Gang Yu Liu

Molecular order at the surface of an organic monolayer studied by low energy helium diffraction

We demonstrate that the surface structure of organic monolayers can be determined by low energy helium diffraction at low surface temperatures. This uniquely surface‐sensitive and nondestructive technique shows that the CH3‐terminated surface of a monolayer of docosane thiol (CH3(CH2)21SH) on Au(111) is composed of small, ordered domains (lattice constant 5.01±0.02 A), a large fraction of which share a common orientation. The helium diffraction intensities decrease monotonically with increasing temperature and vanish around 100 K, due to thermal motion of the CH3 groups. Surface order is observed for chains as short as ten carbons (CH3(CH2)9SH) but a shorter chain, (CH3(CH2)5SH), gave no diffraction.

Superlattice structure at the surface of a monolayer of octadecanethiol self‐assembled on Au(111)

We report direct evidence of a unit mesh containing more than one hydrocarbon chain at the surface of a self‐assembled monolayer of long‐chain n‐alkanethiols. Our helium diffraction measurements for a monolayer of n‐octadecanethiol on Au(111) are consistent with a rectangular primitive unit mesh of dimensions 8.68×10.02 A containing four crystallographically distinct hydrocarbon chains. This packing arrangement can also be described as a c(4×2) superlattice with respect to the fundamental simple hexagonal [(√3×√3)R30°] array of lattice parameter 5.01 A previously observed for monolayers of other n‐alkanethiols on gold. No temperature‐dependent phase behavior is observed in the temperature range where surface diffraction is measurable (30–100 K) and cycling up to temperatures as high as 50 °C caused no observable change in the diffraction. It is proposed that this larger unit mesh is the result of a patterned arrangement of rotations of the hydrocarbon chains about their molecular axes. This patterned arrangement must be different than the herringbone structure expected by simple analogy to bulk n‐alkanes.

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

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 A. 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 A at intermediate chain length [CH3(CH2)13SH/Au(111)/mica], decreasing to 26 A at longer [CH3(CH2)21SH/Au(111)/mica] and 41 A at shorter [CH3(CH2)9SH/Au(111)/mica] chain lengths. No phase transitions could be detected at the surfaces of these monolay...

AN UNEXPECTED PACKING OF FLUORINATED N-ALKANE THIOLS ON AU(111) : A COMBINED ATOMIC FORCE MICROSCOPY AND X-RAY DIFFRACTION STUDY

Atomic force microscopy (AFM) and grazing incidence x‐ray diffraction (GIXD) have been used to study the structure of self‐assembled monolayers of CF3(CF2)n(CH2)2SH (n=11, 7, and 5) on the Au(111) surface. Surprisingly, although the nearest‐neighbor fluorinated alkane thiol distance is very close to the lattice constant of a commensurate p(2×2) structure, the close‐packed rows of molecules are rotated ∼30° with respect to the underlying gold lattice. That packing is incommensurate or at most only close to the high‐order commensurate c(7×7) structure. The relative orientation of the organic monolayer and the Au(111) substrate has been determined unambiguously both with GIXD, and by AFM, taking advantage of an earlier finding (Ref. ) that AFM tips can reversibly displace the thiol molecules under high loads. In addition, we demonstrate that the two techniques provide complementary information on the order and the domain structures of these monolayers.

Substrate dependence of the surface structure and chain packing of docosyl mercaptan self-assembled on the (111), (110), and (100) faces of single crystal gold

Low‐energy helium diffraction has been used to study the surface structure, chain packing, and thermal vibrations of docosyl mercaptan [CH3(CH2)21SH] self‐assembled on single crystal Au(111), (110), and (100). The docosyl mercaptan molecules form monolayers with different periodicity on the different surfaces of gold. On Au(111) at low temperatures (≤100 K), the terminal methyl groups of the docosyl mercaptan molecules form domains of a hexagonal lattice with a unit mesh constant of 5.01±0.02 A. The sulfur head groups are arranged in a commensurate (√3×√3)R30° structure and are believed to adsorb on the triple hollow sites of the Au(111) lattice. The unit mesh parameters for CH3(CH2)21S/Au(110) are a=b=4.99±0.08 A and α=109.5°, suggesting that the chemisorbed sulfur atoms remove the ‘‘missing row’’ reconstruction of the Au(110) surface and form a commensurate c(2×2) lattice. The adsorption of docosyl mercaptan molecules on a Au(100) surface results in a complicated diffraction pattern. Analysis of the dat...

Structural defects in self‐assembled organic monolayers via combined atomic beam and x‐ray diffraction

We present the results of a combined He atom and x‐ray diffraction study of CH3(CH2)n−1SH monolayers self assembled on Au(111) surfaces. By combining these two complementary probes, we have characterized both the surface and the interior structure of the monolayers. In both cases, we find the same structure containing four molecules per unit mesh. However, we demonstrate that there are significant differences in both the diffraction linewidths and the dependence of the linewidth upon chain length for these two techniques.

Low energy helium diffraction studies of CH3Br overlayers physisorbed on C(0001), NaCl(001), and LiF(001) surfaces

Low energy helium diffraction has been used to determine the unit mesh parameters of overlayers of CH3Br physisorbed on C(0001), NaCl(001), and LiF(001) at ≊35 K. CH3Br forms a uniaxially commensurate overlayer on C(0001) with unit mesh parameters 4.26 A×6.75 A. On NaCl(001), CH3Br forms a high coverage and a low coverage phase. The high coverage phase is incommensurate and has unit mesh parameters 4.54 A×6.73 A, whereas the low coverage phase is commensurate with a ((2)1/2×3(2)1/2)R45° unit mesh. The structure of CH3Br/LiF(001) is essentially the same as that of the high coverage phase of CH3Br/NaCl(001) with unit mesh parameters 4.52 A×6.71 A. The unit mesh parameters (with the exception of low coverage CH3Br/NaCl ) are very similar to the lattice parameters of the a‐b [or (001)] plane of bulk crystalline CH3Br at ≊153 K. By analogy with the bulk crystal, it is likely that there are two molecules per unit mesh and that the CH3Br dipoles are nearly perpendicular to the surface and antiferroelectrically o...

Low energy helium atom scattering from HCl monolayers physisorbed on graphite

Abstract Low energy helium atom scattering has been used to characterize monolayers of HCl adsorbed onto single crystals of graphite. The unit cell of this system is described by real space lattice vectors 3.80 A in length, separated by 120°. The monolayers lattice is rotated 30° with respect to the substrate. The attenuation of the He beam scattered in the specular direction due to the thermal motion of the HCl molecules perpendicular to the surface, can be described as due to a collection of Einstein oscillators with a common, characteristic frequency of 7.8 × 10 12 rad s . A polar plot i.e. a plot of the specular intensity as a function of the incident angle, has been compared to the predictions of a close-coupled scattering program, that uses realistic He-surface interaction potentials which include the contributions from the atom-adlayer and atom-substrate interactions, along with the relevant three-body interaction terms. The existing HeHCl spherically averaged potentials do not give good agreement with our data although reducing the well depth by 8.5% improves the agreement slightly. Since the HeHCl interaction in the “T” geometry is less strong than that along the molecular axis (and therefore than the spherically averaged interaction) our measurements may suggest that the HCl molecules are preferentially aligned with their dipole axes parallel to the surface.