Nanosphere Lithography : A Powerful Method for the Controlled Manufacturing of Nanomaterials

*e never-ending race towards miniaturization of devices induced an intense research in the manufacturing processes of the components of those devices. However, the complexity of the process combined with high equipment costs makes the conventional lithographic techniques unfavorable formany researchers.*rough years, nanosphere lithography (NSL) attracted growing interest due to its compatibility with wafer-scale processes as well as its potential to manufacture a wide variety of homogeneous one-, two-, or three-dimensional nanostructures.*is method combines the advantages of both top-down and bottom-up approaches and is based on a two-step process: (1) the preparation of a colloidal crystal mask (CCM) made of nanospheres and (2) the deposition of the desiredmaterial through themask.*emask is then removed and the layer keeps the ordered patterning of themask interstices. Many groups have been working to improve the quality of the CCMs.*roughout this review, we compare the major deposition techniques to manufacture the CCMs (focusing on 2D polystyrene nanospheres lattices), with respect to their advantages and drawbacks. In traditional NSL, the pattern is usually limited to triangular structures. However, new strategies have been developed to build up more complex architectures and will also be discussed.


Introduction
More than years have passed since the famous lecture " ere's plenty of room at the bottom" by Feynman [ ].In his talk, Feynman explored apparently simple and elegant possibilities of working at the atomic scale-possibilities that would have startling results.He touched upon ideas such as miniature writing, seeing and moving atoms, the prospect of designing molecules one atom at a time, and the challenges involved in developing miniature machines.Devices operating on this scale are expected to provide a number of breakthrough applications like more powerful computers and increased data storage due to more e cient and smaller components.Applications are also expected in many elds such as solar cells, (molecular) electronics, and biosensors.
To cite Feynman, the question is now, "How do we write small?"ere are two well-known major routes for nanofabrication, called the "top-down" and the "bottom-up" approaches [ -].
. .Top-Down.In the top-down approach, thin lms or bulk materials are scaled down to create nanodevices.is can be done by using various techniques such as precision engineering and lithography and has been developed and re ned by the semiconductor industry over the past years.Indeed, the never-ending race towards miniaturization of devices (mobile, camera, etc.) induced an intense research in the manufacturing processes of the basic components of these devices.Subtractive and additive transfers lead to di erent nanoarchitectured materials.
In parallel writing, the whole pattern is made simultaneously using a mask, which dictates the features to be reproduced.It is easy to understand that sequential lithography is slower as the pattern is written point by point on the resist surface and is therefore more expensive.
Optical lithographic processes are limited in nal resolution by the wavelength of the light used to expose the photoresist [ ]. e use of short wavelengths and reduction optics between the mask and substrate has allowed a reduction in features size.Extending these processes to wavelengths in deep UV and so X-ray brings increasing technical di culty.Alternative techniques, such as electron beam lithography, have been developed to overcome the limitations of optical light.While optical lithography is mainly intended for patterns of a few micrometers to a few microns, the electron beam lithography allows for its part to achieve submicron resolutions far.Under ideal conditions, it is possible to reach a sub-nm resolution.
Each of these conventional fabrication tools, except for so X-ray lithography, is commercially available (though expensive).ese techniques are highly developed and optimized for semiconductor fabrication.e costs of purchasing, installing, and maintaining the tools they require limit their application in areas other than microelectronics.ese tools are also o en incompatible with nonstandard problems in fabrication (e.g., coatings on nonplanar surfaces).
Over the years, many lithographic and patterning techniques have been proposed to overcome these disadvantages.
e concept of so lithography was introduced in by Whiteside's group at Harvard [ ].It refers to a set of methods [ , , ] (microcontact printing, replica molding, microtransfer molding, etc.) for generating or replicating structures by using a patterned elastomer (e.g., polydimethylsiloxane (PDMS)) as a mask, stamp, or mold.ese methods can be carried out by hand in an ordinary laboratory.Conventional photolithography must take place in clean-room facility devoid of dust and dirt; if a piece of dust lands on the mask, it will create an unwanted spot on the pattern.As a result, the device being fabricated may fail.So lithography is generally more forgiving because the PDMS stamp is elastic.If a piece of dust gets trapped between the stamp and the surface, the stamp will compress over the top of the particle but maintain contact with the surface.us, the pattern will be reproduced correctly except for where the contaminant is trapped.
