Combining lithographic tools and functional self-assembled
monolayers
Citation for published version (APA):
Herzer, N. (2010). Combining lithographic tools and functional self-assembled monolayers. Technische Universiteit Eindhoven. https://doi.org/10.6100/IR675367
DOI:
10.6100/IR675367
Document status and date: Published: 01/01/2010
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Combining lithographic tools and functional
self-assembled monolayers
PROEFSCHRIFT
ter verkrijging van de graad van doctor aan de Technische Universiteit Eindhoven, op gezag van de rector magnificus, prof.dr.ir. C.J. van Duijn, voor een
commissie aangewezen door het College voor
Promoties in het openbaar te verdedigen
op donderdag 1 juli 2010 om 14.00 uur
door
Nicole Herzer
Dit proefschrift is goedgekeurd door de promotoren: prof.dr. U.S. Schubert
en
prof.dr. J.-F. Gohy Copromotor: Dr. S. Hoeppener
Kerncommissie:
prof.dr. U.S. Schubert (Technische Universiteit Eindhoven) prof.dr. J.-F. Gohy (Technische Universiteit Eindhoven) Dr. S. Hoeppener (Friedrich-Schiller-University Jena)
prof.dr. J. Rädler (Ludwig-Maximillians-University München) prof.dr. G.J. Vansco (University of Twente)
prof.dr. P.M. Koenraad (Technische Universiteit Eindhoven) Overige commissieleden:
Dr. U.C. Fischer (Westfälische Wilhelms-University Münster)
This research has been financially supported by the Dutch Organization for Scientific Research, NWO (VICI award for U.S. Schubert).
Cover design: Nicole Herzer
Printing: PrintPartners Ipskamp, Enschede, The Netherlands
Combining lithographic tools and functional self-assembled monolayers by Nicole Herzer Eindhoven: Technische Universiteit Eindhoven 2010
A catalogue record is available from the Eindhoven University of Technology Library. ISBN: 978-90-386-2259-0
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CHAPTER 1
PHOTOLITHOGRAPHY AND SURFACE REACTIONS ON SELF-ASSEMBLED MONOLAYERS 1
1.1. Introduction 2
1.2. Precursors for silicon-oxide based substrates 4
1.3. Photolithography of self-assembled monolayers 9
1.3.1. Degradation of self-assembled monolayers 10
1.3.2. Photodegraded surface templates of self-assembled monolayers 18
1.3.3. Photochemistry of self-assembled monolayers 20
1.4. Surface chemistry 25
1.5. Multifunctional surfaces 33
1.6. Conclusions 35
1.7. Aim and scope of the thesis 36
1.8. References 38
CHAPTER 2 PHOTOPATTERNING OF N-OCTADECYLTRICHLOROSILANE MONOLAYERS 47
2.1. Introduction 48
2.2. UV-ozone patterning of OTS monolayers 49
2.3. Sample target substrates with reduced spot size for MALDI-TOF MS based on patterned self-assembled monolayers 54
2.3.1. Fabrication of MALDI-TOF MS sample substrates 55
2.3.2. MALDI-TOF MS investigations 57
2.4. Fabrication of PEDOT-OTS patterned ITO substrates 60
2.4.1. OTS-PEDOT patterns 62
2.4.2. Fabrication of a patterned OLED 65
2.5. Multifunctional surfaces 66
2.5.1. Functionalization by pipetting, surface modification reactions and electro-oxidative lithography 67
2.5.2. Site-selective self-assembly of nanomaterials on multifunctional surfaces 76
2.6. Conclusions 82
2.7. Experimental section 84
CHEMICAL NANOSTRUCTURES OF MULTIFUNCTIONAL SELF-ASSEMBLED
MONOLAYERS 93
3.1. Introduction 94
3.2. Nanometer-sized patterns on glass 95
3.3. TOF-SIMS imaging of OTS-bromine nanopatterns on silicon 105
3.4. Conclusions 107
3.5. Experimental section 108
3.6. References 110
CHAPTER 4
SELF-ASSEMBLED MONOLAYERS FOR CELL ADHESION 113
4.1. Introduction 114
4.2. Trichlorosiloxane functionalized mPEG surfaces 115 4.3. Preparation and characterization of monofunctionalized surfaces 119
4.4. Preparation of charged surfaces 121
4.5. Physico-chemical properties of the functionalized surfaces 127 4.6. Preliminary cell adhesion experiments on self-assembled monolayers 129
4.7. Conclusions 131
4.8. Experimental section 131
4.9. References 135
CHAPTER 5
ELECTRO-OXIDATION OF N-OCTADECYLTRICHLOROSILANE MONOLAYERS
IN A DROPLET OF WATER 139
5.1. Introduction 140
5.2. Experimental setup and theoretical background 140
5.3. In-situ water contact angle measurements 143
5.4. Ex-situ investigations of the oxidized surfaces 146 5.5. Correlation with oxidations performed on the nanometer scale 152
5.6. Conclusions 153
5.7. Experimental section 154
SAMENVATTING 160
ABBREVIATIONS 164
CURRICULUM VITAE 166
LIST OF PUBLICATIONS 167
CHAPTER 1
PHOTOLITHOGRAPHY AND SURFACE REACTIONS ON
SELF-ASSEMBLED MONOLAYERS
ABSTRACT
Self-assembled monolayers have received increased attention over the last decades since their invention almost 30 years ago. Soon it was discovered that they can be used as resist materials and are compatible with conventional lithographic techniques commonly used in silicon semiconductor industry. Besides of these standard structuring processes self-assembled monolayers additionally provide other attractive tools to introduce also addressability into the patterns by selective functionalization with reactive precursor molecules and/or by applying suitable surface reactions. The following chapter focuses on photolithography with self-assembled monolayers being used as resist materials, which includes photodegradation as well as photochemical reactions. Furthermore, an overview over the functionalization schemes of self-assembled monolayers by chemical surface reactions as well as precursor synthesis is provided. These methods can be applied on non-structured as well as on non-structured surfaces to prepare substrates with tunable surface properties. In particular the chemical activation of surfaces is addressed by a large number of functionalization concepts which are introduced according to the class of chemical reaction that has been applied. These approaches provide a toolbox that can be used to introduce tailor-made surface functionalities to various systems. Finally, effective strategies to implement a diversity of chemical functionalities on one substrate are highlighted.
Parts of this chapter have been published or will be published:
C. Haensch, N. Herzer, S. Hoeppener, U.S. Schubert, Scanning probe microscopy as a tool for the fabrication of structured surfaces in nanostructured surfaces (Ed.: L. Chi), chapter 2, Wiley-VCH 2010, vol. 8, pp. 49124. N. Herzer, S. Hoeppener, U.S. Schubert, Chem. Comm. 2010, in press.
1.1. INTRODUCTION
Photolithography is an established and frequently used technique for the fabrication of micro- and nanometer-sized structures. The photoresists are being utilized to generally protect materials, e.g., semiconductor materials. Selective exposure of the resist material induces changes into the resist material that can be used to develop the resist and to provide a structured resist mask that can be used, e.g., to selectively etch semiconducting substrates.[1] Depending on the applied photoresist two main patterning modes can be used: the positive or the negative mode. Thereby, the properties of the resist material are of special importance. They are not only determining the possible structuring mode, but are also important for the quality of the resulting structures as their stability determines, e.g., the obtainable aspect ratios and resolution of the etched structures.[1-4] Thus the development of suitable resists by optimizing the polymer formulations is an active field of research.[5-7]
The obtainable feature sizes that can be produced with a resist material depend critically on the required thickness of a resist. Besides of the numerical aperture of the illumination system and the used wavelength of the light, the depth of focus (DOF) plays an important role for the obtainable resolution of the photolithographic process which is described by the modified Rayleigh equation.[8,9] 2 ( ) ( ) resist d DOF NA n , (1)
where λ is the wavelength of the used light, NA is the numerical aperture of the illumination setup, nresist is the refractive index of the resist material and d is the thickness of the resist.
