Preparation, characterization, and electrical properties of a self-assembled meso-pyridyl porphyrin monolayer on gold surfaces

Article abstract of DOI:10.1071/CH05176

A meso-pyridyl porphyrin, 5-(4-(2-(4-(S-acetylthiomethyl)phenyl)ethynyl)phenyl)-10,15,20-tris(4-pyridyl) porphyrin was synthesized by coupling of 5-{4-ethynyl}phenyl-10,15,20-tris(4-pyridyl) porphyrin with 4-(S-acetylthiomethyl)-4-iodobenzene under mild palladium coupling conditions. The meso-pyridyl porphyrin was used for the preparation of self assembled monolayers on gold substrates. The gold substrates were made by the thermal evaporation of gold on oxidized Si(100) surfaces. The self-assembled monolayers (SAMs) were characterized using ground state UV absorption and X-ray photoelectron spectroscopic (XPS) techniques and also by scanning probe microscopy (SPM) techniques. The UV-absorption spectra of the porphyrin monolayer exhibited a 10 nm red shift in the Soret band compared to the porphyrin in CHCl₃ solution. This indicates that the porphyrin molecules are aligned on the gold substrate in a side-by-side orientation. The typical shifts in the binding energy by XPS revealed that the chemisorption of the thiolate species of the porphyrin to the surface occurs through a strong sulfur-gold bonding mechanism. Basic hexagonal (√3×√3) R 30° well ordered self-assembled monolayers of meso-pyridyl porphyrin was observed by molecular-resolution atomic force microscopy (AFM). Room temperature current-voltage (I-V) spectra by scanning tunneling spectroscopy (STS) at varying set point current were collected to study the electronic transport properties of the monolayers on the gold surface. CSIRO 2005.

Full text of DOI:10.1071/CH05176

Full Paper
CSIRO PUBLISHING
Aust. J. Chem. 2005, 58, 810–816
www.publish.csiro.au/journals/ajc
Preparation, Characterization, and Electrical Properties
of a Self-Assembled meso-Pyridyl Porphyrin Monolayer
on Gold Surfaces
Amarchand Sathyapalan,A,B Anup Lohani,C Sangita Santra,D Saurabh Goyal,A
M. Ravikanth,D Soumyo Mukherji,E and V. Ramgopal RaoA,F
A Department of Electrical Engineering, Indian Institute of Technology (IIT), Mumbai, 400076, India.
B Current address: Department of Materials Science and Engineering, Nanyang Technological
University, Singapore.
C Central Surface Analytical Facility, Indian Institute of Technology (IIT), Mumbai, 400076, India.
D Department of Chemistry, Indian Institute of Technology (IIT), Mumbai, 400076, India.
E School of Biosciences and Bioengineering, Indian Institute of Technology (IIT), Mumbai, 400076, India.
F Corresponding author. Email: rrao@ee.iitb.ac.in
A meso-pyridyl porphyrin, 5-(4-(2-(4-(S-acetylthiomethyl)phenyl)ethynyl)phenyl)-10,15,20-tris(4-pyridyl) por-
phyrin was synthesized by coupling of 5-{4-ethynyl}phenyl-10,15,20-tris(4-pyridyl) porphyrin with 4-(S-
acetylthiomethyl)-4-iodobenzene under mild palladium coupling conditions. The meso-pyridyl porphyrin was used
for the preparation of self assembled monolayers on gold substrates. The gold substrates were made by the thermal
evaporation of gold on oxidized Si(100) surfaces. The self-assembled monolayers (SAMs) were characterized using
ground state UV absorption and X-ray photoelectron spectroscopic (XPS) techniques and also by scanning probe
microscopy (SPM) techniques.The UV-absorption spectra of the porphyrin monolayer exhibited a 10 nm red shift in
the Soret band compared to the porphyrin in CHCl3 solution.This indicates that the porphyrin molecules are aligned
on the gold substrate in a side-by-side orientation. The typical shifts in the binding energy by XPS revealed that the
chemisorption of the thiolate species of the porphyrin to the surface occurs through a strong sulfur–gold bonding
√
√
mechanism. Basic hexagonal ( 3 × 3) R 30◦ well ordered self-assembled monolayers of meso-pyridyl porphyrin
was observed by molecular-resolution atomic force microscopy (AFM). Room temperature current–voltage (I–V)
spectra by scanning tunneling spectroscopy (STS) at varying set point current were collected to study the electronic
transport properties of the monolayers on the gold surface.
