RU2342316C2 - Highly productive method of depositing carbon nanotubes and composite films - Google Patents

Abstract

FIELD: nanotechnology.

SUBSTANCE: present invention pertains to means of forming a structure based on carbon nanotubes and can be used when making flexible optically transparent conducting coatings, flexible small-scale integration integrated circuits, test structures for a scanning probe microscope, resistive sensors etc. A drop of colloid solution of carbon nanotubes, containing a surface active substance in quantity, sufficient for stabilisation of the colloidal state of the solution, as well as 5-60 vol.% glycerine, sufficient for formation of a drop colloidal solution of a give size, are deposited on a given point on a substrate. For this purpose, the colloidal solution is put into the device, which transfers it to the printing head of a jet-printer. The device is in form of a catridge or a system for continuous supply of the colloidal solution. The substrate is moved using a system, in form of a paper-moving mechanism of a jet printer, or in form of a compact disc movement system. Removal of all components of the colloidal solution except carbon nanotubes, is achieved by evaporation and/or washing in a solvent. To obtain a composite film, carbon nanotubes are deposited on a substrate, in turns, with liquid components of a polymer or simultaneously with the liquid components of the polymer. The liquid components of the polymer are solidified while fusing their microdroplets, or under the effect of electromagnetic radiation, heating or as a result of evaporation of volatile components under atmospheric conditions. Viscosity of liquid components of the polymer corresponds to the microhydraulic system of the jet-printer. The liquid components of the polymer are deposited on the substrate by transferring them to the printing head of the jet printer. The method is characterised by high output and resolving power of depositing carbon nanotubes and composite films on a substrate and reduced cost.

EFFECT: highly productive and cheap method.

2 cl, 2 ex

Description

The invention relates to means for forming structures based on carbon nanotubes.

One of the most widely used methods for the formation of structures based on carbon nanotubes (CNTs) is the deposition of nanotubes on the surface from a colloidal solution (colloid). The reproducibility and functionality of the structures obtained by this method can vary significantly depending on the following factors:

- the geometry and functionality of structures previously formed on the surface and / or formed after applying CNTs to the surface;

- the use of the effects of electrophoresis and dielectrophoresis when applying CNTs;

- the stability of the colloidal solution, determined by the coagulation time of CNTs;

- the size of the applied colloid droplets and the accuracy of their positioning on the surface.

To date, the following methods are used to deposit a colloid on a surface. Historically, the first and still the most widely used is the manual method, which involves the preparation of a colloidal solution containing CNTs, the capture of a certain amount of colloid (usually with a medical syringe or pipette dispenser) and the subsequent application of a drop to the desired location of the substrate [1]. This method is considered as a prototype of the invention. With sufficient operator qualifications and the use of appropriate optical systems, this method allows the application of colloid droplets from 100-200 microns in size with a positioning accuracy of about 200 microns. The relatively large droplet size leads to a proportional decrease in the resolution of the method and an increase in the amount of contaminants transferred from the solution. In addition to the relatively large droplet size and low positioning accuracy, the disadvantages of the method include low productivity and reproducibility.

An alternative to the manual method is the use of specialized installations, such as the Automated CNT microinjection system proposed by Chinese researchers [2], or the American Jetlab [3]. These installations, due to their specialization, provide high resolution and reproducibility, however, they have a high cost and are not universal, since they are not focused on high-performance surface treatment of large areas.

For the formation of CNT composite films, centrifugation [4] and lamination methods are mainly used, the disadvantages of which include:

- a limitation on the thickness of the applied films, which leads in particular to limitations on optical transparency and flexibility;

- restriction on the length of the processed substrates in the case of centrifugation.

The purpose of the invention is to increase the productivity, resolution and versatility of the method of applying CNTs and CNT films to a surface, reducing its cost, reducing the size of colloid droplets and the amount of contaminants transferred from the solution.

This is achieved by the fact that the microhydraulic system of a serial inkjet printer is used to form colloid droplets and apply them to the surface. For this, a colloidal solution of carbon nanotubes is introduced into the printer cartridge instead of ink. This colloidal solution must satisfy the following basic conditions:

- the absence of coagulation of nanotubes in solution for a sufficiently long time;

- compliance of the viscosity of the colloidal solution with the parameters of the microhydraulic system of the printer.

To fulfill the first condition, a surfactant is introduced into the colloidal solution in an amount that ensures stabilization of the colloidal state of the solution. The required viscosity is achieved by introducing into the composition of the colloidal solution from 5 to 60% volume concentration of glycerol.

In order to increase the surface area that can be processed in one operation cycle and to simplify the operation of refilling the colloidal solution, the printer can be equipped with a standard continuous ink supply system or an additionally developed working fluid supply system.

