Recent studies indicate the important role of nanostructures (including monolayer films) in physics, chemistry, materials science, electronics, biology, medicine, etc. Therefore, the question of self-ordering of molecules on different substrates are of particular interests. This work is devoted to some aspects of self-assembling of organic molecules on atomically-flat surfaces.
Monolayers of n-alkanes (CnH2n + 2 with n = 16, 24, 30, 34, 48, 50, 60) and their binary mixtures (C16H34/C34H70, C24H50/C48H98) were obtained on atomically-flat surfaces of highly oriented pyrolytic graphite and reconstructed Au (111) surface. Formation of monolayers were performed by deposition from solution. Liquid n-tetradecane was used as universal solvent.
STM-observations of monolayers of medium-chain alkanes reveals lamellar structures with rectangular packing of molecules that correspond to the traditional for n-alkanes on graphite Grozhek model. However in such monolayers STM-contrast features cannot be explained by Grozek model These features include: blurred lamellas boundaries; nonmonotonic modulation of CH2-groups contrast along the main axes of the molecules.
STM-investigation of monolayers of long chain alkanes C48H98 C50H102 C60H122 reveals oblique packages with deviation in the orientation of molecules from the crystallographic direction <100>, which is in contradiction with the Grozek model. The global restructuring was studied for C60 monolayers. It was established that the restructuring is governed by removing of the surface stress in the monolayer.
For short-chain C16H34 molecules solidification effect at temperature significantly higher than its melting temperature was found. STM-investigation shows that C16H34 monolayers have lamellar structures with oblique packing of molecules that contradicts to the Grozhek model. Furthermore, the melting temperature of the C16H34 monolayers was found.
Monolayers of binary mixtures C16H34/C34H70 on graphite and C24H50/C48H98 С25H52/C50H102 on graphite and Au(111) were investigated by STM. It was found that C16H34/C34H70 monolayers have nematic lamellar structures with pinhole defects. Pinholes corresponds incorporation of C16H34 pairs in C34H70 lamellas. Long-term STM-scanning reveals that the appearance and disappearance of defects is stochastic. Thus, the monolayer is in dynamic equilibrium with C16H34/C34H70 solution. Binary mixtures C24H50/C48H98 С25H52/C50H102 form highly ordered monolayers with nematic phase with no signs of lamellas on the graphite and Au(111) surfaces. The formation of nematic phase is caused by the balance (i) of incompatibility between the periods of the alkanes alkyl chain (0.251 nm) and the hexagonal structure of the graphite lattice (0.246 nm) and (ii) of the mismatch between the lengths of the C24H50 (~6.17 nm) molecules and one C48H98 molecule (~6.10nm). In C24H50/C48H98 monolayers, the lateral interaction between molecules dominates the interaction of the substrate molecule, which causes increased mobility of molecules along adsorption furrows.
For the n-alkane/graphite systems, distances between molecules in adjacent lamellae were estimated. The estimations shows that the anomalies observed in monolayers are caused by the deformation of the monolayer (compression or tension), due to the incommensurability between the period of the alkyl chains of the molecules and the period of the graphite substrate. It is shown that the monolayers of binary mixtures in nematic phase have anomalously low friction coefficients compared to the friction coefficients of the monolayers of pure components.
The hydrophilicity of Au (111) surfaces functionalized with aliphatic thiols has been studied. It is shown that increasing of the polarity of functional groups of molecules (OH-> COOH-> SH-> CH3-) to increase wettability, whereas, with increasing alkyl chain length, hydrophilicity decreases. The effect of increasing the hydrophobicity of the surface functionalized by SH-groups is revealed. Increasing of hydrophobicity is caused by molecules dimerization in presence of oxygen.
A technology for synthesis of Au nanoparticles in deuterated water (D2O) was developed. This technology allows to study the structure and morphology of the stabilizing layer on the surface of nanoparticles using the low-angle neutron scattering method. The developed technology was tested for Turkevich nanoparticles (r = 8 nm). It has been established that the stabilizing citrate shell of nanoparticles contains D2O molecules even after they dry on the surface of the substrates, and its thickness is ~1.25nm.
The kinetics of adsorption of aliphatic thiols on the surface of Au nanoparticles was studied. The empirical dependences of concentration and type of functional group of the modifier thiol molecules on substitution time of citrate shell were obtained.
