Formation Kinetics of Mixed Self-Assembled Monolayers of

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Formation Kinetics of Mixed Self-Assembled Monolayers of Alkanethiols on GaAs (100) Vivien Lacour, Khalid Moumanis, Walid M. Hassen, Celine Elie-Caille, Therese Leblois, and Jan J Dubowski Langmuir, Just Accepted Manuscript • Publication Date (Web): 22 Oct 2017 Downloaded from http://pubs.acs.org on October 23, 2017

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Formation Kinetics of Mixed Self-Assembled Monolayers of Alkanethiols on GaAs (100) Vivien Lacour†,‡, Khalid Moumanis†, Walid Hassen†, Céline Elie-Caille‡, Thérèse Leblois‡, Jan J. Dubowski†,* †

Laboratory for Quantum Semiconductors and Photon-Based BioNanotechnology,

Interdisciplinary Institute for Technological Innovation (3IT), CNRS UMI-3463, 3000 boul. de l’Université, Université de Sherbrooke, Sherbrooke, Québec J1K 0A5, Canada ‡

FEMTO-ST Institute, UMR 6184 CNRS, Université de Bourgogne Franche-Comté, 15B, Av

des Montboucons, 25030 Besançon, France

*Corresponding author. E-mail: [email protected].

ABSTRACT

We report on the formation kinetics of mixed self-assembled monolayers (SAMs) comprising 16-mercaptohexadecanoic acid (MHDA) and 11-mercapto-1-undecanol (MUDO) thiols on GaAs(100) substrates. These compounds were selected for their potential in constructing highly selective and efficient architectures for biosensing applications. Molecular composition and quality of one-compound and mixed SAMs were determined by the Fourier transform infrared

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absorption spectroscopy measurements. The formation of enhanced quality mixed SAMs was investigated as a function of molecular composition of the thiols mixture and the proportion of ethanol/water solvent used during their arrangement. Furthermore, formation of mixed SAMs has been carried out by successive immersion of MHDA SAMs in MUDO thiol solutions, and MUDO SAMs in MHDA thiol solution, through the process involving thiol-thiol substitution. Our results, in addition to confirming that water-ethanol based solvents improve the packing density of single thiol monolayers, demonstrate attractive role of water-ethanol solvents in formation of a superior quality mixed SAMs.

1.

Introduction Numerous recent studies have demonstrated the use of GaAs as a base material for biosensing

exploring different methods of transduction delivered by high electronic mobility transistors (HEMT),1, 2 molecular controlled semiconductor resistor (MOCSER) devices,3 surface plasmon resonance (SPR) sensors,4 as well as Hall effect,5 photoluminescence,6,

7, 8

integrated GaAs

fluorescence sensor and emitter9 and acoustic10 devices. The emergence of GaAs in the field of biosensing implies deep investigation and characterization of biosensing architectures to achieve well-controlled and -oriented biological receptors. In the case of an immunosensor, the biological interface involves antibodies that are preferably attached by a covalent link to the sensor surface. Alkanethiol self-assembled monolayers (SAMs) are attractive linkers for protein adsorption due to tunability of their chain lengths and availability of different terminal groups11 (-OH, -COOH, CH3, NH2, -biotin, etc…). Thiol-based linker is the most widely used SAM to functionalize metal surface.12 It has also been reported that SAMs of thiols could be formed on GaAs surfaces


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with a comparable quality to those formed on Au substrates.13,

14, 15

Due to their ability to

passivate GaAs substrates, alkanethiol SAMs have also been studied for building microelectronic and photonic devices.16, 17 Mixed alkanethiols provide attractive biosensing architectures, e.g., with enhanced ability to immobilize antibodies, such as those reported for Au substrates.18 Mixed SAMs typically constitute two compounds, one compound with a functional headgroup to covalently link bioreceptors, and another compound for dilution that is supposed to prevent non-specific surface adsorptions.18, 19, 20 The role of a thiol diluent is also to reduce the surface concentration of the main component and, thus, prevent steric hindrance due to binding of antibodies at excessive density. The steric hindrance effect was observed, for instance, for mixed thiol functionalized GaAs substrates designed to immobilize protein as homogeneous layers.21, 22 The formation of mixed SAM on Au showed a dependence on the presence of different terminal groups, chain length, relative ratio of components and wetting properties of the surface.23,

24

A binary

monolayer may adopt more complex conformation rather than a homogeneously distributed thiolate, and it could form macro- or microscopically separated islands.23 In that context, it has been attested that, generally, the concentration of mixed SAM components on the surface is not equal to their ratio in a solution.24, 25 Furthermore, numerous studies have reported the impact of solvents on self-assembly of thiols, indicating that polar solvents facilitate formation of wellordered and more dense monolayers than low polarity solvents.26,

27, 28

Some studies have

demonstrated formation of dense alkanethiol SAMs on Au28 and GaAs29,

30

substrates if

deposition was carried out in ethanol-water solutions. In this paper, we investigate kinetics of formation of mixed alkanethiol SAMs precipitating from ethanol and ethanol-water solutions on GaAs (001) substrates. The information about


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composition, density and conformation of the investigated SAMs was deduced from the Fourier Transform Infrared (FTIR) absorption spectroscopy measurements. The role of water was investigated on the process of formation of high quality SAMs mixed of 16mercaptohexadecanoic acid (MHDA) and –OH terminated 11-mercapto-1-undecanol (MUDO) thiolates. We carried out a semi-quantitative analysis to determine optimal MHDA/MUDO ratio for achieving best quality alkyl chain length and terminal group dependent SAM. The interest in having MHDA thiols is that, after an activation process involving formation of highly reactive esters, they provide amide bonds with proteins, while the MUDO thiols are expected to prevent non-specific interactions due to their significantly weaker affinity with proteins.31

2.