Moreover, so lithography can produce nanostructures in a wide range of materials, including the complex organic molecules needed for biological studies [ ]. e technique can print or mold pattern on curved or planar surfaces, but the technology is not ideal for making the structures required for complex nanoelectronics.Currently all integrated circuits consist of stacked layers of di erent materials.Deformations of the so PDMS stamp can produce small errors in the replicated pattern and a misalignment of the pattern with any underlying patterns previously fabricated.erefore, so lithography is not well suited for fabricating structures with multiple layers that must stack precisely on top of one another.
Researchers have found ways, however, to correct this shortcoming-at least in part-by employing a rigid stamp instead of an elastic one.
In a technique called step and ash imprint lithography [ ], photolithography is used to etch a pattern into a quartz plate, yielding a rigid bas-relief master.e master is then pressed against a thin lm of liquid polymer, which lls masters recesses.Exposition to ultraviolet light solidi es the polymer to create the desired replica.A related technique called nanoimprint lithography [ ] also employs a rigid master but uses a lm of polymer that has been heated to a temperature near its melting point to facilitate the embossing process.
Scanning probe techniques [ ] are also potential tools for nanofabrication.Depending on the sharpness of the pen, the resolution of writing can vary from macroscopic feature sizes down to the atomic sale.ese scanning probe techniques are based on the changes undergone by species present on the surface of a substrate.
In practice, such changes can be induced by mechanical, electrical, magnetic, or chemical means.One of the most striking examples of nanoscale fabrication by scanning probe lithography has been the precise positioning of atoms with an STM tip [ ]. Manipulations of atoms or particles by scanning probe lithography techniques will be rather classi ed in the bottom-up approach.
ese tools seem well suited for applications in research but will require substantial development (simultaneous writing patterns with multiple probes [ ]) before they can be used for patterning large areas in manufacturing.e characteristics of all these lithography techniques are summarized in Table .Many reviews are focused on the evolution of these new "unconventional" approaches to nanofabrication [ , , ].
. .Bottom-Up.Bottom-up approach uses small and simple building blocks (atoms, molecules, nanoparticles, etc.) that will self-assemble into larger, more complex structures.Compared with top-down route, this is more like a chemical or biological approach and it has the potential to make complex D structures cheaply and in large quantities.ese methods can easily make the smallest nanostructures-easily reaching sub-nm dimensions-and do so inexpensively.
Self-assembly (although the distinction is not always made in the literature, the terms self-assembly and selforganization should be used, respectively, to describe structures formed close-to-equilibrium and far-from-equilibrium dissipative processes involving energy/matter ow.In some cases, the words are used interchangeably and the confusion is still frequent in the literature.) is de ned as the spontaneous (without the assistance of external forces or spatial constraints) organization of two (or more) components into larger aggregates using covalent and/or noncovalent bonds [ ].
Materials fabricated using this approach include selfassembled monolayers [ ] or nanostructures that selfassemble from block copolymers [ , -] and that can Recent advances in the fabrication of functional nanostructures using self-assembly include self-assembled [ ] magnetic nanoparticles [ ]. Crystalline areas of magnetic nanocrystals can store large amount of information.However, an annealing step is usually necessary to enhance the magnetic properties and may give rise to coalescence of the nanoparticles [ ]. Nontemplated self-assembly, while being attractive for its minimalist use of materials and energy (compared to conventional lithography), is not widely used for nanofabrication.Indeed, self-assembly is prone to producing defects, and the perfect periodicity of self-assembled structures from nanoscale components is generally limited to micrometer-sized areas [ ].Moreover, precise spatial positioning and nanostructure shape control are di cult to implement.
. .Top-Down, Bottom-Up, or Both?Considering the previously described synthesis routes (Sections .and .), the following conclusions can be drawn.e complexity of the preparation process combined with high initial equipment costs makes the conventional lithographic techniques unfavorable for many researchers.
Most of the unconventional lithographic techniques developed to date, however, require the assistance of conventional lithographic techniques, such as photolithography, to design and manufacture the masks or masters.So lithography techniques, although they present many advantages, do require the production of a single mask (which will be used several times) by conventional lithographic techniques.Scanning probe lithography techniques are of course too slow for mass production.Bottom-up techniques, although they easily reach sub-nanometers, may present some disadvantages.Self-assembly has its own problem of regularity and repeatability.Indeed, control of the density and shape of the nanostructures is di cult to achieve, which is rather annoying especially in case of magnetic data storage.
By the way, the dimensions that can now be controlled by either approach (Figure ) are of similar order, and this

Dimensional scale
Year 1940 1950 1960 1970 1980 1990  leads to exciting new hybrid methods including nanosphere lithography towards which our choice was directed.

