The application of nanoparticles as quantum dots in nanoelectronics demands their arrangement in ordered arrays. Shape controlled self-assembly is a challenge due to the difficulties of obtaining proper self-assembling parameters, such as solvent concentration, organic ligands, and nanoparticle size. In this article, hard magnetic FePt nanoparticles were synthesized using a combination approach of reduction and thermal decomposition. The nanoparticles are about 4.5 nm and appeared as truncated octahedral enclosed by the
{100} and {111}
crystal facets of fcc structure. The nanoparticles are of hexagonal close packing and orient randomly in the self-assembly nanoarrays.  By diluting the solution for large-area self-assembly, monolayer, submonolayer, and multilayer nanorings of FePt nanoparticles were formed. The nanoring formation is determined by hydrodynamics, surface effects, and interaction between the FePt nanoparticles and substrates

Fang, Jiye

He, Jibao

Huynh, Tuyet-Anh

Kennedy, Trevor J

O\u27Connor, Charles J.

Stokes, Kevin L.

Zhou, Weilie L

English

University of New Orleans

University of New Orleans ScholarWorks@UNO Physics Faculty Publications Department of Physics 2003 Self-assembly of FePt nanoparticles into nanorings Weilie L. Zhou Jibao He Jiye Fang Tuyet-Anh Huynh Trevor J. Kennedy See next page for additional authors Follow this and additional works at: https://scholarworks.uno.edu/phys_facpubs  Part of the Nanoscience and Nanotechnology Commons, and the Physics Commons Recommended Citation J. Appl. Phys. 93, 7340 (2003) This Article is brought to you for free and open access by the Department of Physics at ScholarWorks@UNO. It has been accepted for inclusion in Physics Faculty Publications by an authorized administrator of ScholarWorks@UNO. For more information, please contact scholarworks@uno.edu. Authors Weilie L. Zhou, Jibao He, Jiye Fang, Tuyet-Anh Huynh, Trevor J. Kennedy, Kevin L. Stokes, and Charles J. O'Connor This article is available at ScholarWorks@UNO: https://scholarworks.uno.edu/phys_facpubs/5 Self-assembly of FePt nanoparticles into nanoringsWeilie L. Zhou,a) Jibao He, Jiye Fang, Tuyet-Anh Huynh, Trevor J. Kennedy,Kevin L. Stokes, and Charles J. O’ConnorAdvanced Materials Research Institute, University of New Orleans, New Orleans, Louisiana 70148~Presented on 13 November 2002!The application of nanoparticles as quantum dots in nanoelectronics demands their arrangement inordered arrays. Shape controlled self-assembly is a challenge due to the difficulties of obtainingproper self-assembling parameters, such as solvent concentration, organic ligands, and nanoparticlesize. In this article, hard magnetic FePt nanoparticles were synthesized using a combinationapproach of reduction and thermal decomposition. The nanoparticles are about 4.5 nm and appearedas truncated octahedral enclosed by the $100% and $111% crystal facets of fcc structure. Thenanoparticles are of hexagonal close packing and orient randomly in the self-assembly nanoarrays.By diluting the solution for large-area self-assembly, monolayer, submonolayer, and multilayernanorings of FePt nanoparticles were formed. The nanoring formation is determined byhydrodynamics, surface effects, and interaction between the FePt nanoparticles and substrates.