Perspective on the Interfacial Reduction Reaction

Ag Nanoparticles-Induced Metallic Conductivity in Thin Films of 2D Metal-Organic Framework Cu3(HHTP)2

Two-dimensional (2D) metal-organic frameworks (MOFs) are usually associated with higher electrical conductivity and charge carrier mobility when compared with 3D MOFs. However, attaining metallic conduction in such systems through synthetic or postsynthetic modifications is extremely challenging. Herein, we present the fabrication of thin films of a 2D MOF, Cu3(HHTP)2 (HHTP = 2,3,6,7,10,11-hexahydroxytriphenylene), decorated with silver nanoparticles (AgNPs) exhibiting significant conductivity enhancement at room temperature. Variable-temperature electrical transport measurements across the low-temperature (200 K) to high-temperature (373 K) regime evidenced metallic conduction. Interestingly, thin films of a 3D MOF, CuTCNQ (TCNQ = 7,7,8,8-tetracyanoquinodimethane), upon decoration with AgNPs, disclosed a converse trend. The origin of such distinctive observations on AgNPs@Cu3(HHTP)2 and AgNPs@CuTCNQ systems was comprehended by using first-principles density functional theory (DFT) calculations and attributed to an interfacial electronic effect. Our work sheds new light on rationally designing synthetic modifications in thin films of MOFs to tune the electrical transport property.

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

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.

Direct Layer-by-Layer Growth of Crystalline Ag-TCNQ Thin Films on Functionalized Au Substrate: How Critical Is the pH of Ag(I) Solution?

Wet-chemical fabrication of a crystalline Ag-TCNQ (TCNQ = 7,7,8,8-tetracyanoquinodimethane) thin film on non-Ag substrate is challenging whereby the chemistry was powered by photon energy and/or electrical energy. We report for the first time, direct chemical growth of a Ag-TCNQ thin film on a functionalized Au substrate by employing the layer-by-layer (LbL) approach at ambient reaction conditions. Various Ag(I) salt precursors previously realized to be unsuitable for the fabrication of Ag-TCNQ thin films on non-Ag substrates ultimately gave rise to dense and uniform thin films of Ag-TCNQ. The crucial knob regulating the direct formation of the thin films of Ag-TCNQ was identified to be the pH of the respective Ag(I) solutions.

Emergent Interface in Heterostructured Thin Films of Cu(II) and Cu(I) Coordination Polymers

In this work we report fabrication of high-quality AB- and BA-type heterostructured thin films of cubic Cu(II) (A-type) and tetragonal Cu(I) (B-type) coordination polymers (CPs) on the functionalized Au substrate by the layer-by-layer method. Successful growth of Cu(I)-CP on top of Cu(II)-CP was assigned to be due to the interfacial reduction reaction (IRR). Notably, electrical transport measurements across AB- and BA-type heterostructured thin films revealed rectification of current in opposite directions. We have attributed such an interestingly new observation to the formation of a well-defined interface of Cu(II)-CP and Cu(I)-CP resembling a p-n junction-a hitherto unreported phenomenon that is anticipated to open enormous opportunities for the heterostructured thin films of CPs, likewise celebrated interfaces of oxide heterostructures.