Branched coordination multilayers on gold

Article abstract of DOI:10.1021/ja0556676

A C₃-symmetric tridentate hexahydroxamate ligand molecule was specially synthesized and used for coordination self-assembly of branched multilayers on Au surfaces precoated with a self-assembled monolayer (SAM) of ligand anchors. Layer-by-layer (LbL) growth of multilayers via metal-organic coordination using Zr⁴⁺ ions proceeds with high regularity, adding one molecular layer in each step, as shown by ellipsometry, wettability, UV-vis spectroscopy, and atomic force microscopy (AFM). The branched multilayer films display improved stiffness, as well as a unique defect self-repair capability, attributed to cross-linking in the layers and lateral expansion over defects during multilayer growth. Transmetalation, i.e., exposure of Zr ⁴⁺-based assemblies to Hf⁴⁺ ions, was used to evaluate the cross-linking. Conductive atomic force microscopy (AFM) was used to probe the electrical properties of the multilayers, revealing excellent dielectric behavior. The special properties of the branched layers were emphasized by comparison with analogous multilayers prepared similarly using linear (tetrahydroxamate) ligand molecules. The process of defect annihilation by bridging over defective areas, attributed to lateral expansion via the excess bishydroxamate groups, was demonstrated by introduction of artificial defects in the anchor monolayer, followed by assembly of two layers of either the linear or the branched molecule. Analysis of selective binding of Au nanoparticles (NPs) to unblocked defects emphasized the superior repair mechanism in the branched layers with respect to the linear ones.

Full text of DOI:10.1021/ja0556676

Published on Web 11/24/2005
Branched Coordination Multilayers on Gold
Meni Wanunu,† Alexander Vaskevich,† Sidney R. Cohen,‡ Hagai Cohen,‡
Rina Arad-Yellin,§ Abraham Shanzer,§ and Israel Rubinstein*,†
Contribution from the Department of Materials and Interfaces, Department of Chemical
Research Support, and Department of Organic Chemistry, Weizmann Institute of Science,
RehoVot 76100, Israel
Received August 18, 2005; E-mail: israel.rubinstein@weizmann.ac.il
Abstract: A C3-symmetric tridentate hexahydroxamate ligand molecule was specially synthesized and used
for coordination self-assembly of branched multilayers on Au surfaces precoated with a self-assembled
monolayer (SAM) of ligand anchors. Layer-by-layer (LbL) growth of multilayers via metal-organic
coordination using Zr4+ ions proceeds with high regularity, adding one molecular layer in each step, as
shown by ellipsometry, wettability, UV-vis spectroscopy, and atomic force microscopy (AFM). The branched
multilayer films display improved stiffness, as well as a unique defect self-repair capability, attributed to
cross-linking in the layers and lateral expansion over defects during multilayer growth. Transmetalation,
i.e., exposure of Zr4+-based assemblies to Hf4+ ions, was used to evaluate the cross-linking. Conductive
atomic force microscopy (AFM) was used to probe the electrical properties of the multilayers, revealing
excellent dielectric behavior. The special properties of the branched layers were emphasized by comparison
with analogous multilayers prepared similarly using linear (tetrahydroxamate) ligand molecules. The process
of defect annihilation by bridging over defective areas, attributed to lateral expansion via the excess
bishydroxamate groups, was demonstrated by introduction of artificial defects in the anchor monolayer,
followed by assembly of two layers of either the linear or the branched molecule. Analysis of selective
binding of Au nanoparticles (NPs) to unblocked defects emphasized the superior repair mechanism in the
branched layers with respect to the linear ones.
later adopted to polymers by Decher and co-workers.10,11
Hydrogen-bonding interactions have also shown utility in the
Introduction
Layer-by-layer (LbL) construction of multilayer films on
surfaces offers several advantages with respect to other deposi-
tion schemes, providing superior control over film thickness
and composition in the vertical direction, which determine the
multilayer properties. Several systems of this kind have been
developed, distinguished by the type of interaction between the
layers. The Langmuir-Blodgett technique for the generation
of multilayers is the earliest reported form of molecularly
controlled deposition.1 Covalent assembly from solution was
presented by Sagiv and coworkers, who utilized bifunctional
organosilanes to prepare multilayers in a LbL fashion.2 Other
types of covalent multilayer assembly schemes were developed
later.3-8 Electrostatic self-assembly, a commonly used technique
for the generation of thin films, was first realized by Iler9 and
construction of multilayers.12-14 LbL multilayer formation by
metal ion coordination using bifunctional ligand building blocks
has been studied in a variety of systems, including mercapto-
carboxylate/Cu(II),15 diphosphonate/M(IV),16-23 amine/Pt(II),24
Co(II)/diisocyanide,25 Ru3-cluster/bipyridyl,26 tetrahydroxamate/
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† Department of Materials and Interfaces.
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‡ Department of Chemical Research Support.
§ Department of Organic Chemistry.
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J. AM. CHEM. SOC. 2005, 127, 17877-17887
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A R T I C L E S
Wanunu et al.
M(IV),27-29 and other systems.30,31 Metal ion coordination was
also demonstrated in ligand-containing polyelectrolyte multilayer
films.32 The importance of precise control over film thickness
and composition was recently highlighted by the fabrication of
several-nanometer-thick multilayers used as dielectrics for
organic thin film transistors.33
In the present work, branched multilayers exhibiting molec-
ular-scale control over film thickness and a high degree of
homogeneity are presented. The distinct features deriving from
this construction are expressed in the structural and mechanical
properties of the films. The branched films are based on a
custom-synthesized C3-symmetrical hexahydroxamate tridentate
molecule consisting of three bis-hydroxamate ion-binding sites.
We show that the branched building block can yield a regular
multilayer growth mode, similar to that of analogous linear
(tetrahydroxamate) building blocks, the latter providing two bis-
hydroxamate binding sites. LbL multilayers were prepared by
coordination self-assembly on Au substrates using Zr4+ ions,
and their morphology, mechanical, and electrical properties have
been investigated and compared to those of the analogous linear
multilayers.
A unique feature of the branching is possible lateral expansion
via the excess functional groups, thus providing an effective
defect self-repair mechanism during multilayer growth. To dem-
onstrate this effect a mixed self-assembled monolayer (SAM)
comprising the anchor ligand and alkanedithiol molecules, the
latter serving as simulated defects, was constructed. Further
assembly of either branched or linear coordination building
blocks, followed by binding of Au nanoparticles (NPs) to
remaining dithiol defects, revealed an efficient defect elimination
mechanism in the branched multilayer.
