Monolithic High Performance Surface Anchored MetalâOrganic

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Monolithic high performance SURMOF Bragg Reflector for Optical Sensing Jianxi Liu, Engelbert Redel, Stefan Walheim, Zhengbang Wang, Vanessa Oberst, Jinxuan Liu, Stefan Heissler, Alexander Welle, Markus Moosmann, Torsten Scherer, Michael Bruns, Hartmut Gliemann, and Christof Wöll Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/cm503908g • Publication Date (Web): 22 Feb 2015 Downloaded from http://pubs.acs.org on March 2, 2015

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Chemistry of Materials

Monolithic high performance SURMOF Bragg Reflector for Optical Sensing Jianxi Liu, Engelbert Redel*, Stefan Walheim, Zhengbang Wang, Vanessa Oberst, Jinxuan Liu, Stefan Heissler, Alexander Welle, Markus Moosmann, Torsten Scherer, Michael Bruns, Hartmut Gliemann, Christof Wöll J. Liu, Dr. E. Redel*, Z. Wang, Dr. J. Liu, S. Heissler, Dr. A. Welle, Dr. H. Gliemann, Prof. Dr. C. Wöll Institute of Functional Interfaces (IFG), Karlsruhe Institute of Technology (KIT), Hermann-vonHelmholtz-Platz 1, 76344 Eggenstein-Leopoldshafen, Germany V. Oberst, Dr. M. Bruns, Institute of Applied Materials (IAM), Karlsruhe Institute of Technology (KIT), Hermann-vonHelmholtz-Platz 1, 76344 Eggenstein-Leopoldshafen, Germany Dr. S. Walheim, Markus Moosmann, Dr. T. Scherer Institute for Nanotechnology (INT), Karlsruhe Institute of Technology (KIT), Hermann-vonHelmholtz-Platz 1, 76344 Eggenstein-Leopoldshafen, Germany Dr. A. Welle, Dr. M. Bruns Karlsruhe Nano Micro Facility (KNMF), Karlsruhe Institute of Technology (KIT), Hermannvon-Helmholtz-Platz 1, 76344 Eggenstein-Leopoldshafen, Germany E-Mail: [email protected]

ABSTRACT We report the fabrication of monolithic dielectric mirrors by stacking layers of metal-organic frameworks (MOFs) and indium tin oxide (ITO). Such Hybrid Photonic Band-Gap (PBG) Materials exhibit high optical quality (reflectivities of 80 %) and are colour tuneable over the whole visible range. While the ITO deposition is accomplished by using a conventional sputter process, the highly porous MOF layers are deposited using liquid-phase epitaxy (LPE), therefore yielding crystalline, continuous and monolithic HKUST-1 SURMOF thin films with high optical performance. We demonstrate the optical sensing capabilities of these monolithic and porous Bragg-stacks by investigating the chemo-responsive optical properties (PBG shift and modulation of the intensity of the PBG maximum) upon the exposure to different organic solvents.

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INTRODUCTION Bragg Stacks (BS) are one-dimensional photonic crystals fabricated by alternately stacking of transparent layers with different refractive index (RI). Such Photonic Band-Gap (PBG) materials consist typically of two different metal oxides,1 but also other materials such as polymers2 have been successfully used. A particularly interesting class of materials for the fabrication of such 1D PBG materials are metal-organic frameworks (MOFs), since these materials are highly porous and can be further functionalized in order to specifically bind target analytes. They consist of inorganic metal (or metal/oxo) nodes connected by organic linkers.3,4,5,6,7 In addition, the RI of MOF-materials can be tailored to be exceptionally low – a fact which makes these materials ideally suited as components for PBG materials, e.g. in the field of optical sensing. In previous works polycrystalline MOF-based PBG were fabricated by spin-coating of ZIF-8 MOFnanoparticles on TiO2 porous layers, yielding particulate porous bragg-stacks (BS) which changed their optical properties upon loading with different alcohols. 8 Besides 1D PBG hybrid surface-grafted MOF thin films photonic materials, 3D MOF (HKUST-1 infiltrated inverse opal) photonic crystals have been already reported.9,10 Furthermore, there are numerous literature examples of different porous metal oxide photonic crystals and multi-layered thin film assemblies made out of various nanoparticle composition11,12,13 also with regard to specific optical,14 and optoelectronic15 device applications. Due to the possibility to control the SURMOF thickness rather accurately by adjusting the number of deposition LPE-cycles, the resulting optical multi-layered systems (PBG-material) exhibit a high optical quality and PBG reflectivity. The value measured here, around ≈ 80%, is the highest reflectivity for a 1D PBG Hybrid Stack reported so far. The monolithic nature of our 1D-BS also allows exploiting the favourable mechanical properties of SURMOF-materials (high