Azuma et al. demonstrated that thin photoresist layers improve the resolution and lead to an improvement of proximity effects.[10]
In 1991 Calvert et al. reported the possibility to use self-assembled monolayers as resist layers.[11] Such layers have a thickness of just a few nanometers and represent therefore attractive systems to tune the resolution of photolithographic methods.
Two main categories of self-assembled monolayers, either thiol or silane based monolayers, are commonly used. Thiol based monolayers were introduced by Nuzzo et al.[12] in 1983 and silane based monolayers were introduced by Sagiv[13] in 1980. While thiol based monolayers react with gold or silver and form highly ordered self-assembled monolayers on both substrates, silane based monolayers can be used to coat technologically important substrates,
i.e., silicon wafers, glass and activated metal surfaces, like Al. This chapter will focus on
silicon-oxide based substrates, i.e., glass, silicon wafers, quartz, and their modification with mainly silane based self-assembled monolayers. For an overview of the use of thiol based
monolayers the reader is referred to reviews of, e.g., Ulman,[14,15] Everhart,[16] Whitesides et
al.,[17] Woodruff[18] and Mizutani.[19]
The use of silane based monolayers has important advantages. One of these advantages is the high stability of the monolayers because of the covalent network formed between the surface and the silane molecules consisting ideally of two bonds between the molecules and an additional bond formed with the surface. These intermolecular bonds can be used to form a laterally polymerized network resulting in an improved stability of the monolayers. This high stability allows in particular to perform further modification steps at elevated temperatures up to 210 ºC without affecting the monolayer.[20-2122] Besides of being used as resist materials self-assembled monolayers can also be actively used to modify the electronic properties of the support material. Peor et al. studied different monolayers self-assembled on SiO2 surfaces
to investigate their influence on the electronic properties of the silicon. Alkyl-, benzyl-, chloromethylbenzyl-, chlorobenzyl-, bromobenzyl- and iodobenzyltrichlorosilanes were self-assembled and studied by Kelvin Probe techniques. It was found that the major parameters for tuning the electronic properties are the coverage of the substrate and the molecular dipole moment of the molecules.[23] Rittner et al. investigated the electrical properties of
self-assembled monolayers on hydroxylated silicon surfaces. The authors utilized C18 alkyl chains
bearing methyl, thiol, thiophene, phenoxy and biphenyl end-groups. Of interest were here the insulating properties of the monolayers and their breakdown voltages. Iodine doping increases the conductivity, which suggests the possibility to fabricate a nanomolecular transistor by using the functional end-group as an active layer for the deposition of a conductive layer on the self-assembled monolayer dielectric layer.[24] Li et al. self-assembled ferrocene containing monolayers onto silicon-oxide and investigated the capacity and conductance of these systems. These monolayers are in particular interesting for applications
in memory devices, because they can be reversibly charged.[25] Self-assembled
monolayer/ceramic bilayer coatings play an important role for the protection for silicon devices or other electronic applications.[26] Furthermore, self-assembled monolayers on glass substrates could be investigated by optical techniques such as fluorescence spectroscopy. Lee
et al. reported the preparation of spot arrays for protein synthesis by patterning various silane
monolayers through a photoresist and subsequent perfluorination of the structured areas followed by amination. The authors coupled 9-fluorenylmethyl chloroformate (Fmoc) protected amino acids onto the glass surface. After fluorescent-labeling they were able to verify the completion of the reaction by fluorescence imaging. Afterwards the authors prepared a model library of biotin labeled amino acids and successfully replicated the
α-enzymatic digestion.[27] This approach represents an example where the functionality of self-assembled monolayers is combined with structuring techniques that integrate a next level of complexity into the device. The one spot arrays allow, e.g., a parallel screening and an integrated analysis of the coupling reactions, as well as permit, moreover, to decrease the amount of required analyte material for a successful analysis due to the small feature sizes. Thus, the integration of functional self-assembled monolayers into pattern frameworks represents a promising approach to create small functional structures. For such strategies the careful design of the self-assembled monolayers, as well as the possibility to structure them, plays an important role.
In this chapter it is intended to present an overview of used silane based monolayers as well as to summarize the synthesis of tailor-made precursor molecules that can be self-assembled onto silicon-oxide substrates. It will be discussed how such systems can be patterned by means of photolithographic approaches. Thereby, the self-assembled monolayers can be used as resist materials or they can be directly patterned by utilizing photochemical reactions. A variety of possibilities to introduce desired functionalities into monolayer systems is provided by the use of chemical surface reactions that tremendously expand the range of obtainable chemical functions. This concept will be discussed and a variety of chemical reaction mechanisms for the tailoring of surface properties and functionalities will be summarized. Illustrative examples will be highlighted by some recently reported examples. Even though a large variety of approaches to introduce surface functionalities has been described during the last decades, the fabrication of multifunctional surface patterns with more than two functionalities, in particular integrated on one substrate, still represents a major challenge. However, the integration of a large diversity of functionalities doubtlessly will allow the design of devices and functional surfaces with a higher degree of complexity and applicability. Therefore, the first attempts to tackle this challenge will be summarized in the last part of this chapter.
1.2. PRECURSORS FOR SILICON-OXIDE BASED SUBSTRATES
To obtain tailor-made functional self-assembled monolayers different strategies can be used. The preparation of self-assembled monolayers on silicon-oxide surfaces can be directly performed by utilizing either silane based precursor molecules, which self-assemble on glass or silicon-oxide,[13] or alternatively by the activation of the substrate with SiCl4 and HNEt2,
approach for the formation of self-assembled monolayers is the hydrogenation of silicon substrates and the subsequent reaction with alkene functionalized molecules.[30]
Each of these strategies has advantages and disadvantages that require a careful tuning of the functionalization conditions. Commercially available silane based precursor molecules are mostly functionalized with trichlorosilane, trimethoxysilane or triethoxysilane groups, which react with the surface to form a covalent network. In this respect the trichlorosilane is the most reactive precursor, while trimethoxy- and triethoxysilanes are less reactive. A large variety of functional moieties can be implemented into the silane based monolayers by utilizing the terminal groups of the silane precursors. However, these functional groups have to be compatible with certain criteria. The introduced functional groups should not interact with the surface and/or should not react with the silane groups to avoid the formation of multilayers or the destruction of the silane group, respectively. Furthermore, the spacer, usually an alkyl chain, plays an important role. Longer alkyl chains result in the formation of more stable and densely packed self-assembled monolayers.[13,31] Most of the patterning approaches concentrate on the use of commercially available silane based molecules such as
n-octadecyltrichlorosilane (OTS), 11-bromo undecyltrichlorosilane,
1H,1H,2H,2H-perfluoro-decyltrichlorosilane, N-[3-(trimethoxysilyl)propyl]ethylenediamine,
(3-aminopropyl)tri-methoxysilane (APTMS), (3-aminopropyl)triethoxysilane (APTES), N-(6-aminohexyl)-3-aminopropyltrimethoxysilane (AHAPS), (3-mercaptopropyl)trimethoxysilane (MPTMS) and poly(ethylene glycol) (PEG) silanes (Scheme 1.1).
