MATERIALS CHARACTERIZATION 42:101–109 (1999)© Elsevier Science Inc., 1999. All rights reserved. 1044-5803/99/$–see front matter655 Avenue of the Americas, New York, NY 10010 PII S1044-5803(98)00050-3

101

S

elf-Assembled Superlattices of Size- and Shape-Selected Nanocrystals: Interdigitative and Gear Molecular Assembling Models

Z. L. Wang

School of Materials Science and Engineering, Georgia Institute of Technology,Atlanta, Georgia 30332-0245

Size and shape selected nanocrystals behave as fundamental building blocks that can beused to construct nanocrystal assembled superlattices. This is a new state of materials thathas orders in atomistic and nanocrystal length-scales. The nanocrystals are passivated withorganic molecules (call thiolates) that not only protect them from coalescing but act as themolecular bonds for forming the superlattice structure. The interparticle distance is adjust-able, possibly resulting in tunable electric, optical, and transport properties. In this article,nanocrystal superlattices (NCSs) constructed by truncated octahedral Ag particles are stud-ied to illustrate the bundling and interdigitation of the passivation thiolates in forming thedirectional molecular bonds in NCSs. The {111}

s

facets observed in the NCSs suggest thekey roles played by the molecular bonding energy in forming the surfaces of NCSs. The for-mation of twins in the NCSs supports the “gear” assembling model of the nanocrystals. Thegear assembling is likely to have higher energy than the interdigitative assembling.

© Elsevier Science Inc., 1999. All rights reserved.

INTRODUCTION

Nanophase and nanoparticle materials, as acomponent of nanotechnology in the 21stcentury, have attracted a great deal of at-tention in the technical community. Theunique properties of nanophase materialsare determined not only by their intrinsicatomistic-scale structure, but also by the in-terparticle interaction. The role played byparticle size is comparable, in some cases,to the particle chemical composition, addinganother flexible parameter for designingand controlling their behavior. Nanoparticlespossess physical and chemical functionsspecificity and selectivity; thus they can beideal building blocks for two- and three-di-mensional cluster self-assembled superlat-tice structures, in which the particles be-have as well-defined molecular matter andthey are arranged with long-range transla-tional and even orientational order.

A number of researchers have success-fully fabricated

self-assembly passivated nano-crystal superlattices

(NCSs) or

nanocrystal ar-rays

(NCA) of metal, semiconductor, andoxide clusters [1-8], which are a new formof materials with fundamental interestsand technological importance. Self-assem-bled arrays involve self-organization intomonolayers, thin films, and superlattices ofsize-selected nanoclusters encapsulated inprotective compact organic coating. Bychanging the length of the molecular chains,quantum transitions and insulator to con-ductor transition could be introduced, re-sulting in tunable electronic, optical, andtransport properties [9].

Following the author’s previous reviewon NCS [10], this article reviews our cur-rent research effort in characterizing nano-crystal self-assembled materials. The focus ison the interparticle molecular bonds and themechanism for creating defects in NCSs. In-

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Z. L. Wang

terdigitative and “gear” assembling modelswill be shown as the two fundamental as-sembling configurations of the passivatingmolecules.

THREE-DIMENSIONAL SELF-ASSEMBLING OF NANOCRYSTALS

NCSs are characterized by unprecedentedsize-uniformity and translation and evenorientation order, created through a multi-stage processing strategy involving self-

assembly, annealing, etching of defects, re-versible passivation by self-assembled mono-layers, and macroscopic separation by sizeof the resulting assemblies [2, 3]. Particlesthat can be self-assembled usually have sizessmaller than 10nm; it is in this size rangethat many exciting and unusual physicalproperties are enhanced. The nanocrystalsare passivated by a monolayer of long-chain organic molecules, such as SR, whereR

5

n

2

C

n

H

2n

1

1

, n

5

4, 6, 8, 12 . . ., calledthiolates. The thiolate serves not only as theprotection for the particles to avoid direct

FIG. 1. (a) Low magnification and (b) high magnification TEM images of three-dimensional superlattices assem-bled by Ag nanocrystals. The NCSs have a fcc lattice. The nanocrystals were synthesized by an aerosol method [3].

Superlattices of Nanocrystals

103

contact between the particles with a conse-quence of coalescing, but also provides ameans for interparticle bonding.

