WO2004074173A1

WO2004074173A1 - Method of forming quantum layer and patterned structure by multiple dip-coating process

Info

Publication number: WO2004074173A1
Application number: PCT/KR2004/000350
Authority: WO
Inventors: Ki-Bum Kim; Tae-Sik Yoon
Original assignee: Seoul National University Industry Foundation
Priority date: 2003-02-20
Filing date: 2004-02-20
Publication date: 2004-09-02

Abstract

The present invention relates to a method of adsorbing quantum dots in a colloid solution on a substrate using a multiple dip-coating process to increase the surface area coverage of quantum dots on the substrate to a level of single layer. Further, this invention relates to a method of forming a uniform pattern of quantum dot clusters on a substrate.

Description

METHOD OFFORMINGQUANTUMLAYERAND PATTERNED STRUCTUREBYMULTIPLEDIP-COATINGPROCESS

BACKGROUND OFTHEINVENTION

1. Field of the Invention

The present invention relates to a nanotechnology in which quantum dots each of which has a size ranged from a few nanometers to a few ten nanometers are arranged on a substrate. Further, this invention is directed to a nanotechnology in which quantum dots each of which has a size ranged from a few nanometers to a few ten nanometers are arranged on a substrate on which a pattern is formed.

2. Description of the Related Art Quantum dot is a material having a size ranged from a few nanometers to a few ten nanometers, and has optical (A. P. Alivistos. Science, 217, 933(1996)), magnetic (T. Yogi et al., IEEE Trans. Magn. 26, 2271 (1990), and J. F. Smyth, Science, 258, 414 (1992)), and electrical (K. K. Likharev, Proceedings of the IEEE., 87 (4), 606 (1999)) properties, which are different from those of material in the form of bulk. These physical properties of quantum dot are varied with the material forming quantum dot and the size of quantum dot.

The quantum dots may be used in the form of powder or colloid. It may be also used to manufacture a highly integrated device or a device having a special function by arranging these quantum dots on a substrate. Many tries for employing a quantum dot layer in flash memory device, optical device, magnetic storage device and the like have been made. As the devices which have been recently researched, there are an optical device (A. P. Alivistos. Science, 217, 933(1996)) having a light emitting wavelength which is controllable depending on the size of quantum dot, and having an outstanding quantum efficiency; a next generation high density magnetic recording media (T. Yogi et al., IEEE Trans. Magn. 26, 2271 (1990), and J. F. Smyth, Science, 258, 414 (1992)) having a storage capacitance of 1 TB/in² or more; and a single electron transistor and single electron memory device (K. K. Likharev, Proceedings of the IEEE., 87 (4), 606 (1999)), using the coulomb blockade effect of charges stored in the quantum dot(M. H. Devoret, and H. Grabert, Single Charge Tunneling, Plenum Press). There are various methods of forming quantum dots. Among such methods, a colloid method, which forms quantum dots using a chemical method, is recently focused (C. B. Murray et al., Annu. Rev. Mater. Sci., 30, 545(2000)). A colloid solution is a solution where particles each of which has a size ranged from a few nanometers to a few ten nanometers are not agglomerated but are uniformly dispersed in a slovent.

The colloid method for forming quantum dots is as follows. Quantum dots each of which has a size ranged from a few nanometers to a few ten nanometers are formed. To prevent the respective quantum dots from being agglomerated by van der Waals force, surfactant is chemically capped on the surface of quantum dot. The resulting product is precipitated in a solution to obtain the quantum dots in the form of powder (which is called "colloidization"). These power-typed quantum dots are dissolved in a solvent to prepare a quantum dot colloid solution. At present, it has become possible to produce quantum dot colloid solutions having a constant size of a few nanometers (deviation of about 4%) by a chemical method from various materials of semiconductor, metal, metal oxide or the like, such as CdE(E=S, Se, Te)(C. B. Murray, D. J. Norris, and M. G. Bawendi, J. Am. Chem. Soα, 115, 8706 (1993)), Au(R. P. Andres et al., Science, 273, 1690(1996)), FePt(S. Sun, C. B. Murray, D. Weller, L. Folks and A. Moser, Science, 287, 1989(2000)), Co, CoO(S. Sun, and C. B. Murray, J. Appl. Phys., 85(8), 4325 (1999)).

