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

Patent metadata

Field	Value
Publication number	US-11635449-B2
Application number	US-202217722603-A
Country	US
Kind code	B2
Filing date	Apr 18, 2022
Priority date	Aug 6, 2018
Publication date	Apr 25, 2023
Grant date	Apr 25, 2023

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. An apparatus configured to determine a mechanical property of a viscoelastic sample with an atomic-force-microscope (AFM) hardware, the apparatus comprising:
an excitation electronic circuitry configured to generate an excitation force signal that includes multiple sinusoidal signals having respectively-corresponding distinct frequencies covering at least one decade in a frequency space, wherein amplitudes of said multiple sinusoidal signals are varied between a maximum value and a minimum value, each of the maximum and minimum values being the same for all of said multiple sinusoidal signals;
a position-detecting system configured to detect a deflection of a cantilever of a cantilevered probe of the AFM hardware and to generate data representing the deflection;
a feedback electronic circuitry operably connected with the position-detecting system and configured to monitor both a mean value of said excitation force signal and an oscillatory component of said first excitation force signal and to generate a feedback output representing both a mean value of said excitation force signal and an oscillatory component of said first excitation force signal;
an electro-mechanical sub-system cooperated with the excitation electronic circuitry and the feedback electronic circuitry and configured:
when the cantilever is deflected by a pre-determined amount from a nominal orientation thereof, to cause a mechanical oscillation between the sample and the probe in response to the excitation force signal transferred from the excitation electronic circuitry;
and
upon receiving the feedback output, to maintain said one of the sample and the probe in a position, with respect to the other of the sample and the probe, in which an area of contact between a tip of the probe and a surface of the sample is kept substantially constant during said mechanical oscillation;

a programmable processor in operable communication with at least position-detecting system and programmed:
to suspend an operation of the electro-mechanical sub-system for a relaxation period of time sufficient for relaxation of a creep of a surface of the sample that is caused by repositioning of said one of the sample and the probe with respect to the other of the sample and the probe; and
to acquire said data from the position-detecting system to determine a viscoelastic parameter after the relaxation period of time has lapsed.

2. The apparatus according to claim 1 , wherein said excitation electronic circuitry is configured to generate said multiple sinusoidal signals
a) without pre-determined phase shifts with respect to one another to produce the excitation force signal having the maximum value only at a beginning of a pre-determined time period and the minimum value only at an end of said time period, or
b) with pre-determined phase shifts with respect to one another to produce the excitation force signal having multiple amplitude peaks within said time period.

3. The apparatus according to claim 1 , wherein said electro-mechanical sub-system is further configured to reposition one of the sample and the cantilevered probe until the cantilever of the cantilevered probe is deflected by the pre-determined amount from the nominal orientation of the cantilever.

4. The apparatus according to claim 1 , wherein said electro-mechanical sub-system is further configured to maintain the sample and the probe in such a mutual position, with respect to one another, in which an average sample-loading force generated by the probe is kept substantially constant during said mechanical oscillation.

5. The apparatus according to claim 1 , wherein the programmable processor is further configured to transfer the excitation force signal from the excitation electronic circuitry to the electro-mechanical sub-system.

6. The apparatus according to claim 1 ,
wherein said feedback electronic circuitry is configured to compensate for the creep of the surface of the sample.

7. The apparatus according to claim 1 , further comprising a recording device in operable communication with the programmable processor and configured to produce an output that is perceivable by a user and that represents said viscoelastic parameter.

8. The apparatus according to claim 1 , wherein the programmable processor is configured to control the excitation electronic circuitry to adjust one or more of maximum and minimum values of an amplitude and a phase of the oscillatory component of the excitation force signal thereby modulating a sample-loading force generated by the probe.

9. The apparatus according to claim 1 , wherein the apparatus is configured to determine said viscoelastic parameter at a set of frequencies that cover said at least one decade in a frequency space in absence of using either a lock-in detection or a Fast-Fourier Transform based analysis.

10. A method for determining a mechanical property of a viscoelastic sample with an atomic-force-microscope (AFM) hardware, the method comprising:
with the use of the apparatus according to claim 1 :
monitoring, with electronic circuitry of the apparatus, an operation of the apparatus at a reference frequency to correct for a change in an area of contact between a tip of a cantilevered probe of the AFM hardware and a surface of the sample, wherein said change is caused by creep of the surface, and
measuring, at a set of frequencies that cover at least one decade in a frequency space, a viscoelastic parameter of a surface of said sample.

11. The method according to claim 10 , wherein said measuring includes measuring, at the set of frequencies that are necessarily within a range from 0.001 Hz to 1,000 Hz, the viscoelastic parameter of the surface of said sample in absence of using either a lock-in detection or a Fast-Fourier-Transform based analysis.

12. The method according to claim 10 , further comprising:
while maintaining the area of contact to remain substantially constant, repositioning the probe of the apparatus towards the surface of the viscoelastic sample until a cantilever of the cantilevered probe is deflected by a pre-determined amount from a nominal orientation of the cantilever as determined by a position-detecting system of the apparatus.

13. The method according to claim 12 , wherein said maintaining the area of contact to remain substantially constant includes 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 cantilevered probe and the surface of the sample.

14. The method according to claim 10 , further comprising:
during a first period of time acquiring, from a sensor of the electronic circuitry of the apparatus, a first set of electrical signals at a frequency from said set of frequencies to determine a depth of deformation 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 apparatus, a second set of electrical signals at a reference frequency to compensate for the 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.

15. The method according to claim 14 , wherein said acquiring the first set of electrical signals and said acquiring the second set of electrical signals are processes alternating with one another.

16. The method according to claim 14 , wherein the reference frequency is not included in said set of frequencies.

17

Assignees

• Bruker Nano Inc

Inventors

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

Classifications

• G01N2203/0094

  Visco-elasticity · CPC title

• G01Q60/363

  Contact-mode AFM · CPC title

• G01N2203/0075

  Strain-stress relations or elastic constants · CPC title

• G01N3/42

  by performing impressions under a steady load by indentors, e.g. sphere, pyramid (G01N3/54 takes precedence) · CPC title

• G01N2203/0005

  Repeated or cyclic · CPC title