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Exploring the Synergy of AFM Resonance Oscillatory Modes

In this interview, Sergei Magonov, President of NT-MDT Development, talks about the latest developments in the capabilities of NT-MDT’s range of AFMs, including new oscillatory resonance modes, and a new thermally-stable microscope cabinet, which allow expanded application opportunities.

To start off with, please can you give us an introduction to your role at NT-MDT?

In the spring of 2011, NT-MDT created a research unit, NT-MDT Development, based in Tempe, AZ, to bring together the expertise of small team of researchers. The team includes myself, as a long-term practitioner in SPM applications, John Alexander, who is a guru in SPM instrumentation with 30 years of experience in this field, and Sergey Belikov, an expert in mathematics, control theory and software design.

From the viewpoint of NT-MDT, this was an excellent opportunity to strengthen their instrumentation line, and to advance SPM applications with their microscopes. From the viewpoint of the individuals on the team, this was a nice chance to implement our passion for the SPM field in a highly creative environment, working with like-minded thinkers. These mutual expectations are currently being fulfilled in the fruitful everyday interactions with NT-MDT team in Russia.

We spoke a couple of years ago about the development of NT-MDT's Hybrid Mode, which you have recently been granted a patent for - can you tell us a bit about why this AFM mode is novel and unique, and how it has been developed over the last two years?

Hybrid Mode is actually the name of our practical implementation of a non-resonant oscillatory mode, which is one of the backbone operations in AFM, in addition to contact mode and the resonant oscillatory modes.

The basic idea of Hybrid mode was expressed in Elings and Gurley’s 1989 U.S. patent on jumping mode.

With the development of AFM electronics, different practical realizations of the concept have emerged, such as Pulsed Force mode, Peak Force mode, Quantitative Imaging, and Fast Force Mapping.

The novelty of Hybrid mode comes in the way we record the probe signals, filter them (if needed) and perform a quantitative on-line extraction of various local materials properties.

We are continuously developing Hybrid mode and its applications. A quantitative mapping of elastic modulus with spatial resolution of the lamellar width (10-20 nm) is routinely demonstrated for polymers. Currently, we are working on studies of the viscoelastic response of these materials. Electrical measurements are also possible - thermoelectric mapping of the Seebeck coefficient has been realized in Hybrid mode, and several other AFM-based electric modes are under development.

Hybrid mode is also useful for a combination of AFM visualization with spectroscopic (e.g. Raman scattering) studies. In other words, Hybrid mode has become the essential complimentary addition to other AFM modes.

A comparison of phase and frequency images of semi-fluorinated alkanes (F14H20) layer on graphite. Click for a larger image.

A comparison of phase and frequency images of semi-fluorinated alkanes (F14H20) layer on graphite. Click for a larger image.

What other innovations have you made in the range of AFM modes you offer recently?"

My long-term cooperation with Sergey Belikov (since we were working together at Veeco Instruments) has to a large extent focused on the theoretical understanding of AFM operation, and the nature of different modes. He has developed this theoretical background by solving the equations describing the tip-sample interactions, and came up with the related classification of AFM resonant oscillatory modes.

In particular, these interactions are described by two equations with four variables: the frequency, amplitude, and phase of the probe, and the sample topography. Therefore, two of the probe variables should be fixed, allowing solutions which include the sample topography and the remaining probe parameter.

This is actually what happens in AFM resonant modes. From a practical side, one of them - amplitude modulation with phase imaging, or AM-PI (originally known as Tapping mode) - is the most well-known resonant mode for studies in ambient conditions. In this mode, the driving frequency (near or at probe resonance) is fixed, and the probe amplitude is fixed at a set-point level. Height (topography) and phase images are the data recorded in this mode,

The other resonant mode, known as frequency modulation or FM (phase and frequency are fixed), is the main choice for measurements in vacuum. For a number of years, this mode has also been applied for studies in air and liquid by a few research groups using home-made microscopes, primarily in Japan.

