Supporting Information Self-Assembled Monolayers ...

Supporting Information Self-Assembled Monolayers Get Their Final Finish via a Quasi-Langmuir-Blodgett Transfer Christian Meltzer1,2, Hanno Dietrich4,5, Dirk Zahn 3,4,5, Wolfgang Peukert1,2,3 and Björn Braunschweig*,1,2,3 1

Institute of Particle Technology (LFG), Friedrich-Alexander-Universität Erlangen-Nürnberg (FAU), Cauerstrasse 4, 91058 Erlangen, Germany

2

Erlangen Graduate School in Advanced Optical Technologies (SAOT), Friedrich-AlexanderUniversität Erlangen-Nürnberg (FAU), Paul-Gordan-Strasse 6, 91052 Erlangen, Germany 3

Cluster of Excellence – Engineering of Advanced Material (EAM), Friedrich-Alexander-

Universität Erlangen-Nürnberg (FAU), Nägelsbachstrasse 49b, 91058 Erlangen, Germany 4

Computer-Chemie-Centrum and Interdisciplinary Center for Molecular Materials, FriedrichAlexander-Universität Erlangen-Nürnberg (FAU), Nägelsbachstrasse 25, 91052 Erlangen, Germany

5

Chair of Theoretical Chemistry, Friedrich-Alexander-Universität Erlangen-Nürnberg (FAU), Nägelsbachstrasse 25, 91052 Erlangen, Germany

1.

Influence of the experimental geometry on the sum-frequency signal

In vibrational sum-frequency generation (SFG), a photon with the sum-frequency of the two incident beams is generated at an interface of two centrosymmetric materials. Although SFG is an inherently nonlinear optical process, the SFG intensity depends also on linear optical properties such as the reflectance and/or transmission coefficients of all participating light waves at the interface of interest. The latter has to be taken into account by introducing an effective Fresnel coefficient Fyyz when the SFG intensity for ssp polarization at an interface that is isotropic within the interfacial plane is described:1,2 2

(2)

2

ISF (ωIR ) ∝ |Fyyz | |χyyz | I(ωVIS ) I(ωIR )

(S1)

(2)

Here χyyz is the second-order electric susceptibility of the material, I(ωVIS ) is the intensity of the visible laser beam and I(ωIR ) the intensity of the infrared laser beam. The effective Fresnel coefficient Fyyz can be calculated by modeling reflection and transmission behavior of a thin dielectric film placed between two infinite media based on the Fresnel coefficients:3 Fyyz = Ly (ωSF ) Ly (ωVIS ) Lz (ωIR ) 2n1 (ωSF ) cos βSF 1 SF SF 1 (ωSF ) cos β1 +n2 (ωSF ) cos β2

(S3)

2n1 (ωVIS ) cos βvis 1 vis n1 (ωVIS ) cos βvis 1 +n2 (ωVIS ) cos β2

(S3)

Ly (ωSF ) = n Ly (ωVIS ) =

(S2)

Lz (ωIR ) = n

IR 2n2 (ωIR ) cos βIR 1 sin β1 IR IR 1 (ωIR ) cos β2 +n2 (ωIR ) cos β1

n (ω )

1 IR ( n′(ω ) ) IR

(S4)

With β1 being the angles of incident of sum-frequency, visible and IR beams, while β2 denote the angles of the refracted beams defined via Snell’s law n1 (ω) sin(β1 )= n2 (ω) sin(β2 ). n1

and n2 are the refractive indices of the bulk materials, while n’ is the refractive index of the interfacial layer. The setup for in situ SFG spectroscopy of SAM formation is shown in Figure S2. Here, the material 1 with the respective refractive index n1 is given by the bulk α-Al2 O3 phase, the thin interfacial layer consists of ODPA and material 2 is either the bulk 2-propanol phase or air. For in situ studies we need to assume propanol while for de-wetted samples, air as the second phase SF/VIS/IR

has to be assumed. The refractive indices of these materials are: nair nVIS α - Al