The quantum dots with a uniform size, formed by a chemical method, are added in a colloid solution, and are adsorbed on a substrate by a dip coating process. The resulting substrate is taken out from the solution, and the solvent is then evaporated to obtain a substrate on which quantum dots are adsorbed. The adsorbed quantum dots are spontaneously uniformly-arranged on the substrate (self-assembly).

^• In other words, as the solvent of the colloid solution is evaporated, the quantum dots in the colloid solution form a quantum dot cluster having a spontaneously uniform array at room temperature. The quantum dot clusters form a close-packed single layer structure over a region of a few hundred nanometers or a superlattice structure in which quantum dot having a size of a few nanometers acts as crystal lattice(C. B. Murray, D. J. Norris, and M. G. Bawendi, J. Am. Chem. Soc, 115, 8706 (1993)/ R. P. Andres et al, Science, 273, 1690(1996)/ S. Sun, C. B. Murray, D. Weller, L. Folks and A. Moser, Science, 287, 1989(2000)/ S. Sun, and C. B. Murray, J. Appl. Phys., 85(8), 4325 (1999)/ B. A. Korgel and D. Fitzmaurice, Phys. Rev. Lett. 80, 3531(1998)).

By arranging quantum dots on a large-sized wafer using the properties of quantum dots, it becomes possible to apply quantum dots to various quantum dot application devices.

Until now, the array of quantum dots is usually confirmed by dropping a colloid solution on a substrate using a spoit. This spoit process is however difficult to form a colloidal thin film with a uniform thickness on a large-sized substrate. Thus, it fails to form the uniform array of quantum dots on a substrate. As a process for arranging quantum dots having a high productivity suitable for the device application, researchers have worked on a spin-coating process (Y.-K. Hong, H. Kim, G. Lee, W. Kim, J.-I. Park, J. Cheon, and J.-Y. Koo, Appl. Phys. Lett. 80, 844 (2002)), and a Langmuir-Blodgett thin film-forming process (S. Huang, G. Tsutsui, H. Sakaue, S. Shingubara, and T. Takahagi, J. Vac. Sci. Technol. B 19, 2045 (2001)). However, the spin-coating process and the Langmuir-Blodgett thin film-forming process have a drawback in that they are not easy to optimize the process conditions. To improve the integrity of devices on the substrate, the present invention increases the surface area coverage of quantum dots adsorbed on a substrate. The present invention provides a process for improving the surface area coverage of quantum dots through a multi-adsorption process where a dip-coating process is repeatedly performed, and the use of said improving process for forming a pattern of quantum dot clusters.

SUMMARY OF THE INVENTION

The present invention relates to a multi-adsorption process for forming quantum dots on a substrate, with the uniform array of quantum dots, by solving said problems.

It is an object of this invention to form a uniform array of quantum dots each of which has a size ranged from a few nanometers to a few ten nanometers on a substrate by repeatedly performing the adsorption of a dip-coating process.

It is another object of the present invention to form quantum dots uniformly in the form of single layer or multi-layer on a substrate. It is a further object of the present invention to form a pattern of quantum dot clusters as uniformly arranged, using the present invention's process and to apply such pattern to electric or magnetic devices.

To apply the electrical, optical and magnetic properties of a material sized in nanometer to a device, it is essential to develop a method of forming quantum dots uniformly arranged on a substrate. According to the present invention, the substrate includes all semiconductor substrates, and various metal and inorganic/organic substrates. Further, it also includes a surface-treated substrate to be able to react with surfactant that is sterically stabilizing quantum dots.

Accordingly, the present invention relates to a multi-adsorption process in which the adsorption of a dip-coating process is repeatedly carried out for maximizing the surface area coverage of quantum dots, with a uniform array of the quantum dots on substrate. Especially, in the present invention, as a bath containing a quantum dot colloid solution ascends, a substrate is dipped in the bath, and then the bath descends at a constant angle and velocity. Thus, the quantum dots in the colloid solution are adsorbed on a substrate. The substrate is then taken out from the solution, and the solvent is evaporated to obtain a uniform array of quantum dots on the substrate. However, since the surface area coverage of quantum dots is below a constant value due to a random sequential adsorption phenomenon ( < 56 % )(G. Y. Onoda and E. G. Linger, Phys. F.ev. A 33, 715 (1986); J. Feder and I. Giaever, J. Colloid Interface Sci. 78, 144 (1980)), this dip-coating process is repeatedly carried out to remarkably increase the surface area coverage of quantum dots on the substrate.

