Surface-Induced Heterogeneity Analysis of an Alkanethiol Monolayer

Article pubs.acs.org/JPCC

Surface-Induced Heterogeneity Analysis of an Alkanethiol Monolayer on Microcrystalline Copper Surface Using Sum Frequency Generation Imaging Microscopy Ming Fang and Steven Baldelli* Department of Chemistry, University of Houston, Houston, Texas 77204-5003, United States S Supporting Information *

ABSTRACT: An octadecanethiol (ODT) self-assembled monolayer on microcrystalline copper is investigated by sum frequency generation (SFG) imaging microscopy and electron backscattering diffraction (EBSD). The strong SFG signal contrast across the domain boundary indicates the existence of grain structures on copper surface, which is further verified by EBSD measurements. The nonresonant contribution of SFG response shows anisotropy with respect to the in-plane rotation of the sample relative to the surface normal and varies in each crystal domain area. The resonant contribution of the monolayer, such as the amplitude ratio of CH3-sym/ CH3-asym, is azimuthally nearly isotropic. Since the zzz tensor component of nonlinear susceptibility dominates, the resonant part of SFG spectra on the metal surface does not show any anisotropy. Further, a strong correlation between the local metal structures with the top monolayer packing behaviors is identified based on the statistical distribution analysis. Using the methyl group as an illustrative case, the variations in tilt angle of methyl group for different crystal grains, visualized in the SFG image, suggest that the underneath local grain structure contributes significantly to the overall monolayer packing behaviors measured on the macroscale.

1. INTRODUCTION The studies of the thin organic film structure on metal surfaces are of particular interest because of its relevance in a wide variety of technological functionalities, such as wettability, corrosion inhibition, biosensors, and nanotechnology.1−8 Various techniques including IR adsorption spectroscopy,9,10 X-ray photoelectron spectroscopy,11,12 contact angle measurement,10,13 ultrafast spectroscopy,14,15 and scanning tunneling microscopy16,17 have been employed to investigate such systems. The most studied system is self-assembled monolayers (SAMs) on metal surfaces due to their simple chemical adsorption, well-ordered structure, and monolayer thickness.18 An important result from previous studies is that the surface functionalities and performance of SAMs is highly dependent on the microscopic ordering structure of the monolayer on a metal surface.19−22 A molecular level investigation of SAMs on heterogeneous metal surface to elucidate how the degree of microscopic structure of metal surface modifies monolayer packing behavior is of critical importance to determine the performance of SAMs that play a key role in many disciplines. Various sized crystal domains with different crystallographic orientations naturally exist on most metal surfaces.23,24 A major area of interest is how the featured surface atom arrangements control the adsorption geometry of SAMs. A common strategy to investigate this correlation is to compare the properties of SAMs on different well-defined single-crystal surface.25,26 Sum © XXXX American Chemical Society

frequency generation vibrational spectroscopy (SFG-VS), as a vibrational spectroscopy technique, which has an excellent chemical selectivity and interfacial sensitivity, has been extensively applied to probe such possible existence of metal surface induced ordering.21,27,28 However, evaluating different single crystal grains requires multiple measurements with careful preparation and characterization of each crystal surface under the same experimental conditions, which is time-consuming. In addition to these experimental difficulties, the crystal boundaries between different crystallographic grains,29 which play an important role in determining surface reactivity, are difficult to probe based on spectroscopy methods alone and not available in single crystal studies. Therefore, to investigate the molecular behavior of metal surfaces with various grains structures and to determine information about the grain boundaries, a more direct measurement method is imperative. SFG imaging has been recognized in recent years as a useful technique to provide a promising and practical solution to these problems. SFG imaging microscopy as a vibrational spectroscopy technique applied in microscope mode directly probes the local molecular information, in relation to the underneath metal structure of a Received: September 17, 2016 Revised: December 17, 2016

A

DOI: 10.1021/acs.jpcc.6b09403 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C

Figure 1. Schematic drawing of laboratory fixed axis system (A), molecular fixed axis systems (B), and molecular orientation angle (C).

