Nanocrystal-metal oxide complex comprising at least two different surfactants and method for preparing the same

Disclosed herein is a nanocrystal-metal oxide complex. The nanocrystal of the nanocrystal-metal oxide complex is substituted with two or more different types of surfactants which are miscible with a metal oxide precursor and enable maintenance of luminescent and electrical properties of the nanocrystal. The nanocrystal-metal oxide complex exhibits superior optical and chemical stability and secures high luminescent efficiency of the nanocrystal. Accordingly, when the nanocrystal-metal oxide complex is used as a luminescent material of an electroluminescent device, it can improve luminescent efficiency and reliability of products. Further disclosed herein is a method for preparing the nanocrystal-metal oxide complex.

CROSS REFERENCE TO RELATED APPLICATION

This non-provisional application claims priority to Korean Patent Application No. 2007-40384 filed on Apr. 25, 2007, and all the benefits accruing therefrom under 35 U.S.C. §119(a), the content of which is herein incorporated by reference in its entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a nanocrystal-metal oxide complex and a method for preparing the complex. More specifically, the present invention relates to a nanocrystal-metal oxide complex with improved luminescent efficiency, superior optical stability and excellent chemical stability which comprises a nanocrystal and metal oxide substituted with two or more surfactants, and a method for preparing the complex.

2. Description of the Related Art

A semiconductor nanocrystal is a crystalline material generally having a particle size (i.e., a particle diameter) of a few nanometers up to about 10 nm, and consists of a cluster of several hundred to several thousand atoms. Such a small-sized semiconductor nanocrystal has a large surface area per unit volume, and therefore most of the constituent atoms of the nanocrystal are present on or near the surface of the nanocrystal. Based on this characteristic structure, a semiconductor nanocrystal exhibits quantum confinement effects and shows electrical, magnetic, optical, chemical and mechanical properties that differ from those inherent to the constituent atoms of the nanocrystal, or from bulk properties of the constituent atoms of the nanocrystal.

Control over the physical size and composition of semiconductor nanocrystals enables the control of the properties of the nanocrystals. Accordingly, semiconductor nanocrystals can be utilized in a variety of applications including: luminescent devices such as light-emitting diodes (“LEDs”), electroluminescence (“EL”) devices, lasers, holographic devices, and sensors; and electronic devices such as solar cells and photodetectors. For various applications, nanocrystals must be incorporated in an appropriate matrix. Accordingly, nanocrystals must exhibit excellent dispersibility and formability in a medium (e.g., a solution) as well as superior luminescent properties.

Nanocrystals are generally prepared by a wet chemistry technique wherein a precursor material is added to a coordinating organic solvent to grow nanocrystals to the desired size. In a wet chemistry technique, as nanocrystals are grown, the organic solvent can coordinate to the surface of the nanocrystals, thus acting as a dispersant for the nanocrystals. Accordingly, the organic solvent allows the semiconductor nanocrystals to grow to nanometer-scale. The wet chemistry technique has an advantage in that nanocrystals having a variety of sizes can be uniformly prepared by appropriately controlling the concentration of precursors used, the kind of organic solvents, and preparation temperature and time, and the like. Also, according to the wet chemistry technique, since nanocrystals have a large surface area per unit volume due to their extremely small size, they are vulnerable to surface defects readily undergo aggregation. The surface defects act as energy traps between energy bandgaps, thereby disadvantageously causing a deterioration in luminescent efficiency.

