Comprehensive investigation of self-assembled monolayer formation on ferromagnetic thin film surfaces

Article abstract of DOI:10.1021/ja800278a

We report a simple, universal method for forming high surface coverage SAMs on ferromagnetic thin (≤100 nm) films of Ni, Co, and Fe. Unlike previous reports, our technique is broadly applicable to different types of SAMs and surface types. Our data constitutes the first comprehensive examination of SAM formation on three different ferromagnetic surface types using two different surface-binding chemistries (thiol and isocyanide) under three different preparation conditions: (1) SAM formation on electroreduced films using a newly developed electroreduction approach, (2) SAM formation on freshly evaporated surfaces in the glovebox, and (3) SAM formation on films exposed to atmospheric conditions beforehand. The extent of SAM formation for all three conditions was probed by cyclic voltammetry for surfaces functionalized with either (11-thiolundecyl)ferrocene (Fc-(CH₂)₁₁-SH) or (11-isocyanoundecyl)ferrocene (Fc-(CH₂)₁₁-NC). SAM formation was also probed for straight-chain molecules, hexadecanethiol and hexadecaneisocyanide, with contact angle measurements, X-ray photoelectron spectroscopy, and reflection-absorption infrared spectroscopy (RAIRS). The results show that high surface coverage SAMs with low surface-oxide content can be achieved for thin, evaporated Ni and Co films using our electroreduction process with thiols. The extent of SAM formation on electroreduced films is comparable to what has been observed for SAMs/Au and to what we observe for SAMs/Ni, Co, and Fe samples prepared in the glovebox.

Full text of DOI:10.1021/ja800278a

Published on Web 07/03/2008
Comprehensive Investigation of Self-Assembled Monolayer
Formation on Ferromagnetic Thin Film Surfaces
Paul G. Hoertz, Jeremy R. Niskala, Peng Dai, Hayden T. Black, and Wei You*
Department of Chemistry, UniVersity of North Carolina at Chapel Hill,
Chapel Hill, North Carolina 27599-3290
Received January 12, 2008; E-mail: wyou@email.unc.edu
Abstract: We report a simple, universal method for forming high surface coverage SAMs on ferromagnetic
thin (e100 nm) films of Ni, Co, and Fe. Unlike previous reports, our technique is broadly applicable to
different types of SAMs and surface types. Our data constitutes the first comprehensive examination of
SAM formation on three different ferromagnetic surface types using two different surface-binding chemistries
(thiol and isocyanide) under three different preparation conditions: (1) SAM formation on electroreduced
films using a newly developed electroreduction approach, (2) SAM formation on freshly evaporated surfaces
in the glovebox, and (3) SAM formation on films exposed to atmospheric conditions beforehand. The extent
of SAM formation for all three conditions was probed by cyclic voltammetry for surfaces functionalized with
either (11-thiolundecyl)ferrocene (Fc-(CH2)11-SH) or (11-isocyanoundecyl)ferrocene (Fc-(CH2)11-NC). SAM
formation was also probed for straight-chain molecules, hexadecanethiol and hexadecaneisocyanide, with
contact angle measurements, X-ray photoelectron spectroscopy, and reflection-absorption infrared
spectroscopy (RAIRS). The results show that high surface coverage SAMs with low surface-oxide content
can be achieved for thin, evaporated Ni and Co films using our electroreduction process with thiols. The
extent of SAM formation on electroreduced films is comparable to what has been observed for SAMs/Au
and to what we observe for SAMs/Ni, Co, and Fe samples prepared in the glovebox.
Introduction
literature reports involving SAMs on Ni, Co, and Fe have
utilized carboxylates, phosphonates, or silanes for binding to
Self-assembled monolayers (SAMs) have been widely studied
and well-characterized on gold and other inert metal surfaces
such as Pt, Ag, and Pd.1,2 Surface attachment is typically carried
out by using alkyl or aryl thiols but has also been achieved via
isocyanides, selenols, and so on.1–4 Self-assembled monolayers
are also known to form readily on various metal/native oxide
surfaces, such as Al2O3, TiO2, ZrO2, SiO2, and Fe2O3, typically
using carboxylate, phosphonate, hydroxamate, or silane surface
attachment chemistries.5–9 Self-assembly of alkyl/aryl thiols on
pristine, oxide-free transition metal surfaces, such as Ni, Co,
and Fe, however, has been met with limited success mainly due
to the presence of the native oxide which inhibits the formation
of direct sulfur-metal linkages.10 As a result, the majority of
the native oxide surface, while those that involve bare surfaces
typically rely on gas-phase adsorption of surfactants.7,8,11–13 In
a few reports, the native oxide of Ni and Fe has been attempted
to be removed by electrochemical reduction of the oxide layer
under acidic or basic aqueous conditions.10,14–17 Surprisingly,
the electrochemical reduction of the surface and transfer of the
substrate to the SAM molecule solution has typically been
carried out under atmospheric conditions in spite of the fact
that reformation of the native oxide is known to be rapid.10,11,14
In two of these reports, the electroreduction was performed
under either acidic16,17 or basic15 aqueous conditions, while
SAM formation was carried out either in situ15 or by pulling
the substrate through a top layer of neat thiol.16,17 In both
instances, the broad applicability of the technique is significantly
limited by the requirement that the SAM molecules be either
slightly miscible15 or completely immiscible16,17 with the
aqueous layer.
(1) Smith, R. K.; Lewis, P. A.; Weiss, P. S. Prog. Surf. Sci. 2004, 75, 1.
(2) Love, J. C.; Estroff, L. A.; Kriebel, J. K.; Nuzzo, R. G.; Whitesides,
G. M. Chem. ReV. 2005, 105, 1103.
(3) Henderson, J. I.; Feng, S.; Bein, T.; Kubiak, C. P. Langmuir 2000,
16, 6183.
(4) Monnell, J. D.; Stapleton, J. J.; Jackiw, J. J.; Dunbar, T.; Reinerth,
W. A.; Dirk, S. M.; Tour, J. M.; Allara, D. L.; Weiss, P. S. J. Phys.
Chem. B 2004, 108, 9834.
(11) Lee, Y.; Morales, G. M.; Yu, L. Angew. Chem., Int. Ed. 2005, 44,
4228.
(5) Hoertz, P. G.; Mallouk, T. E. Inorg. Chem. 2005, 44, 6828.
(6) Onclin, S.; Ravoo, B. J.; Reinhoudt, D. N. Angew. Chem., Int. Ed.
2005, 44, 6282.
(12) Fadeev, A. Y.; McCarthy, T. J. J. Am. Chem. Soc. 1999, 121, 12184.
(13) Kataby, G.; Prozorov, T.; Koltypin, Y.; Cohen, H.; Sukenik, C. N.;
Ulman, A.; Gedanken, A. Langmuir 1997, 13, 6151.
(14) Mekhalif, Z.; Laffineur, F.; Couturier, N.; Delhalle, J. Langmuir 2003,
19, 637.
