Programming structure into 3D nanomaterials - CiteSeerX

Programming structure into 3D nanomaterials Programming three dimensional nanostructures into materials is becoming increasingly important given the need for ever more highly functional solids. Applications for materials with complex programmed structures include solar energy harvesting, energy storage, molecular separation, sensors, pharmaceutical agent delivery, nanoreactors and advanced optical devices. Here we discuss examples of molecular and optical routes to program the structure of three-dimensional nanomaterials with exquisite control over nanomorphology and the resultant properties and conclude with a discussion of the opportunities and challenges of such an approach. Dara Van Gough1,2,3, Abigail T. Juhl1,2,3 and Paul V. Braun1,2,3* 1Department of

Materials Science and Engineering, University of Illinois at Urbana-Champaign, Urbana, IL 61874, USA Institute 3Frederick Seitz Materials Research Laboratory *Email: [email protected] 2Beckman

The self- and directed assembly of materials has been of

interference are being utilized to create 3D nanostructured solids

considerable interest for a number of years, however only recently

with almost limitless structural complexity. Optical interference

has it become possible to program structure and properties on the

has the unique ability to create virtually any periodic structure

nanoscale with the precision required for a number of important

through control of the phase, intensity, polarization and direction

applications. It is now possible to define, with nanometer accuracy,

of the interfering laser beams, although to date, the minimum

the complete 3D structure of functional solids using various

characteristic dimension of structures formed via optical

templating approaches. Rapid progress has been made in designing

interference is greater than those formed via molecular-based

molecular and colloidal templates for inorganic solids at length

templating strategies.

scales ranging from a few to 100s of nanometers. Despite the many with increasing complexity, often the resultant self-assembled

Molecularly Directed Synthesis of Nanomaterials

structures and templates formed from these materials can not

Nanostructured materials have attracted considerable interest

generate the desired complex nanostructure. In such cases, other

for numerous applications, including catalysis1, drug delivery2,

structure programming approaches, such as multibeam optical

energy storage3,4 and molecular sieves5,6, in part because their

recent advances in forming molecular and colloidal building blocks

28

JUNE 2009 | VOLUME 12 | NUMBER 6

ISSN:1369 7021 © Elsevier Ltd 2009

Programming structure into 3D nanomaterials

(a)

(b)

(c)

(d)

REVIEW

Fig. 1 Transmission electron microscope (TEM) images: (a) low magnification image of the inverse hexagonal block copolymer structure containing platinum nanoparticles; (b) high magnification image and electron diffraction pattern of (a); (c) mesoporous Pt-C nanocomposite obtained from pyrolysis of (a); (d) mesoporous Pt after removal of the carbon. (Adapted with permission from3. © 2008 American Association for the Advancement of Science.)

characteristic pore size can be defined over a size range similar to many

Two common pathways to program the structure of

macromolecules, they have a high surface to volume ratio and the pores

mesostructured materials using block copolymers are the isolation

can be chemically functionalized to modify their properities2.

of the inorganic precursors within one phase of the block copolymer

One of the first approaches to forming nanostructured solids was

structure followed by conversion of these precursors to the

based on the dealloying of a less noble metal from a bimetallic alloy.

desired mineral, and the assembly of appropriately functionalized

This was first shown by M. Raney in the 1920s with the discovery of

nanoparticles within one phase of the block copolymer structure. For

Raney

Ni7

and the dealloying process has since been expanded to

example, mesostructured TiO2 and NbO2 were formed by casting a

include systems such as Ag-Au8 and Ni-Zn9. The dealloying concept is a

block copolymer film from a solution of mineral precursors and the

useful strategy for creating nanostructured materials, however, it offers

block copolymer. After heat treatment, well ordered, crystalline metal

only limited control over pore size and geometry and the resultant pore

oxide films were obtained4.

structure is disordered. Recently, there has been considerable progress in programming the

In another example, the formation of mesostructured platinum starting from a high volume fraction of platinum nanoparticles within a

self-assembled structure of block copolymers10,11 and lyotropic liquid

block copolymer composite was recently demonstrated3. In this work,

crystals12-14, resulting in organic templates, which can now be used to

platinum nanoparticles, functionalized such that they were soluble

direct the formation of nanostructured materials including ceramics,

in organic solvents and in one block domain of the block copolymer

metals, semiconductors and polymers with long-range order and

structure, and a block copolymer were dissolved in an organic solvent.

characteristic length scales ranging from a few to 100s of nanometers.

