Source: https://pubs.rsc.org/en/content/articlehtml/2019/me/c8me00112j
Timestamp: 2019-04-26 04:13:20+00:00

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The ability to produce structural color from inherently colorless materials, similar to that in butterfly wings and beetle shells, has attracted considerable research interest over the last three decades. Despite their extraordinary properties and performances, the field of structural colors based on inherently functional 2D materials only took off recently. In this minireview, we highlight the diversity of 2D materials utilized for achieving structural coloration in different architectures. We summarize the large tunability of photonic architectures based on 2D materials and emphasize their extraordinary dynamic response induced by external stimuli. Subsequently, recent strategies to tailor their properties with molecular and structural approaches are discussed. Finally, we point out promising future directions in this emerging field.
Pirmin Ganter obtained his PhD degree under the supervision of Bettina V. Lotsch from the University of Munich (LMU Munich) in 2018. He is currently working as a scientist in the Lotsch group at the Max Planck Institute for Solid State Research. His research interests include the synthesis, assembly and functionalization of 2D materials for various applications.
Bettina V. Lotsch received her Ph.D. from LMU Munich in 2006. After a postdoctoral stay at the University of Toronto she was appointed associate professor at LMU Munich. Since 2017 she has been Director at the Max Planck Institute for Solid State Research in Stuttgart. Her research interests are at the interface between solid-state chemistry, nanochemistry, and molecular chemistry and include porous frameworks and 2D materials for sensing and energy conversion. Bettina was named Fellow of the Royal Society of Chemistry in 2014 and is recipient of an ERC Starting grant (2014) and the EU-40 Materials Prize 2017 from the European Materials Research Society (EMRS).
As 2D materials are coming of age, their richness in composition, structure and properties offer a unique platform for the design of tailor-made nanoscale building blocks for functional devices. Combining the chemical scope, diverse optical properties and stimuli-responsive nature of 2D materials and their ensembles with the recent advancement in liquid-assisted assembly strategies, 2D materials have emerged as versatile building blocks in thin film-based photonic architectures, including Fabry–Pérot interference filters and 1D photonic crystals. To impart such architectures with maximum functionality, design strategies range from molecular level approaches such as ion exchange and intercalation, to morphology engineering such as porosity tuning. The integration of 2D materials into photonic architectures has opened up new horizons in the realization of smart devices, ranging from vapor and pressure sensors to functional surfaces allowing for the touchless tracking of finger motions. On a more fundamental level, thin films exhibiting tunable structural color allow for the observation of otherwise optically silent processes with the naked eye, such as intercalation into 2D materials. Cast into photonic architectures, the unique versatility of 2D materials and their molecularly engineered counterparts can be harvested to push the limits of label-free sensing, anticounterfeiting, radiation shielding, photovoltaics, display technology, and beyond.
Fig. 1 Structural color in nature and everyday life serve as inspiration for artificial photonic architectures: a) interference color of a soap bubble, b) Tmesisternus isabellae beetle shell showing structural color,2 c) Morpho butterfly wing.1 d) Common photonic architectures realized in the laboratory to achieve structural coloration.21 From left to right: thin film on silicon substrate, 1D PC, 2D PC and 3D PC. e) Visualization of Bragg–Snell law. Panel a) reprinted and adapted with permission from Wikimedia Commons, Copyright 2007 Brocken Inaglory. Panel b) reprinted with permission from ref. 2, Copyright 2017 Springer Nature. Panel c) Reprinted with permission from ref. 1, copyright 2018 American Chemical Society.
Fig. 2 Stimulus sensing with a photonic thin film. A change caused by a stimulus in the refractive index a), or layer thickness b), or both c), can result in a change of the displayed structural color d). In the case of c), which is the default situation in nanosheet-based thin films, the change in the layer thickness is much more pronounced compared to the decrease of the refractive index. Therefore, an overall redshift in the reflectance spectrum is observed, d).
Although there are excellent recent reviews on structural colors and their applications,7,8,10–12 2D materials appear only as minor points in these reviews as most of the research in this field is still carried out with nanoparticles and polymers despite considerable drawbacks compared to 2D materials.
With this review, we thus shine a light on photonic nanostructures based on 2D materials in order to reveal their potential as colorimetric sensors. We first give an overview of the tunable structural colors realized with a large variety of 2D materials. As nearly all of the 2D material based structures are able to change their thickness by swelling, we highlight recent examples of stimuli responsive behaviour enabling the tracking of otherwise optically silent processes. This is followed by strategies to tailor their swelling behaviour leading to rationally designed sensors and micron scale patterns of nanosheets. In the last section, promising future directions are presented for structural colors based on such 2D materials. With this review we thus illustrate the enormous potential of 2D materials as tailorable building blocks for nanostructured color sensors and beyond.
