DOI: 10.1002/chem.201204263

Tuning Molecular Self-Assembly Toward Intriguing NanomaterialArchitectures

Ju Young Lee,[a] Byung Hee Hong,[b] Dong Young Kim,[a] Daniel R. Mason,[a, c]

Jung Woo Lee,[a] Young Chun,[a] and Kwang S. Kim*[a]

Harnessing the self-assembly of organic molecules for thesynthesis of arbitrarily structured nanomaterials with diversephysical and chemical properties is undoubtedly one of themost important goals of nanotechnology research. Althoughperformed with ease in Nature, our ability to artificiallyshape-engineer self-assembled materials in the laboratoryremains limited to a known set of precursor molecules thatassemble to a few well-known topologies.[1–4] Currently lack-ing are materials from which a diverse range of structuraltopologies can be chosen and extracted from the synthesisprocess by careful control of the external parameters of thegrowth conditions.

Self-assembly of organic molecules is largely guided bythe interplay of intermolecular noncovalent interactions.The strength of these interactions is comparable across boththe solution and solid phase. This results in a dynamic equi-librium that may enable the shapes of self-assembled struc-tures to be controlled and optimized into their thermody-namically or kinetically favored morphologies.[1–4] Here weshow the shape control of electro- and photochemicallyactive calix[4]hydroquinone (CHQ) into nanoplates, -poly-gons and -tubes and their dynamic conversion into nano-spheres and -hemispheres through a subsequent anisotropicgrowth phase to form thermodynamically more stable struc-tures. We propose the growth mechanism of CHQ nano-structures based on thermodynamic and kinetic model stud-ies and theoretical simulations. This understanding providesa route to shape-engineering of new nanomaterials. TheseCHQ nanostructures can be implemented as nanolenses forhigh-resolution optical imaging[5,6] or form dielectric tem-

plates for a diverse range of metal-coated highly tunableplasmonic materials without the need for additional reduc-ing agents.

The self-assembly of CHQ molecules shows the structuralversatility of calixarene motifs capable of forming variousintermolecular structures when combined with solvent andguest molecules.[7–9] CHQ consists of four hydroquinone sub-units where there are four inner�OH groups to stabilize thecone shape of CHQ through the circular proton-tunnelingresonance; the other �OH groups and aromatic rings con-tribute to intermolecular interactions (see the SupportingInformation). Thus, a CHQ molecule has eight hydrogenbond donors, eight receptors, and four p–p stacking pairsleading to the formation of self-assembled supramolecularstructures.[9] Figure 1 shows the CHQ nanostructures with

various morphologies obtained by changing the tempera-ture, diffusion rate, or evaporation rate. CHQ molecules dis-solved in a 1:1 acetone/water solution self-assemble as thesolution evaporates. The polygonal films and plates form ina rapid evaporation process of the solution in the initialstage. On the other hand, slow evaporation assists the for-mation of diverse and distinctive nanostructures, such asnanotubes,[9] nanospheres and nanolenses.[5] Once CHQnanostructures, such as tubes and plates grow, a subsequentre-assembling process takes place leading to nanolenses andnanospheres.

[a] Dr. J. Y. Lee,+ Dr. D. Y. Kim, Dr. D. R. Mason, J. W. Lee, Y. Chun,Prof. K. S. KimDepartment of ChemistryPohang University of Science and TechnologyPohang, 790-784 (Korea)E-mail : kim@postech.ac.kr

[b] Prof. B. H. Hong+

Department of Chemistry, Seoul National UniversitySeoul, 151-747 (Korea)

[c] Dr. D. R. MasonPresent address: Photonic Systems LaboratorySchool of EECS, Seoul National University, Seoul 151-744 (Korea)

[+] These authors contributed equally to this work.

Supporting information for this article is available on the WWWunder http://dx.doi.org/10.1002/chem.201204263.

Figure 1. Morphological transformations of CHQ nanostructures.

