Resonant Electron Heating and Molecular Phonon Cooling in Single ${\mathrm{C}}_{60}$ Junctions

We study heating and heat dissipation of a single ${\mathrm{C}}_{60}$ molecule in the junction of a scanning tunneling microscope by measuring the electron current required to thermally decompose the fullerene cage. The power for decomposition varies with electron energy and reflects the molecular resonance structure. When the scanning tunneling microscope tip contacts the fullerene the molecule can sustain much larger currents. Transport simulations explain these effects by molecular heating due to resonant electron-phonon coupling and molecular cooling by vibrational decay into the tip upon contact formation.

Figure 1

(a) STM image of a 0.2 monolayer film of ${\mathrm{C}}_{60}$ on Cu(110) ($I_t=1.1\mathrm{nA}$, $V_s=1.75\mathrm{V}$). The intramolecular structure (left inset; $I_t=1.0\mathrm{nA}$, $V_s=2.25\mathrm{V}$ [29]) reveals an intrinsic asymmetry consistent with the adsorption orientation from density functional theory simulations (right inset). (b) Differential conductivity spectrum of ${\mathrm{C}}_{60}$ ($R_{\mathrm{junct}}=2.2\mathrm{G}\Omega$, lock-in amplifier: $V_{\mathrm{ac}}=20\mathrm{mV}$ rms). (c) Conductance and current vs $Z_{\mathrm{appr}}$ plot on top of a ${\mathrm{C}}_{60}$ molecule ($V_s=0.5\mathrm{V}$). The shaded area indicates the contact regime.

Figure 2

$I(Z_{\mathrm{appr}})$ plots showing molecular decomposition in (a) tunnel ($V_s=2.0\mathrm{V}$) and (b) contact regime ($V_s=1.0\mathrm{V}$). (c) Scanning tunneling spectroscopy spectrum of a molecule before and (d) after an $I(Z_{\mathrm{appr}})$ plot like in panel (a). The resonance structure disappears, thus indicating destruction of the fullerene cage. (e) ${\mathrm{C}}_{60}$ island before and (f) after performing $I(Z_{\mathrm{appr}})$ events on the molecules marked in (e). After this, all the marked molecules show a lower height and a $dI/dV$ spectrum similar to panel (d) ($I_t=1.0\mathrm{nA}$, $V_s=2.25\mathrm{V}$).

Figure 3

(a) Statistical average of $I_{\mathrm{dec}}$ vs $V_s$ with two sigma error bars. All the events considered (180) correspond to degradation as in Fig. 2, and were done on fullerenes surrounded by unperturbed ${\mathrm{C}}_{60}$ molecules. (b) Bias dependence of $P_{\mathrm{dec}}$ (squares) and distance to contact $Z$ (diamonds). For events in the tunnel regime, $Z_{\mathrm{dec}}$ is obtained by linear extrapolation. Shaded areas indicate decomposition in the contact regime. Dashed lines represent normalized $dI/d{V}_s$ plots.

Figure 4

Results of theoretical simulations: (a) $T_m$ vs $V_s$ for the indicated distances to contact. The oscillations of $T_m$ are associated to the molecular resonances (dashed line). (b) The values $I_{\mathrm{dec}}$ (diamonds, continuous line) and $P_{\mathrm{dec}}$ (squares, dark broken line) are marked for a threshold temperature of 1650 K of the ${\mathrm{C}}_{60}$ molecule. The shaded area marks the contact regime. (c) Ratio of the phonon absorption rate ($A^{\mathrm{tip}}$) to the total absorption rate in the junction ($A^{\mathrm{tot}}$) vs tip-molecule distance (tip-molecule contact is defined as 0 Å) at 0.4 V. (d) $T_m$ at 0.4 V vs tip-molecule distance with (lower line) and without (upper line) electron-hole pair formation in the tip.