Manuscript received: 25 July 2005.
Final version: 23 November 2005.
Introduction
flexible to adopt any conformation required for specific
function.[8]
Self-assembled monolayers (SAMs) are highly promising in
constructing well-ordered molecular architectures on metal
and semiconductor surfaces.[1,2] In this context, the organiza-
tion of molecules into highly organized functional arrays is
an area of significant research interest, especially the self-
assembly of organo thiols on metal surfaces, because of
their potential applications in the general area of molecu-
lar electronics.[3–6,31] These ordered assemblies have recently
attracted great interest for the formation of nanoscale molec-
ular devices by virtue of their optical, photoelectrochemical,
and chemical properties.[7] Porphyrins are large macrocy-
cles with potential applications in almost every discipline
of science (such as electroactive molecules for charge
storage, light harvesting complexes and photosynthetic
reaction centres, and for molecular electronics, including
electronics and photonic devices) and are sufficiently
Tour et al. have explored the possibility of porphyrins
having thiol and dithiol linkers for phenomena related to
molecular electronics such as in direct electrical contact, con-
ductance switches in nanopores, and tunneling behavior in
break junctions.[9,30] Lindsey and co-workers pioneered the
work on the synthesis of a series of porphyrins with thiol
linkers and incorporated with various metal ions, and used
them for the preparation of ordered SAMs for molecular
electronics applications such as memory devices.[10–15] Por-
phyrins having meso-pyridyl groups are useful to prepare
water-soluble porphyrins, which can bind with biological
molecules such as DNA without further surface modifi-
cation. Interestingly, and to the best of our knowledge,
there is no report on SAMs formed from meso-pyridyl
porphyrins.
© CSIRO 2005
10.1071/CH05176
0004-9425/05/110810

Pyridyl Porphyrin Monolayer on Gold Surfaces
811
CHO
CHO
N
In this work, a meso-pyridyl porphyrin having a
thiol linker such as 5-(4-(2-(4-(S-acetylthiomethyl)phenyl)
ethynyl)phenyl)-10,15,20-tris-(4-pyridyl) porphyrin 3 was
synthesized and used to investigate the formation of self-
assembled monolayers on gold substrates.These novel SAMs
were systematically characterized by various methods in view
of their potential applications in molecular electronics. The
techniques used to investigate the SAMs include ground state
UV-absorption, X-ray photoelectron spectroscopy (XPS),
high resolution atomic force microscopic (AFM) techniques,
and also by scanning tunneling microscopy/spectroscopy
(STM/STS). TheAFM studies revealed that the meso-pyridyl
porphyrin 3 forms well ordered SAMs on gold substrates.The
electronic characteristics, measured using STS, showed that
this monolayer of porphyrin has a non-linear I–V behavior,
similar to a semiconductor junction.
ꢀ
3
ꢀ
4
N
H
C
C
Propionic acid
Reflux for 2 h
H3C Si CH3
CH3
N
NH
N
N
CH3
C
C
Si CH3 ꢀ five other
N
porphyrins
CH3
HN
Experimental
N
1
All chemicals used were procured from Aldrich or Merck, unless oth-
erwise specified. Anhydrous dichloromethane (99.8%) and anhydrous
ethanol were obtained fromAldrich. Ultra pure water with a resistivity of
18.2 Mꢀ cm was used throughout the experiment wherever necessary.
The ground-state absorption spectra were obtained with Perkin–Elmer
Lambda 35 UV/vis spectrophotometer. The X-ray photoelectron spectra
for porphyrin SAMs were obtained on a Thermo VG Scientific Multi-
lab 2000. The SPM (AFM, STM/STS) studies were obtained by using
Nanoscope IV instruments (Digital Instruments).
