WO2010062127A2

WO2010062127A2 - Nanoparticle assembly-based switching device

Info

Publication number: WO2010062127A2
Application number: PCT/KR2009/007026
Authority: WO
Inventors: Tae Hee Kim; Jin Woo Cheon; Jung-Tak Jang
Original assignee: Ewha University-Industry Collaboration Foundation; Industry-Academic Cooperation Foundation, Yonsei University
Priority date: 2008-11-27
Filing date: 2009-11-27
Publication date: 2010-06-03
Also published as: WO2010062127A3; KR20100061405A; KR101210548B1

Abstract

The present invention relates to a switching device fabricated by using nanoparticles and a preparation method thereof. The present invention ensures the mass production of switching devices (e.g., memristor) by use of nanoparticles that exhibits reversible switching behavior at current of less than mA and at room temperature (25°C) 〧 250°C in a more convenient and economical manner. Predictions of the high potential of memristors based on nanoparticle assemblies are supported by the tremendous versatility to tune the electrical behavior of nanoparticles by controlling their nanoscale characteristics such as size, composition, dimension, surface area, and chemical potential.

Description

NANOPARTICLE ASSEMBLY-BASED SWITCHING DEVICE

BACKGROUND OF THE INVENTION FIELD OF THE INVENTION

The present invention relates to a switching device comprising a nanoparticle assembly fabricated by using nanoparticles and a preparation method thereof.

DESCRIPTION OF THE RELATED ART The recent reports on the use of memristors, a contraction of "memory resistors", have opened up new possibilities in the development of computer systems that would have non-volatile random access memory (1. Chua, L. O. IEEE Trans. Circuit Theory 1971, 18, 507; Strukov, D. B., Snider, G. S., Stewart, D. R., Williams, R. S. Nature 2008, 453, 80; Yang, J. J., Pickett, M. D., Li, X., Ohlberg, D. A. A., Stewart, D. R., Williams, R. S. Nat. Nanotechnol. 2008, 3, 429). The reason that the memristor differs from other circuit elements is that it carries a memory of its past. Based on this unique characteristic of memristors, one would expect this to lead to the enhancement of emerging memory technologies that would minimize the time for boot-up processes and, consequently, energy consumption. Williams et a/, were the first to demonstrate the realization of a memristor composed of a thin (5 nm) titanium dioxide film between Pt electrodes(2). The device uses neither magnetic flux, as the theoretical memristor model suggests, nor does it store charges, like a capacitor does. Instead the new memristor was found to achieve a resistance that is dependent on a time-varying current by using a chemical mechanism. When an electric field is applied, a charge-carrier drift arises on a nanometer scale caused by changing the boundary between the high-resistance layer of pure titanium dioxide and the low-resistance layer of titanium oxide via positively charged oxygen holes. The on and off states can be maintained much longer if the voltage does not exceed a specific threshold. The titanium dioxide based system, discovered by Williams et ai, serves as a genera model for memristive electrical switching in nanoscale thin film oxide devices. Nevertheless, the underlying physical details of memristors are still being debated.

Moreover, several issues, including the design strategies and requirements of new nanodevices based on the memristic concept, have not been fully answered.

Another important question in this area is whether memristic systems are limited to thin film devices or if they can be comprised of different kinds of nanostructured systems including nanoparticles.

Throughout this application, various publications and patents are referred and citations are provided in parentheses. The disclosures of these publications and patents in their entities are hereby incorporated by references into this application in order to fully describe this invention and the state of the art to which this invention pertains.

SUMMARY OF THE INVENTION

The present inventors have made intensive researches to develop a switching device comprising a nanoparticle assembly with reversible switching effects in a reproducible manner.

Accordingly, it is an object of this invention to provide a switching device. It is another object of this invention to provide a method for preparing a switching device.

Other objects and advantages of the present invention will become apparent from the following detailed description together with the appended claims and drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

Fig. 1 schematically represents a four probe configuration for measuring current induced switching effects of nanoparticle assemblies. Hg. 2 represents procedures for preparing the switching device of the present invention.

Hg. 3 shows TEM (Transmission Electron Microscopy) images of nanoparticles for switching devices prepared by pyrolysis. a-d) 7 nm, 9 nm, 12 nm and 15 nm MnFe₂O₄ nanoparticles; e-h) 7 nm, 9 nm, 12 nm and 15 nm Fe₃O₄ nanoparticles; i-l)

7 nm, 9 nm, 12 nm and 15 nm CoFe₂O₄ nanoparticles; m-p) 7 nm, 9 nm, 12 nm and

15 nm NiFe₂O₄ nanoparticles.

Hg. 4 shows a High-resolution (HR) TEM image and X-ray diffraction (XRD) analysis of 12 nm Fe₃O₄ nanoparticles. a) HR-TEM image; and b) XRD patterns of 12 nm Fe₃O₄ nanoparticles. The HR-TEM image shows that Fe₃O₄ nanoparticles have single crystallinity. Fe₃O₄ nanoparticles (black line) have well matched XRD patterns to bulk Fe₃O₄ (red line).

Hg. 5 shows TEM images of Zn_xM_1-xFe₂0₄ (M = Mn²⁺, Fe²⁺, Co²⁺, Ni²⁺) nanoparticles for switching devices, a) Zn_xMni._xFe₂0₄ (x = 0, 0.1, 0.2, 0.3, 0.4, 0.8); b) Zn_xFe_2-x0₄ (x = 0, 0.1, 0.2, 0.3, 0.4, 0.8); c) Zn_{0 3}Co_{0 7}Fe₂O₄; and d) Zn₀₃Ni_0-7Fe₂O₄.

Figs. 6a-6b is TEM images of nanoparticles having heterostructures (core- shell) for switching devices, a) CoFe₂O₄(S⁾Fe₃O₄, b) CoFe₂O₄@ N JFe₂O₄, c) CoFe₂O₄@MnFe₂O₄, d) CoFe₂O₄@Zn₀.₄Mn₀.₆Fe₂O₄, e) CoFe₂0₄@Zn_o.₄Fe₂.₆0₄, f) MnFe₂O₄(S⁾CoFe₂O₄, g) MnFe₂O₄(S⁾Fe₃O₄, h) Fe₃O₄(S⁾NiFe₂O₄, i) MnFe₂O₄@Zn₀.₄Mn₀.₆Fe₂O₄, j) MnFe₂O₄@Zn₀.₄Fe₂.₅O₄, k) Zn_o.₄Mn_o.₆Fe₂0₄@CoFe₂0_4, I) Zno.₄Fe₂.₆0₄(S⁾Fe₃0₄₍ and m) Fe₃O₄@Zn₀.₄Fe₂.₆O₄.

Hg. 7 shows TEM images of nanoparticles having one-dimensional structure for switching devices, a) TiO₂, b) Wi₈O₄₉, and c) Mn₃O₄. Rg. 8 represents Infrared Radiation (IR) spectroscopy results of nanoparticles for switching devices that were surface-trimmed in tetramethylammonium hydroxide (TMAOH) solution (red peaks, before surface trimming; black peaks, after surface trimming). After surface trimming, the strength of -CH₂- stretching peaks at 2900 cm^"1 and -C=O stretching peaks at 1700-1500 cm^"1 was greatly decreased, demonstrating that the surface of nanoparticles was trimmed.