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elasticity, Young modulus ~ 10 GPa.).16 Because of their chemical flexibility in terms of the possibility to add further functionalities, e.g. for optical or light induced switching17 or for charge transport properties18 such 1D PBG SURMOF stacks provide the basis for the fabrication of photonic, color-coded chemical sensing devices. MOFs, also referred to as porous coordination polymers (PCPs), were originally developed to provide ultrahigh surface area for large gas storage capacities19 for light gases (e.g. H2, CH4). More recently, in addition to gas storage and separation, more advanced applications including drug delivery, chemical sensing, light harvesting or carbon capture20,21,22 have been reported. Here we focus on the optical and photonic properties of MOF-coatings. With temperature stabilities typically exceeding 200°C the mechanical3, dielectric23 and optical24 properties of MOF coatings provide the basis for numerous potential applications. Loading the pores with appropriate organic semiconductors can change electrical properties25 and optical functionality can be added by a chemical modification of the MOF constituents (e.g. for the optically-triggered release of guest molecules).17 Particularly interesting is the field of optical sensing26. Numerous procedures have been proposed to prepare MOF thin layers, which are needed for optical applications, including coating with a dispersion of MOF powder (nano) particles.27 Here, we focus on epitaxial-grown, surface-grafted HKUST-1 MOF layers (SURMOFs). They are monolithic, highly crystalline and exhibit low defect densities18 as well as excellent mechanical stability.16 In the present paper we demonstrate that HKUST-I (or [Cu3(BTC)2]) SURMOFs are well suited for the fabrication of PBG-materials. The solvent-free, empty HKUST-1 had an RI of n =1.39 (at 200°C)24, at a wavelength of 750 nm, which is in good agreement with the static dielectric constant (n ≈ 1.30) predicted by Seifert and co-workers.28 Exposing the activated and empty HKUST-1 thin films to moisture/EtOH atmosphere, the


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Refractive Index (n) increases to n = 1.55-1.624 and is the same for the freshly LPE sprayed monolithic stacks here. (see Fig.S1) RESULTS and DISCUSSION The fabrication of the monolithic, hybrid SURMOF-based Bragg-Stacks (BS) starts with growing a HKUST-I SURMOF with thicknesses in the range of 80-135 nm on a modified Sisubstrate using Liquid Phase Epitaxy (LPE) (for details see the Supporting Information and Figure S5a-e for additional HR-SEM and FIB cross-sections). Subsequently, an ITO-layer with a thickness of 62-85 nm was sputter-deposited. The density of OH-groups on the sputterdeposited ITO-layers is sufficient to grow a second SURMOF layer by LPE. This process (sputter-deposition of ITO, LPE growth of SURMOF) can be repeated until the desired number of Bragg bilayers is reached. The thickness of the individual SURMOF-layers can be adjusted by changing the number of cycles of the LPE process. For further details on the PBG multilayer fabrication see the Experimental Section. A comparison of the experimental optical reflectivity data (Figure 1a and Figure 1b) with reflectivities simulated for 3 and 5 bilayers PBG systems (see Figure 1b) reveal that the SURMOF-based PBG systems exhibit an exceptional optical quality. In Figure 1c we show a cross-sectional SEM image of a three-layers bragg-stack with 3 SURMOFs with a thickness of 115 nm each sandwiched between ITO layers of 85 nm thickness. The corresponding X-ray diffraction (XRD) data (Figure 3a and Figure 3b) show the typical HKUST-1 diffraction pattern.


