Nanoscale dynamic mechanical analysis via atomic force microscopy (AFM-nDMA)

Patent metadata

Field	Value
Publication number	US-11029330-B2
Application number	US-201916530725-A
Country	US
Kind code	B2
Filing date	Aug 2, 2019
Priority date	Aug 6, 2018
Publication date	Jun 8, 2021
Grant date	Jun 8, 2021

Abstract

An atomic-force-microscope-based apparatus and method including hardware and software, configured to collect, in a dynamic fashion, and analyze data representing mechanical properties of soft materials on a nanoscale, to map viscoelastic properties of a soft-material sample. The use of the apparatus as an addition to the existing atomic-force microscope device.

First claim

The invention claimed is:

1. A method for determining a mechanical property of a soft viscoelastic sample with an atomic-force-microscope (AFM)-based system, the method comprising:
repositioning a probe of the system towards a surface of the sample until a cantilever of the probe is deflected by a pre-determined amount from a nominal orientation of the cantilever;
modifying said repositioning to maintain at least one of
i) an average sample-loading force, generated by the probe, and
ii) an area of contact between a tip of the probe and the surface to be substantially constant;

measuring, at a set of pre-defined frequencies, a viscoelastic parameter of the surface while compensating for at least one of a creep of the surface and a spatial drift of the system; and
producing an output, perceivable by a user and representing said viscoelastic parameter as a function of at least one of variable conditions of said measuring.

2. The method according to claim 1 , wherein said measuring is carried out simultaneously at multiple frequencies from said set of pre-defined frequencies.

3. The method according to claim 1 , wherein said modifying includes modulating a sample-loading force applied by the probe to the sample at a given excitation frequency from said set of pre-defined frequencies.

4. The method according to claim 1 , wherein said modifying includes maintaining the average sample-loading force substantially constant while a separation between the surface and a base of the probe is being modulated.

5. The method according to claim 1 , wherein said measuring the viscoelastic parameter includes performing dual-channel demodulation of operation of the system to carry out at least one of:
simultaneously measuring both a force of excitation imposed on the sample by the probe and a deformation of the surface caused by the force of excitation, and
avoiding reiterative calibration of the system.

6. The method according to claim 1 , further comprising holding an operation of the system for a period of time sufficient for relaxation of the creep of the surface that was caused by said repositioning.

7. The method according to claim 5 , wherein said performing dual-channel demodulation includes combining first and second data respectively received, during said measuring, from a first sensor of electronic circuitry of the system and a second sensor of the electronic circuitry of the system,
wherein the first data represent a position of the probe with respect to the surface and the second data represent a degree of deflection of the cantilever of the probe from the nominal orientation.

8. The method according to claim 5 , wherein said performing dual-channel demodulation includes introducing correction of at least one of drift-induced changes and creep-induced changes in signal data that have been received from at least one of two channels of data-acquisition electronic circuitry of the system.

9. The method according to claim 1 , further comprising continuously monitoring, with at least one of a first electronic circuitry and a second electronic circuitry of the system, an operation of the system at a reference frequency to correct for a change in the area of contact caused by the creep of the surface.

10. The method according to claim 9 , wherein the continuously monitoring includes continuously monitoring with only one of the first electronic circuitry and the second electronic circuitry, and further comprising acquiring calibration data representing signal from another of the first electronic circuitry and the second electronic circuitry obtained from a calibration sample having a hard surface.

11. The method according to claim 9 , further comprising compensating for the change in the area of contact caused by the creep of the surface, wherein said compensating includes at least one of
i) accounting for the change in the area of contact while calculating the viscoelastic parameter with a programmable processor of the system, the programmable processor being operably connected with the AFM; and
ii) repositioning the probe to compensate for said change.

12. The method according to claim 9 , wherein said reference frequency is not included in the set of pre-defined frequencies.

13. The method according to claim 1 , wherein said measuring includes
during a first period of time acquiring, from a sensor of electronic circuitry of the system, a first set of electrical signals at a frequency from the set of pre-defined frequencies to determine a degree of indentation of the surface with the tip of the probe, and
during a second period of time acquiring, from the sensor of the electronic circuitry of the system, a second set of electrical signals at a reference frequency to compensate for a change in the area of contact caused by the creep of the surface,
wherein the sensor includes at least one of a deflection sensor and a sensor configured to measure a position of the probe with respect to the surface.

14. The method according to claim 13 , wherein the reference frequency is not included in the set of pre-defined frequencies.

15. The method according to claim 13 , wherein said acquiring the first set of electrical signals and said acquiring the second set of electrical signals are processes alternated with one another.

16. The method according to claim 13 , further comprising compensating for the change in the area of contact based on determining a change in dynamic stiffness of the contact between the tip of the probe and the surface of the sample.

17. The method according to claim 3 , wherein said modulating the sample-loading force is performed by adjusting an amplitude and a phase of each oscillatory component of the sample-loading force at each given excitation frequency from the set of pre-defined frequencies to a respectively-corresponding target value, while said adjusting depends on response of a material of the sample to an applied modulated sample-loading force.

18. An apparatus configured to determine a mechanical property of a surface of a viscoelastic sample with an atomic-force-microscope (AFM) hardware, the apparatus comprising:
an excitation electronic circuitry configured to generate a first oscillatory signal at at least one frequency, the first oscillatory signal including any of a low frequency signal, a multi-frequency signal, and a mixed frequency signal,
wherein the low frequency signal is a single sinusoidal signal at a frequency of up to 1,000 Hz, the multi-frequency signal is a signal at multiple frequencies, and the mixed frequency signal is a combination of multiple sinusoidal signals at corresponding distinct frequencies;

an electro-mechanical sub-system in operable cooperation with the excitation electronic circuitry, the electro-mechanical system configured
to reposition one of the sample and a cantilevered probe of the AFM with respect to another of the sample and the probe until a point where a cantilever of the cantilevered probe is deflected by a pre-determined amount from a nominal orientation of the cantilever;
to maintain the probe in a position, with respect to the surface of the sample, in which at least one of
i) an average sample-loading force, generated by the cantilevered probe, and
ii) an area of contact between a tip of the cantilevered probe and the surface of the sample is kept substantially constant; and

to cause a mechanical oscillation of one of the sample and the cantilevered probe with respect to another of the sample and the cantilevered probe by transferring said first oscillatory signal to the elect

Assignees

• Bruker Nano Inc

Inventors

• Osechinskiy Sergey
• Ruiter Anthonius
• Pittenger Bede
• Syed-Amanulla Syed-Asif

Classifications

• G01Q60/363

  Contact-mode AFM · CPC title

• G01Q10/065 Primary

  Feedback mechanisms, i.e. wherein the signal for driving the probe is modified by a signal coming from the probe itself · CPC title

• G01N2203/0676

  Force, weight, load, energy, speed or acceleration · CPC title

• G01N2203/0075

  Strain-stress relations or elastic constants · CPC title

• G01N2203/0019

  Compressive · CPC title