Nanosphere Lithography
Nanosphere lithography (NSL) is a promising inexpensive fabrication tool for producing regular and homogenous arrays of nanoparticles with di erent sizes.
is method combines the advantages of both top-down and bottom-up approaches.
e process is divided into two steps (Figure ), the rst of which is mask preparation.
e at substrate is coated with a suspension containing monodisperse spherical colloids (e.g., polystyrene) a er a chemical treatment to enhance its hydrophilic character.Upon drying, a hexagonal-close-packed (HCP) monolayer or bilayer, called a colloidal crystal mask [ ] (CCM), is formed.
is mask is then used to selectively pattern the substrate thanks to the deposition of the material of interest through the interstices of the ordered beads.
e subsequent removal of the mask (li -o ) by sonication in an adequate solvent or by stripping leaves an array of ordered nanodots on the surface of the substrate.An annealing step is sometimes necessary to crystallize the sample or/and induce a crystallographic phase change.e NSL is o en considered as a hybrid between the bottom-up approach (due to the self-organization of the colloidal spheres in a HCP lattice) and the top-down approach (due to the obtention of dots/structured layers such as in a conventional lithography technique).

Substrate
Nanosphere lithography is also known as colloidal lithography [ , ] or natural lithography.
Fischer and Zingsheim [ ] are the rst to report, in , the formation of an ordered monolayer on a glass plate.ey simply deposited a suspension of colloidal spheres with a diameter of nm and allowed it to evaporate.ey obtained small-area particle monolayers.e term of "naturally" assembled polystyrene (PS) latex nanospheres was then introduced.However, the focus of their work was the replication of submicroscopic patterns using visible light and not fabrication of lithographic colloidal masks.
A year later, Deckman and Dunsmuir [ ] extend the scope of the approach of Fischer showing that a monolayer of nanospheres can be used both as a "deposit material" or as a lithographic mask.e deposition is either random or with ordered arrays over the entire surface of a macroscopic substrate [ ]. e coating procedure developed to produce microcrystalline arrays is based on a spin coating process.Several point defects or dislocations are present in the tens of micrometers scale.Because the mask preparation process is obtained by self-assembly phenomenon rather than by photolithography, they named this strategy "natural lithography." In the s, the method is renamed by the group of Hulteen and Van Duyne [ ] and is still nowadays commonly called "nanosphere lithography".eir work is not only focused on mono-(single) layers (SL) but extends to double layers (DL) which leads to the formation of smaller dots corresponding to the small openings that remain in a closed packed structure (Figure ).
Van Duyne's group [ , ] has also intensively investigated the plasmon resonance properties of metallic patterns with the ultimate goal of developing biosensors based on surface enhanced Raman spectroscopy.
Besides, several groups have experimentally and theoretically studied the behavior of colloidal suspensions in order to understand their stability as well as the mechanisms of formation of the mask [ ].
rough years, nanosphere lithography attracted growing interest due to its potential to manufacture a wide variety of one-, two-, or three-dimensional nanostructures [ , -]. Figure highlights the increasing interest for NSL since .
. .Designing Monolayers.In recent years, many groups have been working on di erent strategies to continually improve the quality of the crystal masks.e major methods will be brie y described below, focusing only on two-dimensional polystyrene nanospheres lattices.
. . .Self-Assembly during Solvent Evaporation.Evaporation methods are based on solvent evaporation from a droplet of dilute colloidal nanosphere suspension deposited on a substrate.e evaporation of the solvent induces the formation of a meniscus between the particles and therefore attractive capillary forces give rise to the self-assembly of the particles.Denkov and his team [ ] conducted pioneering work on self-ordering of colloidal particles by direct observation of colloids.ey found that attractive capillary force and convective transport of the nanospheres are the main factors that dominate the self-assembly process, while the ordering Constant monitoring of the evaporation rate as well as ne-tuning of the withdrawal speed of the substrate are key parameters to achieve large D ordered arrays (Figure ).However, the particles layers are formed on both sides of the substrate, thereby increasing the amount of suspension used.
To overcome this drawback, Nagao et al. [ ] engineered a hybrid method between evaporation and dip coating.A drop of colloidal suspension is placed on the substrate, which is therea er inclined vertically until the suspension is Journal of Nanomaterials e spin coating of a colloidal suspension onto a substrate can accelerate the evaporation of the solvent [ , ].
e quality and thickness of colloidal templates are greatly a ected by spin speed, size of the nanospheres, wettability, and so forth.