BRIEF DESCRIPTION OF THE DRAWING

The above and other objects, features and other advantages of the present invention will be more clearly understood from the following detailed description taken in conjunction with the accompanying drawings, in which:

FIG. 1 is a schematic view of the quantum dots sterically stabilized by surfactant, used in the present invention;

FIG. 2a is a photograph illustrating a superlattice structure of the quantum dots according to the present invention, and FIG. 2b is a photograph illustrating a close-packed single layer structure of the quantum dots according to the present invention;

FIG. 3 is a graph illustrating the interaction energy between two quantum dots having a Hamaker constant of 300 kT, a radius of 5 nm and a surfactant length of 1.5 nm, according to the present invention;

FIG. 4 is a schematic view illustrating the adsorption of quantum dots on a substrate according to the present invention;

FIG. 5 is a schematic view of the multi-adsorption method according to the present invention;

FIGs. 6a and 6b are graphs showing the surface area coverage of quantum dots as theoretically calculated, in the multi-adsorption method according to the present invention (in FIG. 6a, =0.2 and in FIG. 6b, α=0.5);

FIG. 7 is photographs illustrating the surface area coverage of quantum dots according to the number of dip-coatings on a silicon(Si) substrate in a colloid solution where γ-Fe₂0₃ quantum dots are dispersed in octane solvent;

FIG. 8 is photographs illustrating the surface area coverage of quantum dots according to the number of dip-coatings on a silicon dioxide (Si0₂) substrate in a colloid solution where γ-Fe₂0₃ quantum dots are dispersed in octane solvent;

FIG. 9 is a graph illustrating surface area the coverage of quantum dots according to the number of dip-coatings on a silicon substrate and a SiQ₂ substrate in a colloid solution where γ-Fe₂0₃ quantum dots are dispersed in octane solvent;

FIG. 10 is photographs illustrating the surface area coverage of quantum dots according to the number of dip-coatings on a silicon substrate in a colloid solution where CdSe quantum dots are dispersed in octane solvent;

FIG. 11 is photographs illustrating the surface area coverage of quantum dots according to the number of dip-coatings on a PEDOT (poly 3,4-ethylenedioxythiopene) substrate in a colloid solution where CdSe quantum dots are dispersed in octane solvent;

FIG. 12 is a graph illustrating the surface area coverage of quantum dots according to the number of dip-coatings on a silicon substrate and a PEDOT (poly 3,4-ethylenedioxythiopene) substrate in a colloid solution where CdSe quantum dots are dispersed in octane solvent;

FIG. 13 is photographs illustrating the surface area coverage of quantum dots according to the number of dip-coatings on a silicon substrate in a colloid solution where FePt quantum dots are dispersed in octane solvent;

FIG. 14 is a graph illustrating the surface area coverage of quantum dots according to the number of dip-coatings on a silicon substrate in a colloid solution where FePt quantum dots are dispersed in octane solvent;

FIG. 15 is a graph illustrating the decomposition of γ-Fe₂0₃ quantum dots adsorbed on a Si substrate depending on the time during which the substrate is again dipped in a colloid solution;

FIGs. 16a to 16c are schematic views illustrating a method of forming a pattern of quantum dot clusters. Specifically, FIG. 16a illustrates that a pattern is formed on a substrate using a photoresist material, FIG. 16b illustrates that the patterned substrate is multiple dip-coaled using a colloid solution to form quantum dot clusters, and FIG. 16c illustrates that a pattern of quantum dot clusters is formed by removing the photoresist material from the substrate; and

FIG. 17 is a photograph showing a pattern of CdSe quantum dot clusters formed on the Si substrate according to the same method as in FIG. 16.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention relates to a method for arranging quantum dots on a substrate by an adsorption phenomenon of a dip-coating process where the substrate is dipped in a colloid solution containing dispersed quantum dots, and is then taken out from the solution after the quantum dots are adsorbed on the substrate.

In other words, to overcome a drawback that the surface area coverage of quantum dots does not reach a level of uniform single layer when the quantum dots dispersed in a colloid solution are adsorbed by once performing a dip-coating process, a multi-adsorption method repeatedly performing a dip-coating process is used to enhance the surface area coverage of quantum dots up to a level of single layer. By using the multi-adsorption method, it is possible to form a multi-layer structure as well as a single layer structure of quantum dots.