polycrystalline metal.30−33 Recently, it has been shown that SFG imaging studies of copper grains structures are possible, which allows a direct correlation between local molecule information and the underlying copper grain structure.34 It is also proposed that the spectral fitting parameter obtained from the local SFG spectra can be used to reconstruct the corresponding chemical map of the monolayer and to determine the conformation order of the monolayer in each crystal domain with distinct crystallographic orientation. However, such a proposal requires further substantiation and addressing about whether phase variation in different azimuthal angles would influence the interpretation of the SFG spectra since the SFG spectra are strongly dependent on the azimuthal angle. Furthermore, the existence of the lateral order of the monolayer needs to be further addressed. In the present work, the azimuthal angle dependences studies of SFG imaging on a microcrystalline copper and Cu(111) surface is presented to examine whether the surface symmetry of crystalline copper surface would induce an orientation order as well as in-plane anisotropy in the ODT monolayer. Based on the SFG measurements, it shows a nearly isotropy distribution for the resonant part of SFG response, in contrast to an anisotropy distribution for the SFG nonresonant background. The tilt angle distribution of methyl group in the monolayer on each individual crystal grains is analyzed to further verify the monolayer heterogeneity. In addition, correlations between the molecular behaviors with the underneath copper grain structure (in particular, the crystallographic orientation) is identified.

nitrogen gas. The SFG cell is mounted on an x-y-z plus tip/tilt 5 axis stage for the SFG imaging experiment. SFG imaging setup was described previously.35,36 A picosecond pulsed Nd:YAG laser (EKSPLA) with a 20 Hz repetition rate provides the 1064 nm beam serving as the pump beam for SFG, as well as the pump beam of the tunable IR beam from 2000 to 4000 cm−1 that is generated from the optical parametric generator/amplifier (OPG/OPA). The SFG imaging is set in a reflection configuration with the incidence angles of the IR beam and 1064 pump beams at 70° and 60° from the surface normal, respectively. The emission angle of the SFG beam is calculated to be 62.1° from the surface normal. The polarization of the 1064 nm beam and the tunable IR beam are both p-polarized (parallel to the incidence plane) set by a polarizer, and the polarization of the SFG signal is considered to be p-polarized (p-polarized SFG signal dominates the SFG signal). This polarization combination is assigned as PPP. A Roper Scientific CCD camera with 1024 × 1024 pixel array is used for collecting the SFG image on a grating by using a 10× objective lens for magnification and a tube lens for collimation. The SFG image is more or less affected by the variations of the beam profile of IR and 1064 nm beams, which are considered to be uniform in the region of interest. Each SFG image is 1024 × 1024 pixels and 1 pixel corresponds to approximate 1 μm distance on the surface. The spatial resolution is approximately 2 μm. The image acquisition time for single wavelength image is around 2 min. The SFG beam size is around 2 mm. The SFG beam wavelength (IR wavenumber range 2750−3050 cm−1) is around 808 nm. The sample azimuth angle (Ω) which is respect to surface normal is also demonstrated in Figure 1A. The absolute number of the azimuth angle is arbitrary. 2.2. Sum Frequency Generation Spectroscopy. There are a number of reviews, which introduce the theoretical background of SFG and its applications.37−39 As shown in eq 1, SFG is a second-order nonlinear optical process which involves two laser beams spatially and temporally overlapped on the surface, to induce a second order polarization, P(2) SF . Here, I1064nm and IIR are the intensity of the visible and infrared laser pulses, 40 respectively, and χ(2) The eff is the second order susceptibility. second order nonlinear optical process is forbidden in a medium with inversion symmetry under the electric dipole approximation, but is active at the interface where the inversion symmetry is broken.41 Thus, SFG is a highly surface sensitive technique.

2. EXPERIMENTAL SECTION 2.1. Sample Preparation. The copper sample is prepared as described elsewhere.34 A 1 cm2 polycrystalline rectangular copper sheet (Goodfellow, 99.99%) of 3 mm thickness is employed as the sample in this work. One side of the copper surface is polished down to 0.1 μm by diamond paste with a mirror-like finish. The polycrystalline copper is transferred to a quartz tube furnace and annealed with argon (300 sccm) and hydrogen (30 sccm) at a temperature of 1050 °C for 3 h. For Cu (111), a 1 cm2 rectangular one sided polished Cu (111) sheet (MTI Corporation) of 1 mm thickness is employed. The copper samples including polycrystalline copper and Cu (111) are annealed with argon (300 sccm) and hydrogen (30 sccm) at a temperature of 600 °C for 1 h. Upon annealing, copper samples are cooled down to room temperature, under the argon and hydrogen flow atmosphere, and immediately immersed into a 5 mM ODT/ethanol solution. After 2 h, the sample is rinsed with ethanol, and dried under nitrogen flow in the glovebox to prevent the oxidation of copper. All the SFG measurements are performed in the SFG cell filled with