In an attempt to overcome this, preparation of a nanocrystal-metal oxide complex in which nanocrystals are dispersed in a transparent metal oxide matrix has been proposed as a way to improve the stability of nanocrystals by preventing oxidation or aggregation of nanocrystals, both of which phenomena results from outside stimulus. The surface of nanocrystals prepared by the wet chemistry process is surrounded by an organic surfactant. Based on this, and to allow the nanocrystals to be dispersed in the metal oxide matrix, some methods have been suggested in which the materials coordinated to the surface of the nanocrystals are substituted with a surfactant, which is compatible with metal oxide precursors. For example, methods for preparing nanocrystal-metal oxide complexes by substituting the surface of nanocrystals with an alkyl silane-based surfactant and mixing the nanocrystals with a metal oxide precursor are known, wherein the alkyl silane-based surfactant has a terminal group (such as thiol (—SH), amino (—NH2) or carboxy (—COOH)) capable of binding to the surface of nanocrystals at one end, and a Si(OR)3terminal group at the other end. International Patent Publication No. WO 2005/049711 discloses a method for preparing nanocrystal-metal oxide complexes by substituting the surface of nanocrystals with a surfactant which has at least one group (e.g., —SH, —NH2or —COOH) capable of at one end binding to the surface of nanocrystals, and a hydrophilic group (e.g., —OH, —COOH, —NH2, —PO3H2, —SO3H or —CN) at one other end, that is capable of interacting with the solvent.

However, these conventional methods, such as methods for preparing nanocrystal-metal oxide complexes wherein the surface of nanocrystals is substituted with one type of surfactant, have a difficulty in dispersing nanocrystals in a metal oxide matrix while luminescent and electrical properties of the nanocrystals are maintained. In addition, there is a limitation that the nanocrystal-metal oxide complexes prepared by the methods has poor luminescent property and stability.

BRIEF SUMMARY OF THE INVENTION

Therefore, in view of the above problems of the prior art, in an embodiment, a nanocrystal-metal oxide complex with improved luminescent efficiency as well as superior optical stability and excellent chemical stability, is provided.

In another embodiment, a method for preparing the nanocrystal-metal oxide complex is also provided.

In accordance with an embodiment, there is provided a nanocrystal-metal oxide complex comprising a nanocrystal and metal oxide wherein the surface of the nanocrystal is substituted with two or more different surfactants.

In an embodiment, one of the surfactants is a hydrophilic surfactant and the other an alkyl amine surfactant.

In an embodiment, the hydrophilic surfactant is at least one compound represented by Formula 1 below:
An-(Rm)—Bl(1)

wherein A is selected from the group consisting of thiol, amino, carboxylic acid, phosphonic acid, phosphine oxide, nitrile, and thiocyanate;

B is selected from the group consisting of hydroxyl, carboxylic acid, amino, phosphonic acid, and sulfonic acid;

R is at least one selected from the group consisting of hydrocarbon including C1-30alkyl and C6-30aryl, and C2-30polyethers including ethylene oxide and propylene oxide;

n and l are each independently an integer of one or greater; and

m is an integer from 1 to 22.

In another embodiment, the nanocrystal is composed of one selected from the group consisting of Group II-VI, Group II-V, Group III-VI, Group III-V, Group IV-VI, Group I-III-VI, Group II-IV-VI and Group II-IV-V semiconductor compounds, and alloys and combinations thereof.

In accordance with still another embodiment, there is provided a method for preparing a nanocrystal-metal oxide complex comprising: (a) reacting a nanocrystal with two or more different surfactants to substitute the surface of the nanocrystal with the surfactants; and (b) mixing the nanocrystal with a metal oxide precursor, a solvent and water, followed by drying.

In accordance with yet another embodiment, there is provided an electronic device comprising the nanocrystal-metal oxide complex.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention will now be described in greater detail with reference to the accompanying drawings.

It will be understood that when an element is referred to as being “on” another element, it can be directly on the other element or intervening elements can be present therebetween. In contrast, when an element is referred to as being “disposed on” or “formed on” another element, the elements are understood to be in at least partial contact with each other, unless otherwise specified.

The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention. As used herein, the singular forms “a”, “an” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. The use of the terms “first”, “second”, and the like do not imply any particular order but are included to identify individual elements. It will be further understood that the terms “comprises” and/or “comprising,” or “includes” and/or “including” when used in this specification, specify the presence of stated features, regions, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, regions, integers, steps, operations, elements, components, and/or groups thereof.

In the drawings, like reference numerals in the drawings denote like elements and the thicknesses of layers and regions are exaggerated for clarity.

