Conventional Nanoindentation in Self-Assembled Monolayers Deposited on Gold and Silver Substrates

Peer reviewe

Conventional Nanoindentation in Self-Assembled Monolayers Deposited on Gold and Silver Substrates

Self-assembled monolayers (SAMs) are promising materials for micromechanical applications. However, characterization of mechanical properties of monolayers is challenging for standard nanoindentation, and new efficient analysis techniques are needed. Hereby, a conventional nanoindentation method has been combined in a unique way with efficient data analysis based on consumed energy calculation and load-displacement data. The procedure has been applied on SAMs of 4,4 -biphenyldithiol (BPDT) on Au, 1-tetradecanethiol (TDT), and 1-hexadecanethiol (HDT) on Au and Ag substrates being the first study where SAMs of the same thiols on different substrates are analyzed by nanoindentation providing a new insight into the substrate effects. Unlike TDT and HDT SAMs, which are found to strongly enhance the homogeneity and stiffness of the underlying substrate, the BPDT covered Au substrate appears softer in mechanical response. In the case of TDT and HDT SAMs on Ag the structures are softer showing also faster relaxation than the corresponding structures on Au substrate. The proposed procedure enables a fast and efficient way of assessing the complex behaviour of SAM modified substrates. As a consequence, the results are relevant to practical issues dependent on layer activity and toughness

How penetrable are thioalkyl self-assembled monolayers?

The depth of penetration of photolytically generated, gas-phase O( 3P) atoms into thioalkyl self-assembled monolayers (SAMs) has been investigated. Custom-synthesized, site-selectively deuterated SAMs were prepared on Au substrates and characterized by scanning tunneling microscopy (STM). Relative yields of gas-phase OD were detected by laser-induced fluorescence (LIF). Reaction was suppressed at the terminal CD3 by the higher abstraction barriers for primary D atoms, yielding only 16 ± 3% of the total OD. The C2 (first secondary) site is the individually most reactive (42 ± 5%). The remaining significant contribution (42 ± 4%) from positions as deep as C3-C6 is a considerable surprise when compared with previous related experiments using higher-energy O+ ion projectiles and detecting OH- products. The apparent greater penetrability of the SAM layer found here may have prior theoretical support. Furthermore, we show that NO2 damages the surfaces but that C12 SAMs are considerably more resistant than C6 SAMs

Functionalizing hydrogen-bonded surface networks with self-assembled monolayers

One of the central challenges in nanotechnology is the development of flexible and efficient methods for creating ordered structures with nanometre precision over an extended length scale. Supramolecular self- assembly on surfaces offers attractive features in this regard: it is a 'bottom- up' approach and thus allows the simple and rapid creation of surface assemblies(1,2), which are readily tuned through the choice of molecular building blocks used and stabilized by hydrogen bonding(3-8), van der Waals interactions(9), pi-pi bonding(10,11) or metal coordination(12,13) between the blocks. Assemblies in the form of two- dimensional open networks(3,9,10,13-17) are of particular interest for possible applications because well- defined pores can be used for the precise localization and confinement of guest entities such as molecules or clusters, which can add functionality to the supramolecular network. Another widely used method for producing surface structures involves self- assembled monolayers (SAMs)(18), which have introduced unprecedented flexibility in our ability to tailor interfaces and generate patterned surfaces(19-22). But SAMs are part of a top-down technology that is limited in terms of the spatial resolution that can be achieved. We therefore rationalized that a particularly powerful fabrication platform might be realized by combining non- covalent self- assembly of porous networks and SAMs, with the former providing nanometre- scale precision and the latter allowing versatile functionalization. Here we show that the two strategies can indeed be combined to create integrated network SAM hybrid systems that are sufficiently robust for further processing. We show that the supramolecular network and the SAM can both be deposited from solution, which should enable the widespread and flexible use of this combined fabrication method.</p