Surface-anchored Metal-organic Framework Research Articles

Impact of Surface Functionalization and Deposition Method on Cu-BDC surMOF Formation, Morphology, Crystallinity, and Stability.

For direct integration into device architectures, surface-anchored metal-organic framework (surMOF) thin films are attractive systems for a wide variety of electronic, photonic, sensing, and gas storage applications. This research systematically investigates the effect of deposition method and surface functionalization on the film formation of a copper paddle-wheel-based surMOF. Solution-phase layer-by-layer (LBL) immersion and LBL spray deposition methods are employed to deposit copper benzene-1,4-dicarboxylate (Cu-BDC) on gold substrates functionalized with carboxyl- and hydroxyl-terminated alkanethiol self-assembled monolayers (SAMs). A difference in crystal orientation is observed by atomic force microscopy and X-ray diffractometry based on surface functionalization for films deposited by the LBL immersion method but not for spray-deposited films. Cu-BDC crystallites with a strong preferred orientation perpendicular to the substrate were observed for the films deposited by the LBL immersion method on carboxyl-terminated SAMs. These crystals could be removed upon testing adhesive properties, whereas all other Cu-BDC surMOF film structures demonstrated excellent adhesive properties. Additionally, film stability upon exposure to water or heat was investigated. Ellipsometric data provide insight into film formation elucidating 7 and 14 Å average thicknesses per deposition cycle for films deposited by the immersion method on 11-mercapto-1-undecanol (MUD) and 16-mercaptohexadecanoic acid (MHDA), respectively. In contrast, the films deposited by the spray method are thicker with the same average thickness per deposition cycle (21 Å) for both SAMs. While the spray method takes less time to grow thicker films, it produces similar crystallite structures, regardless of the surface functionalization. This research is fundamental to understanding the impact of deposition method and surface functionalization on surMOF film growth and to provide strategies for the preparation of high-quality surMOFs.

Tunable J-type aggregation of silicon phthalocyanines in a surface-anchored metal-organic framework thin film.

Organic chromophores and semiconductors, like anthracene, pentacene, perylene, and porphyrin, are prone to aggregation, and their packing in the solid state is often hard to predict and difficult to control. As the condensed phase structures of these chromophores and semiconductors are of crucial importance for their optoelectronic functionality, strategies to control their assembly and provide new structural motifs are important. One such approach uses metal-organic frameworks (MOFs); the organic chromophore is converted into a linker and connected by metal ions or nodes. The spatial arrangement of the organic linkers can be well-defined in a MOF, and hence optoelectronic functions can be adjusted accordingly. We have used such a strategy to assemble a phthalocyanine chromophore and illustrated that the electronic inter-phthalocyanine coupling can be rationally tuned by introducing bulky side grounds to increase steric hindrance. We have designed new phthalocyanine linkers and using a layer-by-layer liquid-phase epitaxy strategy thin films of phthalocyanine-based MOFs have been fabricated and their photophysical properties explored. It was found that increasing the steric hindrance around the phthalocyanine reduced the effect of J-aggregation in the thin film structures.

Solution Atomic Layer Deposition of Smooth, Continuous, Crystalline Metal–Organic Framework Thin Films

For the first time, a procedure has been established for the growth of surface-anchored metal-organic framework (SURMOF) copper(II) benzene-1,4-dicarboxylate (Cu-BDC) thin films of thickness control with single molecule accuracy. For this, we exploit the novel method solution atomic layer deposition (sALD). The sALD growth rate has been determined at 4.5 Å per cycle. The compact and dense SURMOF films grown at room temperature by sALD possess a vastly superior film thickness uniformity than those deposited by conventional solution-based techniques, such as dipping and spraying while featuring clear crystallinity from 100 nm thickness. The highly controlled layer-by-layer growth mechanism of sALD proves crucial to prevent unwanted side reactions such as Ostwald ripening or detrimental island growth, ensuring continuous Cu-BDC film coverage. This successful demonstration of sALD-grown compact continuous Cu-BDC SURMOF films is a paradigm change and provides a key advancement enabling a multitude of applications that require continuous and ultrathin coatings while maintaining tight film thickness specifications, which were previously unattainable with conventional solution-based growth methods.

