# Patent

## Title
Iron chalcogenide nanocomposite and method for preparing same

## Abstract

The present invention relates to an iron chalcogenide nanocomposite with photoluminescent properties. The present invention also relates to a method for preparing the iron chalcogenide nanocomposite. The method includes (a) dissolving a Fe precursor in an organic solvent to form a Fe solution, (b) dissolving a chalcogen powder or a chalcogen precursor in an organic solvent to form a chalcogen solution, (c) dropwise injecting the Fe solution into the chalcogen solution to prepare a mixture solution in which an iron chalcogenide is formed, and (d) purifying the iron chalcogenide from the mixture solution.

## Background
TECHNICAL FIELD
The present invention relates to an iron chalcogenide nanocomposite with photoluminescent properties and a method for preparing the same.
BACKGROUND ART
Transition metal chalcogenides have received attention in various applications, including thermoelectrics, magnetic semiconductors, superconductors, quantum dots, sensors, and photovoltaics because of their unique structures and electrical properties. A significant number of high-quality metal chalcogenide nanostructures have been prepared employing colloidal synthetic approach. Most previous studies have focused on Groups 2-4 semiconductor nanostructures, such as CdSe, CdS, Zns, ZnSe, CdTe, CdSe, Ag2Se, NiSe, and Ag2S and have investigated their optical properties.
However, other potential semiconducting metal chalcogenide nanostructures, especially materials having phase-selectivity problems in the synthesis, have not been investigated until now. For example, iron selenides have two stoichiometric phases, FeSe and FeSe2. FeSe forms a tetragonal or hexagonal crystal, whereas FeSe2 forms a crystal in the cubic or rhombic (marcasite-type) structure. Both phases have Fe2+ ions and their phase-selective syntheses are challenging. Such iron selenides have excellent conductive, optical, electrical, and magnetic properties with a direct band gap (1.23 eV) and can be semiconductors or even superconductors with the characteristics of ferromagnetic/ferrimagnetic metals. For example, PbO-type FeSe exists only in a narrow synthesis zone of 50.6 to 51.0%. It is known that iron elements become superconductors when they have very accurate compositional and specific structural features. NiAs-type FeSe is also a very rare 1:1 Fe—Se compound, exists in a synthesis zone of 42.0 to 50.5%, and is not a superconductor. However, only few iron selenides are known so far. Despite many efforts to synthesize PbO-type FeSe nanoparticles, studies on the optical properties of other potential semiconducting iron chalcogenide nanocomposites still remain in the early stages. Furthermore, attempts to synthesize highly photoluminescent iron chalcogenide nanocomposites have not been successful.
DETAILED DESCRIPTION OF THE INVENTION
Problems to be Solved by the Invention
One object of the present invention is to provide an iron chalcogenide nanocomposite with photoluminescent properties.
A further object of the present invention is to provide a method for preparing an iron chalcogenide nanocomposite.
Means for Solving the Problems
One aspect of the present invention provides an iron chalcogenide nanocomposite with photoluminescent properties.
A further aspect of the present invention provides a method for preparing an iron chalcogenide nanocomposite, including (a) dissolving a Fe precursor in an organic solvent to form a Fe solution, (b) dissolving a chalcogen powder or a chalcogen precursor in an organic solvent to form a chalcogen solution, (c) dropwise injecting the Fe solution into the chalcogen solution to prepare a mixture solution in which an iron chalcogenide is formed, and (d) purifying the iron chalcogenide from the mixture solution.
Effects of the Invention
The iron chalcogenide nanocomposite of the present invention has excellent photoluminescent properties even without heavy metals. Therefore, the iron chalcogenide nanocomposite of the present invention can be considered an environmentally friendly alternative to conventional photoluminescent materials and is effectively applicable to various fields, including semiconductors. In addition, it is expected that the method of the present invention will be applied to the synthesis of other metal chalcogenides.



BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 schematically shows a method for synthesizing FeSex (x=1, 2) according to one embodiment of the present invention.
FIG. 2 shows (a) solutions of FeSe and FeSe2 nanoparticles and (b) photoluminescence of the nanoparticles.
FIG. 3 shows the results of TEM-EDX for FeSe and FeSe2 (top) and XRD patterns of FeSe (black curve) and FeSe2 (red curve) (bottom).
FIG. 4 is a histogram showing the size distribution of FeSe synthesized in the Examples section.
FIG. 5 is a histogram showing the size distribution of FeSe2 synthesized in the Examples section.
FIG. 6 shows TEM images of (a) FeSe and (b) FeSe2 (insets show STEM images, scale bars represent 200 nm), and HR-TEM images of (c) FeSe and (d) FeSe2.
FIG. 7 shows a, b, and c) TEM images of a FeSe nanocomposite (at +60°, 0°, and −51° with respect to the X-axis, respectively); (d, e, f) TEM images of a FeSe2 nanocomposite (at +57°, 0°, and −54°, respectively); 1)-6) 3D TEM morphological images of a FeSe nanocomposite at different viewing angles; and 7)-12) TEM morphological images of a FeSe2 nanocomposite at different viewing angles.
FIGS. 8 to 11 shows TEM images of FeSe (left) and FeSe2 nanocomposites (right) after heating for different times (0 h, 0.5 h, 1 h, and 1.5 h) during their synthesis.
FIG. 12 shows the results of thermogravimetric analysis (TGA) for FeSe in nitrogen gas.
FIG. 13 shows the results of thermogravimetric analysis (TGA) for FeSe2 in nitrogen gas.
FIG. 14 shows the UV and PL intensities of FeSe and FeSe2.
FIG. 15 shows fluorescence images of FeSe (right) and FeSe2 (left) in bottles.
FIG. 16 shows fluorescence lifetime spectra of FeSe and FeSe2.
FIG. 17 shows the dependence of (αhν)1/2 of synthesized FeSe and FeSe2 nanocomposites on hν.
FIG. 18 shows absorbance and fluorescence spectra of FeSe, which were recorded at different reaction times (0.5 and 2 hours).
FIG. 19 shows absorbance and fluorescence spectra of FeSe2, which were recorded at different reaction times (0.5 and 2 hours).



