Patent Application
20130284598
This application claims priority under 35 U.S.C. §119 from GB Patent Application No. 1207463.9 filed Apr. 30, 2012, the entire contents of which are incorporated herein by reference.
1. Field of the Invention
The invention relates to the field of methods and apparatuses for positioning nano-objects with aspect ratios.
2. Description of the Related Art
The controlled synthesis of nano-objects (i.e., nanoscale objects or nanoparticles, sized between 1 and 100 nanometers(nm)) in the form of spheres, rods or wires, etc., has led to a variety of applications in a host of scientific research areas. Bottom up synthesis leads to mono-crystalline nanoparticles and enables the fabrication of multi-component structures. Their structural properties often provide unique or superior performance of the particles in comparison to their top down-fabricated counterparts. A wide spectrum of applications, e.g. in integrated devices, are available if precise placement and alignment relative to neighboring particles or other functional structures on a substrate can be possible. Ideally, it is desirable to obtain both precise placement and alignment simultaneously at high packing density with placement accuracy on the order of the nanoparticle diameter, typically of 5-50 nm, so far, an unresolved challenge.
According to one aspect of the present invention, a method is provided for positioning nano-objects on a surface. The method includes: providing a first surface and a second surface in a position facing each other, where one or more of the surfaces exhibits one or more positioning structures having dimensions on the nanoscale; providing an ionic liquid suspension of the nano-objects between the two surfaces, where the suspension includes two electrical double layers each formed at an interface with a respective one of the two surfaces, and the surfaces have electrical charges of the same sign; enabling the nano-objects in the suspension to position according to a potential energy resulting from the electrical charge of the two surfaces; and depositing one or more of the nano-objects on the first surface according to the positioning structures by shifting the minima of the potential energy towards the first surface.
According to another aspect of the present invention, an apparatus is provided for implementing a method for positioning nano-objects on a surface. The apparatus includes: a first surface and a second surface, in a position facing each other, where one or more of the two surfaces has positioning structures with dimensions on the nanoscale; an ionic liquid suspension of nano-objects between the two surface, where the suspension includes two electrical double layers each formed at an interface with a respective one of the two surface, and the surfaces have electrical charges of the same sign; and a positioning means coupled to the first surface and/or the second surface, where the positioning means is configured to move the first surface relative to the second surface during operation.
The present invention provides a new methodology, which makes it possible to precisely orient and place charged nano-objects at desired positions on a target substrate of choice. Present methods rely only on the charge of the confining surfaces and the liquid, possibly the particles too, which allows for the placement of a wide range of particles ranging from micro-meter long nanowires, all the way down to DNA and proteins. Particles can be neutral or charged, dielectric or metal, etc. These methods advantageously apply in particular to high aspect ratio nanoparticles like nanowire, opening up a way to exploit the functionality of these complex bottom-up derived objects. They can be aligned to existing structures on the substrate enabling device integration. The method works in parallel and high throughput values can be achieved. In addition, the positioning steps can be repeated on top of already assembled items to build up complex three dimensional (3D) functional circuits.
According to a first aspect, the present invention is embodied as a method for positioning nano-objects, on a surface, the method includes: providing two surfaces including a first surface and a second surface in position facing each other, where at least one of the two surfaces exhibits one or more positioning structures having dimensions on the nanoscale; and providing a ionic liquid suspension of the nano-objects between the two surfaces, the suspension including two electrical double layers formed, each, at an interface with a respective one of the two surfaces, the electrical surface charges of the two surfaces being of a same sign; enabling nano-objects in the suspension position according to a potential energy resulting from the electrical charge of the two surfaces; and depositing one or more of the nano-objects on the first surface according to the positioning structures, by shifting minima of the potential energy towards the first surface.
In embodiments, depositing includes reducing a distance between the surfaces, so that the minima of the potential energy are shifted towards the first surface. The distance is preferably reduced to less than 200 nm, more preferably less than 100 nm.
