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Covalent molecular assembly: Construction of ultrathin multilayer films by a two-dimensional fabrication method
Journal of Colloid and Interface Science 392 (2013) 158–166
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Journal of Colloid and Interface Science www.elsevier.com/locate/jcis
Covalent molecular assembly: Construction of ultrathin multilayer films by a two-dimensional fabrication method Zhou Ruitao 1, M.P. Srinivasan ⇑ Department of Chemical and Biomolecular Engineering, National University of Singapore, Singapore
a r t i c l e
i n f o
Article history: Received 30 May 2012 Accepted 23 July 2012 Available online 11 October 2012 Keywords: Multilayer Cross-linking Covalent layer-by-layer
a b s t r a c t A two-dimensional fabrication method was employed to assemble ultrathin multilayer films with specific three-dimensional structures by making use of interlayer and intralayer covalent bonding. The films were assembled in a layer-wise fashion on a silicon surface using bi- and multi-functional molecules as building blocks and strengthened by lateral cross-linking. The fabrication process could be controlled at the sub-nano-scale with the roughness of the surface after deposition of each layer within 0.2 nm. The film showed better resistance to harsh environments than randomly cross-linked or linearly linked films of comparable thickness. The combination of covalent LbL assembly and lateral cross-linking has significant potential as a method of fabrication for assembling nano-structures for a variety of applications. Ó 2012 Elsevier Inc. All rights reserved.
1. Introduction Ultra-thin multilayer films have attracted substantial interest recently due to their many applications, such as surface protection [1], membrane modification [2,3], drug delivery [4,5], and bio-sensors [6,7]. Layer-by-layer (LbL) self-assembly is a simple and versatile method to fabricate ultra-thin multilayer films with controlled thickness and composition. In LbL assembly, the molecules are adsorbed and assembled on the substrate through different interactions, such as physical adsorption, electrostatic interaction [8–10], hydrogen bonding [11,12], charge-transfer interaction [13], and metal–ligand interaction [14,15]. This deposition process can be repeated numerous times until desired number of films and thickness are obtained. Functional molecules and nanoparticles can also be incorporated thereby enabling utilities such as antibacterial protection [16] and electrochromism [17], nano-sensors [18], and nanoreactors [11,19,20]. Despite their extensive usage and potential applications, stability of multilayer films is not satisfactory due to the fragility of the inter- and intra-layer bondings. Typically, the different layers are held together by weak intermolecular interactions, and they can be solved by organic solvent or be destroyed in high pH solution [21], which will limit their applications. Cross-linking is a convenient and effective way to strengthen the multilayer films. Many approaches have been attempted to ⇑ Corresponding author. Address: Block E5-02-24, 4 Engineering Drive 4, National University of Singapore, Singapore 117585, Singapore. E-mail addresses: [email protected] (R. Zhou), [email protected] (M.P. Srinivasan). 1 Address: Block E5-04-32, 4 Engineering Drive 4, National University of Singapore, Singapore 117585, Singapore. 0021-9797/$ - see front matter Ó 2012 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.jcis.2012.07.061
induce cross-linking reaction in the multilayer films. Zhang and Cao [13] built a cross-linked charge-transfer multilayer film containing diazo resin by UV irradiation, which was stable in polar solvents. Shao et al. [22] fabricated a long-lifetime light-emitting multilayer film by thermally-induced cross-linking. Stable freestanding multilayer films strengthened by catalyst [23] and electron beam [24] induced cross-linking reactions were obtained by peeling them from the substrate in H2O/DMF/ZnCl2 and HF, respectively. Photo irradiation was also frequently used to induce cross-linking in both multilayer [25] and monolayer films [26]. Cross-linking improves the intrinsic strength of the multilayer films; however, the interactions between the multilayer films and the substrate are still weak due to their non-covalent properties [22,24], which could limit the applications of functional multilayer films. Therefore, there is a need for improving assembly techniques that may involve intra-layer as well as layer-to-layer links. Furthermore, versatile three-dimensional structures that cater to stability and architectural requirements are desired for construction of nanodevices that may need many assembly steps. Covalent LbL assembly [27,28] is a powerful method to fabricate stable multilayer films in which covalent binding serves as the driving force for molecular assembly. Similar to the concept of click chemistry [29], but more versatile, covalent LbL assembly is a simple method to build nanostructures efficiently without the need for isolation of the product. The multilayer films can be stable in harsh environments due to the strong binding between different layers and between the film and the substrate. This stability facilitates treatment of films to remove weakly adsorbed or physisorbed molecules [30], thereby giving rise to more precise and ordered structures. Furthermore, the films not only can be used to immobilize
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Scheme 1. Fabrication process of lateral cross-linked multilayer films 1: surface of silicon wafer; 2: after deposition of APhS; 3: after deposition of TC; 4: after deposition of OD; 5: after deposition of TFAA; 6: after demethylation reaction; 7: after cross-linking reaction (LCMF1); 8: multilayer film with two cross-linked layers (LCMF2).
functional molecules [31], but also could endure further reactions during the additional manipulations. To provide additional robustness, covalent multilayer films can be further strengthened by cross-linking. Furthermore, cross-linking could be used as a structural fabrication tool. In addition to the
vertical fabrication of LbL assembly by interlayer covalent binding, cross-linking reactions at specific layers can be regarded as a lateral fabrication process. A combination of vertical and lateral fabrication could be a feasible method to fabricate complex devices with three-dimensional structures. The enhanced stability pro-
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Scheme 2. LbL cross-linked multilayer film (LbL CMF).
films with more than one cross-linked layer at predesigned positions has significance due to its potential to facilitate assembly of three-dimensional structures at the nanoscale. In this work, two different methods were explored to build multilayer films with cross-linked structure. In the first approach, the cross-linked multilayer film was built by covalent LbL assembly using multifunctional monomers [34]. This film was expected to have a structure wherein the constituent molecules are randomly cross-linked with each other. The second approach involved a combination of vertical and lateral covalent bonding. A linear multilayer film was firstly built by covalent LbL assembly; subsequently, groups at specific layers were activated and linked with each other laterally by a cross-linking reagent. Multilayer films with one and two crosslinked layers were fabricated, and they demonstrated better stability than the linearly linked multilayer film [28]. In particular, the lateral cross-linked multilayer film (LCMF) has controlled threedimensional structures compared to the disordered structures of the conventional, randomly cross-linked multilayer films [25,35], and it may find more applications in the field of nano-fabrication and assembly of robust thin films. For example, it can serve as robust matrices with cavities to host nanoparticles, thereby facilitating the fabrication of complex nanodevices. The improved resistance of the film to hostile environments can be exploited for protective coatings on micro electrical devices. 2. Experiments 2.1. Materials p-Aminophenyltrimethoxysilane (APhS, Gelest), terephthaloyl chloride (TC, Aldrich), o-diasidine (OD, Aldrich), trifluoro acetic anhydride (TFAA, Alfa Aesar), boron trichloride-methyl sulfide complex (BTCl, Aldrich), 1,4-diisocyanotobutane (DCB, Aldrich), 1,4-phenylene diamine (PDA, Aldrich), 1,2,4,5-tetra aminobenzene tetra hydrochloride (TAB, Fluka), and thionyl chloride (from Merck) were used as received. Triethylamine (GC grade, FLuka), acetone (AR, Merck), toluene (HPLC grade, Tedia), tetrahydrofuran (HPLC grade, Tedia), dimethyl acetamide (HPLC grade, Tedia), sulfuric acid (GR grade, Fischer), and hydrogen peroxide (30%, Merck) were used without further purification. Silicon wafers with a natural oxide layer were obtained from Engage Electronics Pte Ltd., Singapore. 2.2. Pretreatment of silicon wafer Silicon wafers were rinsed with acetone and then treated with piranha solution (7:3 v/v (H2SO4/H2O2)) at 75 °C for 50 min (caution: piranha solution is strongly oxidative and should be handled with extreme care). Then, the wafers were rinsed copiously with water and dried in vacuum at 60 °C for 1 h. The surface-activated wafers were used immediately. 2.3. Lateral cross-linking
Scheme 3. Linear multilayer film.
