DO’s and DON’Ts with metal crystals

Metal leaf springs, typically made out of higher melting point refractories, do not fixate a crystal in a rigid manner. They thereby prevent a lot of damage to the crystal since they allow for thermal expansion during annealing.

When a metal crystal is delivered and introduced into UHV, it carries with it a few manolayers of water and hydrocarbons that are adsorbed from ambient. Since carbon is bulk soluble in most metals to some degree, it is normally wise to initiate the preparation of a metal crystal by prolonged sputtering, i.e. several monolayer equivalents, followed by a stepwise, but gentle ramping up of the anneal temperature to prevent the carbon layer from diffusing into the bulk during annealing.

Occasionally a dust particle will settle on the polished surface of the crystal prior to mounting in the experimental setup. To remove it, dry nitrogen should be used and should be sufficient to blow such particulates off the surface. In a few remote cases, the particle will still stick to the surface and cannot be removed. In that case, acetone p.A. can be used to clear the surface. Drop a single droplet of acetone p.A. on the polished surface, and blow it off the polished surface under an angle using dry nitrogen before it has a chance to dry and form stains. Do not use ethanol, since it has the tendency to leave a hazy, organic residue on the surface that will then need to be removed through extensive sputtering and annealing.

When a crystal is delivered it carries with it a few monolayers of adsorbates from ambient, typically water and some hydrocarbons. The carbonaceous content of these adsorbates will be annealed into the near surface portion of the bulk of the crystal if the crystal is immediately annealed to very high temperatures prior to any sputtering. After such an initial cycle, it will become virtually impossible to remove it through sputtering and annealing as it spreads further through the bulk with every anneal cycle. Silver, with a small, but non-negligible bulk solubilty for carbon, is a notorious case where this has a tendency to happen, and it can in cases be wiser to repolish a crystal to fully remove the carbon-poisoned layer of the crystal instead of persisting in removing the impurity through extensive sputtering/annealing.

The most common kind of damage we observe in crystals that are offered for repolishing are large plastic deformations around places where the crystal was previously clamped or held in place. It is important to realize that crystal holders and components clamping crystals are subject to considerable temperature variations when a crystal is annealed. As a result thermal expansion occurs and this can cause something like a thick plate, screw head or other fixating element that is loosely holding the crystal in place at room temperature, to turn into a virtual death trap for the delicate single crystal when it is heated and expands. It is important to allow for thermal expansion of the crystal prefereably through leaf spring clamps, and where this effect cannot be prevented and a hard fixation is required, to design a crystal using one of several options that we offer like ledges or slits in a crystal to confine the deformation to a region of the crystal that is not under investigation.

Although at first nothing appears wrong with the idea, and the metal crystal will come out of the ultrasound in one piece, it can immediately be trashed after this treatment. The ultrasound couples into the crystal lattice and tends to induce polycrystallinity in the surface region of the crystal. The degree to which this occurs depends on the material and the details of the ultrasound treatment. Once the polycrystalline region is formed, annealing it will lead to a further recrystallization in the surface region.

A cotton tip contains many particulates with an abrasive action that are substantially larger than those that are used to polish the surface of a crystal. Whereas you may get away with using a cotton tip on most harder materials, on softer, lower melting point metals, they will do irreparable damage.