9752 J. Am. Chem. Soc., Vol. 122, No. 40, 2000
Skulason and Frisbie
formed in the microcontact, and statistical sampling may be used
to estimate the rupture force associated with a single bond.10
In this article, we focus on this second approach for detecting
single chemical bond forces. We have chosen to study the
interaction of Au probes with SAMs terminated with S-
containing functional groups, because Au-S interactions are
strong [reported Au-thiolate bond dissociation energies are 120
kJ/mol (1.2 eV/bond),11 roughly a third of a typical C-C bond]
and facile.12 We have prepared both S-acetate-13 and thiol-
terminated alkyl phosphonic acids, reagents I and III, respec-
tively, which will adsorb to metal oxide surfaces such as In-
doped Sn2O3 (ITO) and AlOx via the phosphonic acid group,
leaving the S-containing tail group exposed (Scheme 1). Alkyl
phosphonic acids bind to metal oxides with roughly 3 × 10-10
mol/cm2 coverages depending on the tail group.14 The binding
is believed to involve an ester linkage of the phosphonic acid
with free hydroxyl groups on the substrate.15 We have also
prepared O-acetate- and hydroxyl-terminated alkyl phosphonic
acids, reagents II and IV. Monolayers of II and IV serve as
control samples in our microcontact rupture experiments,
because no S is present in these films and therefore specific
interactions with Au probes are not expected. Note that reagents
I and II are identical except for the substitution of 1 atom,
namely O for S; the same is true for reagents III and IV.
Our intention was to measure rupture forces associated with
discrete Au-thiolate linkages. One article previously reported
that the rupture strength of this bond is 1.4 nN.4a We show here
that the mean rupture forces associated with our tip-SAM
microcontacts are less than 1 nN, making it unlikely that our
microcontact rupture experiments involve breaking Au-thiolate
bonds. However, we do detect a 100 pN force quantum in the
rupture force distributions for microcontacts to I and III. Based
on energetic arguments, we have assigned this force to the
abstraction of Au atoms from the surface of the AFM tip.
Before this work, the strongest specific interactions that have
been probed in tip-SAM microcontacts, excluding biological
interactions such as DNA duplex formation, are hydrogen bonds
such as between amide-modified tips and substrates.4c H-bonds
have energies in the range of 10-40 kJ/mol (100-400 meV/
bond),16 but no direct evidence for rupture of discrete H-bonds
in tip-SAM microcontact pulloff experiments has been reported.
As far as we are aware, our studies represent the first direct
detection of discrete (nonbiological) bonds associated with tip-
SAM microcontacts.
Experimental Section
Materials. 11-Bromoundecanol (98%), p-toluenesulfonic acid mono-
hydrate (98.5%), hexanethiol (95%), triethyl phosphite (98%), potassium
thioacetate (98%), and trimethylbromosilane (98%) were obtained from
Aldrich (Milwaukee, WI) and dihydropyran (99%) was obtained from
Chimica (Geel, Belgium). All solvents were of spectroscopic quality.
Toluene was shaken with sulfuric acid and distilled from CaCl2, and
CH2Cl2 was distilled from P2O5. Absolute ethanol was obtained from
Aaper Alcohol and Chemical Co. (Shelbyville, KY). Gold (99.999%)
was obtained from W. E. Mowrey Co (St. Paul, MN). Aluminum
(99.999%) was obtained from Alfa Esar (Ward Hill, MA). Indium-
tin oxide (Rs e 100 Ω) coated (∼20 nm) glass slides (25 × 75 × 0.9
mm) were obtained from Delta Technologies Ltd. (Stillwater, MN).
Water (18 MΩ) was filtered using a Barnstead system.
Monolayer Preparation. For IR measurements, glass slides (25 ×
75 × 1 mm) were cleaned in boiling 5:1:1 H2O/H2O2/NH4OH, rinsed
with distilled water and absolute ethanol, and dried with flowing N2.
The slides were then coated either with 5 nm of Cr followed by 100
nm of Au, or with 100 nm of Al. The native oxide formed on Al-
coated slides (hereafter referred to as AlOx) was cleaned further in
100-W Ar (500 mTorr) plasma for 30 min. Slides were then immersed
in a 1 mM tetrahydrofuran (THF)/AcOH (50:1) solution of the
respective reagent for 12 h, followed by removal from the solution,
thorough rinsing with the same solvent mixture, and drying in flowing
N2. The bilayer of I or III formed on Au-coated slides was washed
with 2% tetramethylammonium hydroxide [(CH3)4N+OH-] in ethanol,
5% acetic acid in ethanol, and finally a copious amount of ethanol, to
yield the monolayer. For force measurements, ITO17 slides were cut
into 10 × 10 mm pieces, which were cleaned in 100-W Ar plasma for
20 min followed by immersion in 1 mM THF/AcOH (50:1) solution
of the respective reagent for 5 min. The substrates were then removed
from the solution, rinsed thoroughly with the same solvent mixture,
dried with flowing N2, and used immediately.