Experimental

2.1

Chemicals

Undoped (semi-insulating) double side polished, 617 µm thick, GaAs (100) ± 0.5° wafers (AXT, Inc) were used for Fourier-transform infrared (FTIR) transmission measurements. Semiconductor grade OptiClear (National Diagnostics, USA), acetone (ACP Chemicals, Canada), anhydrous ethanol (EtOH, Brampton, Canada) and ammonium hydroxide (28 %, Anachemia, Canada) were used as received. Degassed ethanol solution (typically 250 mL) was prepared by flushing with a 3 SCFH high-purity nitrogen (99.9995 %) stream (Praxair, Canada) for 3h. 16-mercaptohexadecanoic acid (MHDA, 90 %), 11-mercapto-1-undecanol (MUDO, 97 %) and 11-mercaptoundecanoic acid (MUDA, 95 %) were purchased from Sigma Aldrich (Canada). N-hydroxysuccinimide (NHS) and 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide hydrochloride (EDC) included in the Amine Coupling Kit (GE Healthcare Life Sciences) were


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diluted in deionized water (18.2 MΩ resistance) at 0.1 M for NHS and 0.4M for EDC. After solubilization, reagents were separately aliquoted in 250 µL tube and stored at -20 °C.

2.2

Sample and monolayer preparation

Standard protocol for samples preparation. Prior to SAM deposition, 4 x 4 mm² samples of GaAs (100) were cleaned in ultrasonic bath sequentially with OptiClear, acetone and EtOH for 5 min each. After drying, samples were immersed in concentrated ammonium hydroxide for 2 min to remove native oxides. Then, the samples were rinsed with deoxygenated EtOH and immediately incubated in 2 mM thiol solutions. Alkanethiols (MHDA, MUDO and mixture of both) were dissolved in degassed anhydrous ethanol and then sonicated for few minutes to get a homogenized solution. For mixed SAMs experiments, MHDA and MUDO molecules were mixed together in ethanol solution at different ratios, while total concentration was kept at 2 mM. The kinetics of self-assembling was investigated at room temperature by incubating the samples in darkness kept from 15 min to 72 h. After immersion, all samples were rinsed thoroughly with anhydrous ethanol followed by an ultrasonic cleaning for 30 s in ethanol to remove, as much as possible, all physisorbed thiols. Finally, samples were dried under nitrogen gas and immediately stored in dried, clean and sterilized individual Eppendorf Tubes©. Samples were kept in this condition for less than 30 min before the characterization. To investigate the influence of water environment on formation of SAMs, pure alkanethiols (MHDA, MUDO, MUDA) were dissolved in ethanol:water 90:10, 80:20, 67:33 and 50:50 (v/v) solutions, while mixtures of MHDA and MUDO were dissolved only in ethanol:water 2:1 (v/v) solution. A series of MHDA SAM coated samples were prepared for X-ray photoelectron spectroscopy (XPS) analysis of surface impurities and compared to those in a reference GaAs sample. After 20 hours incubation


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in thiol solutions, samples were rinsed with deoxygenated EtOH. Next, they were placed in 1.5 ml Eppendorf tubes filled with deoxygenated EtOH and sonicated for 30 s to remove physiosorbed thiol molecules. Following that, the samples were dried with ultra-pure nitrogen gas and transported in a bag filled with ultra-pure nitrogen for installation in a XPS chamber. The reference sample, after standard cleaning, ammonium hydroxide etching and rinsing with deoxygenated EtOH, was left for 20 hours in deoxygenated EtOH, after which it was dried with ultra-pure nitrogen gas and transported in a bag, same as the thiolated samples. Activation process. SAM coated samples were immersed for 30 min in mixed NHS (0.1 M) and EDC (0.4 M). Aliquoted reagents of EDC and NHS were thawed and the solution was used directly after mixing both reagents (unstable over time). After activation, unreacted NHS and EDC were removed by rinsing the samples with ultrapure water and drying with gas nitrogen for immediate characterization. Thiol replacement procedure. In order to investigate a thiol replacement phenomenon in our (MHDA)x(MUDO)1-x mixtures, FTIR absorption spectroscopy was carried out for MHDA SAM samples immersed in MUDO thiol solutions, and MUDO SAM samples immersed in MHDA thiol solutions. MHDA coated GaAs samples were prepared according to the standard thiolation procedure (20 h of immersion) and then incubated for 1 h, 2 h, 6 h and 25 h in MUDO thiol solutions (concentration was kept at 2 mM). Absorbance of CH2as vibrations was plotted as a function of the incubation time.