Fig. 9 represents a SEM (scanning electron microscopy) image (a) and a photograph (b) of the 7 nm Fe₃O₄ nanopartide assembly pellet formed by pressing. Fig. 10 represents results of the current induced switching (CIS) effect analysis of the Fe₃O₄ nanopartide assembly not surface-trimmed. The nanopartide assembly containing 15 nm or 12 nm Fe₃O₄ nanoparticles not surface-trimmed shows no the current induced hysteric (switching) behavior.

Fig. 11 shows V-I characteristics (CIS effect) measured at RT (295 K) for 7 nm Fe₃O₄ nanopartide assemblies. Transition from low (R_ON) to high (R_OFF) resistance with (R_OFF/R_ON = ca. 20) occurred by applying dc current-bias sweeping toward positive (0 A → +2OxIO^"9 A), numbered 1-2, 6, and toward negative current

(+2OxIO^"9 A → -2OxIO^"9 A), numbered 3-5.

Fig. 12 shows V-I characteristics (CIS effect) measured for 12nm Fe₃O₄ nanopartide assemblies. The current induced hysteric behavior of 12 nm Fe₃O₄ nanopartide assemblies in the form of pellets appears only at lower temperatures. At

210 K, the switching transition of 12 nm Fe₃O₄ nanopartide assemblies appears at a current of ±7^χ lO^"9 A. In contrast, at RT (295 K) this material shows a typical tunneling conductance behavior. Fig. 13 shows V-I characteristics (CIS effect) measured for 9 nm and 15 nm

Fe₃O₄ nanopartide assemblies, respectively. The nanopartide assemblies with 9 nm

Fe₃O₄ nanoparticles represented the switching effect at room temperature and those with 15 nm Fe₃O₄ nanoparticles at 200 K.

Fig. 14 represents temperature-dependent resistivity of 15 nm, 12 nm and 9 nm Fe₃O₄ nanopartide assemblies. It is observed that p increases with decreasing T

1/ /^*r and log p is approximately proportional to ^v . According to the results, resistive switching appears in the shaded region where p > 50 M Ω *cm. Due to the measurement limitation (Keithley 2182 nanovoltmeter), the resistivity of the 7 nm Fe₃O₄ nanopartide assemblies was not obtainable. It is likely that the increased resistivity of smaller particles is a result of "nano-size" effects associated with increased surface to volume ratios.

Rg. 15 represents V-I characteristics (CIS effect) measured for 12 nm MnFe₂O₄, 7 nm CoFe₂O₄ and 7 nm NiFe₂O₄ nanopartide assemblies. The nanopartide assemblies containing 12 nm MnFe₂O₄, 7 nm CoFe₂O₄ and 7 nm NiFe₂O₄ exhibited current induced switching effects at 220 K, 265 K and 270 K, respectively.

Fig. 16 represents V-I characteristics (CIS effect) measured for heterostructured (core-shell) napopartide assemblies. The heterostructured (core- shell structure) 12 nm CoFe₂O₄(S⁾Fe₃O₄, 12 nm CoFe₂O₄@ Mn Fe₂O₄ and 12 nm CoFe₂O₄(S⁾NiFe₂O₄ were analyzed to show current induced switching effects at 240 K, 235 K and 175 K, respectively.

Fig. 17 represents V-I characteristics (CIS effect) measured for Fe₃O₄ nanopartide assemblies with applying external magnetic field (7 kG). It could be appreciated on the basis of these results that controlling the strength and direction of external magnetic field may permit to control resistivity and current at which a switching behavior occurs.

Fig. 18 represents simulation of a memristive device which has both time- dependent resistance (/?) and capacitance (Q. (a) The doped region ( IΨ) / grain boundary (Z.) as a function of time, (b) Current (red line) and voltage (blue line) as a function of time. The shaded areas indicate the low resistance state (R_0N) of the device when the charge saturation is established, labeled 1 and 4. The voltage remains almost constant while the current varies in a sinusoidal manner during that process. On the other hand, the other areas (labeled 3 and 6) are related to the high resistance state (R_OFF) during the refresh charge process, (c) V-I hysteresis obtained from the simulation of equation 5. Numbers of 1-6 are corresponding each other through out figures 18a, 18b, and 18c. The applied current is

All the axes are dimensionless with current, voltage, and time expressed in units of i₀ = 12Ox IO^"9 A, V₀ = 1 V, and t₀ = 2 n/ω₀ = 0.01 s. Other parameters are; R_0N = 10⁷ 2 , R_OFF/RON

ε_Λ = 50, A = 10^"7 cm² where ε₀ is the permittivity of vacuum. In c, a slightly asymmetric shape of hysteresis is caused by non-zero initial charge accumulation Δ #<= -0.1/O/ω₀). The hysteresis loop is perfectly symmetric when q₀ is zero.

DETAILED DESCRIPTION OF THIS INVETNION

In one aspect of this invention, there is provided a switching device, which comprises a nanoparticle assembly containing a plurality of nanoparticles. The term used herein "switching device" means a device exhibiting a switching effect (behavior) by applying a current, voltage or magnetic field.

The switching device has a R_0N (resistance at ON state) or R_0FF (a resistance at OFF state) state at certain current or voltage value. Preferably, the present switching device exhibits a reversible switching behavior by applying current of less than 1 A, more preferably less than 1 mA, still more preferably less than 100 nA. Specifically, the present switching device exhibits a reversible switching behavior by applying current of 1 A-IOO nA.

According to a preferred embodiment, the present switching device has a R_OFF/R_ON value of more than 1, more preferably more than 5 and still more preferably more than 10, most preferably more than 20. Specifically, the present switching device has a R_OFF/R_ON value of 5-30. In considering time-dependent changes, the R_OFF/R_ON value of the present switching device is much more meaningful, as indicated in Further Discussion.

The reversible switching behavior of the present switching device may be shown at a wide range of temperature. According to the present invention, the temperature at which the reversible switching behavior is exhibited may be controlled by controlling either the size or the composition of nanoparticles embodied in the switching device.

According to a preferred embodiment, the reversible switching behavior of the switching device is exhibited at room temperature (25 ⁰C) ± 250 ⁰C, more preferably at room temperature ± 100 ⁰C, still more preferably at room temperature ± 50 ⁰C, most preferably at room temperature. To the best of our knowledge, this is the first room temperature observation of reversible switching behavior in a nanoparticle system.

The nanoparticle assembly is constructed by aggregation or arrangement of nanoparticles. In the nanoparticle assembly, the nanoparticles are not needed to be arranged with a specific distance (space). The nanoparticles are required to be arranged with a space as close as for allowing current flow. Preferably, the nanoparticle assembly has a plurality of nanoparticles that are arranged with a space of no more than 10 nm, more preferably no more than 5 nm, most preferably no more than 2 nm.

The nanoparticles useful in the present switching device are chalcogen-type compounds, pnicogen-type compounds, carbon Group-type compounds, boron Group-type compounds, metals, alloys or multi-component hybrid structured nanoparticles thereof.

