US20120047610A1 - Cantilever-based optical interface force microscope - Google Patents
Cantilever-based optical interface force microscope Download PDFInfo
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- US20120047610A1 US20120047610A1 US13/286,059 US201113286059A US2012047610A1 US 20120047610 A1 US20120047610 A1 US 20120047610A1 US 201113286059 A US201113286059 A US 201113286059A US 2012047610 A1 US2012047610 A1 US 2012047610A1
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- cantilever
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01Q—SCANNING-PROBE TECHNIQUES OR APPARATUS; APPLICATIONS OF SCANNING-PROBE TECHNIQUES, e.g. SCANNING PROBE MICROSCOPY [SPM]
- G01Q30/00—Auxiliary means serving to assist or improve the scanning probe techniques or apparatus, e.g. display or data processing devices
- G01Q30/08—Means for establishing or regulating a desired environmental condition within a sample chamber
- G01Q30/12—Fluid environment
- G01Q30/14—Liquid environment
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01Q—SCANNING-PROBE TECHNIQUES OR APPARATUS; APPLICATIONS OF SCANNING-PROBE TECHNIQUES, e.g. SCANNING PROBE MICROSCOPY [SPM]
- G01Q20/00—Monitoring the movement or position of the probe
- G01Q20/02—Monitoring the movement or position of the probe by optical means
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01Q—SCANNING-PROBE TECHNIQUES OR APPARATUS; APPLICATIONS OF SCANNING-PROBE TECHNIQUES, e.g. SCANNING PROBE MICROSCOPY [SPM]
- G01Q60/00—Particular types of SPM [Scanning Probe Microscopy] or microscopes; Essential components thereof
- G01Q60/24—AFM [Atomic Force Microscopy] or apparatus therefor, e.g. AFM probes
- G01Q60/32—AC mode
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01Q—SCANNING-PROBE TECHNIQUES OR APPARATUS; APPLICATIONS OF SCANNING-PROBE TECHNIQUES, e.g. SCANNING PROBE MICROSCOPY [SPM]
- G01Q60/00—Particular types of SPM [Scanning Probe Microscopy] or microscopes; Essential components thereof
- G01Q60/24—AFM [Atomic Force Microscopy] or apparatus therefor, e.g. AFM probes
- G01Q60/36—DC mode
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01Q—SCANNING-PROBE TECHNIQUES OR APPARATUS; APPLICATIONS OF SCANNING-PROBE TECHNIQUES, e.g. SCANNING PROBE MICROSCOPY [SPM]
- G01Q60/00—Particular types of SPM [Scanning Probe Microscopy] or microscopes; Essential components thereof
- G01Q60/24—AFM [Atomic Force Microscopy] or apparatus therefor, e.g. AFM probes
- G01Q60/38—Probes, their manufacture, or their related instrumentation, e.g. holders
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01Q—SCANNING-PROBE TECHNIQUES OR APPARATUS; APPLICATIONS OF SCANNING-PROBE TECHNIQUES, e.g. SCANNING PROBE MICROSCOPY [SPM]
- G01Q70/00—General aspects of SPM probes, their manufacture or their related instrumentation, insofar as they are not specially adapted to a single SPM technique covered by group G01Q60/00
- G01Q70/08—Probe characteristics
- G01Q70/10—Shape or taper
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- B—PERFORMING OPERATIONS; TRANSPORTING
- B82—NANOTECHNOLOGY
- B82Y—SPECIFIC USES OR APPLICATIONS OF NANOSTRUCTURES; MEASUREMENT OR ANALYSIS OF NANOSTRUCTURES; MANUFACTURE OR TREATMENT OF NANOSTRUCTURES
- B82Y35/00—Methods or apparatus for measurement or analysis of nanostructures
Definitions
- a method comprises positioning a sample substance in proximity to an optical fiber probe suspended from a cantilever.
- An interfacial force between the sample substance and the optical fiber probe may cause the cantilever to deflect.
- the method may further comprise detecting an optical beam reflected from the cantilever and, in response to a movement of the optical beam reflected from the cantilever, applying a voltage to a semiconductive circuit element abutting at least one surface of the cantilever.
- the semiconductive circuit element may reduce deflection of the cantilever.
- the voltage may indicate the strength of the interfacial force between the sample substance and the optical fiber probe.
- an apparatus comprises an optical detector configured to detect an optical beam reflected from a cantilever.
- the apparatus may further comprise a probe suspended from the cantilever and a tray configured to (i) laterally modulate a sample substance in an x-axis direction in relation to the probe, and (ii) move the sample substance in a z-axis direction in relation to the probe.
- the apparatus may further comprise a feedback controller communicatively coupled to the optical detector and a semiconductive circuit element abutting at least one surface of the cantilever.
- the cantilever may be configured to deflect in response to a normal interfacial force and a lateral friction force between the sample substance and the optical fiber probe.
- the COIFM may comprise a cantilever with an optical fiber probe to measure interfacial forces in a liquid environment.
- the optical fiber probe may have a sufficient length to allow the free end of the optical fiber probe to penetrate a fluid surrounding a sample substance while the cantilever remains suspended above the fluid.
- the COIFM may prevent the electrical signals of the feedback loop from affecting the interfacial interactions between the probe and the sample substance.
- the COIFM may obtain accurate measurements of intermolecular interactions in a liquid environment.
- FIGS. 2A to 2D are graphs that illustrate the relationships of example electrical signals in a COIFM, according to certain embodiments.
- cantilever 14 may comprise a linear member having a fixed end attached to a support 26 and a free end that is not attached to a support. In some embodiments, cantilever 14 may project horizontally from support 26 . The application of a force to the free end of cantilever 14 may cause the free end of cantilever 14 to move in the z-axis direction, resulting in deflection of cantilever 14 . The application of a force to the free end of cantilever 14 may cause a torque and/or stress (e.g., shear stress, compression, and/or tension) in one or more portions of cantilever 14 . In some embodiments, cantilever 14 may comprise a circuit element 28 communicatively coupled to a feedback controller 20 that prevents and/or reduces the deflection of cantilever 14 .
