Dip Pen Nanolithography Stamp Tip - Nano Letters (ACS Publications)

Dip pen nanolithography (DPN),1 invented in 1999, is an atomic force ..... the quality of the dendrimer patterns (data not shown) was not comparable t...
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NANO LETTERS

Dip Pen Nanolithography Stamp Tip Hua Zhang,* Robert Elghanian, Nabil A. Amro, Sandeep Disawal, and Ray Eby

2004 Vol. 4, No. 9 1649-1655

NanoInk Inc., 1335 West Randolph Street, Chicago, Illinois 60607 Received May 30, 2004; Revised Manuscript Received July 14, 2004

ABSTRACT A simple and novel method for fabricating poly(dimethylsiloxane) (PDMS)-coated dip pen nanolithography (DPN) stamp tips was developed. These kinds of tips absorb chemicals (“inks”) easily and allow one to generate molecule-based patterns in a conventional DPN experiment. The generated patterns also can be imaged with the same DPN stamp tips. This method is a type of scanning probe contact printing but provides the ability to generate higher resolution structures than one can obtain with the conventional technique, which thus far has only enabled micron scale patterning. Sub-100 nm resolution patterning with 16-mercaptohexadecanoic acid (MHA) as an ink is demonstrated with these novel tips and is comparable to what one can obtain with a conventional ink-coated Si3N4 probe tip. Proof-of-concept is also demonstrated with 1-octadecanethiol (ODT), dendrimers and cystamine as inks.

Introduction. Dip pen nanolithography (DPN),1 invented in 1999, is an atomic force microscopy (AFM)-based directwrite lithographic method for depositing “inks” such as small organic molecules,2 polymers,3 biomolecules,4 sol-gel precursors,5 metal salts,6 and nanoparticles7 onto a solid substrate with an AFM tip used as a “pen”. The direct-write capabilities of DPN allow one to generate soft-hard composites and multi-“ink” nanostructures from the sub-100 nm to many micrometer length scale with high nanostructure alignment and registration. Recently, this method has been transitioned from a serial to parallel writing tool by using cantilever arrays consisting of multiple pens (as many as 10 000 thus far).8 DPN is becoming a powerful complement to the suite of conventional lithographic and stamping tools9-12 now available to researchers interested in nanoscience and technology. In principle, any materials can be deposited by DPN with suitable tips, substrates, and optimal experimental conditions (humidity, temperature, etc.). The two key factors impacting a DPN experiment are (i) the tip-coating process, and (ii) “ink” diffusion from a coated tip onto a solid substrate, arising from the ink/tip and ink/substrate adsorption or interaction. Up until now, commercial Si3N4 probe tips have been used in DPN experiments as “pens” to deposit “inks” and generate nanoscale patterns on a surface. In the case of making biomolecular nanopatterns, such as DNA4b and proteins,4c,d in order to increase the tip-coating efficiency, a silane-modified Si3N4 or thiol-modified gold (Au)-coated Si3N4 tip is used. These methods serve to increase the adsorption of biomolecules via an increased ink-tip interaction. Recently, a tip entirely made of poly(dimethylsiloxane) (PDMS) was fabricated and used to create 1-octadecanethiol (ODT) nanopatterns.13 This novel method was a hybrid * Corresponding author. Telephone: 312-850-0610. Fax: 312-829-4069. E-mail: [email protected]. 10.1021/nl049185o CCC: $27.50 Published on Web 08/12/2004