Most of the literature on spin coating of colloidal suspension is experimentally based.Even if recent papers are more detailed, spin coating protocols described in the literature are varied and sometimes imprecise [ -].
erefore a major goal for further progress in NSL is the development of experimental protocols to control the ordering of particles on solid substrates and to get large well-ordered structures.
However, the spin coating has a signi cant potential for mass production and scaling-up because of its rapid implementation and its compatibility with wafer-scale processes.
For those reasons, we studied by an experimental design approach the in uence of some spin coating parameters on the degree of ordering of the nanosphere layers (Figure ) [ ].We highlighted that, to get large hexagonal-closepacked ordered areas, we need to quickly spin coat the suspension at high rotation speed.We also identi ed adequate spin coating parameters to manufacture large, defectfree areas reaching up to m 2 (with PS nanospheres of nm diameter) [ ]. ese results agree with studies on spin coating of colloidal suspensions [ , ].
As theoretically shown by Kralchevsky and Denkov [ ] interparticle capillary forces arise between spherical particles, which are partially immersed in a liquid on a horizontal solid substrate (Figure ).As the liquid becomes thinner, the liquid surface deformation increases giving rise to increased capillary forces.e next step begins with the motion of more colloids that are driven toward the nucleus and is attributed to a convective ux, which compensates the evaporated solvent in the already ordered array, hence dragging particles suspended in the thicker layers towards the thinner regions.
e newcomers remain attached to the domains, pressed by a hydrodynamic pressure and captured by capillary forces.
e onset of ordering process coincides with the moment when the thickness of the liquid layer becomes smaller than the particle diameter [ ].
e ordering is therefore highly related to the thickness of the evaporating lm. e lm thickness has been demonstrated to be inversely proportional to the spin speed [ , ], which corroborates our results.Moreover, Rehg and Higgins [ ] and Dushkin et al. [ ] both concluded that rapid evaporation rate is better at assembling more uniform monolayers and colloidal crystals.Our results con rm that the rst acceleration in the spin coating process is the critical parameter to reach high % of HCP organization.
Chen  is setup ensures that the pressure remains constant as the substrate is being withdrawn.Macroscopic areas can be patterned with nanospheres.
Controlled Evaporation.e rst step of the process (colloidal monolayer formation) is similar to that of the Langmuir-Blodgett technique.However, the oating monolayer is le free without action of any barrier to modify its assembly, which greatly simpli es the apparatus.Moreover, instead of being slowly withdrawn from the suspension, the substrate is le immersed during the liquid evaporation [ , ].
By slightly tilting the substrate, large HCP domains (Figure ) with size greater than m 2 (PS nanospheres with nm diameter) were manufactured [ ].
(ii) Liquid-Liquid interface Self-assembly may also occur at the interface between two nonmiscible liquids [ ]. anks to carefully prepared surface conditions, nanospheres assemble at the interface.Once they are trapped at the interface, they self-organize into a monolayer.Subsequently, the monolayer is deposited on the substrate by a removal procedure or evaporation of one phase.

Journal of Nanomaterials
. . .Electrophoretic Deposition.In electrophoretic deposition, a colloidal suspension is con ned between two electrodes.An applied electric eld induces the migration of the particles (Figure ) and self-assembly occurs at electrode interfaces (Figure ). is method is therefore limited to conductive substrates (e.g., indium tin oxide (ITO) glass).Two-[ ] and three-dimensional [ ] colloids crystals have been studied both in a DC eld [ -] or an AC eld [ , ].
e use of AC eld allows manipulation of almost any type of particle in any type of media and has the advantage of permitting high eld strengths without causing water electrolysis [ ].
. . .Prepatterned Substrates.Template-assisted selfassembly of colloids is usually employed to fabricate types of arrangements which are di cult or impossible to fabricate with traditional self-assembly process.Indeed, physically patterned substrates are produced by conventional lithographic techniques (optical lithography [ ] and electron beam lithography).By changing the shape of the patterned geometry and the ratio size to particle size (Figures and ), it is possible to control the packing order of the colloidal crystals [ , ].
On the other side, a selective deposition of the colloids may occur on chemically patterned (Figure ) substrates [ ,  ].
Depending on the nature of the interaction between the particles and the template surface, the assembly process is carried out by di erent methods (evaporation, spin coating, or electrophoretic deposition (Figure )).

Applications of NSL
Assembling colloidal micro-nanoparticles into D ordered arrangements presents a high potential for applications in many di erent elds.