(2) 2 (2) 2 ISF ∝ |PSF | ∝ |χeff | I1064 nmIIR

(1)

The second-order susceptibility χ(2) eff shown in eq 2 contains (2) both a nonresonant contribution χNR and a resonant B


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Figure 2. Unprocessed SFG image (IR wavenumber: 3050 cm−1) of the copper surface with the ODT monolayer at the azimuth angle of 0° (I), 60° (II), 120° (III), 180°(IV), 240°(V), and 300°(VI), respectively. SFG images are taken with the PPP polarization. The red frame marks the GBs in 0° (I).

Figure 3. SFG image (left) and EBSD map (right) of copper surface with ODT self-assembled monolayer.

contribution χ(2) R , which relates the induced second-order polarization response with the intensity of the incident beams. χ(2) NR is the nonresonant contribution typically attributed to electronic excitations of the substrate and the adsorbate. χ(2) R is associated with resonant vibrations of the adsorbate molecular layer and is significantly increased when the frequency of an incident infrared beam (ωIR) is coincident with a specific molecular vibrational mode, q. The laboratory-fixed coordinates λ (x,y,z) is defined in Figure 1A. Z denotes the surface normal, and all the three beams propagate in the xz plane. The χ(2) eff , which is defined in λ (x,y,z), correlates the molecular microscopic hyperpolarizability β(2) in the molecular coordinate system λ′ (a,b,c) through the ensemble average over all possible molecular orientations.42 The molecular-fixed coordinates for the methyl group are defined as depicted in Figure 1B. The c− axis is taken to coincide with the C3 symmetry axis of the

methyl group, and the a−c plane is one of the C−C−H planes of the methyl group.21 The molecular orientation including tilt angle (θ), azimuth angle (φ), and rotation angle (ψ) are defined in Figure 1C. N, Aq, ωIR, ωq, and Γq are the surface molecular density, amplitude, frequency of the IR beam, resonant frequency, and the damping constant of the qth SFG active vibrational mode, respectively. Eq 2 serves as the basis equation for the nonlinear model fitting of SFG spectra used for orientation analysis and mapping results. (2) χeff = χR (2) + χNR (2) =

∑ q

Aq ωq − ωIR − i Γq

+ ANR eiϕ (2)

SFG spectra are complicated by the convolution between χ(2) R (2) and χ(2) NR shown in eq 3. The ε and δ denote the phase of χR (2) (2) and χNR respectively, and the phase difference between χR and C


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Figure 4. Polar plots of the azimuthal dependence of the SFG intensity (3050, 2875, and 2965 cm−1). Panels A−I show the results obtained at the domains A−I as shown in Figure 2, respectively.

χ(2) NR is the relative nonresonant phase ϕ. Due to the nearly free electrons in the surface region, the nonresonant contribution from the metal and semiconductor is typically large, resulting in a complex SFG line shape.38,43 (2) 2 (2) (2) ISF ∝ |χNR | + |χR(2) |2 + 2|χNR ||χR |cos[ε − δ]

previous studies,34 such variation in the SFG intensity between different domains is attributed to the variation of crystallographic orientation in each individual crystal domain on the copper surface. To support this ascertainment, EBSD measurements are conducted to determine the specific crystallographic orientations in each domain on the copper sample. From the comparison between the SFG images with the EBSD images shown in Figure 3, it can be clearly seen that the brightness contrast of each domain area in the SFG image corresponds to different crystal grain structures as revealed by the EBSD measurement. The three adjacent (223) domains in the EBSD map have same crystal facet in different azimuthal orientation. Thus, corresponding color contrast could be observed across the domain boundary. As mentioned above, the SFG intensity of the crystalline grains contains the azimuthal anisotropy with respect to the surface normal.34 To further analyze the intrinsic nature of such azimuthal anisotropy of the SFG response in each individual crystal domain, a full range (0°−360°) of azimuthal angle measurements of SFG intensity of all 9 domains is performed with a step increment of 20°. Figure 4 shows the azimuthal angle dependence on SFG intensity (3050, 2875, and 2965

(3)

2.3. EBSD Measurement. The EBSD measurement is performed employing a FEI Quanta 600 FE-SEM. The imaging area is 1350 × 1500 μm. The step size is 15 μm, which results in a matrix of 90 × 100 points. The image acquisition time is approximately 3 h.

