Dip-pen nanolithography

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Classic DPN mechanism: Molecular ink diffusing from a nanoscale tip to a surface through a water meniscus. ClassicDPN mech.png
Classic DPN mechanism: Molecular ink diffusing from a nanoscale tip to a surface through a water meniscus.

Dip pen nanolithography (DPN) is a scanning probe lithography technique where an atomic force microscope (AFM) tip is used to create patterns directly on a range of substances with a variety of inks. [1] A common example of this technique is exemplified by the use of alkane thiolates to imprint onto a gold surface. [2] This technique allows surface patterning on scales of under 100  nanometers. DPN is the nanotechnology analog of the dip pen (also called the quill pen), where the tip of an atomic force microscope cantilever acts as a "pen", which is coated with a chemical compound or mixture acting as an "ink", and put in contact with a substrate, the "paper". [3]

Contents

DPN enables direct deposition of nanoscale materials onto a substrate in a flexible manner. Recent advances have demonstrated massively parallel patterning using two-dimensional arrays of 55,000 tips.

Applications of this technology currently range through chemistry, materials science, and the life sciences, and include such work as ultra high density biological nanoarrays, and additive photomask repair. [4]

Development

The uncontrollable transfer of a molecular "ink" from a coated AFM tip to a substrate was first reported by Jaschke and Butt in 1995, [5] but they erroneously concluded that alkanethiols could not be transferred to gold substrates to form stable nanostructures. A research group at Northwestern University, US led by Chad Mirkin independently studied the process and determined that under the appropriate conditions, molecules could be transferred to a wide variety of surfaces to create stable chemically-adsorbed monolayers in a high resolution lithographic process they termed "DPN". [6] Mirkin and his coworkers hold the patents on this process, [7] and the patterning technique has expanded to include liquid "inks". It is important to note that "liquid inks" are governed by a very different deposition mechanism when compared to "molecular inks".

Deposition materials

Molecular inks

Molecular inks are typically composed of small molecules that are coated onto a DPN tip and are delivered to the surface through a water meniscus.[ citation needed ] In order to coat the tips, one can either vapor coat the tip or dip the tips into a dilute solution containing the molecular ink. If one dip-coats the tips, the solvent must be removed prior to deposition. The deposition rate of a molecular ink is dependent on the diffusion rate of the molecule, which is different for each molecule. The size of the feature is controlled by the tip/surface dwell-time (ranging from milliseconds to seconds) and the size of the water meniscus, which is determined by the humidity conditions (assuming the tip's radius of curvature is much smaller than the meniscus).

Examples

  • Alkane thiols written to gold
  • Silanes (solid phase) written to glass or silicon

Liquid inks

Liquid ink deposition mechanism DPN liquid deposition.png
Liquid ink deposition mechanism

Liquid inks can be any material that is liquid at deposition conditions. The liquid deposition properties are determined by the interactions between the liquid and the tip, the liquid and the surface, and the viscosity of the liquid itself. These interactions limit the minimum feature size of the liquid ink to about 1 micrometre, depending on the contact angle of the liquid. Higher viscosities offer greater control over feature size and are desirable. Unlike molecular inks, it is possible to perform multiplexed depositions using a carrier liquid. For example, using a viscous buffer, it is possible to directly deposit multiple proteins simultaneously.

Examples

Applications

In order to define a good DPN application, it is important to understand what DPN can do that other techniques cannot. Direct-write techniques, like contact printing, can pattern multiple biological materials but it cannot create features with subcellular resolution. Many high-resolution lithography methods can pattern at sub-micrometre resolution, but these require high-cost equipment that were not designed for biomolecule deposition and cell culture. Microcontact printing can print biomolecules at ambient conditions, but it cannot pattern multiple materials with nanoscale registry.

Industrial applications

The following are some examples of how DPN is being applied to potential products.