- a torque and/or stress e.g., shear stress, compression, and/or tension
- cantilever 14 may comprise circuit element 28 that is communicatively coupled to feedback controller 20 .
- circuit element 28 comprises a semiconductor stack such as, for example, a zinc oxide stack.
- Circuit element 28 may be positioned near the fixed end (e.g., base) of cantilever 14 .
- circuit element 28 may act as a bimorph that controls (e.g., prevents and/or reduces) the vertical displacement of the free end of cantilever 14 .
- Feedback controller 20 may use circuit element 28 to provide voltage activated force feedback of cantilever 14 .
- the optical fiber probe 36 has a trunk diameter 44 from seventy (70) to one hundred and eighty (180) ⁇ m. In particular embodiments, the optical fiber probe 36 has a trunk diameter 44 from one hundred and twenty (120) to one hundred and thirty (130) ⁇ m. In some embodiments, the optical fiber probe 36 has a length 40 from one to two centimeters (cm). The free end of the optical fiber probe 36 may be sharpened to form a pointed end 46 . In some embodiments, the pointed end 46 of the optical fiber probe 36 has a diameter from fifty (50) to one hundred and fifty (150) nanometers (nm). In particular embodiments, the pointed end 46 of the optical fiber probe 36 has a diameter from eighty (80) to one hundred and twenty (120) nm.
- an optical fiber probe 36 may comprise any suitable type of optical fiber.
- the optical fiber used to form the optical fiber probe 36 may be uncoated.
- a coated optical fiber may be selected, and the coating may then be stripped from at least a portion of the optical fiber.
- the coating of the optical fiber may be removed by any suitable technique such as, for example, by using a wire stripping device.
- the optical fiber may have any suitable trunk diameter 44 .
- the trunk diameter 44 of the uncoated optical fiber may be from seventy (70) to one hundred and eighty (180) ⁇ m.
- the trunk diameter 44 may be from one hundred and twenty (120) to one hundred and thirty (130) ⁇ m.
- the feedback controller 20 may be configured to measure a DC component of the electrical signal 53 .
- the DC component may be converted, using a conversion factor, to lead to a normal force value.
- the lock-in amplifier 74 may be configured to measure an AC component of the electrical signal 53 .
- Lock-in amplifier 74 may measure amplitude and/or phase of the AC component or related information (e.g in-phase and quadrature components) at a driving frequency of the lateral modulator 72 .
- the AC component may be converted, using a conversion factor, to lead to a friction force value.
- the relationship of normal force and friction force may be represented by the equation:
- V stack may represent the applied voltage 48 to circuit element 28
- a may represent ⁇ proportional constant
- k z may represent a spring constant
- L cant may represent the length of cantilever 14
- L tip may represent the probe length
- F z and F x may represent the normal and friction forces, respectively. Therefore, the normal force conversion factor may be 2 ⁇ k z L cant /3 ⁇ , and the friction force conversion factor may be ⁇ k z L cant 2 /3 ⁇ L tip .
- chains of water molecules may form between probe 36 and tray 80 on piezotube 22 .
- tray 80 on piezotube 22 is in proximity to the silicon tip of probe 36 , the normal forces and friction forces caused by the water chains may oscillate.
- the force response of the water chains may resemble the force response of a polymer (as opposed to the force response of a spring).
- the water molecules confined between probe 36 and tray 80 on piezotube 22 may form a bundle of water chains through hydrogen bonding.
- the length of each chain may be approximated by a model called “freely jointed chain” (FJC), in which the individual segments of each water chain move randomly.
- FJC freely jointed chain
- the FJC model may be expressed by the following equation:
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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.
Description
- This application claims the benefit of U.S. Non-Provisional application Ser. No. 12/757,542, titled CANTILEVER-BASED OPTICAL INTERFACE FORCE MICROSCOPE, filed on Apr. 9, 2010, which is hereby incorporated by reference in its entirety.
- The present invention relates generally to interfacial force microscopy and more specifically to a cantilever-based optical interfacial force microscope.
- Traditional microscope systems are generally unable to measure intermolecular interactions accurately and cost effectively. One type of microscope system is the atomic force microscope (AFM), which has been used to image and/or measure the topography of various surfaces. AFM's, however, suffer from a mechanical instability that prevents the accurate measurement of intermolecular interactions. In particular, AFM's are generally unable to control tip snap-in during tip approach and/or tip snap-off during tip retraction. As a result, AFM's are generally unable to detect intermediate states of various intermolecular interactions such as, for example, the capillary forces between two silicon surfaces.
- Another type of microscope system is the interfacial force microscope (IFM). Traditional IFM's use an electrical detection process to measure various surface phenomena. IFM's, however, have not been widely used due to the low sensitivity and technical complexity of their electrical detection process. Thus, traditional microscope systems have generally been unable to measure intermolecular interactions accurately and cost effectively.
- In accordance with the present disclosure, the disadvantages and problems associated with prior microscope systems have been substantially reduced or eliminated.
- In some embodiments, an apparatus comprises an optical detector configured to detect an optical beam reflected from a cantilever. The apparatus may further comprise an optical fiber probe suspended from the cantilever. The apparatus may further comprise a piezotube configured to move a sample substance in proximity to the optical fiber probe, the cantilever configured to deflect in response to an interfacial force between the sample substance and the optical fiber probe. The apparatus may further comprise a feedback controller communicatively coupled to the optical detector and a semiconductive circuit element abutting at least one surface of the cantilever. In response to detecting a movement of the optical beam reflected from the cantilever, the feedback controller may apply a voltage to the semiconductive circuit element, which may cause the semiconductive circuit element to reduce deflection of the cantilever. The voltage applied by the feedback controller may indicate the strength of the interfacial force between the sample substance and the optical fiber probe.
