Transcript Reading about Molecular Electronics Devices Qingling Hang Contents 1. Characterization of single molecules Molecules with one metal atom Molecules with two metal atoms 2.

Reading about Molecular Electronics
Devices
Qingling Hang
Contents
1. Characterization of single molecules
Molecules with one metal atom
Molecules with two metal atoms
2. Molecular logic gates
3. Molecular memory
Electronic Characterization of Single Molecules
with One Metal Atom
Figure 1 The molecules used in this study
and their electronic properties. a, Structure
of [Co(tpy-(CH2)5-SH)2]2+ (where tpy(CH2)5-SH
is
4'-(5-mercaptopentyl)2,2':6',2"-terpyridinyl) and [Co(tpy-SH)2]2+
(where tpy-SH is 4'-(mercapto)-2,2':6',2"terpyridinyl). The scale bars show the
lengths of the molecules as calculated by
energy
minimization.
b,
Cyclic
voltammogram of [Co(tpy-SH)2]2+ in 0.1 M
tetra-n-butylammonium
hexafluorophosphate/acetonitrile showing
the Co2+/Co3+ redox peak. c, I–V curves
of a [Co(tpy-(CH2)5-SH)2]2+ singleelectron transistor at different gate voltages
(Vg) from -0.4 V (red) to -1.0 V (black) with
Vg -0.15 V. Upper inset, a topographic
atomic force microscope image of the
electrodes with a gap (scale bar, 100 nm).
Lower inset, a schematic diagram of the
device.
JIWOONG PARK, ABHAY N. PASUPATHY, JONAS I. GOLDSMITH, CONNIE CHANG, YUVAL YAISH, JASON R. PETTA,
MARIE RINKOSKI, JAMES P. SETHNA, HÉCTOR D. ABRUÑA, PAUL L. MCEUEN & DANIEL C. RALPH, Nature 417, 722 (2002).
Differential Conductance ( әI/әV)
Figure 2 Colour-scale plots of differential
conductance ( әI/әV) as a function of the bias
voltage (V) and the gate voltage (Vg) for three
different [Co(tpy-(CH2)5-SH)2] single-electron
transistors at zero magnetic field. Black
represents zero conductance and white the
maximum conductance. The maxima of the
scales are 5 nS in a, 10 nS in b, and 500 nS in
c. The әI/әV values were acquired by
numerically differentiating individual I–V curves.
Magnetic-field Dependence of the
Tunnelling Spectrum
Figure 3 Magnetic-field dependence of the tunnelling spectrum of a [Co(tpy-(CH2)5-SH)2] singleelectron transistor. a, Differential conductance plot of the device shown in Fig. 2a at a magnetic
field of 6 T. There is an extra level (indicated with the triangle) owing to the Zeeman splitting of
the lowest energy level of Co2+. The arrows denote the spin of the tunnelling electron. b,
Magnitude of the Zeeman splitting as a function of magnetic field.
Kondo effect in Molecules
Figure 4 Devices made using the shorter
molecule, [Co(tpy-SH)2]2+, exhibit the Kondo
effect. a, Breaking trace of a gold wire with
adsorbed [Co(tpy-SH)2]2+ at 1.5 K. After the
wire is broken the current level suddenly
increases (red dot) owing to the incorporation of
a molecule in the gap. This is not seen for bare
gold wires. b, Differential conductance of a
[Co(tpy-SH)2]2+ device at 1.5 K showing a
Kondo peak. The inset shows әI/ әV plots for
bare gold point contacts for comparison. c, The
temperature dependence of the Kondo peak for
the device shown in b. The inset shows the V =
0 conductance as a function of temperature.
The peak height decreases approximately
logarithmically with temperature and vanishes
around 20 K. d, Magnetic-field dependence of
the Kondo peak. The peak splitting varies
linearly with magnetic field.
