# A Brief Review on the Recent Experimental Advances in Thermal Rectification at the Nanoscale
## 2 Experimental Study
### Thermal Rectification in Vanadium Dioxide Beams
Figure 9: (**A,B**) Heat flow \(Q\) as a function of temperature difference \(\Delta T\) across the VO\({}_{2}\) beams at 300K and 385K, respectively. Different signs (\(+\)) and (\(-\)) of heat flow represent different heat flux directions. Reprinted with permission from ACS Publications. because the lattice distortion is less than 1% in VO\({}_{2}\) crystals, which causes perfect acoustic matching and greatly reduces the phonon coupling resistance. Under these circumstances, the difference between the electron-phonon cooling rate and the phonon-electron cooling rate explains the cause of thermal rectification near the interface. Moreover, the observation of thermal rectification requires a distinct asymmetry throughout the mixed phase, which can be verified by a scattering type scanning near-field optical microscope (s-SNOM). The principle of s-SNOM is that the regions of the metallic phase can cause a higher s-SNOM amplitude signal than the insulating phase due to its larger effective tip\(-\)sample polarizability. The mixed phases can be directly observed by comparing the s-SNOM signal amplitudes between the metallic and insulating phases. Figure 10 compares the s-SNOM images at 300K and 345K. It shows that at 300K the narrow end of the tapered VO\({}_{2}\) beam is metallic while the wide end is insulating. When the temperature is heated to 345 K (above the transition temperature around 340 K), this notable amplitude difference between two phases disappears. Similarly, no difference in signal amplitude was noticed in a VO\({}_{2}\) beam with a uniform cross section. This demonstrates that the asymmetry of the internal mixed phase due to the tapered structure is the cause of the thermal rectification effect. In this work, a thermal rectification device controlled by ambient temperature was realized. The maximum thermal rectification coefficient was observed to be 28% at 320 K, and the thermal rectification effect disappeared when the temperature exceeded the transition temperature at 340 K. This study provides a new research direction in which the interplay between the metallic and insulating phases can realize active thermal control. Moreover, VO\({}_{2}\) has a wide range of applications due to its unique phase change behavior. For example, [PERSON] et al. has experimentally demonstrated a radiative thermal rectifier in the far-field regime with a thin film VO\({}_{2}\) deposited on the silicon wafer [47]. [PERSON] and [PERSON] et al. designed and implemented a novel, all-solid-state thermal switching device by nanostructured phase transformation of VO\({}_{2}\) thin film [48]. Figure 10: **(A)** Atomic force microscope image obtained via tapping mode on a suspended tapered VO\({}_{2}\) beam. **(B,C)** Near-field amplitude signal obtained at second harmonic demodulation on the same tapered VO\({}_{2}\) beam at 300K and 345K, respectively. **(D)** Relative height as a function of beam length at the line profile drawn across the tapered region in **(A)**. **(E,F)** Relative amplitude as a function of beam length at the line profile drawn across the tapered region in **(B,C)**. Reprinted with permission from ACS Publications.