Self-assembly of colloidal one-dimensional nanocrystals

This journal is©The Royal Society of Chemistry 2014 Chem. Soc. Rev., 2014, 43, 2301--2323 | 2301

Cite this: Chem. Soc. Rev., 2014,

43, 2301

Self-assembly of colloidal one-dimensionalnanocrystals

Shuang-Yuan Zhang,† Michelle D. Regulacio† and Ming-Yong Han*

The ability of nanoscopic materials to self-organize into large-scale assembly structures that exhibit

unique collective properties has opened up new and exciting opportunities in the field of

nanotechnology. Although earlier work on nanoscale self-assembly has focused on colloidal spherical

nanocrystals as building blocks, there has been significant interest in recent years in the self-assembly of

colloidal nanocrystals having well-defined facets or anisotropic shapes. In this review, particular

attention is drawn to anisotropic one-dimensional (1D) nanocrystals, notably nanorods and nanowires,

which can be arranged into a multitude of higher-order assembly structures. Different strategies have

been developed to realize self-assembly of colloidal 1D nanocrystals and these are highlighted in the

first part of this review. Self-assembly can take place (1) on substrates through evaporation control,

external field facilitation and template use; (2) at interfaces, such as the liquid–liquid and the gas–liquid

interface; and (3) in solutions via chemical bonding, depletion attraction forces and linker-mediated

interactions. The choice of a self-assembly approach is pivotal to achieving the desired assembly

configuration with properties that can be exploited for functional device applications. In the subsequent

sections, the various assembly structures that have been created through 1D nanocrystal self-assembly

are presented. These organized structures are broadly categorized into non-close-packed and close-

packed configurations, and are further classified based on the different types of 1D nanocrystal

alignment (side-by-side and end-to-end), orientation (horizontal and vertical) and ordering (nematic and

smectic), and depending on the dimensionality of the structure (2D and 3D). The conditions under

which different types of arrangements are achieved are also discussed.

1. Introduction

Nanotechnology has clearly become one of the fastest develop-ing research areas as evidenced by the tremendous progressthat has been made since its emergence as a field of study in

Institute of Materials Research and Engineering, Agency for Science, Technology and

Research, 3 Research Link, Singapore 117602. E-mail: [email protected];

Fax: +65 68720785; Tel: +65 68741987

Shuang-Yuan Zhang

Shuang-Yuan Zhang received hisPhD in Integrative Sciences andEngineering from the National Uni-versity of Singapore. He presentlyworks as a research scientist at theInstitute of Materials Research andEngineering under the Agencyfor Science, Technology andResearch (A*STAR) in Singapore.His research is centered on thesolution-based synthesis and self-assembly of functional nano-structures and their correspondingthermal, optical, magnetic, andcatalytic applications.

Michelle D. Regulacio

Michelle D. Regulacio received herPhD in Chemistry from GeorgetownUniversity. She was a postdoctoralresearch associate in the Depart-ment of Chemistry at PrincetonUniversity before joining theInstitute of Materials Researchand Engineering in Singapore as aresearch scientist. Her researchfocuses on the colloidal chemicalsynthesis and potential techno-logical applications of inorganicnanoscale materials, which includealloyed semiconductor nanocrystalsand hybrid nanostructures.

† These authors contributed equally to this work.

Received 4th November 2013

DOI: 10.1039/c3cs60397k

www.rsc.org/csr

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2302 | Chem. Soc. Rev., 2014, 43, 2301--2323 This journal is©The Royal Society of Chemistry 2014

the 1980s. This interdisciplinary field has paved the way for thecreation of novel nanoscopic materials that are potentiallyuseful in a wide range of applications that span across variousareas of science and technology. Nanometer-sized materials aretypically fabricated using either the top-down or the bottom-upstrategy.1,2 The top-down approach involves crafting of nano-scopic features by controlled removal of materials from largersolids, often through costly lithographic techniques, whereasthe bottom-up approach constructs nanoscale structures fromsmaller components like atoms and molecules. One of the mostextensively employed bottom-up methods is the colloidalchemical synthetic route, which is a simple, inexpensivemethod that can readily be adopted in standard laboratorysettings.3–8 With this synthetic approach, colloidal nanocrystal-line materials of varying sizes, shapes and compositions havebeen successfully prepared and their properties studied.9–14 Anintriguing discovery is that colloidal nanocrystals can sponta-neously arrange themselves to form larger structures througha process called self-assembly.15–22 This has enabled theconstruction of amazingly complex structures that are notaccessible via top-down fabrication techniques. Moreover, theself-assembled structures have been found to exhibit uniquecollective properties that differ from the properties displayed bythe constituent building blocks.23 Further studies revealed thatthe structures built from self-assembly show potential utilityin the areas of photonics,24–26 plasmonics,27–32 and SERS,33–39 andare also promising materials for magnetic,40–43 electronic,44,45

photovoltaic,44,46,47 biomedical,48,49 sensing,50–52 and cata-lytic8,53–55 applications. These interesting findings have naturallyopened up an entire new avenue of research that has attractedthe attention of many scientists worldwide.

In nature, self-assembly has been observed in many bio-logical systems, such as the case of the tobacco mosaic virus(TMV).56 A TMV particle, which is 300 nm in length and 18 nmin diameter, has a rod-shaped helical structure consisting of2130 identical protein subunits and a single strand of RNA.57

The protein subunits self-assemble into the rod-like structureby forming a helical sheath around the RNA strand. The self-assembly process is mediated by various weak non-covalentinteractions between the components, and the use of identical

subunits limits the set of interactions that are necessary to formthe structure correctly. In many ways, self-assembly of colloidalnanocrystals is similar to self-assembly in natural systems. Thediverse array of sophisticated structures that are generatedthrough self-assembly in nature has inspired scientists todevelop new ways in engineering the formation of higher-order structures from colloidal nanocrystals. Different typesof interactions have been exploited to promote and facilitatethe self-organization of colloidal nanoscale units.58–66 In somecases, biological interactions have been utilized to emulatenature’s self-assembly strategy.67

In order to produce well-ordered assembly structures, it is ofkey importance that the nanoscale building blocks are uniformin size and shape (i.e., monodisperse). Significant advances incolloidal nanocrystal synthesis coupled with size-sorting pro-cessing techniques (e.g. size-selective precipitation) haveenabled the preparation of high-quality nanocrystals with asize distribution below 5%.68–71 Colloidal nanocrystals withwell-controlled size can now be routinely prepared for a vastarray of materials, which include metals, metal oxides, andmetal chalcogenide semiconductors.72,73 Oftentimes, the nano-crystals that are synthesized are in the form of spheres (or dots).Owing to the simplicity of their shape and their well-developedsynthetic protocols, spherical nanocrystals are the most fre-quently used elementary components in nanoscale self-assembly.74–88 These nanoscopic units usually self-assembleinto a compact structure through either the cubic (ccp) or thehexagonal (hcp) close-packing arrangement of spheres.89,90

While positional ordering is necessary, orientational orderingis not required when assembling spherical nanocrystals. Bycontrast, the self-organization of nanocrystals having well-defined facets or anisotropic shapes warrants orientationalordering of the building blocks, and this restrictive require-ment poses several challenges. Nevertheless, there has beengrowing scientific interest in the use of non-spherical nano-crystals for self-assembly, and this is because their geometricalfeatures allow for the creation of a much richer variety of assemblystructures from which novel properties may emerge.91–107 Forexample, high-quality In2O3 nanooctahedra have been shown toproduce hexagonally packed structures as well as zigzag andpentagram patterns when self-assembled.108 Meanwhile, highlyuniform triangular nanoplates of LaF3 have been reported to self-organize into two types of assembly configuration through eitheredge-to-edge or face-to-face orientation.109 Another notable exam-ple is the self-assembly of monodisperse colloidal octapod-shapednanocrystals, with each octapod having eight CdS pods and aCdSe center.110,111 Under appropriate conditions, they have beenobserved to self-assemble via two sequential steps. First, theoctapods interlock to form linear chains with their centerswell-aligned along the chain axis. Subsequently, the chainsself-assemble side-by-side into porous 3D structures that canbe welded and potentially used as ion sensors or as porouselectrodes.

In this review, the spotlight is focused on the self-assemblyof colloidal nanocrystals with anisotropic one-dimensional (1D)structure. Particular attention is given to 1D nanocrystalsMing-Yong Han

Ming-Yong Han obtained his PhDin Chemistry from Jilin University.He was with IBM and IndianaUniversity before his current jointappointment as senior scientistwith the Institute of MaterialsResearch & Engineering andNational University of Singapore.His research addresses problemsat the interface of nanoscience,nanotechnology, biotechnologyand optoelectronics.

Review Article Chem Soc Rev

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having rod-like and wire-like morphologies, which are morecommonly known as nanorods and nanowires, respectively.The morphological features and unique characteristics dis-played by these 1D nanostructures have been shown to beadvantageous in many important applications. For instance,the elongated shape of nanorods is highly attractive for photo-voltaic and photocatalytic applications because it facilitatescharge transport and efficiently promotes charge separa-tion.112–114 Meanwhile, nanowires, with their high aspect ratio,are seen as a powerful class of nanomaterials that offer sub-stantial opportunities for the fabrication of novel electronic,optoelectronic, and sensing devices.115,116 The use of nanorodsand nanowires as building blocks for the construction of higherorder structures undoubtedly holds enormous potential inproviding revolutionary advancement in many areas of technology.However, due to the anisotropic shape of 1D nanocrystals, theirassembly into well-ordered structures requires both positionaland orientational ordering of individual building blocks,and oftentimes, specific techniques are necessary to facilitatetheir self-organization. In the first part of this review, wehighlight the different assembly strategies that have beensuccessfully employed in organizing colloidal 1D nanocrystalson substrates, at interfaces, and in solutions (Fig. 1). Eachassembly technique is discussed with accompanying examplesas well as illustrations to aid the reader in visualizing theprocesses described. The section that follows features thevarious assembly structures that have been generated through1D nanocrystal self-assembly (Fig. 2). These structures arebroadly categorized into non-close-packed and close-packedconfigurations, and are further classified based on the differenttypes of 1D nanocrystal alignment, orientation and ordering,

and depending on the dimensionality of the structure. Theconditions under which different types of arrangements areachieved are examined. Finally, the main conclusions andoutlook are presented in the last section.

