Reversible Nanopatterning on Self-Assembled Monolayers on Gold

Apr 5, 2008 - The reversible fabrication of positive and negative nanopatterns on 1-hexadecanethiol (HDT) self-assembled monolayers (SAMs) on Au(111) ...
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J. Phys. Chem. C 2008, 112, 6597-6604

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ARTICLES Reversible Nanopatterning on Self-Assembled Monolayers on Gold Zhikun Zheng, Menglong Yang, and Bailin Zhang* State Key Laboratory of Electroanalytical Chemistry, Changchun Institute of Applied Chemistry and Graduate School of Chinese Academy of Sciences, Chinese Academy of Sciences, Changchun 130022, People’s Republic of China ReceiVed: September 25, 2007; In Final Form: February 11, 2008

The reversible fabrication of positive and negative nanopatterns on 1-hexadecanethiol (HDT) self-assembled monolayers (SAMs) on Au(111) was realized by bias-assisted atomic force microscopy (AFM) nanolithography using an ethanol-ink tip. The formation of positive and negative nanopatterns via the bias-assisted nanolithography depends solely on the polarity of the applied bias, and their writing speeds can reach 800 µm/s and go beyond 1000 µm/s, respectively. The composition of the positive nanopatterns is gold oxide and the nanometer-scale gold oxide can be reduced by ethanol to gold, as proved by X-ray photoelectron spectroscopy (XPS) analysis, forming the negative nanopatterns which can be refilled with HDT to recover the SAMs. The inked material of ethanol acts as a reductant which is transferred to the substrate for the local chemical reactions like that in dip-pen nanolithography (DPN). The negative nanopatterns can be used as templates, for example, for the immobilization of magnetic nanoparticles. Interestingly, we found that the nanometer-scale gold oxide was very stable on hydrophobic HDT/Au(111) in air, whereas on hydrophilic SAMs it decomposed soon and resulted in the formation of the negative nanopattern. In addition, the effect of bias on the nanolithography was investigated.

1. Introduction The atomic force microscope (AFM) is one of the most promising tools for nanofabrication for its feasible nanomanipulation and high-resolution imaging capabilities.1 Generally, AFM nanolithography includes mechanical- and bias-assisted techniques. Typical mechanical-assisted techniques include dippen nanolithography (DPN),2 nanoshaving/nanografting,3 thermomechanical writing,4 mechanical indentation, and plowing.5 Among them, DPN, using inked tips to deliver materials to substrates with nanometer precision, is a simple but powerful technique for nanofabrication.2 In comparison with the mechanical-assisted lithographic techniques, bias-assisted nanolithography provides an additional way for the electric control and characterization. Depending on the magnitude of bias, substrate materials, and environments, the application of the bias can lead to local anodic oxidation of various substrates,1 surface group conversion of self-assembled monolayers (SAMs) on silicon,6 electrochemical removal of SAMs on gold,7 and nanoscale explosion and shock wave propagation.8 Recently, Zhao and Uosaki reported that only negative patterns were obtained by bias-assisted nanolithography on hydrophobic thiol SAMs at both positive and negative bias in toluene containing water,7 whereas Jang et al. reported negative patterns on hydrophilic thiol SAMs and positive patterns on hydrophobic thiol SAMs by bias-assisted nanolithography at a negative tip bias in air.9 Obviously, there is controversy between these results, and * Corresponding author. Phone and Fax: +86 431 85262430. E-mail: [email protected].

additionally, only positive or negative nanopatterns were fabricated in each process in their experiments. Recently, bias-assisted nanolithography has been demonstrated to have a great combining capability with DPN.9-11 Liu et al. fabricated metallic and semiconductor lines by applying an appropriate bias between silicon substrate and metal salts coated tip.10 Cai and Ocko fabricated multiple chemical patterns by writing trialkoxysilanes with zero bias on the oxidized regions made by the biased tip on octadecyltrichlorosilane-coated silicon wafer using electropen nanolithography.11 However, their fabrication processes are irreversible. A good reversibility, that is, the realization of construction (writing), elimination (erasing), and reconstruction (rewriting) of nanofeatures in one process, is really necessary in some application of nanotechniques such as reversible ultrahigh density data storage.4,12 Developing a reversible strategy can strongly upgrade the controllability and applicability of a nanolithography technique. Recently, a “molecular eraser” for DPN was developed by Jang et al.9 Erase or repair of hydrophilic SAMs-based nanopatterns was realized by bias-assisted nanolithography in combination with DPN.9 In this paper, we demonstrate that both positive and negative nanopatterns can be fabricated on HDT SAMs on Au(111) by bias-assisted nanolithography. The controllable formation of positive and negative nanopatterns depends on the polarity of the applied bias. The chemical composition of the positive nanopatterns is gold oxide, and the nanometer-scale gold oxide can be reduced by ethanol to gold, as proved by X-ray photoelectron spectroscopy (XPS) analysis, forming the negative nanopatterns which can be refilled with HDT to recover the