2.3

FTIR, XPS and opical density measurements

Attenuation total reflection FTIR measurements. Attenuated total reflection FTIR spectrometer (ATR-FTIR, ABB Bomen) was used to analyze powder of thiols. Spectrometer was


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equipped with an ATR Diamond kit and IR signal was measured with a deuterated triglycine sulfate (DTGS) thermal detector. The spectra were taken at 4 cm−1 resolution with the accumulation of 64 scans over 4000-600 cm−1 spectral range. Before analysis, the diamond crystal was cleaned consecutively with acetone and ethanol. Transmission FTIR measurements. Transmission spectra of chemically functionalized GaAs samples were recorded with Bruker Vertex 70v spectrometer equipped with a RockSolid interferometer and a wide range Globar IR source covering 6000 to 10 cm-1. The signal was collected with a liquid nitrogen cooled MCT (Mercury Cadmium Telluride) IR detector. The probing spot size was approximately 4 mm in diameter, the spectral resolution was set to 4 cm-1, all measurements were carried out under vacuum and spectra were collected with 512 scans. Spectra of SAMs were subtracted from the spectrum of a freshly etched GaAs (100) sample. This operation was performed to get the data only from SAM, and not from a spectrum containing the substrate signal and possibly the signal corresponding to atmospheric contaminants. The instrument wavelength precision is around 0.01 cm-1 (see Supporting Information for an example of the procedure applied for extracting information about νCH2 stretching modes of MHDA SAM from the FTIR spectra). For the thiol replacement experiment (discussed in Sec. 3.4), a homemade Automated Rotation Wheel (ARW) accessory was employed to carry out measurements of up to 12 samples, without venting the system. Spectra, averaged over 1024 scans, were collected after a 24 h evacuation with a 2 min waiting time between the onset of the measurements for consecutive samples. This approach offers the possibility to acquire within short time multiple samples with a high number of scans and without residual interference due to atmospheric water and carbon dioxide.


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IR spectra post processing. The FTIR spectrometer was interfaced with a computer using OPUS Bruker Optics software that provided all tools necessary for spectral analysis and processing of experimental data. Spectra were leveled out using a manual baseline correction function and taking care to select points outside of the absorption bands and peaks. Peaks were fitted by using 60/40 Gaussian/Lorentzian function as described by Parikh and Allara.32 XPS measurements. The analysis of samples for the presence of surface impurities was carried out in a vacuum chamber of the XPS (Kratos Analytical, AXIS Ultra DLD) system with a base pressure of 1x10-9 Torr. The data were collected for a takeoff angle of 60° with respect to the surface normal. The analysed area was an oval of dimensions 300 µm x 700 µm. The surface survey scans were observed with a 150 W Al Kα source operating in constant energy modes at 160 eV pass energy. A charge neutraliser was used on all samples to compensate for the charging effect. Charge corrections were done using the adventitious saturated hydrocarbon at the peak energy of 285.0 eV. The XPS results were analyzed by CASA XPS 2.3.18. The relative sensitivity factors used for quantification purpose are the experimental values given by Kratos Analytical for their instruments. Optical density measurements. Optical density (OD) of thiol solutions in ethanol and mixtures of ethanol and water were measured in triplicate with a Fisher Scientific spectrophotometer operating at 600 nm wavelength with a 40 nm bandwidth. Thiols were prepared in EtOH:H2O mixtures at 70:30, 65:35, 60:40, 55:45, 50:50, 45:55, 40:60 (v/v) and mixed using a vortex mixer before measurements.


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3.

Results and Discussion To identify specific IR peaks for the thiols of interest, we carried out FTIR absorption

measurements of MHDA and MUDO powders, and related spectra were compared to those of MHDA and MUDO SAMs on GaAs.

3.1

Thiol components and related FTIR peaks in powders and SAMs

The spectra of MHDA powder and SAM phase are shown in Figure 1a and 1b respectively, while those of MUDO powder and SAM phase are shown in Figure 1c and 1d, respectively. These data help to determine specific peaks of each chemical group of thiol molecules. The IR peak assignments are listed in Table 1, where positions for SAMs taken from literature are compared with powder spectra and SAMs obtained in this work. The powder spectra showed νCH2 stretching mode peaks located at 2911.5 and 2849 cm-1 for MHDA and 2914.9 and 2849 cm-1 for MUDO, assigned to respectively asymmetric and symmetric vibrations. In the SAM phase, the MUDO peaks are located at slightly higher frequencies of 2924.4 and 2852.2 cm-1 in comparison to those of the MHDA frequencies of 2921 and 2850.3 cm-1. This suggests a poorer crystallinity of the MUDO SAMs.33 The natural tendency of short alkyl chain thiols towards exhibiting a relatively weaker conformational ordering and packing compared to long alkyl chain thiols due to decreased intermolecular interactions has been evidenced in literature.15,

34

Thus, a short chain SAM could contain a relatively larger number of defects,

resulting in a less efficient passivation of the GaAs surface,35 as suggested by the presence of a νAs-O peak at 800.6 cm-1 observed in the spectra of etched GaAs36, 37 and in the MUDO SAM spectra reported in Fig. 1b. We note that the appearance of this peak has been observed on a


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clean GaAs (001) surface after 8-day storage in the ultra-high-vacuum chamber.36 The MHDA SAM spectrum shows the shift of the νC=O stretch band to lower frequencies from 1694.8 (powder spectrum) to 1562.1 – 1457.3 cm-1. This shift reveals the ionization of the carboxylic acid to corresponding νCOO- asymmetric and symmetric stretches. MHDA carboxylic acid is deprotonated in a neutral environment with pH greater than the pKa value of 6.85, as reported for MHDA SAM on gold.38 Méthivier et al. observed also a deprotonation of MUDA molecules adsorbed on Au when immersed and rinsed in ethanol.39 A weak feature at 1695.27 cm-1 appears corresponding to a small quantity of unionized species.33,

40

For SAM spectra, OH bending

modes are screened by H2O vibrations, therefore the νC-OH, νC-O and νC-S features are not visible. The presence of a small amount of those bonds on the surface, combined with a weak intensity of related vibrations, makes these absorption features undistinguishable from the background or hidden by unintended absorptions due to, e.g., residual water or carbon dioxide.