© 2004 American Chemical Society

between conventional contact printing procedures and DPN and has been termed “scanning probe contact printing”. However, the highest resolution structures obtained thus far have been 330 nm, which is well above what can be achieved in a DPN experiment (∼15 nm).2a Since the probe was completely made of polymeric materials13 and lacked the reflective gold layer typically on the back of an AFM cantilever, AFM feedback control and performance became limited. The DPN resolution of the PDMS probe cannot reach sub-100 nm due to a large radius of the specifically fabricated PDMS tip. Herein we describe a novel, effective, and simple method for fabricating a PDMS-coated Si3N4 tip called a DPN stamp tip, which preserves the advantages and overcomes some of the shortcomings of the PDMS probes mentioned above. Importantly, individual DPN pattern features generated with a DPN stamp tip are able to reach sub-100 nm, comparable with the results obtained with a Si3N4 tip. Experimental Section. Chemicals. Ammonium hydroxide, hydrogen peroxide (30%), ethanol, and methanol were purchased from Fisher Scientific (Fairlawn, NJ). 16-Mercaptohexadecanoic acid (MHA), 1-octadecanethiol (ODT), and cystamine dihydrochloride were purchased from Aldrich (Milwaukee, WI). Starburst PAMAM dendrimer-G6.0 (surface: 100% OH, solvent: methanol, Conc: 11.46% w/w) (referred to as “G6-OH”) was purchased from Dendritech Inc. (Midland, MI). Silicone elastomer and silicone elastomer curing agent (Sylgard 184 silicone elastomer kit) were purchased from Dow-Corning Corporation (Midland, MI). All chemicals were used without further purification. Substrate Preparation. An oxidized silicon wafer (∼70 nm of oxide), purchased from Silicon Quest Int. (Santa Clara, CA), was cut into 1 × 1 cm2 squares. After being ultrasonicated in acetone for 10 min and rinsed with Milli-Q water, the Si/SiOx substrates were immersed into a boiling solution

Scheme 1. Process for Fabricating a DPN Stamp Tip

of ammonium hydroxide and hydrogen peroxide (V(NH4OH)/V(H2O2)/V(H2O) ) 1:1:5) for 1 h. The cleaned substrates were rinsed with Milli-Q water and dried with N2, and then immediately used as substrates for patterning dendrimer G6-OH. Gold substrates were prepared by placing the cleaned Si/SiOx substrates into a thermal evaporator chamber and coating them with a 1 nm Ti adhesion layer, then subsequently coating with 10 nm gold under vacuum conditions (pressure < 1 × 10-7 mbar).2d,f,h Dip Pen Nanolithography (DPN) and AFM Imaging. Fabrication of DPN Stamp Tip. Si3N4 probes (k ) 0.10 N/m, NanoInk, Inc., Chicago, IL; or k ) 0.05 N/m, TM Microscopes, Sunnyvale, CA) were cleaned using the same procedure for cleaning Si/SiOx substrates as mentioned above. The silicone elastomer and the silicone elastomer curing agent were completely mixed at a ratio of 5:1-15:1 (w/w) (typically 10:1 was used), referred to as mixture 1. The inkwells (NanoInk, Inc., Chicago, IL; see http:// www.nanoink.net) were filled with mixture 1 and then were put onto the sample stage of an NSCRIPTOR system (NanoInk, Inc., Chicago, IL). A cleaned Si3N4 tip, which was mounted into a tip holder, was moved to touch the droplet of mixture 1 in an inkwell (see Figure S1 in Supporting Information), retained for about 10 s, and then retracted. The mixture 1-coated Si3N4 tip was removed from the tip holder and cured at 60-70 °C for 12 h (Scheme 1). The obtained DPN stamp tip was used for all DPN experiments. Nanopatterns Using DPN Stamp Tips. Nanopatterns of MHA, ODT and cystamine on gold, and dendrimer G6-OH on gold and Si/SiOx, were generated with a DPN stamp tip coated with these chemicals by being immersed in the respective solution (6.8 mM MHA, 10 mM ODT, 10 mM cystamine in ethanol, and G6-OH in methanol; W/W ) 5.73 mg/g) for ∼15 s, then dried with compressed difluoroethane (Dust-off, Ted Pella, Inc., Redding, CA). The DPN experiments were carried out under ambient conditions (set point ) 0.1 nN, 22-24 °C, 30-36% relative humidity), except in 60% relative humidity for writing G6-OH, by using an NSCRIPTOR system or an AutoProbe CP AFM (TM Microscopes, Sunnyvale, CA), the latter combined with a commercial lithography software package (DPNWrite, NanoInk Inc., Chicago, IL). Wet Chemical Etching. The 10 nm thick gold substrates, patterned with MHA, were immersed in a ferri/ferrocyanide etching solution (a 1:1:1:1 (v:v:v:v) aqueous mixture of 0.1 M Na2S2O3, 1.0 M KOH, 0.01 M K3Fe(CN)6, and 0.001 M K4Fe(CN)6) for ∼20 min under constant stirring to remove 1650

Figure 1. A typical SEM image of a DPN stamp tip.

