US20140161972A1

US20140161972A1 - Method for forming conductive film at room temperature

Info

Publication number: US20140161972A1
Application number: US13/709,006
Authority: US
Inventors: Teng-yuan Dong; Chen-ni Chen
Original assignee: National Sun Yat Sen University
Current assignee: National Sun Yat Sen University
Priority date: 2012-12-09
Filing date: 2012-12-09
Publication date: 2014-06-12

Abstract

A method for forming a conductive film at room temperature is provided and includes steps of: adding AgNO₃into a first solution of dodecanoic acid; dropping n-butylamine and a diluted aqueous solution of a reducing agent into the first solution in turn, so as to initially obtain the dodecanoate-protected silver nanoparticles as a capping ligand; then using cyclohexane as a solvent to apply the silver nanoparticles onto a surface of a substrate to form a patterned film of silver nanoparticles; and finally immersing the substrate into a high concentrated aqueous solution of a reducing agent to chemically reduce the patterned film of silver nanoparticles into a conductive silver film. Thus, a patterned film or circuit can be conveniently and rapidly formed, and the silver nanoparticles can be applied to flexible substrates with low material cost and temperature sensitivity.

Description

FIELD OF THE INVENTION

The present invention relates to a method for forming a conductive film at room temperature, and more particularly to a method using a reducing agent to chemically reduce a film of dodecanoate-protected silver nanoparticles pre-coated on the flexible substrate into a conductive silver film at room temperature.

BACKGROUND OF THE INVENTION

Low-cost liquid direct printing technology can be applied to a variety of patterned and deposition processes, such as processes of integrated circuits (IC), glass substrate circuits of large-size liquid crystal display (LCD), surface circuits of LED wafer, repair of IC open circuits, electronic tags or radio frequency identification (RFID), etc. Traditionally, plating and etching to form a conductive patterned circuit are generally done by lithography processes. However, due to the many complex steps required to construct a layer of circuit, it is relatively time-consuming and the manufacturing cost is higher. Therefore, for the related industry, it is necessary to simplify the process and reduce the manufacturing cost of the direct printing techniques, wherein the technique is to quickly convert the metal nanoparticles into the low-resistance metal conductor to form a conductive circuit after removing the solvent and heat treatment.
Nowadays, one of the direct printing technologies is to firstly mix metal nanoparticles and a solvent as a printing ink, and then spray the printing ink onto a substrate to form a conductive circuit by an inkjet method, wherein the metal nanoparticles is preferably selected from silver. Silver is more suitable than gold for being used in this art, because silver is the metal having the highest conductivity among all the metals. And, gold has excessive material cost which affects its applicability in the field of electronics.
For example, the inventors of the present patent previously disclosed a paper named “One-step synthesis of uniform silver nanoparticles capped by saturated decanoate: direct spray printing ink to form metallic silver films” published in the journal, Physical Chemistry Chemical Physics (2009, vol. 11, p. 6269-6275), on May 27, 2009, wherein it disclosed that the decanoate-protected silver nanoparticles (C₉H₁₉COO₂—Ag) is used as a printing ink. However, the stability of the operation of the decanoate-protected silver nanoparticles at room temperature is only maintained for one day because of desorption by the carbon dioxide and other decomposition fragments. Therefore, there is a need to change other protective agents to stabilize the produced silver nanoparticles, or that will significantly affect the application of the silver nanoparticles in the direct printing technology. Furthermore, the decanoate-protected silver nanoparticles described above is coated on a rigid silicon substrate, and reduce the coating film of the silver nanoparticles into a high conductive silver film by using the high-temperature calcination above 150° C. However, the process of using high-temperature calcination also cannot be applied to the non-heat-resistant flexible plastic substrate and it takes longer process time. And, adding the hydrogen in the high temperature to process reduction reaction will increase the safety risk.
It is therefore necessary to provide a method for forming a conductive film at room temperature to solve the problem of the conventional technology.

