US20080105560A1

US20080105560A1 - Method for Preparing Nano Metallic Particles

Info

Publication number: US20080105560A1
Application number: US11/674,507
Authority: US
Inventors: Yu Tsan Tseng; Cheng Hsuan Lin; Po Ling Shiao
Original assignee: Industrial Technology Research Institute ITRI
Current assignee: Industrial Technology Research Institute ITRI
Priority date: 2006-11-08
Filing date: 2007-02-13
Publication date: 2008-05-08
Also published as: TW200821404A; TWI339221B

Abstract

A method for preparing nano metallic particles comprises the steps of dipping a conductive substrate in an electroplating solution containing metallic ions and performing an electroplating process to form the nano metallic particles on the conductive substrate by the reduction reaction of the metallic ions. The nano metallic particles can be used as a catalyst to perform a chemical vapor deposition process to form carbon nanotubes on the conductive substrate. Subsequently, fluorescent material can be positioned on the carbon nanotubes to form a light-emitting device. When a predetermined voltage is applied between the conductive substrate and the fluorescent material, the carbon nanotubes on the conductive substrate emit electrons due to the point discharge effect, and the electrons bombard the fluorescent material to emit light beams.

Description

BACKGROUND OF THE INVENTION

(A) Field of the Invention
The present invention relates to a method for preparing nano metallic particles, and more particularly, to a method for preparing nano metallic particles by reducing metallic ions with an electroplating process.
(B) Descriptions of the Related Art
Recently researchers have developed many nano metallic particle array technologies based on different principles, for example, electron beam writing method, anode alumina template method, micro-contact printing method and block macromolecular template method.
Although the electron beam writing method (see Adv. Mater. 2003, 15, 49, Vol. 16, p. 3246, 2001) may randomly and precisely implant nano metallic particles, the writing process is quite time consuming and is not suitable for mass production processes requiring efficiency and large area. In additions, it is necessary for the electron beam writing method to use a complicated lithography etching process, and the manufacturing cost of mass production and large area is quite expensive.
In the anode alumina template method (see Appl. Phys. Lett., Vol. 75, p. 367, 1999), a prefabricated mold is used to press a small cylindrical hole array on an aluminum substrate with high purity, then the aluminum substrate with patterned surface is dipped into a chemical electroplating solution as an anode, so as to perform a single crystal deposition of alumina. Because the surface of the aluminum substrate has round holes, the expitaxy speed of the alumina is different, and a cylindrical hole array is formed. However, the anode alumina template method is suitable only for pure aluminum substrates, and the growing of the alumina must be performed in high-temperature chemical solutions.
In the micro-contact printing method (see Appl. Phys Lett., 76, 2071, 2000), a LIGA is used to fabricate a mold (used as a stamp), a solution containing metal catalyst is used as the ink, and the metal catalyst solution is printed on the surface of the substrate with a principle of stamping. However, the micro-contact printing method is limited by the scale of the conventional LIGA process, and it is impossible to use the metal catalyst as a nano-level array (it can only be used as a micron-level array). Moreover, local metal aggregation tends to occur in the micro-contact printing method.
In the block macromolecular template method (see Japan patent publications JP2003342012-A and US patent publications US 20030185985-A1), the pattern is formed on the substrate with self-assembly of the block macromolecules, a component of the block macromolecule is selected to be etched with UV or RIE, and the self-assembled pattern is transferred to the next material. However, in order to increase the aspect ratio of the pattern, several layers of different materials are required as the transferring layers, and a plurality of transferring processes is performed, so as to improve the aspect ratio of the hole structure to the applicable scope. After the hole with aspect ratio is finished, the metal catalyst is deposited in the hole with high aspect ratio by using depositions technique, and finally the transferring layer on the substrate is cleaned, thus forming the nano metallic particles in the nano hole on the substrate. The block macromolecular template method is similar to the semiconductor lithography etching process, and the multi-layer structure and differential etching rates are used, so the transferring process is too complex and the productions cost is quite high, and therefore it has no applicable industry value.