According to a method of forming a multi-layer structure of quantum dots, the dip-coating process is repeated to form a single layer of quantum dots on a substrate; the single layer of quantum dots is surface-treated or other thin film, such as semiconductor, metal, ceramic or organic thin film, is deposited on the single layer of quantum dots and then the dip-coating process is again repeated to form a single layer of quantum dots thereon. By repeating this method of forming a single layer of quantum dots, it is possible to form a multi-layer of quantum dots.

In FIG. 1, the quantum dots sterically stabilized by surfactant perform Brownian movement when they are in a colloid solution. If a substrate is dipped in the colloid solution, the quantum dots are adsorbed on the substrate by van der Waals attraction force. When the substrate is taken out from the solution, the quantum dots are condensed on the substrate while the solvent is evaporated. At this time, if the deviation in the sizes of the quantum dots is small within a few %, the quantum dots form a regular array such as quantum dot hep (hexagonal close-packed) single layer or superlattice structure as shown in FIG. 2. This is because attraction energy acts between quantum dots and thus the room temperature thermal energy provides an annealing energy for the formation of a uniform array.

In FIG. 1, energy u(r) of spherical quantum dots sterically stabilized by surfactant and having the same size is determined as a sum of the van der Waals force E_vdw and the steric repulsion E_steric between the surfactants as shown in the following equation 1.

Equation 1

<^r) = ^EvdW + ^E steric

The van der Waals force E_vdw and the steric repulsion E_steric in equation 1 are respectively expressed by the following equations 2 and 3. Equation 2

Equation 3

Wherein R is a radius of quantum dot; C is a distance between the centers of quantum dots; δ is a length of surfactant; σ is a surface density of the surfactant on the surface of quantum dot; and A is a Hamaker constant indicating the inherent constant of substance. For instance, when the surface of quantum dot having a Hamaker constant of 300 kT and a radius of 5 nm is capped with a surfactant having a length of 1.5 nm at an interval of 0.43 nm, energy acting between the quantum dots is drawn as shown in the graph of FIG. 3. In FIG. 3, coupling energy u(r) acting between the quantum dots having the same size is 6 kT, which is similar to the room temperature thermal energy at an equilibrium position (C ~ 12.5 nm). Accordingly, if quantum dots are arranged on the substrate at room temperature, they are condensed to their thermodynamic equilibrium positions so that quantum dot close-packed single layer is formed.

In the meanwhile, as shown in FIG. 4, the adsorption of quantum dot on the substrate is determined by the van der Waals attraction energy between the quantum dot and the substrate. The van der Waals energy E_SP between the quantum dot having a radius of R and the surface can be expressed by the following equation 4:

Equation

In the above equation 4, A is a Hamaker constant between the substrate and the quantum dot. Accordingly, assuming that in an Au quantum dot, an energy between the substrate and the quantum dot is a minimum value when the Hamaker constant is the same and a distance 'C between the center of the quantum dot and the substrate is R+δ, the energy 'E' between the substrate and the quantum dot is approximated to 49 kT by the equation 4. Accordingly, the quantum dots are adsorbed on the substrate during a dip-coating process by the bonding force between the quantum dot and the substrate, and as the solvent is evaporated, a cluster of quantum dots is formed due to the bonding force between the quantum dots.

The dip-coating process allows quantum dots to be adsorbed and the evaporation of solvent enables to form a cluster of quantum dots having a long range ordering. However, the surface area coverage of quantum dots, obtainable by a single adsorption method, shows values less than 56% due to random sequential adsorption phenomenon. As a method to overcome the aforementioned disadvantages, another embodiment of the present invention provides to repeatedly perform a dip-coating process so that a single layer of quantum dots where the surface area coverage of quantum dots is maintained at a very high level, is formed by the multi-adsorption phenomenon.

FIG. 5 is a view schematically illustrating a flow of the multi-adsorption method. In the first dip-coating process, the quantum dots in the solution are adsorbed on the substrate, and as the solvent is evaporated from the solution, a cluster of quantum dots is formed. In the second dip-coating process, quantum dots are again adsorbed on vacant sites of the substrate, and as the solvent is evaporated from the solution, a cluster of quantum dots is again formed. By repeating the aforementioned processes, the surface area coverage of quantum dots adsorbed on the substrate increases.

