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1 DOI: 1.138/NNANO Experimental Demonstration of a Single-Molecule Electric Motor Heather L. Tierney, Colin J. Murphy, April D. Jewell, Ashleigh E. Baber, Erin V. Iski, Harout Y. Khodaverdian, Allister F. McGuire, Nikolai Klebanov & E. Charles H. Sykes Imaging Molecular Rotation Supplemental Figure S1 shows STM images of an individual BuSMe thioether molecular rotor on a Cu(111) surface. When imaging under non-perturbative scanning conditions and at a temperature of 5 K, the molecule is static and appears as a crescent-shaped protrusion in the STM image as shown in panel a. Panel b contains an image of the BuSMe molecular rotor at 8 K, again under non-perturbative scanning conditions. At this elevated temperature the molecule rotates via fast interconversion between six equivalent orientations dictated by the threefold symmetry of the underlying hexagonal structure of the Cu(111) surface. The molecular rotation occurs much faster than the timescale of STM imaging (ca. 1 min per image) thus; the molecule appears as a hexagon due to the time averaged appearance of all six orientations. Supplemental Figure S1. STM images showing the thermal activation of a BuSMe molecular rotor on a Cu(111) surface. The molecule appears as a crescent-shaped protrusion when it is (a) stationary at 5 K and (b) hexagonal in shape at 8 K due to rotation faster than the time-scale of STM imaging. Scale bar = 1 nm. (Imaging conditions: 5 pa, 1 mv.) NATURE NANOTECHNOLOGY 1

2 DOI: 1.138/NNANO Quantification of Rotational Rate and Directionality As BuSMe is an asymmetric molecule, if the STM tip was carefully placed asymmetrically to the side of one of the six lobes of the rotating molecule during I vs. t (tunnelling current vs. time) spectroscopy measurements, the six states could be distinguished (as shown in Figure 2 in the main paper). To first approximation, if the tip is positioned near the periphery of the molecule, the rotating butyl tail contributes the majority of the tunnelling current. Therefore, the highest tunnelling current would correspond to the butyl tail orientation nearest the tip, and successively lower tunnelling currents would correspond to the butyl tail located farther from the stationary STM tip. This simplified explanation neglects the orientations of the molecule with the butyl tail farthest from the tip in which the methyl group is nearest the STM tip. This leads to changes in tunnelling current that reflect the relative proximities of both the butyl and methyl groups to the tip as shown in Figure S2. Supplemental Figure S2. Schematic of the six orientations of BuSMe with respect to the stationary STM tip (indicated by point x ) during acquisition of I vs. t spectra. The position of the butyl group is the main contributor to the tunnelling current, however, levels 3, 4 and 5 are altered due to the proximity of the methyl group when the butyl tail is at positions farthest from the tip. 2 2 NATURE NANOTECHNOLOGY

3 DOI: 1.138/NNANO SUPPLEMENTARY INFORMATION Despite the complexity of these absolute rotor orientation assignments, in terms of tracking the dynamics of motion, and in particular detecting directional motion, knowledge of the progression of a set of discrete levels is almost as valuable as an absolute assignment of the progression of orientation states. In simpler terms, directional motion would lead to a periodic, reproducible series of discrete current states, whereas random motion would yield a random progression. 1,2 The I vs. t spectra generated with the feedback loop disabled contained thousands of data points and hundreds to thousands of tunnelling current changes, each representing a molecular reorientation, or hop. Each spectrum was initially visually examined and a current range for each molecular orientation was defined. Then consecutive points were compared and each hop was labeled as positive (+) or negative (-) depending on the user defined sequence of orientational hops. Each I vs. t spectrum was assessed and the output was reported as the total number of (+) and (-) hops which could be equated to rotations in opposite directions (i.e. anti-clockwise or clockwise). Thermally-Driven Motion Our previous work investigated the energetic barrier (torsional potential) for individual molecular rotors using Arrhenius measurements, in which the rate of rotation was monitored as a function of sample temperature. 1,2 These measurements for BuSMe on Cu(111) yielded an average activation barrier (E) of 74 ± 6 J/mol and an attempt frequency (A) of 2 x 1 7 ±.4 Hz. As would be expected from the second law of thermodynamics, it was found that this type of thermal excitation gave a random progression of rotational hops NATURE NANOTECHNOLOGY 3