One more resonant mode, in which the phase and amplitude are fixed, has been mentioned in literature but not well developed. In this mode, which we denote as amplitude modulation with frequency imaging, or AM-FI, the phase is held at 90 degrees. Therefore, the probe is driven at its effective resonance, which changes due to the tip-sample interactions. By analogy to the regular amplitude modulation mode, AM-PI, the probe amplitude is kept at the set-point level by a z-servo, which tracks the sample topography, and a frequency image is captured in place of the phase image.

The above mentioned resonant modes are now implemented in NT-MDT microscopes, with the development of a new controller by John Alexander. We have performed studies to verify these modes, and early results have been presented in the recently published NT-MDT Application Note "Exploring Imaging in Oscillatory Resonance AFM Modes: Backgrounds and Applications”.

What are the main applications that will see a benefit from the availability of these modes?

The described oscillatory AFM modes utilize different probe responses of the same tip-sample forces in­teractions. Besides this general similarity, they have also specific features. For example, in AM-PI mode, a peak force is largest at a set-point amplitude around half the amplitude of free oscillation, and the force decreases as probe deviates from the drive frequency at smaller set-point amplitudes.

The situation is different in AM-FI, in which the peak force monotonically increases as the set-point amplitude is lowered, as the drive is always performed at the effective resonant frequency.

Differences like these influence the contrast in the image obtained - for a number of examples we found that frequency images revealed more features than phase images. Furthermore, our theoretical estimates show that changes in frequency are more directly related to the elastic modulus of the sample, and this was also confirmed in our experiments.

Until now, the applications of FM mode both in air and under liquid have been mostly focused on atomic-scale imaging, and precise studies of tip-sample force interactions. We are looking at FM applications in a broader way. Using fine control of the tip-sample interactions, particularly in the true attractive force regime, we can achieve low-force imaging of weakly bonded surface structures, and have already achieved some results in this area.

Another advantage from imaging in AM-FI and FM modes is the recording of quantitative dissipation maps, which are calculated using the appropriate formulae and displayed simultaneously with other images. The dissipation contrast reveals local differences in probe interactions with various components of complex samples, which can be useful for the examination of viscoelastic properties of materials on small scales.

Height and frequency images of a lamellar layer of C242H486 alkanes on graphite.

Height and frequency images of a lamellar layer of C242H486 alkanes on graphite. Click for a larger image.

You will be giving a talk at MRS this year entitled "Synergy of AFM Resonance Oscillatory Modes" - can you give us a quick summary of the presentation, and talk about what is meant by the "synergy" of these techniques?

My experience in SPM applications, which has mainly been to do with developing with advances in instrumentation, has taught me that comprehensive characterization of materials is achieved only if a researcher is aware of the capabilities of different modes, and uses these techniques in synergy.

For example, the AFM-based electric modes benefit from single-pass studies, in which signatures of electrostatic force interactions are recorded at multiple frequencies, to learn about surface potential and capacitance gradients with high sensitivity and high spatial resolution.

Yet in some studies this technique might create unwanted sample charging, which can be avoided using the two-pass method (lift technique) that, in general, provides inferior results.

Therefore, a combined approach making use of both techniques provides definite advantages.

Regarding the resonant modes, they also each emphasize different aspects of tip-sample interactions, and can expand the range of the material characterization best when they are used in combination, together with contact mode and Hybrid mode as well.

Examples illustrating this point will be presented at the SPM Symposium at the coming MRS meeting. In the talk, the practical results will be given, together with the introduction and classification of AFM resonant modes.

What are the benefits to the average AFM user, of expanding the range of available resonance modes beyond the common Tapping Mode?

I don’t like talking about “average” AFM users, as I think everyone who is using this technique should be motivated to learn as much as possible about its real capabilities, above the average skills required to get the images.

I remember during my time at Digital Instruments when phase imaging became available, it took quite a bit of time for Dr. Virgil Elings (co-founder) to convince the application scientists to use this new capability. Although phase imaging is 20 years old now, and has been proven to be a very useful technique for compositional mapping, even the origin of the phase contrast and its interpretation are still far from being properly understood.

The frequency images which are recorded in AM-FI mode can be also very useful for the same purpose. In a number of examples, the frequency contrast can be quite different to the phase contrast, and can be more detailed in comparison. I believe such findings will be really enjoyed by the motivated researchers who keep their eyes open for the novel opportunities AFM continuously offers.

