Surface structure and thermal motion of n-alkane thiols self-assembled on Au(111) studied by low energy helium diffraction

Abstract

Low energy helium diffraction has been used to study the packing and thermal motion of the terminal CH3 groups of monolayers of n-alkane thiols self-assembled on Au(111)/mica films and a Au(111) single crystal surface. At low temperatures (<100 K), the terminal CH3 groups are arranged in domains containing a hexagonal lattice with a lattice constant of 5.01 Å. As the length of the carbon chain is shortened, an abrupt decrease in the diffraction peak intensities is observed for CH3(CH2)9SH/Au(111)/mica, and no diffraction is observed for CH3(CH2)5SH/Au(111)/mica. This is indicative of a sudden decrease in surface order at around ten carbon atoms per chain. A semi-quantitative estimation of the average domain size of each monolayer surface shows a maximum of 46 Å at intermediate chain length [CH3(CH2)13SH/Au(111)/mica], decreasing to 26 Å at longer [CH3(CH2)21SH/Au(111)/mica] and 41 Å at shorter [CH3(CH2)9SH/Au(111)/mica] chain lengths. No phase transitions could be detected at the surfaces of these monolayers from 35 K to 100 K, but as expected for a soft material, the thermal motion of the n-alkane thiol molecules increases with increasing surface temperature and reduces the diffraction intensities to zero at around 100 K. The relative mean square displacements of the surface CH3 groups along the directions perpendicular and parallel to the surface have been calculated from the temperature dependence of the diffraction peak intensities using the standard Debye–Waller formalism. The measured values are in good agreement with the results from a recent molecular dynamics simulation. [J. Hautman and M. Klein, J. Chem. Phys. 93, 7483 (1990).]

Helium atom diffraction measurements of the surface structure and vibrational dynamics of CH3–Si(111) and CD3–Si(111) surfaces

“…47 Well depths for a variety of alkanethiol self-assembled monolayers that possess terminal methyl groups have been calculated and the results range from 6.8 to 8.1 meV depending on odd/even effects of the alkane chain length. 7,55 The average well depth of 6.75 meV for He-CH 3 -Si͑111͒ is thus in excellent agreement with that reported for other, similar systems.…”

“…The well depths determined from the intercepts were D H = 7.5Ϯ 2.6 meV and D D = 6.0Ϯ 3.9 meV, which are in agreement with other methyl-terminated systems. 7,47 We note that the well depths are equal within the precision of the measurement, as expected, since isotopic substitution does not alter the Hesurface attractive potential. In subsequent sections, unless otherwise stated we will use the experimentally determined values of the well depth, noting that the use of the average value of 6.75 meV changes the results of our calculations by less than 5%.…”

“…The perpendicular surface motion for the organic crystal is a low-energy mode involving a shear vertical motion of the entire guanidinium sulfonate sheet. The perpendicular MSDs of various chain length SAMs have been measured 7,13,52 and the thermal motion in the z-direction of such systems is about an order of magnitude greater than for CH 3 -terminated Si͑111͒, even though both systems project terminal methyl groups. For alkanethiol SAMs, low-energy librational modes of the whole chain are the primary contributors to the measured meansquare displacements.…”

“…Several synthetic routes have allowed the fabrication of high-coverage atomic and molecular layers on a variety of metallic and semiconducting surfaces, [1][2][3][4][5] with self-assembled monolayers ͑SAMs͒ produced by thiol linkages to Au͑111͒ comprising the most ubiquitous and advanced methodology. 2,[6][7][8][9][10][11][12][13][14][15] Such SAMs have evolved into a sophisticated technology with applications in nanoscience and biotechnology. [16][17][18][19] With the success of the SAM/Au system, strategies for creating molecular layers on semiconductor substrates have been the focus of ongoing research.…”

Of interfaces and spin transport across nanoscale layers

“…Early experimental He diffraction studies established a ( √ 3 × √ 3)R30 • structure for alkylthiolates on Au(111) with one molecule per surface unit cell resulting in a 1/3 monolayer coverage [106,107]. Later He diffraction and grazing incidence x-ray diffraction (GXRD) studies found a 3 × 2 √ 3 superstructure † .…”