Identifying the forces responsible for self-organization of nanostructures at crystal surfaces

Abstract

the contact width, and L is the nanotube length. These quantities are dif®cult to measure independently and calculations are modeldependent. As a guide, we apply a JKR model for the contact of cylinders 25 and ®nd a contact width of 3 nm for a tube radius of 13.5 nm. Our measurements of 0.006 N m -1 for the friction force per unit length is then consistent with a shear stress of 2 3 10 6 Pa. This can be compared with a value of 5 3 10 6 Pa, as inferred from AFM tip/graphite measurements 16 . To compare rolling and sliding in a single tube, we can calculate the force (4 nN for L 590 nm) and that would be needed to slide tube B, which in fact rolls. Finally, we note that the area under the lateral force trace is a direct measure of energy loss in rolling. For tube B, we measure an energy loss of 8 6 3 3 10 2 16 J per revolution. The sliding energy loss expected for this distance (85 nm) can be calculated using the frictional force of 4 nN, yielding 3 3 10 2 16 J.When we compare our lateral force measurements for sliding and rolling cases, we ®nd that the stick peaks in rolling are higher than the lateral force needed to sustain sliding, and that the energy cost for rolling is larger than that of the sliding cases. Why should the nanotubes roll? We speculate that, owing to the size and surface features of the rolling nanotubes, a stick peak for sliding in side-on pushing might exist that is larger than the threshold for rolling. Atomic-scale substrate interactions may also play a roll as we have observed this characteristic rolling only on graphite. Rolling behaviour has been accompanied by a preferential, threefold, inplane orientation that indicates intimate nanotube/graphite contact, and perhaps lattice registry. Rolling may occur only when both the nanotube and the underlying graphite have long-range order. In these cases that there may be a barrier for sliding which is larger than that for rolling and may preclude the direct measurement of sliding friction 7 . M

Two-Dimensional Nanotemplates as Surface Cues for the Controlled Assembly of Organic Molecules

“…TMA was deposited on Cu(110) at 300 K, a temperature high enough to ensure the presence of mobile Cu adatoms that catalyse the deprotonation of carboxylate groups. The deposition resulted in 1D [-Cu-TMA-Cu-] chains running along the [1][2][3][4][5][6][7][8][9][10] direction of Cu (110). The 1D anisotropy of the substrate is transferred to the resulting 1D chains.…”

“…30). The superstructure is constituted of discrete pores and rotated by 15 ± 4 • with respect to the [1][2][3][4][5][6][7][8][9][10] direction of the underlying Ag(111) surface and shows a periodicity of 3.3 nm. The pores have a diameter of 1.2 nm and a depth of 0.12 nm (these dimensions are underestimated because of the tip-surface convolution due to the tip finite size).…”

“…Upon annealing at 350 K the system evolves to an assembly consisting of 16 TMA molecules arranged in square patterns like molecular Fig. 34 1,3,5-tricarboxylic benzoic acid (trimesic acid, TMA) molecules and Fe atoms represent the primary units employed for the formation of secondary chiral mononuclear (Fe(TMA) 4 ) complexes. The complexes are antecedents for tertiary polynuclear nanogrids which are in turn the supramolecular motifs for the assembly of homochiral nanocavity arrays.…”

“…In this context, the bottom-up approach is emerging as a truly promising method for the controlled growth and processing of nanostructured materials. This approach is based on the concept of (organic) self-assembly [3] or (inorganic) selforganization [4,5].…”

Monte Carlo study of the elastic interaction in heteroepitaxial growth

“…Among these processes, there is one type of growth characterized by the presence of long-range elastic interactions that play a special role [4]. Molecular Beam Epitaxy (MBE) under the effect of elastic interactions has been a topic of recent research, with many applications, specially in the fabrication of low dimensional nanostructures, like quantum dots (QD) and quantum wires [2,5,6]. One challenging question concerning this topic is how, and by which mechanism, island organization [7,8] occurs.…”