SUMMARY OF THE INVENTION

One aspect of the present invention provides a method for preparing nano metallic particles by using an electroplating process to reduce metallic ions to nano metallic particles.
A method for preparing nano metallic particles according to this aspect comprises the steps of dipping a conductive substrate in an electroplating solution containing metallic ions and performing an electroplating process to form the nano metallic particles on the conductive substrate by the reduction reaction of the metallic ions.
The conventional nano metallic particle array technologies all have disadvantages concerning complicated fabrication processes and high fabrication and time costs. The present invention provides a direct method for preparing nano metallic particles with lower costs and larger process windows to control the distribution and size of the nano particles, requiring only a surface treating process (e.g., bombarding the surface of the conductive substrate with plasma) to be performed on the conductive substrate without the complicated fabrication process.
Further, the surface roughness of the conductive substrate after the surface treating process is of nano scale. According to the present invention, 15 the potential range applied during the electroplating process is designed to be close to the standard reduction reaction potential of the metallic ions, thereby controlling the nucleation sites. When nucleation sites are generated, the cyclic number of the electroplating process is adjusted to control the growing size of the nano metallic particles, and therefore nano metallic particles with controllable size can be randomly formed on the conductive substrate. In addition, lithographic technology can be used to prefabricate conductive and non-conductive regions on the conductive substrate, and diversified nano metallic particle array layouts can be fabricated in the present invention.

BRIEF DESCRIPTIONS OF THE DRAWINGS

The objectives and advantages of the present inventions will become apparent upon reading the following descriptions and upon reference to the accompanying drawings in which:

FIGS. 1( a) and 1(b) illustrate methods for preparing nano metallic particles according to the present invention;

FIGS. 2( a) to 3(c) show the effect of surface roughness on the nucleation and the growing mechanism;

FIGS. 4( a) and 4(b) illustrate the effect of the electroplating mode on the nucleation and the growing mechanism;

FIGS. 5( a) to 6(b) show the effect of the surface structure of the conductive substrate on the nucleation and the growing mechanism;

FIG. 7 illustrates a surface image of the conductive substrate after the surface treating process according to the present invention;

FIGS. 8( a) to 8(c) are scanning electron microscope images of the nano metallic particles prepared by the cyclic voltammery electroplating method according to the present invention;

FIGS. 9( a) to 9(c) are scanning electron microscope images of the nano metallic particles prepared by the cyclic voltammery electroplating method according to the present invention;

FIGS. 10( a) to 10(c) are scanning electron microscope images of the carbon nanotubes prepared according to the present invention;

FIGS. 11 (a) and 11(b) illustrate a diode light-emitting device prepared according to the present invention; and

FIG. 12 illustrates a triode light-emitting device prepared according to the present invention.

