In recent years, functional electronic nanomaterials have made significant strides from advancements in the interplay of physics, chemistry, materials science, and computational research. However, synthetically tunable electronic materials are a long-standing, but elusive, technological goal. More recently, metal–organic frameworks (MOFs), a class of nanoporous, hybrid inorganic–organic crystalline solids, have garnered attention as a novel class of electronic nanomaterials. The aim of this perspective is to (i) highlight the charge transport behavior of recently discovered (2017–2019) electronic MOFs and (ii) recommend future directions for improvement of intrinsically and extrinsically conductive MOFs for MOF-based electronics.
Although innovations in physicochemical, nanomaterials, and computational research have enabled the development of functional electronic nanomaterials, achieving synthetically tunable electronic nanomaterials is a challenging technological goal. Inorganic semiconductors have outstanding properties, in part, due to the long-range order achievable in crystalline materials, but limited synthetic flexibility. Alternatively, organic polymers offer chemical tunability and low-cost fabrication techniques such as roll-to-roll processing, but they have poor mobility due in part to disorder, and there are long-term stability issues. While these materials dominate the landscape of the modern electronics industry, the future of innovation in electronics relies on the availability of novel hybrid materials and their processing and fabrication into devices.
Recently, metal–organic frameworks (MOFs), a class of nanoporous, hybrid inorganic–organic crystalline solids, have garnered attention as a novel class of electronic nanomaterials.1,2 MOFs are composed of metal ions linked to multitopic organic linkers by coordination bonds. They self-assemble under mild conditions to form crystalline materials with stable nanoporosity. MOFs combine the advantages of the long-range order found in inorganic conductors and the synthetic tunability of organic semiconductors. Unlike other nanomaterials, the unparalleled synthetic and chemical tunability of MOFs enables the formation of various pore sizes, guest–host interactions, and other physicochemical behavior. This high degree of modularity opens doors to potential applications in electronic devices for electrocatalysis,3–8 environmental sensors,9–15 high performance electrodes,16 and tunable electronics15,17–23 (Fig. 1). Furthermore, MOFs can be grown on surfaces by straightforward chemical methods, such as layer-by-layer deposition,24 Langmuir–Blodgett methods,25 and air–liquid interface methods.26
However, two challenges hinder the poor electrical conductivity in MOFs.27–30 In fact, only a handful of intrinsically conducting MOFs are known.1,25,27,31–36 Sheberla et al. reported a MOF with a graphene-like structure Ni3(2,3,6,7,10,11-hexaiminotriphenylene)2 [Ni3(HITP)2], which currently holds the record for MOF ohmic conductivity at 40 S cm−1.36 First, most MOFs lack building blocks to facilitate charge transport. The criteria include metal ions or clusters that contain easily accessible electrons and holes and organic ligands that are redox-active yet form stable radicals. In most MOFs, there is ineffective communication between redox-active ligands since σ-bonded metal cluster nodes prevent through-bond charge transport. Second, the porosity of MOFs further impedes charge transport. The spatial distances between ligands limit the possibility of π-stacking interactions, and the large size of the ligands prevents metal-to-metal charge transfer. While porosity often leads to low mobility of free charge carriers and thus low electrical conductivity, porosity also enables the introduction of guest molecules into the framework, a mechanism responsible for a majority of interesting MOF behaviors. To take full advantage of the potential of MOFs, scientists must design structures that delicately balance the conductivity and porosity of these nanomaterials.37–41
While MOFs may not replace conventional semiconductors, the fundamental understanding we gain from conductive MOFs may enhance improvements in the semiconductor industry. Importantly, this advancement of knowledge from conductive MOF platforms can expand the set of tools available to the semiconductor industry. Therefore, the aim of this perspective is to highlight the charge transport behavior of intrinsically and extrinsically conductive MOFs from the last two years (2017–2019). Each category of MOFs is further analyzed according to the type of charge transport—through-bond or through-space. In each discussion, we will focus on the structure–property relationships and the interplay between concentration and mobility of charges with electrical conductivity. We will also discuss novel processing and fabrication techniques, where applicable. Excluded from this perspective are work on proton or ionic conductivity and in-depth discussions of work from prior to 2017, which are highlighted elsewhere.27,37,42