The various strategies for multilayer construction present
advantages as well as disadvantages. For example, LbL deposi-
tion of polyelectrolyte films is widely used, benefiting from the
abundance of charged polymers and other building blocks.34
However, such assemblies are known to present an irregular
morphology, resulting in interdigitation of the layers and typic-
ally an increased film permeability with increasing number of
layers.35 On the other hand, multilayers composed of bifunc-
tional molecules are presumed to grow in a lamellar mode. Such
schemes were utilized, e.g., in the construction of asymmetric
multilayers, displaying nonlinear optical properties.36 However,
unavoidable pinholes, adsorbed impurities, surface roughness,
grain boundaries, and inherent tilt of various molecules, lead
to deleterious accumulation of defects with added layers, limiting
the applicability of LbL multilayers.37,38 For example, electronic
applications of multilayer films require pinhole-free layers to
avoid short-circuits between top and bottom contacts.39,40
A possible approach to overcoming the defect accumulation
problem is introduction of branching in the layers, thus providing
a self-healing mechanism and the possibility of bridging over
defects. This, however, may lead to an irregular film structure,
as branching is likely to promote uncontrolled growth. Several
approaches have been demonstrated to the preparation of
hyperbranched polymer films on surfaces for sensing, control
Experimental Section
Chemicals and Materials. All solvents and reagents were analytical
grade and used as received. Chloroform (AR, Biolab) was passed
through a column of activated basic alumina prior to use. Octadecyl-
mercaptan (OM) (98%, Aldrich) was recrystallized from ethanol prior
to use. Water was triply distilled. Samples were dried using 0.2 µm
PTFE-filtered household N2 (>99%, from liquid N2).
Au Substrates. Transmission surface plasmon resonance (T-SPR)
kinetic measurements47 were performed on 5.0 nm Au island films
prepared by evaporation (at 0.005 nm/sec) on silanized glass and
annealed at 200 °C for 20 h.48 The substrates in all other experiments
were semitransparent, continuous Au films (∼17 nm thick), prepared
by evaporation (at 0.05 nm/sec) on aminosilane-treated glass or quartz
and annealed at 200 °C for 20 h.49
Adsorption Conditions. Figure 1a shows the building blocks 1-3
used in this work. Figure 1b shows the construction scheme used for
coordination self-assembly of multilayers of 1. SAMs of 2 were
prepared by overnight adsorption in a 3 mM solution of 2 in 1:1 EtOH/
CHCl3.50 Binding of Zr4+ or Hf4+ was carried out by 1 h immersion in
a fresh 1 mM ethanolic solution of Zr(acac)4 or Hf(acac)4, respectively,
followed by rinsing in EtOH and drying under N2.29 Transmetalation
experiments were performed according to the sequences in Table 2,
with rinsing and drying following each adsorption step. Binding of 1
was accomplished by overnight immersion in a 3 mM methanolic
solution of 1, followed by rinsing in MeOH and drying under N2.
Binding of 3 was performed by overnight immersion in a 3 mM
ethanolic solution of 3, followed by rinsing in EtOH and drying under
N2.29 Binding of Cu2+ ions for blank experiments was accomplished
by 1 h immersion in a 1 mM ethanolic solution of Cu(ClO4)2.
over functional group density, and corrosion protection.41-45
A
recent report on film construction by nonlinearly directed
supramolecular recognition showed ordered multilayer growth,
but rather low film homogeneity, as seen by atomic force
microscopy (AFM).46
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Sasaki, Y. Angew. Chem., Int. Ed. 2003, 42, 2912-2915.
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Vaskevich, A.; Rubinstein, I. Langmuir 2004, 20, 10727-10733.
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Branched Coordination Multilayers on Gold
A R T I C L E S
Figure 1. (a) Chemical structures of molecules 1, 2, and 3. (b) Schematic presentation of the stepwise construction of a Zr4+-coordinated multilayer of 1
on a SAM of 2. Only one of several possible binding modes of 1 is shown (see text).
Ellipsometry. Ellipsometric measurements were carried out with a
Rudolph Research Auto-EL IV null ellipsometer, at an angle of
incidence φ ) 70° and a wavelength λ ) 632.8 nm. The same four
points were measured on each sample before and after self-assembly.
Contact Angle (CA) Measurements. Advancing water CAs were
measured using a computerized CA meter (KSV Instruments, Finland).
Data collection and analysis were carried out using the manufacturer’s
CAM100 software. CAs were measured on three different spots in each
sample.
Transmission UV-Vis Spectroscopy. Transmission spectra were
obtained with a Varian Cary 50 UV-vis spectrophotometer. T-SPR
adsorption kinetics47 were measured by placing the Au island substrate
in a quartz cell filled with methanol, followed by injection of a
concentrated methanolic solution of 1 to make a final concentration of
3 mM. A similar experiment was performed for Zr4+ binding by
injecting a concentrated solution of Zr(acac)4 in ethanol, to make a
final concentration of 1 mM. All other measurements were carried out
in air, using a homemade holder designed for reproducibility of the
sampled spot. Spectra were recorded using air as baseline.
Atomic Force Microscopy (AFM). Appropriate Si cantilever probes
(Mikromasch, Estonia) were selected for each type of experiment.
Dynamic mode AFM imaging of multilayers was performed using a
PicoSPM instrument (Molecular Imaging, U.S.A.), using NSC12
cantilever probes with a resonant frequency of ca. 120 kHz. The effect
of normal load on shear and wear of the multilayers was measured
with a Solver P47 AFM (NT-MDT, Zelenograd, Russia). A CSC12
(tip A) probe was used with normal force constant of 1.5 N/m, as
evaluated using the unloaded resonance technique,51 with cantilever
length and width evaluated from SEM micrographs. For these measure-
ments, a 2.5 µm2 region was first imaged in the dynamic mode at 1
Hz, then a 700 nm2 window in the middle of the larger area was scanned
at 2 Hz in the contact mode, while recording both lateral and total
normal force. Following each scan in the contact mode, the larger area
was again scanned in the dynamic mode. The procedure was repeated
at increasing normal forces until the film was worn away, as evidenced
by exposure of the underlying Au substrate over a significant part of
the scanned region. The normal film wear force was determined as the
force required to completely wear the film. Relative frictional forces
were determined by comparing the lateral deflection signal levels in
right-going and left-going friction traces in the acquired images.
Measurements were made using a single probe and unchanged laser
alignment, to ensure a valid comparison between the different films.
The results for the multilayer films were compared with those for a
SAM of OM on a similar Au substrate. Film thickness was determined
by AFM imaging of the depth of the worn areas, prepared as described
above. The reported AFM film thicknesses were calculated by averaging
10-15 line scans through the worn film. Conductive AFM measure-
ments were carried out at 296 ( 1K and 40 ( 10% relative humidity,
using a Pt-coated Si tip (NSC14/Pt) at a feedback contact force of ∼2
nN. The tip was grounded and the sample bias was controlled using
the STM circuitry of the PicoSPM. I-V curves were obtained on 20
points in different regions of each sample. The same tip was used for
all multilayer samples.
