Structured Layer of Rhenium Dye on SiO2 and TiO2 Surfaces by

May 5, 2014 - †Department of Materials Science and Engineering, and ‡Department of Chemistry, University of Wisconsin, Madison, Wisconsin 53706, ...
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Structured Layer of Rhenium Dye on SiO2 and TiO2 Surfaces by Langmuir−Blodgett Technique Yongho Joo,† Josef W. Spalenka,† Kyle M. McElhinny,† Samantha K. Schmitt,† Paul G. Evans,† and Padma Gopalan*,†,‡ †

Department of Materials Science and Engineering, and ‡Department of Chemistry, University of Wisconsin, Madison, Wisconsin 53706, United States S Supporting Information *

ABSTRACT: We demonstrate the Langmuir−Blodgett assembly of two rhenium−bipyridine complexes containing a flexible or an aromatic bridge, and transfer of the monolayer to SiO2 and single crystal TiO2 substrates. Both of the complexes (ReEC and Re2TC) have a hydrophilic carboxylic acid group, which preferentially anchors into the water subphase, and forms stable monolayers at surface pressures up to 40 mN/m. The optimum conditions for the formation of complete monolayers of both ReEC and Re2TC were identified through characterization of the morphology by atomic force microscopy (AFM), the thickness by ellipsometry, and the surface coverage by X-ray photoelectron spectroscopy (XPS). X-ray reflectivity measurements (XRR) are consistent with the orientation of the molecules normal to the substrate, and their extension to close to their calculated maximum length. Parameters derived from XRR analysis show that there is a higher packing density for Re2TC monolayers than for ReEC monolayers, attributable to the more rigid bridge in the Re2TC molecule.

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INTRODUCTION Bottom-up self-assembly processes for the uniform deposition of monolayers or controlled multilayers have been investigated to develop a basic understanding of the relationship between the molecular structure and ordering at interfaces. At the same time, these studies have led to advances in optical devices,1 photonics,2 synthesis of ultrathin polymer films,3 and electronic devices4 where structure and ordering at interfaces play a key role in device properties. There are many different strategies to fabricating ultrathin films, such as spin-coating,5 dip-coating,6 layer-by-layer deposition,7 the Langmuir−Blodgett (LB) technique,8,9 and self-assembled monolayer techniques.10 Of these methods, the LB method offers assembly of the molecules at an interface using compressive forces to drive close packing. LB is traditionally used to prepare ultrathin films of amphiphilic molecules at the air/water interface, which are then transferred on to a variety of substrates such as TiO2, glass, © 2014 American Chemical Society

mica, or SiO2. LB allows the packing, orientation, and thickness of the films to be controlled. The degree of molecular orientation, uniformity, and thickness are all in turn related to the performance, reliability, and lower costs in the resulting devices/applications. From the development of the LBtechnique in 1930s, and its initial focus on fatty acids, the scope and applicability of LB have been tremendously expanded to a range of functional films for electro-optics/ nonlinear optics,11 magnetic and electrically conductive layers,12 organic field effect transistors,13 as well as for biochemical studies of bacteriorhodopsin.14 Some of the molecules being assembled by LB do not strictly meet the amphiphilicity15 requirement, nor are they organic molecules;16 Received: February 14, 2014 Revised: May 1, 2014 Published: May 5, 2014 6104

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Figure 1. (a) Structure of the dye complexes linked on the solid substrate (TiO2 and SiO2), and (b) the molecular structure of three rhenium− bipyridine complexes.

