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Vacuum Ultraviolet Treatment of Acid- and Ester-Terminated SelfAssembled Monolayers: Chemical Conversions and Friction Reduction Ahmed Ibrahim Abdelhamid Soliman, Toru Utsunomiya, Takashi Ichii, and Hiroyuki Sugimura Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.7b04327 • Publication Date (Web): 16 Feb 2018 Downloaded from http://pubs.acs.org on February 17, 2018

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Vacuum Ultraviolet Treatment of Acid- and EsterTerminated Self-Assembled Monolayers: Chemical Conversions and Friction Reduction Ahmed I. A. Soliman, Toru Utsunomiya, Takashi Ichii, Hiroyuki Sugimura* Department of Materials Science and Engineering, Kyoto University, Yoshida-hommachi, Sakyo-ku, Kyoto 606-8501, Japan

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KEYWORDS: COOH- and COOCH3-terminated Self-assembled monolayers (SAMs), UV hydrosilylation, 172-nm VUV light irradiation in a high-vacuum environment (HV-VUV), Trimming of oxygenated groups, Reductive photolithography, Friction reduction.

ABSTRACT

We have prepared COOH- and COOCH3 -terminated self-assembled monolayers (SAMs) from undec-10-enoic acid (UDA) and methyl undec-10-enoate (MUDO) molecules on hydrogenterminated silicon (H-Si) substrates through ultraviolet (UV) irradiation. The as-prepared UDAand MUDO-SAMs were exposed to 172-nm vacuum UV (VUV) light in a high vacuum environment (HV, < 10-3 Pa) for different periods. The presence of COO components at the surfaces of these SAMs without prior oxidation would simplify the understanding of the origin of the chemical conversions and the changes of surface properties, as the prior oxidation would change the surface properties and generate different oxygenated groups. After the HV-VUV treatment, the abundance of COOH and COOCH3 components of these SAMs decreased without significant dissociation of their C-C backbones. Degradation of these components occurred through dissociating their C-O bonds resulting in different C=O components. Also, the occurrence of Norrish type pathways resulted in a slight decrease of carbon content and produced CH3 components. We have applied the HV-VUV lithography to control the abundance of COOH and COOCH3 components in well-defined areas, and to investigate the friction differences between the irradiated and masked areas. The irradiated areas exhibited lower friction than the masked areas without observing significant height contrasts between these areas. The reduction in friction was attributed to the conversion of the COOH and COOCH3 components to less adhesive components such as C=O and CH3. These experiments suggest HV-VUV treatments as


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an approach for low damage dry surface modifications and reductive lithographic techniques at surfaces terminated by acid and ester groups.

INTRODUCTION Nowadays, surface modification techniques have acquired much attention for drawing the properties of polymers and self-assembled monolayers (SAMs). Such modifications allow these polymers and SAMs to be used in new applications such as electronic and information devices.1– 4

Photomodification techniques, where the surfaces are exposed to light sources such as

ultraviolet (UV) light, are commonly used for modifying the surfaces of polymers and SAMs, due to their ability to confine the modifications to the surface with negligible influences on the bulk.2–7 Many reports have discussed the usage of UV light, especially vacuum UV (VUV) light for surface modification and lithography.4–7 The irradiation environment and light photon energy influence the induced chemical conversions at the irradiated surfaces.6–8 In an ambient environment, the surface modification of organics through VUV treatment has been widely reported, in which their surfaces were functionalized with different oxygenated groups such as C-O, C=O and COO groups.2,4–9 VUV light can generate several oxidative species, such as ozone and atomic oxygen [O], by exciting the molecular oxygen. These oxidative species (O) can oxidize and degrade the irradiated organics. The functionalization and degradation of the organics during exposure to VUV light and (O) species (VUV/(O)) decreased with decreasing concentrations of oxygen in the irradiation environments.8,10 VUV/(O) treatment is widely used in lithographic applications, where the chemical and surface properties of the irradiated areas are changed, but those of the masked areas are preserved.7,8,11,12 In the absence of reactive species in the irradiation environment, the chemical conversions at the irradiated surfaces depend on the photon energy of the employed light.2,6,9,13 VUV light of ³


