Improved electrical conductivity of Co(II) and Cu(II) ladder polymers in the fabrication of photoresponsive Schottky devices

Abstract

In this work, two new Cu(II) and Co(II) based coordination polymers (CPs), [Co₂(bpd)₂(nac)₂]·2CH₃OH·H₂O (Co-CP) and [Cu₂(bpd)₂(nac)₂]·2CH₃CN·2H₂O (Cu-CP) respectively, have been synthesized using a bidentate pyridyl ligand, N,N′-bis(1-pyridine-4-yl-ethylidene) (bpd), linker with the less explored 3-(1-naphthyl)acrylic acid (nac) ligand appended at the metal centre to fulfil the molecular geometry. Here, Co-CP forms a one-dimensional (1D) ladder polymer, whereas Cu-CP consists of a combination of 1D chains and 1D ladder polymers. Interestingly, both the CPs exhibit semiconducting behaviour with increased conductivity upon illumination, signifying the photosensitive nature. However, Cu-CP reveals better conductivity as compared to Co-CP. This is obvious from the field emission electron microscopy (FESEM) study, where Cu-CP with flower-like morphology shows a higher surface area with respect to the rod-shaped morphology of Co-CP, resulting in higher charge transport. To the best of our knowledge, Co/Cu based CPs showing photosensitivity seem to be scarce. Thus, this study opens a new avenue in the fabrication of photoresponsive electronic devices.

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Introduction

The dramatic revolution of laboratory to land applications of materials chemistry propagates at an extraordinary pace. Application of materials has directly been governed by the structural architecture and morphology. In this regard, coordination polymers (CPs)^1–9 have attracted materials researchers mostly because of their unique molecular structures and excellent stabilities. These hybrid molecular systems are constructed through inorganic metal ions or metal clusters and organic ligands.^10–15 Hence, the molecular properties as well as applications are also regulated by the nature of the metal centers and ligands. A combination of organic components (O-donor and N-donor ligands) is commonly exploited to achieve structural varieties and desired molecular properties.^16–18 In these coordination systems, metal salts or organic linkers are judiciously selected to confer the application field. In fact, there is an essential relationship between the structural architecture, properties and utilities of the materials. Rational construction and scientific judgment during the engineering of these crystalline materials make them easier to characterize and illustrate important structure–property relationships. In the construction of higher dimensional supramolecular architecture, various supramolecular interactions such as hydrogen bonding, π⋯π, C–H⋯π, cation⋯π, anion⋯π, halogen⋯halogen, halogen⋯π and van der Waals interactions play the crucial role.^19–21 Sometimes, these self-assembled structures are entirely unlike and more proficient than the unassembled forms. The discussed inorganic–organic hybrid materials are extremely applicable in the territory of gas sorption, molecular storage and separation, ion exchange, electrochemical catalysis, energy technology, drug delivery, temperature-dependent magnetism, sensing and detection of noxious ions and emerging analytes, proton conductance, electrical conductivity, electronic device fabrication, etc.^22–34

Of these, utilization of CPs in electrical conductivity and device fabrication is particularly important as these CPs can be explored to overcome the energy crisis situation and for technological interest. However, in CPs, the poor extended electronic coupling among the metal nodes and the ligands impedes competent charge transport.^35–38 The consequential electronic bands display nominal dispersion, which indicates that the electrons are strongly localized over the whole lattice. The band dispersion determines the efficacy of the compound to transmit electrical current. Therefore, the crucial facet is to adjust the electronic connection among the metal centers and the organic ligands to enhance delocalization for higher conductivity.^39–41 In this regard, a substantive number of CPs based on d¹⁰ metal ions (Zn²⁺/Cd²⁺) have been reported and those behave as semiconducting materials. Some of the materials have also exhibited photosensitivity properties, i.e. enhancement of conductivity upon illumination.³⁷ However, varying metal ions in tailor-fit synthesis, we can easily regulate the semiconducting nature as well as the photosensitivity behavior of CPs. In this facet, CPs based on Co(II)/Cu(II) metal ions with semiconducting properties have not been much explored and remain more elusive in regard to photosensitivity.^8,40

