The kinetics and mechanism for the solution-phase adsorption of n-alkanethiols onto gold to form self-assembled monolayers (SAMs) have been monitored in situ using atomic force microscopy (AFM). Time-dependent AFM images reveal detailed structural information about the adsorbed layer during its growth. In 2-butanol, CH3(CH2)17SH molecules initially adsorb on gold with the molecular axis of their hydrocarbon chains oriented parallel to the surface. As the surface coverage increases to near saturation, a two-dimensional phase transition occurs and produces islands composed of molecules with their hydrocarbon axis oriented ∼30° from the surface normal. Continued exposure to the thiol solution results in a greater number of these islands and the growth of these nuclei until a SAM is formed with a commensurate (∛×∛)R30° structure. The growth of the lying-down phase follows a first-order Langmuir adsorption isotherm, while the phase transition is best described by a second-order reaction. The kinetics of the self-assembly process also depends on the chain length of the alkanethiol and the cleanness of the gold surface. Longer-chained thiols, such as CH3(CH2)17O(CH2)19SH, formed complete SAMs more rapidly than did shorter-chained thiols, such as CH3(CH2)17SH. The physisorbed, lying-down phase for CH3(CH2)17O(CH2)19SH was less homogeneous and its two-dimensional phase transition was more complicated than for CH3(CH2)17SH and CH3(CH2)21SH, as the CH3(CH2)17O(CH2)19SH molecules adopt multiple conformations. Of these, the two dominant ones are an all-trans, and another where the hydrocarbon chain adopts an all-trans conformation except for a gauche bond on both sides of the ether unit. These conformers coexist on the surface during the initial adsorption and its transition to the standing-up phase, but change to the all-trans structure in the complete SAM.

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Two new methods have been developed to precisely position gold nanoparticles on surfaces. The surface-active nanoparticles have a shell of a mixed monolayer comprised of alkanethiol and alkanedithiol molecules to anchor particles to gold surfaces via sulfur−gold chemisorption. In the first method, regions of an alkanethiol self-assembled monolayer (SAM) are shaved by the AFM tip under high force in a solution containing nanoparticles. Nanoparticles then adsorb onto the exposed areas defined by the shaving track of the tip. In a second method, the AFM tip is coated with nanoparticles. Under low force, AFM images are acquired and the nanoparticles remain on the tip. When higher load is applied, areas of the SAM matrix are uncovered and nanoparticles are deposited following the scanning track of the AFM tip. Thus, the 3D positions of the nanoparticles are precisely controlled. The nanostructures are characterized in situ with the same tip at reduced load. Individual particles within the nanopatterns can be resolved by AFM. In both methods, the matrix SAM effectively resists the nonspecific binding of nanoparticles, and prevents lateral diffusion of nanoparticles. The high spatial precision offered by AFM lithography is advantageous for fabrication of nanoparticle-based nanodevices.Metal nanoparticles exhibit size-dependent optical, 1 electronic, 2,3 and catalytic properties, 4 which have great potential for engineering new materials and sensors. 5,6 Prospective applications for nanoparticles include miniature electronic devices, 7-9 spin coatings, 10 and biosensing. [11][12][13][14] Prototype devices in molecular electronics, which incorporate gold nanoparticles as components, include single-electron transistors, 8,15-17 single-electron charging devices, photonic switches, 18 and quantum dots. 19 The 3D positions of nanoparticles on surfaces must be controlled precisely-hopefully at the level of individual particles. To make micro-and nanoscale devices functional, nanoparticles must be aligned precisely in nanowires 20,21 and nanoparticles must be positioned precisely at the gap of metal-insulator-metal junctions. 22 Methods are continuously being developed and improved for directing the organization of metal nanoparticles into thin film layers, 23,24 nanocrystal arrays, 25 and superlattices. 26 Hexagonal ordering has been achieved using approaches such