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172 nm can dissociate the C-O bonds of ethers and alcohols from their excited states (n®s* electronic transitions).2,14 The ≥ 172-nm VUV does not significantly dissociate the C-C skeletons of the irradiated surfaces, as the inducing of s®s* electronic transitions requires VUV light of £ 160 nm.15,16 The exposure of ³ 172-nm VUV light to polyethylene and polypropylene polymers did not significantly deplete these polymers.2,13,15,16 It has been reported that VUV exposure to poly(methyl methacrylate) (PMMA) in low pressure or inert environments cause depletion by dissociating the ester branches.10,13,14 The C=O containing organics are also dissociated under VUV light irradiation through Norrish type pathways, as VUV light can induce the n®s*, n®p* and p®p* electronic transitions.10,13,17–19 The exposure of VUV light to unsaturated polymers induces cross-linking reactions, thereby curing their surface.10,13,14,20 We previously reported the ability to trim the oxygenated groups from the VUV/(O)-modified SAMs through exposure to 172-nm VUV light in a high vacuum environment (HV). The C-C skeleton of these SAMs was not significantly degraded.6,9 The fabrication of well-defined areas terminated with oxygenated groups was performed via HV-VUV treatment of VUV/(O)modified SAMs from the slits of a photomask resulting in different friction between the irradiated and masked areas.9 Although UV/(O) and VUV/(O) treatments can functionalize the irradiated surfaces by generating different oxygenated groups, the type and abundance of these oxygenated groups were not easily controllable. Moreover, the VUV/(O) treatment of SAMs prepared on hydrogenterminated silicon (H-Si) substrate has been reported to damage the irradiated monolayers and the silicon substrate as well.5,6,8,9 The surface of the Si substrate was oxidized to SiO2 insulator components.4,5,8 To clarify the effects of the HV-VUV treatment on surfaces terminated with oxygenated groups, monofunctional-terminated SAMs on H-Si are expected to be good


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representative samples. The complexity of the presence of different oxygenated groups and the oxidation of silicon substrates after prior VUV/(O) treatment were avoided through using these SAMs, which would simplify the understanding of the origin of chemical conversions and the influence of these conversions on the surface properties after the HV-VUV treatment. Herein, undec-10-enoic acid (UDA) and methyl undec-10-enoate (MUDO) were used for preparation of COOH- and COOCH3-terminated SAMs through UV light irradiation, respectively. Formation of COOH- and COOCH3-terminated SAMs on H-Si substrate has been discussed in several reports, and the produced SAMs were highly-dense, environmentally stable, and well-defined.21–30 The water contact angle (WCA) values, thickness values, and morphological structure of the as-prepared UDA- and MUDO-SAMs were characterized by WCA goniometer, spectroscopic ellipsometry, and atomic force microscopy (AFM), respectively. The chemical constituents were characterized by attenuated total reflection-Fourier transform infrared spectroscopy (ATR-FTIR) and X-ray photoelectron spectroscopy (XPS). These SAMs were irradiated by 172-nm VUV light in an HV environment over 3600-sec. Chemical conversions and surface properties were followed, and the expected routes for these conversions were evaluated. Furthermore, we investigated the lithographic usage of HV-VUV treatment in fabricating the surface properties of these SAMs in well-defined domains. The friction differences between the irradiated and masked areas were investigated by using lateral force microscopy (LFM). Such dry and effective surface modification techniques may pave the way for low-damage reductive lithographic processing. EXPERIMENTAL METHODS Formation of UDA- and MUDO-SAMs. Formation of UDA- and MUDO-SAMs on the Si substrate was performed through UV light irradiation of the H-Si substrate in the presence of