The multi-adsorption phenomenon of the dip-coating process can be calculated theoretically as follows. Prior to the calculation, the following items are assumed: (1) quantum dots in solution are adsorbed on the substrate with a specific value of the surface area coverage; (2) quantum dots are adsorbed in a single particle state when they are adsorbed on the substrate (G. Ge and L. Brus, J. Phys. Chem. B 104, 9573 (2000)); and (3) the maximum surface area coverage during the adsorption cycle of quantum dots to the substrate appears below a constant value (< 56%) due to random sequential adsorption.

On the basis of these assumptions, the surface area coverage (Ox) of the quantum dots formed after the adsorption can be expressed by the following equation 5:

Equation 5

Wherein α means a surface area coverage by a single adsorption; according to Langmuir adsorption behavior, θ₀ means a maximum jamming limit in a single adsorption, determined by random sequential adsorption; K_a is an adsorption constant; K_d is a desorption constant; and C is a concentration of quantum dots in a colloid solution.

In the first dip-coating process, a cluster of quantum dots is formed on the substrate. In the second dip-coating process, the cluster of the quantum dots is partly decomposed, so that the surface area coverage of the quantum dots formed in the first dip-coating process can be expressed by the following equation 6:

Equation 6 θ[ = Θ₁ - AΘ = (1-K^ήθ, = βθ₁

Wherein θ^ is a surface area coverage of the quantum dot cluster as formed in the first dip-coating process, left after the decomposition in the second dip-coating process; &_rfc is a decomposition rate coefficient of quantum dot cluster; t is a dipping time; and β is a stability coefficient that is a barometer indicating how stable a cluster of quantum dots is. In the second dip-coating process, the cluster of quantum dots as formed in the first dip-coating is partly decomposed and at the same time quantum dots are again adsorbed on the surface of the substrate to form a cluster. The surface area coverage (θ₂ ^new)of the quantum dots as newly adsorbed on the substrate in the second dip-coating process can be expressed by the following equation 7. Thus, the surface area coverage (θ₂) of the quantum dots as finally formed on the substrate after the second dip-coating process can be expressed by the following equation 8:

Equation 7

Equation 8

The above equation can be generalized with respect to the number (n) of dip-coatings and be expressed by the following equation 9:

Equation 9

The above equation 9 expresses the surface area coverage of the quantum dots as finally left on the substrate depending on the number of dip-coatings. This theoretical calculation of the multi-adsorption method means that the final surface area coverage of quantum dots is determined by the surface area coverage (α) due to a single adsorption, the stability coefficient (β) of a quantum dot cluster, and the number (n) of dip-coatings.

FIGs. 6a and 6b are graphs showing the surface area coverage of quantum dots with varying α, β and n. As can be seen from the results of FIGs. 6a and 6b, as the number (n) of dip-coatings increases, the surface area coverage of quantum dots increases too. Where the quantum dot cluster is not decomposed in the solution, i.e. if β = 1, the surface area coverage of quantum dots becomes 100% theoretically.

FIG. 7 is photographs illustrating the surface area coverage of quantum dots according to the number of dip-coatings on a silicon substrate in a colloid solution where γ-Fe₂0₃ quantum dots are dispersed in octane solvent. As can be seen from the photographs of FIG. 7, as the number of dip-coatings increases, the surface area coverage of quantum dots increases. In particular, in case of five times or more dip coatings, the surface area coverage increases remarkably. The quantum dot cluster shows a shape in which several clusters grow individually and are then coalesced. Herein, the concentration of quantum dots in octane solvent, the dipping rale, the dipping time, etc., can be varied.

FIG. 8 is photographs illustrating the surface area coverage of quantum dots according to the number of dip-coatings on a silicon dioxide (Si0₂) substrate in a colloid solution where γ-Fe₂0₃ quantum dots are dispersed in octane solvent. As the number of dip-coatings increases, the surface area coverage of quantum dots increases.

