US20050009079A1

US20050009079A1 - Monodisperse nanoparticle containing thin films via self-assembly

Info

Publication number: US20050009079A1
Application number: US10/912,266
Authority: US
Inventors: Simone Anders; Shouheng Sun
Original assignee: Individual
Current assignee: Individual
Priority date: 2002-02-20
Filing date: 2004-08-05
Publication date: 2005-01-13
Also published as: US20030157325A1; US6805904B2

Abstract

A method and structure that forms a multilayer nanoparticle thin film assembly begins by functionalizing a substrate with functional molecules. Next, the invention replaces a stabilizer on a bottom surface of the first nanoparticles with the functional molecules via surface ligand exchange to make a first nanoparticle layer on the substrate. The invention then replaces the stabilizer on a top surface of the first nanoparticle layer with functional molecules via surface ligand exchange. The invention replaces the stabilizer on a bottom surface of the second nanoparticles with the functional molecules via surface ligand exchange to make a second nanoparticle layer on the first nanoparticle layer. Lastly, the invention repeats the previous steps and forms additional nanoparticle layers.

Description

BACKGROUND OF THE INVENTION

1. Field of the Invention
The present invention relates to layer-by-layer assembly of nanoparticles, particularly magnetic nanoparticles, into smooth, thin nanoparticles-containing films on a variety of sized and shaped substrates.
2. Description of the Related Art
Self-assembly of functional objects with controlled structure and lateral dimensions is of fundamental and technological interest. Self-assembly is a naturally-occurring process, an evident example being biological molecules, which self-organize into various living structures. The molecules will combine in various manners in this self-assembly process via weak van der Waals interactions, hydrogen bond interactions, or strong ionic and covalent bond interactions. The idea of this molecule-based, self-assembly approach has been applied to the fabrication of artificial structures with controlled functionality. A variety of patented processes based on ionic absorptions to form polymeric/organic films, organic/inorganic hybrid films, and chemical interactions to form polymeric/organic films, or organic/inorganic hybrid films have been issued.
Nanoparticles contain hundreds to thousands of single molecules or atoms and have long been recognized as having enhanced chemical and physical properties compared to their bulk forms. It was recently realized that these nanoparticles with uniformity in size, shape and internal structure could be used as unique building blocks to fabricate nanoparticles-based functional structures. Generally, this nanoparticles-based self-assembly is governed by the nature of the interactions exhibited among the stabilized particles. Various monodisperse nanoparticle materials, including polymers, semiconductors, and metals, have been tested for use in building self-assembly nanoscale devices.
Advances in magnetic recording technology have driven the development of new magnetic nanoparticle-based media/devices and have increased the need for uniformity in both particle size and particle magnetics. Self-assembly of magnetic nanoparticles may offer a suitable approach to such media. In these nanoparticulate films, grains (nanoparticles) are uniform and encapsulated in a non-magnetic coating which minimizes exchange coupling between adjacent grains. Such a new paradigm may present a magnetic recording medium, possibly supporting areal storage densities beyond Terabits per square inch. Until now, the chemical processes to control the assembly thickness and lateral dimension are mainly applied to non-magnetic nanoparticles. Controlled chemical assemblies of magnetic tianoparticles for magnetic recording applications are rare, and most of those processes focus on tape applications.
The present invention focuses on a process to assemble monodisperse nanoparticles on functionalized substrates and the structure resulting therefrom.