BEST MODE FOR CARRYING OUT THE INVENTION
One aspect of the present invention is directed to an iron chalcogenide nanocomposite with photoluminescent properties.
Preferably, the nanocomposite has a NiAs-type phase crystal structure.
The chalcogen is preferably S, Se or Te and the iron chalcogenide is more preferably FeSe or FeSe2.
A further aspect of the present invention is directed to a method for preparing an iron chalcogenide nanocomposite, including (a) dissolving a Fe precursor in an organic solvent to form a Fe solution, (b) dissolving a chalcogen powder or a chalcogen precursor in an organic solvent to form a chalcogen solution, (c) dropwise injecting the Fe solution into the chalcogen solution to prepare a mixture solution in which an iron chalcogenide is formed, and (d) purifying the iron chalcogenide from the mixture solution. According to the method of the present invention, an iron chalcogenide is synthesized by individually preparing two different solutions, i.e. an iron solution and a chalcogen solution, and mixing the solutions by injection under heating to prepare a mixture solution.
In step (a) or (b), the organic solvent is preferably heated to 100 to 140° C.
The organic solvent is preferably selected from the group consisting of ether-based compounds (CnOCn, Cn: hydrocarbon, 4≦n≦30), hydrocarbons (CnH2n+2, 7≦n≦30), unsaturated hydrocarbons (CnH2n, 7≦n≦30), and organic acids (CnCOOH, Cn: hydrocarbon, 5≦n≦30).
The ether-based compounds are more preferably selected from the group consisting of trioctylphosphine oxide (TOPO), alkylphosphines, octyl ether, benzyl ether, and phenyl ether. The hydrocarbons are preferably selected from the group consisting of hexadecane, heptadecane, and octadecane. The unsaturated hydrocarbons are preferably selected from the group consisting of octene, heptadecene, and octadecene. The organic acids are preferably selected from the group consisting of oleic acid, lauric acid, stearic acid, mysteric acid, and hexadecanoic acid.
In step (c), the Fe solution is injected dropwise into the chalcogen solution, followed by heating to 250 to 400° C. to prepare a mixture solution in which an iron chalcogenide is formed. When the heating is performed within the temperature range defined above, a uniformly textured composite can be prepared from the highly volatile chalcogen powder or chalcogen precursor and precursor components, such as iron ions. In the Examples Section that follows, the heating was performed at 330° C.
In step (c), a surfactant is preferably added to and mixed with the mixed solution. The use of the surfactant avoids unwanted precipitation and facilitates collection of the final product. The surfactant is more preferably an organic acid rather than a general-purpose surfactant, such as oleylamine, octadecylamine or trioctylphosphine. The organic acid is selected from the group consisting of oleic acid, lauric acid, stearic acid, mysteric acid, hexadecanoic acid, and mixtures thereof. The use of the surfactant facilitates the collection of the final product on the inner wall of a glass flask at a high temperature, unlike an existing synthetic method employing a general-purpose surfactant.
Preferably, the Fe precursor and the chalcogen or chalcogen precursor are used in a molar ratio of 1:1-2. Iron chalcogenides with different photoluminescent properties may be prepared by varying the amounts of the precursor components.
The chalcogen is S, Se or Te and the iron chalcogenide is more preferably FeSe or FeSe2.
MODE FOR CARRYING OUT THE INVENTION
The present invention will be explained in more detail with reference to the following examples. These examples are provided for illustrative purposes only and are not intended to limit the scope of the present invention.
EXAMPLES
Materials:
Iron (II) acetylacetonate (Fe(acac)2), Rhodamine B (RhB 98%), a selenium powder (Se average diameter 100 mesh), octadecene (ODE, 98%), oleic acid (OA, 98%), and oleylamine (OLA, technical grade 98%) (Sigma-Aldrich) were prepared (O2 was removed by vacuum suction at room temperature for 2 h before addition of OA and OLA to the solution).
Apparatuses:
High-resolution transmission electron microscopy (HR-TEM) images and energy dispersive spectra (EDS) were recorded with a JEOL JEM-3010 microscope (Boston, USA) operating at an accelerating voltage of 200 kV.
TEM samples were prepared by drop casting a dilute solution of a nanostructure in hexane or toluene on an ultrafine carbon-coated copper grid. Three-dimensional transmission electron microscopy (3D-TEM) images were characterized through electronic states in a TEM field (EM912Ω, Carl zeiss, Germany) operating at 120 kV. All bright field (BF)-TEM images were obtained as zero-loss filtered images.
Scanning electron microscopy (SEM) images were obtained using a Hitachi S-4700 FE-SEM at a voltage of 20 KV.
XRD samples were collected using Cu—Kα radiation by drop casting or dip coating solutions of FeSex (x=1, 2) nanostructures on glass substrates.
Fluorescence microscopy images were obtained by depositing a drop of a colloidal solution on a glass substrate and covering the drop with a cover slip (Leica, Heidelberg, Germany).
Photoluminescence (PL) intensities were measured using a fluorescence spectrophotometer (Hitachi F-7000, Japan).
The absorbance values of nanocomposites were measured using a UV/Vis spectrophotometer (Scinco, 5310, Korea).
The fluorescence lifetimes (τ) of samples were measured using a light emitting diode (TM-200 LED strobe Lifetime spectrophotometer 3113, PTI Inc., USA) at an excitation wavelength of 380 nm.
Preparation of FeSex (x=1, 2) Nanocomposites:
All syntheses were performed using Schlenk line systems. A three-neck round bottom flask was placed in a heating mantle (EMA 1000/CEB1, Barnstead/Electrothermal, Britain), and then one neck of the flask was connected to a condenser and the other two necks were capped by septa. A mantle heating system was used for temperature control.
Synthesis of FeSex (x=1)
First, 0.04 mM Fe(acac)2, 2 mL of OA, and 7 mL of ODE were mixed together at room temperature and heated at 120° C. for 2 h to achieve complete dissolution. The solution turned deep red in color. The solution was called “Solution A”.
Next, 1 mL of ODE and a 0.04 mM fine Se powder were mixed in a separate flask and allowed to stand in a vacuum pump for 0.5 h while maintaining the temperature at 120° C. until the metal ions were completely dissolved. Unreacted reactants were removed by nitrogen gas purging to prevent unwanted oxidation in the flask. The solution was called “Solution B”. Solution B was heated at 330° C. for ≧1 h, cooled down, and maintained at 120° C. for ≧1 h. The solution gradually became colorless, revealing complete dissolution of the Se powder in the ODE.
Thereafter, a calculated amount of Solution B was rapidly injected into Solution A with vigorous stirring through a syringe. The mixture solution gradually turned back to colorless (see FIG. 2a). After 0.5 h, the mixture solution was cooled to 110° C., left standing under vacuum for 20 min, and heated at 330° C. for 2 h. Then, the heating mantle was removed, a reaction tube was connected to the flask to cool the mixture solution at room temperature, the final mixture solution was transferred to a test tube, and 5 mL of hexane and 15 mL of ethanol were added thereto. The mixture was centrifuged at 8500 rpm for 0.5 h. The clear supernatant was discarded and the precipitate was dispersed in hexane. FeSe as the final product was purified several times and dispersed in hexane.