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Figure 1. a) Photographs of PBG SURMOF/ITO hybrid materials with different thickness, (stripes are caused only by the light source). b) calculated/predicted and experimentally determined reflectance of a 5 bilayer PBG consisting of HKUST-1/ITO ranging from 80 nm to 135 nm and from 62 nm to 85 nm, respectively. c) HR-SEM cross-sections of multi-layered 3 bilayer and 5 bilayer HKUST-1/ITO 1D PBG hybrid materials.

In all cases the predicted and measured reflectivities agree very well. To achieve a maximum reflectance of a distinct PBG position, the layer thicknesses have been chosen such that the HKUST-1/ITO satisfies quarter-wave requirements. In our HKUST-1/ITO tuneable PBG materials ITO represents the high-refractive-index component (n ≈ 2.0) and HKUST-1 the low-refractive-index one (n ≈ 1.52), for details see also the Supporting Information. Figure S1 shows the spectroscopic ellipsometry (SE) measurements of the different refractive indices comprising the multi-layered SURMOF hybrid PBG material. The optical thickness of the employed HKUST-1 und ITO thin films is equal to the product of


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the geometric thickness and the refractive index of the employed layers. The photographs shown in Figure 1a reveal a very high optical quality, without any defects like comets and/or striations, which are commonly observed for PGB multilayers, fabricated using sol-gel or spin-coating processes. The resulting very high optical quality and particular a PBG reflectivity reaching about 80% of our HKUST-1/ITO monolithic BS multilayers exceeds the results presented in previous reports on such PBD systems, such as e.g. of ZIF-8/Pt29 and ZIF-8/TiO28 , so far the maximum reported MOF-based reflectivity is 55%.29 Figure 2a and 2b exploit the PBG modulation/shifting of the engineered HKUST-1/ITO PBG hybrid material at variable angles of incidence. By starting with a high reflectance above 80% at around 650 nm at 75° incidence the PBG shows an angle dependence decrease of the reflectance maximum and blue shift of the PBG to smaller values in all three samples by lowering the angle of incidence stepwise with a 5° increment during the angular reflectance measurements (see also the Supporting Material Information, Fig.S3-S4). Iridescence can be simply demonstrated by tilting the macroscopic samples (see, Figure 2a) at different angles. Figure 2c shows a 3D reconstruction of a HKUST-1/ITO bilayer based on specific ion signals from SIMS depth profiling. For this analysis a 500×500 µm2 region was continuously analyzed using the Bi3+ ion source, while a concentric area of 800×800 µm2 was eroded with a 2 keV oxygen sputter beam up to a final dose density of 6.3×1017 cm-2 ions. In the ToF-SIMS images in Figure 2c the HKUST-1 layers is depicted as Cu, with 65Cu and Cu2 signals characterizing the MOF layers and with In3O+, together with other InxOy species characterizing the ITO layers. SIMS depth profiling confirmed a smooth, homogeneous and continuous layer system. Details and other multilayered systems e.g. 5 bilayer systems are given in the Supporting Information, see Figure S2.


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Figure 2. a) 3D plot of the PBG angular dependence of a 5 bilayer HKUST/ITO PBG material from 25° to 75° incidence with 5° increment. b) Contour Plot of the respective 3D plot of the PBG angularmodulation. c) SIMS depth profile of a 2 bilayer HKUST-1/ITO stack on silicon substrate. Top left, blue: Si+; top right, green: Cu+; lower left, orange: In3O+; lower right: combined overlay.

XRD data have been recorded for all samples after each bilayer growing step performed in the materials process, e.g. after the HKUST-1 deposition on the previous sputtered ITO thin films. XRD data (Figures 3a and 3b) still indicate the presence of crystalline and oriented HKUST-1 thin films after each spraying cycle on sputtered and crystalline ITO. XRD data of HKUST-1 grown on both ITO and Si substrate and ITO thin films is demonstrated in Figure 3a. This proves that epitaxial growth of crystalline HKUST-1 has the same orientation on both used substrates/layers ITO and Si. During epitaxial growth of HKUST-1 on sputtered and crystalline ITO an additional peak ITO (211) at 21.44˚ appears and increases by intensity with the number of bilayers, see also Figure 3b.