DETAILED DESCRIPTIONS OF THE INVENTION

FIGS. 1( a) and 1(b) illustrate methods for preparing nano metallic particles 16 according to the present invention. A conductive substrate 12 is dipped in an electroplating solution 20 containing metallic ions 22, and an electroplating process (e.g., cyclic voltammery electroplating process) is then performed to form nano metallic particles 16 on the conductive substrate 12 by the reduction reaction of the metallic ions 22. Preferably, the size of the nano metallic particles 16 is between 1 nm and 150 nm. Preferably, the conductive substrate 12 includes indium-tin-oxide (ITO) with the lattice size between 5 nm and 500 nm. The electroplating solution 20 may include nickel nitrate, nickel sulfate or nickel chloride, and the nano s metallic particles 16 may be nickel metallic particle. In addition, the electroplating solution 20 may contain magnetic metallic ions such as iron ions or cobalt ion, and the nano metallic particles 16 may be magnetic metallic particles such as iron particles or cobalt particles.
Referring to FIG. 1( b), a plurality of conductive regions 14A and non-conductive regions 14B are formed on the conductive substrate 12 by lithography technology, and the nano metallic particles 16 selectively grow on the conductive region 14A. The surface roughness of the conductive region 14A of the conductive substrate 12 is preferably of nano scale (e.g., between 5 nm and 10 μm). If the surface roughness of the conductive substrate 12 is too small, a surface treating process (e.g., polishing process or plasma bombarding process) can be performed on the surface of the conductive substrate 12 before the cyclic voltammery electroplating process is performed, such that the surface roughness of the conductive substrate 12 is of nano scale.
FIGS. 2( a) to 3(c) show the effect of the surface roughness on the nucleation and the growing mechanism. Because the surface roughness of the conductive substrate 12 is of nano scale, the reduction reaction of the metallic ions 22 may selectively grow on a specific surface during the electroplating process, for example, at the ITO grain edge of the conductive substrate 12. According to the present invention, the applied potential is set to be close to the standard reduction reaction potential of the metallic ions 22 to control the generation of the nucleation sites. After the nucleation sites are generated, the circulation number is set to control the growth of the crystals so as to obtain the nano metallic particles 16 with uniform size, as shown in FIGS. 2( a) and 3(a). Consequently, the nano metallic particles 16 with controllable distribution and size are randomly formed on the conductive substrate 12.
In contrast, if the electroplating reaction is performed on a uniform metallic surface, e.g., a flat copper surface prepared by sputtering, because the surface roughness is quite small, during the electroplating reaction, the reduction reaction of the metallic ions 22 is performed on the flat copper surface substantially without “position selectivity” during the electroplating process since the surface roughness is very small, and even forms stacking layers of atoms. In this manner, it is impossible to prepare nano metallic particles 16 with nano distribution, as shown in FIGS. 2( b), 3(b), and 3(c).
FIGS. 4( a) and 4(b) illustrate the effect of the electroplating mode on the nucleation and the growing mechanism. As shown in FIG. 4( a), the present invention uses the cyclic voltammery electroplating process to prepare the nano metallic particles 16 such that the nano metallic particles 16 selectively grow on the grain edge of the conductive substrate 12. On the contrary, if the conductive substrate 12 with the same surface roughness distribution is used and different electroplating modes are adopted (for example direct current is used to perform the electroplating reaction), the nucleation sites tend to be non-uniformly distributed, thus resulting in the metal local aggregation. Since the activation of the electroplating reaction requires the electromotive force or the potential of the electroplating system to achieve the reduction potential of the metal to be electroplated. However, the electroplating solution 20 includes species with different concentrations and the “mass transfer” of the species also affects the electroplating reaction, so the direct current of the electroplating system cannot effectively control the electroplating reaction (including electroplating amount and position). Consequently, the nucleation sites tend to be non-uniformly distributed, which results in the metallic local aggregation, as shown in FIG. 4( b).
FIGS. 5( a) to 6(b) shows the effect of the surface structure of the conductive substrate 12 on the nucleation and the growing mechanism. FIG. 5( a) shows a surface structure prepared by sputtering silver (Ag) on a wafer, and the surface structure is quite flat. Therefore, during the nickel electroplating reaction, the reduction reaction of the metallic ions 22 is performed on the flat Ag surface substantially without “position selectivity”, as shown in FIG. 5( b). Particularly, nickel particles are even formed on the flat Ag surface by stacking layers of atoms, so the nickel particles cannot be prepared in a nano distributed manner, as shown in FIGS. 3( b) and 3(c). FIG. 6( a) shows a surface structure obtained by sputtering gold (Au) on a wafer. Similarly, it is clear that the uniformity on reduction growth of the metallic ions 22 is quite high, and the metal layer is substantially formed by stacking layers of atoms, so it is impossible to form nano metallic particles distributed by distances of tens to hundreds of nanometers, as shown in FIG. 6( b).
FIG. 7 illustrates a surface image of the conductive substrate 12 after the surface treating process according to the present invention. It is clear that the conductive substrate 12 made of ITO of has a nano-scale surface roughness.
