Fabrication of an Active Electronic Device Using a Hetero-bimetallic Coordination Polymer

A nickel(II)/lead(II) coordination polymer [(NCS)Pb(H2O)LNi(NCS)]n {H2L = N,N′-bis(3-methoxysalicylidene)propane-1,3-diamine} has been synthesized and characterized. The band gap (3.18 eV) calculated from Tauc’s plot suggests the semiconducting nature of the complex. The material has a photosensitivity of 5.76, indicating its applicability in the fabrication of photosensitive devices. The complex has been successfully applied in a technologically challenging thin-film photosensitive Schottky device.


Optical Analysis
In this study, the UV-Vis absorption spectrum of the complex has been recorded for the deposited thin films of the complex preparing a well dispersion in DMF, in the range 200-1000 nm. The absorption spectrum of the complex depicts strong energy absorption in the visible region at ~352 nm. Optical band gap has been calculated by using Tauc's equation (equation S1). 1

……(S1)
Where is the absorption coefficient, γ is the frequency of light, h is the Plank's constant, A is a constant taken as 1, E g is the band gap energy, n is the nature of transition. Here S3 the value of n is used to determine the type of transition (n= 0.5 for direct and n=2 for indirect transition). The logarithm of equation S1 is given by: To find the value of n we differentiate equation S2 with respect to hto get: From the plot of d[ln(h)]/d(h) versus h given in Figure S1 we get a discontinuity from which an approximate value of energy (Eg) 2 can be estimated. Putting this approximate value of E g in equation S2 we plot a graph of ln(h) versus ln (h-E g ) which is shown in the inset of Figure S1. The slope of the linear fit graph gives the value of n~0.5 which suggests that the synthesized material has direct allowed transition.

Conductivity of the crystal
Here, to calculate the conductivity of the crystal, we have performed the currentvoltage measurement at corresponding applied bias voltage sequentially within the limit ±10 V with two ohmic contacts. I-V curve of the crystal is shown in Figure S3. The conductivity of the crystal has been estimated as 4.1 x 10 -4 S.cm -1 . Figure S3: Current-Voltage characteristic curve of the crystal.

Dielectric Measurements
The dielectric constant of the material has been measured on pellet. The synthesized complex has been pelletized in to a disc of diameter 7.32 mm and thickness 1.8 mm. Figure S4 gives the curve showing the variation of the capacitance (C) as a function of the frequency (f) at constant bias potential. The room temperature capacitance of the complex is shown to be frequency dependent at relatively low frequencies. The capacitance decreases with the increase in frequency and becomes saturated at higher frequency. The capacitance at saturation is 4.33 x 10 -10 F. From the saturated value of capacitance, dielectric constant of the complexes was calculated employing following equation 3 : Where, is the permittivity of free space, is the dielectric constant of the complex, is the capacitance (at saturation), and is the thickness and effective area of the pellet. Using the above formula the relative dielectric constant ( ) of the material has been estimated and found to be 10.53. S6 Figure S4: Capacitance (C) versus frequency (f) curve at constant bias potential.

IR, UV-Vis and Fluorescence spectra
Distinct bands due to azomethine (C=N) group at 1635 cm -1 is routinely noticed in the IR spectrum of the complex. 4 There are two strong bands at 2042 cm -1 and 2112 cm -1 in the complex indicating the presence of terminal (N-bonded) and end to end bridged thiocyanate 5 respectively which are also evident from the crystal structure determination.
Electronic spectrum of the complex displays two absorption bands at 584 nm and 814 nm. The band at 584 nm is assigned as 3 T 1g (F) 3 A 2g (F) whereas band at 814 nm is assigned as S7 This is assigned as intra-ligand (π-π * ) fluorescence. 8 Mean lifetime (г avg ) of the exited state is listed in Table S2. Decay profile is shown in Figure S5.  Figure S5: Lifetime decay profile of the complex.

PXRD
PXRD pattern of the thin film of the complex is in good agreement with the simulated XRD pattern obtained from single crystal X-ray diffraction, indicating consistency of the sample in the thin film ( Figure S6). PXRD pattern of the bulk is also in good agreement with the S8 simulated XRD pattern, indicating purity of bulk material ( Figure S7). Simulated pattern of the complex is calculated from the single crystal structural data (Cifs) using the CCDC Mercury software.

Thermo-gravimetric analysis
The thermal behaviour of the complex is studied in a dynamic nitrogen atmosphere (20 mL min -1 ) at a heating rate of 10°C min -1 using thermo-gravimetric (TG) technique. The thermogravimetric plot for the complex is given in Figure S8. The complex is stable up to ~80 °C. A weight loss of 2.23% for the complex in the temperature range 50-80 °C corresponds to the loss of a water molecule (calc. 2.21%). The dehydrated complex is stable up to ~230°C. The complex is then decomposed as indicated by huge weight loss with increasing temperature. S10 Figure S8: Thermal analysis of the complex.