Cantilever biosensor functionalized with 4 different proteins Cantilever biosensor.png
Cantilever biosensor functionalized with 4 different proteins
  1. Biosensor Functionalization – Directly place multiple capture domains on a single biosensor device
  2. Nanoscale Sensor Fabrication – Small, high-value sensors that can detect multiple targets [16]
  3. Nanoscale Protein Chips – High-density protein arrays with increased sensitivity

Emerging applications

Cell engineering

DPN is emerging as a powerful research tool for manipulating cells at subcellular resolution [17] [18]

  • Stem cell differentiation
  • Subcellular drug delivery
  • Cell sorting
  • Surface gradients
  • Subcellular ECM protein patterns
  • Cell adhesion

Rapid prototyping

SEM image of DPN fabricated gold metastructure arrays. Large meta SEM.png
SEM image of DPN fabricated gold metastructure arrays.
  • Plasmonics and Metamaterials
  • Cell and tissue screening

Properties

Direct write

DPN is a direct write technique so it can be used for top-down and bottom-up lithography applications. In top-down work, the tips are used to deliver an etch resist to a surface, which is followed by a standard etching process. [19] In bottom-up applications, the material of interest is delivered directly to the surface via the tips.

Gold on silicon metastructure fabricated with top-down DPN methods Square circ metastruc.png
Gold on silicon metastructure fabricated with top-down DPN methods

Unique advantages

Thermal dip pen lithography

A heated probe tip version of Dip Pen Lithography has also been demonstrated, thermal Dip Pen Lithography (tDPL), to deposit nanoparticles. [22] Semiconductor, magnetic, metallic, or optically active nanoparticles can be written to a substrate via this method. The particles are suspended in a Poly(methyl methacrylate) (PMMA) or equivalent polymer matrix, and heated by the probe tip until they begin to flow. The probe tip acts as a nano-pen, and can pattern nanoparticles into a programmed structure. Depending on the size of the nanoparticles, resolutions of 78–400 nm were attained. An O2 plasma etch can be used to remove the PMMA matrix, and in the case of Iron Oxide nanoparticles, further reduce the resolution of lines to 10 nm. [22] Advantages unique to tDPL are that it is a maskless additive process that can achieve very narrow resolutions, it can also easily write many types of nanoparticles without requiring special solution preparation techniques. However there are limitations to this method. The nanoparticles must be smaller than the radius of gyration of the polymer, in the case of PMMA this is about 6 nm. Additionally, as nanoparticles increase in size viscosity increases, slowing the process. For a pure polymer deposition speeds of 200 μm/s are achievable. Adding nanoparticles reduces speeds to 2 μm/s, but is still faster than regular Dip Pen Lithography. [22]

Beam pen lithography

A two dimensional array of (PDMS) deformable transparent pyramid shaped tips are coated with an opaque layer of metal. The metal is then removed from the very tip of the pyramid, leaving an aperture for light to pass through. The array is then scanned across a surface and light is directed to the base of each pyramid via a micromirror array, which funnels the light toward the tip. Depending on the distance between the tips and the surface, light interacts with the surface in a near-field or far-field fashion, allowing sub-diffraction scale features (100 nm features with 400 nm light) or larger features to be fabricated. [23]

Common misconceptions

Direct comparisons to other techniques

Streptavidin (4 nm thickness) deposited using microcontact printing Sarfus.SoftLitho.Streptavidin.jpg
Streptavidin (4 nm thickness) deposited using microcontact printing

The criticism most often directed at DPN is the patterning speed. The reason for this has more to do with how it is compared to other techniques rather than any inherent weaknesses. For example, the soft lithography method, microcontact printing (μCP), is the current standard for low cost, bench-top micro and nanoscale patterning, so it is easy to understand why DPN is compared directly to microcontact printing. The problem is that the comparisons are usually based upon applications that are strongly suited to μCP, instead of comparing them to some neutral application. μCP has the ability to pattern one material over a large area in a single stamping step, just as photolithography can pattern over a large area in a single exposure. Of course DPN is slow when it is compared to the strength of another technique. DPN is a maskless direct write technique that can be used to create multiple patterns of varying size, shape, and feature resolution, all on a single substrate. No one would try to apply microcontact printing to such a project because then it would never be worth the time and money required to fabricate each master stamp for each new pattern. Even if they did, microcontact printing would not be capable of aligning multiple materials from multiple stamps with nanoscale registry. [24] The best way to understand this misconception is to think about the different ways to apply photolithography and e-beam lithography. No one would try to use e-beam to solve a photolithography problem and then claim e-beam to be "too slow". Directly compared to photolithography's large area patterning capabilities, e-beam lithography is slow and yet, e-beam instruments can be found in every lab and nanofab in the world. The reason for this is because e-beam has unique capabilities that cannot be matched by photolithography, just as DPN has unique capabilities that cannot be matched by microcontact printing.

