Semina No

CHAPTER 6

Methods of Self-Assembling inFabrication of Nanodevices

V. Shklover,1 H. Hofmann21Laboratory of Crystallography, Swiss Federal Institute of Technology, Zrich, Switzerland2Laboratory of Powder Technology, Swiss Federal Institute of Technology, Lausanne, Switzerland

CONTENTS

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1822. Self-Assembly of Nanoparticles into Nanoaggregates . . . . . . . . . . . . 184

2.1. Self-Assembly Based on Molecular Recognition of Ligands . . . . 1842.2. Template-Assisted Self-Assembly Using Physical Confinement

and Capillary Forces . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1842.3. Layer-by-Layer Assembling . . . . . . . . . . . . . . . . . . . . . . . 1852.4. Self-Assembly on Large Colloids as Templates Using

Spray-Drying Method . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1852.5. Self-Assembly of Nanofibers by Polymer-Controlled

Mineralization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1852.6. Prediction of New Crystalline States for Self-Assembled

Nanocrystalline Aggregates . . . . . . . . . . . . . . . . . . . . . . . . 1853. Self-Assembly of Nanoparticles into Quasi-1D Arrays . . . . . . . . . . . 186

3.1. Carbon Nanotube Templated Self-Assembly . . . . . . . . . . . . . . 1863.2. Self-Assembly on LiMo3Se3 Nanowires . . . . . . . . . . . . . . . . . 1863.3. Template-Assisted Self-Assembly of Nanoparticles into

1D Arrays Using Physical Confinement and Capillary Forces . . 1874. Self-Assembly of 2D and 3D Nanocrystalline Arrays on 2D Substrates 187

4.1. Self-Assembly Using Gravity Sedimentation . . . . . . . . . . . . . . 1874.2. Self-Assembly of Nanoparticles During Solvent Evaporation . . . 1884.3. Self-Assembly Inside the Microchannels . . . . . . . . . . . . . . . . 1894.4. Self-Assembly of Colloidal Nanoparticles Between Two Plates . . 1904.5. Self-Assembly of Nanoparticles Monolayers on Vertical

Substrates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1904.6. Self-Assembly of Nanoarrays of Controlled Thickness on

Vertical Substrates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1914.7. Self-Assembly Using Shear Flow . . . . . . . . . . . . . . . . . . . . . 192

ISBN: 1-58883-075-6Copyright 2006 by American Scientific PublishersAll rights of reproduction in any form reserved.

181

Handbook of SemiconductorNanostructures and Nanodevices

Edited by A. A. Balandin and K. L. WangVolume 2: Pages (181213)

182 Methods of Self-Assembling in Fabrication of Nanodevices

4.8. Shear Alignment of Self-Assembled Arrays . . . . . . . . . . . . . . 1934.9. Self-Assembly Using Physical Confinement and Attractive

Capillary Forces . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1934.10. Self-Assembly on Templated Substrates . . . . . . . . . . . . . . . . . 1964.11. Template Assisted Self-Assembly Using Physical Confinement

and Capillary Forces . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1974.12. Self-Assembly Nanosphere Lithography . . . . . . . . . . . . . . . . . 1974.13. Self-Assembling of Nanoparticles on Phase-Separated Diblock

Copolymers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1984.14. Self-Assembly, Influenced by Dynamics of Colloidal

Organization During Solvent Evaporation . . . . . . . . . . . . . . . 1994.15. Self-Assembly as a Result of Progressive Condensation of

Intermediate Structural Building Blocks . . . . . . . . . . . . . . . . 1994.16. Assembling of Colloidal Particles Under Electric Field or

Magnetic Field . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2005. Nanodevices . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 201

5.1. Nanosensors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2015.2. Optical and Electronic Nanodevices . . . . . . . . . . . . . . . . . . . 203

6. Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 210References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 211

1. INTRODUCTIONThe term self-assembly is understood and used differently in many fields of science fromliving cell biology to the evolution of galaxies. We use self-assembly as applied to theformation processes that involve nanoparticles as preexisting components, are (not obliga-tory) reversible, and can be controlled by proper component design [1]. We will considerthe formation processes which lead to the ordered periodic structures. Self-assembly is nowa practical method of nanotechnology to design ensembles of nanoparticles for many appli-cations. The objective of this review is to make available a set of parameters that couldbe changed easily to enable the researcher to manipulate the self-assembling particles intonanoensembles or nanodevices. That is why we have to consider the self-assembly of nano-particles that involve regulated influence of environment such as crystallization on surfaces,boundaries, and template-assisted self-assembly. The discussion of the methods of fabricationof nanoparticles themselves will be not discussed although we realize that this fabrication ofnanoparticles is sometimes key to the self-assembly.The enormous potential of nanotechnology for industry can be explained by the size

dependence of many important physical and chemical properties at some scale because theproperties of individual atoms cannot be directly correlated with the properties of bulkmaterials. Different regimes of understanding of physical and chemical properties could besuggested: bulk, nanoparticles/nanostructures, atomic clusters. The given in [2] systematicof size-dependent properties includes optical [36], magnetic [7, 8], catalytic [912], thermo-dynamic [13, 14], electrochemical [15], and electric transport properties [1618].The opportunities of self-assembly approach to the design of three-dimensional (3D) pho-

tonic crystals are especially important. The idea on periodic 3D dielectric structureswith electromagnetic band gap due to refractive index modulation, which can permit dis-

tinct laser modes, was formulated by Yablonovitch [19] and John [20]. Yablonovitch has alsoshown that this concept is observed in such experimental systems as hexagonally close-packedglass or polystyrene spheres [19]. As a result of further studies, the use of photonic crystalsfor direction, control, splitting, guiding, and localization of light was suggested. Photoniccrystals have a dominant role in recent investigations toward elaboration of all-optical inte-grated circuits for future communication and computing systems [21]. The resolution thatcan be routinely achieved by lithographic techniques is at the level of several micrometers,and new emerging methods require expensive installations. Because of these limitations, theself-assembling approach remains the most practically important method of fabrication of

Methods of Self-Assembling in Fabrication of Nanodevices 183

periodic structures for photonic applications. Electron-beam and scanning probe lithographypattern the substrate sequentially and therefore this becomes very time consuming.Many examples show that nanoconfinement allows for new opportunities in organizing,

ordering, or assembling nanoparticles. This can lead to materials with qualitatively newphysical and chemical properties. For example, the traditional devices cannot be used todesign sharp waveguide bends without significant signal loss. Two-dimensional photonicnanostructures consisting of periodic 2D arrays of columns of dielectric material can serveas efficient waveguides around a 90 bend [22, 23].Development of the self-assembly approach presents a so-called bottom-up approach,

where a new, functional material or device can be constructed using self-assembly of sim-ple constituent nanoscale elements into an ordered nanocrystalline array, which can lead tosignificant achievements in nanotechnology. While the fabrication techniques of current com-mercial importance such as lithography belong in the top-down category, a bottom-upapproach may offer a number of advantages. The main advantages of bottom-up approachare experimental simplicity, possibility of 3D assembly and potential for inexpensive massproduction [24]. The three goals of the bottom-up approach [25] are (1) self-assembly ofnanometer-scale structures from nanocrystals dispersed in solution, (2) self-organization ofthese structures on technologically relevant substrates, (3) processing of organized arraysinto structures suitable for practical applications.The bottom-up approach is very prospective for medicine and biotechnology because func-

tional biological objects are basically constructed after the same bottom-up principle (see,e.g., Ref. [26]) but with the biological molecular elements as constituent building blocks. Thedesign of tiny (biological function) imitating sensors and devices, artificial and biocompatibletissues, as well as new efficient highly specific small-dose drugs are possible applications ofthe self-assembly approach.The realization of self-assembly of nanoparticles into ordered arrays for design on nan-

odevices can be divided into the following steps:

1. Understanding the interaction between nanoparticles and the main physicochemicalvariables governing formation of functional nanocrystalline arrays.

2. Understanding of function and functional requirements for the desired device.3. Realization of the desired functional structure at the nanolevel, construction of the

nanodevice.

We tried to extract these three directions of interest during the work on this review.In this review, we concentrate in this review on the methods of self-assembling using nano-

particles, leaving aside a large field of publications concerning self-assembly of nanotubes(see, e.g., Ref. [27]), supramolecular self-assembly (see, e.g., Ref. [27, 28]), and self-assemblyof polypeptides and DNA (see, e.g., Ref. [2931]). The practical interest to nanosizedcolloidal particles can be explained among others by the fact that the wavelength of thevisible light coincides with colloidal size range, allowing for opportunities for design of opti-cal switches, chemical sensors, and photonic band gap materials [32]. The importance ofparticles in constructing photonic devices can also be explained by the difficulties in thefabrication of periodic 3D PBG crystals using conventional microlithographic techniques.Self-assembling molecules, macromolecules, molecular assemblies, and supramolecules areoutsides the scope of this chapter.Besides, we consciously limited self-assembly processes leading to the formation of peri-

odically ordered nanocrystalline arrays, starting from ordered aggregates, ordering intoone-dimensional (1D) templates, but our principal concern was with 2D and 3D orderingon planar or nonplanar substrates. Therefore, we divide the examples of contributions ofself-assembly phenomenon in nanodesign into three groups:

1. Self-assembly of particles into nanoaggregates.2. Self-assembly of nanoparticles on quasi-1D templates (such as nanotubes).3. Self-assembly of nanoparticles on 2D planar or not planar substrates.

