Self-assembled Templates Research Articles

Structural Changes during the Conversion Reaction of Tungsten Oxide Electrodes with Tailored, Mesoscale Porosity.

In-plane tungsten oxide nanostructures, including hexagonally patterned cylinders and holes in a matrix, were fabricated via sequential infiltration synthesis (SIS) on self-assembled block copolymer templates. Using the tailored morphology and porosity of these model electrodes with in situ grazing incidence small-angle X-ray scattering, the intrinsic structural change of nanoscale active materials during the conversion reaction of WO3 + 6Li ↔ W + 3Li2O was investigated at controlled electrochemical conditions. Reversible electrode volume expansion and contraction was observed during lithiation and delithiation cycles, respectively. The potential where the electrode's thickness expansion started was ∼1.6 V, which is close to the thermodynamically expected one for the conversion reaction of WO3 with lithium (1.65 V). The temporal evolution of the electrode volume at constant electrode potentials revealed high overpotential for bulk lithiation and slow conversion reaction kinetics, despite the tailored porosity of the SIS electrodes. Oxide cylinders showed a smaller overall electrode thickness change, likely due to unconstrained lateral volume change, as compared to a matrix with holes. On the other hand, better connectivity and guided volume change of the latter electrode morphology provided improved cycling stability. In addition, heterogeneity in an electrode, from internal pores and density gradients, was found to aggravate the fragmentation of the electrode during the conversion reaction. Insights into oxide conversion reaction kinetics and the relationship between electrode mesostructure and cycling behavior obtained from this study can help guide the more rational design of conversion electrodes for high-performing batteries.

Biomimetic design of functional plasmonic surfaces based on polydopamine

Polydopamine is a bioinspired multifunctional material that is emerging in the last years for applications in the biomedical, energy, environmental and catalytic fields. Whereas the conformal coating of solid objects by polydopamine is a well assessed process, the development of polydopamine thin films is less explored and their potential advantages have still to be fully unveiled. Herein, we describe a green and straightforward fabrication of composite membranes based on polydopamine thin films self-assembled at the air/water interface, and two different biopolymers, bacterial cellulose and silk fibroin. Furthermore, the abundant phenolic hydroxyl groups enable the in situ Au nanoparticle decoration on the polydopamine side, providing the membrane surface with plasmonic features. The surface patterning of polydopamine layers using self-assembled templating structures is also demonstrated, opening up new perspectives for biocompatible functional membranes with tunable optical and spectroscopic properties. Lastly, we demonstrate that the obtained functional surfaces are promising tools for the fabrication of SERS substrates. By bridging dopamine chemistry with natural biopolymers and 2D photonic crystals, the results obtained within this paper establish an excellent platform for designing 2D nanoarchitectures with multiple functions.

Role of gelatin and chitosan cross-linked aqueous template in controlling the size of lithium titanium oxide

Significant safety advantages of lithium titanium oxide (LTO) over currently used graphite for lithium-ion batteries have been attracting scientists to develop novel synthetic methods of this anode material in order to combine with another cathode. This study utilizes self-sacrificing cross-linked aqueous templates of gelatin and chitosan polymer to control lithium titanium oxide (LTO) morphology and microstructure. Gelatin and chitosan self-assembled aqueous template containing LTO precursors has been evaporated at 110 °C and then calcined at 750 °C in a muffle furnace to synthesize white color LTO powder. Various techniques such as X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), Energy dispersive x-ray spectroscopy (EDS), Fourier transform infrared spectroscopy (FT-IR), Raman spectroscopy and UV–visible spectroscopy were used to characterize the synthesized LTO powders. Both XRD and EDS spectrum confirm the coating of gelatin and chitosan derived carbon species on the surface of LTO materials. The detailed characterization reveals that increasing the amount of gelatin to the mass ratio of gelatin and chitosan reduced LTO particle sizes. Thus, a size controlled carbon coating LTO preparation strategy have been established via gelatin and chitosan cross-linked aqueous template in controlling the morphology and microstructure of LTO material.