3. RESULTS AND DISCUSSIONS 3.1. SFG Imaging Study on a Microcrystalline Copper Surface. Figure 2(I−VI) shows six SFG images at 3050 cm−1, with azimuthal angles Ω varying from 0° to 360°. Since 3050 cm−1 is far from the resonant region of ODT molecules, the SFG intensity mainly comes from the copper substrate. It is observed that at least 9 domains with significant SFG signal difference and domain size ranging from 50 to 300 μm, are identified in the SFG images and labeled in A−I. Based on D


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The Journal of Physical Chemistry C cm−1) from domains A−I depicted in Figure 3. Since local crystallographic orientation varies, each crystal domain contains its own featured anisotropy pattern. The nonresonant SFG signal dominates the observed anisotropy patterns at 3050 cm−1, where no peak corresponding to the ODT monolayer exists. Hence, such anisotropy pattern is attributed to the underneath local copper structure. However, the SFG intensity located in the resonant region of ODT molecules (2875 and 2965 cm−1) presents the similar anisotropy pattern as the anisotropy patterns at 3050 cm−1. Figure 5 shows the local SFG spectra of domain C and domain D at the two azimuthal angles, 80° and 140°(arbitrary

nonresonant contributions, such anisotropy of SFG spectra may only exist on the substrate surface or the adsorbed molecular layer. Rotation of the sample changes the relative phase between the substrate and the monolayer contribution, which (2) causes the cross term 2|χ(2) NR∥χR |cos[ε − ϕ] in eq 3 to change sign and generate the anisotropic SFG spectra. Similar results were shown in Wang’s SFG work about the monolayer on a single crystal Z-cut quartz surface.46 With the rotation of quartz substrate around the surface normal, the line shape of SFG spectrum shifts dramatically. It is possible to separate the resonant from nonresonant anisotropy by analyzing the anisotropy of the resonant peak amplitudes versus the off resonant signal. Another possibility is attributed to the epitaxial arrangement of the monolayer induced by the underneath substrate structure where both nonresonant and resonant contribution have exactly the sample rotational anisotropy. These measurements are discussed below. Such epitaxial arrangement includes bonding sites geometry,27 azimuthal anisotropy of the methyl groups,28 and lateral structure of the monolayer.21 3.2. SFG Spectra on Cu(111). To elucidate the intrinsic mechanism for such anisotropy of SFG spectra, Cu(111) serves as a homogeneous reference sample because of its well-defined surface atomic arrangement. Figure 6 shows a series of SFG

Figure 5. SFG spectra (2800−3050 cm−1) of the copper surface with an ODT self-assembled monolayer at two azimuthal angles 80° and 140°. (A) and (B) represent the local SFG spectra of domain C and domain D from Figure 2, respectively. The blue symbols describe the data points and the solid lines are line fits based on eq 2.

choice), respectively. The two peaks observed around 2875 and 2930 cm−1 are assigned to the symmetric stretching vibration, Fermi resonance of the methyl group, respectively. Another two weak peaks locate around 2850 and 2910 cm−1 are assigned to the symmetric stretching vibration and Fermi resonance vibration of methylene groups, respectively. The broad peak around 2965 cm−1 is assigned to the combination of two modes: out-of-plane and in-plane asymmetric stretching vibration mode of methyl group.44,45 The SFG spectra vary from domain C to domain D with inspection of Figure 5. The spectral features, including the nonresonant background, peak intensity, and line-shape are dramatically different between domain C with domain D. The fitting parameters are given in Table S-1. This suggests that the local structure is different on each crystal domain. It can be explained by the interaction of the ODT monolayer with two individual crystal domains might be different. However, this explanation is uncertain, since the nonresonant contribution is significantly different and strong enough to perturb the SFG spectrum. To clarify this, the resonant contribution needs to be isolated by curve fitting the SFG spectrum. Upon closer observation, the SFG spectra also show anisotropy upon rotation of the sample around the z-axis. Considering the SFG intensity contains both resonant and