SUMMARY OF THE INVENTION

A primary object of the present invention is to provide a method for forming a conductive film at room temperature, which includes steps of: firstly adding AgNO₃into a solution of dodecanoic acid; sequentially dropping n-butylamine as a silver-ion ligand and a diluted aqueous solution of a reducing agent to reduce silver ions into silver nanoparticles to initially obtain silver nanoparticles stably protected by dodecanoate as a capping ligand; then using cyclohexane as a long-term stabilizing solvent to spin-coat or print silver nanoparticles using dodecanoate as a capping ligand onto the surface of the substrate to form a patterned silver nanoparticle film; finally immersing the substrate into a high concentrated aqueous solution of a reducing agent to chemically reduce the patterned film of silver nanoparticles into a conductive silver film. This method can not only be used to conveniently and rapidly print the patterned film or circuit on the substrate, but also significantly increase the application potential of silver nanoparticles on the flexible substrate (i.e. PET substrate) which is non-heat-resistant and low cost.
A secondary object of the present invention is to provide a method for forming a conductive film at room temperature, which includes steps of: firstly using cyclohexane containing 0.5 wt % dodecanoic acid as a solvent to mix with silver nanoparticles using dodecanoate as a capping ligand into a long-term stabilizing preservation solvent (30-day preservation period) with 5.0 to 15.0 wt % silver nanoparticles, so that silver nanoparticles in the cyclohexane solution containing 0.5 wt % dodecanoic acid is unable to generate mutual aggregation and condensation, and the size uniformity of the nanoparticles can be kept. Therefore, it can improve the printing quality of silver nanoparticles applied to inkjet printing process.
To achieve the above object, the present invention provides a method for forming a conductive film at room temperature, which comprises following steps of:

- (a) adding AgNO₃into a non-polar solvent containing dodecanoic acid to be a first mixture;
- (b) dropping n-butylamine into the first mixture as a ligand of silver ions of silver nitrate to be a second mixture;
- (c) dropping a diluted aqueous solution of a reducing agent into the second mixture to be a third mixture, wherein the reducing agent reduces silver ion into silver nanoparticles and dodecanoate group of dodecanoic acid is used as a capping ligand to be around and protect the silver nanoparticles;
- (d) separating the dodecanoate-protected silver nanoparticles from the third mixture;
- (e) using cyclohexane as a solvent to mix with the dodecanoate-protected silver nanoparticles to be a fourth mixture;
- (f) applying the fourth mixture onto a surface of a substrate to form a film of the dodecanoate-protected silver nanoparticles; and
- (g) immersing the substrate into an aqueous solution of a reducing agent to chemically reduce the film of silver nanoparticles into a conductive silver film;
- wherein the processes in the steps (a) to (g) are all performed at room temperature.

In one embodiment of the present invention, in the step (a), the molar ratio of the silver ions of AgNO₃and the dodecanoate group of dodecanoic acid is 1:2.
In one embodiment of the present invention, in the step (a), the non-polar solvent is toluene.
In one embodiment of the present invention, in the step (b), the molar ratio of the silver ions of AgNO₃and n-butylamine is 1:2.
In one embodiment of the present invention, in the step (c), the diluted aqueous solution of the reducing agent is selected from a diluted aqueous solution of hydrazine, ascorbic acid, NaBH₄or dimethylformamide (DMF).
In one embodiment of the present invention, in the step (c), the diluted aqueous solution of the reducing agent is the diluted aqueous solution of hydrazine, and the molar ratio of the silver ions of AgNO₃and hydrazine of the diluted aqueous solution of hydrazine is 2:1.
In one embodiment of the present invention, in the step (c), the range of average particle diameter of the dodecanoate-protected silver nanoparticles is 6.20±0.57 nanometer (nm).
In one embodiment of the present invention, in the step (d), firstly adding acetone into the third mixture to deposit the dodecanoate-protected silver nanoparticles, and then adopting methanol and acetone to wash, centrifuge and dry by reduced pressure condensation, in order to obtain the dodecanoate-protected silver nanoparticles.
In one embodiment of the present invention, in the step (e), the fourth mixture is: firstly using cyclohexane containing 0.5 wt % dodecanoate acid as a solvent to mix with the silver nanoparticles in the step (d) into the fourth mixture which has 5.0 to 15.0 wt % silver nanoparticles, wherein the fourth mixture is a long-term stabilizing preservation solvent (30-day preservation period) preferably having 10.0 wt % silver nanoparticles.
In one embodiment of the present invention, in the step (f), the fourth mixture is selected to be applied onto the surface of the substrate by spin-coating or inkjet printing.
In one embodiment of the present invention, in the step (f), the substrate is selected from the group consisting of a flexible plastic substrate, a glass substrate and a silicon wafer substrate; wherein the flexible plastic substrate is a polyethylene terephthalate (PET) substrate.
In one embodiment of the present invention, in the step (f), the aqueous solution of the reducing agent is selected from an aqueous solution of hydrazine, ascorbic acid, NaBH₄or dimethylformamide (DMF).
In one embodiment of the present invention, in the step (f), the aqueous solution of the reducing agent is an aqueous solution of hydrazine, and the hydrazine concentration of the aqueous solution of hydrazine is between 70 and 90 wt %, such as 80 wt %.
In one embodiment of the present invention, the processes in the steps (a) to (g) are all performed at room temperature in the range between 20 and 30° C., such as 25° C. In one embodiment of the present invention, after the step (g), the method further comprises: (g1) applying a heat-treatment to the conductive silver film at 100° C.
In one embodiment of the present invention, after the step (g), the method further comprises: (h) repeating the steps (f) and (g) to further stack and form another conductive silver film on the original conductive silver film.
In one embodiment of the present invention, after the step (h), the method further comprises: (h1) applying a heat-treatment to two of the stacked conductive silver films at 100° C.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of steps (a) to (d) of a method for forming a conductive film at room temperature according to a first embodiment of the present invention;