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UDA and MUDO precursors, respectively. A phosphorous doped n-type Si (111) wafer (with resistivity of 1-10 Ω cm) was sonicated in ethanol (Nacalai Tesque, 99.5%) and in ultrapure water (UPW) for 20 min each, in that order. Afterwards, the substrate was VUV/(O) irradiated for 20 min to remove the adsorbed organic contaminations, and the VUV light was generated from a Xe excimer lamp (Ushio Inc., UER 20–172 V, l = 172 nm). The cleaned Si substrate was immersed in HF (5 %, Stella Chemif) for 5 min at room temperature, and then in NH4F (40 %, Daikin) for 30 sec at 80 °C to remove SiO2 layers, resulting in the H-Si substrate. The H-Si substrate was dipped into a photocell containing deoxygenated mesitylene solution (0.5 M) of UDA (Tokyo Chemical Industry (TCI), 98 %) or MUDO (TCI, 96 %) precursors. The substrate was UV irradiated for 1 h using a high-pressure mercury lamp (REX-250, Asahi Spectra). The irradiation process was performed in N2 purging, and the light intensity was 100 mW/cm2. The UDA treated samples were post-cleaned through sonication in mesitylene, acidified water, and in UPW for 5 min each, in that order. The MUDO treated samples were sonicated in mesitylene, ethanol, and in UPW for 5 min, in that order. HV-VUV treatment and patterning of SAMs. SAMs were transferred to a vacuum chamber where a Xe-excimer lamp (similar specification as the previous lamp) was fixed, as illustrated in Figure 1. After the pressure in the chamber reached < 10-3 Pa, VUV light irradiation was performed for different periods. The distance between the lamp window and sample was fixed at 10 mm. The light intensity at this distance was measured in the N2 environment to be » 13.8 mWcm-2. To perform the HV-VUV photopatterning, the samples were covered by a photomask (100-nm thick Cr patterns on a 2-mm thick quartz plate; 93% transparency at 172nm), as illustrated in Figure 1. HV-VUV treatment was performed through the slits of the photomask for 300 sec.


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Figure 1. Schematic illustration of the HV-VUV treatment apparatus and the patterning process. Analytical Methods. FTIR (Digilab Japan Co. Ltd., Excalibur FTS-3000) with a liquidnitrogen−cooled mercury cadmium telluride (HgCdTe) detector was used to investigate the functional groups of the as-prepared SAMs and after the HV-VUV treatment. The ATR mode was applied using GATR (Harrick Scientific Products, Inc.) made of Ge crystal, and the reflection angle was set at 65°. The spectral resolution and scan cycles were 4 cm-1 and 1024, respectively. During the measurements, the samples were pressed to the Ge crystal. XPS (ESCA3400 system, Kratos Analytical, Mg Kα X-ray source) was used to evaluate the chemical constituents of samples qualitatively and quantitatively. Measurements were performed at a pressure of < 5×10-6 Pa, and the obtained spectra were referenced to the Si-Si peak at 99.5 eV.5,6,8 WCA values of the as-prepared SAMs and after HV-VUV treatment were measured using a static contact angle meter (Kyowa Interface Science CA-X Co., DM 500). The volume of the water droplet was 1.8 µL. A spectroscopic ellipsometer (Otsuka Electronics FE-5000) was used to examine the ellipsometric thicknesses of the molecular layer using a model of air/organic


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film/Si. The refractive index of SiO2 was adopted as that of the monolayers. The SiO2 and the monolayers were assumed to be transparent across the measured wavelength range of 400-800 nm.5,6,31,32 The incident angle was set at 70°. AFM (MFP-3D, Oxford Instruments) was used to examine the surface morphology of the fabricated surfaces by applying AC dynamic mode using aluminum backside coated Si probes (SI-DF-40, Hitachi Hi-tech Co. Ltd.). Successful HV-VUV patterning was evaluated by determining the local frictional properties of samples after patterning using LFM with a platinum (Pt) coated probe (PPP-CONTSCPt, NANOSENSORS). The LFM characterizations were performed in an ambient environment, where the humidity was 34% (± 2).

RESULTS AND DISCUSSION Formation of UDA- and MUDO-SAMs. Figure 2 shows the ATR-FTIR spectra of UDA- and MUDO-SAMs obtained through UV light irradiation. The ATR-FTIR spectrum of UDA-SAM shows absorption bands at 2920 cm-1 and 2851 cm-1 as illustrated in Figure 2A, which correspond to the na(CH2) and ns(CH2) vibrations, respectively. The ATR-FTIR spectrum of MUDO-SAM shows absorption bands at 2957 cm-1, 2872 cm-1, 2924 cm-1, and 2853 cm-1, which are attributed to the na(CH3), ns(CH3), na(CH2), and ns(CH2) vibrations, respectively. The positions of na(CH2) and ns(CH2) absorption bands indicate that the UDA-SAM may be more compact than the MUDO-SAM.27,33 In UDA-SAM, the absorption band at 1464-1472 cm-1 is attributed to the ds(CH2) absorption band, and the absorption band at the 1412 cm-1 might be attributed to the d(O-H) absorption band as illustrated in Figure 2B.22 The ds(CH3) absorption band of MUDO-SAM allocates at 1377 cm-1, whereas the da(CH3) absorption band, which was expected to be observed at the range