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At an early stage of tubular crystallization, CHQ mole-cules in the solution phase tend to nucleate into individualtubular structures with strong 1D hydrogen bonds (HB; Fig-ure 2 a).[10] Atomic force microscope (AFM) investigation ofthese 1D structures evaporated onto a SiO2 substrate shows

the smallest height of approximately 2 nm (Figure 2 b), indi-cating an isolated single CHQ organic nanotube (Figure 2 a,inset).[11]

As the crystallization proceeds, p–p stacking interac-tions[12,13] between individual nanotubes lead to the forma-

Figure 3. Formation of CHQ nanotube crystals. a) Temperature dependence of the aspect ratio of CHQ nanotube crystals. b) Illustration representing theformation process of a small bundle. A single tube is formed only by a H bond between tubular units, whereas a bundle is formed by p–p interaction be-tween single tubes (see the X-ray structure in ref. [9]). Thus, one unit cell consists of four CHQ molecules (N= 4n). The number of unit cells along thelongitudinal axis and the transverse direction are l and w, respectively (see the Supporting Information). c) Profile of the total Gibbs free energy (DG)for the CHQ nanotube nucleation depending on the aspect ratio q (1/w) and the number of CHQ monomers, n. d) Volume energies (DGHB

V , DGp�pV ; left)

and the surface energies (DGHBS , DGp�p

S ; right) of the HB and p–p faces depending on aspect ratio for a specific assembling unit.

Figure 2. SEM and AFM images of CHQ nanotubes. a) SEM image of CHQ nano-tubes spun on a SiO2 substrate at 4000 rpm after 3 days of growth. The molecularstructure of a short single nanotube is shown in the inset. b) AFM image of an iso-lated single CHQ nanotube (height: 2 nm) in (a). c) SEM image of CHQ nanotubebundles lying on the surfaces of CHQ nanotube crystals. d) AFM image taken fromthe dotted area in inset of (c). e) SEM image showing the rectangular parallelepipedshapes of CHQ nanotube crystals. Inset: the incomplete CHQ nanotube bundles inthe process of reassembling and curing. f) AFM image of a CHQ nanotube bundleevaporated onto a SiO2 substrate.

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tion of bundles.[14] Scanning electron microscopy (SEM) andAFM images show that the small bundles of a few nano-tubes are epitaxially attached to the incomplete surface ofgrowing crystals (Figure 2 c and d).[15] These small CHQnanotube bundles continuously reassemble and cure the de-fects on the surface of the crystals (Figure 2 e inset). As aresult, self-assembling CHQ nanotubes form rectangularparallelepiped nanocrystals (Figure 2 e and f). A lower con-centration of CHQ and shorter growth time usually produ-ces thinner nanotube crystals. For example, spinning of0.05 mm solution at 4000 rpm for 40 s forms organic nano-tube crystals as thin as 2–5 nm (Figure 2 b).

The shape of the CHQ nanotubes can be controlled bythe growth temperature. The aspect ratios (q= l/w) of thenanotubes increase by up to a few hundred at low tempera-tures (Figure 3 a).[16] Based on the thermodynamic and kinet-ic model studies and theoretical simulations on the growthof microscale and bulk organic crystals,[17,18] the anisotropicgrowth of a rectangular parallelepiped crystal is modeled byconsidering the directionality and the strength of the inter-molecular interactions (Figure 3 b). According to the classi-cal nucleation theory,[19] the formation free energy (DG) de-pends on the free energy gained by the intermolecular inter-action in transferring the assembling units from liquid tosolid (DGV) and the free energy needed to create solid–liquid interfaces (DGS).[20] To explain the thermodynamicstabilities of 1D crystal structures, the change in the Gibbsfree energy with respect to the aspect ratios (q) of crystals,DG(q), is evaluated depending on the number of self-assem-bling units (N). Thus, DGV(q)=�2ep–pN2/3q1/3�eHBN2/3q�2/3 +(eHB + 2ep–p)N, and DGS(q)=4gp–pN2/3q1/3 +2gHBN2/3q�2/3. Thedifferences in chemical potential between the assemblingunit in the solution phase and in crystals in the direction ofHB and p–p faces are represented as eHB and ep–p (<0), re-spectively. The surface energy of the solid–liquid interfacefor the HB and p–p faces are represented as gHB and gp–p