We did not consider the not less interesting studies on nanoparticles aggregation where noperiodic ordering of nanoparticles occurs (see Refs. [33, 34]).


The objective of this chapter is not to provide a comprehensive review of all availablestudies comprising self-assembly as a main or important step in the fabrication of new nano-structures or nanodevices. Instead, we try to present the most characteristic investigationsto provide the reader with the necessary tools to navigate the ocean of current publicationson nanotechnology.According to the review title, we have focused on the formation of organized nano-

arrays and on nanodevices, based on organized nanoarrays, and did not consider applicationsexploiting the physical properties of individual nanoparticles. Additionally, we will not discussthe studies of nanocrystalline arrays with only local ordering (colloidal glasses), discussedfor example in Ref. [35] review.We had some difficulties systematizing the known approaches based of the self-assembly

of nanoparticles. We realized that the essential part of the presented examples of the use ofself-assembly for the formation of ordered periodic nanoarays have simple evidences of morethan one characteristic features. For example, the crystallization due to solvent evaporationis often performed using templated substrates. This kind of difficulty was also encounteredin a recent review article on self-organization of colloids under external control [60]. Wehope that we have finally found a system that exploits different methods of design withinthe self-assembly approach.

2. SELF-ASSEMBLY OF NANOPARTICLES INTO NANOAGGREGATES

2.1. Self-Assembly Based on Molecular Recognition of Ligands

The search for new possibilities in control of self-assembling parameters led to the develop-ment of more sophisticated methods based on the functionalization of nanoparticulate build-ing blocks with oligonucleotides [3638]. The recognition and selective binding of primaryDNA-nanoparticle associate to other DNA sequence and the subsequent agglomeration ofthe particles in a controlled fashion was used for fabrication of satellite structure. Thistype of structure involves smaller nanoparticles located around larger ones, forming a kindof ordered surface structure. In these studies, the recognition ability of oligonucleotides wasused for direct construction of a nanoassembly from nanoparticles of two different types. Asit was pointed out in Ref. [39], this approach can be use for nanoparticles of different sizeand different chemical nature, such as CdS, CdSe, Pt, or Ag.Further advancement in the self-assembly of nanoparticles into nanoaggregates was

the development of bricks and mortar strategy, originating from the use of functional-ized Au colloids as bricks and specially prepared polymer diaminotriazine-functionalizedpolystyrene as mortar [40]. Both discrete microspheres and network structures are highlyordered on the molecular, nanometer and micrometer levels and can be used as buildingblocks, or precursors, for further nanoparticle assembly.

2.2. Template-Assisted Self-Assembly Using Physical Confinementand Capillary Forces

The assembly of nanoparticles using a combination of physical confinement and capillaryforces (see 4.9) can be used for the formation of spherical colloid aggregates with thetemplate-assisted self-assembly (TASA). On the first step, the colloidal ring aggregates haveto be constructed in the template holes. Two approaches were suggested for further stabi-lization of the fabricated aggregates as independent particles:

1. Annealing at the temperatures slightly higher than the temperature of the glass tran-sition of Tg of the polymer (93 C for PS). After removal of the photoresist, the PSbeads aggregates can be released by sonication. The percentage of the uniformly shapednanoaggregates can be high, about 75%.

2. Adding UV-curable pro-polymer to the colloidal suspension before assembling, whichcould serve as a glue to stabilize the particles within the aggregate.

TASA using combination of physical confinement and attractive capillary forces allows forcontrol of the size, shape, and structure of the aggregates assembled from spherical colloids


[41]. The control parameters include the shape of the templates (cylindrical, prism shaped),dimensions of the template holes (depth, hole diameter), the relation between the diameterof particles and holes, and colloid concentration. Using this method, a high overall yield ofthe aggregates (usually 90%) could be stressed.The formation of heteroaggregates can be reached through the stepwise application of

the TASA process to nanoparticles of different size and composition on the same surface,enabling new properties for the constructed aggregates [41].

2.3. Layer-by-Layer Assembling

The layer-by-layer assembly (consecutive adsorption onto core particle) [42] was used forfabrication of quantum dot (QD) labeled biofunctional beads [43]. The first step was deposi-tion of the primer polyelectrolyte film onto 925-nm-diameter PS spheres, providing a uniformpositively charged surface. The negatively charged CdTe nanocrystals 3.54 nm in diametercarrying carboxyl groups (COO were then in alternation with further polyelectrolyte lay-ers. The finalIgG layer provided biospecifity to a QD-tagged PS bead. TEM study allowedfor observation of clearly resolved lattice planes of CdSe QD and an estimate of the averagePE /CdTe QD film thickness of 10 nm.

2.4. Self-Assembly on Large Colloids as Templates UsingSpray-Drying Method

Good controllability of the pore size and morphology of porous silica particles were achievedwith a spray-drying method using PS beads and silica colloids [44]. The pore size can beeasy controlled by the size of colloidal particles. A posteriori high-temperature treatment isnecessary to remove the PS nanoparticles. The pores are arranged, as in hexagonal packing,and exhibit high periodicity, indicating that the self-organization occurs spontaneously dur-ing the solvent evaporation. The pore surface appearance was close to hemispherical. Therelationship between the size of the templating PS beads and silica particles was found toinfluence the self-organization of the porous structure. This agrees with known data for col-loids and films on planar support. The surface coverage of CdSe particles, determined usingsingle-particle light scattering (SPLS), is higher than calculated using a close-packed modelof CdSe nanocrystals on the modified PS surface. This was explained by partial infiltrationof CdSe nanoparticles into the film.

2.5. Self-Assembly of Nanofibers by Polymer-Controlled Mineralization

Highly ordered funnel-like BaCrO4 superstructures were produced by using sodium poly-acrylate as a structure-directing agent in the mineralization process [45]. The morphology ofthe fibers (length, axial ratio, opening angle, mutual packing) can be controlled by temper-ature, pH, and concentration. Isomorphous BaCrO4 structure shows the same self-repeatednucleation and growth pattern but with smaller cone angle and less cohesion between theelementary nanofibers.

2.6. Prediction of New Crystalline States for Self-AssembledNanocrystalline Aggregates

The genetic algorithm for the prediction of the structure of self-assembly products (creatingnew structures by combining characteristics of parent structures) is reported in Ref. [46].The packing density was used as the ultimate figure of merit and binding energy as a fitnessfunction. The ABC, ABC2, AB2C2, ABC3, ABC4, and fused AB were studied, covering therange from RB/RA and RC/RA size ratios from 0.2 to 0.9. The ternary systems were foundto be higher on packing density than phase-separated systems. The consideration of ABstoichiometries gave several well-known phases ZnS, NaCl/NiAs and CsCl for RA/RB 022,0.41, and 0.73, respectively. New self-assembled (AB)2 structure was found in the RA/RBrange of 0.40.65 with large spheres forming impenetrable layers and small spheres pairedin well separated zigzag rows.


3. SELF-ASSEMBLY OF NANOPARTICLES INTO QUASI-1D ARRAYSThe design of structures, where nanoparticles are ordered on some quasi-1D substrates, isone of the interesting directions of nanotechnology and may lead to new materials withpractical and interesting properties. Nanoparticles/nanowires composites can be used forformation of soluble electrically conducting films, which could be deposited from solution.Fabrication of nanostructures, where the nanoparticles are integrated into a conductingnetwork with quasi-1D support, has great potential for photochemical devices, solar cells,batteries, chemical sensors, and electrochemical catalysts [4752]. The review article [53]contains the discussion of the fabrication of 1D nanocrystalline arrays by templating againstalready existing nanostructures. We will consider several examples. The densification of thenanoparticle arrays on the template surface is one of the important questions of quasi-1Darray fabrication.

3.1. Carbon Nanotube Templated Self-Assembly

Carbon nanotubes were used for templating the self-assembly of gold nanocrystals [25].During the assembly, an intense dark-red color of chloroform dispersion of 6-nm gold nano-crystals decreased in intensity when mixing with carbon nanotubes and could be an indicationof binding between the nanotubes and gold nanocrystals. The TEM patterns show a nearlymonolayer coverage of gold nanocrystals adsorbed at the surface of the carbon nanotube.Another product of the self-assembling of gold nanoparticles on the single-walled carbonnanotube (SWNT) was characterized with TEM. The stabilization of gold nanocrystals asa monolayer on the surface of fullerene C60 was also reported in Ref. [54]. Using the self-assembling gold nanoparticles on carbon nanotube, the nanowire with the length up 10 mwere obtained, though, in this case, the absorbed gold nanoparticles are separated from eachother. In the separate step, the nanotubes were annealed at 300 C, and originally, sphericalgold nanoparticles had changed their shape and coalesced because of melting on the surfaceof nanotubes [24]. By this means, the very important third goal of the bottom-up approachwas achieved, namely, the processing of organized arrays into structures suitable for practicalapplications (see Section 1).