- In other embodiments, a method comprises positioning a sample substance in proximity to an optical fiber probe suspended from a cantilever. An interfacial force between the sample substance and the optical fiber probe may cause the cantilever to deflect. The method may further comprise detecting an optical beam reflected from the cantilever and, in response to a movement of the optical beam reflected from the cantilever, applying a voltage to a semiconductive circuit element abutting at least one surface of the cantilever. In response to the voltage, the semiconductive circuit element may reduce deflection of the cantilever. The voltage may indicate the strength of the interfacial force between the sample substance and the optical fiber probe.
- In yet other embodiments, an apparatus comprises an optical detector configured to detect an optical beam reflected from a cantilever. The apparatus may further comprise a probe suspended from the cantilever and a tray configured to (i) laterally modulate a sample substance in an x-axis direction in relation to the probe, and (ii) move the sample substance in a z-axis direction in relation to the probe. The apparatus may further comprise a feedback controller communicatively coupled to the optical detector and a semiconductive circuit element abutting at least one surface of the cantilever. The cantilever may be configured to deflect in response to a normal interfacial force and a lateral friction force between the sample substance and the optical fiber probe. In response to detecting a movement of the optical beam reflected from the cantilever, the feedback controller may be configured to apply a voltage to the semiconductive circuit element. In response to the voltage from the feedback controller, the semiconductive circuit element may be configured to reduce deflection of the cantilever. The voltage applied by the feedback controller may indicate the strength of the normal interfacial force and the lateral friction force between the sample substance and the optical fiber probe.
- In one embodiment, a method of measuring normal and friction forces with an interfacial force microscope includes positioning a sample substance on a piezotube such that the sample substance may be positioned in proximity to a probe suspended from a cantilever. Furthermore, a molecular force between the sample substance and the probe may cause the cantilever to deflect. The method may further include engendering lateral modulation and vertical movement of the piezotube relative to the probe. The method may include detecting cantilever deflection and converting the cantilever deflection into an electrical signal. The method may further include measuring both an AC and a DC component of the electrical signal and converting the AC and DC components into a friction force value and a normal force value, respectively.
- In another embodiment, an interfacial force microscope includes a piezotube, a cantilever comprising a probe and being configured to be in proximity to the piezotube, a detector configured to detect deflection of the cantilever, and a feedback loop coupled between the detector on the one hand, and the cantilever and a lock-in amplifier on the other hand. The piezotube may be configured to move a sample substance vertically and horizontally. The cantilever may be configured to deflect in response to a molecular force acting between a sample substance on the piezotube and the probe. The detector may be configured to convert the detection of the cantilever into an electrical signal, wherein the electrical signal may comprise a normal force component and a friction force component. The lock-in amplifier may be operably connected to the piezotube.
- The present disclosure provides various technical advantages. Various embodiments may have none, some, or all of these advantages. One advantage is that a cantilever-based optical interfacial force microscope (COIFM) may employ an optical detection technique and a feedback loop to self-balance a cantilever configured to sense interfacial forces in a sample substance. The configuration of the feedback loop and cantilever may provide enhanced sensitivity of the COIFM to interfacial forces.
- Another advantage is that the COIFM may comprise a cantilever with an optical fiber probe to measure interfacial forces in a liquid environment. The optical fiber probe may have a sufficient length to allow the free end of the optical fiber probe to penetrate a fluid surrounding a sample substance while the cantilever remains suspended above the fluid. By keeping the cantilever suspended above the fluid, the COIFM may prevent the electrical signals of the feedback loop from affecting the interfacial interactions between the probe and the sample substance. Thus, the COIFM may obtain accurate measurements of intermolecular interactions in a liquid environment.
- Yet another advantage is that the COIFM may be configured to laterally modulate a sample substance (e.g., water) while measuring interfacial forces. In some embodiments, the COIFM may measure the normal forces and/or friction forces caused by interfacial liquid structures in ambient environments. Understanding such forces may permit the design of micro-electro-mechanical system (MEMS) devices that reliably operate in humid and/or wet environments.
- Other advantages of the present disclosure will be readily apparent to one skilled in the art from the description and the appended claims.
