Monolayers and nanoparticles on nickel silicide for molecular electronics

Article abstract of DOI:10.1063/1.2190458

We describe the use of nickel silicide as an electrode in molecular electronics applications. Formation of monolayers of aliphatic and aromatic hydrocarbons on nickel silicide is demonstrated, and these monolayers are used to link CdSe nanocrystals to the

Full text of DOI:10.1063/1.2190458

Monolayers and nanoparticles on nickel silicide for molecular electronics
Eike Marx, Marco Chiesa, Malin Borg, Stefan Bengtsson, Robert J. Less, Paul R. Raithby, Christopher J. B. Ford
,
and Neil C. Greenham
Citation: Applied Physics Letters 88, 143107 (2006); doi: 10.1063/1.2190458
View online: http://dx.doi.org/10.1063/1.2190458
View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/88/14?ver=pdfcov
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APPLIED PHYSICS LETTERS 88, 143107 ͑2006͒
Monolayers and nanoparticles on nickel silicide for molecular electronics
Eike Marx and Marco Chiesa
Cavendish Laboratory, J. J. Thomson Avenue, Cambridge CB3 0HE, United Kingdom
Malin Borg and Stefan Bengtsson
Department of Microtechnology and Nanoscience, Chalmers University of Technology,
SE-412 96 Göteborg, Sweden
Robert J. Less and Paul R. Raithby
Department of Chemistry, University of Bath, Bath BA2 7AY, United Kingdom
a͒
Christopher J. B. Ford and Neil C. Greenham
Cavendish Laboratory, J. J. Thomson Avenue, Cambridge CB3 0HE, United Kingdom
͑
Received 24 October 2005; accepted 14 February 2006; published online 3 April 2006͒
We describe the use of nickel silicide as an electrode in molecular electronics applications.
Formation of monolayers of aliphatic and aromatic hydrocarbons on nickel silicide is demonstrated,
and these monolayers are used to link CdSe nanocrystals to the substrate. Using the conjugated
linker molecule 1,4-ethynylphenyl-2Ј-nitro-1-benzene-dithiolate, scanning tunneling spectroscopy
measurements at 120 K show evidence of Coulomb blockade and resonant tunneling behavior
associated with the nanocrystals. These measurements demonstrate the feasibility of using nickel
silicide as an electrode in molecular electronic devices. © 2006 American Institute of Physics.
͓
DOI: 10.1063/1.2190458͔
Electronic devices using organic molecules or inorganic
nanoparticles as the active element represent the ultimate in
miniaturization, but many challenges remain in interfacing
its structure is shown in the inset of Fig. 2. The substrates
were immersed in a solution containing 0.1 mM of the thio-
acetate derivative of EP2NO in tetrahydrofuran ͑THF͒ and
2
1
large numbers of these devices to macroscopic circuits. Sili-
500:1 aqueous ammonia ͑volume/volume͒ for 24 h, followed
by thorough rinsing with THF. For comparison, the insulat-
ing aliphatic molecule 1,6-hexanedithiol was also assembled
on a separate substrate. CdSe nanocrystals of diameter
con processing technology provides a well-established
method to produce arrays of electrode structures which
2
might be used to contact nanoscale circuit elements. How-
8
,9
ever, the silicon surface forms a native oxide which acts as a
barrier to electron transport, and which makes it difficult to
assemble molecules in such a way that they are in direct
electrical contact with the underlying semiconductor. By