the gold layer from the exposed regions of the gold substrate.2d,f,h After rinsing with Milli-Q H2O, the etched substrates were immersed into a 0.5% (v/v) aqueous HF solution for 10-15 s to remove the 1 nm Ti layer. Then the substrates were rinsed with Milli-Q H2O and dried with N2. AFM Imaging. All AFM topographic and lateral force images of DPN patterns were obtained with the same coated DPN stamp tip (except when specifically annotated). The etched patterns were imaged with a bare Si3N4 tip (k ) 0.10 N/m, NanoInk, Inc., Chicago, IL; or k ) 0.05 N/m, TM Microscopes, Sunnyvale, CA) in contact mode. Results and Discussion. Fabrication of Dip Pen Nanolithography (DPN) Stamp Tips. The process for fabricating a DPN stamp tip is shown in Scheme 1. By using an AFM to control the movement of a Si3N4 tip, one can coat the tip with PDMS precursor mixture 1 (see the Experimental Section and Figure S1 in Supporting Information). The controlled retraction of the AFM cantilever can generate a sharp poly(dimethylsiloxane) (PDMS) tip rather than a blob. A typical SEM image of the fabricated PDMS-coated DPN stamp tip (Figure 1) shows that the tip radius is about 80 nm, which is a little larger than that of the normal Si3N4 due to the PDMS coating layer. The fabricated DPN stamp tip was tested by imaging a gold substrate (see Figure S2 in Supporting Information), indicating that the effective tip of the PDMS-coated probe, used for AFM imaging and also for DPN process (shown in the following text), is the apex (“tip”) of the coated PDMS layer. Nanopatterns Generated with DPN Stamp Tips. Nanopatterns of different shapes and geometries can be fabricated using this type of DPN stamp tips. In addition, a unique advantage of these stamp tips is that one can even generate hollow molecular nanostructures, such as dots and lines with hollow interiors. This effect allows the fabrication of complex molecular patterns with precise positioning of different types of molecules, which may find potential applications in chemical sensor fabrication and molecular electronics. Figure 2 shows the internally hollow dots, i.e., circular rings, generated on a gold substrate with a 16-mercaptohexadecanoic acid (MHA)-coated DPN stamp tip. The line width of the circles is 125 ( 10 nm, which is not dependent on the tip-substrate contact time. The central holes of the hollow rings show a decrease in diameter from 850 to 300 Nano Lett., Vol. 4, No. 9, 2004

Figure 2. Topography (A) and LFM (B) images of patterned MHA nanocircles on gold, imaged with the same MHA-coated DPN stamp tip. The tip-substrate contact times and the sizes of the internal holes are (1) 5.0 s, 850 nm; (2) 4.5 s, 820 nm; (3) 4.0 s, 800 nm; (4) 3.5 s, 720 nm; (5) 3.0 s, 600 nm; (6) 2.5 s, 560 nm; (7) 2.0 s, 450 nm; (8) 1.5 s, 340 nm; (9) 1.0 s, 300 nm, respectively. (C) High resolution 3D-LFM image of MHA nanocircles.

nm with a decreasing tip-substrate contact time from 5 to 1 s. The hollows arise specifically from the use of the DPN stamp tip. This effect is attributed to the fact that the DPN stamp tip can typically hold a much larger amount of “ink” than the conventional Si3N4 tip (see Figure S3 in Supporting Information). When the stamp tip is immersed into a MHA solution, both solvent (ethanol) and MHA are absorbed into the PDMS DPN stamp tip. Due to solvent evaporation, the outer surface of the PDMS DPN stamp tip quickly dries, while the inner part of the tip remains wet for a much longer time. When such a coated DPN stamp tip contacts a gold substrate, not only MHA but also the solvent (ethanol) are transferred onto the gold substrate. Dried MHA on the outer surface of the tip transfers directly onto the gold substrate to create the outer ring structure of the hollow, whereas the diffusion of MHA and solvent (6.8 mM in ethanol) from inside of the “spongy” tip generates the inner hole, which remains filled with solvent. Since the tip-substrate contact time is very short (from 1 to 5 s), there is little MHA in the central hollow. (Note that in a thiol solution, it usually takes several to tens of hours to form a closely packed SAM on gold.) Hollow lines can also be generated with a DPN stamp tip (Figure 3A, B). The hollow width decreases with increased writing speed, since there is less solvent transferred onto Au at a higher writing speed. The hollows can even be filled by another material, e.g., 1-octadecanethiol (ODT), via DPN or other procedures, resulting in two-component molecular nanostructures, which could be a good system for fundamental study in the field of nanoscale tribology. For example, after creating a DPN pattern with lines and accompanying hollows, the sample was further immersed into 1 mM ODT ethanolic solution for 30 s (note that under these conditions there is nearly no exchange of MHA with ODT). The resulting patterns, imaged with a bare Si3N4 tip (Figure 3C), clearly show that MHA/ODT composite patterns show Nano Lett., Vol. 4, No. 9, 2004