K2CO3
THF/CH3OH
Reflux
N
N
NH
N
Synthesis
C
CH
N
To synthesize the desired meso-pyridyl porphyrin having a thiol linker
3, the porphyrin building block having ethynyl phenyl functional
group at the meso-position 2 is required which was synthesized over
sequence of two steps (Scheme 1). The porphyrin 1 was synthesized
by condensing 4-pyridine carboxaldehyde, 4-trimethylsilyl ethynyl ben-
zaldehyde, and pyrrole in propionic acid at refluxing temperature for
2 h followed by silica gel column chromatographic purification. The
deprotection of trimethylsilyl group was carried out using K2CO3 in
THF/CH3OH and afforded porphyrin 2 in 66% yield as purple solid.
The desired porphyrin 3 was then prepared by coupling of 2 with 4-(S-
acetylthiomethyl)-4-iodobenzene in toluene/triethylamine (5/1) under
Lindsey’s mild palladium coupling conditions (Scheme 2). All com-
pounds were characterized by NMR, mass, ground state absorption,
and fluorescence spectroscopic techniques. The absorption spectra of
porphyrin 1 recorded in CHCl3 showed four Q-bands at 514, 548, 588,
and 644 nm and a strong Soret band at 422 nm.
HN
N
2
Scheme 1. Synthesis of meso-pyridyl porphyrins 1 and 2.
(31600), 548 (12000), 588 (10500), 644 (5190). Fluorescence λem/nm
(CH2Cl2, λex 429) 656, 717 (Φ 0.032).
5-(4-Ethynyl)phenyl-10,15,20-tris(4-pyridyl) Porphyrin 2
Porphyrin 1 (50 mg, 0.07 mmol) was dissolved in THF (20 mL) and
K2CO3 in CH3OH (97 mg, 0.70 mmol, in 7 mL of CH3OH) was added to
it. The reaction mixture was refluxed for overnight. The deprotection of
trimethylsilyl group was followed byTLC.After completion of the reac-
tion as judged by TLC, the solvent was removed on rotary evaporator
under vacuum and the purification by silica gel column chromatog-
raphy using CH2Cl2/CH3OH (91/9) and afforded 2 as a purple solid
(30 mg, 66%). mp > 300◦C. δH (CDCl3) −2.89 (2H, s, NH), 3.34 (1H, s,
CCH), 7.93 (2H, d, J 8.05, ArH), 8.16 (8H, m, 3,5-pyridyl 6H + 2HAr),
8.89 (10H, m, 4H β-pyrrole + 6H 2,6 pyridyl), 9.06 (4H, d, β-pyrrole, J
5.86). (Found ES-MS m/z 641.93. C43H27N7 requires 641.74). λmax/nm
(ε/dm3 mol−1 cm−1) (CH2Cl2) 417 (380000), 513 (20800), 547 (7860),
588 (6810), 644 (2950). Fluorescence λem/nm (CH2Cl2, λex 429) 652,
714 (Φ 0.033).
