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Electron based single molecule measurements and artificial single molecule motors Laura Orian Molecular fluctuations and kinetic processes Scuola di Dottorato in scienze Molecolari AA 2012-2013 Electrons have a number fo features that make them suitable probes at the very short length scales relevant for single–molecule measurement: •Small mass: quantum mechanical tunneling behaviour (sub-Angstrom sensitivity) •Coupling of tunneling with inelastic processes: measurement of vibrational and other energy levels Disadvantages: •Observed areas are typically small (less than 1 µ2): difficult to stablish micro-macro scopic features relations •Difficult sample preparation The most widely used techniques to measure the molecular structure and its interaction with the surroundings are STM (Scanning Tunneling Microscopy) and TEM (Transmission Electron Microscopy). STM Molecules are adsorbed on a conductive surface and an atomically sharp metallic probe is rastered across the surface while a bias (1 V) is applied and a tunneling current (pA to nA) is measured. The current decreases exponentially with the distance between the tip and the surface and is measurable up to distances of 1 nm. •Topography of surface features •Single molecules and atoms resolution at 0.1 nm / 0.1 pm The current depends also on the electronic structure of the molecule and thus allows to quantify energy levels within the molecule. TEM In TEM molecules are deposited on a thin electron transparent surface and a beam of high Energy electrons (100keV) is directed thorugh the sample. Unscattered electrons are Collected on a CCD, producing a negative image of the strong electron scatterers. Scattering intensity depends on crystallographic order and atomic number of the scatterer. So typically for proteins, uranyl salts are used to enhance the scattering. Typical resolution is 1/0.1 1/0 1 nm. nm STM is used to quantify packing density and structure of two different halogenated phenols on a Cu{111} surface. Both regular structure and defects are evident in both frames. TEM image and Fourier transform of a nanocrystal superlattice composed of PbS and Pd particles. Again, direct d imaging allows ll d f defects to be b observed in addition to lattice structure. TEM image of a onedimensional lattice of endohedral lanthanide fullerenes in a single-walled carbon nanotube. Inset shows positions of individual lanthanide atoms within fullerenes. Electronic spectroscopy measurements One of the most frequently performed measurements quantifies single molecule conductance. This measurement places stringent restrictions on electrode positioning since it requires that opposite ends of the molecule bind to two terminals of a macroscopic circuit that can be used to measure current. The approach pp to build this device is the scanningg p probe ggeometryy ((sub angstrom g precision)) p Break junction technique: the scanning probe (STM) tip is pressed into the surface and then slowly withdrawn. The metal junction narrows and breaks. If the surface is covered with dithiolated molecules the break junction is bridged by one or more of them. Electronic break junction can be used to quantify the conductance of single molecules. As a gold contact is slowly broken, quantized decreases in conductance are observed, first corresponding to changes in the geometry of the gold contact (a,b), then a set of smaller quantized peaks (c,d) resulting from one, two, or three conductive molecules (here dipyridine) bridging the junction. At slightly larger distances, the junction ceases to exhibit conductance. Electron transport through single conjugated molecules bridging between metal electrodes ChemPhysChem 13, 1116 (2012) An all-electric single molecule motor ACS Nano 4(11) 6681 (2010) Molecular motor: (supra)-molecules able to convert energy into continuous directional motion of one molecular component relative to another. •Two metastable conformations are required separated by a energy barrier, whose height must be several times larger than kBT to prevent thermal fluctuations from setting the molecule into random motion. •The transition must be unidirectional. IDEA: simultaneous driving and detection of the motion of a single molecule motor at the nanoscale! •The rotating moiety has a permanent electric dipole moment •The molecule is conjugated and suspended between two metallic contacts above a gate electrode E By modulating the electric field generated by the gate, the dipole rotor can be driven to rotate with a speed controlled by the frequency of the gate field. As it rotates, the rotor repeatedly switches between two stable states, each corresponding to a planar conformation Rotating where the molecule is fully conjugated. moiety axle connector electrode Rotational barrier p profile: DFT calculations Conductance: Green’s function method They are function of (rotor) and (anchoring groups), defined With respect to the vertical electric field. U r ( ) sin 2 ( ) C cos ( ) 4 The conductance can be used to measure rotation! Torque exerted by an electric field to rotate a dipole p dU ( ) p ( ) p E(t ) sin d Which must exceed the restoring torque in the molecule: dU r ( ) ( ) U 0r sin( 2 ) d r Thermal fluctuations cannot be neglected, they are dominant for nanoscale devices, The dynamics of this motor at temperatures above the quantum level splitting are described by the Langevin equation: d 2 (t ) d (t ) r p I ( ) ( , t ) R (t ) 2 dt dt I moment of inertia Friction coefficient (coupling of molecular motion to the electrodes and interaction between the dipole and the metallic surfaces) R(t) Gaussian distribution of width 2kBT The motor is unidirectional and can perform work except when = 0 and 90 as the rotor is in the top or bottom dead center corresponding to the minimum or maximum of the barrier potential, where the direction of motion is random. The direction of the motion depends on the orientation of the molecule in the junction. This motor prototype is still operational at 77 K, but at room temperature it freely rotates, even in the absence of a driving force: a higher potential barrier is required! Improvement: it is predicted to work at room temperature!