Hollow colloidal spheres are a type of special functional materials that possess a large fraction of empty spaces inside, intact shells, and uniform morphologies. Due to the existence of the hollow core, the surface area of the hollow spheres is much larger, while density is significantly lower than that of their solid counterparts with the same material and size. One result is that the performance of the hollow spheres in catalysis , , , , water treatment , chemical sensors , , , , and energy storage  is dramatically improved. On the other hand, the features of a large fraction of space inside and thin shells endow them with the ability to not only load various substances but also change the mechanical response of materials, which can be used as imaging contrast agent , , drug delivery carriers , , , electrode materials , , nanoreactors , , , , , , , and pressure sensors , to name a few.
In the last few years, several excellent reviews have appeared on hollow spheres or hollow micro-nanostructures , , , , , , , . However, they focused either on the synthesis with specific materials or morphologies or on the aforementioned traditional applications such as catalysis, biomedicine, nanoreactors, energy storage, and so on. There is no review that emphasizes the applications of hollow spheres as building blocks for fabrication of novel nanostructures and nanophotonics, which are inspired by the unique features of hollow spheres – empty cores and uniform shells. The structural features of hollow spheres determine their properties, while the properties of hollow spheres determine their applications. For instance, hollow spheres with dense shells can prevent liquid from infiltrating in the cores to overcome the refractive index (RI) matching problem, resulting in enhancing RI contrast to improve system performance , , . Furthermore, inside the shell of hollow spheres, whispering gallery modes (WGMs) can be generated to enhance light management , . Moreover, the existence of air core of hollow spheres results in low effective index for tuning reflectivity , . In this review, we devote special attention to the very recent progresses on these novel applications of the hollow spheres. Especially, emphasis will be placed on what nanostructures with novel topologies, ordered or disordered (amorphous) self-assembled nanostructures with new functionalities can be realized based on hollow spheres as building blocks. This review starts with a brief overview of the strategies for synthesis of hollow spheres. Next, self-assembly techniques are summarized including fabricating two-dimensional (2D) or three-dimensional (3D) ordered colloidal crystals or amorphous colloidal nanostructures. Afterwards, we mainly focus on the applications of the hollow spheres in different fields such as nanosphere lithography (NSL), colloidal photonic crystals (CPCs), WGM resonators, and coating materials for tuning surface reflectivity. Finally, we will discuss the current challenges and perspectives of the applications of hollow spheres.
2 Synthesis of hollow microspheres
Motivated by the unique properties, many researchers have been devoted to the synthesis of hollow microspheres with rational design for various desired applications. Until now, three main strategies have been used for the synthesis of hollow spheres, such as hard-templating, soft-templating, and self-templating methods.
For the hard-templating approach, as shown in Figure 1, in a typical process, the templates are prepared first, in which rigid monodisperse micro-nanospheres are popularly used, such as polymer beads, silica, carbon, ceramics, or metal colloidal spheres. Desired material is then coated on the outer surface of the templates. The hollow spheres are subsequently obtained after selectively removing the templates. The empty core size of hollow spheres is determined by the size of the template, while the shell thickness is mainly determined by the coating process. SiO2 and polymer [polystyrene (PS), poly (methyl methacrylate) (PMMA)] colloidal spheres are more often used as hard templates to fabricate monodisperse organic and inorganic hollow spheres because of their high uniformity and convenient production. Compared to polymer shell hollow spheres, inorganic hollow spheres are more common, more diverse, and more stable, which endows inorganic hollow spheres with special properties in optics, optoelectronics, magnetics, photoelectrochemistry, mechanics, and catalysis , . The most common coating material is SiO2 , because it has several versatile properties. First of all, SiO2 is hydrophilic and negatively charged, resulting in preventing the aggregation of the colloidal spheres. Second, SiO2 is also versatile for tuning porosity, optical transparency, biocompatibility, and mechanical stability , . Among the core particles, polymer cores such as PS colloidal spheres are widely used due to their high monodispersity and quantity production. For instance, Xia et al.  used a direct coating method (Stöber method) to produce silica-coated PS core-shell spheres (PS@SiO2) with a uniform silica shell, where it requires positively charged PS spheres to get smooth and homogeneous core-shell structures. Directly using negatively charged PS spheres cannot result in a uniform shell without modification of their surfaces , , , , , . Marlow and Deng  reported a modified method that can directly produce monodisperse hollow silica sphere with smooth shell using negatively charged polystyrene spheres as hard templates when tetraethyl orthosilicate (TEOS) is replaced with vinyltrimethoxysilane. Several typical monodisperse hollow spheres synthesized by hard-templating method and their feature parameters are listed in Table 1.
For the soft-templating strategy, emulsion droplets, gas bubbles, and vesicles are usually employed as templates. Although the soft-templating approach does not involve removing template process, it lacks control over the morphology and monodispersity of the products compared with the hard-templating method.
Self-templating is a direct way to synthesize hollow spheres without the need of additional templates. The advantages of the self-templating method are significantly reduced production cost, simplified synthesis procedures, and easy mass production. However, there are only several specific compositions suitable for the self-templating approach, which strictly limits its application field.