X-ray Photoelectron Spectroscopy (XPS). XPS measurements were
carried out with a Kratos AXIS-HS system, using monochromatized
Al (KR) X-ray source (hυ ) 1486.6 eV). To minimize beam-induced
damage, a low dose was maintained, using a relatively low beam flux
(5 mA emission current at 15 keV) and medium energy resolution (pass
energy of 80 eV). The attenuated Au signal was used to calculate the
thickness of the multilayer films, using an electron mean free path λ
) 3.3 nm, a value typically reported for thin organic films.52
Au Nanoparticle (NP) Blocking Experiments. Mixed SAMs of 2
and 1,10-decanedithiol (DDT) were prepared by 1 h immersion of a
SAM of 2 into a 1 mM ethanolic DDT solution. Subsequent coordina-
tion binding of two layers of either 1 or 3 was then carried out under
conditions as detailed above. Au NPs (5 nm average core diameter)
stabilized with tetraoctylammonium bromide (TOAB) were prepared
using the procedure of Brust et al., i.e., biphasic reduction of HAuCl4.53
The resulting NPs were bound to the different types of films by
overnight immersion in a ∼1 mg/mL solution of the NPs in toluene,
followed by copious rinsing, 30 s of sonication, and a 20 min of
immersion in stirred toluene.
Results and Discussion
Synthesis of 1. The synthesis of the C3-symmetric tridentate
hexahydroxamate ligand 1 (Figure 1a) is outlined in Figure 2a,
and the detailed synthesis is described in the Supporting
Information. Starting from 4-ketopimelic acid, the reaction
(52) Briggs, D., Seah, M. P., Eds. Practical Surface Analysis, 2nd ed.; Wiley:
New York, 1990; Vol. 1.
(51) Sader, J. E.; Chon, J. W. M.; Mulvaney, P. ReV. Sci. Instrum. 1999, 70,
3967-3969.
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795-797.
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A R T I C L E S
Wanunu et al.
Figure 3. (a)Transmission UV-vis spectra (in air) of a Au island film
following the initial steps in multilayer assembly in the following
order: bare Au (bottom curve), a SAM of 2, binding of Zr(acac)4, 1, and
Zr(acac)4 (top curve). (b) In situ T-SPR adsorption kinetics (change in the
extinction at 590 nm) of 1 (I) and subsequent adsorption of Zr(acac)4 (II)
onto an ultrathin Au island film derivatized with a Zr4+-coordinated SAM
of 2. Inset: Overnight adsorption kinetics of 1, as in I.
is the expected thickness increment per layer in a coordinated
multilayer of 1 assuming a perpendicular orientation of the
ligands.
Branched Coordination Multilayers. The construction of
coordination multilayers of 1 on Au is outlined schematically
in Figure 1b. The first step is functionalization of the surface
with hydroxamate groups, accomplished by preparing a SAM
of 2 (the anchor layer) on Au, as described previously.50
Introduction of tetravalent metal ions (e.g., Zr4+) followed by
binding of ligand 1 provides the first branched layer. A branched
coordination multilayer is achieved by subsequent alternate
binding of the metal ions and ligand 1,55 as shown previously
for analogous linear (nonbranched) coordination multilayers.27-29
The branched multilayers grow in a highly regular manner, as
shown below. Substitution of acetylacetonate (acac) groups with
hydroxamates has been observed previously both on Au
surface29 and in solution.50
Figure 2. (a) Synthetic route to the preparation of 1. (b) Molecular model
of the 1/Zr4+ 1:3 complex.
The initial steps in multilayer growth were followed by T-SPR
spectroscopy using an ultrathin Au island film on glass as the
substrate.47 Figure 3a presents transmission UV-vis spectra
taken after each step, starting with the bare Au and ending with
one Zr4+-coordinated layer of 1. The spectra show increase of
the Au surface plasmon (SP) band intensity in each binding
step, indicating successful film deposition. The T-SPR results
are in agreement with previous data on a similar system.56
proceeded in eight steps with an overall yield of 15%. The
structure of 1 was confirmed by FAB/HRMS, ES/MS, 1H NMR,
and FTIR (see the Supporting Information). The chelating ability
of 1 was tested by precipitation from methanol upon addition
of Cu(ClO4)2 (blue precipitate), FeCl3 (red precipitate), or ZrCl4
(white precipitate), yielding insoluble complexes and oligomeric
products.
An energy-minimized model of 1 coordinated to a Zr4+ ion
at each bis-hydroxamate binding site is shown in Figure 2b.54
The projection height for Zr4+-coordinated 1 is 1.28 nm, which
(55) To verify that multilayer growth requires a tetravalent ion (Zr4+, Hf4+),
Cu2+ ions were bound to a SAM of 2, followed by sequential immersion
into 1 and Cu2+ solution. No growth was observed following three such
immersion sequences, as monitored by ellipsometry.
(56) Doron-Mor, I.; Cohen, H.; Barkay, Z.; Shanzer, A.; Vaskevich, A.;
Rubinstein, I. Chem. Eur. J. 2005, 11, 5555-5562.
(54) The model was drawn using PC Spartan Pro, Wavefunction, Inc.
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Branched Coordination Multilayers on Gold
A R T I C L E S
Table 1. XPS Atomic Concentration Percent at Different Takeoff
Angles for a 5-Layer and 10-Layer Zr4+-Coordinated Multilayer of
1 Assembled on a SAM of 2 on Au
takeoff
system
angle (deg)
C
O
N
Zr
S
Au
Au/2/(Zr4+/1)5
Au/2/(Zr4+/1)5
Au/2/(Zr4+/1)10
Au/2/(Zr4+/1)10
90
35
90
35
51.5 25.8 11.8 3.1 0.9 6.3
54.3 26.8 11.4 3.3 0.4 3.4
56.6 26.7 11.3 2.9 0.9 1.4
57.5 27.9 11.2 2.8 0.4 0.4
by replacement of surface hydroxamate groups with the more
hydrophobic acac ligand (see Figure 1b). No significant decrease
in the contact angles is observed with increasing number of
layers, suggesting that the surface roughness is preserved.62
Multilayer growth was monitored by XPS during LbL
assembly of Zr4+-coordinated multilayers of 1 on a SAM of 2
on Au. Table 1 shows XPS atomic concentrations for 5-layer
and 10-layer Zr4+-coordinated multilayers. The results indicate
formation of a uniform overlayer structure, deduced from the
lack of angle dependence for the overlayer atoms (i.e., excluding
S and Au) for both the 5-layer and 10-layer multilayers. The
observed N/Zr ratio in the different measurements was 3.5-
4.0. In a saturated, noncross-linked multilayer of 1/Zr4+ the
repeat unit is a molecule of 1 bound to two Zr4+ ions, i.e., one
Zr4+ connecting to the previous layer and the other bound to a
free arm, leaving the third arm available for binding the next
layer. This gives an N/Zr ratio of 4.5, i.e., 12-28% higher than
the observed value. This difference cannot be attributed to cross-
linking, which would increase the ratio due to sharing of Zr4+
ions between neighboring molecules and is therefore assumed
to indicate some residual Zr4+ accumulated in the multilayer.29
Overall, the XPS results confirm the formation of a spatially
homogeneous multilayer structure.