hence, there is extensive flexibility in the applicability of the LB method. There are many applications where interfacial charge transfer processes are important. These include photocatalysis,17 photoelectrochemistry,18 dye-sensitized solar cells (DSSC),19 molecular electronics,20 and light emitting diodes.21 In devices such as dye-sensitized solar cells,19 photoexcitation of the dye sensitizers adsorbed on the surface injects electrons into the conduction band of inorganic semiconductors, and the device performance, to a large extent, is governed by the interfacial electron transfer process. While development of new dyes or organic materials has played a key role in advances in these fields, there is a growing realization that for every molecule there is probably an optimized geometry/arrangement at these interfaces that might lead to optimal charge transfer characteristics. One of the commonly used dyes in DSSC is a metal− bipyridine complex,22 which is used on inorganic semiconductors such as TiO2,23 ZnO,24 and SnO225 for complex sensitization. Upon photoexcitation, these metal−bipyridine complexes are excited to the singlet metal-to-ligand charge transfer state, and the excited electrons are injected into inorganic semiconductors. The rate of electron injection at the interface strongly depends on the electronic coupling between the surface-bound sensitizer and inorganic semiconductor,26,27 which in turn is affected by chemical composition of the anchoring group,28 and the nature of the molecular bridge that links the sensitizer to the surface.29 Studies based on 2D IR spectroscopy30 show that a model donor molecule can have multiple structural subpopulations on nanocrystalline TiO2, leading to different electron transfer rates. The difference in dynamics can be explained by differences in coupling strengths for the conformers. While spectroscopic tools are still being developed to identify these arrangements, from these results it is clear that by controlling the orientation and packing/ aggregation of the molecules at the interface kinetics of charge transfer might be dictated. Here, we outline the use of LB to create ordered monolayers of two metal−bipyridine complexes on SiO2 and TiO2. We show through a combination of X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM), and X-ray reflectivity (XRR) characterization that the dyes are oriented perpendicular to the substrate. The dye is a rhenium− bipyridine complex where the molecular bridge to the

carboxylic acid anchoring group to the oxide is either an aliphatic chain (ReEC) or a bithiophene (Re2TC). Both of these molecules have the potential to form stable LB films at the air−water interfaces, and the change in the bridge characteristics from aliphatic to aromatic alters the driving forces for packing such as cohesive and repulsive forces between the various components of the dye, stiffness of the molecule, and hence the characteristics of the isotherm.

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EXPERIMENTAL SECTION

Materials. All chemicals were purchased from Sigma-Aldrich and used without further purification unless otherwise noted. Single crystal TiO2 and SiO2/Si substrates were bought from CrysTec Inc. (Germany) and Montco Silicon Technologies Inc. (U.S.), respectively. Substrate Preparation. The Si/SiO2 substrates were cleaned using piranha acid (7:3 H2SO4:H2O2; Caution: Reacts violently with organic compounds!), for 20 min, followed by rinsing with deionized (DI) water. The single crystal TiO2 (rutile, (110)) substrates were cleaned with acetone and rinsed with DI water. LB Trough. Surface pressure−area (π−A) isotherms of the monolayer were measured by a Langmuir−Blodgett trough (KSV NIMA Medium size KN 2002) at 23 °C with a Wilhelmy balance (Platinum plate). The ReEC and Re2TC solution (1 mg/mL CHCl3) was spread on water subphase (Milli Q water with resistively ca. 18.2 MΩ cm). Compression of the monolayer was started after 30 min to allow the CHCl3 to evaporate completely. The initial trough area was 273 cm2, and the compression rate was 10 mm/min. The monolayer was transferred via the Langmuir−Blodgett vertical dipping technique onto Si/SiO2 and single crystal TiO2 substrates. X-ray Photoelectron Spectroscopy (XPS). Structured layers of ReEC and Re2TC were characterized by XPS, with a PerkinElmer 5400 ESCA spectrometer Phi model 5400 using a Mg X-ray source (power 300 W, accelerating voltage 15 kV), at a takeoff angle 45° from the surface normal. The hemispherical energy analyzer was operated in the hybrid mode with a 1 mm × 3.5 mm selected area aperture. Survey spectra were collected at constant pass energy of 89.45 eV with a scan step size of 1.0 eV. High-resolution multiplex spectra were collected with pass energy of 35.75 eV and a step size of 0.05 eV. The resulting spectra were analyzed using AugerScan software. Integrated intensity and peak positions were analyzed by fitting multiplex spectra with Voigt functions after a baseline correction of the raw data. Atomic Force Microscopy (AFM). The surface morphology and the degree of alignment of the ReEC and Re2TC molecules were imaged using a Nanoscope III Multimode atomic force microscope (Digital Instruments). Tapping mode was utilized for the AFM measurement. A triangular cantilever with an integral pyramidal Si3N4 6105