Single-molecule Transistors with Two Metal - Atoms Molecules
Figure 1 Fabrication of single-molecule transistors
incorporating individual divanadium molecules. Top
left, the structure of [(N,N',N"-trimethyl-1,4,7triazacyclononane)2V2(CN)4(µ-C4N4)] (the V2
molecule) as determined by X-ray crystallography;
red, grey and blue spheres represent respectively V,
C and N atoms. Top right, the schematic
representation of this molecule. Main panel,
scanning electron microscope image (false colour)
of the metallic electrodes fabricated by electron
beam lithography and the electromigration-induced
break-junction technique. The image shows two
gold electrodes separated by 1 nm above an
aluminium pad, which is covered with an 3-nmthick layer of aluminium oxide. The whole structure
was defined on a silicon wafer. The bright yellow
regions correspond to a gold bridge with a
thickness of 15 nm and a minimum lateral size of
100 nm. The paler yellow regions represent
portions of the gold electrodes with a thickness of
100 nm. Main panel inset, schematic diagram of a
single-V2 transistor.
WENJIE LIANG, MATTHEW P. SHORES, MARC BOCKRATH, JEFFREY R. LONG & HONGKUN PARK, Nature 417, 725 (2002).
Differential Conductance (әI/әV)
Figure 2 Plots of differential conductance
(әI/ әV) as a function of bias voltage (V)
and gate voltage (Vg) obtained from two
different single-V2 transistors D1 (a) and
D2 (b). Both measurements were
performed at T = 300 mK. The әI/ә V
values are represented by the colour
scale, which changes in a, from dark red
(0) to bright yellow (1.55e2/h) and in b,
from dark red (0) to bright yellow (1.3
e2/h). The value of e2/h is 38.8 µS or
(25.8 kΩ ) - 1. The labels I and II mark two
conductance-gap regions, and the
diagrams indicate the charge and spin
states of the V2 molecule in each region.
Transport Data in an Applied Magnetic
Field
Figure 3 Transport data obtained from
single-V2 transistors in an applied
magnetic field (B). a, A әI/әV plot as a
function of V and B obtained from D1 at Vg
= -0.1 V and at T = 300 mK. The әI/әV
values are represented by a colour scale
that varies from dark red (0) to bright yellow
(1.3 e2/h). b, A әI/әV plot as a function of
V and Vg obtained from D2 at B = 8 T and
at T = 300 mK. White arrows indicate the
two әI/әV peaks that arise from a Zeeman
splitting. To clearly illustrate weak Zeemansplit features, the colour scale has been
changed from that in Fig. 2a and varies
from dark red (0) to bright yellow (0.55
e2/h).
Temperature-dependent Transport Data
Figure 4 Temperature-dependent transport data from device
D3. a, A plot of conductance (G) versus V with Vg =-2.25 V at
various
temperatures.
The
temperatures
of
the
measurements (in K) are T = 0.3, 1.0, 2.0, 3.1, 4.2, 6.3, 9.0,
14 and 20, in order of decreasing peak height. Inset, a әI/әV
plot as a function of V and Vg at T = 300 mK. The colour
scale changes from dark red (0) to bright yellow (0.55 e2/h). b,
The Kondo temperature (TK: filled red circles) and the Kondo
peak width determined by the full-width at half-maximum
(open blue circles) plotted against ε/Γ in a logarithmic scale.
Here - ε is the energy of the localized electron measured
relative to the Fermi level of the metal, and Γ is the level
width due to the tunnel coupling to the metallic electrodes.
Measurements of the әI/әV peak widths and slopes that
define conductance gaps (inset in a) show that Γ is ~30 mV
and the gate coupling is a= Cg/Ce = 30 meV Vg-1 (Cg is the
capacitance to the gate, and Ce is the total capacitance).
This value of a allows the conversion of Vg to ε, because ε =
a(Vc - Vg). We estimate that the values of ε and Γ are
accurate to within 20%. Red and blue lines are proportional
to exp(-3 ε / Γ) and exp(-1.3 ε / Γ ), respectively. As ε / Γ
exceeds 2, TK and the peak widths approach respective
asymptotic values. Inset, plot of the Kondo peak height (GK)
as a function of temperature at = 0.43 .
Molecular Logic Gates
Figure 1. (A) Top view of a linear array of six devices, shown approximately to scale. The wires were a few microns in diameter, and each pad
was a few hundred microns across to facilitate making an electrical connection to the device. (B) Side view cross section of a single device junction.