Si NH NH2 MeO MeO OMe Si SH MeO OMe MeO Si NH2 OMe MeO MeO Si N H NH2 MeO MeO OMe Br Si Cl Cl Cl CH3 Si Cl Cl Cl n-Octadecyltrichlorosilane (OTS) 11-Bromo undecyltrichlorosilane N-[3-(Trimethoxysilyl)propyl]ethylenediamine
1H,1H,2H,2H-Perfluorodecyltrichlorosilane (3-Aminopropyl)trimethoxysilane (APTMS)
Si NH2 OEt EtO OEt (3-Aminopropyl)triethoxysilane (APTES) (3-Mercaptopropyl)trimethoxysilane (MPTMS) N-(6-Aminohexyl)-3-aminopropyltrimethoxysilane (AHAPS) CF3 CF2 CF2 CF2 CF2 CF2 CF2 CF2 Si Cl Cl Cl
Scheme 1.1 Schematic representation of the mainly used, commercially available silane
Frequently OTS and 1H,1H,2H,2H-perfluorodecyltrichlorosilane have been used in literature as passivation layers, because of the hydrophobic and chemically inert properties of the formed layers.[32,33] 11-Bromo undecyltrichlorosilane has been applied mostly to carry out surface reactions, such as substitution reactions, or has been applied as initiator for polymerization reactions.[34-38] Amino terminated self-assembled monolayers have been used for the binding of negatively charged nanoobjects,[39-42] as well as for surface reactions, such as, Schiff base reactions,[43-45] or amidations.[46-49] MPTMS is a well studied precursor
molecule utilized for the binding of nanoobjects, i.e., nanoparticles,[50,51] or
biomolecules.[52,53] PEG silanes have been extensively studied as protein repellent materials, which are of special importance for biorelated systems and find applications in cell and protein micropatterning.[54-56] It is also of significant importance for research related to the development of biosensors, lab-on-a-chip devices, tissue engineering, fundamental cell biology studies, drug screening or medical diagnostics.[57-60] Nevertheless, there is also significant interest in further enlarging the availability of precursor molecules.
Two different strategies can be used to synthesize a large variety of functionalized monolayers. The first approach concentrates on the synthesis of new precursor molecules that provide tailor-made functional groups, whereas the second strategy utilizes the implementation of new functionalities by the chemical modification of existing precursor layers.
Mainly three different approaches for the synthesis of precursor molecules are reported in literature to synthesize, e.g., trichlorosilane precursor molecules, which are summarized in Scheme 1.2. R OH n R SiCl3 n R Br n R CH n CH 2 1. (C4H9)3P 2. Mg 3. SiCl4 1. Mg 2. SiCl4 HSiCl3 cat. H2PtCl6
Scheme 1.2 Schematic representation of the syntheses routes to obtain trichlorosilane based
functional molecules.
The first synthetic approach to obtain precursors for silane based monolayers is described by Netzer et al. The authors started with a 10-undecenyl alcohol and converted the hydroxyl group into a chloride group. Afterwards the chloride group was activated by Mg and
subsequently tetrachlorosilane was added to form the trichlorosilane group. The extension of the length of the alkyl chain can be performed by adding oxirane or oxetane to introduce either two or three methylene groups to the magnesium activated molecule.[61] Other possible approaches are the synthesis of alkene functionalized molecules, which can be further converted to silane moieties by adding HSiCl3 and H2PtCl6 as a catalyst, or using bromine
functionalized molecules, which can be converted to silane molecules by adding Mg and subsequent addition of SiCl4 similar to the above described approach. Maoz et al. showed the
synthesis of trans-13-docosenyltrichlorosilane[62] and Wasserman et al. reported the synthesis of methyl 11-(trichlorosilyl)decanoate by a platinum catalyzed method, and the synthesis of 16-heptadecenyltrichlorosilane and 10-undecenyltrichlorosilane by a magnesium activated route.[63]
Based on these general strategies, a number of functional precursor molecules have been synthesized and are summarized in Table 1. The synthesis of cyano-, bromo-, thiocyanato- and thioaceto terminated C16 trichlorosilanes was introduced by Balachander et al. using a
platinum catalyst mediated process.[34] Additionally, the synthesis of iodo-, bromo-,
chloroacetate- and benzyl bromide terminated C16 trichlorosilanes has been established by the
same method.[64] Phtalocyanine molecules have been functionalized with an
alkyltrichlorosilane group by the platinum catalyst pathway and could be self-assembled on silicon substrates. These molecules were proposed to have potential applications in display technology, chemical sensors or photoconducting devices.[65] An 11-(3-thienyl)undecenyl-trichlorosilane was synthesized by the platinum catalyst method and was used for the self-assembly onto silicon-oxide surfaces. These self-assembled monolayers were proposed to be used for the polymerization of thiophene to form conductive layers.[66] In addition, a maleimido terminated alkyl trichlorosilane molecule has been synthesized by the platinum catalyst method. The maleimido group could be utilized for the covalent binding of nucleophilic heterocycles, alkylthiols or amines, which opens the possibility to attach a wide range of molecules onto the surface.[67] The synthesis of C10-12 alkyl chains terminated with a
functional hydroxyl group and a diphenylphosphine (PPh2) group was also reported; the
compounds were used for the self-assembly on silicon-oxide surfaces via the activation of the
surface by SiCl4 and diethyl amine, which resulted in the attachment of the
hydroxyalkylphoshine. The PPh2 end-group could be used as ligand for Rh catalysts on the
surface. Alternatively, the reaction of the Rh catalyst can be performed prior to the self-assembly process. The covalently attached Rh catalyst was tested for the hydrogenation of tolan.[68]
Method Molecule Reference Mg Cl3Si-(CH2)11- CH=CH-(CH2)7-CH3 Cl3Si-(CH2)9- CH=CH2 [62] [61] Platinum Cl3Si-(CH2)16-CN Cl3Si-(CH2)16-Br Cl3Si-(CH2)16-SCN Cl3Si-(CH2)16-SCOCH3 Cl3Si-(CH2)16-I Cl3Si-(CH2)16-OCOCH2Cl Cl3Si-(CH2)12-Ph-CH2Br
Phtalocyanine functionalized trichlorosilanes Cl3Si-(CH2)15-CH=CH2 Cl3Si-(CH2)9- CH=CH2 Cl Si Cl Cl n S N O O (CH2)11 Cl3Si [34] [34,64] [34] [34] [64] [64] [64] [65] [63] [63] [66] [67] Substitution N N+ Cl (CH2)3 (EtO)3Si N N+ C4H9 Cl (CH2)3 (EtO)3Si [69] [69] Surface activation via HNEt2 and
SiCl4
HO-(CH2)n-PPh2
n = 10-12
[68]
Synthesis via NEt3
and SiCl4 Cl3Si-O-(CH2CH2O)n-Ha Mn = 600 g/mol Mn = 1000 g/mol [70] [71]
Table 1.1 Schematic representation of the structures of the synthesized precursor molecules
(a idealized structure).
Zhang et al. demonstrated the formation of a PEG silane by the reaction of PEG with tetrachlorosilane. It was demonstrated that these layers can effectively suppress plasma protein adsorption and cell attachment on these surfaces.[70] Sharma et al. reported the development of ultrathin, uniform and protein resistant PEG films, which were stable under
PEG in anhydrous toluene, and successive addition of triethylamine. Afterwards tetrachlorosilane was added and the hydroxyl groups were converted to trichlorosiloxane groups. Different undesired site-reactions, e.g., the functionalization of PEG hydroxyl groups as well as the reaction of the tetrachlorosilane with several PEG chains, multilayer formation, might occur during this reaction. However, the PEG silane was shown to form homogenous films on silicon-oxide surfaces with roughnesses lower than 1 nm by optimization of the self-assembly conditions.[71] Chi et al. synthesized an imidazolium chloride functionalized triethoxysilane by the reaction of methylimidazole with a triethoxysilane functionalized alkyl chloride. The authors investigated the use of self-assembled monolayers formed by this precursor for the control of the wettability of silicon-oxide substrates by anion exchange. The water contact angle could be varied from 28° to 42° only by the exchange of the counter ion from chloride to PF6-.[69]
The availability of tailor-made functional groups for the binding and stabilization of nanoobjects or to perform subsequent chemical modification reactions is regarded as a major advantage of the use of tailor-made precursor molecules. The whole range of chemical
interactions, e.g., electrostatic and covalent binding, hydrogen bonding,
hydrophilic/hydrophobic interactions, complex formation, etc. are available to attach and stabilize nanomaterials to the structures. Moreover, the use of self-assembled monolayers turned out to be a versatile tool to implement a large variety of chemical functionalities that provides access to chemical surface reactions that can be performed also on the micro- and nanometer scale.
