Dip-Pen Nanolithography

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					Dip-Pen Nanolithography

           Microstructures have always been limited by the size of the components that
make up a device.
“Dip-Pen Nanolithography” (DPN) was developed by Northwestern University as a new
tool for preparing nanostructures. The invention of DPN, which has created the world's
smallest pen, will catalyze many advances in the emerging areas of nanotechnology,
mechanical and molecule-based electronics. Specifically, DPN is the missing link in the
nanotechnology arena that will allow development of smaller, lighter weight, faster, and
more reliably produced:

   1) electronic circuits and devices,
   2) high-density storage materials,
   3) sensory structures, and
   4) micro electro mechanical devices.

DPN is a unique modification of atomic force microscope (AFM) instrumentation. This
direct-write technique offers high-resolution patterning capabilities for a number of
molecular and biomolecular „inks‟ on a variety of substrate types such as metals,
semiconductors, and monolayer functionalized surfaces. The ability to achieve precise
alignment registration of multiple patterns is an additional advantage earned by using an
AFM tip to write, as well as read nanoscopic features on a surface. These attributes of
DPN make it a valuable tool for studying fundamental issues in colloid, surface science,
and nanotechnology, for instance diffusion and capillarity on a surface at the nanometer
level, organization and crystallization of particles onto chemical or biomolecular
templates, monolayer etching resists for semiconductors, and nanometer-sized tethered
polymer structures.


Working of DPN
        Dip-Pen Nanolithography (DPN) is a scanning probe nanopatterning technique in
which an AFM tip is used to deliver molecules to a surface via a solvent meniscus, which
naturally forms in the ambient atmosphere. When an AFM is typically used, the device‟s
stylus is positioned on or close to the specimen surface. This minute distance allows
interaction of an AFM cantilever with the sample surface. As the AFM tip is moved over
the surface, the device‟s movements are translated into topological information of the
sample. However, a side effect of the minute tip-to-surface separation exists; water vapor
condenses from the surrounding air and forms a droplet between the tip and the substrate.
DPN takes advantage of the before mentioned side effect to create patterns on a desired
substrate. A standard AFM cantilever is coated with a single type of organic species;
proteins, oligonucleotides, etc. The modified stylus equipped AFM uses the water
droplet, formed at the tip-substrate interface, as a medium to transport the coated
molecules to the sample surface. (Figure 6).
Figure 1. Illustration of molecular deposit of DPN tip. The desired
molecules are deposited through a water meniscus medium to the surface.



Using the DPN technique, numerous grids of proteins, magnetic nanoparticles, and
DNA can be created (Figure 2).




   Figure 2. Images of dots (2a) and lines (2b) of magnetic nanoparticles
                            created using DPN.
Although DPN allows a great deal of flexibility with respect to pattern designs and
deposition molecules, there is a fundamental limitation; an individual stylus can only be
coated with one type of molecule or one combination of molecules. Therefore, only one-
component patterns can be produced per run. If other molecules or molecular combina-
tions are desired, theAFM tip must be replaced. Likewise, there are a limited number of
species on the tip. Once those molecules are used up, the tip must be recoated.

        To address the abovementioned issues, DPN version 2 will hypothetically work
like a fountain pen (Figure 3a). The device, currently being developed group, consists of
reservoirs of desired molecules in solution. The inks then flow to a customized AFM tip.
The reservoirs can be switched on the fly, by an incorporated micro fluidic valve. The
only limit for species deposition will be the speed at which the solution reaches the tip.
At this time, the actual characteristics of the “Fountain-Pen Nanolithography” (FPN) tip,
channels, and reservoirs are in the design process. Various geometries, materials, and
fabrication processes are being tested for possible prototype development (Figure 3b).
Therefore, the future FPN will depend on µF channel properties to dictate the device‟s
performance.




  Figure 3a) Illustration of possible FPN device. Microfludic channels will
         transport molecular inks to the tip for species deposition.




  b) Illustration of Massively Parallel Multi-tip Nanoscale Manipulator with
                               Fluidic capability.
An important requirement for creating stable nanostructures is that the transported
molecules anchor themselves to the substrate via chemisorption. When T-substituted
alkanethiols are patterned on a gold substrate, a monolayer is formed in which the thiol
headgroups form relatively strong bonds to the gold and the alkane chains extend roughly
perpendicular to surface.