4 DOI: 1.138/NNANO Quantification of the Angle of Each Rotational Hop I vs. t spectra for all combinations of molecular rotor and STM tip chirality were analyzed and the rotational angle hop distribution was plotted. The pairs of histograms for STM tips 1 and 4 in Figure S3 reveal that the rotors with the highest directionality take more single 6 hops than those of the opposite chirality, which undergo all hops with almost equal probability and spin at faster rates. The pair of histograms for STM tip 3 illustrates that electrical excitation by an achiral tip induces an almost equal occurrence of all hop angles and hence no directional rotation. The near identical rotational rates also support the assignment of STM tip #3 as achiral. Tip #1 (Chiral) S Rotor: R Rotor: ⁰ -1⁰ -6⁰ 6⁰ 1⁰ 18⁰ -18⁰ -1⁰ -6⁰ 6⁰ 1⁰ 18⁰ Rotor Direction Rate (%) (Hz) S -5. ±.3 3 ± 1 R.2 ±.3 9 ± NATURE NANOTECHNOLOGY

5 DOI: 1.138/NNANO SUPPLEMENTARY INFORMATION Tip #3 (Achiral) S Rotor: R Rotor: ⁰ -1⁰ -6⁰ 6⁰ 1⁰ 18⁰ ⁰ -1⁰ -6⁰ 6⁰ 1⁰ 18⁰ Rotor Direction Rate (%) (Hz) S -.3 ±.1 14 ± 9 R -.3 ±.1 13 ± 8 Tip #4 (Chiral) S Rotor: R Rotor: ⁰ -1⁰ -6⁰ 6⁰ 1⁰ 18⁰ ⁰ -1⁰ -6⁰ 6⁰ 1⁰ 18⁰ Rotor Direction Rate (%) (Hz) S -.3 ±.1 1 ± 3 R 1.2 ±.2 8 ± Supplemental Figure S3. Histograms showing the experimentally measured distribution of rotational hop angles for the motion of both chiralites of molecular rotor and for both chiral and achiral STM tips. Positive % direction NATURE NANOTECHNOLOGY 5

6 DOI: 1.138/NNANO indicates anticlockwise rotation and vice versa. Butyl Methyl Sulphide Purity Verification with Mass Spectra Mass spectra acquired with an Extorr Residual Gas Analyzer XT quadrupole mass spectrometer. Mass sweep was taken from 4 to 11 amu (Supplemental Figure S4). Peaks 4 amu occurred due to typical background gases in the UHV chamber (including the peak at 4 amu from Ar used to sputter clean the Cu surface). Supplemental Figure S4: Mass Spectrum of BuSMe for purity analysis. Supplementary Movie Movie illustrating how the tunnelling current levels of the I vs. t curves relate to the rotational orientation of the molecular rotor on the surface. 6 NATURE NANOTECHNOLOGY

7 DOI: 1.138/NNANO SUPPLEMENTARY INFORMATION References 1 Baber, A.E., Tierney, H.L., & Sykes, C.H., A Quantitative Single-Molecule Study of Thioether Molecular Rotors. ACS Nano 2, (8). 2 Jewell, A.D. et al., Time-Resolved Studies of Individual Molecular Rotors. J. Phys.-Condes. Matter. 22, (1). 3 Astumian, R.D., Thermodynamics and Kinetics of a Brownian Motor. Science 276, (1997). 4 Astumian, R.D. & Bier, M., Fluctuation Driven Ratchets - Molecular Motors. Phys. Rev. Lett. 72, (1994). 5 Derenyi, I. & Astumian, R.D., Intrawell Relaxation Time: The Limit of the Adiabatic Approximation. Phys. Rev. Lett. 82, (1999). NATURE NANOTECHNOLOGY 7