DPN) Is a Type of Scanning Probe Lithography Method Wherein the Tip of an Atomic Force Microscope Or AFM Draws Desired Patterns Onto a Substrate Directly

Module-7_Unit-5_NSNT

Dip-Pen Nanolithography

Introduction Dip pen nanolithography (DPN) is a type of scanning probe lithography method wherein the tip of an atomic force microscope or AFM draws desired patterns onto a substrate directly. This technique can be used on various types of substrates and different types of inks can be used to draw the features. The most common use of this process is in imprinting alkane thiolates on a gold substrate. Patterns of upto 100nm can be drawn using this technique. Dip pen nanolithography is nanoscaled analogous to dip pen. In DPN, tip of AFM cantilever works as ‘pen’ which is coated with the compound (which needs to be deposited) as ‘ink’. This tip is positioned over the substrate (paper) to ‘write’ the pattern. Nanomaterials can be directly deposited over the substrate of choice by using this technique. This technique has also been scaled for simultaneous patterning using 2-D arrays of 55000 tips. This technique has been used in chemistry, materials science, life science, etc. The applications are as ultrahigh density biological nano-arrays, repairing additive photomask, etc.

Figure 1 Typical DPN technique. Ink containing the nanomaterial diffuses from the tip via a water meniscus.

Development Jaschke and Butt, first reported the uncontrolled transfer of the molecular ink from AFM tip, in 1995. They had concluded that it was not possible to create stable nanostructures by transferring alkanethiols on a gold substrate. Chad Mirkin while working at Northwestern University, investigated this process independently. He concluded that under specific conditions, chemically adsorbed stable monolayers could be created on a substrate by transferring the molecules in a controlled manner. They described the process as a high resolution lithography and termed it as DPN. Mirkin and coworkers got this process patented. Since then the technique has been expanded to include liquid inks as well. Noticeably, the deposition mechanisms involved in liquid inks are different from those for molecular inks.

Deposition Materials a. Molecular Inks Molecular inks essentially comprise small molecules which are coated on the DPN tip. These molecules are transferred to the surface of the substrate via a water meniscus. The tip can be coated by two means: either the required molecules can be vapour deposited on it, or the tip can be dipped (dip coating) in a solution of the molecular ink. When dip coating is used for coating the tip, the solvent must be removed before beginning the deposition process. The rate of deposition of a molecular ink is controlled by the diffusion rate of the molecules, and it is molecule specific (implying that it is different for different

molecules). The pattern size is determined by the dwell time of tip/surface and may vary from few milliseconds to a few seconds. Another factor controlling the size of the pattern is the size of water meniscus, which in turn is influenced by humidity (when the radius of curvature of the tip is much smaller than the meniscus). The characteristics of these inks are:  The process is driven by water meniscus (although exceptions can be there)  Resolution is of the order of 50 to 2000 nanometers  No multiplexed deposition  Molecular inks might be specific to the substrate Typical examples include alkane thiols patterned over gold substrates, and patterning silanes (solid pastes) over glass and silicon substrates. b. Liquid Inks Any material which is liquid under deposition conditions can be deposited as liquid inks. The deposition mechanisms depend upon the interaction between - ink and tip, liquid (ink) and substrate, and viscosity of the liquid. Owing to these interactions, the minimum size of the pattern is limited to ~1micron, depending upon the contact angle of liquid. High viscosity of liquid provides more control over the size of the pattern, thus, inks with high viscosities are preferred. In contrast to the molecular ink, multiplexed deposition is possible by using a carrier liquid. For instance, with a viscous buffer, multiple proteins can be simultaneously deposited.

Figure 2 Typical deposition mechanism of a liquid ink.

 Resolution is of the order of 1-10 microns  Multiplexed depositions are possible  Lesser dependency over ink or substrate  Highly viscous materials can be directly deposited Typical examples include pattering of proteins, peptides and DNA. In addition to these, hydrogels, conductive ink, lipids, liquid phase silanes, etc., can also be patterned.

Applications For differentiating between good and bad DPN applications, understanding the capabilities and limitations of this technique is important. Direct write processes (e.g., contact printing) can be used for patterning multiple biomaterials, however, its resolution is poor and fails to form patterns at subcellular resolutions. Several high resolution lithographic techniques are available which can be used to create features with submicron resolutions, but they necessitate expensive high end equipments, not developed for biomolecule depositions and cell culturing. Another technique called ‘microcontact printing’ can be used to create

biomolecule patterns under ambient conditions, however, multiple patterns cannot be developed with it. The following subsections discuss various applications of DPN technique.