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Figure 1. Infrared spectra of pure MHDA and MUDO thiol powders (a and c) and SAMs on GaAs (001) substrates (b and d). Powders and monolayers were analyzed respectively by FTIR-ATR and FTIR transmission.


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Table 1. Methylene (CH2) and carboxyl (COOH) absorption peaks positions for MHDA and MUDO powders and SAMs formed on GaAs (001). MHDA - Wavenumber (cm-1) SAM our data

SAM literatureb

powder

OH str

3050

~3034

ω-CH2 asym

2930

2931

α-CH2 asym

2925

2925

CH2 asym

2919

2911.5

ω-CH2 sym FR

2900

2900

α-CH2 sym FR

2890

2890

CH2 sym FR

2890

-

ω-CH2 sym

2860

-

α-CH2 sym

2862

2870

CH2 sym

2850

2849

1741, 1767

1755

C=O str, acycl dimer

1718, 1745

-

C=O str cycl dimer

1699-1728

1695

1695

COO- asym

1550-1555

-

1562

COO- sym

1423-1430

-

1457

Vibrationa

C=O monomer

C-OH str

str,

2921

MUDO - Wavenumber (cm-1) SAM our data

SAM literaturec

powder

3425.3

3420.8

29192927

2914.9

2924.4

28522855

2849

2852.2

10601082

1060

2880

2850.3

1473 1473 1467 CH2 def a α- and ω-methylene are adjacent respectively to the thiol and the carboxylic acid group, FR: Fermi resonance, asym: asymmetric, sym: symmetric, str: stretching. b Arnold et al.33. c Frutos et al.41 and Morales Cruz et al.40


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3.2 Impurities in SAM coated samples XPS measurements, employed to analyze surface impurities in MHDA SAM coated GaAs (001) samples, revealed that the XPS peaks were dominated, as expected, by C, O, Ga and As atoms (see Supporting Information). In addition, we detected the presence of Ba at < 2 at.%. We speculate that this impurity originates from bulk GaAs, in agreement with the results of background impurities detected in this material.42 The presence of C 1s peak in the etched GaAs sample (31 at.%) has been assigned to the adventitious saturated hydrocarbon that has frequently been observed in such material.43 However, the significantly greater concentration of C in the sample coated with MHDA SAM formed in ethanol (~ 41 at.%) and, especially, in the sample with MHDA SAM formed in ethanol:water at 2:1 (v/v) solution (~ 50 at.%) is consistent with the increased contribution from hydrocarbons present in MHDA SAMs, and the increased surface coverage achieved in the presence of water, as reported by us earlier.29 In addition, we detected the presence of a small amount of Zn in samples with SAMs fabricated in the ethanol/water solution (0.37 at.%). This has prompted us to suspect that this impurity could be attributed to the presence in deionised water used in our experiments. Indeed, it has been reported that Zn could leak from glass and plastic labware,44 and to be attracted especially by COOH terminated thiols.45 The relatively small concentration of Ba and Zn impurities detected with XPS measurements suggests that they play a negligible role in SAM formation, however a detailed study is required to verify this hypothesis.

3.3 Mixed MHDA/MUDO monolayers on GaAs substrate


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The characterization of mixed SAM of MHDA and MUDO by monitoring the proportion of the features corresponding to νC=O vibrations of the –COOH molecules is challenging. However, considering the difference of chain length of both compounds, the absorbance of νCH2 asymmetric and symmetric stretch vibrations should give an idea of the proportion of MHDA and MUDO, with the position of related peaks indicating the conformation and organization of a mixed SAM structure. We evaluated formation of (MHDA)x(MUDO)1-x SAMs based on the FTIR absorbance and positions of the methylene absorption peaks. The absorbance of (MHDA)x(MUDO)1-x mixed SAM in function of the concentration, x, is plotted in Fig. 2a. The results indicate a two-slope behavior, with an inflection point around x = 0.5, which seems to be related to a qualitatively different organization kinetics of predominantly either short or long chain thiol mixtures. It seems that the shorter thiol (MUDO) adsorbs faster on the surface leaving less space for adsorption of COOH terminated thiols. In contrast, when the MHDA become predominant, the carbon presence increases at a faster rate. For comparison, we plot in this figure the n dependent absorption curve for methyl-terminated SAMs (n is equivalent to the number of CH2 in the molecule) taken from Marshall et al.,32 34where CH2 asymmetric absorbance at n = 16 is normalized over the absorbance of pure MHDA SAM obtained in this work. The linear curve obtained in that case exhibits a weaker slope than our results for x ≥ 0.5. This difference could be related to the presence of different terminal groups that play role in the organization of SAM CH2 chains as reported by Ding et al.46 They claimed that hydrophobic terminal groups will provide a more ordered and a closely packed SAM explained by a small tilted angle of the methylene chains to the substrate normal, which is in contrast to hydrophilic terminal groups providing an inclined organization of the chains constituting the SAM. A greater number of defects in mixed SAMs is expected to result in a weaker FTIR absorption. Thus, the increased


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contribution from MHDA molecules is expected to provide a better packed (organized) SAM. Thus, the higher the ratio of long-chain to short-chain thiols the better crystallinity and enhanced quality SAM could be expected. Figure 2 illustrates a dependence of the absorbance and position of the νCH2as peak on chemical composition of the investigated thiol solutions. Each experimental point in this case was obtained for a 20-h incubated sample. The shift of νCH2 to lower frequencies with x approaching 1 (100 % MHDA) is likely related to an increasing contribution from a better-organized SAM. It can also be seen that due a relatively weak absorbance of MUDO, the fitting procedure for MUDO dominated SAMs (x < 0.5) is charged with greater error bars.