1.3. PHOTOLITHOGRAPHY OF SELF-ASSEMBLED MONOLAYERS
The combination of self-assembled monolayers and lithography techniques is an area of extensive research.[39,72-77] The potential applications of such nano- or micropattern range from biology to electronics and include, e.g., sensors, electronic devices, drug screening platforms, lab-on-chip devices, etc. The available pool of lithographic methods is large and includes methods such as soft lithography,[78-80] nanoimprint lithography,[80] local anodic oxidation,[81] electro-oxidative lithography,[80-82] e-beam lithography,[80,83] and photolitho-graphy.[80]
In particular the use of photolithography is an interesting approach, because it is a well-established method in semiconductor industry. Furthermore, it is an easy as well as fast method and essentially can be combined with all precursor molecules previously introduced.
The principle of this method is the placement of a photomask on top of the self-assembled monolayer functionalized surface and the subsequent irradiation with UV light (Scheme 1.3).
Substrate SAM Photomask UV-light Degradation Photochemistry
Scheme 1.3 Schematic representation of UV patterning process with self-assembled
monolayers as resist.
The self-assembled monolayers can be locally removed or can be photochemically converted in the non-covered areas. The degradation process of the monolayers and the involved photochemistry will be discussed in the following section.
1.3.1. DEGRADATION OF SELF-ASSEMBLED MONOLAYERS
Photolithography in general utilizes the light induced inscription of structures into a photoresist. Calvert et al. introduced in 1991 the use of self-assembled monolayers as
alternative resist materials.[11] Different self-assembled monolayers consisting of
phenyltrichlorosilane, benzyltrichlorosilane, APTMS, N-(2-aminoethyl-3-aminopropyl)tri-methoxysilane or (tridecafluoro-1,1,2,2-tetrahydrooctyl)-1-dimethylchloro-silane were used for the deep ultraviolet (DUV) photopatterning (Scheme 1.4). Irradiation was performed with different UV light sources operating at different wavelengths, such as, pulsed krypton fluoride (248 nm), argon fluoride (193 nm) lasers or high and low pressure mercury lamps. Fourier transform mass spectrometry of the photopatterned substrates revealed the presence of C6H6 mass fragments for the phenyltrichlorosilane and benzyltrichlorosilane monolayers,
which suggested the cleavage of the aromatic ring during the exposure to light. The exposure is also associated with a decrease of the water contact angle observed in the photopatterned areas.
Si O Si O Si Deep UV-light Substrate Phenyltrichlorosilane film Photomask Si O Si O Si H2O Si O Si O Si OH Si O Si O Si O Si Si Cl Cl Cl • •
Scheme 1.4 Schematic representation of DUV photopatterning of phenyltrichlorosilane and
subsequent attachment of the same precursor molecule to the exposed area (see ref. [11]).
The chemically modified areas formed in the illuminated areas were subsequently used to self-assemble a new monolayer in the exposed surface areas. This approach was introduced to fabricate in-plane bifunctional monolayer structures by partial illumination of the initial monolayer and subsequent self-assembly of other precursor molecules. Such patterns were used, e.g., to site-selectively attach fluorescein isothiocyanate and cells.[11]
The photolithographic degradation of the monolayers depends on a number of parameters,
i.e., the intensity and wavelength of the light source, the irradiation time, the distance of the
photomask and the substrate, as well as the atmosphere the self-assembled monolayer is exposed to during the irradiation process.
Calvert et al. found that the wavelength of the light source plays a crucial role. Phenyltrichlorosilane and benzyltrichlorosilane monolayers, which both exhibit a strong absorption at 193 nm, were irradiated with a 193 nm and 248 nm light source, respectively. At both irradiation wavelengths the monolayer could be removed, but it was observed that the required power was 8000 times lower when a wavelength of 193 nm was used instead of 248 nm. Thus, the active absorption of the molecular precursors enhances the degradation of the monolayer and, moreover, reduces the required patterning times.[11] OTS self-assembled monolayers have been prepared on different substrates, which provide silicon-oxide terminated surfaces, such as quartz glass or thermally oxidized silicon substrates. The OTS was patterned by a focused laser beam at a wavelength of 514 nm. The obtained line widths were found to be below the dimension of the laser focus spot of 2.5 µm indicating a nonlinear dependence of the patterning process on the laser intensity. These experiments furthermore
suggested that the monolayer degradation proceeds due to a photothermal excitation mechanism, which means that the decomposition of the monolayer takes place by a laser induced local temperature increase.[84]
254 nm DUV photolithography was applied on different self-assembled organosilane monolayers, such as, 7-octenyldimethylchlorosilane, 5-hexenyldimethylchlorosilane and 4-aminobutyl-trimethoxysilane. The removal of the organosilane self-assembled monolayers was additionally investigated by infrared (IR) spectroscopy which revealed that the methylene vibration signals vanished after 30 minutes irradiation time.[85] Interestingly, chemical transformations of the self-assembled monolayers were even observed if the irradiation time was reduced, although the monolayer was not completely degraded. In this case the formation of polar groups was observed. These polar groups could be used to self-assemble additional self-self-assembled monolayers to form stable bilayer structures.[86] Large area micropatterning by vacuum ultraviolet (VUV) light irradiation at a wavelength of 172 nm was performed on OTS. The degradation process was monitored by water contact angle investigations, ellipsometry as well as X-ray photoelectron spectroscopy (XPS) and indicated that a fast degradation process took place within 5 minutes.[87] A dependence of the
irradiation time with respect to the used wavelength could also be observed. As an example, a 25 minutes shorter irradiation time was required to pattern OTS by utilizing a wavelength of 172 nm compared to an irradiation wavelength of 254 nm. Sugimura et al. investigated the photopatterning of n-octadecyltrimethoxysilane (ODS) by 172 nm UV light by means of XPS, water contact angle investigations and friction force microscopy (FFM) for different irradiation times. The C1s region of the XPS spectrum revealed a strong signal for the monolayer. After irradiation for 10 minutes the intensity of this signal was significantly decreased. After 20 minutes treatment only minor further changes were observed. Tailing of the signal indicated the presence of aldehydes and carboxylic acid species, which was interpreted as a degradation of the monolayer via carbon oxygen species. The carbon signal did not vanish completely; however, reference measurements on a cleaned Si substrate revealed similar amounts of carbon species, e.g., because of contamination of the substrates. The FFM was performed also on different irradiated samples which showed the increasing friction contrast after longer treatment times. Micrometer lines with a width of less than 1 μm could be prepared.[88]
Also the atmosphere in the photoreactor chamber was found to be an influencing factor in the degradation process. In particular for non-adsorbing monolayers oxygen plays an important role for the removal of the self-assembled monolayers as oxygen absorbs the UV light and forms radical species which react with the self-assembled monolayers. Therefore, the
influence of the atmosphere in the reaction chamber was investigated under different aspects. Kim et al. explored the transformations of an ODS monolayer upon the exposure to UV light in the presence of an oxygen atmosphere. A setup was used that permit the variation of the distance of the light source and the substrate. This allowed the investigation of the role of the UV light for the degradation of an ODS self-assembled monolayer in the presence of active oxygen species. The change of the distance between the light source and the substrate was used to create a situation where the UV light was completely absorbed by the oxygen molecules within the irradiation chamber before it could reach the substrate. The irradiated surfaces were characterized by AFM, XPS, water contact angle goniometry and ellipsometry. It was found that the ODS molecules form polar functional groups under irradiation, such as, carboxylic acids, aldehydes and alcohols, due to chemical reactions with the active oxygen molecules.[86] The results demonstrated that the UV light is required for the formation of oxygen radicals which are neccessary for the oxidation process of the monolayer; however, it was also elucidated that the UV light was not directly mandatory for the oxidation and degradation of the monolayer. Hong et al. showed that the irradiation time can be reduced from 1200 s to 300 s by purging the photochamber above the photomask with nitrogen (Scheme 1.5). Photomask Excimer lamp VUV light N2 N2 N2 N2 N2 N2 N2 N2 N2 Air Self-assembled monolayer Substrate Quartz window
Scheme 1.5 Schematic setup of the VUV photolithography process for prevention of UV light
absorption by oxygen above the photomask.