B. Charge transport fundamentals
Contextualizing MOFs in terms of molecular charge transport and solid state band theory serves to inform design principles that can be used to build electronically conductive MOFs. Specifically, while traditional molecular principles describe the local chemistry of the metal environment, the extended crystal structure of MOFs can be readily described by principles adapted from solid state band theory.41 Here, we will briefly outline the main principles of charge carrier density, charge mobility, through-bond transport, and through-space transport, and how they influence the electrical conductivity of MOFs. Thorough descriptions of molecular charge transfer and solid state band theory are outlined elsewhere.27,37–41,43,44
Optimizing electrical conductivity (σ) depends on maximizing charge carrier density (n) and charge mobility (μ). Carriers can be either electrons (e) or holes (h), and their mobility depends on the energy levels available throughout a lattice. This basic relationship is mathematically described in the following equation:
Equation (1) fails to describe factors in increasing charge carrier density and mobility, since this is a complex notion that can be approached via various energetic arguments. At its core, charge carrier density is the result of an excess of electrons in relatively high energy states or an excess of accessible relatively low energy states (holes). These states arise from the chemical bonds formed in the material. In a simple inorganic solid, such as doped silicon, the presence of charge carriers can be perceived as a mismatch in electronic valency. However, with molecular solids such as MOFs, the available states must be rationalized with more complex molecular orbital theory. Charge mobility modulates the electron drift velocity and is the result of the charge carrier response to an external potential. This response is closely tied to the energy states at various positions in the lattice. In a simple inorganic solid, these states are largely the result of lattice geometry, but for a complex and nanoporous molecular solid such as a MOF, these states must be rationalized in the context of the molecular orbitals, which form in a unit cell of the lattice. For a charge to propagate through a molecular solid, there must be overlapping orbitals, i.e., energetically similar states that are in close spatial proximity. To summarize, charge carrier density is the result of available electronic states, and charge mobility is the result of the energetic compatibility of spatially adjacent electronic states. More specifically, if there is a significant energetic minimum somewhere in the lattice, electrons will be unlikely to leave the minimum. This is why in an ionic solid such as NaCl, in which there is a large energy difference between the highest occupied molecular orbitals and the lowest unoccupied orbitals of alternating adjacent atoms, there is often low electronic conductivity.
Solid state band theory is an expansion of molecular orbital theory. In band theory, when multiple identical atoms or molecules are arranged in close proximity, their orbitals overlap. Like in a molecular bond, a solid state congregation is the result of compatible orbitals overlapping. However, unlike a solitary molecule, in a solid, there are no discrete energies; instead, bands of continuous states are formed. However, while there are continuous bands, there may also be bands of quantum mechanically forbidden energy states called bandgaps. The major implications of the band structure arise in the charge density of the material and its correlation to activation energy (Ea). Ea is the energy difference between the Fermi level (EF) and that of the valence band maximum (VBM) or the conduction band minimum (CBM),
To maximize the charge density, minimizing Ea is favorable, as shown by using the Arrhenius equation,
where n0 is a prefactor, k is the Boltzmann constant, and T is absolute temperature. For intrinsic (undoped) semiconductors, EF is located halfway between the conduction band (CB) and the valence band (VB). In other words, minimizing the bandgap (Eg), or the difference between the CB and the VB, is favorable by redox matching components of semiconductors. On the other hand, in extrinsic (doped) semiconductors, the dopant shifts the EF toward the band edge of either the VB or CB, yielding p- or n-type semiconductors, respectively. Overall, minimizing Eg and Ea maximizes the electrical conductivity.