The quantum dots used in FIGs. 7 and 8 are γ-Fe₂0₃ quantum dots capped with oleic acid and having a size of 8.6 nm, and they were multi-adsorbed on Si substrate and Si0₂ substrate in a colloid solution where the quantum dots were dispersed in octane solvent. At this time, the concentration of the quantum dots in the octane solvent was 2.4xl0¹³/cc, the dipping rate was 0.1 mm/sec, and the dipping time was 100 seconds. The solvent was evaporated at room temperature after the substrate on which the quantum dots had been adsorbed were taken out from the solution.

FIG. 9 is a graph illustrating the surface area coverage of quantum dots according to the number of dip-coatings on a silicon substrate and a Si0₂ substrate in a colloid solution where γ-Fe₂0₃ quantum dots are dispersed in octane solvent. As the number of dip-coatings increases, the surface area coverage of quantum dots increases. In particular, in case of five times or more dip-coatings, the surface area coverage increases remarkably. In case of 15 times dip-coatings on a silicon substrate, the surface area coverage increases up to 76%. Theoretically, if the quantum dot cluster is not decomposed in the solution, the surface area coverage of quantum dots can be increased to 100% using a multi-adsorption method, by increasing the long range ordering of the cluster.

FIG. 10 is photographs illustrating the surface area coverage of quantum dots according to the number of dip-coatings on a silicon substrate in a colloid solution where CdSe quantum dots are dispersed in octane solvent. As the number of dip-coatings increases, the surface area coverage of the quantum dots increases.

FIG. 11 is photographs illustrating the surface area coverage of quantum dots according to the number of dip-coatings on a PEDOT (poly

3,4-ethylenedioxythiopene) substrate in a colloid solution where CdSe quantum dots are dispersed in octane solvent. Likewise the above cases, as the number of dip-coatings increases, the surface area coverage of quantum dots increases.

FIG. 12 is a graph illustrating the surface area coverage of quantum dots according to the number of dip-coatings on a Si substrate and a PEDOT (poly 3,4-ethylenedioxythiopene) substrate in a colloid solution where CdSe quantum dots are dispersed in octane solvent. This process was carried out under the conditions that the concentration of the quantum dots in the octane solvent was 100 mg of quantum dots/8ml of solvent, the dipping rate was 0.1 mm/sec, and the dipping time was 100 seconds. The solvent was evaporated at room temperature after the substrate on which the quantum dots had been adsorbed were taken out from the solution.

FIG. 13 is photographs illustrating the surface area coverage of quantum dots according to the number of dip-coatings on a Si substrate in a colloid solution where FePt quantum dots are dispersed in octane solvent. Likewise the above cases, as the number of dip-coatings increases, the surface area coverage of quantum dots increases. In case of five times or more dip-coatings, the surface area coverage is saturated.

FIG. 14 is a graph illustrating the surface area coverage of quantum dots according to the number of dip-coatings on a Si substrate in a colloid solution where FePt quantum dots are dispersed in octane solvent. This process was carried out under the conditions that the concentration of the quantum dots in the octane solvent was 2 mg of quantum dots/lml of solvent, the dipping rate was 0.1 mm/sec, and the dipping time was 100 seconds. The solvent was evaporated at room temperature after the substrate on which the quantum dots had been adsorbed were taken out from the solution.

FIG. 15 is a graph illustrating the decomposition of γ-Fe₂0₃ quantum dots adsorbed on a Si substrate depending on the time during which the Si substrate having the γ-Fe₂0₃ quantum dots adsorbed thereon is dipped in a colloid solution. As can be seen from the graph of FIG. 15, the γ-Fe₂0₃ quantum dots adsorbed on the Si substrate are hardly decomposed even in the colloid solution. Also, it can be known that the surface area coverage of the quantum dots adsorbed on the substrate is maintained without change. This explains that the repetition of the dip-coating process increases the surface area coverage of quantum dots on the substrate.

FIGs. 16a to 16c are schematic views illustrating a method of forming a pattern of quantum dot clusters. Specifically, FIG. 16a illustrates that a pattern is formed on a substrate using a photoresist material; FIG. 16b illustrates that quantum dot clusters are formed on the patterned substrate by multiple dip-coating the substrate using a colloid solution; and FIG. 16c illustrates that a pattern of quantum dot clusters is formed on the substrate by removing the photoresist material from the substrate. The pattern structure of the uniform quantum dot clusters, as shown in FIG. 16c, can be applied to various electrical and magnetic devices.