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Figure 2. Isotherms of surface pressure and mean molecular area of ReEC (a) and Re2TC (b) spread from chloroform solution with concentration of 1.0 mg/mL (compression rate: 10 mm/min, T = 23 ± 1 °C). Points a and a′ are the transition points from liquid to solid phase. tip was used. The typical imaging force was of the order of 10−9 N. Approximate thickness and roughness of thin film were determined. Ellipsometry. The thickness of monolayers of ReEC and Re2TC dye molecules was measured using a Rudolph Research Auto EL ellipsometer using a refractive index of 1.99 for the organic layers.31 Three wavelengths, 632.8, 546, and 405 nm, with an angle of incidence of 70° were utilized for the measurement. Five spots were measured for each sample. Thickness was calculated using FilmEllipse software version 1.1 (Scientific Co. Inc.) from measurements of ellipsometric angles ψ and Δ made at all three wavelengths. X-ray Reflectivity (XRR). X-ray reflectivity measurements were performed with a Panalytical X’Pert MRD in a forward scattering geometry. The width of the incident beam was measured by the knife edge method to be 0.68 mm. Lead shielding was employed between the X-ray source and the sample to minimize contributions to the background intensity from air scattering. Soller slits were mounted before the detector to achieve a favorable combination of background intensity rejection and wide angular acceptance. Synthesis. Detailed synthesis and characterization of the Re1C, ReEC, and Re2TC molecules are reported in a previous paper of Paoprasert et al.32

carboxylic acid, anchored in the water subphase. The isotherm typically consists of distinct phases, which are identified by the presence of discontinuities. How pronounced these transition points are typically depends on the type and size of the molecules being examined, which in turn dictates the scale of attractive or repulsive forces that drive the assembly. From the isotherms, the monolayers for both molecules were quite stable until a high pressure of ∼40 mN/m. The mean-molecular area reaches 50 Å2 at the onset point, at which the molecular arrangement of ReEC transitions from the gas to liquid phase. At 10 mN/m SP (point (a) in Figure 2a), the mean molecular area was 35 Å2, which is close to the calculated cross-sectional area of the ReEC configuration in which the long axis of the molecule is standing up. On the basis of the bond angle and bond lengths described in a previous study by Dominey et al.,33 the calculated dimensions for the two molecules are shown in Figure 3. Therefore, we expect close to a monolayer of ReEC at 10 mN/m SP on the water subphase.34,35 For Re2TC (Figure 2b), the mean-molecular area at the transition point from the gas to the liquid phase was 55 Å2. On further compression as the SP increases to 20 mN/m (point (a′) in Figure 2b), mean

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RESULTS AND DISCUSSION LB Isotherm. Initial LB studies were conducted with the molecule Re1C (Figure 1a), which has direct connectivity between the Re center and the carboxylic acid anchoring group, as Re1C is one of the most commonly used dyes in DSSC. However, the LB of Re1C was unsuccessful as the surface pressure (SP) in the isotherm curve (Supporting Information Figure S1) changes continuously for over 1 h and did not stabilize. In fact, Re1C is solubile in water due to lack of a hydrophobic bridge making it unsuitable for the LB assembly. Hence, we focused the remaining studies on ReEC (Figure 2a) and Re2TC (Figure 2b). The concentrations of the solutions were kept in the dilute regime by using a 1 mg/mL solution to minimize aggregation in the starting solution.13 Concentrations between 0.5 and 2 mg/mL gave similar isotherms, confirming that this is sufficiently dilute to minimize aggregates in the starting solution (Supporting Information Figure S2). In fact, the UV−vis spectrum for both of the molecules in the 0.5−2 mg/mL range does not change as a function of concentration, and no significant shifts in the λmax were observed (Supporting Information Figure S3). It should be noted that these dyes do not fall into the “truly amphiphilic” class of molecules because the headgroup is polar with partial positive charge on the pyridine nitrogens and negative charges on the chloride ligand. However, the carboxylic acid group is relatively more hydrophilic; hence for both of these molecules, the headgroup composed of rhenium bipyridine complex was expected to orient away from the water and tail group consisting of

Figure 3. Fully extended dimensions of (a) ReEC and (b) Re2TC molecule (carbon is gray, chlorine is purple, rhenium is green, hydrogen is white, oxygen is red, nitrogen is blue, and sulfur is yellow). The maximum length and the top−down cross-sectional area were calculated on the basis of diffraction studies from ref 31. 6106