It is noteworthy that from the synthesis strategies mentioned above, only the hard-templating approach can offer hollow spheres with controllable morphology and narrow size distribution (uniformity), which can meet the strict requirements for producing ordered colloidal sphere arrays for NSL and CPCs. Further details on the state-of-the-art of synthesis of hollow spheres can be found in these reviews , , .
3 From colloidal spheres to colloidal arrays
Applications of the colloidal particles are strongly dependent on whether and how the colloidal particles pack. Self-assembly strategies provide a powerful tool to readily organize the colloidal spheres into ordered 2D or 3D nanostructures. Spherical nanoparticles are most often used as building blocks for the formation of high structural and optical quality colloidal crystals due to their highly isotropic features and easy stacking. To obtain high-quality colloidal crystals, the main limitation is that the colloidal spheres must be sufficiently monodisperse. Moreover, some interactions should be considered when the colloidal particles are dispersed in various solvents such as van der Waals forces, steric repulsion, and even Coulombic repulsion for charged colloids. The interactions govern both the stability of the colloids in suspension and the crystallization of the colloids, which has been demonstrated by theory and experiment , , . While the assembly of colloidal spheres into ordered structures needs to meet various assembly requirements, disordered (amorphous) assembly structures can be obtained more easily as the amorphous assembled colloidal spheres do not require long-range order. We will introduce only some self-assembly strategies to produce self-assembled colloidal structures (either ordered or disordered structures) to meet the applications mentioned in this review. More details on self-assembly that cover all facts of colloidal assembly can be found elsewhere , , , .
3.1 2D colloidal crystals
Monodisperse colloidal spheres can form a hexagonal close-packed monolayer of colloidal crystals through a number of different self-assembly methods. Several representative methods are summarized in Figure 2, including dip-coating, spin-coating, electrophoretic deposition, and interface self-assembly (Langmuir-Blodgett).
The pioneering work on fabrication of 2D colloidal crystals was developed by Nagayama et al. using dip-coating method , as shown in Figure 2A, in which attractive capillary forces play a key role to organize the spheres into a hexagonal monolayer close-packed array of colloidal spheres. The capillary forces result from the meniscus formed around the spheres. When the thickness of the liquid layer is very close to the diameter of the colloidal spheres, the nucleus of the colloidal crystal is generated. By continuous evaporation of the liquid, more colloids are driven towards this nucleus by a convective transport. Large-area monolayer arrays form with ordered structures via a manner of analogous epitaxial growth due to the attractive capillary forces. The quality of the obtained 2D crystals strongly relies on the evaporation rate, which is normally controlled by a step motor for lifting up the wettable substrate from the colloidal suspension with a very slow rate, or via strict control temperature for controlling the evaporation rate.
After the development of dip-coating method, spin-coating technique  was also proposed to accelerate solvent evaporation. The critical condition is that the substrate should be wetted completely by colloidal suspension to obtain high-quality 2D colloidal crystals (see Figure 2B). Several research groups have contributed many efforts to the spin-coating strategy to quickly produce colloidal crystals on a large scale. For example, so far, wafer-size monolayer colloidal crystals can be fabricated by spin-coating in minutes, as developed by Jiang’s group . Here, the colloidal spheres are spun in non-volatile solvents and are cured by UV irradiation after casting to enhance mechanical stability. This can be extended to create multilayer colloidal crystals.
Electrophoretic deposition is an alternative method to fabricate monolayer colloidal crystals. Colloidal suspension is confined between two parallel indium tin oxide glass slides. Colloidal spheres are driven by applied electric field to form 2D colloidal arrays due to the electrohydrodynamic interaction between the spheres, as shown in Figure 2C , . Electrophoretic deposition can also realize selective deposition on a predesigned patterned substrate and even 3D colloidal crystal .
Compared to the aforementioned methods, self-assembly at the gas/liquid interface (see Figure 2D) is more widely used because, for this process, no expensive equipment is needed, and colloids at the interface exclusively form a monolayer without variation in the layer thickness. In a typical procedure, a droplet of colloidal suspension is spread onto the water surface by a micropipette or needle-dripping method , , resulting in a monolayer 2D colloidal film floating on the water surface, which can subsequently be transferred onto the desired substrate. The Langmuir-Blodgett (LB) method , ,  also belongs to the category of self-assembly at gas interface strategies, but special equipment is needed. Two hard barriers are employed to compress the floating colloidal particles to form close-packed monolayer film. In addition, binary 2D colloidal crystals can be also fabricated by the self-assembly at air/water interface , .
3.2 3D colloidal crystals
The process of colloidal spheres self-assembling into 3D ordered structures has been studied for three decades. Early theoretical calculations have demonstrated that the colloidal spheres are favored to form face-centered cubic (fcc) structures as fcc packing is slightly more stable than hexagonal close-packed (hcp) structure. However, the small difference in free energy between fcc and hcp results in the self-assembled colloidal crystals always being a mixture of these two structures . A rich variety of methods have been developed to realize 3D colloidal crystals with high quality, including sedimentation, dip-coating, spin-coating, shear force driven, and vertical deposition , , . Most of them are based on evaporation-driven convective assembly. Here, we will only focus on vertical deposition method (also called convective self-assembly method) , which is, to date, the most widely used one to produce high-quality 3D colloidal crystals because it is low cost and large scale. During the assembly process, the evaporation of the liquid (generally, water or ethanol) drives the spheres to arrange in the meniscus formed between a vertical substrate, the suspension and air. The thickness of the colloidal crystals can be precisely controlled by adjusting the concentration of the colloidal suspension. However, this method is limited to the size of the colloidal spheres used. Normally, the diameter of the polymer spheres is larger than 1 μm, but for silica spheres, it is less than 500 nm. Recently, vertical deposition method has been successfully extended by a suitably modified method, which allows it to fabricate crystals using large spheres (diameter larger than 1 μm) .