2. Self-assembly methods

In comparison with isotropic spherical nanocrystals, 1D nano-crystals are more challenging to self-assemble due to theirinherent anisotropic structure. Fig. 1 shows a summary of thedifferent assembly techniques that have been successfullyutilized in assembling colloidal 1D nanocrystals into higher-order structures. The formation of organized structures cantake place on a substrate surface, at an interface, or even in thebulk solution, and the choice of an assembly technique isimportant in achieving the desired structure having propertiesthat can be potentially useful for functional device applications.A more detailed discussion on each of these techniques ispresented in the following subsections.

2.1 Self-assembly on substrates

2.1.1 Evaporation-mediated assembly. The formation ofordered self-assembled nanostructures was first observed in the1980s using transmission electron microscopy (TEM) when adispersion of Fe1�xCx particles in benzine (i.e., petroleum ether)was deposited onto a carbon-coated copper TEM grid and sub-sequently dried in air.117 The Fe1�xCx particles were oxidized toiron oxide in air and were found to form 3D close-packedstructures of monodisperse spherical particles on the amorphouscarbon film upon drying. The solvent evaporation rate is believed

Fig. 1 Randomly arranged colloidal 1D nanocrystals (shown in the center) can be self-assembled to form ordered structures on substrates (A–C), atinterfaces (D and E) and in solutions (F–H) through a variety of self-assembly techniques.

Chem Soc Rev Review Article

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to be critical to the formation of these well-ordered structures.In the succeeding years, this simple assembly approach, whichwas conveniently termed as evaporation-mediated or drying-assisted assembly, has been widely used in the self-assembly ofspherical as well as anisotropic nanocrystals on solid sub-strates.118,119 A schematic representation of evaporation-mediated self-assembly of nanorods on a substrate that is basedon the method of drop-casting is shown in Fig. 3A.120 In general,when a drop of nanocrystal dispersion is deposited onto a clean,flat substrate (e.g. silicon, silicon nitride) and the solvent issubsequently evaporated in a controlled manner, the relativelyweak attractive forces (e.g. van der Waals forces, dipole–dipoleinteractions) between the dispersed nanocrystals become apparentas the volume of the droplet is reduced, driving nanocrystals toself-organize.121 Furthermore, forces such as electrostatic repul-sive forces, hydrophobic interactions (associated with cappingligands), capillary forces, and entropic depletion interactionscan each play a role and mediate the assembly formation.122,123

In assembling colloidal 1D nanocrystals, a very slow eva-poration process that takes several hours to complete is usuallyadopted as this has been found to be beneficial in increasingthe packing order and size of the assembled structure. Fig. 3Bshows a schematic illustration of a more tightly controlledevaporation setup that has been used in the self-assembly ofcolloidal nanorods on a wide range of substrates.124 Theevaporation chamber is supplied with a controlled flow of dry

nitrogen to enable control of the drying rate. To ensure aconstant solvent evaporation rate, the nanocrystal dispersionis maintained at a particular temperature, which is typicallybetween room temperature and 60 1C depending on the chosensolvent’s boiling point and the desired evaporation rate.Temperatures above 60 1C have been reported to producedisordered structures as the evaporation is too fast for anorganized assembly to occur.125

The nanocrystal aggregation and ordering mechanisms havebeen explained using thermodynamics126 and coarse-grainedlattice-gas models,127,128 where the formation of the finalordered structure is thought to be influenced by several factors,including temperature, nanocrystal concentration, nature ofsolvent, and nanocrystal size, among others. Proper control ofthese critical parameters is therefore necessary for a successfulself-assembly. The effects of nanocrystal concentration andsolvent nature on the evaporation-mediated self-assembly ofcolloidal nanorods of cadmium chalcogenide semiconductors(CdS and CdSe) have been investigated by Ryan and co-workers.120

When the nanorod concentration is very low, random depositionof the nanorods on the substrate was observed. In this case, thenanorods are very far apart that the attractive forces between themare too weak to trigger self-assembly. A concentration that is toohigh, however, has only led to short-range ordering, suggestingthat the rods are too close that the repulsive forces between thembecome significant. Thus, for a highly ordered assembly, there

Fig. 2 Colloidal 1D nanoscale building blocks (shown in the center) can be self-organized to produce a variety of non-close-packed (A and B) andclose-packed (C–H) configurations. The 3D needle-like geometry shown in (H) is adapted with permission from ref. 220. Copyright 2012, AAAS.


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exists an optimal concentration window such that the inter-roddistances are small enough that the attractive forces between thenanorods far outweigh the repulsive forces. In choosing anappropriate solvent, properties such as volatility, polarity anddielectric permittivity should be taken into consideration. Becausethe drying rate is largely dependent on the volatility of the solvent,the use of a highly volatile solvent (i.e., having low boiling point) isnot recommended as this would lead to evaporation that is toofast. The choice of solvent is also restricted by its ability toeffectively disperse the nanorods as poorly solvated nanorodshave a high tendency to randomly agglomerate. A non-polarsolvent (e.g. toluene, cyclohexane) is well suited for colloidalnanorods that are surface-passivated with organic surfactants/ligands having large hydrophobic moieties (e.g. trioctylphosphine,oleic acid). Lastly, a solvent with a dielectric constant (er) lowerthan 4 is desirable as the strength of the electrostatic forces thatfacilitate the assembly would be considerably screened in asolvent with higher permittivity. For this reason, chloroform(er = 5), although non-polar, is not a good solvent forevaporation-mediated assembly of hydrophobically coated nano-rods. Other key parameters that have been shown to affectnanorod self-assembly are the concentration and nature of the

stabilizing surfactant (capping ligand), the nanorod aspect ratio,and the interaction energy between the nanorods and thesubstrate.124,125,129

Evaporation-mediated self-assembly can also be effected bycapillary action using an immersion-based setup that is sche-matically illustrated in Fig. 3C. El-Sayed and co-workers havedemonstrated the use of this technique in producing orderedself-assemblies of Au nanorods.130 First, a vertically positionedcarbon-coated copper TEM grid was partially immersed in anaqueous Au nanorod solution (B100 mL), creating a thin film ofsolution on the substrate (copper grid). The solvent (water) isthen slowly evaporated, driving a convective transfer of nano-rods from the bulk of the solution to the thin film. Theevaporation of water from the thin film increases the capillaryforces between the nanorods and builds up a pressure gradient,which produces an influx of water and nanorods from the bulksolution toward the thin film. The solvent flux compensates forthe evaporated solvent from the film whereas the nanocrystalflux causes nanocrystal accumulation in the film. Continuousnanocrystal flux fills up the space between the substrate andthe film surface, which leads to the formation of layered self-assembled structures in most regions of the substrate. It was

Fig. 3 Evaporation-mediated assembly on substrates. (A) Schematic illustration of evaporation-mediated assembly based on the simple drop-castingtechnique. The progression of the nanorod assembly at different stages of droplet drying is shown. Reproduced from ref. 120. (B) Schematic diagram of amore tightly controlled evaporation-mediated assembly setup. A short-walled container filled with nanorod solution (with the substrate at the bottom) isset on a hotplate, and placed inside an evaporation chamber. A controlled flow of dry nitrogen is supplied to control the drying rate. Reproduced withpermission from ref. 124. Copyright 2010 American Chemical Society. (C) Schematic depiction of an immersion-based setup used in evaporation-mediated assembly. (i) A copper TEM grid is partially immersed in an aqueous nanorod solution. (ii) A thin film of solution forms on the grid. Evaporation ofwater from the thin film increases the capillary forces, causing an influx of water and nanorods toward the thin film. (iii) The accumulation of nanorodsleads to the formation of self-assembled structures. Adapted with permission from ref. 130. Copyright 2000 American Chemical Society.


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shown that this immersion-based evaporation-mediated assemblytechnique can produce high-quality multidimensional assembliesunder appropriate conditions. Several factors, which include thenanorod concentration, the free surfactant concentration, theionic strength of solution, and the position of the grid in thesolution, can be varied in order to obtain organized assemblies.

2.1.2 External-field-directed assembly. The use of an appliedexternal field (e.g. electric, magnetic) in facilitating the assembly ofcolloidal nanostructures has been actively explored over the pastdecade. The ability of an external field to direct the alignment ofnanocrystals is especially beneficial for the assembly of anisotropicnanocrystals, which requires orientational ordering aside frompositional ordering of individual building blocks. Under the influ-ence of an external field, 1D nanocrystals align with their long-itudinal axis oriented along the direction of the field lines.

Electric field. For colloidal 1D nanocrystals that carry a netcharge and/or a permanent electric dipole moment, an electric

field is the perfect external stimulus for directing their assem-bly.131–134 In the presence of an applied electric field (E),alignment is achieved when the total electric dipole moment(d = d1 + d0, where d1 and d0 are the induced and permanentelectric dipole moment, respectively) gives an alignment energy(Ualign = E � d) that is strong enough to overcome the room-temperature thermal excitation energy (kBT = 26 meV, where kB

is the Boltzmann constant) that would otherwise randomize thedipole orientation.133 Thus, there is a minimum value ofelectric field strength (Emin) that is necessary for significantalignment to occur.