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Figure 4. Schematic illustration of the different phases in the Langmuir−Blodgett assembly process (left), and the corresponding AFM characterization (height and phase images) of the surface morphology for (a) ReEC and (b) Re2TC at four different surface pressures of (i) 5 mN/ m, (ii) 10 mN/m, (iii) 20 mN/m, and (iv) 40 mN/m along with the surface roughness measurements by AFM.

molecular area measured was 32.2 Å2, which is in good agreement with the calculated cross-sectional area in Figure 3. The points (a) and (a′) were defined as the points of intersection of the slopes of the liquid- and solid-phase regions of the isotherm. Hence, 20 mN/m SP seems to be the optimum condition to form a monolayer of Re2TC on the water subphase. Upon further compression beyond the points (a) and (a′), the SP increases sharply. Both ReEC and Re2TC molecules start to aggregate and form disordered multilayers as the mean molecular area goes below the cross-sectional area of the molecules. Morphology of the Films. The morphologies of the thin films assembled at SP of 5, 10, 20, and 40 mN/m were studied by AFM after transfer of the assembled molecules to a

hydrophilic SiO2 substrate. At 5 mN/m for ReEC (Figure 4ai) and 5−10 mN/m for Re2TC (Figure 4bi,ii), which corresponds to the liquid phase in LB, the bare substrate was visible due to incomplete coverage. The AFM images (Figure 4aii, biii) at the transition from the liquid to the solid phase (SP = 10 mN/m for ReEC and SP = 20 mN/m for Re2TC) show complete coverage. Note that the images showing complete coverage correspond to the formation of the monolayer at points a and a′ on the isotherms. AFM measurements of the roughness of the transferred films show the smoothest surface for films prepared at points a and a′ for ReEC and Re2TC, respectively, which is consistent with a complete monolayer coverage. While the SP values at which the transition to liquid phase occurs for Re2TC and ReEC were similar, the liquid phase for 6107

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Figure 5. Correlation between the thickness (−) of the LB-film after transfer to SiO2 substrate, measured by ellipsometry, its roughness (- - -) measured by AFM, and the surface pressure of assembly for (a) ReEC and (b) Re2TC. The “☆” represents thickness data after transfer to a single crystal TiO2 substrate.

Figure 6. XPS spectra of ReEC (a) and Re2TC (b) films on SiO2 substrate as a function of the surface pressure of the assembly.

Re2TC extends over a wider range of SP, and the onset of liquid to solid phase transition occurs at a much higher pressure of 20 mN/min as compared to 10 mN/min. This indicates that interaction between the organometallic head groups probably drives the transition to the liquid phase, and the difference in the nature of the bridge groups drives the solid phase transition. It is also apparent that the aliphatic bridges interact more strongly as the conformational flexibility leads to faster rearrangement of the chains. The rigid aromatic units in the Re2TC, on the other hand, required higher pressures to rearrange; however, given the large head groups even at these high pressures, the π−π interactions between the bithiophene units are unlikely to play a dominant role in the packing. The quality of the deposited monolayer on a solid support was determined by calculating the transfer ratio of the LB deposition, which is defined as the ratio between the decrease in monolayer area during a deposition stroke and the area of the substrate. Under the optimum deposition conditions, the transfer ratio was close to 0.97 for both ReEC and Re2TC. The close-to-unity transfer ratio indicates quantitative transfer and that the morphology of the films deposited onto the substrate was representative of the morphology at the air/water interface. Upon further compression beyond the points a and a′, the SP increases sharply for both ReEC and Re2TC as the molecules aggregate and form disordered multilayers. From the AFM analysis, a distinct difference in the type of aggregation at these

high pressures for the two molecules was observed. In both cases, the headgroup is larger in cross section than the tail; however, the driving forces for assembly are different. On the basis of the AFM images, we hypothesize that a multilayer micellar morphology is formed for ReEC (Figure 4a,iv),36 whereas beyond point a′, Re2TC forms rod-like aggregates (Figure 4b,iv).37 While formation of micellar aggregates is typically observed at lower surface pressures for surfactant molecules that have a short hydrophobic tail, the aggregates formed here are driven by the combination of a large headgroup and the short flexible aliphatic bridges, which can accommodate the large curvature as well as the H-bonding between the carboxylic acid groups. However, for Re2TC the rigidity of the aromatic bridge is unable to adapt to the curvature in the micelles, driving the rod-like aggregates instead. This is similar to what is commonly observed during the self-assembly of rigid rod oligomers38,39 or polymers in bulk as well as in LB films. Measurement of thickness of the film by ellipsometry and of the roughness by AFM can provide insight into monolayer or multilayer formation. As expected, the thickness of the films increases for both molecules with increasing SP. The thickness values of 1.7 nm for ReEC film (SP = 10 mN/m) and 2 nm for Re2TC film (SP = 20 mN/m) are in good agreement with the calculated extended length of the molecule (Figure 5a, b). The surface roughness is lowest at a SP of 10 mN/m for ReEC, and 6108