In this pattern-forming method, a pattern is first formed on the substrate using a photoresist material and light, ion beam or electron beam, the pattern including a substrate-exposed region and a photoresist-coated region. Afterwards, the patterned substrate is dipped in a colloid solution containing quantum dots to adsorb the quantum dots on the substrate. The substrate is then taken out from the colloid solution, and the solvent is evaporated from the substrate to form the quantum dots on the substrate. The substrate having the quantum dots thereon is again dipped in the colloid solution containing quantum dots to adsorb the quantum dots on the substrate. Afterwards, the substrate is again taken out from the colloid solution, and the solvent is evaporated to form the quantum dots on the patterned substrate and to thereby increase the surface area coverage of the quantum dots. After that, the photoresist-coated region is removed from the substrate to form a uniform pattern of quantum dot clusters with a large surface area coverage.

Also, even if not shown in the drawings, a pattern of quantum dot clusters can be formed by the following method: (1) a substrate is dipped in a colloid solution containing quantum dots to adsorb the quantum dots on the substrate; (2) the substrate is taken out from the colloid solution; (3) the solvent is evaporated to form the quantum dots on the substrate; (4) the substrate having the quantum dots thereon is again dipped in the colloid solution containing quantum dots to adsorb the quantum dots on the substrate; (5) the substrate is again taken out from the colloid solution; (6) the solvent is evaporated to form a quantum dot layer on the substrate and to thereby increase the surface area coverage of the quantum dots; (7) a mask selectively shielding the quantum dot layer is formed on the quantum dot layer; and (8) plasma is irradiated onto the shielded region and the quantum dot layer-exposed region to etch the quantum dot layer so that a uniform pattern of quantum dot clusters with a large surface area coverage can be formed.

FIG. 17 is a photograph showing a pattern structure of CdSe quantum dot clusters according to the same method as in FIG. 16. From the photograph of FIG. 17, it can be known that a uniform pattern of the CdSe quantum dot clusters is formed.

The colloid solutions used in the above embodiments are colloid solutions where quantum dots are dispersed in polar solvent, but it will be apparent to those skilled in the art that a colloid solution where quantum dots are dispersed in non-polar solvent can be also used.

Also, it will be apparent to those skilled in the art that gold substrate, gold film-deposited substrate or gold film-coated substrate can be used as a substrate. Although not shown in the drawings, a multi-layer of quantum dots can be formed by the following method: (1) a single layer of quantum dots is first formed by repeatedly performing the dip-coating process, as stated above; (2) the single layer of quantum dots is surface-treated or other film such as semiconductor, metal, ceramic or organic thin film is deposited on the single layer of quantum dots; (3) the dip-coating process is repeatedly performed to again form a single layer of quantum dots thereon, so that a multi-layer of quantum dots can be formed.

By using a phenomenon that quantum dots are adsorbed on a substrate in a dip-coating process, it is possible to form a uniform pattern of quantum dot clusters on the substrate. By repeatedly performing the dip-coating process, it is possible to remarkably increase the surface area coverage of quantum dots. In other words, this enables us to form a single layer of quantum dots or a multi-layer of quantum dots.

As stated above, the adsorption and multi-adsorption methods of quantum dots using a dip-coating process enable us to form a uniform layer of quantum dots with a very high surface area coverage, on various substrates, and they are the processes with a very high payability. Thus, the processes are advantageous in device applications.

When the present invention's methods are used for forming a pattern of quantum dot clusters, it is possible to form a uniform pattern of quantum dot clusters with a very high surface area coverage. Also, the pattern formed by the methods can be applied to various optical, magnetic and electrical devices.

Claims

WHAT IS CLAIMED IS:

1. A method of arranging quantum dots on a substrate, comprising the steps of: dipping the substrate in a colloid solution containing quantum dots to adsorb the quantum dots on the substrate; taking out the substrate from the colloid solution; evaporating solvent from the substrate to form the quantum dots on the substrate; again dipping the substrate having the quantum dots thereon in a colloid solution containing quantum dots to adsorb the quantum dots on the substrate; again taking out the substrate from the colloid solution; and evaporating solvent from the substrate to form the quantum dots on the substrate and to thereby increase the surface area coverage of the quantum dots.

2. The method of claim 1, wherein the quantum dots are in a colloidal state and each of the quantum dots has a size ranged from a few nanometers to a few ten nanometers.

