X-ray reflectivity studies of liquid metal and alloy surfaces

Abstract

Surface-induced atomic layering at the liquid/vapor interface in liquid metals has been observed using x-ray reflectivity on sputtered clean surfaces under ultrahigh vacuum conditions. A well-defined quasi-Bragg peak is obtained for surfaces of elemental Ga and a Ga-In alloy at large wave vectors q z ϳ2.3-2.5 Å Ϫ1 . These results are an unambiguous indication of atomic layering with an interlayer spacing dϳ2/q z ϭ2.5-2.7 Å. For liquid Ga, the amplitude of the electron-density oscillations, which is significantly underestimated by existing theory and molecular simulation, decays with a characteristic length of 6 Å, which is twice that of Hg. Results on the alloy show a clear enrichment of indium at the topmost surface layer, consistent with the Gibbs adsorption rule. The enrichment consists of a single monolayer, with subsequent layers at the bulk eutectic composition. In order to suppress mechanically excited surface waves, the measurements were performed on thin liquid metal films (Ͻ0.5 mm deep͒, which leads to a macroscopically curved surface due to the large surface tensions in liquid metals. The experimental challenges posed by measurements on curved surfaces and the techniques that were developed are discussed in detail. ͓S0163-1829͑97͒01324-6͔

Temperature- and potential-dependent structure of the mercury-electrolyte interface

“…This surface layering was first verified experimentally at the liquid-air interfaces of Hg [3] and Ga [4] by x-ray reflectivity (XRR) measurements. Subsequent experimental studies demonstrated such layering also for numerous other liquid metals [4][5][6][7][8][9] and liquid metal alloys [10][11][12][13][14][15].…”

Atomic-Scale Surface Demixing in a Eutectic Liquid BiSn Alloy

“…Over the last decade XR revealed the long-predicted surface-induced atomic layering at the liquid-vapor interface for a number of elemental liquid metals [7][8][9][10][11]. Resonant XR near an absorption edge resolved the density profile of each component in GaIn [2], HgAu [3] and BiIn [4] liquid binary alloys. The enhancement of the concentration of the low-surfacetension component was invariably found to be confined to the topmost surface monolayer, with subsequent layers having the composition of the bulk, in accord with the simplest, and widely used, interpretation of the Gibbs rule.…”

Growth of atomically flat nanofilms and surface superstructures of intrinsic liquid alloys

“…At ⌰ Ga = 5, the QS structure suddenly disappeared, and the typical depth of holes decreased to about 2.7 Å. This value is close to the distance between nearest neighbor atoms in bulk liquid Ga ͑2.56 Å͒ 19 and also the spacing between nearest atomic layers ͑2.5-2.7 Å͒ at liquidvapor interfaces of Ga, 12 In, 20 and eutectic In:Ga alloy, 12 suggesting formation of atomic layers with the 5 ϫ 5 structure and no trace of the ␣ -Ga͑001͒ structure.…”

“…11 During further growth of Ga, soluble In atoms diffuse into Ga, resulting in the formation of an outermost In ML. 12,13 To change the morphology of grown Ga from droplets to a flat film, the surface energy of Si ͑ Si Ϸ 1400 dyn/ cm at 30°C͒ 14 must be larger than the sum of energy of the film surface fs and energy of the Si-Ga interface Si-Ga . Segregation of In should reduce fs , since the surface energy of In ͓575 dyn/ cm ͑unnaturally assuming that it is liquid at 30°C͒ 15 and 650 dyn/ cm ͑solid͒ 16 ͔ is lower than that of Ga ͓718 dyn/ cm ͑liquid at 30°C 15 ͒ and 800 dyn/ cm ͑solid͒ 16 ͔.…”