3.3 Disordered (amorphous) self-assembled colloidal structures
Normally, spray coating ,  or layer-by-layer ,  is a commonly used approach to produce amorphous colloidal spheres with large area in a simple way. Achieving ordered self-assembled colloidal crystals requires monodispersity of the spheres and stability of the packed colloids, while for producing disordered assembled nanostructures, these criteria are not necessary.
4 From self-assembled colloidal sphere nanostructures to applications
The self-assembled colloidal structures have numerous applications in nano- and micrometer-scale materials science. For example, 2D ordered colloidal sphere arrays are usually used as masks for creating regular arrays of nanometer-sized features with a technique called colloidal (or nanosphere) lithography. Three-dimensional ordered colloidal sphere arrays are also being routinely used in the field of photonic band gap materials and sensors. While typically order is necessary for photonic structures, disordered (amorphous) colloidal sphere structures can still strongly affect light transport. In this section, we will devote special attention to the applications of hollow spheres in colloidal lithography, CPCs, visible Mie scattering for structural coloration, and tuning surface reflectivity.
4.1 Hollow nanospheres lithography
NSL, also known as colloidal lithography, uses either an individual colloidal sphere or a self-assembled monolayer or binary colloidal arrays to create (patterning) nanostructures . This is an inexpensive, facile, efficient, and reproducible technique to rapidly fabricate large-area patterning nanostructures compared to conventional lithography such as electron beam lithography or focused ion beam milling , , , , , . In a typical process, close-packed colloidal sphere arrays are first prepared, serving as physical masks or templates. Nanostructures are then created through a multistep fabrication process treatment involving evaporation, deposition, imprinting, and/or etching , . Until now, numerous nanostructures such as nanocrescents , nanotriangles , nanohoneycomb , nanobowl , , , nanorings , , , nanopillars , , and nanowells  have been achieved using colloidal spheres as building blocks. Very recently, NSL has been expanded to create more complex nanostructures by incorporating multiple-angle deposition , in-plane rotation etching , or oblique etching . However, the achievements of NSL mentioned above are obtained using solid spheres as mask building blocks and involving complex fabrication processes. As compared to the solid counterparts, hollow spheres exhibit an empty core, which offers an opportunity for directly fabricating novel nanostructures when combing reactive ion etching (RIE) approach. RIE is an anisotropic etching process , and the morphology change of the hollow spheres is determined by the etching time. As a result, the morphology of hollow spheres can be transformed into a unique anisotropic nanostructure. In this section, we will focus on the recent efforts and developments in hollow nanosphere lithography (HNSL) for novel 2D and 3D nanostructures creation. The topic of traditional NSL based on solid spheres as building blocks is beyond the scope of this paper. Instead, we will focus on the new progress of HNSL using hollow sphere as building blocks. More details on the traditional NSL based on solid spheres can be found in other review papers , , , .
4.1.1 Direct fabrication of monodisperse nanorings from hollow spheres as building blocks using RIE
The existing traditional NSL processes still involve multiple and sophisticated fabrication steps. In addition and more importantly, they leave the created nanostructures anchored to the substrate, preventing them from being directly dispersible in an aqueous medium. This strongly limits the application scope and possibilities of subsequent surface functionalization. Therefore, there are still challenges for developing NSL technique to create redispersible nanostructures without involving multistep. Inspired by the empty core of hollow spheres, Zhong et al.  recently developed a new class of NL to directly fabricate monodisperse and dispersible silica nanorings by means of RIE of hollow SiO2 spheres. Compared to conventional NSL, the method does not involve any intermediate steps to generate an etch mask. First, a colloidal crystal monolayer of hollow silica spheres was prepared on a substrate by assembly at the air/water interface . After that, sulfur hexafluoride (SF6) RIE etch was applied. This etching step gradually transforms the hollow silica spheres into rings. Figure 1A (SEM images) and 1B (AFM scans) shows the morphology change of the hollow silica spheres during the etching process.