© 2003 American Institute of Physics. @DOI: 10.1063/1.1540045#Nanoparticles show many unique optical, electric, mag-netic, and catalytic properties that are greatly different fromboth the corresponding bulk materials and the individual at-oms of which they are composed.1–6 The application of thesenanoparticles as quantum dots in nanoelectronics demandstheir regular arrangement in ordered one- ~1D!, two- ~2D!,and three-dimensional ~3D! arrays.7 Various kinds of 1D, 2D,and 3D ordered arrays have been achieved by the self-assembling method.4–11 The self-assembling arrays can betuned by nanoparticle size, organic ligand, solvent concen-tration, etc. It is essential to manipulate the nanoarrays tovarious kinds of shapes and patterns for the nanoelectronicsapplications.12Fe–Pt alloys have been studied for decades because oftheir important applications in permanent magnetism.13–17Self-assembling FePt nanoparticles have attracted more re-searchers due to their excellent hard magnetic properties andpotential application on ultrahigh density magnetic recordingmedia.18–20 In this article, we report direct self-assembly ofFePt nanoparticles into monolayer, submonolayer, andmultilayer nanorings.The FePt nanocrystallites were prepared by followingSun’s method using a combination approach of reduction andthermal decomposition in the presence of oleic acid and oleylamine.17–20 A slightly modified synthetic procedure is as fol-lows: Under airless conditions, platinum acetylacetonate~Alfa, 99 mg, 0.25 mmol!, 1,2-tetradecanediol @Aldrich; 192mg ~purity: 90%!, 0.75 mmol# and dioctylether ~20 ml! weremixed and heated to 100 °C. Oleic acid @Aldrich, 0.09 ml~purity: 90%!, 0.25 mmol#, oleyl amine @Aldrich, 0.12 ml~purity: 70%!, 0.25 mmol#, and Fe(CO)5 ~Aldrich, 0.065 ml,0.50 mmol! were added successively. The mixture was re-fluxed at 297 °C for 30 min and then naturally cooled downto room temperature under the flow of argon gas. At thismoment, nanocrystalline FePt was formed as a seed. Subse-quently, similar amounts of platinum acetylacetonate, 1,2-tetradecanediol, oleic acid, oleyl amine, and Fe(CO)5 wereadded into the flask using the same procedure when the sys-tem was heated up again. After the refluxing continued foradditional 30 min, the mixture cooled down to room tem-perature under an argon atmosphere. The nanoparticles werefirst precipitated by adding a combination of polar solvents~7.5 ml of hexyl alcohol120.0 ml of ethanol! and followedby centrifugation. The precipitate ~FePt! can be redispersedinto hexane followed by a size-selection treatment. The se-lected FePt nanoparticles were eventually redispersed in amixed solvent of (95% octane15% octanol). The suspen-sion was then diluted by using octane, followed by one dropon a transmission electron microscopy ~TEM! grid andevaporated in a vacuum for self-assembling nanorings.Monodisperse FePt nanoparticles arrays can be obtainedafter the size selection. Figure 1~a! shows ~111! ordered ar-rays, in which the nanoparticles have hexagonal close pack-ing. The as-synthesized FePt nanocrystals have a chemicallydisordered face-centered cubic ~fcc! A1 phase.21,22 The crys-tal structure can be determined by selected-area diffractionpattern ~SAD! as shown in Fig. 1~b!. The ratio of Fe and Ptconcentration is close to 1:1 by EDS analysis. The nanocrys-tals appeared as polyhedral shapes. The truncated octahedralnanoparticles were dominated in the synthesis as shown inthe insert in Fig. 1~a!. The truncated octahedral is enclosedby the $100% and $111% crystal facets of the fcc structure. Thesize of the nanoparticles is about 4.5 nm. The orientations ofnanocrystals in the self-assembling nanoarrays are quite ran-dom, which is different from the Ag self-assembling arraysreported by Harfenist et al.,5 in which the Ag nanoparticlesoriented to the same orientation with the nanocrystal lattice.The orientations of the nanoarrays in our samples seem tohave less relationships with those of single nanocrystals.The nanorings were obtained by simply diluting the sus-pension for large-area self-assembling and dropping it ona!Author to whom correspondence should be addressed; electronic mail:wzhou@uno.eduJOURNAL OF APPLIED PHYSICS VOLUME 93, NUMBER 10 15 MAY 200373400021-8979/2003/93(10)/7340/3/$20.00 © 2003 American Institute of PhysicsDownloaded 31 Mar 2011 to 137.30.164.181. Redistribution subject to AIP license or copyright; see http://jap.aip.org/about/rights_and_permissionsTEM