3.2. Self-Assembly on LiMo3Se3 Nanowires

We will discuss the kind of nanoparticles/nanowires composites, based on LiMo3Se3nanowires, where it has been possible to obtain a dense location of nanoparticles on thesurface of the 1D nanoarray acting as a template.The LiMo3Se3 nanowires have a diameter of only 0.85 nm (the diameter of single-and

multiwalled nanotubes is 12 and 225 nm, respectively), bundles of LiMo3Se3 wires havemetallic conductivity and are soluble in polar organic solvents [47]. The LiMo3Se3 nanowires,decorated with ligated 6-nm CdSe nanoparticles and 13-nm Au nanoparticles, were obtainedfrom LiMo3Se3- and trioctophosphineoxide-ligated CdSe and citrate-ligated Au particlesin DMSO, respectively [47]. Dependence of the product morphology on the nanoparticlesconcentration was studied for CdSe.LiMo3Se3 composite. At low CdSe content, under thehomogeneous reaction conditions, the CdSe nanoparticles are randomly distributed overthe nanowires surface and the morphology of the LiMo3Se3 polymer was unchanged. Atan excess of CdSe nanoparticles, at heterogeneous reaction conditions, the LiMo3Se3 wiresare entirely and uniformly coated with CdSe nanoparticles, the nanowires play the role oftemplate for anisotropic condensation of CdSe nanoparticles. As should be expected, theLiMo3Se3 network becomes less regular as a result of the formation of Au.LiMo3Se3 com-posite because of the larger size of Au nanoparticles (13 nm, which means that surface areaof Au nanoparticle is about five times that of CdSe nanoparticle). The Au nanoparticlesare incorporated into nanowires what could mean stronger wire-particle interaction than ofCdSe nanoparticles. Study of the washing of composites CdSe.LiMo3Se3 and Au.LiMo3Se3in organic solvent indicates that covalent bonding is most likely interaction between nano-particles and LiMo3Se3. The Au.LiMo3Se3 strands are likely bond together electrostati-cally. The temperature dependence of conductivity indicates metallic behavior for films of


LiMo3Se3 and Au.LiMo3Se3 over a temperature interval of 5 K to 300 K, whereas films ofCdSe.LiMo3Se3 have metallic character only at high temperature. Below 60 K (low CdSecontent) and 120 K (high CdSe), these films exhibit semiconducting characteristics. Possi-ble interpretation of temperature dependence of conductivity leads to the conclusion thatconductivity of CdSe.LiMo3Se3 is via framework even at high CdSe content [47].

3.3. Template-Assisted Self-Assembly of Nanoparticles into 1D ArraysUsing Physical Confinement and Capillary Forces

Self-assembling of monodispersed spherical particles into 1D arrays using a combinationof physical confinement and capillary forces on 2D templates (see Section 4.9) has greatpotential for fabrication of 1D arrays of different configurations and with well-predictedstructure [53, 5557]. For example, the assembling of very small 150-nm spherical colloidsinto a linear chain using a template with parallel trenches was performed. The zigzag chainscan be obtained by using templates with cylindrical holes, where the diameter of the holesexceeds the diameter of the beads. Interestingly, that in the last case the PS beads in thesame layer were not in physical contact and the aggregation was accomplished by the contactof the beads from the upper level. The preparation of helical 1D nanoarrays is also possible[53]. The interplay between the dimensions and shape of the particles and holes can beused to increase the complexity of the 1D motives with gradual transition to 2D and 3Dstructures. Several nanomotives were produced, illustrating the possibility of the controlof the individual chains morphology by the length of the channel. Incomplete filling ofthe trenches is the common defect of the aggregates. The limit on the feature and sizeof aggregation products depends on the density of used colloidal particles, temperature,and rate of the crystallization process. All these parameters influence the balance of forcesgoverning nanocrystalline array formation. The application of an external magnetic field ormagnetic nanoparticles is another means of advancing 1D nanoarray assembly [56].

4. SELF-ASSEMBLY OF 2D AND 3D NANOCRYSTALLINE ARRAYSON 2D SUBSTRATES

The systematization used in this section is, of course, only one of the possible approaches.Very often the described processes are combinations or overlaps of several processes. Forexample, self-assembling in physically confined cells can proceed during solvent evaporation,self-organization under external field is carried out as templated crystallization, and solventevaporation is one of the important driving force of different other methods of self-assembly.We did not separate, for example, the crystallization of negatively charged nanoparticles asindependent types of self-assembling because this process is considered in other parts of thereview as a constituent part of the other self-assembly processes.

4.1. Self-Assembly Using Gravity Sedimentation

Sedimentation in a gravitational field, from dispersions, is the most natural way to producenanocrystalline arrays among other methods of self-assembly. But, noted in Ref. [58], thesedimentation procedure is not that simple and comprises several processes such as gravita-tional settling, Brownian motion, and crystallization, including both nucleation and growthstages. Self-assembling using sedimentation is most commonly applied to silica colloids dueto the high density of amorphous silica [58]. The sedimentation process normally leads tothe formation of cubic close-packed (ccp) structures.The crystallization process in a system of sedimenting silica colloids was studied using

confocal microscopy [59]. Upon increasing sediment thickness, crystallization starts in theliquid layer at the bottom of the sample container. Crystallization in this first layer proceedsvia first-order transition. Crystal growth in the second layer proceeds epitaxially on the topof first 2D organized layer. The stacking errors can be reduced if the crystal growth rate isreduced or by use of the template with the symmetry of the face-centered cubic (fcc) (111)plane [60].


Using slow sedimentation of the charge-stabilized 260-nm silica colloids, the regular 3Dfcc superlattices were obtained showing photonic band gap in the visible spectrum range[61]. The faceted contacts could be observed because of applied sintering of the sample at600 C, most likely due to formation of siloxane-bonding formation between free hydroxylgroups. The sintering also affects the photonic properties of colloidal crystals. The slowsedimentation method was used for assembling photonic crystal from SiO2 nanoparticleswith the diameter of 295 nm used for study of lasing properties [62] (see Section 5.2.7).Photonic crystals built of ZnO colloidal spheres of different diamters from 116 to 602 nm

were fabricated using the sedimentation self-assembly process [22]. The solution was droppedonto Si or glass substrates typically at 160 C. Optical microscopy shows no differencebetween the layers formed on Si or glass substrates. Domain size is in the range of sev-eral millimeters. Temperature and solvent play an important role in this type of particleself-assembly. Layers, assembled at low temperature, exhibited no periodicity observable inSEM. However, layers deposited at high temperature had bad adhesion to the substrate.Best layers were produced from the reaction solution. Cross-sectional SEM patterns indicatefcc structure of deposited layers.The application of the sedimentation method in the formation of opal using core-shell

Au@SiO2 nanoparticles [6365] does not differentiate from the deposition of the nor-mal nanoparticles due to the same nature of the particles surface. The plasmon resonanceand Bragg diffraction in these systems obey different mechanisms. The Au@SiO2 nano-particles were sedimented using 3-(trimethoxysilyl)propyl methacrylate followed by tetram-ethoxysilane (TMOS) injected and continued centrifuging. The hydrolysis of the TMOS andoligomeric silane groups connected the colloidal opal leads to good-ordered structures thatwe characterized by electron microscopy.Generally, the sedimentation method is quite slow and does not allow a sufficient control

over either the surface morphology or the thickness of the resulting nanocrystalline array,see for example [6668].

4.2. Self-Assembly of Nanoparticles During Solvent Evaporation

Crystallization during solvent evaporation is one of the frequently used techniques for 2Dand 3D assembling of colloidal crystals, discussed in Ref. [69]. Advantages of this techniqueare (1) simplicity, (2) low cost, (3) possibility to grow millimeter-sized arrays, and (4) possibil-ity to control thickness of the array by varying initial suspension concentration. The shortageof crystallization during solvent evaporation is that the crystallographic 3D orientation ofthe crystallized array with regard to the substrate could not be reached.The narrow distribution of nanocrystal size is important for self-assembly of colloidal

nanocrystals into ordered superlattice. If the dispersion of nanocrystal size does not exceed5%, then the self-assembly of even very small nanocrystals can be achieved (see Ref. [35]).Successful self-assembly into supercrystal of even very small 3.6-nm CdSe nanocrystals with-out any fractioning or size sorting was peformed [70].However, the observation of perfect ordering attests to a narrow nanoparticle size distribu-

tion. For example, the PbSe quantum dots, which, depending on the synthesis temperature,can have different diameter from 3 to 8 nm, assemble into ordered close-packed arrays. Theordering confirms that the QD PbSe have well-controlled size and shape. The clear hexago-nal structure of the individual QD PbSe indicates that the synthesis method used producessingle-crystalline nanoparticles free from internal defects [71].The structure of the self-assembled aggregate depends on the rate of solvent evapora-

tion (destabilization). At slower destabilization, the particles form close-packed amorphousaggregate. Slow destabilization by evaporation from the mixture of solvents results in orderedcolloidal crystals [35]. The initial stages of the growth of CdSe colloidal crystal were followedwith electron microscopy. The incomplete {111} pyramids are the most common shape ofthe colloidal crystal.The formation of very good ordered square-symmetric arrays was observed for monodis-

perse maghemite Fe2O3 nanoparticles, deposited onto the TEM Cu grid from hexane oraqueous dispersion and then dried under vacuum at 100 C, according to TEM studies [72].