FIG. 2 is a schematic diagram of steps (e) to (g) of the method for forming the conductive film at room temperature according to the first embodiment of the present invention; and

FIG. 3 is a schematic diagram of steps (e) to (g) of a method for forming a conductive film at room temperature according to a second embodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The structure and the technical means adopted by the present invention to achieve the above and other objects can be best understood by referring to the following detailed description of the preferred embodiments and the accompanying drawings. Furthermore, directional terms described by the present invention, such as upper, lower, front, back, left, right, inner, outer, side, longitudinal/vertical, transverse/horizontal, and etc., are only directions by referring to the accompanying drawings, and thus the used directional terms are used to describe and understand the present invention, but the present invention is not limited thereto.
Please refer to FIGS. 1 to 3, a method for forming a conductive film at room temperature according to a first embodiment of the present invention mainly comprises the following steps: (a) adding AgNO₃into a non-polar solvent containing dodecanoic acid (C₁₁H₂₃COOH, also known as n-dodecanoic acid) to be a first mixture; (b) dropping n-butylamine into the first mixture as a ligand of silver ions of silver nitrate to be a second mixture; (c) dropping a diluted aqueous solution of hydrazine (N₂H₄.H₂O) into the second mixture to be a third mixture, wherein the hydrazine reduces silver ion into silver nanoparticles (NPs) and the dodecanoate group of dodecanoic acid is used as a capping ligand to be around and protect the silver nanoparticles; (d) separating the dodecanoate-protected silver nanoparticles (Ag—C₁₁H₂₃CO₂) from the third mixture; (e) using cyclohexane as a solvent to mix with the dodecanoate-protected silver nanoparticles to be a fourth mixture; (f) coating the fourth mixture onto a surface of a substrate by spin-coating to form a film of dodecanoate-protected silver nanoparticles; and (g) immersing the substrate into an aqueous solution of hydrazine to chemically reduce the silver nanoparticles into a conductive silver film; wherein the processes in the steps (a) to (g) are all performed at room temperature. FIGS. 1-3 of the present invention will be used to describe the implementation details and reaction principles of each step of the first and second embodiments.
Please refer to FIG. 1, the first step (a) of the method for forming a conductive film at room temperature according to the first embodiment of the present invention is to add AgNO₃into a non-polar solvent containing dodecanoic acid to be a first mixture. In the step, the present invention executes some sub-steps of: adopting 31.69 ml of toluene as a non-polar solvent and adding 3.3387 g (16.67 mmol) Idodecanoic acid into toluene; stirring it until it is dissolved, then adding 1.4156 g (8.33 mmol) of AgNO₃, which contains 0.25 mol/L Ag⁺, wherein AgNO₃in the first mixture is used as a precursor of silver ions, and in the following step (c), dodecanoic acid in the first mixture will be used as a capping ligand of the silver nanoparticles. However, in the step, dodecanoic acid doesn't yet react with AgNO₃. Furthermore, in the step (a), the molar ratio of the silver ions of AgNO₃and the dodecanoate group of dodecanoic acid substantially maintains at 1:2. The non-polar solvent preferably is selected from toluene, but it can still be selected from other similar non-polar solvents which have equivalent effect.
Please referring to FIG. 1, the following step (b) of the method for forming a conductive film at room temperature according to the first embodiment of the present invention is to drop n-butylamine into the first mixture as a ligand of silver ions of AgNO₃to be a second mixture. In the step, the present invention executes the way of dropping per second to add the 1.6473 ml (16.67 mmol) of n-butylamine. After finishing dropping within 2.5 minutes, the second mixture then continues to react for 3.5 minutes and it gradually turned milky turbid suspension in the process of n-butylamine is temporarily used as a ligand of silver ions to separate silver ions from AgNO₃firstly. And, the status of the milky turbid suspension shows that the second mixture includes the complex formed by coordinating silver ions and n-butylamine. In the step (b), the molar ratio of the silver ions of AgNO₃and n-butylamine substantially maintains at 1:2.
