Negative Thermal Expansion in Nanosolids

Nanosolids usually exhibit a variety of peculiar physical features due to the size effect. The unique surface electronic states and coordination structures of nanosolids make them particularly important as promising functional materials. After several decades of research effort on the preparation processes and formation mechanisms of nanomaterials, the attention of nano- science has been shifted to their functionalization and utilization. In the development of nanodevices, the thermal expansion matching between nanosized components is becoming increasingly important for the selection of units and design of nanodevices. In nanosolids, particularities of bonding features and coordination environments lead to size-dependent thermal expansion behavior that is significantly different from the behavior of their bulk counterparts. Thus, size tuning becomes one of the most efficient techniques in tailoring lattice thermal expansion. Unlike the traditional tailoring methods like chemical doping, the modification of chemical bonds and lattice vibration modes mainly contributing to the abnormal thermal expansion of nanosolids can be realized by adjustment of local coordination on the surface and surface/interface lattice strain. With the introduction of the nanosizing effect, the functional properties of nanosolids can be thoroughly remolded, which provides a huge space for functional applications of negative thermal expansion (NTE) nanosolids. However, understanding the origin of novel thermal expansion in nanosolids remains a challenging issue because of the lack of knowledge of precise atomic arrangements at both long-range and local structure levels. In this Account, by virtue of various advanced characterization techniques, we provide a comprehensive understanding at the atomic level of the abnormal thermal expansion behaviors in nanosized PbTiO3-based compounds, oxides, fluorides, and bimetallic alloys. Our results demonstrate that nanoscale structural features can be used to alter the spontaneous polarization, surficial/interfacial coordination, local lattice symmetry, and elemental distribution, resulting in the crossover of thermal expansion from the bulk and the generation of zero thermal expansion (ZTE). Furthermore, structural peculiarities in nanosolids, e.g., the lack of long-range coherence, abnormal surficial/interfacial bonding, lattice imperfection, and distribution of local phases, open the door for local-scale manipulations of the physical properties of electronic structure and lattice vibration during adjustment of thermal expansion. For the development of nanodevices with high thermostability, atomic-level information on the nanostructure thermal evolution provides a guideline for intelligent designs of the functional components and matrix. Understanding of the structural transformation in nanosolids will help future exploration of functional nanomaterials based on short-range atomistic design and optimization.