Intaglio Nanotemplates Based on Atomic Force Microscopy for

ARTICLE pubs.acs.org/JPCC

Intaglio Nanotemplates Based on Atomic Force Microscopy for Ferroelectric Nanodots Jong Yeog Son,† Yun-Sok Shin,*,‡ Seung-Woo Song,‡ Young-Han Shin,*,§ and Hyun Myung Jang‡ †

Department of Applied Physics, College of Applied Science, Kyung Hee University, Suwon 446-701, Korea Department of Materials Science and Engineering and Division of Advanced Materials Science, Pohang University of Science and Technology, Pohang 790-784, Korea § Departments of Physics, Chemistry and EHSRC, University of Ulsan, Ulsan 680-749, Korea ‡

ABSTRACT: We demonstrate the nanointaglio process to form a nanotemplate and a series of nanoscale grooves using atomic force microscopy. Nanopores with a diameter of 20 nm were hoed on the surface of a SrTiO3 thin film epitaxially grown on a Nb-doped SrTiO3 substrate using a diamond tip. For the demonstration of the nanointaglio process, a nanotrench was formed as an intaglio with a length of approximately 150 nm and a width of approximately 20 nm. The nanointaglio process enables the fabrication of a nanotemplate with a highest packed hexagonal array, with a density of approximately 9.3  1014/m2. A ferroelectric poly(vinylidene fluoride-ran-trifluoroethylene) (PVDF TrFE) nanodot with a diameter of about 20 nm was formed by filling a nanopore with a PVDF TrFE solution, whose ferroelectric properties were confirmed using piezoelectric force microscopy.

’ INTRODUCTION Nanotemplates require well-aligned nanomaterials for the formation of nanodots and nanowires; some possible options are anodizing aluminum oxide (AAO) and diblock copolymers.1 9 For example, nanotemplates have been used to fabricate nanomaterials with highly packed, hexagonal patterns consisting of various materials such as metals, semiconductors, and insulators.8 A recent nanotemplate study reported the formation of nanomaterials with diameters as small as 10 nm, much improved over those from self-assembly methods.2,3 High performances of nanotemplates in the contexts of uniformity and alignment of each component, such as nanopores, have yet to be demonstrated in the micrometer range. Atomic force microscopy (AFM) has been widely used for surface observations, scratch nanolithography, and dip-pen nanolithography (DPN).10 18 Scratch nanolithography typically requires repeat dragging of the material surface to pattern nanostructures such as nanowires or nanosculptures.11,14 The successive dragging in scratch nanolithography limits the accuracy of the positioning and the uniformity of the nanostructures. In contrast to soft nanolithography, the DPN technique enables the formation of nanostructures with controllable sizes and precise positioning, as the AFM instrument can handle an extremely small tip with nanoscale accuracy.9,13 In this work, we demonstrate a nanointaglio technique with a hard AFM tip for use on the surface of SrTiO3 thin films19 grown on Nb-doped SrTiO3 substrates. Nanopores and nanotrenches were formed through a hoeing process using a diamond AFM tip. A highly packed nanotemplate was prepared with a density of approximately 9.3  1014/m2, forming a hexagonal array of nanoscale pores with diameters of approximately 10 nm and an r 2011 American Chemical Society

interpore distance of approximately 50 nm. For the demonstration of a polymer nanodot, a ferroelectric poly(vinylidene fluorideran-trifluoroethylene) (PVDF TrFE) nanodot with a diameter of about 20 nm was formed by filling a nanopore with a PVDF TrFE solution. Note that the nanointaglio technique based on AFM can provide nanotemplates to fabricate various polymer nanostructures with the precision of AFM.

