Simulation of a Pt/benzene molecular junction by J. C. Cuevas
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The field of molecular electronics
was developed following a nice idea of two researchers (Aviram and
Ratner). They suggested that a
single-molecule can serve as an ultimately small
electronic-device, a diode, while the function of
the device (may it be a diode, memory switch or transistor)
can be controlled by the structure of the molecule.
By using the
rich structural variety of organic and inorganic molecules we are looking
for the relation between the structure of a suspended molecule between two metallic electrodes
and the properties of electronic transport through this structure.
From the point of view of chemical physics, molecular junctions provide the opportunity to learn
about the structure and dynamics of molecules not as isolated entities but as a part of an open system
( an electronic circuit ) out of equilibrium. For example, the interaction of molecular
vibrations with the electrons that cross the molecule is a major issue in this context.
In a general perspective we use the molecular junctions as a test-bed for exploring new ways for
manipulating electronic current at the atomic scale. Molecules are good candidates for this task
thanks to the structural diversity offered by chemical synthesis as well as their unique mechanical and electronic properties.
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Mechanically Controllable Break Junction (MCBJ) Technique
We use the mechanically-controllable break-junction technique which works in the following way.
A metal wire with a notch is attached to a flexible substrate. Then a chamber around the
sample is pumped and the system is cooled by liquid helium to ~4.2 K.
At this stage most of the unwanted contaminations freeze on the chamber surface (cryogenic vacuum).
After cooling, the wire is broken at the notch by bending the substrate and we have
two freshly exposed electrodes in ultra-clean conditions.
At this point molecules can be introduced to the metallic junction.
Thanks to the bending mechanism,
the distance between the electrodes can be controlled in a
sub-atomic accuracy, and we can adjust the gap to have a molecule
size. In case that a molecular bridge is formed it is possible
to stabilize it or even stretch the molecule in a
controlled way (e.g.
D. Djukic et al. PRB 71. 161402 (2005),
O. Tal et al. PRB 80, 085427 (2009) ).
The two metallic electrodes
can be squeezed towards each other by relaxing the bent substrate.
If this procedure is done properly a new molecular junction can be formed by breaking again
the contact between the electrodes. Repeating this procedure many times when measuring electron
transport properties allows us to collect statistical data on hundreds or even thousands of different molecular junctions.
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Mechanically Controllable Break Junction
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Gold Conductance traces
Conductance Histogram of an Au junction
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Conductance Traces
Conductance traces are obtained by
measuring the junction conductance while slowly increasing the
electrode distance. For example, several conductance traces of a gold junction are presented on the left figure.
A gold atom is known to have a conductance of 1G0 where G0= 2e2/h is the conductance of a fully
open single conductance channel. For a gold atom suspended between two gold electrodes the
current is carried by a single conductance channel associated with the s valence orbital of
the gold atom. The abrupt jumps to lower conductance values observed in the top figure to
the left appear when the number of gold atoms in the narrowest part of the wire is reduced
during stretching. The long plateau in the end of the traces (near 1G0) takes place when the wire
cross-section is reduced to a single atom and a chain of gold atoms is formed.
As we break and reform
the junction, the conductance traces are varied. The high
stability o f the MCBJ allows us to measure
thousands of such traces and use them
to construct conductance histograms that
indicate the number of times that a certain conductance value was measured. These
histograms can tell us what the most probable conductance values are ( e.g. when stretching
the junction). For example, the fingerprint of a clean platinum junction is a histogram with a main
peak at 1.6 G0 and low contribution between this peak and the tunneling tail at low conductance
(left figure, solid curve). Here, unlike in the case of gold, contributions from both the s and
d valence orbitals lead to total conductance which is higher than 1G0 for the narrowest metallic constriction.
When a molecule is
introduced between the electrodes
the conductance histogram changes
substantially, allowing us to
identify the event and learn about the typical most-probable conductance of the molecular junction. For example, the figure to the left shows the difference between a conductance histogram of a platinum junction, to the one produced when benzene molecules are introduced (gray).
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Inelastic Spectroscopy
During the
measurements of inelastic
spectroscopy the differential conductance (dI/dV)
is measured as a function of the applied voltage (V) across
a molecular junction. Inelastic spectroscopy provides information about inelastic electron
transport processes that can take place as
a result of excitation of molecular vibrations, spin flips, etc. The
top figure on the right provides an example of a
measured spectrum. In the presented case, when the voltage
difference between the metal electrodes reaches a threshold given by
a vibration mode energy (hω) the electrons have enough energy to
excite the molecular vibration mode.
At this voltage (eV =
hω) a step in the differential
conductance appears due to the change in the
overall transmission probability (e.g. at 40 mV in the right
figure ). The interpretation of the step in the differential conductance signal in terms of inelastic
changes induced by a molecular vibration mode can be confirmed by measurements of isotopes
derivatives. The change in mass is expressed as a shift in vibration energy.
For
each type of molecular junction we can collect many dI/dV
spectra where each measurement is performed on&
nbsp; a newly formed junction. The
figure on the bottom right
presents the number of times that a vibration
mode with certain energy was
detected for four different types
of molecular junctions. The unique distribution of the
vibration-mode energies for the four cases
reveals an individual signature for
each molecular junction.
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Differential conductance (benzene on Pt)
M. Kigushi et al. PRL 101, 046801 (2008)
dI/dV Peak Histograms for different molecules
O. Tal et al. PRB 80, 085427 (2009)
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Length histogram of an Au junction
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Length Histograms
When breaking a metal junction
of platinum, iridium or gold, chains of single
atoms can be
formed before complete detachment.
These chains can be observed in length
histograms. Such histograms present
the number of times that a certain
plateau length appears in an ensemble of conductance traces. Here the peaks in the left figure
represent chains of 1,2 and 3 gold atoms. Length histograms and the combination of length and
conductance histograms can teach us about the structure of molecular junctions.
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Shot Noise Measurements
Shot-noise measurements
are used to analyze the electronic
transmission channels available
for electrons to cross an atomic or molecular junction. So by measuring the level of shot
noise, one can gain insight into electronic properties that are not accessible by standard
conductance measurements.
Current shot noise results from
time-dependent fluctuations in the electrical current that take
place because the current is not smooth but it is
a flow of discrete electrons that can either be
transmitted or backscattered. The noise level of a point
contact ( e.g. atomic or molecular junction) is determined by the
number of transmission channels in which electrons can cross the junction and their transmission
probabilities. In the right figure three sets of noise measurements vs. current bias are presented together
with a fitting to the expression for noise power for an arbitrary set of transmission probabilities.
Each set was measured on a different Pt/water junction.
Since
shot noise provides information about the decomposition of
the total conductance in terms of
individual transmission channels it can help us to determine the molecule orientation, the nature
of its binding to the electrodes and can be very useful in relating theory to measurements.
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Shot noise measurements of a Pt/H2O molecular junction
O. Tal et al. PRL 100, 196804 (2008)
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