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135 nm HKUST-1 // 100 nm ITO (222)

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First layer HKUST-1@Si

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Figure 3. a) Out-of-plane X-ray diffraction patterns of the sputtered ITO, and HKUST-1 SURMOF thin films deposited on both ITO and Si substrates. b) Out-of-plane X-ray diffraction patterns of HKUST-1 thin films deposited on Si substrate, ITO with 3 and 5 bilayers of HKUST-1/ITO. The patterns are compared with a simulated powder reference. Each pattern is normalized with regard to the (222) diffraction peak.

Figure 3a shows the X-ray diffraction patterns of different layers of HKUST-1 thin films, deposited firstly on Si substrate, as the first and low RI-layer and then on sputtered ITO after the first bilayer. As the first and low RI layer of HKUST-1 was deposited on Si substrate, no peak of ITO is shown in the pattern. After the first bilayer of sputtered ITO high RI thin film another low RI layer HKUST-1 was deposited. As shown in Figure 3b the (211) ITO peak appears and increases in intensity with the numbers of bilayers, as shown with 3 and 5 bilayers in the multilayered HKUST-1/ITO PBG material. In a next step we can exploit the porous nature of the HKUST-1 SURMOFs inside our PBG multilayers to change the optical constant as well as the respective optical thickness of the individual SURMOF thin films. The significant change in refractive index (n) resulting from loading guest molecules in the pores of the HKUST-1 molecular framework24 directly allows the realization of a sensor and at the same time the quantitatively study of sorption behaviour of the MOF material with high accuracy (timeresolved) in-situ. The results shown in Figure 4a demonstrate a pronounced sensitivity to liquid/immersed (in direct contact) polar ethanol (EtOH) solution. While performing sorption


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behavior e.g. also with other organic solvents (see Figure 4a and Table1) we observed a significant red shift of 20 to 40 nm (depending of the respective organic solvent) of the PBG as well as a reduction of the maximum reflectance by around 20 %, see Figure 4a. This process (DRY-WET-SORBED) is fully reversible and cycleable. After all pores have been filled (i) with the analyte (organic solvent, see Table 1), the system reached a minimum of the PBG reflectance. After desorption (ii) of the excess organic solvent the reflectance increased again and finally (iii) the PBG shifted back to its original starting position, see Figure 4a.

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Figure 4. Optical Sensing Approach: (a) Glove-box experiment with EtOH immersion: When the dried sample is immersed in EtOH a pronounced redshift can be observed, which slowly (within a few seconds, depending on the BP of the organic solvent) recovers when the sample dries out. This process is fully reversible and cycleable. When the pores are filled, the higher index of refraction of the filled low index part of the stack lowers the maximal reflectivity and the amplitude of the spectral modulation (higher reflectivity in the minima). (b) PBG blue edge change after heating at ≈ 200°C for 1-2 h, complete dry/activated HKUST-1 SURMOF material: the removal of the water (or residual solvent molecules) leads to a lower reflectance and to a 20% intensity decrease as well as in comparison the loaded 5 bilayer stack with EtOH solution. Table1. Optical sensing experiments demonstrating a significant red shift of the PBG by immersing the stacks into different organic solvents with a different RI index* at 20°C and λ=660 nm. Immersion Solvent / RI* EtOH / 1.3598 i-PrOH /1.3809 THF / 1.407 Cyclohexan / 1.4241 Toluol / 1.4874

PBG Start Max. 658 nm 658 nm 657 nm 658 nm 657 nm


PBG (Red) shift 685 (27 nm) 689 (31 nm) 681 (24 nm) 684 (26 nm) 695 (38 nm)