2 O3

=1.8;

nIR α - Al

2 O3

=1.7,

nSF =1.38; 2-propanol

nVIS =1.38; 2-propanol

=1.0, nSF α - Al

2 O3

=1.8;

nIR =1.37 2-propanol

for

λSF =650 nm, λVIS =800 nm and λIR =3500 nm.3–5 For the thin dielectric layer a refractive index of n’ = 1.4 is assumed which is typically used in literature for such systems. 6 The relevant angles VIS IR in our geometry are: βSF 1 =55°, β1 =54° and β1 =60°. air Based on this the Fresnel coefficient for both states can be calculated: |Fyyz | = 1.3 and 2−𝑝𝑟𝑜𝑝 |𝐹𝑦𝑦𝑧 | = 6.3. To allow a comparison of the intensities of spectra taken with the SAM in

contact with 2-propanol and in contact with air, all 2-propanol spectra shown are divided by a 2

2−𝑝𝑟𝑜𝑝 𝑎𝑖𝑟 correction factor of |𝐹𝑦𝑦𝑧 ⁄𝐹𝑦𝑦𝑧 | = 23.5. In case of vibrational amplitudes the correction 2−𝑝𝑟𝑜𝑝 𝑎𝑖𝑟 factor is |𝐹𝑦𝑦𝑧 ⁄𝐹𝑦𝑦𝑧 | = 4.8.

The very high effective Fresnel coefficient can strongly influence the result of the comparison 2

2−𝑝𝑟𝑜𝑝 𝑎𝑖𝑟 made in Figure 3 of the main article. Therefore a sensitivity analysis of |𝐹𝑦𝑦𝑧 ⁄𝐹𝑦𝑦𝑧 | on

crucial parameters was performed to avoid that the comparison of intensities with the SAM being in contact with 2-propanol and air is driven by overestimating the influence of the experimental geometry.

The main uncertainty in determining the effective Fresnel coefficient is given by the incident angles which are determined by a simple geometrical method with a measurement error of 1°. To 2−Prop

evaluate the influence of this uncertainty, the dependence of |Fyyz

2

air ⁄Fyyz | on the incident

angle of the VIS and the IR beam is shown in Figure S1. The working point for the measurements presented (marked with the black arrow in Figure S1) 2−Prop

is located on a plateau of |Fyyz

2

air ⁄Fyyz | > 20 ranging from βIR 1 = 55° to 90° and from

βVIS 1 = 52° to 90°. As the distance of the working point towards the edge of this plateau is 5° for the IR and 2° for the VIS beam, one can be confident that the intensity correction performed does not wrongly influence the results and their interpretation due to the uncertainty in determining the incident angles. Furthermore we studied the influence of the refractive index of 2-Propanol. Assuming 2−Prop

n2-Propanol = 1.45 for all beams results in |Fyyz 2−Prop

|Fyyz

2

2

air ⁄Fyyz | = 35.8 and n2-Propanol = 1.35 in

air ⁄Fyyz | = 17.8. Thus the same conclusion can be drawn as for the incident angles.

2

2−𝑃𝑟𝑜𝑝 𝑎𝑖𝑟 Figure S1 Dependence of the intensity correction factor |𝐹𝑦𝑦𝑧 ⁄𝐹𝑦𝑦𝑧 | (color code) on the

incident angle of the visible and infrared laser beam on the prism surface. The tip of the black arrow marks the working point for the measurements presented.

2. Measuring cell for in situ studies of SAM formation To study the formation of ODPA SAMs on α − Al2 O3 (0001) in situ, a custom-made sample cell as depicted in Figure S2 was used. The sample surface can be probed from the bottom through a α − Al2 O3 prism. Due to the very low absorbance of α − Al2 O3 within the relevant wavelength range of 800nm for the VIS, 3500nm for the IR and 650 nm for the sum-frequency beam the sample surface can be probed without high losses. With the liquid phase on top of the sample surface, good contact of the surface with the ODPA solution can be ensured. To inject the ODPA solution into the sample cell, a glass plug was removed and the solution was injected with an acid cleaned glass syringe. The sample cell has a volume of approximately 10 ml. To remove the sample solution, a large amount of it is withdrawn with the help of a

syringe. Afterwards the glass cover is removed to allow the drying of the sample with help of a stream of nitrogen (purity 99.999%).