Fig. 4A and B schematically show examples of experimentalsetups that have been used in aligning colloidal nanorods ofwurtzite-phase cadmium chalcogenides under direct current(DC) electric fields.131,133 In general, a nonconducting flat sub-strate (e.g. Si3N4) is positioned between parallel electrodes thatare arranged either in a coplanar manner (Fig. 4A)133 or in a top-down manner (Fig. 4B).131 The colloidal nanorod dispersion is

Fig. 4 External-field-directed assembly on substrates. Schematic representations of the experimental setups used in the assembly of colloidal 1Dnanocrystals under the influence of (A and B) electric fields and (C) magnetic field. (A) A coplanar arrangement of electrodes (Au) results in nanorods thatare aligned in the plane of the substrate. The TEM image shows CdSe nanorods (shown as red bars) aligned along the electric field streamlines (shown inyellow). Adapted with permission from ref. 133. Copyright 2006 American Chemical Society. (B) A top-down arrangement of electrodes yields nanorodsthat are oriented perpendicular to the substrate. The TEM image shows vertically oriented CdS nanorods. Adapted with permission from ref. 131.Copyright 2006 American Chemical Society. (C) A glass substrate is vertically inserted into a dispersion of ellipsoidal particles, and a magnet is placedabove the dispersion. The SEM images show the resulting close-packed 3D assembly structure of magnetically active g-Fe2O3–SiO2 core–shell ellipsoids.Adapted with permission from ref. 138. Copyright 2009 Wiley-VCH.


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deposited onto the substrate and the solvent is slowly evaporatedover a time period (usually several hours) during which a directvoltage of sufficient magnitude is applied. Binary cadmiumchalcogenides possess a noncentrosymmetric wurtzite latticethat gives rise to a permanent electric dipole moment thatincreases with the nanorod volume.132 When placed under theinfluence of an external electric field, the nanorods experience atorque that rotates them to align their long axis with the fielddirection (i.e., perpendicular to the faces of the electrodes). Therods can align over large areas by following the field streamlinesthroughout the electrode gap.134 The placement of the electrodesdetermines the field direction and thus dictates the final orien-tation of the nanorods with respect to the substrate that isbetween the electrodes. When the electrodes are arranged in acoplanar manner, the rods align in the plane of the substrate asshown in Fig. 4A. By contrast, a top-down arrangement of theelectrodes results in nanorods that are oriented perpendicular tothe substrate as seen in Fig. 4B. Meanwhile, controlled evapora-tion of solvent assists in the close-packing of the orientednanorods and the degree of positional order is dependent onthe evaporation rate. A higher degree of positional order isobserved for a slower rate of solvent evaporation.131

Alternating current (AC) electric fields can also be employedto direct the formation of 1D nanocrystal assemblies usingsimilar experimental setups as those described above but withalternating voltage applied to the electrodes. An advantage ofusing an AC electric field is that it avoids the interference ofelectro-osmotic and electrochemical effects that are presentwhen DC is used.121 Mayer and co-workers have demonstratedthe assembly of Au nanowires dispersed in a dielectric mediumupon application of an AC electric field using coplanar inter-digitated electrodes.135 The metallic nanowires are easily polar-ized in the electric field due to charge separation at thenanowire surface. The nanowire alignment process can beinfluenced by the magnitude as well as the frequency of thealternating voltage. A shorter alignment time is observed whenthe magnitude and frequency of the alternating voltage areincreased.

Magnetic field. For anisotropic nanostructures that are mag-netically active (i.e., with permanent or field-induced magneticdipole moment), an external magnetic field can be utilized tofacilitate self-assembly by forcing these building blocks to alignwith their long axis parallel to the magnetic field lines.136 Theposition of the magnet relative to the substrate dictates theorientation of the particles whereas the strength of the mag-netic dipolar interactions between the particles influences thepositional order.137 The magnetic field strength can beadjusted by altering the distance between the magnet and thedispersion. Song and co-workers have demonstrated the use ofan applied magnetic field (produced by a RuFeB magnet) tocontrol the orientation of ellipsoidal core–shell g-Fe2O3–SiO2

particles during solvent evaporation (Fig. 4C).138 A close-packed3D array of ellipsoidal particles with both orientational andpositional order was realized. For nanorods and nanowires,magnetic facilitations have been shown to produce side-by-side

(e.g. FePt nanorod rail-tracks)139 and end-to-end (e.g. Ni nano-wire chains)140 self-assemblies.

2.1.3 Template-assisted assembly. Another way to facilitateself-assembly of colloidal nanocrystals on substrates is toutilize templates. Most templates are surface-modified solidsubstrates containing geometrically constrained sites and/orchemically functionalized regions that can induce the selectivedeposition of nanocrystals and enable the formation of nano-crystal assemblies.141–150 In creating templates from solidsubstrates, the substrate surface is often patterned with precisespatial and chemical control through techniques such asoptical and electron beam lithography and microcontactprinting. However, it should be noted that templates, in theirbroadest sense, can also be in the form of microorganisms(e.g. bacteria, viruses),151,152 microstructures (e.g. carbon nano-tubes),153,154 and polymers (e.g. block copolymers)155–163 asthese can also provide a platform onto which nanocrystalscan be arranged into organized structures. While there issignificant literature on the use of microorganisms and micro-structures as templates for self-assembly of spherical nano-crystals, their use for 1D nanocrystal assembly is rarelyreported.147 Thus, we focus our review on templates that arebased on patterned solid substrates and block copolymers,which have been considerably utilized for 1D nanocrystalassembly.

Geometrically defined templates. Templates with geometri-cally constrained features (e.g. pores, channels), when suitablydesigned, can allow the creation of different assembly patterns.The design of the template needs to be in accordance with thestructural properties of the building blocks. For 1D nano-crystals, the template feature size and morphology can imposestrict restrictions on the way these anisotropic nanocrystalsdeposit onto the substrate. Block copolymer-based templates,with their nanoscopic domains, can provide the necessarystructural framework to obtain 1D nanocrystal assemblies.156–163

For example, a block copolymer thin film with nanoscopicchannels lying parallel to an underlying substrate has beenshown to align CdSe nanorods along the channel walls asshown schematically in Fig. 5A.156 A film flotation methodwas used where the channels of the template were exposed tothe surface of the aqueous nanorod dispersion for 10 to 12 h.After flotation, the CdSe nanorods were found to accumulatewithin the channels and on the surface of the template. Rinsingwith water enabled the removal of the nanorods from thetemplate surface, leaving behind the nanorods that are con-fined within the channels. The length of the nanorods relativeto the channel width is critical in controlling the orientationand the lateral position of the rods. To align CdSe nanorodswith average lengths of 25 and 35 nm, a channel width of 30 nmwas used. A channel width that is too small would prevent theconfinement of the rods within the channels while an exces-sively large width would lead to disordered assembly. A tem-plate having cylindrical domains can also be constructed fromblock copolymers, and this has been shown useful in assem-bling nanorods through pore confinement.156 Furthermore, the


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cylindrically confined domains of block copolymers can beused to direct the self-assembly of nanorods into 1D nano-strings with either end-to-end organization or side-by-sidetwisted arrangement under appropriate conditions.162

Chemically patterned templates. The use of solid substrateswith chemically functionalized surfaces has also displayedgreat potential for self-assembly of colloidal nanocrystals.142

For instance, a chemically patterned template whose surfacehas been designed with regions having different binding affi-nities has been used to assemble suitably functionalized col-loidal Au nanorods with precise position and orientationcontrol.149 Although promising, this type of assembly techni-que is only applicable to specific binding combinations. A moregeneric approach is to make use of templates with patternedwettability whereby selective deposition is based on the inter-actions between the dispersion medium (i.e., the solvent) andthe patterned template surface.144 This technique has beeneffectively employed by Bao and co-workers in assembling

water-dispersible Pd nanorods on solid substrates as schema-tically illustrated in Fig. 5B.145 First, a drop of the aqueous Pdnanorod dispersion is deposited onto a template, which hasbeen patterned into alternating hydrophobic and hydrophilicstripes through microcontact printing. The nanorods are thenallowed to settle near the template surface and the excessdispersion is withdrawn through a pipette, leaving behinddiscrete tiny droplets that are selectively deposited on thehydrophilic portions of the template. Once the water hascompletely evaporated, nanorod assemblies that are confinedwithin the hydrophilic regions are obtained. To control theorientational order, the width of the hydrophilic stripes relativeto the nanorod length is adjusted. Nanorod alignment isrealized when the width of the hydrophilic stripes is less thanthe length of the nanorods. In addition to water, other liquidswith different wettability can also be employed as dispersionmedium but factors such as surface properties, and shape anddimensions of the template features will need to be adjustedaccordingly to obtain the desired assembly.

Fig. 5 Template-assisted assembly on substrates. Schematic illustrations of template-assisted assembly of colloidal nanorods based on (A) geometricconfinement, (B) patterned wettability, and (C) textured surface. (A) A template with nanoscopic channels is utilized to confine colloidal nanorods withinthe channel walls. Adapted with permission from ref. 156. Copyright 2006 American Chemical Society. (B) A template with surface patterned intohydrophilic and hydrophobic regions is used to confine and align colloidal nanorods within the hydrophilic stripes. The inset shows that if the width of thehydrophilic stripes is greater than the rod length, nanorod alignment is not achieved. Adapted with permission from ref. 145. Copyright 2006 Wiley-VCH.(C) A textured template is used to facilitate the ordering of nanorods by regulating the slow evaporation of solvent. Reproduced with permission fromref. 164. Copyright 2007 American Chemical Society.