Preferably, the chalcogen-type compound is M^a _xA_z, M^a _xM^b _yA_z (M^a is one or more elements selected from the group consisting of Group 1 elements, Group 2 elements, Group 13 elements, Group 14 elements, transition metal elements, Lanthanide Group elements and Actinide Group elements; M^b is one or more elements selected from the group consisting of Group 1 elements, Group 2 elements, Group 13-15 elements, Group 17 elements, transition metal elements, Lanthanide Group elements and Actinide Group elements; A is one or more elements selected from the group consisting of O, S, Se, Te and Po; 0<x<16, 0<y<16, 0<z<8), or a multi-component hybrid structure thereof.

More preferably, the chalcogen-type compound is M^a _xA_z, M^a _xM^b _yA_z (M^a is one or more elements selected from the group consisting of transition metal elements, Lanthanide Group elements and Actinide Group elements; M^b is one or more elements selected from the group consisting of Group 1 elements, Group 2 elements, Group 13-15 elements, Group 17 elements, transition metal elements, Lanthanide Group elements and Actinide Group elements; A is one or more elements selected from the group consisting of O, S, Se, Te and Po; 0<x<16, 0<y<16, 0<z<8), or a multi-component hybrid structure thereof.

Preferably, the pnicogen-type compound is M^C _XA₂, M^c _xM^d _yA_z (M^c is one or more elements selected from the group consisting of Group 1 elements, Group 2 elements, Group 13 elements, Group 14 elements, transition metal elements, Lanthanide Group elements and Actinide Group elements; M^d is one or more elements selected from the group consisting of Group 1 elements, Group 2 elements, Group 13-14 elements, Group 16-17 elements, transition metal elements, Lanthanide Group elements and Actinide Group elements; A is one or more elements selected from the group consisting of N, P, As, Sb and Bi; 0<x<24, 0<y<24, 0<z<8), or a multi-component hybrid structure thereof. More preferably, the pnicogen-type compound is M^C _XA₂, M^c _xM^d _yA_z (M^c is one or more elements selected from the group consisting of transition metal elements, Lanthanide Group elements and Actinide Group elements; M^d is one or more elements selected from the group consisting of Group 1 elements, Group 2 elements, Group 13-14 elements, Group 16-17 elements, transition metal elements, Lanthanide Group elements and Actinide Group elements; A is one or more elements selected from the group consisting of N, P, As, Sb and Bi; 0<x<24, 0<y<24, 0<z<8), or a multi-component hybrid structure thereof.

Preferably, the carbon Group-type compound is M^e _xA_z, M^e _xM^f _yA_z (M^e is one or more elements selected from the group consisting of Group 1 elements, Group 2 elements, Group 13 elements, Group 14 elements, transition metal elements, Lanthanide Group elements and Actinide Group elements; M^f is one or more elements selected from the group consisting of Group 1 elements, Group 2 elements, Group 13 elements, Group 15-17 elements, transition metal elements, Lanthanide Group elements and Actinide Group elements; A is one or more elements selected from the group consisting of C, Si, Ge, Sn and Pb; 0<x<32, 0<y<32, 0<z<8), or a multi-component hybrid structure thereof.

Preferably, the boron Group-type compound is M⁹ _XA_Z, M⁹ _xM^h _yA_z (M³ is one or more elements selected from the group consisting of Group 1 elements, Group 2 elements, Group 13 elements, Group 14 elements, transition metal elements, Lanthanide Group elements and Actinide Group elements; M^h is one or more elements selected from the group consisting of Group 1 elements, Group 2 elements, Group 14-17 elements, transition metal elements, Lanthanide Group elements and Actinide Group elements; A is one or more elements selected from the group consisting of B, Al, Ga, In and Tl; 0<x<40, 0<y<40, 0<z<8), or a multi-component hybrid structure thereof.

According to a preferred embodiment, the metal for nanoparticles is alkali metal, alkaline earth metal, transition metal, Lanthanide Group metal and Actinide Group metal or a multi-component hybrid structure thereof. More preferably, the metal is transition metal (Ti, V, Cr, Mn, Fe, Co, Ni, Cu, or Ru), Lanthanide Group metal (Ce, Pr, Nd, Pm, Sm, Gd, Eu, Tb, Dy, Ho, Er, Tm, Yb, or Lu), Actinide Group metal (Th, Pa, U, Np, Pu, Am, Dm, Bk, Cf, Es, Fm, Md, No or Lr) or a multi- component hybrid structure thereof. According to a preferred embodiment, the alloy for nanoparticles is M^e _xM^f _y,

M^e _xM^fyM⁹ _z (M^e is one or more elements selected from the group consisting of transition metal elements, Lanthanide Group elements and Actinide Group elements; M^f and M⁹ are one or more elements selected from the group consisting of Group 1 metal elements, Group 2 metal elements, Group 13-17 elements, transition metal elements, Lanthanide Group elements and Actinide Group elements; 0<x<20, 0<y<20, 0<z<20), or a multi-component hybrid structure thereof.

More preferably, the alloy for nanoparticles is M^e _xM^f _y, M^e _xM^f _yM⁹ _z (M^e is one or more elements selected from the group consisting of transition metal elements (Ba, Cr, Mn, Fe, Co, Ni, Cu, Zn, Nb, Mo, Zr, Te, W, Pd, Ag, Pt and Au), Lanthanide Group elements (Ce, Pr, Nd, Pm, Sm, Gd, Eu, Tb, Dy, Ho, Er, Tm, Yb, and Lu) and Actinide Group elements (Th, Pa, U, Np, Pu, Am, Dm, Bk, Cf, Es, Fm, Md, No, and Lr); M^f and M⁹ are one or more elements selected from the group consisting of Group 1 metal elements, Group 2 metal elements, Group 13-17 elements, transition metal elements, Lanthanide Group elements and Actinide Group elements; 0<x<20, 0<y<20, 0<z<20), or a multi-component hybrid structure thereof.

More preferably, the nanoparticles for the switching device are M^a _xO_z, M^a _xM^b _yO_z (M^a is one or more elements selected from the group consisting of transition metal elements, Lanthanide Group elements and Actinide Group elements; M^b is one or more elements selected from the group consisting of Group 1 elements, Group 2 elements, Group 13-15 elements, Group 17 elements, transition metal elements, Lanthanide Group elements and Actinide Group elements; 0<x<16, 0<y<16, 0<z<8), or a multi-component hybrid structure thereof; most preferably M_xFe_yO_z (M is one or more transition metal elements selected from the group consisting of Zn, Mn, Fe, Co and Ni; 0<x<8, 0<y<8, 0<z<8), Zn_wM_xFe_yO_z (M is one or more transition metal elements selected from the group consisting of Zn, Mn, Fe, Co and Ni; 0<w<8, 0<x<8, 0<y<8, 0<z<8) , or a multi-component hybrid structure thereof.

The multi-component hybrid structure for the switching device comprises at least two materials selected from the group consisting of chalcogen-type compounds, pnicogen-type compounds, carbon Group-type compounds, boron Group-type compounds, metals and alloys. Alternatively, the multi-component hybrid structure comprises at least one material selected from the group consisting of chalcogen-type compounds, pnicogen-type compounds, carbon Group-type compounds, boron Group-type compounds, metals and alloys, and other materials. The form of the multi-component hybrid structure may be core-shell, core-multi shell, heterodimer, trimer, multimer, bar code or co-axial rod, but not limited to. Preferably, the multi- component hybrid structure comprises at least one of chalcogen-type compounds or pnicogen-type compounds.

