# Recent Advances in Research from Nanoparticle to Nano-Assembly: A Review
## 3 Stimuli Dependent Techniques
### Physical Stimuli-Based Self-Assembly of Nanomaterials
#### 3.2.4 Temperature-Induced Self-Assembly
Upon decreasing the temperature further, a NaZn13-type lattice was observed as shown in Figure 8c. As shown in Figure 8d-g, [PERSON] et al. demonstrated the SA approach using plasmonic AgNPs, where the NPs intrinsically had the thermally responsive organic coating to facilitate the SA [121]. The spatial distribution of the AgNPs can be reversibly switched on account of the dynamic SA resulting from temperature-driven changes of the organic coating shape. The TEM analysis of the samples presented interesting results where the Lm phase, the measured interlayer distance was 8.5 \(\pm\) 0.4 nm (cooled), while for Iso the interparticle distance was 7.3 \(\pm\) 0.8 nm (heated). A model was developed to understand the occurrence of the SA in the NPs, and it was noted that while the Iso phase presented a homogenous distribution of promesogenic and alkyl ligands, these entities were primarily located in between the layers for the Sm structure [121]. Such a spatial rearrangement of the NP metallic core assists in the formation of reversible SA structures. While these are the representative examples for the formation of SA by modulating the temperature, we present a close look into the recently developed technique based on the CSNE in the subsequent sections. Fundamentally, the CSNE is a technology where the homogenous solution of NPs is subjected to a temperature gradient to yield precise NAs. The temperature gradient assists in the migration of the NPs from the hot end of the system to the cold end through a phenomenon termed the soret effect [42; 122]. Since the entire process can be carried out using cryo-environments (or low temperatures), the process of obtaining NAs through this technology is called CSNE and the thus-obtained assemblies are termed cryosorets. The schematic representation of the steps involved in the formation of the cryosorets is presented in Figure 9a, where the capability of low temperatures to yield cryosorets at faster rates is captured conceptually [42; 122]. That is, while the process of obtaining cryosorets using \(-80\)\({}^{\circ}\)C takes 30 min, the same process is completed in 3 min for a \(-196\)\({}^{\circ}\)C temperature. In other words, at lower temperatures, the thermal gradient can overcome the electrostatic repulsion exhibited by the citrate-capped metallic NPs, resulting in precise nano-assemblies. It has been observed that while the freshly prepared (via the conventional Turkevich method using citrate as reducing and capping agents) plasmonic AgNPs are subjected to such low temperatures, precise NPs can be obtained, enabling researchers to tune the number of NPs per assembly by modulating the time of exposure to temperature gradient [42; 122]. Figure 8: (**a**) lower critical solution temperature (LCST)-induced aggregation in water of individually coated magnetite poly(N-isopropylacrylamide) NPs. Adapted with permission from reference [119]. (**b**) The thermoresponsive tuning of plasmonic properties of charged colloidal AuNPs where the switching between the disassembled and assembled states in response to temperature change is observed. The changes in the LSPR are also shown. Adapted with permission from reference [120]. (**c**) Self-assembly of 8 nm CdSe and 3.1 nm PbS nanocrystals at different temperatures (c1) 40 \({}^{\circ}\)C (c2) 65 \({}^{\circ}\)C and (c3) 85 \({}^{\circ}\)C. Observation of the binary phases assembled at different temperatures. Typical electron microscopy images of binary superlattices isostructural with NaZn\({}_{13}\), AlB\({}_{2}\), and NaCl compounds are presented. Adapted with permission from reference [22]. Representative TEM image of thermally annealed Ag@LN NPs presenting the patterns of (**d**) Iso and (**e**) Lm structures, scale bar, 20 nm. (**f**) Scheme of nanoparticle arrangements in Iso and (**g**) Lm structures as deduced from SAXRD and TEM. Adapted with permission from reference [121].