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of 1435-1450 cm-1, overlapped with the ds(CH2) absorption band that was at 1464 cm-1.22,34 The broadband in the region of 1500-1700 cm-1 was ascribed to the bending vibrations of adsorbed moisture. This absorption band was not attributed to the n(C=C) vibrations of unreacted precursors because the spectra did not exhibit the distinctive band n(C-H) of vinyl structure, which was expected to be at > 3000 cm-1, as illustrated in Figure S1 (Supporting Information).34 The stretching absorption band of C=O components of UDA- and MUDO-SAMs appeared at 1715 cm-1 and 1742 cm-1, respectively. The band position of the C=O region was consistent with the previously reported values of COOH- and COOCH3-terminated monolayers, which indicates successful immobilization of UDA- and MUDO precursors without further reactions such as hydrolysis or anhydrous formation.21–23,25,27,33,35 The C-O and Si-C absorption bands were expected to be located in the region of 1130-1300 cm-1. The absorption bands of Si-O, Si-O-Si and Si-OH components were observed at wavenumbers of < 1130 cm-1 as illustrated in Figure S1 (Supporting Information), and the presence of these bands was attributed to the hydrolysis of the unhydrosilylated H-Si sites.22,25 The n(O-H) absorption bands were observed at wavenumbers of > 3000 cm-1, and the presence of these OH components was attributed to the presence of COOH components -in the case of UDA-SAM-, Si-OH components, and absorbed moisture.22,31,36 The presence of the stretching bands of C=O and C-O components and the absence of C=C-H components in UDA-SAMs suggest the occurrence of C=C hydrosilylation. Similarly, the presence of both C=O and CH3, and absence of C=C-H components indicates that the attachment of MUDO precursors occurred through the C=C hydrosilylation. Hence, both COOH and COOCH3 components of the used precursors did not react with H-Si, which is consistent with the previous reports.21–23,25


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Figure 2. (A) ATR-FTIR spectra of alkyl region with n(CH3) and n(CH2) bands of UDA- and MUDO-SAMs. (B) Carbonyl, alkyl, and oxide region of UDA- and MUDO-SAMs. Figure 3 shows the XPS C1s, O1s, and Si2p spectra of UDA- and MUDO-SAMs. The C1s spectrum of UDA-SAM (Figure 3A) was deconvoluted into three carbon components of C-C, CH2 adjacent to COOH, and COOH at binding energies of 285.0 eV, 286.8 eV, and 289.3 eV, respectively. The areas under these deconvoluted peaks are consistent with the abundance ratio of the corresponding components in the assembled UDA molecules. The XPS O1s spectrum (Figure 3B) was deconvoluted into three components of O-Si, C=O, and O-C at binding energies of 531.9 eV, 532.9 eV, and 533.9 eV, respectively.22 These XPS spectra illustrate the presence of COOH and C-O components in UDA-SAM, which is consistent with the ATR-FTIR results of UDA-SAM. The deconvolution of the XPS C1s spectrum of MUDO-SAM is shown in Figure 3C, in which different deconvoluted peaks of C-C, CH2 adjacent to COO, H3C-O, and COO appeared at binding energies of 285.0 eV, 286.3 eV, 287.6 eV, and 289.5 eV, respectively. The abundance of


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these components agrees with those of MUDO molecules as indicated from the areas under their corresponding deconvoluted peaks. The deconvolution of the O1s spectrum, which is illustrated in Figure 3D, revealed the presence of O-Si, O=C, and O-C components at 531.9 eV, 532.9 eV, and 534.3 eV, respectively. These results also illustrate the presence of C=O, O-CH3, and C-O components, which supports the observations from the ATR-FTIR results of MUDO-SAM. The presence of Si-O components in both UDA- and MUDO-SAMs was attributed to the hydrolysis of the unhydrosilylated H-Si sites, as indicated from ATR-FTIR spectra illustrated in Figure S1(Supporting Information).22 The higher carbon content of UDA-SAM (30%) than that of MUDO-SAM (23%) was not attributed to the presence of UDA dimers due to the absence of =C-H and C=C bands in the ATR-FTIR spectra of UDA-SAM. Therefore, the bulky COOCH3 groups at the surface of MUDO-SAM are expected to restrict the dense assembling of MUDO molecules. In other words, the prepared UDA-SAM is denser than MUDO-SAM based on the positions of n(CH2) absorption bands and the high carbon content. The SiO2 components were not observed in the XPS Si2p spectra of UDA- and MUDO-SAMs (Figure 3E), indicating that the UDA- and MUDO-SAMs protect the silicon surface from oxidation.