Charge. transport, conductivity and Seebeck coefficient in pristine and TCNQ loaded preferentially grown metal-organic framework films

This investigation on metal-organic framework (MOF) HUKUST-1 films focuses on comparing the undoped pristine state and with the case of doping by TCNQ infiltration of the MOF pore structure. We have determined the temperature dependent charge transport and p-type conductivity for HKUST-1 films. Furthermore, the electrical conductivity and the current–voltage characteristics have been characterized in detail. Because the most common forms of MOFs, bulk MOF powders, do not lend themselves easily to electrical characterization investigations, here in this study the electrical measurements were performed on dense, compact surface-anchored metal-organic framework (SURMOF) films. These monolithic, well-defined, and (001) preferentially oriented MOF thin films are grown using quasi-liquid phase epitaxy (LPE) on specially functionalized silicon or borosilicate glass substrates. In addition to the pristine SURMOF films also the effect of loading these porous thin films with TCNQ has been investigated. Positive charge carrier conduction and a strong anisotropy in electrical conduction was observed for highly oriented SURMOF films and corroborated with Seebeck coefficient measurements. Van der Pauw four-point Hall sample measurements provide important insight into the electrical behavior of such porous and hybrid organic–inorganic crystalline materials, which renders them attractive for potential use in microelectronic and optoelectronic devices and thermoelectric applications.

Broadband Dielectric Spectroscopic Detection of Ethanol: A Side-by-Side Comparison of ZnO and HKUST-1 MOFs as Sensing Media

The most common gas sensors are based on chemically induced changes in electrical resistivity and necessarily involve making imperfect electrical contacts to the sensing materials, which introduce errors into the measurements. We leverage thermal- and chemical-induced changes in microwave propagation characteristics (i.e., S-parameters) to compare ZnO and surface-anchored metal–organic-framework (HKUST-1 MOF) thin films as sensing materials for detecting ethanol vapor, a typical volatile organic compound (VOC), at low temperatures. We show that the microwave propagation technique can detect ethanol at relatively low temperatures (<100 °C), and afford new mechanistic insights that are inaccessible with the traditional dc-resistance-based measurements. In addition, the metrological technique avoids the inimical measurand distortions due to parasitic electrical effects inherent in the conductometric volatile organic compound detection.

Microwave Dielectric Spectroscopy Study of MOF Clathrates in Volatile Organic Compounds Detection

Metal-organic frameworks (MOFs) are porous defective crystalline compounds that can form clathrates. In principle, the defects in these materials behave like the defects in other metal oxides such as ZnO, which are used as metal-oxide semiconductor gas sensors. Traditional chemical resistivity based MOS gas sensors require electrical contact to the sensing materials. These contacts are imperfect and will subsequently introduce errors into the measurements. In this paper, we will demonstrate the feasibility of using contactless broadband dielectric spectroscopy (BDS)-based metrology in gas monitoring that avoids distortions in the reported resistivity values due to probe use, and parasitic errors (i.e., tool-measurand interactions). Specifically, we will show how radio frequency (RF) propagation characteristics can be applied to study discrete processes involved in HKUST-1 SURMOF (Surface Anchored Metal-Organic Framework) sensing and detection of gases and volatile organic compounds (VOCs). HKUST-1 MOF films, also known as Cu3(BTC)2 (BTC: benzenetricarboxylicacid), consist of metal ions connected by organic ligands, forming highly ordered porous structures and are commonly grown by a layer-by-layer (LBL) liquid phase epitaxy (LPE) technique. We intend to use BDS to investigate the initial oxidization of MOF gas sensing material in air and nitrogen at temperatures below 200 °C, and to show that the technique yields new mechanistic insights that are unobtainable with the conventional resistance-based measurements.