SUMMARY OF THE INVENTION

In view of the foregoing and other problems, disadvantages, and drawbacks of conventional magnetic storage devices, the present invention has been devised, and it is an object of the present invention, to provide a structure and method for a process that forms a multilayer nanoparticle thin film assembly. The process begins by functionalizing a substrate with functional molecules. Next, the invention replaces a stabilizer on a bottom surface of the first nanoparticles with the functional molecules via surface ligand exchange to make a first nanoparticle layer on the substrate. The invention then replaces the stabilizer on a top surface of the first nanoparticle layer with functional molecules via surface ligand exchange. The invention replaces the stabilizer on a bottom surface of the second nanoparticles with the functional molecules via surface ligand exchange to make a second nanoparticle layer on the first nanoparticle layer. Lastly, the invention repeats the previous steps and forms additional nanoparticle layers. The substrate can include either glass, quartz, ceramics, silicon, silicon oxide, or carbon. The functionalization of the substrate can include coating, a layer of the functional molecules on the substrate by dipping the substrate, into the molecule solution; or, it can include spin coating a layer of the functional molecule in the substrate. The functional molecules can either include monomeric or polymeric molecules; or it can include COOR, CONR₂, NH₂, SH, OH; or, at least two functional groups represented as in H₂N—R—NH₂. The R comprises a common organic or inorganic chain. The stabilizer can be either H₂O, RCOOR₁, RCONR₁, RNH₂, RSH, RCN, ROH, or R₄N⁺, where R and R₁represent common organic chains. After the functionalizing process, the invention performs a particle dispersion of aqueous alcoholic, ether, or hydrocarbon solvent. Next, the invention rinses the substrate with solvent and dries the substrate. The first nanoparticles and the second nanoparticles can be the same, or they can be different.
The invention also includes a process which forms a multilayer nanoparticle thin film assembly. First, the invention flnctionalizes a substrate with functional molecules. Next, the invention replaces a stabilizer on a bottom surface of the first nanoparticles with the functional molecules via surface ligand exchange to make a first nanoparticle layer on the substrate. The invention then replaces the stabilizer on a top surface of the first nanoparticle layer with functional molecules via surface ligand exchange. The invention replaces the stabilizer on a bottom surface of the second nanoparticles with the functional molecules via surface ligand exchange to make a second nanoparticle layer on the first nanoparticle layer. The invention repeats the process to form additional nanoparticle layers and then thermally anneals the multilayer nanoparticle thin film assembly to modify the magnetic and chemical properties of the thin film assembly. The thermally annealing process is performed under an inert atmosphere comprising either nitrogen, argon, or helium. This process is also performed under a reducing atmosphere that includes either hydrogen atmosphere, nitrogen and hydrogen atmosphere, argon and hydrogen atmosphere, or helium and hydrogen atmosphere. The thermally annealing process is also performed under a reactive gas atmosphere. The reactive gas comprises either air, oxygen, hydrocarbon, hydrogen sulfide, halogen or thermally annealing forms, respectively, being one of a granular metal oxide film, a metal carbide film, a metal sulfide film, or a metal haloride film.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other objects, aspects and advantages will be better understood from the following detailed description of preferred embodiments of the invention with reference to the drawings, in which:
FIG. 1A is a schematic diagram of an example of solution phase liquid exchange;
FIG. 1B is an image of polymer-stabilized cobalt nanoparticles;
FIGS. 2A and 2B are schematic diagrams showing a nanoparticle assembly formation on a substrate via surface ligand exchange;
FIG. 3 is a schematic diagram showing layer-to-layer assembled nanoparticle film;
FIG. 4 is an image of one layer assembly of nanoparticles via polyethylenimine on a silicon oxide surface;
FIG. 5 is an image of one layer assembly of nanoparticles via polyvinylpyrolidone on a carbon surface;
FIG. 6 is an image of a three-layer assembly of nanoparticles via PEI on a silicon oxide surface;
FIG. 7 shows NEXAFS spectra for one and seven layer films of 8 nm particles;
FIG. 8 is an in-plane hysteresis loop of a three layer PEI/FePt assembly measured using a superconducting quantum interference device (SQUID);
FIG. 9 is a diagram of a magneto-resistive read-back signals from written bit transitions in a thermally annealed three layer assembly of 4 mn FePt nanoparticles; and
FIG. 10 is a flowchart showing the processing with the invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION