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Figure 3. Deconvoluted XPS C1s (A,C) and O1s (B,D) spectra of UDA-SAM (A,B) and MUDO-SAM (C,D). (E) XPS Si2p spectra of UDA- and MUDO-SAMs. Figure 4A shows clear terraces of H-Si separated by monoatomic steps of approximately ~ 0.3 nm in height, which is consistent with the reported morphological structure of H-Si (111) substrate.5,6,9,31,32 The topography of UDA-SAM without post-cleaning in acidified water is shown in Figure 4B, where the terraces were significantly contaminated with UDA aggregates. As previously reported, these aggregations at the surface of UDA-SAM were hydrogen-bonded


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UDA precursors with the terminal carboxyl groups of the assembled molecules.22,23,25,27 Cleaning the contaminated sample in acidified water was sufficient for removing the UDA aggregates, as illustrated in Figure 4C. Therefore, the high carbon content of UDA-SAM was attributed to having a higher density than MUDO-SAM. Figure 4D shows the clear terraces without any distortions or aggregations of MUDO-SAM. These topographic images demonstrate that the UDA and MUDO molecules were uniformly assembled on the terraces of the H-Si substrates through UV hydrosilylation.25,30

Figure 4. AFM topographic images of (A) H-Si, (B) UDA-SAM before cleaning in acidified water, (C) UDA-SAM after cleaning in acidified water, and (D) MUDO-SAM. The WCA values of UDA-SAM before and after cleaning in acidified water were 70.3º and 62.8°, respectively. The decrease of WCA values after cleaning in acidified water also confirms that the usage of acidified water was sufficient for removing the H-bonded UDA aggregations. The obtained WCA value of UDA-SAM after cleaning is in good agreement with the reported values of COOH-terminated monolayers on H-Si substrate.28,30,35 Recently, Guo et al. reported


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that the packing density and the formation of water-COOH composite structure through hydrogen bonding between the COOH groups and water would enhance the hydrophobicity of COOH-terminated SAMs.37 The WCA value of MUDO-SAM was 71.0°, and this value is in good agreement with the reported values of ester-terminated SAMs.30,33 The ellipsometric thickness values of UDA- and MUDO-SAMs were 2.09 nm and 1.93 nm, respectively, and these values were consistent with the expected thickness values.30,31 HV-VUV Trimming of the COO Components. The ATR-FTIR spectrum at the alkyl region of UDA-SAM, which is illustrated in Figure 5A, shows bands at 2926 cm-1 and 2855 cm-1, and these bands were assigned to the na(CH2) and ns(CH2), respectively. Compared with UDA-SAM before the HV-VUV treatment, as illustrated in Figure S2 (Supporting Information), these bands shifted to higher wavenumbers, indicating that the order of UDA-SAM may be disturbed after the HV-VUV treatment.9,38 A new absorption band at 2957 cm-1 was observed in the spectrum after the HV-VUV treatment, and this band was attributed to na(CH3).9 The presence of this band indicates that the CH3 components were generated during the HV-VUV treatment. Also, the absorption band of ds(CH3) was clearly observed at 1377 cm-1 as illustrated in Figure S3 (Supporting Information).The ATR-FTIR spectrum of MUDO-SAM after the HV-VUV treatment shows the absorption bands of na(CH3), ns(CH3), na(CH2), and ns(CH2) at 2957 cm-1, 2872 cm-1, 2926 cm-1, and 2857 cm-1, respectively. The slight shift in the na(CH2) and ns(CH2) bands after the HV-VUV treatment, as illustrated in Figure S2 (Supporting Information), may be due to the disorder of the assembled MUDO molecules.9,38 The C=O absorption bands of UDA- and MUDO-SAMs were shifted to 1711 cm-1 and 1730 cm-1, as illustrated in Figure 5B, respectively. The shift in the C=O absorption bands was likely due to the chemical conversions that occurred in the C=O containing components. The Si-O


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components may not be influenced by HV-VUV treatment, as the absorption bands corresponding to these components were still mentioned at wavenumber < 1130 cm-1, as illustrated in Figure S3 (Supporting Information). The ratio between the areas under the n(C=O) and n(C-H) bands (An(C=O)/An(C-H)) of UDA- and MUDO-SAMs significantly decreased after the HV-VUV treatment, demonstrating that the abundance of the C=O components significantly decreased.9 The chemical changes after different HV-VUV irradiation periods were investigated by XPS to confirm and enhance our understanding of the expected chemical conversions during HV-VUV treatment.