5-(4-(2-Trimethylsilyl)ethynyl)phenyl-10,15,20-tris(4-pyridyl)
Porphyrin 1
Samples of 4-(2-(trimethylsilyl)ethynyl)benzaldehyde (500 mg,
2.47 mmol), pyridine-4-carboxaldehyde (0.70 mL, 7.41 mmol) and pyr-
role (0.68 mL, 9.88 mmol) were dissolved in propionic acid (75 mL)
and refluxed for 2 h. The propionic acid was removed under vacuum
and the crude compound was washed thoroughly with warm water and
dried. The crude black compound was passed through silica gel column
using CH2Cl2/CH3OH (90/10) to remove the undesired polymeric com-
pounds. TLC analysis after passage through a filtration column showed
the formation of six porphyrins in this condensation as expected. The
crude compound was subjected twice to silica gel column chromatog-
raphy and the desired compound 1 was collected as fifth band using
CH2Cl2/CH3OH (92/8). The solvent was removed on rotary evaporator
and afforded 1 as a purple solid in 4% yield (66 mg). mp > 300◦C. δH
(CDCl3) −2.90 (2H, s, NH), 0.38 (9H, s, 3CH3), 7.90 (2H, d, J 8.4, Ar),
8.21 (8H, m, 3,5-pyridyl 6H + 2H Ar), 8.89 (6H, m, 2,6-pyridyl), 9.06
(8H, s, β-pyrrole). (Found ES-MS m/z 714.20. C46H35N7Si requires
713.91). λmax/nm (ε/dm3 mol−1 cm−1) (CH2Cl2) 419 (364000), 513
5-(4-(2-(4-(S-Acetylthiomethyl)phenyl)ethynyl)phenyl)-
10,15,20-tris(4-pyridyl) Porphyrin 3
A
mixture of porphyrin 2 (10 mg, 0.015 mmol) and 4-(S-
acetylthiomethyl)-4-iodobenzene (22 mg, 0.078 mmol) were dissolved
in toluene/triethylamine (20 mL/5 mL) and stirred under argon

812
A. Sathyapalan et al.
N
0.5
0.4
meso-Porphyrin (422 nm)
N
NH
N
meso-Porphyrin SAM
(432 nm)
0.3
ꢀ
C
CH
I
CH2SCOCH3
N
HN
0.2
0.1
2
N
Pd2(dba)3, AsPh3
toluene, TEA
0.0
N2 atm, 35°C
ꢁ0.1
N
300
400
500
600
700
Wavelength (nm)
Fig. 1. Ground-state absorption spectra of meso-pyridyl porphyrin 3
on Au and 3 in solvent.
N
NH
C
C
CH2SCOCH3
N
4 mL vial. Concentrated 60 µL sulfuric acid was then added and the
solution was incubated for 2 h for deprotection of the thiol moiety.
A freshly cleaned gold substrate (gold thermally deposited on quartz
or silicon) was immersed at room temperature for a period of 21 h,
under dark and often purged with N2. After completion of the pro-
cess, the samples were taken out of the solvents and washed with
pure dichloromethane and methanol, and dried under N2, and used
immediately for the characterization.
HN
N
N
3
In the base-promoted technique, 1 mg of meso-pyridyl porphyrin
3 was dissolved in a mixture of dichloromethane/methanol (2/1 by
volume) in a 4 mL vial.The thiol moiety was deprotected from the meso-
pyridyl porphyrin 3 by adding 40 µL of concentrated ammonia solution
flushed with N2 and incubated for ∼2 h. Freshly cleaned gold substrate
was then introduced into the reaction mixture at room temperature and
kept it for 21–22 h, introduced N2 and kept under dark during immer-
sion. After the assembly, the samples were removed from the solutions,
rinsed thoroughly with methanol, acetone, and dicholoromethane, and
dried under a stream of high purity N2.
Scheme 2. Synthesis of meso-pyridyl porphyrin 3.
atmosphere for 10 min. Triphenylarsine (5.7 mg, 0.018 mmol) followed
by Pd2(dba)3 (2.1 mg, 0.0022 mmol) were added and the reaction mix-
ture was stirred for 12 h at 35◦C. The progress of the reaction was
monitored with TLC. After complete disappearance of 2, the reaction
was stopped and the solvent was removed on rotary evaporator under
vacuum. Column chromatography on silica gel using CH2Cl2/CH3OH
(91/9) gave the required compound 3 as a purple solid in 47% yield
(6 mg). mp > 300◦C. δH (CDCl3) −2.87 (2H, s, NH), 2.06 (3H, s, CH3),
4.15 (2H, s, CH2), 7.54 (8H, m, ArH), 7.75 (6H, bs, 3,5-pyridyl), 8.19
(6H, bs, 2,5-pyridyl), 8.87 (4H, bs, β-pyrrole), 9.08 (4H, bs, β-pyrrole).
(Found ES-MS m/z 805.14. C52H35N7OS requires 805.96). λmax/nm
(ε/dm3 mol−1 cm−1) (CH2Cl2) 422 (168000), 514 (9550), 548 (4120),
588 (3380), 644 (1730). Fluorescence λem/nm (CH2Cl2, λex 429) 650,
713 (Φ 0.071).
