The effect of roughness on the T3-dewetting of molecular hydrogen

Abstract

Triple-point dewetting is a well-known behaviour of molecular hydrogen and other van der Waals systems like noble gases on a solid substrate. Recent theoretical and experimental investigations (Phys. Rev. Lett. 88 (2002) 55702) suggest that it is caused primarily by the roughness of the substrate. Strain induced due to the mismatch of the lattice constant of the substrate and the growing layers of the adsorbed materials is increased by the micro-roughness of the substrate which eventually leads to the growth of only a thin solid film of the adsorbate. The dominating role of the substrate roughness is demonstrated, e.g., by ellipsometric measurements on smooth Si surfaces (rms 0.15 nm), where a thicker solid hydrogen film than the 3 monolayers on "usual" substrates is observed. We present a way to modify and improve the surface quality of snbstrates for such wetting studies of solid van der Waals films. substrate-adsorbate interaction. Recently, a more realistic calculation has been done by Esztemann et al. [4], in which the roughness profile of the substrate is taken into account. The computed results OF that investigation predict a significant variation of the film thickness on substrates with different roughnessas, i.e., the adsorbed film will be thicker on the smoother substrate, and for a certain range of substrate strength ( 1 . 5~ R 6 3 ) it will diverge as the roughness goes to zero.In this work we present results of adsorbed HI on a Si-wafer with an average roughness (ms) of 0.15 nm. Its surface is distinctly smoother than the previously used evaporated gold substrates on glass [3,4], which had an rms roughness of 1.3 nrn and more. We use an elIipsometry setup, as shown in Fig. 1, to measure the Hz film thickness on the Si substrate. The sample is mounted vertically in a Cu cell with windows for optical access to the sample surface. The whole assembly is placed inside an optical flow cryostat. A laser diode provides a very stable laser beam which passes through a polarizer and compensator before entering the cryostat. After reflection on the sampIe the beam leaves the cryostat, passes through an analyser and is detected by a photodiode.0921-4526/03/% -see front matter 0

Triple-point wetting of molecular hydrogen isotopes

“…In all the experimental runs presented here, first an adsorption isotherm was taken at a temperature well above the triple temperature of the respective adsorbate, as in earlier experiments [21][22][23]28]. A sufficient amount of gas was condensed into the system that gas-liquid coexistence was reached, and bulk liquid formed at the bottom of the sample cell.…”

“…SPS has been applied for measuring film thicknesses on the surface of Au [23], while ellipsometry has been used for measurements on the surface of Si [28]. The substrate was mounted vertically inside a copper cell.…”

“…In our measurements, we used two well-known techniques for determining the film thickness, namely surface plasmon spectroscopy (SPS) and ellipsometry. SPS has been applied for measuring film thicknesses on the surface of Au [23], while ellipsometry has been used for measurements on the surface of Si [28]. The substrate was mounted vertically inside a copper cell.…”

“…The typical rms roughness of an Au substrate is measured to be about 1.3 nm; for commercial Si wafer surfaces it is ∼0.15 nm [28], one order of magnitude smoother. The van der Waals interactions for H 2 on Au and Si are nearly the same [40,[56][57][58].…”

“…Films of H 2 -D 2 mixtures [22,23] and thick quench-condensed films [24][25][26][27] have been investigated via these methods. A few measurements [28,29] have also been performed on the surface of Si. There are hints that strain in the solid films, induced by surface roughness, is important for T 3 dewetting, in addition to the lattice mismatch effects mentioned above.…”