Figure 6. SFG spectra (2800−3050 cm−1) of the Cu(111) surface with an ODT self-assembled monolayer for azimuthal angles varying from 0° to 60° (offset:100).

spectra for azimuthal angle ranging from 0° to 90° on a Cu(111) surface covered with an ODT self-assembled monolayer. In comparison of these SFG spectra, the peak position and spectral line-shape, as well as nonresonant background strongly depend on the azimuthal angle. More quantitative information about the monolayer epitaxial arrangement is obtained by fitting the SFG spectra with the resonant amplitude and comparing at different azimuthal angles. Previous studies demonstrated that the SFG technique determines the tilt angle of the terminal methyl group of ODT E


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The polar map is fitted by eq 4 with the solid line as shown in Figure 8.49 A and C are the anisotropic and isotropic contributions to the SFG intensity, respectively, and B is the phase correction. Fitting results are tabulated below (Table 1).

on copper surface based on the amplitude ratio of CH3‑sym/ CH3‑asym.47,48 The symmetric/asymmetric stretching amplitude ratio of the methyl group and nonresonant amplitude are obtained from the fitting to the series of SFG spectra in Figure 6 according to eq 2 and plotted against the azimuthal angle as shown in Figure 7. The symmetric/asymmetric stretching

Table 1. Fitting Parameter for Data in Figure 8 2880 cm−1 3000 cm−1

B (rad)

C (a.u.)

2.46 ± 0.02 2.49 ± 0.03

4.09 ± 0.04 5.89 ± 0.05

B values of 2880 cm−1 with 3000 cm−1 are identical, which denotes that the 3-fold symmetry at 2880 cm−1 is the same with the pattern at 3000 cm−1. Such results preclude the possibility that the observed SFG anisotropy at 2880 cm−1 partially originates from the anisotropic orientation distribution of the tilted alkyl chains. If the monolayer indeed exhibits a rotational angle anisotropy in the XY plane, such anisotropy transforms differently in azimuthal angles compared with the substrate. It would give a different anisotropy pattern near the ∼2880 cm−1 resonance compared with near the ∼3000 cm−1 off-resonance. Unless the distribution along the azimuthal angles of terminal methyl groups is taken to be a sum of three delta functions shifted by 120° in registry with the 3-fold symmetry of the Cu(111) surface, and no torsional motion is allowed. Moreover, one can see that the anisotropic term A from these two wavenumber agrees with each other quantitatively, indicating that such anisotropy pattern is only attributed to the underneath local copper structure. In contrast, the isotropic term C is significantly different from each other. Such difference further suggests that the SFG response of monolayer is isotropic. To describe the molecular orientation of monolayers, C3v symmetry is assigned for the methyl group. There are 11 nonvanishing components for the second-order microscopic susceptibility, namely, βaac, βbbc, βccc,βaca, βbcb, βcaa, βcbb, βaaa, βbba, βabb, βbab.50,51 For the r+ mode, there are three second-order microscopic susceptibilities, which are βaac = βbbc, βccc. For the r− mode, there are eight second-order microscopic susceptibilities, which are

Figure 7. (A) Nonlinear background and methyl symmetric/ asymmetric stretching amplitude ratio of ODT/Cu(111) for azimuthal angles varying from 10° to 130°. The step width is 10°.

amplitude ratio is nearly isotropic, while the nonlinear background presents an obvious dependence of azimuthal angle. The symmetric stretching amplitude shown in Figure S1 presents the similar isotropic result. These results indicate that the molecular orientation is independent of the azimuthal angle. Figure 8 shows a comparison of the azimuthal angle dependence of the SFG intensity between 2880 cm−1 (methyl symmetric stretching mode) with 3000 cm−1. The SFG signal is measured in the PPP polarization as a function of azimuthal angle in the increment of 20°. The SFG intensity at 2880 and 3000 cm−1 both shows a clear 3-fold symmetry. Considering only the nonresonant component contributes the SFG intensity at 3000 cm−1, the 3-fold symmetry at 3000 cm−1 indeed reflects the effective symmetry of the Cu(111) substrate. ISF = [A × sin(3ϕ + B) + C ]2