The particles were stabilized against aggregation with a layer of oleic acid. The exchange ofoleic acid for block copolypeptide results in the stabilization of the nanoparticles into smallaggregates.The progress in nanostructures fabrication using slow evaporation of thiol-stabilized gold

and silver nanoparticles can be found in the review in Ref. [24]. Results of these studiesconfirm importance of metal particles monodispersity for fabrication of 2D or 3D arrays. Insome cases, however, interesting phenomena of ordering of nanoparticles of two differentsizes can be observed [73, 74]. Patterns of thiol-derivatized gold nanocrystals, spontaneouslyorganized into a well-ordered 2D array were observed. Larger Au particles form a hexagonalarray with smaller particles occupying the trigonal interstices. According to HRTEM, the{111} planes of the larger particles are parallel to the film plane and exhibit twinning oficosahedral and decahedral particles. No twinning was observed for smaller particles, whichcould be described as truncated cubooctahedra. The local stoichiometry of the observedordered bimodal region is AB2 (A designates large and B small particles).Interestingly, the similarity between bimodal structure of nanoscope Au array, that is

observed in other colloidal crystals (in micrometer scale) with ordered intermetallics, canbe observed in the AlB2 [24] or basal plane of the intermetallic CaCu5, which also has A:Bration of 1:2 [73]. According to estimations [75], the AB2 colloidal crystals are only stablein the range 0482 < RB/RA < 0624 (the ratio of the radii of two components) [75] andphase diagram consideration indicates particular stability of the array with RB/RA 058.The coexistence of spatially resolved monodisperse and bimodal rafts in the product of self-organization of Au nanoparticles of two different sizes is remarkable and was explained byentropy-driven crystallization [73].Other organized bimodal nanostructures such as AB, AB3, AB4, and AB13 are also possible

[74]. When nA/nB = 1 (local number ratio of two components) and 027 < RA/RB < 0425,the AB superlattice can be formed, which could be considered as an analogue of NaClstucture. The metallic cores on the SEM patterns of fabricated bimodal nanoarrays are notdirectly touching because the particles are surrounded by the capping thiol groups.The first-ordered mixed Au/Ag superlattices were obtained when gold particles were in

large excess (e.g., Au:Ag > 10:1). The largest raft of this array type (typically embeddedinto random alloy array) has dimensions of 150 nm 150 nm, significantly less than the sizeof bimodal AB rafts of 1 m 1 m. The more dark particles on Fig. 24 with diameter of8 nm form a hexagonal array; the less dark particles with diameter of 5 nm are situated inthe trigonal interstices. By this means, the local stoichiometry of the nanoarray correspondsto AB2 structure. The distances between the nanoparticles are 1.5 nm, determined by thedimensions of the C10 thiol radicals bonded to the particles surfaces. The high precisionEDX analysis (using a 0.7-nm electron probe) shows strong correlations of Au signal with8-nm separations and of Ag signals with 5-nm separations, confirming the local ratio ofnAu/nAg = 05. The closeness of the RA/RB ratio to upper limit of the predicted [75] range of0482 < RB/RA < 0624 could be explained by additional electronic stabilization of the arrayby electron transfer between Au and Ag particles via tunneling mechanism at the observedinterparticle separations [24, 74].The ordered, nanoparticle assemblies may exhibit interesting electronic properties, dis-

cussed in Ref. [24], including Coulomb staircase charging behavior and conductivity by acti-vated electron hopping. The interplanar separations between the particles can be changed byvariation of attached groups or linker molecules what opens the way to a new class of mate-rials with tunable properties. As an example, the metal-insulator transitions were observedby the change of the thiol chain length, as it was shown by the studies of optical propertiesof Langmuir single layer films [76]. When propanediol is used as a linker, the metal nano-particles are close enough to have the properties of the continuous metallic film, as it wasshown by investigations of UV-vis spectra of array of gold nanoparticles [24].

4.3. Self-Assembly Inside the Microchannels

The geometrically confined silica colloidal crystals were grown inside rectangular microchan-nels of different depths and on different substrates using directed evaporation induced


self-assembly. As observed from the SEM patterns, crystallization takes place within thechannels, indicating that capillary forces were driving the process. The SEM top view showsthe hexagonal packing of the spheres. The spatial coherence along the channels is of theorder of the hundreds of micrometers to, sometimes, millimeters. The consideration oftransversal and longitudinal patterns confirms fcc structure rather than random packing ofthe obtained colloidal array. The consideration of the SEM patterns also shows that the[112] crystalline direction is perpendicular to the walls, and the [110] direction is parallel tothem. This was also confirmed by computer simulations. According to Ref. [77], the resultingcrystallographic orientation of designed nanocrystalline arrays is the product of a complexprocess including capillary, convection, and gravitational forces, as well as particleparticleand particlewall interactions. The particles tend to create high-density crystalline surfacesin contact with the flat walls or substrate and explain observed formation of oriented fccstructure in the channels.

4.4. Self-Assembly of Colloidal Nanoparticles Between Two Plates

In Ref. [78], the self-assembly of spherical nanoparticles between two quartz plates wasused for fabrication of two kinds of periodic nanocrystalline materials. In the first kind ofmaterials, nanocrystalline arrays are fabricated from spherical poly-N-iropropylacrylamide(PNIPAM) spheres, the diameter of which changes with the temperature due to knownvolume response effect of PNIPAM. For example, the diameter of PNIPAM sphere changesfrom 100 nm at 40 C to 300 nm at 10 C. Optical properties of resulted nanocrystallinearrays are discussed in [78]. The second kind of new materials are nanocrystalline arraysof PS spheres assembled in the NIPAM solution and further UV-polymerized to improvethe mechanical properties due to locks in the ordering. The polymerized films shrink andswell continuously between 10 and 35 C, with the change of the lattice spacing and thecorresponding change of diffraction pattern. Variation of temperature can be used for tuningdiffraction wavelength with the temperature. This means that the film functions as controlledtunable optical filter. The diffraction wavelength can be changed by varying the temperatureor the angle of incidence. The film can act as a tunable wavelength reflector at a constantangle of incidence.The crystallization of 842 nm PS spheres between two quartz plates was used for the study

of the structural transitions within the nanocrystalline array subjected to lateral compressionwithin the wedge [79]. The methods of videomicroscopy and SEM were used. The depen-dence of the microstructure of the crystallized array on the local height of the wedge wasstudied. The sequence of the closest-packed structures as function of the cell height waspredicted and basically confirmed by the experimental observations. The observed bucklingprocess between layers of quadratic symmetry with n and n+ 1 layers was interpreted asfcc stacks with periodic patterns of stacking faults.The crystallization of suspension of 326-nm PS nanoparticles between two parallel glass

walls separated by 28 m was used in the study of the possible role of like-charge attractionsin the formation of colloidal crystallites [80]. The structure and dynamics of these metastablecrystals indicate strong, long-range interactions between similarly charged particles. Theseinteractions might influence practical properties of colloidal suspensions.

4.5. Self-Assembly of Nanoparticles Monolayers on Vertical Substrates

Linear continuous growth of nanoparticle monolayers during the substrate movement in theopposite direction but with the same rate as the movement of the growing array was usedto fabricate 2D nanocrystalline arrays [81]. Water evaporation is the driving force of thisfabrication approach. The quality of the grown arrays was dependent on the monodispersityof the nanocrystalline suspension, as it was demonstrated by comparing morphologies of thearrays, fabricated from nanocrystalline arrays with 2.8% and 1% polydispersity. The follow-ing conditions of the growth process have to be met: the suspension quality, atmospherearound the cell, substrate quality, cell cleanness, and wettability. The particles have to bemonodisperse and have to absorbed onto the substrate. This self-assembly approach exploitscapillary forces and is similar to Langmuir-Blodgett technique; however, the layer and its


structure are forming simultaneously on the solid substrate and a continuous formation ofthe layers is established.The quantitative analysis was applied to control the process parameters. The following

expressions for the rate of the array growth vc and withdraw rate of the substrate vw weresuggested [81]:

vc =lje

h1 1 (1)

vw =l

0605je

kd1 (2)

where l is experimentally determined constant, je is the water evaporation rate, h is thethickness of the particle array, = NwVw is the particle volume fraction, 1 = NwVwis the water volume fraction, 1 is the array density ( is the porosity of the array), k isnumber of layers, and d is particle diameter.