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Textured templates. Templates with textured surface, suchas highly oriented pyrolytic graphite (HOPG) and topographi-cally patterned polydimethylsiloxane (PDMS) stamp, have beenshown to facilitate the formation of close-packed nanorodassemblies.148,164 This is done by trapping the nanorod disper-sion between the textured template and a smooth surface(e.g. silicon wafer), followed by controlled drying (Fig. 5C).The textured surface allows for a slow and regulated solventevaporation, which in turn facilitates the ordering of thenanorods. This technique can be employed to produce large-scale well-ordered assemblies of vertically oriented nanorods,without the need for external fields.

2.2 Self-assembly at interfaces

Interfacial assembly of particles has been investigated for morethan a century. It is commonly observed in Pickering emulsions(e.g. homogenized milk), where solid particles spontaneouslyadsorb to the interface between two immiscible liquids to keepthe emulsion droplets from coalescing.165,166 This phenomenonwas later exploited as a technique to self-assemble colloidalmicroscopic particles and, more recently, nanocrystals at fluidinterfaces.167,168 The high interfacial energy that arises whentwo immiscible fluid phases are in contact is lowered whenparticles adsorb to the fluid–fluid interface. The decrease ininterfacial energy (DE) drives the assembly process, which isfurther mediated by interfacial deformation-induced capillaryforces that pull the particles together.60,169 For micron-sizedparticles, DE is several orders of magnitude larger than thethermal energy (kBT) that causes fluctuations, leading to irre-versible adsorption of particles to the interface.170 However, fornanometer-sized particles, DE is comparable to the thermal

excitation energy, resulting in particles that are only weaklyadsorbed to the interface. In this case, a size-dependent particleexchange occurs at the interface, where nanoparticles that arerelatively larger in size displace the smaller ones at a rate that isconsistent with their adsorption energies.171,172 Needless tosay, assemblies that are formed from larger particles are morestable. In addition to particle size, the particle shape, thewettability of the particle surface and the interparticle interac-tions are also important factors that determine the stability ofthe interfacial particle assembly.170,173,174

2.2.1 Liquid–liquid interfacial assembly. Russell andco-workers have investigated the behavior of colloidal CdSenanorods at liquid–liquid interfaces and noted that because ofshape anisotropy, nanorods behave differently from isotropicnanocrystals.175 A Pickering emulsion is generated by addingwater to a dispersion of hydrophobically coated CdSe nano-rods in toluene, followed by vigorous shaking (Fig. 6A). Thefluorescence confocal microscopy image of the water dropletsin toluene showed fluorescent circles, which confirms theadsorption of the fluorescent CdSe nanorods to the toluene–water interface. Through small-angle neutron scattering, it wasfound that the assemblies formed at the interface are sheet-likewith a thickness that is in good agreement with the thickness ofa sheet of rods that are oriented parallel to the interface. This isconsistent with theoretical arguments, which predict that isolatednanorods will preferentially align their long axis parallel to theplane of the interface as this orientation results in larger interfacialcoverage per nanorod and much lower interfacial energy ascompared to perpendicular alignment.167 When a nanorod-stabilized water droplet is dried on a carbon-coated coppergrid, it was found that the orientation and packing of the

Fig. 6 Self-assembly at interfaces. Schematic depictions of self-assembly of colloidal nanorods at (A) liquid–liquid and (B) gas–liquid interface.(A) A Pickering emulsion is created by adding water to a dispersion of nanorods in toluene followed by vigorous shaking. The emulsion droplets arestabilized through the adsorption of nanorods to the toluene–water interface. (B) Assembly of nanorods at the air–water interface using the Langmuir–Blodgett (LB) technique. Compression using a set of barriers results in assembly of nanorods that are aligned parallel to the barriers.


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nanorods vary across the droplet as a result of in-plane com-pression. Most of the nanorods were seen oriented parallel tothe interface, self-assembled into a smectic phase at theperiphery of the dried droplet, where the concentration of thenanorods is lowest. Nanorods that are oriented perpendicularto the interface were observed particularly at the center of thedried droplet, where the nanorod concentration is too high thatthe rods are forced to achieve the densest packing configura-tion to effectively minimize the interfacial energy. By controllingthe nanocrystal concentration and aspect ratio, and the inter-facial energy between liquids and nanocrystals, the orientationand packing structures of 1D nanocrystals at liquid–liquidinterfaces can be varied.175

2.2.2 Gas–liquid interfacial assembly. Colloidal 1D nano-crystals can also be assembled at gas–liquid interfaces, and theresulting assembly can be carefully transferred onto a solidsubstrate through the Langmuir–Blodgett (LB) technique.A schematic illustration of a typical nanorod assembly experi-ment using an LB apparatus (i.e., LB trough) is shown in Fig. 6B.First, a dispersion of hydrophobically passivated nanorods in anorganic solvent is allowed to spread evenly on the water surfaceof the LB trough, creating a monolayer of randomly orientednanorods at the air–water interface. Note that the hydrophobiccoating is necessary to prevent aggregation and to make surethat the nanorods float on the water surface.33,176 The nanorodsare then compressed slowly using a set of barriers, with thesurface pressure monitored. The compression generates high-density assemblies of nanorods that are aligned parallel to thetrough barriers. In some cases, the final assembly structure canbe tuned by controlling the compression process.177 Theresulting assemblies can be collected onto a desired substrate(e.g. TEM grid, silicon wafer) using either vertical or horizontallift-off. Perhaps the major advantage of the LB technique is itsability to produce 1D nanocrystal assemblies over large areas withultrahigh packing density. Large-scale high-density nanoscaleassemblies can be utilized in the fabrication of integratedfunctional nanodevices.178

2.3 Self-assembly in solutions

In terms of ease of manipulation, solution-based assembly isthe most facile way to assemble colloidal 1D nanocrystals intoorganized structures. As opposed to other assembly techniques,this approach is not reliant on external directors (e.g. appliedfield, template), does not require special equipment or compli-cated experimental setups, and does not necessitate extensivecontrol of drying conditions. Its complexity, however, lies in thedelicate interplay between the various interactions among theentities that are present in the solution. Each interaction worksin a unique way and can be very specific toward differentcombinations of nanocrystals, capping ligands and solvents.Complete control over these interactions may be difficultto achieve but they can be adequately modulated throughproper selection of solution components. Although the specificinteractions that drive the assembly vary for different solution-based assemblies, the assembly process can be briefly describedby a general mechanism. Typically, small self-assemblies are

initially formed due to interactions between solution constitu-ents and the collisions caused by Brownian motion. Thesesmall assemblies serve as nucleation sites, which subsequentlydraw neighboring building blocks together, and eventuallygrow into larger ordered structures.179,180 This process is akinto crystallization and can be influenced by factors such asconcentration and temperature. A selective change in solventquality (e.g. addition of a nonsolvent) can also facilitate theabove process by causing destabilization of the colloidal nano-crystal solution.180–182

2.3.1 Chemical-bond-directed assembly. In a recent study,Han and co-workers have shown that chemical bonding inter-action can regulate the solution-phase self-assembly of CoPnanowires (Fig. 7A).183 The nanowires are first synthesized insolution at 320 1C in the presence of organic capping ligands.After reaction completion, the reaction solution is allowed tocool to a lower temperature (20 1C or 0 1C) and subsequentlyaged at this temperature for at least 2 hours. The resultingassembly structure was found to be dependent on the agingtemperature. Large-scale self-assembly of vertically orientednanowires is produced when the reaction solution is aged at20 1C. By contrast, nanowires that are assembled horizontallyinto single-layered sheets are obtained when the reactionsolution is aged at 0 1C in a refrigerator. Thermal treatment,ultrasonication and addition of organic solvents (e.g. toluene,hexane) are not able to dismantle the assembled structures,indicating the existence of a strong attractive force betweenadjacent nanowires. It should be noted that without sub-sequent aging, randomly aligned nanorods are obtained, whichsuggests that the assembly process takes place in solutionduring aging. Further investigations revealed that upon agingof the reaction solution, the capping ligands dynamicallyadsorb/desorb from the nanowire surface, and this allowsneighboring nanowires to directly interact, leading to irrever-sible formation of Co–P chemical bonds between exposed Coand P atoms of adjacent wires (inset of Fig. 7A). The Co–Pchemical bonding is facet-specific (i.e., oriented attachmentalong the (200) planes), forcing the nanowires to assemble withboth positional and orientational order.

2.3.2 Depletion-force-driven assembly. Through depletionattraction forces, Manna and co-workers have successfullyassembled core–shell CdSe–CdS semiconductor nanorodsdirectly in solution.184 As schematically depicted in Fig. 7B,introduction of an additive to a colloidal solution of nanorodsinduces an inward force, known as depletion force, whichtriggers the assembly process. The additive molecules, whichare highly miscible with the solvent, draw the solvent away fromthe rods, and the elimination of solvent from the regionbetween the rods gives rise to depletion forces that push therods toward each other. The assembly formation can bemonitored visually by the appearance of turbidity in an initiallytransparent solution. Once formed, the resulting assemblystructure can be deposited onto a substrate of choice. Largemolecules such as long chain fatty acids (e.g. oleic acid) andpolymers (e.g. poly(ethyleneglycol) methacrylate) have been usedas an additive in the self-assembly of nanorods in nonpolar


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solvents (e.g. toluene).184,185 The concentration and size of thenanorods as well as those of the additive are parameters that canbe adjusted to tune the strength of depletion attraction forces.

2.3.3 Linker-assisted assembly. To assist the self-assemblyof nanorods in solutions, a commonly employed strategy is tomake use of specific linkers which serve as a bridge thatconnects the nanorods together. The placement of these linkerson the nanorod surface (end or side) can influence the finalassembly structure.

