Nanoscale observation of delayering in alkane films

Abstract

-Tapping-mode Atomic Force Microscopy and synchrotron X-ray scattering measurements on dotriacontane (n-C32H66 or C32) films adsorbed on SiO2-coated Si(100) wafers reveal a narrow temperature range near the bulk C32 melting point T b in which a monolayer phase of C32 molecules oriented perpendicular to surface is stable. This monolayer phase undergoes a delayering transition to a three-dimensional (3D) fluid phase on heating to just above T b and to a solid 3D phase on cooling below T b . An equilibrium phase diagram provides a useful framework for interpreting the unusual spreading and receding of the monolayer observed in transitions to and from the respective 3D phases.

Structure and phase transitions of monolayers of intermediate-length n-alkanes on graphite studied by neutron diffraction and molecular dynamics simulation

“…37 In particular, nanoparticles of C32 supported on a solid surface are believed to undergo a transition from an orthorhombic structure at low temperature to a nearly hexagonal phase in which the molecules diffuse rotationally about their long axis and translate parallel to it. 3,38 Quasielastic neutron scattering measurements and MD simulations are consistent with both of these types of molecular diffusive motion occurring in the C24 monolayer smectic phase along with motions associated with conformational changes in the molecules. 4 The neutron diffraction and MD results that we have presented here indicate that these diffusive motions occur within a 2D lamellar structure characterized by a shorter coherence length than the low-temperature crystalline phase.…”

“…2 It may also help explain whether or not complete wetting of the solid film occurs at a higher coverage. 3 Knowledge of the monolayer structure is also essential for understanding its dynamical properties, including both vibratory 4 and diffusive 5,6 motion.…”

“…3,24,25 Of these systems, probably the best characterized are monolayers of C32 and C24 on high-quality graphite basal-plane surfaces that have been investigated by scanning tunneling microscopy ͑STM͒, [10][11][12][13][14] x-ray diffraction, 15 and neutron scattering. [16][17][18][19][20][21] Graphite substrates also have the advantage that relatively reliable empirical atom-atom potentials are available for representing the molecule/graphite basal-plane interaction.…”

Localized diffusive motion on two different time scales in solid alkane nanoparticles

“…When these samples are thermally cycled in air above the bulk melting point, there is a tendency for the nanoparticles to grow at the expense of the partial perpendicular layers. This behavior is consistent with a Stranski-Krastanov growth mode at equilibrium in which preferentially oriented bulk particles nucleate above the parallel layers of the film [7]. Our samples of C32 nanoparticles were made by dipcoating an acid-cleaned, electronic-grade Si(100) wafer (5 cm diameter and 0.3 mm thick polished on both sides) in a solution of C32 dissolved in heptane (C7) [7,14].…”

“…The structure [3] and associated dynamics [4][5][6] of these so-called "rotator" phases has remained an active area of research as normal alkane chains are the basic building blocks of a wide variety of soft matter, including polymers, liquid crystals, and lipids [1]. More recently, interest in lubricants, surfactants, and lipid membranes [1] has led to the investigation of rotator phases in restricted geometries such as alkane nanoparticles [7] and monolayer alkane films [8].…”