The two surfaces provided are designed to have an asymmetrical electrical charge, so that each of the two surfaces exhibits a same electrical charge sign and the second surface has a higher electrical charge than the first surface.
Nano-objects provided have an aspect ratio, preferably higher than 2:1, more preferably higher than 5:1; the positioning structures provided include one or more grooves extending parallel to an average plane of the first surface or an average plane of the second surface; and enabling nano-objects position according to the potential energy further includes letting the nano-objects orient according to the potential energy.
According to embodiments, the first surface provided is the surface of a layer of a removable material provided on a substrate and preferably including a polymer such as polyphthalaldehyde.
The method further includes, prior to providing the two surfaces, creating the positioning structures in the layer of the removable material, preferably by a thermal scanning probe lithography technique.
In embodiments, the method further includes, after depositing the nano-objects, removing the removable material to transfer one or more nano-objects deposited on the first surface to the substrate.
In embodiments, removing the removable material includes evaporating the removable material, where the removable material is preferably a polymer, the polymer being evaporated at a temperature above the ceiling temperature of the polymer.
The method further includes, after removing the removable material, providing a new layer of material on top of the deposited nano-objects and repeating the steps of: providing the two surfaces and the ionic liquid suspension; enabling nano-objects position; and depositing, where the two surfaces now includes a surface of the new layer of material as a new first surface.
In embodiments, the method further includes depositing the removable material onto the substrate, prior to providing the surfaces, and depositing the removable material preferably includes spin casting a polyphthalaldehyde film onto the substrate.
In variants, depositing the removable material includes depositing the removable material onto both the substrate and one or more pre-existing structures such as electrodes or pads on the substrate.
The method further includes dragging the suspension of nano-objects, for example a water-based suspension, into and/or from a gap between the two surfaces, the gap being preferably less than 200 nm, and dragging is preferably carried out by way of capillary and/or electrophoretic forces.
According to embodiments, depositing the nano-objects includes reducing a distance between the surfaces, so that the minima of the potential energy are shifted towards the first surface, and reducing the distance between the surfaces includes moving the first surface relatively to the second surface, perpendicularly to an average plane of one of the two surfaces, and where the second surface preferably includes one or more of the positioning structures.
The second surface provided is tilted with respect to the first surface and depositing the nano-objects includes reducing a distance between the surfaces, so that the minima of the potential energy are shifted towards the first surface, where reducing the distance includes moving the first surface relatively to the second surface, parallel to an average plane of the first surface.
According to another aspect, the invention is embodied as an apparatus, adapted for implementing the method according to any one of the above embodiments, the apparatus including: two surfaces in a position facing each other: a first surface and a second surface, where at least one of the two surfaces has positioning structures with dimensions on the nanoscale; a ionic liquid suspension of nano-objects between the two surfaces, the suspension including two electrical double layers formed, each, at an interface with a respective one of the two surfaces, the electrical surface charges of the two surfaces being of a same sign; and positioning means coupled to the first surface and/or the second surface, the positioning means configured to move the first surface relatively to the second surface, in operation.
Methods and apparatuses embodying the present invention will now be described, by way of non-limiting examples, and in reference to the accompanying drawings.
The following description is of general embodiments of the present invention and high level variants. Referring generally to
First, first surface 15 and second surface 17 are placed in position facing each other. At least one of the surfaces, for example surface 15, exhibits positioning structures 16. In variants, second surface 17 or both surfaces can be provided with such structures. Positioning structures 16 have dimensions on the nanoscale, i.e., at least one characteristic dimension thereof (e.g., a diameter or principal length) is between 1 and 100 nm.
Second, ionic liquid suspension 30 of the nano-objects is confined between surfaces 15 and 17. The ionic liquid, for example be a water-based suspension, is dragged into the gap between surfaces 15 and 17. The gap is preferably less than 200 nm. Dragging the liquid can be carried out by way of capillary and/or electrophoretic forces. In variants, one can squeeze a droplet of liquid between the two surfaces, etc.