Molecular linkers. For Au nanorods, a,o-alkanedithiols(e.g. 1,5-pentanedithiol), with two thiol groups at opposite endsof the aliphatic chain, have been used to bridge the nanorods inan end-to-end fashion as illustrated in Fig. 7C.186 Here, theends of the Au nanorods, which are dominated by {111} planes,are brought together through covalent thiol linkage due tothe preferential binding of thiols to Au(111). Multifunctional

molecules (e.g. 11-mercaptoundecanoic acid, 4-mercaptophenol,cysteine, glutathione), which carry a terminal thiol group(–SH) and at least one terminal hydrogen-bonding functionalgroup (e.g. –COOH, –OH, –NH2), are also useful molecularconnectors for Au nanorods in solution.187–189 The thiol end ofthe molecule binds to the Au nanorods while the appendedhydrogen-bonding groups assist in the assembly of the rodsthrough intermolecular hydrogen bonding. The solution pHhas been observed to affect the assembly efficiency in accor-dance with the hydrogen bonding theory,189 and in certaincases, it has also been shown to dictate the final assemblypattern (end-to-end or side-by-side).188 The use of adipic acid,an alkanoic acid having two carboxyl groups at opposite endsof the alkyl chain, as a linker for side-by-side assembly ofcolloidal Au nanorods has also been demonstrated.190 The Aunanorods are first coated with a cationic surfactant, whichgives them a net positive charge in solution. At sufficiently

Fig. 7 Self-assembly in solutions. (A) Schematic diagram of the chemical bonding-directed self-assembly of CoP nanowires in solution. The finalassembly structure is dependent on the aging temperature. The inset shows the facet-specific chemical bond formation between Co and P atoms ofadjacent wires. Adapted with permission from ref. 183. Copyright 2012 Wiley-VCH. (B) Sketch depicting the depletion-force-driven assembly of colloidalcore–shell CdSe–CdS nanorods. Introduction of an additive to the nanorod solution causes the emergence of depletion forces that push the rods towardeach other. Reproduced with permission from ref. 184. Copyright 2010 American Chemical Society. (C) Scheme showing the end-to-end assembly of Aunanorods using a,o-alkanedithiols as molecular linkers. The thiols selectively bind onto the Au(111) ends and bridge two neighboring rods to form dimers,which eventually assemble into chains. Reproduced with permission from ref. 186. Copyright 2006 American Chemical Society.


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basic pH, adipic acid is completely deprotonated and attachesto the positively charged nanorod surface through electro-static interactions, and this drives the assembly process.The linker-assisted self-assembly technique has also beenemployed for systems other than Au nanorods. For example,Millo et al. have reported the self-assembly of CdSe nanorodsusing 1,4-phenylenediamine as a molecular linker, which cancovalently bind to the surface atoms of the nanorods throughits two amine groups.191

Biological linkers. Exploiting biological interactions isanother means of driving linker-assisted nanorod assembliesin solutions. Biomolecules, such as nucleic acids and pro-teins, have been instrumental in directing the self-assembly ofnanorods through biological interactions. For instance, thebase-pairing interactions (i.e., hybridization) in nucleic acidsor oligonucleotides have been used to program the self-assembly of Au nanorods in solutions.192–195 The most widelyexamined nucleic acid-based linker is deoxyribonucleic acid(DNA). A common strategy is to first functionalize the surfaceof the Au nanorods with thiolated single DNA strands viaAu–thiol linkage. The DNA-modified nanorods are then incu-bated with the complementary DNA strands that contain ashort self-complementary recognition sequence at the end(termed as ‘‘sticky end’’), which serves to connect the nano-rods together.196 In a study by Mirkin and co-workers, DNAhybridization has been shown to induce the ordering of Aunanorods into 2D sheets, which subsequently reorganize intoa 3D structure.194 The rods preferentially assemble with theirlong axis parallel to each other as this type of arrangementmaximizes the DNA linker interactions. More recently, Tanget al. have found that the DNA-mediated assembly and dis-assembly of Au nanorods can be controlled by varying thesolution temperature.195 This finding has led to the realiza-tion of reversible plasmonic circular dichroism responses,which can be useful in the fabrication of ultrasensitive sensorsand in the creation of intelligent materials with unique opticalresponses.

Rege et al. have demonstrated the use of cysteine-containingelastin-like polypeptides in generating optically responsive Aunanorod assemblies. The optical response can be manipulatedbased on exposure to near-infrared (NIR) light, which induces areversible change in the polypeptide conformation.197 Murphyand co-workers have utilized the extraordinarily high bindingaffinity of the protein streptavidin toward biotin (a B-vitamin)in both end-to-end and side-by-side assembly of Au nano-rods.198,199 The biotinylated Au nanorods are linked togetherby streptavidin through its multiple biotin binding sites. Inanother example, streptavidin–biotin conjugation has made itpossible to assemble Au-tipped CdSe nanorods into uniqueflower-like arrangements.200 Meanwhile, Tan and co-workershave used antibody–antigen biorecognition to assemble Aunanorods into long chain-like structures.201 This further illus-trates the utility of biomolecular recognition in guiding theassembly of nanoscopic materials, which can be exploited inthe fabrication of biomolecule-based nanoscale devices.

3. Self-assembly structures

In contrast with spherical nanocrystals, which often self-assemble into highly ordered cubic (ccp) and hexagonal (hcp)close-packed structures, 1D nanocrystals, due to their elon-gated morphology, can produce a much larger diversity ofstructures when self-assembled. These structures range fromsimple non-close-packed configurations such as 1D chains andstripes to more complex close-packed structures such as 2Dsheets and 3D double-domed cylinders. In this section, weprovide a summary of the various assembly structures thathave been achieved from the self-organization of colloidal 1Dnanocrystals (Fig. 2).

3.1 Non-close-packed structures

Non-close-packed assembly structures are created when colloidal1D nanocrystals arrange themselves into loosely packed struc-tures having distinct shapes such as rings and flowers, or whenthey align to form 1D structures such as chains and stripes,which can reach several micrometers in length. In these struc-tures, the 1D building blocks are organized in either one of thesetwo types of alignment: side-by-side or end-to-end. In certaincases, a combination of both types of alignment is observed.

3.1.1 Side-by-side alignment. Side-by-side assembly struc-tures are typically in the form of stripe patterns, which are alsocalled ribbons or rail-tracks in the literature. Each stripe is asingle-layer 1D structure, which is composed of 1D buildingblocks that are packed parallel to each other with their long axisoriented perpendicular to the packing direction (Fig. 8A). These1D stripes can extend to mesoscopic length scales and areusually positioned independent of each other, with substantialamount of space in between them. While most stripes arenearly linear, there are some that exhibit pronounced degreeof bending. Displayed in Fig. 8B and C are examples of side-by-side assembly structures that have been reported by Yang andco-workers.202 These well-defined stripe structures are formedfrom spontaneous self-assembly of ultrathin nanorods ofNb2O5 (Fig. 8B) and ZnO (Fig. 8C) in solution in the presenceof excess organic surfactants. Nanorods of other transitionmetal oxides, such as TiO2 and CoO, have also been shown toself-assemble into arrays of stripes.185,202,203 The TiO2 stripestructures can reach lengths of over 20 mm, with each stripeconsisting of thousands of nanorods that are packed in a side-by-side manner.185

According to a study by Korgel et al. using CdS nanorods,side-by-side alignment of nanorods will be more favored thanend-to-end because the energetic forces (e.g. dipole–dipole inter-action, van der Waals force) between nanorods that are arrangedside-by-side are considerably stronger than those between rodsthat are aligned end-to-end.204 This is attributed to the muchshorter center-to-center distance between neighboring rodswhen they are placed side-by-side than when positioned end-to-end. Because side-by-side alignment is more energeticallyfavored, it is believed that end-to-end assembly structures arenot equilibrium structures. Nevertheless, it was observed thatwhen a dispersion of CdS nanorods in chloroform is drop-cast


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onto a substrate at submonolayer coverage and subsequentlydried, the nanorods primarily align in an end-to-end fashion toform a network of kinetically limited stripe-like patterns, withonly a few nanorods packed side-by-side within the stripes. Theformation of these kinetically trapped structures is found tooccur via spinodal decomposition during the solvent evaporationprocess, but this is observed only when the dispersion is drop-cast at relatively low surface coverage (1 nanorod per 630 nm2).Nanorods drop-cast at higher surface coverage were found topack in a side-by-side fashion. To achieve preferential formationof end-to-end assembly structures, several techniques have beendeveloped, as summarized in the succeeding subsection.

3.1.2 End-to-end alignment. Fig. 8D shows pictorial repre-sentations of examples of assembly structures that can beformed through end-to-end alignment of colloidal 1D nano-crystals. An effective way of creating end-to-end assemblies is toselectively functionalize the end faces (or tips) of the 1Dnanocrystals with molecular linkers that can bring the endsof neighboring nanocrystals together in solution (Fig. 7C).Chain-like structures are the most common form that resultsfrom this strategy.186–188,198 For example, Au nanorods havebeen assembled into chains through thiol linkages with the useof 1,9-nonanedithiol as linkers (Fig. 8E).186 The chain lengthcan be adjusted by varying the concentration of the dithiollinker. Thioalkylcarboxylic acids such as 3-mercaptopropionicacid have also been used as linkers to direct the chaining of Aunanorods in solution.187 The thiol moiety preferentially bindsto the ends of the rods, which then assemble in a longitudinalfashion through cooperative hydrogen bonding between thecarboxyl functionalities. Chaining of nanorods can also bedirected through hybridization of nucleic acids or oligonucleo-tides. Au nanorods that are selectively functionalized at theends with mercaptoalkyloligonucleotide have been found toassemble in an end-to-end manner upon the addition of thecomplementary oligonucleotide target.193 On the other hand,the protein streptavidin has not only been useful in chainformation but has also been instrumental in the organizationof biotinylated Au-tipped CdSe nanorods into flower-like struc-tures (Fig. 8F).200 This unique arrangement is due to conjuga-tion of the rods to a central point, which is made possible bythe multiple biotin binding sites of streptavidin.