Disclosed herein is a nanocrystal-metal oxide complex in which the surface of a nanocrystal is substituted with two or more different surfactants. One of the surfactants is a hydrophilic surfactant and the other is an alkyl amine surfactant.

FIG. 1is a schematic diagram showing the structure of a nanocrystal-metal oxide complex according to one embodiment of the present invention. As shown inFIG. 1, the nanocrystal-metal oxide complex100is characterized in that the surface of the nanocrystal101is substituted with two or more surfactants of different types. That is, the nanocrystal of the nanocrystal-metal oxide complex is coordinated to both of: a hydrophilic surfactant that can react to form hydrophilic surfactant structures110that are miscible with and reactive toward a metal oxide precursor, and with which the hydrophilic surfactant can form metal oxide structures120; and an alkyl amine surfactant130which protects the surface and allows the luminescent and electrical properties of the nanocrystal to be maintained. Accordingly, the nanocrystal-metal oxide complex100has advantages of maintaining the luminescent and electrical properties of the nanocrystal, while exhibiting superior stability and formability. As used herein, “stability” refers generally to both structural stability, in which the integrity of the structure of the nanocrystal-metal oxide complex is maintained when challenged with various environmental conditions such as, for example, thermal, humidity, pH, electrical, radiation (e.g., light), mechanical (e.g., abrasion), and the like; and to property stability, in which the nanoparticle exhibits consistent appearance properties (e.g., color) or performance properties (e.g., luminescence, mechanical, electrical, and the like) when subject to the above environmental conditions. “Formability”, as used herein, means ability of the nanocrystal-metal oxide complex to be formed, alone or in combination with other components, into articles of different shapes and dimensions, and by suitable processes. The surface of the nanocrystal can in this way be appropriately and efficiently coordinated to by the surfactant without any significant defects, and the alkyl amine surfactant controls a dry speed by rendering a solvent to be slowly evaporated. Based on these characteristics, the nanocrystal-metal oxide complex can be prepared in a monolith form.

In an embodiment, the hydrophilic surfactant is represented by Formula 1 below:
An-(Rm)—Bl(1)

wherein A is selected from the group consisting of thiol, amino, carboxylic acid, phosphonic acid, phosphine oxide, nitrile, and thiocyanate;

B is selected from the group consisting of hydroxyl, carboxylic acid, amino, phosphonic acid, and sulfonic acid;

R is at least one selected from the group consisting of a hydrocarbon including C1-30alkylene and C6-30arylene, and C2-30polyether units including ethylene oxide and propylene oxide;

n and l are each independently an integer of one or greater; and

m is an integer from 1 to 22.

The nanocrystal-metal oxide complex comprises metal oxides. Exemplary metal oxides include, for example, TiO2, ZnO, SiO2, SnO2, WO3, Ta2O3, BaTiO3, BaZrO3, ZrO2, HfO2, Al2O3, Y2O3, ZrSiO4, Fe2O3, Fe3O4, CeO, CrO3and a mixture thereof, but the nanocrystal-metal oxide complex is should not be considered as limited to these metal oxides. The nanocrystal-metal oxide complex can be prepared in a powder, thin film, or monolith form.

The nanocrystal constituting the nanocrystal-metal oxide complex includes nanocrystals (e.g., metal nanocrystals and semiconductor nanocrystals) that can be prepared by wet chemistry processes. Materials for semiconductor nanocrystals can be selected from the group consisting of Group II-VI, Group II-V, Group III-VI, Group III-V, Group IV-VI, Group I-III-VI, Group II-IV-VI and Group II-IV-V semiconductor compounds, alloys thereof, and combinations thereof.

Where two or more nanocrystal materials are present as the nanocrystal, they can be partially localized (i.e., can have a layered, core-shell, or other composite structure) or can be present in admixture or alloy form. The size of nanocrystal is not especially limited, but can be, in an embodiment from 2 nm to 20 nm, specifically from 3 nm to 15 nm.