’ EXPERIMENTAL SECTION Epitaxial SrTiO3 thin films with thicknesses of approximately 5 and 8 nm were deposited on Nb-doped SrTiO3 substrates using pulsed laser deposition (PLD). A commercially available 1-in.-diameter SrTiO3 target was used for deposition at a base pressure of ∼10 7 Torr. To produce an atomically flat surface on the Nb-doped SrTiO3 substrates (Nb doping level ≈ 1 wt % and resistivity ≈ 0.001 Ω cm), the surfaces of the SrTiO3 substrates were etched in a dilute HF solution and annealed for 1 h at 1000 °C. During PLD, the growth temperature and oxygen partial pressure were maintained at 800 °C and 0.1 mTorr, respectively. The surfaces of the SrTiO 3 thin films (5 and 8 nm) were hoed using a diamond AFM tip with a tip radius smaller than 10 nm, as confirmed by scanning electron microscopy. During the hoeing process, the diamond AFM tip (force constant k ≈ 3.5 N/m and tip diameter ≈ 10 nm) was oscillated with an amplitude of 50 nm at a frequency of 75 kHz. The morphology of the nanointaglio pattern was observed using the atomic force microscope with a sharp tip with a radius smaller than 5 nm. Received: March 31, 2011 Revised: June 7, 2011 Published: June 20, 2011 14077

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Figure 1. (a) AFM image of the SrTiO3 thin film on a Nb-doped SrTiO3 substrate with uniform terraces at intervals of approximately 100 nm. Scale bar = 200 nm. (b) SEM image of a diamond tip with a diameter less than 10 nm. Scale bar = 20 nm.

Figure 2. Schematic drawings of the nanointaglio process for a SrTiO3 thin film on a Nb-doped SrTiO3 substrate. (a) Nanointaglio by the hoeing process with a diamond AFM tip that was oscillated with an amplitude of A. (b) For the hoeing process, the AFM tip was set below the surface of the SrTiO3 thin film at a depth of d. (c) AFM measurements were conducted to monitor the current through the nanointaglio structures of the SrTiO3 substrate. (d) Nanopore array as a nanotemplate.

To confirm the exposed surface of the Nb-doped SrTiO3 after the nanointaglio process, conducting AFM (CAFM) measurements were performed with an applied bias of 0.2 V at the CAFM tip (Nb-doped Si, force constant k ≈ 6 N/m and tip radius ≈ 3 5 nm). A flat surface with a roughness of 0.3 nm and uniformity of the terraces at intervals of approximately 100 nm were confirmed in a 5-nm-thick epitaxial SrTiO3 thin film by AFM (Figure 1a). The flat surface and the uniform terraces were crucial to the surface deformation caused by the hoeing process, in which a diamond AFM tip with a diameter smaller than 20 nm was used, as shown in the scanning electron microscope (SEM) image in Figure 1b.

’ RESULTS AND DISCUSSION Schematic drawings of the nanointaglio technique applied to the surface of a SrTiO3 thin film deposited on a Nb-doped SrTiO3 substrate using a hoeing process with a diamond AFM tip are presented in Figure 2. The insulating SrTiO3 thin film was epitaxially grown on the metallic Nb-doped SrTiO3 single-crystal substrate. To hoe a part of the surface of the SrTiO3 thin film, the AFM tip was oscillated with an amplitude A and to a depth d from