Herein, we report two Co(II) and Cu(II) based one-dimensional (1D) CPs [Co₂(bpd)₂(nac)₂]·2CH₃OH·H₂O (Co-CP) and [Cu₂(bpd)₂(nac)₂]·2CH₃CN·2H₂O (Cu-CP) which have been utilized to fabricate metal–semiconductor (MS) junction Schottky devices (SDs). The current–voltage (I–V) characteristics of the fabricated SDs were measured to estimate the diode parameters, such as on/off ratio, photosensitivity, ideality factor, barrier height, and series resistance. The study reveals that both the CPs exhibit higher charge conduction upon illumination signifying the photosensitive nature. The charge transport parameters including mobility and lifetime of the charge carriers were also investigated with the help of the space–charge limited current (SCLC) theory, which demonstrates the improved device performance of Co-CP and Cu-CP. However, Cu-CP exhibits better conductivity with respect to Co-CP. This observation is also supported by a field emission scanning electron microscope (FESEM) study. Flower-like morphology of Cu-CP offers higher surface area as compared to rod-shaped Co-CP and exhibits increased electrical conductivity.

Results and discussion

Structural description of [Co₂(bpd)₂(nac)₂]·2CH₃OH·H₂O (Co-CP)

The single crystal X-ray diffraction (SCXRD) study reveals that Co-CP crystallizes in the monoclinic space group C2/c. The asymmetric unit consists of one Co(II) centre, two nac ligands and a bpd ligand. The asymmetric unit further contains one each of CH₃OH and H₂O molecules in the lattice (Fig. S1, ESI†). The Co(II) centre adopts a six-coordinated octahedral geometry (Fig. 1). The equatorial plane is defined by two O atoms of a chelating nac ligand and two O atoms of two bridging nac ligands. To complete the octahedron, two N atoms from two bpd ligands take the axial positions. Here also, two Co(II) centres and two bridging carboxylate groups form a dinuclear 8-membered [Cu₂(O₂CC)₂] secondary building unit (SBU) with Co⋯Co distance of 4.3064 Å. Thereby, a pair of bpd ligands are aligned parallel via π⋯π stacking contacts with the centroid–centroid distance of 3.986 Å (Fig. 1). A crystallographic inversion centre is also found in the middle of the ring. The SBUs and a pair of bpd ligands are combined to generate a 1D ladder polymer. Further, the 1D ladder polymers undergo extensive π⋯π stacking interactions with the centroid–centroid distance of 3.766–3.849 Å to form a 3D supramolecular assembly. In the solid-state structure, if the π⋯π interactions are considered as joining the ladder polymers, they would generate a 2D layer structure. The aligned bpd pairs act as pillars which join the 2D layers into a doubly interpenetrated 3D supramolecular pillar-layered network of cubic topology (Fig. 2 and Fig. S2, ESI†).

Fig. 1 Dimeric SBU of Co-CP with coordination environment around Co(II) centres. Carbon: gray; nitrogen: blue; oxygen: red; cobalt: navy. Hydrogen atoms and solvent molecules are not shown for clarity. Dashed lines represent the π⋯π contacts between bpd ligands.

Fig. 2 Doubly interpenetrated 3D supramolecular pillar-layered structure of Co-CP.

Structural description of [Cu₂(bpd)₂(nac)₂]·2CH₃CN·2H₂O (Cu-CP)

The SCXRD study reveals that Cu-CP also crystallizes in the monoclinic space group C2/c. The asymmetric unit contains two parts (Fig. S3, ESI†). The first part consists of a Cu(II) centre (Cu1), one bpd ligand and two nac ligands. However, the second part comprises a Cu(II) centre (Cu2), half a bpd ligand and one nac ligand. The asymmetric unit further consists of one each of CH₃CN and H₂O molecules in the lattice (Fig. S3, ESI†). All the Cu(II) centres adopt six-coordinated octahedral geometries (Fig. 3). The equatorial plane of the octahedron is formed by two O atoms from a chelating nac ligand and two O atoms from two nac ligands. Two N atoms of two bpd ligands occupy the axial positions. Here, two Cu(II) centres and bridging carboxylate groups form a 8-membered dinuclear [Cu₂(O₂CC)₂] secondary building unit (SBU) with Cu⋯Cu distance of 3.4843(6) Å. A pair of bpd ligands are aligned via face-to-face π⋯π stacking interactions (centroid–centroid distance 3.776 Å) (Fig. 3). A crystallographic inversion centre is present in the middle of the [Cu₂(O₂CC)₂] ring. A 1D ladder polymer is obtained by the combination of SBUs and a pair of bpd ligands (Fig. 4).