Three-dimensional assembling of nano-crystals can form large size bulk crystallinematerials. Shown in Fig. 1a is a TEM imageof powder NCSs, whose sizes can be largerthan 5–10

m

m. A high magnification imageof the NCSs (Fig. 1b) reveals the array lat-tices formed by the nanocrystals, whereeach dark dot is a row of nanocrystals. Thecrystallography of the nanocrystal assem-bly can be directly determined by the im-age, where the crystal A is oriented alongthe [110]

s

of the face-centered cubic (

fcc

)[the subscript s stands for the superstruc-ture of the NCS], and the crystal B is ori-ented along [100]

s

of the

fcc

. The 3-D struc-ture of the NCS is

fcc

. It is also noticed that

the NCSs have faceted shape and the pre-ferred faces are {111}

s

and {100}

s

.The faceted structure of the NCSs is

clearly shown in the image given in Fig. 2,in which the {111}

s

faces of this

fcc

struc-tured Ag NCS are unambiguously pre-sented. It is well known that the {111} facesin

fcc

are the most densely packed plane,resulting in the lowest surface energy. Thisatomistic scale structural principle alsoholds in nanocrystal assembling, showingthe strong intermolecular interaction (viavan der Waals force) among the surfacepassivated molecules as well as betweenthe nanocrystals. Therefore, the nanocrys-tals actually behave like molecular matterand their assembling is not only a geomet-rical matching (see the next section), butalso a bonding energy governed structure.

FIG. 2. TEM image of {111}s faceted nanocrystal superlattices assembled by Ag nanocrystals. The NCS is orientedalong {110}s, the preferred orientation of this type of NCSs.

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Z. L. Wang

INTERDIGITATION OF MOLECULAR BONDS IN NANOCRYSTAL

SELF-ASSEMBLING

If the nanocrystals can be taken as thebuilding blocks, their 3-D assembling is un-avoidably affected by the particle shape. Inthis section, the self-assembling of trun-cated octahedral Ag nanocrystals is takenas an example to illustrate the roles playedby the particle shape in NCSs [2, 10]. Ourmain goal is to illustrate the formation ofdirectional, interparticle molecular bondsby the thiolates.

Figure 3a is a TEM image recorded froman Ag NCS deposited on a carbon sub-

strate. The Ag nanocrystals have a trun-cated octahedral shape and they are ori-ented along the [110] of the Ag atomiclattice in the image, along which four [111]and two [100] facets are imaged edge-on.The unit cell of the NCS is also orientedalong [110]

s

of

fcc

. Therefore, the orienta-tion relationship between the Ag particlesand the nanocrystal lattice is [110]

i

[110]sand [001]

i

[110]

s

. Accordingly, a structuremodel for the NCS is built (Fig. 3b) inwhich the nanocrystals are oriented follow-ing an assembling principle of

face-to-face

[10]. The orientational order is observed forthe first time in this NCS system.

A careful examination of the image

FIG. 3. (a) [110]s TEM image of a 3-D fcc assembled Ag nanocrystal superlattice. The nanocrystals are truncated oc-tahedra and they have orientational symmetry. (b) The 3-D fcc self-assembling model of the NCS (the specimenwas provided by Drs. S. A. Harfenist and R. L. Whetten).

Superlattices of Nanocrystals

105

shown in Fig. 3a indicates there are somewhite spots in the image, corresponding toopen channels formed by the thiolate mole-cules. This suggests that the thiolates aretethered on the faces of the nanocrystalsand they are likely to be erected on the sur-face. The shortest distance between theface-to-face {100} facets of the two adjacentparticles is only 1.5—2nm, almost equal tothe 1.5nm chain length of the thiolate mole-cules used for passivating the Ag nanocrys-tals. Therefore, the thiolate molecules teth-ered on the facets of the nanocrystals arelikely to interpenetrate, forming the

inter-digitative bonds

. This model is supported bythe image contrast displayed in Fig. 3a. Adirect observation of the thiolates can beprovided by the energy-filtered TEM (EF-TEM) [11].

The EF-TEM relies on the principle ofelectron energy-loss spectroscopy (EELS)and the images (or diffraction patterns) areformed by electrons with specific energy-losses [12]. If the electrons which have ex-cited the carbon K ionization edge are se-lected for forming the image, the imagecontrast is approximately proportional tothe thickness projected carbon atoms in thespecimen, providing a direct chemical mapof carbon. The thiolate molecules are com-posed of mainly carbon, thus the EF-TEMof the carbon K edge can give the distribu-tion of the thiolates around the nanocrys-tals. For this analysis, Ag NCSs are depos-ited on an amorphous SiO

x

substrate andthe effects from the substrate can be re-moved by processing the experimental im-ages acquired pre- and post-edge.