A (a.u.) 1.83 ± 0.05 1.88 ± 0.07

βaca = βbcb , βcaa = βcbb βaaa = −βbba = −βabb = −βbab

(4)

(5)

Figure 8. Polar plots of the azimuthal dependence based on the SFG intensity for the ODT monolayer on Cu(111) at (A) 2880 cm−1, (B) 3000 cm−1 monolayer. Black dots are experimental results; solid red line are fitting results based on eq 4. F


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The Journal of Physical Chemistry C Previous probe scanning microscopy results demonstrate that the thiol monolayer assembling on the metal surfaces composes nanometer size oriented grains.19,20 Consequently, in nanometer scale the monolayer is not isotropic with respect to the surface normal. Based on this consideration, 8 nonvanishing (2) (2) (2) macroscopic susceptibility terms, that are, χ(2) zzz , χxxz , χxzx , χzxx , (2) (2) (2) (2) χzxz , χxzz , χzzx , and χxxx exist for the PPP polarization combination. Corresponding effective susceptibility of the resonant mode can be written as

(2) (2) χeff, = Ns[Fzzz⟨χzzz ⟩] r−

= 2Ns × Fzzz × [βaca ⟨cos θ ⟩ − βaca ⟨cos3 θ ⟩]

Hence, an azimuthal isotropy of SFG resonant intensity from monolayer is observed, regardless of isotropic random azimuthal orientation or locked orientation corresponding with the copper substrate existing in the monolayer. Vice versa, SFG measurements are indeterministic in the specific epitaxial arrangement existing in the monolayer/metal system; however, the tilt angle (θ) information could be reliably determined from spectral fitting with the amplitude ratio of CH3-sym/CH3-asym of SFG spectrum. Corresponding equation to calculate the effective susceptibilities for methyl symmetric stretching and asymmetric stretching vibrational mode are listed in eq 10 and eq 11. Considering the metal surface always contributes a very large screen effect to the electric field parallel to the surface, this conclusion is capable to be applied on other monolayer/metal system. To further elucidate the isotropic resonant amplitude of ODT, SFG imaging data (shown in Figure S2) of ODT on microcrystalline copper surface with azimuthal angles 80° and 240° is evaluated by employing statistical methods represented in the form of histogram distributions. The excellent agreement between these two histograms of amplitude ratio is illustrated in Figure 9. The two ratio distributions match each other when

(2) (2) (2) (2) (2) χeff = Ns[Fzzz⟨χzzz ⟩ − Fxxz⟨χxxz ⟩ − Fxzx⟨χxzx ⟩ + Fzxx⟨χzxx ⟩ (2) (2) (2) (2) + Fzxz⟨χzxz ⟩ − Fxzz⟨χxzz ⟩ + Fzzx⟨χzzx ⟩ − Fxxx⟨χxxx ⟩

(6)

Since the variation in the incident angle and refractive indices between the SFG beam with 1064 beam is negligible, the (2) Fxzx⟨χ(2) xzx ⟩ is approximately same with the Fzxx⟨χzxx ⟩. This also (2) (2) applies between Fzxz⟨χzxz ⟩ with Fxzz⟨χxzz ⟩. Thus, the third, fourth, fifth, and sixth term in eq 6 cancel each other out. According to IR selection rules, only the vibrational modes with components projected along the surface normal are observed.52,53 The x, y components of the IR electric field are screened at the metal surface, so the Fzzx and Fxxx are approximately zero. These approximations simplify the analysis and consider only the nonzero elements which are the first and second term as follows: (2) (2) (2) χeff = N[Fzzz⟨χzzz ⟩ − Fxxz⟨χxxz ⟩]

(11)

(7)

If assuming that the monolayer has an isotropic rotation angle (ψ) distribution, thus for the symmetric stretching vibrational mode, (2) χxxz, = βaac[⟨cos3 θ ⟩ + ⟨cos θ ⟩⟨cos2 φ⟩ − ⟨cos3 θ ⟩ r+