Figure 2. (a) Absorbance and (b) position of νCH2as peaks versus composition x of (MHDA)x (MUDO)1-x mixed thiol solutions in EtOH:H2O at 2:1 (v/v). Absorbance results are compared to data from Marshall et al.34 The enhancement of carbonyl vibration peak intensities with increasing x could be used, potentially, to quantify the absorption originating from MHDA SAMs. These moieties, however, were either ionized or hidden by residual water vibrations. Unlike IR reflection-adsorption


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spectroscopy,33, 40 polarization-absorption spectroscopy39, 47 or grazing angle48 methods applied to Au surfaces, where the high sensitivity to molecular layer could be reached, the FTIR absorption of SAM-GaAs interfaces was not sufficiently accurate for this purpose. Therefore, to evaluate the number of available –COOH groups, we modified chemically MHDA SAMs designed to enhance absorption from νC=O vibrations.

Figure 3. (a) Infrared spectra of activated COOH group of MHDA and (MHDA)0.5(MUDO)0.5 SAMs on GaAs and (b) absorbance of νC=O and νCH2as versus x. N-hydroxysuccinimide (NHS) is the most common reagent applied to activate carboxylates, which is designed to get amine-reactive crosslinking.49, 50 This activation reagent is often used in combination with an EDC reagent to get a highly reactive acidic intermediate. Such cross-linker has frequently been used to immobilize antibodies. In our case, an EDC/NHS mixture was used to get enhanced absorbance from C=O vibrations, knowing that the νC=O of carboxylic acid overlaps with that of the activated ester. In Figure 3a, this feature appears at ~ 1740 cm-1 for both MHDA and mixed (MHDA)0.5(MUDO)0.5. Based on the data plotted in Figure 3b, the absorbance ratios of νCH2as to νC=O for x = 0.1, 0.5 and 1 were estimated at 83 %, 74 % and 51 %, respectively. This indicates that an increased contribution from activated carboxyl was


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obtained with the MHDA presence increasing in the monolayer. The ester molecules, due to their small size, may react more efficiently with carboxyl groups when the proportion of MHDA to MUDO is weaker. Indeed, it has been reported that in pure COOH terminated SAMs the ester formation could hardly yield full surface activation.49 Monitoring of the amount of carboxylic acid in the SAM by this way is therefore not obvious because the number of activated groups doesn’t correspond to the number of MHDA in the SAM. Indeed, when the concentration of MHDA in a solution decreases 10 times (from x = 1 to 0.1), the amount of the activated group decreases only 1.86 times. This suggests that the efficiency of the activation procedure increases with decreasing x, and even for a relatively low proportion of MHDA in the mixed SAM, the number of reactive sites (e.g. for protein binding) remains relatively large.

3.3

Water effect on formation of hydroxyl and carboxylic acid terminated alkanethiol

monolayers

Figure 4. νCH2 region of MHDA, MUDO and MUDA monolayers formed in EtOH and EtOH:H2O (2:1) mixtures (a), and a schematic idea of enhanced density MUDA, MUDO and MHDA SAMs formed in EtOH:H2O (2:1) solutions (b).


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Figure 4 shows FTIR spectra of the methylene region for MUDA, MUDO and MHDA alkanethiol SAMs obtained in either ethanol or ethanol/water (2:1 v/v) solutions. Three samples for each of these processes were measured, which allowed determining experimental errors reported in Table 2. It can be seen that peak position wavenumbers of SAMs incubated in water solutions decreased, depending on the molecules, by 1.2 to 2.9 cm-1. Furthermore, an increase of the asymmetric and symmetric absorbance peaks by 2x, 2.1x and 1.34x is observed, respectively, for MHDA, MUDO and MUDA monolayers grown in ethanol/water solutions. Comparable molecules were studied to understand the effect of the terminal group and the chain length of molecules in formation of SAM in water/ethanol. MHDA and MUDA (same –COOH terminated group) molecules were chosen to compare the chain length impact on this phenomenon, whereas MUDO (-OH terminal group) and MUDA were selected to understand the end-group effect. For the same –COOH terminal groups, the longer chain (c16) MHDA SAMs incubated in an ethanol/water solution exhibit a greater increase of the absorbance than the c11 chain MUDA SAMs. This underlines the role of intermolecular forces between alkane chains in formation of high quality SAMs, in agreement with the observation that the intermolecular forces become increasingly important in longer alkyl chains.51 On the other hand, it is known that molecules with longer hydrocarbon chains tend to be non-polar and their solubility in water decreases. Thus, the structure of alkanethiol SAMs could be influenced by the water moderated interactions between hydrocarbon chains through the van der Waals forces that, under optimized conditions, lead to densely packed monolayers with lower density of gauche conformations.27


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Table 2. Absorbance and wavenumber values of MHDA, MUDO, and MUDA νCH2as selfassembled on GaAs in EtOH and EtOH:H2O (2:1) νCH2as Absorbance (x10-4) νCH2as Wavenumber (cm-1)

