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 junctiona hitherto unreported
phenomenon that is anticipated to open enormous opportunities for
the heterostructured thin films of CPs, likewise celebrated interfaces
of oxide heterostructures.
Chemical reactions involving oxidation and reduction processes at interfaces may vary from those in conventional liquid-phase or solid-phase reactions and could influence the overall outcome. This article primarily features a study on metal−ligand coordination at the solid−liquid interface. Of particular mention is the spontaneous reduction of Cu(II) to Cu(I) at a solid−liquid interface without the need of any extraneous reducing agent, unlike in the liquidphase reaction whereby no reduction of Cu(II) to Cu(I) took place. As a consequence of the interfacial reduction reaction (IRR), thin films of Cu-TCNQ (tetracyanoquinodimethane) and Cu-HCF (hexacyanoferrate) were successfully deposited onto a thiol-functionalized Au substrate via a layer-by-layer (LbL) method. IRR is anticipated to be useful in generating new functional and stimuli-responsive materials, which are otherwise difficult to achieve via conventional liquid-phase reactions.
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Depositing thin films of pristine metal–organic
framework
(MOF) on top of a lattice-matched and molecularly doped MOF could
provide a new path for generating electronic heterostructures of MOFs
with well-defined interfaces. Herein, the Cu3BTC2 (top-layer)/TCNQ@Cu3BTC2 (bottom-layer) system
is fabricated by sequential deposition on a functionalized Au substrate,
and clear-cut rectification of electrical current across the thin
film was observed at room-temperature. Interestingly, the electrical
current rectification ratio (RR) was found to be significantly influenced
by the effect of temperature (400 K), resulting in a remarkable figure
in the domain of MOFs.
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.
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