Another means of promoting the formation of end-to-endassemblies is to selectively coat the nanorod ends with hydro-phobic polymers while the rest of the rod surface is coveredwith hydrophilic ligands. This produces amphiphilic nanorodsthat can preferentially assemble into chains or rings dependingon the solvent quality.205 Meanwhile, Zubarev and co-workershave demonstrated that water droplets in air can mediate theassembly of polystyrene-coated Au nanorods during solventevaporation, leading to well-defined ring-like organization ofnanorods along the edge of the droplet (Fig. 8G).206 The use ofexternal fields has also been shown to produce end-to-endassembly structures as observed in the magnetic-field-inducedchaining of Ni nanowires and the electric-field-directed micro-string formation of CdS nanowires and nanorods.140,207 Inaddition, a variety of templates (e.g. surface-modified sub-strates, block copolymers, carbon nanotubes) have been founduseful in assisting the end-to-end alignment of colloidal 1Dnanocrystals.143,154,163

3.2 Close-packed structures

Close-packed assembly structures refer to the configurationsthat are formed when colloidal 1D nanocrystals are arrangedvery close to each other, usually with a high degree of orienta-tional order. These structures may be classified in three ways.First, depending on how the constituent nanocrystals alignrelative to the substrate onto which they are deposited, theirorientation can be described as either horizontal or vertical.

Fig. 8 Side-by-side vs. end-to-end alignment. Schematic representationsand TEM images of assembly structures based on (A–C) side-by-side and(D–G) end-to-end alignments. The TEM images in (B and C) show well-defined stripe structures that are formed from the side-by-side assemblyof ultrathin nanorods of (B) Nb2O5 and (C) ZnO. Reproduced with permis-sion from ref. 202. Copyright 2009 American Chemical Society. The TEMimages in (E–G) display the different end-to-end assembly structures thatare created by (E) linker-assisted chaining of Au nanorods, (F) protein-binding-directed flower-like organization of Au-tipped CdSe nanorodsand (G) water-droplet-mediated ring formation of Au nanorods. Figures (E)and (F) are adapted with permission from ref. 186 and 200, respectively.Copyright 2006 American Chemical Society. Figure (G) is reproduced withpermission from ref. 206. Copyright 2007 Wiley-VCH.


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Second, when classified based on the type of ordering, thestructures can be described as either nematic or smectic,similar to the arrangement of elongated molecules in liquidcrystals. Third, in terms of dimensionality, close-packed assemblystructures can be classified as either 2D or 3D. All these arediscussed more thoroughly in the subsections below.

3.2.1 Horizontal vs. vertical orientation. The TEM images inFig. 9A and B both show a monolayer structure of tightly packedCoP nanowires that have been assembled through chemicalbonding interactions in solution (discussed in Section 2.3).183

The difference between the two assembly structures is themanner in which the nanowires are oriented with respect tothe substrate onto which they are deposited. In Fig. 9A, thenanowires lay parallel to the plane of the substrate, and this isreferred to as horizontal orientation. Fig. 9B, on the otherhand, shows nanowires that are ‘‘standing’’ on the substrate,which indicates vertical orientation. In this case, the nano-wires are aligned with their long axis oriented perpendicularto the substrate, making them appear as dots when viewedtop-down through TEM. When an assembled monolayer ofvertically oriented 1D nanocrystals is highly ordered, such asthe case in Fig. 9B and D, a 2D hexagonal pattern is observed.This pattern is reminiscent of the hexagonal array of wax cellsconstructed by honey bees, and is thus commonly referred toas honeycomb structure. A series of tilted TEM images can betaken to confirm that the honeycomb structure is composed of

vertically oriented 1D nanocrystals and not of sphericalnanocrystals.

The orientation of 1D nanocrystals on substrates is criticalfor most device applications; thus it is important to know theconditions under which different orientations can be observed.Kral et al. have predicted the preferred orientation of colloidalsemiconductor nanorods with intrinsic electric dipoles (e.g.CdS, CdSe) on substrates using theoretical models, whichconsider the interaction between nanorods and their couplingto the substrates.208 On the basis of their theoretical analysis, asingle nanorod with a high aspect ratio will preferentially orientin a horizontal manner because its side-to-substrate coupling(i.e., van der Waals interaction between the side of the rod andthe substrate) is significantly stronger than its face-to-substratecoupling (i.e., van der Waals interaction between the end face ofthe rod and the substrate). Meanwhile, vertical orientation ispredicted to be stable only when the nanorod aspect ratio is lessthan 1.75, with which the face-to-substrate coupling dominates.However, in large nanorod domains, the attractive forcesbetween neighboring nanorods can contribute to the competi-tion between the horizontal and vertical rod orientation. Whenthese are included in the equation, the critical aspect ratiobelow which nanorods favor vertical orientation is calculated tobe higher than 1.75. For a monolayer assembly consisting of103 nanorods, it is predicted that vertical orientation will beachieved by nanorods with a critical aspect ratio of B2.3. Thesetheoretical predictions are fairly consistent with publishedexperimental results. Colloidal cadmium chalcogenide nano-rods with high aspect ratios have been observed to self-assemble horizontally on substrates when the solvent in whichthey are dispersed is allowed to evaporate.134,209 As an example,the TEM image in Fig. 9C shows core–shell CdSe–CdS nanorodswith an aspect ratio of 13.6 in horizontal orientation. On theother hand, for cadmium chalcogenide nanorods with relativelylower aspect ratios, vertical orientation is observed particularlyat sufficiently high concentrations.120,129 A concentration thatis too low leads to horizontally oriented nanorods, whereas atsome intermediate concentration, a combination of horizon-tally and vertically oriented nanorods is obtained. Fig. 9Dshows a TEM image of a well-ordered self-assembly of verticallyoriented CdS nanorods with an aspect ratio of B3. The tendencyof low-aspect-ratio nanorods to vertically orient more readilythan those with higher aspect ratios has also been attributed tokinetic effects as the rotational diffusion constant of nanorods isstrongly size-dependent.124

In many nanocrystal-based device applications, such as solarcells and magnetic memory devices, the 2D honeycomb structureformed from vertically oriented 1D nanocrystals is considered asthe ideal assembly geometry. This compact hexagonal sheet con-figuration allows the collective properties of densely packednanorods to be harnessed on a large scale, which is desirablefor device fabrication. Consequently, a variety of strategies havebeen developed to achieve vertical orientation even for nanorodswith high aspect ratios. For instance, it has been reported thatsolvent evaporation, when conducted at elevated temperatures,124

on the surface of warm water,134 or under the conditions of

Fig. 9 Horizontal vs. vertical orientation. (A and B) TEM images of assem-bly structures of CoP nanowires that are in (A) horizontal and (B) verticalorientation. Pictorial representations are shown in the insets. Adapted withpermission from ref. 183. Copyright 2012 Wiley-VCH. (C) TEM image ofhorizontally oriented core–shell CdSe–CdS nanorods with an aspect ratioof 13.6. Adapted with permission from ref. 134. Copyright 2007 AmericanChemical Society. (D) TEM image of vertically oriented CdS nanorods withan aspect ratio of B3. Reproduced with permission from ref. 129. Copy-right 2008 American Chemical Society.


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continuously decreasing solvent quality,180 can lead to verticalnanorod orientation. Furthermore, external electric fields131,132,134

(Fig. 4B) and textured templates148,164 (Fig. 5C) have been found toassist in vertically orienting 1D nanocrystals during assembly.If the assemblies are to be integrated in devices, it would benecessary to assess their quality in terms of the orientationdistribution of nanorods in the assembly structure. Usinggrazing-incidence wide-angle X-ray diffraction, Alivisatos andco-workers were able to quantify the percentage of verticallyoriented CdS nanorods in large-scale assembly structures.124 Inthis technique, knowledge of the crystal lattice orientation isutilized to extract information on the nanorod orientation. Inthe case of CdS, which has a wurtzite crystal lattice, the longaxis of the nanorod coincides with the c-axis or the [001]direction in the rod’s lattice. For CdS nanorods that areoriented vertically, the diffraction from (002) planes falls atthe top of the Bragg ring, where the rod orientation angle (o)approaches 01. Integration of the diffraction intensity as afunction of o along the circumference of the (002) Bragg ringgives the orientation distribution function, from which thepercentage of vertically oriented nanorods can be determined.

For 1D nanocrystals in horizontal orientation, it is desirablethat the 1D building blocks have their long axis orientedparallel to a common direction so as to maximize the attractiveinteractions between them. An effective way to achieve this isthrough the Langmuir–Blodgett (LB) technique (Fig. 6B), bywhich 1D nanocrystals align with their long axis parallel to thetrough barriers upon compression at the air–water interface.This can also be accomplished by application of external fields,which force 1D nanocrystals to orient their long axis along thefield direction (Fig. 4A and C). A high degree of orientational

order is often seen in the assemblies produced using the above-mentioned techniques. In the case of the LB technique, it isalso possible to manipulate the inter-nanocrystal spacing bycontrolling the compression process.

3.2.2 Nematic vs. smectic ordering. Colloidal 1D nano-crystals exhibit two general types of close-packed ordering,which are named based on the two liquid crystal phases thatthey resemble: nematic and smectic. Nematic ordering ischaracterized by 1D nanocrystals that possess long-range orien-tational order but have no positional order (i.e., no periodicityin all dimensions). This can be seen in Fig. 10A and B,125,210

where the nanorods have their long axis pointed to a commondirection but there is no correlation in their position. On theother hand, in smectic ordering, the 1D nanocrystals possess adegree of positional order (along at least one spatial dimen-sion) in addition to orientational order. This type of ordering isexhibited by the nanorod assemblies shown in Fig. 10C and D.An important feature that easily distinguishes smectic fromnematic ordering is the presence of well-defined stripes that areclosely packed parallel to one another. The distinct stripepatterns seen in smectic ordering is a consequence of theexistence of both positional and orientational order. Eachstripe is a row of positionally ordered nanorods that are packedside by side with their long axis oriented perpendicular to thestripe direction.

