Ind. Eng. Chem. Res. 2008, 47, 9195–9200
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Anomalous Crystal Growth on TiO2 Thin Film Induced by the AFM Tip Nan Yao and King Lun Yeung* Department of Chemical Engineering, the Hong Kong UniVersity of Science and Technology, Clear Water Bay, Kowloon, Hong Kong, People’s Republic of China
Tip-induced nucleation and crystallization have been observed in polymer thin films. This work shows for the first time that it is possible to induce a rapid crystal growth on amorphous inorganic thin film using the AFM tip. The force applied on the AFM tip exerts an enormous pressure on the surface of an amorphous titania thin film. The movement of the tip displaced the spin-coated titania sol and deformed the film. The V-shaped grooves left by the passage of the tip measured 65 nm in width and penetrated 5 nm into the 15 nm thick film. Heat treatment in air at 723 K crystallizes the TiO2. Large TiO2 crystallized on the grooves compared to the surrounding film indicating an anomalous rapid crystal growth. It is speculated that the enormous tip pressure caused a local densification and structural rearrangement of the titania that presage nucleation resulting in a faster crystallization. surface coatings,29,30clean energy production,31,32 and pollution treatments.32–44
Introduction The scanning probe microscope not only observes but can also actively intervene and manipulate surfaces and surface phenomena at the nanometer scale.1–3 Several scanning probebased lithography (SPL) methods have been developed in recent decades. The early approach uses the tip to mechanically sculpt the surface by physically displacing the material with the tip.4 This method is often used for soft materials to limit the damage to the probe tip during the lithography. Materials including soft, malleable metals and thin films, polymers (i.e., photoresist), and self-assembledmonolayeroforganicmolecules(i.e.,nanoshaving)5,6 were successfully sculpted by scanning tunneling microscopy (STM) and atomic force microscopy (AFM). The probe tip can nudge atoms and molecules on surfaces7 and can also induce chemical reactions for binding the molecules to the surface.8 The probe tip was used as a “nib” in the “dip-pen” nanolithography to “write” an ink of solvable materials on surfaces. Surface-active molecules,9,10 biomolecules,11–13 monomers,14 and inorganic sol15 were patterned on surfaces using this method. The intense electric field generated by the tip was used to locally melt and modify surfaces creating indentations and bumps on gold and silver.16,17 Dagata et al.18 exploit the enormously high electric field to induce local anodic oxidation of a silicon surface. The process is similar to conventional electrochemical anodization but with the probe tip acting as the cathode and the ambient moisture serving as the electrolyte.19 This method was used to create nanometer oxide patterns on Si,20 Al,21 Ti,22 metal silicide,23 SiN,24 III-V semiconductor,25 and even diamond surface.26 The AFM tip-induced nucleation and crystallization had been reported for polymer thin films.27,28 The objective of this work is to investigate whether it is possible to induce a similar phenomenon on an inorganic thin film. Titania crystallizes at moderate to high temperatures to brookite, anatase, and rutile phases and was selected for the present study. Titanium dioxide (TiO2) has many unique properties and finds applications in * To whom correspondence should be addressed. E-mail: kekyeung@ ust.hk.
Experimental Section The TiO2 coating sol (TIP-50) was prepared by vigorous mixing of a titanium isopropoxide (98%, ACROS) solution in ethanol with a 45 mM HNO3 (Fisher Scientific) in ethanol solution. The resulting sol was spin-coated on a clean Si(100) wafer at 6000 rpm (Specialty Coating System P-6204) to produce a thin, uniform film. The samples were kept at ambient temperature and humidity in a clean container until use. Imaging and lithography were done using Nanoscope IIIR AFM from Digital Instruments. The surface was imaged by tapping-mode AFM using a silicon cantilevered tip from Nanosensors, whereas lithography was carried out in contact-mode AFM using a SiN2 tip. Nanoscript software guided the movement of the tip across the surface during the lithography. A tip force of 0.032 N was applied on the film surface at a scan rate of 0.2 µm · s-1. The sample was heat-treated at 723 K after lithography to crystallize TiO2. The elemental composition of the sample was analyzed by X-ray photoelectron spectroscopy (XPS, Physical Electronics PHI 5600) with the monochromatic Al KR X-ray (350 W) at a shallow incident angle of 45°. Results and Discussions The surface morphology of spin-coated titania thin film on silicon was imaged by tapping-mode AFM and is shown in Figure 1. The spin-coated film displays a surface roughness of less than 2 nm (Figure 1a) and consists of small, uniform-sized spherical particles that have a diameter of 20 ( 5 nm (Figure 1b). The thickness of the film was measured by depth profiling. The surface elemental composition was monitored as layers of the film were etched away by sputtering. Figure 1c plots the surface elemental composition as a function of sputtering time. The concentration profiles of silicon (Si2p) and titanium (Ti2p) suggest a diffused interface between the titania thin film and silicon substrate, which is unlikely considering the low processing temperature. The broad interface is a sputtering artifact caused by the redeposition of sputtered-off titanium and silicon on the surface. A titania film thickness of 15 nm was obtained after accounting for the sputtering profile of titanium and silicon
10.1021/ie071181h CCC: $40.75 2008 American Chemical Society Published on Web 01/24/2008
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Figure 1. (a and b) Tapping-mode AFM images of titania thin film on silicon and (c) elemental composition along the thickness of the sample plotted as a function of the sputtering time. (Note: scan size is in micrometers.)
