaminant species are, how reactive the surface is, as well as its surface area (powders) and how porous it is. The following cleaning procedures are standard:

  • For low contamination, gentle heating to 250 - 300 °C for 30 - 60 mins.
  • For high levels of contamination, Ar+ ion sputtering can be used to physically remove (etch) material from the surface. Note that this also introduces disorder to the surface, and is not generally suitable if subsequent ARPES and/or LEED measurements are required.
  • Annealing up to 1300 °C is available, and can be performed up to 900 °C under partial gas pressures by arrangement. Annealing a (gently) sputtered sample can help to reorder surfaces, depending on the chemistry of the surface. Annealing to high temperatures may also be required to remove strongly bonded contaminant species. Partial gas pressures during annealing help to limit the preferential loss of reactive constituents, e.g. maintaining the correct oxygen stoichiometry in oxides.

In situ cleaving or exfoliation

Alternatively, it is possible to reveal a fresh (pristine) sample surface in situ, either by physically cleaving the sample, or via mechanical exfoliation. Both of these approaches are available in the NanoESCA at room temperature. It is currently not possible to cleave at low temperature. Cleaving and/or exfoliation is usually straightforward on layered (e.g. 2D) materials, but may be impossible for hard, 3D materials. Surfaces obtained this way are usually acceptable for LEED and ARPES, though it is common to attempt multiple cleaves before finding a suitable surface. Note that surfaces may still reconstruct, and multiple cleavage planes are often available, so remember you are still measuring a surface, which may or may not be representative of the bulk.