25th Anniversary Article: What Can Be Done with the Langmuir‐Blodgett Method? Recent Developments and its Critical Role in Materials Science

The Langmuir‐Blodgett (LB) technique is known as an elegant method for fabrication of well‐defined layered structures with molecular level precision. Since its discovery the LB method has made an indispensable contribution to surface science, physical chemistry, materials chemistry and nanotechnology. However, recent trends in research might suggest the decline of the LB method as alternate methods for film fabrication such as layer‐by‐layer (LbL) assembly have emerged. Is LB film technology obsolete? This review is presented in order to challenge this preposterous question. In this review, we summarize recent research on LB and related methods including (i) advanced design for LB films, (ii) LB film as a medium for supramolecular chemistry, (iii) LB technique for nanofabrication and (iv) LB involving advanced nanomaterials. Finally, a comparison between LB and LbL techniques is made. The latter reveals the crucial role played by LB techniques in basic surface science, current advanced material sciences and nanotechnologies.     

Self-assembly route for photonic crystals with a bandgap in the visible region

Three-dimensional photonic crystals, or periodic materials, that do not allow the propagation of photons in all directions with a wavelength in the visible region have not been experimentally fabricated, despite there being several potential structures and the interesting applications and physics that this would lead to. We show using computer simulations that two structures that would enable a bandgap in the visible region, diamond and pyrochlore, can be self-assembled in one crystal structure from a binary colloidal dispersion. In our approach, these two structures are obtained as the large (Mg) and small (Cu) sphere components of the colloidal analogue of the MgCu(2) Laves phase, whose growth can be selected and directed using appropriate wall patterning. The method requires that the particles consist of different materials, so that one of them can be removed selectively after drying (for example, by burning or dissolution). Photonic calculations show that gaps appear at relatively low frequencies indicating that they are robust and open for modest contrast, enabling fabrication from more materials.     

In‐Plane Direct‐Write Assembly of Iridescent Colloidal Crystals

Materials made by directed self‐assembly of colloids can exhibit a rich spectrum of optical phenomena, including photonic bandgaps, coherent scattering, collective plasmonic resonance, and wave guiding. The assembly of colloidal particles with spatial selectivity is critical for studying these phenomena and for practical device fabrication. While there are well‐established techniques for patterning colloidal crystals, these often require multiple steps including the fabrication of a physical template for masking, etching, stamping, or directing dewetting. Here, the direct‐writing of colloidal suspensions is presented as a technique for fabrication of iridescent colloidal crystals in arbitrary 2D patterns. Leveraging the principles of convective assembly, the process can be optimized for high writing speeds (≈600 µm s^?1) at mild process temperature (30 °C) while maintaining long‐range (cm‐scale) order in the colloidal crystals. The crystals exhibit structural color by grating diffraction, and analysis of diffraction allows particle size, relative grain size, and grain orientation to be deduced. The effect of write trajectory on particle ordering is discussed and insights for developing 3D printing techniques for colloidal crystals via layer‐wise printing and sintering are provided.     

Growth and time dependent alignment of KCl crystals in Hemoglobin LB monolayer

Nature and organism often use the biomineralization technique to build up various highly regular structures such as bone, teeth, kidney stone etc., and recently this becomes the strategy to design and synthesis of novel biocomposite materials. We report here the controlled crystallization of KCl in Langmuir and Langmuir Blodgett (LB) monolayer of Hemoglobin (Hb) at ambient condition. The nucleation and growth of KCl crystals in Hb monolayer has temporal and KCl concentration dependency. The growth of KCl crystals in LB film of Hb has distinct behavior in the alignment of crystals from linear to fractal like structures depending on growth time. The crystallographic identity of the biomineralized KCl crystal is confirmed from HR-TEM, XRD, and from powder diffraction simulation. Our results substantiated that the template of Langmuir monolayer of proteins plays a crucial role in biomineralization as well as in designing and synthesizing of novel biocomposite materials.     