vided by the lateral fabrication process may help to immobilize and assemble nanodevices [19], for they can endure more harsh reacting conditions and severe operating environments. Especially, the lateral cross-linked multilayer film has a designed threedimensional structure, so it can provide a new method to immobilize nanoparticles. For example, they can provide a robust matrix for nanoparticle synthesis that is conventionally carried out in electrostatically assembled LbL systems [32]. In contrast to the cross-linked monolayer or multilayer films that are typically assembled [33], fabrication of robust multilayer
2.3.1. Covalent LbL assembly The surface-activated silicon wafers were immersed in APhS (4 mM in toluene) for 3 h in order to derivatize the surface with amine groups [36–38]. Lateral cross-linking: After the assembly of the first layer, TC (having two acyl chloride groups) and OD (having two amine groups) were deposited in succession on the APhS-covered surface to enable formation of amide bonds between the layers. The depositions were conducted by immersing the derivatized surfaces in 8 mM TC or OD in tetrahydrofuran (THF). A few drops of triethylamine were added to the solution as catalyst for the reaction. After
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Fig. 1. The thicknesses of the lateral cross-linked multilayer film (LCMF1) after sequential deposition of each layer (with reference to Scheme 1).
After the assembly of each layer on the surface, the samples were rinsed copiously and then ultrasonically washed in DMAc for 15 min and then rinsed with THF to remove the physisorbed molecules on the surface. Subsequently, the samples were blown dry with nitrogen and then dried in vacuum at 60 °C for 1 h.
Amine APhS
2.3.2. Activation of lateral groups The demethylation reaction used to convert the methoxy groups of OD to hydroxyl groups was carried out according to literature [39]. Multilayer films containing the OD layer were immersed in BTCl (10 mM in THF) for 3 h, sonicated in a solution of THF and de-ionized water (1:1, v/v) for 5 min, and then rinsed copiously with THF. The samples were blown dry with nitrogen and then dried in vacuum at 60 °C for 1 h.
Amide TC
2.3.3. Lateral cross-linking Cross-linking between the hydroxyls of the demethylated OD was carried out using DCB as the linking agent. The samples were immersed in DCB (4.5 mM solution in THF) for 30 min and then cleaned and dried using the same methods described above. Scheme 1 shows the assembly process of cross-linked multilayer films with one and two OD layers.
OD
2.4. LbL cross-linking
TFAA
408
406
404
402
400
398
396
394
B.E.(eV)
After the assembly of the first APhS layer, TC and TAB (having four amine groups) were alternately deposited on the surface for three cycles. The experimental conditions were the same as those for the deposition of the TC and OD pair. The presence of four amine groups in TAB facilitates cross-linking between adjacent moieties between layers. Scheme 2 shows the expected structure after deposition. 2.5. Linear multilayer film
Fig. 2. N1s XPS spectrum of the multilayer film (structures 2–5 in Scheme 1).
the deposition of TC and before the deposition of OD, the samples were pretreated in 20% thionyl chloride in THF solution for 30 min and then rinsed copiously with THF. All the processes were carried out at room temperature in an argon atmosphere. Each deposition was carried out with a 3-h exposure of the substrate to the deposition species. After the TC and OD were alternately assembled on the surface for several cycles, the samples were immersed in 14 mM TFAA in THF solution, and few drops of triethylamine were added to the solution to act as catalyst.
After the assembly of the first APhS layer, TC and PDA (having two amine groups) were alternately deposited on the surface for three cycles. The experimental conditions were the same as those for the deposition of the TC and OD pair (Scheme 3). Since PDA has only two amine groups, it is more likely to facilitate linear growth without cross-linking. 2.6. Characterization of the multilayer films Ellipsometry (WVASE 32, J.A. Woollam Co. Inc.) was used to characterize the thickness of the multilayer films. Scanning spectra were acquired over the wave length range of 250–600 nm with a fixed incidence angle at 70°. A Cauchy model was used to fit the
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Reflection %
30
25
APhs st 1 TC st 1 OD nd 2 TC nd 2 OD 20
200
210
220
230
240
250
Wavelength nm. Fig. 3. UV–vis spectrum of the multilayer film with two OD layers (LCMF2).
3. Results and discussion 3.1. Construction of lateral cross-linked multilayer films (LCMF) by vertical and lateral covalent links
TFAA
Demethylation
292
290
288
286
284
282
280
B.E. (eV) Fig. 4. C1s XPS spectrum of the LCMF1 before and after cross-linking.
layer thickness with the refractive index fixed at 1.47. Each sample was measured at three different points. The surfaces of the multilayer films were characterized by X-ray photoelectron spectroscopy (XPS) using a Kratos Analytical AXIS HSi spectrometer (XPS) with a monochromatized Al Ka X-ray source (1486.71-eV photons) at a constant dwell time of 100 ms and pass energy of 40 eV. The C1s hydrocarbon peak at 284.6 eV was adopted as reference for all binding energies. Surface roughness and morphologies were investigated by a Nanoscope III atomic force microscope (AFM). All tests were conducted in tapping mode, and a monolithic silicon tip was used. The drive frequency was 330 ± 50 Hz, and the scan rate was 1.0–1.2 Hz. UV–visible reflection spectra were recorded on a Shimadzu UV3600 spectrophotometer with the scanning wave length ranging from 700 nm to 200 nm. 2.7. Stability test The stability tests were carried out by immersing the sample in hot water at 90 °C and in 17.5 mM sodium hydroxide solution. The film thickness was measured by ellipsometry after 15, 30, 60, and 120 min of immersion.