According to a preferred embodiment, the nanoparticle for the switching device is at least one of metals having an oxidation number of no less than 1.

According to a preferred embodiment, the nanoparticle is in a size of 1-1000 nm, more preferably 2-500 nm, still more preferably 5-50 nm.

The nanoparticle for the switching device may be in any form. Preferably, the nanoparticle has (i) a zero-dimensional structure selected from the group consisting of a sphere, a core-shell and a multi-core shell structure; (ii) a one-dimensional structure selected from the group consisting of a rod, a barcode, a core-shell coaxial rod and a multi-core shell coaxial rod structure; (iii) a two-dimensional structure selected from the group consisting of a sheet, a layer and a multi-component sheet structure; or (iv) a three-dimensional structure selected from the group consisting of a branched structure, a dendrite structure, a dumbbell and a multi-pod structure.

According to a preferred embodiment, the nanoparticle has a surface trimmed to remove organic materials attached thereon.

The switching device of the present invention may be used to a wide variety of applications, for example, DRAM (Dynamic Random Access Memory), EEPROM (Electrically Erasable Programmable Read-only Memory), SRAM (Static Random Access Memory), PRAM (Phase change Random Access Memory), RRAM (Resistance Random Access Memory), MRAM (Magnetoresistive Random Access Memory), FRAM(Ferroelectric Random Access Memory), CBRAM (Conductive Bridging Random Access Memory), memristor and spintronics devices. Most preferably, the switching device of the present invention is a memristor.

In another aspect of this invention, there is provided a method for preparing a switching device, which comprises the steps of: (a) preparing nanoparticles; (b) forming a nanoparticle assembly using the nanoparticles; and (c) connecting to the nanoparticle assembly a means for applying a current, voltage or magnetic field. Preferably, the step (a) for preparing nanoparticles is carried out in a gas phase or a liquid phase (e.g., aqueous solution, organic solvent and multi-solution system), more preferably organic solvent.

In the case of using organic solvent, a reaction mixture containing a metal precursor and a surfactant or a surfactant-containing solvent are prepared and subjected to pyrolysis at 50-600⁰C.

The metal precursor includes any metal precursor known to one of skill in the art, preferably a metal precursor having oxidation number of more than 0.

The metal precursor comprises at least one metal element selected from the group consisting of transition metal elements, Lanthanide Group elements, Actinide

Group elements and Group 13-14 elements, preferably a metal nitrate-based compound, a metal sulfate-based compound, a metal fluoroacetoacetate-based compound, a metal acetylacetonate, a metal halide-based compound (MX_a, where M is at least one selected from the group consisting of transition metal elements, Lanthanide Group elements, Actinide Group elements and Group 13-14 elements; X

= F, Cl, Br, or I, and 0<a<5), a metal perchlorate-based compound, a metal sulfamate-based compound, a metal carboxylate, a metal stea rate-based compound, an organometallic compound, or a multi-component hybrid structure thereof.

The organometallic compound is M_xL_y (M is at least one selected from the group consisting of transition metal elements, Lanthanide Group elements, Actinide Group elements and Group 13-14 elements; L is at least one ligand to coordinate with metals; 0<x<10, 0<y<120), or a multi-component hybrid structure thereof.

The surfactant useful in the synthesis of the nanoparticle is an organic acid, an organic amine, alkane thiol, phosphonic acid, trioctylphosphine oxide, tributyl phosphine, alkyl phosphate, alkyl sulfate or tetraalkylammonium halide. More preferably, the surfactant is oleic acid, lauric acid, stearic acid, mysteric acid, hexadecanoic acid, oleyl amine, lauryl amine, trioctyl amine, dioctyl amine, hexadecyl amine, dodecane thiol, hexadecane thiol, heptadecane thiol, tetradecyl phosphonic acid, octadecyl phosphonic acid or trioctylphosphine oxide.

The solvent useful in the synthesis of the nanoparticle is an ether-based compound, hydrocarbon, organic acid, organic amine, alkane thiol, phosphonic acid, alkyl phosphine oxide, tributyl phosphine, alkyl sulfate, alkyl phosphate or tetraalkyl ammonium halide. More preferably, the solvent is octyl ether, benzyl ether, phenyl ether, hexadecane, heptadecane, octadecane, oleic acid, lauric acid, stearic acid, mysteric acid, hexadecanoic acid, oleyl amine, trioctyl amine, dioctyl amine, hexadecyl amine, dodecane thiol hexadecane thiol or heptadecane thiol.

According to the present invention, it is possible that the size of the nanoparticles is controlled by adjusting a concentration of the surfactant, an amount of the solvent, a reaction temperature or a reaction time. The surfactant and the solvent are introduced into the reaction mixture in the amount 1-100 fold higher than the metal precursor.

According to a preferred embodiment, the step (a) is performed with no use of oxidants or reductants. The nanoparticles synthesized may be applicable to various fields such as magnetic resonance imaging agents and data storage (iron oxide nanoparticles), photocatalyst and sensor (titanium oxide nanoparticles), photocatalyst and desulfurization sorbents (tungsten oxide nanoparticles) and ceramic condenser electrode, chemical catalyst and soft magnet (manganese oxide nanoparticles).

The nanoparticles synthesized in step (a) may be further surface-treated to improve their switching effects. The surface treatment includes a removal of organic ligands (e.g., surfactants) on the surface of nanoparticles (i.e., trimming the surface of nanoparticles) or an additional coating. The preferable surface treatment is to treat with an alkali solution to remove surfactants on the surface of nanoparticles. Preferably, the alkali solution for the surface treatment includes a alkali compound selected from the group consisting of alkylammonium, alkylammonium hydroxide, alkylammonium halide, alkylphosphine, alkylphosphine hydroxide and alkylphosphine halide [wherein alkyl is C_nH_2n+I (0<n<5)], more preferably alkylammonium and alkylammonium hydroxide, still more preferably tetramethylammonium hydroxide and tetraethylammonium hydroxide.

The alkali compound may be used in a polar solvent such as alcohols, dimethyl sulfoxide, dimethyl formamide and water. The most preferable polar solvent is alcohols. The alkali compound may be dissolved in the polar solvent with a concentration of 0.001-10 M, preferably 0.1-5 M.

According to a preferred embodiment, the method further comprises the step of (aθ trimming a surface of nanoparticles to remove organic materials attached thereon, such that a switching effect of the switching device is enhanced. More preferably, the trimming is carried out in the presence of an alkali solution. Most preferably, the trimming is carried out by sonication.

According to the present method, the formation of the nanoparticle assembly in the step (b) is carried out by pressing, LB (Langmuir Blodgett), LBL (layer by layer), print, self-assembly or solution evaporation. Preferably, the nanoparticles are assembled under pressure not to induce deformation of the nanoparticles. More preferably, the formation of the nanoparticle assembly is carried out by pressing under a pressure of more than 100 Pa, still more preferably 140-180 Pa. The period of time for pressing is not specifically limited; preferably more than 1 min, more preferably more than 5 min.