     A) AFM image showing lattice-resolved monolayer of octadecanethiol
                         patterned on gold via DPN.




Attributes of DPN
              Creating nanostructures using DPN is a single step process, which does not
require the use of resists. Using a conventional Atomic Force Microscope it is possible to
achieve ultra-high resolution features–as small as 15 nm linewidths and ~ 5 nm spatial
resolution, Figure 4a. For nanotechnology applications, it is not only important to pattern
molecules in high resolution, but also to functionalize surfaces with patterns of two or
more components.

        One of the most important attributes of DPN is that because the same device is
       used to image and write a pattern, patterns of multiple molecular inks can be
       formed on the same substrate in very high alignment, Figure 4b.
      A) Ultra-high resolution pattern of mercaptohexadecanoic acid on
                           atomically flat gold surface.
     B) DPN generated multi-component nanostructure with two aligned
                            alkanethiol patterns.



Applications of DPN
         We are currently using DPN to probe fundamental surface science questions as
well as to create technologically relevant nanostructures. Part of the process of
investigating these technological applications requires that we develop methods, which
will allow parallel patterning in addition to the serial capabilities of DPN.


      Applications of DPN can be classified as:



                   DPN technology could be used to create many small-scale sensors
                    and power assemblies mounted on a single chip for use on micro-
                    satellites or mounted within an unmanned aerospace vehicle (UAV).
                    The savings in launch weight provides for significant savings in
                    launch costs.
            DPN is currently using to probe fundamental surface science
             questions as well as to create technologically relevant
             nanostructures.

            It catalyzes many advances in the emerging areas of nanotechnology
             and molecule-based electronics. This advance will enhance the
             possibility of future Air Force weapon systems becoming smaller,
             lighter, and less expensive.

            It is used to produce solid-state nanoresists, organic and bioorganic
             circuits, nanoprinted catalysts etc.




Figure 5. Some of the potential applications of DPN, center: micro
                 fabricated multiple AFM probe.
A Multipen Plotter for Parallel Patterning

        Initial DPN experiments have involved a single AFM probe for formation of
patterns on a substrate in a serial fashion. Creation of many patterns in duplicate is thus a
slow process. The throughput of DPN patterning may be significantly increased if a large
and dense array of DPN pens is used to create features in parallel. Initial experiments in
which two DPN pens operated in parallel were used to demonstrate this concept.
Significantly, it has been determined that the DPN linewidth is not a strong function of
the contact force within a limited range.
         This important characteristic of DPN eliminates the need to perfectly align the
pen arrays to the substrate surfaces, and therefore, the need for complicated multiple pen
feedback system. Current efforts in this area are focused on the design and testing of a
prototype microfabricated 32-pen array.




                       A) Schematic of two-pen DPN plotter.




Patterning on Semiconductor Surfaces via DPN
      Using DPN a method of patterning high resolution (sub-100 nm) organic patterns on
silicon oxide (SiOx) and gallium arsenide (GaAs) surfaces has been developed. The
choice of molecular ink is crucial to successful patterning. The surface coating agents
that are commonly employed for these types of substrates (trichloro or trialkoxysilanes)
are rapidly hydrolysed under standard DPN conditions (30% humidity) and thus
polymerize on the AFM tip before they can be transferred to the surface.
DPN-Generated Templates for Combinatorial Fabrication and
Study of New Particle-Based Materials.
        DPN is well suited for nanofabrication of customized structures in arrays
consisting of several to thousands components which can be combinatorially screened for
a certain process, for instance catalysis or cell adhesion. DPN allows one to
systematically vary the lattice parameters of a 2-dimensional chemical or biochemical
template array, including spacing, dot size, orientation, and chemical composition.
Subsequent selective interaction, for instance, of certain particles with the template can
be used to initiate the process of 3-dimensional colloidal crystal growth. The ability to
form these types of crystals based on particles of a size on the order of the wavelength of
light with a high degree of control over the lattice parameters and integrity of the
structure is extremely important for the study and fabrication of photonic bandgap
materials for use in optical communications devices.

				
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