Scheme 1 shows the sequence of formation of successive molecular layers on APhS derivatized Si. Multilayer films with one (LCMF1) and two cross-linked layers (LCMF2) were fabricated. Results from the ellipsometry shows that the thicknesses of LCMF1 (Fig. 1) and LCMF2 (Supporting information – Fig. S1) increased linearly with each deposition until the OD layer. The thickness of the film decreased slightly after deposition of TFAA. The reason may be that some adsorbed molecules (e.g., due to formation of hydrogen bonding between hydrolyzed acyl chloride and the triethylamine (catalyst)) in the films are replaced by the smaller TFAA molecules. This inference is supported by the observed deposition of TFAA validated by the formation of amide bonds in the XPS spectrum (Fig. 2). After demethylation, further decrease in film thickness may be attributed to the removal of the lateral methyl groups in the OD layer. Incorporation DCB (validated from XPS spectra (Fig. 5)) and the ensuing cross-linking reaction would have caused the subsequent increase in film thickness. The lessthan-expected thickness of OD layer (3.6 Å measured with a refractive index of 1.47), compared to the molecular length of 11.4 Å for the OD (calculated by Chem3D Ultra (Cambridgesoft Corporation) in minimizing energy mode) may be due to absence of dense compaction of OD because of its nonlinear shape. Fig. 2 shows the N1s XPS spectra obtained after the deposition of each layer. Deposition of APhS on the silicon surface is confirmed by the main peak at 398.7 eV corresponding to the presence of free amine. There is also another small peak at 400.8 eV ascribed to the protonated amine due to side reactions [40]. A strong peak at 399.5 eV appeared and the original strong peak at 398.7 eV almost vanished after deposition of TC, which indicates the conversion of free amine to amide and formation of amide bonds between the APhS and TC layers. Subsequently, the N1s peak at 398.7 eV increased again after OD was assembled onto the TC layer, indicating that free amine groups in the film increased due to introduction of OD. The small increase in the free amine peak is consistent with the smaller thickness of the OD layer than that of the TC layer. When TFAA molecules were deposited on the OD layer, the N1s peak at 398.7 eV decreased sharply due to the formation of amide bonds between OD and TFAA layers at the expense of free amines of OD.
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N 1s
F 1s
Demethylation
Crosslinking
408
406
404
402
400
398
396
394
392
694
692
690
B.E. (eV)
688
686
684
682
680
B.E.(eV)
(a) N 1s
F 1s
Demethylation
Crosslinking
408
406
404
402
400
398
396
394
392
B.E. (eV)
694
692
690
688
686
684
682
680
B.E.(eV)
(b) Fig. 5. N1s and F1s XPS spectrum of the cross-linked film before and after cross-linking reaction (a: LCMF1; b: LCMF2).
The UV–visible spectra (Fig. 3) of the LCMF2 during each of the assembly steps show a continuous decrease in reflection at 224 nm with the increase in number of layers due to the absorption by phenyl groups in APhS, TC and OD and provides evidence of film deposition consistent with the ellipsometry results.
The demethylation reaction was characterized by C1s XPS spectrum (Fig. 4). The C1s peak at 286.1 eV (corresponding to the C–O group) decreased after demethylation due to the removal of lateral methyl groups of the OD, which is in agreement with the expected changes in the film. After demethylation reaction, a smaller C1s
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Fig. 6. Section images of multilayer films in each fabrication process (length: 1 lm, height: 1.5 nm) and surface plot of the multilayer film with two cross-linked layers (area: 1 lm 1 lm, height: 1 nm).
peak at 286.1 eV persisted because of the newly formed C–OH group (Scheme (Structure 6)) near the same binding energy. However, the number of the C–OH groups is only half of the number of C–O groups before demethylation (one C–O–C group equals to two C–O groups). The changes of C1s peak at 286.1 eV by demethylation are consistent with the structural conversion of OD from lateral methoxy groups to hydroxyl groups. Fig. 5 shows the XPS spectra after cross-linking. The ratios of nitrogen to fluorine (N/F) in LCMF1 before and after cross-linking were 4.3 and 6.1, respectively (1.3 and 1.7 respectively in theory according to Schemes 1 and 7), and those for LCMF2 before and after cross-linking were 2.73 and 3.14, respectively (1.7 and 3.3, respectively, in theory according to Schemes 1 and 8). The films
showed higher N/F ratios than expected, which may be due to the loosely compacted OD layer and the side reactions of the APhS layer. The additional presence of DCB after cross-linking led to the increase in N/F ratio. However, for LCMF2, the ratio of hydroxyl groups in different OD layers that have reacted could not be specified due to difficulties in resolving the difference in the cross-linking reactions between the two OD layers. Urethane bonds are formed when the two isocyanate groups of DCB react with the hydroxyl groups in OD layer. If one isocyanate group has not reacted, it will be hydrolyzed and decomposed in the air and new amine groups will be formed. Absence of a new amine peak after crosslinking suggests that most DCB molecules have reacted with two functional groups of OD. Therefore, all the vertically aligned molec-
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APhS 1st TC 1st OD 2nd TC 2nd OD TFAA Demethylation Cross-linking
RMS of the surface (nm) (LCMF1)
RMS of the surface (nm) (LCMF2)
0.102 0.086 0.085 – – 0.104 0.114 0.159
0.052 0.054 0.095 0.075 0.107 0.088 0.089 0.085
Relative thickness 100%
Layer
After crosslinking
1.0
Analyzing area: LCMF1, 300 nm 300 nm; LCMF2, 1 lm 1 lm.
0.8
0.6
Before crosslinking 0.4
0.2
0.0 0
20
40
60
80
100
120
o Time /min (90 C in water)
Fig. 9. Thickness of film with two cross-linked layers before and after hydrolysis.
Intensity
Fig. 7. Thickness of the LbL cross-linked (B) and linear multilayer films (C).
13000 12500 12000 11500 11000 10500 10000 9500 9000 8500 8000 7500 7000 6500 6000 5500
Relative Thickness 100%
1.0
Lateral crosslinked multilayer film (LCMF2)
0.8
0.6
0.4
LBL crosslinked multilayer film (B) 0.2
Linear multilayer film (C) 0.0 0
20 40 60 80 100 120 140 160 180 200 220 240
Time min-1 Fig. 10. Thickness of multilayer films during successive aqueous and alkaline hydrolysis (the films were hydrolyzed in hot water at 90 °C for the first 2 h and then eroded in 17.5 mM NaOH solution for another 2 h subsequently).
3.2. Construction of cross-linked multilayer film with multi-functional moieties 408
406
404
402
400
398
396
394
B.E. (eV) Fig. 8. N1s XPS spectrum of the LbL cross-linked multilayer film (LbL CMF).
ular chains on the surface have been effectively linked by lateral connections within the OD layer, thus confirming the efficiency of DCB as a cross-linking reagent for the OD layer. The efficiency may be ascribed to the dilute solution used (4.5 mM) that provided sufficient free space between OD chains and the suitable length of the DCB molecule. The surface topography of the LCMF2 during assembly was investigated by AFM (Fig. 6). The surface of the film after deposition of each layer was planar with roughness of ranging from 0.1 to 0.2 nm, indicating that the surfaces are even and uniform. The difference in roughness between LCMF1 and LCMF2 (Table 1) is mainly due to the APhS layer that may be on the account of side reactions when silane is used as the modifier of the silicon surface [41]. The small roughness values indicate that this can be controlled to sub-nano-scales by covalent LbL assembly.