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Table 1. Atomic Coverage of ReEC on Si/SiO2 and Single Crystal TiO2 Substrate Calculated from Cl2p, N1s, and Re4f Peaks with Si2p or Ti2p Peaks as Internal Reference

a

surface pressure (mN/m)

Cl(2p) (atoms/cm2)

N(1s) (atoms/cm2)

Re(4f) (atoms/cm2)

averagea (molecules/nm2)

5 10 20 40 10 (on TiO2)

× × × × ×

× × × × ×

× × × × ×

1.34 1.84 2.51 4.00 2.17

1.15 1.46 2.11 3.91 2.05

14

1.80 2.58 3.39 4.80 2.67

10 1014 1014 1014 1014

14

10 1014 1014 1014 1014

1.08 1.35 2.02 3.26 1.85

14

10 1014 1014 1014 1014

Surface coverage of each complex is the average of the atomic coverage of elements present on the surface.

Table 2. Atomic Coverage of Re2TC on Si/SiO2 and Single Crystal TiO2 Substrate Calculated from Cl2p, N1s, S2p, and Re4f Peaks with Si2p or Ti2p Peaks as Internal Reference

a

surface pressure (mN/m)

Cl(2p) (atoms/cm2)

N(1s) (atoms/cm2)

Re(4f) (atoms/cm2)

S(2p) (atoms/cm2)

averagea (molecules/nm2)

5 10 20 40 20 (onTiO2)

× × × × ×

× × × × ×

× × × × ×

× × × × ×

1.27 1.68 2.27 3.89 2.26

1.02 1.41 2.05 3.88 2.26

14

10 1014 1014 1014 1014

1.50 2.37 2.87 4.60 2.57

14

10 1014 1014 1014 1014

1.09 1.15 1.91 3.21 2.05

14

10 1014 1014 1014 1014

1.48 1.79 2.28 3.22 3.11

14

10 1014 1014 1014 1014

Surface coverage of each complex is the average of the atomic coverage of elements present on the surface.

SCI(2p) = 0.891, SRe(4f) = 3.961, SN(1s) = 0.477, SSi(2p) = 0.399, and STi(2p) = 1.798.42,43 Surface coverage is defined as the number of molecules per unit area on the surface. Surface coverage was calculated by averaging the molecules/nm2 determined from the Cl(2p), Re(4f), and N(1s) peaks (Table 1). From the XPS data, the concentration of ReEC molecules in films transferred at 10 mN/m SP was 1.84 molecules/nm2. This value is close to a monolayer coverage, as the estimated cross-sectional area of the vertically oriented ReEC molecule on the substrate is ∼0.4 nm2 (Figure 3). The estimated dimensions give a theoretical maximum coverage of ∼2.5 molecules/nm2 for a monolayer. However, if we account for the repulsive forces between the molecules with partial ionic character, in close proximity, the value of surface coverage is expected to be much lower in the range of ∼2−2.5 molecules/nm2. For Re2TC, the surface coverage calculated from XPS was 2.27 molecules/nm2 (Table 2) at ∼20 mN/m SP. The higher surface coverage for Re2TC as compared to ReEC correlates with the observations in the isotherm curve where higher pressures were required due to the rigid nature of the thiophene bridge and the extended liquidphase regime. The LB assembly and transfer processes were initially studied using Si/SiO2 substrates to facilitate characterization of the transferred monolayers. However, a well-ordered dye monolayer on TiO2 would be required for interfacial charge transfer studies. Hence, we chose a smooth single crystalline TiO2 substrate as the LB transfer substrate. TiO2 has strong interactions with carboxylic acid groups,32 which facilitates binding of ReEC and Re2TC. Deposition was achieved by LB on a single crystal TiO2 substrate (RMS roughness = 0.12 nm). The optimal condition for SP and solution concentration that were developed for the SiO2 substrates were used to transfer molecular layers to the TiO2 substrates. Complete monolayer formation was obtained for ReEC at ∼10 mN/m SP and for Re2TC at ∼20 mN/m SP on the single crystal TiO2 substrates. Further, the surface coverage measured by XPS on TiO2 (Tables 1 and 2) was similar to those obtained on SiO2 substrates. Together the XPS, AFM, and LB isotherm indicate the formation of a monolayer for the two molecules under these optimized conditions.