To monitor the efficiency of charge transport, charge mobility must also be evaluated. While both hopping transport and band transport modes occur in MOFs, the goal to optimize the latter charge transport mode is ideal. Optimizing charge mobility requires maximizing charge carrier delocalization and minimizing effective mass (m*),
where e is the elemental charge and τ is the mean time between two charge-scattering events. Charge-scattering sites, such as disorder, defects, impurities, or grain boundaries, can minimize τ and ultimately μ. Minimizing effective mass is correlated with a good spatial and energetic overlap between orbitals of appropriate symmetry.
Therefore, the major alterations in the MOF design to engender conductivity are (i) increasing charge carrier concentration via intrinsic chemical composition, chemical doping, or redox doping, and (ii) increasing charge mobility via structural moieties or guest molecules, promoting through-bond or through-space charge transport. These will be discussed in Secs. II and III.
C. Charge transport mechanisms in MOFs
Both hopping and band transport pathways are lacking in many MOFs. To understand the conductivity of these materials, chemists have applied two charge transport pathways in MOFs—through-bond and through-space (Fig. 2).
1. Through-bond charge transport
In the through-bond conduction mechanism, charge transport is favored when there is a spatial and energetic overlap of metal and ligand orbitals involved in covalent bonding. Charges move through continuous chains of covalent and coordination bonds in the material. This has been observed in MOFs with redox-active linkers and metal ions,34,45 in addition to 2D MOFs with extended π-conjugation.36 There is also, typically, a temperature dependence,46 but this correlation varies depending on the strength of coupling among the components of the material. For example, through-bond tunneling, which is an uncommon phenomenon in MOFs, is characterized by weak temperature dependence.47 Guest molecules, if strongly coordinated to the MOF framework, also serve as another example of through-bond charge transport, which exhibits low temperature dependence.1,15,21
2. Through-space charge transport
In the through-space conduction mechanism, charge transport occurs through noncovalent interactions (i.e., π–π stacking).48 In MOFs, this mode of charge conduction is observed between redox-active molecular fragments fixed by metal–ligand bonds with interplanar distances of less than 3.5 Å.49 This was first observed in a library of isostructural 3D helical columnar frameworks, in which the shorter interligand stacking distance resulted in a better orbital overlap of the ligands and, therefore, two orders of magnitude higher room temperature conductivity.50
D. Potential solutions
To circumvent the two challenges (e.g., lack of charge-facilitating building blocks and presence of porosity in MOFs), two main strategies have been implemented. First, since 2009, chemists have designed intrinsically conductive architectures via synthesizing MOFs with electroactive ligands or metal ions in specific topologies. This enables long-range charge delocalization (i.e., resonance) through space (π–π stacking). Second, extrinsically conductive MOFs have been achieved by the introduction of guest molecules. This strategy activates long-range charge delocalization through bonds, through space, or through redox behavior of the electroactive building blocks.
II. MOFs WITH INTRINSIC CONDUCTIVITY
In the context of this review, we will define intrinsically conductive MOFs as MOFs that exhibit electrical conductivity in the absence of charge transfer pathways or charge carriers made accessible via guest molecules or other external sources. Note that we are not considering the introduction of free charge carriers via electrochemical redox doping as an external source.
MOFs with intrinsic conductivity, such as all MOFs, can conceivably be engineered via all the principles laid out in Sec. I D. Section II specifically outlines two strategies that have successfully produced intrinsically conductive MOFs: (i) 2D MOFs with an extended π network of free charge carriers exhibiting through-bond charge transport and (ii) 3D MOFs in which metal ions provide free charge carriers exhibiting through-bond charge transport. While these systems were successful fundamentally, their behavior indicates significant hurdles for using intrinsically conductive MOFs in electronics. Both systems exhibit widely varying conductivity based on environmental contamination and structural defects. This variation shows the incredible tunability of these materials, and also the strict degree of control engineers must exercise when using them for electronics applications.
