Atomic force microscopy measurements of topography and friction on dotriacontane films adsorbed on a SiO2 surface

Abstract

We report comprehensive atomic force microscopy ͑AFM͒ measurements at room temperature of the nanoscale topography and lateral friction on the surface of thin solid films of an intermediate-length normal alkane, dotriacontane ͑n-C 32 H 66 ͒, adsorbed onto a SiO 2 surface. Our topographic and frictional images, recorded simultaneously in the contact mode, reveal a multilayer structure in which one to two layers of molecules adsorb adjacent to the SiO 2 surface oriented with their long axis parallel to the interface followed by partial layers of molecules oriented perpendicular to the surface. The thicknesses of the parallel and perpendicular layers that we measured with the AFM agree with those inferred from previous x-ray specular reflectivity measurements on similarly prepared samples. We also observe bulk dotriacontane particles and, in contrast with our previous measurements, are able to determine their location. Above a minimum size, the bulk particles are separated from islands of perpendicularly oriented molecules by regions of exposed parallel layers that most likely extend underneath the particles. We find that the lateral friction is sensitive to the molecular orientation in the underlying crystalline film and can be used effectively with topographic measurements to resolve uncertainties in the film structure. We measure the same lateral friction on top of the bulk particles as on the perpendicular layers, a value that is about 2.5 times smaller than on a parallel layer. Scans on top of parallel layers indicate a constant height but reveal domains having different sublevels of friction. We explain this by the domains having different azimuthal orientations of the molecules.

Nanoscale observation of delayering in alkane films

“…Nanoscale observation of delayering in alkane films X-ray diffraction [11] and contact mode AFM [12] measurements indicate that these particles have an orthorhombic structure in which the C32 molecules are aligned perpendicular to the SiO 2 surface. Because these mesa-shaped orthorhombic particles appear on annealing and retain their structure on cooling to room temperature as well as in a subsequent heating cycle, we believe they represent the equilibrium structure at room temperature.…”

“…X-ray specular reflectivity [11] and contact mode AFM measurements [12] at room temperature indicate that these perpendicular monolayer islands reside on one to two layers of C32 molecules oriented with their long axis parallel to the SiO 2 surface. Because of their larger area of contact, the parallel molecules are presumably bound more strongly by van der Waals forces to the SiO 2 surface than the perpendicular molecules are to the parallel layers.…”

“…The image shows an island of adsorbed C32 having a "dragonfly" shape. Topographic cross-sections indicate the height of this feature to be ∼ 4.2 nm after calibration using the AFM contact mode [12,13] and X-ray specular reflectivity measurements [11]. Because this height is approximately equal to the all-trans length of the C32 molecule, we interpret the dragonfly feature as consisting of a single layer of molecules oriented with their long axis perpendicular to the surface.…”

“…Using silicon cantilevers with a resonance frequency typically of ∼ 33 kHz, we simultaneously recorded AFM images of topography and phase angle (cantilever oscillation relative to the drive) as a function of temperature. The samples reported on here were made by dip-coating an acid-cleaned, electronicgrade Si(100) substrate in a solution of C32 dissolved in heptane (C7) [11,12] with alkane purity > 99.9%. The Si(100) wafers typically had native oxide coatings with thickness in the range 12-25Å [11].…”

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

“…1(a): partial layers in which the rod-shaped molecules are oriented with their long axis perpendicular to the SiO 2 surface (labeled y and z), the so-called perpendicular layers, coexisting with preferentially oriented nanoparticles (labeled {) [13]. Both the perpendicular layers and the nanoparticles reside on one or two layers of molecules aligned parallel to the SiO 2 surface (the dark areas labeled x), the so-called parallel layers.…”