Commensurate-incommensurate transition and strain relief patterns in monolayer C60 on Cd(0001)

Abstract

We have studied the commensurate-incommensurate phase transition, rotation epitaxy, and strain relief patterns in ${\mathrm{C}}_{60}$ overlayers grown on Cd(0001) with a low-temperature scanning tunneling microscopy. When deposited at low temperature (\ensuremath{\sim}200 K), ${\mathrm{C}}_{60}$ molecules form the 10\ifmmode\times\else\texttimes\fi{}10 high-order commensurate (HOC) phase with two different half-unit cells, resembling the $\mathrm{Si}(111)\text{\ensuremath{-}}7\ifmmode\times\else\texttimes\fi{}7$ surface. Postannealing at room temperature (RT) results in the transition from HOC phase to incommensurate phase, in which strain relaxation takes place in the form of periodic vacancies and ${\mathrm{C}}_{60}$ heptamer arrays. In the case of RT deposition, parallel stripe domain walls appeared in the commensurate $2\sqrt{3}\ifmmode\times\else\texttimes\fi{}2\sqrt{3}$ R30\ifmmode^\circ\else\textdegree\fi{} phase. These results provide essential information for understanding the strain relaxation mechanism, and the role of substrate temperatures in the process of ${\mathrm{C}}_{60}$ thin films growth.