Figure S2 Schematic drawing of the sample cell used to study in situ formation kinetics of ODPA on α − Al2 O3 (0001). The bottom part of the cell (dark grey) is fabricated out of Teflon, while the cover and the plug are made out of Duran glass.

3. Details of performed fitting procedure The sum-frequency signal consists of non-resonant and resonant parts of the second-order electric susceptibility. The resonant contribution is due to excitation of vibrational modes from interfacial molecules. Assuming homogeneous line broadening only, the resonant part of the (2)

second-order electric susceptibility χyyz of the vibrational bands can be described as Lorentzian curves:2

2

(2)

(2)

2

𝐼𝑆𝐹 (𝜔𝐼𝑅 ) ∝ |𝐹𝑦𝑦𝑧 | |𝜒𝑁𝑅 + ∑𝑞 𝜒𝑞 | 𝐼(𝜔𝑉𝐼𝑆 ) 𝐼(𝜔𝐼𝑅 ) 𝐴𝑞 exp(𝑖 𝜑𝑞 )

(2)

with 𝜒𝑞 = 𝜔

(S5) (S6)

𝑞 −𝜔𝐼𝑅 +𝑖Γ𝑞

(2)

Here 𝜒𝑁𝑅 describes the non-resonant contributions, Aq the amplitude, 𝜔𝑞 the resonance frequency and Γ𝑞 the bandwidth of the vibrational mode q. 𝜔𝐼𝑅 is the frequency of the infrared laser beam. To determine the spectral contributions of the different vibrational modes, the measured SFG spectra are typically fitted with a linear combination of Lorentzian peaks. In the measurements presented here, the spectra of ODPA in contact with air were fitted based on this procedure with four Lorentzian curves. The fit parameters and results are summarized in Table S1. For the SAM being in contact with 2-propanol solutions, the FR-AS peak (see main text) consists of at least 5 bands: two methyl Fermi resonance peaks and two methyl asymmetric stretching vibration peaks one originating from ODPA and one from 2-propanol respectively. In addition, a methine vibrational band from interfacial 2-Propanol is located in this wavelength region.7 As fairly arbitrary results may arise when one very broad spectral feature is fitted by 5 independent vibrational bands, we used a single Voigt-profile 𝑒

𝑉(𝜔𝐼𝑅 ) = (

−

(𝜔𝑞 −𝜔𝐼𝑅 ) ⁄ (2𝜎2 )

𝜎 √2𝜋

∗

𝛾 2

𝜋 ((𝜔𝑞 −𝜔𝐼𝑅 ) + 𝛾2 )

) (𝜔𝑞 − 𝜔𝐼𝑅 )

(S7)

to fit the methyl contributions of the FR-AS peak and added three Lorentzian curves, one for the methine peak, one for 𝐶𝐻2𝑠𝑠 and one for 𝐶𝐻3𝑠𝑠 contributions from ODPA. An overview of the used fit parameters as well as the results of the fitting procedure for the spectra shown in Figure 3 of the main article and in Figure S3 is given in Table S2. Table S3 gives fit parameters for the spectra shown in Figure S4.

Table S1

Overview of fit parameters for ODPA SAMs in contact with air. The spectra were fitted with a

linear combination of 4 Lorentzian peaks. A negative amplitude accounts for a phase shift of π. The samples 0.5 h dried directly/c-reduced are those depicted in Figure 3b of the main article. Samples 15 h dried directly/c-reduced are depicted in Figure S4 c to d.

Sample

Bandwidth / cm-1

Resonance freq. / cm-1

Amplitude / a.u.

