Nanofiber composite, method of manufacturing the same, and field effect transistor including the same

A nanofiber composite including a nanofiber formed of a hydrophobic polymer, a nanowire formed of a conductive or semiconductive organic material that is oriented in the nanofiber in the longitudinal direction of the nanofiber, and an ionic active material.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority under 35 U.S.C. §119 to Korean Patent Application No. 10-2010-0003133, filed on Jan. 13, 2010, in the Korean Intellectual Property Office, the contents of which is incorporated herein in its entirety by reference.

BACKGROUND

Example embodiments relate to a nanofiber composite, and more particularly, to a nanofiber composite including a conductive or semiconductive organic material, a method of manufacturing the nanofiber composite, and a field effect transistor including the same.

2. Description of the Related Art

Nanowires have a large specific surface area per unit volume. Accordingly, they may contribute to improving the performances of devices and also reducing the size thereof, and thus much research is being conducted into replacing two-dimensional thin films in general devices with nanowires.

Organic material-based nanowires may be simpler and less inexpensively manufactured when compared to inorganic material-based nanowires and may be applied to a flexible substrate. Electrospinning is an efficient way to manufacture nanofibers including organic nanowires having a diameter ranging from several ten to several hundred nanometers.

Conventionally, electrospinning of a conductive or semiconductor polymer has been conducted by dissolving the conductive or semiconductor polymer in a solvent. A nanofiber formed by electrospinning of conductive or semiconductor polymer has suitable electrical properties. However, since conductive or semiconductor polymers are sensitive to moisture and oxygen in air, physical properties of a nanofiber formed of conductive or semiconductor polymers may be changed by the ambient humidity or oxygen, or electrical properties of the nanofiber may deteriorate with time.

SUMMARY

Provided are a nanofiber composite including a conductive or semiconductive organic material and having improved processability without loss of physical and electrical properties due to ambient factors, a method of manufacturing the nanofiber composite, and a field effect transistor including the nanofiber composite.

According to example embodiments, a nanofiber composite includes: a nanofiber formed of a hydrophobic polymer; at least one nanowire formed of a conductive or semiconductive organic material that is oriented in the nanofiber in a longitudinal direction of the nanofiber; and an ionic active material in the at least one nanowire and in the nanofiber

According to example embodiments, a method of manufacturing a nanofiber composite includes: preparing a composition by dissolving a conductive or semiconductive organic material, a hydrophobic polymer, and an ionic active material in a solvent; and forming a nanofiber composite from the composition, wherein the nanofiber composite includes a nanofiber formed of the hydrophobic polymer and at least one nanowire formed of the conductive or semiconductive organic material that is oriented in the nanofiber in a longitudinal direction of the nanofiber.

According to example embodiments, a field effect transistor includes: a gate electrode; source and drain electrodes electrically insulated from the gate electrode and separated from each other; and a channel formed of a nanofiber composite connecting the source and drain electrodes.

DETAILED DESCRIPTION

Hereinafter, a nanofiber composite, a method of manufacturing the same, and a field effect transistor including the same according to example embodiments will be described in detail by explanation with reference to the attached drawings. Like reference numerals refer to the like elements throughout. In the drawings, the thicknesses of layers and regions are exaggerated for clarity. Example embodiments may, however, be embodied in many different forms and should not be construed as being limited to example embodiments set forth herein; rather, these example embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the concept of example embodiments to those of ordinary skill in the art.

FIG. 1Ais a perspective view of a nanofiber composite10including a conductive or semiconductive organic material and a hydrophobic polymer according to example embodiments, andFIG. 1Bis a cross-sectional view of the nanofiber composite10ofFIG. 1A. Referring toFIGS. 1A and 1B, the conductive or semiconductive organic material is oriented in a nanofiber11formed of a hydrophobic polymer in a longitudinal direction of the nanofiber11to form a nanowire13. A plurality of nanowires13are formed of the conductive or semiconductive organic material in the nanofiber11formed of the hydrophobic polymer. The nanofiber11formed of the hydrophobic polymer and the nanowire13formed of the conductive or semiconductive organic material may further include an ionic active material15.

FIG. 2Ais a perspective view of a nanofiber composite20including a conductive or semiconductive organic material and a hydrophobic polymer according to another example embodiments, andFIG. 2Bis a cross-sectional view of the nanofiber composite ofFIG. 2A. Referring toFIGS. 2A and 2B, a single nanowire23is formed of the conductive or semiconductive organic material at the center of a nanofiber21formed of the hydrophobic polymer, so that the nanofiber composite20has a core-shell structure. The single nanowire23includes an ionic active material25.