Two multilayer films fabricated by LbL assembly with procedures similar to those discussed above are shown as Schemes 2 and 3 (Supporting information – Schematic S1). To fabricate these films, molecules with amine groups (TAB and PDA) and molecules with acyl chloride groups (TC) were deposited on the surface alternately for three cycles. For the cross-linked film (LbL CMF, Scheme 2), TAB with four amine groups was used. One TAB can react with four TC molecules that can facilitate formation of cross-linked structures after a three-cycle deposition. For the linear film (Scheme 3), PDA with two amine groups was used. The cross-linked and the linear films have comparative thickness, which is about 40 nm after three cycles of deposition (Fig. 7). The similarity in thickness is due to the comparable molecular lengths of the monomers. The cross-linked film was investigated by N1s XPS spectrum (Fig. 8). The ratio of nitrogen atoms in amine groups (398.7 eV) and amide groups (399.37 eV) was found to be 0.68, which is larger than that expected of a perfect cross-linked structure with a theoretical amine/amide ratio of 0.18. This may be attributed to steric hindrance, which may not permit all four of the amines in TAB to form amides. Therefore, it can be inferred that the cross-linked
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multilayer film has less cross-linking bonds than that illustrated in Scheme 2. 3.3. Stability of the cross-linked films 3.3.1. Comparison of the stability of the lateral cross-linked film (LCMF2) before and after cross-linking The stability of the LCMF2 before and after the cross-linking reactions was investigated by immersion of the film-laden substrate in hot water at 90 °C for 2 h. The thickness of the films was measured during the hydrolyzation test at periodic intervals (Fig. 9). Before cross-linking, the thickness of the film decreased by 35% within 30 min and then showed a slower rate of decrease. The decrease may be due to the loss of binding between APhS and the substrate that would result in loss of film material. This loss may be aggravated by the possibility that APhS molecules may only react with the hydroxyl groups of the silicon surface with one or two methoxy groups due to the side reactions and steric resistance [37,40]. On the other hand, the cross-linked film showed much better performance, losing about 12% thickness over 120 min. The film was strengthened at least by two factors: first, the molecules on the surface are bound by additional lateral cross-linking bonds, so that they cannot be easily deprived off the surface; second, water is more efficiently blocked out of the surface by three-dimensional network formed by cross-linking reaction. 3.3.2. Comparison of the stability of the cross-linked and linear films The stability of the two cross-linked films (LCMF2 and LbL CMF) was compared for film thickness by ellipsometry with the linear film (Scheme 3) by hydrolyzing the films successively in hot water at 90 °C for 2 h and in NaOH (17.5 mM) solution for an additional 2 h (Fig. 10). Ether and amide bonds in the film can be slightly hydrolyzed in hot water, and the silicon surface can be eroded by sodium hydroxide solution. Therefore, the test environment is really a severe condition. The two cross-linked films showed better stability than the linear film in hot water, and the lateral crosslinked film showed better stability than the LbL cross-linked film as well as the linear film in the NaOH solution. The LCMF2 structure was able to resist erosion to a greater extent on account of its higher degree of cross-linking that enabled more effective blockage of hydroxyl ions. The prevalence of cross-linking would have also enabled the film to retain its integrity even in the presence of ongoing erosion. Furthermore, as inferred from the XPS spectra for the LbL cross-linked film (Fig. 8), the effect of the incomplete cross-linking due to steric hindrance may also have contributed to the decrease in stability relative to that of the laterally cross-linked film. 4. Conclusions Ultrathin multilayer films were fabricated by covalent LbL assembly, in which molecules with multifunctional groups were used to obtain cross-linked structures. Interlayer and intralayer covalent bonds formed by use of appropriate functional molecules and fabrication methods yielded film structures with lateral and vertical links. The surfaces of the films were even and uniform with a small roughness of less than 0.2 nm. The lateral assembly has higher cross-linking efficiency and showed better resistance to erosion than the LbL cross-linked film in 17.5 mM NaOH solution. Both forms of cross-linked films showed better resistance to hot aqueous and alkaline environments than the linearly linked films, thereby attesting to the advantage of cross-linking to improve robustness.
Acknowledgment The authors thank the National University of Singapore for providing financial support for this project and a research scholarship for Zhou Ruitao.
Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.jcis.2012.07.061. References [1] D.V. Andreeva, E.V. Skorb, D.G. Shchukin, ACS Appl. Mater. Interfaces 2 (2010) 1954–1962. [2] M.L. Bruening, D.M. Dotzauer, P. Jain, L. Ouyang, G.L. Baker, Langmuir 24 (2008) 7663–7673. [3] H. Wang, B.D. Ratner, E.H. Sage, S.Y. Jiang, J. Phys. Chem. C 111 (2007) 8504– 8508. [4] S. Hirsjarvi, Y. Qiao, A. Royere, J. Bibette, J.P. Benoit, Eur. J. Pharm. Biopharm. 76 (2010) 200–207. [5] A.L. Becker, A.P.R. Johnston, F. Caruso, Small 6 (2010) 1836–1852. [6] Z. Chang, M.A. Chen, H. Fan, K. Zhao, S.Q. Zhuang, P.G. He, Y.Z. Fang, Electrochim. Acta 53 (2008) 2939–2945. [7] S. Firdoz, F. Ma, X. Yue, Z. Dai, A. Kumar, B. Jiang, Talanta 83 (2010) 269–273. [8] X.Y. Li, P.W. Fan, X.L. Tuo, Y.N. He, X.G. Wang, Thin Solid Films 517 (2009) 2055–2062. [9] D. Chen, S. Huang, C. Zhang, W.Z. Wang, T.X. Liu, Thin Solid Films 518 (2010) 7081–7085. [10] F. Rivadulla, C. Mateo-Mateo, M.A. Correa-Duarte, J. Am. Chem. Soc. 132 (2010) 3751–3755. [11] W. Zhang, A. Zhang, Y. Guan, Y. Zhang, X.X. Zhu, J. Mater. Chem. 21 (2011) 548– 555. [12] R. Zirbs, F. Kienberger, P. Hinterdorfer, W.H. Binder, Langmuir 21 (2005) 8414– 8421. [13] Y.J. Zhang, W.X. Cao, Langmuir 17 (2001) 5021–5024. [14] F.X. Zhang, M.P. Srinivasan, Langmuir 23 (2007) 10102–10108. [15] J.R. Siqueira, F.N. Crespilho, V. Zucolotto, O.N. Oliveira, Electrochem. Commun. 9 (2007) 2676–2680. [16] T. He, V. Chan, J. Biomed. Mater. Res. A 95A (2010) 454–464. [17] R. Montazami, V. Jain, J.R. Heflin, Electrochim. Acta 56 (2010) 990–994. [18] J. Han, X. Xu, X. Rao, M. Wei, D.G. Evans, X. Duan, J. Mater. Chem. 21 (2011) 2126–2130. [19] T. Komatsu, H. Terada, N. Kobayashi, Chem. Eur. J. 17 (2011) 1849–1854. [20] H. Qiu, L. Bednarova, W.Y. Lee, Appl. Catal. A 314 (2006) 200–207. [21] J.J. Harris, P.M. DeRose, M.L. Bruening, J. Am. Chem. Soc. 121 (1999) 1978– 1979. [22] Y. Shao, X. Gong, A.J. Heeger, M. Liu, A.K.Y. Jen, Adv. Mater. 