Soft Langmuir–Blodgett Technique for Hard Nanomaterials

Materials and their assemblies of dimensions down to a few nanometers have attracted considerable scientific interest in physical, chemical, and biological sciences because of unique properties not available in their bulk counterparts. The Langmuir–Blodgett (LB) technique allows rigid nanomaterials to be aligned in particular structures through a flexible assembly process at liquid interfaces. In this review, we summarize the development of assembly of hard nanomaterials using soft LB techniques. An initial summary of the basic features of nanomaterials will include dimension‐related effects, synthesis, characterization, and analysis, and will be followed by examples of LB assemblies of nanomaterials described according to their morphology: nanoparticles, nanorods, nanowires, nanotubes, and nanosheets. Some of the nanomaterials have been fabricated in orientation‐controlled morphologies, and have been incorporated into prototype devices for gas sensing and photocurrent transport. In the final part of this review, the challenges remaining for LB techniques of hard nanomaterials will be overviewed, and will include a comparison with the widely‐used LB technique involving soft materials.     

Vertical cell assembly of colloidal crystal films with controllable thickness

Formation of colloidal crystal films (CCFs) is the first step in the fabrication of inverse colloidal crystals, a class of three-dimensionally ordered, macroporous materials. A new vertical cell assembly method is described that yields CCFs with thicknesses up to 100 µm. The cell consists of two microscope cover glasses separated by a thin polymeric spacer. The lower edge of the cell is placed in a colloidal dispersion. Particles are transported to the top of the cell by capillary force and self-assemble into a CCF as the solvent evaporates. This novel vertical cell assembly method is well suited for fabrication of large area CCFs with controllable thickness that could be further processed into ICC filters for size-based separations of molecules and particles.     

Structured Porous Materials via Colloidal Crystal Templating: From Inorganic Oxides to Metals

The formation of nanostructured materials by using colloidal crystals as templates is a relatively new but rapidly growing area of materials science. Colloid crystalline templates are three‐dimensional close‐packed crystals of submicrometer spheres, whose long‐ranged ordered structure is replicated in a solid matrix, to yield materials with ordered pores. These materials hold promise for use as photonic crystals, advanced catalysts, and in a variety of other applications. Here we review the wide range of materials that have been made following the original synthesis of structured porous silica. This method has been recently modified to produce porous metals.     

大面积3D有序介孔二氧化钛薄膜光子晶体制备与性能研究

介绍了大面积有序反蛋白石结构介孔二氧化钛薄膜光子晶体制备与性能研究的进展.为了保证二氧化钛骨架结构的稳定性和有序度,从而使氧化钛介孔薄膜达到大面积结构均匀,在介孔薄膜制备过程中采用了几种新的工艺方法,其中包括二氧化硅晶体模板的应用和用NaOH溶液代替常用的HF溶液作为模板去除剂.制备的介孔二氧化钛薄膜光子晶体的面积达到厘米尺寸,二氧化钛骨架的填充率达到17.4%,薄膜制备过程中的收缩率<3%.薄膜透射光谱研究结果表明,这种大面积3D有序的反蛋白石结构介孔二氧化钛薄膜具有非常优良的光子带隙特性,有望成为一类具有非常好的发展和应用前景的光子晶体材料.     

Colloidal Self‐Assembly Meets Nanofabrication: From Two‐Dimensional Colloidal Crystals to Nanostructure Arrays

Self‐assembly of colloidal microspheres or nanospheres is an effective strategy for fabrication of ordered nanostructures. By combination of colloidal self‐assembly with nanofabrication techniques, two‐dimensional (2D) colloidal crystals have been employed as masks or templates for evaporation, deposition, etching, and imprinting, etc. These methods are defined as “colloidal lithography”, which is now recognized as a facile, inexpensive, and repeatable nanofabrication technique. This paper presents an overview of 2D colloidal crystals and nanostructure arrays fabricated by colloidal lithography. First, different methods for fabricating self‐assembled 2D colloidal crystals and complex 2D colloidal crystal structures are summarized. After that, according to the nanofabrication strategy employed in colloidal lithography, related works are reviewed as colloidal‐crystal‐assisted evaporation, deposition, etching, imprinting, and dewetting, respectively.     

The Control of Colloidal Grain Boundaries through Evaporative Vertical Self‐Assembly

Self‐assembly continuously gains attention as an excellent method to create novel nanoscale structures with a wide range of applications in photonics, optoelectronics, biomedical engineering, and heat transfer applications. However, self‐assembly is governed by a diversity of complex interparticle forces that cause fabricating defectless large scale (>1 cm) colloidal crystals, or opals, to be a daunting challenge. Despite numerous efforts to find an optimal method that offers the perfect colloidal crystal by minimizing defects, it has been difficult to provide physical interpretations that govern the development of defects such as grain boundaries. This study reports the control over grain domains and intentional defect characteristics that develop during evaporative vertical deposition. The degree of particle crystallinity and evaporation conditions is shown to govern the grain domain characteristics, such as shapes and sizes. In particular, the grains fabricated with 300 and 600 nm sphere diameters can be tuned into single‐column structures exceeding ≈1 mm by elevating heating temperature up to 93 °C. The understanding of self‐assembly physics presented in this work will enable the fabrication of novel self‐assembled structures with high periodicity and offer fundamental groundworks for developing large‐scale crack‐free structures.     