Since the nanowires13and23formed of the conductive or semiconductive organic materials are oriented in the longitudinal direction of the nanofibers11and21, a mobility of a charge carrier is improved. In addition, since the conductive or semiconductive organic materials13and23are surrounded by the nanofibers11and21, external moisture or oxygen are blocked, and thus, the electrical performance of the conductive or semiconductive organic materials13and23may be stably maintained. In addition, the nanowires13and23may have improved electrical conductivity due to the ionic active materials15and25.

Hereinafter, elements forming the nanofiber composites described with reference toFIGS. 1A,1B,2A, and2B will be described in detail.

The conductive or semiconductive organic materials13and23may be a high molecular weight material, intermediate molecular weight material, or low molecular weight material. The conductive or semiconductor high molecular weight material may include poly-3-hexylthiophene, polyaniline, polypyrrole, poly paraphenylene vinylene, polyacetylene, poly[9,9(dioctylfluorenyl-2,7-diyl)-co-(9-hexyl-3,6-carbazole)], poly[9,9(dioctyl fluorenyl-2,7-diyl)-co-(1,4-diphenylenevinylene-2-methoxy-5-{2-ethylhexyloxy}-benzene)], and poly[{2-methoxy-5-(2-ethylhexyl oxy}-1,4-(1-cyanovinylenephenylene))-co-{2,5-bis(N,N′-diphenylamino)-1,4-phenylene}].

The conductive or semiconductor intermediate molecular weight material or low molecular weight material may include pentacene, tetracene, antracene, rubrene, parylene, coronen, α-sexithiophene, copper phthalocyanine, merocyanine, perylenetetracarboxyldiimide derivatives, carbon nanotube (CNT), and graphene.

The hydrophobic polymers11and21may protect the conductive or semiconductive organic materials13and23from external environment such as moisture and may have insulating property.

The ionic active material may improve electrical properties such as conductivity and/or semiconducting property of the nanofiber composites10and20. The ionic active material may be an ionic liquid or ionomer. The ionomer may be an anionic ionomer, cationic ionomer, or bipolar ionomer.

Anions of the ionic liquid may include substituted or unsubstituted sulfonate-based anion, cyanate-based anion, thiocyanate-based anion, and carboxylate-based anion.

The ionomer may include an ethylene acrylic acid copolymer, a polyurethane ionomer having a polytrimethylene oxide bond, and an α-olefin copolymer ionomer having ethylene, propylene, 1-butene, 1-pentene, 1-hexene, 1-heptene, 3-methyl-1-butene, or 4-methyl-1-pentene as α-olefin.

FIG. 3is a flowchart illustrating a method of manufacturing a nanofiber composite.

Referring toFIG. 3, a conductive or semiconductive organic material, a hydrophobic polymer, and an ionic active material are completely dissolved in a solvent to prepare a composition for spinning in operation S10. In this regard, the composition for spinning is maintained at a temperature in the range of about 40 to about 70° C.

The conductive or semiconductive organic material may be a high molecular weight material, intermediate molecular weight material, or a low molecular weight material. The conductive or semiconductive high molecular weight material may include at least one selected from the group consisting of poly-3-hexylthiophene, polyaniline, polypyrrole, poly paraphenylene vinylene, polyacetylene, poly[9,9(dioctylfluorenyl-2,7-diyl)-co-(9-hexyl-3,6-carbazole)], poly[9,9(dioctyl fluorenyl-2,7-diyl)-co-(1,4-diphenylenevinylene-2-methoxy-5-{2-ethylhexyloxy}-benzene)] and poly[{2-methoxy-5-(2-ethylhexyl oxy}-1,4-(1-cyanovinylenephenylene))-co-{2,5-bis(N,N′-diphenylamino)-1,4-phenylene}].

The conductive or semiconductor intermediate molecular weight material or low molecular weight material may include pentacene, tetracene, antracene, rubrene, parylene, coronen, α-sexithiophene, copper phthalocyanine, merocyanine, perylenetetracarboxyldiimide derivatives, carbon nanotube (CNT), and graphene.

While the solvent is evaporated during electrospinning, the intermediate molecular weight material or low molecular weight material is self-aligned in a specific direction when transformed from a liquid state to a solid state to form a nanowire. In addition, since the nanofiber is formed while the composition is being stretched in a specific direction by a high electrical field during the electrospinning, the nanowire may be easily formed from the organic material in the hydrophobic polymer nanofiber. The nanowire formed of the organic material may provide an efficient pathway for a charge carrier.

The solvent dissolving the high molecular weight material may include chloroform, chlorobenzene, tetrahydrofuran, toluene, acetone, ethanol, methanol, dimethylformamide, dimethylsulfoxide, benzene, dioxane, cyclohexane, acetic acid, and water. The solvent may be a mixture of at least two of the foregoing. For example, a mixture solvent including chloroform and chlorobenzene may be used.

The ionic active material may be an ionic liquid or ionomer. The ionomer may be an anionic ionomer, cationic ionomer or bipolar ionomer.

