Atomic force acoustic microscopy

Last updated October 15, 2019

Atomic force acoustic microscopy (AFAM) is a type of scanning probe microscopy (SPM). It is a combination of acoustics and atomic force microscopy. The principal difference between AFAM and other forms of SPM is the addition of a transducer at the bottom of the sample which induces longitudinal out-of-plane vibrations in the specimen. These vibrations are sensed by a cantilever and tip called a probe. The figure shown here is the clear schematic of AFAM principle here B is the magnified version of the tip and sample placed on the transducer and tip having some optical coating generally gold coating to reflect the laser light on to the photodiode.

Scanning probe microscope (SPM) is a branch of microscopy that forms images of surfaces using a physical probe that scans the specimen. SPM was founded in 1981, with the invention of the scanning tunneling microscope, an instrument for imaging surfaces at the atomic level. The first successful scanning tunneling microscope experiment was done by Binnig and Rohrer. The key to their success was using a feedback loop to regulate gap distance between the sample and the probe.

Acoustic microscopy is microscopy that employs very high or ultra high frequency ultrasound. Acoustic microscopes operate non-destructively and penetrate most solid materials to make visible images of internal features, including defects such as cracks, delaminations and voids.

A transducer is a device that converts energy from one form to another. Usually a transducer converts a signal in one form of energy to a signal in another.

An elastic modulus is a quantity that measures an object or substance's resistance to being deformed elastically when a stress is applied to it. The elastic modulus of an object is defined as the slope of its stress–strain curve in the elastic deformation region: A stiffer material will have a higher elastic modulus. An elastic modulus has the form:

In materials science, shear modulus or modulus of rigidity, denoted by G, or sometimes S or μ, is defined as the ratio of shear stress to the shear strain:

The frequency used sweeps from some few kHz to MHz, keeping the sine wave amplitude constant. The sine longitudinal waves are sensed by the probe, and the deflection of the probe is detected by laser light focused on to a position sensitive photodiode (PSPD). This deflection of the reflected laser beam from the cantilever (probe) indicates the flexural and torsional parameters of the specimen. The high-frequency signal is sent to a lock-in amplifier and correlated with the reference signal sent by the signal generator to form the AFAM image.

A lock-in amplifier is a type of amplifier that can extract a signal with a known carrier wave from an extremely noisy environment. Depending on the dynamic reserve of the instrument, signals up to 1 million times smaller than noise components, potentially fairly close by in frequency, can still be reliably detected. It is essentially a homodyne detector followed by low-pass filter that is often adjustable in cut-off frequency and filter order. Whereas traditional lock-in amplifiers use analog frequency mixers and RC filters for the demodulation, state-of-the-art instruments have both steps implemented by fast digital signal processing, for example, on an FPGA. Usually sine and cosine demodulation is performed simultaneously, which is sometimes also referred to as dual-phase demodulation. This allows the extraction of the in-phase and the quadrature component that can then be transferred into polar coordinates, i.e. amplitude and phase, or further processed as real and imaginary part of a complex number.

Since the development of atomic force microscopy many modes and related techniques have emerged. Ultrasonic force microscopy, ultrasonic atomic force microscopy, scanning acoustic force microscopy and AFAM all come under the branch of near-field microscopy techniques called contact resonance force microscopy (CRFM). CRFM techniques depend principally on the calculation of contact resonance frequencies and how they shift with variations (like precipitates and matrix) in the sample.

Resonance phenomenon in which a vibrating system or external force drives another system to oscillate with greater amplitude at specific frequencies

Resonance describes the phenomena of amplification that occurs when the frequency of a periodically applied force is in harmonic proportion to a natural frequency of the system on which it acts. When an oscillating force is applied at the resonant frequency of another system, the system will oscillate at a higher amplitude than when the same force is applied at other, non-resonant frequencies.

History

Atomic force acoustic microscopy (AFAM) was originally developed by Rabe and Arnold ^[1] from the Fraunhofer Institute of Nondestructive Testing in 1994. The technique is now used for qualitative and quantitative measurements of the local elastic properties of materials. AFAM was used by Anish Kumar et al.^[2]^[3] to map the precipitates in the polycrystalline materials.

Principle

In the AFAM setup the sample is coupled to a piezoelectric transducer. This emits longitudinal acoustic waves into the sample, causing out-of-plane vibrations in the sample's surface. The vibrations are transmitted into the cantilever via the sensor tip. The cantilever vibrations are measured by a 4-section photo-diode and evaluated by a lock-in amplifier. This setup can be used either to acquire cantilever vibration spectra or to take acoustic images. The latter are maps of cantilever amplitudes on a fixed excitation frequency near the resonance. A contact-mode topography image is acquired simultaneously with the acoustic one.

The frequency range employed covers the flexural modes of the cantilever from 10 kHz up to 5 MHz, with an average frequency of around 3 MHz. It can be used to map the elastic modulus variations between the precipitates and matrix of a material, such that even the elastic properties of the thin films can be determined. It can be used in air, vacuum and liquid media.