In this sense, FM mode offers our users various possibilities to explore this technique for imaging at different force levels, particularly in the true attractive force regime. Our goal as a microscope manufacturer is to make the resonant modes easy to run - currently, we are improving the automated controls for all resonance modes, so that users can focus mostly on data generation and analysis.


NT-MDT's new fan-free thermally stable cabinet for AFM.

What other developments have been made at NT-MDT recently?

Besides the described efforts in the development of AFM modes, the overall performance of the scanning probe microscope is also under our continuous attention.

When we joined NT-MDT, it was nice to find out that a gentle probe engagement routine was under development. This feature is now routinely used in our microscopes, and it prevents unwanted tip and sample damage during the probe’s approach to a sample.

Furthermore, it is commonly recognized that the best SPM performance is achieved in a quiet environment, free of acoustic and vibrational noise. Therefore, protective acoustic hoods and various means of vibration isolation are often used.

However, less care is devoted to stabilization of the microscope temperature, which means that thermal drift over the course of a measurement can be significant, particularly for longer measurements.

Therefore, we have designed a fan-free thermal cabinet, which ensures not only very quiet, but also temperature-stable environment for NT-MDT microscopes. The sample temperature is kept slightly above room temperature, and the sample temperature varies around the set-point less than 10 mK. This is realized with precise electronic control of the heaters embedded into the cabinet. At such conditions, temperature drift is quite low: below 0.2 nm/min, and the temperature perturbations caused by opening the cabinet door to change the sample or probe are short-lived.

There are multiple benefits to the microscope performance under low thermal drift conditions. Routine sub-100 nm and atomic-scale imaging becomes possible with a low scanning rate (1-2 Hz), which allows precise manipulation of the topography feedback control. This is important for profiling of highly corrugated technological surfaces, where slow scanning rates are usually applied.

Repeatedly imaging the same small locations using different modes is also much easier in the thermally-stable cabinet. Long-term, automated and non-attended measurements are greatly improved as well. This is only a short list of the rewarding AFM applications, which we will expand upon further in the future.

Other than your own presentation, what will NT-MDT's key focus points be at the upcoming MRS Fall Meeting in Boston?

At the exhibition this year we will have a booth with two systems. Our main goal will be to increase researchers’ awareness about the continuously expanding capabilities of our microscopes, and the high quality of the obtained results.

We will introduce a new controller that enables an extended set of the resonant modes, and will demonstrate operation in these modes. In addition, we will continue to promote the combined AFM/Raman system based on our NTEGRA Spectra optical set-up and DXR Raman microscope from Thermo Fisher Scientific.

About Sergei Magonov

Sergei MagonovSergei Magonov has received MS (1975) and PhD (1978) degrees at Moscow Institute of Physics and Technology, and he started his scientific career in the USSR Academy of Sciences working in polymer research.

During his research visit to Germany in 1988, Sergei started his activities first in STM and later in AFM, later as the Director of STM/AFM Laboratory in Materials Research Center of Freiburg University.

In 1995 he became an application scientist at Digital Instruments in Santa Barbara CA. He stayed with this company, which was acquired by Veeco Instruments, until 2007. In 2007-2011 Sergei worked as an application scientist at Agilent Technologies. Since 2011, he has served as President of NT-MDT Development Inc.

Sergei is author and co-author of 1 book, 15 reviews and chapters and over 200 per-reviewed articles and 6 US patents mostly in the field of scanning probe microscopy.

Disclaimer: The views expressed here are those of the interviewee and do not necessarily represent the views of Limited (T/A) AZoNetwork, the owner and operator of this website. This disclaimer forms part of the Terms and Conditions of use of this website.

Will Soutter

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Will Soutter

Will has a B.Sc. in Chemistry from the University of Durham, and a M.Sc. in Green Chemistry from the University of York. Naturally, Will is our resident Chemistry expert but, a love of science and the internet makes Will the all-rounder of the team. In his spare time Will likes to play the drums, cook and brew cider.


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