Long-range orientational order is common to both nematicand smectic ordering. As mentioned earlier, orientational ordercan be forcibly achieved through compression with a set ofbarriers at the air–water interface (i.e., the LB technique).However, the use of this technique does not always result inpositional order, which is present in smectic but not in nematicordering. The assembly behavior of 1D nanocrystals uponcompression is highly dependent on their aspect ratio.176 Whencompressed, 1D nanocrystals with high aspect ratios, typicallynanowires, prefer to order into the nematic phase arrange-ment.33,178 Meanwhile, for some low-aspect-ratio nanorods,different types of ordering can arise at different stages ofcompression in response to the change in surface pressure.For example, the assembly formed from BaCrO4 nanorods withan aspect ratio of 4 has been observed to undergo phaseevolution – from isotropic to 2D nematic to 2D smectic to 3Dnematic – with increasing surface pressure, as shown inFig. 11A–D.177 This demonstrates how the final assemblystructure can be varied by altering the packing density throughcompression. Prior to compression, an isotropic state isobserved, where the nanorods assume all possible orientation(Fig. 11A). During the compression process, the nanorod den-sity increases as the inter-rod separation distance decreases.A transition from isotropic to 2D nematic phase (Fig. 11B) isobserved at the early stage of compression as it becomesincreasingly difficult for the rods to point in random directionswith the increase in density. The transition to nematic phaseoccurs in accordance with Onsager theory, which predicts thatnematic ordering will arise from hard-core repulsive inter-actions between rod-like particles if the density is sufficientlyhigh.211 The orientation of rods parallel to each other is favored

Fig. 10 Nematic vs. smectic ordering. Electron microscopy images ofcolloidal nanorods that exhibit (A and B) nematic and (C and D) smecticordering. Some of the rods are highlighted in yellow as a guide to the eye.White arrows are used to indicate the direction to which the nanorods arepointed. (A) and (C) are TEM images of core–shell CdSe–CdS nanorods.Reproduced with permission from ref. 210. Copyright 2009 AmericanChemical Society. (B) and (D) are SEM images of Au nanorods. Reproducedwith permission from ref. 125. Copyright 2008 Wiley-VCH.


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as it maximizes the translational entropy by minimizing theexcluded volume per nanorod in the assembly structure, andthe net gain in translational entropy compensates for the lossof orientational entropy. With further compression, the densityincreases significantly, which leads to 2D smectic ordering(Fig. 11C). The observation of a nematic to smectic phasetransition at relatively higher densities is consistent withFrenkel’s computer simulations for a system of hard rods withpurely repulsive interactions despite the fact that attractiveinteractions are present in the experimentally obtained systemof BaCrO4 nanorods.212 This density-driven phase evolution isnot usually observed in other real systems, such as in the caseof low-aspect-ratio Au nanorods, which exhibit much strongerattractive inter-rod interactions than BaCrO4 nanorods.176

When the compression is too strong, the BaCrO4 nanorodsorganize into the 3D nematic configuration (Fig. 11D).177 Thisresults from the breaking of the monolayer into multilayers asthe critical buckling surface pressure is reached. The nematicarrangement in this 3D assembly structure is frequently dis-rupted by disclinations (Fig. 11E). Disclinations are rotationaldefects in orientationally ordered structures that can arisenaturally during phase transitions. The disclination shown inFig. 11E resembles a type of rotational defect in nematic liquid

crystals. A similar defect has been observed by Alivisatos et al.for CdSe nanorods when a highly concentrated nanorod dis-persion is allowed to dry on a substrate.209 To minimize thetotal elastic energy during drying, the rods in the vicinity of thedefect tend to break into branches. The TEM image in Fig. 11Fshows that there are six branches of CdSe nanorods around thedisclination core. Each branch corresponds to a distinct regionwhere nanorods are oriented parallel to a common direction.This implies that there is nematic ordering within the branchesbut it vanishes at the core, where an isotropic state exists.

Orientational order can also be achieved when anisotropic1D nanocrystals are assembled under the influence of electricfields. As demonstrated by Manna et al., the presence of apermanent electric dipole moment in core–shell CdSe–CdSnanorods can be exploited to induce the orientational order-ing of nanorods using electric fields.134 The nanorods canalign over microscale areas along the field direction through-out the electrode gap. Inspection through SEM revealed thatboth nematic and smectic ordering of nanorods are obtained.A magnified view of a selected region where nanorods are innematic arrangement is shown in Fig. 11G, while a zoom-inview that displays an area of nanorods in smectic configu-ration can be seen in Fig. 11H. However, the conditions under

Fig. 11 Orientational order. (A–D) Compression-induced ordering of BaCrO4 nanorods. The TEM images reveal the occurrence of phase evolution –from (A) isotropic to (B) 2D nematic to (C) 2D smectic to (D) 3D nematic – with increasing surface pressure. White arrows indicate the direction to whichthe rods are pointed. Insets in (B) and (C) are Fourier transform of the corresponding image, showing the regularity of the inter-rod distances.Reproduced with permission from ref. 177. Copyright 2001 American Chemical Society. (E and F) TEM images of nanorod assemblies showingdisclinations or defects. Shown are nanorods of (E) BaCrO4 (ref. 177) and (F) CdSe. Reproduced with permission from ref. 209. Copyright 2003Wiley-VCH. (G and H) Orientational ordering of nanorods under the influence of electric fields. The TEM images show assembly structures of core–shellCdSe–CdS nanorods in (G) nematic and (H) smectic arrangement. White arrows indicate the field direction. Some of the rods are highlighted in red as aguide to the eye. Adapted with permission from ref. 134. Copyright 2007 American Chemical Society.


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which only one type of ordering is present have yet to beinvestigated.

3.2.3 2D vs. 3D structure. The assembly structure that isformed when colloidal 1D nanocrystals self-organize into asingle layer of densely packed building blocks is oftendescribed as a 2D monolayer or sheet-like structure. For a 2Dmonolayer assembly of monodisperse colloidal nanorods, thesheet thickness can be estimated from the rod length if the rodsare vertically oriented or from the rod diameter if the rods arein horizontal orientation. Fig. 12A shows an example of a 2Dmonolayer assembly structure of vertically oriented CdS nano-rods that was obtained by controlled evaporation of a nanoroddispersion.120 When a number of these 2D monolayers arestacked one over the other, a 3D multilayer structure is pro-duced. The SEM image displayed in Fig. 12B shows a 3Dmultilayer structure that consists of four layers of verticallyoriented CdS nanorods. Each layer by itself is an individual 2Dsheet having a hexagonal (or honeycomb) pattern, which istypical for well ordered 2D monolayer assemblies of vertically

oriented nanorods. Even the rods at the edges of each layer arehighly ordered, suggesting that each 2D monolayer sheet ispreassembled during drying before depositing sequentially toform 3D multilayers. The generation of 3D multilayers (i.e.,multiple sheet assembly) is usually observed when the nano-crystal concentration is sufficiently high and when the solventevaporation rate is strictly controlled.120,131,213 When 3D multi-layer assembly structures of vertically oriented nanorods areviewed top-down in transmission mode, characteristic Moireinterference patterns can be seen. For instance, the TEM imageof a 3D bilayer structure of vertically oriented CdS nanorods inFig. 12C shows a Moire interference pattern that results fromthe AB stacking of two identical 2D hexagonal sheets. Thesuperimposition of the two 2D sheets is more clearly seen inthe schematic representation in Fig. 12D. A comprehensivestudy by Ryan and co-workers using scanning TEM (STEM)has revealed that the formation of Moire patterns arises fromrotational offsets between the overlapping 2D monolayersheets.214 A range of distinct Moire patterns has been observedand each pattern has been indexed to a specific rotationalmisorientation angle. The occurrence of the Moire pattern isnot seen when there is no rotational offset or when therotational offset is at exact multiples of 301. However, thepresence of lateral shift even when there is no rotational offsetcan give rise to the formation of different patterns.

The creation of 3D multilayer structures with distinctpatterns can also be achieved by stacking of 2D sheets ofhorizontally oriented nanowires through the LB technique.For instance, the fabrication of bilayer assembly structureswith mesh-like or crisscross patterns using ultrathin nanowireswith very high aspect ratios has been successfully demon-strated. First, a 2D monolayer sheet of closely packed nano-wires (Fig. 12E) is assembled at the air–water interface at anoptimal surface pressure and transferred to a substrate by LBdeposition. The crossed bilayer structure (Fig. 12F) is thencreated by depositing a second 2D sheet of nanowires perpendi-cular to the first sheet. This simple assembly strategy hasenabled the construction of large-scale high-density crossedstructures of ZnSe, Te and Ag2Te nanowires with potentialapplications in electronics and photonics.215,216 A series ofdifferent crisscross patterns can be formed by varying the crossangle between the two 2D monolayer sheets. When more thantwo 2D sheets of horizontally oriented nanowires are depositedon top of one another in a layer-by-layer fashion, a 3D multi-layer structure with woodpile-like configuration is obtained.This type of structure has been recently fabricated by Ling andco-workers using Ag nanowires to achieve a highly activesurface-enhanced Raman scattering (SERS) substrate for sen-sing applications.217

In addition to the 3D multilayer structures discussed above,a range of different 3D assembly configurations have beenrealized in the past few years. The construction of close-packed 3D structures with well-defined geometries, such asspheres, cylinders and needles, has been made possiblethrough manipulation of the surface chemistry of colloidalnanorods. For instance, Kumacheva et al. have demonstrated

Fig. 12 2D vs. 3D structure. SEM images of (A) 2D monolayer and (B) 3Dmultilayer assembly structures consisting of vertically oriented CdS nano-rods. Reproduced from ref. 120. (C) TEM image showing a Moire inter-ference pattern formed by AB stacking of two layers of vertically orientedCdS nanorods. (D) Schematic depiction of the pattern shown in (C).Reproduced with permission from ref. 131. Copyright 2006 AmericanChemical Society. TEM images of (E) 2D monolayer and (F) 3D bilayerassembly structures that are composed of horizontally oriented Ag2Snanowires. Reproduced with permission from ref. 216. Copyright 2010American Chemical Society.