(7) Yee, C.; Kataby, G.; Ulman, A.; Prozorov, T.; White, H.; King, A.;
Rafailovich, M.; Sokolov, J.; Gedanken, A. Langmuir 1999, 15, 7111.
(8) Helmy, R.; Wenslow, R. W.; Fadeev, A. Y. J. Am. Chem. Soc. 2004,
126, 7595.
(15) Bengio, S.; Fonticelli, M.; Benitez, G.; Creus, A. H.; Carro, P.;
Ascolani, H.; Zampieri, G.; Blum, B.; Salvarezza, R. C. J. Phys. Chem.
B 2005, 109, 23450.
(9) Folkers, J. P.; Gorman, C. B.; Laibinis, P. E.; Buchholz, S.; Whitesides,
G. M.; Nuzzo, R. G. Langmuir 1995, 11, 813.
(16) Stratmann, M. AdV. Mater. 1990, 2, 191.
(17) Volmer, M.; Stratmann, M.; Viefhaus, H. Surf. Interface Anal. 1990,
16, 278.
(10) Mekhalif, Z.; Riga, J.; Pireaux, J.-J.; Delhalle, J. Langmuir 1997, 13,
2285.
9
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2008 American Chemical Society
J. AM. CHEM. SOC. 2008, 130, 9763–9772 9763

A R T I C L E S
Hoertz et al.
Figure 1. Left: three-chamber design. A, electroreduction; B, rinsing; C, SAM formation. Right: structures of molecules studied.
were performed with an MBraun evaporator contained within an
MBraun nitrogen glovebox such that freshly evaporated thin films
could be removed from the evaporator chamber under a nitrogen
atmosphere. Thicknesses of films were determined using a quartz
crystal microbalance located just below the substrate holder in the
evaporator. Tooling factors were determined by measuring the true
thicknesses of films with a profilometer; the films were deposited
through a TEM grid (Ted Pella, Item No. 1GC50, 50 square mesh)
shadow mask that was secured to the surface with Kapton tape.
X-ray diffraction spectra were obtained for the M/Ti/SiOx/Si thin
films (100 nm for Ni, Co, and Fe; 50 nm for Au; 25 nm Ti for all
cases) using a Rigaku multiplex powder diffractometer with Ni-
filtered Cu KR radiation (1.5418 Å) over the range 2θ ) 10-85°.
The evaporated metal films were observed to grow with either a
111 texture (Au, Ni, and Co) or 110 texture (Fe).21 The grain
sizes for the thin films were deduced from the X-ray diffraction
data using the Scherer equation: Ni (7.0 Å), Co (5.1 Å), Fe (8.6
Å), and Au (9.3 Å).
For SAMs prepared in the glovebox, a 1 or 10 mM SAM solution
in ethanol (100%, Pharmco) was placed in a vial with septum-
containing screw cap (VWR part 15900-008), purged with Argon
for 1 h, and then brought into the glovebox (O2 levels e2 ppm).
The freshly evaporated M/Ti/SiOx/Si film was immediately sub-
merged in the SAM solution and allowed to react inside of the
glovebox (time was typically for 24-48 h). For SAMs prepared
under atmospheric conditions, the M/Ti/SiOx/Si film was removed
from the glovebox and exposed to atmospheric conditions for at
least 30 min prior to submersion in the SAM solution and stored
under atmospheric conditions.
For SAMs prepared via electroreduction, the M/Ti/SiOx/Si film was
removed from the glovebox and secured to a glass microscope slide
(typically substrate size ) 1.5 × 2.5 cm) using UV-epoxy (Epotek
OG116-31). UV irradiation was carried out by exposing both sides of
the sample for ∼10 min; through-glass irradiation was performed by
placing the substrate on a small, clean plastic Petri dish at an angle of
∼15°. The purpose of the UV-epoxy is to ensure that the applied
potential during the electroreduction step is provided to the exposed
metal surface and not to the sides and bottom of the silicon substrate;
lower SAM surface coverages were observed when the M/Ti/SiOx/Si
sample was not secured to glass with UV-epoxy especially in the cases
of Co and Fe. A custom-built (Quark Glass, Inc.), three-chambered
piece of glassware was used to perform the electroreduction procedure
under an argon atmosphere (Figure 1). The SAM solution (20 mL)
was prepared and placed in the right-most chamber, deionized water
(30 mL) was placed in the central chamber, and aqueous electrolyte
was placed in the left-most chamber (20 mL). 24/40 rubber septa were
used to seal the openings of the three chambers. For the left-most
chamber, a wire soldered to a Ni-plated alligator clip was inserted
through the rubber septum to make an electrical connection to the
reference electrode. A Ni-plated alligator clip was soldered to a PTFE-
Herein, we report a simple, universal method for forming
high surface coverage SAMs on ferromagnetic thin (e100 nm)
films of Ni, Co, and Fe. Unlike previous reports, our technique
is broadly applicable to several different SAMs and surface
types. Our data constitutes the first comprehensive examination
of SAM formation on three different ferromagnetic surface types
using two different surface-binding chemistries (thiol and
isocyanide) and three different preparation conditions: glovebox,
electroreduction, and atmosphere. Previous reports have sug-
gestedthatisocyanidesandthiolsbindwelltoNisurfaces.10,11,14,18,19
Literature reports for SAMs on Co and Fe are extremely limited
but suggest that thiolated SAMs can bind to these surfaces.16,17,20
For glovebox samples, the thin films are removed from the
evaporator under an inert atmosphere and submerged in SAM
molecule solutions to avoid native oxide formation. For
atmosphere samples, the thin films are removed from the
glovebox and SAM formation is carried out after native oxide
formation. For electroreduction samples, the native oxide is
removed electrochemically under an argon atmosphere and the
thin film is then transferred to the SAM molecule solution all
under an inert atmosphere. A key advancement is the ability to
utilize electroreduction techniques on thin films as opposed to
bulk metal samples. Thin films are a highly desirable platform
for incorporating self-assembled monolayer junctions into large-
scale molecular electronic devices. In this paper, we focus on
ways to control and minimize the oxide content at the
SAM-metal interface because the electronic structure of this
interface will inevitably influence the tunneling behavior of the
molecular junction in molecular electronic devices.20
Experimental Section
Preparation of SAMs on Ferromagnetic Surfaces. Nickel, iron,
and cobalt metal films (50 or 100 nm) were prepared by thermal
evaporation (Pinitial ) 2-4 × 10-6 mbar; Pdeposition ) 2-8 × 10-6
mbar) on top of a 25 nm titanium adhesion layer (2 Å/s) on a SiOx/
Si wafer (Si <100>, 1 µm thick oxide on one side, Addison
Engineering). Ni, Fe, and Co were deposited at 1-1.5 Å/s. The
evaporation boats were purchased from RD Mathis; the specific
boat for Ni, Co, and Fe evaporation was an alumina-coated tungsten
boat (type S1AO-W to avoid alloying of the metals with tungsten),
while that for Ti and Au were tungsten boats (type S4-015 W and
S42-015W, respectively). Gold (99.999%), nickel (99.98%), cobalt
(99.95%), and iron (99.95%) pellets (1/8” × 1/8”) were purchased
from Kurt J. Lesker, Inc. Titanium slugs (99.995%, 3.175 mm ×
3.175 mm) were purchased from Alfa Aesar. Thermal evaporations
(18) Vogt, A. D.; Han, T.; Beebe, T. P. Langmuir 1997, 13, 3397.