The nanocrystal can have a core-shell structure wherein a shell surrounds and partially or fully encases one or more cores, the shell being composed of a material having a large band gap that is greater than that of the core. Exemplary shell materials can include ZnS or ZnSe. The nanocrystal core is a nanocrystal material selected from the group consisting of CdS, CdSe, CdTe, ZnS, ZnSe, ZnTe, HgS, HgSe, HgTe, GaN, GaP, GaAs, InP, InAs, InSb, AlN, AlP, AlAs, AlSb, SiC, Fe, Pt, Ni, Co, Al, Ag, Au, Cu, FePt, Si, Ge, PbS, PbSe, PbTe, alloys thereof, and combinations thereof. The nanocrystal shell is a second nanocrystal material selected from the group consisting of ZnS, ZnSe, ZnTe, CdS, CdSe, CdTe, HgS, HgSe, HgTe, AlN, AlP, AlAs, AlSb, GaN, GaP, GaAs, GaSb, GaSe, InN, InP, InAs, InSb, TlN, TlP, TlAs, TlSb, PbS, PbSe, PbTe, and alloys and combinations thereof. Where a core-shell structure is used, the nanocrystal materials of the core and shell are not identical. More than one shell can be present for a single core, where subsequent shells are formed to surround and encase, in whole or in part, the previously formed shell, where the subsequently formed shell has a larger band-gap than the previously formed shell.

The nanocrystal of the nanocrystal-metal oxide complex can have various shapes depending on reaction conditions. Specifically, the nanocrystal has a shape selected from the group consisting of spheres, tetrahedrons, cylinders, rods, triangles, discs, tripods, tetrapods, cubes, boxes, stars, and tubes, but is not necessarily limited thereto. The nanocrystal can efficiently emit light in a visible region of the spectrum and in other regions including ultraviolet and infrared regions.

Prior to surface substitution, the surface of the nanocrystal is selectively coordinated to by an organic material (e.g., a solvent). The coordination of the nanocrystal surface to the organic material contributes to improvement in suspension stability (i.e., stability of the suspended nanocrystals in a solution or dispersion) and prevention of aggregation between nanocrystals, by forming a layer at the surface of the nanocrystal. The organic material comprises a solvent, which is used in the process of nanocrystal growth. Examples of the organic material include, but are not limited thereto C6-C22alkanes and alkenes having a terminal carboxylic acid (COOH) group; C6-C22alkanes and alkenes having a terminal phosphonic acid (—PO3H2) group; alkanes and alkenes having a phosphine oxide (—PO) group; C6-C22alkanes and alkenes having a terminal sulfinic (SOOH) group; and C6-C22alkanes and alkenes having a terminal amino (—NH2) group. Examples of specific organic materials include oleic acid, stearic acid, palmitic acid, hexylphosphonic acid, n-octylphosphonic acid, tetradecylphosphonic acid, octadecylphosphonic acid, n-octyl amine, and hexadecyl amine.

In another embodiment, a method for preparing a nanocrystal-metal oxide complex is disclosed.FIG. 2is a schematic diagram illustrating a method for preparing a nanocrystal-metal oxide complex according to one embodiment. The method comprises, in an embodiment, addition of a combination of metal oxide precursor220, metal oxide precursor capping agent221, ethanol222as solvent, and water223, to an alkylamine solution of the nanocrystal-surfactant complex200comprising a nanocrystal201with hydrophilic surfactant210and alkylamine surfactant230The method will be explained in greater detail with reference to the accompanying drawings.

According to the method, first, a nanocrystal is reacted with two or more types of surfactants to displace the solvent layer at the surface of the nanocrystal with the surfactants (step a). Generally, nanocrystals can be surrounded by a hydrophobic surfactant adhering to the surface of the nanocrystal; and in an embodiment, such a nanocrystal can be used. The nanocrystal surface-substituted with two or more types of surfactants (i.e., the nanocrystal-surfactant complex200), or a solution of the nanocrystal-surfactant complex200is mixed with (i.e., has added to it) a metal oxide precursor220, a solvent222and water223, followed by drying, to induce a crosslink reaction (Step b). A capping agent221having a terminal Si(OR)3group can be added to improve the dispersibility and stability of the resulting nanocrystal-metal oxide complex. In an embodiment, the drying of the nanocrystal-metal oxide complex is carried out at 60° C. to 150° C. to improve the hardness of the nanocrystal-metal oxide complex.