Fig. 3 Coordination environment around Cu(II) centres of Cu-CP. Carbon: gray; nitrogen: blue; oxygen: red; copper: brown. Hydrogen atoms and solvent molecules are not shown for clarity. Dashed lines represent the π⋯π stacking interactions between bpd ligands.

Fig. 4 The 3D supramolecular network of Cu-CP comprising 1D ladder polymers and 1D linear chains.

On the other hand, the Cu2 centre is surrounded by four O atoms of two carboxylate groups from two individual chelating nac ligands forming the equatorial plane, while two N atoms from two individual bpd ligands complete the octahedral geometry. The connectivity of the Cu2 centre and nac ligand leads to the formation of a 1D linear chain. Further, 1D ladder polymers and 1D linear chains are assembled to form 3D supramolecular frameworks by the arrangement of extensive face-to-face π⋯π stacking interactions (centroid–centroid distance 3.834 Å), and C–H⋯π interactions with edge-to-face distance of 3.023–3.160 Å (Fig. 4 and Fig. S4, ESI†).

Surface morphological studies

The surface morphologies of the Co-CP and Cu-CP were investigated using FESEM micrographs and are displayed in Fig. 5 and 6. These kinds of microstructures of the polymers not only help to study shapes and sizes, but also show the characteristics of the MS junction that is formed while fabricating the Schottky diodes. It is observed from the images that the Co-CP are rod-shaped (Fig. 5), while the Cu-CP are flower-like (Fig. 6). The flower-like shapes offer higher surface area, contributing to the higher contact area and larger electrical conduction.⁴² This study prompts us to investigate further the electrical conductivity of the materials.

Fig. 5 FESEM micrographs of Co-CP.

Fig. 6 FESEM micrographs of Cu-CP.

Optical properties

The optical spectra of Co-CP and Cu-CP were recorded to estimate their optical band gaps. To deduce the band gaps, Tauc's equation (eqn (1))⁴³ was employed, which is given by:

(αhν)ⁿ = A(hν − E_g) (1)

where α is the absorption coefficient, E_g is the bandgap, h is Planck's constant, ν is frequency, A is a constant, and n = 2 and 1/2 corresponding to the allowed direct and indirect optical transitions, respectively. By extrapolating the linear region of the plot (αhν)²vs. hν to α = 0, as displayed in Fig. S11 (ESI†), the direct optical band gaps of Co-CP and Cu-CP were evaluated as 3.48 eV and 3.46 eV respectively, which suggest the intermediate band gap region.³¹

Device fabrication method

To study the electrical characteristics of Co-CP and Cu-CP, the MS junction SDs based on these CPs were fabricated. In a typical method, first ITO-coated glass substrates were cleaned using acetone, followed by 2-propanol and distilled water. Then, on the top of the cleaned indium tin oxide (ITO) coated substrate, a thin film of Co-CP/Cu-CP was deposited by the spin coating unit. Subsequently, the as-prepared films were dried in a vacuum oven for 1 h. Next, as a metallic contact, aluminium (Al) was deposited onto the coated film using a vacuum coating unit (12A4D, HINDHIVAC) under an atmospheric pressure of 4.7 × 10⁻⁶ torr in the deposition method. In this process, a typical quad punch-hole shadow mask was used to control the effective diode area of the Schottky device as 7.065 × 10⁻⁶ m⁻². The schematic diagram of the Al/CPs/ITO structure is portrayed in Scheme 1.

Scheme 1 Schematic diagram of the fabricated Al/CPs/ITO SDs.