First, freestanding cross-linked polymer nanoparticle lms can be used as ltration membranes to separate small proteins or gold nanoparticles and present the great advantage of a narrow pore size distribution that is never seen in the present polymer membranes [ ].
Furthermore, the D HCP monolayer can be used as mask or template to generate patterns of functional materials on  Although the preparation of a colloidal mask is easy and cheap, the resulting pattern is limited to triangular structures, which can be a signi cant drawback, compared to conventional lithographic techniques.Indeed, in many applications, the material properties and target applications are highly dependent on the shape of the patterned species.To overcome this disadvantage and build new architectures, various strategies have been suggested and will also brie y discussed in this review.
. .Nanodots.Nanoparticle arrays can simply be generated by deposition on the substrate through the mask, which is usually positioned normal to the propagation direction of a material vapor.
e particles obtained a er removal of the mask have, in general, approximate triangular shape (Figure (a)) and are distributed in hexagonal lattices at the substrate surface.
Among all applications of those structures, localized surface plasmon resonance (LSPR) [ ] is one active area of great interest.LSPR plays a major role in new developments of nanophotonics, such as surface enhanced Raman scattering [ , ] (SERS) substrates or biosensors [ ]. Haynes and Van Duyne [ ] investigated the relationship between particles sizes, shape, interparticle spacing, and LSPR.ey worked on hexagonally ordered triangular Ag dot arrays obtained with single-layered colloidal masks or Ag nanoparticle arrays obtained by double-layered colloidal masks (Figure ).
ey demonstrated that the absorption band of the metallic nanostructures can be easily tuned from visible to near IR.
Another important area of the NSL is the design of ordered nanostructures for ultrahigh density magnetic recording [ , -].Compared with conventional techniques for fabricating magnetic ordered nanostructures, NSL is inexpensive and allows high-output fabrication.In general, reducing the feature dimensions to lower length scales has revealed novel magnetic properties [ ].For example, nanoscale magnetic materials o en exhibit superparamagnetic behavior, which hinders the miniaturization race.
NSL allows a ne-tuning of the size of the manufactured nanostructures.Indeed, a number of posttreatment methods (e.g., heating (Figure ) and etching) have been developed in order to modify the interstitial structure of the colloidal mask.
Haynes and coworkers [ ] also pioneered angle-resolved colloidal lithography, which consists in the deposition of materials at the nonzero incident angle ( ) of the vapor beam with respect to the normal direction of the substrate.is technique not only reduced the dimension of the nanodots but also modi ed the shape from equilaterals to isosceles triangles (Figure ).
Recently, Wang and coworkers broadened the scope of angle-resolved colloidal lithography by developing a stepwise strategy to generate heterogeneous binary particles (Fig- . .Nanodisks-Nanorings.A modi ed colloidal lithography (hole-mask colloidal lithography [ ]) enables the preparation of metal nanodisk arrays a er several deposition/etching steps.
e essential new feature of hole-mask colloidal lithography compared to the original colloidal lithography is the presence of a sacri cial layer combined with a thin lm mask with nanoholes ("hole mask").A sacri cial polymer lm (e.g., PMMA) is spin-coated onto a at surface (e.g., glass slide and Si wafer). is is followed by deposition of a colloidal solution containing PS bead.A thin lm, which is resistant to reactive oxygen plasma etching, is then deposited onto the surface (and not under the nanospheres).e PS beads    nanospheres.e periodicity can be tuned by using di erent nanosphere sizes and the distance between neighboring columns could be controlled by experiment parameters (e.g., pressure) during process (Figure ).
By changing the corresponding target in the PLD process, this strategy can be used to synthesize similar nanocolumns

Conclusions
e complexity of the preparation process combined with high initial equipment costs made the conventional lithographic techniques unfavorable for many researchers.Moreover, most of the nonconventional lithographic techniques Journal of Nanomaterials developed to date require the assistance of conventional lithographic techniques, such as photolithography, to design and manufacture the masks or masters.