Of the two or more different kinds of surfactants used for surface substitution of the nanocrystal, one is a hydrophilic surfactant and the other is an alkyl amine surfactant. The hydrophilic surfactant can be used singly or in combination thereof. The hydrophilic surfactant allows the nanocrystal to be miscible with and reactive toward a metal oxide precursor. The alkyl amine surfactant protects the surface of the nanocrystal, thereby maintaining the luminescent and electrical properties of the nanocrystal.

Where the mixture of the nanocrystal and the metal oxide precursor solvent is dried rapidly, cracks in the nanocrystal-metal oxide complex can be created, making it impossible to prepare the nanocrystal-metal oxide complex in a monolith form. However, since the alkyl amine surfactant can be selected to adjust the drying speed to a desired low level, a nanocrystal-metal oxide complex can thereby be prepared in a monolith form. Accordingly, the method allows preparation of a nanocrystal-metal oxide complex in any form which is suitable for an intended use of the complex.

The surface substitution of the nanocrystal with at least one hydrophilic surfactant and an alkyl amine surfactant is carried out by treating the nanocrystal surface with a solution containing a hydrophilic surfactant, an alkyl amine surfactant, and a solvent. Examples of the solvent include, but are not limited to C1-20alkyl alcohol, acetone, ethyl acetate, dichloromethane, chloroform, dimethylformamide, tetrahydrofuran, dimethylsulfoxide, pyridine, C1-20alkyl amine, and a mixture thereof.

As the hydrophilic surfactant miscible with a metal oxide precursor, any hydrophilic surfactant can be used without particular limitation so long as it has a functional group capable of being bound to the surface of the nanocrystal at one end of the hydrophilic surfactant molecule, such as —SH, —NH2, —COOH, —PO3H2, —PO, —CN or —SCN; and a hydrophilic functional group such as —OH, —COOH, —NH2, —PO3H2, —SO3H or —CN at the other end of the hydrophilic surfactant molecule. The hydrophilic surfactant can be a compound represented by the following Formula 1, but is not limited to thereto:
An-(Rm)—Bl(1)

wherein A is selected from the group consisting of thiol, amino, carboxylic acid, phosphonic acid, phosphine oxide, nitrile and thiocyanate;

B is selected from the group consisting of hydroxyl, carboxylic acid, amino, phosphonic acid and sulfonic acid;

R is at least one selected from the group consisting of hydrocarbon including C1-30alkyl and C6-30aryl; and a C2-30polyether including ethylene oxide and propylene oxide;

n and l are each independently an integer of one or greater; and

m is an integer from 1 to 22.

The alkyl amine surfactant reduces or eliminates defects of the surface of the nanocrystal-metal oxide complex, which result from surface substitution of the nanocrystal with a surfactant having a terminal hydrophilic group, which can allow maintaining luminescent and electrical properties of the nanocrystal. In addition, the alkyl amine surfactant acts as a drying control chemical additive, allowing the catalyst and metal oxide precursors for polymerizing the complex to be dried slowly. Accordingly, by use of the nanocrystal substituted with both a hydrophilic surfactant and an alkyl amine surfactant, a nanocrystal-metal oxide complex with superior luminescent and electrical properties can be prepared in a monolith form as well as in a powder or thin film form.

The nanocrystal can be prepared by any process known in the art. For example, in an embodiment, the nanocrystal is prepared by adding a Group V or VI precursor to a mixed system of a solvent and a dispersant containing a Group II, III, or IV precursor, and reacting the mixture.

The metal precursor that can be used is selected from a metal alkoxide, metal halide, or metal hydroxide.

In another embodiment, an electronic device comprises the nanocrystal-metal oxide complex. The nanocrystal-metal oxide complex can be utilized in various applications including displays (e.g., plasma display panels (“PDPs”) and luminescent diodes (“LEDs”)), lasers, linear photodiodes, sensors (e.g., biosensors) reacting with a target material to emit light, and photovoltaic devices.

