Direct detection of force gradient using atomic force microscopy with very small oscillation amplitude

Using a quartz, which has a self-sensing capability, simple configuration of atomic force microscopy (AFM) is realized, and because of its high stiffness frequency-modulation (FM) operations with a small oscillation amplitude below 100 pm is possible. The small amplitude AFM operation enhances sensitivity of short range forces. Moreover, a force gradient can be directly detected from force vs. distance measurements. We carried out FM-AFM at low temperature (LT) in ultra-high vacuum using a quartz length-extension resonator with a tungsten tip (Spring constant and resonant frequency of the resonator are 540 000 N/m and 1 MHz, respectively) (An et al., APL \textbf{76}, 133114 '05). The system was simply made by attaching an extra electrode to the tip-holder of our LT-scanning tunneling microscope cooled by $^{3}$He (APL \textbf{88}, 113115 '06). Direct detection of the force gradient was performed on the Si(111) 7x7 surface using very small oscillation amplitude of 70 pm at 2.4 K.