Crystal Structure and Band-Gap Engineering of a Semiconducting

Feb 12, 2018 - Department of Materials Science and Engineering, Nagoya Institute of Technology, Gokiso, Showa, Nagoya , Aichi 466-8555 , Japan. ‡ Ce...
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Communication Cite This: Inorg. Chem. 2018, 57, 2373−2376

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Crystal Structure and Band-Gap Engineering of a Semiconducting Coordination Polymer Consisting of Copper(I) Bromide and a Bridging Acceptor Ligand Takashi Okubo,*,† Kento Himoto,† Koki Tanishima,† Sanshiro Fukuda,† Yusuke Noda,‡ Masanobu Nakayama,‡,§ Kunihisa Sugimoto,# Masahiko Maekawa,† and Takayoshi Kuroda-Sowa† †

Faculty of Science and Engineering, Kindai University, 3-4-1 Kowakae, Higashi-Osaka, Osaka 577-8502, Japan Department of Materials Science and Engineering, Nagoya Institute of Technology, Gokiso, Showa, Nagoya, Aichi 466-8555, Japan ‡ Center for Materials Research by Information Integration, Research and Services Division of Materials Data and Integrated System, National Institute for Materials Science, 1-2-1 Sengen, Tsukuba, Ibaraki 305-0047 Japan # Research and Utilization Division, Japan Synchrotron Radiation Research Institute, Kouto, Sayo-cho, Sayo-gun, Hyogo 679-5198, Japan §

S Supporting Information *

dithiocarbamate complexes and revealed that the CuIBr frameworks show good carrier-transport properties with relatively high carrier mobilities,9 which are comparable to those of organic semiconductors. Another approach to developing semiconducting CPs would be to combine a donor unit including metal ions and a bridging acceptor ligand. In this case, it is expected that the overlap between the HOMO of the metal ion and the LUMO of the bridging ligand produces energy bands for conducting pathways to increase the carrier mobility and a narrow band gap to increase the charge-carrier density in the CP. In this context, copper halides are good candidates as building blocks for the donor unit to develop semiconducting CPs because copper halide infinite networks have the potential to transport the carriers because of relatively high carrier mobilities, and the HOMO levels are suitable for overlap with the LUMOs of π-conjugated molecules (Figures S1 and S2). In this paper, we demonstrate that a combination of CuIBr and a bridging acceptor ligand, 1,2,4,5-tetrazine (ttz), with a low LUMO level can produce a new 3D semiconducting CP, [Cu2Br2(ttz)]n (1), with a higher carrier mobility and a smaller band gap, based on the formation of the energy bands generated by mixing of HOMOs and LUMOs, than those of [CuBr(pyz)]n (2; pyz = pyrazine),10 including a ligand pyz with a higher LUMO level, as shown in Figure S2. We further present a crystal structure, conducting properties, and a band structure calculated by a first-principles calculation method with a comparison to those of 2. 1 was synthesized by the reaction of CuBr and ttz.11 An acetonitrile/acetone solution of CuBr was added to an acetone solution of ttz. Black single crystals of 1 were obtained by the slow diffusion of hexane to the reaction mixture at 40 °C in 2 days (yield 44%). Single-crystal X-ray analysis of 1 revealed the formation of a CP, [Cu2Br2(ttz)]n, having an infinite 3D structure. Figure 1a shows the 3D view along the b axis of the CP. The Cu ion had the typical tetrahedral coordination geometry of CuI ions, bound by two bridging Br anions and two N atoms of ttz. The Cu1−N1

ABSTRACT: A new semiconducting 3D coordination polymer, [Cu2Br2(ttz)]n (1), with an acceptor bridging ligand, 1,2,4,5-tetrazine (ttz), was synthesized. The complex shows large absorption bands extending to the near-IR region, indicating a small band gap in the coordination polymer. This complex shows higher conductivity than those of [CuBr(pyz)]n (2), including pyrazine (pyz) with a higher lowest unoccupied molecular orbital level. We performed density functional theory band calculations using the VASP program to understand the electronic states and conducting paths of the coordination polymer.