The EF-TEM was performed for the AgNCS oriented along {110}

s

, which is the op-timum orientation for imaging thiolate dis-tribution between the particles. The EF-TEM image acquired using the carbon Kedge from a Ag NCS gives an interestingcontrast feature (Fig. 4a). The projected car-bon density between the particles shows acontrast pattern that is the strongest be-tween the A and B types of particles, whilethe contrast is lower between the A and Cor B and C types of particles. To interpretthis phenomenon, we first construct the

[110]

s

projection of the NCS based on the3-D model given in Fig. 3b, and the result isshown in Fig. 4b. From the structural pointof view, the molecular bonds tend to paral-lely align the facets on which they are teth-ered. For the nanocrystals A and B assem-bled by facing the [100] faces, in addition tothe carbon density contributed by the inter-digitated thiolates passivated on the [100]facets (which are edge-on while viewedalong [110]

s

), the thiolates passivated onthe four [111] planes (not edge-on) alsocontribute to the projected carbon densityalthough the [111] faces are at an anglewith the projection direction. Therefore, theprojected density of the thiolate moleculesbetween particles A and B is expected to behigher than that between A and C (or B and

FIG. 4. (a) Energy filtered TEM image of the fcc struc-tured Ag NCS recorded using the electrons after ex-citing the carbon K ionization edge, showing thedistribution of the thiolates molecules around thenanocrystals. The orientation of the NCS is [110]s andthe 3-D self assembling model is given in Fig. 3b. (b) Amodel of thiolate distribution on the surfaces of Agnanocrystals, which supports the interdigitative mo-lecular bonding in the NCS.

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Z. L. Wang

C) if the size of {111} faces is the same as thefact of {100} as well as the density of the thi-olate passivation is the same on both {111}and {100}. Considering that the resolutionof the EF-TEM is on the order of

z

2nm, thechannels formed by the bundled thiolatesmay not be resolved in this type of images.

THE “GEAR” MODEL OF NANOCRYSTAL SELF-ASSEMBLY AND TWIN STRUCTURE

As reported previously, NCS has many as-pects analogous to the atomic level struc-ture, and it can have a variety of defectstructures [10]. One of the common defectsis a twin (Fig. 5). In this

fcc

structured NCSof Ag nanocrystals dominated by truncatedoctahedra, the twin and stacking planes are{111}

s

as indicated by arrowheads. To un-derstand this structure, two principles needto be observed in reference to the theoreti-cal calculation of Luedtke and Landman[13]. First, from the geometrical point ofview, the NCSs are assembled by aligningthe nanocrystals

face-to-face

. Thus, the {111}

facets can be interconnected naturally witheither the {111} facets of the {100} facets, thelatter, if possible, results in a rotation of thesuperlattice, possibly leading to the forma-tion of a {111}

s

twin. Secondly, the thiolatesare bundled on the facets of the Ag nano-crystals, the assembling of the nanocrystalscan be achieved by a “gear” model, inwhich the bundled thiolates are the “teeth”of the gear and the two nanocrystals are as-sembled simply by filling the space, result-ing in a rotation in the particle orientation.This type of assembling is simply a geomet-rical matching between the nanocrystals,while the van der Waals molecular interac-tion dominates the bundling of the thiolatesbelonging to the same particle. The twinmodel is thus given in Fig. 6, in which thetwo {111}

s

planes of the NCSs forming thetwin has an angle of 109.4

8

, in agreementwith the experimental image (Fig. 5), whilethe {111} faces of the nanocrystals on theleft hand side is not exactly parallel to the{100} faces of those on the right-hand side.This again suggests the erected bundling ofthe thiolates on the nanocrystal surfaces.

FIG. 5. TEM image of an fcc Ag NCS with twinned structure, where the unit cells are indicated on both sides of thetwin plane. There is a slight glide parallel to the twin plane.