The surfaces and the liquid are designed, such that the suspension includes two electrical double layers (or EDLs, also called double layer). Each of the EDLs is formed at an interface with a respective surface. Two EDL systems arise because of the two surface-liquid interfaces involved. EDLs are known and have been the subject of many research papers in the past decades. An EDL appears at the surface of an object (solid object or particle, or even a liquid droplet) when placed in contact with a liquid. A “double layer” refers to two parallel layers of charges next to the object surface. The first layer refers to the surface charge (either positive or negative), that includes ions adsorbed directly onto the object due to a host of chemical interactions between the surface and the liquid. The second (diffuse) layer includes ions, which arise in reaction to the first layer. These ions electrically screen the first layer and are attracted to the surface charge via the coulomb force. Rather than being firmly anchored to the first layer, the second layer is diffuse (and is thus called the diffuse layer) and the free ions it includes move in the liquid under the influence of both the electric attractions and thermal motion. The second layer; therefore, refers to the liquid.
Thus, surfaces 15 and 17 each present a surface charge, i.e., the “first” layer of the respective EDL is charged. Each of the surfaces exhibits the same electrical charge sign. Preferably, the charge is asymmetric, i.e., second surface 17 has a higher electrical charge than first surface 15. As a result, the nano-objects in the suspension are stabilized by charge in suspension (or at least interact therewith, by way of entropic/electric effects) and thus, can also be “charged”. Therefore, they do not deposit on either of the two surfaces. The potential energy, as experienced by a nano-object in the suspension, which results from the charge of the surfaces, typically exceeds the thermal energy of this object and thus, prevents it from depositing. Note that an uncharged particle disturbs the cloud of ions responsible for the built-up of the potential. Therefore, a dielectric particle also experiences a force due to entropic reasons. Consequently, present methods also work for dielectric particles.
The potential energy, as experienced by the particles, results from the charged surfaces and the reaction of the liquid (containing ions). This potential essentially controls the nano-objects. The concentration of ions determines the range of the potential, that is, how far it reaches into the liquid. The charge of the nano-objects can be refined by adding charged surfactants to the ionic solution, which will self-assemble around the particle and provide the charge. The nano-objects, i.e., particles, can also be chemically modified by attaching charged molecules covalently on the particles surface, i.e. thiols on gold or silanes on SiOx surfaces. The charge of such molecules can be modified by controlling the pH of the water solution, as can be the charge of the surfaces, etc.
Nano-objects in the suspension will spontaneously position (and possible orient) according to the potential energy resulting from the electrical charge of the surfaces. This potential energy has a non-flat profile, whose shape is notably determined by the positioning structures. An estimated potential energy contour surface 31 is represented in
Finally, nano-objects can be deposited on first surface 15, according to the positioning structures, by shifting minima 32 of the potential energy towards first surface 15. Namely, a force field is applied which allows the nano-objects to overcome the electrostatic potential barriers imposed by first surface 15 (i.e., the lower charge surface). As a result, particles deposit on first surface 15, according to positioning structures 16. Particles adjust their position and orientation before and during deposition.
Referring to
A number of parameters will impact the potential experienced by the particles. The range of the potentials is determined by the ionic concentration in the solution. This range will also determine to which resolution the topographic features can determine the potential. If the range is large, small features in the topography will not be reflected in the potential. Therefore, if the range is short, the potential has higher resolution and will improve the precision of the placement process. The minimum range is given by the minimal achievable separation between the surfaces which ensures transfer of the particles. Therefore, the distance d is reduced to values as small as possible, e.g., below 200 nm. In some cases, this distance will need to be reduced to less than 100 nm, as exemplified later. At such separation distances, capillary and/or electrophoretic forces can be used to drag the liquid.