MHDA in EtOH

8.44 ± 1.34

2921.8 ± 1.0

MHDA in EtOH/H2O (2:1)

17.40 ± 3.16

2918.9 ± 1.0

MUDO in EtOH

2.34 ± 0.37

2923.9 ± 2.0

MUDO in EtOH/H2O (2:1)

4.85 ± 0.70

2921.9 ± 0.4

MUDA in EtOH

2.71 ± 0.34

2923.7 ± 1.9

MUDA in EtOH/H2O (2:1)

3.62 ± 0.74

2923.4 ± 2.2

Results reported in Table 2 suggest that –OH terminated thiols (MUDO) exhibit slightly less dense monolayers when grown in ethanol compared to –COOH terminated molecules. The influence of water on the quality of synthesized SAMs is more significant for MUDO SAMs, showing a 2X enhanced FTIR absorbance, while MUDA SAMs show only 1.3X increased absorbance. This could be related to the total energy of hydrogen bonding interactions for carboxylic acids being greater than that for compounds containing hydroxyl group, because they are both hydrogen-bond acceptors (-C=O) and donors (-OH). For water-free solutions, Nuzzo et al. claimed that 50 % of neighboring COOH groups are bonded,48 whereas Li et al. suggested that up 100 % of such molecules could become bonded.52 These authors claimed that the formation of head to head COOH dimers lead to a highly oriented SAM, although Arnold et al.33 argued that this favorable outcome is obtained for SAMs prepared in low concentration of molecules, whereas high concentration (≥ 0.5 mM) will lead to a disordered SAM. Furthermore, Kim et al.53 suggest that carboxylic acid terminated thiols are more stable due to additional lateral hydrogen bonding of their terminal group. At high carboxylic acid concentration in the


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SAM and in presence of humidity, simulations carried out by Szori et al.54 show formation of clusters of adsorbed water molecules bridging between COOH groups. Thus, in an aqueous solution, carboxylic acid can interact more freely with water, forming so called hydrogen-bond barrier on top of an SAM layer, induced by intermolecular H-bonding between COOH groups and water molecules (see Figure 4). This barrier could be a limitation for thiol adsorption, partly explaining a weaker increase of carbon vibrations when MUDA SAM is formed in EtOH:H2O solution compared to MUDO SAM. The role of water environment has previously been reported during formation of –COOH terminated MHDA SAMs (2.38X enhanced FTIR absorbance).29 The almost similar influence of water has been observed for the same c16 chain length -CH3 terminated HDT SAMs (1.73X enhanced FTIR absorbance).30 This illustrates that water plays also important role in moderating intermolecular interactions extending beyond those involving thiol terminal groups.

Figure 5. Dependence of FTIR absorbance and wavenumber of νCH2as peaks on the concentration of H2O in EtOH/thiolate solutions employed for formation of MHDA (a), and MUDO (b) SAMs.


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Dependence of the FTIR absorbance and wavenumber of νCH2as peaks for MHDA and MUDO SAMs on the concentration of H2O in EtOH thiolate solutions is shown in Figure 5. It can be seen that the increasing absorbance is correlated with a shift towards lower frequencies of the methylene peak in proportion to the percentage of water in thiolate solutions used for both molecules. The absorbance of MHDA and MUDO SAM νCH2 peaks increases quasiquadratically with the concentration of H2O, until a maximum is reached near 35 % for MHDA SAMs, and near 50 % for MUDO SAMs. The improvement of crystal structure of SAM, indicated by a decreasing wavenumber of the νCH2 peak position, follows a similar behavior. The decreased absorbance observed in this work for MHDA SAM incubated in EtOH/H2O solvent comprising 50 % of H2O (see Figure 5a) seems coinciding with a loss of the thiol solubility for H2O concentrations exceeding 35 % (see Figure S3 in Supporting Information). In contrast, the MUDO SAM absorbance increases for all the investigated solutions, including that of EtOH:H2O at 50 %:50 % (see Figure 5b). Indeed, the MUDO molecules are more polar and do not seem to lose their solubility up to 60 % of water (see Supporting Information). The role of water in the formation of increased quality SAMs on GaAs, in addition to moderating the inter-alkane chain29, 55 and thiol-substrate interactions, could also be related to the modification of the chemical state of the GaAs surface involving dissolution of As-oxides that are formed on the surface of this material.29, 30, 56


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Langmuir

Figure 6. FTIR absorbance (a) and position of νCH2as peaks (b) versus concentration, x, of (MHDA)x(MUDO)1-x SAMs in EtOH:H2O at 2:1 (v/v) solutions. The FTIR νCH2as absorbance and the peak positions for SAMs consisting of (MHDA)x(MUDO)1-x mixtures formed in EtOH:H2O at 2:1 (v/v) solution are shown in Figure 6. It can be seen that MUDO SAMs, characterized by the absorbance of ~7.5 x 10–4, form from water solutions containing up to 50 % of MHDA. The presence of water seems to enhance the process of MUDO monolayer formation, probably leaving fewer interstices for the absorption of slower immobilizing MHDA molecules. Above the inflection point, defined by x = 0.5, mixed SAMs are formed with increasing contribution of MHDA, as indicated by the absorbance increasing to 1.95 x 10-3 in the EtOH:H2O solution. The increase of the absorbance observed for SAMs incubated in an EtOH/H2O solution, in comparison to that in EtOH, seems to be related to the formation of an enhanced density MHDA network that dominates formation of a mixed SAM. A decreasing energy of νCH2as vibrations observed in Figure b for x ≥ 0.5 indicates formation of an increased quality SAM. In Figure 7 we present a schematic idea of the contribution from MUDO and MHDA molecules to the formation of mixed SAMs in either EtOH or EtOH:H2O at 2:1 (v/v) solutions. It