Figure 3. Topography (A) and LFM (B) images of patterned MHA hollow lines on gold, imaged with the same MHA-coated DPN stamp tip. The writing speeds are 0.10, 0.15, 0.20, 0.25, 0.30, 0.35, 0.40, and 0.45 µm/s (from left to right), respectively. LFM image of the MHA hollow lines, passivated with ODT, imaged with a bare Si3N4 tip (C), and section analysis (D) of the white line in (C).

distinct contrast due to higher friction of MHA molecules (Figure 3D). This result provides evidence that the nanoscale hollows generated with the DPN stamp tip are quite different from the surrounding MHA patterns (consisting of MHA SAM). Note that the differences in measured hollow widths in Figure 3B and C arise from a variation in tip-sample contact areas, i.e., due to the difference in the effective tip radii (Figure 3B was imaged with a DPN stamp tip but Figure 3C with a bare Si3N4 tip). Normally, in a conventional DPN experiment, generating hollow nanostructures requires many steps of DPN writing.14 1651

Figure 4. Topographic images (A, C) of the etched gold nanofeatures, imaged with a bare Si3N4 tip. (B, D) Height profiles of the white lines in A and C, respectively.

In contrast, by using a DPN stamp tip, only one DPN holding/writing step is needed to create hollow nanostructures. Therefore, these special DPN stamp tips open up a novel method of creating hollow nanostructures by simultaneous transfer of molecules of interest and their solvent (“liquid” ink). Interestingly, we found that after a MHA-coated DPN stamp tip is used continuously for 1-2 h (depending on the amount of coated “ink”) or letting the MHA-coated DPN stamp tip completely dry in air, the solvent within the PDMS layer will completely evaporate, and thus, further patterning with the same tip yields “normal” structures that do not have internal hollows (see Figure S4 in Supporting Information). After immersing these non hollow-patterned gold substrates in a gold etchant and then a diluted HF solution (see Experimental Section),2d,f,h gold nanostructures were obtained (Figure 4). The measured heights are 14.1 ( 1.1 nm for dots (Figure 4B) and 12.5 ( 2.0 nm for lines (Figure 4D). These results demonstrated that the MHA patterns with a DPN stamp tip can be used as gold etch resists to subsequently create gold nanostructures, as consistent with other DPN experiments.2d,f,h Taken together, we have demonstrated that the PDMS stamp tip can not only serve as a DPN pen to directly write molecular patterns but also image the generated pattern; in addition, the DPN stamp tip can be used to generate an interesting type of hollow nanostructure. As a lithographic method, one of the most important parameters pertains to how small nanostructures can be made. Herein, we show that with a DPN stamp tip, the generated nanostructures can reach sub-100 nm, comparable to the conventional DPN method.1,2a,b Figure 5 shows the topography and LFM images of patterned MHA dots on Au with different tip-substrate contact times. When the contact time changes from 2.00 to 0.125 s, the patterned MHA dot size decreases from 470 to 60 nm. The patterned MHA line width also changes with the writing speed. Fast writing speed results in narrow line 1652

Figure 5. Topography (A) and LFM (B) images of patterned MHA dots on gold, imaged with the same MHA-coated DPN stamp tip. The tip-substrate contact times and the sizes of the dots are (1) 2.00 s, 470 nm; (2) 1.75 s, 430 nm; (3) 1.50 s, 400 nm; (4) 1.25 s, 340 nm; (5) 1.00 s, 300 nm; (6) 0.75 s, 240 nm; (7) 0.50 s, 215 nm; (8) 0.25 s, 105 nm; (9) 0.125 s, 60 nm, respectively.