(19) Friend, C. M.; Stein, J.; Muetterties, E. L. J. Am. Chem. Soc. 1981,
103, 767.
(20) Caruso, A. N.; Wang, L. G.; Jaswal, S. S.; Tsymbal, E. Y.; Dowben,
P. A. J. Mater. Sci. 2006, 41, 6198.
(21) Ghebouli, B.; Cherif, S.-M.; Layadi, A.; Helifa, B.; Boudissa, M. J.
Magn. Magn. Mater. 2007, 312, 194.
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Self-Assembled Monolayer Formation
A R T I C L E S
coated stainless steel rod (1/16 in. diameter, McMaster-Carr) for an
electrical connection to the platinum gauze counter electrode. A bent
(∼30°), PTFE-coated stainless steel rod (1/16 in. diameter) with
soldered, Ni-plated alligator clip was inserted through the rubber
septum located at the central chamber opening to make an electrical
connection to the working M/Ti/SiOx/Si/epoxy/glass electrode. The
chambers were purged with argon for 2 h prior to the electroreduction
step by using three 10” stainless steel needles that penetrate through
the rubber septa and are submerged in the solution within each
chamber; a short needle was placed in one of the septa to avoid pressure
build-up. During electroreduction, purging was continued only at the
SAM solution chamber. During the entire purging process, the M/Ti/
SiOx/Si/epoxy/glass electrode was submerged in the central chamber’s
deionized water to discourage exposure to semivolatile SAMs like
hexadecanethiol. This process was used to avoid gas-phase hexade-
canethiol from depositing on the metal surface prior to electroreduction.
Following electroreduction and working electrode transfer into the
SAM solution, purging was continued for the entire duration of the
SAM formation.
The applied potential during the electroreduction was provided
by a Bioanalytical Systems (BAS) Epsilon potentiostat, and currents
were measured as a function of time in the controlled electrolysis
mode. The applied potential was increased until the initial current
reached 2-3 mA. For Ni and Fe, the electrolyte was 0.1 M NaOH.
For Co, the electrolyte was chosen to be 0.1 M KH2PO4 (pH ) 8,
adjusted with NaOH (aq)) and the applied potential was typically
-0.95 - (-1.0 V) versus Ag/AgCl. Significant etching of the Co
film was evident when 0.1 M NaOH (aq) was used as the
electrolyte; etching was not apparent by eye with 0.1 M KH2PO4
(pH ) 8). The electroreduction time was typically 10 min. After
electroreduction, the working electrode was transferred to the middle
chamber for rinsing of the electrode using the rigid, PTFE-coated
stainless steel rod and then transferred to the SAM solution for
SAM formation. At the completion of SAM formation, the working
electrode was rinsed. All samples (i.e., glovebox, electroreduction,
and atmospheric samples) were rinsed well with ethanol followed
by acetone, sonicated for 30 s in THF, then rinsed again with
acetone and ethanol, and then dried in a nitrogen stream. After
characterization by CV and/or contact angle measurements, SAM
samples were stored in a nitrogen atmosphere glovebox (1-2 ppm
O2) in a parafilm-encapsulated plastic Petri dish.
angle data reported are averages of 4-8 contact angles; standard
deviations fall within the range (0.5-2.0.
X-ray photoelectron spectroscopy (XPS) measurements were
carried out at the Chapel Hill Analytical and Nanofabrication
Laboratory (CHANL) at UNC using a Kratos Analytical Axis Ultra
spectrometer with monochromatized X-ray Al KR radiation (1486.6
eV). Survey scans were performed with a step size of 1 eV and a
pass energy of 80 eV, while region scans were performed with a
step size of 0.1 eV and a pass energy of 20 eV. The peaks of region
scans were fit with a Gaussian-Lorentzian product function that
was weighted 30% Lorentzian. The binding energies of the O 1s,
C 1s, and S 2p XPS peaks were referenced to metal 2p3/2
photoelectron peaks at 852.7 (Ni), 778.3 (Co), and 707 eV (Fe)
for oxide-free metal surfaces.22 Surfaces functionalized with
hexadecanethiol (10 mM SAM molecule solution concentration)
were prepared under glovebox, electroreduction, and atmosphere
conditions. The stability of the SAM and the surface beneath the
SAM was studied as a function of exposure time to the ambient.
The first data point was attained by removing the samples from
their storage conditions and transporting the samples to the XPS
location (total loading time ∼30 min-1 h). Electroreduction and
glovebox samples were stored under a nitrogen atmosphere in a
glovebox. Atmospheric samples were removed from their SAM
molecule solutions and rinsed with solvent an hour prior to XPS
analysis.
AFM images were collected using a Multimode IIIa Atomic
Force Microscope (Veeco Metrology Group). The microscope was
operated in tapping mode at ambient conditions (T ) 21 °C, RH
) 45%), using silicon cantilevers (Mikromasch, Part No. NSC14/
no Al) with resonance frequencies of approximately 160 kHz and
tip radii less than 10 nm. To ensure accuracy, multiple images were
taken of the same sample but in different areas.
Reflection-absorption infrared spectroscopy (RAIRS) was con-
ducted at the Nanochemistry Laser and Vibrational Spectroscopy
Laboratory located at North Carolina State University in the
Department of Chemistry. Spectra were recorded using a Bio-Rad-
Digilab FTS-3000 Fourier transform infrared (FT-IR) spectrometer
using a Varian Universal variable grazing angle reflectance attach-
ment with a ZnSe polarizer having a normal spectral window of
650-7500 cm-1. The infrared light was focused onto the photo-
diodeofaliquidnitrogen-cooled,narrowbandmercury-cadmium-telluride
(MCT) detector with a normal spectral response of 650-7000 cm-1
.