- For a more complete understanding of the present disclosure and its advantages, reference is now made to the following description taken in conjunction with the accompanying drawings, in which:
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FIG. 1 illustrates a cantilever-based optical interfacial force microscope (COIFM), according to certain embodiments; -
FIGS. 2A to 2D are graphs that illustrate the relationships of example electrical signals in a COIFM, according to certain embodiments; -
FIGS. 3A and 3B illustrate the formation of an optical fiber probe for a COIFM, according to certain embodiments; -
FIG. 4 illustrates a COIFM configured to analyze interfacial liquid structures by laterally modulating a sample substance, according to certain embodiments; and -
FIG. 5 illustrates the lateral modulation of a tray in a COIFM, according to certain embodiments. -
FIG. 1 illustrates a cantilever-based optical interfacial force microscope (COIFM) 10, according to certain embodiments.COIFM 10 may be configured to detect and/or measure the interfacial forces between molecules in asample substance 12.COIFM 10 may employ an optical detection technique and a feedback loop to self-balance acantilever 14 that senses interfacial forces in thesample substance 12. The configuration of the feedback loop andcantilever 14 may provide enhanced sensitivity of COIFM 10 to interfacial forces. In some embodiments,COIFM 10 may unveil structural and mechanical information regarding asample substance 12 at the molecular level.COIFM 10 may comprise at least onelight source 16,cantilever 14,optical detector 18,feedback controller 20, andpiezotube 22. -
Light source 16 may emit anoptical beam 24 towardscantilever 14.Optical beam 24 fromlight source 16 may reflect off at least one surface ofcantilever 14.Optical detector 18 may be positioned to receiveoptical beam 24 reflected fromcantilever 14. Ascantilever 14 is deflected, causing the unsupported end ofcantilever 14 to move in the z-axis direction, the angle of reflection ofoptical beam 24 may change. Based at least in part on the angle of reflection ofoptical beam 24 fromcantilever 14,COIFM 10 may determine the position ofcantilever 14. -
Light source 16 may comprise any suitable source of electromagnetic radiation. In some embodiments,light source 16 may comprise a laser such as, for example, a semiconductor laser, a solid state laser, a gas laser, a chemical laser, an excimer laser, and/or any suitable type of laser. In other embodiments,light source 16 may comprise a light-emitting diode and/or lamp emitting a low-divergenceoptical beam 24. - As noted above,
light source 16 may emitoptical beam 24 towardscantilever 14.Cantilever 14 may comprise a linear member having a fixed end attached to asupport 26 and a free end that is not attached to a support. In some embodiments,cantilever 14 may project horizontally fromsupport 26. The application of a force to the free end ofcantilever 14 may cause the free end ofcantilever 14 to move in the z-axis direction, resulting in deflection ofcantilever 14. The application of a force to the free end ofcantilever 14 may cause a torque and/or stress (e.g., shear stress, compression, and/or tension) in one or more portions ofcantilever 14. In some embodiments,cantilever 14 may comprise acircuit element 28 communicatively coupled to afeedback controller 20 that prevents and/or reduces the deflection ofcantilever 14. -
Cantilever 14 may comprise any suitable type of structural member. In some embodiments,cantilever 14 may comprise a semiconductive material such as, for example, a doped and/or undoped silicon material. In particular embodiments,cantilever 14 may comprise phosphorus doped silicon and/or boron doped silicon.Cantilever 14 may have any suitable dimensions. In some embodiments,cantilever 14 has alength 30 from eighty (80) to one hundred and eighty (180) micrometers (μm). In particular embodiments,cantilever 14 has alength 30 from one hundred and twenty (120) to one hundred and thirty (130) μm. In some embodiments,cantilever 14 has athickness 32 from two (2) to six (6) μm. In particular embodiments,cantilever 14 has athickness 32 from three (3) to five (5) μm. In some embodiments,cantilever 14 has a width from forty (40) to seventy (70) μm. In particular embodiments,cantilever 14 has a width from fifty (50) to sixty (60) μm. - As noted above,
cantilever 14 may comprisecircuit element 28 that is communicatively coupled tofeedback controller 20. In some embodiments,circuit element 28 comprises a semiconductor stack such as, for example, a zinc oxide stack.Circuit element 28 may be positioned near the fixed end (e.g., base) ofcantilever 14. In conjunction withcantilever 14,circuit element 28 may act as a bimorph that controls (e.g., prevents and/or reduces) the vertical displacement of the free end ofcantilever 14.Feedback controller 20 may usecircuit element 28 to provide voltage activated force feedback ofcantilever 14. In some embodiments,feedback controller 20 may usecircuit element 28 for self-sensing ofcantilever 14, for statically deflecting and/or reducing deflection of the free end ofcantilever 14, and/or for oscillating and/or reducing oscillation ofcantilever 14. -
Cantilever 14 inCOIFM 10 may be configured to measure intermolecular interactions for various sample substances. In some embodiments,cantilever 14 comprises aprobe 36 affixed to the free end ofcantilever 14. A sample substance may be positioned onpiezotube 22 in proximity to probe 36. Intermolecular interactions betweenprobe 36 and thesample substance 12 may exert a force oncantilever 14, causing a slight deflection ofcantilever 14.Optical detector 18 may detect the deflection ofcantilever 14. In response to the deflection,feedback controller 20 may adjust thevoltage 48 applied tocircuit element 28 in order to reduce and/or prevent further deflection ofcantilever 14. Based on thevoltage 48 required to prevent and/or reduce the deflection ofcantilever 14,COIFM 10 may determine the interfacial forces betweenprobe 36 and thesample substance 12. This information may be used to analyze characteristics ofsample substances 12 such as, for example, interfacial adhesion, interfacial liquid structures, and/or measurements of chemical interactions. - Probe 36 of
cantilever 14 may be any suitable type of probe. In some embodiments,probe 36 may be a semiconductive tip that protrudes vertically from the free end of a horizontally positionedcantilever 14. In such embodiments,probe 36 may be a pyramid-shaped tip that comprises a silicon material. The pyramid-shaped tip may resemble a spike and/or may have any suitable dimensions. For example, the pyramid-shaped tip may have a height from fifteen (15) to twenty (20) μm. - In other embodiments,