10.1021/jp077684i CCC: $40.75 © 2008 American Chemical Society Published on Web 04/05/2008

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Zheng et al. 2.4. XPS Analysis. XPS analysis was performed using an ESCALAB 250 from Thermo VG Scientific equipped with an Al KR or Mg KR monochromatic source with 120 µm spot. The spectra were referenced to metal gold (4f7/2) at a binding energy (BE) of 84.0 eV. 3. Results and Discussion

Figure 1. Three-dimensional AFM topographic image of six lettershaped nanopatterns with alternate positive and negative features on HDT/Au(111) fabricated by bias-assisted nanolithography at a bias of 3.0 and -3.0 V, respectively, under a relative humidity (RH) of 73% ( 5%.

SAMs. More importantly, we demonstrate a strategy for the reversible fabrication of positive and negative nanopatterns on HDT/Au(111) in air via bias-assisted nanolithography with a reductant inked tip. In addition, the effects of bias, writing speed, and terminal group of the matrix SAMs on bias-assisted nanolithography were investigated. 2. Experimental Methods 2.1. Materials. 11-Mercaptoundecanoic acid (HS(CH2)10COOH, MUA), 16-mercaptohexadecanoic acid (HS(CH2)15COOH, MHA), 1-hexadecanethiol (CH3(CH2)15SH, HDT), and 11-mercapto-1-undecanol (HS(CH2)11OH, MUD) were used as received from Aldrich. R-Fe2O3 hydrosol was synthesized according to Huo et al.’s method.13 2.2. Preparation of SAMs on Au(111) Substrate. Gold single-crystal bead was prepared by melting one end of a gold wire in a hydrogen-oxygen flame. After being annealed in the flame and quenched in Milli-Q water, it was immersed in a 1.0 mM ethanol solution of thiols at room temperature for at least 24 h. Prior to use, the SAMs were rinsed copiously with ethanol and blown dry with high-purity nitrogen (99.999%). 2.3. Bias-Assisted and Dip-Pen Nanolithography. Commercially available Cr/Pt-coated AFM tips (BS-Cont E probe with a spring constant of 0.2 N/m and a tip curvature radius of about 25 nm, Innovative Solutions Bulgaria Ltd. or CSG 11/Pt with a spring constant of 0.1 N/m and a tip curvature radius of about 35 nm, NT-MDT Co.; both were coated with 5 nm chromium then 25 nm platinum) were cleaned by dipping in “piranha” solution (a mixture of 30% H2O2 and concentrated H2SO4 by 3:7 (v/v). Caution! Piranha solution reacts violently with organic materials) for 30 min at room temperature, rinsed copiously with Milli-Q water, ethanol, and then blown dry with nitrogen. No apparent difference was observed for different tips in this experiment. AFM (SPA-400, SII., Japan) was used for nanolithography in a vector scan mode at a force of about 1.0 nN and a dc bias voltage (in all this experiments means the substrate relative to the probe) and subsequent imaging in contact mode with a force of 0.1 nN. For DPN experiments, the newly cleaned tip was dipped in neat ethanol for 30 s and then dried in air. This was repeated at least three times just before use. All experiments were performed under ambient conditions at a relative humidity (RH) of 61% ( 2%, and all line features were fabricated at a scan rate of 0.1 µm/s except otherwise noted. All data of the height and width (full width at half-maximum) represent the average of at least six measurements.