21 (2009) 1972– 1975. [23] P. Ott, K. Trenkenschuh, J. Gensel, A. Fery, A. Laschewsky, Langmuir 26 (2010) 18182–18188. [24] I. Amin, M. Steenackers, N. Zhang, A. Beyer, X.H. Zhang, T. Pirzer, T. Hugel, R. Jordan, A. Golzhauser, Polymer Carpets Small 6 (2010) 1623–1630. [25] J.Y. Chen, L. Huang, L.M. Ying, G.B. Luo, X.S. Zhao, W.X. Cao, Langmuir 15 (1999) 7208–7212. [26] A. Aoki, T. Miyashita, Polymer 42 (2001) 7307–7311. [27] F.X. Zhang, M.P. Srinivasan, in: 10th Int. Conf. Organized Mol. Films, 2003, pp. 509–514. [28] N.M. Adarnczyk, A.A. Dameron, S.M. George, Langmuir 24 (2008) 2081–2089. [29] N. Akeroyd, B. Klumperman, Eur. Polym. J. 47 (2011) 1207–1231. [30] F. Zhang, Z. Jia, M.P. Srinivasan, Langmuir 21 (2005) 3389–3395. [31] S. Jayaraman, D. Rajarathnam, M.P. Srinivasan, Mater. Sci. Eng. B 168 (2010) 45–54. [32] S.Y. Gao, D.Q. Yuan, J. Lu, R. Cao, J. Colloid Interface Sci. 341 (2010) 320–325. [33] S.H. Hsu, D.N. Reinhoudt, J. Huskens, A.H. Velders, J. Mater. Chem. 21 (2011) 2428–2444. [34] F.R. Mayo, J. Polym. Sci. 55 (1961) 65. [35] H. Yang, D. Lazos, M. Ulbricht, J. Appl. Polym. Sci. 97 (2005) 158–164. [36] S.R. Puniredd, M.P. Srinivasan, Langmuir 22 (2006) 4092–4099. [37] G.S. Caravajal, D.E. Leyden, G.R. Quinting, G.E. Maciel, Anal. Chem. 60 (1988) 1776–1786. [38] E.T. Vandenberg, L. Bertilsson, B. Liedberg, K. Uvdal, R. Erlandsson, H. Elwing, I. Lundstrom, J. Colloid Interface Sci. 147 (1991) 103–118. [39] P.G. Williard, C.B. Fryhle, Tetrahedron Lett. 21 (1980) 3731–3734. [40] D. Kowalczyk, S. Slomkowski, M.M. Chehimi, M. Delamar, Int. J. Adhes. Adhes. 16 (1996) 227–232. [41] L.D. White, C.P. Tripp, J. Colloid Interface Sci. 232 (2000) 400–407.
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Journal of Colloid and Interface Science www.elsevier.com/locate/jcis
Covalent molecular assembly: Construction of ultrathin multilayer films by a two-dimensional fabrication method Zhou Ruitao 1, M.P. Srinivasan ⇑ Department of Chemical and Biomolecular Engineering, National University of Singapore, Singapore
a r t i c l e
i n f o
Article history: Received 30 May 2012 Accepted 23 July 2012 Available online 11 October 2012 Keywords: Multilayer Cross-linking Covalent layer-by-layer
a b s t r a c t A two-dimensional fabrication method was employed to assemble ultrathin multilayer films with specific three-dimensional structures by making use of interlayer and intralayer covalent bonding. The films were assembled in a layer-wise fashion on a silicon surface using bi- and multi-functional molecules as building blocks and strengthened by lateral cross-linking. The fabrication process could be controlled at the sub-nano-scale with the roughness of the surface after deposition of each layer within 0.2 nm. The film showed better resistance to harsh environments than randomly cross-linked or linearly linked films of comparable thickness. The combination of covalent LbL assembly and lateral cross-linking has significant potential as a method of fabrication for assembling nano-structures for a variety of applications. Ó 2012 Elsevier Inc. All rights reserved.
1. Introduction Ultra-thin multilayer films have attracted substantial interest recently due to their many applications, such as surface protection [1], membrane modification [2,3], drug delivery [4,5], and bio-sensors [6,7]. Layer-by-layer (LbL) self-assembly is a simple and versatile method to fabricate ultra-thin multilayer films with controlled thickness and composition. In LbL assembly, the molecules are adsorbed and assembled on the substrate through different interactions, such as physical adsorption, electrostatic interaction [8–10], hydrogen bonding [11,12], charge-transfer interaction [13], and metal–ligand interaction [14,15]. This deposition process can be repeated numerous times until desired number of films and thickness are obtained. Functional molecules and nanoparticles can also be incorporated thereby enabling utilities such as antibacterial protection [16] and electrochromism [17], nano-sensors [18], and nanoreactors [11,19,20]. Despite their extensive usage and potential applications, stability of multilayer films is not satisfactory due to the fragility of the inter- and intra-layer bondings. Typically, the different layers are held together by weak intermolecular interactions, and they can be solved by organic solvent or be destroyed in high pH solution [21], which will limit their applications. Cross-linking is a convenient and effective way to strengthen the multilayer films. Many approaches have been attempted to ⇑ Corresponding author. Address: Block E5-02-24, 4 Engineering Drive 4, National University of Singapore, Singapore 117585, Singapore. E-mail addresses: [email protected] (R. Zhou), [email protected] (M.P. Srinivasan). 1 Address: Block E5-04-32, 4 Engineering Drive 4, National University of Singapore, Singapore 117585, Singapore. 0021-9797/$ - see front matter Ó 2012 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.jcis.2012.07.061
induce cross-linking reaction in the multilayer films. Zhang and Cao [13] built a cross-linked charge-transfer multilayer film containing diazo resin by UV irradiation, which was stable in polar solvents. Shao et al. [22] fabricated a long-lifetime light-emitting multilayer film by thermally-induced cross-linking. Stable freestanding multilayer films strengthened by catalyst [23] and electron beam [24] induced cross-linking reactions were obtained by peeling them from the substrate in H2O/DMF/ZnCl2 and HF, respectively. Photo irradiation was also frequently used to induce cross-linking in both multilayer [25] and monolayer films [26]. Cross-linking improves the intrinsic strength of the multilayer films; however, the interactions between the multilayer films and the substrate are still weak due to their non-covalent properties [22,24], which could limit the applications of functional multilayer films. Therefore, there is a need for improving assembly techniques that may involve intra-layer as well as layer-to-layer links. Furthermore, versatile three-dimensional structures that cater to stability and architectural requirements are desired for construction of nanodevices that may need many assembly steps. Covalent LbL assembly [27,28] is a powerful method to fabricate stable multilayer films in which covalent binding serves as the driving force for molecular assembly. Similar to the concept of click chemistry [29], but more versatile, covalent LbL assembly is a simple method to build nanostructures efficiently without the need for isolation of the product. The multilayer films can be stable in harsh environments due to the strong binding between different layers and between the film and the substrate. This stability facilitates treatment of films to remove weakly adsorbed or physisorbed molecules [30], thereby giving rise to more precise and ordered structures. Furthermore, the films not only can be used to immobilize
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Scheme 1. Fabrication process of lateral cross-linked multilayer films 1: surface of silicon wafer; 2: after deposition of APhS; 3: after deposition of TC; 4: after deposition of OD; 5: after deposition of TFAA; 6: after demethylation reaction; 7: after cross-linking reaction (LCMF1); 8: multilayer film with two cross-linked layers (LCMF2).
