text: A qPlus sensor is used in many ultra-high vacuum nc-AFMs. The sensor was originally made from a quartz tuning fork from a wristwatch. In contrast to a quartz tuning fork sensor that consists of two coupled tines that oscillate opposed to each other, a qPlus sensor has only one tine that oscillates. The tuning fork is glued to a mount such that one tine of the tuning fork is immobilised, a tungsten wire, etched to have a sharp apex, is then glued to the free prong. The sensor was invented in 1996 by physicist Franz J. Giessibl. The AFM deflection signal is generated by the piezoelectric effect, and can be read from the two electrodes on the tuning fork.
  As the tungsten tip wire is conductive, the sensor can be used for combined STM/nc-AFM operation. The tip can either be electrically connected to one of tuning fork electrodes, or to a separate thin (~30μm diameter) gold wire. The advantage of the separate wire is that it can reduce crosstalk between the tunnel current and the deflection channels, however the wire will have its own resonance, which can affect the resonant properties of the sensor. New versions of the qPlus sensor with one or several integrated service electrodes as proposed in reference  and implemented in  solve that problem. The Bergman reaction has recently been imaged by the IBM group in Zurich using such a qPlus sensor with integrated STM electrode.
  The sensor has a much higher stiffness than silicon microcantilevers, ~1800 N/m (tip placement further down the tine can lead to higher stiffness's ~2600 N/m). This higher stiffness allows higher forces before snap to contact instabilities. The resonant frequency of a qPlus sensor is typically lower than that of a silicon microcantilever, ~25 kHz (Watch tuning forks have a resonant frequency of 32,768 Hz before tip placement). Several factors (in particular detector noise and eigenfrequency) affect the speed of operation. qPlus sensors with long tip wires that approach the length of the sensor display a movement of the apex which is no longer perpendicular to the surface, thus probing the forces in a different direction to expected.