Figure 4. Advancing water contact angles during multilayer construction:
a SAM of 2 (square) followed by alternate exposure to Zr(acac)4 (triangles)
and 1 (circles).
Formation of a well-organized LbL multilayer requires
optimization of the adsorption conditions, to ensure binding of
a single, complete layer in each step. Therefore, the binding
kinetics of 1 onto a Zr4+-terminated surface and of Zr4+ onto a
1-terminated surface were investigated by in situ monitoring
of the change in the SP band intensity measured at a single
wavelength (Figure 3b).47,57 Upon injection of a solution of 1
into the cell (curve I), a sharp rise in the Au SP intensity is
observed, followed by a slower, steady rise which stabilizes
after 17-20 h (Figure 3b, inset), indicating that overnight
immersion is necessary for obtaining a complete monolayer of
1. The binding kinetics of Zr4+ ions to the hydroxamate-
terminated layer of 1 are shown in curve II of Figure 3b.
Complete binding of Zr4+ to 1 is achieved much faster than
complete binding of 1, as indicated by the relatively fast
stabilization of the SP extinction intensity. This, together with
XPS results (see below), indicates that 1 h adsorption is
sufficient for complete coordination of Zr4+ to the surface.
The morphology of multilayer films of 1 was investigated
by dynamic mode AFM, as shown in Figure 5. The semitrans-
parent Au substrates are {111}-textured and consist of
200-500 nm wide Au single-crystal grains, as previously
described.49 Figure 5a shows a 3 × 3 µm2 AFM image obtained
after removal of a square window from the multilayer film using
the AFM tip (see the Experimental Section). The image shows
that the film morphology is superimposed on the grain structure
of the Au substrate. Cross sections such as the one shown in
Figure 5b, performed on films with varying number of layers,
show a uniform thickness increment of 1.3 nm per layer, in
good agreement with film thickness measurements using other
techniques (see below). A higher resolution image (Figure 5c)
shows that the multilayer film is rather smooth, the concavities
arising from grain boundaries of the Au substrate (see arrows).
RMS roughness analysis was performed on individual Au grains,
showing a relatively constant roughness value of 0.5-0.6 nm
(Figure 5d), not significantly higher than the roughness of the
bare Au. This is in agreement with the contact angle results
(Figure 4) discussed above.
There is a large difference (more than an order-of-magnitude)
in the time required for formation of complete layers of 1 and
of Zr4+, even though the chemistry is the same (bishydrox-
amate-Zr4+ coordination upon removal of two acac ligands)
and mass-transport effects are negligible for a monolayer cover-
age. The results therefore suggest two-step adsorption kinetics
for the formation of a monolayer of 1: the majority of molecules
in the layer (60-70%) are bound in the first several minutes,
followed by slow insertion of the remaining molecules, eventu-
ally forming a compact monolayer in several hours. This mode
has been observed with other monolayer systems, rationalized
by the ability of the molecules in the layer to rearrange, exposing
further surface binding sites.58-61 The difference in the observed
adsorption kinetics of 1 and Zr4+ may therefore be explained
by the slower rearrangement of 1 compared to that of Zr(acac)4.
Advancing water CA data for the multilayer construction are
shown in Figure 4. The CAs are sensitive to the different binding
steps, showing an oscillatory behavior27,29 between 18 ( 4° and
36 ( 3° for hydroxamate-terminated and Zr4+-terminated
surfaces, respectively, indicating regular multilayer growth. The
decreased hydrophilicity following Zr4+ binding is explained
The regularity of multilayer growth was monitored optically
by transmission UV-vis spectroscopy and ellipsometry. Mol-
ecule 1 absorbs light in the far UV region (see inset of Figure
6a) showing a peak at 208 nm with a molar extinction coefficient
ꢀ208 ) 2.9 × 104 M-1 cm-1, measured in aqueous solution. The
high ꢀ value enabled monitoring the LbL assembly of a
(57) Kalyuzhny, G.; Schneeweiss, M. A.; Shanzer, A.; Vaskevich, A.; Rubinstein,
I. J. Am. Chem. Soc. 2001, 123, 3177-3178.
(58) Bain, C. D.; Whitesides, G. M. J. Am. Chem. Soc. 1989, 111, 321-335.
(59) Chen, S. H.; Frank, C. W. Langmuir 1989, 5, 978-987.
(60) Wasserman, S. R.; Tao, Y. T.; Whitesides, G. M. Langmuir 1989, 5, 1074-
1087.
(61) Tillman, N.; Ulman, A.; Schildkraut, J. S.; Penner, T. L. J. Am. Chem.
Soc. 1988, 110, 6136-6144.
(62) Wenzel, R. N. Ind. Eng. Chem. 1936, 28, 988-994.
9
J. AM. CHEM. SOC. VOL. 127, NO. 50, 2005 17881

A R T I C L E S
Wanunu et al.
Figure 5. (a) Dynamic mode AFM image of a window made by the AFM
tip in a 10-layer multilayer of 1 on a SAM of 2 on Au. (b) Line sections
through the window in (a), showing a film thickness of ∼14 nm. (c) Higher
resolution AFM image of an intact 10-layer film; the arrows indicate grain
boundaries of the Au film (z-scale ) 8 nm). (d) RMS roughness values for
multilayers of 1, calculated from AFM images (calculations performed on
200 × 200 nm2 areas of single Au domains).
coordination multilayer of 1 on semitransparent, Au-coated
quartz substrates by transmission UV-vis spectroscopy.49 Figure
6a shows differential extinction spectra (after subtracting the
spectrum of the bare Au substrate) during construction of eight
layers of 1. The presence of the multilayer on the metallic
substrate results in a decreased overall reflection, seen as a
decreased extinction in the visible-near UV range.63 In the far
UV region, this effect is outweighed by absorption of the ligand
1, as evidenced by the stepwise increase in the film extinction
with additional layers of 1. Figure 6b presents plots of the
differential extinction at two wavelengths, 208 nm (the absorp-
tion maximum of 1) and 633 nm (nonabsorbing film), showing
regular increases and decreases of the extinction, respectively,
corresponding to regular buildup of the multilayer.