Preferably, present positioning methods are applied to nano-objects 20 having an aspect ratio. The positioning structures can be grooves 16 (or any elongated structures, or more generally structures reflecting the symmetry of the nano-objects), extending parallel to average plane 15a of surface 15. Thus, nano-objects having an aspect ratio will position and orient according to the potential energy, i.e., according to the grooves. As illustrated in
Referring to
Material 14 preferably includes polymer chains, which are able to unzip upon suitable stimulation (energetic or chemical modification event, protonation, etc.). There, film 14 can be stimulated via nano-probe 52 for triggering an unzipping reaction of polymer chains. The polymer material can include polymer chains, for which an energetic or chemical modification event triggers the unzipping reaction. Typically, stimulating a first chemical modification or degradation event triggers a partial or total unzipping effect. Thus, patterning steps need to include proper stimulation, typically by heating the layer of material 14 via probe 50, such that a suitable modification event occurs in a polymer chain of the polymer material. Probe 50, 52 should be designed, e.g., connected to an electrical circuit, to allow for heating of the probe during a controlled time and at a controlled temperature. As discussed above, the polymer material preferably includes poly-phthalaldehydes. An organocatalytic approach to the polymerization of phthalaldehyde is preferred, e.g., using dimeric 1-tert-butyl-2,2,4,4,4-pentakis(dimethylamino)-2Λ5,4Λ5-catenadi(phosphazene) (P2-t-Bu) phosphazene base as an anionic catalysts in presence of an alcoholic initiator. For example, a resulting polymer (including ˜200 monomer units equivalent to a molecular weight of 27 kDa) possesses a low ceiling temperature and facilitates the ability to create permanent patterns by selective thermolysis, using a heated probe. With such materials, deep patterns can be written with very little indentation force applied to the probe tip. This minimizes pattern distortion that results from indenting or displacing the material. Furthermore, polymeric chains can be made of an arbitrary length which offers substantial flexibility in tuning the material properties, such as the glass temperature and solvent resistance. An additional advantage is that no fine-tuning of intermolecular forces is required at variance with materials requiring stabilization from a secondary structure, such as hydrogen bonds.
In variants, material 14 can include a polymer material where molecules are cross-linked via intermolecular bonds. Such molecules can conveniently desorb when patterning the polymer material with heated nano-probe 50, 52. An average molecular mass of the molecules is preferably between 100 Da and 2000 Da, and more preferably in the range from 150 Da to 1000 Da, which offers enhanced desorbing properties. The film can be cross-linked via intermolecular bonds, such as van der Waals forces or Hydrogen bonds. When probe 52, suitably heated, is urged against the surface of film 14, and interacts with it, the interaction is likely to desorb one or more molecules. The probe temperature and the exposure time of the probe to the surface can be suitably adjusted to optimize desorption of molecules.
Material 14 can be deposited onto the substrate using known methods, e.g., by spin casting the material, e.g., a polyphthalaldehyde film, onto the substrate.
Referring to
Once material 14 has been removed, i.e., once the objects have been transferred to substrate 11, a new layer of material (not necessarily the same removable material) can be provided on top of already deposited nano-objects, and the above steps repeat, in order to build complex architectures of nano-objects. This is illustrated in
Referring to
So far, positioning structures have been essentially contemplated on receiving surface 15. However, variants are possible, as illustrated in
As further illustrated in
Surfaces are charged naturally in response to the contact with a liquid. Additional chemical means can be involved, e.g., dissociating groups on the surface. If necessary, these surface charges can even be supported by an external electric field. Thus, an electrical control means can optionally be provided. The additional electric field can support the asymmetry of the charged surfaces. Fields on the order of delta V/d are typically needed, i.e. on the order of ˜0.1 V/100 nm. Electrical control means can notably be used to help moving potential energy minima towards the receiving surface.
More generally, apparatus 100 can further include any feature in respect of the methods as contemplated in an embodiment of the present invention and described herein.
The above embodiments have been described in reference to the accompanying drawings. In preferred embodiments, several combinations of the above features can be contemplated. A detailed example is given below.
The specific embodiment of the present invention discussed in this section is especially suited for placement of high aspect ratio nano-objects. Capillary-based assembly does not work for such particles because the high densities at the three-phase contact line lead to the formation of close packed configurations, which hinder an aligned positioning. Therefore, it is preferred to use trapping forces as discussed in the previous sections to trap and pre-align the nano-objects in preferred directions, which are determined by the positioning structures. From these trapped states, the particles are then approached towards the target surface and finally brought into adhesive contact by approaching the confining surfaces.