Upon solvent evaporation, the block copolymer directed the assembly of the platinum nanoparticles into a mesoporous structure and the

3D Block Copolymer Templating

block copolymer was removed or converted to carbon (Fig. 1).

3D Block copolymer templating takes advantage of phases formed by self-assembly of large macromolecules to direct the formation of

Lyotropic Liquid Crystal Templating

inorganic phases over a broad range of sizes. Rich block copolymer phase

Lyotropic liquid crystal templating is the term typically used when a

behaviors have been observed experimentally10,11 and theoretically10,11,15-17

self-assembled surfactant based structure, as compared to a structure

and these phases have been used to template a wide variety of structures

formed by a block copolymer, is used to impart a nanoscale periodic

into both organic and inorganic materials, including oxide4,18-20 and

structure, typically with characteristic dimensions of 1-10 nm, into a

carbon21 monoliths as well as semiconducting22-24, metallic3 and oxide25

second material phase. It is an attractive route for the synthesis of

nanoparticle assemblies. Applications suggested to take advantage of

mesoporous materials due to its demonstrated potential to directly

inorganic structures templated by block copolymers include fuel cell

provide sub-nanometer control of the pore diameter and packing within

membranes3, battery electrodes26 and novel optical devices27,28.

a broad range of possible materials29.

JUNE 2009 | VOLUME 12 | NUMBER 6

29

REVIEW

Programming structure into 3D nanomaterials

Fig. 2 Schematic of lyotropic liquid crystal templating showing a TEM image of CdS overlaid with a schematic of the hexagonally arrayed mesopores; the pores are highlighted by red circles. Upper left: an overlay of the self-assembled surfactant structure showing the hydrophobic domain (red) forming a pore and the hydrophilic domain (blue) containing the mineralized CdS. (Adapted with permission from41. © 1996 Nature Publishing Group.)

Fig. 3 Schematic portraying the mineralization of mesoporous ZnS onto a sacrificial silica or poly(styrene) core. Sacrificial core colloids are dispersed in a lyotropic liquid crystal with zinc acetate and thioacetamide and aged to mineralize ZnS. After washing away the lyotropic liquid crystal and core etching, a mesoporous hollow sphere is obtained. (Reprinted with permission from45. © 2005 American Chemical Society.)

mesostructured ZnS hollow sphere (Fig. 4), with the diameter controlled by the size of the core colloid. Gold nanoparticle containing ZnS “nanorattles” were formed by encapsulation of gold nanoparticles within the sacrificial colloidal cores

Lyotropic liquid crystal templating takes advantage of a self-

followed by the subsequent mineralization of mesoporous ZnS onto this

assembled structure formed by a mixture of small molecule amphiphiles

sacrificial core (Fig. 4). For successful MHS formation, the interaction

and a solvent, typically water, to control the formation of an inorganic

between the surface of the core template and both the lyotropic liquid

phase within the hydrophilic region of the liquid crystal (Fig. 2). The

crystal template and the shell precursors is crucial. Only with specific

resultant nanostructure is determined by the small molecule amphiphile,

core particle surface chemistries were ZnS MHS formed46. It was found

the ratio of solvent to amphiphile, and temperature; considerable information is known about the phase diagrams of various amphiphile-

(a)

(b)

(c)