probe 36 may be anoptical fiber probe 36. The use of anoptical fiber probe 36 may allow COIFM 10 to measure interfacial interactions in liquid environments. Theoptical fiber probe 36 may have asufficient length 40 to allow the free end of theoptical fiber probe 36 to penetrate a fluid 42 surrounding asample substance 12 whilecantilever 14 remains suspended abovefluid 42. By keepingcantilever 14 suspended abovefluid 42,COIFM 10 prevents the electrical signals of the force feedback loop from affecting the interfacial interactions between theoptical fiber probe 36 and thesample substance 12. In other words, by keepingcantilever 14 and force feedback loop isolated fromfluid 42,COIFM 10 may obtain accurate measurements of intermolecular interactions associated with thesample substance 12. - The
optical fiber probe 36 may comprise any suitable type of optical fiber. For example, theoptical fiber probe 36 may comprise a glass fiber, a plastic fiber, and/or any suitable type of optical fiber. One end of theoptical fiber probe 36 may be affixed to cantilever 14 while the other end (i.e., the free end) of theoptical fiber probe 36 is not affixed to any structure. Theoptical fiber probe 36 may be affixed to cantilever 14 using any suitable technique. For example, an end of theoptical fiber probe 36 may be affixed to cantilever 14 with a thermosetting polymer such as, for example, epoxy. Theoptical fiber probe 36 may have any suitable dimensions. In some embodiments, theoptical fiber probe 36 has atrunk diameter 44 from seventy (70) to one hundred and eighty (180) μm. In particular embodiments, theoptical fiber probe 36 has atrunk diameter 44 from one hundred and twenty (120) to one hundred and thirty (130) μm. In some embodiments, theoptical fiber probe 36 has alength 40 from one to two centimeters (cm). The free end of theoptical fiber probe 36 may be sharpened to form apointed end 46. In some embodiments, thepointed end 46 of theoptical fiber probe 36 has a diameter from fifty (50) to one hundred and fifty (150) nanometers (nm). In particular embodiments, thepointed end 46 of theoptical fiber probe 36 has a diameter from eighty (80) to one hundred and twenty (120) nm. - In some embodiments,
probe 36 may comprise a wire having a sharpened tip. The tip of the wire may be sharpened according to any suitable technique such as, for example, chemical etching.Probe 36 may comprise any suitable type of wire. For example, probe 36 may comprise tungsten, titanium, chromium, and/or any suitable material. - In some embodiments,
probe 36 may be coated with one or more layers of material to insulateprobe 36 from liquid. A coating may be deposited overprobe 36,cantilever 14, and/or bothprobe 36 andcantilever 14. The coating may prevent the electrical signals of the force feedback loop inCOIFM 10 from affecting the interfacial interactions betweenprobe 36 and thesample substance 12. For example, whereprobe 36 is a pyramid-shaped silicon tip that extends fromcantilever 14, a coating onprobe 36 and/orcantilever 14 may allow COIFM 10 to measure interfacial interactions in a liquid environment. To enhance the resolution and/or sensitivity ofCOIFM 10, the coating may not cover the apex of the tip ofprobe 36. The coating may comprise any suitable insulating material. For example, the coating may comprise an elastomer (e.g., silicone elastomer, polyisoprene, polyurethane, etc.), a polymer, a polyimide, and/or any suitable material. - As noted above, interfacial forces between
probe 36 and thesample substance 12 may cause some deflection ofcantilever 14, which may cause a change in the reflection ofoptical beam 24 fromcantilever 14.Optical detector 18 may detect the movement ofoptical beam 24 reflected fromcantilever 14. In some embodiments,optical detector 18 outputs tofeedback controller 20 an electrical signal indicating the amount of deflection ofcantilever 14.Optical detector 18 may be any suitable device that senses the presence and/or movement ofoptical beam 24.Optical detector 18 may comprise a transducer that converts an optical signal into an electrical signal. In some embodiments,optical detector 18 may comprise one or more laser detectors, photomultipliers, photodiodes, thermopile detectors, and/or pyroelectric energy detectors. -
Feedback controller 20 may receive fromoptical detector 18 an electrical signal that indicates the deflection ofcantilever 14. In response to the electrical signal,feedback controller 20 may adjust thevoltage 48 applied tocircuit element 28 oncantilever 14 in order to prevent and/or reduce the deflection ofcantilever 14. Thevoltage 48 that is output fromfeedback controller 20 may be based at least in part on avoltage 50 associated with aset point 51 and avoltage 52 fromoptical detector 18. In some embodiments,feedback controller 20 may causecircuit element 28 to create a torque oncantilever 14 in order to achieve a zeroerror voltage 53. -
Feedback controller 20 may comprise any suitable type of controller. For example,feedback controller 20 may be a digital controller, an analog controller, a linear gain controller, and/or a non-linear gain controller. In some embodiments,feedback controller 20 may be a proportional integral derivative (PID) controller. Thevoltage 48 required fromfeedback controller 20 to prevent and/or reduce the deflection ofcantilever 14 may indicate the strength of the interfacial forces between thesample substance 12 andprobe 36. - The
sample substance 12 may be positioned onpiezotube 22 inCOIFM 10.Piezotube 22 may be coupled to a z-axis controller 54 and/or anamplifier 60, which may cause piezotube 22 to move thesample substance 12 closer to and/or further fromprobe 36. Thus, piezotube 22 may move thesample substance 12 in the z-axis direction. The interfacial forces measured byCOIFM 10 may depend on the distance between the free end ofprobe 36 and thesample substance 12. -
Piezotube 22 may be any suitable type of piezoelectric actuator.Piezotube 22 may comprise a ceramic and/or crystalline material that, in response to an electric field, changes in size. This property may allow piezotube 22 to position thesample substance 12 with accuracy (e.g., better than micrometer precision) in relation to probe 36 inCOIFM 10.Piezotube 22 may be any suitable type of piezoelectric actuator such as, for example, a direct piezo actuator and/or an amplified piezo actuator. -