Absorption Spectroscopy
The ground state UV-visible spectra were measured on a Perkin–Elmer
Lambda 35 spectrophotometer at room temperature in the range of 300–
750 nm. Fig. 1 shows the typical absorption spectrum of the porphyrin
3 self-assembled on gold substrates on quartz. The Au substrates were
scanned before using it for forming the SAMs to obtain background
spectrum for the substrate’s absorption due to gold and glass. The back-
ground spectrum was subtracted from the absorption spectrum of aSAM
of porphyrin to get the porphyrin monolayer absorption.
Preparation of Porphyrin SAM on Gold
Goldsubstrates(thickness∼120 nm)werepreparedbythermalevap-
oration onto RCA-cleaned, thermally oxidized Si(100) wafers (Virginia
Semiconductor, Fredricksburg, VA) and quartz pieces (1 × 2 cm) that
were first coated with a layer of chromium (20 nm) under 2 × 10−6 torr
vacuum. The gold coated wafers were cut into pieces as per requirement
of the studies and stored in pure ethanol or propanol for future use. Prior
to SAM formation, gold substrates were rinsed with deionized water,
dried in a stream of ultrahigh purity N2, and held in a freshly prepared
Piranha solution for 15 min. The surfaces were then rinsed thoroughly
with de-ionized water, ethanol, propanol, and acetone, and then dried in
a flowing ultrahigh purity N2 gas. The freshly cleaned substrates were
used for making the SAMs.
Porphyrin SAMs were prepared by the two most reliable acid-
or base-promoted methods described elsewhere[16,17,30] by Tour and
co-workers. They have proved in their experiments with self-assembled
monolayersofthio-organics, thatbothmethodscouldbeusedformaking
the SAMs.
XPS Measurements
The XPS data for the meso-pyridyl porphyrin 3 SAMs were collected
using a Thermo VG Scientific Multilab 2000 system equipped with
a MgKα X-ray (1253.60 eV) source, a hemispherical electron energy
analyzer (CLAM 4) having a nine-channel detector. The main XPS
chamber was maintained at a base pressure of less than 4 × 10−10 mbar.
The samples were introduced by using a fast-transfer mechanism com-
prised of a long rod and an intermediate pumping stage. The total time
required for the introduction of the sample was ∼10 min. Full survey (0–
1000 eV) was carried out with pass-energy of 50 eV and step size of 1 eV.
Elemental scans were acquired with a pass-energy of 50 eV, step size
of 0.05 eV, which corresponds to a spectral resolution, ꢁE1/2 ≈ 0.85 eV
and dwell time of 50 ms. The survey and high-resolution spectra were
obtained by averaging 10 or 25 scans, with a dwell time of 50 ms per
point and scan, respectively. All XPS spectra were acquired at 15 kV
and 150W. High-resolution spectra were obtained for S 2p, N 1s, C 1s,
In the acid-promoted method, 1 mg of the compound 3 was dissolved
in a solvent mixture of dichloromethane/methanol (2/1 by volume) in a

Pyridyl Porphyrin Monolayer on Gold Surfaces
813
(a)
800 000
700 000
600 000
500 000
400 000
300 000
200 000
100 000
0
Au 4f7/2
Au 4f5/2
O 1s
Au 4s
Au 4p1
Au 4p3
Au 4d3/2
N 1s
Au 4d5/2
C 1s
S 2p3
Au 5s
Au 5p1
Au 5p3
1000
800
600
400
200
0
Binding energy (eV)
(b)
(c)
464 000
295 000
462 000
460 000
458 000
456 000
454 000
294 000
293 000
292 000
291 000
290 000
452 000
410
168 166 164 162 160 158
405
400
395
390
Binding energy (eV)
Binding energy (eV)
Fig. 2. (a) Survey XPS of the meso-pyridyl porphyrin 3 SAM on Au. (b) High-resolution
scan of the S 2p3/2 spectral region. (c) High resolution N 1s spectrum.
and O 1s lines for the SAMs. To compensate for energy shifts due to
Absorption Spectroscopy
possible surface charging effects, all of the XPS peak positions were
referenced to the adventitious C 1s peak at 284.6 eV.