Figure 5. ATR-FTIR spectra of (A) alkyl and (B) n(C=O) regions of UDA- and MUDO-SAMs after 300-sec HV-VUV treatment. Figure 6A shows the deconvoluted XPS C1s spectrum of UDA-SAM after 300-sec HV-VUV treatment, where the intensity of COOH components significantly decreased. The intensity of C-C components did not significantly change. A new peak occurred at ~ 288.0 eV, and this peak was attributed to the C=O components.5,6,9 These chemical conversions were also reflected in the O1s spectra (Figure 6B), where the O-C component significantly decreased from the UDA-SAM


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(Figure 3B). The COOH components were not fully eliminated, and the O=C components in the XPS O1s spectra represent the C=O of COOH components and the newly produced C=O components.6,9,14 These observations may explain the slight shift in the n(C=O) band of UDASAM from 1715 cm-1 to 1711 cm-1 after the HV-VUV treatment, as this n(C=O) band may represent the different C=O-containing components, as previously reported.9 Similarly, the COO components of MUDO-SAM decreased after 300-sec HV-VUV treatment, and new deconvoluted peak of C=O was observed at ~ 288.1 eV, as illustrated in Figure 6C. The abundance of O-C components decreased after the HV-VUV treatment, as illustrated in Figure 6D, whereas the O-Si components were not significantly changed after HV-VUV treatment. The n(C=O) band at 1730 cm-1 was expected to represent the different photochemically formed C=O components.9 Figure S4 (Supporting Information) shows the XPS C1s, O1s, and Si2p spectra of UDA- and MUDO-SAMs before and after different periods of HV-VUV treatment. The XPS C1s spectra illustrated that the components of COO and C-O were strongly influenced by the HV-VUV treatment, but the C-C components were not apparently influenced as illustrated Figures S4A and S4D (Supporting Information). The XPS O1s spectra in Figures S4B and S4E (Supporting Information) revealed the decrease in FWHM of the observed peaks after the HVVUV treatment. These O1s spectra demonstrate the decrease in abundance of O-C components, which was also indicated from the deconvoluted O1s spectra before and after HV-VUV treatment (Figures 3 and 6). The Si components were not visibly influenced by the HV-VUV treatment, which confirmed the stability of all Si components during HV-VUV treatment as illustrated in Figure S4C and S4F (Supporting Information).


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Figure 6. XPS C1s and O1s spectra of UDA- and MUDO-SAMs after 300-sec HV-VUV treatment. Quantitative analysis of the changes in carbon content versus the HV-VUV treatment time is presented in Figure S5 (Supporting Information), in which the carbon content of UDA- and MUDO-SAMs slightly decreased after the HV-VUV treatment. Such a small decrease in carbon content confirms the stability of the C-C skeleton during the HV-VUV treatment. Figure 7 shows the changes in the Aoxygenated-carbon /Atotal carbon of UDA- and MUDO-SAMs after different irradiation times, as calculated from eq 1. The Aoxygenated-carbon represents the area under all carbon components that directly attached to oxygen atoms, while AC1s or Atotal carbon represent the area under the XPS C1s spectrum. The AC-C and ACH2–COO represent the areas under the deconvoluted C-C and CH2–COO peaks of the XPS C1s spectra illustrated in Figures 3 and 6. The decrease in


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the oxygenated carbon of MUDO-SAM after HV-VUV treatment was greater than in UDASAM. Aoxygenated-carbon AC1s -( AC-C +ACH2 –COO ) = Atotal carbon AC1s

(1)