Investigation of Aging Effects in Thermoelectric Surface Anchored Metal-Organic-Framework (SURMOFs) Films Using Nanoindentation

Thin films of Metal-organic-framework (MOF) materials are deposited using a layer-by-layer (LBL) liquid phase epitaxy (LPE) technique. The highly oriented Surface Anchored Metal-Organic Framework (SURMOFs) of Cu3(BTC)2 (BTC: benzenetricarboxylicacid), known as HKUST-1 MOF were grown on Si (100) substrates. MOF materials are designed scaffold-like compounds that consist of metal ions connected by organic ligands, forming highly ordered porous structures. MOF materials found their way to various applications including gas storage due to high storage capacity and device applications in thermoelectrics and microelectronics [1,2]. The thermoelectric application potential of HKUST-1 SURMOF films is based on their large Seebeck coefficient and the premise that the electrical properties of the otherwise insulating MOF films can be custom tailored by loading the porous MOF structure with metal clusters. For modulating the electrical conductivity of MOF films the most effective method is to infiltrate guest molecules inside the porous framework to transport ions, and hence to modulate the electrical properties of the host framework. With nanoindentation testing of the fresh and aged pristine and TCNQ loaded samples, we demonstrate that aging effects impacted the mechanical properties of the films. The aged pristine samples hardness is 0.29 GPA as compared to the fresh as-grown pristine sample of 0.16 GPA at depth of indentation/film thickness (Similarly for the TCNQ loaded samples the hardness of the aged and fresh samples is 0.22 and 0.05 GPA respectively. As hybrid organic-inorganic thermoelectric materials HKUST-1 SURMOF films are subject to aging effects. References Mazure and A.J. Auberton-Herve, Proc. of 35th European Solid-State Device Research Conf. ESSDERC, p. 29 (2005)A. Langdo, M. T. Currie, Z.-Y. Cheng, J.G. Fiorenza, M. Erdtmann, G. Braithwaite, C.W. Leitz, C.J. Vineis, J. A. Carlin, A. Lochtefeld, M. T. Bulsara, I. Lauer, D.A. Antoniadis, M. Somerville, Solid-State Electronics, 48 , 1357 (2004). Figure 1

Analysis of Pristine and Tcnq-Loaded MOF FILMS By Contactless Microwave Dielectric Spectroscopy Technique

Metal-organic frameworks (MOFs) are porous and crystalline Coordination Network Compounds (CNCs), also referred to as porous coordination polymers (PCPs), which consist of inorganic metal (or metal/oxo) clusters connected by organic linkers. Surface Anchored Metal-organic-framework (SURMOF) films consist of quasi-liquid phase epitaxially grown highly ordered crystalline MOF films. For this study we used MOF Cu3(BTC)2 (BTC: benzene tricarboxylate) films, also known as HKUST-1 (Hong Kong University of Science and Technology-1), which were infiltrated with tetracyano-quinodimethane (TCNQ) guest molecules to modulate the charge transport characteristics. HKUST-1 MOF thin films have been grown using a liquid phase epitaxy (LPE) spray method in which the metal-containing solution [1 mmol Cu(OAc)2] and the linker solution [0.1 mmol BTC] are sprayed alternately on modified silicon substrates. Subsequently, the samples were immersed into an ethanoic TCNQ solution.We report on localized charge transfer processes at microwave frequencies of porous metal-organic framework (MOF) HKUST-1 Cu2(BTC)3 samples. As part of this study we will compare measurements done in the HKUST-1 pristine form of the material with open pores and HKUST-1, where the pores have been infiltrated with tetracyano-quinodimethane (TCNQ), resulting in a hybrid structure with enhanced electrical conductivity and a significant measurable effect in the microwave propagation characteristics [1]. We present data and analysis from an evaluation of pristine and loaded HKUST-1 SURMOF films using the contactless broadband microwave dielectric spectroscopy (BDS). The technique enables the accurate operando evaluation of the electrical and magnetic properties of materials without compromising the kinetic conditions of the experiment. The BDS method is uniquely suitable for investigating charge transfer reactions, regardless of the nature of the charge carriers, i.e., electrons and holes [2]. As such, these material state-sensitive parameters could be used to monitor charge transfer changes in the SURMOF and SURMOF-TCNQ hybrid analytes.[1] G. M. Espallargas and E. Coronado, “Magnetic functionalities in MOFs: from the framework to the pore”, Chem. Soc. Rev., 2018,47, 533, DOI: 10.1039/c7cs00653e[2] A. M. Wernbacher et al, Operando Electrical Conductivity and Complex Permittivity Study on Vanadia Oxidation Catalysts, J. Phys. Chem. C 2019, 123, 8005−8017, DOI: 10.1021/acs.jpcc.8b07417

