Download Electron based single molecule measurements and artificial single

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!