(>0), respectively (see the Supporting Information).Figure 3 c shows the energy profiles of the crystallization

into CHQ nanotubes depending on the aspect ratio and thenumber of CHQ molecules. The minimum value of DG canbe found from d(DG(q))/dq=0, which corresponds to q* ofapproximately 5, where the nucleation occurs. When thenumber of CHQ molecules in the cluster is larger than thatof the number of molecules in the critical nucleus (n* ~7),crystals start growing. The steady state growth shape is notdetermined by the thermodynamic stability since the crystalsgrow by kinetics rather than by thermodynamics. To under-stand the shape evolution of the CHQ nanotube, we plottedthe volume energy and the surface energy of the HB andthe p–p faces for a given N in Figure 3 d. There is a muchlarger energy gain for growth in the direction of HB faceswith increasing q than that in the direction of p–p stackingfaces with decreasing q (Figure 3 d, left). In addition, the in-terfacial free energy barrier of p–p faces increases slowly asq increases, whereas that of HB interaction faces rapidly in-creases as q decreases (Figure 3 d, right). As a result, thenucleated crystals evolve towards long and thin nanotubes

with higher q. On the other hand, as the temperature in-creases, the chemical potential for the crystallization in thedirection of HB faces decreases much more than that in thedirection of p–p faces, and thus, the aspect ratio of thenanotubes decreases with increasing temperature.

As the temperature changes, the crystal structures are de-stabilized, resulting in morphology conversions into differenttypes of nanostructures (Figure 4).[21] For example, when thecrystals grown at room temperature are at 40 8C in an aque-ous environment for 3 h, CHQ molecules released from thedestabilized crystals form nanospheres (Figure 4 d). Thismorphology change proceeds even at room temperature,though it takes longer. These nanospheres can be separatedinto uniform size by filtration, sonication and centrifugation(Figure 4 e). The CHQ nanospheres are stable in aqueoussolution as well as dry conditions, and moreover, they are

Figure 4. Anisotropic growth of CHQ nanospheres. a)–c) SEM images ofspherical particles attached to the CHQ nanotubes. d) SEM image ofnanospheres springing from a nanotube crystal grown at room tempera-ture and subsequently heated at 40 8C for 3 h. e) SEM image of the mon-odisperse CHQ nanospheres acquired by filtration and centrifugation.f) Gibbs free energy (DG) for the anisotropic formation of CHQ nano-spheres depending on q’ and the number of CHQ monomers, n. g) Aschematic representation of free energy for the anisotropic growth ofCHQ nanospheres from CHQ monomers. h), i) SEM images of two-di-mensional disks on the CHQ hexagonal plates. j) SEM image of CHQnanolenses separated from the film structures. Hollow films show tracesof the formation and separation of CHQ lenses. Inset: CHQ lens separat-ed from a film; scale bar: 1 mm.

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impervious to high temperatures of up to approximately200 8C. The formation process of CHQ nanospheres interest-ingly takes place on the surfaces of the CHQ nanotube crys-tals anisotropically (Figure 4 b), unlike growth through iso-tropic spherical expansion.[22] As a result, their growth inter-mediaries show interesting cut-sphere shapes involvingdome-, hemisphere-, and superhemisphere structures, whichhave been utilized to study subwavelength nano-optical phe-nomena.[5]

To understand the anisotropic formation of CHQ nano-spheres, the kinetic barriers for anisotropic nucleation arecompared to those for isotropic nucleation. The infrared(IR) and X-ray diffraction (XRD) analyses and ab initio cal-culations suggest that the amorphous spherical morphologyis formed by van der Waals interactions and p–p interac-tions between the CHQ hexamer based units (see the Sup-porting Information). The Gibbs free energy (DG) for thenucleation of CHQ nanospheres depends on the ratio of di-ameter-to-height of the cut-sphere shape (q’) and thenumber of CHQ molecules (Figure 4 f). When q’�1/2, the

activation barrier for nucleation is the lowest. The isotropicformation of a nanosphere with the highest interfacial freeenergy barrier is represented by q’= 1. This kinetic barrier isalso higher than that of CHQ nanotube crystals (Figures 4 fand 3 c). Thus, instead of the self-assembling pathway forthe direct formation of CHQ nanospheres from solvatedmolecules, kinetics drives the pathway towards the crystalli-zation of CHQ nanotubes prior to nanospheres. Then, ther-modynamics ultimately drives CHQ nanotubes to convertinto nanospheres. In this process, CHQ nanospheres nucle-ate and grow anisotropically on the surface of crystals toreduce the surface energy barrier. Thus, the self-assemblingpathways of the anisotropic formation of CHQ nanospheresby CHQ nanotubes are more energetically favorable thanthose of the isotropic formation (Figure 4 g).