4.6. Self-Assembly of Nanoarrays of Controlled Thicknesson Vertical Substrates

Using this approach, the SiO2 colloidal arrays with dimensions of up to 1 cm in length wereobtained using capillary forces for organization of colloids, ranging in diameter from 260to 384 nm [82]. The vertical setup was used but was unnecessary to withdraw the substratefrom the suspension. The natural evaporation rate was used. Both the ordering of colloidalparticles parallel to the substrate plane and normal to the film are important for opticalproperties of the films. Particle diameter d and particle volume fraction were found to beimportant for this deposition process. For the same volume fraction, a solution of large par-ticles produces fewer layers. However, more concentrated suspensions lead to thicker films.The formation of thicker films requires multiple depositions because film quality degradeswith the increase of the suspension concentration above 4%. As it was noted in Ref. [82],consideration for only SEM patterns does not allow for distinguishing between fcc andhcp structures, although the theoretical study indicates that fcc structure is more stablefor noninteracting particle arrays. The homogeneous color of obtained films confirms thesingle-crystal character and uniform orientation of the (111) domains related to the substratesurface. Two types of vacancies are observed in obtained colloidal arrays: sphere vacanciesafter every 10 m and vertical cracks observed after every 10 m. The reason for crack-ing could be both (1) metal deposition for SEM and high-vacuum conditions during SEMand (2) drying-induced shrinkage of the film.The possibility to construct films of precisely controlled thickness is a great advantage

of this method of self-assembly. The variation from several layers to hundreds of layers ispossible by variation of colloidal concentration and sphere size. The model developed byNagayama et al. for self-assembly of monolayer colloidal films (see Refs. [81, 8385]) wasadapted to the formation of multilayer films leading to the following dependence for thethickness of multilayer [82]:

k = L0605d1 (3)

where k is the number of layer, L is the meniscus height, is the ratio between the velocityof the particle in solution and the fluid velocity, d is the sphere diameter, and is theparticle volume fraction in the suspension. The independence of the final film thickness onthe solvent evaporation rate draws attention.Observed dependence of the peak position in the optical spectrum at normal incidence

agrees with the results for arrays obtained by the gravity-sedimentation method. The positionof this peak can be found using the sphere diameter and effective refraction index of themedia [82]:

max = 2neffd111 (4)


where d111 = 2/3D is interplanar separation in direction [111], and D is sphere

diameter.Peak positions, predicted from Eq. (4) agree well with experimental values. The observed

increase of the peak width with the increase of the film thickness is similar to Debye-Scherrerbroadening of X-ray diffraction peaks.In the process of controlled drying, the 2D hexagonal close-packed crystal of large L

silica spheres (RL = 203 nm) were grown on a vertical glass substrate at a growth rate of1 to 2 mm per day and used as templates for layer-by-layer deposition of small S silicaor polystyrene particles (RS = 110 nm for silica, 97 nm for PS) [86]. Depending on thevolume fraction and size ration = RS/RL, small particles arranged in periodic arrays. Therepetition of the deposition of the layer of large particles leads to binary colloidal crystals.Results of the fabrication of the binary colloidal crystals in a narrow range of = 0.48to 0.54 are reported in [86]. Three types of binary colloidal crystals with stoichiometry LS,LS2, and LS3 were fabricated and characterized with electron microscopy. For size ratiosclose to 0.5, colloidal crystals of the stoichiometry LS2 were grown. Both increase (LS3 anddecrease (LS) of the amount of small spheres led to unexpected crystal symmetries [60].The self-organization of small particles of the second layer can be considered as the resultof several processes: geometrical packing requirements, minimization of the surface energyof the drying film, and surface forces due to the curved menisci [86].

4.7. Self-Assembly Using Shear Flow

The flow velocity gradient (e.g., shear rate) generates shear stress in a suspension, leadingto formation of different microstructures. Strong shear flow can lead to the formation ofmicrostructure with a long-range order in resulted colloidal crystal, depending on the direc-tion of the streamline and shear gradient. On the basis of the concept that an abrupt releaseof the high rate shear flow may result in a well-oriented colloidal crystal, if the nonequi-librium ordering at high-rate shear flow is close to the equilibrium crystal structure (shearquenching), the series of highly ordered photonic structures was fabricated using apparatusdescribed in detail in Ref. [87]. The [111] direction of the shear-induced structures was nor-mal to the capillary axis and [110] direction was parallel to the capillary axis. The angulardependence of the transmission spectrum of the obtained array contains two sharp dips, 1 and 2, shifting to different directions during the change of the incidence angle. Thefollowing expression was used for the determination of the angular positions of the Braggwavelength:

Bragg = 2ncs0Ghkl/Ghkl2 (5)

where Bragg is the Bragg wavelength in air, n is the refractive index of the suspension, s0 isthe unit vector parallel to the incident light in the suspension, Ghkl is the reciprocal vectorof the hkl reciprocal lattice point. The refractive index was found using the expression

nc = nw1 %+ np% (6)

where nw is the refractive index of water (1.33) and np is that of polystyrene (1.59). The 1can be explained as diffraction from (111) planes and 2 as diffraction from (002) planes.

At the same time, the dip 1 could originate also from diffraction from the hcp spacingsaligned as a random stacking or hcp stacking. The uniform change of the color during therotation of the designed colloidal array indicates its single-crystalline character.The more detailed description of the formation of colloidal crystal under shear flow is sug-

gested [87]. When the shear rates are lower than G1 (1 s1, the steady-state microstructureof the suspension is polycrystalline. The ordering of suspension is a function of the shearat the rates above G1. At high shear rate G2 (of the order of 6,000 s1 complete loss ofordering occurs (shear melting). The shear quenching technique applied in [87] is in contrastto the shear annealing technique of transformation of polycrystalline suspensions into largersingle-crystalline arrays.


4.8. Shear Alignment of Self-Assembled Arrays

To increase the domain size and alignment of an initial nanoarray, obtained by crystallizationfrom octanol between two glasses (separated by 10 m), the shearing force was applied bymoving the moving bottom glass with respect to the top back and forth at a frequency ofabout 0.5 Hz [88]. The Bragg spots from the probe laser were monitored during the shearingprocess. The change of scattered light intensity from six to continuous ring indicated meltingof the initial nanoarray. The removal of the shear led to recovery of the of the six Braggspots in the orientation indicating long-range order parallel to the substrate. The resultingstructure demonstrates generally high degree of ordering, with some defects explained byevaporation of the octanol after emoval of the top glass. The hexagonal layering throughoutthe whole structure can be seen. The intensities of the six Bragg spots were measured as afunction of the incident angle between the laser beam and the normal to the glass substrate.The presence of four maxima of different intensity of two main peaks draws attention.The Bragg spots can be analyzed by considering each layer of colloidal crystal as a 2Ddiffraction grating with the pitch equal to d

3/2, where d is the sphere diameter. The

observed diffraction can be described as

sin ' + sin' % = 0n G

(7)

where ' is the angle of incidence, % is the angle of the diffracted Bragg spot beam measuredfrom the zero-order transmitted beam, n is the refractive index of the water, 0 is theincident wavelength, and G is the grating pitch. The different height of two main peaksmay indicate the presence of more than one type of fcc motives in the sample, estimated as60% and 40%. The reduction of the transmittance at 1,941 nm is larger for the thicker film(115 m) than for the thinner film (10 m). The transformation of the pure fcc structure ofthe colloidal crystal after shear into twinned crystal is remarkable [60].

4.9. Self-Assembly Using Physical Confinement and AttractiveCapillary Forces

The detailed scheme of the setup of practical method for rapid formation of ordered crys-talline arrays of colloidal particles with the areas of 1 cm2 and controlled thickness canbe found for examples in Refs. [89, 90]. The freestanding films can be also be produced bythis method when using UV- or thermally cross-linkable polymers. As it was emphasized inRef. [89], the method is simple, can be used for relatively large areas, and allows controlover the film thickness of the obtained arrays and number of the layers in the film. Thenumber of layers is determined by the ratio between the cell thickness and the diameter ofthe particles. The possibility to get large-area crystalline arrays is especially important forthe practical applications because long-range order is critical for the performance of filmsas optical devices. The depth of the channel, necessary to provide the fluid flow, determinesthe minimum diameter of the particles that could be assembled with this method. However,the decrease of the depth of the channels influences also the rate of the packing. The SEMpatterns indicate the conservation of the same ccp structure of the film with its thickness.Three forces determine this process of self-assembly [41]:

1. The capillary force due to meniscus of the liquid slug (plays major role).2. Gravitational force due to density difference between the particle and dispersion (neg-

ligible for polymer particles).3. Electrostatic force between the particle and the substrate. Brownian motion can be

important for particles smaller than 1 m.The ABC sequence of the densest layers of designed crystals can be assigned on thebasis of SEM studies. The investigations of optical spectra show that the (111) planes are par-allel to both the film and substrates surfaces. The sonication plays important role, enablingthe formation of the extended homogeneous single crystalline array with the ccp structure.A mixture of ccp arrays of 480-nm PS beads with different orientation of crystalline domains


5 mmD

A

B

C

Figure 1. SEM pattern, showing structure of fcc nanocrystalline arrays, constructed from 255-nm Seradyn PS col-loidal nanoparticles ([email protected], www.seradyn.com) using combination of physical confinement and capillaryforces. The termination of the nanocrystalline array by the crystal faces {111} (plane ABC) and {110} (plane ABD)is shown. The crack parallel to the faces {110} could be also seen (plane (CBD)).

was obtained when the crystallization was performed in the absence of continuous sonica-tion [90].We included in this review two examples of nanocrystalline arrays that we fabricated using

a combination of physical confinement and capillary forces. The rational crystallographicorientation of the faces and major tracks of the nanocrystalline array is shown on Fig. 1. Wehave also obtained the nanocrystalline array built of domains D1 and D2 with ccp structurebut with a different support orientation (Fig. 2). The domains D1 are oriented with (111)faces parallel to the film surface, while domains D2 have the (001) face parallel to the filmsurface. Domains D1 form the largest part of the nanocrystalline array. The ccp structurewith the highest density (111) outer surface is reported normally for colloidal crystallinearrays. The formation of the mixture of ccp domains with different orientations was noted,for example, with crystallization of 480-nm PS beads under shear-flow conditions but withoutsonication [90]. The presence of domains, terminated from the surface by the faces (002)and not by the densest faces (111) could be explained by several reasons:

1. Domains D2 do not correspond to equilibrium structure of NCA1 (with respect to theapplied shear-flow crystallization approach). Their presence could be observed becausethe crystallization process is not completed. More prolonged crystallization may leadto uniform orientation of the densest PS crystalline faces of all domains with respectto the support (only D1 domains will be present).