As schematically illustrated in Figure 3C, the formation mechanism of the nanorings can be explained by a simple model. The model is placed in a Cartesian coordinate system with the center of the hollow sphere as the origin and the y-axis as the etching direction. In the plasma environment, the reactive species are accelerated perpendicularly to the sample in the unidirectional electric field. This results in the top surface of the hollow spheres being preferentially etched at initial etching stage, leading to the formation of a hole on the top surface of the hollow spheres. Because of the mask effect of the sphere itself, the etching of the outside bottom surface of the hollow spheres is negligible. When the top surface is opened, the bottom surface starts being etched from the inside because the reactive species can penetrate inside the hole in the top surface of the hollow spheres. At the same time, the inside bottom surface of the hollow spheres is also being etched, resulting in formation of two holes in the top and bottom surfaces of the hollow spheres, respectively. Once there exists a hole on the bottom surface, the etching occurs at both top and bottom surfaces along the shell to the equator simultaneously, which is indicated by two arrows in Figure 3C, resulting in the morphology of the hollow spheres finally transforming into ring-shaped structures. Moreover, the resulting silica nanorings can be readily redispersed in solution and subsequently serve as universal template for the synthesis of core-shell nanoring topology. As an example, Au nanoparticles (NPs) were assembled on the silica core (nanoring) surface. By controlling the degree of Au NP coverage, the plasmonic resonance properties of the final composite nanorings can be tuned. For a dense surface coating, a novel silica-Au core-shell ring-shaped nanostructure is created, which possesses high-quality factor resonances in the near-infrared demonstrated by spectroscopic measurements and finite difference time domain simulations, as shown in Figure 3D.
4.1.2 Direct fabrication of complex 3D nanostructures from hollow sphere colloidal crystals
Tremendous efforts have been made in developing NSL, but they are still based on mono- or bilayer solid sphere colloidal array as template to create nanopatterning. The most resulting nanostructures are still (monolayer) 2D patterning, not more than two layers. This is limited by the used building blocks of solid spheres. It still remains a challenge to fabricate complex 3D nanostructures using the traditional NSL. Very recently, Zhong et al.  have expanded the capability of hollow sphere lithography, which allows to directly fabricate complex 3D hierarchical nanostructures based on hollow sphere colloidal crystals (HSCCs) without the need for an external mask. The obtained nanostructures have features as small as ~30 nm, which is comparable to the resolution obtained with electron-beam lithography.
The formation mechanism of these complex 3D hierarchical nanostructures can be explained by an fcc unit cell and a series of typical morphological evolutions of the hollow spheres in different layers during the etching process, as shown in Figure 4A and B, respectively. As mentioned above, during the RIE process, the reactive species are vertically accelerated to the sample surface, resulting in the top surface of the spheres being etched preferentially , , . Simultaneously, the first-layer sphere array serves as shadow mask for the layer below as the reactive species can go through the interstices of the spheres. For the first layer of hollow spheres, the morphology of the spheres is transformed from hollow sphere to bowl-shaped, then ring-shaped, splitting ring-shaped, to finally form nanodot structures. For the second layer, when the top surface of the first-layer spheres is broken, triangle-shaped openings appear in the top surface of the second-layer hollow sphere at the same time due to the shadow effect from the interstices between spheres of the first layer, as described in Figure 4B (b, second). Subsequently, the triangle-shaped openings also were created on the bottom surfaces of the second-layer spheres during the bottom surfaces of the first-layer spheres broken because the reactive species can penetrate inside the hollow spheres from the broken part. However, once two holes formed on the top and bottom surfaces of the first-layer spheres, the reactive species can go through both the original triangle interstices and the broken spheres themselves of the first-layer spheres, which serve as a mask for the lower layer spheres, resulting in more complex morphologies (cage-like nanostructures) created in the second-layer spheres. Furthermore, the resulting complex nanostructures in the second layer also act as a mask for the third layer. For the third layer, what happened with the second layer is repeated, but there is a delayed etching as the etching rate decreases with increasing penetration depth for the reactive species, as illustrated in the Figure 2B, the third row. This series of structural evolution was also confirmed by SEM images in Figure 4C. Indeed, while the morphological changes in the first and second layers discussed above are already in effect, it is clear that the third layer remains unaffected (as shown in Figure 4C-2′) as the reactive species do not reach the third layer at the initial stage of etching due to the drastic reduction in flow rate of the reactive species inside colloidal crystal structures .
4.2 Hollow spheres as building blocks for nanophotonics
Besides being templates for NSL as abovementioned, self-assembled colloidal sphere structures have another important application, normally in nanophotonics. Of course, their functionalities are strongly determined by not only the constituent materials but also the long-term, mesoscopic order (periodic or non-periodic structures). For instance, colloidal microsphere systems support strong WGM resonances that operate light in the visible range for dramatically enhancing light absorption , . Moreover, 3D ordered colloidal crystals have been widely used in photonic crystals, termed colloidal photonic crystals or inverse opals , , , , , . While disordered (amorphous) colloidal sphere structures are a burgeoning area in the self-assembled colloidal spheres, they herald a new era of photonic structural materials due to their wide applications in random laser , , Anderson localization , structural coloration , or anti-reflection . Hollow spheres are a special case of colloidal spheres, having an empty space inside and uniform dense shell. In introducing these unique features into self-assembled hollow sphere structures, novel and fascinating properties have also emerged. In this section, we will focus on the recent progress and achievements of the self-assembled hollow spheres with ordered or disordered structures.
