Zero Thermal Expansion in a Nanostructured Inorganic-Organic Hybrid Crystal Y. Zhang, 1, * Z. Islam, 2 Y. Ren, 2 P. A. Parilla, 1 S. P. Ahrenkiel, 1 P. L. Lee, 2 A. Mascarenhas, 1 M. J. McNevin, 3 I. Naumov, 4 H.-X. Fu, 4 X.-Y. Huang, 5 and J. Li 5 1 National Renewable Energy Laboratory, 1617 Cole Boulevard, Golden, Colorado 80401, USA 2 Advanced Photon Source, Argonne National Laboratory, 9700 S. Cass Avenue, Argonne, Illinois 60439, USA 3 Department of Chemistry and Biochemistry, University of Colorado, Boulder, Colorado 80309, USA 4 Department of Physics, University of Arkansas, Fayetteville, Arkansas 72701, USA 5 Department of Chemistry and Chemical Biology, Rutgers, The State University of New Jersey, Piscataway, New Jersey 08854, USA (Received 9 May 2007; published 19 November 2007) There are very few materials that exhibit zero thermal expansion (ZTE), and of these even fewer are appropriate for electronic and optoelectronic applications. We find that a multifunctional crystalline hybrid inorganic-organic semiconductor, -ZnTeen 0:5 (en denotes ethylenediamine), shows uniaxial ZTE in a very broad temperature range of 4 – 400 K, and concurrently possesses superior electronic and optical properties. The ZTE behavior is a result of compensation of contraction and expansion of different segments along the inorganic-organic stacking axis. This work suggests an alternative route to designing materials in a nanoscopic scale with ZTE or any desired positive or negative thermal expansion (PTE or NTE), which is supported by preliminary data for ZnTepda 0:5 (pda denotes 1,3-propanediamine) with a larger molecule. DOI: 10.1103/PhysRevLett.99.215901 PACS numbers: 65.40.De, 61.10.i, 61.46.w, 63.22.+m The majority of the materials that are known to exhibit near zero thermal expansion (ZTE) in a broad temperature range are oxides and usually insulators, with a few non- oxide exceptions: e.g., YbGaGe being metallic [1], and some Prussian Blue analogues being ferromagnetic [2,3]. Traditional interests in ZTE materials have largely been in areas such as optics, heat-engine components, and kitch- enwares. ZTE materials with applications in nonconven- tional areas such as electronics and optoelectronics are rare. Materials with ZTE may be found in three categories: (1) With isotropic ZTE, often achieved by alloying com- monly available positive thermal expansion (PTE) and less available negative thermal expansion (NTE) materials. These materials are mostly oxides with open network structures [4 – 6], but they can also be ferromagnetic ma- terials such as INVAR [4] and some members of the Prussian Blue family [2]. (2) With anisotropic thermal expansion (TE) but overall ZTE in volume, including lithium aluminum silicates [4] and the recently found metallic YbGaGe [1]. (3) With anisotropic TE and ZTE in one or two dimensions, but with nonzero TE in the other dimension(s) as well as in volume. The materials studied in this work belong to the last category. ZTE materials in category (2) are typically used in the form of polycrystal- line aggregates, and so as the temperature changes, internal mesoscopic cracking is often a major concern [4]. When a ZTE material with mesoscopic domains is used as an active medium in electronics or optoelectronics, conductivity also suffers from the existence of grain boundaries. Therefore, if isotropic ZTE is not available, uniaxial or biaxial ZTE is preferred and is adequate for numerous applications, such as solid state laser cavities, semiconductor lasers, and x-ray monochromators and metrologies. A TE coefficient in the range of jj < 2  10 6 K 1 is generally considered as ‘‘very low TE’’ [4]. While the Prussian Blue analogue shows small isotropic TE with an average  1:5  10 6 K 1 between 4 and 300 K [2], the hybrid crystal studied here offers uniaxial but significantly smaller TE with jj < 4:3  10 7 K 1 in a broader temperature range of 4– 400 K. More significantly, the hybrid approach offers a new route to designing materials with any desired TE. Hybrid nanocomposites have lately received a great deal of interest in both fundamental science and applications [7,8]. The materials, MQL x (MQ for II –VI semiconduc- tor, L for organic molecule), to be investigated here belong to a new family of multifunctional hybrid crystals that consist of subnanometer inorganic building blocks inter- connected or coordinated by small organic molecules [9,10]. They can be classified into three groups: 3-D, 2-D, and 1-D structures, depending on the bonding situ- ation between the inorganic and organic components. Here we focus on the 3-D structures, which are inorganic- organic superlattices with a well-defined stacking axis. It is along this axis that the unusual TE is found. The struc- tures have been shown to be fully ordered structures with- out the physical and chemical fluctuations typically found in other hybrid materials and nanostructures [9 –11]; they have also been shown, both experimentally and theoreti- cally, to possess unique electronic and optical properties [9,10,12 –14] that are highly desirable for applications in optoelectronics, including a massive band gap blueshift (  1:4 eV) from that of the inorganic semiconductor [10,13], and exceedingly strong band-edge excitonic ab- sorption (up to 10 6 cm 1 )[13,14]. Together with low- weight and the flexibility of the organic material, these properties make them very promising candidates as active PRL 99, 215901 (2007) PHYSICAL REVIEW LETTERS week ending 23 NOVEMBER 2007 0031-9007= 07=99(21)=215901(4) 215901-1  2007 The American Physical Society