Nanoparticles generally exhibit a uniformity in size, shape and internal structure and are, therefore, useful as unique building blocks to fabricate nanoparticle-based functional structures. Self-assembly of nanoparticles offers an easy approach to such structures. Generally, this nanoparticle-based, self-assembly is governed by the nature of the interactions exhibited between the stabilized particles and the substrates. Various monodisperse nanoparticle materials, including polymers, semiconductors, and metals, have been tested in self-assembly processes to build nanoscale devices.
The present invention focuses on a process of assembling nanoparticles, especially magnetic nanoparticles, on a functionalized solid substrate. The process involves “functionalization” of a substrate (coating a substrate with functional molecules); replacement of the stabilizer on the bottom surface of a nanoparticle with functional molecules (via surface ligand exchange) to make one layer of nanoparticle assembly on a substrate; replacement of the stabilizer on the top surface of the nanoparticle with functional molecules (again via surface ligand exchange); and replacement of the stabilizer on the bottom surface of a new nanoparticle with functional molecules (via surface ligand exchange) to make a second layer of nanoparticle assembly on the substrate. The process is repeated to make a multilayered nanoparticle assembly.
To “functionalize” the substrate, the substrate is dipped into a functional organic/polymer solution and then washed and dried. Next, the functionalized substrate is dipped into a nanoparticle dispersion (specifically, a magnetic nanoparticle dispersion) to form the first layer nanoparticle assembly. This process is repeated to form the multilayered nanoparticle assembly.
Thermal annealing is applied to control physical properties of the assembly. This layer-by-layer assembly approach is easily applied to any size and shape substrate, including hard plates of glass, silicon, aluminum (for hard disk) and soft films of plastics (for tape). Therefore, the invention offers a convenient fabrication procedure for nanoparticle assemblies with controlled lateral dimension and thickness.
The invention is based on molecule assisted nanoparticle assembly. Molecules can be monomeric bifunctional molecules (such as Cl₃Si-R-X, X-RX, with R being normal organic/inorganic chain and X=NR₂, COOR, SH, CN, OH,), polymeric molecules (such as polyvinylpyrrolidone (PVP), polyethylenimine (PEI), etc., containing multiple NH₂, COOR CONR, SH, OH units), and/or bimolecular DNA and protein. The substrate upon which the nanoparticles are formed can be any solid substrate and can have any size or shape. The substrate can be formed of any kind of material, including hard plates, glass, silicon, or aluminum (for hard disk) or soft plates, plastics (for tape), etc.
Functionalized substrates are prepared by absorbing a layer of functional molecules onto the substrates. In these cases, molecular-substrate interactions exist which result in a layer of molecular thin film. The interactions can be any kind from van der Waals, hydrogen bonding to chemical bonding interactions. For example, binding of the trichlorosilane (Cl₃Si-R) head groups to the hydrophilic surface is well-known. Hydrolysis of the Si—Cl bonds is caused by traces of water absorbed at the hydrophilic substrate surface. The whole absorption process can be looked at as an in-situ formation of polysiloxane that is connected to surface silanol groups via Si—O—Si bonds. Another example of the interactions includes alkylthiols (RSH) bonding to the gold surface via gold-thiolate (Au—SR) linkages. Polyethylenimine (PEI) can attach a hydrophilic surface via ionic bonding interactions, while polyvinylpyrrolidone (PVP) molecules can attach to a surface via van der Waals and hydrogen bonding interactions.
The functional molecules are coated onto a substrate by immersing the substrate into the molecule solution. Surface chemical/physical interactions between the molecule and the substrate occur. As the substrate is withdrawn from the solution, a continuous film is generated on the substrate. Extra molecules that adhere to the substrate can be washed away with fresh solvent, leaving a thin molecular layer or thin molecule film that can be used directly for nanoparticle deposition. The functionalized substrate can be thermally treated to facilitate chemical bonding among the molecules and to increase the assembly strength.
Forming a molecule-assisted nanoparticle assembly on a solid substrate involves using ligand exchange reactions to exchange the stabilizers bound to the particles with the molecules that selectively bind to the substrate. This ligand exchange reaction is illustrated in FIG. 1A. A polymer 11 (for example Polyvinylpyrrolidone, PVP) replaces a long chain surfactant, such as oleic acid, of the 8 nm cobalt nanoparticles 10 in solution to give polymer stabilized nanoparticle dispersion. As a result, the particle assembly on a solid substrate adopts the polymer-like chain pattern 12. This example demonstrates a solution phase polymer (PVP) molecule replacement of a monomeric molecule (oleic acid) around each cobalt nanoparticle to form PVP stabilized cobalt nanoparticles. FIG. 1B is a TEM image of such polymer-stabilized cobalt nanoparticles that indicates that the particles are buried into the polymer media with good dispersion. Note that the substrate can have any shape, including a plane shape, a sphere, a V-tip, an ellipsoid, a cylinder, etc.