grids. Figure 2~a! is a monolayer nanoring of FePtnanoparticles. The nanoparticles inside the ring also formedordered arrays. The diameter of the nanoring is about 550 nmand the thickness of the wall is about 30 nm. The diameterand wall thickness of the nanorings vary from each other.Few nanoparticles can be seen both inside the ring and theoutside. The submonolayer rings were often observed. Thediameters of the nanoring in Fig. 2~b! is 420 nm and thethickness of the wall is about 100 nm. The FePt nanopar-ticles in the second layer occupied the two-fold saddle andthree-fold hollow sites of the first layer arrays and formedsuperlattice and ring packing toward the inner rim of thenanoring.23Multilayer nanorings were first observed as shown inFig. 3~a!, in which the diameter of the nanoring is 800 nmand thickness of the wall is more than 400 nm. The first threelayers can be clearly identified as denoted by arrows 1, 2,and 3 in Fig. 3~a!. The nanoring becomes darker toward theinner rim, which implies multilayer packing was formed. Itseems that upper layer nanoparticles were pushed out towardthe rim and formed multilayer packing during self-assembly.Again, only a few extra nanoparticles were observed insideand outside the ring. In our experiment, the suspension con-centration was the main parameter to control the formationof nanorings. It is hard to tell the exact concentration forforming the nanorings. However, the swamplike patternwould always be observed before further diluting the suspen-sion and obtaining ring structure. Figure 3~b! is a swamplikeself-assembling pattern of FePt nanoparticles. The size of the‘‘swamps’’ varies from each other, but close to the ring di-ameter and the concentration for forming swamp pattern isclose to the critical concentration for forming ring structureas well.Ring formation is a complex process.24–26 It is deter-mined by hydrodynamics, surface effects, interaction be-tween self-assembling subunits, and substrates. When nano-particle solutions were dropped on the carbon grid, the liquidthin film was believed to have formed.24 As the liquid thin-film thickness reached a critical value, holes opened in orderto restore the film to its equilibrium thickness. The holesnucleated and grew bigger due to evaporation-driven insta-bility. The advance of the growing hole rims in the solventfilm was stopped when the radial force outward was nolonger sufficient to overcome the frictional effects of thenanoparticles which had been collected in the rim. The holesnucleated and pushed nanoparticles outward and left fewnanoparticles inside and outside the rings, and friction effectsof nanoparticles also kept the thickness of the upper layerwall of the nanoparticle ring thinner than the lower layer inmultilayer ring formation. In our observation, different sizesand configurations of nanoring patterns were observed. Theconcentration of the suspension may have been very criticalfor forming nanoparticle rings in our system. The high con-centration of the suspension generally induced the formationof large-area self-assembly or swamplike patterns. Theproper concentration generated ring ‘‘seeds’’ for nanoparticlering formation. The concentration of local ligands, evapora-tion rate of the solution, and the interaction between thenanoparticles and substrate control the configurations of thenanorings. The monolayer nanoring formation can be easilyexplained by the aforementioned theory. For the submono-layer and multilayer rings, the local concentration of the so-lution may have caused the nanoparticles to form 3D pack-ing. As the liquid thin film reached a critical thickness, theholes were formed and the advance of the growing holesafter the solvent evaporation pushed the multilayer arraysoutward and formed multilayer packing. As the radial forceFIG. 