Characterization of SAMs on Ferromagnetic Surfaces. Sur-
face coverage measurements were made by performing cyclic
voltammetry in a three-electrode cell with Fc-(CH2)11-SH or Fc-
(CH2)11-NC derivatized M/Ti/SiOx/Si as the working electrode. The
electroactive area of the working electrode was defined by a viton
O-ring (7 mm outer diameter) placed in a 7 mm hole of a 1 mm
thick Delrin spacer. A 1” inner diameter viton O-ring was then
placed about the Delrin hole followed by a glass joint (Chemglass,
part CG-124-05). The entire arrangement was held together by a
clamp (Chemglass, part CG 150-06). The electrolyte solution (0.1
M tetrabutylammonium hexafluorophosphate in dry, distilled tet-
rahydrofuran) was then placed inside the glass joint along with a
platinum gauze counterelectrode and a BAS nonaqueous reference
electrode (Ag wire in 0.01 M AgNO3/0.1 M tetrabutylammonium
hexafluorophosphate in acetonitrile). External electrical contact to
the working electrode was made via an alligator clip. The dryness
of the THF electrolyte was critical for attaining good CV data with
Co films and was less critical for Ni and Fe. As a result, all
components of the apparatus were dried with a heat gun prior to
CV experiments performed on Co films. Cyclic voltammograms
were attained by scanning the potential from -0.2 to 0.4 V and
back at a particular scan rate (typically 0.2 or 0.5 V/s). Integration
of the area under the redox waves was performed after subtracting
away the baseline current that results from nonfaradaic processes.
The angle of incidence used was 70° with respect to the surface
normal. An infrared polarizer was used to obtain p-polarized light.
The spectrometer and attachment where purged with dry com-
pressed air to reduce the possibility of atmospheric water or CO2
contamination of the spectra and samples. The spectra presented
are an average of 256 scans. All spectra were recorded at room
temperature, approximately 23 ( 0.5 °C, with a resolution of 2
cm-1
.
Great care was taken to ensure that the SAMs being characterized
were chemisorbed and not physisorbed species. Physisorbed
molecules were removed by 30 s of sonication in THF. For
glovebox and electroreduction samples, sonication did not result
in a lowering of the surface coverage and/or contact angle,
suggesting a prevalence of chemisorbed SAMs. For SAMs prepared
under atmospheric conditions, however, sonication typically resulted
in a significant lowering of the surface coverage and/or contact
angle. For example, the contact angle for HDT/Co(ox) was ∼120°
prior to sonication and decreased to 70-90° after sonication.
Presumably, hydrogen-bonded/physisorbed molecules are removed
during sonication in THF.
Attempts were made to probe the presence of pinholes in SAM
layers on magnetic surfaces by performing cyclic voltammetry in
the presence of a solution-phase redox couple. Attempts were also
Contact angle measurements were performed using a CAM 200
optical contact angle meter (KSV Instruments, Ltd.). A 5 µL drop
of deionized water was placed atop the SAM/surface. The contact
(22) Moulder, J. F. Handbook of X-Ray Photoelectron Spectroscopy; Perkin-
Elmer Corporation: Waltham, MA, 1992.
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J. AM. CHEM. SOC. VOL. 130, NO. 30, 2008 9765

A R T I C L E S
Hoertz et al.
Table 1. Surface Coverages of Fc-(CH2)11-SH Adsorbed on Ni, Co, and Fe Thin Films under Glovebox (GB), Electroreduction (ER)a, and
Atmospheric (Atm) Conditions
surface coverage, ×10-10 mol/cm2
Co
Ni
ER
Fe
ER
concn,
mM
time,
days
GB
Atm
GB
ER
Atm
GB
Atm
1
2
9
2
9
3.7(0.1
2.6(0.5
4.7(0.5
6.0(1.8
2.9(0.4
1.1(0.4
1.7(1.0
2.7(0.4
1.6(0.4
1.8(0.4
3.4(2.3
5.4(3.0
1.8(0.4
1.5(0.2
4.6(1.6
17.5(7.8
2.9(1.0
5.0(3.5
0.6(0.1
1.1(0.2
2.3(1.5
1.8(1.0
1.2(0.2
2.3(0.4
0.8 (0.1
0.5 (0.3
1.1 (0.5
0.05(0.01
10
2.7(0.4
4.7(0.7
a Electroreductions were performed at the minimum applied potential that allowed for maintenance of >1 mA reductive currents for the entire 10 min
duration. Following electroreduction, SAM formation was carried out for a 12-18 h period due to reusage of the three-chambered cell.
Table 2. Surface Coverages of Fc-(CH2)11-NC Adsorbed on Ni, Co, and Fe Thin Films under Glovebox (GB), Electroreduction (ER), and
Atmospheric (Atm) Conditionsa,b
surface coverage, ×10-10 mol/cm2
Co
Ni
Fe
concn,
mM
GB
ER
Atm
GB
ER
Atm
GB
ER
Atm
1
10
0.90(0.1 0.54(0.2 2.1(0.3(0.37(0.05) 0.23(0.05
1.7 (0.5 1.8 (0.6 3.5(0.8(1.4 (0.7)
0
0.65(0.5 (0)
0
0.19(0.02 0.12(0.02(0.15(0.03)
0.54(0.10 0.21(0.05 0.55(0.09(0) 0.67(0.08 0.34(0.05 0.61(0.10(0.85(0.08)
a Data in parenthesis involve a 2 day exposure to the solution of isocyanide. The remainder of the glovebox and atmospheric data involve >5 day
exposures. Electroreduction exposures were 12-18 h. b Electroreductions were performed at the minimum applied potential that allowed for maintenance
of >1 mA reductive currents for the entire 10 min duration.
acidic conditions.23 Previous literature reports involving
made to quantify the thickness of the SAM layer by spectroscopic
ellipsometry. Please see Supporting Information for details.
electroreduction of Ni and Fe surfaces have almost exclu-
sively been carried out using acidic conditions with bulk
metal samples. Extensive etching with bulk metals is not
prohibitive, but is highly problematic with evaporated thin
films. The electrolyte for Ni and Fe was 0.1 M NaOH (aq),
while that for Co was 0.1 M KH2PO4 (aq, pH ) 8); Co films
became visibly etched during exposure to basic aqueous
conditions under both argon and atmospheric conditions. The
applied potential during the electroreduction was critical for
optimizing SAM surface coverages (Vide infra). After elec-
troreduction of the working electrode for 10 min, the substrate
was immediately transferred to the second chamber for
rinsing with deionized water and then transferred to a third
chamber containing the ethanolic solution of either thiol or
isocyanide molecules. The contents of each chamber were
purged with argon for 2 h prior to electroreduction and the
cell was kept under argon during the entire duration of SAM
formation to prevent reoxidation of the metal surface.