4.2.1 Applications of ordered self-assembled hollow sphere structures
Colloidal crystals are also called CPCs, which have been widely employed as a class of photonic crystals to manipulate light propagation in the visible range due to the existence of periodic refractive indices, forming a photonic band gap (PBG) . PBG provides opportunities for modulation of optical density of states (DOS) to amplify light and matter interaction. Compared to the photonic crystals obtained by top-down approaches, CPCs are fabricated by direct bottom up self-assembly of spherical building blocks, which is low cost, scalable, and convenient , , , . Although various optical devices based on CPCs have been demonstrated including inhibition or enhancement of spontaneous emission , low-threshold lasing , , lossless waveguiding , and sensing , , to name a few, improving the performances and developing the novel functionalities of the CPCs are never ending. Hollow sphere CPCs (HSCPCs) provide a new avenue to further improve the performances and increase the functional diversity of the CPCs. Pioneer works on HSCPCs were reported by Asher’s and Xia’s groups. Asher and Xu  used hollow polymeric spheres as building blocks to create CPCs by self-assembly, in which the diameter and shell thickness of the hollow polymeric spheres can be tuned over a wide range by the hard-templating method. Xia et al.  reported HSCPCs consisting of hollow SiO2 spheres, in which the hollow SiO2 spheres were synthesized by Stöber method  using monodisperse polystyrene spheres as templates. However, they did not further study the unique properties of such CPCs, which are caused by their specific hollow features. Together with hollow polymeric spheres, a silica shell on hollow spheres is the most commonly used building blocks, due to their organic solvents resistance, thermal stability, and easy surface functionalization. Clays et al.  studied the optical properties of the hollow silica sphere CPCs after infiltrating different organic liquids. They further found that such a system can overcome the RI matching problem with liquid infiltrating, unlike with the traditional solid sphere CPCs or inverse opals. The dense shell of the hollow SiO2 spheres can prevent liquid from infiltrating, leading to increasing the RI contrast. RI matching is a severe problem that limits the use of the traditional CPCs in some specific conditions such as under outdoor varying weather conditions or under aqueous environment for real-time sensing. In addition, the existence of WGMs inside the shell of hollow spheres has also been demonstrated for light management to improve the performance of optoelectronic devices. In this subsection, we will review a number of elaborations of hollow sphere CPCs, such as engineering defect modes in CPCs and defect mode lasing, patterned CPCs for real-time fluorescence detection and optomicrofluidics, selective sensing of the CPCs, robust sealed CPCs with ultrastable PBG and WGM resonators for light management.
22.214.171.124 Engineering planar defects in CPCs and defect pass mode laser
Analogous to doping in semiconductors, defect modes can be introduced into photonic crystals to create allowed states (the pass band) within the forbidden band (PBG), resulting from the DOS redistribution (enhancing the DOS) in the defect mode , . Introducing defects into photonic crystals in a controllable manner is as important as that of dopants in semiconductors, because it can provide more functionalities for photonic crystals. For instance, waveguides with bend radii on the order of the wavelength without significant loss of signal, low-power all-optical switching, and low-threshold laser can be achieved by introducing suitable defects into photonic crystals , . Normally, the defect modes can be created in photonic crystals by locally disrupting the periodicity or the RI of the photonic crystals , . An interesting special defect mode for CPCs is a planar defect, because they are technically easy to create. When a planar defect is introduced in CPCs, it results in localized states in the stop band (the PBG), leading to a pronounced dip in the corresponding reflectance spectra. So far, many techniques have been developed to introduce a planer defect into CPCs, such as Langmuir-Blodgett , , , , chemical vapor deposition , , spin-coating , transfer printing , , , and sputtering , . However, most of the reported results are not satisfying as they cannot produce a sufficiently pronounced passband within the PBG.
Recently, Zhong et al.  developed a facile way that can significantly improve the quality of the introduced defect mode pass band. Typically, high-quality 2D monolayer of SiO2 spheres (guest spheres) was first fabricated by self-assembly at the air/water interface using surfactant as “soft barrier” to compress the individual microspheres into long-range hexagonal arrays, as shown in the inset of Figure 5A. Then the floating monolayer acting as defect layer was transferred onto a prepared CPC consisting of silica-coated polystyrene spheres (PS@SiO2 core-shell spheres, host spheres). Finally, after deposition of a second CPC of host core-shell spheres on the surface of the defect layer, sandwiched CPCs are obtained, as shown in Figure 5A. Compared to the previous methods to introduce planar defects into CPCs, this method results in much more pronounced pass bands in the PBG of the CPCs. More importantly, the full width at half maximum of the obtained pass band is only 16 nm, as can be seen in Figure 5B. To understand how the defects affect the optical properties of the sandwiched CPCs, different defect modes were obtained and studied by adjusting the size of guest spheres and/or transforming the material nature of host spheres from PS@SiO2 core-shell spheres to hollow SiO2 spheres by calcination, as shown in Figure 5C.
As discussed above, insertion of defect modes into PhCs in a controlled manner can provide more opportunities to diversify their applications, especially in photonic crystal lasers , , . Until now, lasers based on photonic crystal cavities (defect modes) have been almost exclusively demonstrated by top-down fabricated PhCs. Only a few reported results demonstrate lasing in defect mode based on bottom-up engineered CPCs, as the quality of the defect mode created by bottom-up methods is usually not good enough to allow lasing in the pass band (defect mode). Most of CPC lasers are based on the photonic band edge effect , .