Ellipsometric measurements during construction of the mul-
tilayers showed a typical stepwise decrease in ∆ accompanying
addition of molecular layers of 1.27,29,64 Multilayer growth,
expressed as increase in film thickness, was evaluated by
four independent techniques, i.e., ellipsometry, transmission
UV-vis spectroscopy, AFM profilometry,29,65 and XPS mea-
surements66 (Figure 6c). The ellipsometric and spectroscopic
measurements were both analyzed at a wavelength of 632.8 nm,
using a multilayer refractive index of nf ) 1.50, kf ) 0,
established in numerous previous studies.27,64,67 For analysis of
the spectroscopic data, the thickness of the bare Au substrate
Figure 6. (a) Differential extinction spectra (measured in air) during the
construction of eight layers of 1 on a SAM of 2 on transparent, continuous
15-nm-thick Au film on quartz. The gray spectrum corresponds to the SAM
of 2. Inset: Absorption spectrum of 20 µM 1 in water (ꢀ ) 2.9 × 104 M-1
cm-1). (b) Differential extinctions at 208 nm (full circles) and at 633 nm
(open circles), showing increasing extinction at 208 nm (absorbance of 1)
and decreasing extinction at 633 nm (change in reflection). Full squares
correspond to a blank experiment at 633 nm, in which the regular adsorption
steps on a SAM of 2 were replaced by immersion in the pure solvents (no
1 or Zr(acac)4). (c) Film thickness values of coordination multilayers of 1
on a SAM of 2 on Au, obtained by (1) ellipsometry, (2) transmission
spectroscopy, (3) AFM profilometry ((1 nm), and (4) XPS (using λ ) 3.3
nm). Optical calculations were performed at λ ) 632.8 nm, using a film
refractive index nf ) 1.50, kf ) 0. The first point (square) is the ellipsometric
thickness of a SAM of 2.
was initially adjusted by fitting the experimental Au spectrum
to a calculated glass/Au model, showing a good fit of the
spectrum (see the Supporting Information).68 A 3-layer model
consisting of glass (n, k ) 1.52, 0), Au69 (n, k ) 0.12, 3.29),
and the film (n, k ) 1.50, 0) was then applied in order to
calculate the multilayer film thickness.70
As seen in Figure 6c, the film thickness values obtained by
the four different methods are practically identical, showing
highly regular film growth. Results from AFM and XPS are
(63) Heavens, O. S. Optical Properties of Thin Solid Films; Dover Publica-
tions: New York, 1991.
(64) Lee, H.; Kepley, L. J.; Hong, H. G.; Akhter, S.; Mallouk, T. E. J. Phys.
Chem. 1988, 92, 2597-2601.
(68) The experimental transmission spectrum of the bare Au was found to
correspond to 21 nm Au, deviating from the nominal evaporation thickness
(15 nm), as previously discussed in ref 49.
(65) Wanunu, M.; Popovitz-Biro, R.; Cohen, H.; Vaskevich, A.; Rubinstein, I.
J. Am. Chem. Soc. 2005, 127, 9207-9215.
(66) The overlayer thickness was calculated from the ratio of the total overlayer
atomic signals to the Au substrate signal, using standard electron attenuation
considerations.
(67) Bakiamoh, S. B.; Blanchard, G. J. Langmuir 1999, 15, 6379-6385.
(69) Handbook of Optical Materials; Weber, M. J., Ed.; CRC Press: Boca Raton,
FL, 2003.
(70) Differential transmission data were simulated by modeling the layers using
Tfcalc software.
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Branched Coordination Multilayers on Gold
A R T I C L E S
Table 2. XPS Atomic Concentration Percent at Normal Takeoff
Angles for Transmetalation Experiments
identical within experimental error, giving an average multilayer
thickness increment of 1.3 nm ( 5% per layer. The results
obtained by the two optical methods coincide within the
experimental errors of the XPS and AFM data, confirming the
appropriateness of the nf value chosen for the multilayer film.71
The sample-to-sample variability in the slope of thickness versus
number of layers is negligible, indicating excellent reproduc-
ibility. The observed 1.3 nm thickness increment per layer
coincides with the expected height of a perpendicularly oriented
molecular layer of 1 (see Figure 2b).
adsorption
sequence
C
O
N
Zr
Hf
S
Au
Experiment A
34.6 16.3 2.6 1.2
35.0 15.5 2.4
2/Zr4+
2/Hf4+
2.0 43.0
1.1 2.0 43.6
2/Zr4+/Hf4+
37.0 16.6 2.4 0.3 1.3 2.5 40.3
Experiment B
33.3 17.3 3.5 1.8
33.2 19.6 3.8 1.6 0.4 2.3 39.1
2/Zr4+/3/Zr4+
2.3 41.8
2/Zr4+/3/Zr4+/Hf4+
Experiment C
The increase of the extinction at 208 nm accompanying
multilayer construction was exploited for estimating the surface
density (γ) of 1 in each layer, using the relationship: γ ) 6.02
× 106 Ar/ꢀ, where γ is in molecules/nm2, r is the roughness
factor of the substrate,72 ꢀ is the molar extinction coefficient of
1 (we used the value of ꢀ measured in water), and Α is the
differential extinction per layer. This calculation gave a surface
density of 1.3 molecules/nm2, or 0.79 nm2 per molecule of 1 in
each layer, taking into account the roughness factor of the Au
substrate (r ) 1.39).49 Since the real part of the refractive index
of 1 in the UV region (corresponding to changes in film
reflectivity) is not known, a true quantitative determination of
the density of 1 in each layer is not possible, and the surface
density calculated above is a lower limit. A more accurate
determination of the multilayer absorbance requires simultaneous
measurement of the transmission and reflection. Nevertheless,
considering the measured thickness increment of 1.3 nm per
layer (Figure 6c) that matches the perpendicular height of 1,
the absorbance results are consistent with formation of compact,
vertically oriented layers of 1.
Figure 1b depicts binding of molecule 1 to the underlying
layer through one bis-hydroxamate arm, leaving the other
two arms available for further coordination. However, attach-
ment via two bis-hydroxamate arms bound to the underlying
layer leaving one free arm is also possible. The latter mode
imparts lateral cross-linking of the underlying layer, while the
former mode (as in Figure 1b) provides the possibility of lateral
cross-linking in the new layer, as well as lateral expansion over
voids. Despite the different possible binding modes, branching
and cross-linking, the end result is a highly regular multilayer
growth with a constant, molecule-length thickness increment
per layer.
2/(Zr4+/3)2/Zr4+
33.8 21.3 4.6 2.2
1.4 36.7
2/(Zr4+/3)2/Zr4+/Hf4+ 34.1 21.8 4.7 2.1 0.3 1.3 35.7
Experiment D
36.2 15.5 4.1 1.6
37.5 17.8 3.4 1.3 0.8 2.1 37.0
2/Zr4+/1/Zr4+
2.0 40.5
2/Zr4+/1/Zr4+/Hf4+
Experiment E
2/(Zr4+/1)2/Zr4+
43.1 24.0 5.9 2.7
1.3 22.9
2/(Zr4+/1)2/Zr4+/Hf4+ 43.2 25.8 5.8 2.6 0.5 1.4 20.6
enabling the use of Hf4+ for transmetalation in the film, i.e.,
for probing the stability of metal ions in the film with respect
to exchange with a different metal ion.