A. In-plane charge transport in 2D MOFs
Two-dimensional MOFs are generally built from electron-rich linkers and square planar metal ions. When the electron-rich linkers are oxidized, a large number of charge carriers delocalize through the bonds. The hexagonal lattices and stacking orientation of these MOFs enable through-bond charge transport. The stacking conformation in fully eclipsed, staggered, or slipped-parallel modes varies depending on the ligand composition. While oxygen- and nitrogen-based ligands generally form eclipsed and slipped-parallel conformations, sulfur-based ligands often yield eclipsed layers. Recently, Pathak et al. demonstrated high electrical conductivity with a metal–sulfur plane integrated within a MOF.51
While several examples of 2D MOFs have been investigated from 2012 to 2015,26,35,36,52–56 the first one to exhibit metallic behavior at low temperatures was developed by the Marinescu group in 2017. They synthesized and characterized a cobalt-based dithiolene MOF ([Co3(THT)2]3−) with square planar cobalt centers bridged by 2,3,6,7,10,11-triphenylenehexathiolate (THT) [Fig. 3(a)].57 The 2D structure contains a conjugated π orbital system across the organic linker and a high orbital overlap between the d orbitals of cobalt with the π system of the organic linker, resulting in long-range charge delocalization. Interestingly, this framework transitions from exhibiting semiconducting to metallic behavior upon cooling to below 150 K. Based on calculations, a monolayer of this material should be metallic. Semiconductive characteristics arise at higher temperatures from the presence of planar defects, in contrast to the metallic behavior of a defect-free material. Recently, the same group showed that an iron analog of the MOF exhibited similar room temperature metallic conductivity induced by oxidation.58
There are significant changes in the conductivity of both bulk material and thin films of [Co3(THT)2]3− under varying temperature, film thickness, and degree of solvation. Generally, both the pressed pellets and films of [Co3(THT)2]3− exhibited a form of second order temperature-dependent resistivity. There are striking variations in the pressed pellet resistivity, with one pellet showing a maximum resistivity of ∼22.5 MΩ cm, while thinner films exhibit 105× less resistivity at the same temperatures [Fig. 3(b), yellow scaled down 105×]. Both thinner and desolvated films exhibit metallic behavior at higher temperatures [Fig. 3(c)]. Furthermore, PXRD data show some minimal structural changes in the material at various temperatures, implying that this transition is not due to a change in the crystal structure. These results suggest that the behavior of this material is highly dependent on sample quality and environmental conditions.
Further work conducted by the same group on an analogous material, FeTHT, shows a similar environmental dependence.58 FeTHT notably exhibited its transition from metallic to semiconducting at higher temperatures as the material was oxidized by ambient conditions. The results published by Dinca et al. have also demonstrated the significant role of anisotropy in the metallic behavior of single crystal 2D MOFs.59,60
From a practical standpoint, this variation is extremely problematic when it comes to fabricating these materials into devices. On the other hand, this variation shows the promise of these materials in sensing or other stimuli dependent electronics applications. In order to take advantage of these promising 2D MOFs, methods to reliably control the ordering of layers and film thickness must be developed.
B. Conducting 3D MOFs based on metal ions providing mobile charge carriers
In 2018, the Long group oxidatively doped an iron-based MOF, which consequently exhibited nearly a billion fold increase without a significant structural change.61 They enabled through-bond charge delocalization by coordinating metal ions to π-conjugated organic ligands. This is in contrast to σ-bonded metal clusters, which prevent electronic communication. [Fe(tri)2] has a caged diamondoid structure and is composed of octahedral Fe(II) nodes stabilized by 1,2,3-triazolate (tri) linkers (Fig. 4).