Structural Coloration with Nonclose‐Packed Array of Bidisperse Colloidal Particles

Colloidal crystals and glasses have their own photonic effects. Colloidal crystals show high reflectivity at narrowband, whereas colloidal glasses show low reflectivity at broadband. To compromise the opposite optical properties, a simple means is suggested to control the colloidal arrangement between crystal and glass by employing two different sizes of silica particles with repulsive interparticle potential. Monodisperse silica particles with repulsive potential spontaneously form crystalline structure at volume fraction far below 0.74. When two different sizes of silica particles coexist, the arrangement of silica particles is significantly influenced by two parameters: size contrast and mixing ratio. When the size contrast is small, a long‐range order is partially conserved in the entire mixing ratio, resulting in a pronounced reflectance peak and brilliant structural color. When the size contrast is large, the long‐range order is rapidly reduced along with mixing ratio. Nevertheless, a short‐range order survives, which causes low reflectivity at a broad wavelength, developing faint structural colors. These findings offer an insight into controlling the colloidal arrangements and provide a simple way to tune the optical property of colloidal arrays for structural coloration.     

Layer‐by‐Layer Approach to (2+1)D Photonic Crystal Superlattice with Enhanced Crystalline Integrity

Large‐area polystyrene (PS) colloidal monolayers with high mechanical strength are created by a combination of the air/water interface self‐assembly and the solvent vapor annealing technique. Layer‐by‐layer (LBL) stacking of these colloidal monolayers leads to the formation of (2+1)D photonic crystal superlattice with enhanced crystalline integrity. By manipulating the diameter of PS spheres and the repetition period of the colloidal monolayers, flexible control in structure and stop band position of the (2+1)D photonic crystal superlattice has been realized, which may afford new opportunities for engineering photonic bandgap materials. Furthermore, an enhancement of 97.3% on light output power of a GaN‐based light emitting diode is demonstrated when such a (2+1)D photonic crystal superlattice employed as a back reflector. The performance enhancement is attributed to the photonic bandgap enhancement and good angle‐independence of the (2+1)D photonic crystal superlattice.     

Inside Front Cover: Line Defects Embedded in Three‐Dimensional Photonic Crystals (Adv. Mater. 15/2005)

The inside cover illustrates an approach to creating line defects embedded in the interior of a self‐assembled photonic crystal, as reported by Zhao and co‐workers on p. 1917. Photoresist patterns are first constructed on the surface of a silica opal film by conventional optical photolithography. After regrowth of the silica colloidal crystal, photoresist line defects are successfully introduced into the self‐assembled silica colloidal crystal. Further processing results in an inverse opal with air‐core line defects embedded in its interior, which provides a prototype for future optical waveguide devices based on self‐assembled three‐dimensional photonic crystals.     

Magnetic assembly of nonmagnetic particles into photonic crystal structures

We report the rapid formation of photonic crystal structures by assembly of uniform nonmagnetic colloidal particles in ferrofluids using external magnetic fields. Magnetic manipulation of nonmagnetic particles with size down to a few hundred nanometers, suitable building blocks for producing photonic crystals with band gaps located in the visible regime, has been difficult due to their weak magnetic dipole moment. Increasing the dipole moment of magnetic holes has been limited by the instability of ferrofluids toward aggregation at high concentration or under strong magnetic field. By taking advantage of the superior stability of highly surface-charged magnetite nanocrystal-based ferrofluids, in this paper we have been able to successfully assemble 185 nm nonmagnetic polymer beads into photonic crystal structures, from 1D chains to 3D assemblies as determined by the interplay of magnetic dipole force and packing force. In a strong magnetic field with large field gradient, 3D photonic crystals with high reflectance (83%) in the visible range can be rapidly produced within several minutes, making this general strategy promising for fast creation of large-area photonic crystals using nonmagnetic particles as building blocks.     