COIFM 10 may be configured to measure intermolecular interactions associated with any suitable type ofsample substance 12. For example, thesample substance 12 may comprise one or more biological substances such as, for example, proteins, ligands, cellular systems, and/or bacterial systems. As another example,sample substance 12 may comprise a liquid (e.g., water), which may allow COIFM 10 to measure interfacial fluid structures. As yet another example,sample substance 12 may be a solid, gaseous, and/or plasma substance. - In operation,
COIFM 10 may be used to measure intermolecular interactions in asample substance 12. Thesample substance 12 may be positioned onpiezotube 22 inCOIFM 10.Piezotube 22 may be positioned in proximity to probe 36 suspended from the free end ofcantilever 14 inCOIFM 10. WhenCOIFM 10 is activated,light source 16 may emitoptical beam 24 towardscantilever 14, which may reflectoptical beam 24 towardsoptical detector 18. -
COIFM 10 may actuate piezotube 22 in the z-axis direction such that thesample substance 12 onpiezotube 22 moves closer to probe 36. The interfacial forces between the molecules in thesample substance 12 andprobe 36 may causeprobe 36 to move closer to or further from thesample substance 12, which may cause a slight deflection ofcantilever 14. The deflection ofcantilever 14 may causeoptical beam 24 reflected fromcantilever 14 to move. The movement ofoptical beam 24 may be detected by optical sensor, which may, in response, transmit an electrical signal tofeedback controller 20. In response to the electrical signal from optical sensor,feedback controller 20 may apply avoltage 48 tocircuit element 28 affixed to cantilever 14. By applying avoltage 48 tocircuit element 28,feedback controller 20 may prevent and/or reduce the deflection ofcantilever 14. Based at least in part on the amount ofvoltage 48 required to prevent and/or reduce the deflection ofcantilever 14,COIFM 10 may determine and/or indicate the strength of the interfacial forces in thesample substance 12. -
FIGS. 2A to 2D are graphs that illustrate the relationship of example electrical signals inCOIFM 10, according to certain embodiments. Thex-axis 202 of each graph represents time and the y-axis 204 of each graph represents a respective voltage in the feedback loop inCOIFM 10. Electrical signals inCOIFM 10 may be adjusted to determine the time resolution ofCOIFM 10. For example, as illustrated inFIG. 2A , when thesample substance 12 is not in proximity to probe 36,COIFM 10 may apply a square wave voltage with a particular amplitude (e.g., 0.2 V) and frequency (e.g., 10 Hz) to the set-point voltage (Vset point) 206. As illustrated inFIG. 2B ,feedback controller 20 may be operable to configure the preamp output (VA-B) 208 to follow the square wave by applying appropriate voltages (Vstack) 210 tocircuit element 28 affixed to cantilever 14. The square wave may causecircuit element 28 to create a torque oncantilever 14 in order to achieve a zero error voltage (Verror)) 212, as illustrated inFIG. 2C . Thus,feedback controller 20 may be configured to optimize the transient response to achieve the appropriate time response forCOIFM 10. As illustrated inFIG. 2D ,COIFM 10 may, in some embodiments, have a practical time resolution that is between one and two milliseconds (ms). - Although particular voltage levels and time resolutions are described above, it should be understood that
COIFM 10 may be configured to operate with any suitable voltage levels and time resolutions. -
FIGS. 3A and 3B illustrate the formation of anoptical fiber probe 36 forCOIFM 10, according to certain embodiments. In some embodiments, apointed end 46 may be formed on theoptical fiber probe 36 by an acid etching technique. - As noted above, an
optical fiber probe 36 may comprise any suitable type of optical fiber. In some embodiments, the optical fiber used to form theoptical fiber probe 36 may be uncoated. In other embodiment, a coated optical fiber may be selected, and the coating may then be stripped from at least a portion of the optical fiber. The coating of the optical fiber may be removed by any suitable technique such as, for example, by using a wire stripping device. - The optical fiber may have any
suitable trunk diameter 44. In some embodiments, thetrunk diameter 44 of the uncoated optical fiber may be from seventy (70) to one hundred and eighty (180) μm. In particular embodiments, thetrunk diameter 44 may be from one hundred and twenty (120) to one hundred and thirty (130) μm. - To form a
pointed end 46 on theoptical fiber probe 36, an uncoated optical fiber may be positioned vertically in acontainer 62.Container 62 may be any suitable type of container such as, for example, an acid resistant beaker. Once theoptical fiber probe 36 is positioned incontainer 62, anacid 64 may be added tocontainer 62. A sufficient quantity ofacid 64 may be added such thatacid 64 immerses the free end of theoptical fiber probe 36.Acid 64 may be any suitable type ofacid 64 such as, for example, a monoprotic acid and/or a polyprotic acid. In some embodiments,acid 64 may be a mineral acid, a sulfonic acid, and/or a carboxylic acid. In particular embodiments,acid 64 may be a hydrofluoric acid and/or a hydrochloric acid. - After
acid 64 is added tocontainer 62, a solvent 66 may be added tocontainer 62.Solvent 66 may be less dense and/or immiscible inacid 64. Consequently, solvent 66 may form a separate layer of fluid overacid 64. The layer of solvent 66 may serve as a protective barrier to theoptical fiber probe 36 so that only a controlled portion of theoptical fiber probe 36 is dissolved and/or sharpened byacid 64. -
Solvent 66 may be any suitable type of solvent 66 that is less dense thanacid 64 and/or immiscible inacid 64. For example, solvent 66 may be an aromatic hydrocarbon such as, for example, toluene and/or benzene. As another example, solvent 66 may be hexane and/or cyclohexane. - In some embodiments,
acid 64 incontainer 62 may form ameniscus 68 on theoptical fiber probe 36.Meniscus 68 may recede asacid 64 dissolves the material (e.g., glass) in the optical fiber. Due to the formation ofmeniscus 68, more material (e.g., glass) may be dissolved at the immersed (e.g., distal) end of the optical fiber, which may result in the continuous narrowing of the optical fiber to create apointed end 46. Thepointed end 46 of the optical fiber may have anysuitable diameter 70. In some embodiments, thediameter 70 of thepointed end 46 may be from fifty (50) to one hundred and fifty (150) nm. - The
optical fiber probe 36 may be left incontainer 62 for any suitable period of time (e.g., sixty minutes, ninety minutes, etc.) to form thepointed end 46. Once thepointed end 46 is formed, theoptical fiber probe 36 may be removed fromcontainer 62 and cleaned. In some embodiments, thepointed end 46 of theoptical fiber probe 36 may be polished and/or annealed. Annealing may align the molecules in thepointed end 46 of theoptical fiber probe 36 to enhance the accuracy of measurements byCOIFM 10. - Although an acid etching technique is described above, it should be understood that any suitable technique may be used to form the