(Keynote) Electromagnetic Field Stimulated MOFs

Electromagnetic, external stimuli can be used to switch Metal-Organic Framework (MOF) materials and to achieve control over the pores of these materials on different ways. We designed MOFs with switchable moieties in their side-chains, such as azobenzene, to be switchable under irradiation of UV/Vis light. Additionally, we developed devices to be able to measure the impact of electromagnetic fields in-situ. We focused on surface anchored MOF (SURMOF) thin films, deposited on asymmetric, porous α-Al2O3 supports as gas separation membrane layers. It is possible to control the gas transport properties of the materials using, but also using electric fields, and measure changes gas separation (permeability and selectivity) of binary mixtures in-situ, using the Wicke-Kallenbach technique. The light switchable systems were investigated on their CO2 separation performance: By switching the azobenzene between cis and trans isomers inside the pore windows, it was possible to switch dipolar moments, thus changing the adsorption properties towards the quadrupolar moment of CO2. Using electric fields of 500 V/mm we could show on the prominent example of ZIF-8, a zeolitic imidazolate framework, that it has a strong impact on the gas transport properties, especially looking at the molecular sieving. The mechanistic of the electric field influence on the ZIF is as follows: The electric field distorts the lattice of the MOF, shifting the Zn2+ ions, lowering the symmetry of the unit cell. This effect leads to polarized polymophs of the structure, sharpening the molecular sieving through a stiffening effect. The purification of Propylene/Propane, a highly desired separation for the plastics industry, could be increased through the electric field stimuli by 33%.References Knebel, A. et al. Defibrillation of soft porous metal-organic frameworks with electric fields. Science 358, 347–351; 10.1126/science.aal2456 (2017).Wang, Z. et al. Tunable molecular separation by nanoporous membranes. Nat. Commun. 7, 13872; 10.1038/ncomms13872 (2016).Müller, K. et al. Switching Thin Films of Azobenzene-Containing Metal-Organic Frameworks with Visible Light. Chem. Eur. J. 23, 5434–5438; 10.1002/chem.201700989 (2017).Knebel, A. et al. Azobenzene Guest Molecules as Light-Switchable CO 2 Valves in an Ultrathin UiO-67 Membrane. Chem. Mater. 29, 3111–3117; 10.1021/acs.chemmater.7b00147 (2017).Tan, N. Y. et al. Investigation of the terahertz vibrational modes of ZIF-8 and ZIF-90 with terahertz time-domain spectroscopy. Chem Commun 51, 16037–16040; 10.1039/c5cc06455d (2015). Figure 1

Exciton Coupling and Conformational Changes Impacting the Excited State Properties of Metal Organic Frameworks.

In recent years, the photophysical properties of crystalline metal-organic frameworks (MOFs) have become increasingly relevant for their potential application in light-emitting devices, photovoltaics, nonlinear optics and sensing. The availability of high-quality experimental data for such systems makes them ideally suited for a validation of quantum mechanical simulations, aiming at an in-depth atomistic understanding of photophysical phenomena. Here we present a computational DFT study of the absorption and emission characteristics of a Zn-based surface-anchored metal-organic framework (Zn-SURMOF-2) containing anthracenedibenzoic acid (ADB) as linker. Combining band-structure and cluster-based simulations on ADB chromophores in various conformations and aggregation states, we are able to provide a detailed explanation of the experimentally observed photophysical properties of Zn-ADB SURMOF-2: The unexpected (weak) red-shift of the absorption maxima upon incorporating ADB chromophores into SURMOF-2 can be explained by a combination of excitonic coupling effects with conformational changes of the chromophores already in their ground state. As far as the unusually large red-shift of the emission of Zn-ADB SURMOF-2 is concerned, based on our simulations, we attribute it to a modification of the exciton coupling compared to conventional H-aggregates, which results from a relative slip of the centers of neighboring chromophores upon incorporation in Zn-ADB SURMOF-2.