Single‐Stimulus‐Induced Modulation of Multiple Optical Properties

Stimuli‐responsive smart optical materials hold great promise for applications in active optics, display, sensing, energy conversion, military camouflage, and artificial intelligence. However, their applications are greatly restricted by the difficulty of tuning different optical properties within the same material, especially by a single stimulus. Here, magnetic modulations of multiple optical properties are demonstrated in a crystalline colloidal array (CCA) of magnetic nanorods. Small‐angle X‐ray scattering studies reveal that these nanorods form an unusual monoclinic crystal in concentrated suspensions. The CCA exhibits optical anisotropy in the form of a photonic bandgap and birefringence, thus enabling magnetic tuning of the structural color and transmittance at a rate of 50 Hz. As a proof‐of‐concept, it is further demonstrated that the fabrication of a multifunctional device for display, anticounterfeiting, and smart‐window applications based on this multiple magneto‐optical effect. The study not only provides a new model system for understanding colloidal assembly, but also opens up opportunities for new applications of smart optical materials for various purposes.     

Introducing Defects in 3D Photonic Crystals: State of the Art

3D photonic crystals (PhCs) and photonic bandgap (PBG) materials have attracted considerable scientific and technological interest. In order to provide functionality to PhCs, the introduction of controlled defects is necessary; the importance of defects in PhCs is comparable to that of dopants in semiconductors. Over the past few years, significant advances have been achieved through a diverse set of fabrication techniques. While for some routes to 3D PhCs, such as conventional lithography, the incorporation of defects is relatively straightforward; other methods, for example, self‐assembly of colloidal crystals (CCs) or holography, require new external methods for defect incorporation. In this review, we will cover the state of the art in the design and fabrication of defects within 3D PhCs. The figure displays a fluorescence laser scanning confocal microscopy image of a y‐splitter defect formed through two‐photon polymerization within a CC.     

Inverse Face‐Centered Cubic Thin Film Photonic Crystals

Recent work on a technique for fabricating inverse fcc photonic crystals from a colloidal system of monodisperse microspheres and titania nanoparticles is described. The technique can be used to produce photonic crystals with other background materials that are available as nanoparticles. The Figure shows the Moiré pattern of a typical photonic crystal sample.     

非晶无序光子晶体结构色机理及其应用

结构色是一种由光学尺度的微纳结构与光相互作用形成干涉、衍射或散射而产生颜色的物理生色效应。与化学生色不同,结构色由于没有色素或者染料的参与,因此没有颜色褪色的现象,同时能够避免使用染料和色素带来的环境污染。目前结构色材料受到研究者和应用开发人员的广泛关注,大量的研究发现结构色可以来源于光子晶体与非晶光子晶体两种结构。光子晶体由规整的周期性结构组成,产生的颜色鲜艳却具有明显的角度依赖性。而非晶光子晶体,其"自身缺陷"导致的短程有序结构具备了各向同性的光子带隙、非虹彩效应、光局域化等特点,赋予了材料柔和亮丽不随角度变化的显色效果,可控的激光效应以及优良的发光效率,从而更能满足材料领域对光散射和光传输等方面的特殊需求。对非晶光子晶体的概念和结构,与可见光作用产生颜色的原理,以及制备非晶光子晶体的不同方法(平板刻蚀法、胶体颗粒自组装法、模板法、相分离法)做了详细的讨论,并对非晶光子晶体产生的结构色效应在光电器件、功能涂料和纺织材料等多个领域中的应用进行了展望。     

Polymer‐Mediated Nanoparticle Assembly: Structural Control and Applications

Nanoparticle–polymer composites are diverse and versatile functional materials, with applications ranging from electronic device fabrication to catalysis. This review focuses on the use of chemical design to control the structural attributes of polymer‐mediated assembly of nanoparticles. We will illustrate the use of designed particles and polymers to create nanocomposites featuring interesting and pragmatic structures and properties. We will also describe applications of these engineered materials.     

CdS、Ag^+掺杂改性纳米TiO_2光催化降解有机物研究

在理论分析的基础上对纳米Ti02分别进行掺CdS和掺银改性实验研究。采用XRD和粒度分析技术对改性后的样品进行表征,并以掺杂CdS、Ag^＋纳米Ti02进行光催化降解甲基橙模拟实验。结果表明,掺杂CdS、Ag^＋纳米Ti02在可见光范围内降解有机物的效率有较大提高。