Reading about Molecular Electronics Devices

1
Reading about Molecular Electronics Devices
Qingling Hang
2
Contents

• Characterization of single molecules
• Molecules with one metal atom
• Molecules with two metal atoms
• Molecular logic gates
• Molecular memory

3
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)22 (where tpy-(CH2)5-SH is
4'-(5-mercaptopentyl)-2,2'6',2"-terpyridinyl)
and Co(tpy-SH)22 (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)22 in 0.1 M
tetra-n-butylammonium hexafluorophosphate/acetonit
rile showing the Co2/Co3 redox peak. c, IV
curves of a Co(tpy-(CH2)5-SH)22
single-electron 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).
4
Differential Conductance ( ?I/?V)
Figure 2 Colour-scale plots of differential
conductance ( ?I/?V) 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 ?I/?V values
were acquired by numerically differentiating
individual IV curves.
5
Magnetic-field Dependence of the Tunnelling
Spectrum
Figure 3 Magnetic-field dependence of the
tunnelling spectrum of a Co(tpy-(CH2)5-SH)2
single-electron 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.
6
Kondo effect in Molecules
Figure 4 Devices made using the shorter molecule,
Co(tpy-SH)22, exhibit the Kondo effect. a,
Breaking trace of a gold wire with adsorbed
Co(tpy-SH)22 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)22
device at 1.5 K showing a Kondo peak. The inset
shows ?I/ ?V 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.
7
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-1,4,7-triazacyclononane)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-nm-thick 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).
8
Differential Conductance (?I/?V)
Figure 2 Plots of differential conductance (?I/
?V) 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
?I/? 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 kO ) - 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.
9
Transport Data in an Applied Magnetic Field
Figure 3 Transport data obtained from single-V2
transistors in an applied magnetic field (B). a,
A ?I/?V plot as a function of V and B obtained
from D1 at Vg -0.1 V and at T 300 mK. The
?I/?V values are represented by a colour scale
that varies from dark red (0) to bright yellow
(1.3 e2/h). b, A ?I/?V 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 ?I/?V peaks
that arise from a Zeeman splitting. To clearly
illustrate weak Zeeman-split 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).
10
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 ?I/?V 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 e/G in a logarithmic scale. Here
- e is the energy of the localized electron
measured relative to the Fermi level of the
metal, and G is the level width due to the tunnel
coupling to the metallic electrodes. Measurements
of the ?I/?V peak widths and slopes that define
conductance gaps (inset in a) show that G 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 e, because e
a(Vc - Vg). We estimate that the values of e and
G are accurate to within 20. Red and blue lines
are proportional to exp(-3 e / G) and exp(-1.3 e
/ G ), respectively. As e / G 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 .
11
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).
12
R(1) Rotaxane Molecule
Figure 2. A drawing of the R(1) rotaxane molecule
used here.
13
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.
14
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
three-input 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."
15
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).
16
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.
17
Typical IV Curves of Molecular Devices
Fig. 3. Typical IV curves of molecular devices.
(a), (b), and (c) correspond to molecules a, b,
and c shown in Fig. 2, respectively.
18
Read/Write of Molecules
Fig. 4. (a) IV 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.