Very recently, Zhong et al.  further experimentally and theoretically demonstrated, for the first time, lasing oscillation within the defect mode pass band of the self-assembled CPCs. The middle layer of the large spheres (guest) was modified by laser dye (Rhodamine B, RhB) to serve as a defect layer and a gain medium simultaneously, as shown in Figure 6A. A pronounced pass band with very narrow line width (~11.7 nm) is observed in the reflectance spectrum at a wavelength of ~591 nm, as shown in Figure 6B. When using low energy to excite the sample, a broad emission spectrum of RhB molecules was observed around a wavelength of 572 nm. While increasing the pump energy, a narrow peak suddenly appears at the long wavelength shoulder of the broad fluorescence spectrum when the excitation energy reached 14.3 μJ (see Figure 6C, left panel). Upon further increasing the excitation energy, the line width of the emission spectrum dramatically narrows while the amplitude is simultaneously enhanced. The narrowest line width of the emission line is 0.77 nm (see Figure 6C, right), when increasing the excitation energy. Furthermore, the simulations reveal that the relative low-threshold lasing exhibited by the sandwich structure can uniquely be attributed to the efficient coupling of the spontaneous emission of the dye to the defect mode of the doped crystals.
126.96.36.199 Patterned superhydrophobic/superhydrophilic surfaces of hollow sphere CPCs for real-time fluorescence detection
It is well known that creating superhydrophobic surfaces should combine surface roughness with low surface energy . So far, there is no single chemical modification known that can produce a smooth surface with superhydrophobicity. Self-assembled CPCs provide an opportunity to obtain superhydrophobic surfaces with a suitable surface modification due to their inherent surface roughness. Combing the unique optical properties of CPCs with the specific surface wettability not only broadens the application range of CPCs in various optical devices but also improves their performances , , . For instance, an ultrasensitive fluorescence sensor based on hydrophilic/hydrophobic patterned CPCs has been developed by fabricating hydrophilic CPC dots on a hydrophobic substrate using the inkjet printing technique . Such patterned CPCs are inspired by the hydrophilic/hydrophobic patterned back of beetle that has an ability to collect and enrich water from fog . Moreover, CPCs with extremely wetting patterns have been used for microfluidics because the flow only wets and moves along the hydrophilic regions. However, these patterned CPCs are not suitable for real-time sensing as their optical properties are dramatically diminished when water penetrates the building blocks of solid colloidal spheres resulting in the RI index matching problem. How to improve the performances of the CPC devices for real-time detection is still a challenge. As mentioned above, using hollow spheres as building blocks to replace the solid colloidal spheres provides an effective way to overcome the RI matching problem. Hollow sphere CPCs show remarkable advantages and unique properties in this content.
Recently, Zhong et al.  developed a hollow sphere CPC system for real-time fluorescence detection in aqueous system. Because the dense shell of the hollow sphere prevents liquid from infiltrating the inner cavity, liquid can only replace the air between the microspheres but not inside. Thus, hollow sphere CPCs exhibit the ability to maintain a high RI contrast after the infiltration of water (physiological environment), leading to exceptionally strong photonic band gap effects and fluorescence enhancement, which is essential for real-time fluorescence detection in aqueous system. In addition, they introduced superhydrophilic patterns on the superhydrophobic hollow spheres CPCs, as shown in Figure 7A. Oxygen plasma etching was employed to create superhydrophilic patterns on the superhydrophobic hollow sphere CPCs. The related structural properties of the hollow sphere building blocks and the patterned hollow sphere CPCs are shown in Figure 7B. Figure 7C shows the shift and amplitude enhancement of the reflectance peak of the superhydrophilic regions of HSCPCs after water infiltration. The shift is attributed to the effective RI increase, while the amplified enhancement refers to the relative RI contrast increase. After water infiltration into the voids in HSCPCs, the RI of the voids among sphere changes from 1 (air) to 1.33 (water), while that of the core of the hollow sphere still remains air (dense shell preventing water infiltration). This results in both effective indices and RI contrast increase. The patterned hydrophilic regions (pinning centers) in an otherwise hydrophobic environment were used to strongly confine and concentrate water-soluble analytes, again leading to a significant (local) increase in fluorescence, as shown in Figure 7D. The orthogonal nature of the PBG and enrichment effect allows for a multiplicative effect, resulting in an increase of fluorescence, allowing enhanced real-time fluorescence detection without any sample preparation.
Incorporation of high-performance optical devices into microfluidics has recently attracted great attention due to the emerging unique properties. Such a system is also termed optomicrofluidics, which combines the advantages of optics with microfluidics to realize rapid in situ sensing of analytes. CPCs in particular are promising candidates to be integrated into optomicrofluidic systems. However, the traditional CPCs always consist of solid spheres that result in an inherent RI matching problem when the voids in CPCs are filled with water. This drawback leads to significantly diminishing or even completely vanishing optical properties of CPCs. Zhong et al.  introduced a superhydrophilic channel into superhydrophobic HSCPCs, which form an optomicrofluidic device. This hollow-sphere-based optomicrofluidics results in stronger a PBG effect rather than in RI matching. Figure 8A shows the fabrication process. By combining simple surface modification and selective oxygen plasma etching, an integrated HSCPC optomicrofluidic systems can be achieved. A typical cross-section SEM image of HSCPCs is shown in Figure 8B. The aqueous solution is only penetrating the superhydrophilic channel (yellow part) due to the surrounding superhydrophobic confinement effect, as shown in Figure 8C (left). The superhydrophilic 3D virtual channel is further confirmed by two-photon fluorescence microscopy, as shown in Figure 8C (right). This also indicates that oxygen plasma reaches the bottom layer of the HSCPCs after 20-s etching. Moreover, proof of concept, real-time specific bioassays with high sensitivity were realized in such an optomicrofluidic system that makes post-treatment drying procedures redundant.