3.1. Simultaneous Fabrication of Positive and Negative Nanopatterns. Positive and negative nanopatterns were continuously fabricated on hydrophobic 1-hexadecanethiol (HDT)/ Au(111) just by switching the polarity of bias, as shown in Figure 1. Positive and negative features were fabricated at a bias of 3.0 and -3.0 V, respectively. The height of the positive nanopatterns was about 1.2 ( 0.2 nm, and the depth of the negative nanopatterns was about 0.9 ( 0.2 nm. The formation of negative patterns in Figure 1 can be reasonably explained by the removal of the adsorbed HDT molecules with the application of negative bias since the gold-sulfur bond was broken in a cathodic process14 as has been proved in the biasassisted nanolithography.7,15 As for the big difference between the depth of the negative patterns and the thickness of HDT monolayers (∼2.06 nm),16 it may be attributed to the adsorption of unremoved HDT molecules and contaminants (from reaction residual or air) onto the fresh gold because of the high surface energy of such fresh gold and some extent deformation of the surrounding HDT SAMs under the load of the tip.7 Recently, Xie et al. reported the formation of positive and negative nanopatterns on silicon first by formation of microscale droplets via dilute hydrofluoride etching, then conversion of the droplets to acidic thin layers by AFM probe scanning, and then subsequent fabrication using bias-assisted nanolithography in the aqueous layers.17 The process is not reversible. To understand the chemical composition of the positive nanofeatures, a positive pattern of about 100 µm × 100 µm was fabricated and used for XPS analysis. Spectra at unpatterned areas and patterned areas on HDT/Au(111) were obtained as shown in Figure 2, parts a and b, respectively. The peak of S 2p appears at a BE of 162.0 eV in Figure 2a,18 which confirms the existence of HDT SAMs in the unpatterned areas in this particular case. The appearance of the C 1s peak at a BE of 284.9 eV, Au 4f at a BE of 84.0 (4f7/2) and 87.7 (4f5/2) eV, and O 1s at a BE of 529.1 eV in Figure 2a are assigned to the alkyl chain of the HDT SAMs, Au(111) substrate, and chemisorbed oxygen,19-21 respectively. In comparison with only one doublet (4f7/2 and 4f5/2) of the Au 4f spectra in the unpatterned areas, two doublets are seen in the Au 4f spectra in the patterned areas as shown in Figure 2b. The first doublet is located at 85.9 and 89.6 eV, indicating the generation of gold oxide based on previous reports.19 The formation of Au2O3 suggests that the fabrication process on HDT/Au(111) with a positive sample bias is an electrochemical process,22 which is the same as that happens on other metal and semiconductor substrates.1 The second doublet is located at 84.0 and 87.7 eV, which can be assigned to underlying gold substrate under gold oxide in the patterned area and the unoxidized gold substrate around the patterned area but within the spot size of XPS (about 120 µm) as well as gold from partial decomposition of gold oxide since gold oxide is unstable in air.23 The appearance of O 1s at a BE of 530.1 eV in the patterned areas further confirms the generation of gold oxide.19b The ratio of O/Au atoms in the gold oxide phase was calculated from the ratio of the integrated intensities of the Au 4f7/2 peak at a BE of 85.9 eV to O 1s peak in Figure 2b and divided by their sensitivity factors, and found to be 1.6, slightly greater than the stoichiometric ratio of 1.5

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Figure 2. XPS spectra of O 1s, C 1s, S 2p, and Au 4f of HDT/Au(111) in unpatterned areas (a) and patterned areas fabricated by bias-assisted nanolithography at positive bias before (b) and after (c) ethanol immersion.

Figure 3. AFM topographic (top) and friction force (bottom) images: (a) HDT/Au(111), (b) positive nanopatterns fabricated by bias-assisted nanolithography at a bias of 2.6 V under a relative humidity (RH) of 73% ( 5%, (c) negative nanopatterns after subsequent 2 h of immersion of the sample in ethanol, and (d) recovered HDT SAMs after 30 min of further immersion in 1.0 mM HDT ethanol solution. The scan range is 2.5 µm.

for Au2O3. The difference can be ascribed to the existence of chemisorbed oxygen in the O 1s spectra in Figure 2b. No S was detected in Figure 2b, indicating the content of S in patterned areas decreased greatly after the patterning. The disappearance of S might be ascribed to the oxidation of S in HDT to volatile substances by the bias. The appearance of the C 1s peak at a BE of 284.8 eV in Figure 2b can be ascribed to the HDT SAMs around the patterned areas but within the XPS spot, as well as the carbon-containing contaminants from air or remnants of the SAMs after patterning. The above XPS analysis proves the composition of the positive nanopatterns is Au2O3. Generally, Au2O3 is unstable; it can be reduced by organic air contaminants or ethanol to gold.22,23 To test the stability of the nanoscale Au2O3, the following experiment was performed. Figure 3 shows a series

of AFM height and force images of results, respectively. Figure 3a is the initial surface feature of HDT SAMs on Au(111), and Figure 3b reveals positive nanopatterns (in letter shapes) fabricated by bias-assisted nanolithography at a bias of 2.6 V. The height and width of the line patterns from the height image of Figure 3b were estimated to be 1.1 ( 0.1 and 56.5 ( 1.5 nm, respectively. Interestingly, the nanoscale gold oxide on the hydrophobic HDT/Au(111) was very stable in air; no obvious change was observed within 72 h (see Figures S1 and S2 in the Supporting Information). The stability of the nanoscale gold oxide depends on the amphiphiles of the matrix SAMs (vide infra). The hydrophobicity or hydrophilicity of the matrix SAMs may cause the local environment difference, i.e., hydrophobic surrounding may reduce the migration possibility of organic contaminants in air to the gold oxide and thus stabilize it. The

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Figure 4. AFM topographic image of lines obtained after about 2 h of immersion of the sample fabricated at a bias of 6.0 V in ethanol (a), and then 30 min in 1.0 mM MHA ethanol solution and 2 min in R-Fe2O3 hydrosol (b), subsequently. The scan range is 6 µm. The inset in (b) is a high-resolution image of the line in (b), and the scale bar is 90 nm.