Intercalation of Porphyrin‐Based SURMOF in Layered Eu(III) Hydroxide: An Approach Toward Symbimetic Hybrid Materials

AbstractA strategy for rational design of synergetic hybrid materials exploiting stabilization of intercalated layered matrices via coordination bonding is described. A new hybrid material is assembled through subsequent intercalation of the surface‐anchored metal–organic framework (SURMOF) components, zinc acetate and 5,10,15,20‐tetrakis(4‐carboxyphenyl)‐porphyrin‐zinc(II) (ZnTCPP), into the layered europium(III) hydroxychloride (LEuH). The formation of the SURMOF clusters intercalated in LEuH is confirmed by X‐ray diffraction, FTIR and Raman spectroscopy, and BET nitrogen absorption methods. The catalytic function of the SURMOF/LEuH hybrid and its components in the model reaction of hydrolysis of bis(4‐nitrophenyl) phosphate in the acidic solution is studied by UV–vis and MALDI‐TOF spectroscopy. Both the non‐intercalated matrix and the MOF powder are inactive and unstable in the substrate solution. Unlike its components, the SURMOF/LEuH hybrid exhibits synergetic catalytic activity increasing with the amount of the intercalated compounds because of the mutual stabilization of the components through coordination interactions. The results provide a basis for symbimetic (mimicking the symbiotic behavior in biological systems) hybrid materials, in which stabilization of functional units in the intercalated structure translates into a synergy of useful properties.

Introducing electrical conductivity to metal-organic framework thin films by templated polymerization of methyl propiolate.

We herein present a case study on the templated, Pd-catalyzed polymerization reaction of methyl propiolate in the confined pore space of three different surface anchored metal-organic framework (SURMOF) systems in order to introduce electrical conductivity to MOF thin films and provide predictions for potential device integrations. To gain comprehensive insight into the influence of the template on polymerization, we chose Cu(bpdc), Cu2(bdc)2(dabco) and HKUST-1 because of their different types of pore channels, 1D, quasi-1D and 3D, and their free pore volumes. Well-defined MOF thin films were prepared using layer-by-layer deposition, which allows for the application of several characterization techniques not applicable for conventional powder MOFs. With SEM, AFM, XRD, MALDI-ToF/MS, ToF-SIMS and QCM, we were able to investigate the behaviour of the polymer formation. For lower dimensional pore channels, we find a depot-like release of monomeric units leading to top-layer formation determined by desorption kinetics, whereas for the 3D channels, quick release of an excess amount of monomers was observed and polymerization proceeds perfectly. Despite polymerization issues, control over the maximum chain lengths and the molecular weight distribution was achieved depending on the dimensionality of the pore systems. For the HKUST-1 system, polymerization was optimized and we were able to measure the electrical conductivity introduced by the conjugated polymer inside the channels.

Synthesis, Transfer, and Gas Separation Characteristics of MOF-Templated Polymer Membranes.

This paper discusses the potential of polymer networks, templated by crystalline metal–organic framework (MOF), as novel selective layer material in thin film composite membranes. The ability to create mechanically stable membranes with an ultra-thin selective layer of advanced polymer materials is highly desirable in membrane technology. Here, we describe a novel polymeric membrane, which is synthesized via the conversion of a surface anchored metal–organic framework (SURMOF) into a surface anchored gel (SURGEL). The SURGEL membranes combine the high variability in the building blocks and the possibility to control the network topology and membrane thickness of the SURMOF synthesis with high mechanical and chemical stability of polymers. Next to the material design, the transfer of membranes to suitable supports is also usually a challenging task, due to the fragile nature of the ultra-thin films. To overcome this issue, we utilized a porous support on top of the membrane, which is mechanically stable enough to allow for the easy membrane transfer from the synthesis substrate to the final membrane support. To demonstrate the potential for gas separation of the synthesized SURGEL membranes, as well as the suitability of the transfer method, we determined the permeance for eight gases with different kinetic diameters.

Metal-Organic Framework-Templated Biomaterials: Recent Progress in Synthesis, Functionalization, and Applications.