Figure 7. Changes in the ratio of Aoxygenated-carbon /Atotal carbon of UDA- and MUDO-SAMs versus different HV-VUV treatment periods. Based on the ATR-FTIR and XPS results, the C-O bonds of the COO components in UDAand MUDO-SAMs are expected to be dissociated under the HV-VUV treatment, and new different C=O components are expected to be produced. Norrish type pathways are expected to occur in both COOH and COOCH3 components, which explains the slight decrease in carbon content as illustrated in Figure S5 (Supporting Information).6,9,10,13,17,18,39–41 Production of different C=O components especially ketones may explain the shift in the n(C=O) band illustrated in Figures 2B and 5B, and observing the C=O peaks in Figures 3 and 6.. The slight decrease in carbon content demonstrated that the C-C skeletons of SAMs are stable during the


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HV-VUV treatment. Furthermore, inducing Norrish type I suggests the possibility of CH3 formation, but this pathway is not the predominant, as the C=O components were observed in both ATR-FTIR and XPS results. According to previous reports, C=C components may be produced through hydrogen abstraction during the HV-VUV treatment, and such C=C components can undergo cross-linking reactions.6,9,10,13,14 However, the absorption bands of C=C or C=C-H components were not observed from ATR-FTIR spectra after HV-VUV treatment, as illustrated in Figure S3 (Supporting Information) and the insignificant change in the FWHM of the C-C peaks at 285.0 eV, as illustrated in Figures S4A and S4D (Supporting Information). Ellipsometric thickness values of UDA- and MUDO-SAMs slightly decreased after 3600-sec HV-VUV treatment to 1.99 nm and 1.86 nm, respectively. The WCA values of UDA- and MUDO-SAMs increased to > 80° after 300-sec HV-VUV treatment, and decreased to ~ 63° after 3600-sec HV-VUV treatment. The slight decrease in thickness values after the HV-VUV treatment may be attributed to the dissociation of COO components and disordering of the assembled molecules. The increase in WCA values at shorter periods of HV-VUV treatment resulted from the dissociation of the hydrophilic COO components. After elongation of the HVVUV treatment time, the decrease in WCA values may be due to inducing cross-linking reactions and disordering of the assembled molecules.6 The formation of cross-links and disordering of the assembled molecules would reduce the hydrophobic endings, which may be exposed to water droplets. Ellipsometric thickness values of thin films are calculated based on their refractive indices, and changes in these indices influenced the calculated thickness. The HV-VUV treatment is expected to change the thickness of SAMs due to different chemical conversions such as trimming of oxygenated groups and formation of cross-links.10,38,42


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Friction reduction. We developed a simple method to fabricate the COO components of the UDA- and MUDO-SAMs in well-defined areas through HV-VUV treatment from the slits of a photomask, as illustrated in Figure 1. These patterned samples were investigated by LFM, which can demonstrate the differences in the friction between the irradiated and masked areas. Figure 8 shows the topographic and corresponding lateral trace images of UDA- and MUDO-SAMs after 300-sec HV-VUV patterning. The lateral trace images (Figures 8B and 8D) show well-defined dark circle patterns of the irradiated area, while the corresponding topographic images (Figures 8A and 8C) illustrate the insignificant height differences between these areas. The observed bright contrasts illustrated from the lateral trace images, which are the masked areas, reflect the high friction of the acid or ester-terminated areas. Figure S6 (Supporting Information) shows the topographic and the corresponding lateral trace images of UDA- and MUDO-SAMs after 300sec patterning using a photomask with different sized slits. The irradiated areas exhibited lower friction than the masked areas without distinguishable height differences, which is consistent with Figure 8. As previously described from ATR-FTIR and XPS results, the adhesive COOH and COOCH3 components were converted to different less adhesive components such as C=O and CH3 components.6,9 Consequently, the dark contrast at the exposed areas indicates these conversions at the COOH and COOCH3 components. Additionally, the HV-VUV treatment did not dissociate the carbon skeleton (C-C) of these SAMs, which explains the absence of significant height differences between the irradiated and masked areas. The terminal group is known to be the key factor in analyzing friction properties.9,43–48 Using LFM, several studies have yielded higher friction coefficients for COOH-terminated SAM than for CH3-terminated SAM.8,49,50 Furthermore, it has been reported that the magnitude of the