Fig. 3 Modulation of structural colors from 2D materials. For all panels a formula of the nanosheet composition and top and side view of the respective nanosheet structure is shown. The TiO6 octahedra are light blue, SbO6 octrahedra dark blue, PO4 tetrahedra red, Li0.2Sn0.8S2 octahedra orange, TaO6 octahedra green, Mg1−xAlxO6 octahedra dark green, SiO4 tetrahedra black and (MgxLiy)O4F2 octahedra purple. The tunable structural colors result from variations in the spin-coating speed or repetition except of a). a) Lepidocrocite-type Ti0.87O20.52− forming a highly oriented liquid crystal in the presence of a magnetic field. Depending on the nanosheet concentration, the interlayer spacing changes and hence, the structural color.65 b) H3Sb3P2O14/SiO2 1D PCs with different nanosheet layer thicknesses including a SEM cross-section image to highlight the morphology difference of the nanoparticle (NP) and nanosheet layer.41 c) Lithium tin sulfide (LTS) 1D PCs with different nanosheet layer thicknesses. As seen in the SEM cross-section compared to H3Sb3P2O14/SiO2 1D PCs a lower number of total layers is sufficient for achieving a high reflectance due to the higher refractive index contrast in the LTS/SiO2 1D PCs.55 d) SEM cross-section of a H1−xTBAxTaP2O8 thin film on a silicon substrate and images of films with different thicknesses.56 e) Mg–Al–NO3 LDH/TiO2 1D PCs with adjustable structural colors.59 f) LAPONITE®/TiO2 1D PCs with different layers thickness and representative SEM cross-section image of the sample. As a structural model a closely related hectorite layer is depicted instead of LAPONITE®.40 Panel a) reprinted and adapted with permission from ref. 65, Copyright 2016 Springer Nature. Panel b) reprinted and adapted with permission from ref. 41, Copyright 2015 John Wiley and Sons. Panel c) reprinted and adapted with permission from ref. 55, Copyright 2018 John Wiley and Sons. Panel d) reprinted and adapted with permission from ref. 56, Copyright 2017 John Wiley and Sons. Panel e) reprinted and adapted with permission from ref. 59, Copyright 2012 the Royal Society of Chemistry. Panel f) reprinted and adapted with permission from ref. 40, Copyright 2008 John Wiley and Sons.
a MMO mixed metal oxide.
In terms of functionality, stimuli responsive sensors based on 2D materials can be grouped into different categories depending on the stimuli they can detect, e.g. vapor, liquid, temperature, and mechanically or magnetically responsive sensors.7,10 In the following, we will highlight the different types of photonic sensors based on 2D materials, putting special emphasis on humidity, ion and vapor sensors, as these are currently the most investigated ones.
Fig. 4 Dynamic structural color changes in photonic architectures based on 2D materials, induced through various stimuli (see text). a) Variation in structural color of a graphene nanoplatelet interference sensor under different axial strain values;67 b) sensing of temperature with a H0.52−xTMAxTi0.87O2 liquid crystal65 and c) with Mg–Al–NO3 LDH/TiO2 1D PC.59 d) Sensing of the magnetic flux direction65 and e) pH with H0.52−xTMAxTi0.87O2 liquid crystals.65 f) Humidity sensing with a graphene oxide thin film on a silicon substrate59 and g) with a H3Sb3P2O14/SiO2 1D PC.58 In g) also the reversible transparency switching is shown, which happens due to the cancellation of refractive index contrast upon water infiltration of the structure. In addition, due to their fast response time and high sensitivity to moisture the H3Sb3P2O14/SiO2 1D PC can be utilized to detect human finger motions in a touchless fashion, h).41 Panel a) reprinted and adapted with permission from ref. 67, Copyright 2017 The Royal Society of Chemistry. Panel b), d) and e) reprinted and adapted with permission from ref. 65, Copyright 2016 Springer Nature. Panel c) reprinted and adapted with permission from ref. 59, Copyright 2012 The Royal Society of Chemistry. Panel f) reprinted with permission from ref. 58, Copyright 2015 American Chemical Society. Panel g) and h) reprinted and adapted with permission from ref. 41, Copyright 2015 John Wiley and Sons.