Fabrication of Temperature- and Alcohol-Responsive Photonic Crystal Hydrogel and Its Application for Sustained Drug Release.

Herein, crack-free photonic crystal templates with enhanced color contrast were first demonstrated by the coassembly of polystyrene (PS) microspheres and graphene oxide (GO). Then, photonic crystal hydrogels (PCHs) with quick responses to temperature and alcohol solution concentration changes were fabricated by photopolymerization of monomers in the gaps of the self-assembled colloidal crystal templates. The structural color of the PCHs changed from yellow to blue within 120 s as the temperature rose from 25 to 40 °C, whereas upon a decrease in temperature from 40 to 25 °C, the structural color changed from blue to yellow. The structural color of the PCHs also shows an obvious response with the concentration of alcohol solution ranging from 40 to 100 wt %. The quick responses of the PCHs' structural color to changes in temperature and alcohol solution concentration are attributed to the temperature sensitivity of poly(N-isopropylacrylamide) and preferential adsorption and swelling of the alcohol solution for the polymer chains. Furthermore, moxifloxacin (Mox) was loaded into PCHs by hydrogel swelling and exhibited sustained released by increasing the temperature. The sustained release process was facilely monitored by observing the corresponding color changes in real time. The rapid and visible response offers the fabricated PCHs great potential application prospects in the semiquantitative analysis of alcohol concentration and intelligent drug delivery.

CsPbBr3 microarrays with tunable periodicity, optoelectronic and field emission properties using self-assembled polystyrene template and co-evaporation method.

The booming growth of all inorganic cesium lead halide perovskites in optoelectronic applications has prompted extensive research interest in the fabrication of ordered nanostructures or microarrays for enhanced device performances. However, the high cost and complexity of commercial lithographic approaches impede the facile fabrication of perovskite microarrays. Herein, CsPbBr3 microarrays with tunable periodicities have been fabricated using a self-assembled polystyrene nanosphere template and a co-evaporation method. The periodicity of CsPbBr3 microarrays is precisely manipulated by simply modifying the size of polystyrene nanospheres. These microarrays are beneficial for light harvesting, leading to better light absorption ability and prolonged photoinduced carrier lifetime. The longest average carrier lifetime of 58.3 ns is obtained for CsPbBr3 microarrays with a periodicity of 1.0 μm. More importantly, the periodic structures of CsPbBr3 microarrays result in a tunable density of emitter tips in field emission devices. Compared to compact CsPbBr3 films, a 68.2% decrease of the turn-on field is observed for CsPbBr3 microarrays when the periodicity is 150 nm. The higher density of emitter tips leads to larger local field enhancement, and hence the largest field enhancement factor of 3346.6. Finally, a good emission current stability for CsPbBr3 microarray-based field emission devices has been demonstrated.

Metal/nitrogen co-doped hollow carbon nanorods derived from self-assembly organic nanostructure for wide bandwidth electromagnetic wave absorption

Converting carbonaceous material into heteroatom-doped hollow nanomaterial is a promise route to realize lightweight and high-performance electromagnetic wave absorption (EMA). In this work, a series of metal/nitrogen co-doped hollow carbon nanorods (M-NHCRs, M represent as Fe, Co, Ni) were successfully fabricated via a facile self-assembly template strategy, which refers to self-assembling of 9,10-dibromoanthracene (DBA), surficial modification with polydopamine (PDA) and metal ions, and controlled pyrolysis. Among these M-NHCRs, Co-NHCR exhibits remarkable EMA performance, where the optimal reflection loss (RL) reaches −58.6 dB at a loading ratio of 5 wt%. Moreover, the effective absorption bandwidth (EAB) gets to 7.65 GHz. It is considered that the high-performance EMA should be attributed to the unique hollow structure, excellent conductance loss and improved polarization relaxation. This research not only provides a new idea for the design of lightweight electromagnetic absorbents with broadband efficient absorption, but also broadens the way of synthesizing hollow nanomaterials. • Hollow carbon nanorod was constructed by thermally removable self-assembly template. • Doping by trace of metal obviously enhances electromagnetic absorption performance. • Effective absorption bandwidth (EAB) gets to 7.65 GHz at low loading ratio.