⟨cos2 φ⟩] + βccc[⟨cos θ ⟩ − ⟨cos3 θ ⟩ − ⟨cos θ⟩⟨cos2 φ⟩ + ⟨cos3 θ ⟩⟨cos2 φ⟩]

(8)

For the asymmetric stretching vibrational mode, (2) χxxz, = 2βaca [⟨cos3 θ ⟩ + ⟨cos θ ⟩⟨cos2 φ⟩ r−

− ⟨cos3 θ⟩⟨cos2 φ⟩ − ⟨cos θ ⟩]

Figure 9. Histogram of amplitude ratios (CH3‑sym/CH3‑asym) of methyl group on microcrystalline copper surface with azimuthal angle 80° (red) and 240° (green), respectively. The corresponding SFG image is shown in Figure S2.

(9)

From inspection of the above analysis, the χ(2) xxz component contains the azimuth angle (φ) dependence. Since no azimuthal anisotropy of the symmetric stretching vibrational mode and asymmetric stretching vibrational mode appears, it is concluded that the χ(2) xxz makes a limited contribution in the effective susceptibility χ(2) eff . The reason is the enhanced electric field, which is normal to the surface, while the parallel component of electric field is screened due to the boundary conditions of the electric field on metal surface.54,55 The 2 calculated value for |Fzzz⟨χ(2) zzz ⟩| is ten times larger than | (2) 2 Fxxz⟨χxxz ⟩| (detailed simulation results are shown in Figure S3). Previous SFG work about metal surfaces also proved that the isotropic tensor component Fzzz⟨χ(2) zzz ⟩ dominates the SFG signal on the metal surface.56,57 For the symmetric stretching vibrational mode,

the sample azimuthal angle shifts from 80° to 240°. This can be understood that the azimuthal angle transformation does not influence the interpretation of SFG spectrum, as well as the conformational order of the monolayer. Past studies have deduced the anisotropic orientation distribution by monitoring the molecular SFG response as a function of the surface azimuthal angle.21,58−60 These results demonstrate that the molecules have a well-defined anisotropic arrangement following the structure of substrate, however, there are where the substrates are all oxides with relatively low electric conductivity. Thus, the screening effect to the electric field parallel with the surface from the silicon substrate is not as strong as copper, which causes the lateral order of the monolayer to be easily revealed. Previous studies from probe scanning microscopes demonstrate monolayer self-assembles on metal surface composed nanometer size oriented domain grain structures.19,20 As in the

(2) (2) χeff, = Ns[Fzzz⟨χzzz ⟩] r+

= Ns × Fzzz × [βaac⟨cos θ⟩ + βccc⟨cos3 θ⟩ − βaac⟨cos3 θ⟩]

(10)

For the asymmetric stretching vibrational mode, G


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Figure 10. (I) Representative SFG image (2980 cm−1) of ODT on microcrystalline copper surface at azimuthal angle 80°. (II) Corresponding EBSD map with the color coded orientation map of the scanned area. (III) Corresponding histogram of amplitude ratio (sym/asym) of methyl group on copper at azimuthal angle 80°.

case of optically flat copper surfaces, many local nanometer size structures exist, such as terraces, steps, and defects. As such, domains consisting of these nanometer size structures are smaller than single imaging pixels, there is a “superposition” of these oriented structures. That may also be why an isotropic SFG response for the monolayer may be observed instead of the expected anisotropic response. Observing such domains is beyond the scope of SFG imaging. Nevertheless, SFG imaging provides a reliable spectroscopic approach to chemically visualize the heterogeneous metal surfaces at a micrometer scale. Since the interpretation of the SFG spectrum is independent from the azimuthal angle distribution of the molecules, the molecular tilt angle for molecules in different crystal grain could be determined and compared. This would provide tremendous new information on the structure and interactions of these molecules at a given crystal grain. 3.3. Tilt Angle of Methyl Group and Histogram Analysis. Besides providing information about the copper substrate, SFG can also retrieve information on the tilt angle, namely, through the examination of the amplitude ratio of different vibrational modes. A representative SFG image (2980 cm−1) at azimuthal angle 80° is shown in Figure 10I, with the corresponding EBSD image of the same area in Figure 10II. Four domains, each with different crystal indices, can be identified in the SFG image and EBSD image, and are labeled in Figure 10I (A−D). Comparing the EBSD and SFG maps, it