(d)

solvent systems12-14,30,31, greatly simplifying the selection of components. Since the pioneering work by researchers at Mobile, who first synthesized well ordered, mesoporous SiO25,32,33 using lyotropic liquid crystal templating, a wide variety of mesoporous materials have been synthesized in lyotropic liquid crystals, including metals (e.g., Pt34 and Pd35), metal alloys (e.g., Pt-Ru36 and Pt-Ni36), oxides (e.g., SiO237,38, TiO237, NbO237) and semiconductors (e.g., ZnS39, CdS39-41, CdTe40). Further functionality has been programmed into mesoporous materials by wrapping them around a secondary template to form mesporous hollow spheres. These mesoporous spheres have been formed in a variety of materials, including silica42, aluminosilicates43, sulfides44-46 and organic-inorganic composites47,48. Such mesoporous hollow spheres are interesting for applications including catalysis, selective transport44 and drug delivery49. Previously, for example, we presented a “Double Direct Templating”44-46 approach to synthesize ZnS mesoporous hollow spheres (MHS) by combining lyotropic liquid crystal templating with a secondary, sacrificial colloid template. ZnS was heterogeneously nucleated onto a sacrificial core colloid within the hydrophilic regions of a hexagonal lyotropic liquid crystal (Fig. 3). Removal of the sacrificial core by solvent etching results in a periodically

30

JUNE 2009 | VOLUME 12 | NUMBER 6

Fig. 4 TEM images of ZnS mesoporous hollow spheres (MHS) after sacrificial core etching: (a) a damaged MHS after sacrificial core etching; (b) a higher magnification image and FFT of the boxed region in (a); (c) a MHS nanocontainer containing gold nanoparticles; (d) a higher magnification image of the boxed region in (c). (Reprinted with permission from45. © 2005 American Chemical Society.)

Programming structure into 3D nanomaterials

REVIEW

flurbiprofen was loaded into the silica MHS. Most of the flurbiprofen

(a)

released by the silica MHS occurred over 24 hours, compared to days for mesoporous silica; the authors propose it will be possible to control the release profile of the drug through surface functionalization of amine groups on the silica MHS surfaces42. Emulsion Templating Although not central to this review given the low degree of “programming” currently possible via emulsion templating, it is (b)

(c)

(d)

worth including a short discussion of this approach. Emulsions, surfactant-stabilized colloidal mixtures of immiscible liquids, have been used for a number of years in the synthesis of nanostructured materials. This approach has found utility in controlling the size and morphology of both organic and inorganic materials into an assortment of nanostructures, including spheres, rods, flat sheets50 and porous solids51-54. For example, platinum nanowire networks (ca. 2 nm diameter) were synthesized by the reduction of a platinum salt in the presence of worm-like micelle networks formed by cetyl trimethylammonium

Fig. 5 (a) Schematic representation of the interaction between different molecular species and the mesoporous ZnS shell: (i) Amplex Ultra-Red (a fluorogen) and H2O2 (not shown) pass through the MHS and undergo a soybean peroxidase-catalyzed reaction to yield resorufin, a fluorescent dye; (ii) papain, a protease, was sterically blocked from entering the MHS and digesting the soybean peroxidase; (iii) NaN3 readily entered the MHS to irreversibly inhibit soybean peroxidase. Transmitted (top) and fluorescence (bottom) optical microscope images of ZnS MHS containing soybean peroxidase exposed to (b) Amplex Ultra-Red and H2O2, (c) Amplex Ultra-Red and H2O2 following exposure to papain and (d) Amplex Ultra-Red and H2O2 following exposure to sodium azide. The scalebar corresponds to all optical images. (Adapted with permission from44. © 2009 American Chemical Society.)

bromide (CTAB) in water and chloroform55. It was observed that the nanostructure of Pt could be varied from nanowires protruding from ca. 5-10 nm Pt nanoparticles to hyperbranched Pt nanowire networks as a function of stirring rate. Another interesting route makes use of emulsion systems to fabricate ordered and disordered porous materials51-54,56. In a typical example, this synthetic approach utilizes emulsion droplets to create spherical pores in a continuous phase. For example, gelling a solution that contains a titanium alkoxide mixed with emulsified oil forms porous TiO2. Following a heat treatment step, a porous, inorganic TiO2 phase is formed56.

that carboxylic acid groups on the surface of the core colloids promote due to a combination of metal ion complexation and favorable

Optically Directed Synthesis of Nanostructured Materials

interfacial interactions between the carboxylic acid groups and the

In many cases, self-assembly, even of rather complexly designed

lyotropic liquid crystal46.