188.8.131.52 Hollow mesoporous silica spheres CPCs for selective sensing
In the past years, chemical sensing by CPCs and their derivative (inverse opal) has been realized by the shift of the reflectance spectra (PBG peak) , , , , , . When the solvents infiltrate into the voids of CPCs, the color of CPCs will change because the average RI of the entire CPCs is increased due to the replacement of air with solvent. However, the traditional CPC sensors cannot clearly distinguish between chemical species with similar refractive indices because of the small PBG shift. Recently, Li et al.  reported novel monodisperse hollow mesoporous silica spheres (HMSS) with high surface area and a narrow size distribution to assemble HMSS-CPCs, as shown in Figure 9A and B. The HMSS-CPCs show two reflection peaks, which is different from the single reflection peak of the traditional solid sphere CPCs. An interesting phenomenon was observed that remarkable PBG shifts occur for the long-wavelength reflection peak, while the short-wavelength reflection peak remains nearly constant when liquid infiltrated. Even for homologues or isomers with small RI difference, the shift of the first PBG is still significant, as shown in Figure 9C and D. The high sensitivity and selectivity of HMSS-CPCs is attributed to the hollow and mesoporous structure, which features increased specific surface and improve adsorption capabilities for the analytes.
184.108.40.206 Sealed HCPCs for ultrastable PBG and robust mechanical properties
The applications of HSCPCs mentioned above all utilize the tunable properties of the PBG of the CPCs, which are also causing the instability of the CPCs because of their inherent porosity. This means that the air among spheres of the CPCs can be readily replaced by liquid (changing the RI), which gives rise to the PBG shift. In other words, the porosity of the CPCs results in their PBG sensitive to the working environment. In addition, the porosity of the conventional CPCs also lowers their mechanical strength because there are only van der Waals interactions to connect all spheres. However, some special applications require not only an ultrastable PBG without wavelength shifting and deterioration but also robust mechanical strength. More recently, Zhong et al.  proposed a strategy to create sealed CPCs of hollow spheres, which is inspired by the limpet’s shell. The fabrication process is shown in Figure 10A. First, TEOS precursor was introduced into the voids of the prepared PS@SiO2 core-shell CPCs. The subsequent hydrolysis of TEOS results in the formation of silica-sealed PS@SiO2 core-shell CPCs with an overlayer of silica. Thickness can be controlled by the spin-coating speed. Finally, removal of PS cores by calcination leads to silica-sealed hollow SiO2 CPCs, as shown in Figure 10B. The resulting sealed CPCs show not only an ultrastable PBG under high pressure (Figure 10C) but also enhanced mechanical robustness demonstrated by a series of tests with extreme conditions, as shown in Figure 10D. Furthermore, photonic crystal lasing is achieved in such sealed HSCPCs at a constant wavelength with the sealed sample whether dry in air or submerged in water.
220.127.116.11 WGMs in hollow sphere structures
When the size of the hollow spheres matches the optical wavelength scale, it can confine and guide light along the shell of hollow spheres rather than directly pass through the shell, resulting in forming a closed light path and generating resonances at certain frequencies. This new phenomenon is generally described as a WGM, which gives rise to the coupling of incident light into particular resonant modes to enhance light trapping to substantially improve light absorption.
Cui et al.  have demonstrated broadband light absorption enhancement in spherical shell nanostructures due to the existence of WGM resonances. Figure 11A shows a cross-section SEM image of a monolayer of hollow spherical nanostructures. Figure 11B shows the measured absorption spectra of the sample with and without hollow spherical structures on top, in which the sample with nanostructure significantly enhances the light absorption more than the sample without hollow spherical nanostructures over a broad region. To better understand the enhancement of broadband light absorption, the authors further investigated the amplitude of the WGM electric-field patterns, as shown in Figure 11C, which corresponds to different order resonances of a single hollow sphere. These resonant modes contribute to the enhancement of broadband absorption. The absorption spectra are further broadened by optimizing the parameters of the hollow spherical structures such as size, shell thickness, and the number of layers. A similar effect has also been reported by Yin and co-workers , in which they employed self-assembled hollow ZnO sphere arrays to create WGM resonators to trap sunlight for improving absorption in silicon thin film solar cells. A 9.3% enhancement in the short circuit current density can be achieved theoretically by optimizing the diameter, shell thickness, and the periodicity of the hollow sphere arrays.