Depending on the substrates being processed, it is possible to use either a standard paper-broaching mechanism of the printer (in the case of substrates with sufficient flexibility), or a standard system for supplying CDs for printing (in the case of hard substrates having planar dimensions up to 120 mm), or additionally developed substrate feeding system. In the case of creating a specialized substrate supply system, the allowable geometric dimensions of the substrates can be expanded, and the positioning accuracy of the substrates can be increased, which is important in tasks involving the combination of structures formed at different stages of the process.

In all these cases, standard or additionally developed printer software and graphic editors can be used to specify the geometric location of the colloid droplets applied to the surface and control the application process.

In all the cases described above, the term “additionally developed” means something that does not apply to standard systems of a serial inkjet printer or to standard systems for providing it, but is designed specifically for the purpose of implementing the method of depositing carbon nanotubes and composite films based on carbon nanotubes.

To form composite films based on carbon nanotubes, it is necessary to introduce the corresponding liquid polymer components into part of the printer cartridges. The viscosity of the liquid components of the polymer should also correspond to the parameter of the microhydraulic system of the printer. The composite is formed by alternately applying layers of carbon nanotubes and a polymer. The curing of the polymer can occur either when the microdroplets of its liquid components merge, or under the action of the corresponding electromagnetic radiation, heating, or as a result of evaporation of the volatile component in the atmosphere. In the resulting composite film, it is possible to achieve the concentration of CNT components in a layer of minimum thickness (monolayer), which will contribute in particular to an increase in the optical transparency of the film.

The size of the droplets formed by the microhydraulic system of the printer, within certain limits, can be controlled by the viscosity of the working fluid. For the piezoelectric type printheads that are currently most widely used in inkjet printers, droplet formation does not occur at a viscosity less than a certain critical value. An increase in the viscosity leads first to an increase in the size of the droplets formed, and then again to a decrease. With a sufficiently high viscosity of the working fluid, the formation process stops again. The minimum size of the formed colloid droplets is about 5-10 microns in diameter, which corresponds to a volume of about 10 ^-12 l.

The method provides: application speed - up to 120 thousand drops per second; the accuracy of the positioning of the droplets relative to each other is up to 10 μm in the direction of rapid movement of the print head of the printer (scanning direction), and about 20-30 μm in the perpendicular direction. The total surface area processed in one cycle of operation is limited only by the capacity of the tanks of the continuous fluid supply system and for standard supply systems is about several tens of square meters.

Concrete example

A colloidal solution of carbon nanotubes of a given concentration is prepared, which has sufficient stability and the required viscosity. To do this, 0.1% of the mass fraction of the “CTAB” surface-active substance (surfactant) is dissolved in distilled water, then the required amount of nanotube material is added and ultrasonic treatment is carried out for many hours in order to achieve the colloidal state of the system. After adding 20% of the volume fraction of glycerol to the colloid, mechanical stirring and repeated ultrasonic treatment are performed.

The colloidal solution obtained in this way is introduced into one or several containers of the continuous ink supply system, which in turn ensures its supply to the print heads of the Epson Stylus Photo R220 inkjet printer. This printer model provides printing on A4 paper or on the surface of CDs. A silicon wafer with a diameter of up to 120 mm is placed in the CD tray. If the plate has a smaller diameter and / or several separate parts of the cut plate are used, then a standard CD can be used as a carrier base. The thickness of the silicon wafers used in microelectronics is of the order of 300 μm, and the gap between the nozzles of the print head of the printer and the compact disc in the tray allows such wafers to be placed on the surface of the disc.

A matrix of planar structures is preliminarily formed on the surface of the silicon wafer using standard photolithography methods, each of which includes a gate electrode located below the silicon oxide layer, conductive paths and busbar electrodes located above the oxide, connecting all conductive paths of the same polarity with the corresponding common contact pads, brought to the edge of the silicon plates. In the process of processing the plate with a colloidal solution, a sinusoidal alternating voltage of 1 MHz and an amplitude of 10 V is applied to the bus electrodes, due to which, when a microdroplet of a colloid gets on the conductive tracks of one of the elements, nanotubes suspended in a drop, due to the effect of dielectrophoresis, electrically connect these conductive tracks . The intensity of the process of electrical shorting of the conductive tracks can be monitored in situ by the drop in circuit resistance. By varying the concentration of nanotubes in the colloid, one can influence the probabilities of obtaining structures with single nanotubes and with networks of nanotubes between conducting paths. After processing the silicon substrate in the manner described above, it is washed in acetone and annealed at 350-500 ° C in order to remove surfactant molecules, glycerol and other contaminants. Then the plate is scribed onto individual crystals. The elements thus obtained can serve, in particular, as mock-ups of field effect transistors based on carbon nanotubes (in the event that single semiconductor single-layer nanotubes are planted) or mock-ups of resistive gas sensors.