In this context, nanosphere lithography emerged as a hybrid (between top-down and bottom-up approaches) and high-output nanostructure process: (i) to produce nanoobjects, exhibiting similar features in terms of size, shape, and interface in order to exhibit similar (e.g., chemical, magnetic,etc.)properties; (ii) to manufacture long range ordered arrays of nanostructures: all nanostructures are equidistant and have the same environment; (iii) to control the density of these nanostructures in order to isolate or otherwise increase their number on a given surface; (iv) to o er a certain exibility in the choice of materials, both for the substrate and the nanostructures; (v) to ful ll the above conditions in a short time and at reasonable price.
roughout this review, we compared the various deposition techniques that can be used to manufacture the colloidal crystal masks, with respect to their advantages and drawbacks.Nanosphere lithography was here clearly evidenced as a powerful process to manufacture various arrays of periodic structures with a wide potential of interest for the eld of nano-and microfabrication, as highlighted by the unexhaustive list of architectures/applications manufactured by NSL.
of single-layer (SL) and double-layer (DL) nanosphere masks and the corresponding periodic particle array PPA surfaces (dotted line = unit cell)."Reprinted with permission from [ ].Copyright ( ) American Vacuum Society." papers/year related to NSL, as retrieved by Sci nder Scholar (July ). and quality of the obtained arrays are considerably a ected by the rates of solvent evaporation [ ].By controlling the temperature and the humidity of the system, Micheletto et al. [ ] prepared colloidal monolayers on a slightly tilted substrate.Since then, other tests were performed successfully [ , ].Low control over the process o en yields mixed multilayers structures. . . .Dip Coating.Based on their work on convective assembly of colloidal nanospheres during solvent evaporation, Nagayama et al. [ , ] developed a dip-coating procedure for formation of D colloidal templates.
) SEM micrograph of a PS monolayer ( nm diameter) containing defects of multilayers and voids."Reprinted with permission from [ ].Copyright ( ) American Chemical Society."; (b) Tapping-mode atomic force microscopy micrograph of PS monolayer."Reprinted with permission from [ ].Copyright ( ) American Institute of Physics." 3 m F : SEM micrograph of a PS nanoparticles ( nm diameter) monolayer."Reprinted from [ ].Copyright ( ), with permission from Elsevier." dried.However, many defects are still present in the network (Figure ).Multilayers can be obtained by repeating the operation. . . .Spin Coating.
et al. [ ] recently provided lots of useful information for the e cient and controllable fabrication of largescale and uniform monolayer or multilayer colloid-crystal lms with PS nanospheres of various diameters by spincoating technology.ey improved the understanding of the relationship between ratios of monolayer coverage areas and spin speed/acceleration.
or three-dimensional organization of PS nanospheres (depending on the spin coating parameters).Typical SEM micrographs of organized PS nanospheres showing in (a) large HCP area, (b) small HCP area, and (c) multilayers.Scale bars are m."Reprinted from [ forces in ordering of spherical particles in wetting lm on substrate.is a capillary force between particles trapped in thin liquid lm whose thickness is around the particle diameter.is a hydrodynamic force, which drags the immersed particles toward the thinner regions. is force is caused by the hydrodynamic ux ( ) of the solvent, which compensates the solvent evaporated from thinner regions."Reprinted from [ ] Copyright ( ), with permission from Elsevier." F : SEM micrograph showing large area of self-assembled PS ( nm diameter) monolayer.e scale bare is m."Reprinted from [ ].Copyright ( ), with permission from Elsevier". . . .Self-Assembly at Interface of Two Media (i) Air-Liquid interface Langmuir-Blodgett Coating.Langmuir-Blodgett coating refers to the preparation and transfer of nanosphere monolayers from liquid-gas interface onto a solid substrate by controlled withdrawal of the substrate.rough use of surfactants [ ] or through surface modi cation of the nanospheres [ ] in order to enhance their hydrophobicity, a D crystal lm is formed on the liquid surface.e resulting oating monolayer is compressed by a barrier that promotes the assembly of the nanospheres in close-packed lattices.
of PS pattern produced by DC electrophoretic deposition in the presence of an UV illumination motif.e inset shows the overall appearance of the pattern."Reprinted by permission from Macmillan Publishers Ltd.Reference [ ], copyright () ".
assembly on a physically patterned substrate.F : SEM micrographs showing large area of polystyrene spheres on patterned photoresist template.Inset shows magni ed image of the channel."Reprinted with permission from [ ].Copyright ( ) American Chemical Society." Two fundamentally di erent strategies are available, namely, the self-assembly on chemically or physically (Figure ) patterned substrates.Both methods involve an additional preliminary step in the process and may consequently increase the time and the cost of preparation.