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Furthermore, theoretical simulations taking in account PBG predictions for the empty (activated/dried HKUST-1) as well as the sprayed (solvent-loaded HKUST-1) multi-layered thin films, which predicts a lowering of reflectance of around 10 % and a red shift of around 20 nm (see Figure S6a). Reflection losses are arising from additional/in excess absorbed water/solvent molecules with interactions within the SURMOF pores as well as an increase of the RI of the porous SURMOF constituent and the solvent layer on the BS. The increase in the optical thickness of 10-15 nm for each SURMOF thin-film within the BS when they are immersed into the organic solvent based on PBG prediction must be due to an increase of the RI (see also Figure S6b). Previous work on the loading of HKUST-1 has shown that this rather rigid lattice does not expand substantially upon loading with small molecules.30,31 This property is in contrast to most organic polymers, for which loading with organic molecules leads to a swelling accompanied by a geometric thickness change, e.g. the swelling of a thin polymer film in toluene vapor.32 We consider the absence of pronounced geometrical changes upon loading, as a substantial advantage of SURMOFs since the built-up of mechanical stress within the BS hetero-structures during loading is avoided, see also Figure S8. Our in situ AFM measurements show that the thickness of the 5 bilayer BS does not change upon immersion into EtOH from 987 nm (Air) to 985 nm (in EtOH). Activation and drying of the HKUST-1/ITO BS leads to a pronounced change in their PBG behaviour, including a significant reduction of scattered light at the blue edge of the PBG, which is consistent with the removal of excess water/solvent molecules from the HKUST-1 SURMOF crystal structure, see Figure 6b. The heating/activation process thus results in a improved optical quality of the BS, thereby increasing fringe visibility and generating sharp PBG edges. These effects have been simulated by PBG calculation, where we can show that the PBG position is not


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shifting, but the reflectance increases as well as the visibility of the fringes, see Figures S7a-b. We speculate that the heating/activation process leads to a change of the polycrystalline structure of the HKUST-1 MOF or of the sputtered ITO layer toward a more dense/homogenous morphology. A more compact SURMOF or ITO thin film within the BS, results in a smaller amount of defect-scattering. The further optimization of the annealing procedure is expected to lead to a further improvement of the BS optical quality. Optical sorption and desorption studies can be described in a dynamic way. The modulation and shifting of the PBG (activated/dried and filled HKUST-1 pores) is in consistence with our prediction/simulation, see Figures S6a and S6b and Figures S7a and S7b in the Supporting Information. Differences between the experimental and calculated reflectance spectra (e.g. empty and filled pores of the respective HKUST-1 SURMOF thin film) can be explained by variations in the sputtered ITO and LPE sprayed HKUST-1 film thickness. Additional effects must also be taken into account, like the development of surface- and eventually interface roughness between adjacent layers, as well as scattering and reflection losses. Prior to the measurements, all experimental reflectance spectra were normalized to a reference, a clean Si-wafer under in-situ conditions in the glove-box under N2 atmosphere. CONCLUSION Here we demonstrate that the combination of a LPE spraying process yielding high quality SURMOFs with an ITO sputtering process is suited to fabricate high quality optical devices on a centimetre scale. In our hybrid systems the MOF-material, HKUST-1 or [Cu3(BTC)2], acts as the low RI index material and the sputtered ITO as the high RI index material. By modifying the individual layer thicknesses the resulting 1D SURMOF-hybrid PBG material can be colour tuned


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over the whole visible range. The PBG SURMOF-based hybrid multilayers presented here also provide a tuneable platform with regard to future chemo-responsive optical sensing devices. Our results demonstrate repeatable the loading and unloading of specific guest molecules inside the molecular framework material results in pronounced changes of the PBG optical properties: a pronounced reflection peak shift (≈ 20-40 nm depending on the organic molecule), which can be easily monitored by simple colorimetric methods or e.g. by the naked eye. This was further proofed by in situ AFM investigations which show that virtually no thickness change occurs during the sorption/desorption process. The 5 bilayer HKUST-1/ITO monolithic BS-System, mounted on a Si substrate features an outstanding optical quality with low scattering losses which is manifested by a reflectance of around ≈ 80% at the max. PBG wavelength. Our BSSystem comprising HKSUT-1 is a first example of a monolithic grown photonic-based SURMOF device and could be further extended to other (more water stable) HCP monolithic SURMOF systems. Due to their high optical quality, SURMOF from LPE processing are suitable on large scales and industrial device applications, and we foresee also the realisation of larger-scaled systems, thus providing the basis for the development of robust SURMOF-based, photonic color-coded chemical sensors and nanophotonic devices.