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oordination polymers (CPs) or metal−organic frameworks, consisting of metal ions and bridging organic ligands, have attracted considerable interest as new organic− inorganic hybrid materials with polymeric structures that show characteristic functionalities based on the unique infinite structures connected by coordination bonds such as gas adsorption,1 catalysis,2 magnetic properties,3 and electronic conductivities.4 The design and creation of semiconducting CPs is one of the most attractive subjects in this field because these CPs have a potential application in electronic devices such as ion batteries,5 solar cells,6 light-emitting diodes,7 and field-effect transistor devices.8 Most of the CPs, however, are insulators because their highest occupied molecular orbitals (HOMOs) or lowest unoccupied molecular orbitals (LUMOs) are localized on the metal centers or bridging ligands because of the mismatch of the energy levels for each component. One of the strategies for designing semiconducting CPs is to piece together several components of metal ions and bridging ligands with similar HOMO or LUMO levels, which would create energy-band structures important for the carrier transport of holes and electrons in semiconductors because of overlap of d orbitals of metal ions and HOMOs or LUMOs of bridging ligands. On the basis of this strategy, we developed semiconducting CPs consisting of copper(I) bromide (CuIBr) and copper(II) © 2018 American Chemical Society

Received: November 17, 2017 Published: February 12, 2018 2373

DOI: 10.1021/acs.inorgchem.7b02923 Inorg. Chem. 2018, 57, 2373−2376

Communication

Inorganic Chemistry

around 1400 nm. The high absorption in the visible region around 750 nm can be attributed to the MLCT transition because neither CuIBr nor ttz showed light absorption in this region. For determination of the energy band gaps (Eg) of 1 and 2, the Kubelka−Munk plots of ( f(R) E)1/2 versus E were employed (Figure 2b).14 Eg corresponds to the intersection point between the baseline along the energy axis and a line extrapolated from the linear portion of the threshold. Thus, the Eg values of 1 and 2 were determined to be 0.71 and 2.29 eV, respectively, indicating that the drastic decrease in Eg for 1 is due to the existence of lower conduction bands formed by mixing of the d orbitals of the CuI ion and the LUMO of ttz. The direct-current (dc) electrical resistivities of powderpressed pellet samples of 1 and 2 sandwiched by brass electrodes (diameter, 13 mm) were measured at 300 K with an increase in the applied voltage to 200 V. In the low-voltage region, the current densities (J) for 1 and 2 increased in proportion to the applied voltage according to Ohm’s law (Figure 3a). The

Figure 1. (a) Perspective view of the 3D framework of 1 viewed along the b axis. (b) 2D sheet structure consisting of 1D zigzag CuIBr chains along the b axis and bridging ttz ligands. (c) 1D zigzag chain parallel to the c axis, which connects the 2D sheets of part b. H atoms are omitted for clarity. Color code: red, Cu; orange, Br; white, C; blue, N.

distance was 2.001(2) Å, which was slightly shorter than those of the other CuIBr complexes with heterocyclic ligands such as [CuBr(pyz)]n [2.046(4) Å], [Cu3Br3(Tri)]n [2.075(4) Å av.],12 and [CuBr(Qnz)]n [2.116(6) and 2.020(6) Å].12 The shorter Cu−N distance must have been due to the larger π-backdonation caused by the lower LUMO level of the ttz ligand. In order to confirm the oxidation state of the Cu ion, we performed bond-valence-sum (BVS) calculations.13 The estimated BVS value for the Cu ion was 1.30, which was larger than those of most of the copper(I) complexes such as [CuBr(pyz)]n (1.16), [Cu3Br3(Tri)]n (1.13 av.), and [CuBr(Qnz)]n (1.10). This indicates that the oxidation state of the Cu ion in this complex was electron-deficient rather than those of typical CuI ions due to the strong acceptor ligand, ttz. Figure 1b shows a 2D sheet structure on the ab plane included in the 3D CP 1. The infinite Cu1−Br1 chains along the b axis were bridged by ttz ligands to construct the 2D sheet. Figure 1c is the 1D chain along the c axis. Cu1−Br1 chains were bridged by the ttz ligand to connect the 3D framework. Figure 2 shows the diffuse-reflectance spectra of 1 and 2. Most of the copper(I) complexes with heterocyclic N-donor ligands

Figure 3. (a) J−V curves of powder-pressed pellet samples of 1 and 2 obtained by dc conductivity measurements. (b) Complex impedance Z′−Z″ plots of 1. The solid lines represent the fit using a circuit equivalent to that in the inset. (c) Arrhenius plot of σ1 (bulk) calculated from R1.