The assembly of nanoparticles into a macroscopic two-dimensional array on a functionalized substrate is depicted in FIG. 2A. More specifically, FIG. 2A is a schematic diagram showing a nanoparticle assembly formation on a substrate via surface ligand exchange. Item L1 (22) is the stabilizer initially attached to the nanoparticle 20. The stabilizer 22 keeps the nanoparticles from aggregating or oxidizing and, as would be known by one ordinarily skilled in the art, can be any suitable substance. For example, the stabilizer could be H₂O, RCOOR₁, RCONR₁, RNH₂, RSH, RCN, ROH, R₄N⁺, where R and R₁represent common organic chains, etc.
Item L2 (21) is a functional chemical molecule that has strong chemical absorption tendencies to the nanoparticles and replaces L1 to give L2 coated nanoparticles. The functional chemical molecules can be any substance that is more attractive to the nanoparticles than the stabilizer. Therefore, the functional molecules can be monomeric and polymeric molecules. The functional molecules can be in functional groups, including COOR, CONR₂, NH₂, SH, and OH and, more specifically, can be in at least two functional groups represented as H₂N-R-NH₂, where R is a common organic or inorganic chain.
Item 23 is the substrate and can comprise glass, quartz, ceramics, silicon, silicon oxide, carbon, etc. As discussed above, the substrate 23 is dipped into a solution containing functional molecules 21, or functional molecules 21 are spin coated onto the substrate 23. Then, the substrate 23 is optionally rinsed and dried. More specifically, an optional particle dispersion is performed using a dispersion of solvent (e.g., H₂O, alcohol, ether, hydrocarbon, etc.). Next, the substrate 23 is dipped into a solution containing nanoparticles 20 stabilized by the stabilizer 22. As shown, the lower surface of the nanoparticles 20 exchange the functional molecules 21 in place of the stabilizers 22 to form a first layer of nanoparticles 20 on the substrate 21. The replacement leads to a monolayer of nanoparticles 26, as shown in FIG. 2B.
As shown in FIG. 2B, after the first layer 26 of nanoparticles is formed, the ligands 22 on top of the nanoparticles 20 are replaced with L2 ligands (21) by dipping the substrate again into the functional molecule solution (or spin coating the functional molecules thereon). Replacement of the top L1 ligands with L2 (bifunctional or multiple functional ligand) leads to an L2 based ligand layer that can be used to adhere another layer 25 of nanoparticles over the first layer 26 of nanoparticles. Repeating this process can lead to an even greater number of layers in the multilayered nanoparticle assembly.
A schematic diagram of a layer-by-layer assembled nanoparticle film formed via PEI is shown in FIG. 3. Item 30 represents the substrate, items 31 and 33 represent the ligand layers, and items 32 and 34 represent the nanoparticle layers. Nanoparticle layer 1 (NPLI) 32 and nanoparticle layer 2 (NPL2) 34 can be the same nanoparticles, or they may be different. For example, if both nanoparticle layer 1 (32) and nanoparticle layer 2 (34) are FePt nanoparticles, then an FePt nanoparticle thin film is formed. If nanoparticle layer 1 (32) is cobalt nanoparticles, while nanoparticle layer 2 (34) is FePt nanoparticles, then a layered nanocomposite Co/FePt film is formed. If nanoparticle layer 1 (32) is non-magnetic nanoparticles, such as Cu or Ag nanoparticles, and nanoparticle layer 2 (34) is magnetic particles, such as Co or Fe nanoparticles, then the layered structure may be used as an Fe GMR (giant magneto-resistive) device.
In one embodiment, a silicon membrane was immersed into the chloroform solution of PEI for about 30 seconds, then withdrawn from the solution and dipped into ethanol solvent to wash off extra PEI on the substrate surface and dried. The functionalized substrate was immersed into the hexane dispersion of 6 nm Fe₅₀Pt₅₀for 30 seconds, withdrawn from the dispersion, washed with fresh hexane and dried. FIG. 4 is a TEM image of one layer of 6 nm Fe₅₀Pt₅₀nanoparticles self-assembled on a PEI-modified silicone oxide, with film composition being SiO/PEI/Fe₅₀Pt₅₀, produced by the previous process.
Another example of forming one layer of Co nanoparticle assembly via PVP is described as follows. An amorphous carbon membrane was dipped into the dichloromethane solution of PVP, withdrawn from the solution and dried. The PVP modified membrane was then dipped into the hexane dispersion of 8 nm cobalt nanoparticles for 30 seconds, withdrawn from the dispersion, washed with fresh hexane and dried. FIG. 5, is the TEM image of one layer of 8 nm cobalt nanoparticles self-assembled on a PVP-modified carbon surface, with film composition being SiO/P VP/Co, produced by the previous process.
A third example of forming three layers of FePt nanoparticle assembly via PEI is as follows. A silicon oxide substrate was immersed into the chloroform solution of PEI for about 30 seconds, then withdrawn from the solution and dipped into ethanol solvent to wash off extra PEI on the substrate surface and dried. The PEI functionalized substrate was immersed into the hexane dispersion of 6 nm Fe₅₀Pt₅₀for 30 seconds, withdrawn from the dispersion, washed with fresh hexane and dried. The coated substrate was immersed into the chloroform solution of PEI for about 30 seconds, then withdrawn from the solution, rinsed with fresh ethanol and dried. The substrate was, once again, immersed into the hexane dispersion of 6 nm Fe₅₀Pt₅₀for 30 seconds, withdrawn from the dispersion, rinsed with fresh hexane and dried. By repeating the coating of PEI and FePt, the three layers of 6 nm Fe₅₀Pt₅₀nanoparticle assembly was made. By repeating this inventive two-step process in a cyclic fashion, layer-by-layer assembled polymer/nanoparticle multilayer films can be obtained. FIG. 6 is the TEM image of three layers of 6 nm Fe₅₀Pt₅₀nanoparticles self-assembled on a PEI-modified silicone oxide surface, produced by the previous process. The film is alternatively layered with PEI and FePt in the form of sub/PEI/FePt/PEI/FePt/PEI/FePt., FIG. 6.