Figure 5. Schematic illustration of the process for the reversible nanofabrication on HDT/Au(111) by bias-assisted nanolithography with an ethanol-ink tip. The route a f b, i.e., the oxidation, is irreversible, whereas routes a T c and b T c are reversible by bias-assisted nanolithography using a chemical material inked tip (HDT or ethanolink tip) or with HDT or ethanol solution immersion.

stability of the Au2O3 on hydrophobic HDT/Au(111) reduces greatly the possibility of assigning the Au 4f at a BE of 84.0 and 87.7 eV in the patterned areas in Figure 2b to be gold from Au2O3 decomposition. After (10-120 min typically) immersion of the sample in ethanol solution, positive nanopatterns disappeared and negative nanopatterns appeared as shown in Figure 3c. The depth and base width of the line patterns was estimated to be 1.0 ( 0.2 and 30 ( 1.5 nm from the height image of Figure 3c, respectively. The difference in width between the positive and negative pattern in Figure 3, parts b and c, can be attributed to the lateral growth of Au2O3 and tip imaging convolution. When the sample was immersed further in 1.0 mM HDT ethanol solution, negative nanopatterns disappeared and the uniform HDT SAMs were recovered as shown in Figure 3d. This implies that the nanoscale gold oxide can be reduced to gold by ethanol and the pattering process is reversible. For further confirmation that gold oxide can be reduced to gold by ethanol, XPS analysis was performed as shown in Figure 2c. In comparison with two doublets in the Au 4f spectra before ethanol immersion, only one doublet located at a BE of 84.0 and 87.7 eV was observed after the immersion, which indicated the reduction of gold oxide to gold. The appearance of C 1s at a BE of 284.8 eV and O 1s at a BE of 529.1 eV can be attributed to the big XPS spot and the contaminations (from reaction products, immersion liquid, and air) chemically adsorbed on the surface of newly exposed gold21 which may partially account for the lower depth of the negative nanopatterns in Figure 3c than the thickness of HDT SAMs.

Zheng et al. An exciting application of nanopatterns is in the use as templates for functional modification. Several groups reported the fabrication of template on SAMs on gold by replacement lithography3,24 or by exposure of gold substrate.25 The result in Figure 3 indicated that Au2O3 could be reduced by ethanol into newly exposed gold substrate. In other words, Au2O3-based positive nanotemplates22,26 can be converted into gold-based negative nanotemplates by ethanol reduction. Here, we demonstrate the negative nanopatterns can be used as nanotemplate, and for proof-of-concept experiment, magnetic nanostructures were created on the patterned areas. The negative lines in Figure 4a were obtained after about 2 h of immersion in ethanol of the sample fabricated at a bias of 6.0 V and were used as templates for creating magnetic nanostructures. Then, the sample was immersed in 1.0 mM MHA ethanol solution for about 30 min, and then 2 min in R-Fe2O3 hydrosol, to form magnetic nanostructures as shown in Figure 4b. Obviously, the topography of the patterned lines changed from negative groove into positive protuberance with heights of 7-12 nm. The inset in Figure 4b shows a high-resolution image of the lines; nanoparticles can be clearly seen from the image. These results confirm that R-Fe2O3 nanoparticles were immobilized on the patterned areas. Similarly, a negative bias generated negative pattern can also be used as a template (see Figure S3 in the Supporting Information). 3.2. Reversible Nanopatterning. Enlightened by the results above as well as the outstanding ability of DPN for precise delivering of ink materials to substrates,2 we believe that reversible nanofabrication of both positive and negative patterns could be achieved by bias-assisted nanolithography with an ethanol-ink tip. The strategy is schematically illustrated in Figure 5. With an ethanol-inked bias-assisted tip, two positive (I and II) and two negative (III and IV) point nanopatterns were first fabricated at positive and negative bias (6.0 (I), 5.5 (II), -5.5 (III), and -6.0 V (IV)), respectively, as shown in Figure 6a. The height/depth and diameter of the positive dots and negative pits were estimated to be 15.4 and 53 nm (position I), 11.6 and 36 nm (position II), 1.2 and 41 nm (position III), and 1.2 and 43 nm (position IV), respectively. The dot I and pit IV are used as references. The dot at position II was not a round one, which might be caused by the mechanical and thermal drifts of the scanner. With DPN at zero bias on the position II, the positive dot was erased and a negative pit (∼1.3 nm in depth) appeared as shown in Figure 6b. When a tip without ethanol ink was used and other conditions were the same, no surface topographic change was observed (see Figure S4 in the Supporting Information). The formation of the negative pit was ascribed to the reduction of the gold oxide to gold by ethanol transferred from the tip. When the tip was located at the center of the negative pit at position III and then a bias of 5.5 V was applied for 0.2 s, a positive dot was formed as shown in Figure 6c, which is ascribed to the oxidation of the gold substrate to gold oxide. The height and width of the dot were estimated to be 14 and 40 nm, respectively. A very small part of previous pit was not fully covered by the dot, and this may be attributed to the mechanical and thermal drift of the scanner or the shape change of the tip. This dot can also be erased by DPN as shown in Figure 6d. A continuous decrease in the size of the dot at position I was observed from Figure 6a-d, which might be caused by the partial reduction of the Au2O3 by ethanol during repeated alignments of the tip. No change of pit IV was observed. These results indicate that positive and negative nanopatterns are fabricated reversibly on HDT/Au(111) using the bias-assisted nanolithography with an ethanol-ink tip.