macromolecules, can not program the required structure for a given

the high-fidelity formation of the mesoporous ZnS shells, presumably

Key elements of the programming procedure for making mesoporous

application and a different approach is needed. In particular, a number of

hollow spheres are mild, room temperature mineralization and sacrificial

unique optical interference approaches have been applied to program the

core etching procedures. Biologically active nanoreactors were obtained

structure of 3D solids with ever increasing complexity over length scales

by encapsulating an enzyme within mesoporous ZnS shells; soybean

ranging from 10s of nanometers to micrometers. Interference lithography

peroxidase was chosen as a model enzyme since it is larger than the

is the interaction between two or more coherent light sources to

3 nm pores in the ZnS shells and thus even after removal of the silica

create spatially periodic regions of high and low light intensity within a

cores, the enzyme remained encapsulated44 (Fig. 5). Small molecules

photosensitive media. The periodicity is related to the wavelength of the

were demonstrated to still be able to permeate the mesoporous wall,

light source and refractive index of the photosensitive media, placing

while large molecules were efficiently excluded (Fig. 5).

some limits on the minimum and maximum possible characteristic

In a second example, silica MHS were synthesized and the controlled release of flurbiprofen from these structures was studied42. In this

dimensions, but improvements in lasers, photosensitive media and new design tools are increasing the number of possible structures57-60.

work, the acid catalyzed condensation of tetraethyl orthosilicate and

One primary advantage of interference lithography over

3-aminopropyltrimethoxysilane was templated by a lyotropic liquid

conventional photolithography or electron beam lithography is

crystal during ultrasonication, which created amphiphile stabilized

that 3D structures can be written in a single exposure. Berger et al.

bubbles; the MHS formed around these bubbles. After synthesis,

first theorized the possibility of creating 3D nanostructures using

JUNE 2009 | VOLUME 12 | NUMBER 6

31

REVIEW

Programming structure into 3D nanomaterials

Direct 3D Optical Patterning of Inorganic Materials Pure polymer nanostructures have a number of limitations including low thermal stability and generally uninteresting optical and electrical properties, thus there is significant interest in directly programming the structure of inorganic materials in 3D at the nanoscale. Until a few years ago, it remained an open question whether inorganics can be patterned in 3D via similar optical interference procedures as the polymer photoresists previously described. The common approach is to infill a 3D nanostructured polymer template with an inorganic material Fig. 6 Scanning electron microscope (SEM) image of a SU-8 photonic crystal created by four beam interference lithography. (Adapted with permission from62. © 2000 Nature Publishing Group.)

interference lithography in

199761. This

was followed three years later

by the holographic production of photonic crystals active in the visible region of the electromagnetic spectrum62. The resultant nanostructure

and subsequently remove the polymer to create an inverted, inorganic structure66-68; however, it is intriguing to consider the possibility of directly patterning the inorganic material through the use of inorganic or inorganic/organic hybrid photoresists due to the limitations in the compatibility of polymers to many infilling procedures. Until recently, there has been difficulty in creating inorganic

was face centered cubic-like and was recorded in SU-8 photoresist using

photoresists that can record nanoscale features, rendering them

four 355 nm laser beams (Fig. 6).

incompatible with optical interference lithography. Some of the

Typically, a polymer photoresist such as SU-8 serves as a template

promising recent work in designing high resolution inorganic photoresists

into which this pattern is recorded, resulting in the production of

involves silica-like poly(methylsilsesquioxane)s (PMSSQ)69,70 and

defect-free nanostructures through photochemical transformations,

chalcogenide glasses, such as arsenic selenium tellurium (AsSeTe)71 and

which enable the selective removal of material from either the regions

arsenic sulfide (As2S3)72,73. 3D PMSSQ, AsSeTe and As2S3 structures

of constructive or destructive interference. It is nearly standard to

have been formed through multiphoton direct laser writing, a process

use organic photoresists to synthesize one, two, or three-dimensional

whereby the focal point of a highly focused, pulsed laser is rastered in 3D

nanostructures via interference lithography; however, there is a need to

throughout the interior of the inorganic photoresist in a predefined 3D

not only program structure, but also to place an arbitrary material, not

pattern, followed by development of the unexposed material (Fig. 7).

just polymer photoresist, into either the high or low intensity volumes

It has now been demonstrated that PMSSQ-based resists can be

with nanoscale accuracy. Further discussion will focus on cutting-edge

patterned via one- and two-photon proximity field nanopatterning

technologies that utilize interference lithography to rapidly generate

resulting in a wide variety of periodic nanoscale structures with high

unique nanostructures including inorganic photoresists, liquid crystal

thermal stability and low dielectric constant70. Examples of these

polymer composites and nanoparticle-containing photoresists.

structures can be seen in Fig. 8. There is every reason to believe the optical interference based patterning approaches can be extended to

Phase Mask Interference Lithography

other photosensitive glasses such as the ones discussed here.