The integration of a porous crystalline framework with soft polymers to create novel biomaterials has tremendous potential yet remains very challenging to date. Metal-organic framework (MOF)-templated polymers (MTPs) have emerged as persistent modular materials that can be tailored for desired biofunctions. These represent a novel class of hierarchically structured assemblies that combine the advantages of MOFs (precisely controlled structure, enormous diversity in framework topology, and high porosity) with the intrinsic behaviors of polymers (soft texture, flexibility, biocompatibility, and improved stability under physiological conditions). Transformation of surface-anchored MOFs (SURMOFs) via orthogonal covalent cross-linking yields surface-anchored polymeric gels (SURGELs) that open up exciting new opportunities to create soft nanoporous materials. SURGELs overcome the main drawbacks of SURMOFs, such as their limited stability under physiological conditions and their potential to release toxic metal ions, a substantial problem for applications in life sciences. MOF (SURMOF)-templated polymerization processes control the synthesis on a molecular level. Additionally, the morphology of the original MOF crystal template is replicated in the final network polymers. The MOF-templated polymerization can be induced by light, a catalyst, or temperature using several types of reactions, including thiol-ene, metal-free alkyne-azide click reactions, and Glaser-Hay coupling. In the case of photoinduced reactions, the cross-linking process can be locally confined, allowing control of the macroscopic patterning of the resulting network polymer. The use of layer-by-layer (lbl) techniques in the SURMOF synthesis serves the purpose of precise, layer-selective incorporation of functionalities via the combination of the postsynthetic modification and heteroepitaxy strategies. Transforming the functionalized SURMOF into a SURGEL allows the fabrication of polymers with desired bioactive functions at the internal or external surfaces. This Account highlights our ongoing research and inspiring progress in transforming SURMOFs into persistent, modular nanoporous materials tailored with biofunctions. Using cell culture studies, we present various aspects of SURGEL materials, such as the ability to deliver bioactive molecules to adhering cells on SURGEL surfaces, applications to advanced drug delivery systems, the ability to tune cell adhesion via surface modification, and the development of porphyrin-based SURGEL thin films with antimicrobial properties. Then we critically examine the challenges and limitations of current systems and discuss future research directions and new approaches for advancing MOF-templated biocompatible materials, emphasizing the need to include responsive and adaptive functionalities into the system. We emphasize that the hierarchical structure, ranging from the molecular to the macroscopic scale, allows for optimization of the material properties across all length scales relevant for cell-material interactions.

Highly Efficient One-Dimensional Triplet Exciton Transport in a Palladium-Porphyrin-Based Surface-Anchored Metal-Organic Framework.

Efficient photon-harvesting materials require easy-to-deposit materials exhibiting good absorption and excited-state transport properties. We demonstrate an organic thin-film material system, a palladium-porphyrin-based surface-anchored metal-organic framework (SURMOF) thin film that meets these requirements. Systematic investigations using transient absorption spectroscopy confirm that triplets are very mobile within single crystalline domains; a detailed analysis reveals a triplet transfer rate on the order of 1010 s-1. The crystalline nature of the SURMOFs also allows a thorough theoretical analysis using the density functional theory. The theoretical results reveal that the intermolecular exciton transfer can be described by a Dexter electron exchange mechanism that is considerably enhanced by virtual charge-transfer exciton intermediates. On the basis of the photophysical results, we predict exciton diffusion lengths on the order of several micrometers in perfectly ordered, single-crystalline SURMOFs. In the presently available samples, strong interactions of excitons with domain boundaries present in these metal-organic thin films limit the diffusion length to the diameter of these two-dimensional grains, which amount to about 100 nm. Our results demonstrate high potential of SURMOFs for light-harvesting applications.

Influence of Tcnq Loading on the Mechanical Properties of Metal-Organic Framework Films