Connection to atomic force microscopy

DPN evolved directly from AFM so it is not a surprise that people often assume that any commercial AFM can perform DPN experiments. In fact, DPN does not require an AFM, and an AFM does not necessarily have real DPN capabilities. There is an excellent analogy with scanning electron microscopy (SEM) and electron beam (E-beam) lithography. E-beam evolved directly from SEM technology and both use a focused electron beam, but it is not possible to perform modern E-beam lithography experiments on a SEM that lacks the proper lithography hardware and software components.

It is also important to consider one of the unique characteristics of DPN, namely its force independence. With virtually all ink/substrate combinations, the same feature size will be patterned no matter how hard the tip is pressing down against the surface. [25] As long as robust SiN tips are used, there is no need for complicated feedback electronics, no need for lasers, no need for quad photo-diodes, and no need for an AFM.

See also

Related Research Articles

<span class="mw-page-title-main">Nanotechnology</span> Field of science involving control of matter on atomic and (supra)molecular scales

Nanotechnology was defined by the National Nanotechnology Initiative as the manipulation of matter with at least one dimension sized from 1 to 100 nanometers (nm). At this scale, commonly known as the nanoscale, surface area and quantum mechanical effects become important in describing properties of matter. The definition of nanotechnology is inclusive of all types of research and technologies that deal with these special properties. It is therefore common to see the plural form "nanotechnologies" as well as "nanoscale technologies" to refer to the broad range of research and applications whose common trait is size. An earlier description of nanotechnology referred to the particular technological goal of precisely manipulating atoms and molecules for fabrication of macroscale products, also now referred to as molecular nanotechnology.

<span class="mw-page-title-main">Atomic force microscopy</span> Type of microscopy

Atomic force microscopy (AFM) or scanning force microscopy (SFM) is a very-high-resolution type of scanning probe microscopy (SPM), with demonstrated resolution on the order of fractions of a nanometer, more than 1000 times better than the optical diffraction limit.

<span class="mw-page-title-main">Electron-beam lithography</span> Lithographic technique that uses a scanning beam of electrons

Electron-beam lithography is the practice of scanning a focused beam of electrons to draw custom shapes on a surface covered with an electron-sensitive film called a resist (exposing). The electron beam changes the solubility of the resist, enabling selective removal of either the exposed or non-exposed regions of the resist by immersing it in a solvent (developing). The purpose, as with photolithography, is to create very small structures in the resist that can subsequently be transferred to the substrate material, often by etching.

<span class="mw-page-title-main">Soft lithography</span> Techniques that create structures using stamps

In technology, soft lithography is a family of techniques for fabricating or replicating structures using "elastomeric stamps, molds, and conformable photomasks". It is called "soft" because it uses elastomeric materials, most notably PDMS.

<span class="mw-page-title-main">Self-assembled monolayer</span>

Self-assembled monolayers (SAM) of organic molecules are molecular assemblies formed spontaneously on surfaces by adsorption and are organized into more or less large ordered domains. In some cases molecules that form the monolayer do not interact strongly with the substrate. This is the case for instance of the two-dimensional supramolecular networks of e.g. perylenetetracarboxylic dianhydride (PTCDA) on gold or of e.g. porphyrins on highly oriented pyrolitic graphite (HOPG). In other cases the molecules possess a head group that has a strong affinity to the substrate and anchors the molecule to it. Such a SAM consisting of a head group, tail and functional end group is depicted in Figure 1. Common head groups include thiols, silanes, phosphonates, etc.

Masklesslithography (MPL) is a photomask-less photolithography-like technology used to project or focal-spot write the image pattern onto a chemical resist-coated substrate by means of UV radiation or electron beam.

Nanolithography (NL) is a growing field of techniques within nanotechnology dealing with the engineering of nanometer-scale structures on various materials.

<span class="mw-page-title-main">Nanochemistry</span> Combination of chemistry and nanoscience

Nanochemistry is an emerging sub-discipline of the chemical and material sciences that deals with the development of new methods for creating nanoscale materials. The term "nanochemistry" was first used by Ozin in 1992 as 'the uses of chemical synthesis to reproducibly afford nanomaterials from the atom "up", contrary to the nanoengineering and nanophysics approach that operates from the bulk "down"'. Nanochemistry focuses on solid-state chemistry that emphasizes synthesis of building blocks that are dependent on size, surface, shape, and defect properties, rather than the actual production of matter. Atomic and molecular properties mainly deal with the degrees of freedom of atoms in the periodic table. However, nanochemistry introduced other degrees of freedom that controls material's behaviors by transformation into solutions. Nanoscale objects exhibit novel material properties, largely as a consequence of their finite small size. Several chemical modifications on nanometer-scaled structures approve size dependent effects.

