The field of ambient chemical nanolithography technologies was born in 1999 with two papers. One paper was based on the dip pen concept where to achieve the smallest writing dimensions a dry molecular ink was coated on an atomic force microscopy like probe [1]. This method was called dip pen nanolithography (DPN). A second paper was based on the concept of a fountain pen [2] and fully employed atomic force microscopy
(AFM) and its feedback mechanisms but the writing element used was a nanopipette. This paper focuses on fountain pen nanolithography (FPN) and its evolution into a powerful generally applicable chemical writing method on many length scales with many surfaces and utilizing a wide variety of inks including gases.
The technique in general uses a quartz cantilevered nanopipette with a tip highly exposed to the optical axis as seen in Figure 1.
Such an exposed tip has great advantages over other probe technologies where the tip is generally obscured from above by the cantilever
Controlled Writing On Rough Real Substrates
The above writing was affected on generally flat surfaces. In Figure 4 the ability to accomplish complex tasks on rough surfaces with pressure control are shown. An example is the filling of a photonic band gap crystal with the fluorescent labelled protein BSA (Figure 4a).The subsequent near-field optical fluorescent image is shown in Figure 4b. Photonic band gap crystals guide light and are very sensitive to perturbations in their structure. The ability to affect such perturbations selectively using FPN is an important attribute that has many potential applications in photonics.An additional example also shown in Figure 4 is the demonstration of filling a deep trench with a suspension of 1.4 nm gold particles. The task accomplished in this case was the filling of a via with pure gold so that an effective contact could be made with the copper contact shown at the base (Figure 4c). The high aspect ratio of the nanopipette probes are very effective in this task and the results of the filling operation are shown in Figure 4 f and g.
Figure 4: Controlled nanoinjection in holes. (a) SEM image of florescent protein deposition in a single hole of a 1.5 micron photonic band gap with pressure control. (b) On-line near-field optical characterization of the fluorescent bovine serum albumin (BSA) that filled the holes of this silicon photonic band gap material. (c) Illustration of the injection to a 0.5 X 0.5 µ2 hole. The challenge was to fill this via with pure metallic nano particles. (d) 3-D AFM image of the hole before the injection. (e) Height profile along the blue line indicated in panel d shows that the depth of the hole to be 2.3µm. (f) 3-D AFM image of the hole after the voltage injection of 1.4 gold nanoparticles in methanol solution. (g) Height profile along the 1.4 µm green line indicated in panel f shows the depth of the hole to be 1.4 µm after injection.
Control of the Writing Process
The use of differential pressure in the controlled evolution of an ink is clearly shown in Figure 5. In Figure 5a the use of pressure to evolve gas in an aqueous medium is clearly evident while in Figure 5b and c, slits in a polymethylmethacrylate (PMMA) layer were accomplished with a nanopipette through which acetone vapor was effectively effused. The depth of the etching of the PMMA is clearly dependent on the speed of the relative motion of the nanopipette over the surface. These are the first examples of AFM controlled gas chemistry on a surface [9].
In addition, such pressure effects are of crucial importance in the AFM controlled deposition of macroscopic objects such as single cells and shown in [10].
Figure 5: Nanowriting with gases. (a) Using pressure to evolve air in an aqueous medium. (b) AFM image of slits in a PMMA layer etched using acetone vapor delivery through an 800nm pipette. The velocity of the tip scanning was changed as indicated above each line. (b) Cross section through the lines in panel b.
Set-Point:
In Figure 6 another AFM parameter is altered and its effect on the line widths of different materials on different surfaces are compared. In Figure 6a, 50nm silver particles were written with the same pipette and conditions except that the set-point of the nanopipette was altered. It can clearly be seen that the larger force between the nanopipette and the surface led to higher particle concentration on both the Au and SiO2 surfaces across which the writing took place. An electrical AFM probe was placed on these lines with a second contact on the Au surface and the conductivity was measured. The written line at the extreme left of this image visibly had the largest concentration of particles and showed the highest conductivity. In Figure 6 b and c the ultimate resolution of writing BSA in aqueous media, using non-contact mode on a glass surface with low set point is shown and the line widths are between 50-100 nm. Note that such FPN protein writing [11] can be accomplished without protein modification which is difficult to accomplish with DPN.