3. The method of claim 1, wherein the substrate is of one selected from the group consisting of metal, ceramic, semiconductor and organic material.

4. The method of claim 1, wherein the quantum dots contained in the colloid solution are dispersed in polar solvent or non-polar solvent.

5. A method of arranging quantum dots on a substrate, comprising the steps of: dipping the substrate in a colloid solution containing quantum dots to adsorb the quantum dots on the substrate; taking out the substrate from the colloid solution; and evaporating solvent from the substrate to form the quantum dots on the substrate, said steps being repeatedly performed five times or more, thereby increasing the surface area coverage of the quantum dots.

6. The method of claim 5, wherein the quantum dots are in a colloidal state and each of the quantum dots has a size ranged from a few nanometers to a few ten nanometers.

7. The method of claim 5, wherein the substrate is of one selected from the group consisting of metal, ceramic, semiconductor and organic material.

8. The method of claim 5, wherein the quantum dots contained in the colloid solution are dispersed in polar solvent or non-polar solvent.

9. A method of forming a pattern of quantum dot clusters on a substrate, comprising the steps of: forming a pattern on the substrate using a photoresist material and light, ion beam or electron beam, the pattern including a substrate-exposed region and a photoresist-coated region; dipping the substrate having the pattern thereon in a colloid solution containing quantum dots to adsorb the quantum dots on the substrate; taking out the substrate from the colloid solution; evaporating solvent from the substrate to form the quantum dots on the substrate; again dipping the substrate having the quantum dots thereon in a colloid solution containing quantum dots to adsorb the quantum dots on the substrate; again taking out the substrate from the colloid solution; evaporating solvent from the substrate to form the quantum dots on the substrate and to thereby increase the surface area coverage of the quantum dots; and removing the photoresist-coated region from the substrate.

10. The method of claim 9, wherein the quantum dots are in a colloidal state and each of the quantum dots has a size ranged from a few nanometers to a few ten nanometers.

11. The method of claim 9, wherein the substrate is of one selected from the group consisting of metal, ceramic, semiconductor and organic material.

12. A method of forming a pattern of quantum dot clusters on a substrate, comprising the steps of: dipping the substrate in a colloid solution containing quantum dots to adsorb the quantum dots on the substrate; taking out the substrate from the colloid solution; evaporating solvent from the substrate to form the quantum dots on the substrate; again dipping the substrate having the quantum dots thereon in a colloid solution containing quantum dots to adsorb the quantum dots on the substrate; again taking out the substrate from the colloid solution; evaporating solvent from the substrate to form a quantum dot layer on the substrate and to thereby increase the surface area coverage of the quantum dots; forming a mask selectively shielding the quantum dot layer on the substrate; and irradiating plasma onto the shielded region and the quantum dot layer-exposed region of the substrate to etch the quantum dot layer of the quantum dots-exposing region.

13. The method of claim 12, wherein the quantum dots are in a colloidal state and each of the quantum dots has a size ranged from a few nanometers to a few ten nanometers.

14. The method of claim 12, wherein the substrate is of one selected from the group consisting of metal, ceramic, semiconductor and organic material.

15. A method of forming a multi-layer of quantum dots on a substrate, comprising the steps of: dipping the substrate in a colloid solution containing quantum dots to adsorb the quantum dots on the substrate; taking out the substrate from the colloid solution; evaporating solvent from the substrate to form the quantum dots on the substrate; again dipping the substrate having the quantum dots thereon in a colloid solution containing quantum dots to adsorb the quantum dots on the substrate; again taking out the substrate from the colloid solution; evaporating solvent from the substrate to form a single layer of quantum dots and to thereby increase the surface area coverage of the quantum dots; performing a surface treatment of the substrate on which the single layer of quantum dots is formed, or depositing a thin film of one selected from the group consisting of semiconductor, metal, ceramic and organic material, on the single layer of quantum dots; dipping the substrate having the thin film thereon in a colloid solution containing quantum dots to adsorb the quantum dots on the substrate; taking out the substrate from the colloid solution; evaporating solvent from the substrate to form the quantum dots on the substrate; again dipping the substrate having the quantum dots thereon in a colloid solution containing quantum dots to adsorb the quantum dots on the substrate; again taking out the substrate from the colloid solution; and evaporating solvent from the substrate to form the quantum dots on the substrate and to thereby increase the surface area coverage of the quantum dots.