The formation of the film-like structures on the CHQcrystals causes the CHQ molecules to accumulate betweenfilm and crystal rather than diffusing directly into solutionafter their release from the crystal surface. This induces ahyper-concentration of CHQ molecules and a short diffusion

Figure 5. Metal nanoshells/semishells and the predicted tuning of their localized surface plasmon resonances. a) TEM images of Pt nanoshells. b) TEMimage of a Ag nanoshell. c) Electron diffraction pattern of a Pt nanoshell. d), e) Normalized extinction coefficients. d) D =50 nm dielectric hemisphere(of refractive index nd =1.5) coated with successively increasing thickness (t) of gold (t=5, 10, 15, 20, 25, 30, 40 and 50 nm, as indicated; permittivity ofgold is taken from ref. [26]); the bottom surface of the hemisphere is uncoated, and the entire semishell structure is immersed in air. e) A dielectric hemi-sphere with a gold coating of thickness t=10 nm and successively increasing diameters (D =50, 60, 70, 80, 90 and 100 nm, as indicated). f), g) Curvesdemonstrating the tunability of the LSPR resonance with respect to thickness of the gold coating (f) and diameter of the dielectric core (g) for both di-ACHTUNGTRENNUNGelectric hemispheres (^) and full spheres (*).

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distance, and hence leads to fast nucleation and growth oflens-shaped structures. In the self-assembly process of aCHQ nanolens, disk-shaped structures are formed prior tothree dimensional growth into CHQ nanolens structures(Figure 4 g and h). The curvature of the lens surface isformed through re-assembling of CHQ molecules under thedisk. After formation of the plano-convex lens with near-perfect spherical curvature, they can be separated from thefilm substrate (Figure 4 i). The CHQ nanolens is very stablewhen it is isolated and dried in air. However, the CHQnanolenses existing in aqueous solution are dynamically con-verted into CHQ nanospheres, which are the most stableamong the CHQ nanostructures.[5]

Using these CHQ nanospheres and nanolenses as tem-plates,[23] we synthesized platinum and silver nanoshells (Fig-ure 5 a–c and the Supporting Information) with the reducingpower of CHQ itself. The same method could be applied toproduce nanoshells made from other types of noblemetals.[24] The nanoshells and semishells would exhibit inter-esting optical and chemical properties depending on thethickness and the size/shape of the metal layers (Figure 5 d–g). Variation of the film thickness is predicted to offer tuna-bility of their localized surface plasmon resonance (LSPR)over the spectral range of about 570–900 nm, whereas in-creasing the diameter of the dielectric core is predicted tooffer tunability over the range of about 660–960 nm. Assuch, the versatility of shape engineering of self-assembledCHQ nanostructures could be a useful means of producingdielectric templates for a wide range of highly tunable nano-shell or nanocup plasmonic structures.

The thermodynamic and kinetic interpretation of the for-mation of CHQ nanostructures provides not only a morecomplete understanding of the formation mechanism of or-ganic nanostructures but also insight into the useful strategyof synthesizing organic self-assembled nanomaterials withdesired morphologies and physical properties. Further, di-verse shaped CHQ nanostructures with the intrinsic proper-ties of CHQ itself are expected to have a wide range of ap-plications to nano-electronic and -optical devices,[25] biolo-gy,[26,27] and catalysis.[28]

Acknowledgements

This work was supported by NRF (National Honor Scientist Program:2010-0020414, WCU: R32-2008-000-10180-0), Ministry of Education, Sci-ence and Technology (2010-0028075, 2010-0081966, 2011-0006268, 2009-0083540) and KISTI (KSC-2011-G3-02).

Keywords: nanostructures · organic nanomaterials ·plasmon resonance · self-assembly · self-engineering

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Received: November 30, 2012Revised: April 22, 2013

Published online: May 24, 2013

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