Figure 6: Controlling The Line Dimensions With Different Set-points. (a) AFM image of lines written with different set-points with an ink of 50nm silver particles between the interface of a gold and SiO2 substrate. The highest conductivity was measured on the line with the largest concentration of nanoparticles as seen in the AFM image. (b) deposition of unmodified BSA protein on glass in non-contact mode with low set-point. (c) Line scan of the lines in panel b. showing a width: 50-100nm.
Conclusion
The generality of the writing process with FPN as has been noted, is based on the manipulation of the wetting process on a nanometric scale. This is a multifaceted problem both from the practical aspects of the FPN and from the fundamental aspects of such processes on a nanoscale. An excellent review has recently appeared [12].
From the point of view of the practical aspects of FPN, there are the issues of how a liquid column emanating from the tip of a pipette wets the surface that is to be written on. However, there is also the question of the wetting behavior of the nanopipette itself. For example, how does the surface on the inside walls of the nanopipette behave vis a vis the solvent that will be used for the writing? Or, what is the nature of the outer walls of the pipette?
>In addition, from the perspective of the surface there is certainly the issue of whether the surface is hydrophobic or hydrophyllic or represents some intermediate state. But even these basic questions have to be complexed on the nanoscale with the roughness of the surface. Basically, roughness amplifies wetting behavior and such wetting behavior is best described by the contact angle. The contact angle is defined conventionally as the angle at the point where a liquid/vapor interface meets a solid surface (see Figure 7a). It quantifies the wettability of the solid surface. Thus, the issues of FPN nanowriting are at the forefront of nanotechnology both from an experimental and computational perspective. Thus, there is enormous fundamental work that can be accomplished in this area using the FPN technologies described in this paper..
Figure 7b and 7c show the effect of such wetting using an aqueous solution of 20% silver nitrate deposited on gold with 200 nm nanopipettes. A wetting agent BYK348 was added to the solution. When the concentration of the wetting agent was 0.003% wt. a contact angle of 65 with the gold substrate was measured and for such a contact angle lines of 80-110nm width were obtained as shown in Figure 7b. This was less than the orifice of the pipette used. On the other hand, the same pipette with the same solution but with 0.005% wt. of BYK348 gave a contact angle of 50+80 with the gold substrate and the lines were 600-700nm in width (see Figure 7c), more than the nanopippete aperture. Finally, when the concentration of the wetting agent was 0.001% wt, the contact angle was 78and no writing took place.
Finally, we should note that chemical characterization of the process of FPN is of great importance as we move forward. Thus, nanopipettes and all glass based probes, that have the attribute of exposed probe tips, allow with the SPM platforms we have employed a completely clear optical axis from above and below for full integration with techniques such as Raman spectroscopy. Using on-line Raman one can understand the chemical changes occurring during the writing but one can also understand both the chemical structure of the written pattern. A most appropriate example is a recent initial study that has shown that carbon nanotubes written by FPN on silicon oxide sitting on a bulk silicon substrate are highly oriented [13]. Further work on fully quantifying this effect is underway in our laboratories which has import both in the deposition of other forms of nanorods and in controlled polymer deposition that has been used to create polymer lenses for nanobiochip applications [14].
Figure 7: Controlling The Width of FPN Written Lines With Different Contact Angle. (a) illustration of the contact angle between a surface and a liquid droplet. (b,c) AFM images of the deposition of 20% wt. silver nitride in aqueous solution on a gold substrate with a 200 nm orifice nanopipette (b) 0.003% wt. of the wetting agent BYK-348 was added. This solution gave a contact angle of 65+50 with the gold substrate. The resulting lines had a width of 80-110 nm, less than the nanopippete orifice. (c) 0.005% wt. of BYK-348 wetting agent was added. The solution gave a contact angle of 50+80 with the gold substrate. The lines were 600-700nm width, more than the nanopippete aperture.
Summary
As can be seen, the application of FPN is diverse and has already matured in many important directions. However, the future is also very bright and some of the seeds of that future are already either in the germination form or are actively prevalent in the writings of those who have actively considered the technique. An example of the former is the controlled deposition of catalytic particles [15]. An example of the latter is the important
chromatography that was already noted earlier in the literature [16]. This could combine nanodeposition with chemical separation. Finally, the AFM systems and probes used in the work reported in this article are also able to be inserted into scanning electron microscopes and SEM/FIB systems [17]. Thus, the ability to, with AFM control, deposit gases or multiple gases with a high degree of local control could materially affect the quality of electron beam based lithographic methodology which presently has to fill the vacuum chamber of the SEM/FIB with specific gases for writing with electron and ion beams.
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Last modified on Thursday, 22 March 2018 15:55