Bulk Metal-organic-framework (MOF) materials are designed scaffold-like compounds that consist of metal ions connected by organic ligands, forming highly ordered porous structures. Bulk MOFs yield typically powders with micron sized crystallites, which limit their applications. Many practical applications require instead suitably uniform MOF films. In recent years, efforts to grow highly oriented Surface Anchored Metal-Organic Framework (SURMOF) films of Cu3(BTC)2 (BTC: benzenetricarboxylic acid), known as HKUST-1 MOF, have been successful by utilizing a liquid phase epitaxy (LPE) spray method on surface functionalized gold coated silicon substrates. Originally MOFs were intended for gas storage due to high storage capacity inside the porous MOF bulk material. New potential device applications in thermoelectrics and microelectronics [1,2] were explored based on the premise that the electrical properties of the otherwise insulating MOF can be custom tailored by loading the porous MOF structure with metal clusters. For modulating the electrical conductivity of MOF films the most effective method is to infiltrate guest molecules inside the porous framework to transport ions, and hence to modulate the electrical properties of the host framework. Early work focused on the mechanical properties of pristine SURMOF films [3,4]. In this study, we have investigated the influence of loading pristine highly oriented MOF films with TCNQ on the mechanical properties by utilizing nanoindentation methods to measure the hardness and modulus of SURMOF films with and without TCNQ loading. Our findings reveal that undoped pristine SURMOF samples are relatively harder with an intrinsic hardness of ~ 1.6 GPa in the upper third of the MOF film. The hardness of the pristine SURMOF samples is steadily increasing as a function of the penetration depth. The hardness is approaching the substrate Si hardness as the indenter tip approaches the substrate. The measured modulus values of the pristine SURMOF samples are also steadily increasing as a function of the indentation depth. However, the TCNQ loaded SURMOF samples differ markedly and do not show a substrate effect. Interestingly the loaded SURMOF films are much softer and exhibit a constant hardness as a function of depth of ~ 0.5 GPa. As the indenter approaches the Si substrate there is no measurable change in the constant hardness and an absence of a substrate effect. The modulus values of TCNQ loaded SURMOF films is also significantly lower compared with the pristine sample and remains constant as a function of indenter penetration depth. Thus, loading the porous scaffolding of SURMOF films with TCNQ did not only alter the electrical conductivity but triggers marked changes in the mechanical properties, i.e., softening effect. This softening effect in TCNQ infiltrated MOF films can be accounted for by a widening of the lattice constant und the concomitant introduction of lattice defects that weaken the crystal and lead to a much lower hardness.

Seebeck Coefficient Measurements of Pristine and Tcnq Loaded HKUST-1 Thin Films

Seebeck meaurements have been performed on i) quasi-liquid epitaxially highly oriented HKUST-1 Surface Anchored Metal-Organic Framework (SURMOF) films and for comparison on ii) random polycrystalline HKUST-1 thin films; pristine as well as loaded with tetracyanoquinodimethane (TCNQ). However, the horizontal Seebeck coefficient of highly oriented (001) SURMOF films is practically around 0 µV/K as there is no measurable horizontal carrier transport parallel to the SURMOF surface. Only in the polycrystalline and pristine HKUST-1 thin films exhibit a high and positive Seebeck coefficient of 422.30 µV/K at 330 K. For the TCNQ loaded polycrystalline HKUST-1 thin films a lowering of the Seebeck coefficient of 388.80 µV/K at 330 K. The high Seebeck coefficients of these random polycrystalline MOF films demonstrates promising application potential of MOF films in future thermoelectric and electronic devices working highly efficient in the RT (Room Temperature) region. X. Chen, Z. Wang, Z. M. Hassan, P. Lin, K. Zhang, H. Baumgart, E. Redel ECS Journal of Solid State Science and Technology 2017 6(4), 1-4. Figure 1