Laser micro/nanomachining technology for soft matter

Laser micro/nanomachining technology for soft matter achieves the purpose of fabricating the spherical structures of soft matter by combing laser-assisted mechanical injection and controllable self-assembly, which has significant advantages in comparison with conventional methods like droplet microfluidics. In this study, the effects of laser parameters such as laser energy, beam size, and irradiation position on the injection are investigated. It is found that there also exists one upper limit of the laser energy, and if the laser irradiation is too strong, it can introduce a convection flow of liquid crystal rather than trigger off the injection of guest microdroplets. Thus, the laser injection can be achieved in a specific energy range of the laser irradiation. By manipulating the laser beam with a smaller size, the guest water microdroplets can be injected at the preselected location on the surface of a host liquid crystal droplet. In addition, the influences of material parameters such as the surfactant concentration, the material type and phase state of liquid crystal on the laser-assisted mechanical injection, and the size of the injected guest droplet are investigated. It is found that the liquid crystal droplet with higher surfactant concentration requires less energy from the laser irradiation to generate enough mechanical force to trigger off the injection. Because under the same temperature increment, the liquid crystal droplet with higher ion concentration enjoys a stronger surface tension gradient. By comparing several different types of liquid crystals, it is found the injection of guest droplets into a host with a higher elastic constant liquid crystal can be more difficult. The influences of the material type of liquid crystal and the concentration of surfactant on the critical size of guest microdroplets are summarized. Finally, the defect lines of liquid crystal are introduced as the self-assembly template, through which microdroplets of liquid crystal with the sophisticated spherical structure are fabricated. The self-assembly kinetic behaviors of guest droplets in the defect line are analyzed. The laser micro/nanomachining technology of soft matter can be applied to the extreme processing and application development of 3D spherical structures in the fields of optoelectronics, photonics, and biomedicine.

Self-Assembly of [3]Catenane and [4]Catenane Based on Neutral Organometallic Scaffolds

Transition metal-mediated templating and self-assembly have shown great potential to construct mechanically interlocked molecules. Herein, we describe the formation of the bimetallic [3]catenane and [4]catenane based on neutral organometallic scaffolds via the orthogonality of platinum-to-oxygen coordination-driven self-assembly and copper(I) template–directed strategy of a [2]pseudorotaxane. The structures of these bimetallic [3]catenane and [4]catenane were characterized by multinuclear NMR {1H and 31P} spectroscopy, electrospray ionization time-of-flight mass spectrometry (ESI-TOF-MS), and PM6 semiempirical molecular orbital theoretical calculations. In addition, single-crystal X-ray analyses of the [3]catenane revealed two asymmetric [2]pseudorotaxane units inside the metallacycle. It was discovered that tubular structures were formed through the stacking of individual [3]catenane molecules driven by the strong π–π interactions.

Highly Elastic Interconnected Porous Hydrogels through Self‐Assembled Templating for Solar Water Purification

AbstractInterfacial evaporation using porous hydrogels has demonstrated highly effective solar evaporation performance under natural sunlight to ensure an affordable clean water supply. However, it remains challenging to realize scalable and ready‐to‐use hydrogel materials with durable mechanical properties. Here, self‐assembled templating (SAT) is developed as a simple yet effective method to fabricate large‐scale elastic hydrogel evaporators with excellent desalination performance. The highly interconnected porous structure of the hydrogels with low tortuosity and tunable pore size enables high level of tunability on the water transport rate. With superior elasticity, the porous hydrogels are easy to process with a rapid shape recovery after being rolled, folded, and twisted over hundred times, and exhibit highly effective and stable evaporation with an evaporation rate of ≈2.8 kg m−2 h−1 and ≈90 % solar‐to‐vapor efficiency. It is anticipated that this SAT strategy, without the typical need for freeze‐drying, will accelerate the industrialization of hydrogel solar evaporators for practical applications.