Synthesis of FeSex (x=2)
FeSe2 was synthesized in the same manner as in the synthesis of FeSex (x=1), except that 2 mL of Solution B was further added to prepare a mixture solution.
Results
The procedure for the synthesis of FeSex is schematically shown in FIG. 1. Results of the FeSex (x=1, 2) nanocomposites will be explained with reference to the accompanying drawings.
FIG. 2 shows images of the synthesized nanocomposites. Dropwise injection of Solution A into Solution B resulted in a color change of the mixture solutions to yellowish or yellowish brown during heating ((a) of FIG. 2), demonstrating the preparation of the final FeSex (x=1, 2). The FeSex (x=1, 2) nanocomposites showed photoluminescent properties when irradiated with ultraviolet light at a 365 nm wavelength ((b) of FIG. 2).
FIG. 3 shows the results of TEM-EDX for the synthesized FeSe and FeSe2 (top) and XRD patterns of the FeSe and FeSe2 (bottom). Particularly, the TEM-EDX analysis revealed that the element ratios of Fe:Se in the nanocomposites were 1:1.05 (for FeSe) and 1:2.17 (FeSe2). These results indicate that the compositions of the FeSex (x=1, 2) nanocomposites were determined by the molar ratio of the precursor components. All diffraction peaks in the XRD patterns of FeSe (JCPDS 03-0533) and FeSe2 (JCPDS21-0432) were well associated with the reference crystal structures, and the diffraction peaks of the FeSe2 nanoparticles were stronger and more discernible than those of the FeSe nanoparticles, which was attributed to the crystallinities of the nanocomposites. Since the crystallinity is greatly improved with increasing amount of the Se precursor, the amount of the Se precursor is believed to determine the crystallinity as well as the kind of the final product.
FIGS. 4 and 5 show the size distributions of the synthesized FeSe and FeSe2, respectively. As can be seen from FIGS. 4 and 5, the average size of the FeSe nanocomposite was slightly larger than that of the FeSe2 nanocomposites. The diameters of the FeSe and FeSe2 nanocomposites were 36±4.1 nm and 32±5.3 nm, respectively. Generally, the crystallinity of a nanomaterial is determined from the phase change of a precursor or heating conditions (e.g., heating temperature and time) during reaction. From the above results, it is believed that this size difference was due to the amount of the Se element and the phase separation mechanism during synthesis because the two nanocomposites had uniform size distributions and were synthesized under the same nucleation and crystal growth conditions. Further, since the heating was controlled under the exactly same conditions, the amount of the Se precursor used is considered an important factor that improves the crystallinity.
FIG. 6 shows the results of transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HR-TEM) for the synthesized FeSe and FeSe2 nanocomposites. FIG. 7 shows three-dimensional transmission electron microscopy (3D-TEM) images of the FeSe and FeSe2 nanocomposites, confirming that the nanocomposites had hexagonal phase crystal structures. The white lines shown in c and d of FIG. 6 represent the spacing between the adjacent lattices during crystal growth of the nanoparticles, showing that the inner planes correspond to the FeSe (002) and FeSe2 (011) distances.
FIGS. 8 to 11 show a series of TEM images of the nanocomposites grown in the individual synthesis steps. The final products correspond to hexagonal NiAs-type phase crystal structures. The slight difference in structure between the FeSe and FeSe2 nanocomposites appears to be associated with the element used in different amounts.
FIGS. 12 and 13 show the results of thermogravimetric analysis (TGA) for FeSe and FeSe2 in nitrogen gas. Thermogravimetric analysis was performed by heating from room temperature to 600° C. (FeSe) and 800° C. (FeSe2) at a rate of 15° C./min. As a result, the decomposition onset temperatures of the FeSe2 and FeSe2 nanocomposites were approximately 155° C. and 247° C., respectively, and the decomposition completion temperatures of the FeSe2 and FeSe2 nanocomposites were approximately 605° C. and 807° C., respectively. The nanocomposites underwent weight losses of about 80% after final decomposition. When comparing such decomposition processes, the FeSe2 nanocomposite was more stable than the FeSe nanocomposite.
FIGS. 14 to 17 show the optical and electrical properties of the FeSe and FeSe2 nanocomposites. Referring to FIG. 14, the absorption bands of the nanocomposites were observed at wavelengths of about 300 nm, and strong photoluminescence (PL) bands of the FeSe and FeSe2 nanocomposites in n-hexane solutions at room temperature were measured at wavelengths of 447 nm and 462 nm, respectively. FIG. 15 shows fluorescence images of solutions of the nanoparticles in bottles. Referring to FIG. 15, the FeSex nanocomposites showed strong luminescence irrespective of their kind. Referring to FIG. 16, the FeSex nanocomposites had full widths at half maximum of 100 nm, which is a typical wavelength contributing to the size distribution of the nanocomposites. The bandwidths of the nanocomposites were relatively narrow. When Rhodamine B was used as a standard, the quantum yields of the nanocomposites were approximately 20% (FeSe) and 16% (FeSe2). The fluorescence lifetimes (τ) of the nanocomposites were 3.07±0.023 ns (FeSe) and 2.54±0.018 ns (FeSe2). Based on the optical observation of the nanoparticles, the bandwidths of the nanoparticles were estimated from the relationship between (αhν)1/2 and hν, where α is the absorbance, h is the Planck's constant, and u is the frequency (FIG. 17). As a result, the energy bandwidths of the FeSex nanocomposites were found to be in the range of 2.25-2.51 eV, which were slightly broader than that of the reference. Such results are believed to arise from the nanosize effects and crystal structures of the nanocomposites.
FIGS. 18 and 19 show the PL properties of the FeSe and FeSe2 nanoparticles during growth. The PL spectra of the nanoparticles showed gradual red shifts with increasing reaction time, implying the growth of the nanoparticles.
As described above, the FeSex (x=1, 2) nanocomposites had strong fluorescence peaks at wavelengths of 447 nm and 462 nm, respectively, and diameters of about 30 nm that correspond to the bandwidths of 2.25-2.51 eV, demonstrating new crystal characteristics of the nanocomposites. In conclusion, the FeSex nanocomposites exhibited excellent photoluminescent properties even without heavy metals (Cd, Pb). Therefore, the FeSex (x=1, 2) nanocomposites can be considered environmentally friendly alternatives to conventional photoluminescent materials.
INDUSTRIAL APPLICABILITY
The nanocomposite of the present invention exhibits high photoluminescence even without heavy metals. Therefore, the nanocomposite of the present invention can be considered an environmentally friendly alternative to conventional photoluminescent materials. In addition, the nanocomposite of the present invention appears to be useful for better band gap engineering of semiconductor nanoparticles. Furthermore, the synthetic approach is expected to offer tremendous potential to control the synthesis of other metal chalcogenides.