2. The presence of D2 domains is dictated by hydrodynamic reason because the orien-tation of D2 domains could offer a broader channel for liquid flow because of moreappropriate orientation of tetrahedral and octahedral holes inside of the ccp structure.

500 nm 500 nm

Figure 2. SEM pattern of the nanocrystalline arrays fabricated from 431 nm Seradyn PS colloidal nanoparticles([email protected], www.seradyn.com) using combination of physical confinement and capillary forces. Array hasccp structure. Two crystal domains D1 and D2 are shown, terminated with the face (111) (D1, left) and face (001)(D2, right). Domains D2 form minor part of the nanocrystalline array. The unit cell parameter of the ccp layera = 571 nm of the ccp unit cell (space group Fm3m) could be calculated from the from the diagonal of the unitcell as a = d1/2

2 or as a = d2/

3 from projection (111) or measured directly on the projection (001).


3. The presence of domains D2 is due to local violations of the general balance of theforces governing shear-flow crystallization of NCA1 because of small values of theshear-flow velocity, small gradient of shear-flow velocity, and the nonregularity of aris-ing crystallized arrays.

The convenience of the controlling of the crystallization process using combination of thephysical confinement and attractive capillary forces made a major contribution to studies ofthe dependence of optical properties (stop-band attenuation) on the thickness and defectsin fabricated photonic crystals. Study of the dependence of attenuation on the film thickness[91] using 220-nm PS beads in the gaskets separated by 2- to 22-m-thick films, when thenumber of (111) planes was increased from 13 to 127 layers, has shown that:

1. Attenuation increases monotonically from 1 dB to 21 dB.2. Bandwidth and attenuation at the frequencies below stop band was also increased.

The performed simulation in Ref. [91] of the dependence of the transmittance on the wave-length shows several remarkable properties (first of all stronger attenuation at the stop-bandposition), which could be explained by the presence of defects in real photonic crystal.Controlled generation of defect states in the photonic band-gap structure by doping them

with the colloid of different size is one of the challenges of nanotechnology. The creationof the well-defined defects inside of 3D-photonic crystal may allow propagation of thephonons at defined frequency [92]. The doping impurities can be added to the suspensionbefore the crystallization process. Several studies are devoted to the influence of (1) ratiobetween the diameters of host and dopant and (2) dopant concentration on the crystal-lization of the binary colloid suspension. In Ref. [92], monodispersed 155-nm PS dopantparticles were introduced to the aqueous suspension of monodispersed 230-nm PS particlesat the doping level from 0% to 17%. The analysis of resulted transmittance spectra show:

1. Position of the minimum of transmittance spectra at 573 nm does not change signifi-cantly until doping level of 9%.

2. The attenuation of the minimum increases monotonically when doping level increasesfrom 0% to 17%.

3. Decrease of transmittance was observed also at the frequencies smaller than band gap.

The defects of nanoarray due to the doping were investigated in detail with SEM. Increase ofthe degree of doping led to the development of dislocations in addition to the point defectsand decrease of the crystalline domain size. The following equation, based on Zachariasenapproach of dynamical X-ray diffraction, relates the transmittance T to the thickness t ofthe nanocrystal or to the number of (111) planes contributing to the diffraction [93, 94]:

T = +cosht/t0,2 (8)

where

t0 = +w min2 sin ',/-. , (9)

Plotting the dependence of the transmittance on the number of (111) planes clearly indicatesthat the increase of the doping level leads to the reduction of the number of diffracting(111) planes [92].Using this powerful method of self-assembly, the ordered 3D arrays of core-shell colloids

Au@SiO2 consisting of the Au cores with diameter of 50 nm with the SiO2 shells of differentthickness t were prepared [57]. The position of the plasmon peak of noncoated Au particlesin aqueous suspension, located at 540 nm, was not sensitive to the coating thickness (at t =70, 80, and 100 nm). The transmission spectra of the assembles of Au@SiO2 nanoparticlesexhibit two peaks at plasmon frequency and because of Bragg diffraction that could overlapdepending on the shell thickness.


4.10. Self-Assembly on Templated Substrates

The controllable linkage of nanoparticles to the surface of already existing structure (tem-plate) would be the next step toward self-assembly of more complex structures. The use ofgeometrical constrains to prevent spontaneous 3D agglomeration is a natural and effectiveway to control the aggregation of nanoparticles. A number of groups are contributing tothe studies of templated self-assembly. The methods of templated self-assembly of colloidalnanoparticles have been recently reviewed by Xia and coworkers [41].Patterning surfaces may be performed chemically or mechanically. A very interesting

approach was suggested in Ref. [60] to consider self-assembly in the presence of constrainsthat modify the thermodynamic behavior of the colloids, such as walls or confining bound-aries, as self-assembly under an external field. Using the patterned surfaces with non-densestarrangements (symmetries) of fcc packing, such as (100) or (110), the corresponding orien-tations of colloidal crystals, called colloidal epitaxy, related to the substrate surface can begenerated [32, 60]. According to Ref. [32], the stacking sequence can be completely dictatedby the template. When using the templates with a square-symmetric fcc (100) arrangementof holes, only monomolecular coverage (n = 1) followed the template pattern [69]. Whenthe coverage fills or exceeds one single layer, the reconstruction to the hexagonal packingwas observed, possibly because the reconstructed structure is less defective or because theeffects of growth during compacting grow field or capillary effects favor the ccp structure.The translation of the square-symmetric template structure to the colloidal array was

observed for pillar-shaped templates. The symmetrically ordered vacancies distribution wasobserved for n= 1. The formation of crystalline fcc (100) regions was observed for depositionof n 2 layers. The amount of fcc (111) domains was increasing with the increase of thecrystal thickness and fcc domains prevail at n 4.The possibility for achieving 3D non-close-packed growth using crystallization during sol-

vent evaporation with templates and a pitch comparable to the nanoparticle size is describedin Ref. [69]. The Si colloids were arranged in a cubic bcc structure (packing density k = 068)by template-induced colloidal deposition during solvent evaporation in a vertical setup. Thegravitational field may play a crucial role in template-directed convective assembly.The colloidal crystals with simple cubic (sc) packing (packing density k = 052) can be

obtained using the templates with increased pillars and subsequent removal of the templateby etching.The titling of the substrate by a small angle of 210 (angle between the surface normal

and gravity) was suggested to enforce the gravity to act as an additional force toward thetemplate and to induce defect-free arrangement of the particles [69]. By this means, thesedimentation can be completed before drying and close-packed crystals can be orientedwith regard to the substrate.Template-assisted self-assembly for patterning and alignment of self-assembled meso-

porous materials is one of the active areas directed to fabricating new microelectronic devicesand sensors [95].High-resolution TEM study of hexagonal mesoporous materials indicates clear hexagonal

arrangement of pores with uniform size [96]. The pore walls exhibit no structural ordering.The ED pattern shows a sharp diffraction reflection up to the sixth order, demonstratingperfect long-range ordering of the mesophase. The removal of surfactant does not affectframework structure. The pore structure can be described as a hexagonal array of uniform1D channels.Using the 1,2-bis(trimethoxysilyl)ethane (BTME) as a silica source and cetyltrimethylam-

monium chloride (CTAC) as a surfactant, the ordered arrays of self-assembled periodichybrid organic-inorganic silicate crystals on glass substrates were obtained prepatterned withgold triangles. The self-assembled silicate crystals have octahedral morphology and containuniformly distributed organic groups within the silicate framework [96]. The cubic structure(space group Pm3m) was confirmed by XRD and TEM studies. The nucleation and growthof mesophase crystals follows the micropatterning. Two preferred orientations of octahedralsilicate crystals on triangle patterns were observed, parallel and antiparallel. In both cases,the {111} faces are parallel to the substrate surface. The orientation alignment is one of the


crucial issues of these materials. Well-shaped crystals were formed after more than 3 hoursof reaction. The particles, grown during 1 hour, have a spherical shape and more than oneparticle can be located on the triangular motive of the micropattern. The SEM study indi-cates spherical morphology of particles of mesoporous material on the edges and corners oftriangles. Very interesting results were obtained for the growth on lined micropatterns. Thegrowth of closely packed crystals on the lines with 3, 6, >8, and >10 m in width leads toa formation of one row, two contacting rows, two separate rows, and rows both in middleand on the edges of lines, respectively. The influence of the line width of the underlay-ing micropattern on the morphology of the self-assembled crystal and self-assembled crystalarrays was demonstrated using electron microscopy. This could be interpreted as a tendencyfor preferred nucleation of the self-assembled crystals on the edges of the micropatterns.Importantly, that the crystals, self-assembled on the lines, do not exhibit well-shaped formand do not possess preferred crystalline orientation. Therefore, the geometry of micropat-tern influences the crystal morphology of the self-assembled crystals and can be consideredas new option in monitoring of crystal growth.