functional molecules [31], but also could endure further reactions during the additional manipulations. To provide additional robustness, covalent multilayer films can be further strengthened by cross-linking. Furthermore, cross-linking could be used as a structural fabrication tool. In addition to the
vertical fabrication of LbL assembly by interlayer covalent binding, cross-linking reactions at specific layers can be regarded as a lateral fabrication process. A combination of vertical and lateral fabrication could be a feasible method to fabricate complex devices with three-dimensional structures. The enhanced stability pro-
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Scheme 2. LbL cross-linked multilayer film (LbL CMF).
films with more than one cross-linked layer at predesigned positions has significance due to its potential to facilitate assembly of three-dimensional structures at the nanoscale. In this work, two different methods were explored to build multilayer films with cross-linked structure. In the first approach, the cross-linked multilayer film was built by covalent LbL assembly using multifunctional monomers [34]. This film was expected to have a structure wherein the constituent molecules are randomly cross-linked with each other. The second approach involved a combination of vertical and lateral covalent bonding. A linear multilayer film was firstly built by covalent LbL assembly; subsequently, groups at specific layers were activated and linked with each other laterally by a cross-linking reagent. Multilayer films with one and two crosslinked layers were fabricated, and they demonstrated better stability than the linearly linked multilayer film [28]. In particular, the lateral cross-linked multilayer film (LCMF) has controlled threedimensional structures compared to the disordered structures of the conventional, randomly cross-linked multilayer films [25,35], and it may find more applications in the field of nano-fabrication and assembly of robust thin films. For example, it can serve as robust matrices with cavities to host nanoparticles, thereby facilitating the fabrication of complex nanodevices. The improved resistance of the film to hostile environments can be exploited for protective coatings on micro electrical devices. 2. Experiments 2.1. Materials p-Aminophenyltrimethoxysilane (APhS, Gelest), terephthaloyl chloride (TC, Aldrich), o-diasidine (OD, Aldrich), trifluoro acetic anhydride (TFAA, Alfa Aesar), boron trichloride-methyl sulfide complex (BTCl, Aldrich), 1,4-diisocyanotobutane (DCB, Aldrich), 1,4-phenylene diamine (PDA, Aldrich), 1,2,4,5-tetra aminobenzene tetra hydrochloride (TAB, Fluka), and thionyl chloride (from Merck) were used as received. Triethylamine (GC grade, FLuka), acetone (AR, Merck), toluene (HPLC grade, Tedia), tetrahydrofuran (HPLC grade, Tedia), dimethyl acetamide (HPLC grade, Tedia), sulfuric acid (GR grade, Fischer), and hydrogen peroxide (30%, Merck) were used without further purification. Silicon wafers with a natural oxide layer were obtained from Engage Electronics Pte Ltd., Singapore. 2.2. Pretreatment of silicon wafer Silicon wafers were rinsed with acetone and then treated with piranha solution (7:3 v/v (H2SO4/H2O2)) at 75 °C for 50 min (caution: piranha solution is strongly oxidative and should be handled with extreme care). Then, the wafers were rinsed copiously with water and dried in vacuum at 60 °C for 1 h. The surface-activated wafers were used immediately. 2.3. Lateral cross-linking
Scheme 3. Linear multilayer film.
vided by the lateral fabrication process may help to immobilize and assemble nanodevices [19], for they can endure more harsh reacting conditions and severe operating environments. Especially, the lateral cross-linked multilayer film has a designed threedimensional structure, so it can provide a new method to immobilize nanoparticles. For example, they can provide a robust matrix for nanoparticle synthesis that is conventionally carried out in electrostatically assembled LbL systems [32]. In contrast to the cross-linked monolayer or multilayer films that are typically assembled [33], fabrication of robust multilayer
2.3.1. Covalent LbL assembly The surface-activated silicon wafers were immersed in APhS (4 mM in toluene) for 3 h in order to derivatize the surface with amine groups [36–38]. Lateral cross-linking: After the assembly of the first layer, TC (having two acyl chloride groups) and OD (having two amine groups) were deposited in succession on the APhS-covered surface to enable formation of amide bonds between the layers. The depositions were conducted by immersing the derivatized surfaces in 8 mM TC or OD in tetrahydrofuran (THF). A few drops of triethylamine were added to the solution as catalyst for the reaction. After
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161
Fig. 1. The thicknesses of the lateral cross-linked multilayer film (LCMF1) after sequential deposition of each layer (with reference to Scheme 1).
After the assembly of each layer on the surface, the samples were rinsed copiously and then ultrasonically washed in DMAc for 15 min and then rinsed with THF to remove the physisorbed molecules on the surface. Subsequently, the samples were blown dry with nitrogen and then dried in vacuum at 60 °C for 1 h.
Amine APhS
2.3.2. Activation of lateral groups The demethylation reaction used to convert the methoxy groups of OD to hydroxyl groups was carried out according to literature [39]. Multilayer films containing the OD layer were immersed in BTCl (10 mM in THF) for 3 h, sonicated in a solution of THF and de-ionized water (1:1, v/v) for 5 min, and then rinsed copiously with THF. The samples were blown dry with nitrogen and then dried in vacuum at 60 °C for 1 h.
Amide TC
2.3.3. Lateral cross-linking Cross-linking between the hydroxyls of the demethylated OD was carried out using DCB as the linking agent. The samples were immersed in DCB (4.5 mM solution in THF) for 30 min and then cleaned and dried using the same methods described above. Scheme 1 shows the assembly process of cross-linked multilayer films with one and two OD layers.
OD
2.4. LbL cross-linking
TFAA
408
406
404
402
400
398
396
394
B.E.(eV)
After the assembly of the first APhS layer, TC and TAB (having four amine groups) were alternately deposited on the surface for three cycles. The experimental conditions were the same as those for the deposition of the TC and OD pair. The presence of four amine groups in TAB facilitates cross-linking between adjacent moieties between layers. Scheme 2 shows the expected structure after deposition. 2.5. Linear multilayer film
Fig. 2. N1s XPS spectrum of the multilayer film (structures 2–5 in Scheme 1).
the deposition of TC and before the deposition of OD, the samples were pretreated in 20% thionyl chloride in THF solution for 30 min and then rinsed copiously with THF. All the processes were carried out at room temperature in an argon atmosphere. Each deposition was carried out with a 3-h exposure of the substrate to the deposition species. After the TC and OD were alternately assembled on the surface for several cycles, the samples were immersed in 14 mM TFAA in THF solution, and few drops of triethylamine were added to the solution to act as catalyst.