In order to control the three-dimensional nanostructures created by power, geometry and the number of laser beams. This can be rather

Holographically Triggered Polymerization Induced Phase Separation

complex, so rather than a many beam laser setup, it is attractive

Optical interference can program structures of increasing complexity

to use a phase mask to generate the multiple interfering beams, an

through careful design of the photosensitive media. For example,

interference lithography, it is important to regulate the polarization,

approach termed proximity field

nanopatterning63.

complex nanostructures can be formed by optically triggering a phase separation of a two or more component system using interfering beams

placed into conformal contact with a photoresist. Upon exposure, the

of light. Usually a monomer begins to polymerize in the high intensity

relief structures in the elastomeric phase mask cause light diffraction

regions of the interference pattern while the second phase sequesters

and subsequent interference within the

32

In this method, relief

structures are formed into a transparent elastomeric mask, which is

photoresist64.

In a variant of this

into the low intensity regions. The most widely investigated example of

approach, the photoresist is directly imprinted with a relief structure,

this process are holographic polymer dispersed liquid crystals (HPDLCs),

the mask is removed and similar results are achieved65. This method

which start as a homogeneous mixture of monomer, free-radical

has the ability to create nanoscale features in 3D using only a simple

photoinitiator and liquid crystal74. Upon optical exposure, the monomer

1-step exposure process. Theoretically, almost any 3D structure can

begins to polymerize in the regions of constructive interference, which

be programmed through design of the phase mask; however, current

reduces the solubility of the liquid crystal in those areas. This causes

limitations in the design of phase masks have prevented this.

the liquid crystal to sequester into droplets in the regions of destructive

JUNE 2009 | VOLUME 12 | NUMBER 6

Programming structure into 3D nanomaterials

(a)

REVIEW

(c)

(b)

Fig. 7 SEM images of woodpile structures formed by direct laser writing within inorganic photoresists. (a) An 8 layer organic/inorganic hybrid PMSSQ structure. (Adapted with permission from 69. © 2008 Wiley-VCH.) (b) A 4 layer woodpile structure formed from AsSeTe, a photosensitive chalcogenide glass. (Adapted with permission from71. © 2000 American Institute of Physics.) (c) An all inorganic As2S3 structure with lattice constant of 700 nm, inset is a focused ion beam cross-section of the sample. (Adapted with permission from73. © 2007 ACS Publications.)

interference. The exact morphology and connectivity of these droplets is

interesting nanostructures. If interference lithography is performed

a function of both the exposure conditions and material system.

on a homogenous resist of bare 20 nm silica nanoparticles, monomer, liquid crystal and photoinitiator, the nanoparticles typically sequester

Interference Lithography with Nanoparticles

into the regions of destructive interference, the liquid crystal rich

A significant advancement from all-organic HPDLCs was the

areas. But what if a reactive functionalization is placed on the surface

interference lithography programmed assembly of nanoparticles. Vaia

of the nanoparticles? We recently studied the positioning of 20 nm

et al. first demonstrated the ability to assemble nanoparticles through

silica particles that were functionalized with methacryloxypropyl-

photo-induced free-radical polymerization75. In this method 5 nm

trimethoxysilane (MPTMS) and pentyltriethoxysilane (PTES) and

gold nanoparticles, free radical photoinitiator and monomer are used

compared the results with unfunctionalized silica nanoparticles78.

to record a 1D interference pattern from a 532 nm laser. When the

The unfunctionalized particles tended to stay within the low

laser exposure begins, polymerization occurs in the regions of high

intensity, liquid crystal regions as predicted, but the particles

intensity. Monomer diffuses towards these regions of high intensity

functionalized with MPTMS and PTES had a tendency to remain in the

and is incorporated into the polymer phase, leaving the nanoparticles

polymer rich or high intensity regions of the sample. It should also be

congregated in regions of low intensity (Fig. 9). This procedure was also

noted that the PTES particles were agglomerated in the polymer rich

demonstrated with 260 nm poly(styrene) spheres75, clay powder75,

regions, while the MPTMS functionalized particles were well dispersed

luminescent nanoparticles76 and silica nanoparticles77.

throughout the polymeric volume. This result can be seen in Fig. 10.