4.11. Template Assisted Self-Assembly Using Physical Confinementand Capillary Forces

The complex structures fabricated with TASA on templates patterned with relief structureswith self-assembly method using a combination of physical confinement and attractive cap-illary forces (see Section 4.9), are discussed in review article [41]. Isolated nanocrystallineaggregates and 1D nanocrystalline arrays of different configurations can be constructed withthis method. The capillary forces because of the presence of liquid meniscus play the mostimportant role and are determining the success of the TASA. Sedimentation is less impor-tant. Because the density of particles is not sufficiently high, only a very small part of thetemplate holes can be filled with this mechanism.An example of 2D crystallization of spherical colloids on a channel-comprised template

with uniform arrays, consisting of both linear and zigzag structure, is discussed in Ref. [41].The character of the structure depends on the relation between the width of the channelW and particle diameter d. The zigzag structures are formed when d


The triangular nanoparticles occupy only 7.2% of the substrate area [2]. The increase ofthe concentration of the nanospheres suspension can lead to self-assembling of a doublelayer of nanospheres, which means smaller density of sixfold intersticies. The diameter ofhexagonal nanoparticle a and interparticle separation dip are

a =(

3 1 13

)D and dip = D (11)

The hexagonal nanoparticles cover 2.2% of the substrate area under double layer. Inaccordance with the theory of close packing, the self-assembling third-nanosphere layerin the ABA manner retains the array of hexagonal particles; the self-assembling third-nanosphere layer in ABC manner blocks all mask holes.The extended studies of NSL give rise to more sophisticated nano-ring creation techniques,

new NSF structure motive, and typifies the deposition of Ni. The angle-resolved NSL, whennew NSL structure is forming by varying the angle between the substrate and the beamof deposition material, led to the change in shape and size of approachable area of thesubstrate [2].Self-assembly using NSL was applied for growing photonic crystals based on periodic

arrays of aligned carbon nanotubes [101]. On the first step, the self-assembling of commer-cially available PS nanospheres (diameter up to 500 nm) on Si substrate was performedand then a highly ordered large-area monolayer was brought from the substrate 1 onto awater surface and deposited onto substrate 2. Nanoarrays are subsequently used as a maskfor deposition of catalysis (Fe, Co, or Ni), the PS spheres were removed and honeycombnanotube arrays were grown using PECVD. The quality (straightness) of grown nanotubescould be improved by either better care of the PS beads removal from the substrate afterNi deposition or by better growth control [101].The resulting 2D array of carbon nanotubes show both diffraction effects and a photonic

band gap in the visible frequency range. The honeycomb array has triangular reciprocallattice. In 3D reciprocal lattice, the array consists of parallel G lines, each is normal tothe corresponding reciprocal lattice plane and going through reciprocal lattice points G =mb1 +mb2, where b1 and b2 are basis vectors of triangular reciprocal lattice. The diffractionpatterns of the fabricated array were recorded using blue and green laser light; dependenceof the intensity of the spots on their order was studied.According to Yablonovich [19], periodic arrays with the dielectric constant different from

environment leads not only to Bragg scattering but also to opening the frequency gaps atthe Bragg reflections points (i.e., at the Brillouin zone boundaries). If such gaps occur in alldirections of propagation, photonic spectrum has an absolute gap and total reflection of lightof given frequency is observed. The honeycomb array of rods with large dielectric constanta in material with low dielectric constant b has to produce a photonic-band structure andabsolute gaps at low filling. The experiments in the microwave range of 27 to 75 GHz usinga collimated microwave beam on two-dimensional graphite-like photonic crystals (1.5 mmdiameter, 5 cm long alumina cylinders with 95 inserted in a dielectric foam with 5)[102] and infrared frequency ranges [103] support this theory.

4.13. Self-Assembling of Nanoparticles on Phase-SeparatedDiblock Copolymers

Patterns of block copolymers can be used as a template for the directed self-assembly ofthio-passivated nanoparticles [104]. This kind of patterning based on different affinities oftwo polymer components to the adsorption of particles, has high resolution on the orderof 1 nm and utilizes commercially available polymers. The selectivity of covering of goldparticles for polystyrene PS phase of phase-separated polystyrene-block-methyl metacry-late (PMMA) block copolymer is higher than 99%. In Ref. [104], the TEM patterns ofalkanethiol-coated 12-nm gold nanocrystals deposited on a phase-separated polystyrene-b-PMMA block copolymer film were recorded. Dense coverage resulted from deposition frommore concentrated solutions. At higher concentrations, the particles are close packed on thePS phase but can also be observed on the PMMA phase. The increase of gold particle size


to 5 nm also leads to nonselective covering. The more energetically favorable interaction ofgold nanocrystals with nonpolar PS than with polar PMMA moieties is a possible explanationfor this kind of surface ordering. The possibility to control the surface morphology of theblock copolymer is an important question. Application of an electric field during annealingis one of the methods for controlling the precise alignment of the polymer.

4.14. Self-Assembly, Influenced by Dynamics of Colloidal OrganizationDuring Solvent Evaporation

Mobility of hard nanospheres on a surface during deposition can influence the equilibriumstructure of the resulting colloidal structure. For example, zero-surface diffusion leads todisordered monolayers on 2D substrates, while high-surface mobility leads to close-packedhexagonal arrays. When the thermocapillary forces influence the assembling process duringsolvent evaporation, one can expect diverse microstructures such as polygonal networks,rings that can be explained by involvement of dispersed nanoparticles to convection flowsdue to evaporation [105, 106].Formation of self-assembled periodic hexagonal networks (or 2D honeycomb structures)

was observed during drop casting of the dispersion of monodisperse 3.5 nm diameter ster-ically stabilized, gold nanoparticles onto 2D substrates [107]. The morphology of the self-assembly depends on the nanocrystals concentration in the dispersion. The convective flow,which produces a hexagonal network (due to Marangoni instability), occurs below somecritical particle concentration. The nanocrystals coalesce into ring structures at high concen-trations (e.g., 1.67 g/L for gold particles in chloroform suspension). At concentration below12 102 g/L, no network formation was observed. The morphology of the self-assembledstructures also depends on the nanocrystal size, size distribution, and temperature. The ringstructures are significantly thicker than the honeycomb networks (25 and 15 nm, respec-tively) [107].The so-called Marangoni number Ma determines so-called Marangoni convection, which

gives rise to hexagonal honeycomb networks of convective flow cells when Ma > Mac = 80and convective motion sets in:

Ma = 2T.Td345

(12)

where 2T is the liquid surface tension, .T is the temperature difference across the fluid filmof the thickness d 3 is density, 6 is kinematic viscosity, and 5 is the thermal diffusitivity.This instability in the fluid film leads to a steady-state flow pattern of hexagonally organizedcells. Formation of periodic hexagonal motives was observed not only for gold but also fornickel nanocrystals [107].

4.15. Self-Assembly as a Result of Progressive Condensation ofIntermediate Structural Building Blocks

This review is devoted to the processes of self-assembly using hard nanoparticles as primarybuilding blocks. We are not considering cases of essential change (growth) of nanoparticlesduring ordering. Nevertheless, we included a very short discussion of some examples ofprogressive condensation of intermediate structural building blocks (hereafter referred toas progressive condensation) for the reason that (1) this method also deals with self-assembling from suspensions of colloidal particles; (2) well-ordered arrays can result fromprogressive condensation; and (3) the character of nanodevices, produced by progressivecondensation, is very similar to that obtained by self-assembly of not-changed nanoparticles[108]. The concept of progressive condensation in the obtaining ordered nanocrystallinearrays was illustrated by the formation of the regular hexagonal arrays of uniform TiO2anatase nanocrystallites, whose structure was proved by HRTEM study. The resulting close-packed hexagonal superlattice can be characterized by the spacing between the centersof adjacent nanocrystallites of about 14 nm, where the size of individual nanocrystallite is13 nm. Less-ordered tetragonal arrays were observed as a result of similar progressive con-densation of TiO2 colloids but including step hydrothermal growth and at lower pH in [109].


The self-assembled nanocrystalline films were obtained by deposition of thin TiO2 films usingdoctor-blade technique.