Highly Elastic Interconnected Porous Hydrogels through Self-Assembled Templating for Solar Water Purification.

Interfacial evaporation using porous hydrogels has demonstrated highly effective solar evaporation performance under natural sunlight to ensure an affordable clean water supply. However, it remains challenging to realize scalable and ready-to-use hydrogel materials with durable mechanical properties. Here, self-assembled templating (SAT) is developed as a simple yet effective method to fabricate large-scale elastic hydrogel evaporators with excellent desalination performance. The highly interconnected porous structure of the hydrogels with low tortuosity and tunable pore size enables high level of tunability on the water transport rate. With superior elasticity, the porous hydrogels are easy to process with a rapid shape recovery after being rolled, folded, and twisted over hundred times, and exhibit highly effective and stable evaporation with an evaporation rate of ≈2.8 kg m-2 h-1 and ≈90 % solar-to-vapor efficiency. It is anticipated that this SAT strategy, without the typical need for freeze-drying, will accelerate the industrialization of hydrogel solar evaporators for practical applications.

Iron rich self-assembly micelles on the Doce River continental shelf

After the Fundão iron ore mining dam rupture in November 2015, yellow/ocher emulsions never before reported on the continental shelf adjacent to the Doce River began to be seen, both in coalesced and foam forms. XRD analyses pointed to a prevailing composition of iron and kaolinite with a substantial contribution of an organic-metallic compound, measured in multiple periods over 2 years of sampling. Optical microscopy images allowed the identification of micelles composed of nanoparticles of iron oxyhydroxide making up this emulsion. The generation of dendritic snowflake-shaped microcrystals on fiber filters after water sample filtration and heating confirmed the presence of micelles composed of iron oxyhydroxide nanoparticles enveloped by organic polymers. After losing water, the micelles may act as a self-assembly template seed, where the polymer acts in the oriented adsorption of nanoparticles according to their crystallographic structure. The study brought to light the distinct behavior of a portion of the tailings material, which has already been reported to not have the same flocculation process as the clay minerals previously found in the suspended particulate material (SPM) before the dam rupture.

Gas sensor based on cobalt-doped 3D inverse opal SnO2 for air quality monitoring

In this research, a semiconductor metal oxide (SMO)-based gas sensor was designed for the ultrasensitive and tunably selective detection of formaldehyde and acetone. Cobalt-doped 3D inverse opal SnO2 multilayer films (3D IO Co-SnO2 MFs) used as sensing materials were prepared with the ultrasonic nebulizing deposition (UND) method combined with a self-assembly template. The 3D IO Co-SnO2 (Co/Sn = 1:24 atom%) MF-based sensor exhibited a much higher response and lower limit of detection to the volatile organic compounds (VOC) gases because of the larger specific surface area, effective gas accessibility, high catalytic activity, and increased chemisorbed oxygen species generated by the elevation of the Fermi level and the narrowing of the band gap. More importantly, the 3D IO Co-SnO2 MF-based sensor showed dual-model gas sensing characteristics for selectively detecting formaldehyde and acetone at 200 °C and 225 °C, respectively, because of the difference of the VOC catalytic conversion and surface oxidative reaction rate that was dependent on the operating temperature.