Probes used for AFAM are made up of silicon nitride (Si₃N₄) or silicon (Si). Cantilevers with low spring constants (0.01-0.5 N/m) for soft materials and high spring constants (42-50 N/m) for hard materials are used. Within the probe structure, the cantilever and tip material may not be same. Tips are usually manufactured using anisotropic etching or vapor deposition. The probe is placed at an angle around 11-15 degrees from the horizontal axis.

Two models are used for the calculations in AFAM: the cantilever dynamics model and the contact mechanics model. Using these two models the elastic properties of the materials can be determined. All the calculations are done using LabView software. The frequency of the eigen modes of the cantilever depends, amongst other parameters, on the stiffness of the tip-sample contact and on the contact radius, which in turn are both a function of the Young's modulus of the sample and the tip, the tip radius, the load exerted by the tip, and the geometry of the surface. Such a technique allows one to determine the Young's modulus from the contact stiffness with a resolution of a few tens of nanometers, mode sensitivity is about 5%.

Models

For calculation of the elastic properties of the materials we need to consider two models:^[4] cantilever dynamic model - calculation of the k* (contact stiffness); and Hertz contact model - contact mechanics - calculation of the reduced elastic modulus (E*) of the sample considering the contact area.

Procedure to calculate the elastic properties of various materials

Use of the two models mentioned above will take us to correct determination of the various elastic properties for various materials. The steps needed to be considered for the calculation are:

Acquire the contact resonances for any two bending modes.
The two modes can be acquired separately or simultaneously. Importance of simultaneous acquisition has been demonstrated by Phani et al.^[5]
By measuring the contact-resonance frequencies of two modes, one can write two equations containing two unknown values L1 and k*. By plotting k* as a function of the tip position (L1/L) for the two modes, one obtains two curves, the cross-point of which yields a unique value of k* of the system using both modes.
Using the Hertz contact model, k* can be converted to E*. As accurate measurement of R of the tip is very difficult; measurement on a reference sample is carried out to eliminate the requirement of knowing the value of R. The reference sample may be an amorphous material or a single crystal.

Advantages over other SPM processes

Frequency shifts are easier to measure accurately than absolute amplitudes or phase.
Can be used in air as well as liquid environment like (in a droplet).
Can test any type of material.
Atomic level resolution.
Flaw characterization and detection of hidden structures can be done.
Quantitative characterization of nano material layers.
Quantitative and qualitative measurements at the nano scale.
Damping measurements at nano level which can give actual idea of crack initiation and propagation which are very important in the case of structural materials.

Related Research Articles

Atomic force microscopy (AFM) or scanning force microscopy (SFM) is a very-high-resolution type of scanning probe microscopy (SPM), with demonstrated resolution on the order of fractions of a nanometer, more than 1000 times better than the optical diffraction limit. This capability resulted in the discovery in 1997, of a new cellular structure at the plasma membrane named porosome, the universal secretory machinery in cells, thereby establishing a new field in biology, nanocellbiology.

Magnetic force microscopy (MFM) is a variety of atomic force microscopy, in which a sharp magnetized tip scans a magnetic sample; the tip-sample magnetic interactions are detected and used to reconstruct the magnetic structure of the sample surface. Many kinds of magnetic interactions are measured by MFM, including magnetic dipole–dipole interaction. MFM scanning often uses non-contact AFM (NC-AFM) mode.

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Chemical force microscopy (CFM) is a variation of atomic force microscopy (AFM) which has become a versatile tool for characterization of materials surfaces. With AFM, structural morphology is probed using simple tapping or contact modes that utilize van der Waals interactions between tip and sample to maintain a constant probe deflection amplitude (constant force mode) or maintain height while measuring tip deflection (constant height mode). CFM, on the other hand, uses chemical interactions between functionalized probe tip and sample. Choice chemistry is typically gold-coated tip and surface with R-SH thiols attached, R being the functional groups of interest. CFM enables the ability to determine the chemical nature of surfaces, irrespective of their specific morphology, and facilitates studies of basic chemical bonding enthalpy and surface energy. Typically, CFM is limited by thermal vibrations within the cantilever holding the probe. This limits force measurement resolution to ~1 pN which is still very suitable considering weak COOH/CH₃ interactions are ~20 pN per pair. Hydrophobicity is used as the primary example throughout this consideration of CFM, but certainly any type of bonding can be probed with this method.

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Piezoresponse force microscopy (PFM) is a variant of atomic force microscopy (AFM) that allows imaging and manipulation of piezoelectric/ferroelectric materials domains. This is achieved by bringing a sharp conductive probe into contact with a ferroelectric surface and applying an alternating current (AC) bias to the probe tip in order to excite deformation of the sample through the converse piezoelectric effect (CPE). The resulting deflection of the probe cantilever is detected through standard split photodiode detector methods and then demodulated by use of a lock-in amplifier (LiA). In this way topography and ferroelectric domains can be imaged simultaneously with high resolution.

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Nanosensors is a brand of SPM and AFM probes for atomic force microscopy (AFM) and scanning probe microscopy (SPM).