An anion of the ionic liquid may include substituted or unsubstituted sulfonate-based anion, cyanate-based anion, thiocyanate-based anion, and carboxylate-based anion.

The ionomer may include an ethylene acrylic acid copolymer, a polyurethane ionomer having a polytrimethylene oxide bond, and an α-olefin copolymer ionomer having ethylene, propylene, 1-butene, 1-pentene, 1-hexene, 1-heptene, 3-methyl-1-butene, or 4-methyl-1-pentene as α-olefin.

A nanofiber composite is prepared using the composition for spinning by electrospinning in operation S20.FIG. 4schematically shows an electrospinning device. The composition for spinning filled in an injector31is pressed and discharged out of a nozzle33at a constant rate using an injector pump32. In this regard, the composition for spinning is maintained at a temperature in the range of about 40 to about 70° C. When a droplet of the composition is formed out of the nozzle33, the composition is electrospun to a collector37by applying a voltage in the range of about 10 to about 20 KV to the nozzle33using a power supply unit35to form a nanofiber composite. Optionally, the collector37is rotated at a higher speed, or a voltage is applied between two electrodes28connected to the collector37, so that the nanofiber composite is formed in a single direction.

In the nanofiber composite prepared as described above, the conductive or semiconductive organic material is oriented in a nanofiber formed of a hydrophobic polymer in a longitudinal direction of the nanofiber to form a plurality of nanowires, and the ionic active material is dispersed in the nanowire and nanofiber as described with reference toFIGS. 1A and 1B.

Alternatively, a nanofiber composite having a core-shell structure may be prepared by electrospinning using a double nozzle in the electrospinning device. In other words, the nanofiber composite having a core including the conductive or semiconductive organic material in which the ionic active material is dispersed and a shell including the hydrophobic polymer may be prepared by spinning a composition including the conductive or semiconductive organic material and the ionic active material via an inner nozzle and the hydrophobic polymer via an outer nozzle.

Meanwhile, the nanofiber composite may also be prepared using the composition for spinning by wet spinning, conjugate spinning, melt blown spinning, or flash spinning.

FIG. 5is a cross-sectional view of a field effect transistor including a nanofiber composite as a channel. Referring toFIG. 5, an insulating layer120is formed on a gate electrode110of a substrate100. Source/drain electrodes130are formed of conductive materials on the insulating layer120. The source/drain electrodes130are spaced apart from each other. A nanofiber composite140is formed to connect the source/drain electrodes130.

The nanofiber composite140may be the nanofiber composite described with reference toFIGS. 1A,1B,2A, and2B above. In other words, the nanofiber composite including the nanowire that is formed by elongating the conductive or semiconductive organic material in the hydrophobic polymer and the ionic active material that is dispersed in the nanowire and the nanofiber, or in the nanowire, may be used as a channel of a transistor. As described above, the conductive or semiconductive organic material may be a high molecular weight material, intermediate molecular weight material, or a low molecular weight material.

A part of the nanofiber formed of the hydrophobic polymer of the nanofiber composite140may be selectively removed to electrically connect the source/drain electrodes130and the nanowire formed of the conductive or semiconductive organic material. If the nanofiber composite140includes a plurality of nanowires, a part of the nanowire formed of the conductive or semiconductive organic material may be exposed to outside of the nanofiber composite140, and the nanofiber composite140may be electrically connected to the source/drain electrodes130via the exposed portion of the nanowire.

A transistor including a substantially one-dimensional channel formed of the nanofiber composite may have a higher mobility of a charge carrier and/or a low power consumption compared to a transistor including a substantially two-dimensional channel.

Typically, the conductive or semiconductive organic material is sensitive to moisture, and thus electrical performance of the conductive or semiconductive organic material can be significantly decreased with time when exposed to the external agents. According to example embodiments, however, electrical performance of the nanofiber composite may be stably maintained because the hydrophobic polymer protects the conductive or semiconductive organic material from external agents. Thus, the nanofiber composite may be used as a channel of a transistor without demanding an additional encapsulation process for protecting the conductive or semiconductive organic material.

InFIG. 5, the gate electrode110is disposed below the source/drain electrodes130. However, the nanofiber composite may be applied to a channel of a transistor with various structures. For example, the gate electrode110may be disposed on the source/drain electrodes130.

Hereinafter, the preparation of the nanofiber composite and morphology and electrical properties of the nanofiber composite will be described in detail.

EXPERIMENTAL EXAMPLE 1

10 mg of poly-3-hexylthiophene and 50 mg of polystyrene were mixed with a solvent including 0.3 ml of chlorobenzene and 0.2 ml of chloroform at 55□ to prepare a composition for spinning. The composition was filled in an injector and discharged from a nozzle using an injector pump at a constant rate of 0.4 ml/h. In order not to precipitate poly-3-hexylthiophene contained in the composition, the temperature of the composition in the injector was maintained at 55° C. When a droplet of the composition was formed out of the nozzle of the injector, the composition was electrospun to a collector by applying a voltage of 15 KV thereto using a power supply unit to prepare a nanofiber composite having a diameter in the range of several ten to several hundred nanometers (nm). Meanwhile, source/drain electrodes of a transistor were disposed on the collector such that the nanofiber composite formed a channel between the source/drain electrodes of the transistor.