This prevents the absorption of the VUV light by oxygen. The part between photomask and sample was flushed by air to provide a large amount of active oxygen near the sample for the degradation process. In this case the VUV light can interact with the surface as well as with the oxygen molecules.[89]
The influence of the distance of the photomask and the surface (Scheme 1.6) was also investigated. It could be demonstrated that larger distances of the photomask to the substrate promoted the photochemical reaction of the self-assembled monolayer but resulted also in broader feature sizes which could be caused by Fresnel diffraction or photoelectrons generated by the absorption of VUV photons in the self-assembled monolayer.[90,91] Also the irradiation time was observed to decrease for an increased gap distance for the photopatterning process due to an improved supply of active oxygen to the reaction center and a better removal of the reaction products from the reaction center.
variable distance
Scheme 1.6 Schematic representation of the variable gap approach to photopattern
monolayers.
The studies also suggested that the ODS self-assembled monolayer reacts with active oxygen under formation of carboxylic acid groups; this assumption was supported by XPS investigations. After prolonged irradiation times the ODS self-assembled monolayer was
observed to be converted to volatile species such as H2O and CO2 which leave the
surface.[90,91]
Ye et al. have performed additional investigations to understand the degradation process of the monolayer. The authors irradiated an ODS monolayer with UV light using a wavelength of mainly 254 nm. They investigated the degradation process by means of contact angle goniometry, FT-IR spectroscopy, XPS and subsequent labeling experiments. The main results of their investigations revealed that the degradation of the monolayer proceeded mainly by ground state atomic oxygen formation by UV irradiation of ozone. Due to the fact that the corresponding water contact angle decreased significantly but did not completely wet the surface, it was suggested that polar and unpolar functional moieties were created on the surface. Attempts to use FT-IR spectroscopy revealed a decrease of the methylene vibration, but exhibited also a complex kinetic of this process. The XPS results showed the decrease of
the carbon content, but oxygenated carbon species could not be clearly identified. Labeling experiments could prove the presence of some (<5%) functional polar groups. Thus, the degradation process was proposed to gradually proceed from the head groups. Based on these results Ye et al. proposed a model as a possible mechanism of the degradation process which is schematically depicted in Scheme 1.7.
OH CO2H CO2H CHO CO 2H CHO OH ● CHO O ● ● OH CHO O B1 B2 A
Scheme 1.7 Schematic representation of the proposed mechanisms possibly involved in the
UV degradation process of alkylsiloxane monolayers according to Ye et al.[93] A) Formation of polar functional groups, B1) formation of polar functional groups and radicals and B2) recombination of radicals.
The possible degradation mechanism might not be restricted only to the reaction of the terminal groups of the hydrocarbon chains and their conversion into polar functional groups,
i.e., carboxylic acids, alcohols, aldehyds (Scheme 1.7, A), but also radical formation might be
involved in the photodegradation process (Scheme 1.7, B1). These radicals could recombine subsequently and form unpolar species (Scheme 1.7, B2) which would explain the results on the non-complete wetting of the structures,[92] which might be also caused by remaining alkyl
chains.
Additional investigations of the degradation process use the VUV light photopatterning of alkene self-assembled monolayers, which were covalently attached onto hydrogenated silicon surfaces. Irradiation of 1-alkenes for 30 minutes demonstrated in XPS investigations the presence of shorter alkyl chains and the formation of carbonyl end-groups due to the generation of O and OH radicals. Additional Kelvin probe force microscopy (KFM) studies revealed that the surface potential of the tested 1-hexadecene and 1-octadecene monolayers
was explained by the formation of the negatively charged carbonyl species on the irradiated regions.[93]
KFM can also be used to investigate surface patterns consisting of two different self-assembled monolayers. Such substrates were fabricated by utilizing the created photodegraded areas to self-assemble a second precursor molecule that exclusively attach on the polar groups and/or the silicon-oxide substrate. A 172 nm VUV exposure system was used to introduce a micropatterning which resulted in the formation of 1 µm wide lines and circles larger than 2 µm.[89] Utilizing this approach bifunctional patterns consisting of ODS/heptadecafluoro-1,1,2,2-tetrahydro-decyl-1-trimethoxysilane (FAS17), ODS/3,3,3-tri-fluoropropyltrimethoxysilane (FAS3), ODS/4-(chloromethyl)phenyltrimethoxysilane (CMPS), ODS/AHAPS, silicon-oxide/FAS3, silicon-oxide/FAS17, silicon-oxide/AHAPS and silicon-oxide/ODS have been prepared and were studied by KFM (Figure 1.1). A surface potential contrast was observed in all cases. It was found that the surface potential of FAS3, FAS 17 and CMPS was lower compared to ODS and the surface potential for AHAPS was higher compared to ODS. All surface potentials were referenced to silicon-oxide. These measurements were in good agreement with surface potentials predicted from dipole moments of the corresponding precursor molecules, except for FAS3 which was explained by the lower density of the formed monolayer.[94-96]
Figure 1.1 KFM image of silicon oxide/ODS pattern. Reproduced from ref. [95].
Field-emission scanning electron microscopy (FE-SEM) and KFM investigations have been utilized to study photopatterned microstructures of FAS17 and ODS. FE-SEM investigations were performed at acceleration voltages below 1 kV of the electron beam as voltages of 5 kV did not show any detectable contrast. The region of ODS appeared to be brighter than the FAS17 areas; additionally performed KFM measurements confirmed that the FAS17 has a 180 mV lower surface potential compared to ODS. The difference in the electronic states between ODS and FAS17 caused by the negatively charged fluorine atoms was proposed as possible origin of this contrast.[97] The lower surface potential of fluorinated monolayers was
also reported by Venkataraman et al. who investigated surface chemical gradients of alkyl and fluorinated monolayers by XPS and KFM.[98]
Several investigations were performed to reduce the feature size of the obtained photopatterns. An ODS self-assembled monolayer was irradiated through a photomask with 157 nm light and nanostructures with less than 200 nm line width could be created by utilizing Fresnel diffraction at the aperture margins. Complex nanostructured patterns were fabricated by utilizing the dependence of the photochemical decomposition on the light intensity distribution at the substrate. The obtained experimental results correlated well with light intensity calculations utilizing the Fresnel equation.[99] Rapid patterning of ODS was demonstrated by a focused laser beam with a wavelength of 514 nm under ambient conditions. By this method line widths down to 100 nm were created with a laser spot diameter of 1.2 µm.[100]
Furthermore, a photocatalytical approach was introduced, which was shown to accelerate the degradation process. Lee et al. demonstrated the photocatalytic patterning of an alkylsiloxane self-assembled monolayer by pressing a quartz slide, which is partially coated with a thin film of TiO2, onto the self-assembled monolayer coated surface and subsequent irradiation
with 254 nm UV light. The patterning process was based on the fact that the decomposition of the self-assembled monolayer proceeds much faster on the areas which were in contact with the TiO2. The obtained patterns showed a line width of 400 nm and were obtained
within 2 minutes. Afterwards a ZrO2 layer was selectively deposited on the pattern by atomic
layer deposition.[101]
In summary, photopatterning of self-assembled monolayers represents a complex process which can be influenced by various parameters. Although photochemistry is being used for a long time the exact mechanisms is not yet completely understood. This is due to different energies required, e.g., to break C-C bonds of aryl or alkyl chains, active absorption of light at different wavelengths, different thermal stabilities, and others. Different groups have tried to obtain insights of the process. E.g., the degradation of alkyl chains and aromatic groups was studied. It was observed that the cleavage of alkyl chains required the presence of active oxygen, while aromatic groups can be cleaved only by UV-light irradiation. Since its introduction UV lithography has been utilized in a large variety of examples and has been developed into a frequently used tool to fabricate structured surfaces. Selected examples include several applications in device fabrication which specifically utilize different properties of the photolithographically patterned self-assembled monolayers.