Surface Coverage. The extent of SAM formation was
probed by either cyclic voltammetry, using either (11-
thiolundecyl)ferrocene (Fc-(CH2)11-SH) or (11-isocyanoun-
decyl)ferrocene (Fc-(CH2)11-NC). The surface coverage
results from cyclic voltammetry for the ferrocene SAMs
(Figure S1) for all three conditions are provided in Tables 1
and 2 and depicted graphically in Figure 2. For comparative
purposes, surface coverage data is also provided for Fc-
(CH2)11-SH and Fc-(CH2)11-NC SAMs formed on freshly
evaporated Au/Ti thin films (Table 3). The peak current from
cyclic voltammograms showed a linear dependence with scan
rate over the range 0.1-1 V/s consistent with a surface-
confined Faradiac process and surface-bound redox species.11
As the data shows, surface coverages for all three metals
were generally higher when Fc-(CH2)11-SH was used,
especially for Co and Fe, which showed vanishingly low
Results and Discussion
SAM Formation. Thin metal films were prepared by thermal
evaporation of 50-100 nm of Ni, Co, or Fe onto a titanium
adhesion layer (25 nm) on SiOx (1 µm)/Si wafers (i.e., M/Ti/
SiOx/Si films). Three different conditions for SAM formation
were explored: (1) transfer of M/Ti/SiOx/Si substrates into an
argon-purged SAM molecule solution immediately following
thermal evaporation of the thin film under glovebox conditions,
(2) a novel electroreduction process as will be discussed below,
or (3) immersion of M/Ti/SiOx/Si substrates into the SAM
molecule solution after exposing the film to atmospheric
conditions for at least 30 min. Under glovebox and electrore-
duction conditions, it is anticipated that minimal oxidation of
the metal surface occurs prior to and/or during SAM formation.
The major difference between these two conditions is that the
e-reduced films begin with native oxides, which are then
removed electrochemically, while the glovebox surfaces are not
exposed to the atmosphere until after SAM formation. In
contrast, surface oxidation is expected to occur prior to and
during SAM formation for samples prepared under atmospheric
conditions. The major advantage of using electroreduction over
glovebox conditions is the ability to make devices under
atmospheric conditions and to then remove surface oxide
electrochemically to allow the formation of good, low-defect
SAMs on the ferromagnetic surface.
Our novel electroreduction process is carried out by
utilizing a custom-built, three-chamber cell that is maintained
under an argon atmosphere the entire time (Figure 1). In the
first chamber, the native oxide is removed by electroreduction
in aqueous electrolyte by means of a three-electrode cell.
The electrolyte for the electroreduction was chosen to be pH
> 7 aqueous solution to allow for the use of thin evaporated
films by avoiding extensive etching of Ni, Co, and Fe under
(23) CRC Handbook of Chemistry and Physics, 88th ed.; Lide, D. R., Ed.;
CRC Press: Boca Raton, FL, 2007-2008.
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9766 J. AM. CHEM. SOC. VOL. 130, NO. 30, 2008

Self-Assembled Monolayer Formation
A R T I C L E S
Figure 2. Surface coverage of SAMs on Ni, Co, and Fe surfaces prepared from 1 mM SAM molecule solutions. Glovebox and atmosphere samples were
prepared over a 2 day period; electroreduction samples were soaked in the SAM molecule solution for 12-18 h.
Table 3. Surface Coverages and Contact Angles on Gold Thin
Films
roughening of the surface over time as corroborated by AFM
measurements.11,12 Interestingly, Co (atmosphere) treated
surface coverage,
×10-10mol/cm2
contact angle,
deg
with 10 mM Fc-(CH2)11-SH over nine days results in more
typical surface coverages on the order of 10-10 mol/cm2. For
both Co and Fe surfaces prepared under atmospheric condi-
tions over a nine day period in 10 mM solutions, the surfaces
become visibly etched and pitted, which may explain the
lower-than-expected surface coverages.
SAM
concn, mM
Fc-(CH2)11-SH
Fc-(CH2)11-NC
C16SH
1
1
1
5.6(0.5
1.8(0.4
111.0
77.5
C16NC
10
Contact Angle. Comparisons between the three methods for
SAM formation were also made with HDT and HDI using
contact angle measurements (Table 3). High contact angles
approaching or equal to that of HDT on Au (111°) were
obtained for HDT on Ni and Co at both 1 and 10 mM SAM
molecule concentrations under glovebox conditions, suggest-
ing excellent SAM formation and alkyl chain alignment. For
Fe, the contact angle for HDT was significantly lower (49°)
at 1 mM concentrations but improved markedly for 10 mM
concentrations (89°). Under electroreduction conditions, 10
mM HDT concentrations were required to achieve ap-
preciably high contact angles for all three surfaces. The
contact angles for HDT on e-reduced films were surprisingly
low when considering the high surface coverages observed
for Fc-(CH2)11-SH under identical conditions and the low
surface oxide content for electroreduced films observed by
XPS (Vide infra). AFM studies (Figure 3) clearly show that
the root-mean-square (rms) surface roughness of postelec-
troreduction Ni and Co films without SAMs increases relative
to that for freshly evaporated thin films (Ni, 1.5 to 2.2 nm;
Co, 0.89 to 1.2 nm). For Fe, the rms surface roughness
decreases slightly after 10 min of electroreduction from 1.7
to 1.6 nm; however, the lower contact angle for HDT on
electroreduced films is most likely a result of higher surface
oxide content according to XPS (Vide infra). Hence, we
attribute the lower contact angles for electroreduced Ni and
Co to increased surface roughness, which disturbs the long-
range crystalline order of the SAM and affects the wetting
behavior. For all three ferromagnetic surfaces, as well as gold,
HDT gave significantly higher contact angles than HDI. This
is consistent with inferior crystallinity of HDI SAMs but
could also be due to lower SAM densities, as suggested by
the ferrocene surface coverage data. It is clear that increasing
the HDI solution concentration results in higher contact
angles in most cases. HDI on Ni gave higher contact angles
than the same on Co and Fe, which further corroborates the
Table 4. Contact Angles for Ni, Co, and Fe Thin Films Soaked in
Ethanolic Hexadecanethiol or Hexadecaneisocyanide Solutions
under Glovebox (GB), Electroreduction (ER)a, and Atmospheric
(Atm) Conditionsd
contact angle, deg
Ni
Co
ER
Fe
concn,
mM
SAM
GB
ER Atm
GB
Atm
GB
ER
Atm
a
C
16SH
1
100.4 68.0 43.9 110.7 63.4
48.5 53.0 43.7
89.2 73.3
76.1
44.3 64.8 51.0
57.8 64.8 77.7
73-83
74-94
b,c
c
10 107.3 92.9 68.8 112.5 79.1
1
10
C16NC
39.1 48.8 35.6 57.6 25.5 43.9
87.8 66.1 49.7 40.1 52.0 44.4
a Electroreductions were performed at -1.2 V vs Ag/AgCl for 10
min. b Contact angles were examined over several days and fluctuated
with time. c Etching of the surface occurs over time. d The contact angle
data reported are averages of 4-8 contact angles; standard deviations
fall within the range (0.5-2.0.
