Persistence of the Au(100)-(1×1) M¯3surface state at the Cr/Au(100) interface: Epitaxial growth of a gold-chromium alloy

Abstract

The evolution of the Au(100)-(1\ifmmode\times\else\texttimes\fi{}1) Tamm surface state with M${\ifmmode\bar\else\textasciimacron\fi{}}_{3}$ symmetry upon deposition of Cr in the range 0--4 monolayers (ML) is studied by angle-resolved photoelectron spectroscopy and low-energy electron diffraction. Deposition of a small amount of Cr (\ensuremath{\sim}0.3 ML) on room-temperature clean Au(100)-(5\ifmmode\times\else\texttimes\fi{}20) quickly converts the reconstructed surface to Au(100)-(1\ifmmode\times\else\texttimes\fi{}1) with the concomitant M${\ifmmode\bar\else\textasciimacron\fi{}}_{3}$ surface-state emission. With increasing Cr coverage the surface state progressively shifts from 1.6 to 2.7 eV binding energy, decreases in intensity, and eventually vanishes near 2 ML. When the substrate temperature is held at 400 K or higher during evaporation, the surface state even subsists up to coverages as high as 3 ML with an intensity comparable to clean Au(100)-(1\ifmmode\times\else\texttimes\fi{}1). We present photoemission data from coevaporated Au-Cr alloys as well as oxidation experiments that demonstrate that this remarkable behavior is the result of surface intermetallic alloy formation. Actually it is found that the first 2 or 3 ML of Cr, deposited on Au(100) at room temperature or above, in-diffuse and occupy Au lattice sites forming an epitaxial substitutional ${\mathrm{Au}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$${\mathrm{Cr}}_{\mathrm{x}}$ (x\ensuremath{\le}0.5) alloy. The latter exhibits a Au-rich surface with Au(100)-(1\ifmmode\times\else\texttimes\fi{}1) patches covering essentially the whole surface for substrates held at 500 K during evaporation, and the highly surface localized Au(100)-(1\ifmmode\times\else\texttimes\fi{}1) M${\ifmmode\bar\else\textasciimacron\fi{}}_{3}$ state, which is derived from in-plane Au 5${d}_{\mathrm{xy}}$ orbitals, persists as a true surface state in a Au(100)-(1\ifmmode\times\else\texttimes\fi{}1) monolayer supported on ${\mathrm{Au}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$${\mathrm{Cr}}_{\mathrm{x}}$(100).