width (Figure 6). These results are consistent with the DPN diffusion model that accounts for the transfer of “ink” molecules onto a substrate.15 The achieved narrowest line width for MHA is 55 nm (line 2 in Figure 6). Compared to a Si3N4 tip, another great advantage of a DPN stamp tip is that the PDMS coating acts as an ink reservoir and easily absorbs different types of DPN inks. Not only MHA but also ODT, cystamine, macromolecules such as dendrimer G6-OH, and even inorganic salt “inks”16 can be used as “inks” to generate patterns on different substrates of interest. For example, ODT molecules can be patterned on gold substrates by following the same protocol for MHA (see Experimental Section and Figure S5 in Supporting Information). Direct write of cystamine with a normal Si3N4 tip poses a challenge, due to its volatility,2h but nanoscale lines of cystamine on Au can be easily patterned with a DPN stamp tip. As shown in Figure 7, the generated line widths and heights are in the range of 200-300 nm and 0.11-0.47 nm, respectively. The topography and LFM images clearly show the cystamine nanopatterns. The measured heights, 0.11-0.22 nm, demonstrate that the thinner patterns of lines (lines 6-8 in Figure 7) consist of a submonolayer or monolayer of cystamine, whereas the thicker lines, 0.380.41 nm (lines 1-5 in Figure 7), consist of multiple layers of cystamine. The packing density of the cystamine patterned lines increases with decreasing DPN writing speed. We should emphasize that the PDMS coating of a DPN stamp tip is compatible with a wide variety of molecules, which has been demonstrated in previous studies involving microcontact printing of different materials with PDMS stamps.11 Among the different types of ink materials, macromolecules such as polymers and biomolecules including DNA and proteins are of particular interest and importance in both fundamental science and technology. Conventional DPN method for patterning these macromolecules still leaves much space for improvement in ink-coating efficiency, throughput capability, etc. In contrast, DPN stamp tips fabricated in this work can successfully achieve a much higher ink-coating efficiency and hence higher throughput Nano Lett., Vol. 4, No. 9, 2004

Figure 7. Topography (A) and LFM (B) images of patterned cystamine nanolines on gold, imaged with the same cystaminecoated DPN stamp tip. The writing speeds and height of the patterned cystamine lines in A (from left to right, i.e., 1-8) are 0.10, 0.15, 0.20, 0.25, 0.30, 0.35, 0.40, 0.45 um/s and 0.41, 0.42, 0.47, 0.44, 0.38, 0.22, 0.17, 0.11 nm, respectively.

Figure 6. Topography (A) and LFM (B, scan direction from left to right; C, scan direction from right to left) images of patterned MHA lines on gold, imaged with the same MHA-coated DPN stamp tip. The writing speeds for lines 1 and 2 are 1.0 and 1.5 µm/s, respectively.

patterning capability due to the larger ink-adsorption capability of the PDMS coating. Herein, we demonstrate that G6OH polymers can be readily patterned on substrates such as Si/SiOx and Au. By using a dendrimer G6-OH-coated DPN stamp tip, hollow dendrimer nanocircles can be generated on a Si/SiOx surface (Figure 8A and B). The height of the dendrimer-derived nanocircles is 4.1 ( 0.2 nm, and the width of outer circular lines is ∼165 nm. The diameter of the internal hollows is ∼300 nm. The dendrimer nanocircles arise from the diffusion of the solvent (in this case, methanol) absorbed into the “spongy” PDMS stamp tip. When the solvent dries after prolonged use of the stamp tip, dendrimer dot arrays are formed without hollow interior structures. By changing the tip-substrate contact time, nanostructures are formed with measurable and regular changes in the patterned height, ranging from 3.7 ( 0.2 nm (Figure 8C) to 1.0 ( 0.2 Nano Lett., Vol. 4, No. 9, 2004

nm (Figure 8E), but with only small changes in the dot size, from 470 nm (Figure 8C) to 435 nm (Figure 8E). This phenomenon arises from the hindrance of lateral diffusion of the large dendrimer molecules on the substrate.4c,d,17 Note that the ideal-sphere diameter of dendrimer G6-OH is ∼6.7 nm.18 The measured height of the DPN dendrimer patterns is less than 6.7 nm due to the strong interaction between the dendrimer and the Si/SiOx substrate.19 Apparently, DPN pattern features generated with shorter tip-substrate contact time create more “squashed” molecular structures (i.e., the molecules are more flattened). When using a longer tipsubstrate contact time, the increased diffusion of dendrimer molecules from the tip onto the substrate causes an increase of the packing density of the dendrimers, affecting layer thickness by decreasing the squashing effect on the molecules. Dendrimer lines were also generated on both Si/SiOx and Au surfaces. Variations in writing speed only changed the height of the dendrimer lines, whereas the line width remained constant (670 nm in Figure 9A, and 480 nm in Figure 9C). These results provide further evidence of hindered diffusion of macromolecules on the patterned substrate,4c,d,17 and a longer tip-substrate contact time 1653