4.16. Assembling of Colloidal Particles Under Electric Fieldor Magnetic Field

An excellent recent review [60] is devoted to the consideration of the aggregation of colloidsunder external fields. We include, in this section, a brief discussion of several studies ofordered periodic array formation under an external field to demonstrate the possible controlparameters and approaches that enable further reproducible fabrication of colloidal arrays.The electrodeposition of monodispersed, charged, colloidal spheres onto substrate

patterned as an array of electroconducting grooves with dimensions commensurate or non-commensurate with the discrete number of colloidal spheres was studied [110]. The elec-trophoresis of 0.58 m PS particles was performed against gravity to avoid sedimentationeffects. No significant difference in the structure of resulting 2D arrays was observed for thenonpatterned ITO surface and substrate patterned with 7.5 m grooves, as it could be seenfrom SEM patterns. A good, ordered, hexagonal close-packed array was observed for thesubstrate patterned with 4.2 m grooves. Discussion of the organization of particles in thegrooves was performed in terms of two processes: electrodeposition driven by Coulombicinteractions of particles with electrode surface and electrodynamic and capillary forces. Theproposed approach has great potential for controlled growth of large-scale colloidal crystals.The electrophoretic deposition is a suitable method for fabrication of 2D and 3D pho-

tonic crystals composed of metal or PS particles [111]. Application of an electrical field canclearly accelerate the sedimentation rate; however, the quality of the films is much worsecompared with the quality of the film prepared by gravity sedimentation. In Ref. [112], theelectrophoretic growth of excellent ordered colloidal crystals was reported by performingthe deposition in a polar, yet nonelectroactive, dispersion medium onto ITO substrate. Thisprevents gas evolution on the electrodes, what is probably brought to the defects and poorquality in earlier attempts to use electrophoretic deposition for design of colloidal crystals.SEM patterns of colloidal crystals, obtained by electrophoretic deposition of 300-nm PSspheres, show that perfect close-packed ordering extends over areas of 10 m. Two kinds ofsphere packings were observed, both belong to fcc structure but have different orientations,(111) and (100), with regards to the substrate surface.It is essential that perfect fcc structure extends in the direction normal to the substrate over

the entire film thickness when looking at the stacking edges. Electrophoretically obtainedfilms show clear photonic stop band in the visible range in the normal incidence transmissionspectra. The electrophoretic technique was further used for impregnation of the 3.54.5 nmCdTe nanocrystals from aqueous colloidal solution into fabricated PS colloidal crystal. A dip,observed in the emission spectrum, correlates with the position of the stop band, indicatinginhibition of the spontaneous emission as a result of a photon density of states [112].Three different interpretations are suggested for explanation of observed aggregation of

PS microspheres on a solid surface induced by an ac or dc electric field [113]:

1. Attractive forces leading to aggregation might originate from an electro-osmotic flowdue to surface charge of the particles. This explanation concerns only the dc electricfield.

2. Electrohydrodynamic flow-producing aggregation is due to the particle-induced distor-tion of the applied dc or ac field in the double layer of ions or counterions at theelectrode surface.

3. Inhomogeneous electric field due to electrode polarization leads to a bulk change indensity.

Results of the study of 2D aggregation of PS beads on the ITO surface support interpreta-tion (2). The particles assemble into hexagonally packed aggregates when frequency 6 waslower than a contact frequency 6c, as it could be seen from micrographs of aggregatesof 1.5-m PS spheres, deposited at E0 = 185 V cm1 and 6 = 400 Hz. Below 6c crystallineaggregates were formed, the density of which increases as the frequency decreases. The size


of the aggregates depends on both the initial density of the powders and on electric fieldparameters. The behavior of the system depends on the competition between the attractiveelectrohydrodynamic forces and repulsive dipoledipole interactions.The change of the ac frequency can be used for tuning particle interactions in nanocrys-

talline arrays that can be used for device fabrication [114]. The assembling of binary colloidalsuspensions into planar superstructures was studied using ac electric fields. Application ofdc or low-frequency ac field (f < 1 kHz) leads to lateral motion of particles and resultsin planar, close-packed clusters. High-frequency (f > 1 kHz) high-amplitude fields lead towidely spaced arrays. The intermediate frequencies, when the attractive and repulsive forcesare in balance, are interesting from what could be seen from the observed cluster formation.At high frequencies (20200 kHz) and low particles concentration (20% area coverage)formation of the chains of alternating particles was observed, while at higher concentrations(80% of area coverage), square or triangular superlattices can be formed, depending onthe relative concentrations of PS and silica spheres. The observation of optical photographsof the planar structures formed by binary suspensions on an electrode in a high-electric fieldwas used. It was shown that interaction energies of particles with different polarizabilitiescan be modulated by electric fields.The method exploring electric-field-induced assembly with photochemical sensitivity of

semiconductors was reported [115]. The change of the current distribution across the ITOelectrode, which could be performed by varying the illumination intensity (below 310 nm),can cause the colloidal particles (2-m PS spheres or submicrometer-sized silica colloids) toseep into lighted regions.The Ni-coated silica, or polymer nanoparticles, form 2D and 3D arrays in electromagne-

torheological fluid [116]. Structural transitions within nanocrystalline arrays can be enabledby titling the magnetic field at the angle ' from the normal direction z and rotating anangle % (see detailed description of experimental set up in Ref. [116]). The body-centeredcubic (bcc) structure of the obtained 3D arrays were recognized using analysis of cross sec-tions of SEM images. The model of formation of different planar lattices in dependence onthe polar angle ' and the azimuthal angle % of the magnetic field was suggested. An interest-ing phenomenon was observed of the structure transition (martensitic) from the structureswith bcc symmetry, formed at zero magnetic field (electric field only), to the structure withfcc symmetry when the magnetic field was applied perpendicular to electric field. Slight shiftof the particles inside the columns is necessary for the realization of this structure transition.

5. NANODEVICESThere are several good reviews concerning physical and chemical properties of nanoarraysrelevant for fabricating different devices (see, e.g., Ref. [53]).We summarized some example of nanodevices, where the self-assembly of nanoparticles

was used. We have concentrated on nanodevices based mainly on the use of nanoparticles.We did not include in the review papers that consider the application of arrays formed byessentially 1D structures such as nanorods, nanowires, or nanotubes.

5.1. Nanosensors

Very high surface-to-volume ratio and the possibility to impart practically important physi-cal properties (e.g., conductivity) make nanocrystalline arrays attractive for different sensorapplications, both as active and passive components (see, e.g., Ref. [53]). Current biologicalsensors are mostly based on optical detection and have several disadvantages, discussed inRef. [117], most relevant of which are complexity and difficulties in miniaturization. Broadlypresent on the sensor market SnO2-based sensors have several shortfalls. These sensors havehigh-power consumption due to the high temperature of operation and do not have suffi-cient chemical sensitivity. The phtalocyanine-based sensors have higher-chemical sensitivitybut poor reproducibility of film formation, are water sensitive, and possess slow responsetime. The sensors based on conducting polymer have several commercial applications butsuffer from insufficient reproducibility, low-specifity, and sensitivity to water [118].


5.1.1. Sensors for Gas Sensing Based on Thiol-StabilizedGold Nanoparticles

The transducer described in Ref. [119], a fundamentally new type of chemiresistors, incor-porates so-called metal-insulator-metal ensemble (MIME), an ensemble of nanometer-sizedmetal nanoparticles, coated by an organic monomolecular layer. Alkanethiol-stabilized goldnanocluster material was used for this device, which is very stable in the solid state, easyto synthesize, and can be easily redispersed in nonpolar solvents at concentrations up to10 wt%. The conductivity of the film depends on the thickness of the ligand shell. Thesensor was fabricated by deposition of a thin film of the Au:C8 (1:1) cluster (eight car-bon atoms in the alkane chain) onto interdigital microelectrode. The airbrush techniquewas used for film deposition from chloroform solution. The solvent was rapidly flashedaway by heating above the chloroform boiling point, leaving a more uniformly texturedfilm. Toluene, chloroform, a hydrogen bonding polar organic, and a very polar inorganic,all at similar vapor pressure, were selected for characterization of this sensor in broad con-centration range. The response to toluene vapor is very large. The sensor is remarkablyinsensitive to water vapor even at high concentrations, which is important for practical appli-cations. Small conductance increase was observed for only very high vapor concentrations ofpropanol. The detection limit for toluene and tetrachloroethylene permits detection below1 ppmv.Two mechanisms of electron transport in the obtained system of gold cluster film are

suggested: (1) tunneling between the metal cores (core diameter is smaller than the deBroglie wavelength of electron in the gold particle core) and (2) hopping along thiol-alkylligand. The conductivity in metallic nanoparticle film can be expressed as [118]:

2 e2:eEc/kT (13)The first term shows that conductivity is very sensitive to corecore separation, leading toa decrease of around one order per angstrom. The activation energy Ec is sensitive to thecharge of the cores and permittivity of the media. For example, the presence of the vaporscan influence both intercore tunneling and electron-hopping mechanisms. The choice of asensor system has to include selection of both the nanoparticles nature and size as well asstabilizing ligands, which should be short to permit sufficient conductivity and carry properfunctional groups.Small aromatic organothiol derivatives HS-C6H4-X were used for stabilization of gold par-

ticles, separated by a dielectric of relative permittivity , in the course of construction of thesensor system, formed on microelectrode pattern using solvent evaporation [118]. Four dif-ferent types of functional groups were used to stabilize the particles. The room-temperatureconductivities between 106 and 102 ;1 cm1 were 711 orders of magnitude lower thanconductivity of the bulk gold of 3 105 ;1 cm1, which shows insulating effect of stabi-lizing ligands. No percolation effects and resulting high conductivity of the ensemble wereobserved. Good repeatability was observed for the samples II and III with regards to polarsolvents (methanol), namely, samples II and III show decrease, while the samples I and IVshow increase in conductivity. H-bonding could play a role in determining the response ofthis sensor system. The ellipsometric studies are consistent with conductivity measurementsand indicate that the decrease of conductivity may be associated with film swelling, and theincrease of conductivity is associated with decrease of particle separation. The response tononpolar organic analytes (pentane, hexane) was found to be less reproducible, the strongestresponse was observed for CH3-derivated sample IV.

5.1.2. Nanotube Field Emission Transistor SensorField-effect transistor (FET), fabricated using semiconducting single-wall carbon nanotubes(nanotube FET, NTFET), is an example

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