Superlattices of Nanocrystals

107

SELF-ASSEMBLING OF MAGNETIC NANOCRYSTALS

Patterned magnetic nanocrystals are of vi-tal interest both scientifically and techno-logically because of their potential applica-tions in information storage, color imaging,bioprocessing, magnetic refrigeration, andferrofluids. In ultra-compact informationstorage, for example, the size of the domaindetermines the limit of storage densitywhile the sharpness of the domain bound-aries is closely related to the media noise.This issue is critically important in the 300Gbit/in

2

information storage predicted forthe 21st century [14]. The noise reductioncan be achieved by the segregation of anon-magnetic phase at the grain bound-aries, thus the media are composed of atleast two materials. The self-assembly pas-sivated nanocrystal superlattice is a poten-tial candidate for solving this problem, inwhich the passivated surfactant serves notonly as an isolation layer but also as a pro-

tection layer for the nano-magnets. Shownbelow is an example of self-assembling ofCoO tetrahedral nanocrystals.

Cobalt oxide nanocrystals were synthe-sized by chemical decomposition of Co

2

(CO)

8

in toluene under oxygen atmosphere. Sodiumbis(2-ethylhexyl) sulfosuccinate (C

20

H

37

O

2

SNa,in short, Na(AOT)) was added as a surfaceactive agent, forming an ordered mono-layer passivation (called the thiolate) overthe nanocrystal surface. The particle sizewas controlled by adjusting the wt.% ratiobetween the precursor and Na(AOT). Theas-prepared solution contained Co andCoO nanoparticles, and pure CoO nano-particles were separated using a techniquereported by Yin and Wang [8, 15].

Figure 7 shows a bright-field TEM imageof the as-formed NCAs on an amorphouscarbon film, in which the well-ordered nano-crystal arrays are seen with translationalorder. It is striking that all of the nanocrys-tals shown here have a narrow size distri-bution. The particles are in the size range of

FIG. 6. (a–c) A “gear” assembling model of the bundled thiolates for forming the twin structure in NCS.

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Z. L. Wang

4–5nm. The nanocrystal structure has beenidentified as CoO with the NaCl structure[8]. More importantly, the particle shapesare dominated by tetrahedra. This is thefirst example of illustrating the orderedself-assembling of tetrahedral nanocrystals.Recently tetrahedra Ag nanocrystals havealso been prepared and their assemblinghas been studied [16].

The strength of the chain molecule is inthe order of 0.1eV, comparable to the ki-netic energy of atom thermal vibration. Akey question here is about the stability andthe phase transformation behavior of theweakly bonded NCA since the meltingpoint of nanoparticles is much lower thanthat of the bulk. Considering the even

lower melting point of the passivating or-ganic molecules, the stability of the super-lattices (not only the structure of nanoc-rystals but also the “crystallinity” of thesuperlattice) above ambient temperature isa serious concern because the NCS is likelyto be used in the areas such as microelec-tronics and data storage. The in-situ behav-ior of monolayer self-assembled NCAs ofcobalt oxide nanoparticles has been ob-served using TEM [17]. The results provedthe high stability of a monolayer assem-bling to temperatures as high as 600

8

C,while the multilayer assembly coalesced at250

8

C. The substrate exhibited strong adhe-sive effect on the stability of the nanocrys-tals, but reaction between nanocrystals and

FIG. 7 TEM image showing the self-assembled monolayer of tetrahedral CoO nanocrystals (specimen provided byDr. J. S. Yin).

Superlattices of Nanocrystals

109

the carbon substrate occured at tempera-tures

z

400

8

C.

CONCLUSION

In this article, nanocrystal superlattices(NCSs) constructed by truncated octahe-dral Ag particles are studied to illustratethe bundling and interdigitation of the pas-sivation thiolates in forming the directionalmolecular bonds in NCSs. The thiolates arebundled and tethered on the faces of thenanocrystals, and tend to align the nano-crystal face-to-face in forming the NCSs.This configuration is favorable geometri-cally and energetically. The {111}

s

facets ob-served in the NCSs show the key roleplayed by the molecular bonding energy informing the surface of NCSs. This resultsuggests that the intermolecular interactionenergy is likely to be much higher than thephonon energy (

z

0.1 eV). The formation oftwins in the NCSs supports the “gear” assem-bling model of the nanocrystals. The gearassembling is likely to have higher energythan the interdigitative assembling. TheNCSs of truncated octahedral Ag particlesis an ideal system for observing these models.

The author is grateful to his collaborators, Drs.J. S. Yin, I. Vezmar, R. L. Whetten, S. A.Harfenist, J. Bentley, and N. D. Evans, for theircontributions to this review. This work wassupported in part by NSF grant DMR-9733160.

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Received June 1998; accepted September 1998.