Smallest Metallic Nanorods Using Physical Vapor Deposition

Physical vapor deposition provides a controllable means of growing two-dimensional metallic thin films and one-dimensional metallic nanorods. While theories exist for the growth of metallic thin films, their counterpart for the growth of metallic nanorods is absent. Because of this absence, the lower limit of the nanorod diameter is theoretically unknown; consequently the experimental pursuit of the smallest nanorods has no clear target. This Letter reports a closed-form theory that defines the diameter of the smallest metallic nanorods using physical vapor deposition. Further, the authors verify the theory using lattice kinetic Monte Carlo simulations and validate the theory using published experimental data. Finally, the authors carry out a series of theory-guided experiments to grow well-separated metallic nanorods of $\sim 10\mathrm{nm}$ in diameter, which are the smallest ever reported using physical vapor deposition.

Figure 1

(a) Schematic of the two modes of nanorod growth, with mode II giving rise to the smallest nanorods; and (b) evolution of a nanorod, corresponding to the boxed one in (a), as a function of time for mode II.

Figure 2

(a) The theoretical distribution $S_n(L)$ for various numbers of layers $n$ in height; the inset shows a comparison of the numerical solution, the closed-form expression, and LKMC simulation results under complete geometrical shadowing as a function of $({\nu }_{3\mathrm{D}}/F{)}^{1/5}$. (b) LKMC simulation results under incomplete geometry shadowing as a function of $({\nu }_{3\mathrm{D}}/F{)}^{1/5}$; the separation of nanorod nuclei $L_s$ is included for comparison, and the incidence angle is 85°. The inset shows nanorods from a LKMC simulation with random nucleation. (c) LKMC simulation results under incomplete geometry shadowing as a function of incidence angle, with either the same $F_e=1\sin 5°\mathrm{nm}/\mathrm{s}$ or the same $F=1\mathrm{nm}/\mathrm{s}$; the separation of nanorod nuclei $L_s$ is included for comparison.

Figure 3

Scanning electron microscopy images of well-separated (a) Cu and (b) Au nanorods at an early stage; the insets with the same scale show the morphologies of substrates.

Figure 4

Scanning electron microscopy images of (a) Cu and (b) Au nanorods at a later stage when nanorods are about 1000 nm long; the insets with the same scale show surface morphologies of nanorods when conventional substrates are used.