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Figure 6. Three-dimensional AFM topographic images on HDT/Au(111) with an ethanol-inked conductive tip. The bias-assisted nanolithography was performed at a sample bias with a tip-substrate contact time of 0.2 s. DPN was performed by two repeat raster scans at a scan rate of 0.8 Hz and a zero bias voltage. (a) Positive dots (marked as I and II) and negative pits (marked as III and IV) fabricated at a sample bias of 6.0 (I), 5.5 (II), -5.5 (III), and -6.6 V (IV); (b) the formation of pit after dot erasing by DPN at position II; (c) the dot created on the pit at position III with a positive sample bias of 5.5 V; (d) the pit after erasing by DPN at position III.

Previously, ethanol has been used as meniscus material in biasassisted nanolithography.27 Unlike the common inks for DPN such as thiol molecules,2 silanes,28 fluorescent dyes,29 polymers,30 solid materials,31 catalysts,32 enzymes,33 DNA,34 and proteins,35 ethanol is volatile, reactive, but low remnant, similar to allyl bromide ink.36 Although it cannot be used for longtime patterning, we choose ethanol instead of other reductants here because we do not want to introduce factitious materials to the negative nanopatterns during the reduction of gold oxide. Both ethanol and its corresponding oxidation product aldehyde22 are volatile, which can finally escape from the surface. Such a topography-based reversibility in our process can quite match the principle of the reversible data storage. In comparison with a typically topography-based reversible data storage process using thermomechanical nanolithography on polymer,4 our process is realized using bias-assisted nanolithography using a chemical material (ethanol) inked tip on SAMs. Both biasassisted nanolithography and chemical material transferring based nanolithography are reliable and versatile nanopatterning techniques for variable substrates.1,2 So the combination of biasassisted nanolithography with chemical material transferring based nanolithography will be more possible to be applied on different materials to develop new processes for different goals. 3.3. Effect of Bias on Bias-Assisted Nanolithography. For the optimization of operation of the lithography, the influences of bias on the formation of the positive and negative nanopatterns were investigated. The high correlativity of straight line fitting in Figure 7a indicates that both Au2O3 line height and

width increase linearly with the applied bias, which can be reasonably explained by a field-induced oxidation model.37 After reduction of the Au2O3 to gold by the immersion in ethanol, positive lines were converted into negative grooves because of the “removal” of HDT SAMs. Here the percentage of the SAMs-removed area, P for brevity, was used to determine the extent of SAMs’ removal on HDT/Au(111) for a given bias (Figure 7b).38 P ) Sa/Sm, where Sa represents the actual removed area (the average width value of the line multiplying the length of the line in the image), Sm represents the maximum removed area (the widest value of the line multiplies the length of the line preset in the vector scan program). It is clear that no HDT molecules can be removed from HDT/Au(111) at a bias of e4.5 V. When the bias was g4.8 V, the Premoved could be higher than 50% and finally reached ca. 90%. The Premoved of 100% was not achieved possibly because the Sm was defined as the widest value of the width of the line multiplying the programmed length of the line in the vector scan and drift of the piezoelectric scanner during fabrication always caused uneven line width and a little shorter line length. With a bias of 4.6 and 4.7 V, the Premoved was less than 50%, and the Premoved at 4.6 V was much higher than that of at 4.7 V which indicated that the fabrication at the bias range was not very stable under our experiment conditions. Here we take the bias at the halfprobability as a critical bias, which is 4.8 V, in this case. More experiments indicate that the critical bias strongly depends on the RH, for example, 6.7 V under an RH of 51% ( 2% and 2.0 V under an RH of 73% ( 5% (Figure S5 in the Supporting Information), respectively; that is, the higher the RH, the lower

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Figure 7. Gold oxide line height ([) and width (]) as a function of bias (a); formation probability for the removal of the adsorbed HDT molecules on HDT/Au(111) (P) after reduction of gold oxide in (a) to gold by ethanol as a function of a positive bias (b); P as a function of negative bias (c).

the critical bias. These results indicate that RH plays a very important role in the bias-assisted nanolithography, which can be reasonably explained by an electrochemical mechanism.7 The dependence of critical bias on the RH is possibly caused by the influences of bias on the formation of a water meniscus. The formation of a water meniscus is a collective result of electricfield-induced condensation and capillary condensation when a bias is applied between the tip and the sample.39 On highly hydrophobic surfaces like HDT/Au(111), the field-induced condensation tends to dominate the development of the meniscus under moderate humidity.40 Only an increased bias can induce the formation of a water meniscus under a decreased RH; in other words, the minimum bias to generate the water meniscus depends reversely on the RH. Since stable water meniscus is necessary to supply enough oxyanions for the stable proceeding of electrochemical reactions in a bias-assisted nanolithography,40 we can reasonably deduce that the critical bias to ensure the