Figure 3 Cantilevel based biomolecules sensor functionalized with 4 different proteins. a. Industrial applications The potential applications of DPN are: - Biosensor functionalization: directly placing several trapping sites on a single biosensor. - Nanosensors: this technique can be used to fabricate small and high value sensors to identify multiple targets. - Nanosized protein chips: highly dense protein arrays having enhanced sensitivity. b. Emerging Applications Cell Engineering: DPN can be sued to manipulate cell with submicron resolutions for research purposes. This has many potential uses such as stem cell differentiation, delivering drugs to specific cells, cell sorting, cell adhesion, etc. Rapid Prototyping: It has applications in plasmonics, metamaterials, cell and tissue screening.

Figure 4 SEM micrograph showing gold metastructure arrays fabricated via DPN.

Properties of DPN It is a direct write process which can be employed for both top-down and bottom-up lithography applications. When used for top-down lithography, tip is employed to put etch resist on the surface, this can be followed by traditional etching. In case of bottom-up, the tip directly deposits the compound on the surface.

Figure 5 Gold on silicon metastrutcure produced via top-down DPN.

Advantages DPN has the following unique advantages:  Direct Placement: This technique directly prints the material over an existing micro- or nano- structure.  Direct Write: Patterns and features can be directly created without any mask. The resolutions achieved vary between 50 nanometers to upto 10 microns.  Biocompatibility: Since the technique can achieve submicron to nanometer resolutions at ambient conditions, it can be effectively used in biological applications as well.  Scalability: It is a force independent technique which can be used for parallel depositions as well.

Thermal Dip Pen Lithography It is a heated tip version of the standard DPN technique. This technique can be used to write semiconductor, magnetic, metallic, or optically active nanoscaled particles onto a substrate. A suspension of the particles, to be deposited, is prepaid with PMMA or any other polymer matrix. This suspension is heated till the nanoparticles begin to flow. The probe tip works like a nanopen which creates patterns of nanoparticles in a programmed manner. The resolution achieved depends upon the dimensions of the nanoparticles and can vary between 78 to 400 nanometers. PMMA can be removed by oxygen plasma etching. The merits of thermal DPN include: - it does not require mask - very low resolutions can be achieved - a variety of nanoparticles can be written without any special solution preparation technique. Its drawbacks are that the nanoparticles must be smaller than the radius of gyration of the polymer (for PMMA, it is 6 nanometer). As the size of the nanoparticles increases, the viscosity also increases, which slows down the process. For example, pure polymer can be deposited at 200 μm/s, whereas the addition of nanoparticle decreases it to 2μm/s. However, thermal DPN is faster than the standard DPN.

Common Misconceptions The most common misconceptions regarding DPN are: a. Patterning speed: This is the most talked feature of this technique. It is usually compared with soft lithographic techniques such as microcontact printing, benchtop microscale and nanoscale patterning, etc. Out of these, microcontact printing is presently used standard in low-cost printing. DPN is most often compared with microcontact printing which is used to pattern one material over a large area in a

single stamping step, analogous to photolithography patterning large areas with single exposure. In comparison to these techniques, DPN is slow. DPN does not use masks and can produce several patterns (on a single substrate) of different sizes, shape, and resolution. Microcontact printing cannot be applied to this scenario as it requires creating separate master stamp for all the features, which would be both time consuming and expensive. Furthermore, microcontact printing cannot achieve the resolutions on the order of nanometers. This misconception can be removed by understanding the application areas and methods of photolithography and electron beam lithography. Since in photolithography application areas, e-beam will obviously be slow, the same thing happens between DPN and microcontact printing. b. Related to Atomic Force Microscopy: Since DPN has direct roots to the AFM, it is only natural for people to think that all commercial AFMs can be used for DPN. Most surprisingly, DPN does not need AFM, and it is not necessary for an AFM to have DPN functionalities. The difference between both these, can be best understood by considering the analogy to SEM and e-beam lithography. E-beam lithography has directly evolved from SEM, and both employ focused e-beam. However, both these techniques are not interchangeable at one another’s application area.

Figure 6 Microcontact printed streptavidin having a thickness of 4nm.

A very significant characteristic of DPN is that it is force independent. Virtually all ink and substrate combinations can be used to draw a feature regardless of the hardness of the tip against the substrate and vice versa. While using robust SiN (silicon nitride) tips, the feedback system, laser, quad photodiodes, AFM, etc., all can be avoided.

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