The process flow of this placement strategy is depicted in
The steps outlined above can be repeated to deposit a second layer of nano-objects on top of the first layer with similar accuracy in position and orientation. In this way, an assembly of different types of particles can be achieved and the functionality of each particle type can be exploited.
As discussed in more details below, a mechanical setup can be constructed, which allows the cover slip to align parallel to the substrate underneath, and to approach with nanometer precision. The setup is preferably designed for high quality optical access and the trapping performance can be studied in-situ. This setup can then be used to study the complex interplay between surface topography, curvature, and charging with the confined nano-particle suspension. The confinement can be varied in-situ due to the movable cover slip and the confinement effects can be studied without varying other parameters.
In summary, embodiments disclosed herein use geometrical confinement in combination with top-down designed topographical features to manipulate the local electrostatic potential in low ionic-strength solutions. A local electrostatic minimum is created which traps and aligns the nano-objects. In a second step, the objects are forced into adhesive contact by approaching the confining surfaces. The position and orientation is further focused by the shape matching topographical features on the receiving substrate. The placement process relies only on the charge of the nanoparticles and the confining surfaces. Any type of charged object can be used, ranging from high aspect ratio nanowires over flexible polymers (like DNA), down to potentially even single proteins. The placement can be precisely registered to underlying functional structures. Several placement steps can be repeated with similar accuracy. In particular, placing high aspect ratio nanowires according to methods described herein leads to a wide range of scientific and economic high impact applications, some of which are discussed below.
The methods discussed above have the following unique features in comparison to conventional placement methods.
First, the placement process is separated into a trapping step and a transfer step. This has several consequences. Elongated or more complex shaped objects can first adapt their planar orientation according to the trapping potential before they are transferred to the substrate surface. The forces acting on the objects are well defined by the shape of the electrostatic potential and the transfer method. This allows for placing fragile pre-assembled objects in a defined state. The separated steps allow for spectroscopically assessing the properties of the captured particle. Depending on the observed properties, decisions can be made as to whether the particle should be positioned or disposed.
Second, use is made of a decomposable polymer as a receiving material and a scanning probe based method to design the guiding topography. The polymer allows for decoupling the placement process from the underlying substrate and the writing method enables registration to underlying features. Combining both aspects, multiple subsequent placement steps can be achieved with precise registry. These unique features can be exploited for a number of applications. Two examples of applications are discussed below.
A first application concerns the positioning of several semiconducting or metallic nanowires on top of two pre-structured pads 12, as illustrated in
In a second application, functional nanowires grown by vapor-liquid-solid growth can be positioned to exploit the functionality of the wires. Functionality can be integrated by controlling the dopant concentrations during growth or building hetero-structures to other materials along the nanowire direction or in the radial direction in the form of core-shell structures. The nanoscale dimension enables the combination of materials with much larger deviations in lattice constants than possible in planar geometry. This enables the production of field effect transistors, light emitting, or harvesting devices, etc., in single nanowires. For example,
In applications, wires of different internal functionality can be integrated into a working circuit which combines single functions to achieve greater functionality. As an example, one can integrate a field effect transistor nanowire to drive a light emitting diode nanowire. Thus, present positioning methods provide a new way to approach the fabrication of the so called ‘nanoprocessor’.
Cover slip 18 can be patterned by optical lithography including a center island of 200-500 μm diameters which is raised by 20-50 μm. The recess of the remaining area can be provided to avoid problems with dirt particles 60 preventing the two surfaces from achieving approach distances below 100 nm.
