Zero Thermal Expansion in a Nanostructured Inorganic-Organic Hybrid Crystal

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, $\beta \mathrm{\text{−}}\mathrm{ZnTe}(\mathrm{en}{)}_{0.5}$ (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 $\mathrm{ZnTe}(\mathrm{pda}{)}_{0.5}$ (pda denotes 1,3-propanediamine) with a larger molecule.

Figure 1

The crystal structure of $\beta \mathrm{\text{−}}\mathrm{ZnTe}(\mathrm{en}{)}_{0.5}$, determined by single-crystal x-ray diffraction. Two-monolayer-thick $(\overline{1}10)$ ZnTe slabs are interconnected by ethylenediamine molecules bonded to Zn atoms. Hydrogen atoms are omitted for clarity. The unit cell, as outlined, contains a pair of ZnTe slabs, rotated 180° with respect to each other about the $b$ axis.

Figure 2

The temperature dependence of the lattice constants, $a$, $b$, and $c$ for $\beta \mathrm{\text{−}}\mathrm{ZnTe}(\mathrm{en}{)}_{0.5}$, measured on both powder and single-crystal samples and on different x-ray diffraction systems. The span of the vertical axis in the three upper panels is $\sim 0.654\%$ of the respective lattice constant.

Figure 3

Atomic vibration amplitudes with respect to average crystallographic sites, along the three principle axes $a$, $b$, and $c$ of $\beta \mathrm{\text{−}}\mathrm{ZnTe}(\mathrm{en}{)}_{0.5}$, for atoms within the unit cell (H atoms are not shown for clarity) defined by their $b$ coordinates. Included are the four lowest optical phonon modes at the $\Gamma$ point.

Figure 4

Raman spectra of $\beta \mathrm{\text{−}}\mathrm{ZnTe}(\mathrm{en}{)}_{0.5}$ measured at 30 and 300 K with cross polarization ($c|a$). The inset shows the 30 K spectra with parallel polarization ($a|a$) and ($b|b$). The equivalent slit width was $0.5{\mathrm{cm}}^{-1}$.