Shruti Mendiratta

Semiconductor Metal–Organic Frameworks: Future Low-Bandgap Materials

Metal-organic frameworks (MOFs) with low density, high porosity, and easy tunability of functionality and structural properties, represent potential candidates for use as semiconductor materials. The rapid development of the semiconductor industry and the continuous miniaturization of feature sizes of integrated circuits toward the nanometer (nm) scale require novel semiconductor materials instead of traditional materials like silicon, germanium, and gallium arsenide etc. MOFs with advantageous properties of both the inorganic and the organic components promise to serve as the next generation of semiconductor materials for the microelectronics industry with the potential to be extremely stable, cheap, and mechanically flexible. Here, a perspective of recent research is provided, regarding the semiconducting properties of MOFs, bandgap studies, and their potential in microelectronic devices.

Semiconductor Behavior of a Three-Dimensional Strontium-Based Metal–Organic Framework

The self-assembly of a three-dimensional strontium-based metal-organic framework [Sr(Hbtc)(H2O)]n (1) was achieved through the reaction of Sr(NO3)2 with a 1,2,4-benzenetricarboxylic acid (1,2,4-H3btc) ligand under hydrothermal conditions. This Sr-based metal-organic framework exhibits remarkable semiconducting behavior, as evidenced by theoretical calculations and experimental measurements. Temperature-dependent DC conductivity, near-room-temperature AC conductivity, diffuse reflection spectra, and photoluminescence spectra provide strong proof that compound 1 shows a band gap of 2.3 eV, which is comparable to that for other commonly available semiconducting materials (e.g., CdSe, CdTe, ZnTe, GaP, etc.). The optimized molecular structure and electronic properties (density of states and band gap energy) of 1 were calculated using density functional theory, and the results are consistent with experimental findings. This is the first report on the semiconducting properties of a strontium-based MOF, which will pave the way for further studies in semiconducting MOFs with interesting potential applications in optoelectronic devices.

Metal–organic frameworks for electronics: emerging second order nonlinear optical and dielectric materials

Abstract Metal–organic frameworks (MOFs) have been intensively studied over the past decade because they represent a new category of hybrid inorganic–organic materials with extensive surface areas, ultrahigh porosity, along with the extraordinary tailorability of structure, shape and dimensions. In this highlight, we summarize the current state of MOF research and report on structure–property relationships for nonlinear optical (NLO) and dielectric applications. We focus on the design principles and structural elements needed to develop potential NLO and low dielectric (low-κ) MOFs with an emphasis on enhancing material performance. In addition, we highlight experimental evidence for the design of devices for low-dielectric applications. These results motivate us to develop better low-dielectric and NLO materials and to perform in-depth studies related to deposition techniques, patterning and the mechanical performance of these materials in the future.

Guest dependent dielectric properties of nickel(II)-based supramolecular networks

Two nickel(II)-based low dielectric supramolecular compounds {[Ni2(bbim)(H2bbim)4]·2CH3COO·CH3CN}2 (1, H2bbim = bisbenzimidazole) and (±)-[Ni(H2bbim)3]·2Cl·2H2O (2) were synthesized and characterized by single-crystal X-ray crystallography. Compound 1 was found to have a dimeric structure with guest molecules such as acetate ions and acetonitrile in its environment, while compound 2 had a monomeric structure with chloride ions and water molecules associated with it. Both compounds were highly thermally stable, especially compound 1, which was stable at temperatures of up to 500 °C. More importantly, compound 1 adopted a sharp different frequency dependent dielectric behaviour when compared with 2. Compound 2 with highly polarizable guest molecules showed a significant higher value of dielectric constant (er′(ω) = 12.6 at 40 Hz) than that of 1 (er′(ω) = 4.76 at 40 Hz), indicating that solvent molecules and counterions play a crucial role in regulating the value of the dielectric constant. This study serves as a good example of the design of both high and low-κ materials with a judicious selection of guest molecules in the supramolecular networks.

