ARTICLE pubs.acs.org/Langmuir
Characterizing the Molecular Order of Phosphonic Acid Self-Assembled Monolayers on Indium Tin Oxide Surfaces Mark D. Losego,† Joshua T. Guske,‡ Alina Efremenko,‡ Jon-Paul Maria,† and Stefan Franzen*,‡ †
Department of Materials Science and Engineering Department of Chemistry North Carolina State University, Raleigh, North Carolina, United States
‡
bS Supporting Information ABSTRACT: Self-assembled monolayers (SAMs) of alkanephosphonic acids with chain lengths between 8 and 18 carbon units were formed on thin films of indium tin oxide (ITO) sputterdeposited on silicon substrates with 400 nm thermally grown SiO2. The silicon substrates, while not intended for use in near-IR or visible optics applications, do provide smooth surfaces that permit systematic engineering of grain size and surface roughness as a function of the sputter pressure. Argon sputter pressures from 4 to 20 mTorr show systematic changes in surface morphology ranging from smooth, micrometer-sized grain structures to 0.95). ITO Surface Structure. As described in the Methods and Materials section, the surface structure of ITO films deposited on SiO2/Si can be manipulated by adjusting the sputter pressure. Figure 3 shows representative AFM images of the ITO surface prepared over a range of sputter pressures. As expected, increases in sputter pressure cause a reduction in the grain size and an increase in the surface roughness, similar to previous findings.30 Figure 4 plots the root-mean-square (rms) surface roughness vs deposition pressure over a 2 μm 2 μm square area. From these AFM images and rms roughness values, we observe a clear onset of surface roughening and small-grained substructure at sputter pressures of 9 to 10 mTorr. At sputter pressures between 12 and
Figure 4. Root mean square (rms) roughness data calculated from 1 μm2 from each of the AFM images shown in Figure 3.
Figure 3. Atomic force microscopy (AFM) images of ITO on Si at deposition pressures ranging from 4 to 20 mTorr: A, 4 mTorr; B, 6 mTorr; C, 8 mTorr; D, 9 mTorr; E, 10 mTorr; F, 12 mTorr; G, 15 mTorr; H, 20 mTorr. 11885
dx.doi.org/10.1021/la201161q |Langmuir 2011, 27, 11883–11888
Langmuir
Figure 5. Summary of NEXAFS data for the deposition pressure series. (A) Magnitude of the peak-to-trough intensity differences from the 90 20° difference spectra as a function of deposition pressure. (B) Molecular tilt angles, calculated from both the σ*C H and σ*C C peaks. The error bars are the standard error of the tilt angle.
15 mTorr, the small-grained morphology is fully realized and surface roughness increases dramatically. Figures 3 and 4 show that it is not possible to completely separate grain size from surface roughness, since the lateral grain size is related to the height of surface features. Figure 5A plots the peak-to-trough magnitudes from the difference spectra for 1-octadecyl phosphonic acids prepared on each of these ITO surfaces. The 90 20° difference spectra are shown in Figure S6 of the Supporting Information. SAMs formed from 1-octadecyl phosphonic acid (ODPA) show consistent order for sputter pressures below 15 mTorr. SAMs deposited on films prepared at 15 and 20 mTorr show reduced order. We attribute this reduction in order to the increase in surface roughness. Calculations of the molecular tilt angle (Figure 5B) also confirm worse packing for ODPA monolayers prepared on the rougher ITO surfaces. Interestingly, despite the dramatic reduction in lateral grain size, ODPA monolayers prepared on the ITO sample deposited at 12 mTorr form well-ordered monolayers with a molecular tilt angle (σ*C C ∼25°) similar to the surfaces with substantially larger lateral grain sizes.
’ DISCUSSION Commercially available ITO coated glass substrates are generally fabricated using conditions that give an optimal combination of high conductivity and visible transparency. Because of the sensitivity of ITO composition and microstructure to deposition parameters, these “optimal” conditions can often be widely different from manufacturer to manufacturer leading to dramatic difference in surface structure and ultimately differences in molecular attachment and self-assembly on their surfaces. We have
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previously reported on this variability in commercially available ITO and its effects on depositing thiol and carboxylate monolayers.13 Our previous study further demonstrated that different lots of ITO from the same manufacturer can bring dramatic differences in ITO surface structure. A recent study by Chockalingam and colleagues20 also corroborated that variation in commercial ITO substrates can impact assembly of phosphonic acid monolayers on ITO. These previous works motivated our current study in which we have aimed to systematically control the ITO surface microstructure by preparing our own ITO films to gain a more direct understanding of the surface features that influence monolayer ordering on ITO. However, because polished glass substrates have relatively large surface roughness, it is difficult to directly control ITO microstructure. For this reason, we conducted the present study using atomically smooth silicon substrates upon which we can systematically engineer the ITO surface. We find that for long alkyl chain phosphonic acids (e.g., 18-octadecyl phosphonic acid) monolayer ordering is maintained even as the lateral grain dimension decreases to