An efficient double template strategy to construct large-area and highly ordered silver “urchin-like” arrays for sensitive SERS analysis

Plasmonic metal nanostructures possessing plentiful gaps or tips have attracted great attention in surface enhanced Raman scattering (SERS) due to their superior performance. Herein, we present an efficient fabrication strategy to construct a large-area and highly ordered 3D silver “urchin-like” array (SULA) based on the ultra-thin alumina membrane (UTAM) mask and the self-assembled polystyrene spheres template. Numerical simulations reveal that the proposed SULA possesses enormous hotspots at the tips of Ag nanocones and in the crevice gaps between the adjacent Ag nanocaps responsible for high sensitivity with an enhancement factor of 1.8 × 106 and excellent reproducibility with a relative standard deviation of 8.3% in SERS analysis. Furthermore, the SULA substrate exhibits great SERS performance for detecting the contaminant crystal violet and thiram in real water environments and excellent linear intensity-concentration dependence relationships. The results indicate the proposed SULAs are promising to be developed as high-performance SERS platforms in qualitative and quantitative trace contaminants in complex water environments.

Templated Colloidal Self-Assembly for Lattice Plasmon Engineering

ConspectusOver the past 30 years, the engineering of plasmonic resonances at the nanoscale has progressed dramatically, with important contributions in a variety of different fields, including chemistry, physics, biology, engineering, and medicine. However, heavy optical losses related to the use of noble metals for the fabrication of plasmonic structures hindered their application in various settings. Recently, an answer to these long-lasting issues emerged in the use of lattice plasmon resonances (LPRs, also called surface lattice resonances), bringing new excitement in the field of plasmonics. Specifically, the organization of plasmonic nanoparticles into ordered arrays enables far-field coupling of the scattered light exploiting the diffraction modes of the array, generating plasmonic resonances with bandwidths as narrow as a few nanometers, corresponding to an increase of over 10-fold in the quality factors compared to localized plasmon resonances. As such, LPRs offer new opportunities to harness light–matter interactions at the nanoscale, while generating renewed interest in the self-assembly of colloidal metal nanoparticles, as a scalable approach to the preparation of such plasmonic arrays. Templated self-assembly emerged as one of the most versatile approaches, being compatible with soft-lithographic techniques such as nanoimprint lithography and amenable to a variety of materials, colloids, and solvents. Templated self-assembly additionally allows the preparation of arrays where the repeating units are composed of multiple self-assembled nanoparticles (i.e., plasmonic clusters). In this system, near-field coupling can be finely tuned, thereby showing promising results in biosensing, catalysis, or plasmonic heating. In this Account, we review the preparation of ordered arrays of clusters of plasmonic nanoparticles. We present various aspects involved in the templated self-assembly of colloidal nanoparticles, with the aim of achieving at the same time close-packed structures within each cavity of the template, and uniform deposition over a large area. We then analyze the optical properties of the prepared substrates. The preparation of hierarchical structures and the possibility of tuning both the internal structure of the cluster and their organization into arrays with different lattice parameters enable control over both near-field and far-field plasmonic coupling. This unique feature of such substrates makes it possible to exploit the interplay between these two types of coupling, for the preparation of versatile functional substrates, expanding the possibilities for the integration of plasmonic arrays into functional devices for various applications. A well-established example is their use for surface-enhanced Raman scattering. On the other hand, optimization of far-field coupling provides access to plasmonic cavities for lasing or refractive index sensing. Despite two decades of fervid scientific research, the preparation and engineering of plasmonic arrays remains a relevant topic, and many directions remain largely unexplored. We conclude with a collection of perspectives and challenges that we find particularly stimulating toward future developments of the field.

Selective adsorption and detection of p-arsanilic acid on MOF-on-MOF heterostructure induced by nitrogen-rich self-assembly template