1.3.2. PHOTODEGRADED SURFACE TEMPLATES OF SELF-ASSEMBLED MONOLAYERS
In particular, the compatibility of structured self-assembled monolayers with wet chemical etching techniques, or the application of electroless metal deposition processes directed by the inscribed structures, have attracted significant attention.
Photopatterned alkyl- and fluoroalkylsilane self-assembled monolayers have been utilized as masks for the wet chemical etching to transfer 2 μm width micropatterns with an edge resolution of 200 nm and 10 nm etching depth.[102] Even higher aspect ratios were obtained for 2 μm wide lines with a depth of 120 nm.[103] Additionally, wet chemical etching was utilized to fabricate a combination of nano- and microstructures. For this purpose photolithography was combined with AFM lithography to obtain micro- and nanopatterns on one substrate. Initially, a self-assembled monolayer was photopatterned obtaining the microstructures, subsequently, an additional nanopattern was created by tip induced AFM lithography on the non-irradiated areas. Afterwards the treated self-assembled monolayers were selectively etched with a mixture of NH4F/H2O2/H2O. The photopatterned structures
revealed 10 μm line width and the micrometer lines were connected by less than 100 nm thick lines fabricated by AFM lithography.[104,105]
Besides of their application as photoresists and etch masks, photopatterned self-assembled monolayers provide additional beneficial possibilities to fabricate functional metal nano- and microstructures. Those metal structures can be in particular interesting for the fabrication of nano- and microelectronic devices. Examples involve the site-selective deposition of Pd/Sn catalyst on photopatterned self-assembled monolayers with a line width of 400 nm and subsequent growth of Cu or Ni metal films by an electroless metal deposition process.[106] The electroless metal deposition process on photopatterned self-assembled monolayers has been implemented into a process to fabricate transistor test structures. Gate structures of 10, 5 and 1 μm could be realized.[85]
Additionally, the photopatterned self-assembled monolayers can be used to implement a second self-assembled monolayer introducing a chemical functionality, such as, amine, which can be used, e.g., for metal deposition or nanoparticle assembly. One example used an APTES/ODS pattern fabricated by photolithography on indium tin oxide (ITO) coated glass slides, which were subsequently applied to fabricate a two dimensional array of Au nanoparticles.[73] Furthermore, APTES/OTS patterns fabricated by photolithography were utilized for the selective Au metallization of the APTES.[72]
Besides of the selective binding properties also the wettability contrast present in photopatterned self-assembled monolayers can be used to fabricate tailor-made surface structures. Examples include, e.g., the photopatterning of ODS self-assembled monolayers by short-wavelength UV irradiation. In this process patterned hydrophilic domains surrounded by hydrophobic areas were created. These patterns were utilized to study the spreading of phospholipid vesicles. The vesicles formed a bilayer structure on the ODS monolayer (Scheme 1.8). O O O O O P O O O N+ POPC Masked OTS Illuminated OTS Boundary region = Moat
Glass or SiO2/Si
Scheme 1.8 Schematic presentation for the proposed morphology of lipid layers on patterned
ODS surfaces. The boundary region between the monolayer and the bilayer region was difficult to characterize and is assumed to reveal an ill-defined region.
The outer edge of the boundary region was used to absorb proteins or vesicles, whereby the inner boundary closer to the bilayer showed no absorption of proteins. These results provided the basics for the construction of complex biomembrane models, as well as for the fabrication of protein patterns.[107]
These selected examples demonstrate the broad applicability of photopatterned self-assembled monolayer systems and show, moreover, the wide diversity of structures and applications that can be targeted by the photochemical patterning of self-assembled monolayer systems. Potential further applications of patterned monolayers are seen, e.g., in microfluidics, sensor technology, wetting driven self-assembly and microelectronics.
1.3.3. PHOTOCHEMISTRY OF SELF-ASSEMBLED MONOLAYERS
While the previously introduced examples mainly concentrated on the local degradation and the removal of the monolayer, the formation of the polar groups during the degradation process can be actively used. Besides using the still not well-understood process (see above), well-designed precursor molecules can be applied that undergo photochemical activation. In the following paragraph this approach is summarized and selected examples for the use of this technique are highlighted.
A possible photochemical patterning approach was introduced by Liu et al., where the photolithographic patterning of a MPTMS was used. Thereby, a photomask was placed on the self-assembled monolayer and the surface was irradiated with UV light with a wavelength of 243 nm. This exposure leads to a transformation of the thiol end-groups to –SO3H terminal
groups in the non-covered regions. The pattern could be utilized for the self-assembly of Au nanoparticles, which selectively self-assembled on the thiol functionalized regions. This approach may find application in the assembly of microelectronic circuits and microbiosensors.[108] In addition, the photochemical transformation of chlorophenyl and
chlorobenzyl groups into aldehyde groups was demonstrated. Brandow et al. reported the self-assembly of p-chlorophenyltrichlorosilane which could be patterned through a photomask by irradiation with UV light (193 nm). Thereby, the chlorine groups on the irradiated region were converted into aldehyde groups. The latter ones were further converted to amines by treatment with NH4OAc and NaBH3CN. Subsequently, the amine moieties were
used for the self-assembly of a Pd(II) catalyst, which promoted the electroless deposition of nickel.[109] In another approach a p-chloromethylphenyltrimethoxysilane was self-assembled on a silicon-oxide surface and irradiated through a photomask with 172 nm VUV light. The
non-covered areas were transformed by changing the –CH2Cl groups into polar
functionalities. Mainly carboxylic acid groups were found to be formed. Thereby, hydrophilic-hydrophobic patterns were created which were used for the self-assembly of ODS onto the created polar functional groups. A spatial resolution down to 1 µm was obtained.[110]
The use of a photoinitiated polymerization on patterned substrates was reported to generate structured polymer films. A 3-(trichlorosilyl)propyl methacrylate was self-assembled and utilized for the photoinitiated polymerization of various acrylates, such as, poly(ethylene glycol) dimethacrylate. The monomer and the photoinitiator were spincoated onto the self-assembled monolayer and irradiated through a photomask with 365 nm UV light. This resulted in the formation of patterned polymer arrays with typical lateral sizes ranging from
600 µm to 7 µm. Revzin et al. demonstrated the incorporation of pH sensitive fluorophores into PEG hydrogels. As the pH sensitivity of the gel immobilized dye was comparable to the properties of aqueous buffer solutions these hydrogel arrays may be utilized in protein and cell adhesion experiments.[111]
Another photochemical pattern was fabricated by Chen et al. The authors demonstrated the UV light patterning of an (aminoethylaminoethyl)phenylsilioxane self-assembled monolayer by irradiation with a wavelength of 193 nm through a photomask. The benzylic C-N bonds were cleaved in the irradiated regions followed by the formation of aldehydes. The intact amines in the non-irradiated areas were used to bind DNA by first reacting the amine with succinimidyl-4-[maleimidophenyl]butyrate to form an ester bond and afterwards thiol functionalized DNA was covalently attached.[43]
A number of research groups have synthesized molecules which combine photosensitive groups with suitable anchor groups such as a triethoxysilyl group which can react with the surface to perform photochemical patterning. Buxboim et al. have designed a molecule containing a surface reactive triethoxysilyl group, a bioactive PEG spacer and a photocleavable 6-nitroveratrylmethyloxycarbonyl group, known as “DAISY” (Scheme 1.9).