of some typical examples of polygonal aggregates that were formed by templating PS beads against D arrays of cylindrical holes.e cylindrical holes used in all experiments are ≅ m.(a) Dimers with .m PS beads; (b) trimers with .m PS beads; (c) square tetramers with .m PS beads, and (d) pentagons with .m PS beads."Reprinted with permission from [ ].Copyright ( ) American Chemical Society." of a chemical patterning experiment by use of a square elastomer stamp as structure for printing.(a) Overview of the sample, (b) zoom on one square.Dimensions of the pattern: squares ( m) 2 ."Reproduced from [ ] with permission of the PCCP Owner Societies." of D colloidal crystals grown by electrophoretic deposition on (a) a patterned and on (b) an unpatterned ITO substrate.Scale bars are m."Reprinted with permission from [ ].Copyright ( ) John Wiley and Sons." a substrate.e deposited materials can be chosen without almost any limitation and the feature size of obtained structures can be controlled by adjusting the particle diameters, ranging from less than ten nanometers [ ] to tens of micrometers [ ].
SEM micrograph of Ag triangular nanodots made with a single-layered colloidal mask.(b) SEM micrograph of Ag nanodots made with a double-layered colloidal mask.e scale bar is m."Reprinted with permission from [ ].Copyright ( of a PS mask annealed in water/EtOH/acetone mixture by (a) , (b) , and (c) microwave pulses.e scale bar is nm."Reprinted with permission from [ ].Copyright ( ) John Wiley and Sons".
of gold nanodots fabricated by angle-resolved colloidal lithography with (a) = ∘ and (b) = ∘ .e scale bar is m."Reprinted with permission from [ ].Copyright ( ) American Chemical Society." are tape-stripped away and the lm is then subjected to plasma treatment, leaving nanoholes in the plasma-resistant lm ("hole mask") resting on the sacri cial PMMA layer.e hole mask can then be used either as a deposition-or etchmask or both.Kasemo and his group fabricated Pt [ ], Pd [ ], Ag [ ] and Al [ ] nanodisks (Figure ) and studied their plasmonic properties.Choi et al. [ ] studied nanopatterned Co/Pt nanodisks for magnetic memory devices.Aizpurua et al. [ ] investigated the optical properties of Au nanorings using another modi ed colloidal lithography, coupled with an Ar ion beam etching step.A red-shi ed localized surface plasmon was observed compared to Au nanodisks of similar size.ose nanorings may also be used as ultrasensitive biosensors [ ].
Schematic illustration of depositing gold and silver onto a hexagonally close-packed spheres monolayer at the incidence angles ( ) of and − ∘ , respectively.e incident beam of gold, the incident beam of silver, and the normal direction of the colloidal template are highlighted by yellow, blue, and black arrows, respectively.(b) SEM image of the resulting heterogeneous binary array.e original locations of PS spheres, gold NPs, and silver NPs are highlighted by black circles, yellow triangles, and blue triangles, respectively.e scale bar is m."Reprinted with permission from [ ].Copyright ( ) American Chemical Society." SEM micrograph of Pt nanodisks.e scale bar is nm."Reprinted with permission from [ ].Copyright ( ) American Chemical Society." (b) AFM D micrograph of a Al nanodisk.Cross-section pro le reveals a rather rough surface."Reprinted with permission from [ ].Copyright ( ) American Chemical Society." A dewetting process also enables the synthesizing of nanorings of various materials (TiO 2 [ ], CdSe Quantum Dots [ ], and proteins [ ]).First, a solution containing the desired material is lled into a colloidal template on a substrate.During the evaporation of solvent at the nanosphere/substrate interface, the solvent molecules around the contact area usually evaporate last, because they are con ned by capillary forces.e materials dissolved or dispersed in the solvent move towards these areas and form ring-like structures under the nanospheres (Figure ). . .Nanoholes.e fabrication of nanoholes is based on the modi cation of colloidal masks by etching or heating treatment.It modulates the size of the interstitial pores between the particles, changing the arrangement from close packing to nonclose packing. is technology allows considerable freedom to control both the feature dimensions and shape of nanoholes by changing the sizes of nanospheres.By lling the empty interspace between nanospheres with target materials, nanohole or nanopore arrays can be produced.Periodic metallic nanohole arrays (Figures and ), as other metallic nanostructures, have attracted great interest because of their important technological applications in nanophotonic devices.Current challenge in this eld lies in the fabrication of nonspherical nanohole arrays (Figure ) due to the fact that the shape of the individual nanoholes o ers an important handle to tune the plasmonic properties.Recently, Yu et al. [ ] developed a facile and new approach to prepare large-scale ordered porous nanoparticles (e.g., gold or Fe 3 O 4 ) lms, by lling the nanoparticles into interstitial voids of the sphere arrays by ltration technique.en the ordered porous structured were formed a er heating and removing the sphere template (Figure ).e obtained structures can be used for membranes, catalysts, and surface enhanced Raman scattering probes. . .Nanorods-Nanowires-Nanopillars-Nanotips.By catalytic deposition or growth, D structured arrays can be fabricated using D colloidal crystals as templates.Metal nanoparticle arrays can be used as catalysts to grow D nanowire arrays.Indeed, Ni nanoparticle arrays have been used to prepare carbon nanotube arrays (Figure ) by plasma-enhanced chemical vapor deposition (PECVD).
