ABSTRACT
Controlled bottom-up fabrication of molecular nanostructures through on-surface reactions of tailor-made precursors is of scientific and technological interest. Recently, on-surface polymerization reactions influenced by precursor self-assembly have been reported. Thus, a fundamental understanding of the reaction process is a prerequisite for controlled formation. Herein, we report on the influence of molecular self-assembly of dibrominated hexaphenylbenzene (Br2-HPB) on the on-surface polymerization reactions on a Au(111) substrate. By using low-temperature scanning tunnelling microscopy (STM), we find that the polymerization of Br2-HPB proceeds while maintaining the long-range ordered self-assembly, despite a decrease in HPB-HPB distance due to debromination and successive covalent bonding of Br2-HPB. From the STM investigations of the polymerization process, we conclude that the polymerization of Br2-HPB is accompanied by molecular rotations to maintain the periodic array of the self-assembled structure, contrary to the conventional understanding of the polymerization of the self-assembled precursor layer.
ABSTRACT
High-Q factor retuned fork (RTF) force sensors made from quartz tuning forks, and the electric circuits for the sensors, were evaluated and optimized to improve the performance of non-contact atomic force microscopy (nc-AFM) performed under ultrahigh vacuum (UHV) conditions. To exploit the high Q factor of the RTF sensor, the oscillation of the RTF sensor was excited at its resonant frequency, using a stray capacitance compensation circuit to cancel the excitation signal leaked through the stray capacitor of the sensor. To improve the signal-to-noise (S/N) ratio in the detected signal, a small capacitor was inserted before the input of an operational (OP) amplifier placed in an UHV chamber, which reduced the output noise from the amplifier. A low-noise, wideband OP amplifier produced a superior S/N ratio, compared with a precision OP amplifier. The thermal vibrational density spectra of the RTF sensors were evaluated using the circuit. The RTF sensor with an effective spring constant value as low as 1000 N/m provided a lower minimum detection limit for force differentiation. A nc-AFM image of a Si(111)-7 × 7 surface was produced with atomic resolution using the RTF sensor in a constant frequency shift mode; tunneling current and energy dissipation images with atomic resolution were also simultaneously produced. The high-Q factor RTF sensor showed potential for the high sensitivity of energy dissipation as small as 1 meV/cycle and the high-resolution analysis of non-conservative force interactions.