converting the top layer of the silicon into a metal silicide, it
is possible to provide a stable, conducting surface which can
ϳ4 nm were synthesized by standard methods and as-
sembled onto the exposed thiol surface by immersing the
substrates in a toluene solution containing 0.016 wt % of
nanocrystals coated by trioctylphosphine oxide for 24 h, fol-
lowed by thorough rinsing. Similar strategies for assembly of
CdSe nanocrystals have previously been demonstrated on
1
0
11
3
both metallic and semiconducting surfaces.
be used as an electrode. This silicidation process is used to
Attenuated total internal reflection Fourier transform in-
frared spectroscopy ͑ATR-FTIR͒ measurements showed
peaks at 2854 and/or 2925 cm− corresponding to the CH
stretching vibrations of the aliphatic and aromatic hydrocar-
bons, respectively. This confirms the presence of the organic
molecules on the surface, and the shifts in these peaks com-
pared with the bulk values suggest that the molecules are
form gate and interconnect structures in current silicon-based
microelectronic devices. In this letter, we show that mono-
layers of thiolated molecules can be formed on nickel silicide
surfaces. These monolayers are used to attach CdSe nano-
crystals to the surface, and the electrical properties of the
resulting films are investigated using scanning tunneling
spectroscopy ͑STS͒.
Nickel disilicide samples were obtained by thermal
evaporation of 50 nm Ni on boron-doped Si ͑100͒ substrates
with a conductivity of 1–10 ⍀ cm, followed by rapid ther-
mal annealing at 800 °C for 3 min under a nitrogen atmo-
sphere. The thickness of the silicide layer was estimated to
be 180 nm. The samples were dipped in buffered hydrofluo-
ric acid to remove any surface oxides, and rinsed in de-
ionized water. The conjugated phenyleneethynylene
1
1
2,13
closely packed.
Photoluminescence peaking at 601 nm
was observed under excitation at 488 nm, confirming the as-
sembly of CdSe nanocrystals on the surface. The emission
spectrum was identical to that of the nanocrystals in solution.
No luminescence was observed in control samples processed
in the same way but using 1-dodecanethiol. Infrared and
photoluminescence spectra are provided as supplementary
14
material. In order to assess coverage, contact angles for
sessile water droplets were measured for various organic
monolayers on nickel. 1-dodecanethiol gave a contact angle
of 99°, less than the value of 110° obtained for self-
oligomer
1,4-ethynylphenyl-4Ј-ethynylphenyl-2Ј-nitro-1-
benzene-dithiolate ͑EP2NO ͒ was chosen for assembly on
2
the surface since its monolayers are known to exhibit inter-
15
assembled monolayers on gold, but significantly more than
4
,5
esting nonlinear properties in molecular electronic devices.
the 68° obtained on the nickel silicide substrate. This indi-
cates that the layers do not have the well-ordered structure
obtained on gold, but nevertheless the coverage is substan-
6
,7
EP2NO was synthesized using literature procedures, and
2
a͒Electronic mail: ncg11@cam.ac.uk
tial. For EP2NO the contact angle of 76° is again signifi-
2
his article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 75.102.71.3
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On: Mon, 24 Nov 2014 20:59:13
0