The versatility of the method proposed in the invention consists in combining relatively high resolution with high performance. This makes it possible to perform a wide range of tasks - from simple coating of flexible polymer sheets of large area with grids of nanotubes to the formation of integrated circuits of low degree of integration. Thus, the invention is potentially in demand in the following areas: creation of flexible optically transparent conductive coatings and flexible integrated circuits of low degree of integration (solar cells, touch panels, flexible displays, electronic “paper”, “smart” coatings and packaging materials, etc.), test structures for scanning probe microscopy, resistive sensors of physical and chemical fields based on nanotubes, structures for studying the electronic properties of CNTs, etc.

An example of a method of applying composite films to a substrate corresponds to the above example and is characterized in that in addition to a colloidal solution of carbon nanotubes, epoxy-resin ED-20 and its PEPA hardener are introduced into separate containers of the continuous ink supply system, which ensures their separate supply to the print heads of an Epson inkjet printer Stylus Photo R220. " Since the printer software associates a color for each nozzle of the print head, the corresponding color image is used by the printer software to control the percentage of epoxy resin and hardener applied to the substrate. Thus, a ratio of hardener to epoxy resin of 0.1-0.06 should be ensured. The supply of epoxy resin and hardener to the substrate is carried out alternately with the deposition of layers of carbon nanotubes.

Information sources

1. Hertel T., Martel R., Avouris Ph. Manipulation of Individual Carbon Nanotubes and Their Interaction with Surfaces. // Journal of Physical Chemistry B 102. p. 910. 1998 - prototype.

2. Qu, S.C .; Fung, C.K.M .; Chan, R.H.M .; Li, W.J. Development of an automated microinjection system for fabrication of carbon nanotube sensors. // Intelligent Control and Automation. WCICA 2004. Fifth World Congress. Volume 6. Issue. 15-19 June 2004. Page (s): 5613-5618 (www2.acae.cuhk.edu.hk/~cmns/papers/icima-2004-kmfung.pdf).

3. Berend-Jan de Gans, Paul C. Duineveld, Ulrich S. Schubert. Inkjet printing of polymers: state of the art and future developments. // Advanced Materials. Vol 16. No.3, pp.203-213, 2004.

4. Patent EP 0379400. Apparatus for manufacturing composite materials by centrifugation, 1990.

Claims (2)

1. The method of applying carbon nanotubes to a substrate, including preparing a colloidal solution of carbon nanotubes, applying a drop of a colloidal solution to a given point on the surface of the substrate, removing by evaporation and / or washing in a solvent all components of the colloidal solution except carbon nanotubes, characterized in that the composition of the colloidal surfactant is introduced into the solution in an amount that ensures stabilization of the colloidal state of the solution, as well as 5-60 vol.% glycerol, providing the formation of droplets of a colloidal solution of a given size, control the process of applying a drop of the prepared colloidal solution to a given point on the surface of the substrate by introducing a colloidal solution into a device that delivers it to the printhead of an inkjet printer and is made, in particular, in the form of cartridges or as a continuous colloidal supply system solution, and moving the substrate by means of a substrate supply system, made in particular in the form of a paper-pulling mechanism of an inkjet print Era, or as a CD feed system. 2. A method of applying carbon nanotubes and composite films to a substrate, including preparing a colloidal solution of carbon nanotubes, applying a droplet of a colloidal solution to a given point on the surface of the substrate, removing by evaporation and / or washing in a solvent all components of the colloidal solution, except carbon nanotubes, and applying to polymer substrate, characterized in that a surfactant is introduced into the composition of the colloidal solution in an amount that ensures stabilization of the colloidal state solution, as well as 5-60 vol.% glycerol, which provides the formation of droplets of a colloidal solution of a given size, control the process of applying a drop of a prepared colloidal solution to a given point on the surface of the substrate by introducing a colloidal solution into a device that delivers it to the print head of an inkjet printer and is made, in particular, in the form of cartridges or in the form of a system of continuous supply of colloidal solution, and the movement of the substrate by means of a substrate supply system, made in particular and, in the form of a paper-drawn mechanism of an inkjet printer or in the form of a compact disc feeding system, carbon nanotubes are applied to the substrate alternately with the liquid components of the polymer or simultaneously with them and solidify the liquid components of the polymer when their microdrops merge with each other or under the influence of electromagnetic radiation, heating or as a result of evaporation of a volatile component in atmospheric conditions, while the viscosity of the liquid polymer components corresponds to the micro-hydraulic system of an inkjet printer, and their application to the substrate is carried out by feeding them to the printhead of an inkjet printer.