at 20 mN/m for Re2TC, both of which correspond to the points (a and a′) of monolayer formation in the LB isotherm.40 The lower thickness, for example, at 5 mN/m SP for both molecules, indicates a disordered or tilted arrangement; hence, the roughness is also higher. At much higher pressures, both thickness and roughness increase due to formation of multilayered micelles or rod-like aggregates with a height of 5−7 nm for ReEC and 7−12 nm for Re2TC. XPS Studies. The surface coverage of the molecules at different surface pressure points on the isotherm was measured by XPS. The appearance of N(1s) peaks from 399 to 404 eV, Cl(2p) peaks from 198 to 204 eV, and Re(4f) peaks from 41 to 49 eV confirms the presence of ReEC on the surface (Figure 6a). For substrates with Re2TC molecules, an additional S(2p) peak from 163 to 168 eV was seen from the two thiophene rings in the molecule (Figure 6b). The concentration of molecules on the surface can be determined from the XPS spectra using the intensity ratio between the elements on the surface and the underlying substrate. The underlying substrate density is well-known and provides a reference for the measurement. The surface coverage of ReEC and Re2TC was determined from the experimental spectra using41,42 ⎛ A ⎞⎛ S ⎞ ΦM,MO2λM,MO2 sin(θ ) e(t / λX,organic sin θ) NX = ⎜ X ⎟⎜ M ⎟ ⎝ AM ⎠⎝ SX ⎠ e(t / λM,organic sin θ) (1)

Here, NX is number of atoms per unit area for element X, AX/ AM is the ratio of integrated peak areas for element X and the substrate peak, and SM/Sx is the ratio of the sensitivity factors of element X and the substrate. Two different types of substrates were used, SiO2 and single crystal TiO2 (M1 = Si and M2 = Ti). The term ΦM,MO2 is the number of M atoms per unit volume in MO2, and λM,MO2 is inelastic mean free path (IMFP) of M photoelectrons in MO2. The layer thickness is t, and λX,organic, λTi,organic, and λSi,organic are the inelastic mean free paths for the electrons emitted from X, Ti, or Si traveling through the organic layer. 41 The angle θ is the takeoff angle of photoelectrons with respect to the sample plane (θ = 45°), and the λSi,SiO2 value was calculated using ΦSi,SiO2 = 5 × 1022 atoms/cm3, ΦTi,TiO2 = 2.6 × 1022 atoms/cm3, Ss(2p) = 0.666, 6109

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Figure 7. (a) Schematic of the box model of Re2TC used for fitting the experimental data. A Re2TC molecule is shown schematically on a silicon substrate. To partition the molecule into boxes, the molecule is split at the bond between the Re complex and the bridge. (b) Example of the electron density as a function of position along the molecule, as approximated by the box model. The dashed black curve is a schematic of the electron density for the example case of perfect interfaces. The red curve is a schematic of the electron density for the example case of interfaces with a root-mean-square (rms) roughness of 6 Å similar to the experimental results. (c) Gradient of the electron density as a function of position along the molecule for the case of rms roughness at each interface of 6 Å.

