In contrast to conventional optical detection AFM, self-sensing or purely electrical detection schemes have not yet been established as reliable and routine techniques for atomic resolution under UHV conditions. The QPlus sensor however has successfully been used at low temperatures (5K). This sensor is now introduced for variable temperature operation (25K - 1500K) in the Omicron VT STM.
The QPlus sensor [1] employs a quartz tuning fork for force detection in non-contact AFM operation mode. One prong of the tuning fork is fixed while the SPM probe tip is mounted to the second prong. It thus acts as a quartz lever transforming it's oscillation into an electrical signal as a result of the piezo-electric effect. The feedback signal is based on frequency shift originating from tip-sample force interaction. A dedicated pre-amplification technique ensures distance control based on the vibrational signal.
The main motivation for the QPlus sensor is to improve AFM resolution for short range forces by the high spring constant of the sensor (approx. 1800 N/m, cantilever typ. a few ten N/m). This allows small oscillation amplitudes in the range of 1nm or below (cantilever typ. 10 nm), which more precisely match the range of the involved (chemical) forces.
Optimal image performance was achieved using conventionally wet-chemically etched tungsten tips, glued onto the tuning fork.. This in combination with the high spring constant of the QPlus sensor also allows the sensor to be used for high performance STM and STS.
Measurements on Si(111) 7x7 show that tunnelling current and vibrational signal are clearly separated. In addition, benchmark measurements on NaCl with a typical corrugation of approx. 10pm prove that resolution on insulation samples is competitive to best cantilever based AFM results.
[1] Franz J. Giessibl, Appl. Phys. Lett. 73, 3956 (1998)
|