0.5 h dried directly c-reduced 𝐶𝐻2𝑠𝑠 𝐶𝐻3𝑠𝑠 𝐶𝐻3𝐹 𝐶𝐻3𝑎𝑠 𝐶𝐻2𝑠𝑠 𝐶𝐻3𝑠𝑠 𝐶𝐻3𝐹 𝐶𝐻3𝑎𝑠 𝐶𝐻2𝑠𝑠 𝐶𝐻3𝑠𝑠 𝐶𝐻3𝐹 𝐶𝐻3𝑎𝑠

6.7 29.3 -24.6 -1.4 2850.3 2875.4 2938.4 2968 15 10.3 11.6 10

8.8 25 -22 -6.8 2850.3 2875.4 2938.4 2968 15 10.3 11.6 10

15h dried directly c-reduced 3.5 30.8 -24.1 -7.9 2850.3 2874.4 2934.7 2968 15 10.3 11.6 10

10.5 24.5 -20.4 -6.6 2850.3 2874.4 2934.7 2968 15 10.3 11.6 10

Table S2

Overview of fitting parameters for ODPA SAMs in contact with 2-propanol. The spectra were

fitted with a linear combination of three Lorentzian peaks and a Voigt profile. A negative amplitude accounts for a phase shift of π. The spectra of the 0.5 h – end of kinetic samples are depicted in Figure 3a and those of the 0.5 h – pure 2-propanol samples are depicted in Figure 3c of the main article.

Sample

Voigt profile

Bandwidth / cm-1

Resonance freq. / cm-1

Amplitude / a.u.

0.5 h – end of kinetic dried directly c-reduced

0.5h – pure 2-propanol dried directly c-reduced

𝐶𝐻2𝑠𝑠

1.2

1.4

0.3

1.2

𝐶𝐻3𝑠𝑠

0.5

0.6

3.0

1.7

𝐶𝐻

0.5

1.2

2.5

1.8

𝐶𝐻2𝑠𝑠

2845.7

2845.7

2845.7

2845.7

𝐶𝐻3𝑠𝑠

2868.3

2869

2872.1

2870.2

𝐶𝐻

2927.4

2919

2937

2932.5

𝐶𝐻2𝑠𝑠

10.5

10.5

10.5

10.5

𝐶𝐻3𝑠𝑠

10.3

10.3

10.3

10.3

𝐶𝐻

15

15

15

15

AV 𝜔𝑉 γV 𝜎𝑉

-3.4 2957.5 2 15.2

-3.2 2957.7 2 15.2

-3.3 2965 2 15.2

-3.2 2963.1 2 15.2

Table S3

Overview of fitting parameters for ODPA SAMs in contact with 2-propanol for a self-

organization time of 15h. The spectra were fitted with a linear combination of three Lorentzian peaks and a Voigt profile. A negative amplitude accounts for a phase shift of π. The spectra and the fits are depicted in Figure S4.

Sample

Voigt profile

Bandwidth / cm-1

Resonance freq. / cm-1

Amplitude / a.u.