Table 2 shows XPS atomic concentrations for transmetalation
experiments in the early stages of construction of multilayers
of the linear molecule 3 and branched molecule 1. Experiment
A corresponds to SAMs of the bis-hydroxamate anchor 2. The
compositions of SAMs of 2 coordinated with either Zr4+ or Hf4+
are consistent with the formation of a 1:1 ligand/ion complex
(theoretical N/M4+ ratio, 2:1; actual, 2.2:1), as previously
demonstrated for a similar bis-hydroxamate monolayer.29 Treat-
ment of a Zr4+-coordinated SAM of 2 with Hf4+ shows that ca.
26% of the Zr4+ ions remain in the layer, i.e., most of the
binding sites in the SAM were exchanged with Hf4+. Note that
the total amount of metal ions in the SAM increased after the
exchange beyond the number of available binding sites, i.e.,
N/(Hf + Zr) < 2, suggesting some nonspecific accumulation
of Hf4+
.
Experiment B corresponds to transmetalation experiments
carried out on one Zr4+-terminated layer of the linear molecule
3. After treatment with Hf4+, an 18% decrease in the Zr/N ratio
was observed. If the layer of 3 is not cross-linked, the Zr4+
ions at the top of the layer would represent ∼50% of the total
Zr4+ ions in the film, hence an 18% decrease in the total Zr/N
ratio would represents a 36% transmetalation of the topmost
layer with Hf4+, much lower than the 74% exchange observed
for a SAM of 2. This result suggests a certain degree of cross-
linking in the layer of 3, as explained below. Experiment C,
which corresponds to transmetalation on two layers of 3
terminated with Zr4+, shows only a 7% decrease in the Zr/N
ratio following treatment with Hf4+. This result, indicating little
transmetalation, suggests more extensive cross-linking in the
second layer of 3. Considering the structure of molecule 3, which
consists of four hydroxamate-terminated arms, cross-linking may
involve tridentate binding of Zr4+ ions using an additional
hydroxamate arm from an adjacent molecule in the layer,
thereby stabilizing the ion binding toward exchange. Cross-
linking involving tetradentate binding is energetically unfavor-
able in this case.
Evaluation of Cross-Linking in the Layers. The suggested
model of multilayer growth using branching molecules implies
that the metal ions participate in attachment of the next layer,
while also enabling lateral cross-linking between neighboring
molecules. This proposed scheme suggests a unique structure,
analogous to that of cross-linked, organometallic polymer
films.73 Various properties of the system, such as robustness,
insulation, or defect management, are directly related to the
extent of cross-linking in the layers, analogous to covalent cross-
linking in organic polymers.
To substantiate the existence of lateral cross-linking in the
multilayers, transmetalation experiments were carried out.
Coordination binding of Hf4+ is similar to that of Zr4+ 28
,
(71) Thickness variations in the ellipsometric thickness on the order of (0.3
nm were observed in the first layer of 1, which may be attributed to
variability in the density and/or orientation of 1 in the first layer.
(72) The roughness factor is defined as the ratio of real to geometric surface
area.
(73) Archer, R. D. Inorganic and Organometallic Polymers; Wiley-VCH: New
York, 2001.
Experiments D and E correspond to similar transmetalation
experiments carried out on films comprising one or two Zr4+
-
9
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A R T I C L E S
Wanunu et al.
Table 3. Normal and Lateral Breakpoint Forces for Coordination
Multilayer Films Composed of Molecule 1 or 3 (Number of Layers
Does Not Include the SAM of the Anchor 2)
breakpoint force
1
3
no. of
layers
normal
(nN)
lateral
(A. U.)
normal
(nN)
lateral
(A. U.)
2
5
7
10
23 ( 1
113 ( 2
169 ( 4
223 ( 5
0.16 ( 0.01
0.35 ( 0.02
0.48 ( 0.04
0.88 ( 0.07
40 ( 1
0.24 ( 0.02
0.39 ( 0.05
135 ( 3
Figure 7. AFM lateral (FL) vs normal (FN) force curves for different
samples obtained in the wearless regime (before the breakpoint) for Zr4+
-
coordinated multilayer films assembled on SAMs of 2 on Au. Similar
measurements on an octadecylmercaptan (OM) SAM on Au are included
as a reference. The legend gives the number of layers of 1 or 3 assembled
on the SAM of 2. Inset: Schematic presentation of the forces recorded in
the measurements.
Figure 8. Conductive AFM I-V curves recorded under ambient conditions
for a 3-layer (a) and 5-layer (b) multilayer of Zr4+/1 and a 5-layer multilayer
of Zr4+/3 (c) on a SAM of 2 on Au.
terminated layers of the branched molecule 1. The results show
a ca. 2% decrease in the Zr/N ratio for one and two layers of 1.
These results suggest extensive cross-linking even for one layer
of 1. In the case of molecule 1, consisting of three bishydrox-
amate groups, cross-linking in the layer involves formation of
a stable tetradentate Zr4+ complex between two adjacent
molecules.
cross-linking is tetradentate for layers of 1 and largely tridentate
for layers of 3; and the extent of cross-linking is higher in
multilayers of 1 due to the excess hydroxamate groups provided
by the branching. This difference in structure and stability is
expected to affect the mechanical properties of the systems in
terms of film stiffness. Hence, the mechanical properties of
branched multilayer films composed of molecule 1 were
investigated using AFM and compared with analogous multi-
layers composed of linear molecule 3, the latter previously
studied by our group.27,29 The lateral force (friction) was
monitored as a function of the applied normal force while
scanning the film with a SiO2-terminated tip (Figure 7, inset).
Figure 7 shows friction (FL) versus normal force (FN) for the
different multilayer films, recorded using forces below the film
breakpoint. All the curves show a linear dependence of the
friction on the normal force, with no specific distinction between
multilayers of 1 and 3.
The transmetalation results may be rationalized by considering
the nature of the metal-organic bond. Binding of Zr4+ to the
SAM of 2 results in bidentate ligation to the metal ion through
one bis-hydroxamate group (plus two weakly bound acac
molecules), evidently enabling facile exchange with Hf4+ ions.