After the assembly of the first APhS layer, TC and PDA (having two amine groups) were alternately deposited on the surface for three cycles. The experimental conditions were the same as those for the deposition of the TC and OD pair (Scheme 3). Since PDA has only two amine groups, it is more likely to facilitate linear growth without cross-linking. 2.6. Characterization of the multilayer films Ellipsometry (WVASE 32, J.A. Woollam Co. Inc.) was used to characterize the thickness of the multilayer films. Scanning spectra were acquired over the wave length range of 250–600 nm with a fixed incidence angle at 70°. A Cauchy model was used to fit the
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Reflection %
30
25
APhs st 1 TC st 1 OD nd 2 TC nd 2 OD 20
200
210
220
230
240
250
Wavelength nm. Fig. 3. UV–vis spectrum of the multilayer film with two OD layers (LCMF2).
3. Results and discussion 3.1. Construction of lateral cross-linked multilayer films (LCMF) by vertical and lateral covalent links
TFAA
Demethylation
292
290
288
286
284
282
280
B.E. (eV) Fig. 4. C1s XPS spectrum of the LCMF1 before and after cross-linking.
layer thickness with the refractive index fixed at 1.47. Each sample was measured at three different points. The surfaces of the multilayer films were characterized by X-ray photoelectron spectroscopy (XPS) using a Kratos Analytical AXIS HSi spectrometer (XPS) with a monochromatized Al Ka X-ray source (1486.71-eV photons) at a constant dwell time of 100 ms and pass energy of 40 eV. The C1s hydrocarbon peak at 284.6 eV was adopted as reference for all binding energies. Surface roughness and morphologies were investigated by a Nanoscope III atomic force microscope (AFM). All tests were conducted in tapping mode, and a monolithic silicon tip was used. The drive frequency was 330 ± 50 Hz, and the scan rate was 1.0–1.2 Hz. UV–visible reflection spectra were recorded on a Shimadzu UV3600 spectrophotometer with the scanning wave length ranging from 700 nm to 200 nm. 2.7. Stability test The stability tests were carried out by immersing the sample in hot water at 90 °C and in 17.5 mM sodium hydroxide solution. The film thickness was measured by ellipsometry after 15, 30, 60, and 120 min of immersion.
Scheme 1 shows the sequence of formation of successive molecular layers on APhS derivatized Si. Multilayer films with one (LCMF1) and two cross-linked layers (LCMF2) were fabricated. Results from the ellipsometry shows that the thicknesses of LCMF1 (Fig. 1) and LCMF2 (Supporting information – Fig. S1) increased linearly with each deposition until the OD layer. The thickness of the film decreased slightly after deposition of TFAA. The reason may be that some adsorbed molecules (e.g., due to formation of hydrogen bonding between hydrolyzed acyl chloride and the triethylamine (catalyst)) in the films are replaced by the smaller TFAA molecules. This inference is supported by the observed deposition of TFAA validated by the formation of amide bonds in the XPS spectrum (Fig. 2). After demethylation, further decrease in film thickness may be attributed to the removal of the lateral methyl groups in the OD layer. Incorporation DCB (validated from XPS spectra (Fig. 5)) and the ensuing cross-linking reaction would have caused the subsequent increase in film thickness. The lessthan-expected thickness of OD layer (3.6 Å measured with a refractive index of 1.47), compared to the molecular length of 11.4 Å for the OD (calculated by Chem3D Ultra (Cambridgesoft Corporation) in minimizing energy mode) may be due to absence of dense compaction of OD because of its nonlinear shape. Fig. 2 shows the N1s XPS spectra obtained after the deposition of each layer. Deposition of APhS on the silicon surface is confirmed by the main peak at 398.7 eV corresponding to the presence of free amine. There is also another small peak at 400.8 eV ascribed to the protonated amine due to side reactions [40]. A strong peak at 399.5 eV appeared and the original strong peak at 398.7 eV almost vanished after deposition of TC, which indicates the conversion of free amine to amide and formation of amide bonds between the APhS and TC layers. Subsequently, the N1s peak at 398.7 eV increased again after OD was assembled onto the TC layer, indicating that free amine groups in the film increased due to introduction of OD. The small increase in the free amine peak is consistent with the smaller thickness of the OD layer than that of the TC layer. When TFAA molecules were deposited on the OD layer, the N1s peak at 398.7 eV decreased sharply due to the formation of amide bonds between OD and TFAA layers at the expense of free amines of OD.
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N 1s
F 1s
Demethylation
Crosslinking
408
406
404
402
400
398
396
394
392
694
692
690
B.E. (eV)
688
686
684
682
680
B.E.(eV)
(a) N 1s
F 1s
Demethylation
Crosslinking
408
406
404
402
400
398
396
394
392
B.E. (eV)
694
692
690
688
686
684
682
680
B.E.(eV)
(b) Fig. 5. N1s and F1s XPS spectrum of the cross-linked film before and after cross-linking reaction (a: LCMF1; b: LCMF2).
The UV–visible spectra (Fig. 3) of the LCMF2 during each of the assembly steps show a continuous decrease in reflection at 224 nm with the increase in number of layers due to the absorption by phenyl groups in APhS, TC and OD and provides evidence of film deposition consistent with the ellipsometry results.
The demethylation reaction was characterized by C1s XPS spectrum (Fig. 4). The C1s peak at 286.1 eV (corresponding to the C–O group) decreased after demethylation due to the removal of lateral methyl groups of the OD, which is in agreement with the expected changes in the film. After demethylation reaction, a smaller C1s
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Fig. 6. Section images of multilayer films in each fabrication process (length: 1 lm, height: 1.5 nm) and surface plot of the multilayer film with two cross-linked layers (area: 1 lm 1 lm, height: 1 nm).
peak at 286.1 eV persisted because of the newly formed C–OH group (Scheme (Structure 6)) near the same binding energy. However, the number of the C–OH groups is only half of the number of C–O groups before demethylation (one C–O–C group equals to two C–O groups). The changes of C1s peak at 286.1 eV by demethylation are consistent with the structural conversion of OD from lateral methoxy groups to hydroxyl groups. Fig. 5 shows the XPS spectra after cross-linking. The ratios of nitrogen to fluorine (N/F) in LCMF1 before and after cross-linking were 4.3 and 6.1, respectively (1.3 and 1.7 respectively in theory according to Schemes 1 and 7), and those for LCMF2 before and after cross-linking were 2.73 and 3.14, respectively (1.7 and 3.3, respectively, in theory according to Schemes 1 and 8). The films
showed higher N/F ratios than expected, which may be due to the loosely compacted OD layer and the side reactions of the APhS layer. The additional presence of DCB after cross-linking led to the increase in N/F ratio. However, for LCMF2, the ratio of hydroxyl groups in different OD layers that have reacted could not be specified due to difficulties in resolving the difference in the cross-linking reactions between the two OD layers. Urethane bonds are formed when the two isocyanate groups of DCB react with the hydroxyl groups in OD layer. If one isocyanate group has not reacted, it will be hydrolyzed and decomposed in the air and new amine groups will be formed. Absence of a new amine peak after crosslinking suggests that most DCB molecules have reacted with two functional groups of OD. Therefore, all the vertically aligned molec-
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APhS 1st TC 1st OD 2nd TC 2nd OD TFAA Demethylation Cross-linking
RMS of the surface (nm) (LCMF1)
RMS of the surface (nm) (LCMF2)
0.102 0.086 0.085 – – 0.104 0.114 0.159
0.052 0.054 0.095 0.075 0.107 0.088 0.089 0.085
Relative thickness 100%
Layer
After crosslinking
1.0
Analyzing area: LCMF1, 300 nm 300 nm; LCMF2, 1 lm 1 lm.