P-arsanilic acid (p-ASA), as a widely used feed additive, is potentially toxic to aquatic organisms and seriously affects the water environment. Therefore, the development of an intelligent multi-functional material for selective simultaneous detection and removal of p-ASA in water has become a top priority. Here, we report one case of novel nitrogen-rich two-dimensional metal–organic framework, Cu(I)-tpp (Htpp = 1-(4-(tetrazol-5-yl))3-(pyrzin-2-yl) pyrazol), which has outstanding fluorescence emission at 655 nm; and nitrogen-rich sites exposed on the surface make it an excellent self-assembly template to induce the formation of heterostructures. According to the target requirements, ZIF-8 is selected to construct the Cu(I)-tpp@ZIF-8 heterostructure by liquid-phase epitaxy ignoring the lattice mismatch problem. And the electronic structure of the Cu(I)-tpp framework is analyzed by electrostatic potential (ESP) to determine the nucleation site of ZIF-8. The prepared Cu(I)-tpp@ZIF-8 heterostructure assembles adsorption and fluorescence sensing properties, exhibiting significant adsorption (qmax = 303.0 mg g−1) and fluorescence quenching response (LOD = 0.4 µg·L−1) for p-ASA. Notably, Cu(I)-tpp@ZIF-8 achieves a special response to p-ASA that is different from other organic arsenic acids, especially compared with its isomer (o-ASA) and roxarsone (ROX). As a smart multifunctional material, Cu(I)-tpp@ZIF-8 realizes the specificity in the simultaneous detection and removal of p-ASA, presenting excellent application prospects in water quality monitoring.

Nanoscale directional etching features and mechanism of HF/HNO3 etchant

Large-scale and periodic silicon nanostructure arrays are an essential component for various functional devices. However, the conventional selective etching mechanism of specific crystal faces for monocrystalline silicon generally achieves slope or vertical morphology, which greatly limits the preparation of other morphology of silicon nanostructures. In this work, the conventional isotropic etchant HF/HNO3 has successfully achieved nanoscale directional etching and cambered morphology on monocrystalline silicon surface for the first time by changing the synchronous process of oxidation and dissolution into an asynchronous process. Based on the self-assembled monolayer colloidal crystal templates, large-area and high-quality nanopore arrays are prepared on the monocrystalline silicon surface. Particularly, the directional etching ability of HF/HNO3 etchant is further confirmed by combining with electron beam lithography and scanning probe lithography technologies. Hence, this nanoscale directional etching mechanism and etchants provide a promising strategy to achieve cambered morphology nanopores on the monocrystalline silicon surface.

Controllable fabrication of polymeric nanowires by NIL technique and self-assembled AAO template for SERS application

A controllable strategy to fabricate the polymeric nanowires with high throughput and low cost is developed by using the thermal nanoimprint lithography (NIL) technique and self-assembled anodic aluminum oxide (AAO) template. The length of polymeric nanowires can be controlled by adjusting the duration of thermal NIL. A fill mechanism of thermoplastic intermediate polymer stamp (IPS) polymer pressed into the AAO nanopores is closely studied. The as-prepared IPS polymeric nanowire-based Surface-Enhanced Raman Scattering (SERS)-active substrate exhibits a remarkable reproducibility. The effective adsorption of the R6G as probe molecule near to hotspots generated at 3D vertically aligned polymeric nanowire SERS active substrates shows extraordinary enhancement of Raman signal with an enhancement factor (EF) of 105–106. The present strategy is of great guiding significance to broaden the use of thermal NIL technique and AAO template for the fabrication of other nanomaterials, especially for the flexible and transparent polymer-based nanomaterials.

Bench-Top Fabrication of Single-Molecule Nanoarrays by DNA Origami Placement.

Large-scale nanoarrays of single biomolecules enable high-throughput assays while unmasking the underlying heterogeneity within ensemble populations. Until recently, creating such grids which combine the advantages of microarrays and single-molecule experiments (SMEs) has been particularly challenging due to the mismatch between the size of these molecules and the resolution of top-down fabrication techniques. DNA origami placement (DOP) combines two powerful techniques to address this issue: (i) DNA origami, which provides a ∼100 nm self-assembled template for single-molecule organization with 5 nm resolution and (ii) top-down lithography, which patterns these DNA nanostructures, transforming them into functional nanodevices via large-scale integration with arbitrary substrates. Presently, this technique relies on state-of-the-art infrastructure and highly trained personnel, making it prohibitively expensive for researchers. Here, we introduce a cleanroom-free, $1 benchtop technique to create meso-to-macro-scale DNA origami nanoarrays using self-assembled colloidal nanoparticles, thereby circumventing the need for top-down fabrication. We report a maximum yield of 74%, 2-fold higher than the statistical limit of 37% imposed on non-specific molecular loading alternatives. Furthermore, we provide a proof-of-principle for the ability of this nanoarray platform to transform traditionally low-throughput, stochastic, single-molecule assays into high-throughput, deterministic ones, without compromising data quality. Our approach has the potential to democratize single-molecule nanoarrays and demonstrates their utility as a tool for biophysical assays and diagnostics.