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the surface of the SrTiO3 thin film, as shown in parts a and b, respectively, of Figure 2. The oscillations caused the AFM tip to strike the surface of the SrTiO3 thin film to a depth of d. In other words, the surface of the SrTiO3 thin film was hoed to a depth of d, producing a flake pile at the circumference. An AFM tip can hoe a nanopore through the surface of the metallic Nb-doped SrTiO3 substrate with a depth greater than the thickness of the SrTiO3 thin film, as demonstrated by measuring the current through the nanopore using a conducting AFM tip. This ability is due to the conductance of the exposed surface of the Nb-doped SrTiO3 substrate (Figure 2c). Furthermore, the nanointaglio process enables the formation of a nanotemplate in the form of a nanopore array, as illustrated in Figure 2d.8 First, nanointaglio was performed on a nanopore and a nanotrench, where the AFM tip was oscillated at a frequency of 75 kHz with A = 100 nm and d = 10 nm (Figure 3). Figure 3 shows AFM images of a nanopore as a function of the number of collisions between the AFM tip and the surface of the SrTiO3 thin film (10, 72, and 521 nm for parts a c, respectively, of Figure 3). The dimension of the nanopore increased because of the cone shape of the AFM tip as a function of the number of collisions until the AFM tip reached d = 10 nm and saturation. The increasing number of collisions caused the circumference of the nanopore to accumulate flakes, as in scratch lithography.11 For d = 10 nm, the nanopore exhibited a diameter of approximately 20 nm and a depth of approximately 10 nm, as shown in Figure 3c. Figure 3d shows a nanotrench with a length of approximately 150 nm and a width of approximately 25 nm produced using an AFM tip scanning speed of approximately 200 nm/s. Note that the nanopore and nanotrench can be ubiquitously used as components of any nanotemplate, such as AAO and diblock copolymer nanotemplates, for the fabrication of nanodots and nanowires.8 For example, we designed a nanotemplate composed of a SrTiO3 thin film grown on a Nb-doped SrTiO3 substrate. Thus, the surface exposure of the Nb-doped SrTiO3 substrate at a nanopore or a nanotrench could be observed using the CAFM method (Figure 3e h).20 CAFM images showed that the pattern was more distinguished with increasing d as the current to the tip of the CAFM from the Nb-doped SrTiO3 substrate increased. Figure 4a shows the depth of the nanopore as a function of the number of collisions with a diamond AFM tip on the surface of a SrTiO3 thin film. The depth of the nanopore nearly reached the 10-nm set point after 500 collisions. In nanointaglio patterning, the AFM tip unintentionally produced flakes near the shoulder and crevices, as shown in Figure 3c,d. The flakes could be effectively removed using an etching process with a HF solution. To demonstrate the etching process, an 8-nm-deep nanopore and a nanotrench were produced on the SrTiO3 thin film. The rate of HF solution etching of SrTiO3 depends on the crystalline orientation (Figure 4b), and the rate for (110) is 1.4 times higher than the rates for (100) or (010). The anisotropic etching rate caused the circular pattern to become rectangular. Parts c and d of Figure 4 show the rectangular nanopore and nanotrench, respectively, after etching in the HF solution for 50 s. Figure 4e,f shows CAFM images for the nanopore and nanotrench, corresponding to the AFM images of Figure 4c,d, where each pair of images shows an identical pattern. The nanointaglio process enables the formation of a nanotemplate with high density. Two nanotemplates were 14078

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Figure 3. Nanointaglio processes for a nanopore and a nanotrench. (a c) AFM images of a nanopore for (a) 10, (b) 72, and (c) 521 collisions of the AFM tip with the surface. The nanopore was 10 nm deep and had a diameter of approximately 20 nm. (d) AFM image of the nanotrench. The nanotrench was approximately 150 nm long and 25 nm wide. (e h) CAFM images of the nanopore and nanotrench corresponding to AFM images of a d, respectively. Sale bar = 20 nm.

Figure 5. Two types of nanotemplates with checkerboard and hexagonal nanopore arrays. (a) AFM image of a checkerboard nanopore array with a density of approximately 2.8  1014/m2. (b) AFM image of a hexagonal nanopore array with a density of approximately 9.3  1014/m2. Scale bar = 40 nm.