Zheng et al. electrochemical reactions is similar to the minimum bias to form the meniscus, being affected by the RH. Similarly, Premoved was used to determine the extent of SAMs’ removal on HDT/Au(111) at a negative bias (Figure 7c). The critical bias is -4.2 V. High scattering of the data points were observed when the bias was more negative than -4.2 V and less negative than -5.7 V, which indicated that the reproducibility was poor for patterning at this bias range. When the bias was more negative than -5.7 V, the Premoved could be higher than 80%, indicating the start of bias range for good patterning. 3.4. Effect of Writing Speed. The biggest limitation of scanning probe lithography is its low throughput. One of most efficient strategies to increase the throughput is to increase the scanning speed capacities. To get the maximum fabrication speed, a series of Au2O3 lines were patterned at a bias of 9.9 V at variable writing speeds as shown in Figure 8a. A maximum writing speed of 800 µm/s is obtained in this case. The measured height and width of the oxide lines are plotted versus writing speed as shown in Figure 8b. The high correlativity of logarithmic curve fitting in Figure 8b indicates that the oxide line height and width decrease logarithmically with writing speed, or in other words, increase logarithmically with the reciprocal of speed. As for the maximum speed for negative bias based biasassisted lithography, Figure 8c shows that it can go beyond 1000 µm/s at a bias of -9.9 V. Faster writing speed has not been tested since the available maximum speed in the vector scan program of our AFM unit is 1000 µm/s. In comparison with the logarithmical dependence of the line width of gold oxide generated at a positive bias on writing speed, the width of the lines generated at negative bias was similar. The results can be reasonably explained. When a positive bias was applied between the tip and the surface, the fabrication process is an electrochemical oxidation process and reaction speed is relatively slow. The oxidation of gold to gold oxide determines the reaction speed when the RH was high enough to get a stable water meniscus. The growth of gold oxide in the longitudinal and lateral directions follows a logarithmical rule, so the fabricated gold oxide line height and width decrease logarithmically with writing speed in this case. When a negative bias was applied, the fabrication process is an electrochemical desorption process of the SAMs and the reaction speed is relatively high. When the electrical field was strong and the water meniscus was stable, the desorbed area of the SAMs lies on tip bias covered effective area and the bias covered area mainly determined by the size of the water meniscus. The similar line width of the lines in Figure 8c was ascribed to the similar water meniscus size under our experiment conditions. 3.5. Effect of the Terminal Group of the SAMs. Different from the high stability of Au2O3 on hydrophobic HDT SAMs, we found that Au2O3 is unstable on hydrophilic SAMs. On the hydrophilic 11-mercaptouncecanoic acid (MUA)/Au(111) (Figure 9a), the initial 3 × 3 array of positive dots (Figure 9b) fabricated at a series of positive bias with different tip-substrate contact time disappeared soon and negative pits appeared as shown in a series subsequent images of Figure 9c-f. Three preexisting dot features (circled) on prepared SAMs were used as marks for reference. If we do not follow the process by repeat imaging (about 15 min), a similar result could be observed as well (see Figure S6 in the Supporting Information). This implies that the disappearance of gold oxide is not due to the tip scratch. A possible explanation was that the Au2O3 was unstable on the hydrophilic MUA/Au(111) in air and changed to gold soon. The base width and the depth of the pits were 75-100 nm and 0.8

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Figure 8. AFM topographic image of lines at a bias of 9.9 (a) and -9.9 V (c) at a writing speed of 10-1000 µm/s under an RH of 90% ( 8%; (b) height ([) and width (]) of the gold oxide line in (a) as a function of writing speed and fitted with direct-logarithm curves. The scan ranges of (a) and (c) are 4.5 and 5.0 µm, respectively.

Figure 9. AFM topographic images of (a) MUA/Au(111) with three pre-existing dots (as the marks indicated by circles), (b) a 3 × 3 array of dots fabricated by bias-assisted nanolithography at 3.8 (the bottom line), 3.9 (the middle line), and 4.0 V (the top line) with a tip-substrate contact time of 0.2 (the right column), 0.5 (the middle column), and 1.0 s (the left column), (c-f) the subsequent images of (b) at the interval of 2.5 min. The scan range and the height scale are 1.2 µm and 1.9 nm, respectively.