15 h – end of kinetic dried directly c-reduced

15 h – pure 2-propanol dried directly c-reduced

𝐶𝐻2𝑠𝑠

0.8

0.8

0.3

1.0

𝐶𝐻3𝑠𝑠

2.4

2.6

3.4

2.7

𝐶𝐻

2.9

3.5

3.1

2.7

𝐶𝐻2𝑠𝑠

2848.6

2848.6

2848.6

2848.6

𝐶𝐻3𝑠𝑠

2871.5

2871.5

2871.5

2871.5

𝐶𝐻

2935.5

2935.5

2935.5

2935.5

𝐶𝐻2𝑠𝑠

10.5

10.5

10.5

10.5

𝐶𝐻3𝑠𝑠

10.3

10.3

10.3

10.3

𝐶𝐻

15

15

15

15

AV 𝜔𝑉 γV 𝜎𝑉

-2.6 2969.3 2 15.2

-2.6 2972.6 2 15.2

-2.8 2968.3 2 15.2

-3.2 2967.5 2 15.2

Figure S3 Fits (black solid lines) and SFG data (dots) for the spectra shown in Figure 3 of the main article. Spectra shown on the right side are dried directly after 0.5 h formation time, those on the left are c-reduced after 0.5 h. (a) shows the spectrum at the end of the kinetic, (b) the spectrum after the ODPA solution concentration was reduced. (c) and (d) show SFG spectra of respective layers after drying in a stream of nitrogen gas and (e) and (d) after reimmersion in pure 2-propanol. The fits were performed as outlined in this chapter of the Supporting Information.

Table S4

Fitting parameters of the spectra shown in Figure 3d of the main article are given. The fits were

performed with 4 Lorentzian profiles as is discussed in this chapter of the Supporting Information. A negative amplitude accounts for a phase shift of π.

Sample

Bandwidth / cm-1

Resonance freq. / cm-1

Amplitude / a.u.

st

𝐶𝐻2𝑠𝑠 𝐶𝐻3𝑠𝑠 𝐶𝐻3𝐹 𝐶𝐻3𝑎𝑠 𝐶𝐻2𝑠𝑠 𝐶𝐻3𝑠𝑠 𝐶𝐻3𝐹 𝐶𝐻3𝑎𝑠 𝐶𝐻2𝑠𝑠 𝐶𝐻3𝑠𝑠 𝐶𝐻3𝐹 𝐶𝐻3𝑎𝑠

Dried 1 time

After 4 cycles

19.7 21.4 -22.2 -0.8 2850.3 2873.3 2938.4 2968 15 10.3 11.6 10

14.6 37.1 -25.9 -2.2 2850.3 2873.3 2938.4 2968 15 10.3 11.6 10

4. SFG spectra after 15 h formation time

Figure S4 Fits (black solid lines) and SFG spectra (dots) for samples with a selforganization time of 15 h. Spectra shown on the right side are dried directly, those on the left are c-reduced. a shows the spectrum at the end of the kinetic, (b) the spectrum after the ODPA solution concentration was reduced. (c) and (d) show the respective layers after drying with a stream of nitrogen and e and f after reimmersion in pure 2-propanol. The fits were performed as outlined in chapter 3 of the Supporting Information.

5. Details on Molecular Dynamics Simulations

5.1 Simulation Conditions

All simulations were carried out using the DLPOLY Classics 1.9 code8 with a 12 Å cutoff for long-range forces and an integration time step of 1 fs. A Berendsen-type thermostat9 with a relaxation time of 0.5 ps was used to maintain a constant temperature of 300 K. All systems were simulated with 2D-periodic boundary conditions. To apply ambient pressure to the solvent layer, a mobile barrier was introduced to the corresponding systems by slight modification of the DLPOLY code, analogous to a pressure stamp in a previous work.10 The simulations were equilibrated until all energetic terms had converged, followed by a production run of 5 ns. For geometric analyses snapshots were taken each picosecond amounting to a total of 5000 frames.

5.2 Force Field Parameters

The force field was a combination of an interatomic potential model for α-Al2O3 published by Sun et. al11 and the Generalized Amber Force Field (GAFF)12 for organic moieties. Van-derWaals interactions were calculated using a 12-6 Lennard-Jones term, except for the interactions within the aluminum oxide (AL-OA, AL-AL and OA-OA), which were treated with Buckingham potentials according to the force field from Sun et al., and are not listed in Table S1. The values in Table S1 for AL and OA are the Lennard-Jones parameters used for mixing of the aluminum oxide atoms with the organic moieties. Beside the introduction of aluminum oxide, the hydroxyl and hydroxide hydrogen atoms (HO, HX) were given additional van-der-Waals parameters to prevent irregular overlapping with highly charged acid and hydroxide oxygen species.