MeO MeO NO2 O O NH O NH Si(OEt)3 O n Self-assembly on silicon surface MeO MeO NO2 O O NH O NH Si O n O O O UV-light 365 nm NH2 O NH Si O n O O O DAISY
Scheme 1.9 Schematic overview of the formation of “DAISY” monolayers and subsequent
formation of amine terminal groups by UV light irradiation.
“DAISY” can be cleaved under amine formation by irradiation with UV light with a wavelength of 365 nm. These molecules were self-assembled on silicon-oxide surfaces and irradiated through a photomask to form amine functionalized groups on the non-covered regions. On the amine functionalized region DNA was immobilized. As a result structures
extracts onto micrometer-scale traps was demonstrated. The presented approach was proposed as a suitable technology to develop biochip areas.[112]
A similar approach of performing photochemical patterning by attaching molecules with photosensitive groups was introduced by Stegmaier et al.. Two photoremovable protecting groups, namely 6-nitroveratryloxycarbonyl and diethylamino-coumarin-4-yl, were attached to an APTES molecule. These molecules were self-assembled on quartz slides and silicon surfaces. By irradiation with UV light an amine functionalized surface was obtained (Scheme 1.11a). Si MeO OMeOMe N H O O O2N MeO OMe Si MeO OMeOMe N H O O O O N Si O O O N H O O O2N MeO OMe Si O O O N H O O O O N Self-assembly on silicon-oxide or quartz slide surface Self-assembly on silicon-oxide or quartz slide surface Si O O O N H2 O O2N MeO OMe -CO2 -CO2 O H O O N 345 nm 412 nm 3. UV -light ir ra diat io n ( 345 n m ) 4. Coupl ing of amine w ith Ale xa fluor 488 1. UV-light irradiation (412 nm)
2. Coupling of amine with Alexa fluor 647
Unprotected amine (B) coupled with Alexa fluor 647 Unprotected amine (A) coupled with Alexa fluor 488
Unprotected amine (A and B) coupled with Alexa fluor 488 und 647 Protected amines (A and B)
A B
b) Patterning and fluorescent labeling
a) Self-assembly and photoclevage of A and B
Scheme 1.11 a) Self-assembly and photocleavage of
(7-(diethylamino)-2-oxo-2H-chromen-4-yl)methyl-3-(triethoxysilyl) propylcarbamate (right) and 3-triethoxsilylpropyl-N-(4,5-dimethoxy-2-nitrobenzyloxycarbonyl) amine (left) and b) photopatterning and subsequent fluorescent labeling.
By self-assembly of a mixture of 3-triethoxsilylpropyl-N-(4,5-dimethoxy-2-nitrobenzyloxycarbonyl) (A) and (7-(diethylamino)-2-oxo-2H-chromen-4-yl)methyl-3-(tri-ethoxysilyl) propylcarbamate (B) molecules and subsequent sequential irradiation with two different wavelengths of UV light bifunctional patterns have been created (Scheme 1.11b).
The photochemical reaction to deprotect the amines was confirmed also by subsequent labeling with fluorescent dyes (Alexa fluor 488 and 647).[113]
Another approach was used for the site-selective protein immobilization.[114] Molecules containing a photoreactive benzylthiocyanate group and a surface reactive trimethoxysilyl group were also introduced. These molecules were self-assembled onto a silicon-oxide surface and the thiocyanate group was isomerized to form an isothiocyanate under UV light irradiation (254 nm) in an inert atmosphere. A pattern was created by UV light irradiation of the thiocyanate functionalized surface, covered by a photomask. The selectively formed isothiocyanate was reacted with propylamine under formation of a thiourea bond (Scheme 1.10).[115] UV-light 254 nm Self-assembly on silicon-oxide surface Si O O O S N Si O O O N S Si O O O N H N H S NH2 Si S N MeO OMeOMe
Scheme 1.10 Schematic overview of the self-assembly of
trimethoxy[4-(thiocyanate-methyl)phenyl]silane on silicon-oxide and subsequent photoisomerization by UV light into isothiocyanate followed by the formation of a thiourea to bind amine functionalized molecules.
Ganesan et al. synthesized a diazoketo functionalized photoactive compound which contained a surface active trimethoxy silane group. By IR spectroscopy, AFM and ellipsometry investigations it was shown that self-assembled monolayers formed on glass and silicon-oxide. Selective UV light irradiation led to carboxylic acid functionalized self-assembled monolayers. The carboxylic acid was used for the immobilization of biotin which was then applied to bind streptavidin.[75]
Yang et al., e.g., have patterned an OTS self-assembled monolayers by synchroton X-ray (0.814 nm) and extreme UV light (13.3 nm) irradiation through a photomask. The presence of oxygen resulted in the formation of hydroxyl and aldehyde functional groups. Patterns with various shapes and sizes (50-0.15 μm) were obtained.[116] This reaction could be also performed with light of longer wavelength. Hong et al. demonstrated the photoreactivity of ODS by irradiation with VUV light of 172 nm wavelength through a photomask. The authors
place. The carboxylic acid moities were further used for the binding of fluorine functionalized silane molecules or (p-chloromethyl)phenyltrimethoxysilane.[117]
Frydman et al. reported the photochemical conversion of a 18-nonadecenyltrichlorosilane self-assembled monolayer into a thiol functionalized monolayer by treatment with H2S and
UV irradiation at 254 nm.[118] The formed disulfide bonds were cleaved by treatment with either NaBH4[119] or BH3 × THF.[120] The created thiol functions were subsequently used to
bind Ag and CdSe nanoparticles[121] to form macroscopic electrodes onto surfaces. This modification scheme was demonstrated to be compatible with electrochemical AFM tip induced oxidation techniques to create, e.g., conductive metallic wires with nanometer resolution, but has not yet been used in photolithographic patterning approaches.[119]
Furthermore, photochemistry can be utilized to induce photoisomerization reactions. Azo-silanes were synthesized and self-assembled on silicon oxide substrates. By irradiation with 360 nm UV light a trans-cis photoisomerization was observed whereas at 450 nm UV light irradiation a conversion of the molecule from the cis-isomer into the trans-isomer was triggered. The changes of the photoisomerization of the molecules were investigated by means of surface plasmon resonance spectroscopy which allowed the detection of very small changes in the thickness of the layers.[122] Stilbene functionalized alkenes were synthesized and self-assembled on a hydrogenated silicon surface. The stilbene was shown to be photoswitchable by UV irradiation as confirmed by AFM and contact angle measurements.[123] Also in this case the changes in the layer thickness were regarded as an indication for the reversible switching of the molecules triggered by light.