Please refer to FIG. 1, the following step (c) of the method for forming a conductive film at room temperature according to the first embodiment of the present invention is to drop a diluted aqueous solution of hydrazine (N₂H₄.H₂O) into the second mixture to be a third mixture. In the step, the present invention executes the way of firstly adding 80 wt % a solution of hydrazine (N₂H₄.H₂O) into 25 ml of a deionized water for diluting and mixing to be a diluted aqueous solution of hydrazine as a chemical redox fluid, which includes 0.2607 g (4.17 mmol) hydrazine. Then, until n-butylamine in the step (b) reacted for 3.5 minutes, executing the way of dropping per second to add the prepared diluted aqueous solution of hydrazine into the second mixture to be a third mixture. The titration takes 15 minutes. After that, the third mixture reacts again for 3 hours, wherein hydrazine reduces silver ions using n-butylamine as a temporary ligand into silver atoms in the metallic state. Silver atoms are further re-aggregated to be silver nanoparticles and the dodecanoate group of dodecanoic acid is used as a capping ligand to be around the silver nanoparticles. In the step (c), the molar ratio of the silver ions of AgNO₃and hydrazine of the diluted aqueous solution of hydrazine is 2:1.
In more detail, in step (c), silver ions using n-butylamine as a temporary ligand can be further reduced into silver atoms of the metallic state by hydrazine. And, a plurality of silver atoms of the metallic state is clustered with each other into nano-scale silver nanoparticles. After clustered, the average particle size of the silver nanoparticles is roughly in the range of 6.20±0.57 nm (nanometer), wherein each silver nanoparticles are actually formed by ten to hundreds of silver atoms clustered together. And, the outermost layer of the silver nanoparticles reacts with dodecanoate to form ionic complex; wherein the outermost layer of the silver atoms will transform into positively charged silver ions to bond with negative charge dodecanoate groups (as shown in the right-most side of FIG. 1). The dodecanoate groups are used as capping ligands to be around and protect the silver nanoparticles (Ag—C₁₁H₂₃CO₂) for avoiding the neighboring silver nanoparticles from clustering with each other again to enlarge the particle size, and further it can effectively restrict the number of silver atoms clustered inside the silver nanoparticles is no longer increased to stabilize the particle size of silver nanoparticles. The advantage of this is that the particle size of the silver nanoparticles can be controlled stably. It can effectively avoid the problem that when forming the patterned circuit in the following steps, the conductive quality of the circuit will be affected because of large conductive particles.
Please refer to FIG. 1, the following step (d) of the method for forming a conductive film at room temperature according to the first embodiment of the present invention is to separate the dodecanoate-protected silver nanoparticles (Ag—C₁₁H₂₃CO₂) from the third mixture. In this step, when the reaction of hydrazine is finished, adding 200 ml of acetone into the third mixture to deposit the dodecanoate-protected silver nanoparticles, followed by using the solution of methanol and acetone to wash, centrifuge and dry by reduced pressure condensation, so that dark blue powder of dodecanoate-protected silver nanoparticles can be obtained. However, the separation method of the present invention is not limited thereto; the present invention can also use other existing separation techniques or other solvents to separate the dodecanoate-protected silver nanoparticles from the third mixture.
