Synthesis and characterization of TiO 2 thin films on organic self-assembled monolayers: Part II. Film formation via an
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R. J. Collins Department of Chemistry, Case Western University, Cleveland, Ohio 44106
M. R. De Guire and A. H. Heuer Department of Materials Science and Engineering, Case Western Reserve University, Cleveland, Ohio 44106
C. N. Sukenik Department of Chemistry, Case Western University, Cleveland, Ohio 44106 (Received 1 August 1994; accepted 17 November 1994)
Crystalline, uniform, adherent, ultrathin films of TiO2 were deposited onto OH-functionalized organic self-assembled monolayers (SAMs) on single-crystalline Si at low-temperature (0.8 nm) decreases in film thickness (i.e., film stripping). The hydrolysis of the TFA was also monitored using XPS, by observing changes in the F and C ^ O carbon signals. In the phosphate buffer hydrolysis, ultrasonic treatment at pH 7.6 for 1 h showed 0.70% F at the surface; after 18 h, 0.12%; and after 25 h, 0.06% F (on the order of baseline noise). Figure 1 shows the carbon XPS spectrum of the hydrolyzed SAM. Curve fitting revealed three chemical states of carbon, with binding energies of 284.6 [(-CH 2 ) B ], 286.4 (-CH 2 Si0 3 ), and 288.4 eV (-CH 2 OH). The carbonyl C peak of the original TFA had completely disappeared. Curve fitting of the oxygen spectrum (Fig. 2) showed two oxygen states: 531.8 eV (oxide/siloxane) and 530.5 eV (CH 2 OH). B. Characterization of TiO2 thin films The XPS results in Figs. 3 and 4 illustrate the evolution of the TiO2 film as a function of immersion time under a controlled anhydrous atmosphere. After 1 h, the Ti spectrum (Fig. 3) was identical to that found in standard XPS spectra of TiO 2 , with peaks at 464.4 (Ti 2pm) and 458.7 eV (Ti 2/73/2).6"8 At early stages of film deposition (10-30 min), the single oxygen XPS peak [Fig. 4(a)] had a binding energy of 532.0 eV,
III. RESULTS AND DISCUSSION 286
A. Characterization of OH-functionalized SAMs The deposited TFA-bearing films on Si substrates showed contact angles for water of 101° (advancing) and 82° (receding). Their ellipsometric thickness was (on 700
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Binding Energy (eV) FIG. 1. Curve-fitting of the carbon XPS spectrum for OHfunctionalized SAMs.
J. Mater. Res., Vol. 10, No. 3, Mar 1995
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H. Shin et al.: Synthesis and characterization of TiO2 thin films on organic self-assembled monolayers: Part II
10Min (a)
30Min 531
535
528 (b)
Binding Energy (eV) FIG. 2. Curve-fitting of the oxygen functionalized SAMs.
XPS
spectrum
for
OH-
characteristic of oxygen in the SiO2/siloxane layers. Figure 4(b) shows a shoulder at 530.0 eV, typical of the T i - 0 bond and therefore indicative of surface attachment of Ti-alkoxide to the OH-functionalized SAMs. As a function of depth from the oxide film surface, the XPS spectra [Figs. 4(c) and 4(d)] clearly show the shift of oxygen binding energy from that characteristic of S i - 0 to that characteristic of Ti-O. Though the oxygen peak attributed to S i - 0 persists even after a 12 h immersion time, we do not think there are major pinholes in either the SAM or the TiO2 layer,
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