Continuous broadband emission from a metal–organic framework as a human-friendly white light source

Natural white light emission from a single component, a novel strontium(II)-based metal–organic framework, {[Sr(ntca)(H2O)2]·H2O}n (1) was achieved. Compound 1 exhibited a remarkable continuous broadband emission with the highest sensitivity at around 550 nm, making it more physically comfortable for the human eye. The continuous spectrum of compound 1 also closely resembles the sunlight spectrum with a CCT of 5451 K, producing a natural white light. An alkaline earth metal (Sr) is used as the metal node in the preparation of the compound which is more environmentally friendly as compared with commonly used lanthanides in luminescent materials. This natural white light emitter with a pleasant effect on human visibility, and without any doping of lanthanide promises to open up new perspectives for the development of high-performance solid-state lighting sources.

A rare doubly nitrato and phenoxido bridged trimetallic CuII complex: EPR, antiferromagnetic coupling and theoretical rationalization

A rare trimetallic CuII complex, [Cu3(L)2(NO3)2] (1) has been afforded incorporating a [CuL] metalloligand, [where H2L = N,N′-bis(salicylidene)-1,3-propanediamine]. In 1, in addition to the double phenoxido bridge, the two terminal CuII atoms are linked to the central CuII by means of syn–anti bridging nitrate anions giving rise to a linear arrangement. Complex 1 exhibits strong antiferromagnetic coupling and shows an χMT value of 0.703 cm3 mol−1 K at 300 K. DFT computational studies reveal that there is a clear magneto-structural correlation between the Cu–O–Cu angle and the JCu–Cu values, which is in accordance with the experimental evidence. The biological effect of 1 on the viability of different human carcinoma cells was evaluated using MTT assay and the results indicate that this complex induces a decrease in cell-population growth of human colorectal carcinoma cells (COLO 205) with apoptosis.

Zn(II)-based metal–organic framework: an exceptionally thermally stable, guest-free low dielectric material

The synthesis of an exceptionally thermally stable, chemically stable, guest-free low-κ dielectric metal–organic coordination framework [Zn2(Hbbim)2(bbim)]n (1, H2bbim = bisbenzimidazole) was achieved under hydrothermal conditions. Structural analysis showed that compound 1 crystallizes in a triclinic space group P1 and possesses a guest-free structure. Compound 1 was found to be a low-κ dielectric material (3.05 at 1 MHz) that was extremely robust towards various solvents. The compound was exceptionally thermally stable and retained its structure at temperatures of up to 450 °C. Temperature-dependent dielectric studies revealed that 1 has a low-dielectric constant that remains stable upon increasing the temperature. This low-dielectric constant was further supported by density functional theory calculations, which showed that the dielectric property can be mainly attributed to electronic polarizability.

High-κ Samarium-Based Metal–Organic Framework for Gate Dielectric Applications

The self-assembly of a samarium-based metal-organic framework [Sm2(bhc)(H2O)6]n (1) in good yield was achieved by reacting Sm(NO3)3·6H2O with benzenehexacarboxylic acid (bhc) in a mixture of H2O-EtOH under hydrothermal conditions. A structural analysis showed that compound 1 crystallized in a space group of Pnmn and adopted a 3D structure with (4,8) connected nets. Temperature dependent dielectric measurements showed that compound 1 behaves as a high dielectric material with a high dielectric constant (κ = 45.1) at 5 kHz and 310 K, which is comparable to the values for some of the most commonly available dielectric inorganic metal oxides such as Sm2O3, Ta2O5, HfO2, and ZrO2. In addition, electrical measurements of 1 revealed an electrical conductivity of about 2.15 × 10-7 S/cm at a frequency of 5 kHz with a low leakage current (Ileakage = 8.13 × 10-12 Amm-2). Dielectric investigations of the Sm-based MOF provide an effective path for the development of high dielectric materials in the future.

CCDC 1492702: Experimental Crystal Structure Determination

Related Article: Shruti Mendiratta, Muhammad Usman, Chun-Chi Chang, Yung-Chi Lee, Jenq-Wei Chen, Maw-Kuen Wu, Ying-Chih Lin, Chao-Ping Hsu, Kuang-Lieh Lu|2017|J.Mater.Chem.C|5|1508|doi:10.1039/C6TC05314A