is more evident that the regions of distinctly different SFG signal correspond to particular grain structures by the EBSD data shown in Figure 10II. To elucidate monolayer heterogeneity in different crystal domains, the evaluation of the SFG imaging data from a SAMs monolayer on such copper surface is carried out by employing statistical methods represented in the form of histogram distributions of the tilt angle of a methyl group. In Figure 10III, the distribution of amplitude ratio (sym/asym) and tilt angle of methyl group are presented to visualize the chemical heterogeneity in each domain. Corresponding statistical analysis result are listed in Table 2. Upon closer inspection of the histogram of the amplitude ratio and tilt angle, each crystal domain has its own Table 2. Statistical Parameters of Distributions of the Methyl (sym/asym) Ratio and Terminal Methyl Group Tilt Angle (°) methyl (sym/asym) ratio

H

domain

Miller index

A B C D

(323) (223) (223) (235)

methyl tilt angle (°)

mean

distribution width

mean

distribution width

0.39 1.91 0.78 1.31

0.51 1.52 0.38 0.95

63 31 34 29

23 17 9 8



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feature ratio distribution, which suggests that the underneath grain structure of the copper surface dramatically influences the molecules packing behavior and causes the tilt angle variation in different regions. These structure dependent relative methyl tilt angle are evident and highlighted on polycrystalline copper surfaces, the molecular packing behavior is strongly structure dependent at the microscopic level. Similar observations have already been suspected by other studies on copper surfaces.61−64 However, they are directly visualized in the SFG imaging. As long as the fitting parameters are reliable, comparing the differences of the molecular orientation order on different grains can provide tremendous new information on the interaction of the molecules with the underneath metal structure. This is the reason for SFG imaging that could be further applied in various fields.

AUTHOR INFORMATION

Corresponding Author

*[email protected]. ORCID

Steven Baldelli: 0000-0002-5747-259X Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This project is supported by the National Science Foundation (CHE-1361885). The authors thank Joon Hee Jang (University of Houston) for developing the curve fitting program. The FESEM acquisition is supported by the NSF grant DBI-0116835, the VP for Research Office, and the TX Eng. Exp. Station. M.F. acknowledges valuable suggestions from Desheng Zheng, Michelle De Leon (University of Houston), and Peipei Gao (Texas A&M University).

4. CONCLUSIONS The structure of an ODT monolayer formed on a microcrystalline copper is studied by the combination of the SFG microscopy and EBSD. SFG imaging technique provides detailed information and understanding of the molecular orientation of adsorbed monolayers on the surface through quantitative measurements and analysis of SFG images. Clear domain structures with obvious brightness contrast reveal existing grain structures on the copper surface, and are confirmed by corresponding EBSD image. The transformation of sample azimuthal angle cause the SFG spectra phase shift, to generate the anisotropy of SFG spectra. The resonant contribution is separated by SFG spectra fitting and confirmed to be constant as the sample azimuthal angle varies, whereas only the isotropic χ(2) zzz component dominates the resonant SFG contribution. The different anisotropy SFG response on each grain reveals the heterogeneity of copper surface with various crystal grains. The heterogeneous molecular packing behavior induced by the underneath copper grain structure is visualized and characterized by the SFG microscopy. This SFG microscopy study illuminates that local metal surface structure modifies the molecular packing behavior. This observation enables the implementation of SFG microscopy as a promising tool for chemical resolving the surface in addition to other spectroscopic techniques, such as Coherent AntiStokes Raman Microscope, and therefore is of great practical interest. Considering the molecular orientations of the surface can significantly influence the chemical performance, the approach to understand and control molecular packing through selection of the surface structure and symmetry would be extremely beneficial. Over all, the application of SFG microscopy could be greatly expanded with ease in many laboratories and industries for quantitative and qualitative measurements and understanding of complex surfaces in energy, material, catalysis, and corrosion research in the future.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.6b09403. Additional experimental information including anisotropy of methyl symmetric stretching amplitude on Cu (111), statistical analysis, corresponding SFG image, and fitting result of SFG spectra (PDF) I


Article

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Surface-Induced Heterogeneity Analysis of an Alkanethiol Monolayer

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