RECOMENDACIONES Y EVALUACION

Single–grain quasicrystal ingots can be synthesised using normal crystal growth techniques e.g. Bridgman, flux-growth. The most common technique used to grow quasicrystal samples is the Czochralski technique. The set–up for this technique is shown in Figure 33, where a seed crystal is pulled at a very slow rate from a melt of the sample composition. The even shape of the ingot is increased by rotating the pulling rod. The temperature of the furnace at the growth front is set so that a meniscus forms at the solid–liquid interface. A sample with a certain crystallographic orientation can be grown using a seed crystal of the same phase and orientation, this is termed homogeneous growth. Quasicrystal samples are cut along the required crystallographic direction from the ingot grown. The quality of the sample structure is then determined using x–ray or electron diffraction.

Figure 33: Set–up used in the Czochralski crystal growth technique. C: crystal grown M: melt U: protecting envelope E: heater, R: pulling rod, N: thin neck, S: seed crystal, K: seed carrier, T: thermocouple, B: susceptor. Reprinted from [133].

The quasicrystal samples are carefully polished to significantly reduce the surface roughness on the macro–scale. The surface is polished with different grade diamond paste (0.25 - 6 µm) and rinsed with methanol between stages. The sample is then cleaned of contaminants using an ultra–sonic bath of methanol. The resulting surface is sufficiently flat for many surface science techniques.

The dried sample is subsequently mounted on a plate designed for transfer and treatment in the vacuum system. The plate is normally made of Mo, Ta or Cu and

the sample is spot welded to the plate with Ta wires or strips.

Quasicrystal samples are treated in–situ to produce a bulk–truncated surface by cycles of sputtering and high–temperature annealing. Contaminants are cleaned from the surface by ion bombardment (generally referred to as sputtering) with noble gas ions (e.g. Ar, Ne). The sputter ion gun is operated at grazing incidence relative to the sample so as to increase the surface flatness and to avoid embedding of sputter gas atoms in the surface layers. The lines containing the noble gas are over–pressured (above atmosphere) to stop diffusion of atmospheric gas into the line. The noble gas pressure is flowed at a high enough pressure to produce an emission current in the order of 10-3A. The ion density and mean free path are optimised to maximise the ion current detected at the sample. Sputtering removes the surface layers to produce a off–stoichiometry, disordered surface. In the case of quasicrystals sputtering has been shown to produce nano–crystallites at the surface [50].

Figure 34: A survey XPS spectrum obtained from the surface of i–Al–Pd–Mn after a sputter cycle of 75 minutes length.

As an example of the effects of UHV preparation on a chemically complex surface, Figure 34 shows an XPS survey scan from the i–Al–Pd–Mn five–fold surface. After being introduced to the UHV environment the surface has been sputtered for 75 min- utes with Ar+ at 2.5 keV. The XPS spectra are used to calculate the atomic fraction

of elements in the sample region scanned by fitting the area of each bulk peak and taking the element sensitivity factors into account assuming an even distribution of elements in this region. The XPS spectrum shows that the chemical composi- tion of the surface has changed from the bulk composition of Al70.5Pd19.5Mn8.5 to Al60.4Pd28.7Mn11.0. The depletion of Al in the surface layers demonstrates that the Ar ions preferentially removes the element with the least mass from the surface. It is also notable that there is only a very small oxygen OKLL signal present. Further sputtering cycles were applied to the sample for a period of half an hour with a lower ion kinetic energy.

Figure 35: A survey XPS spectrum obtained from the surface of i–Al–Pd–Mn after an anneal of 3 hours.

Following sputtering, the sample was then heated in UHV to near melting tem- perature to use the thermal energy to restore bulk composition at the surface. This process is termed annealing the sample. It is carried-out usinge–beam or thermionic heating applied behind the sample plate on the manipulator arm. The initial anneal- ing conditions used were 3 hours at a temperature of 920 K. The time for subsequent anneals was 2 hours. Prolonged annealing enables the formation of larger flat ter- races which are sufficient to produce clear LEED patterns and improve STM scans.

Longer anneals at higher temperatures may yield pits and voids in the terraces. Figure 35 shows a survey spectrum from the surface following the first anneal of 3 hours. There is an increase of the O 1s and OKLL signals and also a small C signal. These contaminants are due to diffusion from the bulk of the sample and from degassing of the surrounding area e.g. the sample plate. The surface is now Al rich with a calculated composition of Al82.2Pd11.5Mn6.3.