Please refer to FIG. 2, the following step (e) of the method for forming a conductive film at room temperature according to the first embodiment of the present invention is to use cyclohexane as a solvent to mix with the dodecanoate-protected silver nanoparticles to be a fourth mixture 10. In the step, the present invention executes the way of firstly using cyclohexane containing 0.5 wt % dodecanoate as a solvent and adding silver nanoparticles of the step (d) into the solvent to mix to be a fourth mixture 10 which has 5.0 to 15.0 wt % silver nanoparticles; wherein the fourth mixture 10 is a long-term stabilizing preservation suspension, which preferably has 10.0 wt % silver nanoparticles. Because the fourth mixture 10 contains 0.5 wt % dodecanoate and cyclohexane, it is benefit to stably preserve silver nanoparticles using dodecanoate as the capping ligand at least about 30 days or more. The dodecanoate-protected silver nanoparticles can be stably suspended in the cyclohexane solvent containing 0.5 wt % dodecanoate, and hence the fourth mixture 10 not only can be used for the storage backup purposes, but also can avoid the problem that silver nanoparticles further cluster to expand the particle diameter during the preservation.
Please refer to FIG. 2, the following step (f) of the method for forming a conductive film at room temperature according to the first embodiment of the present invention is to coat the fourth mixture 10 onto a surface of a substrate 20 by spin coating to form a film 11 of dodecanoate-protected silver nanoparticles. In the embodiment, the fourth mixture 10 is selected to be applied onto the upper surface of the substrate 20 by spin coating via a spin-coating liquid distributor 30; wherein the substrate 20 is selected from the group consisting of a flexible plastic substrate, a glass substrate and a silicon wafer substrate and preferably selected from a flexible plastic substrate, for example, the substrate made of polyethylene terephthalate (PET) of non-heat-resistant and low cost. In this step, the present invention adopts the fourth mixture 10 containing 10 wt % dodecanoate-protected silver nanoparticles (Ag—C₁₁H₂₃CO₂) to execute spin coating wherein the substrate 20 is placed on the a turntable, and spin-coated for 15 seconds at 2000 round per minute, so that the mixture 10 can be uniformly coated on the substrate 20 (e.g., PET substrate). Until the substrate 20 is dried by nitrogen, so that cyclohexane in the fourth mixture is completely volatilized. The surface of the substrate 20 can be formed a film 11 of dodecanoate-protected silver nanoparticles.
Please refer to FIG. 2, the following step (g) of the method for forming a conductive film at room temperature according to the first embodiment of the present invention is to immerse the substrate 20 into an aqueous solution of hydrazine to chemically reduce the film 11 of silver nanoparticles into a conductive silver film 12. In the step, the concentration of hydrazine of the aqueous solution of hydrazine is obviously higher than that used in the step (c); wherein the concentration of hydrazine of the aqueous solution of hydrazine in the step is between 70 and 90 wt %, for example, preferably 80 wt %. In the embodiment, the present invention immerses the coated film 11 of silver nanoparticles of the step (f) into the aqueous solution of hydrazine (N₂H₄) at room temperature of 25° C. for one hour, and then chemically reduce it to obtain a conductive silver film 12. Finally, deionized water is used to wash the surface of the conductive silver film 12 and the surface is blown dried by nitrogen gas (or by wind). In more detail, the hydrazine can reduce silver ions on the surface of the silver nanoparticles in the film 11 of the silver nanoparticles to the silver atoms in the metal state, and desorb dodecanoate from silver nanoparticles. After reducing and desorption, silver nanoparticles only contain silver atoms in the metal state, and closely attach and combine to the upper surface of the substrate 20.
It is worthy to note that the processes in the steps (a) to (g) are all performed at room temperature. The room temperature of the present invention refers to the range between 0 and 50° C., and preferably between 10 and 40° C. and especially between 20 and 30° C., such as 21, 23, 25, 27 or 29° C. Furthermore, the conductive silver film 12 formed by the steps (a) to (g) are specially suitable to be applied to the integrated circuit (IC) on a wafer, a circuit design of a transparent conductive layer of a thin film transistor liquid crystal display (TFT-LCD) or repair of fine-pitch tiny opening defects.