## Claims

1. A method for preparing an iron chalcogenide nanocomposite, comprising
(a) dissolving a Fe precursor in an organic solvent to form a Fe solution,
(b) dissolving a chalcogen powder or a chalcogen precursor in an organic solvent to form a chalcogen solution,
(c) dropwise injecting the Fe solution into the chalcogen solution to prepare a mixture solution in which an iron chalcogenide is formed, and
(d) purifying the iron chalcogenide from the mixture solution.

2. The method according to claim 1, wherein, in step (a) or (b), the organic solvent is heated to 100 to 140° C.

3. The method according to claim 1, wherein the organic solvent used in step (a) or (b) is selected from the group consisting of ether-based compounds (CnOCn, Cn: hydrocarbon, 4≦n≦30), hydrocarbons (CnH2n+2, 7≦n≦30), unsaturated hydrocarbons (CnH2n, 7≦n≦30), and organic acids (CnCOOH, Cn: hydrocarbon, 5≦n≦30).

4. The method according to claim 3, wherein the ether-based compounds are selected from the group consisting of trioctylphosphine oxide (TOPO), alkylphosphines, octyl ether, benzyl ether, and phenyl ether.

5. The method according to claim 3, wherein the hydrocarbons are selected from the group consisting of hexadecane, heptadecane, and octadecane.

6. The method according to claim 3, wherein the unsaturated hydrocarbons are selected from the group consisting of octene, heptadecene, and octadecene.

7. The method according to claim 3, wherein the organic acids are selected from the group consisting of oleic acid, lauric acid, stearic acid, mysteric acid, and hexadecanoic acid.

8. The method according to claim 3, wherein, in step (c), the Fe solution is injected dropwise into the chalcogen solution, followed by heating to 250 to 400° C. to prepare a mixture solution in which an iron chalcogenide is formed.

9. The method according to claim 3, wherein, in step (c), a surfactant is added to and mixed with the mixed solution.

10. The method according to claim 9, wherein the surfactant is an organic acid selected from the group consisting of oleic acid, lauric acid, stearic acid, mysteric acid, and hexadecanoic acid or is a mixture of these organic acids.

11. The method according to claim 3, wherein the Fe precursor and the chalcogen precursor are used in a molar ratio of 1:1-2.

12. The method according to claim 3, wherein the chalcogen is S, Se or Te.

13. The method according to claim 12, wherein the iron chalcogenide is FeSe or FeSe2.