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the formation of 3D hollow spherical assembly structures insolution from colloidal Au nanorods that are amphiphilic,meaning they have both hydrophilic and hydrophobic domains(Fig. 13A).205,218 To render the Au nanorods amphiphilic, theyare first coated with a double layer of cetyl trimethylammoniumbromide (CTAB), which preferentially binds to the longitudinalsides of the rods. Since the two ends of the rods are CTAB-deprived, thiol-terminated polystyrene (PS) molecules are selec-tively attached to both ends making these regions hydrophobicwhile the rest of the rod surface is hydrophilic. In tetrahydro-furan (THF), the amphiphilic Au nanorods are stabilized dueto the favorable interactions between their PS-terminatedends and THF. When water is added to the dispersion, theTHF–water mixture becomes a better solvent for the CTAB-coatedsurface and a poor solvent for the PS-terminated ends, and thistriggers the aggregation of nanorods. Increasing the water con-tent gradually leads to the formation of 3D hollow sphericalstructures that are shown in Fig. 13A. The walls of these spheresare composed of a single layer of closely packed Au nanorods inwhich the side-to-side spacing between the rods is approxi-mately 5.3 nm.

More recently, Cao’s group has reported the fabrication ofmore complex 3D assembly configurations using highly lumi-nescent CdSe–CdS core–shell nanorods as building blocks.219–222

Their strategy involves two major steps as shown schematicallyin Fig. 13B. Water-dispersible nanorod micelles are first pre-pared by mixing a chloroform solution of hydrophobicallycapped nanorods with an aqueous solution of dodecyl trimethyl-ammonium bromide (DTAB), followed by evaporation of chloro-form. The nanorod micelle solution is then injected into a flaskcontaining ethylene glycol under vigorous stirring. This secondstep results in the loss of DTAB molecules into the growthsolution, which leads to the aggregation of nanorods andeventual formation of the 3D assembly structures. TEM tiltingexperiments revealed that the resulting structures have multiplewell-defined supercrystalline domains with configurations andsizes that are dependent on the total number (N) of constituentnanorods in each assembly structure. The 3D double-domedcylindrical structure shown in Fig. 13B is obtained when N is lessthan B80 000. This double-domed cylinder consists of sevendistinct domains where the nanorods within a single domain arearranged parallel to each other while the nanorods of neighbor-ing domains are arranged normal to each other. Interestingly, itwas found that when the nanorods are incubated with octyla-mine prior to micelle preparation, 3D needle-like assemblystructures having only one supercrystalline domain are obtained.A schematic depiction of the elongated needle-shaped structureis shown in Fig. 2H. It has been demonstrated that these 3D

Fig. 13 Other 3D assembly structures. (A) Schematic illustration that shows the formation of 3D hollow spherical assembly structures from amphiphilicAu nanorods. SEM images of the self-assembled structures are also shown. Reprinted with permission from Macmillan Publishers Ltd: Nature Materials(ref. 205), copyright 2007. (B) Schematic depiction of the formation of a 3D double-domed cylindrical structure from core–shell CdSe–CdS nanorodscapped with octadecylphosphonic acid (ODPA) and octylamine (OcAm). Also shown is a representative TEM image of the resulting 3D assemblystructure. Adapted with permission from ref. 220. Copyright 2012, AAAS.


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needles, when unidirectionally aligned and embedded in PDMSthin films, display great promise as downconversion phosphorsfor polarized light-emitting diodes.

4. Summary and outlook

The growth in the number of publications on nanoscale self-assembly over the past decade is proof that this area of study isgenerating continued interest in the scientific community.Although much of the published work is on the self-assemblyof spherical nanoscale building blocks, there is significantinterest in recent years in the self-assembly of anisotropic 1Dnanocrystals, which is the focus of this review. Colloidal 1Dnanocrystals, such as nanorods and nanowires, can be orga-nized into a variety of well-defined assembly structures, and thechoice of an assembly approach is crucial for the formation ofthe desired structure. Different strategies have been developedto realize self-assembly of colloidal 1D nanocrystals on sub-strates, at interfaces, and in solutions. Self-assembly on sub-strates can be achieved through methods that involveevaporation control, external field facilitation and templateuse. Evaporation-mediated assembly has been shown to becapable of creating highly ordered 1D nanocrystal assemblieson a wide range of substrates. However, this approach necessi-tates appropriate drying conditions and warrants careful con-trol of several parameters. External-field-directed assembly hasproven to be effective in producing assemblies with a highdegree of orientational order by forcing 1D nanocrystals toorient their long axis along the field direction. When this iscoupled with controlled evaporation, a high degree of posi-tional order can be observed as well. While there is significantwork on the use of electric field to induce the ordering ofcolloidal nanorods, there have been relatively few reports onmagnetic-field-driven nanorod assembly. An applied magneticfield would be the perfect stimulus for directing the verticalassembly of magnetically active nanorods on substrates, andthe resulting magnetic nanorod array can potentially be used inthe fabrication of high-density magnetic storage devices. Intemplate-assisted assembly, a wide range of materials can beutilized as templates for arranging 1D nanocrystals intoordered structures. An advantage of this technique is thatthe template properties (e.g. feature size, surface functionality)can be specifically tailored to match the nanocrystal properties(e.g. dimensions, surface reactivity) and/or to achieve a parti-cular assembly structure or pattern. As more materials withnovel properties are attracting interest as building blocks fornanoscale self-assembly, it is anticipated that a variety of newtemplates with tailored properties will be developed in the comingyears. In the case of interfacial assembly, the LB technique is seenas a powerful tool for generating large-scale assemblies of 1Dnanocrystals with control over orientational order, packing densityand inter-nanocrystal spacing. This is highly desirable for con-structing macroscopic-scale ultrahigh-density assembly structuresthat can be integrated in functional nanocrystal-based devices.When it comes to ease of manipulation, solution-based assembly

is perhaps the most facile approach as it does not rely on externaldirectors and does not require complicated experimental setupsand strict control of drying conditions. Different types of inter-actions can be exploited to drive and regulate the self-assembly of1D nanocrystals in solutions. However, it is important that thesolution components (e.g. solvent, surfactant, additive) are pro-perly selected as the various interactions among the entities insolution can significantly influence the assembly process.

The different methods that are now available for the self-assembly of 1D nanocrystals have enabled the creation of avariety of assembly configurations that range from looselypacked structures (e.g. 1D chains) to densely packed monolayer(e.g. 2D sheets) and multilayer (e.g. 3D stacked sheets) struc-tures. More complex 3D assembly geometries have also beenrealized. The final assembly structure is primarily a result of the1D nanocrystals’ preferred type of alignment, orientation,ordering and stacking behavior, which can be influenced byseveral factors. Thus, the conditions under which differenttypes of alignment (side-by-side vs. end-to-end), orientation(horizontal vs. vertical), ordering (nematic vs. smectic) andstructure dimensionality (2D vs. 3D) can be achieved are highlycrucial and merit further investigation. Several studies havebeen focused on developing strategies for vertically orienting1D nanocrystals during assembly. This is because the 2Dhexagonal sheet (honeycomb) structure that is formed fromvertically oriented 1D nanocrystals is regarded as the idealassembly configuration for many technological devices, suchas solar cells and magnetic memory devices. Meanwhile, theclose-packed 3D multilayer structures (e.g. woodpile) that aregenerated from the stacking of 2D monolayer sheets of hor-izontally oriented 1D nanocrystals are starting to gain signifi-cant interest as potential components for switching andsensing devices. Very recently, fluorescent nanorods have beenmade to self-assemble into 3D needle-shaped structures, whichcan be used as components of light-emitting diodes. Lookingahead, it is likely that the assembly structures formed from 1Dnanocrystals will progress to more sophisticated architectureswith properties that will cater for a wider range of applications.

As there are continuous efforts to commercialize nanotechnol-ogy, the ability to successfully translate nanofabrication methodsfrom a laboratory to an industrial setting becomes increasinglyimportant. In comparison with top-down lithography-based nano-fabrication, which is still the method of choice in the electronicsindustry, the bottom-up colloidal self-assembly approach canproduce ordered structures with smaller feature dimensions with-out the need for expensive facilities. However, the structures thatare built from self-assembly are prone to defects and it is oftendifficult to achieve a high degree of order that extends over largeareas, which is crucial if the assembled structures are to beintegrated in devices. These issues present challenges that mustbe dealt with in future studies. One possible solution is tocombine the colloidal self-assembly strategy with other existingnanofabrication techniques, such as in the case of colloidallithography. The integration of two or more nanofabricationtechniques may potentially provide the necessary tool for theconstruction of reliable nanomaterial-based devices that can meet


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industrial requirements. Ultimately, the goal is to develop a self-assembly method that is not only simple, reliable and inexpen-sive, but also versatile, readily scalable and compatible withexisting nanofabrication and processing technologies.

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Self-assembly of colloidal one-dimensional nanocrystals