Please refer to FIG. 3, the method for forming a conductive film at room temperature according to the second embodiment of the present invention is illustrated and similar to the first embodiment, so that the second embodiment uses similar terms or numerals of the first embodiment. However, compared with the first embodiment, the second embodiment comprises the following steps: (a) adding AgNO₃into a non-polar solvent containing dodecanoic acid (C₁₁H₂₃COOH) to be a first mixture; (b) dropping n-butylamine into the first mixture as a ligand of silver ions of silver nitrate to be a second mixture; (c) dropping a diluted aqueous solution of hydrazine (N₂H₄.H₂O) into the second mixture to be a third mixture, wherein the hydrazine reduces silver ion into silver nanoparticles and uses the dodecanoate group of dodecanoic acid as a capping ligand to be around and protect the silver nanoparticles; (d) separating the dodecanoate-protected silver nanoparticles (Ag—C₁₁H₂₃CO₂) from the third mixture; (e) using cyclohexane as a solvent to mix with the dodecanoate-protected silver nanoparticles to be a fourth mixture; (f) inkjet printing the fourth mixture onto a surface of a substrate to form a film of dodecanoate-protected silver nanoparticles; and (g) immersing the substrate into an aqueous solution of hydrazine to chemically reduce the silver nanoparticles into a conductive silver film; wherein the processes in the steps (a) to (g) are all performed at room temperature.
In the second embodiment of the present invention, the difference is that: as shown in FIG. 3, the step (f) of the second embodiment further uses the way of inkjet printing to replace that of spin-coating. In the second embodiment of the present invention, the fourth mixture 10 is selected to be applied onto the upper surface of the substrate 20 by inkjet printing via an inkjet liquid distributor 40; wherein the substrate 20 is selected from the group consisting of a flexible plastic substrate, a glass substrate and a silicon wafer substrate and preferably selected from a flexible plastic substrate, for example, the substrate formed by polyethylene terephthalate (PET) of non-heat-resistant and low cost.
In this step (f), the present invention adopts the fourth mixture 10 containing 10 wt % dodecanoate-protected silver nanoparticles (Ag—C₁₁H₂₃CO₂) to inkjet printing; wherein the substrate 20 is placed on the a work platform or a mobile platform, and then the upper surface of the substrate 20 is inkjet-printed by the inkjet liquid distributor 40, so that the mixture 10 can be uniformly inkjet-printed on the substrate 20 (e.g., PET substrate). Until the substrate 20 is dried by nitrogen (or by wind) so that cyclohexane in the fourth mixture 10 is completely volatilized, the surface of the substrate 20 can be formed a film 13 of dodecanoate-protected silver nanoparticles, which can be the patterned shape of circuits. In the following step (g), immerse the substrate 20 into the aqueous solution of hydrazine to reduce the film 13 of silver nanoparticles to the conductive silver film 14. The steps (a) to (e) and (g) of the second embodiment are almost similar to the first embodiment, and therefore the description thereof is no longer to be repeated therein.
On the other hand, according to other embodiments of the present invention, in the step (c) of the first and second embodiments described above, the diluted aqueous solution of hydrazine can be replaced with the diluted aqueous solution of other reducing agents, such as diluted aqueous solution of ascorbic acid, NaBH₄or dimethylformamide (DMF). Meanwhile, the aqueous solution of the hydrazine in the step (g) can be replaced with the aqueous solution of other reducing agents, such as aqueous solution of ascorbic acid, NaBH₄or dimethylformamide (DMF). The above agents are formulated into different concentrations of the reducing agents. The capabilities of electronic conduction of the silver conductive film are obtained by using various reducing agents, as shown in Table 1:

