A Cd(ii)-based MOF as a photosensitive Schottky diode: experimental and theoretical studies

Abstract

A Cd(ii) based 2D metal-organic framework (MOF), [Cd(4-bpd)(SCN)] (1) where 4-bpd = 1,4-bis(4-pyridyl)-2,3-diaza-1,3-butadiene, has been synthesized and characterized by standard methods including single crystal X-ray diffraction analysis. When it is sandwiched between ITO coated glass and Al, 1 shows interesting conduction properties. The I-V characteristics of the ITO/1/Al configuration measured in the dark and under illumination of incident light exhibit a highly non-linear rectifying behavior, which signifies its Schottky diode character. The conductivity of the configuration is 2.90 × 10 S m and 7.16 × 10 S m under dark and photoirradiation conditions, respectively. Different parameters have been analyzed and these indicate that 1 can be a promising candidate for light sensing electronic devices. This material has good sensitivity to the light source when switched on/off. Theoretical calculations have been performed to understand the reason for the enhancement of conductivity under illumination of incident light. It has been found that upon irradiation, slight changes in the bond distances of 4-bpd in 1 occur. These changes are related to the considerable decrease in the energy needed for the allowed electronic transition. This may influence the increase of conductivity along with other factors.

Achieving current rectification ratios ≥ 105 across thin films of coordination polymer

“…This is a huge achievement considering the fact that coordination polymers including MOFs (either in bulk form or in thin lm form) so far have not been able to cross the 10 2 mark in RR values. [27][28][29][30] Motivated by such an unusual observation, we wanted to directly compare the I-V characteristics of our system with those of commercial Si diodes (1N4007 and 1N4733A), and RR values were estimated to be on par (Fig. S8 †)!…”

“…Schottky barriers at the metal-semiconductor interfaces are well known to cause current rectication and all the previous MOF Schottky diodes were demonstrated to primarily originate from specic semiconducting MOF-metal interfaces. [27][28][29][30] To check such a possibility in our system, we have measured inplane and cross-plane I-V characteristics on the doped thin lm using various types of contacts, namely EGaIn, conductive C paste, direct Au tips, and direct Pt tips, having different work function values. However, similar I-V patterns as well as RR values were consistently observed (Fig.…”

“…However, the best performing molecular diodes [23][24][25] with RR values of 10 2 to 10 3 were considered to be inferior to commercial diodes until realization of the 'supramolecular diode' at metal/SAM/metal oxide junctions with an RR value beyond 10 5 , driven by electrostatic interactions. 26 Also, earlier studies on thin lms of MOFs did reveal electrical rectication, specically at indium tin oxide (ITO)/ Cd(II)-MOF/aluminium (Al) junctions; [27][28][29][30] however, RR values were consistently observed to be <10 2 . Thus, to the best of our knowledge, the present work reports the highest value of RR to date in the domain of organic-inorganic hybrid materials like CPs including MOFs.…”

Electrically Conductive Metal–Organic Frameworks

“…In general, higher conductivities in this category appear to correlate with the presence of redox-active components in the MOFs (e.g., linkers featuring metal–dithiolene units, 26 , 64 , 216 conjugated organic cores, 215 , 217 and azo groups 214 , 218 − 221 ). Excitation of mobile carriers from these motifs presumably facilitates conductivity.…”

“… 214 Roy and co-workers made Cd II frameworks with an extended azo-based linker, 1,4-bis(3-pyridyl)-2,3-diaza-1,3-butadiene, with conductivities of 4.53 × 10 –7 S/cm 220 and 2.90 × 10 –6 S/cm. 221 All of these azo materials also exhibited Schottky diode-like behavior in thin film devices, with some increases in conductivity upon photoillumination. They were dense except for one Zn II framework with 4,4′-azobispyridine and 5-hydroxyisophthlate, which displayed a BET surface area of 105.8 m 2 /g.…”