surface coverages with Fc-(CH2)11-NC. This is consistent with
literature findings that isocyanide-functionalized molecules
bind appreciably well to Ni surfaces.11 For Fc-(CH2)11-SH,
the surface coverages for electroreduced surfaces are com-
parable to those prepared in the glovebox. Relative to the
“gold standard”, the surface coverages for glovebox and
electroreduction samples are typically less than a factor of 2
lower. Increasing the Fc-(CH2)11-SH concentration improves
the surface coverage for electroreduced Co, electroreduced
Fe, and glovebox Ni. Atmospheric conditions for Ni and Fe
typically gave surface coverages that were lower than that
for electroreduced or glovebox conditions. The exceptions
are Ni (1 mM, 9 day) and Fe (10 mM, 2 day), which show
higher atmospheric surface coverages than expected. Surpris-
ingly, atmospheric conditions for Co (1 mM, 9 days) gave
coverages that were an order of magnitude (1.8 × 10-9 mol/
cm2) higher than that for glovebox or electroreduction
conditions. We ascribe this to unexpectedly high etching rates
of CoOx/Co by Fc-(CH2)11-SH, which result in significant
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J. AM. CHEM. SOC. VOL. 130, NO. 30, 2008 9767

A R T I C L E S
Hoertz et al.
peaks are assigned to Ni2+(OH)2, NiO, and surface contaminants
(H2O, C-O containing organics, and CO2), respectively.24,25
Bare Co showed similar peaks with the same binding energies
and assignments with similar distributions. Bare Fe showed a
broad (fwhm ) 2.0) O 1s peak at 531.7 eV (% area ) 50.1%)
along with a narrower peak (fwhm ) 1.1) at 530.2 eV (% area
) 49.9%) assigned as Fe2+(OH)2 and Fe3O4, respectively.24 The
metal 2p3/2 spectral regions for all three bare ferromagnetic films
showed evidence of a highly oxidized surface especially in the
case of Fe as indicated by the presence of intense peaks at
slightly higher binding energy than the pristine metal 2p3/2 peaks
at 852.7 eV (Ni), 778.3 eV (Co), and 707 eV (Fe; Figure 5).22
After these samples were Ar ion sputtered for 10 min in situ,
the intensities of the O 1s band decreased significantly by a
factor of ∼7.5 and the metal 2p3/2 spectral regions were
consistent with pristine, nonoxidized metal surfaces. The O 1s
and metal 2p3/2 spectral regions for hexadecanethiol (10 mM)
glovebox and electroreduction samples were then compared to
that of bare and sputtered metal surfaces. The intensities of the
O 1s bands for Ni and Co gloVebox and electroreduction
samples were comparable to that of the sputtered bare surfaces
indicating that these films haVe Very low surface oxygen content.
The oxygen content of glovebox and electroreduced Fe surfaces
was significantly higher and only showed slightly lower oxygen
levels than the bare surface with native oxide. Interestingly, the
electroreduced Fe sample showed lower amounts of Fe3O4 than
glovebox and bare surfaces but greater amounts of Fe2+(OH)2.
The metal 2p3/2 spectral regions corroborate the findings from
the O 1s data: electroreduced Ni and Co show high intensity
pristine metal 2p3/2 peaks with low intensity peaks at 856.0 and
781.5 eV, respectively; glovebox Ni and Co show signs of
shoulders at slightly higher binding energies than the pristine
metal 2p3/2 peaks; and the metal 2p3/2 spectra for glovebox and
electroreduced Fe are practically superimposable with that of
bare Fe. Importantly, the XPS data clearly shows that the
electroreduction process effectively removes oxide/hydroxide
from a surface that has been exposed to atmospheric conditions.
This supports our claim that electroreduction can be utilized as
a process step during the construction of molecular electronic
devices under atmospheric conditions. Low surface oxide/
hydroxide content at the SAM/ferromagnetic surface is expected
to play a crucial role in charge transport in metal-insulator-metal
device architectures.
Figure 3. Atomic force microscopy images of bare nickel (a), electrore-
duced nickel (b), bare cobalt (c), electroreduced cobalt (d), bare iron (e),
and electroreduced iron (f). Scale bar: 500 nm.
claim that isocyanides bind preferably to Ni surfaces. For
Ni and Fe, HDT SAMs did not apparently form well under
atmospheric conditions. Interestingly, the contact angle for
HDT on Co under atmospheric conditions showed time-
dependent behavior; the contact angle decreases from 94 to
74° over a four day period (Figure S2). This is consistent
with etching of the Co surface by HDT over time, which
leads to a rougher film. Over time, crystals form on the
surface and in solution. Centrifugation followed by dissolu-
tion of the crystals in a minimal amount of THF affords a
pale purple solution indicative of a CoII complex, presumably
Co(HDT)x. This observation will be further explored in future
studies and suggests the etching mechanism involves air
oxidation of Co surface atoms followed by complexation by
HDT.
XPS. X-ray photoelectron spectroscopy (XPS) was used to
determine the extent of oxide formation before and after
hexadecanethiol SAM formation for glovebox and electrore-
duction conditions. Samples prepared under atmospheric condi-
tions were studied by XPS to a lesser extent because of inferior
surface coverage and contact angle data. Bare Ni, Co, and Fe
surfaces were analyzed as references. Bare Ni showed a broad
(fwhm ) 2.1) O 1s peak at 531.8 eV (% area ) 64.4%), a
narrower peak (fwhm ) 1.0) at 529.6 eV (% area ) 30.1%),
and a broad peak at 532.8 eV (% area ) 5.5%; Figure 4). These
Time Dependence of Electroreduction. The aforementioned
XPS data was attained after 10 min of electroreduction at the
minimum potential that could maintain the current at >1 mA
for the entire duration. Atomic force microscopy performed on
Ni and Co freshly evaporated surfaces and electroreduced
surfaces without SAMs showed that electroreduction causes a
∼40% increase in surface roughness. This increased roughness
is attributed to etching of the surface during the electroreduction
process. SAMs that are formed on surfaces with roughnesses
on the same order as the length of the molecular monolayer are
expected to have larger defect densities and poor crystallinity.
Thus, it is important to control surface roughening during the
electroreduction process. To this end, we studied electroreduc-
tion as a function of time. First, we examined how the surface
coverage for Fc-(CH2)11-SH varied with the time of the
electroreduction (Figure 6). For Ni, the surface coverage
(24) Bhargava, G.; Gouzman, I.; Chun, C. M.; Ramanarayanan, T. A.;
Bernasek, S. L. Appl. Surf. Sci. 2007, 253, 4322.
(25) Dupin, J.-C.; Gonbeau, D.; Vinatier, P.; Levasseur, A. Phys. Chem.
Chem. Phys. 2000, 2, 1319.
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9768 J. AM. CHEM. SOC. VOL. 130, NO. 30, 2008

Self-Assembled Monolayer Formation
A R T I C L E S
Figure 4. XPS O 1s regional scans for (a) Ni, (b) Co, and (c) Fe bare surfaces (black line), bare surfaces after 10 min in situ Ar ion sputter treatment (black
dashed line), 10 mM hexadecanethiol SAMs prepared in the glovebox (red line), and 10 mM hexadecanethiol SAMs prepared after 10 min electroreduction
(blue line).
