Charge-transfer interface of insulating metal-organic frameworks with metallic conduction

Abstract

Downsizing materials into hetero-structured thin film configurations is an important avenue to capture various interfacial phenomena. Metallic conduction at the interfaces of insulating transition metal oxides and organic molecules are notable examples, though, it remained elusive in the domain of coordination polymers including metal-organic frameworks (MOFs). MOFs are comprised of metal centers connected to organic linkers with an extended coordination geometry and potential void space. Poor orbitals overlap often makes these crystalline solids electrical insulators. Herein, we have fabricated hetero-structured thin film of a Mott and a band insulating MOFs via layer-by-layer method. Electrical transport measurements across the thin film evidenced an interfacial metallic conduction. The origin of such an unusual observation was understood by the first-principles density functional theory calculations; specifically, Bader charge analysis revealed significant accumulation and percolation of charge across the interface. We anticipate similar interfacial effects in other rationally designed hetero-structured thin films of MOFs.

Physical vapor deposition of an oriented metal–organic framework HKUST-1 thin film on an insulating substrate

“…43 Additionally, at the interface of the band insulator Cu(II)-BPyDC (BPyDC = 2,2 0 -bipyridene-4,4 0 -dicarboxylate) and Mott insulator Cu(I)-TCNQ (TCNQ = 7,7,8,8-tetracyanoquinodimethane), metal conduction occurs through charge transfer. 44 Because the proposed method enables the direct growth of MOFs on substrates with a uniform interface without contamination, this study paves the way for a detailed investigation of the structure, composition, and electronic states of interfaces.…”

Insulator-to-metal-like transition in thin films of a biological metal-organic framework

“…The energy-optimized structure was subjected to Rietveld refinement of the XRD pattern of the Cu(Cys) 2 thin film 45 . Preferred orientation was determined by looking at the XRD peak intensities and was applied during the Rietveld refinement 46 , whereby a reasonably fair match between the experimental and calculated XRD patterns was achieved (Supplementary Fig. 12 ).…”

“…Later on, upon increasing the temperature from 300 to 400 K, both in-plane and cross-plane modes, we realized a significant change in the resistance value for the thin film of Cu-TCNQ; and, interestingly, no change in the resistance value for the thin film of Cu-BTC (BTC = 1,3,5-benzenetricarboxylate) 50 . Therefore, it is the intrinsic property of the material, for example, Mott-type (Cu-TCNQ) or band-type (Cu-BTC) insulator 46 , 51 , which played a more important role than the influence of the interfaces. In the present study, successful capture of the IMLT for the Cu(Cys) 2 thin film (thickness ~1.2 μm) in both cross-plane and in-plane modes (consistent change in the resistance by 10 10 fold) clearly endorses that it is not primarily an interface effect.…”

“…23 ) and partial charge density plots (Fig. 5 ) were carefully examined and Bader charge analysis 39 , 46 , 53 was carried out to understand the distribution of charge on various atoms at different temperatures which showed that S atom released and Cu atom accumulated charges at higher temperatures (Supplementary Table 1 ) so that the respective electronic band structures were affected by the overall charge re-distribution across the high-symmetry points (Supplementary Fig. 24 ).…”

Thin Films of MOF-on-Guest@MOF: A Simple Strategy of Designing Electronic Heterostructures

“…The LbL method has also been realized to be an efficient approach for generating various heterostructured thin films of MOFs . However, the challenge remained in rationally designing a heterostructure with a well-defined electronic interface which could have a profound impact in electronic device applications. , In the fabrication of heterostructured thin films, lattice-matching is one of the critical factors which can minimize the defects at the interface, leading to desirable electrical transport properties. Keeping this in mind, what happens if a thin film system is developed by depositing pristine MOF layers on top of molecularly doped MOF layers whereby lattice-mismatch can be avoided?…”