by Professor Charlie Sykes
While molecular machines driven by chemical, light or thermal energies can be
found throughout nature, little progress has been made toward creating synthetic
counterparts. The gap between nature and nanotechnology remains due to the
limited fundamental understanding of the transfer of energy to mechanical motion
at the nanoscale.
Understanding and actuating the rotation of individual molecules on surfaces
is a crucial step towards the development of nanoscale devices such as fluid
pumps, sensors, delay lines, and microwave signaling applications. Recently a
new, stable and robust system of molecular rotors consisting of thioether
molecules (RSR) bound to metal surfaces has offered a method with which to study
the rotation of individual molecules as a function of temperature, molecular
chemistry, proximity of neighboring molecules and electrical current.
Our initial studies used the simple, symmetric thioether dibutyl sulfide.
These molecules adsorb to metal surfaces via the central sulfur atom and
rotation of the alkyl tails occurs around the central S-metal bond. These
molecules appear hexagonal as they rotate due to the superposition of three
equivalent orientations with respect to the hexagonally-packed surface below.
Rotation of these molecules at 80 K occurs faster than the time-scale of
scanning tunneling microscopy (STM) imaging (~2 min/image), so it was not
possible to decouple the superimposed orientations at this temperature. Upon
cooling the system further (to 5 K) it was possible to observe the single
dibutyl sulfide molecules in static positions, as their rotation was halted.
STM image of three molecular rotors, just 1
nanometer wide, spinning at over 1,000,000 times per second when heated to a
temperature of 78 Kelvin (-320 F).
Low-temperature STM allowed the measurement of the rotation of the dibutyl
sulfide molecules as a function of temperature and the quantification of both
the energetic barrier and pre-exponential factor for their motion. In these
measurements the feedback loop (which is normally used to modulate the STM tip
height in order to maintain a constant tunneling current) is turned off and the
tunneling current is monitored with respect to time (I vs. t). By
measuring the rotational rate as the temperature is increased, it is possible to
create Arrhenius plots to further understand the rotational energetics of the
To understand the interaction between the alkyl tails of rotor and the
surface we studied the rotation of thioether molecules as a function of chain
length. Dimethyl-, diethyl-, dibutyl- and dihexyl sulfides were studied, and it
was found that all of the rotors except dimethyl sulfide were static at 7 K.
Each of the molecular species was then heated until they were visibly rotating
within STM images.
Interestingly, the thermal onset to rotation was found to be nearly identical
for studied thioether molecules with alkyl tails of two carbons or more. It is
proposed that this plateau in thermal onset was due to an interplay between
degrees of freedom in the alkyl tail vs. S-metal bond length, which was
supported by subsequent molecular dynamics calculations1,2.
Unlike longer alkyl-chained thioethers, dimethyl sulfide molecules were seen
to rotate too quickly to measure at 7 K. Experimental studies showed a very low
barrier to rotation, as dimethyl sulfide molecules rotated >103 Hz
at 5 K. In order to further understand this molecule's rotation, DFT studies
were used to calculate the rotational barrier. Also, using theoretical methods
the minimum energy adsorption site was determined and the mechanism of rotation
was elucidated. These theoretical results indicate that the rotation of a small,
simple molecule is actually rather complex; as the CH3 groups of
dimethyl sulfide rotate around the Au-S bond, the central S atom precesses
around a surface Au atom2.
STM images showing how a spinning molecular rotor
can be "braked" by physically moving it towards a chain of static
Through a series of single-molecule manipulation experiments, we have
mechanically switched the rotation on and off reversibly by moving the molecules
toward or away from one another. If two rotors are pushed close together, they
stop rotating due to van der Waals attraction between the alkyl chains. One of
the major goals for the field of molecular rotors is creating ordered arrays
with which to study rotational energy propagation.
Our mechanical deactivation of dibutyl sulfide molecules demonstrates that
there will be a complex interplay between sterics and electrostatics that will
mediate the rotational coupling of neighboring molecules. Towards this end, we
have created ordered 2D arrays of dibutyl sulfide rotors on a Ag/Cu(111) surface
alloy. This alloy forms a very regular hexagonal array of hcp and fcc stacked
atoms, and allows the thioether rotors to be precisely self-assembled, with a
spacing of 2.6 nm3,4.
While our previous studies revealed that small amounts of thermal energy are
capable of inducing rotation, thermodynamics dictates that thermal energy alone
cannot be used to perform useful work in the absence of a temperature gradient.
Therefore, for molecules to meet their full potential as components in molecular
machines, methods for coupling them to external sources of energy that
selectively excite the desired motions must be devised.
To this end, we have studied using an electrical current to rotate individual
dibutyl sulfide molecules on command. For these studies the source of energy is
supplied via high energy electrons from the STM tip. It was found that at
temperatures below 8 K the molecules were static and could be stably imaged for
many hours at tunneling voltages less than ±0.35 eV. However, either imaging or
positioning the STM tip over the molecules at biases above ±0.35 eV caused them
to switch between their three distant orientations. With isotopic labeling
experiments we were able to show that the mechanism for this
electrically-induced rotation is a C-H stretch which decays to the rotation of
While these studies are mostly fundamental in nature, they have made great
strides forward for the field of molecular rotors. Fifty years ago Richard
Feynman expressed his dreams to see the miniaturization of useful machines.
Today, while we have the tools to study many of these systems, we still need the
imagination which Feynman displayed in his famous address to realize all of the
potential for the use of nanomachines across every field of science and
While this study and others like it have not put Feynman's "swallowable
surgeons" on the shelves in any stores, we (the scientific research community)
are just beginning to understand the necessary fundamental science toward
achieving these goals. Our studies are continuing in the field of molecular
rotors with chiral molecular rotors5,6 and the influences of chiral scanning probe tips on their
rotation in order to further diversify the utility of this class of single
1. Baber A. E. . et al. ACS Nano 2, 2385-2391 (2008).
2. Tierney, H. L. et al. J. Phys. Chem. C 113, 10913-10920
3. Tierney, H. L. et al. J. Phys. Chem. C 114,
4. Bellisario, D. O.; Baber, A. E.; Tierney,
H. L.; Sykes, E. C. H. J. Phys. Chem C 113, 5895-5898 (2009).
5. Tierney, H. L. et al. Chem. Eur. J. 15, 9678-9681 (2009).
6. Tierney, H. L.; Han, J. W.; Jewell, A. D.; Iski, E. V.; Baber, A.
E.; Sholl, D. S.; Sykes, E. C. H. J. Phys. Chem. C (2010) In Press.
7. Tierney, H. L.; Jewell, A. D.; Baber, A. E.; Iski, E. V.; Sykes,
E. C. H. Submitted to Phys. Rev. Lett. (2010).
Copyright AZoNano.com, Professor Charlie Sykes (Tufts