The most common transduction mechanism in 2D nanosheet-based photonic sensors relies on changes in the dimensionality of the active component.40,41,50,58 Changes in thickness of the nanosheet layer causing the color change are either due to intercalation41,50,58 or ion exchange reactions.40,66,80 The observed changes are typically large as they are maximized due to the preferred alignment of the nanosheets parallel to the substrate. The most common stimulus that is sensed with such architectures is humidity.41,50,55,56,58,71,72 The first example of reversible humidity sensing by a photonic architecture based on 2D materials dates back to the 1990s, where LAPONITE® thin films were assembled by the sequential deposition of nanosheets from colloidal suspensions.50 This was followed later with graphene oxide thin films (Fig. 4f) prepared by dip-coating,58 and by various 1D PCs including H3Sb3P2O14/SiO2 (Fig. 4g),41 H3Sb3P2O14/TiO2,41 HSbP2O8/TiO2,72 LTSLTS/TiO2,55 LTS/SiO255 and silk/silk-TiOx71 1D PCs prepared by spin-coating. Most of these architectures show extremely fast response times on the order of milliseconds to a few seconds and an ultrahigh sensitivity (defined as colorshift (nm)/% RH) to the humidity stimulus.41,50,58,72 Interestingly, for some of the structures, e.g. H3Sb3P2O14/SiO2 and LTS/SiO2 1D PCs, the RI contrast is cancelled at high relative humidity,41,55 and as a consequence, the photonic structure turns transparent. This transparency switching is due to RI matching upon water intercalation into the structure as the high-RI nanosheet layer swells upon water uptake (effective RI decreases) and the textural pores of the low-RI layer are filled with water (effective RI increases).41,55 The combination of fast response times and giant color shift in response to humidity enables the touchless tracking of the motion of a finger across the surface of a 1D PC (Fig. 4h), based on the humid atmosphere surrounding a human finger.41 Touchless tracking of finger motion may be interesting in both touch- and touchless user interfaces for input or as feedback mechanism.
While humidity sensors continue to be of utmost importance, sensors that are capable of detecting and differentiating between volatile organic compounds (VOCs) are likewise of immense practical interest due to their broad application range, e.g. in environmental monitoring and medical diagnostics.10,25,87–94 Photonic structures based on unmodified 2D materials have shown some potential in the field of vapor differentiation, i.e. as photonic noses.68,72 We were able to show that different polar protic vapors could be distinguished from non-polar vapors by recording the time dependent optical shift of a HSbP2O8/TiO2 1D PC.72 Using this combined read-out of spectral shift and response time, even constitutional isomers among the polar and protic vapors could be differentiated (Fig. 5a). In fact, the observed degree of vapor differentiation with a single-element photonic nose is among the best reported so far. The main reason for this high selectivity is due to the acidic nature of the nanosheet layer, which is able to selectively interact with polar protic analytes through hydrogen bonding and acid–base interactions.72 Graphene oxide thin films were also able to distinguish between polar protic vapors (see Fig. 5b).68 For both graphene oxide thin films and HSbP2O8/TiO2 1D PC the sensing response is reversible.
Fig. 5 Dynamic structural color changes caused by gaseous and liquid stimuli in 2D material based structures. Differentiation of polar and protic vapors with HSbP2O8/TiO2 1D PCs, a),72 and graphene oxide thin films, b).68 c) Identification of bulky ionic surfactants and molecules with different functional groups with LAPONITE® thin films and 1D PCs.40 d) Primary alkylamine recognition with H3Sb3P2O14 thin films. The linear increase in the alkyl chain length results in linearly increasing d-spacing and hence, optical shift.82 Panel a) reprinted with permission from ref. 72, Copyright 2016 John Wiley and Sons. Panel b) reprinted and adapted with permission from ref. 68, Copyright 2018 Elsevier. Panel c) reprinted and adapted with permission from ref. 40, Copyright 2008 John Wiley and Sons. Panel d) reprinted and adapted with permission from ref. 82, Copyright 2018 American Chemical Society.
Recently, we were able to extend this theme to the intercalation of various primary and tertiary alkylamine vapors into H3Sb3P2O14 photonic thin films (Fig. 5d).82 As the amines are protonated during intercalation, they are trapped in the interlayer space. Since the layer charge density of H3Sb3P2O14 is higher compared to LAPONITE® layers, we obtained a clearer correlation with the analyte size.40,82 This is due to the fact that the layer charge density governs the orientation and, hence, packing of the charged surfactant in the interlayer space. With increasing number of carbon atoms in the alkylchain we observed a linear increase in the intergallery spacing (i.e. d-values) and, as a consequence, in the optical shift for primary alkyl amines. Moreover, we could differentiate between similarly sized primary and tertiary alkylamines based on the intercalation time.82 Intriguingly, by simulating the time-evolution of the optical spectra, we were able to monitor vertical diffusion of primary alkyamines into H3Sb3P2O14/TiO2 1D PCs in real time. In essence, this allows us study processes occurring at the molecular level like diffusion and layer expansion with a simple macroscopic optical read-out.83 Therefore, optical architectures with 2D materials might help to add to the general understanding of diffusion phenomena and intercalation mechanisms of molecules in 2D materials.