As expected, the pure valence closed shell state of Fe(II) shows extremely low conductivity on the order of 10−12 S/m. However, the conductivity increases by nine orders of magnitude upon oxidation. In the mixed valence form, the nitrogen of the triazolate linkers acts as π acceptors, facilitating charge transfer through the iron node network. The chemical oxidative doping of the MOF (Scheme 1) is accompanied by the transfer of ions into the caged MOF pores, which then yields Fe(tri)2(BF4)x.
One of the major challenges when working with MOFs is their high propensity for defect formation or chemical alteration. Long and co-workers cautiously minimized unintentional or partial oxidation of the MOF during the synthesis, processing, and electrical testing of Fe(tri)2.61 Even in a highly stabilized closed shell valence state, the iron nodes of the MOF can be oxidized while still retaining crystallinity, porosity, and color. Oxidation of even 2.5% of the iron sites of Fe(tri)2 can yield differences in conductivity values. While MOFs offer an ideal platform for understanding complex charge transfer phenomena, it is important to consider how defects affect properties of MOFs.
In the same year, the Long group also reductively doped an iron-based MOF, leading to a 10 000-fold increase in conductivity throughout the reduction without a significant structural change.62 The Fe2(BDP)3 framework is comprised of iron atoms bridged by 1,4-benzenedipyrazolate (BPD) organic linkers (Fig. 5).
Fe2(BDP)3 exhibits conductivity along the iron chains of the MOF. The pyrazolate unit acts as a π-acceptor, facilitating charge transport along the iron chain. Reductive doping of the material can greatly enhance conductivity by five orders of magnitude. The reductive doping of Fe2(BDP) is accompanied by the transfer of potassium counter ions into the MOF pores, yielding KxFe2(BDP), where 0 < x < 2.
The authors used an innovative approach in making single crystal MOF field effect transistors. Using a dilute microcrystalline solution of Fe2(BDP)3 to prevent aggregation, they dropcasted onto pre-patterned electrodes. Using a focused ion beam in the SEM, graphite was deposited to improve the connection between the Fe2(BDP)3 single crystal and the patterned electrodes [Fig. 6(a)]. van der Pauw measurements were performed on an undoped Fe2(BDP)3 MOF FET. The device showed temperature dependent conductivity, typical of a semiconductor. Two point DC electrical measurements were performed on reductively doped MOF FETs [Fig. 6(b)]. The results indicate the source/drain current to be variable with the degree of reductive doping, reaching a maximum when with K.98Fe2(BDP)3 as 0 < x < 2.
Similar work was also performed by Dinca et al. on another iron based MOF. This MOF differed from those mentioned above only by a slight difference in the organic linker. Yet, through redox doping, the conductivity of the MOF varied over 5 orders of magnitude and reached the largest known value for a 3D MOF of 1 S/m.63
III. MOFs WITH EXTRINSIC CONDUCTIVITY
As evidenced by the previous examples, slight changes in the structure and compositions of intrinsically conductive MOFs lead to a dramatic change in their conductivity. To tune and enhance the conductivity of MOFs, MOF chemists have instead post-synthetically modified MOFs with guest molecules to tune their electrical conductivity. These extrinsically conductive MOFs exhibit through-bond charge transport of coordinated cross-linking guests and solvent-induced conductivity. Examples of each strategy is given as follows.