pointed end 46 on theoptical fiber probe 36. For example, thepointed end 46 on theoptical fiber probe 36 may be formed by milling, dry etching, vapor etching, and/or any suitable technique. In some embodiments, thepointed end 46 may be formed on theoptical fiber probe 36 by thermal heating of the optical fiber with a laser (e.g., a carbon dioxide laser). In other embodiments, thepointed end 46 may be formed on theoptical fiber probe 36 by resistive heating. - In some embodiments,
COIFM 10 may be used to analyze interfacial liquid structures in an ambient environment. To analyze interfacial liquid structures,COIFM 10 may measure the normal force and/or the friction force betweenprobe 36 inCOIFM 10 and thesample substance 12. Measuring the normal force may permit COIFM 10 to monitor the adhesion betweenprobe 36 and thesample substance 12. Measuring the friction force may allow COIFM 10 to monitor the ordering of molecules in thesample substance 12. In some embodiments, the friction force may be measured by laterally modulating thesample substance 12 as it is brought into proximity withprobe 36. -
FIG. 4 illustratesCOIFM 10 configured to analyze interfacial liquid structures by laterally modulating thesample substance 12, according to certain embodiments.COIFM 10 may compriselight source 16,cantilever 14,optical detector 18,feedback controller 20, andpiezotube 22, as described above with respect toFIG. 1 .COIFM 10 may further comprise alateral modulator 72 and lock-inamplifier 74 communicatively coupled topiezotube 22 andfeedback controller 20. -
Lateral modulator 72 may be operable to modulatepiezotube 22 in the x-axis and/or y-axis directions (also called lateral modulation).Lateral modulator 72 may comprise a voltage supply that is configured to actuate the modulation ofpiezotube 22.Lateral modulator 72 may be any suitable modulator such as, for example, a piezoelectric actuator. For example, piezotube 22 may comprise a ceramic structure that contracts and/or expands in the x-axis and/or y-axis directions in response to a voltage applied by the voltage supply inlateral modulator 72. -
Lateral modulator 72 may be communicatively coupled to lock-inamplifier 74. Lock-inamplifier 74 may be operable to detect and/or measure the lateral modulation ofpiezotube 22. Lock-inamplifier 74, which may act as a homodyne with a low pass filter, may be operable to extract a signal with a known carrier wave from a noisy environment. Lock-inamplifier 74 may be operable to convert the phase (and/or related information such as in-phase and quadrature components) and amplitude of the extracted signal into a time-varying, low-frequency voltage signal. In some embodiments, lock-inamplifier 74 may be configured to measure phase shift associated with the extracted signal. - In operation,
COIFM 10 may laterally modulate thesample substance 12 to gather information regarding interfacial liquid structures in thesample substance 12. In some embodiments, a sample substance 12 (e.g., a fluid) may be deposited onpiezotube 22.COIFM 10 may then establish a feedback loop betweenoptical detector 18 andcircuit element 28 oncantilever 14.Piezotube 22 may then be actuated in the z-axis direction (i.e., vertically) such that thesample substance 12 is brought near to and/or in contact with the free end ofprobe 36 inCOIFM 10. As thesample substance 12 is brought into proximity withprobe 36, adhesion forces between thesample substance 12 andprobe 36 may causecantilever 14 to deflect.Optical detector 18 may detect the deflection ofcantilever 14. Based on signals fromoptical detector 18 andfeedback controller 20,COIFM 10 may measure the adhesions forces between thesample substance 12 andprobe 36. - For example, and as discussed above, the feedback loop may receive an
electrical signal 53 related to the deflection of thecantilever 14, and theelectrical signal 53 may comprise a normal force component and a friction force component. In one embodiment, the normal force component may comprise a DC component of theelectrical signal 53, and the friction force component may comprise an AC component of theelectrical signal 53. The normal force component may be measured at thefeedback controller 20, while the friction force component may be measured at the lock-inamplifier 74. In some cases, the normal and friction force components may be measured concurrently. For instance, z-axis controller 54 andamplifier 60 may engender movement ofpiezotube 22 in the z- or vertical axis, while lock-inamplifier 74 andlateral modulator 72 may engender lateral modulation ofpiezotube 22. In response to the vertical and lateral modulation of thepiezotube 22, and as a result of molecular force acting between asample 12 placed on thepiezotube 22 and probe 36 of thecantilever 14, thecantilever 14 may deflect. Ascantilever 14 deflects,optical detector 18 may detect the deflection of thecantilever 14 and convert the deflection into anelectrical signal 52. Theelectrical signal 52 may be compared with anelectrical signal 50 from aset point 51 to yield anelectrical signal 53.Feedback controller 20 may be configured to receive theelectrical signal 53 and, as described above, inducecircuit element 28 to counteract the deflection ofcantilever 14. Also as discussed above,feedback controller 20 may be coupled to lock-inamplifier 74. - In one embodiment, the
feedback controller 20 may be configured to measure a DC component of theelectrical signal 53. In some cases, the DC component may be converted, using a conversion factor, to lead to a normal force value. Also, the lock-inamplifier 74 may be configured to measure an AC component of theelectrical signal 53. Lock-inamplifier 74 may measure amplitude and/or phase of the AC component or related information (e.g in-phase and quadrature components) at a driving frequency of thelateral modulator 72. In some cases, the AC component may be converted, using a conversion factor, to lead to a friction force value. The relationship of normal force and friction force may be represented by the equation: -
- In the foregoing equation, Vstack may represent the applied