TABLE 1

			Thickness
Reductant	Reaction time	Resistivity (μΩcm)	(nm)

80 wt % Hydrazine	1	hr	4.80-9.90	60-250
1M ascorbic acid	1	hr	13.4-17.2	110-121
0.005M NaBH₄	1	min	91.2-112.9	175-240
50% DMF	2	hr	249.1-377.0	145-180

In addition, after the step (g), the method can optionally further include a sub-step (g1): applying a heat-treatment to the conductive silver film at 100° C. for 1 hour to density the silver conductive film to enhance the conductivity of the film. For example, the resistivity (μΩcm) can be reduced from 4.80-9.90 μΩcm to 2.05-4.75 μΩcm.
Furthermore, after the step (g) or (g1), the method can optionally further include a step (h): repeating steps (f) and (g) to further stack and form another conductive silver film on the original conductive silver film to enhance the conductivity of the film by increasing the thickness and filling the holes of the first layer of conductive film. And, the results are shown in Table 2 as follows:

TABLE 2

Reductant	Reaction	Resistivity of two layer	Thickness of two
(In the step	time in	of conductivity Ag film	layer of conductivity
h)	the step h	(μΩcm)	Ag film (nm)

80 wt %	1	hr	3.23-4.62	183-349
Hydrazine
1M ascorbic	1	hr	4.34-5.95	203-278
acid
0.005M	1	min	6.50-10.9	181-256
NaBH₄
50% DMF	2	hr	52.3-78.3	165-237

Similarly, after the step (h), the method can optionally further include a sub-step (h1): applying a heat-treatment to the conductive silver film at 100° C. for 1 hour to density the silver conductive film to enhance the conductivity of the film.
As described above, compared with the conditional method of forming a conductive film using decanoate-protected silver nanoparticles at room temperature, the stability of nanoparticles is poor, and high temperature calcination over 150° C. is adopted to reduce the coated film of silver nanoparticles on the rigid silicon substrate to high conductive silver film, and it is not suitable to the flexible plastic substrate of non-heat-resistance and has the shortcomings of the process time-consuming and high safety risk. FIG. 1-3 according the present invention shows that firstly adding AgNO₃into a solution of dodecanoic acid; sequentially dropping n-butylamine as a silver-ion ligand and a diluted aqueous solution of a reducing agent to reduce silver ions into silver nanoparticles to initially obtain dodecanoate-protected silver nanoparticles stably; then using cyclohexane as a long-term stabilizing solvent to spin-coat or print the dodecanoate-protected silver nanoparticles on the surface of the substrate to form a patterned silver nanoparticle film; finally immersing the substrate into a high concentrated aqueous solution of a reducing agent to chemically reduce the patterned film of silver nanoparticles into a conductive silver film. This method can not only be used to conveniently and rapidly print the patterned film or circuit on the substrate, but also significantly increase the application potential of silver nanoparticles on the flexible substrate (ie. PET substrate) which is non-heat-resistant and low cost.
Furthermore, the present invention includes steps of: firstly using cyclohexane containing 0.5 wt % dodecanoic acid as a solvent to mix dodecanoate-protected silver nanoparticles into a long-term stabilizing preservation solvent (30-day preservation period) with 5.0 to 15.0 wt % silver nanoparticles, so that silver nanoparticles in the cyclohexane solution containing 0.5 wt % dodecanoic acid is unable to generate mutual aggregation and condensation, and the size uniformity of the nanoparticles can be kept. Therefore, it can improve the printing quality of silver nanoparticles applied to inkjet printing process.
The present invention has been described with a preferred embodiment thereof and it is understood that many changes and modifications to the described embodiment can be carried out without departing from the scope and the spirit of the invention that is intended to be limited only by the appended claims.