Figure 5. XPS metal 2p regional scans for (a) Ni, (b) Co, and (c) Fe bare surfaces (upper plot, black line), bare surfaces after 10 min in situ Ar ion sputter
treatment (upper plot, red line), 10 mM hexadecanethiol SAMs prepared in the glovebox (lower plot, red line), and 10 mM hexadecanethiol SAMs prepared
after 10 min electroreduction (lower plot, black line).
longer electroreduction times is most likely a result of more
complete surface oxide removal. The surface coverage data is
consistent with O 1s and metal 2p3/2 XPS data performed as a
function of electroreduction time (Figures S3, S4). For Ni,
maximum oxide/hydroxide removal from the surface occurs after
5 min of electroreduction. For Co, 10 min of electroreduction
appears to be necessary to achieve minimal surface oxygen
content. For Fe, the lowest oxygen/oxidized Fe levels occurs at
1 min of electroreduction and increases slightly for longer
electroreduction times, which is consistent with the low variance
of the time-dependent surface coverage data.
Applied Potential Dependence of Electroreduction. The
optimal applied potential used during the 10 min electroreduc-
tion was also studied for Ni, Co, and Fe by determining the
surface coverage of Fc-(CH2)11-SH after submersion of the
electroreduced sample in a 1 mM ethanolic solution (Figure
7). A clear plateau in surface coverage is reached beyond -1.0
V versus Ag/AgCl for both Ni and Co. In the case of Fe, the
surface coverage reaches a maximum at -1.2 V versus Ag/
AgCl and then dramatically plummets. The critical potential
appears to coincide with prevalent hydrogen evolution at the
working electrode; below this potential, bubble formation does
not occur to a large extent. It is known from the literature that
Figure 6. Surface coverages of Fc-(CH2)11-SH as a function of the duration
of electroreduction at -1200 mV vs Ag/AgCl reference electrode for nickel
(black line with squares), cobalt (red line with squares), and iron (blue line
with squares).
increases sharply at t > 30 s and then increases less dramatically
up to 10 min. For Fe, the surface coverage remains fairly
constant as a function of electroreduction time. Surface cover-
ages for Co appear to abide by a sigmoidal function showing a
rapid increase in surface coverage between 5 and 10 min.
Because the surface roughness from AFM only increases by
∼40% for Ni and Co between t ) 0 and t ) 10 min and actually
decreases slightly for Fe, the surface coverage improvement at
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J. AM. CHEM. SOC. VOL. 130, NO. 30, 2008 9769

A R T I C L E S
Hoertz et al.
Figure 8. Reflectance-absorbance infrared spectra for hexadecanethiol on
electroreduced nickel (black line), cobalt (red line), and iron (blue line) as
well as on evaporated gold (green line).
Figure 7. Surface coverages of Fc-(CH2)11-SH as a function of the applied
potential during electroreduction for nickel (black line with squares), cobalt
(red line with squares), and iron (blue line with squares).
surface-bound hexadecanethiol (Figure S5). For electroreduction
samples, however, the absorbances were sample dependent
(Figure S6). For Ni, the absorbance is greatest after 10 min,
consistent with XPS and Fc-(CH2)11-SH surface coverage data.
For Fe, the highest absorbance was observed with the 1 min
electroreduction sample, which is consistent with XPS data
which shows lower surface hydroxide/oxide content for the 1
min sample. For Co, the absorbance decreased considerably as
the electroreduction time was increased from 1 to 5 to 10 min
(Figure S6). This observation may be due to extensive etching
of Co during electroreduction which results in thinner films and
signal attenuation.
hydrogen atoms are facile surface oxide etchants.26–30 Thus, we
propose that the surface oxide is removed by electroreduction
via one or both of the following mechanisms:
NiO ⁄ Ni + H2O + e- f H(ads) ⁄ NiO ⁄ Ni + -OH
2H(ads) ⁄ NiO ⁄ Ni f H2O + Ni0 ⁄ Ni
(1)
Ni(OH)2 ⁄ Ni + 2H2O + 2e- f 2H(ads) ⁄ Ni(OH)2 ⁄ Ni + 2-OH
(2)
2H(ads) ⁄ Ni(OH)2 ⁄ Ni f 2H2O + Ni0 ⁄ Ni
Etching of the Surface. Several observations suggest that
etching of the surface occurs in the cases and Co and Fe under
atmospheric conditions but is undetectable in the case of Ni.
First, crystals form over time on the surface and in solution
when Co and Fe surfaces are submerged in 10 mM solutions
of HDT or Fc-(CH2)11-SH under atmospheric conditions.
Second, the entire ferromagnetic surface becomes visibly etched
over long periods of time (>2 weeks) under these conditions
leaving behind only the titanium adhesion layer. The etching
rates appear to be faster for HDT than Fc-(CH2)11-SH perhaps
due to lower solubility of the metal-HDT adducts. Third, the
Fc-(CH2)11-SH surface coverage data abides by predictable
trends in the case of Ni but not for Co and Fe, which suggests
that etching/surface roughening is playing a role in the latter
cases. It is interesting to note that Nuzzo and co-workers
observed discoloration and pitting when forming short-chain
SAMs (n e 3) on Cu and Ag surfaces and that prolonged
exposure resulted in dissolution of the metal.32
SAM Stability. The stability of SAMs on ferromagnetic
surfaces upon extended exposure to atmospheric conditions is
an important consideration for implementation of these molec-
ular monolayers into real devices. XPS was used to examine
the stability of hexadecanethiol (10 mM) SAMs prepared under
glovebox and electroreduction conditions over the course of
several days to weeks. For glovebox and electroreduction
samples exposed to atmospheric conditions on the 1-2 h time
scale, the XPS data shows a S 2p peak at ∼162 eV consistent
with a metal-bound thiolate species (Figure 9, S7).32 Over time,
the sulfur atom becomes increasingly oxidized to a SdO
containing species (e.g., sulfonate, sulfinate, sulfone), as shown
by the appearance of a S 2p peak at ∼169 eV.32 After five days,
Beyond -1.2 V, the surface coverage on Fe dramatically
plummets to ∼1 × 10-11 mol/cm2. The cause of this may be
due to the fact that at -1.4 V, O2 evolution at the Pt
counterelectrode becomes increasingly rapid. Thus, the solution
O2 concentration increases, allowing for surface reoxidation
either during electroreduction or immediately after the applied
potential is removed. Fe oxidation is known to be quite facile.
The results are consistent with Fe being more susceptible to
reoxidation than both Ni and Co. One way to avoid reoxidation
of the electroreduced surface is to purge the solution rapidly
with argon during the electroreduction process. When this is
performed in the case of -1.4 V for Fe for 10 min, the surface
coverage dramatically improves to 1.4 × 10-10 mol/cm2.
Surface IR. Hexadecanethiol SAM formation on Ni, Co, and
Fe under glovebox and electroreduction conditions was studied
using reflectance-absorbance infrared spectroscopy (RAIRS).