( 0.1 nm, respectively. A negative sample bias can also lead to the removal of MUA from MUA/Au(111) (see Figure S7 in the Supporting Information). Similar phenomena were observed on 11-mercapto-1-undecanol (MUD)/Au(111) (see Figure S7 in the Supporting Information). All the above AFM and XPS experiments prove that a positive enough bias can oxidize the thiol SAMs as well as the gold substrate and results in positive structure of gold oxide over the surrounding SAMs in ambient atmosphere. However, the stability of the gold oxide structure produced by nanolithography depends strongly on the specific environments (such as the content of reductants and hydrophobicity or hydrophilicity of the matrix SAMs). We noted that Zhao and Uosaki7 and Jang et al.9 gave only negative features by bias-assisted nanolithography with positive substrate bias under their experiment conditions. Zhao and Uosaki’s bias-assisted nanolithography was carried out on the surface of ODT/Au(111) in a liquid environment of water-containing toluene.7 The reason for only observation of negative patterns might be ascribed to the pre-existing or electric-field-produced reductive materials in toluene which can decompose gold oxide to gold quickly. Jang’s bias-assisted nanolithography was actually performed on the surface of hydrophilic thiol SAMs on gold.9 The high instability of gold oxide nanopatterns on the hydrophilic SAMs surface may account for the final results of negative patterns. 4. Conclusions In summary, reversible nanopatterning of positive and negative features can be realized on HDT/Au(111) via bias-assisted nanolithography using a reductant as ink material, and we anticipate that the result will show new insight in reversible

high-density data storage. The formation of the positive and negative nanopatterns via bias-assisted nanolithography depends solely on the polarization of the bias. The writing speed of positive and negative nanopatterns can reach 800 and beyond 1000 µm/s, respectively. The chemical composition of the positive nanopattern is gold oxide. and the nanometer-scale gold oxide can be reduced by ethanol to gold, as proved by XPS, forming the negative nanopattern which can be refilled with HDT to recover the SAMs. The nanometer-scale gold oxide has the potential to be used as template and etch resist.41 The negative nanopattern can also be used as a template, and magnetic nanoparticles were immobilized on the template. Both the positive and negative nanopatterns have promising potential to be used as templates for biosensors, biochips, and data storage. Acknowledgment. The financial support from the National Natural Science Foundation of China (Nos. 20675077 and 20735001) is acknowledged. Supporting Information Available: Stability of the positive nanopatterns, construction of magnetic nanostructure on negative bias generated template on HDT/Au(111); effect of relative humidity on the formation probability for the removal of the adsorbed HDT molecules on HDT/Au(111) at a positive bias; control experiment of Figures 6 and 9; bias-assisted nanolithography on hydrophilic SAMs of MUA/Au(111) at a negative sample bias and of 11-mercapto-1-undecanol (MUD)/Au(111). This material is available free of charge via the Internet at http:// pubs.acs.org.

6604 J. Phys. Chem. C, Vol. 112, No. 17, 2008 References and Notes (1) (a) Xie, X. N.; Chung, H. J.; Sow, C. H.; Wee, A. T. S. Mater. Sci. Eng., R 2006, 54, 1. (b) Garcia, R.; Martinez, R. V.; Martinez, J. Chem. Soc. ReV. 2006, 35, 29. (c) Tseng, A. A.; Notargiacomo, A.; Chen, T. P. J. Vac. Sci. Technol., B 2005, 23, 877. (d) Kramer, S.; Fuierer, R. R.; Gorman, C. B. Chem. ReV. 2003, 103, 4367. (2) Piner, P. D.; Zhu, J.; Xu, F.; Hong, S.; Mirkin, C. A. Science 1999, 283, 661. (3) Liu, G. Y.; Xu, S.; Qian, Y. L. Acc. Chem. Res. 2000, 33, 457. (4) Binnig, G.; Despont, M.; Drechsler, U.; Haberle, W.; Lutwyche, M.; Vettiger, P.; Mamin, H. J.; Chui, B. W.; Kenny, T. W. Appl. Phys. Lett. 1999, 74, 1329. (5) Cappella, B.; Sturm, H. J. Appl. Phys. 2002, 91, 506. (6) Maoz, R.; Cohen, S. R.; Sagiv, J. AdV. Mater. 1999, 11, 55. (7) Zhao, J. W.; Uosaki, K. Langmuir 2001, 17, 7784. (8) Xie, X. N.; Chung, H. J.; Sow, C. H.; Adamiak, K.; Wee, A. T. S. J. Am. Chem. Soc. 2005, 127, 15562. (9) Jang, J.-W.; Maspoch, D.; Fujigaya, T.; Mirkin, C. A. Small 2007, 3, 600. (10) Li, Y.; Maynor, B. W.; Liu, J. J. Am. Chem. Soc. 2001, 123, 2105. (11) Cai, Y. G.; Ocko, B. M. J. Am. Chem. Soc. 2005, 127, 16287. (12) Sato, A.; Tsukamotoy, Y. Nature 1993, 363, 431. (13) Huo, L. H.; Li, W.; Lu, L. H.; Cui, H. N.; Xi, S. Q.; Wang, J.; Zhao, B.; Shen, Y. C.; Lu, Z. H. Chem. Mater. 2000, 12, 790. (14) Widrig, C. A.; Chung, C.; Porter, M. D. J. Electroanal. Chem. 1991, 310, 335. (15) Schoer, J. K.; Zamborini, F. P.; Crooks, R. M. J. Phys. Chem. 1996, 100, 11086. (16) Bain, C. D.; Troughton, E. B.; Tao, Y. T.; Evall, J.; Whitesides, G. M.; Nuzzo, R. G. J. Am. Chem. Soc. 1989, 111, 321. (17) Xie, X. N.; Chung, H. J.; Sow, C. H.; Wee, A. T. S. AdV. Funct. Mater. 2007, 17, 919. (18) Nuzzo, R. G.; Zegarski, B. R.; Dubois, L. H. J. Am. Chem. Soc. 1987, 109, 733. (19) (a) Pireaux, J. J.; Liehr, M.; Thiry, P. A.; Delrue, J. P.; Caudano, R. Surf. Sci. 1984, 141, 221. (b) Koslowski, B.; Boyena, H. G.; Wilderotter, C.; Ka¨stle, G.; Ziemann, P.; Wahrenberg, R.; Oelhafen, P. Surf. Sci. 2001, 475, 1. (20) Heister, K.; Zharnikov, M.; Grunze, M.; Johansson, L. S. O. J. Phys. Chem. B 2001, 105, 4058. (21) Krozer, A.; Rodah, M. J. Vac. Sci. Technol., A 1997, 15, 1704.

