Flexible hybrid semiconductors with low thermal conductivity: the role of organic diamines.

Crystalline compounds built up of periodically ordered nanostructured inorganic semiconductor motifs and organic molecules are a new type of hybrid semiconducting materials that are of great fundamental importance and technological relevance. The most appealing feature of these materials is that many favorable properties of each individual component are brought into the hybrid structure by incorporating two distinctly different components into a single crystal lattice. Integration and combination of exceptional transport properties and structural/thermal stability from the inorganic component and superb flexibility and processibility from the organic component can be expected. Additionally, the blending of the inorganic and organic modules in these crystalline hybrid structures takes place at the atomic level and through chemical bonds, and thus is free of the interface issues that are inevitably present in conventional hybrid composite materials. Furthermore, the formation of such hybrid crystals almost always leads to unique and remarkable new features that are not possible for the individual constituents. Some notable examples include organic–inorganic perovskite-like structures and related materials, hybrid metal oxides, and semiconductors composed of zinc blende and wurtzite frameworks. The II/VI based hybrid semiconductor crystal structures (II: Group 12 elements and Mn; VI: Group 16 elements) are composed of one-dimensional (1D) chains or two-dimensional (2D) slabs of II/VI semiconductor fragments that are interconnected or separated by organic amine molecules to form periodic crystal lattices. They are of the general formula [MQ(L)x] (M=Mn, Zn, Cd; Q= S, Se, Te; L= organic amine or diamine; and x= 0.5, 1). The most intriguing observations include extremely strong band-edge absorption (e.g. 10–20 times higher than bulk II/VI andGaAs) and exceedingly large band-gap tunability (0.1–2.0 eV) as a result of very strong structure-induced quantum confinement. Although, according to theoretical calculations, the organic spacers give rise to a very limited effect on the band-gap-related electronic and optical properties, they play a crucial role in the structural, mechanical, and thermal behaviors of these hybrid materials. Herein, we report five crystal structures of 3D-[ZnTe(L)0.5] made of ZnTe single-atomic slabs and longchain diamines, as well as their structural phase transitions, mechanical properties, specific heat capacity, thermal diffusivity, and thermal conductivity. Our analysis shows that crystalline hybrid semiconductors of this type are much lighter and substantially more flexible than their inorganic counterparts. The incorporation of organic molecules into the semiconductor crystal lattices also leads to significantly reduced thermal conductivity that is most desirable for high performance thermoelectric materials with structural integrity. All the compounds were synthesized by solvothermal reactions using alkyldiamines as reactive solvents (for details, see the Supporting Information). The optimal reaction temperatures were found to be 200–210 8C, and the final products are colorless plate-like crystals; the crystal structures were determined by single-crystal X-ray diffraction methods. a-[ZnTe(bda)0.5] (bda= 1,4-butanediamine; 1) and a-[ZnTe(hda)0.5] (hda= 1,6-hexanediamine; 3) are isostructural and crystallized in the orthorhombic space group Pbca (Supporting Information, Table S1). Both structures are composed of single-atomic [ZnTe] slabs interconnected via diamine molecules by Zn N coordinative bonds. Each zinc atom bonds to three tellurium atoms and one nitrogen atom from the alkyldiamine molecule to form a stable tetrahedral conformation. Each tellurium atom connects to three neighboring zinc atoms. Alternating zinc and tellurium atoms form puckered six-membered rings extended in two dimensions (the crystallographic ab plane). The overall three-dimensional structures of 1 and 3 are given in Figures 1a and 2a, respectively. The [ZnTe(ptda)0.5] structure (ptda= 1,5-penta[*] Dr. X. Huang, M. Roushan, Dr. T. J. Emge, Dr. W. Bi, Prof. J. Li Department of Chemistry and Chemical Biology, Rutgers, The State University of New Jersey Piscataway, NJ 08854 (USA) Fax: (+1)732-445-5312 E-mail: [email protected]