Charge Transport, Conductivity, and Carrier Density in Tcnq Loaded MOF Films

Bulk Metal-organic-framework (MOF) films are designed scaffold-like compounds that consist of metal ions connected by organic ligands in the form of crystalline powders, while Surface Anchored Metal-organic-framework (SURMOF) films consist of quasi-liquid phase epitaxially grown highly ordered crystalline films. The ability to conduct electrical charge is one of the most important but least understood properties of MOF films and coordination network materials. Very little work exists on electrical characterization of MOF films, because the early synthesized pristine MOF films exhibited mostly insulating characteristics. Recently, it has been reported that the electrical properties of bulk host MOF films can be modulated by infiltrating guest molecules (metal clusters) inside the porous MOF framework. This renders MOF materials a novel and potentially promising material for microelectronic devices, sensors, and thermoelectrics. For this case study we used MOF Cu3(BTC)2 (BTC: benzene tricarboxylate) films, also known as HKUST-1 (Hong Kong University of Science and Technology-1), which were infiltrated with tetracyano-quinodimethane (TCNQ) guest molecules to modulate the charge transport characteristics. HKUST-1 MOF thin films have been grown using a liquid phase epitaxy (LPE) spray method in which the metal-containing solution [1 mmol Cu(OAc)2] and the linker solution [0.1 mmol BTC] are sprayed alternately on modified silicon substrates. Subsequently, the samples were immersed into an ethanoic TCNQ solution. For benchmarking we have investigated the electrical characteristics of pristine HKUST-1 and TCNQ loaded compact HKUST-1 SURMOF films by Hall-Effect measurements and Seebeck measurements. Both Hall Effect and Seebeck coefficient measurements indicate that electrical conduction in dense compact MOF films involves positive charge carriers. The Hall coefficient, mobility as well as the charge carrier density, the electrical conductivity and the sheet resistance for HKUST-1 MOF films will be discussed.

(Invited) Advances in Testing Inorganic & Hybrid Organic Thermoelectric Thin Films

In this work, a novel Si-based Lab-on-a-Chip test platform is introduced to perform in-plane ZT measurements of various classes of thermoelectric films with a complete set of thermoelectric test structures. The electrical conductivity σ, the Seebeck coefficient S, the thermal conductivity κ, and the power factor PF and the figure of merit ZT values can be experimentally determined in a single simultaneous measurement run as a function of temperature. It is shown that this Lab-on-a-Chip test system can be applied to classic inorganic thermoelectric films such as PbTe and PbSe and also to hybrid inorganic-organic films such as SURMOF films and as well as to conducting organic PEDOT:PSS films. Generally, the thermoelectric film can be deposited in the exposed test area of the chip by using a shadow mask. All the parameters can be measured in the temperature range of 300 K to 370 K with a 1 T magnet field. Applications are demonstrated for all three film material categories. Here, PbTe and PbSe thin films and double-layer PbTe/PbSe films were synthesized by Atomic Layer Deposition (ALD) technology directly on the pre-structured Si chip. A thin Al2O3 protection and diffusion barrier layer was applied to avoid oxidation and degradation of PbTe/PbSe films. The Seebeck coefficient, electrical conductivity and thermal conductivity for PbTe, PbSe and double-layer PbTe/PbSe film were measured simultaneously by applying in-pane ZT chips. Furthermore, this Lab-on-a-Chip test system can be extended to Surface Anchored Metal-organic-framework (SURMOF) films, which consist of quasi-liquid phase epitaxially grown highly ordered crystalline films. MOF films are designed scaffold-like compounds that consist of metal ions connected by organic ligands. Furthermore, the example of a complete thermoelectric characterization of a conductive organic polymer thin film material will be discussed.

Inkjet-Printed Photoluminescent Patterns of Aggregation-Induced-Emission Chromophores on Surface-Anchored Metal-Organic Frameworks.

Organic chromophores that exhibit aggregation-induced emission (AIE) are of interest for applications in displays, lighting, and sensing, because they can maintain efficient emission at high molecular concentrations in the solid state. Such advantages over conventional chromophores could allow thinner conversion layers of AIE chromophores to be realized, with benefits in terms of the efficiency of the optical outcoupling, thermal management, and response times. However, it is difficult to create large-area optical quality thin films of efficiently performing AIE chromophores. Here, we demonstrate that this can be achieved by using a surface-anchored metal-organic framework (SURMOF) thin film coating as a host substrate, into which the tetraphenylethylene (TPE)-based AIE chromophore can be printed. We demonstrate that the SURMOF constrains the AIE-chromophore molecular conformation, affording efficient performance even at low loading densities in the SURMOF. As the loading density of the AIE chromophore in the SURMOF is increased, its absorption and emission spectra are tuned due to increased interaction between AIE molecules, but the high photoluminescent quantum yield (PLQY = 50% for this AIE chromophore) is maintained. Lastly, we demonstrate that patterns of the AIE chromophore with 70 μm feature sizes can be easily created by inkjet printing onto the SURMOF substrate. These results foreshadow novel possibilities for the creation of patterned phosphor thin films utilizing AIE chromophores for display or lighting applications.