Experimental Section All chemicals used in this work are commercially available from different sources and suppliers. HKUST-1 were grown on pretreated/modified Si wafers (Si wafers (100) orientation, polished, thickness: 525 ± 20 µm, speciﬁc resistivity: 8−12 Ω/cm; from Silchem Handelsgesellschaft GmbH, DE). The Si wafers were treated by oxygen plasma (Diener Plasma; gas flow: 50 sccm, mixture: pure O2) for 30 min to remove impurities as well as increase the


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number of OH functional groups and the hydrophilicity on the Si surface before growing the SURMOFs. The HKUST-1 SURMOFs were epitaxially grown on the pretreated Si substrate using the LPE spray method described elsewhere,6 in which the metal-containing solution [1 mmol Cu(OAc)2] and the linker solution [0.1 mmol BTC] are sprayed subsequently on the substrate. The desired thickness can be adjusted using a number of distinct LPE spray cycles; e.g., 20 spray cycles for HKUST-1 results in a total thickness from LPE spraying of 80 nm, 35 spray cycles for HKUST-1 results in a total thickness of 120 nm, and 40 spray cycles for HKUST-1 results in a total thickness of 135 nm, for details see Figure S1b in the SI.

ITO (Indium Tin Oxide SnO2:In2O3 with 10wt% Sn) as the high Refractive Index (n) component was sputtered describing a top down approach from an ITO sputter substrate in a self-build UHV-sputtering chamber. ITO (Indium (10%) Tin (90%) Oxide, Kurt J Lesker, UK) as the high Refractive Index (n) component was deposited using a RF sputtering setup with a 3″ION’X planar magnetron source (TFC, Grafenberg, Germany) mounted on a standard double-cross recipient equipped with pre-sputter shutter and a sample positioner allowing for various working distances (150–200 mm) between magnetron and substrate.

Supporting Information This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author *Dr. Engelbert Redel


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Institute of Functional Interfaces (IFG), Karlsruhe Institute of Technology (KIT), Hermann-vonHelmholtz-Platz 1, 76344 Eggenstein-Leopoldshafen, Germany E-Mail: [email protected]

Funding Sources Karlsruhe Institute of Technology (KIT), Alexander von Humboldt (AvH) Foundation, Chinese Scholarship Council (CSC), Landesstiftung Baden-Württemberg. SPP 1362 of the German Research Foundation (DFG), KNMF Laboratory for Microscopy and Spectroscopy.

ACKNOWLEDGMENT E.R. thanks the Alexander von Humboldt (AvH) Foundation for a Feodor Lynen Postdoctoral Return-Fellowship as well as KIT and CMM for additional funding. J. L. and Z.W. thanks the Chinese Scholarship Council (CSC) for financial aid. E. R.; S.W. and H.G. acknowledge financial support from the Landesstiftung Baden-Württemberg. This work was funded within the priority program SPP 1362 of the German Research Foundation (DFG). The support from the KNMF Laboratory for Microscopy and Spectroscopy is acknowledged.


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The table of contents entry The fabrication of monolithic MOF-based 1D PBG hybrid materials is reported here. By using liquid-phase epitaxy (LPE) highly oriented, crystalline and continuous HKUST-1 thin films were epitaxial grown on intermediate indium tin oxide (ITO) layers with very high optical performance. These SURMOF-based 1D-PBG materials are color tunable over the whole visible range, we further demonstrate optical sensing capabilities employing the chemo-responsive optical responses. Such 1D-PBG monolithic SURMOF stacks provide the basis for the fabrication of upcoming photonic, color-coded chemical sensing devices. Jianxi Liu, Engelbert Redel*, Stefan Walheim, Zhengbang Wang, Vanessa Oberst, Jinxuan Liu, Stefan Heissler, Alexander Welle, Markus Moosmann, Torsten Scherer, Michael Bruns, Hartmut Gliemann, Christof Wöll

Monolithic high performance SURMOF Bragg Reflector for Optical Sensing


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