Modeling the destruction of realistic nanotube emitters: Relative role of charging and temperature

To discriminate between the effect of charge and temperature as the main cause of structural changes in nanotube-based field-electron emitters, we performed molecular dynamics simulations of charged single- and multiwall carbon nanotubes under realistic conditions. Our results indicate that cap-terminated nanotubes disintegrate abruptly under critical emission conditions. Disintegration, involving either desorption of cap fragments or a transformation into carbon chains and graphitic flakes, occurs prior to melting and is accelerated by Coulomb repulsion that destabilizes the charged tip.

Figure 1

(Color online) Distribution of excess charge and temperature in a carbon nanotube electron emitter under realistic emission conditions. (a) Schematic picture of the nanotube emitter in a uniform electric field $E\left(z\right)$. The position $z$ is given with respect to the nanotube tip. (b) Excess charge density along the tube based on results from Ref. 13. (c) Typical steady-state temperature profile of a self-heating nanotube emitter, with the highest temperature $T_{\mathit{max}}$ at the tip.

Figure 2

(Color online) Time evolution of the kinetic temperature $T$ during microcanonical molecular dynamics simulations of charged, hot nanotube emitters. Results for low excess charge $\Delta q=0.11e$/atom in (a) and (b) are compared to results for $\Delta q=0.13e$/atom in (c) and (d). Results for the low initial temperature $T_{\mathit{\text{initial}}}=2000\mathrm{K}$ in (a) and (d) are compared to those for hotter tubes with $T_{\mathit{\text{initial}}}=3000\mathrm{K}$. Structural snapshots at the point of destruction, indicated by an arrow in (b) and (e), are presented in (c) and (f), respectively.

Figure 3

(a) Average binding energy in a model nanotube emitter as a function of the excess charge $\Delta q$. (b) Dependence of the relative potential energy $\Delta U\left(T\right)$ of this system on the temperature $T$ with respect to the value at $T=1000\mathrm{K}$. ● marks the point of disintegration. (c) Dependence of the relative potential energy $\Delta U\left(\Delta q\right)$ of this system on the excess charge $\Delta q$ with respect to the value for the neutral system obtained in the long-time limit of molecular dynamics simulations with the initial temperature $T_{\mathit{\text{initial}}}=2000\mathrm{K}$. (d) Expected lifetime of the emitter as a function of the excess charge $\Delta q$ for different initial temperatures.