A. Through-bond charge transport of coordinated cross-linking guests
In 2017, the Hupp group demonstrated the electrical conductivity of a robust zirconium-based MOF (NU-1000) post-synthetically modified with conjugated polymers. They incorporated pentathiophene oligomers onto the zirconium nodes of NU-1000 via solvent-assisted ligand incorporation (SALI).64 The anchored oligomers then undergo oxidative electropolymerization to create pore-immobilized polythiophene strands. These polymers anchored on the inner surface of the 31 Å diameter channels of NU-1000 and created a possible conduction pathway in the composite structure [Fig. 7(a)]. Little to no additional changes in the MOF porosity occurred as a result of polythiophene anchoring. The BET surface area of nearly 1600 m2 g−1 for the composite MOF material is greater than that observed for other MOF materials and on a par with many conductive carbon materials. The resulting composite displayed an electronic conductivity of up to 1.3 × 10−7 S cm−1 [Fig. 7(b)]. Other conductive MOF-based composites and MOF-derived materials have since been demonstrated with NU-1000.65,66 The same group also reported that an NU-1000 analog, NU-901, can be rendered electronically conductive by physically encapsulating C60, an excellent electron acceptor.66
In 2017, Shiozawa and co-workers demonstrated that a honeycomb framework, Co-MOF-74, doped with tetracyanochinodimethane (TCNQ) enhances its electronic properties.15 This was similar to the work of Allendorf and co-workers, who observed million fold improvement in the conductivity of HKUST-1 upon loading with TCNQ.1,21 Upon infiltrating Co-MOF-74 with TCNQ, the charge transfer between the guest molecule and the host framework led to a 1.5 eV decrease in the optical bandgap and improved electrical transport.
The proposed electrical conduction pathway occurs through the conjugate π orbital of TCNQ bridging the Co nodes in the framework, similar to that observed for HKUST-1 infiltrated with TCNQ.1,21 The MOF yielded temperature-dependent conductivity with a low activation energy of 0.24 eV, but due to the low conductivity values, the authors suggest that the structure of Co-MOF-74 may not be forming a continuous conductive pathway (Fig. 8). However, more theoretical and experimental work is necessary to elucidate the charge transport pathway of this material. Unlike TCNQ, H4TCNQ lacks π-bonds, and introducing them into the framework may underscore the importance of the conjugated π orbitals of TCNQ with the dimeric copper subunits in the framework. It is also unclear whether the authors utilized an inert atmosphere when synthesizing and post-synthetically modifying the MOFs, since exposure to water and air affects the performance of the material. Conducting experiments in a dry environment is important in minimizing defect formation in the MOFs.
B. Solvent-induced MOF conductivity
In 2018, Sun et al. demonstrated the possibility of reversible solvent-induced changes in the structure and electrical conductivity. These changes are due to the coordination and release of solvent molecules in inner-sphere changes at the open metal centers in Fe2(DSBDC), where DSBDC4− is 2,5-disulfidobenzene-1,4-dicarboxylate. The room-temperature electrical conductivity increased by three orders of magnitude with the coordination of N,N-dimethylformamide (DMF) to the open metal sites.32 Although there is distortion associated with the DMF solvent exchange and evacuation, it does not affect the connectivity of the frameworks; the distortions caused are possibly reversible (Fig. 9).67
With Fe2(DSBDC), the major charge transport mechanism is believed to be hole hopping between sulfur and iron within chains. Specifically looking into the DMF soaked Fe2(DSBDC), it is concluded that the solvent does not affect charge mobility but instead modulates charge density in the skeleton of the framework. The charge density increased and electrical conductivity improved due to the electron transfer, generating holes as charge carriers. The likely electron transfer is from the iron centers to the bound DMF molecules; these results stress the importance of redox-matching guest molecules to the framework. Investigating other metal centers bound to the DMF molecules will help elucidate the charge transport pathway between metal centers and solvent molecules.
IV. FUTURE OUTLOOK
Understanding the puzzling complexity of charge transport pathways in MOFs will unlock our ability to enhance the conductivity of MOFs. Although endowed with beneficial properties, MOFs contain flaws related to their building blocks and porosity that must be overcome before incorporating them into electronics. To overcome these material limitations of MOFs, we may find solutions in existing coordination polymers exhibiting high electronic conductivity.68,69 On the processing side, methods still need to be optimized to make ultrathin, homogeneous, and low roughness coatings. Developing novel composites and improving processing techniques present different sets of ongoing challenges.