Current–voltage (I–V) measurement

The electrical properties of the MS junction thin-film devices of Co-CP and Cu-CP were investigated. For this, the current–voltage (I–V) characteristics of the CP based Schottky devices were studied between the applied bias voltage of ±1 V under dark and light conditions. The I–V characteristics of the ITO/CPs/Al configurations presented in Fig. 7 exhibit a nonlinear rectifying behaviour for the SDs. The conductivities of the CPs under dark and light conditions were measured. It has been observed that the charge conduction is higher under illumination for both of the CPs, signifying the photo-responsive nature.

Fig. 7 (a) I–V characteristics curves for Co-CP and Cu-CP based devices. (b) I–V curves on logarithmic scale.

The current vs. voltage characteristics of the CP-based SDs were further discussed using thermionic emission (TE) theory.⁴⁴ Here, the differential method first exploited by Cheung et al. was employed to evaluate different SD parameters.⁴⁵ The current density (J) vs. voltage (V) curves could be written as:⁴⁶

where q is the charge of the electron, k is the Boltzmann constant, T is the temperature, V_D is the potential drop and η is the ideality factor J and J_S are respectively the current density and the current density at saturation, which is also given by:where A* stands for the Richardson constant and ϕ_B is the barrier height. Considering V_D = V − IR_S using the Cheung and Cheung method,⁴⁷eqn (2) could be reduced to the following differential form:Furthermore, considering the function H(J) as eqn (5), the barrier or potential height of the SDs was deduced using the following eqn (6):

H(J) = A_effR_SJ + ηϕ_B (6)

The G(J) vs. J and H(J) vs. J graphs for dark and light conditions are presented in Fig. 8(a–d) respectively. The ideality factor (η) and series resistance (R_S) were determined from the intercept and slope of the linearly fitted plot of G vs. J, respectively. On the other hand, the barrier heights (ϕ_B) for the Co-CP and Cu-CP based devices are determined from the H vs. J plot. The obtained values of the photosensitivity and the dc conductivity under dark and illumination conditions are listed in Table 1, which depicts that Cu-CP is more photo responsive than Co-CP. Also, Cu-CP exhibits higher conductivity as compared to Co-CP.

Fig. 8 G vs. J and H vs. J plots under dark and light conditions for (a and b) Co-CP, and (c and d) Cu-CP based thin-film devices.

Table 1 Charge transport parameters

	Photosensitivity (S)	Conductivity (σ) (S m^−1)
		Dark	Light
Co-CP	0.41	4.78 × 10^−6	6.15 × 10^−6
Cu-CP	0.86	19.97 × 10^−6	70.61 × 10^−6

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The measured ideality factors (η), series resistances (R_S) and barrier heights (ϕ_B) for Co-CP and Cu-CP under both dark and illumination conditions are listed in Table 2. It is observed that, after light soaking, the ideality factor of both of the CP-based devices approached very close to 1, indicating a more ideal device. This is generally due to the reduced photo-carrier recombination at the junction under light irradiation.⁴³ In the presence of light, the series resistances were drastically reduced compared to the dark condition, causing a large increase in the photocurrent. The evaluated values of barrier height also show that under light, the turn-on voltages were slightly decreased. Table 2 manifests the improved diode behaviour for the Cu-CP based diodes. Also, the enhanced performance of both of the devices after light illumination portrays the excellent potential of Co-CP and Cu-CP in the field of various optoelectronic devices.

Table 2 Schottky diode parameters

	Ideality factor (η)		Barrier height (ϕB) (eV)		Series resistance (RS) (kΩ)
	Dark	Light	Dark	Light	Dark	Light
Co-CP	0.89	1.15	0.72	0.71	23.42	16.41
Cu-CP	1.43	1.01	0.70	0.66	4.21	2.60

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The study of different diode parameters led us to thoroughly observe the charge transport properties of the CPs. In this regard, the I–V curves are plotted on the logarithmic scale (Fig. 7b). Fig. 7b reveals three distinguishable slopes, which are marked as region-I, II and III. In region-I, the current follows as I ∝ V, which denotes the ohmic region. In the second region, the slope is close to 2, where the current varies as I ∝ V² and is controlled by the space–charge limited current (SCLC) field (Fig. 9).⁴⁸ The injected carriers increase much higher as proportional to the background carriers. As a result, it spreads all over the space and creates a space–charge field. In region III, the injection level of electrons is much higher and they follow the power-law (I ∝ Vⁿ, where n > 2).