The setup can be characterized using interferometric distance measurements 120, which allows for testing the stability of the setup and the response to the pressures induced by filling with liquid and approaching the confining surfaces. This way, mechanical stability of <1 nm in vertical direction and approach distances below 50 nm can be contemplated. The position and motion of the particles will be detected optically. For gold nanoparticles, the plasmonic response can be exploited using dark field microscopy. For semiconducting, particles scattered light or fluorescence can be detected. The Brownian motion of the particles at these length scales requires exposure times of <1 ms. Optimally, the time resolution of the setup needs to be sufficient to track the motion of single particles. However, for determining the shape of the potentials from the particle positions, a statistical measurement of the positions is sufficient. Preferably, a microscope, including a high speed camera, can be used to enable high fidelity detection path.
In operation, positioning of the substrate is carried out using piezo-motor driven x-y coarse positioning system 102, fine positioning piezo stage 104, and three piezo positioners 106 to align the plane of the sample and cover slip 18. Cover slip 18 is mounted on the holder and can be manually moved in vertical direction 108. Cover slip 18 is etched outside the optical viewing window with recess 18a having a depth of 20-50 μm to accommodate dirt particles and imperfect flatness of the sample. Microscope 110 is used to determine the particle positions using fluorescence or light scattering detection. The orientation of cover slip 18 with respect to the substrate plane is measured using laser interferometer 120.
In variations, apparatuses (and methods) according to embodiments of the present invention can include any one, or several of the features recited in respect of the setup of
An in-situ characterization of the surface and particle potentials developed in the fluidic slit can be useful to understand the observed phenomena. For instance, electrodes can be implemented into the setup to generate lateral electric fields. The zeta potential of the particles can be obtained using a commercial Zetasizer (Malvern Instruments). If the particle potentials are known, the potential of the confining surfaces can be extracted from the particle speed in eletrophoretic/osmotic flow measurements in confined (unstructured) nanoslits. First, the potential of the glass surfaces can be determined using two confining glass surfaces. Using this knowledge, the potential of the confining polymer surface can be determined in a system using a polymer and a glass surface.
Notably, two types of stabilization strategies for the particle solutions can be used. For instance, one can use nanoparticles stabilized by organic surfactants. Nanoparticle solutions of this type are readily available commercially (Nanopartz, US), stabilized e.g. by Cetyl trimethylammonium bromide (CTAB). Also, the surfactants provide a simple way to control the charge density at the polymer surface, because the formation of a mono/multi-layer at the surfaces is expected. This has been corroborated by some experimental results on the stability of CTAB stabilized Au nanorods. Unspecific adsorption on the polymer surface was not observed. The drawback of using organic stabilizers is that they can influence the functional performance after assembly and can; therefore, need to be removed. They can, e.g., induce contact problems, if organic matter remains between the assembled particles and electrodes on the surface. However, in first experiments with gold nanoparticles, this was not observed.
One can also use purely electrostatically stabilized particle solutions in order to avoid organic molecules. It has been shown that the conductivity is enhanced in close packed assemblies of such particles. Methods are known which allow for exchanging the organic stabilizers by ions and works for a wide range of particles.
Both stabilization methods can also be used for stabilizing nanowires in solution. The measured values can be used to feed the simulations described below. They also give initial values to estimate the depth of the trapping potentials and guide the strategy for placing the nanoparticles.
The trapping potential of the system can warrant investigation. One can, for example, rely on the unique patterning capabilities offered by tSPL methods to define topographical structures with high precision in three dimensions. In variants, one can use nano-imprint lithography methods to create such structures with high throughput. The trapping potentials can be determined by measuring the position of the nanoparticles in real space and time. This can be done optically using a high numerical aperture (NA) objective and detecting scattered light from the particles.
Another possible concern is the observation of a curvature induced trapping potential and how it interplays with the topographically induced electrostatic minimum. In a feedback loop with modeling results, the topography which induces the trapping potential and the charge densities can be jointly optimized. This makes it possible to find optimal conditions which provide a stable trapping, e.g., of nano-objects with high aspect ratios.
Theoretical modeling efforts and computer simulations can be carried out using the commercial package COMSOL, in order to understand the effects discussed herein. This allows for understanding the trapping mechanisms including the curvature induced trapping potentials. In addition, the effect of external fields on the trapping potentials can be investigated. Some recipes of how to use COMSOL for related applications are available in the literature. The underlying idea is to solve the nonlinear Poisson-Boltzmann in three dimensions using charge neutrality and constant charge boundary conditions at the interfaces.