First, we need a deeper fundamental understanding of the effect of defects in MOFs on charge transport. For 2D conductive MOFs, it is critical to control the presence of planar defects, which affect the anisotropy of their conductivity. density functional theory (DFT) studies of a 2D MOF [Ni3(HITP)2] suggest that these frameworks are metallic in bulk but semiconducting as a monolayer.70 Foster et al. proposed pillaring the 2D sheets of another analog, Cr3(HITP)2, with a bipyridine spacer (Fig. 10).71 The resulting 3D material minimizes interactions between the planes of the 2D Ni3(HITP)2, thereby favoring the semiconducting behavior. For guest-induced conductive MOFs such as HKUST-1, Co-MOF-74, and NU-1000, a more detailed investigation of the short-range and long-range defect structures (grain boundaries) is important to better understand the origin and details of charge-transport mechanisms.72–74 In principle, the crystalline nature of the parent material frameworks enables more rigorous theoretical investigations and searching for the compositional and structural conditions needed for the development and identification of promising candidates for MOF-based electronics.75
Second, we need improved standardization of experimental and measurement techniques. Before executing electrical conductivity measurements, a critical concern is the physical form in which the MOF sample is measured. Different forms, such as pressed pellets, polycrystalline films, single-domain films, and single crystals, naturally yield varying conductivity values. To evaluate materials, it is critical to confirm and specify what morphologies are of interest. Thus far, the majority of conductivity measurements have been performed on pressed pellets of MOFs, which are prepared by compressing a powder under high pressure. For extrinsically conductive MOFs, such as Co-MOF-74 and Fe2(DSBDC), we need improved reporting of infiltration methods, concentrations, phase (i.e., liquid or vapor), solvent type (i.e., non-coordinating or coordinating with dopants), washing protocol, timing, activation method, purity of dopant, and processing in inert or ambient atmosphere. Standardization of measurement techniques and conditions for electrical measurements, such as light, temperature, humidity, and types of contacts between electrodes and samples, must be elucidated. Since MOF crystals grow in varying sizes and shapes, preparing for and performing electrical measurements on the MOFs becomes difficult. If MOFs are made into pellets, the pressure applied and the method of pellet formation must be reported. Two point probe or van der Pauw measurements are the most common ways of measuring the electrical conductivity of MOFs in a pressed pellet or polycrystalline thin films. To circumvent contact issues, flash-photolysis time-resolved microwave conductivity (FP-TRMC) measurements can be made without the need for electrodes.62,76,77
Third, incorporation of MOFs into devices must be improved. While the deposition of MOFs onto surfaces has been extensively explored,24–26,78–83 inkjet printing nanoscaled MOFs into films has not. While the synthesis and fabrication of nanoscaled MOFs is well-established, coating them onto surfaces is unknown. One potential technique is inkjet printing nanoscaled MOFs into coatings. Synthesis of MOF particle sizes of <200 nm and verification of this via SEM and PXRD must be standardized. Controlling reaction time, concentration, and selection of metal precursors is critical in determining the synthesis, particle size, and reaction kinetics, respectively, of the nanoscaled MOFs. Altogether, these synthetic variables dictate the formation of nanoscaled MOFs and ultimately affect their successful incorporation into devices.
Since the late 2000s, the emergence of MOFs as charge transport materials has advanced in strides due to over 80 seminal experimental and theoretical studies focused on the notion of MOF-based electronic schemes. The same momentum is pushing this field forward a decade later. MOFs have much wider chemical and structural modularity, enabling new and exciting possibilities for controlling the method by which they transport charges. However, despite their popularity, there is much room for improvement in understanding the influence of defects on electrical conductivity, standardization of measurement and experimental techniques, and incorporation of MOFs into devices. We suspect that a continued collaborative effort between synthetic materials and computational chemists will lead to the optimization of conductive MOFs. Only then can MOF-based electronics improve our fundamental understanding in charge transport.
J.J.C., S.M.A., and M.C.S. acknowledge support from the U.S. Department of Energy, Office of Science, Office of Workforce Development for Teachers and Scientists (WDTS) under the Visiting Faculty Program (VFP).