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frictional force was in the order of COOH- > COOCH3- > CH3-terminated SAMs.50–52 The lower friction at the irradiated areas of UDA- and MUDO-SAMs is attributed to the conversion of the COOH and COOCH3 terminals to less adhesive components such as C=O and CH3.6,9,14 Due to the higher friction of COOH terminals than COOCH3 terminals, the observed frictional differences between irradiated and masked areas of UDA-SAM were larger than those of MUDO-SAM as indicated from the line profiles and histograms in Figures 8B, 8D and S6 (Supporting Information), which is in good agreement with the reported frictional properties.49–51 On the other hand, it has been reported that the order and thickness of SAMs strongly influence their friction properties.53–55 SAMs with a chain length shorter than 8 carbon atoms have a higher friction than SAMs with a longer chain length (³ 12 carbon atoms).56,57 The effective interactions between the ordered long-chained assembled molecules result in a blocking layer under the shearing force without disorder, resulting in lower friction.45,50,55 The friction at the surface of disordered SAMs is higher than for ordered SAMs. However, the irradiated areas are expected to be more disordered than the masked areas without significant decrease in their thicknesses.9,38 Therefore, the changes in the thickness and disorder of the irradiated areas did not contribute to the friction decrease in these areas. The cross-linking reactions between the assembled molecules are except to be induced as previously mentioned.6,9,10,13,14,20 The increase of cross-links in poly(2-hydroxyethyl methacrylate) would increase the friction due to the decrease of conformational freedom of their backbones.58,59 Therefore, the formation of crosslinks is not the reason of the friction decrease after HV-VUV treatment. As previously mentioned, the friction of SAMs is expected to increase with decreasing their thickness, disturbing their order and formation of cross-links. Therefore, the chemical conversions at the terminal COO components were the dominant factor in the friction reduction


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at the irradiated areas. Thus, controlling the terminal moieties through the HV-VUV treatment, as presented in this study, may pave the way for low damage reductive patterning, which can be further applied such as for templates for molecular building blocks or sensors for molecular recognition through linkage with terminal moieties.

Figure 8. (A) and (C) are topographic images of UDA- and MUDO-SAMs after 300-sec HVVUV patterning, respectively, and (B) and (D) are the corresponding lateral trace images. CONCLUSIONS In this report, we examined HV-VUV treatment on the chemical constituents, wettability, morphology and friction of UDA- and MUDO-SAMs. The immobilization of UDA and MUDO molecules on H–Si substrate was performed through UV hydrosilylation of their C=C components from their mesitylene solutions. The WCA and thickness values of UDA-SAM were 62.8° and 2.09 nm, respectively. The WCA and thickness of MUDO-SAM were 71.0° and 1.93 nm, respectively. The WCA and thickness values of these SAMs agree with the reported values for UDA- and MUDO-SAMs. Based on the carbon content and position of n(CH2) absorption


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bands, UDA-SAM was expected to be more compact than MUDO-SAM, as the hydrosilylation of MUDO molecules was expected to be influenced by the bulky COOCH3 terminal groups. The HV-VUV treatment to UDA- and MUDO-SAMs can dissociate the C-O bonds of COO components and induce Norrish type pathways without significant dissociation of their C-C skeletons. The surface properties of UDA- and MUDO-SAMs were significantly influenced due to trimming of their COO hydrophilic components, formation of cross-links, and disordering of the assembled molecules. These chemical conversions influenced the wettability and morphology of SAMs. The SAMs were disordered after the HV-VUV treatment. Dissociation of the COO components would increase the hydrophobicity, while the formation of cross-links and disordering of the assembled molecules decrease the wettability. When these SAMs were HV-VUV-irradiated through slits of a photomask, the irradiated areas had lower friction than the masked areas, but the height differences between these areas were insignificant. The changes in friction were attributed to the conversions of the adhesive COOH and COOCH3 components to less adhesive components such as C=O and CH3 components. The insignificant height differences between the irradiated and masked areas may be due to the stability of the carbon skeleton under the HV-VUV treatment. The observed frictional differences between the irradiated and masked areas of UDA-SAM were higher than those of MUDO-SAM, as the COOH groups are more adhesive than COOCH3 groups. Such a dry, effective, and low destructive photolithographic process is promising for surface fabrication and for controlling the properties of hydrophilic surfaces in different patterns. Supporting information Additional ATR-FTIR, XPS and LFM results. Corresponding Author


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*E-mail address: [email protected] Tel: +81-75-753-9131 Acknowledgment This work was partially supported by JSPS KAKENHI Grant Numbers JP24246121 and JP15H02297. A.I.A.S acknowledges the Ministry of Higher Education of Egypt (Cultural Affairs Sector and Missions) for financial support. REFERENCES (1)

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