0.8
0.6
Before crosslinking 0.4
0.2
0.0 0
20
40
60
80
100
120
o Time /min (90 C in water)
Fig. 9. Thickness of film with two cross-linked layers before and after hydrolysis.
Intensity
Fig. 7. Thickness of the LbL cross-linked (B) and linear multilayer films (C).
13000 12500 12000 11500 11000 10500 10000 9500 9000 8500 8000 7500 7000 6500 6000 5500
Relative Thickness 100%
1.0
Lateral crosslinked multilayer film (LCMF2)
0.8
0.6
0.4
LBL crosslinked multilayer film (B) 0.2
Linear multilayer film (C) 0.0 0
20 40 60 80 100 120 140 160 180 200 220 240
Time min-1 Fig. 10. Thickness of multilayer films during successive aqueous and alkaline hydrolysis (the films were hydrolyzed in hot water at 90 °C for the first 2 h and then eroded in 17.5 mM NaOH solution for another 2 h subsequently).
3.2. Construction of cross-linked multilayer film with multi-functional moieties 408
406
404
402
400
398
396
394
B.E. (eV) Fig. 8. N1s XPS spectrum of the LbL cross-linked multilayer film (LbL CMF).
ular chains on the surface have been effectively linked by lateral connections within the OD layer, thus confirming the efficiency of DCB as a cross-linking reagent for the OD layer. The efficiency may be ascribed to the dilute solution used (4.5 mM) that provided sufficient free space between OD chains and the suitable length of the DCB molecule. The surface topography of the LCMF2 during assembly was investigated by AFM (Fig. 6). The surface of the film after deposition of each layer was planar with roughness of ranging from 0.1 to 0.2 nm, indicating that the surfaces are even and uniform. The difference in roughness between LCMF1 and LCMF2 (Table 1) is mainly due to the APhS layer that may be on the account of side reactions when silane is used as the modifier of the silicon surface [41]. The small roughness values indicate that this can be controlled to sub-nano-scales by covalent LbL assembly.
Two multilayer films fabricated by LbL assembly with procedures similar to those discussed above are shown as Schemes 2 and 3 (Supporting information – Schematic S1). To fabricate these films, molecules with amine groups (TAB and PDA) and molecules with acyl chloride groups (TC) were deposited on the surface alternately for three cycles. For the cross-linked film (LbL CMF, Scheme 2), TAB with four amine groups was used. One TAB can react with four TC molecules that can facilitate formation of cross-linked structures after a three-cycle deposition. For the linear film (Scheme 3), PDA with two amine groups was used. The cross-linked and the linear films have comparative thickness, which is about 40 nm after three cycles of deposition (Fig. 7). The similarity in thickness is due to the comparable molecular lengths of the monomers. The cross-linked film was investigated by N1s XPS spectrum (Fig. 8). The ratio of nitrogen atoms in amine groups (398.7 eV) and amide groups (399.37 eV) was found to be 0.68, which is larger than that expected of a perfect cross-linked structure with a theoretical amine/amide ratio of 0.18. This may be attributed to steric hindrance, which may not permit all four of the amines in TAB to form amides. Therefore, it can be inferred that the cross-linked
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multilayer film has less cross-linking bonds than that illustrated in Scheme 2. 3.3. Stability of the cross-linked films 3.3.1. Comparison of the stability of the lateral cross-linked film (LCMF2) before and after cross-linking The stability of the LCMF2 before and after the cross-linking reactions was investigated by immersion of the film-laden substrate in hot water at 90 °C for 2 h. The thickness of the films was measured during the hydrolyzation test at periodic intervals (Fig. 9). Before cross-linking, the thickness of the film decreased by 35% within 30 min and then showed a slower rate of decrease. The decrease may be due to the loss of binding between APhS and the substrate that would result in loss of film material. This loss may be aggravated by the possibility that APhS molecules may only react with the hydroxyl groups of the silicon surface with one or two methoxy groups due to the side reactions and steric resistance [37,40]. On the other hand, the cross-linked film showed much better performance, losing about 12% thickness over 120 min. The film was strengthened at least by two factors: first, the molecules on the surface are bound by additional lateral cross-linking bonds, so that they cannot be easily deprived off the surface; second, water is more efficiently blocked out of the surface by three-dimensional network formed by cross-linking reaction. 3.3.2. Comparison of the stability of the cross-linked and linear films The stability of the two cross-linked films (LCMF2 and LbL CMF) was compared for film thickness by ellipsometry with the linear film (Scheme 3) by hydrolyzing the films successively in hot water at 90 °C for 2 h and in NaOH (17.5 mM) solution for an additional 2 h (Fig. 10). Ether and amide bonds in the film can be slightly hydrolyzed in hot water, and the silicon surface can be eroded by sodium hydroxide solution. Therefore, the test environment is really a severe condition. The two cross-linked films showed better stability than the linear film in hot water, and the lateral crosslinked film showed better stability than the LbL cross-linked film as well as the linear film in the NaOH solution. The LCMF2 structure was able to resist erosion to a greater extent on account of its higher degree of cross-linking that enabled more effective blockage of hydroxyl ions. The prevalence of cross-linking would have also enabled the film to retain its integrity even in the presence of ongoing erosion. Furthermore, as inferred from the XPS spectra for the LbL cross-linked film (Fig. 8), the effect of the incomplete cross-linking due to steric hindrance may also have contributed to the decrease in stability relative to that of the laterally cross-linked film. 4. Conclusions Ultrathin multilayer films were fabricated by covalent LbL assembly, in which molecules with multifunctional groups were used to obtain cross-linked structures. Interlayer and intralayer covalent bonds formed by use of appropriate functional molecules and fabrication methods yielded film structures with lateral and vertical links. The surfaces of the films were even and uniform with a small roughness of less than 0.2 nm. The lateral assembly has higher cross-linking efficiency and showed better resistance to erosion than the LbL cross-linked film in 17.5 mM NaOH solution. Both forms of cross-linked films showed better resistance to hot aqueous and alkaline environments than the linearly linked films, thereby attesting to the advantage of cross-linking to improve robustness.
Acknowledgment The authors thank the National University of Singapore for providing financial support for this project and a research scholarship for Zhou Ruitao.
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