Adsorption of gold on low index planes of rhenium

Abstract

On atomically rough areas of a thermally cleaned rhenium field emitter, adsorbed gold behaves like it does on tungsten. The average work function \\ ̄ gf increases at low average gold coverage \\ ̄ gq due to formation of gold-rhenium dipoles, and at high coverage a structural transformation in the gold layer leads to a \\ ̄ gq-independent work function. Broadly similar behaviour is found for gold on the low-index planes of tungsten, but on low-index rhenium planes gold behaves rather differently. When thermally cleaned at > 2200 K and annealed below 800 K, the work function, φ(clean), of (101̄1̄) takes one of two values 5.25 ± 0.04 eV, and 5.36 ± 0.04 eV, which are tentatively attributed to the two possible structures of this plane. Similar behaviour is expected and observed for (101̄0),but the values taken by φ(clean) are not well defined. Both forms of (101̄1̄) are thought to undergo reconstruction above 800 K forming a single structure with φ(clean) = 5.55 ± 0.03 eV. (112̄0) and (112̄2̄) each have only one possible structure, and in keeping with this, φ(clean) has a single well-defined value for each plane. The flatness of (101̄1̄) and (101̄0) leads to field reduction at their centres which produces an increase in their measured work functions by up to 10%. The initial increase in φ produced by gold condensed at 78 K and spread at low equilibration temperatures T s on (112̄2̄), (101̄1̄) and (112̄0) is attributed to gold-rhenium dipoles, which, on the latter two planes approximate to the Topping model, giving dipoles characterised by μ 0(1011) = 0.1 × 10 −30 C-m with α = 10 Å 3 and μ 0(112̄0) = 0.32 × 10 −30 C-m with α = 22 Å 3, where μ 0 is the zero-coverage dipole moment and α its polarizability. Failure of the Topping model on (112̄2̄) is attributed to its atomically rough structure. No dipole effect is seen on (101̄0). Energy spectroscopy of electrons field emitted at (202̄1̄) and (101̄1̄) demonstrates the non-free character of electrons in rhenium, while the small effect of adsorbed gold strengthens the belief that gold is bound through a greatly broadened 6s level centred 5.6 eV below the Fermi level and the dipolar nature of the bond supports this model. At higher values of T s and \\ ̄ gq gold appears to form states which are well-characterised by a coverage-independent work function. (101̄0), (101̄1̄) and (112̄0) each form two such states, one in the range 2 < \\ ̄ gq < 4 (state 1), and the second at \\ ̄ gq > 4 (state 2). The atomic radii of gold and rhenium are thought to be sufficiently similar to allow the possibility that state 1 is a replication of the Re plane structure by gold. The high work function and thermal stability of state 2, taken together with the observed temperature dependence of the transformation of state 1 to state 2, encourages the belief that state 2 results from atomic rearrangement of state 1 into a close-packed Au(111) structure. State 2 also forms on (112̄2̄) and the absence of state 1 on this plane suggests some surface alloying at coverages below 4 \\ ̄ gq.