The nanoparticle assembly is connected to a means for applying a current, voltage or magnetic field. It is noteworthy that the switching device requires no electrodes. Examples of means for applying a current and/or voltage include a power supply known to one of skill in the art. Examples of means for applying magnetic field include electromagnetic devices known to one of skill in the art.

The method of the switching device provides a large number of passages for electric flow by increasing the contact surface area between nanoparticles with no influence on electrical characteristics of nanoparticle surface via nanoparticle assembly (e.g., peptization). In this regard, the switching device prepared by the present invention shows a dramatically enhanced switching behavior (specifically, reversible switching behavior).

Unlike the conventional switching devices operable at lower temperature, the present invention ensures the mass production of switching devices (e.g., memristor) by use of nanoparticles that exhibits reversible switching behavior at current of less than mA and at room temperature±250°C in a more convenient and economical manner. The present invention enables realization of memories with no requirement of electricity and computer booting. Predictions of the high potential of memristors based on nanoparticle assemblies are supported by the tremendous versatility to tune the electrical behavior of nanoparticles by controlling their nanoscale characteristics such as size, composition, dimension, surface area, and chemical potential, as demonstrated in Examples. The present invention will now be described in further detail by examples. It would be obvious to those skilled in the art that these examples are intended to be more concretely illustrative and the scope of the present invention as set forth in the appended claims is not limited to or by the examples.

EXAMPLES

EXAMPLE 1: Size and composition controlled preparation of metal ferrite (MFe₂O₄, M = Mn²⁺, Fe²⁺, Co²⁺, Ni²⁺) nanoparticles for switching devices

MFe₂O₄ (M = Mn²⁺, Fe²⁺, Co²⁺, Ni²⁺) napoparticles were obtained according to previously suggested methods described in Korean Pat. No. 0604975 and PCT/KR2004/003088 filed by the present inventors. MCI₂ (M = Mn²⁺, Fe²⁺, Co²⁺, Ni²⁺; Aldrich, USA) and Fe(acac)₃ (iron tris-2,4-pentadionate, Aldrich, USA) as precursors of nanoparticles were added to 4 mmol oleic acid (Aldrich, USA) and 4 mmol oleylamine (Aldrich, USA) as capping molecules in trioctylamine (Aldrich, USA). The mixture was incubated at 200 ⁰C and in turn at 300 ⁰C under an argon gas atmosphere, and was then cooled to room temperature. Upon the addition of excess ethanol, the resulting nanoparticle precipitate was resuspended in toluene, yielding a colloidal solution of MFe₂O₄ (M = Mn²⁺, Fe²⁺, Co²⁺, Ni²⁺) napoparticles. The isolated nanoparticles were observed to have a sphere shape with a homogeneous size of 7 nm.

The size of nanoparticles was further controlled by employing varying growth conditions including the amount of reactants (e.g., oleic acid and oleylamine). For example, the size of Fe₃O₄ nanoparticles was tuned from 7-15 nm by varying the ratio of oleic acid and oleylamine. The characteristics of nanoparticles were analyzed by transmission electron microscopy (TEM), high-resolution transmission electron microscopy and x-ray diffraction. As shown in Figs. 3 and 4 representing results of transmission electron microscopy and x-ray diffraction, MFe₂O₄ (M = Mn²⁺, Fe²⁺, Co²⁺, Ni²⁺) napoparticles synthesized have well-controlled size monodispersity (σ < 5 %) and higher crystallinity.

EXAMPLE 2: Size and composition controlled preparation of Zn_xM_{1 x}Fe₂O₄ (M = Mn²⁺, Fe²⁺, Co²⁺, Ni²⁺) nanoparticles for switching devices

Zn²⁺ ion doped Zn_xM_1-XFe₂O₄ (M = Mn²⁺, Fe²⁺, Co²⁺, Ni²⁺) nanoparticles were obtained according to previously suggested methods described in Korean Pat. No.

0604975, PCT/KR2004/003088 and Korean Pat. Appln. No. 2006-0018921 filed by the present inventors. ZnCI₂, MCI₂ (M = Mn²⁺, Fe²⁺, Co²⁺, Ni²⁺) and Fe(acac)₃ as precursors of nanoparticles were added to 20 mmol oleic acid and 20 mmol oleylamine as capping molecules in trioctylamine. The mixture was incubated at 200 ⁰C and in turn at 300 ⁰C under an argon gas atmosphere, and was then cooled to room temperature. Upon the addition of excess ethanol, the resulting nanoparticle precipitate was resuspended in toluene, yielding a colloidal solution. The isolated nanoparticles were shown to have a composition of Zn_{0 4}Mc₆Fe₂O₄ (M = Mn²⁺, Fe²⁺, Co²⁺, Ni²⁺) with a homogeneous size of 15 nm. We also verified that the change in the relative mole ratios of materials of MCI₂ (M = Mn²⁺, Fe²⁺, Co²⁺, Ni²⁺) permits the composition of nanoparticles synthesized to be conveniently controlled. The nanoparticles synthesized were analyzed to have a sphere shape with well-controlled size monodispersity as represented in TEM images of Fig. 5.

EXAMPLE 3: Preparation of nanoparticles with heterostructure (core-shell structure) for switching devices

Metal oxides-containing nanoparticles with heterostructure (15 nm sized core- shell ferrite) were prepared by a seed-medicated growth method according to previously reported methods described in Korean Pat. No. 0604975 and PCT/KR2004/003088 filed by the present inventors.

The core materials with a size of 7 nm were synthesized as procedures described in .Example 1 and heterostructured core-shell nanomaterials with a size of 15 nm were then synthesized using them. The reaction mixture composed of the core materials, MCI₂ (M = Mn²⁺, Fe²⁺, Co²⁺, Ni²⁺), Fe(acac)₃, 4 mmol oleic acid and 4 mmol oleylamine in trioctylamine was incubated at 200 ⁰C and in turn at 300 ⁰C under an argon gas atmosphere. The nanomaterials synthesized have core-shell structure with a size of 15 nm. The isolation of nanomaterials was performed as Example 1.

A variety of nanomaterials with core-shell typed heterostructure were yielded by varying compositions of metal precursors. For example, CoFe₂O₄@ Fe₃O₄, CoFe₂O₄@MnFe₂O₄, CoFe₂O₄(SNiFe₂O, MnFe₂O₄@CoFe₂O₄, MnFe₂O₄@Fe₃O₄, Fe₃O₄@NiFe₂O₄, MnFe₂O₄@Zn₀.₄Mn₀.₆Fe₂O₄, MnFe₂O₄(O⁾Zn_{0 4}Fe_{2 6}O₄, Zn₀.₄Mn₀.₆Fe₂O₄@CoFe₂O₄, Zn₀.₄Fe₂.₆O₄@ Fe₃O₄ and Fe₃O₄@Zn₀.₄Fe₂.₆O₄ in a size of 15 nm were successfully synthesized. The nanomaterials synthesized have a sphere shape with a size monodispersity (σ < 10%) as shown in Fig. 6. EXAMPLE 4: Preparation of metal oxide nanoparticles for switching devices

Metal oxide nanoparticles were synthesized according to previously reported methods described in Korean Pat. No. 0604975 and PCT/KR2004/003088 filed by the present inventors. 0.5 mmole of titanium tetrachloride (Aldrich, USA) was mixed with 0.28 g of oleic acid and 1.7 g of oleylamine and allowed to undergo pyrolysis for 2 min at 290⁰C, finally yielding titanium oxide (TiO₂) nanoparticles. 0.1 mmole of tungsten tetrachloride (Aldrich, USA) was mixed with 1.63 g of oleic acid and 0.54 g of oleylamine and allowed to undergo pyrolysis for 1 hr at 350 ⁰C, giving tungsten oxide (Wi₈O₄₉) nanoparticles. 0.1 mmole of manganese chloride (Aldrich, USA) was mixed with 0.15 g of oleic acid and 1.94 g of oleylamine and allowed to undergo pyrolysis for 1 hr at 350 ⁰C, giving manganese oxide (Mn₃O₄) nanoparticles. The isolation of nanoparticles was carried out as Example 1. The TEM images of TiO₂, Wi₈O₄₉ and Mn₃O₄ were shown in Rg. 7.