Fig. 9 I vs. V ² curves under both dark and light conditions for (a) Co-CP and (b) Cu-CP based thin-film devices.

Following this model, the effective carrier mobility was deduced using the Mott–Gurney equation:⁴⁹

where A_eff, ε₀, ε_r and d are the effective diode area, the free space permittivity, the dielectric constant of the materials and the thickness of the diode, respectively. The dielectric constant (ε_r) was evaluated from the capacitance vs. frequency plot (Fig. 10) by employing the following equation:⁵⁰where C is the capacitance at saturation, d is the thickness of the film, A is the effective area and ε₀ is the permittivity of free space.

Fig. 10 Capacitance versus frequency (C–f) plots of (a) Co-CP and (b) Cu-CP.

Transit time (τ), which is another key parameter to study charge transport across the SD junction,⁵¹ was evaluated using the following equation:

All the deduced values of the diode parameters displayed that the charge conduction of the CPs was improved after light irradiation (Table 3). Also, the higher conductivity (Table 1) of Cu-CP could be accredited to its higher mobility and lower transit or lifetime. It is also to be noted that the diode characteristics as well as the transport parameters of both the Co(II) and Cu(II) CP based SDs demonstrate much enhanced charge transfer properties after light soaking (Scheme S1, ESI†). The transient photocurrent vs. time measurements were performed to study the conducting mechanism of the CPs. It is observed that the conductivity of the CP does not vary with the passage of time (Fig. S12, ESI†). Thus, Co-CP and Cu-CP display immense potential to be employed in photoresponsive device applications.

Table 3 SCLC parameters

	ε r	μ eff (m^2 V^−1 s^−1)		τ (s)
		Dark	Light	Dark	Light
Co-CP	1.15 × 10^−10	6.94 × 10^−7	8.72 × 10^−7	1.66 × 10^−6	1.31 × 10^−6
Cu-CP	2.34 × 10^−10	1.94 × 10^−6	3.70 × 10^−6	5.51 × 10^−7	2.90 × 10^−7

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Conclusion

In summary, the present work analyses an unusual utilization of transition metal ions in the fabrication of CPs. Here, less explored nac acts as an ancillary ligand and bpd links the metal centres to generate 1D chains. The materials have been employed to fabricate semiconducting Schottky devices; through which electrical properties have been analyzed. Interestingly, the electrical conductivities of the materials have been enhanced upon light illumination. Therefore, the synthesized CPs can act as next generation photosensitive electronic materials. Charge transportation and electrical conductivity are also well tuned with the metal nodes present in the structural architecture. This work can be an inspiration for future researchers in the field of materials science.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by UGC-DAE CSR, India (Grant No. CRS/2021-22/02/538 Dated 30/03/2022). B. D. thanks the Indian Institute of Science Education and Research Kolkata for IISER-K PDF fellowship. P. P. R. gratefully acknowledges the financial support of this work by SERB-DST, Govt. of India (Sanction No. EMR/2016/005387, Dated – 24.07.2017). All the authors thank Jadavpur University, Kolkata, India for the instrumental facility.