Another approach used the photochemical reaction of 1-alkenes with hydroxylated surfaces via Markovnikov addition. Thereby, a droplet of 1-alkene was deposited on the surface and on top a TEM grid was placed hold by a quartz slide, which reduced the evaporation of the alkene and allowed the absoption of UV light (185 nm). The sample was irradiated for 10 hours by a mercury lamp and the 1-alkene was covalently attached on the irradiated areas abd could be removed on the covered areas.[124] Those patterns were shown to be suitable for the patterning of microchannels and the site-specific immobilization of DNA. Thereby, 2,2,2-trifluoroethyl undec-10-enoate (TFEE) was used as 1-alkene and patterned as described above. On the patterned TFEE an amine functionalized fluorescent labeled DNA was attached by amidation.[125]
These examples were chosen to demonstrate the large diversity of photochemical reactions that can be applied to obtain tailor-made surface functionalizations. Their use as selective binding sites and wetting structures make these molecular layers a well-suited tool to
implement chemical functionalities into structured areas. In particular photoswitchable molecular layers might be of interest for detection and sensing applications.
However, photochemical reactions are just one possible class of reaction mechanisms that can be potentially used to enlarge the diversity of obtainable functional groups within the monolayers. They frequently require the difficult and time-consuming synthesis of suitable precursor molecules, which have to be optimized also regarding to their ability to form reliable monolayers. As a consequence the use of classical synthetic strategies can be sometimes advantageous for the modification of well-defined standard self-assembled monolayer systems. Possible reaction schemes include, e.g., substitutions, “click” chemistry, esterifications, Schiff base reactions, reductions, oxidations as well as polymerizations. Some examples of chemical surface reactions are discussed in the following paragraph which have partially been already implemented on patterned self-assembled monolayers. Major reaction processes which have been used to modify the surface functionalities are briefly summarized.
1.4. SURFACE CHEMISTRY
Additional possibilities to expand the capabilities of the combination of lithography with functional self-assembled monolayers emerge from the field of surface chemistry, which allows the implementation of a wide range of reaction schemes to obtain different surface functionalizations. These reactions have been partially performed on non-patterned surfaces, but also on the micro- and nanometer scale. Nonetheless, all described reactions are potentially suitable to be implemented also on photolithographically patterned surfaces. An overview of suitable surface reactions is provided in Scheme 1.12.
Br Substitution X N N N R “Click” reaction x= -NHR, -SR, -N3, -SCN, -CN, -SO3H, -SCOCH3 x= N3 Reduction x= N3 NH2 Reduction SH x= SCN X Oxidation x= -OH, -COOH R X O x= NH, O x= -COOH O R O x= -OH N H R O Amidation N H N H X R x= S, O Formation of thio- and ureas
NH R
Schiff base reaction
Esterification
Scheme 1.12 Schematic overview of possible modification schemes that can be implemented
by surface reactions.
The nucleophilic replacement is one attractive possibility to introduce functional groups on monolayer functionalized surfaces. Mainly bromine terminated monolayers are widely used for nucleophilic substitution reactions on surfaces (Figure 1.12) to generate a large diversity of chemically functionalized monolayers. In particular, Balachander et al. demonstrate the
replacement of the bromine end-group by azide or thiocyanate.[34] Furthermore, the
substitution of bromine or chlorine terminated self-assembled monolayers was utilized for the attachment of functional molecules, e.g., decanethiol, n-decylamine, p-nitrothiophenol, glutathione, terpyridines and cysteine or, for the introduction of functional moieties such as thioacetate, sulfonate and nitrile.[35,36,64,126]
The concept of “Click” chemistry is since its introduction by Sharpless et al.[127] a frequently utilized reaction tool in organic chemistry (Scheme 1.12). Since this first report many different reactions have been introduced and examples for the functionalization of surfaces by means of “click” chemistry, mainly utilizing 1,3-dipolar cycloaddition of azides and acetylenes, emerged.[128-131] The first example of utilizing click chemistry to functionalize a
silicon-oxide surface by using azides and acetylenes was reported by Lummerstorfer et al.[132] Rohde et al. described the activation of a hydrogenated surface by chlorination to bind
sodium acetylene. The acetylene functionality was utilized to click an electroactive benzoquinone (Scheme 1.13). The now covalently attached benzoquinone was reduced to a primary amine group by applying a voltage which was used to covalently bind a ferrocene complex via an amide linkage.[133]
H O O N H O N3 Cu(I) catalyst + N N N NH O O O -800 mV N N N NH2 OH O O N N N NH N,N’-diisopropylcarbodiimide a) b) c)
Scheme 1.13 a) Schematic representation of the 1,3 dipolar cylcloaddition of an acytelene
functionalized surface with azide functionalized benzoquinone, b) electrochemical cleavage of the benzoquinone under formation of amine terminated surface and c) coupling of ferrocene to the surface via amide linkage.
Ciampi et al. reported on the covalent immobilization of diacetylene compounds on hydrogenated silicon surfaces via a hydrosilylation procedure. Afterwards the alkyne
end-group was used to click various azide compounds.[134] Furthermore, the clicking of
molecules, i.e., polymers,[135] and dyes,[136] was demonstrated. Click reactions have been also used in combination with photolithographic patterning. Thereby, an n-octyldi-methylchlorosilane monolayer was gradually modified by an UV lamp to generate ozone to form acid groups. These acid groups have been utilized to bind acetylene terminated molecules, which were further used to click peptides via the 1,3-dipolar cycloaddition by formation of triazole rings.[137]
Another possible reaction scheme to bind functional molecules on a surface is the esterification or amidation, which utilizes the reaction of an acid or ester group with a hydroxyl or with an amine group. Either carboxylic acid or ester functionalized molecules can be introduced onto the surface; alternatively, amine or hydroxyl functionalized molecules can be used.[22,43,46-48,126,138] Zhang et al. patterned a self-assembled 1H,1H,2H,2H-perfluorodecyltrichlorosilane monolayer by electron beam lithography. APTMS was subsequently self-assembled on the irradiated spots and further functionalized with biotin by
a reaction of the amine group with an activated acid group attached to the biotin to form an amide bond. By this method nanostructures down to 250 nm have been created.[49]
The efficient attachment of amines to carboxylic acid functionalized surfaces can be performed by the activation with N-hydroxysuccinimide (NHS) (Scheme 1.14).
NHS activation of carboxylic acid Amidation RNH2 N O O OH O O H N O O O O R NH O
Scheme 1.14 Schematic representation of the NHS activation of carboxylic acid for the
binding of amine functionalized molecules.
In this approach NHS reacts with the carboxylic acid group under formation of a succinate ester.[139] This method has been utilized by Fabre et al. who self-assembled an ethyl undecylenate and a 1-decene monolayer on a hydrogenated silicon surface. The ethyl undecylenate self-assembled monolayer was activated with NHS by immersing the substrate into a solution of 1-(3-dimethyl-aminopropyl-3-ethylcarbodiimide) and NHS. Onto the NHS activated ethyl undecylate self-assembled monolayer a 2-aminoethylferrocenylmethylether was covalently attached under formation of the amide bond.[140]
Amine terminated surfaces can be functionalized with aldehyde molecules by Schiff base reactions to form an imine bond or vice versa (Figure 13).[43-45,141] La et al. demonstrated the nanopatterning of self-assembled monolayers by the selective chemical transformation induced by soft X-ray irradiation. For this purpose (3-aminopropyl)diethoxymethylsilane was self-assembled on a silicon-oxide surface and was functionalized with 4-nitrobenzaldehyde or 4-nitrocinnamaldehyde under formation of an imine bond. A nitrosubstituted phenyl-imine self-assembled monolayer was converted into a secondary amine monolayer by selective X-ray irradiation. The non-irradiated areas were afterwards hydrolyzed to amines, which can be further react with a Cy3-tagged oligonucleotide. The selective conversion of the amine with the Cy3-tagged oligonucleotide was proven by fluorescence imaging.[142] Schiff base
reactions have been applied in particular for the binding of biomolecules, which make this reaction scheme in particular interesting for future applications in bioassays, cell studies and others.
Additionally, the formation of thioureas and ureas was used to introduce functional moieties into self-assembled monolayers (Scheme 1.12).[46,143] This chemical reaction could be