(eq 1). This thickness agreed with the AFM and ellipsometry measurements. titanium film thickness ) (TD) ⁄ Ttotal sputter time ) 15 nm
(1)
T - sputter time before Si was detected (T ) 6.25 min) D - total depth (D ) 60 nm) Ttotal sputter time - total sputter time during analysis (Ttotal sputter time ) 25 min) An applied force of 0.032 N to the AFM tip exerted a pressure of about 1012-1013 Pa (eq 2) on the surface of the titania thin film creating the parallel furrows shown in Figure 2. Each line measures 5 µm in length, and the distance between adjacent lines was fixed at 300 nm (Figure 2a) by the lithography software. The particles that decorate the pattern (e.g., A, B, and
Figure 2. (a and b) Contact-mode AFM image of the patterned titania thin film after lithography and (c) a cross-sectional analysis of the patterned thin film. (Note: scan size is in micrometers.)
C in Figure 2b) are mostly along the edges of the lines and are aligned with the pattern. These particles are materials displaced by the tip during the lithography. Since the pressure exerted by the probe tip during the lithography is enormous and the heat
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Figure 4. Tapping-mode AFM image of the patterned thin film after crystallization of anatase TiO2 phase at 723 K. (Note: scan size is in micrometers.)
Figure 3. X-ray photoelectron spectra of (a) Ti2p, (b) Si2p, and (c) O1s obtained from patterned area of the thin film sample after the heat treatment at 723 K.
generated by friction is not negligible, it is believed that the combined heat and pressure are responsible for the transformation of the loose titania sol displaced by the tip into the dense, crystal-like particles observed in the figures. These particles were mobile and often disappeared from the surface after continuous imaging in contact-mode, as they were swept to the periphery of the scan area by the raster movement of the tip. Figure 2c is a cross-sectional slice taken across the pattern (Figure 2a). The width along each line was uniform and measured 65 nm at the surface. It tapered to a depth of about 5 nm into the 15 nm thick titania film. The tapered shape of the furrows was a result of the pyramidal shape of the SiN2 probe tip. Film deformation and densification are expected at the enormous pressure exerted by the probe tip during the lithography. P ) F ⁄ (πr2) P - pressure F - AFM tip force (0.032 N) πr2 - SiN2 AFM tip area, and r is the radius of the tip (20-60 nm)
(2)
The sample was heated in air to 723 K for an hour to crystallize anatase TiO2. The XPS data for the Ti2p, Si2p, and O1s of the calcined sample are displayed in Figure 3. The Ti2p spectrum in Figure 3a contains two peaks at 459 and 465 eV assigned to the core levels of the Ti4+ 2p3/2 and 2p1/2, respectively, corresponding to the deposited thin titania layer. This confirmed the formation of a crystalline TiO2 after the heat treatment. The weak Si2p signal at 102.5 eV belonging to SiO2 was detected only at a high incident X-ray beam angle (Figure 3b) and is believed to originate from the substrate. The O1s signal in Figure 3c deconvolutes to give two peaks at 531.7 and 530.4 eV that belong, respectively, to the Si-O-Ti bond at the film-substrate interface45 and the lattice oxygen of TiO2.45,46 The thin film was re-examined by tapping-mode AFM after the heat treatment. Figure 4 shows the lines carved into the thin film were completely grown over by large particles of 60 ( 10 nm in diameter commensurate to the original width of the furrows. These particles were significantly larger than the surrounding TiO2 grains that have diameters of 12 ( 2 nm. This suggests an accelerated crystallization and growth of TiO2 at the patterned area. The enormous pressure exerted by the tip during the lithography is expected to deform and alter the titania as the tip plowed through the film. The densification could have promoted crystallization as was observed in the case of polymer thin films.27,28 A slight force applied on the AFM tip was sufficient to initiate the crystallization of lamella structure on amorphous polymer thin films. The tip deformed the polymer causing local densification (i.e., supersaturation) and rearrangement of the polymer into nuclei that eventually led to crystallization. Unlike polymers, inorganic materials crystallize at a higher temperature, and therefore the same tip-induced phenomenon could only be observed after the heat treatment. A previous in situ AFM study of silica and silica-alumina gel during aging at ambient conditions showed the grains grew faster along the gel periphery.47 The process occurred mainly through Ostwald ripening with the dissolution of smaller grains and transport of the dissolved species in the hydrated gel. This nanoscale rearrangement could be responsible for the observed anomalous growth, as aging could occur during the slow heating
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Figure 6. Tapping-mode AFM picture taken at the edge of the titania thin film after heat treatment at 723 K. (Note: scan size is in micrometers.)