The chemical nature of the films produced with the invention was studied using near-edge X-ray absorption fine structure spectroscopy (NEXAFS) measured in total electron yield. The data indicate that all films contain a fraction of iron oxide. FIG. 7 shows as an example the NEXAFS spectra for 1 and 7 layer films of 8 nm particles annealed at 580° C. in N₂for 30 min, together with a metallic Fe reference spectrum. The 7 layer film shows a much more metallic character than the 1 layer film. The best fit to the experimental data for a weighted superposition of Fe and Fe₃O₄reference spectra indicates that iron/iron oxide ratio for the 1 layer assembly is 55%/45%, while for the 7 layer assembly, a 75%/25% ratio is obtained. The ratio of the spectra is not identical to the ratio of Fe and Fe₃O₄in the film because of the surface sensitivity of the method. Adjustment of annealing conditions and chemistry led to a reduction of the oxidized Fe content and an increase in metallic Fe present in the ordered L1 ₀phase. This correlates strongly with the increase of the coercivity of the film. A model assuming a metallic FePt core and an oxide shell shows that an oxide thickness of only 0.35 to 0.4 nm is sufficient to explain the observed NEXAFS spectra for samples with various particle sizes between 4 and 8 nm. The relatively large contribution of the oxide shell to the spectra is due to the geometry and the short electron escape depth.
The magnetic properties of the film are controlled by thermal annealing. FIG. 8 is a (SQUID) hysteresis loop of a three layered 4 nm Fe₅₈Pt₄₂nanomagnetic granular film annealed under N₂for 10 min. The film has been annealed at 580° C. for 10 min under one atmospheric nitrogen environment. The coercivity of this film reaches 2400 Oe. The coercivity of the annealed films typically increases with annealing time and temperature, and a minimum temperature of about 500° C. is required to begin forming the ordered L1 ₀phase. An increased number of layers also leads to an increase in the film coercivity. For magnetic recording, thin, high an isotropy layers of small nanoparticles are most desirable. The thinnest layers that showed coercivity greater than 300 Oe (as measured by SQUID) were 1 layer films for 8 in particles and 2 layer films for 4 nm particles. Superconducting quantum interference device (SQUID) measurements show that coercivity as high as 17 kOe can be achieved for 3 layer/4 nm particle thin films (annealed at 800° C. for 5 min. in N₂). The easy axis of the particles is 3D randomly oriented, leading to identical hysteresis loops for in-plane and perpendicular measurements and a remnant magnetization M_r=0.5 M_swhere M_sis the saturation magnetization.
The film is magnetically hard and can support magnetization reversal transitions. FIG. 9 shows magneto-resistive (MR) read back signals from the written bit transitions in a thermally annealed three layer assembly of PEI/4 nm Fe₅₈Pt₄₂nanoparticles. It demonstrates that a FePt nanoparticle assembly as thin as around 12 nm can still support magnetic recording.
A flowchart of the processing of the invention is illustrated in FIG. 10. In item 100, the invention functionalizes a substrate. Next, in item 110, the invention replaces stabilizers on a bottom surface of the first nanoparticles. In item 120, the invention makes a first nanoparticle layer. In item 130, the invention replaces the stabilizer on a top surface of the first nanoparticle layer. In item 140, the invention replaces the stabilizer on a bottom surface of the second nanoparticle with functional molecules. Next, in item 150, the invention repeats the processing steps in items 130 and 140 to form additional nanoparticle layers. Lastly, in item 160, a magnetic thin film is made by thermally annealing the multilayer nanoparticle thin film assembly.
The inventive process involves absorption of polymeric/monomeric functional molecules on a solid substrate and replacement of the stabilizer around the nanoparticles (via ligand exchange) to make a nanoparticle assembly on the substrate. The bi/multiple functional molecules are used again to replace the stabilizer on the top surface of the nanoparticles and to replace the stabilizer of a new group of nanoparticles to form a second nanoparticle layer. The process is repeated to make a multilayered nanoparticle assembly.
The invention is especially useful for forming magnetic recording media and/or magnetic hard disks. The invention also generates a smooth nanoparticle-based film with controls on film dimension, thickness and physical properties. It is useful for forming magnetic recording media, magnetic hard disks, magnetic recording tape, and magneto optical recording media. The multilayered nanoparticle films are also important to the study and fabrication of magneto-resistive devices. The assembly of a small magnetic nanoparticle on the end of a sharp tip, such as an AFM probe, will lead to highly sensitive magnetic sensors for magnetic force sensing at high spatial resolution. The well-controlled monolayer assembly of nanoparticles on various surfaces can also be used as catalysts.
While the invention has been described in terms of preferred embodiments, those skilled in the art will recognize that the invention can be practiced with modification within the spirit and scope of the appended claims.