EXAMPLE 5: Surface trimming of nanoparticles for switching devices

The surface of nanoparticles for switching devices prepared in Examples 1-4 was trimmed. Organic ligands on the nanoparticles were removed by sonication in 1 M tetramethylammonium hydroxide (TMAOH) in butanol. After 15 min sonication in TMAOH solution, the nanoparticles were isolated by centrifugation and washed sequentially with hexane, acetone, and ethanol. Isolated nanoparticles were dried under vacuum at room temperature before nanoparticle assembly pellet preparation. The surface-trimmed nanoparticles were analyzed by Infrared Radiation (IR) spectrophotometry. The nanoparticles not surface-trimmed were measured to show peaks corresponding to long alkyl chains of surfactants surrounding nanoparticles, e.g., -CH₂- stretching peaks at 2900 cm^"1 and -C=O stretching peaks at 1700-1500 cm^'1 (red peaks in Fig. 8). After surface trimming, the strength of -CH₂- stretching peaks at 2900 cm^"1 and -C=O stretching peaks at 1700-1500 cm^"1 was greatly decreased, demonstrating that the surface of nanoparticles was trimmed.

EXAMPLE 6: Nanoparticle assembly pellet preparation and measurement of current induced switching (CIS) effect The nanoparticle assemblies in the form of the compact pellets (0.5^χ l^χ4 mm) were produced using the surface-trimmed nanoparticles by cold-pressing in a die under 160 Pa for 15 min. In order to avoid alteration of the surface properties of the nanoparticles, no heat-treatment step was used in the preparation of the pellets. The shape of the nanoparticle assembly pellets was rectangular: 4 mm long, 1 mm wide and 0.5 mm thick. Their SEM (scanning electron microscopy) image and photographs are shown in Hg. 9. The current induced switching (CIS) effect of the nanoparticle assembly pellets was measured using the circuit shown in Fig. 1. The electrical characteristics were measured by the conventional four-probe configuration with a Keithley 2182 nanovoltmeter and a Keithley 6220 current source. An electrode (for current injection) was made at each end of the pellet (using indium contact) so that current flow was as uniform as possible. The voltage drop across the sample was observed using two other electrodes attached to the surface.

EXAMPLE 7: Current induced switching effect of nanoparticle assembly not surface-trimmed

The current induced switching (CIS) effect of the Fe₃O₄ nanoparticle assembly not surface-trimmed was measured at different temperatures. The Fe₃CX_? nanoparticles not surface-trimmed were revealed to show no switching effect in all temperature conditions and to show a typical tunneling conductance behavior. Fig. 10 represents that the nanoparticle assembly containing 15 nm or 12 nm Fe₃O₄ nanoparticles not surface-trimmed shows no the current induced hysteric (switching) behavior. EXAMPLE 8: Current induced switching effect of nanoparticle assembly surface-trimmed

The current induced switching (CIS) effect of nanoparticle assemblies fabricated with surface-trimmed Fe₃O₄ nanoparticles in a 7-nm size was analyzed. In the case of nanoparticles with a diameter (D) of 7 nm (Rg. 11), a new type of hysteresis is observed at room temperature (RT), as shown in Figure 11. The bistable V-I characteristics, observed in the sample of D = 7 nm, illustrate that the switching properties are directly related to the existence of hysteretic behavior as the current is swept in steps 1 to 6 (corresponding 0 → +I_max → 0 → -I_max → 0). As the current increases from 0 to +I_max, the switching between the low-resistance state (R_0N ~ 2^χlO⁷ Ω) and the high resistance state (R_0FF ~ 4^χlO⁸ Ω) occurs at I = +16^χlO^"9 A. In contrast, ohmic behavior is seen when the current decreases from +I_max to 0. The same behavior is observed when the current direction is reversed. This type of instantaneous switching from the low-resistance (i.e. on-state) to the high-resistance (i.e. off-state) occurs in a typical R_OFF/R_ON ratio of 20:1. It is remarkable that when sweeps are carried out repeatedly, no damage or breakdown of the sample occurs. To the best of our knowledge, this is the first room temperature observation of reversible switching behavior in a nanoparticle system.

It is interesting to observe that the V-I hysteresis behavior described above is dependent on the size of the nanoparticle. For example, the current induced hysteric behavior of nanoparticle assembly pellets of D = 12 nm appears only at lower temperatures (Fig. 12). As the data in Figure 12 show, at 210 K, the switching transition of the D = 12 nm assembly appears at a current of ±7^χlO^*9 A. In contrast, at RT (295 K) this material shows a typical tunneling conductance behavior (Fig. 12). Furthermore, Fe₃O₄ nanoparticle assemblies with a diameter (D) of 9 nm and

15 nm were also analyzed to show a similar switching effect (Fig. 13). The nanoparticle assemblies with 9 nm Fe₃O₄ nanoparticles represented the switching effect at room temperature and those with 15 nm Fe₃O₄ nanoparticles at 200 K. Based on the results of temperature-dependent resistivity (p(T)) measurements for the magnetite nanoparticles with different diameters, it appears that the switching behavior dominates when p value is higher than ca. 50 MΩ *cm (the shaded area of Hg. 14). An increase of p is observed with decreasing D. It is likely that the increased resistivity of smaller particles is a result of "nano-size" effects associated with increased surface to volume ratios (in the case of 7 nm Fe₃O₄ nanoparticles, resistivity is too high to be measured). In other words, by controlling the size of nanoparticles in nanoparticle assemblies, temperature and size for the switching effect of nanoparticle assemblies may be controlled.

EXAMPLE 9: Current induced switching effect of nanoparticle assemblies containing surface-trimmed different types of MFe₂O₄ (M = Mn²⁺, Fe²⁺, Co²⁺, Ni²⁺) or heterostructured (core-shell) nanoparticles

The nanoparticle assemblies containing surface-trimmed MFe₂O₄ (M = Mn²⁺, Fe²⁺, Co²⁺, Ni²⁺) nanoparticles were analyzed for current induced switching effect. As results, it was revealed that the nanoparticle assemblies containing 12 nm MnFe₂O₄, 7 nm CoFe₂O₄ and 7 nm NiFe₂O₄ exhibited current induced switching effects at 220 K, 265 K and 270 K, respectively (Fig. 15). The heterostructured (core-shell structure) 12 nm CoFe₂O₄(O⁾Fe₃O₄, 12 nm CoFe₂O₄(S⁾MnFe₂O₄ and 12 nm CoFe₂O₄(O⁾NiFe₂O₄ were analyzed to show current induced switching effects at 240 K, 235 K and 175 K, respectively (Fig. 16).