Hereinafter, the present invention will be explained in more detail with reference to the following examples. These examples are provided for the purpose of illustration and are not intended to limit the present invention.

EXAMPLES

Preparation Example 1

Preparation of CdSe/CdS.ZnS Nanocrystal

16 g of trioctylamine (TOA), 0.3 g of oleic acid and 0.4 mmol of cadmium oxide were simultaneously placed in a 100 ml-flask equipped with a reflex condenser. The reaction temperature of the mixture was adjusted to 300° C. with stirring to prepare a cadmium precursor solution. Separately, a selenium (Se) powder was dissolved in trioctylphosphine (TOP) to obtain a Se-TOP complex solution (Se concentration: ca. 1 M). 2 ml of the Se-TOP complex solution was rapidly fed to the cadmium precursor solution, followed by stirring for about 4 minutes to prepare a solution of a 2 mM solution of CdSe nanocrystal that emits light at a wavelength of 536 nm.

8 g of TOA, 0.1 g of oleic acid, 0.1 mmol of cadmium oxide and 0.1 mmol of zinc acetate were simultaneously placed in a 100 ml-flask equipped with a reflex condenser. The reaction temperature of the mixture was adjusted to 300° C. with stirring. After the CdSe nanocrystal solution was fed to the reaction mixture, a S-TOP complex solution (1 ml; S concentration: 0.4 M) was slowly added thereto over about 2 min. The reaction was allowed to proceed for about one hour.

After the reaction was completed, the reaction mixture was cooled to room temperature as rapidly as possible. Ethanol (20 ml) as a non-solvent was added to the reaction mixture, and the resulting mixture was centrifuged. The obtained precipitate was separated from the supernatant, and dispersed in toluene (dispersion concentration: 1 wt %) to produce a CdSe/CdS.ZnS nanocrystal that has a diameter of 8 nm and emits light at a wavelength of 594 nm.FIG. 3shows a photoluminescence spectrum of the CdSe/CdS.ZnS nanocrystal.FIG. 3demonstrates that the CdSe/CdS.ZnS nanocrystal thus produced emits light at 594 nm.

Preparation of CdSe/CdS.ZnS Nanocrystal-silica Complex

Ethanol (20 ml) was added to the 1 wt % nanocrystal toluene solution thus prepared. The mixture was centrifuged. The obtained precipitate was separated from the supernatant. Pyridine (5_ml) was added to the precipitate, followed by stirring until the mixture became clear. Hexane (20_ml) was added to the nanocrystal pyridine solution. The resulting mixture was centrifuged. The obtained precipitate was separated from the supernatant. Then, the precipitate was dissolved in 100 μL solution of 6-mercaptohexanol and propylamine (1:1 ratio, v/v) in 5 mL of pyridine, followed by stirring for about two hours.

Hexane (10 ml) was added to the resulting solution to precipitate the CdSe/CdS.ZnS nanocrystal. The obtained precipitate was separated from the supernatant. 200 μL of tetraethoxyorthosilane (TEOS), 100 μL of ethanol, 100 μL of propylamine and 50 μL of water were added to the precipitate, followed by stirring. The reaction mixture was charged in a round mold and dried at room temperature to yield a CdSe/CdS.ZnS nanocrystal-silica complex. The concentration of the nanocrystal can be adjusted within a range 0.01 to 20 vol % according to the amount of the nanocrystals and TEOS.

Preparation of CdSe/CdS.ZnS Nanocrystal-titania Complex

Ethanol was added to the 1 wt % solution of nanocrystal in toluene thus prepared. The mixture was centrifuged. The obtained precipitate was separated from the supernatant. Pyridine (5 ml) was added to the precipitate, followed by stirring until the mixture became clear. Hexane (20_ml) was added to the nanocrystal pyridine solution. The resulting mixture was centrifuged. The obtained precipitate was separated from the supernatant. Then, the precipitate was dissolved in a 100 μL solution of 6-mercaptohexanol and propylamine in pyridine, followed by stirring for about two hours.