Claims

1-26. (Cancelled)

27. A multilayer nanoparticle thin film assembly comprising:

a substrate having a coating of functional molecules;

a first layer of nanoparticles bound to said substrate by said functional molecules;

at least one additional layer of nanoparticles bound to a preceding layer of nanoparticles with said functional molecules.

28. The assembly in claim 27, wherein said substrate has a shape of one of a plane, a sphere, a V-type tip, an ellipsoid and a cylinder.

29. The assembly in claim 27, wherein said substrate comprises one of glass, quartz, ceramics, silicon, silicon oxide, and carbon.

30. The assembly in claim 27, wherein said functional molecules comprise one of monomeric and polymeric molecules.

31. The assembly in claim 27, wherein said functional molecules comprise functional groups including COOR, CONR2, NH2, SH, and OH.

32. The assembly in claim 27, wherein said functional molecules comprise at least two functional groups represented as H2N-R-NH2, where R comprises a common organic or inorganic chain.

33. The assembly in claim 27, wherein said stabilizer comprises one of H2O, RCOOR1, RCONR1, RNH2, RSH, RCN, ROH, R4N+, where R and R1 represent common organic chains.

34. The assembly in claim 27, wherein said first nanoparticles are different than said second nanoparticles.

35. The assembly in claim 27, wherein said first layer of nanoparticles and said additional layer of nanoparticles comprise one of a hard magnetic thin film, a granular metal oxide film, a metal carbide film, a metal sulfide film, and a metal haloride film.

36. The assembly in claim 35, wherein said hard magnetic thin film comprises one of a FePt, CoPt, and CoSm for magnetic recording applications.