These results urge us to reason that not only nanoparticle systems with different compositions but also heterostructured nanoparticle systems may give current induced switching effects, and the control of nanoparticle compositions allows for controlling temperature and size for the switching effect of nanoparticle assemblies.

EXAMPLE 10: Current induced switching effect of surface-trimmed nanoparticle assemblies with applying external magnetic field

The current induced switching effect of the surface-trimmed Fe₃O₄ nanoparticles with a 7-nm size was analyzed with applying external magnetic field (7 kG). As represented in Fig. 17, the V-I hysteresis behavior at low resistivity was measured to be similar to that of the case with no external magnetic field; however, the V-I hysteresis behavior at high resistivity was clearly affected by external magnetic field. The external magnetic field was analyzed to give rise to decrease in resistivity.

It could be appreciated on the basis of these results that controlling the strength and direction of external magnetic field may permit to control resistivity and current at which a switching behavior occurs.

Further Discussion

A plausible explanation for the V-I hysteresis observed for nanoparticle assemblies is based on an extended model for the memristor. In this model, the nanoparticle assembly is simply represented by a 1-dimensional repeating nanoparticle array of Fe₃O₄, which have doped and undoped charge carrier regions separated by a moving boundary. Since this system has almost infinitely alternating repetition of the conducting and insulating parts, the time-dependent capacitance Qή as well as the time-dependent resistance is considered in the model. Another feature of this system is that it is comprised of two charge carriers, Fe³⁺ and Fe²⁺ ions that have different mobilities in the nanoparticle lattices. This results in a different distribution of each carrier and thus an additional time-dependent capacitance ΔQt) across the particle boundaries. In contrast, the initial memristor model proposed in reference 2 consists of a single type of charge carrier drifting in the insulator and requires consideration of only the time dependent resistance.

The time-dependent change of w and associated change of voltage for the model nanoparticle system in response to injection of an alternating current can be simulated by using the following mathematical treatment. In the model, the time- dependent state variable wis the length of the doped region, bounded between zero and L which is the full length of grain boundary region. The voltage drop v(ή is then given by equation 1,

v(t) = R(w,t)i(t) +

C(w,t)

(1) where /is the current and q is the charge. R(w,1) and (\w,t) are the resistance and the capacitance respectively, and they are given by the relationships in equations 2- 4.

where R_0n (RO_FF) is the resistance of the doped (undoped) region, C_ON is the capacitance at w = 0, μ is the average carrier mobility, and ΔC{ή is the additional capacitance caused by the different mobilities of the two carriers and proportional to the imbalanced charge accumulation Δq (= + + - 2 3 Fe Fe Q Q ) around the undoped region. The ztø is assumed to be proportional to the total charge q with a dimensionless proportion coefficient x, which is material and geometry dependent. As the current direction is reversed, the phase of Δq is shifted by π because the sign of charge accumulation also depends on the current direction. Based on equations 1-4, the voltage drop can be given by using equation 5.

In Figures 18a and b are shown the time-dependent changes of w and associated changes of voltage, obtained by simulation using equation 5, when an alternating current is injected. The consistency between the results of V-I hysteresis modeling and the experimental observations (Figure 11) is clearly shown. The model shows that this unusual hysteresis originates from abrupt changes of w (Figures 18a and 18b). When {ή = 0 and n<0 = L₁ the nanopartide assembly is in the low resistance state. As the current becomes increasingly larger, more charge accumulates until it reaches a critical value. At that point, the boundary abruptly moves back to w = 0 in order to relax the charge accumulation and then the resistance is high. It should be noted that the experimentally observed chirality of hysteresis in both bias polarities is counter-clockwise (Figure 18c), whereas the chirality in the previously studied memristor with only time-dependent resistance changes depends on the bias polarity. The difference in the chirality of hysteresis between the two systems can be explained only when the capacitance part in the equation 5 is properly taken into account. The nanopartide system described above is unique in terms of the size of the monodispersity and the mild conditions required for preparing pristine nanopartide surfaces. In previous work on magnetite nanoparticles for which ligand eliminations were performed using high temperature thermal treatment procedures, in contrast to our observations, either only tunneling behavior was shown or the resistive switching was found to appear only at very low temperatures below 120 K. In the latter case, the switching was explained as a bulk effect in the context of the Verwey transition, which is not the case in this work since the hysteresis was observed at much higher temperature (i.e. RT) than T_v.

The approach described above for the development of memristors based on nanopartide assemblies is innovative. The observed memristic effects in resistive switching behavior are nicely accounted for by using a model composed of both resistance and capacitance.

The observations presented above regarding the switching behavior of nanoparticle assemblies serve as the framework for devising new applications to a wide range of electronic devices. Predictions of the high potential of memristors based on nanoparticle assemblies are supported by the tremendous versatility to tune the electrical behavior of nanoparticles by controlling their nanoscale characteristics such as size, composition, dimension, surface area, and chemical potential. It is clear that nanoparticles will serve as key materials for exploring memristic behavior and, perhaps for the fabrication of new devices.

Having described a preferred embodiment of the present invention, it is to be understood that variants and modifications thereof falling within the spirit of the invention may become apparent to those skilled in this art, and the scope of this invention is to be determined by appended claims and their equivalents.

Claims

What is claimed is:

1. A switching device, which comprises a nanoparticle assembly containing a plurality of nanopartides.

2. The switching device according to claim 1, wherein the switching device exhibits a current-, a voltage- or a magnetic field-induced switching behavior.

3. The switching device according to claim 1, wherein the switching device has an operation temperature of room temperature (25⁰C) ± 250⁰C for a reversible switching behavior.

4. The switching device according to claim 1, wherein the switching device shows a reversible switching behavior at a current of less than 1 mA.

5. The switching device according to claim 1, wherein the switching device has a RO_FF/R_ON value of more than 1 (wherein Ro_FF is a resistance at OFF state and R_0N is a resistance at ON state).

6. The switching device according to claim 1, wherein the nanoparticle assembly has a plurality of nanopartides that are arranged with a space of no more than 10 nm.

7. The switching device according to claim 1, wherein the nanopartides are chalcogen-type compounds, pnicogen-type compounds, carbon Group-type compounds, boron Group-type compounds, metals, alloys or multi-component hybrid structured nanopartides thereof.

8. The switching device according to claim 7, wherein the chalcogen-type compound is M^a _xA_z, M³ _xM^b _yA_z (M^a is one or more elements selected from the group consisting of Group 13 elements, Group 14 elements, transition metal elements, Lanthanide Group elements and Actinide Group elements; M^b is one or more elements selected from the group consisting of Group 1 elements, Group 2 elements, Group 13-15 elements, Group 17 elements, transition metal elements, Lanthanide Group elements and Actinide Group elements; A is one or more elements selected from the group consisting of O, S, Se, Te and Po; 0<x<16, 0<y<16, 0<z<8), or a multi-component hybrid structure thereof.