<span class="mw-page-title-main">Microcontact printing</span>

Microcontact printing is a form of soft lithography that uses the relief patterns on a master polydimethylsiloxane (PDMS) stamp or Urethane rubber micro stamp to form patterns of self-assembled monolayers (SAMs) of ink on the surface of a substrate through conformal contact as in the case of nanotransfer printing (nTP). Its applications are wide-ranging including microelectronics, surface chemistry and cell biology.

Scanning probe lithography (SPL) describes a set of nanolithographic methods to pattern material on the nanoscale using scanning probes. It is a direct-write, mask-less approach which bypasses the diffraction limit and can reach resolutions below 10 nm. It is considered an alternative lithographic technology often used in academic and research environments. The term scanning probe lithography was coined after the first patterning experiments with scanning probe microscopes (SPM) in the late 1980s.

Microlithography is a general name for any manufacturing process that can create a minutely patterned thin film of protective materials over a substrate, such as a silicon wafer, in order to protect selected areas of it during subsequent etching, deposition, or implantation operations. The term is normally used for processes that can reliably produce features of microscopic size, such as 10 micrometres or less. The term nanolithography may be used to designate processes that can produce nanoscale features, such as less than 100 nanometres.

Plasmonic nanolithography is a nanolithographic process that utilizes surface plasmon excitations such as surface plasmon polaritons (SPPs) to fabricate nanoscale structures. SPPs, which are surface waves that propagate in between planar dielectric-metal layers in the optical regime, can bypass the diffraction limit on the optical resolution that acts as a bottleneck for conventional photolithography.

<span class="mw-page-title-main">Local oxidation nanolithography</span>

Local oxidation nanolithography (LON) is a tip-based nanofabrication method. It is based on the spatial confinement on an oxidation reaction under the sharp tip of an atomic force microscope.

<span class="mw-page-title-main">Thermal scanning probe lithography</span>

Thermal scanning probe lithography (t-SPL) is a form of scanning probe lithography (SPL) whereby material is structured on the nanoscale using scanning probes, primarily through the application of thermal energy.

<span class="mw-page-title-main">Nanofountain probe</span>

A nanofountain probe (NFP) is a device for 'drawing' micropatterns of liquid chemicals at extremely small resolution. An NFP contains a cantilevered micro-fluidic device terminated in a nanofountain. The embedded microfluidics facilitates rapid and continuous delivery of molecules from the on-chip reservoirs to the fountain tip. When the tip is brought into contact with the substrate, a liquid meniscus forms, providing a path for molecular transport to the substrate. By controlling the geometry of the meniscus through hold time and deposition speed, various inks and biomolecules could be patterned on a surface, with sub 100 nm resolution.

Thermochemical nanolithography (TCNL) or thermochemical scanning probe lithography (tc-SPL) is a scanning probe microscopy-based nanolithography technique which triggers thermally activated chemical reactions to change the chemical functionality or the phase of surfaces. Chemical changes can be written very quickly through rapid probe scanning, since no mass is transferred from the tip to the surface, and writing speed is limited only by the heat transfer rate. TCNL was invented in 2007 by a group at the Georgia Institute of Technology. Riedo and collaborators demonstrated that TCNL can produce local chemical changes with feature sizes down to 12 nm at scan speeds up to 1 mm/s.

Scanning electrochemical microscopy (SECM) is a technique within the broader class of scanning probe microscopy (SPM) that is used to measure the local electrochemical behavior of liquid/solid, liquid/gas and liquid/liquid interfaces. Initial characterization of the technique was credited to University of Texas electrochemist, Allen J. Bard, in 1989. Since then, the theoretical underpinnings have matured to allow widespread use of the technique in chemistry, biology and materials science. Spatially resolved electrochemical signals can be acquired by measuring the current at an ultramicroelectrode (UME) tip as a function of precise tip position over a substrate region of interest. Interpretation of the SECM signal is based on the concept of diffusion-limited current. Two-dimensional raster scan information can be compiled to generate images of surface reactivity and chemical kinetics.