The methods of polymerization induced phase separation

Reactive functionalization of nanoparticles within a system that phase

and nanoparticle addition can be combined to create even more

separates upon polymerization, therefore, offers a means to program

(a)

(b)

(e)

(f)

(c)

(g)

(h)

(d)

Fig. 8 Various nanostructures created by maskless proximity field nanopatterning with PMSSQ based photoresist: (a-c) 3D periodic structures; (d) fibers; (e, f) semi-periodic 3D arrays; (g) ellipsoids; (h) cuboids. (Adapted with permission from70. © 2009 Wiley-VCH.)

JUNE 2009 | VOLUME 12 | NUMBER 6

33

REVIEW

Programming structure into 3D nanomaterials

(a)

(b)

(c)

(d)

Fig. 9 SEM image of 5 nm gold nanoparticles sequestered in the regions of destructive interference in a one dimensional interference pattern. (Reprinted with permission from75. © 2001 Wiley-VCH.)

the sequestration of the nanoparticles into either the constructive or destructive regions formed during interference lithography.

Top-Down Combined with Bottom-Up The methods described up to this point for programming nanostructures have been purely bottom-up (molecular), or top-down (interference lithography). It is interesting to consider approaches that combine both bottom-up and top-down techniques to tailor material placement over many size scales. For example, Li et al. patterned a mixture of block

Fig. 10 TEM images of microtomed cross-sections of HPDLC samples formed using one- dimensional interference lithography: (a) without nanoparticles; (b) with unfunctionalized 10 wt% silica nanoparticles; (c) with 10 wt% silica particles functionalized with PTES; (d) with 10 wt% silica particles functionalized with MPTMS. Nanoparticles within the liquid crystal regions were lost during TEM sample preparation. (Reprinted with permission from78. © 2009 Wiley-VCH.)

copolymer poly(ethylene oxide)-block-poly(caprolactone) and Norland Optical Adhesive 65 (NOA) at 80°C into a one dimensional interference

by blending top-down and bottom-up methods27. Interference

pattern with 200 nm periodicity28. The NOA began to polymerize in the

lithography was used to write a two dimensional hexagonal lattice, with

high intensity regions, which pushed the block copolymer into the low

700 nm periodicity, into a bi-functional sol-gel hybrid material. This

intensity regions. After the grating was cooled to room temperature, the

sample is then coated with a poly(styrene)-block-poly(2-vinylpyridine)

block copolymer regions (100 nm wide) of the sample further segregated

copolymer and annealed to create an array of hexagonally packed

into layers of 20 nm periodicity during crystallization. The mix of the top-

poly(2-vinylpyridine) spherical domains with 100 nm periodicity. Silver

down and bottom-up approaches created a “layer-in-layer hierarchical

nanoparticles, chosen for their resonance in visible wavelengths and

nanostructure” with periodicities as small as tens of nanometers

their chemical affinity for the poly(2-vinylpyridine) domains, were then

(Fig. 11a). Hierarchically ordered plasmonic masks can also be fabricated

deposited. The resulting structure is presented in Fig. 11b.

(a)

(b)

Fig. 11 (a) A TEM image of the layer-in-layer hierarchical nanostructure showing the block copolymer ordering within the one-dimensional interference pattern. The inset contains a schematic of the hierarchical structure overlaid on top of the magnified TEM image. (Adapted with permission from28. © 2007 American Chemical Society.); (b) AFM image of a hierarchically ordered plasmonic mask. The periodic 700 nm hexagonal array was formed by 2D interference lithography and the 100 nm scale array corresponds to the spherical block copolymer domains. The inset is a Fourier transform of the structure. (Adapted with permission from27. © 2009 Wiley-VCH.)