A typical absorption spectrum of the meso-pyridyl porphyrin
3 SAM comparing with 3 in CHCl3 solution is shown in
Fig. 1. The absorption bands of porphyrin 3 SAMs were
broadened and experienced red shifts compared to por-
phyrin 3 in solution. Specifically, the Soret band, which is
observed at 422 nm in solution, was shifted to 432 nm on
solid substrate. It is known in the literature that the relative
orientation of porphyrin molecules in the assembly can be
understood by observing the shift in the Soret band.[18,19,29]
Thus when an edge-to-edge aggregation occurs, the Soret
band shifts towards longer wavelength (red shift) whereas if
face-to-face aggregation occurs, the Soret band experiences
a blue shift.[18,19] The significant red shift in the Soret bands
observed for the meso-pyridyl porphyrin 3 SAMs indicate
that the porphyrins are arranged in a side-by-side orienta-
tion. It was also noted that there was no change in the Soret
band even after the porphyrin 3 SAMs were kept for several
days, indicating that the monolayers are stable.
The presence of meso-pyridyl porphyrin 3 SAMs were determined
by XPS by obtaining the characteristic peaks for N, O, S, and C from
the SAMs along with Au peaks, as shown in Fig. 2. Chemically the
attachment of the thiol to the Au surface is believed to proceed through
strong and stable Au–S bonding mechanism.[10–15,26] XPS shows that
chemisorption of thiols on gold surfaces gives gold–thiolate species.The
relative peak intensities of Au, C, O, S, and N were used to determine
atomic composition.
Scanning Probe Microscopy
The surface morphology was obtained by atomic force microscope and
was performed in ambient laboratory conditions. Atomic resolution
AFM imaging was performed in contact mode at a scan rate of 60 Hz
with 0.4 µm scanner. Si3N4 tipped cantilevers with a spring constant of
0.12 nN nm−1 were used. The electrical properties were studied using
the STS plot modes at room temperature (T 300 K), where the tip was
positioned at the center of the selected scanned area with a constant
tip–sample separation at a point on the surface, and a spectroscopic plot
was acquired and displayed in a scope format.
Results and Discussion
XPS Studies of the SAMs of meso-Pyridyl Porphyrin 3
The SAMs were prepared using meso-pyridyl porphyrin 3
by either acid- or base-promoted techniques as described
elsewhere.[16,17]
The general surface characteristics of thermally evaporated
Au films deposited on Si(100) were examined by XPS before

814
A. Sathyapalan et al.
monolayer deposition to survey the quality of the initial sub-
strates. Measurements on the clean surfaces revealed intense
Au signals near 84, 88, 335, and 353 eV, corresponding to
the 4f7/2, 4f5/2, 4d5/2, and 4d3/2 states, respectively. Weak
signals were also seen from carbon and oxygen. These sig-
nals are an indication of the minor contamination of the
Au surface commonly resulting from adsorption of adven-
titious hydrocarbons and/or water present in the ambient
environment, and they point to the unreliable nature of
the C 1s signal for quantitative determinations of surface
concentrations.
The XPS signatures of the meso-pyridyl porphyrin 3
SAMs were examined to characterize the binding motif of the
monolayer to the Au surface. A representative survey spec-
trum of the SAM is shown in Fig. 2. In the C 1s region, it can
be seen that the binding energy for C 1s is 284.5 eV, which is
consistent with the fact that the majority of the carbon atoms
are sp2 hybridized.[27,28] The spectra of the SAMs exhibit
multiple peaks consistent with the molecular composition of
the porphyrins bearing thiol groups. In particular, in addition
to signals from the bulk Au, the spectra show discrete signals
forSandNfromtheporphyrin.Thehigh-resolutionspectraof
S (Fig. 2b) and N 1s (Fig. 2c) signals are observed at binding
energies of 161.70 0.1 and 399.40 0.1 eV, respectively,
in qualitative agreement with previously reported values for
porphyrins.[3,20–22]
The chemical environment of the S atoms was examined
in detail. Fig. 2b shows a representative high-resolution S 2p
spectrum for the SAM. The binding energy data of sulfur
for the SAM in our case was found to be 161.70 0.1 eV
which is in good agreement with that observed for
neat alkanethiols[3,23–25] and thiol-functionalized porphyrin
monolayers chemisorbed onto Au substrates.[16,20] Thus the
attachment of thiol to theAu surface proceeds through anAu–
S bonding mechanism.[26] The high-resolution XPS spectrum
of the monolayer confirms a layer of chemisorbed thiolate
with an S 2p3/2 species.