Nanosphere lithography (NSL) is an economical technique for generating single-layer hexagonally close packed or similar patterns of nanoscale features. Generally, NSL applies planar ordered arrays of nanometer-sized latex or silica spheres as lithography masks to fabricate nanoparticle arrays. NSL uses self-assembled monolayers of spheres as evaporation masks. These spheres can be deposited using multiple methods including Langmuir-Blodgett, dip coating, spin coating, solvent evaporation, force-assembly, and air-water interface. This method has been used to fabricate arrays of various nanopatterns, including gold nanodots with precisely controlled spacings.

Fluidic force microscopy (FluidFM) is a type of scanning probe microscopy, and is typically used on a standard inverted light microscope.

Electrochemical AFM (EC-AFM) is a particular type of Scanning probe microscopy (SPM), which combines the classical Atomic force microscopy (AFM) together with electrochemical measurements. EC-AFM allows to perform in-situ AFM measurements in an electrochemical cell, in order to investigate the actual changes in the electrode surface morphology during electrochemical reactions. The solid-liquid interface is thus investigated. This technique was developed for the first time in 1996 by Kouzeki et al., who studied amorphous and polycrystalline thin films of Naphthalocyanine on Indium tin oxide in a solution of 0.1 M Potassium chloride (KCl). Unlike the Electrochemical scanning tunneling microscope, previously developed by Itaya and Tomita in 1988, the tip is non-conductive and it is easily steered in a liquid environment.