34

JUNE 2009 | VOLUME 12 | NUMBER 6

Programming structure into 3D nanomaterials

REVIEW

Outlook

much below 100 nm. In many respects, this minimum feature size is

Programmed assembly of nanostructured solids is a promising

limited not by fundamental physics, but rather by the photosensitive

emergent field. The ability to define structure over length scales

materials for three-dimensional patterning. Photolithography has been

ranging from a few to 100s of nanometers in polymers, metals,

demonstrated for planar structures well below 50 nm; achieving this

semiconductors and ceramics has lead to the possibility of a number

level of control in a three-dimensional solid would be a very significant

of powerful applications. There however remain a number of very

achievement. Combinations of optical assembly with molecular

significant challenges moving forward. At smaller length scales, where

self-assembly show great promise, but it remains to be seen if such

molecular-based approaches are primarily used, it is difficult to create

approaches can generate highly functional materials. To date, such

arbitrary structures since even complex macromolecules assemble

hybrid approaches mostly remain laboratory curiosities. Finally, many

into only a limited set of periodic structures. The minimum feature

applications will require scale-up to large volumes of material and

size of the nearly arbitrary set of 3D periodic structures formed by

most examples of programmed assembly to date have been limited to

optical interference remains unanswered; however, currently it is not

laboratory scale studies.

REFERENCES

40. Braun, P. V., et al., Adv Funct Mater (2005) 15 (11), 1745

1.

Zhao, X. S., et al., Mater Today (2006) 9 (3), 32

41. Braun, P. V., et al., Nature (1996) 380 (6572), 325

2.

Vallet-Regi, M., et al., Angew Chem Int Edit (2007) 46 (40), 7548

42. Wang, J. G., et al., Chem Mater (2009) 21 (4), 612

3.

Warren, S. C., et al., Science (2008) 320 (5884), 1748

43. Li, Y. S., et al., Nano Lett (2003) 3 (5), 609

4.

Lee, J., et al., Nat Mater (2008) 7 (3), 222

44. Gough, D. V., et al., Nano Lett (2009) Article ASAP

5.

Kresge, C. T., et al., Nature (1992) 359 (6397), 710

45. Wolosiuk, A., et al., J Am Chem Soc (2005) 127 (47), 16356

6.

Boissiere, C., et al., Chem Mater (2003) 15 (2), 460

46. Son, D., et al., Chem Mater (2009) 21 (4), 628

7.

Raney, M., US Patent (1927) 1,628,190,

47. Djojoputro, H., et al., J Am Chem Soc (2006) 128 (19), 6320

8.

Forty, A. J., and Durkin, P., Philos Mag A (1980) 42 (3), 295

48. Suarez, F. J., et al., Chem Mater (2007) 19 (13), 3096

9.

