J. Phys. Chem. B 2001, 105, 1587-1593
1587
Radiation Damage of Alkoxy and Siloxy Ligands Bonded to Silica D. W. Werst* and E. I. Vinokur Chemistry DiVision, Argonne National Laboratory, Argonne, Illinois 60439 ReceiVed: October 17, 2000; In Final Form: December 7, 2000
The pattern of radiation-induced bond dissociation in alkoxy and siloxy ligands bonded to silica was studied using EPR spectroscopy. Bond-specific damage to ligands was deduced from observation of grafted and physisorbed radicals. Our results show that ligand damage predominates over desorption of ligands from the surface and are consistent with studies of radiation-induced modification of self-assembled monolayers on various types of substrates. Dissociation of C-H bonds is the dominant process, followed by C-C dissociation. In siloxy ligands losses of hydrogen atoms and alkyl groups were both major processes. Direct excitation of dissociative excited states of the organic ligands and ionization are both important damage mechanisms.
1. Introduction The scope of applications involving organic functionalization of inorganic, nanostructured materials is growing at a rapid pace with intense activity in the areas of microelectronics, optoelectronics, electrochemical interfaces, catalysis, separations, and microsensors. The attachment of organic ligands via ionic or covalent bonds to metals, semiconductors, and insulators is motivated by diverse aims that range from surface passivation1-5 to addition of chemical functionality.4 It is one way to shift surface states in and out of the band gap of semiconductor nanocrystals and control the relaxation dynamics of photoexcited electrons and holes.6,7 In general, organic ligands can allow the realization of versatile chemical reagents from metal and semiconductor nanocrystals by enabling them to be dissolved in a solvent, spun into a polymer film, or bound to other nanocrystals or molecules.4 The most investigated applications of functionalized surfaces involve patterning of self-assembled monolayers (SAMs) of organic molecules.8-18 The chemical effects of radiation exposure (from ion beams, electron beams, X-rays, and cosmic radiation) during processing, characterization, and applications of inorganic/organic hybrid materials are poorly understood. Experimental methods used to study SAMs, such as XPS and STM, can cause modification, resulting in data not representative of the pristine films.19 Indeed, the radiation-induced modification of SAMs is the basis for their use as resists in lithography. Few other techniques exist for the facile production of ultrathin, uniform organic films, and thus SAMs are leading candidates for resists in sub-100-nm patterning based on e-beam, X-ray, or scanning probe techniques. The optimization of resist properties of ultrathin organic films (higher contrast, sensitivity, tailored threshold energy, etc.) depends on understanding the physical and chemical changes induced by irradiation. Previous studies of e-beam and X-ray modification of SAMs have evaluated aggregate damage to the films using IR reflection-absorption,10,20-23 XPS,10,19,22-24 X-ray absorption,19,20 wettability,11,20,22,23 ellipsometry,11,22,23 H2 desorption,25 AFM,15-18 and SIMS.26 The most studied SAMs are organic monolayers of closely packed, aligned alkane chains, such as * To whom correspondence should be addressed. E-mail: dwwerst@ anl.gov. Fax: (630) 252-4993.
alkanethiols on metal or semiconducor substrates (for example, hexadecanethiolate on gold) and siloxanes on insulator substrates (for example, octadecylsiloxane on silica). Damage to the molecular constituents of the film results in cross-linking, disordering, increasing unsaturation, an increase in surface energy, and only minor reduction of film thickness. In other words, the surface is not selectively cleaned; the exposed regions are slowly degraded. Either the proximal bond to the surface remains intact or the desorbed ligand becomes entangled in the film (including cross-linking with other radicals). Nevertheless, it was shown to be possible to achieve pattern transfer to the substrate because the damaged area of the film allows easier penetration by a chemical etchant.16 Selective removal of residual carbon from exposed regions by a UV/ozone developing method was also demonstrated.18 These efforts show the promise of e-beam patterning of SAMs with spatial resolution of tens of nanometers. Further improvement in the efficiency of e-beam patterning of SAMs needs fundamental, bond-specific understanding of the radiation-induced chemistry of the grafted molecules that constitute the film. In the present study we adopted an approach different from that of previous studies of e-beam and X-ray modification of ultrathin organic films. We identified bond-specific damage to organic molecules grafted to silica using EPR spectroscopy. This information was revealed by the observation of radicals grafted to the silica and radicals physisorbed on the silica surface after radiation-induced bond scission. We studied isolated molecules and not thin films. Most of the energy deposited by the e-beam in our experiment is absorbed by the silica substrate and not directly by the organic ligands. Similarly, in high-energy (150 keV) e-beam lithography, secondary electrons generated in the substrate cause most of the film damage.20 Likewise, X-ray damage to SAMs is caused by secondary electrons emitted from the substrate as evidenced by the observation that substrates that emit a lower flux of electrons exhibit slower film damage under the same irradiation conditions.27 The systems chosen for our study were mesoporous silica powders derivatized with simple alkoxy and siloxy groups. The high surface areas of the silica powders vastly increase the sensitivity for spectroscopic measurements, and the mesoporosity aids in trapping of free radicals by immobilization on the surface at low temperatures. The choice of relatively simple
10.1021/jp003806u CCC: $20.00 © 2001 American Chemical Society Published on Web 01/30/2001
1588 J. Phys. Chem. B, Vol. 105, No. 8, 2001
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ligands (no more than five carbon atoms) allowed unambiguous assignment of the positions of bond cleavage, while exploring the dependence on molecular structure. These conditions optimized our ability to observe the primary chemical intermediates of radiation-induced ligand damage and to detect the occurrence of any desorption of ligands from the substrate. The latter process, ligand removal, should be easier to detect in the case of isolated ligands than for monolayer films. 2. Experimental Section 2.1. Materials. Two mesoporous silica powders were used in this study. One was Davisil silica gel (grade 643, 300 m2/g) from Aldrich. The second was MCM-41 silica, which we synthesized by two different hydrothermal procedures reported in the literature. One MCM-41 was synthesized using alkyltrimethylammonium (CnH2n+1(CH3)3N+, n ) 14) surfactant, and the reaction mixture was heated to 373 K for 5 days with pH adjustment to 10 with acetic acid.27 A second MCM-41 was synthesized in 2 h at room temperature using alkyltrimethylammonium surfactant, n ) 16.28 Each procedure yielded a product after calcination in air at 540 °C with mesopores slightly greater than 3 nm in diameter. Powder XRD in each case showed a very intense (100) diffraction peak and three weak (110), (200), and (210) peaks, indicative of the well-ordered hexagonal phase with narrow pore size distribution. The BET specific surface areas of the MCM-41 silicas are approximately 1000 m2/g.27 Organic functionalization of the silica surface was carried out by gas-solid reactions using alcohols and monochlorotrialkylsilanes as alkylating agents and silylating agents, respectively. All EPR samples were prepared in 4 mm o.d. Suprasil tubes. The silica powder was first heated to 450 °C for 4 h in a vacuum (