4.2.2 Applications of disordered assembled hollow sphere structures
18.104.22.168 Mie scattering
Compared to the (ordered) periodically self-assembled hollow sphere crystals, amorphous (disordered) ones with no positional long-range order can still significantly affect light propagation such as creating structural color and tuning the surface reflectivity. This is attributed to the unique feature of hollow spheres whose size and shape, the aspect ratio of shell thickness, and inner empty cores can fine-tune the optical properties. For instance, Armes and co-workers  found that their synthetic hollow sphere powders display a blue color when placed on a dark background. It was presumed that the unexpected color is attributed to a single-particle optical effect rather than their ordered close-packed structures.
Retsch et al.  systematically investigated the optical properties of amorphous self-assembled hollow spheres with diameter between 333 nm to 642 nm (but the shell thickness remains constant). As shown in Figure 12, the color can be tuned in a wide range. Unlike color arising from either light absorption by dyes or pigments or coherent scattering arising from long-range ordered colloidal crystals, the authors demonstrated that such colors resulted from single nanoparticle optical effect caused by resonant Mie scattering. The hollow silica spheres with appropriate shell thickness could dramatically increase the transport mean free path length of light, which substantially reduced background scattering. Visible Mie scattering can also be observed for fused hollow spheres with particulate shells . Similar effects have also been reported by Xu et al. for titania hollow spheres with diameter between 380 and 600 nm .
22.214.171.124 Antireflection coatings
Reflection is a phenomenon that occurs when light passes the interface between two media with different refractive indices. For some specific applications, surface reflectivity should be tuned such as solar reflective coating for decreasing or preventing heat radiation or antireflection coating for improving device performances. Hong et al.  reported the utility of hollow SiO2 sphere as coatings to reflect solar radiations. They investigated how the size of hollow spheres and their shell structures influence solar reflectivity. It was shown that the size of the empty core of the hollow spheres has a strong impact on the reflectivity to NIR light whereas the shell itself affects the reflection of UV-blue light.
Reducing reflection has also been achieved by using hollow spheres as coatings due to the hollow feature leading to low RI. Cohen and co-workers  reported the utility of hollow silica spheres in fabricating conformal thin film antireflection coatings on PMMA and glass substrates. The antireflection coatings reduced the reflection of PMMA from 7% to 0.5% while it increased the transmission from 92% to 98% in the UV-visible range. Song et al.  reported multifunctional antireflection coatings based on hollow spheres with silica sol infiltrated. The obtained hollow sphere/silica sol composite coatings not only reduce reflection but also endow other functionalities to the substrates, including high transparency, robust resistance to moisture, high hardness, and antifogging.
5 Conclusion and perspective
Over the past two decades, hollow colloidal spheres have attracted increasing attention due to their fascinating properties and widespread classical applications in nanoreactors, photonics, drug loading and releasing, energy storage, water treatment, and so on. This review specifically highlights the recent developments of hollow spheres in NSL for directly fabricating novel nanostructures, CPCs with more advanced functionalities (e.g. lasing, optomicrofluidics, light management), and amorphous assembled nanostructures for tuning surface reflectivity. These achievements of hollow spheres are attributed to their unique features of empty cores leading to anisotropic etching, dense shell preventing liquid infiltrating resulting in remaining high RI contrast, and inside shell generating WGMs and large ratio of empty core to shell thickness for tuning the effective RI.
Although many efforts have been devoted to the synthesis of monodisperse hollow spheres with such unique properties, there is still a need for one-pot, large-scale, and low-cost synthesis routes that can meet the requirements for applications. Moreover, the toxicity for man and environment should be always a concern. In addressing these challenges, seeking new low-cost and environmental-friendly materials and better understanding of the synthesis mechanism are necessary.
Self-assembled hollow spheres provide another element for realizing these novel applications. Colloidal spheres are widely used self-assembled materials that can produce various nanostructures. These structures result in a variety of functionalities not present in the individual colloids, rendering such self-assembled colloidal spheres attractive for a range of applications, in particular, NSL, wetting, and optics. Although the past few years have witnessed a great progress in self-assembly of colloidal spheres, the preparation of large-area and highly ordered colloidal sphere arrays without defects is still a huge challenge. New methods, which can eliminate the spontaneous defects such as cracks, vacancies, dislocations, and grain boundaries, are urgently needed. Moreover, the crystalline lattice of the CCs created by the traditional self-assembly approach is very limited to the fcc structure. There are still challenges to obtain arbitrary types of crystalline lattices such as triclinic, simple cubic, and body-centered cubic lattices. This is very important towards both improving the optical properties of the CPCs (obtaining the complete PBG) and towards creating novel nanostructures.
Further research should focus on deeply understanding the current synthesis and self-assembly approaches of the hollow spheres. Better understanding these mechanisms can not only boost production of hollow spheres but also easily incorporate hollow spheres into the design and fabrication of novel functional materials and devices. We hope that a broad range of applications based on hollow spheres will emerge soon.
K. Zhong acknowledges the financial support provided by the Internal Funds KU Leuven.
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About the article
Published Online: 2018-02-22
Published in Print: 2018-03-28
Citation Information: Nanophotonics, Volume 7, Issue 4, Pages 693–713, ISSN (Online) 2192-8614, DOI: https://doi.org/10.1515/nanoph-2017-0109.
© 2018 Koen Clays et al., published by De Gruyter, Berlin/Boston. This work is licensed under the Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 License. BY-NC-ND 4.0