Katagiri, A., and Nakata, M., J Electrochem Soc (2003) 150 (9), C585

49. Zhu, Y. F., et al., Angew Chem Int Edit (2005) 44 (32), 5083

10. Bates, F. S., and Fredrickson, G. H., Annu Rev Phys Chem (1990) 41, 525

50. Pileni, M. P., Nat Mater (2003) 2 (3), 145

11. Bates, F. S., Science (1991) 251 (4996), 898

51. Barbetta, A., et al., Chem Commun (2000) (3), 221

12. Israelachvili, J. N., et al., J Chem Soc Farad T 2 (1976) 72, 1525

52. Butler, R., et al., Advanced Materials (2001) 13 (19), 1459

13. Nagarajan, R., and Ruckenstein, E., Langmuir (1991) 7 (12), 2934

53. Manoharan, V. N., et al., Advanced Materials (2001) 13 (6), 447

14. Wennerstrom, H., J Colloid Interf Sci (1979) 68 (3), 589

54. Imhof, A., and Pine, D. J., Advanced Materials (1998) 10 (9), 697

15. Helfand, E., and Wasserman, Z. R., Macromolecules (1976) 9 (6), 879

55. Song, Y., et al., Nano Lett (2007) 7 (12), 3650

16. Helfand, E., Macromolecules (1975) 8 (4), 552

56. Imhof, A., and Pine, D. J., Nature (1997) 389 (6654), 948

17. Helfand, E., and Lauritze.Ji, Macromolecules (1973) 6 (4), 631

57. Rinne, J. W., et al., Opt Express (2008) 16 (2), 663

18. Templin, M., et al., Science (1997) 278 (5344), 1795

58. Rinne, J. W., and Wiltzius, P., Opt Express (2006) 14 (21), 9909

19. Finnefrock, A. C., et al., J Am Chem Soc (2003) 125 (43), 13084

59. Sanchis, L., et al., Appl Phys Lett (2004) 84 (22), 4460

20. Brezesinski, T., et al., Adv Funct Mater (2006) 16 (11), 1433

60. Hakansson, A., et al., Phys Rev Lett (2006) 96 (15),

21. Liang, C. D., et al., Angew Chem Int Edit (2004) 43 (43), 5785

61. Berger, V., et al., Journal of Applied Physics (1997) 82 (1), 60

22. Fogg, D. E., et al., Macromolecules (1997) 30 (3), 417

62. Campbell, M., et al., Nature (2000) 404 (6773), 53

23. Zhang, Q. L., et al., J Am Chem Soc (2006) 128 (12), 3898

63. Jeon, S., et al., Advanced Materials (2004) 16 (15), 1369

24. Lin, Y., et al., Nature (2005) 434 (7029), 55 25. Ba, J. H., et al., Advanced Materials (2005) 17 (20), 2509

64. Jeon, S., et al., Proceedings of the National Academy of Sciences of the United States of America (2004) 101 (34), 12428

26. Tarascon, J. M., and Armand, M., Nature (2001) 414 (6861), 359

65. Jeon, S., et al., Opt. Express (2007) 15 (10), 6358

27. Kim, W. S., et al., Advanced Materials (2009) 21 (19), 1921

66. King, J. S., et al., Applied Physics Letters (2003) 83 (13), 2566

28. Birnkrant, M. J., et al., Nano Letters (2007) 7 (10), 3128

67. Ramanan, V., et al., Applied Physics Letters (2008) 92 (17), 173304

29. Attard, G. S., et al., Nature (1995) 378 (6555), 366

68. King, J. S., et al., Advanced Materials (2006) 18 (12), 1561

30. Mitchell, D. J., et al., J Chem Soc Farad T 1 (1983) 79, 975

69. Jun, Y., et al., Advanced Materials (2008) 20 (3), 606

31. Kunieda, H., et al., J Phys Chem B (1997) 101 (40), 7952

70. George, M. C., et al., Angew Chem Int Edit (2009) 48 (1), 144

32. Beck, J. S., et al., J Am Chem Soc (1992) 114 (27), 10834

71. Feigel, A., et al., Applied Physics Letters (2000) 77 (20), 3221

33. Kresge, C. T., et al., Science and Technology in Catalysis 1994 (1995) 92, 11

72. Feigel, A., et al., Applied Physics Letters (2003) 83 (22), 4480

34. Attard, G. S., et al., Science (1997) 278 (5339), 838

73. Wong, S. H., et al., Chemistry of Materials (2007) 19 (17), 4213

35. Bartlett, P. N., et al., Phys Chem Chem Phys (2002) 4 (15), 3835

74. Tondiglia, V. P., et al., Advanced Materials (2002) 14 (3), 187

36. Yamauchi, Y., et al., Chem Mater (2008) 20 (3), 1004

75. Vaia, R. A., et al., Advanced Materials (2001) 13 (20), 1570

37. Yang, P. D., et al., Science (1998) 282 (5397), 2244

76. Sakhno, O. V., et al., Materials Science and Engineering: C (2008) 28 (1), 28

38. Bagshaw, S. A., et al., Science (1995) 269 (5228), 1242

77. Tomita, Y., et al., Opt. Lett. (2006) 31 (10), 1402

39. Braun, P. V., et al., J Am Chem Soc (1999) 121 (32), 7302

78. Busbee, J. D., et al., Advanced Materials (2009) In press,

JUNE 2009 | VOLUME 12 | NUMBER 6

35