Molecular switches have great potential to convert different forms of energy into mechanical motion; however, their use is often limited by the narrow range of operating conditions. Here we report on the development of bilayer actuator devices using molecular spin crossover materials. Motion of the bilayer cantilever architecture results from the huge spontaneous strain accompanying the spin-state switching. The advantages of using spin crossover complexes here are substantial. The operating conditions used to switch the device can be manipulated through chemical modification, and there are many existing compounds to choose from. Spin crossover materials may be switched by diverse stimuli including light, temperature, pressure, guest molecules and magnetic field, allowing complex input combinations or highly specific operation. We demonstrate the versatility of this approach by fabricating actuators from four different spin crossover materials and by using both thermal variation and light to induce motion in a controlled direction.

Tunneling junctions having the structure Ag TS −S(CH 2 ) n−1 CH 3 // Ga 2 O 3 /EGaIn allow physical−organic studies of charge transport across selfassembled monolayers (SAMs). In ambient conditions, the surface of the liquid metal electrode (EGaIn, 75.5 wt % Ga, 24.5 wt % In, mp 15.7°C) oxidizes and adsorbs-like other high-energy surfaces-adventitious contaminants. The interface between the EGaIn and the SAM thus includes a film of metal oxide, and probably also organic material adsorbed on this film; this interface will influence the properties and operation of the junctions. A combination of structural, chemical, and electrical characterizations leads to four conclusions about Ag TS −S(CH 2 ) n−1 CH 3 // Ga 2 O 3 /EGaIn junctions. (i) The oxide is ∼0.7 nm thick on average, is composed mostly of Ga 2 O 3 , and appears to be self-limiting in its growth. (ii) The structure and composition (but not necessarily the contact area) of the junctions are conserved from junction to junction. (iii) The transport of charge through the junctions is dominated by the alkanethiolate SAM and not by the oxide or by the contaminants. (iv) The interface between the oxide and the eutectic alloy is rough at the micrometer scale. ■ INTRODUCTIONWe, and others, are developing procedures with which to study charge transport across self-assembled monolayers (SAMs). 1−32We have explored two systems, both based on electrodes made of liquid metals (Hg, and a eutectic alloy of gallium and indium, which we abbreviate as EGaIn) and focused on the latter. The latter system has two major components: (i) a SAM supported by a template-stripped silver (Ag TS ) electrode and contacted by (ii) a "top" electrode of EGaIn (75.5 wt % Ga, 24.5 wt % In, mp 15.7°C 33 ) that is a liquid at room temperature and covered with a thin metal oxide film; we refer to these junctions by a nomenclature defined earlier 1 as Ag TS −SR//Ga 2 O 3 /EGaIn, where R is an organic group (which may range in structure from simple n-alkyl groups to more complex functionalities, e.g., aromatics 28 or ferrocenes 13,34,35 ). These junctions are typically formed, characterized, and used in contact with ambient laboratory atmosphere. In these conditions, the surface of EGaIn oxidizes rapidly and spontaneously (for convenience we indicate the composite structure-oxide skin and metal electrode-as "Ga 2 O 3 / EGaIn") and it-as do all other surfaces-adsorbs adventitious contaminants (e.g., water, organic molecules, particles). The electrical resistance, thickness, and heterogeneity of the composite films of metal oxide and contaminants on the surface (and their variability from electrode to electrode, and from junction to junction) have not been characterized: the most serious ambiguity affecting the measurement of charge transport through Ag TS −SR//Ga 2 O 3 /EGaIn junctions is currently the effect of the oxide skin and adventitious contaminants.

This paper applies statistical methods to analyze the large, noisy datasets produced in measurements of tunneling current density (J) through self-assembled monolayers (SAMs) in large-area junctions. It describes and compares the accuracy and precision of procedures for summarizing data for individual SAMs, for comparing two or more SAMs, and for determining the parameters of the Simmons model (β and J 0). For data that contain significant numbers of outliers (i.e. most measurements of charge transport), commonly used statistical techniques-e.g. summarizing data with arithmetic mean and standard deviation, and fitting data using a linear, least-squares algorithm-are prone to large errors. The paper recommends statistical methods that distinguish between real data and artifacts, subject to the assumption that real data (J) are independent and log-normally distributed. Selecting a precise and accurate (conditional on these assumptions) method yields updated values of β and J 0 for charge transport across both odd and even n-alkanethiols (with 99% confidence intervals), and explains that the socalled odd-even effect (for n-alkanethiols on Ag) is largely due to a difference in J 0 between odd and even n-alkanethiols. This conclusion is provisional, in that it depends to some extent on the statistical model assumed, and these assumptions must be tested by future experiments.