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The colloidal probe technique is commonly used to measure interaction forces acting between colloidal particles and/or planar surfaces in air or in solution. This technique relies on the use of an atomic force microscope (AFM). However, instead of a cantilever with a sharp AFM tip, one uses the colloidal probe. The colloidal probe consists of a colloidal particle of few micrometers in diameter that is attached to an AFM cantilever. The colloidal probe technique can be used in the sphere-plane or sphere-sphere geometries. One typically achieves a force resolution between 1 and 100 pN and a distance resolution between 0.5 and 2 nm.

Non-contact atomic force microscopy (nc-AFM), also known as dynamic force microscopy (DFM), is a mode of atomic force microscopy, which itself is a type of scanning probe microscopy. In nc-AFM a sharp probe is moved close to the surface under study, the probe is then raster scanned across the surface, the image is then constructed from the force interactions during the scan. The probe is connected to a resonator, usually a silicon cantilever or a quartz crystal resonator. During measurements the sensor is driven so that it oscillates. The force interactions are measured either by measuring the change in amplitude of the oscillation at a constant frequency just off resonance or by measuring the change in resonant frequency directly using a feedback circuit to always drive the sensor on resonance.

AFM-IR is one of a family of techniques that are derived from a combination of two parent instrumental techniques; infrared spectroscopy and scanning probe microscopy (SPM). The term was first used to denote a method that combined a tuneable free electron laser with an atomic force microscope equipped with a sharp probe that measured the local absorption of infrared light by a sample; it required that the sample be coupled to an infrared-transparent prism and be less than 1μm thick. It improved the spatial resolution of photothermal AFM-based techniques from microns to circa 100 nm.

A probe tip in scanning microscopy literally is a very sharp piece of metal or non-metal, like a sewing needle with a point at one end with nano or sub-nanometer order of dimension. It can interact with up to one molecule or atom of a given surface of a sample that can reveal authentic properties of the surface such as morphology, topography, mapping and electrical properties of a single atom or molecule on the surface of the sample.

References

↑ Rabe, U.; Arnold, W. (21 March 1994). "Acoustic microscopy by atomic force microscopy". Applied Physics Letters. AIP Publishing. 64 (12): 1493–1495. doi:10.1063/1.111869. ISSN 0003-6951.
↑ A.E. Asimov and S.A. Saunin "Atomic Force Acoustic Microscopy as a tool for polymer elastisity analysis" SPM 2002 Proceedings. P.79. ^{[ permanent dead link ]}
↑ Kumar, Anish; Rabe, Ute; Arnold, Walter (18 July 2008). "Mapping of Elastic Stiffness in an α+β Titanium Alloy using Atomic Force Acoustic Microscopy". Japanese Journal of Applied Physics. Japan Society of Applied Physics. 47 (7): 6077–6080. doi:10.1143/jjap.47.6077. ISSN 0021-4922.
↑ "Atomic Force acoustic microscopy", Ute Rabe
↑ Kalyan Phani, M.; Kumar, Anish; Jayakumar, T. (20 May 2014). "Elasticity mapping of delta precipitate in alloy 625 using atomic force acoustic microscopy with a new approach to eliminate the influence of tip condition". Philosophical Magazine Letters. Informa UK Limited. 94 (7): 395–403. doi:10.1080/09500839.2014.920538. ISSN 0950-0839.

External links

AFAM details
Tatami, Junichi; Ohbuchi, Tomoko; Komeya, Katsutoshi; Meguro, Takeshi (2005). "Nanofractography of Alumina by Scanning Probe Microscopy". Key Engineering Materials. Trans Tech Publications. 290: 70–77. doi:10.4028/www.scientific.net/kem.290.70. ISSN 1662-9795.
Contact Resonance imaging ^{[ permanent dead link ]}
Johnson, K. L. (1987). Contact mechanics. Cambridge Cambridgeshire New York: Cambridge University Press. ISBN 978-0-521-34796-9. OCLC 17282784.

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[1] Rabe, U.; Arnold, W. (21 March 1994). "Acoustic microscopy by atomic force microscopy". Applied Physics Letters. AIP Publishing. 64 (12): 1493–1495. doi:10.1063/1.111869. ISSN 0003-6951.

[2] A.E. Asimov and S.A. Saunin "Atomic Force Acoustic Microscopy as a tool for polymer elastisity analysis" SPM 2002 Proceedings. P.79. ^{[ permanent dead link ]}

[3] Kumar, Anish; Rabe, Ute; Arnold, Walter (18 July 2008). "Mapping of Elastic Stiffness in an α+β Titanium Alloy using Atomic Force Acoustic Microscopy". Japanese Journal of Applied Physics. Japan Society of Applied Physics. 47 (7): 6077–6080. doi:10.1143/jjap.47.6077. ISSN 0021-4922.

[4] "Atomic Force acoustic microscopy", Ute Rabe

[5] Kalyan Phani, M.; Kumar, Anish; Jayakumar, T. (20 May 2014). "Elasticity mapping of delta precipitate in alloy 625 using atomic force acoustic microscopy with a new approach to eliminate the influence of tip condition". Philosophical Magazine Letters. Informa UK Limited. 94 (7): 395–403. doi:10.1080/09500839.2014.920538. ISSN 0950-0839.