Figure 5. In situ AFM images of patterned titania thin film during aging at (a) 298, (b) 373, and (c) 423 K. (Note: scan size is in micrometers.)
(1 K · min-1) of the titania thin film. Therefore, the patterned titania thin film was observed with time by in situ AFM in an environmental cell at aging temperatures of 298, 373, and 423 K. Figure 5 shows no evidence of preferred growth at these temperatures, and the pattern remained unchanged. Also, the titania grain growth was negligible. It is also possible that the growth was simply driven by the higher surface energy associated with film edges. In the experiment, half of the silicon surface was coated with 15 nm titania thin film and heat-treated under similar conditions at 723 K. It can be seen from Figure 6 that the TiO2 along the edges were only slightly larger than the average TiO2 grains in the
Figure 7. (a and b) Tapping-mode AFM pictures of titania thin film after overnight immersion in deionized distilled water and (c) after heat treatment at 723 K. (Note: scan size is in micrometers.)
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film and were much smaller than the 60 nm TiO2 observed in Figure 4. It is also important to note that, at 723 K, there is no noticeable growth of SiO2 from the silicon substrate. It is therefore difficult to attribute the appearance of particles within the patterned furrows to the formation of SiO2. The growth of SiO2 on silicon is slow at 723 K and in practice is carried at a much higher temperatures. It was also speculated that the tip may perturb the local water concentration in the titania thin film by either introducing water through the water bridge formed between the probe tip and the surface48 or by displacing water as the film is subjected to enormous compression during the lithography. Local water content could affect the grain growth as water concentration is known to affect the rate of hydrolysis and condensation reactions in sol-gel.49 Figure 7, parts a and b, shows the titania thin film immersed overnight in water displays a rougher surface with coarser grain size of 50 ( 12 nm compared to the original film (Figure 1, parts a and b). Water did indeed promote the growth of titania grain during aging at room temperature. However, Figure 7c shows there is no significant increase in the grain size after crystallization of anatase TiO2 at 723 K. Concluding Remarks This work shows for the first time that it is possible to induce a rapid crystal growth on amorphous inorganic thin film using the AFM tip. The AFM tip was used to exert an enormous local pressure causing film deformation that led to densification and rearrangement into incipient nuclei. Crystallization occurred preferentially along the patterned area during heat treatment resulting in the formation of large crystals. This shows that it is possible to use the AFM to manipulate the crystallization of inorganic materials at the nanometer scale. Acknowledgment The authors gratefully acknowledge the financial supports from the Hong Kong Research Grant Council and the technical support from the Material Preparation and Characterization Facility (MCPF) of the Hong Kong University of Science and Technology. Literature Cited (1) Wouters, D.; Schubert, U. S. Nanolithography and Nanochemistry: Probe-Related Patterning Techniques and Chemical Modification for Nanometer-Sized Devices. Angew. Chem., Int. Ed. 2004, 43, 2480. (2) Loos, J. The Art of SPM: Scanning Probe Microscopy in Materials Science. AdV. Mater. 2005, 17, 1821. (3) Yeung, K. L.; Yao, N. Scanning Probe Microscopy in Catalysis. J. Nanosci. Nanotechnol. 2004, 4, 647. (4) Nyffenegger, R. M.; Penner, R. M. Nanometer-Scale Surface Modification Using the Scanning Probe Microscope: Progress since 1991. Chem. ReV. 1997, 97, 1195. (5) Liu, G.-Y.; Xu, S.; Qian, Y. Nanofabrication of Self-Assembled Monolayers Using Scanning Probe Lithography. Acc. Chem. Res. 2000, 33 (7), 457. (6) Bernard, A.; Delamarche, E.; Schmid, H.; Michel, B.; Bosshard, H. R.; Biebuyck, H. Printing Patterns of Proteins. Langmuir 1998, 14, 2225. (7) Bouju, X.; Joachim, C.; Girard, C.; Sautet, P. Imaging and Moving a Xenon Atom on a Copper (110) Surface with the Tip of a Scanning Tunneling Microscope: A Theoretical Study. Phys. ReV. B 1993, 47 (12), 7454. (8) Dougherty, D. B.; Lee, J.; Yates, J. T. Assembly of Linear Clusters of Iodobenzene Dimers on Cu(110). J. Phys. Chem. B 2006, 110 (41), 20077. (9) Piner, R. D.; Zhu, J.; Xu, F.; Hong, S. H.; Mirkin, C. A. “Dip-Pen” Nanolithography. Science 1999, 283, 661. (10) Hong, S. H.; Mirkin, C. A. A Nanoplotter with Both Parallel and Serial Writing Capabilities. Science 2000, 288, 1808.
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ReceiVed for reView August 31, 2007 ReVised manuscript receiVed November 5, 2007 Accepted November 12, 2007 IE071181H