Zheng et al. (22) Ron, H.; Rubinstein, I. J. Am. Chem. Soc. 1998, 120, 13444. (23) Ron, H.; Matlis, S.; Rubinstein, I. Langmuir 1998, 14, 1116. (24) (a) Zhao, J. W.; Uosaki, K. Nano Lett. 2002, 2, 137. (b) Gorman, C. B.; Carroll, R. L.; He, Y.; Tian, F.; Fuierer, R. Langmuir 2000, 16, 6312. (25) Zhou, D.; Sinniah, K.; Abell, C.; Rayment, T. Angew. Chem., Int. Ed. 2003, 42, 4934. (26) Krysinski, P.; Blanchard, G. J. Bioelectrochemistry 2005, 66, 71. (27) (a) Kim, Y.; Kang, S. K.; Choi, I.; Lee, J.; Yi, J. J. Am. Chem. Soc. 2005, 127, 9380. (b) Tello, M.; Garcia, R.; Gago, J. A. M.; Martinez, N. F.; Gonzalez, M. S. M.; Aballe, L.; Baranov, A.; Gregoratti, L. AdV. Mater. 2005, 17, 1480. (28) Jung, H.; Kulkarni, R.; Collier, C. P. J. Am. Chem. Soc. 2003, 125, 12096. (29) Noy, A.; Miller, A. E.; Klare, J. E.; Weeks, B. L.; Woods, B. W.; DeYoreo, J. J. Nano Lett. 2002, 2, 109. (30) (a) Lim, J.-H.; Mirkin, C. A. AdV. Mater. 2002, 14, 1474. (b) Maynor, B. W.; Filocamo, S. F.; Grinstaff, M. W.; Liu, J. J. Am. Chem. Soc. 2002, 124, 522. (31) Fu, L.; Liu, X. G.; Zhang, Y.; Dravid, V. P.; Mirkin, C. A. Nano Lett. 2003, 3, 757. (32) Li, J.; Lu, C.; Maynor, B.; Huang, S.; Liu, J. Chem. Mater. 2004, 16, 1633. (33) Hyun, J.; Kim, J.; Craig, S. L.; Chilkoti, A. J. Am. Chem. Soc. 2004, 126, 4770. (34) Demers, L. M.; Ginger, D. S.; Park, S.-J.; Li, Z.; Chung, S.-W.; Mirkin, C. A. Science 2002, 296, 1836. (35) Lee, K.-B.; Lim, J.-H.; Mirkin, C. A. J. Am. Chem. Soc. 2003, 125, 5588. (36) Zheng, Z. K.; Yang, M. L.; Liu, Y. Q.; Zhang, B. L. Nanotechnology 2006, 17, 5378. (37) Yang, M.; Zheng, Z.; Liu, Y.; Zhang, B. Nanotechnology 2006, 17, 330. (38) Lewis, M. S.; Gorman, C. B. J. Phys. Chem. B 2004, 108, 8581. (39) (a) Perez-Murano, F.; Martin, C.; Barniol, N.; Kuramochi, H.; Yokoyama, H.; Dagata, J. A. Appl. Phys. Lett. 2003, 82, 3086. (b) Galleja, M.; Tello, M.; Garcia, R. J. Appl. Phys. 2002, 92, 5539. (c) Gomez-Monivas, S.; Saenz, J. J.; Galleja, M.; Garcia, R. Phys. ReV. Lett. 2003, 91, 056101. (40) Yang, M.; Zheng, Z.; Liu, Y.; Zhang, B. J. Phys. Chem. B 2006, 110, 10365. (41) Sharpe, R. B. A.; Burdinski, D.; Huskens, J.; Zandvliet, H. J. W.; Reinhoudt, D. N.; Poelsema, B. J. Am. Chem. Soc. 2006, 128, 15560.