Hexane (10_ml) was added to the resulting solution. The obtained precipitate was separated from the supernatant. 200 μL of titanium butoxide, 100 μL of ethanol, 100 μL of propylamine and 50 μL of water were added to the precipitate, followed by stirring. The reaction mixture was charged in a round mold and dried at room temperature to yield a CdSe/CdS.ZnS nanocrystal-titania complex.

Comparative Example 1

Ethanol was added to the 1 wt % nanocrystal toluene solution thus prepared in Preparation Example 1. The mixture was centrifuged. The obtained precipitate was separated from the supernatant. Pyridine (5_ml) was added to the precipitate, followed by stirring until the mixture became clear. Hexane (20 mL) was added to the nanocrystal pyridine solution. The resulting mixture was centrifuged. The obtained precipitate was separated from the supernatant. Then, the precipitate was dissolved in a 100 μL solution of 6-mercaptohexanol in pyridine, followed by stirring for about two hours. Hexane (10 mL) was added to the resulting solution. The obtained precipitate was separated from the supernatant. 200 μL of TEOS, 100 μL of ethanol, 50 μL of water, and 100 μL of 5-aminopentanol as a catalyst were added to the precipitate, followed by stirring. The reaction mixture was charged in a round mold and dried at room temperature to yield a CdSe/CdS.ZnS nanocrystal-silica complex.

Comparative Example 2

Ethanol was added to the 1 wt % nanocrystal solution in toluene thus prepared in Preparation Example 1. The mixture was centrifuged. The obtained precipitate was separated from the supernatant. To the precipitate were added 100 μL of 5-aminopentanol, 200 μL of TEOS, 100 μL of ethanol and 50 μL of water, followed by stirring. The reaction mixture was charged in a round mold and dried at room temperature to yield a CdSe/CdS.ZnS nanocrystal-silica complex substituted with 5-aminopentanol.

Experimental Example 1

To evaluate the luminescent properties of the nanocrystal-silica complexes prepared in Example 1 and Comparative Examples 1 and 2, the nanocrystal-silica complexes were heated in air at 150° C. and observed by luminescence spectroscopy.FIG. 4is a photograph showing a comparison in luminescent property between the nanocrystal-silica complexes before and after heating. InFIG. 4, the nanocrystal-silica complexes before heating and the nanocrystal-silica complexes after heating are shown at the top and bottom, respectively. The nanocrystal-silica complexes corresponding to Example 1, Comparative Example 1 and Comparative Example 2 are shown from left to right.

As can be seen inFIG. 4, the nanocrystal-silica complex substituted with 6-nercaptohexanol and propanol in Example 1 exhibited substantially similar luminescent properties and shape before and after heating. Conversely, the luminescent properties and volume of the nanocrystal-silica complexes prepared in Comparative Examples 1 and 2 underwent significant reduction in the luminescent properties and volume, as a result of being greatly cracked after heating.

These results indicate that the nanocrystal-silica complex substituted with both a hydrophilic surfactant and an alkyl amine surfactant exhibits superior luminescent properties and thermal stability, as compared to the complex prepared from an amine compound having any other terminal group except for alkyl amine (Comparative Example 1) and the nanocrystal-silica complex substituted with a hydrophilic surfactant only (Comparative Example 2).

As can be seen from the foregoing, since the nanocrystal-metal oxide complex comprises two or more different types of surfactants, it can advantageously maintain luminescent and electrical properties of the nanocrystal, at the same time, exhibits improved thermal stability and optical stability. In addition, according to the method herein, a nanocrystal-metal oxide complex can be prepared in various forms e.g., powder, thin film and monolith.

The nanocrystal-metal oxide complex of the present invention exhibits superior stability as well as high luminescent efficiency. Accordingly, when the nanocrystal-metal oxide complex is applied to an electroluminescent device, it undergoes no deterioration in luminescent properties even at a high driving temperature of the device, thus realizing superior characteristics, as compared to other structural nanocrystal complexes.