9. The switching device according to claim 7, wherein the pnicogen-type compound is M^CA, M^c _xM^d _yA_z (M^c is one or more elements selected from the group consisting of Group 13 elements, Group 14 elements, transition metal elements, Lanthanide Group elements and Actinide Group elements; M^d is one or more elements selected from the group consisting of Group 1 elements, Group 2 elements, Group 13-14 elements, Group 16-17 elements, transition metal elements, Lanthanide Group elements and Actinide Group elements; A is one or more elements selected from the group consisting of N, P, As, Sb and Bi; 0<x<24, 0<y<24, 0<z<8), or a multi- component hybrid structure thereof.

10. The switching device according to claim 7, wherein the carbon Group-type compound is M^e _xA_z, M^e _xM^f _yA_z (M^e is one or more elements selected from the group consisting of Group 13 elements, Group 14 elements, transition metal elements, Lanthanide Group elements and Actinide Group elements; M^f is one or more elements selected from the group consisting of Group 1 elements, Group 2 elements, Group 13-14 elements, Group 16-17 elements, transition metal elements, Lanthanide Group elements and Actinide Group elements; A is one or more elements selected from the group consisting of C, Si, Ge, Sn and Pb; 0<x<32, 0<y<32, 0<z<8), or a multi-component hybrid structure thereof.

11. The switching device according to claim 7, wherein the boron Group-type compound is M⁹ _XA_Z, M⁹ _xM^h _yA_z (M⁹ is one or more elements selected from the group consisting of Group 13 elements, Group 14 elements, transition metal elements, Lanthanide Group elements and Actinide Group elements; M^h is one or more elements selected from the group consisting of Group 1 elements, Group 2 elements, Group 13-14 elements, Group 16-17 elements, transition metal elements, Lanthanide Group elements and Actinide Group elements; A is one or more elements selected from the group consisting of B, Al, Ga, In and Tl; 0<x<40, 0<y<40, 0<z<8), or a multi-component hybrid structure thereof.

12. The switching device according to claim 7, wherein the metal is alkali metal, alkaline earth metal, transition metal, Lanthanide Group metal and Actinide Group metal or a multi-component hybrid structure thereof.

13. The switching device according to claim 7, wherein the alloy is M e^e _x MMf

V

M^e _xM^f _yM⁹ ₂ (M^e is one or more elements selected from the group consisting of transition metal elements, Lanthanide Group elements and Actinide Group elements; M^f and M⁹ are one or more elements selected from the group consisting of Group 1 metal elements, Group 2 metal elements, Group 13-17 elements, transition metal elements, Lanthanide Group elements and Actinide Group elements; 0<x<20, 0<y<20, 0<z<20), or a multi-component hybrid structure thereof.

14. The switching device according to claim 7, wherein the multi-component hybrid structure comprises at least one of chalcogen-type compounds or pnicogen-type compounds.

15. The switching device according to claim 1, wherein the nanoparticle is at least one of elements having an oxidation number of no less than 1.

16. The switching device according to claim 1, wherein the nanoparticle is M^a _xO_z, M^a _xM^byO_z (M^a is one or more elements selected from the group consisting of transition metal elements, Lanthanide Group elements and Actinide Group elements; M^b is one or more elements selected from the group consisting of Group 1 elements, Group 2 elements, Group 13-15 elements, Group 17 elements, transition metal elements, Lanthanide Group elements and Actinide Group elements; 0<x<16, 0<y<16, 0<z<8), or a multi-component hybrid structure thereof.

17. The switching device according to claim 1, wherein the nanoparticle is M_xFe_yO_z (M is one or more transition metal elements selected from the group consisting of Zn,

Mn, Fe, Co and Ni; 0<x<8, 0<y<8, 0<z<8), Zn_wM_xFe_yO_z (M is one or more transition metal elements selected from the group consisting of Zn, Mn, Fe, Co and Ni; 0<w<8, 0<x<8, 0<y<8, 0<z<8) , or a multi-component hybrid structure thereof.

18. The switching device according to claim 1, wherein the nanoparticle has (i) a zero-dimensional structure selected from the group consisting of a sphere, a core- shell and a multi-core shell structure; (ii) a one-dimensional structure selected from the group consisting of a rod, a barcode, a core-shell coaxial rod and a multi-core shell coaxial rod structure; (iii) a two-dimensional structure selected from the group consisting of a sheet, a layer and a multi-component sheet structure; or (iv) a three- dimensional structure selected from the group consisting of a branched structure, a dendrite structure, a dumbbell and a multi-pod structure.

19. The switching device according to claim 1, wherein the nanoparticle is in a size of 2-500 nm.

20. The switching device according to claim 1, wherein the nanoparticle has a surface trimmed to remove organic materials attached thereon.

21. The switching device according to claim 1, wherein the switching device is a memristor.

22. A method for preparing a switching device, which comprises the steps of:

(a) preparing nanoparticles;

(b) forming a nanopartide assembly using the nanoparticles; and

(c) connecting to the nanopartide assembly a means for applying a current, voltage or magnetic field.

23. The method according to claim 22, wherein the step (a) for preparing nanoparticles is carried out in a gas phase or a liquid phase

24. The method according to claim 22, wherein the step (a) for preparing nanoparticles is carried out by pyrolysis of a reaction mixture containing a metal precursor and a surfactant or a surfactant-containing solvent at 50-600⁰C.

25. The method according to claim 24, wherein the metal precursor comprises at least one metal element selected from the group consisting of transition metal elements, Lanthanide Group elements, Actinide Group elements and Group 13-14 elements.

26. The method according to claim 24, wherein the surfactant is an organic acid, an organic amine, alkane thiol, phosphonic acid, trioctylphosphine oxide, tributyl phosphine, alkyl phosphate, alkyl sulfate or tetraalkylammonium halide.

27. The method according to claim 24, wherein the solvent is an ether-based compound, hydrocarbon, organic acid, organic amine or alkane thiol.

28. The method according to claim 24, wherein the nanoparticles are size- controlled by adjusting a concentration of the surfactant, an amount of the solvent, a reaction temperature or a reaction time.

29. The method according to claim 24, wherein the method further comprises the step of (a⁷) trimming a surface of nanoparticles to remove organic materials attached thereon, such that a switching effect of the switching device is enhanced.

30. The method according to claim 29, wherein the trimming is carried out in the presence of an alkali solution.

31. The method according to claim 30, wherein the alkali solution comprises a alkali compound selected from the group consisting of alkylammonium, alkylammonium hydroxide, alkylammonium halide, alkylphosphine, alkylphosphine hydroxide and alkylphosphine halide [wherein alkyl is C_nH_2n+I (0<n<5)].

32. The method according to claim 29, wherein the trimming is carried out by sonication.

33. The method according to claim 22, wherein the formation of the nanoparticle assembly is carried out by pressing, LB (Langmuir Blodgett), LBL (layer by layer), print, self-assembly or solution evaporation.

34. The method according to claim 33, wherein the formation of the nanoparticle assembly is carried out by pressing under a pressure of more than 100 Pa.