Figure 4. (a) Nanopore depth as a function of the number of collisions of the AFM tip with the surface. (b) Etching rates of a SrTiO3 surface depending on crystalline orientation. AFM images of the (c) nanopore and (d) nanotrench after surface etching in HF solution for 50 s. CAFM images of the (e) nanopore and (f) nanotrench. Sale bar = 20 nm.

fabricated: a square and a hexagonal nanopore array, as shown in parts a and b, respectively, of Figure 5. In the square array, each 20-nm-diameter nanopore was located on the vertex of a checkerboard pattern, where the center-to-center distance between nearest nanopores was approximately 50 nm, resulting in a density of approximately 2.8  1014/m2 (Figure 5a). Alternatively, the three nearest 20-nm-diameter nanopores formed the smallest equilateral triangle in the hexagonal array,

where a center-to-center distance of 50 nm between adjacent nanopores resulted in the highest density of approximately 9.3  1014/m2 (Figure 5b). Notably, the nanointaglio processes provided nanotemplates with greater precision and homogeneity than any other reported template method. The nanointaglio technique can be used to form nanodots. For example, a hemispheric PVDF TrFE nanodot with a diameter of about 20 nm was formed by filling a nanopore with a PVDF TrFE solution using dip-pen nanolithography. The PVDF TrFE nanodot was slowly dried for a couple of days at room temperature. The PVDF TrFE nanodot was annealed for 1 h at 140 °C for the crystallization of the nanodot. Parts a and b of Figure 6 show AFM images of the nanopore and the corresponding PVDF TrFE nanodot, respectively. Parts c and d of Figure 6 show piezoelectric force microscopy (PFM, in contact mode) images of the PVDF TrFE nanodot with upward and downward ferroelectric polarizations, respectively, where an external bias of 2 V (2 V) was applied to the PFM tip for the upward (downward) polarization. Figure 6e shows the piezoelectric hysteresis of the PVDF TrFE nanodot as a function of the applied voltage, where the coercive voltage was about 1 V. We postulate that the coercive voltage of about 1 V in the hemispherical PVDF TrFE nanodot with a diameter of 20 nm is due to the lateral scaling effect of ferroelectric nanodots.21,22 Note that the saturation of the ferroelectric hysteresis is not clear at a bias 14079

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Korean Science and Engineering Foundation funded by the Ministry of Education, Science and Technology (Grant R312008-000-10059-0), Republic of Korea. J.Y.S. and Y.H.S. acknowledge financial support from the Priority Research Center Program through NRF funded by the Ministry of Education, Science and Technology (2009-0093818) and from Basic Science Research Program through the National Research Foundation of Korea funded by the Ministry of Education, Science and Technology (2010-0028128).

’ REFERENCES

Figure 6. Ferroelectric PVDF TrFE nanodot formed by filling a nanopore with PVDF TrFE solution. (a) AFM image of a nanopore with a diameter of about 20 nm. (b) AFM image of the hemispherical PVDF TrFE nanodot. PFM images of the PVDF TrFE nanodot with (c) upward and (d) downward polarizations, where biases of 2 and 2 V were applied for upward and downward polarizations, respectively. (e) Ferroelectric hysteresis loop of the PVDF TrFE nanodot as a function of applied voltage. Sale bar = 20 nm.

that might be from leakage current through the PVDF TrFE dot at the bias.

’ CONCLUSIONS In this study, we demonstrated nanointaglio processes based on AFM for nanotemplates. SrTiO3 thin films grown on a Nb-doped SrTiO3 substrate were used in a hoeing process with a diamond AFM tip that was oscillated with an amplitude of 100 nm at a frequency of 75 kHz. The CAFM technique was used to crosscheck the qualities of the nanointaglio structures by monitoring the current through the nanostructures from the Nb-doped SrTiO3 substrate used as a bottom electrode. As a result, a nanotemplate of a hexagonal nanopore array exhibited a density of approximately 9.3  1014/m2. A PVDF TrFE nanodot, which exhibited ferroelectric characteristics, was formed by filling a nanopore obtained by the nanointaglio techniques with a PVDF TrFE solution.

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’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected] (Y.-S.S.), yhshin.at.uou@gmail. com (Y.-H.S.). Tel.: 82-31-201-3770. Fax: 82-31-201-8122.

’ ACKNOWLEDGMENT This work was supported by the Brain Korea 21 Project 2010 and the WCU (World Class University) program through the 14080

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