µARPES Samples

This guide is for preparing bulk crystalline samples for microARPES. 

For exfoliated flake samples, please follow the guidance for nARPES samples.


Guide to mounting and post-cleaving samples.

1) Ensure the sample is conducting (as in all ARPES). 

2) Identify the crystallographic orientation. 

a) by Laue Diffraction.  The ALS has a Laue setup, but we do not always have time to help with training and operation, so it is best if this work is done at the home institution.  For assistance with Laue at ALS, contact Jonathan Denlinger, jddenlinger@lbl.gov.

b) by crystal form. Although crystals can be roughly shaped, the general shape can indicate the orientation of the surface of interest (see figure below). This surface needs to be mounted (ideally) parallel to the sample holder surface.

Figure Credit: Geologyin.com

3) Affix the sample to the sample holder (see figure below).

a) choose epoxy.   We recommend, and provide at the beamline, two kinds:

i) Epotek H20E which is conductive, and its curing can be accelerated by heating on a hot plate.

ii) Loctite EA1C also known as "Torr Seal", "Epoxy Patch" or "Hysol 1C".  Stronger than silver epoxy but has the disadvantage of being insulating.

b) Ensure sample is well secured. Epoxy should wrap around the edges of the sample (but not all the way to bury the entire crystal.

c) Use suspended graphite paint  (not needed for conductive epoxy) to ensure a conducting path.  Sample sides should be coated with a bridging conductive path to the metal sample puck.  Avoid depositing graphite on top surface as much as possible.

4) Glue a post to the sample surface.  Epoxy may be conductive or insulating.  

a) We use small ceramic posts with diameter typically comparable or less than the sample diameter.  But any short, round, stiff rod will do as long as it is vacuum compatible.

b) Epoxy should be wicked onto the post. It is best not to let the epoxy waterfall over the sample edge, in order to avoid contacting the epoxy layer at the sample's bottom.

c) total height must be <10mm but we highly recommend minimizing this to ~5mm to avoid accidental cleaving during sample transport in our vacuum chambers.

d)The sample should be located in the range accessible to the motors, which is roughly 0.75" diameter (microARPES) or 0.5" diameter (nanoARPES)

5) PRACTICE CLEAVING.  We appreciate that samples are precious to you and that you don't want to waste any.  However our instrument time is also precious to both you and us.  If you want to increase the chance of success please practice cleaving your samples in air or in our glovebox. Examine the sample surface in our microscope: are there flat, mirror-like areas larger than our minimum spot size (12µm)?  Did the sample cleave in the expected direction?  Is the epoxy strong enough or did the post just fall off without cleaving the surface?

6) Ideas to help improve cleaving.

a) cleave in our glovebox with your hands instead of in the vacuum chamber. Sometimes it takes more strength to cleave than the wobble stick can provide. Insert sample into vacuum immediately after cleaving/microscopy.

b) cleave at low temperature. Many materials cleave better at <50K than at RT.

c) consider cleaving along a different surface orientation.  What you think is the easy cleavage plane might not be how the crystal cleaves.

d) follow epoxy mixing directions precisely.  Measure the epoxy components as accurately as you can using a digital balance.

e) stronger epoxy.  We sometimes carry Loctite Stycast 2350 (it has a short shelf life) which is the strongest insulating epoxy we are familiar with, that also works well at low temperature.  

f) cure epoxy longer, or at higher temperature. The guidelines for the H20E suggest a range of temperatures/times. We have found hotter + longer leads to a stronger bond

g) scribing crystal in the side before cleaving.  We have heard rumors of this working but we do not have experience ourselves.

h) put multiple samples on one sample holder as in the illustration at the top of this page. This increases the chance of success.

-- avoid so many samples that the posts shadow neighboring samples from the photon beam.

-- keep all samples within the maximum envelope (0.5" for nARPES, 0.75" for µARPES), see figure above.

i) if your samples are rare, consider epoxying a wire to the top post to retain the port that is normally lost to the bottom of the chamber. This can be reglued to the holder for another cleaving attempt.  Note: this must be done very carefully. The wire should not be so long that the dangling post gets caught in our transfer mechanisms. (see picture below). Nor should the dangling post interfere with the xray beam or electron path to the analyzer.