Surface stress in the self-assembly of alkanethiols on gold probed .by a force microscopy technique

Abstract

The extreme sensitivity of scanning force microscopy allows its application to be extended well beyond the probing of surfaces. We discuss micromechanical force microscope cantilevers, with a thin Au receptor layer on one side, as sensors for gas-phase adsorption of alkanethiols, which self-organize into well-ordered, densely packed films. Quantitative data show that surface stress develops during the self-assembly process for alkanethiols of different chain lengths. In particular, we present an analysis of the kinetics of the replacement process of adsorbates by butanethiol molecules, details of the acquisition analysis, and a quantitative calibration technique.Numerous local tip-sample interactions are used to obtain information on sample surface properties such as topography, conductivity, elasticity, friction, and magnetic responses. Recently, the scanning force microscope (SFM) tip has been used to probe the stretching and unfolding of individual molecules [1]. The alternative concept to using the tip of the cantilever as a local sensing element that is scanned over the surface is to use the entire cantilever surface area as a receptor. This technique constitutes a new avenue for further experiments to gather information on specific and nonspecific interactions and reactions on the nanoscale.One concept is the use of "bimetallic" cantilevers to probe temperature changes. This concept was pioneered to follow heat evolution during the catalytic reaction of O 2 and H 2 to form water on a thin Pt layer evaporated on an SFM cantilever [2]. The bimetallic technique was also applied to probe local temperature differences along a biased resistor [3]. It also enabled photothermal spectroscopy to be conducted on a picogram quantity of material [4]. Signals from the cantilever response can be detected with the high sensitivity, speed, and bandwidth of conventional SFM technique. In addition to probing subtle temperature differences, a cantilever-type sensor can additionally be used as a transducer for femtoscale effects in science [5][6][7][8][9][10][11][12][13]. We applied the micromechanical sensor technique to probe the self-organization of alkanethiols -HS−(CH 2 ) n−1 −CH 3 for n = 4, 6, 8, 12, and 14, where n is the number of carbon atoms in the alkyl chain -on Au [14,15]. Self-assembled monolayers (SAMs) are used in applications such as microcontact printing [16] and voltametric microsensors [17], and have recently been applied to molecular host-guest recognition [18]. ExperimentalOur measurement setup, outlined schematically in Fig. 1, is based on a NanoScope II SFM head (Digital Instruments, Santa Barbara CA, USA) which uses the laser beam deflection technique to detect sensor deflection ∆z down to sub-Å levels. The normalized voltage difference ∆V = (V 1 − V 2 )(V 1 + V 2 ) −1 of two segments of a positionsensitive detector (PSD) is converted into a 12-bit value in an analog-to-digital converter and is displayed continuously on a monitor as well as being stored on a computer hard disk. All V-shaped SiN x senso...

Charge redistribution in electrochemically actuated mechanical sensors

“…As already reported [30,31], upon adsorption of thiol the cantilever experiences a downward bending, suggesting that this adsorption is accompanied by a gold surface expansion. The maximum displacement is about 80 nm after 5 h (end of step 1) and 160 nm after step 2 (the initial states correspond to the beginning of adsorption steps 1 and 2, respectively).…”

The electronic structure of metal/alkane thiol self-assembled monolayers/metal junctions for magnetoelectronics applications

“…This indicates that fi lms exhibit some two-dimensional order and again, that the thiol bonding is oriented largely normal to the surface at higher coverages. The order is limited to domains, and the thiol thin fi lms do not form a single crystal of close packed chains, consistent with the scanning tunneling microscope (STM) results [23][24][25][26][27]. This indicates that the adsorption geometry of the multilayer fi lms is drastically different from the ML fi lms (the molecules of the monolayer fi lms are, most likely, lying mostly parallel to the surface as expected [4,10]).…”

Resonating modes of vibrating microcantilevers in liquid

“…This versatile tool has already found application in the field of genomics, 1-3 proteomics, 4,5 food engineering, 6 and chemistry. 7 Surface stress ͑static mode͒ and mass change ͑dynamic mode͒ are the important parameters of interest for microcantilever applied as a sensor. 8 The focus of this article is on the dynamic mode.…”