Rotational transitions in a C${}_{60}$ monolayer on the WO${}_2/$W(110) surface

Variable-temperature scanning tunneling microscopy (STM) is shown to be an effective technique to study two-dimensional phase transitions. Observations show that a monolayer of C${}_{60}$ deposited on an ultrathin WO${}_2$ layer grown on the W(110) surface undergoes a structural phase transition at 259 K, similar in temperature to that of bulk C${}_{60}$. In turn, a kinetic transition has been observed at 220 K, which is significantly higher than that of the bulk C${}_{60}$ crystal (90 K). This difference is attributed to interactions between the molecular overlayer and the substrate, as well as correlation effects within the C${}_{60}$ film. Different types of molecular nanomotion, such as rotation, spinning, and switching between different orientations, have been observed. STM measurements are supported by density functional theory calculations, which provide confirmation of different orientations of C${}_{60}$ on the WO${}_2$ thin film.

Figure 1

STM images of the C${}_{60}$ monolayer at different temperatures. (a) and (b) $12\times 14$ nm${}^2$ STM images acquired at $T=78$ K. (a) Image of quenched C${}_{60}$ film; (b) image of the C${}_{60}$ film obtained by slow cooling at a rate of 100 K/day. Highlighted in black in panels (a) and (b) are the outlines of individual molecules. (c) and (d) $19\times 16$ nm${}^2$ STM images of the same C${}_{60}$ film acquired at $T=315$ K and $T=256$ K, respectively. All molecules in (c) appear as perfect spheres due their fast rotation. Solid black arrows in panel (d) indicate examples of static C${}_{60}$ molecules, solid white arrows show molecules with unresolved orbital structure, and black-outlined and white-outlined arrows point to spinning molecules with a dip or a protrusion at the center, respectively (see text).

Figure 2

Squared probability of finding a static C${}_{60}$ molecule, $p^2$, versus temperature, $T$. Blue (220 K) and red (259 K) arrows indicate the temperatures of kinetic and structural rotational transitions, respectively. In the temperature range from 220 K to 259 K, $p^2$ is fitted by a linear function $p^2=\alpha (T_C-T)$, where $\alpha =0.022\pm 0.002$ K${}^{-1}$ and $T_C=259$ K.

Figure 3

Dynamics of transitions between different molecular states of a C${}_{60}$ monolayer at $T=256$ K. (a)–(d) $19\times 19$ nm${}^2$ STM images of the C${}_{60}$ film acquired at $V_b=1.0$ V and $I_t=0.07$ nA, over a sampling time of 480 sec. Panels (e)–(h) present a zoom of the area marked by the dashed box in panels (a)–(d). The same molecule is indicated by a white circle in (e)–(h) and it can be seen to switch from the medium-conductance state (e) to the high-conductance state (f) and to go from spinning (g) to non-spinning (h). Arrows in panel (g) highlight several molecules that engage in spinning motion, resulting in their ring-shaped appearance. These molecules do not spin in (e) and (f).

Figure 4

(a) $19\times 9$ nm${}^2$ STM image of the C${}_{60}$ film acquired at $V_b=1.0$ V and $I_t$ $=$ 0.07 nA, demonstrating the decoration of the WO${}_2$ rows of the underlying substrate surface by dim C${}_{60}$ molecules, $T=256$ K. (b) Zoom of the area indicated by the dashed box showing a row of spinning molecules.

Figure 5

First row: DFT simulations of the partial charge density distribution of electron states of static C${}_{60}$ molecules facing the substrate by different parts of the molecule. h and p configurations correspond to the molecule facing the substrate by a hexagon or a pentagon, respectively. In h-h and h-p configurations, C${}_{60}$ faces the surface by the C-C bond shared between two hexagons or a hexagon and a pentagon, respectively. Second row: Images obtained by rotating the charge distributions in the top panel in increments of 5${}^{\circ }$. These represent the weighted electron density of a molecule in a particular configuration with the additional degree of rotation around the axis perpendicular to the substrate. Third row: Cross sections through the rotated charge distribution shown in the second row.

Figure 6

STM images of individual spinning molecules with a dip (a) or a protrusion (c) at the center. Panels (b) and (d) correspond to cross sections along the indicated lines in the STM images of the spinning molecules presented in panels (a) and (c). Dashed lines indicate the boundaries of an individual molecule's cross section.