4.00
3.00
2.00
1.00
0
0
1.00
2.00
3.00
4.00
nm
Fig. 3. High resolution atomic force microscopy of meso-pyridyl
porphyrin 3 SAM on gold.
current is due to tunnelling, which gives rise to the observed
behavior.
We also observe that current is almost negligible before
a certain cut-in voltage and rises sharply afterwards. This
can be explained with the help of alignment of the metal’s
Fermi levels with molecular orbitals (HOMO/LUMO levels)
as described by Datta et al.[34] The substrate voltage V at
which the conduction starts is given by following equation[34]
ꢀ
ꢁ
EF − EH EL − EF
eV > min
,
(1)
η
η
where EF is metal’s Fermi energy level, EH and EL are energy
levels for HOMO and LUMO, and η is the ratio of distance
of substrate from the centre of molecule to the distance of
substrate from the tip. With varying set-point currents, we
also observe a change in the cut-in voltage (voltage beyond
which current starts to rise rapidly). This is because of the
variation in the factor η in Eqn (1) above. Assuming EH to
be closer to the Fermi level, the voltage at which conduction
starts is given by (EF − EH)/η.[34] As we increase the set-point
current, the tip–substrate separation decreases resulting in an
increase in η.This gives rise to a decrease in the cut-in voltage
at which conductions starts.The conduction starts at ∼0.92V
when the set point current is 1 nA while the conduction starts
at 0.32V when the set-point current is 5 nA. This is in good
agreement with the theory proposed earlier.[32–34]
Scanning Probe Microscopy
Several atomic force microscopic experiments were per-
formed in order to see the SAMs. High resolution images
were used to visualize the monolayers. The 5-nm resolution
images demonstrated the formation of self-assembled mono-
layers of meso-thio porphyrins on gold surfaces. Basic hexag-
√
√
onal ( 3 × 3) R 30◦ arrangement with highly ordered
monolayers was observed (Fig. 3). In the STS mode elec-
tronic measurements were performed consisting of Pt–Ir/
SAM/metal structure, where the tip was positioned at the cen-
ter of the selected scanned area with a constant tip–sample
separation.
Fig. 4 reports three I–V curves collected at different set
point currents by ramping the bias voltages from −1.5V to
+1.5V and keeping all other parameters constant. Since the
bias applied is kept constant, an increase in the set point
current corresponds to a decrease in the tip–sample height
separation. SAMs on gold substrate show clear non-linear I–V
characteristics, which can be explained due to the presence
of a metal–insulator–metal type of a junction.[35] Hence the
Conclusions
We synthesized meso-pyridyl porphyrin 3 having a thiol
linker and showed its ability to form SAMs on Au surface,
using the Au–S bonding mechanism. The typical shifts in
the binding energy by XPS revealed that the chemisorp-
tion of the thiolate species of the porphyrin to the surface
through a strong sulfur–gold bonding mechanism.The SAMs

Pyridyl Porphyrin Monolayer on Gold Surfaces
815
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Set point ꢂ 1nA
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thus formed were further characterized in terms of orien-
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on gold. The absence of blue-shifted components in the por-
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considerable face-to-face aggregation. So the molecules are
assembled as monolayers which is also demonstrated byAFM
√
√
where a well ordered basic hexagonal ( 3 × 3) R 30◦ struc-
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