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appears that in EtOH solutions, the increasing presence of MHDA in mixed SAM is compensated by a decreasing presence of MUDO. However, in EtOH/H2O solutions, the presence of MUDO thiolates dominate mixed SAMs, until the deposition is carried out from an EtOH/H2O solution containing at least 50 % of MHDA thiol. For solutions with greater concentrations of MHDA, mixed SAMs become rapidly dominated by the presence of MHDA thiolates. This behavior seems to be related to different deposition kinetics and solubility of various size, shape and concentration of molecules involved in the formation of mixed SAM.

Figure 7. Schematic idea of MHDA/MUDO proportion in mixed SAMs versus their ratio in solution for monolayers prepared in EtOH and EtOH:H2O at 2:1 (v/v) solutions.

3.4

Kinetics of MHDA/MUDO mixed SAMs formation

As reported by numerous studies, the mechanism of self-assembling is described by a rapid adsorption regime followed by a slow organization phase where the surface density of thiols continues to increase.57, 58 Compared to Au surfaces, the adsorption kinetics under vacuum could be faster on a clean GaAs surface.59 However, if deposition is carried out from liquid solutions,


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GaAs offers conditions for SAM formation that require a longer period for the initial phase. For instance, McGuiness et al.58 estimated that it takes several hours for 1-octadecanethiol (CH3(CH2)17SH, ODT) initial adsorption on GaAs(100), which compares with several minutes for ODT adsorption on Au. The authors explain this difference by the presence of residual oxides on the GaAs surface that need to be removed before strong S-As or S-Ga bonds could be formed.60, 61

Figure 8. Kinetics of self-assembling MHDA (circles), MHDA:MUDO (1:1) (squares), MHDA:MUDO (1:9) (triangle up) and MUDO (triangle down) on GaAs substrate (a), and derivatives of the respective Langmuir fits over time to illustrate the saturation behavior (b). In Figure 8 we present (a) incubation time dependent FTIR absorbance data for the asymmetric C-H stretching modes characterizing MHDA, MHDA:MUDO (1:1), MHDA:MUDO (1:9) and MUDO, and (b) plots of the time dependent derivatives of the curves describing data points based on the Langmuir adsorption and rearrangement model. The experimental points, for MHDA and MUDO SAMs were fitted with a two-term equation:62, 63, 64


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= 1 −

− (1 − )

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where A0 denotes the limiting absorbance of a completely organized monolayer, Aads and τads are, respectively, the coefficient and the rate constant of the adsorption phase, and τrea is the rate constant of the slow rearrangement phase. The coefficient of the rearrangement (the second phase) is replaced by (1-Aads) to satisfy boundary conditions at t = 0 h. The Aads coefficient for MHDA is equal to 0.51± 0.09, which is close to the 0.43 value reported by Kim et al.62 The rate constant related to the adsorption is 0.11 ± 0.09, and the rearrangement constant is 2.62 ± 0.98. These values are relatively close, respectively, to those of 0.30 and 4.6, reported by Kim et al.62 The fast adsorption rate and low absorbance of MUDO molecules make it more challenging to determine related coefficients by a fitting algorithm. The monolayers formed from different thiol solutions reach saturation levels (A0) between 2.93x10-4 for MUDO and 7.98x10-4 for MHDA molecules. The absorbance at t = 20 h has frequently been considered in literature as the incubation time needed to reach saturation for MHDA SAM (e.g., Ref.

13

, and references

therein). However the results presented in this report suggest that the full saturation may require as long as 40 h. The plot of time dependent derivatives in Figure 8b indicates that the fastest completion of the adsorption phase takes place for MUDO SAM, followed successively by MHDA 10 %, MHDA 100 % and MHDA 50 % SAMs. The large dispersion obtained for the mixed SAMs suggests the presence of competition between adsorption and desorption of different molecules. The completion of the adsorption phase in solutions where thiols are in equal amount is significantly faster in comparison to that in pure MHDA or MUDO solutions. The mixed MHDA:MUDO (1:1) curve does not saturate after 70 h of immersion in the thiol solution. Kim et al. and Baralia et al. showed the substitution of 1-octanedecanethiol (ODT)


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SAM by respectively MUDA53 and MUDO65 on gold substrates. Their results demonstrate a time-dependent process of thiol replacement phenomenon. Kim et al. showed also that the rate of replacement is not the same in the reversed process, where MUDA is replaced by ODT.53 This phenomenon explains why the completion of the adsorption phase is increased when thiols are mixed together at the initial stage. To investigate further this problem, we have designed an experiment where mixed SAMs were formed in a two-step procedure: (1) adsorption of a homogenous SAM (thiol A), and (2) partial replacement in the SAM with a second molecule (thiol B). Figure 9 shows FTIR absorption and νCH2as peak positions for a series of GaAs (001) sample, that following a 20 h deposition of MHDA SAMs, were exposed to a MUDO thiol solution for 1, 2, 6, 25 and 45 hours. This experiment, designed to investigate the kinetics of thiolate replacement, revealed the presence of two distinct phases. The first phase (t ≤ 8 h), describes a fast decreasing absorbance correlated with the peak position shifting toward higher frequencies. This could be explained by the adsorption of MUDO at the expense of MHDA molecules. In the second phase (t > 8 h), a slow decrease of both absorbance and the νCH2as peak position is observed, which suggests the onset of organization of a new thiol architecture. At this stage, the absorbance tends to approach a typical MUDO absorbance value, while the νCH2as peak position continues to decrease. At 45 h, the position of νCH2as at 2922.3 cm-1 corresponds to the lowest value observed for mixed MHDA/MUDO or MUDO SAMs (see Figure 2).