Claims

What is claimed is:

1. A method for forming a conductive film at room temperature, comprising:

(a) adding AgNO₃into a non-polar solvent containing dodecanoic acid to be a first mixture;

(b) dropping n-butylamine into the first mixture as a ligand of silver ions of silver nitrate to be a second mixture;

(c) dropping a diluted aqueous solution of a reducing agent into the second mixture to be a third mixture, wherein the reducing agent reduces silver ion into silver nanoparticles and dodecanoate group of dodecanoic acid is used as a capping ligand to be around and protect the silver nanoparticles;

(d) separating the silver nanoparticles using dodecanoate as the capping ligand from the third mixture;

(e) using cyclohexane as a solvent to mix with silver nanoparticles using dodecanoate as the capping ligand to be a fourth mixture;

(f) applying the fourth mixture onto a surface of a substrate to form a film of silver nanoparticles using dodecanoate as the capping ligand; and

(g) immersing the substrate into an aqueous solution of a reducing agent to chemically reduce the film of silver nanoparticles into a conductive silver film;

wherein the processes in the steps (a) to (g) are all performed at room temperature.

2. The method for forming a conductive film at room temperature according claim 1, wherein in the step (a), the molar ratio of the silver ions of AgNO₃and the dodecanoate group of dodecanoic acid is 1:2.

3. The method for forming a conductive film at room temperature according claim 1, wherein in the step (a), the non-polar solvent is toluene.

4. The method for forming a conductive film at room temperature according claim 1, wherein in the step (b), the molar ratio of the silver ions of AgNO₃and n-butylamine is 1:2.

5. The method for forming a conductive film at room temperature according to claim 1, wherein in the step (c), the diluted aqueous solution of the reducing agent is selected from a diluted aqueous solution of hydrazine, ascorbic acid, NaBH₄or dimethylformamide (DMF).

6. The method for forming a conductive film at room temperature according to claim 5, wherein in the step (c), the diluted aqueous solution of the reducing agent is the diluted aqueous solution of hydrazine, and the molar ratio of the silver ions of AgNO₃and hydrazine of the diluted aqueous solution of hydrazine is 2:1.

7. The method for forming a conductive film at room temperature according ti claim 1, wherein in the step (c), the range of average particle diameter of the dodecanoate-protected silver nanoparticles is 6.20±0.57 nm.

8. The method for forming a conductive film at room temperature according to claim 1, wherein in the step (d), firstly adding acetone into the third mixture to deposit the dodecanoate-protected silver nanoparticles as the capping ligand, and then adopting methanol and acetone to wash, centrifuge and dry by reduced pressure condensation, in order to obtain the dodecanoate-protected silver nanoparticles.

9. The method for forming a conductive film at room temperature according to claim 1, wherein in the step (e), the fourth mixture is treated by firstly using cyclohexane containing 0.5 wt % dodecanoate acid as a solvent to mix silver nanoparticles in the step (d) into the fourth mixture which has 5.0 to 15.0 wt % silver nanoparticles.

10. The method for forming a conductive film at room temperature according to claim 1, wherein in the step (f), the fourth mixture is selected to apply onto the surface of the substrate by spin-coating or inkjet printing.

11. The method for forming a conductive film at room temperature according to claim 1, wherein in the step (f), the substrate is selected from the group consisting of a flexible plastic substrate, a glass substrate and a silicon wafer substrate.

12. The method for forming a conductive film at room temperature according to claim 11, wherein the flexible plastic substrate is a polyethylene terephthalate substrate.

13. The method for forming a conductive film at room temperature according to claim 1, wherein in the step (f), the aqueous solution of the reducing agent is selected from an aqueous solution of hydrazine, ascorbic acid, NaBH₄or dimethylformamide (DMF).

14. The method for forming a conductive film at room temperature according to claim 13, wherein in the step (f), the aqueous solution of the reducing agent is an aqueous solution of hydrazine, and the hydrazine concentration of the aqueous solution of hydrazine is between 70 and 90 wt %.

15. The method for forming a conductive film at room temperature according to claim 1, wherein the processes in the steps (a) to (g) are all performed at room temperature in the range between 20 and 30° C.

16. The method for forming a conductive film at room temperature according to claim 1, wherein after the step (g), the method further comprises:

(g1) applying a heat-treatment to the conductive silver film at 100° C.

17. The method for forming a conductive film at room temperature according to claim 1, wherein after the step (g), the method further comprises:

(h) repeating the steps (f) and (g) to further stack and form another conductive silver film on the original conductive silver film.

18. The method for forming a conductive film at room temperature according to claim 17, wherein after the step (h), the method further comprises:

(h1) applying a heat-treatment to two of the stacked conductive silver film at 100° C.