Each device consisted of a monolayer of molecules sandwiched between two perpendicularly oriented electrodes and contained several million
molecules. (C) The energy level diagram of one of the devices in (A). The Fermi levels (Ef) of the Al electrodes are shown at both ends of the
diagram, and discrete molecular redox energy levels [determined by solution-phase voltammetry measurements of the R(1) rotaxane] are shown in
the middle. The oxidation states are noted with filled circles. The diagonally striped areas between the electrodes and the rotaxane energy levels
are tunneling barriers. The thick barrier is the Al2O3 passivating layer (measured to be 1 to 1.5 nm), and the thin barrier is the Ti-rotaxane interface
(estimated to be 0.5 nm).
C. P. Collier, E. W. Wong, M. Belohradský, F. M. Raymo, J. F. Stoddart, P. J. Kuekes, R. S. Williams, J. R. Heath, Science 285, 391
(1999).
R(1) Rotaxane Molecule
Figure 2. A drawing of the R(1) rotaxane molecule used here.
Operation of the Devices
Figure 3. (A) Current-voltage traces that show the
operation of the devices. As prepared, the molecular
switches are "closed," and the status of the devices is
probed by applying a negative voltage to the bottom
electrode. The switches are "opened" by oxidizing the
molecules at voltages greater than +0.7 V. Finally, the
open switches are again interrogated at negative bias.
The current ratio at 2 V between open and closed states
is between 50 and 70, depending on the specific device.
(B) The same data as presented in (A), but plotted as
the NDOS. When the closed switch is read, two distinct
features are recorded in the NDOS. The first feature
corresponds closely to the states shown in Fig. 1C. A
single feature is observed in the oxidation NDOS.
However, this feature is not a resolved electronic state.
Rather, oxidation at around +0.7 to +0.9 V irreversibly
changes the molecules, so the NDOS falls to 0 as the
resonant tunneling process is quenched. When the
oxidized devices are interrogated at negative voltage,
the electronic states that were observed between 0 and
1 V for a "switch closed" device are now absent.
Experimentally Measured Truth Tables for
Logic Gates
Figure 4. Experimentally measured truth tables for logic
gates configured from linear arrays of molecular switch
junctions. For all logic gates, a low input is held at ground,
and a high input is held at +2 V. Arbitrary high and low
output current levels are assigned on each plot. The
inputs are labeled alphabetically, and one device, labeled
L, was configured as a load impedance on the gate. (A)
The current output of a two-terminal AND gate as a
function of input address, with an accompanying
schematic of how the device was configured. (B) The
current output (plotted on a logarithmic scale) for a threeinput OR gate (solid trace), which was subsequently
reconfigured into a two-input OR gate (dotted line) by
oxidizing input C. For the two-input gate, the same truth
table was measured, but input C was a dummy input. The
[001] address state does raise the output current level, but
not nearly enough to make the output "high."
Molecular Memory
Fig. 1. (a) Optical micrograph of the nanoelectrode array. Inset: AFM image of four Au
nanoelectrodes with a Pd nanowire lying across. (b) Schematic diagram of the Pd/molecular
wires/Au junctions on a Si/SiO2 substrate.
Chao Li, Daihua Zhang, Xiaolei Liu, Song Han, Tao Tang, and Chongwu Zhou, Wendy Fan, Jessica
Koehne,Jie Han, and Meyya Meyyappan, A. M. Rawlett, D. W. Price, and J. M. Tour, Appl. Phys. Lett. 82,
645 (2003).
Different Molecules
Fig. 2. Molecular wires used. Molecules a, b, and c contain redox centers while molecules d and
e do not contain such centers.
Typical I–V Curves of Molecular Devices
Fig. 3. Typical I–V curves of molecular devices. (a), (b), and (c) correspond to molecules a, b, and c
shown in Fig. 2, respectively.
Read/Write of Molecules
Fig. 4. (a) I–V curves recorded after the device containing molecule a was written into states 1 and
0. (b) Retention time measurement. (c) Current recorded after the device was repeated written
into states 1 and 0.