Here, ρj is the electron density of the jth box, zj is the position of the interface between the jth and (j + 1)th boxes, and σj is the RMS roughness of the interface between the jth and (j + 1)th boxes.44 A schematic of this model for the electron density is shown in Figure 7b for the example cases of perfect interfaces (dashed black lines) and for interfaces with 6 Å roughness (red curve). The corresponding gradient in electron density, (dρ(z)/dz), for the example case of 6 Å roughness is shown in blue in Figure 7c. The density dependence of the reflectivity can be simplified to be:45

X-ray Reflectivity (XRR) Studies. X-ray reflectivity (XRR) studies were conducted to obtain insight into the structural ordering of molecules at the interface. XRR characterization was performed using monolayers deposited on silicon substrates. The ReEC and Re2TC films were deposited using surface pressures of 10 and 20 mN/m, respectively. With the assumption that the electron density of the film varies only in the direction perpendicular to the substrate z, the X-ray reflectivity R(Q) of a rough interface calculated from the electron density profile ρ(z) is44,45 1 R(Q ) = RF(Q ) ρ(z → −∞)

∫

dρ(z) iQz e dz dz

2

dρ(z) iQz 1 e dz ρ(z → −∞) dz 2 2 1 = ∑ (ρj + 1 − ρj ) × e−Q σj /2 × eiQzj ρSub j

∫

Here, RF(Q) is the Fresnel reflectivity of the substrate, Q is the X-ray wavevector, and ρ(z → −∞) is defined such that ρ(z → −∞) = ρSubstrate = ρSi.44,45 Although the reflectivity R(Q) can be calculated using an analytic form of ρ(z), the inversion of X-ray reflectivity data to obtain the density profile is more difficult.44 To extract insight into the physical and electronic structure of the monolayer film, a model for ρ(z) was assumed and model parameters adjusted until a fit to the data was achieved. The model for ρ(z) was developed by approximating the structure of the electron density of the molecular layer as a series of boxes of uniform density and by making the kinematic approximation, where refraction and multiple reflections are assumed to make small contributions to the scattered intensity.44,45 A schematic of the box structure for Re2TC is shown in Figure 7a. The interfaces between boxes are assumed to be broadened by error functions. With these assumptions, the expression for ρ(z) becomes ρ(z) = ρSub +

∑ j

For the box model, the molecule was partitioned into two boxes. The top box consists of the rhenium complex and the bottom box of the molecular bridge group. Each box is characterized by its thickness L and its electron density ρ. The box thickness L (Å) was estimated by splitting the molecule at the bond between the Recomplex and the bridge group and taking the corresponding fraction of the calculated maximum length of the molecule. The electron density ρ (e−/Å3) was estimated by counting the number of electrons in each box and dividing it by the volume of each box with the calculated lateral dimensions and thicknesses. The molecular model parameters for ReEC are LTop = 8.3 Å, LBottom = 8.8 Å, ρTop = 0.692 e−/Å3, ρBottom = 0.203 e−/Å3, and ρSubstrate = 0.699 e−/Å3. The model parameters for Re2TC are LTop = 8.3 Å, LBottom = 11.8 Å, ρTop = 0.692 e−/Å3, ρBottom = 0.310 e−/Å3, and ρSubstrate = 0.699 e−/Å3. The RMS roughness of each interface σj is determined during the fitting process.

⎛ z − z j ⎞⎤ ρj + 1 − ρj ⎡ ⎢1 + erf⎜ ⎥ ⎜ 2 σ ⎟⎟⎥ ⎢⎣ 2 ⎝ ⎠ j ⎦ 6110

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Figure 8. X-ray reflectivity data from ReEC (a) and Re2TC (b) samples. The data are fit assuming constant layer thickness and electron density values as derived from the box model. The data are fit by varying monolayer coverage and interface roughnesses.

respectively. For the ReEC film, σTOP = 6.66 Å, σBottom = 7.20 Å, and σSub = 6.60 Å. For the Re2TC film, σTOP = 6.43 Å, σBottom = 7.50 Å, and σSub = 5.88 Å. For reference, σSub = 6.38 Å for a bare silicon substrate. The X-ray reflectivity data confirm the presence of a wellordered monolayer on the surface. The ability to fit the data without adjusting the length and electron density parameters suggests that the molecules are extended close to their calculated maximum length and are oriented with the long axis of the molecule more or less normal to the substrate. The roughness values determined by the fitting process suggest that the roughness of the substrate is translated to the film. The higher percentage of signal arising from the monolayer for Re2TC agrees with the higher surface coverage values extracted from the XPS results and indicates a better packing for the Re2TC sample. The higher coverage observed using XPS and XRR for Re2TC supports the hypothesis that the higher rigidity of the Re2TC molecular bridge allows Re2TC to form well-ordered monolayers. The relative coverage of the two molecules is also in agreement with the static water contact angles of the transferred LB films and of the control sample of disordered dip-coated films. In both cases, the contact angles for LB films were higher than those for dip-coated layers. For ReEC the contact angle increased from 55° to 68°, and for Re2TC from 56° to 73°. Hence, the packing and the anchoring of carboxylic acid group to the substrate interface aid in the orientation of the molecule (Supporting Information Figure S4).