Binding of either 1 or 3 to the SAM of 2 occurs via tetradentate
Zr4+ ligation to two bis-hydroxamate groups, forming a more
stable complex, not amenable kinetically to transmetalation. The
fact that in experiments B-E (Table 2) most of the Zr4+ ions
were not exchanged implies, therefore, that hydroxamate groups
not used for binding to the underlying layer are largely cross-
linked, forming more stable multidentate bonds to Zr4+, most
likely tetradentate and tridentate in the case of molecules 1 and
3, respectively. An additional test experiment (not shown) in
which the opposite ion replacement was investigated, i.e.,
binding of Hf4+ onto two Zr4+-bound layers of 1, followed by
exposure to Zr4+, resulted in similar, inefficient exchange. The
transmetalation experiments therefore indicate that the formed
layers are largely cross-linked. Assembly of the next layer of 1
or 3 requires opening of a fraction of the cross-links, i.e.,
breaking of multidentate coordination bonds. This applies pri-
marily to layers of 3 and less so to layers of 1, as the branching
in the latter furnishes an excess of bishydroxamate groups in
the layers.
Whereas linear friction behavior on a macroscopic scale has
been ascribed to increasing contact area with force due to
plastic74 or elastic75 deformations, continuum mechanical models
predict a nonlinear behavior for a single asperity under elastic
2
contact. For example, a /3 power dependence was observed
for epitaxial C60 monolayers, which conforms to Herzian
elasticity formulas for a spherical tip.76 On the other hand, linear
dependence was also observed on this scale.52,77 Friction is a
complex physicochemical phenomenon, with various casual
factors including surface morphology, viscoelastic properties,
(74) Bowden, F. P.; Moore, A. J. W.; Tabor, D. J. Appl. Phys. 1943, 14, 80-
91.
(75) Greenwood, J. A.; Williamson, J. Proc. R. Soc. London 1966, 295, 300-
Mechanical Properties. As noted above, the structure of
multilayers comprising the branched molecule 1 and the linear
molecule 3 appears to be different in two respects: the lateral
319.
(76) Schwarz, U. D.; Allers, W.; Gensterblum, G.; Wiesendanger, R. Phys. ReV.
B 1995, 52, 14976-14984.
(77) Liu, Y. H.; Wu, T.; Evans, D. F. Langmuir 1994, 10, 2241-2245.
9
17884 J. AM. CHEM. SOC. VOL. 127, NO. 50, 2005

Branched Coordination Multilayers on Gold
A R T I C L E S
Figure 9. Binding of TOAB-capped NPs to a mixed SAM of 2 and DDT on Au, prior to (I) and following binding of two Zr4+-coordinated layers of either
the linear ligand 3 (II) or the branching ligand 1 (III). Corresponding AFM images of I-III (1 × 1 µm2) are shown with line profiles.
surface free energy, lubrication by films, and chemical reactiv-
ity.78 While similarity in the friction behavior (i.e., the FL/FN
slope in Figure 7) between multilayers of 1 and 3 (Figure 7)
could therefore be fortuitous, it is strongly indicative of similar
surface properties, such as surface energy79 and rigidity,80 of
the two hydroxamate-terminated films. Since no significant
frictional variations are observed in different surface regions
during scanning (not shown), the recorded friction is assumed
to reflect the homogeneous mechanical properties of the
multilayer films.81 For comparison, a force/friction curve for a
monolayer of OM is also shown in Figure 7. The friction values
of OM are approximately an order-of-magnitude lower. The
apparently larger shear strength of the hydroxamate-terminated
films could arise both from the higher surface energy and from
the different structure-related mechanical behavior relative to
the highly ordered, methyl-terminated OM surface. Adhesion
measurements carried out with the AFM (not shown) indicated
that only minor differences in surface energy exist between
multilayers of 1 and 3.
differences between linear (3) and branched (1) multilayers: for
the 5- and 10-layer multilayers, FN at breakpoint is larger for
the branched molecule by 180% and 65%, respectively. This
seems to be a direct consequence of the lateral cross-linking,
more prevalent and more stable in the branched multilayers than
in the linear ones, as discussed above.
Electrical Properties. The electrical properties of branched
coordination layers of Zr4+/1 were evaluated using conductive-
tip AFM measurements. Figure 8 shows AFM I-V curves for
three and five coordinated layers of 1 on a SAM of 2 on Au.
The curves show a typical resistance to electron transfer,
exhibiting a symmetric breakdown voltage of ca. (2 V for three
layers of 1 and ca. (6 V for five layers of 1. The results were
highly reproducible on the same spot (when the current was
limited to <10 pA) and at different locations on the sample.
The measured resistance for three layers of 1 is ca. 1012
Ω
(derived from the slope of the I-V curve in the low bias range,
before breakdown), similar to that reported by us for three layers
of 1 coordinated to a top layer of Au NPs.65 This suggests that
the resistance of direct tip-to-films contact is not significantly
The breakpoint forces for the different multilayer films are
given in Table 3. The robustness of the films increases with
the number of layers in all cases. Here the data show distinct
(80) Overney, R. M.; Meyer, E.; Frommer, J.; Brodbeck, D.; Luthi, R.; Howald,
L.; Guntherodt, H. J.; Fujihira, M.; Takano, H.; Gotoh, Y. Nature 1992,
359, 133-135.
(81) Bhushan, B.; Kulkarni, A. V.; Koinkar, V. N.; Boehm, M.; Odoni, L.;
Martelet, C.; Belin, M. Langmuir 1995, 11, 3189-3198.
(78) Bhushan, B. Introduction To Tribology; Wiley: New York, 2002.
(79) Also observed by wettability measurements, not shown here.
9
J. AM. CHEM. SOC. VOL. 127, NO. 50, 2005 17885

A R T I C L E S
Wanunu et al.
different from that achieved via a layer of NPs, in contrast to
results reported for alkanethiol SAMs.82 For comparison, an I-V
curve for a 5-layer multilayer of Zr4+/3 is shown (Figure 8c).
The I-V curve is qualitatively similar to those of the multilayers
of 1, with a breakdown voltage of ca. (3 V.
The electrical data show that the multilayers function as
efficient dielectric films, with a breakdown voltage that can be
tuned by the number of layers. The electric field at breakdown,
calculated using the measured multilayer thicknesses (Figure
6), is 5 MV/cm to 8 MV/cm (for three and five layers of 1,
respectively) and 4.5 MV/cm (for five layers of 3, using a
thickness of ca. 6.5 nm83). These values are of the same order
as breakdown values of ultrathin silicon oxide films,84 as well
as values reported for electrical breakdown of several organic
monolayer systems.85,86 Although the results suggest a higher
breakdown field for multilayers of 1 compared to 3, a definite
conclusion requires additional experiments.