1. ~a! Monodisperse FePt nanocrystal self-assembly arrays. ~b! SADpattern of FePt nanoparticles. The insert is a high-resolution TEM image ofFePt truncated octahedral nanoparticle.FIG. 2. ~a! Monolayer FePt nanoparticle ring. ~b! Submonolayer FePt nano-particle ring.7341J. Appl. Phys., Vol. 93, No. 10, Parts 2 & 3, 15 May 2003 Zhou et al.Downloaded 31 Mar 2011 to 137.30.164.181. Redistribution subject to AIP license or copyright; see http://jap.aip.org/about/rights_and_permissionsoutward was no longer sufficient to overcome the frictionalforces generated from the different layers of nanoparticles,the multilayer rings were formed as denoted by 1, 2, and 3 inFig. 3~a!. This was very hard to identify after accounting forthe three layers since the darker contrast of the inner layerimplies more layer packing of the nanorings was formed. Itis not yet known how much the magnetic moments of thenanoparticles contribute to the ring formation during the self-assembling process. Closely checking for the monolayer na-noring indicated that the distance between two particles wastoo big for the magnetic dipolar interaction. However, therewere magnetic dipolar interactions between the multilayernanoring formations due to close contacts among nanopar-ticles of different layers. It is believed that dipolar interactioncompetes with nondirectional van der Waals interactions atclose range and the ring formation is directed by the mag-netic moments.27 In our case, the multilayer rings are alwaysconcentric circles, which implies that magnetic dipolar inter-action among the nanoparticles may stabilize the nanoringsand make the energy and magnetic preference in themultilayer packing system, which is different from othernonmagnetic nanoparticles.28The subhundred nanometer spin-dependent electronicdevices with nanometer-scale control of material propertiesin all dimensions have been studied by an IBM group.29 Thenanorings formed by FePt magnetic nanoparticles have a po-tential role in the magnetotransport in nanostructured mate-rials. Size and configuration control of the nanorings are un-der tuning, which may provide collective magneticconfigurations for the magnetic nanoparticle rings.30This work is supported by a research grant from Louisi-ana Board of Regents Contract No. NSF/LEQSF ~2001-04!-RII-03.1 J. R. Heath, Science 270, 1315 ~1995!.2 T. S. Ahmadi, Z. L. Wang, T. C. Green, A. H. Henglein, and M. A.El-Sayed, Science 272, 1924 ~1996!; T. S. Ahmadi, Z. L. Wang, A. H.Henglein, and M. A. El-Sayed, Chem. Mater. 8, 1161 ~1996!; S. Link, Z.L. Wang, and M. A. El-Sayed, J. Phys. Chem. B 103, 3529 ~1999!.3 Y. Wang and N. Herron, J. Phys. Chem. 95, 525 ~1991!.4 C. B. Murray, C. R. Kagan, and M. G. Bawendi, Science 270, 1335~1995!.5 S. A. Harfenist, Z. L. Wang, M. A. Alvarez, I. Vezmar, and R. P. Whetten,J. Phys. Chem. 100, 13904 ~1996!.6 L. Motte, F. Billoudet, and J. H. Bawendi, J. Phys. Chem. 98, 8827 ~1994!.7 B. A. Korgel and D. Fitzmaurice, Adv. Mater. 10, 661 ~1998!.8 K. V. Sarathy, G. U. Kulkarni, and C. N. R. 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Sun, Science 290,1131 ~2000!.30 J. Rothman, M. Kla¨ui, L. Lopez-Diaz, C. A. F. Vaz, A. Bleloch, J. A. C.Bland, Z. Cui, and R. Speaks, Phys. Rev. Lett. 86, 1098 ~2001!; S. P. Li,D. Peyrade, M. Natali, A. Lebib, Y. Chen, U. Ebels, L. D. Buda, and K.Ounadjela, ibid. 86, 1102 ~2001!.FIG. 3. ~a! Multilayer FePt nanoparticle ring. ~b! Swamplike pattern of FePtself-assembling arrays.7342 J. Appl. Phys., Vol. 93, No. 10, Parts 2 & 3, 15 May 2003 Zhou et al.Downloaded 31 Mar 2011 to 137.30.164.181. Redistribution subject to AIP license or copyright; see http://jap.aip.org/about/rights_and_permissions

Self-assembly of FePt nanoparticles into nanorings

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Self-assembly of FePt nanoparticles into nanorings

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