Cantilever-based optical interface force microscope

Abstract

A method and an apparatus for detecting a normal force component and a friction force component between a probe and a sample substance using an interfacial force microscope is disclosed herein. According to one embodiment, a method of measuring normal and friction forces with an interfacial force microscope includes positioning a sample substance on a piezotube and in proximity to a probe suspended from a cantilever such that a molecular force between the sample substance and the probe causes the cantilever to deflect. The method may include converting the deflection of the cantilever into an electrical signal comprising a normal force and a friction force component, and measuring the normal and friction force components.

What is claimed is: 
     
         1 . A method of measuring normal and friction forces with an interfacial force microscope, the method comprising:
 positioning a sample substance on a piezotube, the sample substance positioned in proximity to a probe suspended from a cantilever such that a molecular force between the sample substance and the probe causes the cantilever to deflect;   engendering lateral modulation and vertical movement of the piezotube relative to the probe;   detecting cantilever deflection and converting the cantilever deflection into an electrical signal;   measuring an AC component of the electrical signal;   measuring a DC component of the electrical signal; and   converting the measured AC and DC components into a friction force value and a normal force value, respectively.   
     
     
         2 . The method of  claim 1  wherein measuring the AC component comprises measuring an amplitude and/or phase of the electrical signal. 
     
     
         3 . The method of  claim 2  wherein measuring the AC component further comprises using a lock-in amplifier to measure the amplitude and/or phase, and wherein the lock-in amplifier is operably coupled to the piezotube. 
     
     
         4 . The method of  claim 1  wherein measuring the DC component comprises measuring the electrical signal using a feedback controller, wherein the feedback controller is operably coupled to a semiconductive circuit element abutting at least one surface of the cantilever, and wherein the semiconductive circuit element is configured to reduce deflection of the cantilever. 
     
     
         5 . The method of  claim 1  further comprising recording the AC and DC components of the electrical signal as a function of distance, and wherein the distance is related to the movement and/or modulation of the piezotube. 
     
     
         6 . The method of  claim 1  wherein converting the AC component of the electrical signal to a friction force value comprises using a conversion factor proportional to the cantilever length squared and inversely proportional to the probe length. 
     
     
         7 . The method of  claim 1  wherein converting the DC component of the electrical signal to a normal force value comprises using a conversion factor proportional to the cantilever length. 
     
     
         8 . The method of  claim 1  wherein engendering vertical movement of the piezotube comprises moving the piezotube at a variable probe-approaching and retracting speed of approximately 8 nm/s. 
     
     
         9 . The method of  claim 1  wherein engendering lateral modulation of the piezotube comprises modulating at a variable amplitude of approximately 0.6 nm and a variable frequency of approximately 1 kHz. 
     
     
         10 . The method of  claim 1  where in the method steps of measuring the AC component and the DC component are performed concurrently. 
     
     
         11 . An interfacial force microscope comprising:
 a piezotube configured to move a sample substance vertically and to modulate the sample substance laterally;   a cantilever comprising a probe, the cantilever configured to deflect in response to a molecular force acting between a sample substance on the piezotube and the probe, and wherein the probe is configured to be in proximity to the piezotube;   a detector configured to detect deflection of the cantilever and convert the deflection of the cantilever to an electrical signal;   wherein the electrical signal comprises a normal force component and a friction force component; and   a feedback loop coupled to the detector on the one hand, and the cantilever and a lock-in amplifier on the other hand, wherein the lock-in amplifier is operably connected to the piezotube.   
     
     
         12 . The microscope of  claim 11  wherein the normal force component comprises a DC component and the friction force component comprises an AC component. 
     
     
         13 . The microscope of  claim 12  wherein the lock-in amplifier is configured to measure an amplitude and/or phase of the AC component of the electrical signal. 
     
     
         14 . The microscope of  claim 12  wherein the feedback loop comprises a feedback controller, and the feedback controller is configured to measure the DC component of the electrical signal. 
     
     
         15 . The microscope of  claim 12  further comprising a memory element configured to record the AC and DC components of the electrical signal as a function of distance, and wherein the distance is related to the movement and/or modulation of the piezotube. 
     
     
         16 . The microscope of  claim 12  wherein the microscope is configured to convert the AC and DC components of the electrical signal into a friction force value and a normal force value, respectively. 
     
     
         17 . The microscope of  claim 16  wherein the conversion to a friction force value comprises a conversion factor proportional to the cantilever length squared and inversely proportional to the probe length. 
     
     
         18 . The microscope of  claim 16  wherein the conversion to a normal force value comprises a conversion factor proportional to the cantilever length. 
     
     
         19 . The microscope of  claim 11  further comprising a vertical movement amplifier and/or a vertical movement controller coupled to the piezotube. 
     
     
         20 . The microscope of  claim 11  wherein the detector is an optical detector.