FIGS. 8( a) to 8(c) are scanning electron microscope images of the nano metallic particles 16 prepared by the cyclic voltammery electroplating method according to the present invention, and the magnifications are 150, 10,000 and 50,000, respectively. According to the present invention, the cyclic voltammery electroplating method is performed on the conductive substrate 12 (having conductive regions 14A/non-conductive regions 14B) after the surface treating process, the reduction reaction of nickel metallic ions 22 is performed with 200 times of circulation in the potential interval from −0.6 V to −1.0 V so as to obtain nickel nano metallic particles 16 distributed with distances between 100 and 200 nm and with diameter of approximately 60 nm.
FIGS. 9( a) to 9(c) are scanning electron microscope images of the nano metallic particles 18 prepared by the cyclic voltammery electroplating method according to the present invention, with magnifications of 2,000, 10,000, and 50,000, respectively. According to the present invention, the cyclic voltammery electroplating method is performed on the surface of the conductive substrate 12 (having conductive regions 14A/non-conductive regions 14B) after the surface treating process, the reduction reaction of nickel metallic ions 22 is performed with 500 times of circulation in the potential interval from −0.6 V to −0.75 V so as to obtain distribution of the nickel nano metallic particles 16 with distances between 500 and 1000 nm and with the diameter of approximately 120 nm. It is known from the embodiments shown in FIGS. 8( a) to 9(c) that the present invention incorporates the conductive substrate 12 after the surface treating process and the electroplating mode control to obtain the nano metallic particles 16 with the control window of distance distribution and diameter from tens to hundreds of nanometers.
FIGS. 10( a) to 10(c) are scanning electron microscope images of carbon nanotubes prepared according to the present invention. According to the present invention, the conductive substrate 12 shown in FIG. 7 is used to perform the cyclic voltammery electroplating method to form the nano metallic particles 16 on the conductive substrate 12. Subsequently, the nano metallic particles 16 are used as the catalyst to perform a plasma enhanced chemical vapor deposition (PECVD) process so as to prepare the carbon nanotubes with an average pipe diameter of approximately 30 nm, which are arranged in straight manner. Particularly, the reacting gas of the PECVD process may include acetylene and ammonia, and the reacting pressure is approximately 1-10 torr.
FIGS. 11( a) and 11(b) illustrate a diode light-emitting device 30 prepared according to the present invention, which adopts the diode design and may be used as a backing light or a display. According to the present invention, a plurality of spacers 24 are formed on the carbon nanotubes 18, and a fluorescent substrate 26 (including a transparent conductive substrate and a fluorescent material) is then formed on the spacer 24 to complete the light-emitting device 30. When a predetermined voltage (e.g. 350 V) is applied to the conductive substrate 12 and the fluorescent substrate 26, the carbon nanotubes 18 on the conductive substrate 12 emit electrons due to the point discharge effect, and the electrons bombard the fluorescent material of the fluorescent substrate 26 to emit light beams, as shown in FIG. 11( b).
FIG. 12 illustrates a triode light-emitting device 40 prepared according to the present invention, which adopts the triode design. According to the present invention, a dielectric block 32 and a conductive block 34 are formed between the conductive substrate 12 and the spacer 24 so as to form three conductive terminals (i.e. the conductive region 34, the fluorescent substrate 26, and the conductive substrate 12), as shown in FIG. 12.
The conventional nano metallic particle array technologies all have disadvantages concerning complex fabrication processes and high fabricating time costs. The present invention provides a direct method for preparing nano metallic particles 16 with low cost and large control window of distribution and size, wherein a complex fabricating process is not required and only a surface processing (for example bombarding the surface of the conductive substrate with plasma) is performed on the conductive substrate 12. Furthermore, the surface roughness of the conductive substrate 12 after the surface treating process is of nano scale.
According to the present invention, the potential applied during the electroplating process is designed to be close to the standard reduction reaction potential of the metallic ions 22, thereby controlling the generation of the nucleation sites. When the nucleation sites are generated, the circulation number of the electroplating process is adjusted to control the growing rate of the nano metallic particles 16; therefore, nano metallic particles 16 with controllable size can be randomly implanted on the conductive substrate 12. In addition, if lithography technology is used to prefabricate conductive regions 14A/non-conductive regions 14B on the conductive substrate 12, diversified nano metallic particle array layouts can be fabricated according to the present invention.
In addition, in order to avoid the shielding effect of the field emission, it is necessary to separate the carbon nanotubes by a predetermined distance, and the ratio of the tube length and the distance suggested by the reference is approximately 1:1 or 1:2. Generally, the distance between the carbon nanotubes prepared by the polymer self-assembly technique cannot exceed 100 nm, and the application scope is limited. In contrast, the surface roughness (e.g., nucleation sites) can be designed to adjust the implanting distance between the carbon nanotubes to be larger than 100 nm. The layout of the conductive regions 14A/non-conductive regions 14B can also be designed to adjust the implanting distance between the carbon nanotubes in micro-scale, and thus enhance the emission uniformity of carbon nanotubes. As a result, the emission of carbon nanotubes is not limited by the shielding effect of the field emission according to the present invention.
The above-described embodiments of the present inventions are intended to be illustrative only. Numerous alternative embodiments may be devised by those skilled in the art without departing from the scope of the following claims.