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Figure 9. Kinetics of MHDA SAM replacement in MUDO thiol solution monitored by the absorbance and position of the νCH2as peak (a), and comparison of the absorbance of νCH2as and νCOOH vibrations for 0, 6 and 25 hours of immersion (b). Figure 10 shows FTIR absorption and νCH2as peak positions for a series of GaAs (001) samples that, following a 20 h deposition of MUDO SAMs, were exposed to MHDA thiol solutions for 25, 45 and 65 hours. A continuously increasing absorbance versus immersion time is observed in this case. Furthermore, a significant shift of the νCH2as peak position towards low frequencies is observed, indicating formation of a better organized mixed MHDA/MUDO SAM. The phenomenon of replacement taking place for MHDA or MUDO SAMs immersed in the opposite thiol solution does not seem to be “symmetrical”. The 45 h immersion of MHDA coated samples in a MUDO solution resulted in an almost entirely disappeared FTIR absorption related to the COOH stretch vibration, showing also a relatively low value of the νCH2as stretch vibration (see Figure 9). The absorbance of MUDO coated samples, following the immersion in MHDA solution, increased due to the increasing presence of COOH stretch vibrations in the


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mixture (see Figure 10). However, after a 65 h immersion, the νCH2as absorbance has saturated at 4 x 10-4, a value significantly inferior to that corresponding to an MHDA SAM. This illustrates formation of a less dense mixed SAM than that obtained with the MUDO initially coated samples. Clearly, these results demonstrate the feasibility of formation mixed SAMs through the process of controlled substitution. Given a relative flexibility in designing structural composition of mixed SAMs, this approach could lead potentially to the fabrication of advanced architectures attractive, e.g, for biosensing applications.

Figure 10. Kinetics of MUDO SAM replacement in MHDA thiol solution monitored by the absorbance and position of the νCH2as peak (a), and comparison of the absorbance of νCH2as and νCOOH vibrations for 0, 25 and 65 hours of immersion (b).


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Conclusion We have investigated mechanisms of formation of mixed SAMs on GaAs (001) consisting of MHDA and MUDO alkanethiol compounds. The choice of these compounds is dictated by the potential application of MHDA/MUDO SAMs for constructing highly selective and efficient architectures for biosensing applications. The results presented in this paper highlight that enhanced molecular organization of mixed SAM is primarily driven by the water moderated intermolecular reactions. The most densely packed individual and mixed SAMs, as illustrated by the FTIR absorbance and energy of their CH2 vibrations, have been formed for ethanol: water solutions at 2:1 (v/v). The MUDO and MHDA SAMs grown from ethanol: water at 1:1 (v/v) solutions did not show enhanced characteristics, although MUDO SAMs exhibited increased FTIR absorbance. This seems to be related to the limited solubility of the investigated thiols in water. Analysis of mixed MHDA/MUDO SAM indicated a qualitative change of their structure at 50/50 (v/v) of MHDA/MUDO in ethanol or ethanol:water solutions. This seems related to the formation of a non-trivial double phase SAM through adsorption and rearrangement, combined with self-substitution of involved molecules and formation of a quasi-equimolar mixture of thiolates The contribution of –COOH terminated thiols to mixed SAMs was investigated by detecting C=O stretch vibration revealed by preforming the NHS/EDC reaction. This gives an enhanced IR signature of the NHS derivative compounds, and opens up novel possibilities of characterization –COOH terminated monolayers on GaAs. In addition to studying mixed SAMs deposited from (MHDA)x(MUDO)1-x thiol solutions, we have also investigated the thiol replacement phenomenon in formation of mixed SAMs. Given a relative flexibility of SAM formation by


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substitution, it appears that this approach could lead to fabrication of potentially superior quality mixed SAMs with increased number of binding sites used in architectures designed for biosensing applications. Such properties could also be important in offering enhanced protection against oxidation66 or photo-oxidation67 of metallic surfaces.

ASSOCIATED CONTENT Supporting information Example of the procedure applied for extracting information about νCH2 stretching modes in MHDA SAM from FTIR spectra, XPS analysis of impurities in etched GaAs and MHDA SAMs and evaluation of MHDA and MUDO solubility in aqueous ethanol by using optical density measurements.

ACKNOWLEDGMENT Funding for this research was provided by the region Franche-Comte (France), the Canada Research Chair in Quantum Semiconductors, the Natural Sciences and Engineering Research Council of Canada (NSERC) CRD project CRDPJ 452455 - 13 and the NSERC Discovery Grant RGPIN-2015-04448. This work was also partly supported by the French RENATECH network and the Labex ACTION “Integrated Smart System”. The authors thank Jean-Sebastien Binette (Université de Sherbrooke) for making available the ATR-FTIR setup and Sonia Blais (Université de Sherbrooke Centre de Caractérisation de Matériaux) for collecting XPS data.


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