EXPERIMENTAL EXAMPLE 2

10 mg of poly-3-hexylthiophene and 50 mg of polystyrene were mixed with a solvent including 0.01 ml of chlorobenzene and 0.4 ml of chloroform at 55° C. to prepare a composition for spinning. The composition was electrospun to prepare a nanofiber composite in the same manner as in Example 1.

Amounts of the elements contained in the compositions according to Experimental Examples 1 and 2 are shown in Table 1 below.

In Experimental Example 1, 0.3 ml of chlorobenzene and 0.2 ml of chloroform were used. As the ratio of chlorobenzene to chloroform increases, the degree of the nanofiber formation increases. According to Experimental Example 2, when 0.1 ml of chlorobenzene and 0.4 ml of chloroform were used, a long nanofiber having a uniform thickness was formed.FIG. 6is an optical microscopic image of a nanofiber prepared according to Experimental Example 2.

EXPERIMENTAL EXAMPLE 3

10 mg of poly-3-hexylthiophene and 40 mg of polystyrene were mixed with a solvent including 0.4 ml of chlorobenzene and 0.1 ml of chloroform at 55° C. to prepare a composition for spinning. The composition was electrospun to prepare a nanofiber composite in the same manner as in Example 1.

EXPERIMENTAL EXAMPLE 4

5 mg of poly-3-hexylthiophene and 50 mg of polystyrene were mixed with a solvent including 0.4 ml of chlorobenzene and 0.1 ml of chloroform at 55° C. to prepare a composition for spinning. The composition was electrospun to prepare a nanofiber composite in the same manner as in Example 1.

Amounts of the elements contained in the compositions according to Experimental Examples 3 and 4 are shown in Table 2 below.

A nanofiber including small grains was produced when 10 mg of poly-3-hexylthiophene and 40 mg of polystyrene were used according to Experimental Example 3. A uniform nanofiber including less small grains than the nanofiber according to Experimental Example 3 was produced when 5 mg of poly-3-hexylthiophene and 50 mg of polystyrene were used according to Experimental Example 4.FIG. 7is an optical microscopic image of the nanofiber prepared according to Experimental Example 4.

It was identified that the nanofiber had more uniform shape as the ratio of polystyrene to poly-3-hexylthiophene increases referring to Experimental Examples 3 and 4. However, when the ratio of polystyrene to poly-3-hexylthiophene was greater than a predetermined level, electrical properties of nanofiber deteriorated.

FIG. 8is a cross-sectional view of a field effect transistor including a nanofiber composite on electrodes as a channel. Referring toFIG. 8, the nanofiber41is disposed to cross both electrodes43.

FIG. 9is a graph illustrating drain voltages with respect to drain currents of a field effect transistor including a nanofiber composite channel that is formed by electrospinning a composition including poly-3-hexylthiophene and polystyrene at a weight ratio of 1:1. Referring toFIG. 9, as the drain voltage increases, the drain current increases, and as the gate voltage increases, the slope of the graph of drain voltage with respect to drain current increases. Referring to the graph ofFIG. 9illustrating drain voltages with respect to drain currents, the transistor having a nanofiber composite channel shows general characteristics of a transistor.

FIG. 10illustrates gate voltages with respect to channel current of a transistor including a nanofiber composite that is electrospun using a composition prepared by adding 5 mg of 1-butyl-3-methylimidazolium hexafluorophosphate (BMIM-PF6), as an ionic liquid, to a solution prepared by dissolving 10 mg of poly-3-hexylthiophene and 40 mg of polystyrene in a mixed solvent including chlorobenzene and chloroform.

Referring toFIG. 10, in the transistor having the nanofiber channel including the ionic liquid, a channel current is less than 10−7A at a gate voltage of −1 V, and a channel current is greater than 10−5A at a gate voltage of −3 V. Thus, it is identified that the difference between on-current and off-current increases. That is, if the nanofiber composite includes an ionic liquid, off-characteristics of a field effect transistor are significantly improved.

As described above, according to the one or more of the above example embodiments, because the nanowire formed of the conductive or semiconductive organic material is formed in the nanofiber formed of the hydrophobic polymer, conductivity or semiconductor properties of the conductive or semiconductive organic material may not be influenced by the ambient environment including moisture and oxygen.

In addition, since the nanofiber composite includes the ionic active material, electrical characteristics of the field effect transistor using the nanofiber composite may be further improved.