of Au nanorings: (a) top view and (b) ∘ tilted view.Both scale bars are nm."Reprinted with permission from [ ].Copyright ( ) American Chemical Society." F : SEM micrograph of Ni porous ordered arrays.e scale bar is m."Reprinted with permission from [ ]of elliptical Ag porous ordered arrays before (a) and a er (b) nanosphere removal.e scale bar is m."Reprinted with permission from [ ].Copyright ( ) American Chemical Society." Wang and coworkers [ ] prepared aligned ZnO nanorods by vapor-liquid-solid growth guided by Au nanoparticles arrays.Li and coworkers [ ] reported a new approach for manufacturing TiO 2 hcp nanocolumn arrays (Figure (a)) by pulsed laser deposition (PLD) in oxygen on the top of PS

F
: Cross-sectional SEM image (a er removal of the .m polystyrene monolayer template) of a lm prepared from nm gold nanoparticles.e scale bar is nm."Reprinted from [ ] with permission of the Royal Society of Chemistry." 200 nm F : Tilted SEM micrograph of aligned carbon nanotubes (CNTs)."Reprinted with permission from [ ]2 nanocolumn array obtained by PLD using a PS colloidal monolayer as substrate.(a) HR-SEM micrograph observed from the side (the dotted circle highlights the PS sphere)."Adapted with permission from [ ].Copyright ( ) American Chemical Society." (b), (c) Top-view HR-SEM micrograph of TiO 2 nanorods obtained under di erent background gas pressures ((b) .Pa for min and (c) .Pa for min) and subsequent annealing."Adapted with permission from [ ].Copyright ( ) John Wiley and Sons".arrays grown via the "templated growth" route trough a sacri cial layer of polystyrene nanospheres, showing controllable and reversible wetting properties.(a) Cross-section SEM micrograph showing the as-grown ZnO nanowires through the polystyrene nanospheres.(b) SEM micrograph of the ZnO nanowires a er removal of the polystyrene template, highlighting a hexagonal symmetry.e inset scale bar is nm.(c) Reversible hydrophobicity to superhydrophilicity by successive UV illumination and dark storage or annealing (CA: contact angle)."Adapted with permission from [ ].Copyright ( ) American Chemical Society".) SEM micrograph of a cross-section of the silicon tip arrays obtained by wet etching.e inset is the pro le of water droplet on the sample."Reproduced from [ ] with permission of the Royal Society of Chemistry." (b), (c) Respectively side and top SEM micrographs of RIE-etched silicon sample for antire ective applications.e scale bar is nm."Reprinted with permission from [ ]. " (d) SEM micrograph of silicon nanoimprint lithography stamp obtained by RIE etching.(e) SEM micrograph of corresponding imprinted pattern on PMMA.e scale bar is nm."Reprinted with permission from [ ].Copyright ( ) John Wiley and Sons". in di erent materials such as Co 3 O 4 , [ ] ZnO, [ ] Fe 2 O 3 , [ ], or CuO [ ]. ese nanostructures, in addition to intrinsic properties of materials they are made of, may display interesting wettability properties [ ].Our group also investigated the use of nanosphere lithography process to produce density-controlled patterned arrays of well-aligned ZnO nanowires by hydrothermal growth.Reversible superhydrophilicity to hydrophobicity was observed and controlled by successive UV illumination and O 2 annealing (Figure ) [ ]. Two-dimensional colloidal crystals can also be used as protective masks to prepare nanotip/nanopillar arrays by etching procedures such as dry etching by reactive ion etching (RIE) or metal catalytic wet etching [ , ]. RIE steps of certain materials are realized by corresponding ions, for example, polymer and carbon by oxygen ions, but silicon and silica by uorine ions [ ]. Nanotips and nanopillars are of great importance because of their unique potential in a wide variety of applications (Figure ), such as antire ection coatings [ , ], highly hydrophobic surfaces [ ], and mold for nanoimprint lithography [ ].