estimated conductivities for 1 and 2 were 8.17 × 10−10 and 8.30 × 10−16 S cm−1, respectively, in which the conductivity of 1 was 6 orders of magnitude greater than that of 2. In order to avoid the effect of contact resistivity, impedance measurement was carried out using powder-pressed pellet samples of 1 and 2. The plots of Z′ versus Z″ (Nyquist plot) for 1 at different temperatures are shown in Figure 3b, in which the radii of parts of the semicircular arcs decreased with increasing temperature, indicating decreased resistivity in the sample characteristics of semiconductors. ZView software15 was used to fit the experimental impedance data in the frequency range of 200 Hz to 1 MHz for 1. The results obtained by applying the least-squares fitting method to the equivalent model are shown in the inset of Figure 3b. The conductivities of the bulk sample were calculated using R1. The estimated conductivity of the bulk sample at 300 K was 1.92 × 10−7 S cm−1, and the value of the estimated activation energy from the slope in Figure 3c was 0.31 eV, which was almost half of the optical band gap estimated from the absorption spectrum. However, we could not succeed in the fitting of the impedance data of 2 because of the low impedance response (Figure S6). To reveal the electronic structures of 1 and 2, we performed density functional theory (DFT) calculations (PAW16 / HSE0617) using the VASP program.18 The partial densities of states (PDOSs) of 1 and 2 for each atom are plotted in parts a and b of Figure 4, respectively. The atom-specific PDOS plots

Figure 2. (a) Diffuse-reflectance spectra of 1 and 2. (b) Plots of the modified Kubelka−Munk function versus energy of the exciting light.

show high absorption in the visible region based on the metal-toligand charge-transfer (MLCT) transition. 2 also shows a high absorption at 350 nm due to MLCT (Figure 2a). On the other hand, the MLCT bands of 1 were largely shifted to the near-IR (NIR) region with a maximum peak at 1257 nm and a shoulder 2374

DOI: 10.1021/acs.inorgchem.7b02923 Inorg. Chem. 2018, 57, 2373−2376

Inorganic Chemistry

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Communication

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Takashi Okubo: 0000-0003-3430-9498 Yusuke Noda: 0000-0003-0401-6731 Kunihisa Sugimoto: 0000-0002-0103-8153 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was partly supported by a Grant-in-Aid for Science Research from the Ministry of Education, Culture, Sports, Science and Technology of Japan and PRESTO, Japan Science and Technology Agency (JST), Kawaguchi, Saitama, Japan. Part of the work was supported by MEXT-Supported Program for the Strategic Research Foundation at Private Universities 2014− 2018, a subsidy from MEXT and Kindai University. This work was also supported in part by the “Materials Research by Information Integration” Initiative (MI2I) project of the Support Program for Starting Up Innovation Hub from JST.

Figure 4. Electronic PDOSs for 1 (a) and 2 (b). Color code: red, Br; green, Cu; blue, N; pink, C; sky blue, H.

show that the valence bands were composed predominantly of Cu 3d orbitals, but 1 had a larger proportion of Br ions and N atoms of the heterocyclic bridging ligand than 2 did, which indicates mixing of the 3d orbitals of the Cu ion, the 4p orbitals of the Br ion, and the LUMO of ttz for 1. The conduction bands were mainly composed of LUMOs of heterocyclic ligands for both complexes, but 1 had a larger proportion of Cu ions in the conduction band, which was consistent with the existence of strong π-back-donation in this complex. The diagrams of bandenergy dispersion for 1 and 2 are shown in Figures S7 and S8, respectively. The results indicate that both complexes were indirect band-gap semiconductors. The estimated indirect band gaps of 1 and 2 were 0.92 eV (between the Y and S points) and 2.09 eV (between the Γ and D points), respectively, which were similar to those of the optical band gaps estimated by UV−vis− NIR measurements. In Figure S7, the valence-band maximum and conduction-band minimum are located on points Y and S, respectively, indicating an indirect transition-type semiconductor. The estimated effective masses are listed in Table S4, in which the Y → T and S → R directions have the smallest effective masses estimated from the curvatures of the dispersions for holes and electrons, respectively. This indicates that the 1D chain through ttz in Figure 3b became the most effective hole and electron conducting path in the 3D framework in 1. In summary, we synthesized a new semiconducting CP with a narrow band gap by combining CuIBr and the ttz ligand with a low LUMO level. In addition, the band structure was revealed by a DFT calculation.

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The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.7b02923. Experimental details, crystal structures, magnetic data, band structures, and photocurrent transient (PDF) Accession Codes

CCDC 1586000 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing data_ [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033. 2375

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DOI: 10.1021/acs.inorgchem.7b02923 Inorg. Chem. 2018, 57, 2373−2376