Defect Self-Repair. Branching and efficient lateral cross-
linking promoted by the excess hydroxamate groups, as shown
to proceed in multilayer construction using ligand 1, point to
the enticing possibility of lateral growth over defects and voids,
which would act as a unique self-repair mechanism during
multilayer formation. To investigate lateral growth over defects
in LbL assembly of coordination multilayers of 1, the experiment
presented schematically in Figure 9 was carried out. A mixed
SAM of the anchor 2 and 1,10-decanedithiol (DDT) was
prepared on semitransparent Au substrates (see the Experimental
Section). The DDT domains in the mixed SAM serve to simulate
defects in the monolayer of 2, providing sites which are inert
to coordination and further growth of the multilayer. Exposed
DDT domains can be detected by interaction with TOAB-
stabilized Au NPs (5 nm average core diameter), which bind to
the terminal thiol groups but not to molecules 2.87 Two blank
experiments were carried out: (i) Immersion of a SAM of 2 in
a solution of the Au NPs resulted in no binding of NPs to the
hydroxamate-terminated surface, showing that the NPs bind
selectively to DDT molecules in the mixed SAM. (ii) Immersion
of a mixed SAM of 2 and DDT in a Zr4+ solution prior to NP
binding had no effect on the amount of bound NPs, showing
no influence of bis-hydroxamate-Zr4+ coordination on the
thiol-NP reaction. Differential spectra for the blank experiments
are shown in the Supporting Information.
Figure 10. (a) Differential extinction spectra of samples I-III (Figure 9).
(b) Comparison of the NP density (from AFM) and the uncorrected
differential extinction at λ ) 510 nm, for samples II and III.
show a striking difference between II and III in the density of
bound NPs: the surface treated with the linear molecule 3
(image II) shows considerably higher density of NPs than the
surface treated with the branching molecules 1 (image III),
demonstrating the superior defect repair process taking place
in the branched layers. The line profiles through images I-III
(shown under the images) emphasize the different NP densities.
The height of the features, as seen in the line sections, is 5-7
nm, matching the size of the NPs.65 Some large bright spots
(ca. 10 nm high) are visible in images II and III, which are
attributed to aggregated NPs.
AFM image I (Figure 9) shows a nearly full layer of Au NPs
bound to the initial mixed SAM, indicating that the DDT
“defects” are homogeneously distributed over the scanned
surface. When two layers of either the linear (3) or branched
(1) ligand were coordinatively assembled onto the mixed SAM,
the amount of bound NPs was drastically reduced, as seen in
images II and III, respectively, indicating partial blocking of
DDT defect sites by the two coordinated layers. The images
Figure 10a presents differential extinction spectra of samples
I-III (Figure 9), showing an extinction band characteristic of
the localized surface plasmon absorption of Au NPs. The band
intensity decreases from I to III, corresponding to the NP density
in the AFM images. Spectrum I is red-shifted from 510 nm
(spectra II, III) to 540 nm, attributed to the close-packing of
the NPs seen in AFM image I. A summary of the differences
between samples II and III is given in Figure 10b, showing NP
density (obtained by counting NPs in the AFM images) and
differential extinction (from the absorbance maxima). The two
parameters are 5.5 and 3.4 times larger, respectively, in sample
II.88 It is therefore evident from both the microscopic and
macroscopic measurements that branched molecule 1 blocks the
DDT defect sites much more efficiently than linear molecule
3, confirming the effective defect self-repair mechanism in the
branched layer assembly.
(82) Cui, X. D.; Primak, A.; Zarate, X.; Tomfohr, J.; Sankey, O. F.; Moore, A.
L.; Moore, T. A.; Gust, D.; Harris, G.; Lindsay, S. M. Science 2001, 294,
571-574.
(83) Wanunu, M.; Vaskevich, A.; Cohen, H.; Shanzer, A.; Rubinstein, I., 2005,
manuscript in preparation.
(84) Chen, I. C.; Holland, S.; Young, K. K.; Chang, C.; Hu, C. Appl. Phys.
Lett. 1986, 49, 669-671.
(85) Wold, D. J.; Frisbie, D. J. Am. Chem. Soc. 2001, 123, 5549-5556.
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(87) The C10 dithiol was chosen so that its extended length is slightly larger
than the length of 2, to ensure that binding of Au NPs is not sterically
constrained in the mixed SAM.
(88) The AFM results are more reliable since determination of the true
absorbance by the NPs requires correction for reflection losses.
9
17886 J. AM. CHEM. SOC. VOL. 127, NO. 50, 2005

Branched Coordination Multilayers on Gold
A R T I C L E S
Conclusions
demonstrated via efficient masking of intentional thiol defects
introduced into an anchor SAM, showing that two branched
layers blocked Au NP binding to the defect surface thiols much
more efficiently than two analogous linear layers.
Highly controlled LbL construction of ultrathin films coupled
with cross-linking and effective lateral growth over defects is
an intriguing concept which may be important for various
purposes, such as electronic applications requiring nanometer-
scale pinhole-free dielectric layers, as well as formation of
continuous, homogeneous layers on nonplanar and corrugated
surfaces. The special qualities of branched coordination mul-
tilayers demonstrated here may open the way to the development
of improved and more applicable multilayer systems.
A viable approach to the construction of coordination-based
multilayers displaying unique properties has been demonstrated.
The C3-symmetrical ligand 1, possessing three bis-hydroxamate
ligand arms, was synthesized and utilized in the LbL construc-
tion of branched coordination multilayers on semitransparent
Au. Functionalization of the Au surface with a SAM of bis-
hydroxamate anchor molecules, followed by alternate binding
of layers of 1 and Zr4+ ions, produced coordination multilayer
films. The branched multilayers were characterized by a variety
of techniques and their structural, mechanical, and electrical
properties studied. Multilayer growth proceeds in a highly
regular fashion, showing characteristic oscillating wettability
and a constant thickness increment per added layer, correspond-
ing to the size of one perpendicularly oriented molecule. Ex-
tensive intermolecular cross-linking was found in the layers,
indicated by inefficient transmetalation of the coordinated layers
with an equivalent metal ion. The excess hydroxamate groups
introduced by the branching, effecting more pronounced cross-
linking, is suggested to be responsible for the improved
mechanical properties over analogous, linear multilayers. The
electrical properties of the multilayers are typical of highly
efficient, low-leakage dielectric insulators, exhibited by break-
down fields of 5-8 MV/cm.
Acknowledgment. Support from the Israel Science Founda-
tion, Grant No. 307/03, and the Israel Ministry of Science,
Tashtiot Infrastructure Project No. 01-01-01470, is gratefully
acknowledged. M.W. is recipient of a Levy Eshkol Doctoral
Fellowship, Israel Ministry of Science. We thank Dr. Rachel
Persky for the FAB/HRMS measurements.
Supporting Information Available: Synthetic procedures for
the preparation of molecule 1 (including FAB/HRMS and
1H NMR); FTIR, ES/MS spectra of 1; experimental and calcu-
lated extinction spectra of the transparent Au film on aminosi-
lanized glass; differential extinction spectra for blank experi-
ments. This material is available free of charge via the Internet
at http://pubs.acs.org.
The concept of branching in multilayers introduces a space-
filling element which affords lateral expansion, bridging between
spaced molecules, and continuous repair of voids and other
defects during multilayer formation. The latter property was
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