Additional geometric terms and offsets are needed to construct an accurate model of the experimental XRR intensity distributions. The model used for fitting the experimental data was: ISimulated = I0*(G(Q )R(Q ) + e−Q

2

2 /2σDB

) + IBackground

Here, I0 is the incident beam intensity, G(Q) accounts for changes in the beam footprint as the sample is rotated into the incident beam, R(Q) is the model for the reflectivity discussed in the previous paragraph, σDB is the width of the direct beam set by the detector, and IBackground is a uniform background intensity. The difference in monolayer coverage and density was accounted for in the model by using a weighted average of the expected reflected intensity from the bare silicon wafer and from the respective molecule: ISimulated = x*ISimulated,Molecule + (1 − x)*ISimulated,Substrate

Here, x is the percentage of the signal arising from the respective molecule and 0 ≤ x ≤ 1. The simulation was fit to the experimental data by taking the common logarithm of the experimental and simulated intensity and minimizing the least-squares fitting. The reflectivity data from ReEC and Re2TC samples deposited using surface pressures of 10 and 20 mN/m, respectively, are shown in Figure 8. The intensity is shown normalized to the incident beam intensity. The high intensity points at Q < 0.01 Å−1 arise because a portion of the direct beam is incident on the detector at small angles. The local minimum and increase in intensity from 0.01 Å−1 < Q < 0.03 Å−1 (Qc = 0.0314 Å−1) results from changes in the beam footprint with increasing incident angle. As the incident angle increases, a larger fraction of the beam footprint is occupied by the sample, resulting in an increase in the reflected intensity. The broad feature observed at 0.2 Å−1 for ReEC and Re2TC films arises from reflection from the film. Using the model parameters described above, the data were fit using seven parameters: incident beam intensity I0, the width of the direct beam set by the detector σDB, the percentage of the signal arising from the respective molecule x, the RMS roughness of each of the three interfaces σj, and the background intensity IBackground. The layer thicknesses and electron densities are not fit and assume the values determined from the box model. The best fit parameters for each data set are inset in the plot. Figure 8 shows reflectivity data from a ReEC film (a) and a Re2TC film (b). The fit values for the percentage of signal arising from the monolayer covered substrate are 0.74 and 0.96,

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CONCLUSION We have shown that it is feasible to form a structured monolayer of dye molecules on oxide surfaces using LB technique. The two rhenium−bipyridine complexes ReEC and Re2TC studied here are not truly amphiphilic, yet they form stable LB films up to high surface pressures of 40 mN/m. The hydrophilic carboxylic acid anchors the molecule into the water subphase with the organomettallic headgroup facing the air. The presence of a flexible aliphatic linker in ReEC aids in faster packing of the molecules, hence compressing the liquid phase in the isotherm, as compared to the more rigid aromatic link in Re2TC. Given that the cross-sectional dimensions of the headgroup (CO−Re−Cl distance is approximately 4.8 Å) are larger than the effective distance for π−π interactions, the interactions between the aromatic bridges are minimal in Re2TC even when close-packed. Hence, the observed differences between the two molecules are consistent with the 6111

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greater rigidity of the 2TC link in comparison with EC. AFM and XRR characterization shows that the molecules are extended close to their calculated maximum length and are oriented with the long axis of the molecule close to normal to the substrate. These atypical molecules can be assembled in ordered monolayers at the air/water interface and transferred to oxide surfaces, including TiO2 substrates with high transfer ratio, hence opening a path to study the effect of specific molecular orientation or aggregation states on the charge injection dynamics.

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ASSOCIATED CONTENT

S Supporting Information *

Isotherms for ReC, isotherms and UV−vis spectra for different concentrations of ReEC and Re2TC, and the contact angle measurements on the films. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the University of Wisconsin, Materials Research Science and Engineering (NSF grant no. DMR-1121288).

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