Figure 8 shows the IR spectra for electroreduction samples of
hexadecanethiol on Ni, Co, and Fe overlaid with hexadecanethiol
on evaporated Au. For HDT on Ni, Co, Fe, and Au, three peaks
were observed at 2961, 2925, and 2853 cm-1 assigned as the
υas(CH3), υas(CH2), and υs(CH2) stretching modes, respec-
tively.31 For atmospheric and glovebox conditions, nearly
identical spectra with similar absorbances were attained for
(26) Hase, A.; Gibis, R.; Kunzel, H.; Griebenow, U. Appl. Phys. Lett. 1994,
65, 1406.
(27) Eyink, K. G.; Grazulis, L. J. Vac. Sci. Technol., B 2005, 23, 554.
(28) Akatsu, T.; Plossl, A.; Stenzel, H.; Gosele, U. Proc. - Electrochem.
Soc. 2001, 99, 60.
(29) Kagadei, V. A.; Proskurovskii, D. I.; Romas, L. M. Russ. Microelec-
tron. 1998, 27, 91.
(30) Ohashi, T.; Saito, Y.; Maruyama, T.; Nanishi, Y. J. Cryst. Growth
2002, 237-239, 1022.
(31) Nuzzo, R. G.; Dubois, L. H.; Allara, D. L. J. Am. Chem. Soc. 1990,
112, 558.
(32) Laibinis, P. E.; Whitesides, G. M.; Allara, D. L.; Tao, Y.-T.; Parikh,
A. N.; Nuzzo, R. G. J. Am. Chem. Soc. 1991, 113, 7152.
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Self-Assembled Monolayer Formation
A R T I C L E S
Figure 9. XPS S 2p regional scans for hexadecanethiol on electroreduced (a) Ni, (b) Co, and (c) Fe after minimal exposure to atmospheric conditions (black
line) and after 5 days constant exposure to atmospheric conditions (red line).
Figure 10. XPS metal 2p regional scans for hexadecanethiol on electroreduced (a) Ni, (b) Co, and (c) Fe after minimal exposure to atmospheric conditions
(black line) and after five days constant exposure to atmospheric conditions (red line).
Figure 11. XPS metal 2p regional scans for hexadecanethiol SAMs on (a) Ni, (b) Co, and (c) Fe prepared in the glovebox after minimal exposure to
atmospheric conditions (black line), after 8 days constant exposure to atmospheric conditions (red line), and after 15 days constant exposure to atmospheric
conditions (blue line).
the Ni and Co electroreduced samples contain both oxidized
and nonoxidized sulfur with the latter predominating. After eight
days, glovebox Ni and Fe samples contain a mixture of oxidized
and nonoxidized sulfur atoms, while the Co sample’s sulfur
atoms have become almost entirely oxidized.
of the Fe surface atoms. Oxidation of Ni and Co surfaces appears
to occur more slowly for samples prepared in the glovebox than
those prepared by electroreduction by comparing the relative
intensities of the pristine and M(ox) peaks over time (Figure
11). For glovebox Co, the intensity ratio decreases to 1.2 over
8 days, while that for Ni decreases to 1.4. For glovebox Fe, the
surface at t ) 1 h is noticeably less oxidized than the related
electroreduced sample (intensity ratio 0.75 vs 0.61, respectively).
As a result, the surface of the glovebox Fe sample showed signs
of oxidation over time.
Additional insights into the oxidation of the surfaces over
time were attained by analyzing the XPS O 1s spectral region
and fitting the curves to the sum of three Lorentzian-Gaussian
functions centralized at ∼533, ∼531.5, and 529.5 eV (Ni and
Co, 530.1 eV for Fe; Figures S8, S9, Table S1). The peaks at
∼530 eV were attributed to O2--containing species, NiO, CoO,
or Fe3O4, while the broader peak at 533 eV was attributed to
the adsorption of H2O, C-O containing organics, and/or CO2
to the surface.24,25 The intensities of both of these peaks remain
By focusing on the metal 2p spectral regions, the kinetics of
oxidation of the surface metal atoms in the presence of a SAM
overlayer can be attained (Figures 10 and 11). Electroreduced
Co appears to reoxidize more rapidly than electroreduced Ni
over a five day period as observed by the attenuation of the
pristine Co peak at 778.3 eV and growth of an equally intense
broad peak at 782 eV (attributed to oxidized Co, or Co(ox);
Figure 10). For Ni, surface oxidation appears to occur more
slowly as is evident from only a slight increase in the intensity
of the Ni(ox) peak at 856.3 eV. In fact, the intensity ratio for
the pristine Ni peak at 852.7 eV to the Ni(ox) peak at 856.3 eV
only decreases from 1.5 to 1.3 over a five day period. For
electroreduced Fe, the 1 h and 5 day spectra are nearly
superimposable, indicating only a minor change in the oxidation
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A R T I C L E S
Hoertz et al.
fairly constant over extended periods on time in the ambient.
The ∼531.5 eV species were assigned to the presence of
M2+(OH)2 species as previously mentioned. Both glovebox Ni
and Co exhibit a marked increase (∼factor of 3) in the intensity
of the ∼531.5 eV peak over time. Meanwhile, electroreduced
Ni and Fe show negligible changes in intensity of the ∼531.5
eV peak, while glovebox Fe and electroreduced Co show
noticeable but relatively minute intensity increases at this
binding energy over time. Because the O 1s peak intensities at
∼531.5 eV increase over time while those at ∼530 eV remain
constant for Ni and Co, we attribute the increases in intensity
of the metal 2p3/2 spectral region at ∼856.0 and ∼781.5 eV,
respectively, to oxidation of surface atoms to M2+(OH)2 species
for both glovebox and electroreduction cases.
of ferromagnetic surfaces, facilitating the formation of high
surface coverage SAMs on thin, evaporated metal films that
have been pre-exposed to atmospheric conditions. The SAMs
prepared under electroreduction conditions rival those prepared
under inert conditions in a glovebox prior to exposure to the
ambient. Overall, this study comprises an important step toward
the development of molecular electronic devices that utilize self-
assembled monolayers on ferromagnetic surfaces with low
surface-oxide content.
Acknowledgment. This work was supported by the University
of North Carolina at Chapel Hill and the NSF (ECCS-0707315).
We would also like to thank Carrie Donley for help with countless
hours of XPS, Simon E. Lappi for help with conducting RAIRS
experiments, Peter S. White for running XRD samples, and John
Tumbleston of the UNC Physics Department for help with
ellipsometry.
Conclusion
In conclusion, we have examined SAM formation of thiols
and isocyanides on Ni, Co, and Fe thin films under inert and
atmospheric conditions. We found that isocyanides form decent
SAMs on Ni while thiols form good layers on all three surfaces,
especially for Ni and Co. We developed a novel, broadly
applicable electrochemical technique to remove native oxides
Supporting Information Available: Detailed synthesis of all
molecules. This material is available free of charge via the
Internet at http://pubs.acs.org.
JA800278A
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9772 J. AM. CHEM. SOC. VOL. 130, NO. 30, 2008