References

1. S. R. Batten, S. M. Neville and D. R. Turner, Coordination Polymers: Design, Analysis and Application, Royal Society of Chemistry, Cambridge, 2009.
2. H.-C. Zhou and S. Kitagawa, Chem. Soc. Rev., 2014, 43, 5415–5418.
3. S. Halder, A. Dey, A. Bhattacharjee, J. Ortega-Castro, A. Frontera, P. P. Ray and P. Roy, Dalton Trans., 2017, 46, 11239–11249.
4. J. Duan, W. Jin and S. Kitagawa, Chem. Soc. Rev., 2017, 332, 48–74.
5. D. Sun, S. Yuan, H. Wang, H.-F. Lu, S.-Y. Feng and D.-F. Sun, Chem. Commun., 2013, 49, 6152–6154.
6. S. S. Kumar, N. Kurra and H. N. Alshareef, J. Mater. Chem. C, 2016, 4, 215–221.
7. D. Cardenas-Morcoso, E. Vey, M. Heiderscheid, G. Frache and N. D. Boscher, J. Mater. Chem. C, 2022, 10, 2194–2204.
8. F. Ahmed, S. Halder, B. Dutta, S. Islam, C. Sen, S. Kundu, C. Sinha, P. P. Ray and M. H. Mir, New J. Chem., 2017, 41, 11317–11323.
9. B. Dutta, A. Dey, C. Sinha, P. P. Ray and M. H. Mir, Inorg. Chem., 2019, 58, 5419–5422.
10. T. Panda and R. Banerjee, Proc. Natl. Acad. Sci., India, Sect. A, 2014, 84, 331–336.
11. K. Naskar, S. Sil, N. Sahu, B. Dutta, A. M. Z. Slawin, P. P. Ray and C. Sinha, Cryst. Growth Des., 2019, 19, 2632–2641.
12. F. Ahmed, S. R. Ghosh, S. Halder, S. Guin, S. M. Alam, P. P. Ray, A. D. Jana and M. H. Mir, New J. Chem., 2019, 43, 2710–2717.
13. J. J. Vittal, Coord. Chem. Rev., 2007, 251, 1781–1795.
14. M. Dincă and J. R. Long, J. Am. Chem. Soc., 2005, 127, 9376–9377.
15. B. Dutta, A. Dey, K. Naskar, S. Maity, F. Ahmed, S. Islam, C. Sinha, P. Ghosh, P. P. Ray and M. H. Mir, New J. Chem., 2018, 42, 10309–10316.
16. A. A. Talin, A. Centrone, A. C. Ford, M. E. Foster, V. Stavila, P. Haney, R. A. Kinney, V. Szalai, F. El Gabaly, H. P. Yoon, F. Leonard and M. D. Allendorf, Science, 2014, 343, 66–69.
17. B. Dutta, R. Jana, A. K. Bhanja, P. P. Ray, C. Sinha and M. H. Mir, Inorg. Chem., 2019, 58, 2686–2694.
18. T. Komatsu, J. M. Taylor and H. Kitagawa, Inorg. Chem., 2016, 55, 546–548.
19. S. Naaz, B. Debnath, S. Islam, S. Khan, F. Ahmed, B. Dutta, A. D. Jana and M. H. Mir, Cryst. Growth Des., 2022, 22, 1253–1262.
20. P. Ghorai, A. Dey, A. Hazra, B. Dutta, P. Brandão, P. P. Ray, P. Banerjee and A. Saha, Cryst. Growth Des., 2019, 19, 6431–6447.
21. F. Ahmed, B. Dutta and M. H. Mir, Dalton Trans., 2021, 50, 29–38.
22. F. B. de Almeida, M. da Silva Cunha, W. P. Barros, H. A. De Abreu and R. Diniz, J. Phys. Chem. C, 2020, 124, 21103–21112.
23. M. I. Velasco, R. H. Acosta, W. A. Marmisollé, O. Azzaroni and M. Rafti, J. Phys. Chem. C, 2019, 123, 21076–21082.
24. B. Dutta, R. Jana, A. K. Bhanja, P. P. Ray, C. Sinha and M. H. Mir, Inorg. Chem., 2019, 58, 2686–2694.
25. M. L. Hu, M. Abbasi-Azad, B. Habibi, F. Rouhani, H. Moghanni-Bavil-Olyaei, K. G. Liu and A. Morsali, ChemPlusChem, 2020, 85, 2397–2418.
26. Y. Park, J. Jung and M. Chang, Appl. Sci., 2019, 9, 1070.
27. G. Chakraborty, I.-H. Park, R. Medishetty and J. J. Vittal, Chem. Rev., 2021, 121, 3751–3891.
28. T. A. Fernandes, I. F. Costa, P. Jorge, A. C. Sousa, V. André, N. Cerca and A. M. Kirillov, ACS Appl. Mater. Interfaces, 2021, 13, 12836–12844.