References

  1. ↑ Ginger, David S.; Zhang, Hua; Mirkin, Chad A. (2004). "The Evolution of Dip-Pen Nanolithography". Angewandte Chemie International Edition. 43 (1): 30–45. doi:10.1002/anie.200300608. ISSN   1433-7851. PMID   14694469.
  2. ↑ Piner, R. D. (1999). ""Dip-Pen" Nanolithography". Science. 283 (5402): 661–663. doi:10.1126/science.283.5402.661. ISSN   0036-8075. PMID   9924019. S2CID   27011581.
  3. ↑ "DPN – Northwestern – Intro". Northwestern University. Archived from the original on 12 June 2013. Retrieved 7 May 2013.
  4. ↑ Elhadj, Selim; Chernov, Alexander A; De Yoreo, James J (13 February 2008). "Solvent-mediated repair and patterning of surfaces by AFM". Nanotechnology. IOP Publishing. 19 (10): 105304. Bibcode:2008Nanot..19j5304E. doi:10.1088/0957-4484/19/10/105304. ISSN   0957-4484. PMID   21817697. S2CID   22708216.
  5. ↑ Jaschke, M.; Butt, H.-J. (1995). "Deposition of Organic Material by the Tip of a Scanning Force Microscope". Langmuir. 11 (4): 1061–1064. doi:10.1021/la00004a004.
  6. ↑ Piner, R. D.; Zhu, J.; Xu, F.; Hong, S.; Mirkin, C. A. (1999). "Dip Pen Nanolithography". Science. 283 (5402): 661–663. doi:10.1126/science.283.5402.661. PMID   9924019. S2CID   27011581.
  7. ↑ "Dip-Pen Nanolithography". Archived from the original on 12 June 2013. Retrieved 7 May 2013.
  8. ↑ Lee, K.-B. (7 February 2002). "Protein Nanoarrays Generated By Dip-Pen Nanolithography". Science. American Association for the Advancement of Science (AAAS). 295 (5560): 1702–1705. Bibcode:2002Sci...295.1702L. doi: 10.1126/science.1067172 . ISSN   0036-8075. PMID   11834780. S2CID   1050903.
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  10. ↑ Sistiabudi, Rizaldi; Ivanisevic, Albena (2 October 2008). "Dip-Pen Nanolithography of Bioactive Peptides on Collagen-Terminated Retinal Membrane". Advanced Materials. Wiley. 20 (19): 3678–3681. Bibcode:2008AdM....20.3678S. doi:10.1002/adma.200800950. ISSN   0935-9648. S2CID   135957280.
  11. ↑ Demers, L. M. (7 June 2002). "Direct Patterning of Modified Oligonucleotides on Metals and Insulators by Dip-Pen Nanolithography". Science. American Association for the Advancement of Science (AAAS). 296 (5574): 1836–1838. Bibcode:2002Sci...296.1836D. doi:10.1126/science.1071480. ISSN   0036-8075. PMID   12052950. S2CID   25516647.
  12. ↑ Fu; Liu; Zhang; Dravid (2003). "Nanopatterning of "Hard" Magnetic Nanostructures via DPN and a Sol-based Ink". Nano Letters. 3 (6): 757–760. Bibcode:2003NanoL...3..757F. doi:10.1021/nl034172g. S2CID   44215721.
  13. ↑ Su; Aslam; Fu; Wu; Dravid (2004). "Dip-pen nanopatterning of photosensitive conducting polymer using a monomer ink". Appl. Phys. Lett. 84 (21): 4200. Bibcode:2004ApPhL..84.4200S. doi:10.1063/1.1737469. S2CID   7050999.
  14. ↑ Lenhert, Steven; Sun, Peng; Wang, Yuhuang; Fuchs, Harald; Mirkin, Chad A (2007). "Massively parallel dip-pen nanolithography of heterogeneous supported phospholipid multilayer patterns". Small. 3 (1): 71–75. doi:10.1002/smll.200600431. ISSN   1613-6810. PMID   17294472. S2CID   32133498.
  15. ↑ Sekula, Sylwia; Fuchs, Jeanette; Weg-Remers, Susanne; Nagel, Peter; Schuppler, Stefan; et al. (2008). "Multiplexed Lipid Dip-Pen Nanolithography on Subcellular Scales for the Templating of Functional Proteins and Cell Culture". Small. Wiley. 4 (10): 1785–1793. doi:10.1002/smll.200800949. ISSN   1613-6810. PMID   18814174. S2CID   13843962.
  16. ↑ Tang; Shi (2008). "Preparation of gas sensors via DPN". Sensors and Actuators B. 131 (2): 379–383. doi:10.1016/j.snb.2007.11.043.
  17. ↑ Pulsipher, Abigail; Yousaf, Muhammad N. (2 March 2010). "Surface Chemistry and Cell Biological Tools for the Analysis of Cell Adhesion and Migration". ChemBioChem. Wiley. 11 (6): 745–753. doi:10.1002/cbic.200900787. ISSN   1439-4227. PMID   20198673. S2CID   8243543.
  18. ↑ Yousaf, Muhammad N (2009). "Model substrates for studies of cell mobility". Current Opinion in Chemical Biology. Elsevier BV. 13 (5–6): 697–704. doi:10.1016/j.cbpa.2009.10.001. ISSN   1367-5931. PMID   19864174.
  19. ↑ Zhang, Hua; Amro, Nabil A.; Disawal, Sandeep; Elghanian, Robert; Shile, Roger; Fragala, Joseph (2 January 2007). "High-Throughput Dip-Pen-Nanolithography-Based Fabrication of Si Nanostructures". Small. Wiley. 3 (1): 81–85. doi:10.1002/smll.200600393. ISSN   1613-6810. PMID   17294474.
  20. ↑ Maskless lithography
  21. ↑ Nature Chemistry Vol 1, August 2009
  22. 1 2 3 Woo, Dai, King & Sheehan "Maskless Nanoscale Writing of Nanoparticle-Polymer Composites and Nanoparticle Assemblies using Thermal Nanoprobes" NanoLetters (2009)
  23. ↑ Huo, Fengwei; Zheng, Gengfeng; Liao, Xing; Giam, Louise R.; Chai, Jinan; Chen, Xiaodong; Shim, Wooyoung; Mirkin, Chad A. (2010). "Beam pen lithography". Nature Nanotechnology. 5 (9): 637–640. Bibcode:2010NatNa...5..637H. doi:10.1038/nnano.2010.161. PMID   20676088.
  24. ↑ Mei, Ying; Cannizzaro, Christopher; Park, Hyoungshin; Xu, Qiaobing; Bogatyrev, Said R.; Yi, Kevin; Goldman, Nathan; Langer, Robert; Anderson, Daniel G. (2008). "Cell-Compatible, Multicomponent Protein Arrays with Subcellular Feature Resolution". Small. Wiley. 4 (10): 1600–1604. doi:10.1002/smll.200800363. ISSN   1613-6810. PMC   2679812 . PMID   18844310.
  25. ↑ Exceptions exist when printing to soft materials – Maedler, C.; Chada, S.; Cui, X.; Taylor, M.; Yan, M.; La Rosa, A. (2008). "Creation of nanopatterns by local protonation of P4VP via dip pen nanolithography". Journal of Applied Physics. 104 (1): 014311–014311–4. Bibcode:2008JAP...104a4311M. doi:10.1063/1.2953090. ISSN   0021-8979. S2CID   120578436.
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