Infrared Spectroscopy. Infrared spectra were recorded using a
Nicolet MAGNA 550 FT-IR spectrometer equipped with Harrick
Seagull reflectance apparatus and a KRS-5 polarizer. Reflection-
adsorption IR spectra were acquired using p-polarized light incident
angle at 84° relative to the surface normal. Typically, 2048 scans were
acquired at 2 cm-1 resolution.
X-ray Photoelectron Spectroscopy. X-ray photoelectron spectros-
copy studies were performed on a Physical Electronics PHI 5400 fitted
with a 180° spherical capacitor analyzer, using a Mg X-ray source at
300 W. The S2p spectra were recorded from a sampling area of ∼3
mm2 with a takeoff angle of 55° and analyzer pass energy of 35.75
eV. Acquisition times were ∼15 min with a base pressure less than 1
× 10-9 Torr.
Force Measurements. Force measurements were performed with a
Nanoscope III from Digital Instruments (Santa Barbara, CA) equipped
with a fluid cell. Commercially available V-shape Si3N4 cantilevers
with leg length of 200 µm and leg width of 20 µm were used. Both
sides of each cantilever were primed with 3 nm of Cr, followed by 36
nm of Au, deposited by thermal evaporation. Au-coated cantilevers
were used immediately after evaporation. Alternatively, 40 nm of Al
was evaporated on both sides of a cantilever, followed by cleaning in
25-W Ar plasma for 3 min and immersion in 1 mM THF/AcOH (50:
1) solution of the respective reagent for 5 min. The force constant of
each lever was determined by the Cleveland method.18 Resonance
frequencies of coated cantilevers varied from 13.5 to 15.5 kHz, with
the corresponding variations of the force constant between 0.048 and
0.076 N/m. Force measurements were typically performed with a Z
position sweep of 100 nm at a rate 100 nm/s and ∼250 force curves
collected. Force curves were analyzed using routines written in Igor
Pro (Wavemetrics, Lake Oswego, OR). For each force curve, drift and
nonlinearity of the photodetector was corrected by giving the contact
region of the retraction curve a slope of -1. The smoothly varying
(10) The ability to achieve good rupture force statistics has been improved
recently by use of tipless cantilever and a microfabricated array of tips as
the substrate. Many different chemical interactions can be sampled in the
same session by this method. See Green, J.-B. D.; Novoradovsky, A.; Park,
D.; Lee, G. U. Appl. Phys. Lett. 1999, 74, 1489.
(11) (a) Nuzzo, R. G.; Dubois, L. H.; Allara, D. L. J. Am. Chem. Soc.
1990, 112, 558. (b) Nuzzo, R. G.; Zegarski, B. R.; Dubois, L. H. J. Am.
Chem. Soc. 1987, 109, 733.
(12) (a) Ulman, A. Chem. ReV. 1996, 96, 1533. (b) Swalen, J. D.; Allara,
D. L.; Andrade, J. D.; Chandross, E. A.; Garoff, S.; Israelachvili, J.;
McCarthy, T. J.; Murray, R.; Pease, R. F.; Rabolt, J. F.; Wynne, K. J.; Yu,
H. Langmuir 1987, 3, 932. (c) Nuzzo, R. G.; Allara, D. L. J. Am. Chem.
Soc. 1983, 105, 4481.
(13) Tour, J. M.; Jones II, L.; Pearson, D. L.; Lamba, J. J. S.; Burgin, R.
P.; Whitesides, G. M.; Allara, D. L.; Parikh, A. N.; Atre, S. V. J. Am. Chem.
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(14) (a) Gao, W.; Dickinson, L.; Grozinger, C.; Morin, F. G.; Reven, L.
Langmuir 1996, 12, 6429. (b) Gardner, T. J.; Frisbie, C. D.; Wrighton, M.
S. J. Am. Chem. Soc. 1995, 117, 6927. (c) Folkers, J. P.; Gorman, C. B.;
Laibinis, P. E.; Buchholz, S.; Whitesides, G. M. Langmuir 1995, 11, 813.
(15) (a) Farrow, J. B.; Warren, L. J. Colloids Surf. 1989, 34, 255. (b)
Ramsier, R. D.; Henriksen, P. N.; Gent. A. N. Surf. Sci. 1988, 203, 72. (c)
Kuys, K. J.; Roberts, N. K. Colloids Surf. 1987, 24, 1.
(16) (a) Ben-Tal, N.; Sitkoff, D.; Topol, I. A.; Yang, A.-S.; Burt, S. K.;
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(17) Hereafter the term metal oxide (MOx) will be used for both AlOx
and ITO where applicable.
(18) Cleveland, J. P.; Manne, S.; Bocek, D.; Hansma, P. K. ReV. Sci.
Instrum. 1993, 64, 403.