29. M. Moghaddam-Manesh, D. Ghazanfari, E. Sheikhhosseini and M. Akhgar, Appl. Organomet. Chem., 2020, 34, e5543.
30. B. Dutta, R. Jana, C. Sinha, P. P. Ray and M. H. Mir, Inorg. Chem. Front., 2018, 5, 1998–2005.
31. W.-M. Chen, X.-L. Meng, G.-L. Zhuang, Z. Wang, M. Kurmoo, Q.-Q. Zhao, X.-P. Wang, B. Shan, C.-H. Tung and D. Sun, J. Mater. Chem. A, 2017, 5, 13079–13085.
32. L.-L. Han, S.-N. Wang, Z. Jagličić, S.-Y. Zeng, J. Zheng, Z.-H. Li, J.-S. Chen and D. Sun, CrystEngComm, 2015, 17, 1405–1415.
33. X. Gao, S.-S. Zhang, H. Yan, Y.-W. Li, Q.-Y. Liu, X.-P. Wang, C.-H. Tung, H.-Y. Ma and D. Sun, CrystEngComm, 2018, 20, 4905–4909.
34. X.-P. Wang, Y.-Q. Zhao, Z. Jagličić, S.-N. Wang, S.-J. Lin, X.-Y. Li and D. Sun, Dalton Trans., 2015, 44, 11013–11020.
35. B. Dutta, A. Dey, S. Maity, C. Sinha, P. P. Ray and M. H. Mir, ACS Omega, 2018, 3, 12060–12067.
36. V. Stavila, A. A. Talin and M. D. Allendorf, Chem. Soc. Rev., 2014, 43, 5994–6010.
37. S. Roy, A. Dey, P. P. Ray, J. Ortega-Castro, A. Frontera and S. Chattopadhyay, Chem. Commun., 2015, 51, 12974.
38. S. Islam, J. Datta, S. Maity, B. Dutta, S. Khan, P. Ghosh, P. P. Ray and M. H. Mir, Cryst. Growth Des., 2019, 19, 4057–4062.
39. P. Amo-Ochoa, O. Castillo, S. S. Alexandre, L. Welte, P. J. de Pablo, I. Rodriguez-Tapiador, J. Gomez-Herrero and F. Zamora, Inorg. Chem., 2009, 48, 7931–7936.
40. F. Ahmed, J. Datta, S. Khan, B. Dutta, S. Islam, S. Naaz, P. P. Ray and M. H. Mir, New J. Chem., 2020, 44, 9004–9009.
41. B. Dutta, A. Dey, C. Sinha, P. P. Ray and M. H. Mir, Dalton Trans., 2019, 48, 11259–11267.
42. A. R. Padmavathi, P. Sriyutha Murthy, A. Das, P. A. Nishad, R. Pandian and T. S. Rao, Biofouling, 2019, 35, 1007–1025.
43. J. Tauc, R. Grigorovici and A. Vancu, Phys. Status Solidi B, 1966, 15, 627–637.
44. S. M. Sze, Physics of Semiconductor Devices, Wiley, New York, 2nd edn, 1981.
45. S. K. Cheung and N. W. Cheung, Appl. Phys. Lett., 1986, 49, 85–87.
46. E. H. Rhoderick and R. H. Williams, Metal–semiconductor contacts, Clarendon, Oxford, 2nd edn, 1988.
47. M. Das, J. Datta, R. Jana, S. Sil, S. Halder and P. P. Ray, New J. Chem., 2017, 41, 5476–5486.
48. M. Soylu and B. Abay, Phys. E: Low-Dimens. Syst. Nanostructures, 2010, 43, 534–538.
49. I. Ullah, M. Shah, M. Khan and F. Wahab, J. Electron. Mater., 2016, 45, 1175–1183.
50. S. Sil, A. Dey, J. Datta, M. Das, R. Jana, S. Halder, J. Dhar, D. Sanyal and P. P. Ray, Mater. Res. Bull., 2018, 106, 337–345.
51. D. Das, M. Das, S. Sil, P. Sahu and P. P. Ray, ACS Omega, 2022, 7, 26483–26494.

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Footnotes

† Electronic supplementary information (ESI) available: Experimental section, Tables S1–S5, Fig. S1–S12, Scheme S1 and X-ray crystallographic data in CIF format for compounds Co-CP and Cu-CP. CCDC 2203412 (Co-CP) and 2203413 (Cu-CP). For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d2ma00911k

‡ These authors contributed equally.

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