Figure 9. Topography (A) and LFM (B) images of patterned dendrimer G6-OH nanolines on Si/SiOx, imaged with the same dendrimer-coated DPN stamp tip. The writing speed and height of the patterned dendrimer lines in A (from left to right) are 0.10, 0.15, 0.20, 0.25, 0.30, 0.35, 0.40, 0.45 um/s and 6.9, 6.6, 6.0, 5.2, 4.8, 4.5, 4.0, 3.3 nm, respectively. All line widths in A are 67 nm. Topography (C) and LFM (D) images of patterned dendrimer G6OH nanolines on gold, imaged with the same dendrimer-coated DPN stamp tip. The writing speeds and height of the patterned dendrimer lines in C (from left to right) are 0.010, 0.025, 0.050, 0.075, 0.100, 0.125, 0.150, 0.200 um/s; and 1.9, 1.6, 1.4, 1.3, 1.2, 1.1, 1.0, 0.8 nm, respectively. All line widths in C are 480 nm. Figure 8. Topography (A, C, E) and LFM (B, D, F) images of a patterned dendrimer G6-OH nanoarray on Si/SiOx, imaged with the same dendrimer-coated DPN stamp tip. The tip-substrate contact time, height, width, and internal hollow size of the nanocircles (A) are 10 s, 4.1 ( 0.2, 165, and 300 nm, respectively. The tip-substrate contact time, height, and size of the dots are 5 s, 3.7 ( 0.2 nm, 470 nm (C) and 2 s, 1.0 ( 0.2 nm, 430 nm (E), respectively.

resulting in thicker DPN patterns. Comparing the results obtained on Si/SiOx and Au, although the writing speeds (from left to right in Figure 9A) on Si/SiOx are faster than those on Au (from left to right in Figure 9C), the diffusion rate of G6-OH on Si/SiOx is greater than that on Au - not only in the lateral direction (670 nm on Si/SiOx vs 480 nm on Au) but also in the vertical direction (3.3-6.9 nm on Si/SiOx vs 0.8-1.9 nm on gold). This difference arises from the different surface properties of the substrates and a variation in ink-substrate binding interactions. Normally it is not easy to coat a Si3N4 tip with large molecules and then transfer them onto a solid surface. We tried to pattern G6OH by using a Si3N4 tip without PDMS coating layer; however, the quality of the dendrimer patterns (data not shown) was not comparable to that of Figure 9. Clearly, one main advantage of using a DPN stamp tip, which can generate uniform nanopatterns even with macromolecular inks, is that the “spongy” PDMS tip can effectively absorb/ adsorb these large molecules, much like what has been done 1654

with the µCP technique.11 Obviously, the resolution of this lithography method with a DPN stamp tip greatly exceeds that of the conventional µCP method. Conclusions. A novel, simple and effective method was developed for fabricating PDMS-coated DPN stamp tips. These tips have been used to generate DPN nanopatterns of 16-mercaptohexadecanoic acid (MHA), 1-octadecanethiol (ODT), dendrimers, cystamine, and inorganic salts. The advantages of using this kind of tip in a DPN experiment are (1) easy fabrication, low cost, and high output (near 100% yield); (2) the PDMS coating layer acts as an ink reservoir and readily absorbs a wide variety of DPN inks; (3) DPN stamp tips can be used to pattern some of the common inksubstrate combinations that are currently used with the microcontact printing (µCP) technique; (4) the resolution obtained with the DPN stamp tips exceeds that obtained with scanning probe-contact printing and µCP; (5) DPN stamp tips also can pattern liquid (solvent) ink and make internal hollow nanostructures; and (6) DPN stamp tips can be used for AFM imaging after making DPN patterns. These novel DPN stamp tips essentially combine many of the advantages of the DPN (such as high resolution and simultaneous writing and imaging) and µCP techniques (such as excellent chemical compatibility with many different ink materials and high inkadsorption capability). Nano Lett., Vol. 4, No. 9, 2004

Acknowledgment. The authors acknowledge Prof. Chad Mirkin at Northwestern University, Dr. Rongchao Jin at University of Chicago, and Prof. Albena Ivanisevic at Purdue University for editing the manuscript and giving their invaluable comments and suggestions. Dr. Bjoern Rosner is acknowledged for imaging the DPN stamp tip with SEM.

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