143107-2
Marx et al.
Appl. Phys. Lett. 88, 143107 ͑2006͒
FIG. 1. Scanning tunneling spectroscopy measurements at 120 K for 1,6-
hexanedithiol monolayers on nickel silicide at 120 K. Data are shown for
the nickel silicide substrate only ͑labeled with occasional squares͒, with
1
,6-hexanedithiol ͑diamonds͒, and with 1,6-hexanedithiol and CdSe nano-
crystals ͑circles͒. Spectra are averaged over 25 scans from negative to posi-
tive bias at different places on the sample.
FIG. 2. Current-voltage characteristics at 120 K for a monolayer of 1,4-
ethynylphenyl-2Ј-nitro-1-benzene-dithiolate ͑EP2NO ͒ on nickel silicide
2
͑
triangles͒, and for the same monolayer with CdSe nanocrystals ͑circles͒.
cantly larger than that of the substrate, and is consistent with
the presence of a thiol group at the film surface, which is
expected to give more polar behavior than 1-dodecanethiol.
The differential conductance of the latter sample ͑squares͒ is also shown.
Spectra are averaged over 25 scans from negative to positive bias at differ-
ent places on the sample. The inset shows the structure of EP2NO2.
1
0
Further work is necessary to determine the degree of pack-
ing, ordering, and orientation in the organic monolayers, but
these results indicate that the monolayers are reasonably
dense and well packed.
nant tunneling of holes from the tip to the occupied molecu-
lar orbitals of the nanocrystals. There is also some evidence
of a peak around +1.4 V, which is attributed to resonant
tunneling of electrons from the tip to the unoccupied molecu-
lar orbitals of the nanocrystals. STS of CdSe nanocrystals
separated from a metallic substrate by an insulating mono-
Ultrahigh vacuum STS measurements were obtained at
120 K in an Omicron variable-temperature UHV scanning
probe microscope using platinum-iridium scanning tunneling
microscopy ͑STM͒ tips ͑Materials Analytical Services, ra-
dius approximately 50 nm͒. Current-voltage characteristics
were obtained by sweeping the applied voltage between
1
7
layer has been studied in detail by Bakkers et al. Compari-
son with their results suggests that the sample is in the “shell
tunneling” regime, where the tunneling of charges from the
nanocrystals to the substrate is much faster than the tunnel-
ing of charges from the tip onto the nanocrystal. Although
the set point for our measurement implies a smaller tip-
nanocrystal distance than was used by Bakkers et al., since
we use a conjugated monolayer the tunneling rate from nano-
crystal to substrate is likely to be enhanced, consistent with
working in the shell tunneling regime. Enhanced tunneling is
expected due to the presence of relatively low-lying highest
occupied molecular orbital ͑HOMO͒ and lowest unoccupied
±2.5 V at a rate of 500 V/s. STM measurements were per-
formed at a tunneling current of 0.1 nA. STS measurements
were performed at a fixed tip height, set to give a tunneling
current of 0.2 nA at −2 V. Similar parameters were previ-
1
6
ously used by Walzer et al. to study the current-voltage
characteristic of self-assembled monolayers of CdSe nano-
crystals on GaAs surfaces. Current-voltage characteristics
were averaged over 25 scans at different points on the
sample from negative to positive bias unless stated
otherwise.
molecular orbital ͑LUMO͒ levels in EP2NO . We note that
Figure 1 shows the STS spectra of 1,6-hexanedithiol
monolayers on nickel silicide. The spectra for the bare sub-
strate and for the organic monolayer are largely symmetric
about zero bias, show no obvious conductivity gap, and are
consistent with the behavior expected for a metallic substrate
separated from the tip by a dielectric gap. The addition of a
layer of CdSe nanocrystals gives a slight increase in current
at small biases, but does not significantly affect the shape of
the current-voltage characteristic. Surprisingly, no additional
features associated with Coulomb blockade or resonant tun-
neling are observed.
2
there is also the possibility for the charge to become local-
ized on the EP2NO molecule, leading to a more complex
2
three-step tunneling process.
The STS spectra presented so far have been averaged
over 25 scans at different points on the sample. More de-
tailed information about the tunneling process can be ob-
tained by examining individual current-voltage scans for
both positive and negative scan directions, as shown in Fig.
3. Peaks in the conductance spectrum are now much clearer,
since the effects of inhomogeneous broadening due to the
nanocrystal size distribution and the differences in coupling
to the organic monolayer are no longer important. When the
voltage is swept from positive to negative, a sharp peak at
−1.55 V is seen. This peak is significantly broadened when
sweeping from negative to positive. Conversely, the peaks in
forward bias are more prominent when sweeping from nega-
tive to positive. These hysteresis effects are likely to be due
Figure 2 shows the STS spectra of samples using
EP2NO as the linker layer. Addition of the EP2NO layer
2
2
alone gives increased current at low voltages compared with
the substrate and with the hexane-dithiol layer. Addition of
CdSe nanocrystals gives a significant change in the form of
the current-voltage characteristic, with a clear conductance
gap around zero bias, and an asymmetry between forward
and reverse bias. The differential conductance shows a clear
1
8
to rearrangements in the local structure of the monolayer,
his article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 75.102.71.3
peak centered around −1.3 V, which is attributed to the reso-
or in the coupling of the nanocrystals to the monolayer. Cou-
On: Mon, 24 Nov 2014 20:59:13

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Appl. Phys. Lett. 88, 143107 ͑2006͒
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his article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 75.102.71.3
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