Electrostatics were treated with a shifted-force Coulomb sum. To determine the atomistic atomic charges for the phosphonic acid and 2-propanol, the molecule was optimized in Gaussian0913 using the B3LYP functional with a 6-31G(d) basis set. On the optimized geometry, a HartreeFock single point calculation with the IOP(6/33=2,6/41=10,6/42=17) directive was performed to determine the ESP charges. Afterwards, the RESP charges were calculated with Antechamber from the AmberTools Package. Angle, bond and dihedral parameters listed in Tables S2, S3 and S4 were also taken from GAFF. For bond and dihedral angle parameters, no alterations were made. However, the force constants for the angle were enhanced as suggested by Meagher, Redman and Carlson,14 since the geometries of the phosphonic acid anchor groups appeared strongly distorted using the lower GAFF values.

Table S5 Atom types

and nonbonding parameters of the force field.

Type

Description

Mass / u

Charge / e

ε / kJ/mol

σ/Å

C3

Alkyl carbon

12.011

resp

0.45773

3.39968

HC

Alkyl hydrogen

1.008

resp

0.06569

2.64954

P5

Phosphorus

30.9738

resp

0.8368

3.74178

O

PA carbonyl oxygen

15.9994

resp

0.87864

2.95993

OH

PA/iPrOH oxygen of OH group

15.9994

resp

0.88031

3.06648

HO

PA /iPrOH hydrogen of OH group

15.9994

resp

0.19246

0.40000

H1

Hydrogen neighboring OH group

1.008

resp

0.06569

2.4714

OX

Oxygen of hydroxide ion

1.008

-1.2

0.88324

3.06647

HX

Hydrogen of hydroxide ion

1.008

0.2

0.19246

0.40000

AL

Aluminum atom

26.9815

3

6.8785

1.66776

OA

Oxygen atom in Al2O3

15.9994

-2

0.87864

2.95992

Table S6 Bond

parameters of the force field.

k / kJ/mol

r0 / Å

C3

C3

2536.3409

1.535

C3

HC

2822.5263

1.092

C3

P5

2173.1697

1.813

C3

H1

2810.8112

1.09

P5

O

4081.0737

1.481

P5

OH 2687.8017

1.625

OH HO 3092.8129

0.974

OX HX 2205.4000

0.957

Table S7 Angle

parameters of the force field.

k / kJ/mol φ0 / deg C3

C3

C3

528.941

110.63

C3

C3

HC

388.024

110.05

C3

C3

H1

387.940

110.07

C3

C3

P5

513.712

112.32

C3

P5

O

836.800

112.50

C3

P5

OH

836.800

101.56

HC

C3

HC

329.950

108.35

HC

C3

P5

355.975

109.64

H1

C3

OH

426.517

109.88

O

P5

OH

1171.52

115.26

P5

OH HO

369.447

110.14

O

P5

1171.52

115.80

O

Table S8 Dihedral

parameters of the force field

k / kJ/mol φ0 / deg n C3

C3 C3

C3

0.753

0

3

C3

C3 C3

C3

1.046

180

2

C3

C3 C3

C3

0.837

180

1

C3

C3 C3

HC 0.669

0

3

C3

C3 C3

P5

0.651

0

3

C3

C3 P5

O

0.093

0

3

C3

C3 P5

OH 0.093

0

3

C3

P5

OH HO 2.231

0

3

HC

C3 C3

HC 0.628

0

3

HC

C3 C3

P5

0.651

0

3

HC

C3 P5

O

0.093

0

3

HC

C3 P5

OH 0.093

0

3

H1

C3 C3

HC 0.651

0

3

H1

C3 OH HO 0.697

0

3

O

P5

0

3

OH HO 2.231

6. Histograms for fitting CH2 and CH3 signal densities

Figure S5

Histogram data from CH2 (top) and CH3 (bottom) bond vector analysis with fitted Gaussian

functions of MD2.57 in vacuum (red) and in 2-propanol (yellow) as well as MD3.62 in vacuum (blue) and in 2-propanol (cyan).

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