In establishing technical implementation details of the transfer methods, the goal is to optimize the conditions in the fluidic slit in a way that trapped particles can be transferred into adhesive contact with the substrate by external manipulation. How to achieve this can benefit from (but does not depend on) the results obtained in the theoretical modeling and computer simulation work evoked in above. The forces acting between particles and a (planar) surface are given by the well known Derjaguin, Landau, Verwey, and Overbeek (DLVO) theory. The theory predicts that at very small separations the attractive van-der-Waals forces dominate the electrostatic repulsive force and a nano-object can therefore be pulled into contact. However, to approach such distances, the repulsive electrostatic interactions need be overcome. A successful implementation of such a transfer process was demonstrated in the past. For instance, a successful transfer of 80 nm gold nanoparticles was achieved using laser powers ranging from 350 μW to 10 mW, corresponding to (calculated) forces of up to 15 pN. A preferred way of achieving the transfer is to use purely electrostatic forces. This ensures that the trapping and placement steps are only dependent on the charge of the particles and no other physical property. As discussed earlier, an idea is to use asymmetric charge densities on receiving PPA surface 15 and cover slip surface 17. In that case, the potential minimum can be shifted toward the side with the lower potential value.
The electrostatic potential can be calculated analytically assuming constant surface potentials and a planar geometry. The resulting potential ψ between a first surface positioned at d=0 having a surface potential of ⅓ kBT/e (using standard notations) and a second surface at dS=2, 3, 5, and 10 κ−1 (κ−1 being the Debye length) having a surface potential of 1 kBT/e is plotted in,
For large distances, the potential is sufficiently strong to trap certain types of particles. As the distance between the surfaces decreases, the potential barrier diminishes, as seen in
As touched earlier, one can repeat the placement process onto nano-objects assembled in a previous placement step. A question is whether adhesive contact with the first layer is sufficiently stable to allow subsequent coating with PPA. An alternative method to coat the first layer of objects is to float a PPA film from a template surface. Depending on this step, subsequent steps can be carried out identically. If sufficient yield is achieved in the placement process, the stacking can be repeated several times.
A first application consists of positioning a metallic nanowire in a first step across two electrodes or pads, as depicted in
Next, one could want to design a parallel placement and printing scheme to achieve high throughput placement of nano-objects; be it at the price of the placement accuracy. In an implementation, topographical features can be etched into the cover slip using existing dry etch methods. The following sequence can be achieved:
trapping, transfer into adhesive contact,
moving to a new printing position, and
refilling of the gap by electophoretic forces.
This allows for patterning large areas with repeated assemblies of particles. Alternatively, the topography inducing the trapping potentials can be fabricated into the cover-slip (see
In the embodiment of
Concerning
The guiding potentials can be similarly shaped and the transfer to the substrate can be achieved by similar means. Both approaches have in common that the topographic shapes used for trapping can be reused multiple times. In the first case (
As another example, one can pattern a functional circuit from stacked functional nanowires placed in a cross-type fashion and aligned to pre-patterned electrodes on the surface, as in
While the present invention has been described with reference to certain embodiments, it will be understood by those skilled in the art that various changes can be made and equivalents can be substituted without departing from the scope of the present invention. In addition, many modifications can be made to adapt a particular situation or material to the teachings of the present invention without departing from its scope. Therefore, it is intended that the present invention not be limited to the particular embodiments disclosed, but that the present invention will include all embodiments falling within the scope of the appended claims. In that respect, not all the components/steps depicted in the accompanying drawings need be involved, depending on the chosen embodiments. In addition, many other variants not explicitly discussed above can be contemplated. For example, other materials can be used, as well as other separation distances.
| Number | Date | Country | Kind |
|---|---|---|---|
| 1207463.9 | Apr 2012 | GB | national |