Structural and optical properties of transition metal ion-doped ordered mesoporous TiO2 prepared by chelating-assisted evaporation induced self-assembly method

This paper demonstrates the fabrication of highly ordered mesoporous titanium dioxide (OMT) with different transition metals (TM: Fe, Mn, and Ni) by employing a self-assembly surfactant template route. The ordered TM-loaded mesoporous TiO2 (TM-OMT) has uniform mesopores (3–5 nm pore diameter), and a crystalline framework is successfully prepared via soft template strategy by employing Pluronic® F127 triblock copolymer as a chelating and structural directing agent. The structural and optical characteristics of the TM-OMT hybrids have been examined by scanning electron microscopy (SEM), transmission electron microscope (TEM), XRD, N2-sorption isotherms, UV/visible diffuse reflectance spectroscopy, and X-ray photoelectron spectroscopy (XPS) reveals the successful incorporation of TM ions over the highly ordered mesostructured OMT. The XRD pattern evidenced that the obtained TM-doped OMT samples are pure anatase of the crystalline phase. By employing TEM analysis, the average particle size is estimated to be 2–2.7 nm, whereas diffraction patterns have evidenced the polycrystalline features of the mesoporous structure. The bandgap energy of TM-doped mesoporous decreases according to the particular metal and its amount due to the reduced particle size. This cost-effective method provides the exceptional potential to tune the optical features for TM-doped mesoporous TiO2 materials for its optoelectronic applications and show more significant promise for energy conversion technologies.

Influence of Alumina Addition on the Optical Properties and the Thermal Stability of Titania Thin Films and Inverse Opals Produced by Atomic Layer Deposition

TiO2 thin films deposited by atomic layer deposition (ALD) at low temperatures (<100 °C) are, in general, amorphous and exhibit a smaller refractive index in comparison to their crystalline counterparts. Nonetheless, low-temperature ALD is needed when the substrates or templates are based on polymeric materials, as the deposition has to be performed below their glass transition or melting temperatures. This is the case for photonic crystals generated via ALD infiltration of self-assembled polystyrene templates. When heated up, crystal phase transformations take place in the thin films or photonic structures, and the accompanying volume reduction as well as the burn-out of residual impurities can lead to mechanical instability. The introduction of cation doping (e.g., Al or Nb) in bulk TiO2 parts is known to alter phase transitions and to stabilize crystalline phases. In this work, we have developed low-temperature ALD super-cycles to introduce Al2O3 into TiO2 thin films and photonic crystals. The aluminum oxide content was adjusted by varying the TiO2:Al2O3 internal loop ratio within the ALD super-cycle. Both thin films and inverse opal photonic crystal structures were subjected to thermal treatments ranging from 200 to 1200 °C and were characterized by in- and ex-situ X-ray diffraction, spectroscopic ellipsometry, and spectroscopic reflectance measurements. The results show that the introduction of alumina affects the crystallization and phase transition temperatures of titania as well as the optical properties of the inverse opal photonic crystals (iPhC). The thermal stability of the titania iPhCs was increased by the alumina introduction, maintaining their photonic bandgap even after heat treatment at 900 °C and outperforming the pure titania, with the best results being achieved with the super-cycles corresponding to an estimated alumina content of 26 wt.%.