Fig. 6 Controlling the properties of nanosheet sensors by non-covalent functionalization. Effect of ion exchanging layered phosphate thin films with TBA on a) the sensitivity towards humidity, and b) the capability to distinguish between vapors with varying polarity.56 c) Influence of primary alkylamine intercalation on the selectivity of H3Sb3P2O14 thin films.82 d) Area resolved intercalation of primary alkylamines (DA decylamine, BA, butylamine and EA ethylamine) for creating a sensor array on a single H3Sb3P2O14 thin film and identification of vapor through characteristic color patterns with the functionalized thin films.82 e) H1−xTBAxCa2Nb3O10 area resolved control of the interlayer cation by photocatalytic decomposition of the interlayer species under UV-light with a mask. In the development step from the second to third picture the TBA containing areas get washed away.57 f) Micron-scale structures obtained by this technique (photocatalytic nanosheet lithography), utilizing H1−xTBAxCa2Nb3O10 as a negative photoresist.57 Panel a) and b) reprinted and adapted with permission from ref. 56, Copyright 2017 John Wiley and Sons. Panel c) and d) reprinted and adapted with permission from ref. 82, Copyright 2018 American Chemical Society. Panel e) and f) reprinted and adapted with permission from ref. 57, Copyright 2017 John Wiley and Sons.
Fig. 7 Tailoring the functions of 2D material based photonic architectures by changing the composition and modifying the photonic lattice. a) 1D PCs based on LTS/H3Sb3P2O14 nanosheets with a schematic and a cross-section image including an EELS map.55 b) Changing the porosity of 2D materials by templating with organic spheres. SEM cross-section images of different examples: top, LAPONITE®/TiO2 1D PC, middle, all-LAPONITE® 1D PC based on different porosities,80 and bottom, LAPONITE® 3D PC.66 c) Utilizing nanosheet layers as defect layers in resonator structures: In the displayed case a TiO2/SiO2 1D PC with a H3Sb3P2O14 defect (green) containing a dye layer (red) was utilized to construct fluorescence turn-on and -off humidity sensors.84 Panel a) reprinted and adapted with permission from ref. 55, Copyright 2018 John Wiley and Sons. Panel b) reprinted and adapted with permission from ref. 66 and 80, Copyright 2008 American Chemical Society. Panel c) reprinted and adapted with permission from ref. 84, Copyright 2017 John Wiley and Sons.
As illustrated based on the above examples, a fine selection of inorganic 2D materials has already been used as the source of tunable structural color, with many more to come.
And yet, there is plenty of room left for innovation as the field matures. In the final section, some promising future directions will be highlighted (Fig. 8).
Fig. 8 Overview of future directions in photonic nanoarchitectonics based on stimuli-responsive 2D materials.
Although several 2D materials have been integrated into architectures for structural coloration, a plethora of new and existing 2D materials with stimuli-responsive properties are at hand, including for example layered Zintl phases, MXenes and Xenes, metal halogenides, and transition metal (di)chalcogenides.42,44,49,76,77 Introducing these families of compounds with widely differing properties can result in novel functionalities. Moreover, combining different 2D materials with each other, with functional nanoparticles or with polymers in complex photonic architectures is still in its infancy and will unleash complex and enhanced functionality.
A large and essentially unexplored area is the integration of covalently modified nanosheets into photonic architectures. Pre- or post-assembly covalent modification of the nanosheet layers through grafting is an excellent tool to endow them with analyte specific functionality. There are now many reports available describing how to tailor the properties of the nanosheets and parent layered materials by covalent modifications.48,49,89,90,102–106 The reactions are as diverse as the different nanosheet compositions and structures and comprise reactions with thiols, including click reactions, electrophiles, such as alkyl iodides and diazonium salts, isocyanates, epoxides and coordination with metal salts.
In summary, this review highlights a new area in which 2D materials have the potential to excel, but unlike other directions, the use of 2D materials as building blocks for photonic architectures is still in its infancy. We have highlighted the diversity in composition and structures of the 2D materials used for structural coloration, and summarized the dynamic sensing response of 2D materials to various stimuli, which enables the tracking of otherwise optically silent processes. We discussed different strategies to tailor the functionality of photonic architectures based on 2D materials on the atomic scale by ion exchange and intercalation, as well as by changing the composition and design of the photonic architectures. To conclude, the future for creating tunable structural colors based on 2D materials is bright.
Open Access funding provided by the Max Planck Society. Financial support was granted by the Max Planck Society, the University of Munich (LMU), the Center for NanoScience (CeNS), and the Deutsche Forschungsgemeinschaft (DFG) through the Cluster of Excellence Nanosystems Initiative Munich (NIM).
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