voltage 48 tocircuit element 28, a may represent α proportional constant, kz may represent a spring constant, Lcant may represent the length ofcantilever 14, Ltip may represent the probe length, and Fz and Fx may represent the normal and friction forces, respectively. Therefore, the normal force conversion factor may be 2βkzLcant/3α, and the friction force conversion factor may be βkzLcant 2/3αLtip. The derivation and sample calculations of the above equations can be found in the paper: Byung I. Kim et al., Simultaneous Measurement of Normal and Friction Forces Using a Cantilever-Based Optical Interfacial Force Microscope, REVIEW OF SCIENTIFIC INSTRUMENTS 82, 05311 (2011), which is hereby incorporated by reference in its entirety. - By way of example, if the probe is made of 1-10 Ωcm phosphorus doped Si, with a nominal spring constant (kz) and resonance frequency known to be 3N/m and 50 kHz, respectively, and if the cantilever and probe dimensions are measured to be Lcant=485 μm and Ltip=20 μm, respectively. Then, measurements may be taken in ambient conditions with relative humidity of 55%. Tip speed may be chosen to be 10 nm/s, and lateral movement achieved by modulating the sample along the long axis direction of the cantilever with a 1 nm amplitude and a frequency of 100 Hz. Based on these numbers, the amplitude of the AC component may be measured at the lock-in
amplifier 74 and the DC component at thefeedback controller 20. Based on this information, and using the conversion factors disclosed above, the normal force conversion factor may be calculated to be approximately 5 nN/V and the frictional force conversion factor may be calculated to be approximately 60 nN/V. Of course, one of ordinary skill in the art would recognize that based on any multitude of variables, each respective conversion factor could change significantly. - Additionally, a memory element (not shown) may be coupled to the
feedback controller 20 and/or the lock-inamplifier 74 in order to record values measured by each respective element. The measured values may be recorded as a function of distance, wherein the distance is related to the movement ofpiezotube 22 by the z-axis controller 54 and/or theamplifier 60. In some embodiments, the memory element may be coupled internally to the feedback loop. In other cases, the memory element may be external to the microscope and coupled to the feedback loop through any type of wired or wireless connection, as appropriate. - As illustrated in
FIG. 5 , piezotube 22 may be modulated laterally (e.g., in the x-axis and/or y-axis directions) aspiezotube 22 moves thesample substance 12 into contact withprobe 36. As thesample substance 12 approaches and retracts fromprobe 36, lock-inamplifier 74 may detect a voltage signal that indicates the effect of friction forces between thesample substance 12 andprobe 36. In some embodiments,COIFM 10 may indicate and/or record information regarding the normal forces, friction forces, and/or the distance betweenprobe 36 and thesample substance 12. - An example illustrates certain embodiments of
COIFM 10. In some embodiments,COIFM 10 may measure the effect of interfacial water in micro-electro-mechanical system (MEMS) devices. In such devices, the presence of water may hinder the movement and/or function of micro-electro-mechanical structures. Understanding the effects of interfacial water in MEMS devices may enable designing MEMS devices that effectively operate in humid and/or wet environments. - In the present example, water may be deposited on a tray 80 (e.g., silicon substrate) on
piezotube 22 in an ambient environment, as illustrated inFIG. 4 .COIFM 10 may be equipped withprobe 36 that comprises a silicon tip.COIFM 10 may be placed in an enclosure 76 (e.g., an acryl box) having at least oneinlet port 78 for dry nitrogen gas and at least oneinlet port 78 for humid water vapor. Appropriate levels of nitrogen gas and water vapor may then be added toenclosure 76 to control the amount of humidity. - In the present example,
COIFM 10 may establish a feedback loop betweenoptical detector 18 andcircuit element 28 oncantilever 14.Lateral modulator 72 may modulate piezotube 22 in the x-axis and/or y-axis directions aspiezotube 22 moves in the z-axis direction to bring the water into contact withprobe 36.COIFM 10 may measure both the normal forces and the friction forces between the water andprobe 36.COIFM 10 may collect and/or record data aspiezotube 22, while modulating, approaches and retracts fromprobe 36. - In the present example, chains of water molecules may form between
probe 36 andtray 80 onpiezotube 22. Whentray 80 onpiezotube 22 is in proximity to the silicon tip ofprobe 36, the normal forces and friction forces caused by the water chains may oscillate. As the gap distance decreases betweenprobe 36 andtray 80, the force response of the water chains may resemble the force response of a polymer (as opposed to the force response of a spring). - In some embodiments, the water molecules confined between
probe 36 andtray 80 onpiezotube 22 may form a bundle of water chains through hydrogen bonding. The length of each chain may be approximated by a model called “freely jointed chain” (FJC), in which the individual segments of each water chain move randomly. The FJC model may be expressed by the following equation: -
- In the foregoing equation, l may represent the number of water joints, σ may represent the diameter of water, f may represent tip force, n may represent the number of water chains, kB may represent the Boltzmann constant, and T may represent temperature. Applying the FJC model in the present example, the measurements by
COIFM 10 may indicate that, asprobe 36 approachestray 80 onpiezotube 22, the number of water chains betweenprobe 36 and silicon substrate may increase while the number of water joints in each chain may decrease. - Although the foregoing example describes the use of COIFM 10 to measure interfacial forces associated with water chains, it should be understood that
COIFM 10 may be used to measure interfacial forces in any suitable substance. - The present disclosure encompasses all changes, substitutions, variations, alterations and modifications to the example embodiments described herein that a person having ordinary skill in the art would comprehend. Similarly, where appropriate, the appended claims encompass all changes, substitutions, variations, alterations, and modifications to the example embodiments described herein that a person having ordinary skill in the art would comprehend.
Claims (20)
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.
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Also Published As
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US20130298294A1 (en) | 2013-11-07 |
US9091705B2 (en) | 2015-07-28 |
US9140720B2 (en) | 2015-09-22 |
US20130312142A1 (en) | 2013-11-21 |
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