Claims

1. A method for preparing nano metallic particles, comprising the steps of:

dipping a conductive substrate in an electroplating solution containing metallic ions; and

performing an electroplating process to reduce the nano metallic particles on the conductive substrate.

2. The method for preparing nano metallic particles as claimed in claim 1, wherein the surface roughness of the conductive substrate is of nano scale.

3. The method for preparing nano metallic particles as claimed in claim 2, wherein the surface roughness is between 5 nm and 10 μm.

4. The method for preparing nano metallic particles as claimed in claim 1, wherein the conductive substrate includes conductive regions and non-conductive regions.

5. The method for preparing nano metallic particles as claimed in claim 1, wherein the conductive substrate comprises indium tin oxide.

6. The method for preparing nano metallic particles as claimed in claim 1, wherein the metallic ions are magnetic metallic ions.

7. The method for preparing nano metallic particles as claimed in claim 1, wherein the metallic ions are iron ions, cobalt ions or nickel ions.

8. The method for preparing nano metallic particles as claimed in claim 1, further comprising a step of performing a surface roughening process on the surface of the conductive substrate such that the surface roughness of the conductive substrate is of nano scale.

9. The method for preparing nano metallic particles as claimed in claim 8, wherein the surface roughening process is a polishing process or a plasma bombarding process.

10. The method for preparing nano metallic particles as claimed in claim 1, wherein the electroplating process is a cyclic voltammery electroplating process.

11. The method for preparing nano metallic particles as claimed in claim 1, wherein the size of the nano metallic particles is between 1 nm and 150 nm.

12. The method for preparing nano metallic particles as claimed in claim 1, further comprising a step of performing a chemical vapor deposition process by using the nano metallic particles as a catalyst to form carbon nanotubes on the conductive substrate.

13. The method for preparing nano metallic particles as claimed in claim 12, wherein the surface roughness of the conductive substrate is of nano scale.

14. The method for preparing nano metallic particles as claimed in claim 12, wherein the surface roughness of the conductive substrate is between 5 nm and 10 μm.

15. The method for preparing nano metallic particles as claimed in claim 12, wherein the conductive substrate includes conductive regions and non-conductive regions.

16. The method for preparing nano metallic particles as claimed in claim 12, further comprising a polishing process or a plasma bombarding process on the surface of the conductive substrate such that the surface roughness of the conductive substrate is of nano scale.

17. The method for preparing nano metallic particles as claimed in claim 12, wherein the electroplating process is a cyclic voltammery electroplating process.

18. The method for preparing nano metallic particles as claimed in claim 1, further comprising the step of:

performing a chemical vapor deposition process by using the nano metallic particles as a catalyst to form carbon nanotubes on the conductive substrate; and

forming a fluorescent material on the carbon nanotubes to form a light-emitting device.

19. The method for preparing nano metallic particles as claimed in claim 18, wherein the surface roughness of the conductive substrate is of nano scale.

20. The method for preparing nano metallic particles as claimed in claim 18, wherein the surface roughness of the conductive substrate is between 5 nm and 10 μm.

21. The method for preparing nano metallic particles as claimed in claim 18, wherein the conductive substrate includes conductive regions and non-conductive regions.

22. The method for preparing nano metallic particles as claimed in claim 18, further comprising a polishing process or a plasma bombarding process on the surface of the conductive substrate such that the surface roughness of the conductive substrate is of nano scale.

23. The method for preparing nano metallic particles as claimed in claim 18, wherein the electroplating process is a cyclic voltammery electroplating process.