Atomic force microscopy (AFM) is an analytical instrument used for surface mapping, scanning, and line-by-line recording of the surface topography. The instrument runs in two modes of operations: quasistatic mode and dynamic mode.
Image Credit: sanjaya viraj bandara/Shutterstock.com
The frequency modulation AFM (FM-AFM) method provides subatomic spatial resolution and comprises a cantilever with an oscillating frequency varied by the force gradient acting between the tip and the sample.
It is regarded as a precise method for measuring time, frequency, and other physical parameters in surface mapping or scanning with high accuracy.
FM-AFM is characterized by dissipative forces, greater harmonics, signal-frequency shift, noise, signal-to-noise ratio, and continuous work function measurements. Since the introduction of the technique in 1986, it has emerged over the years, distinctly with the introduction of the qPlus sensor as the method that provides subatomic spatial resolution with sub-piconewton sensitivity.
qPlus based AFM is known as the best method to study equipment and materials at ultralow temperatures as it allows examining and navigating even the nonconductive parts of the equipment and materials.
Role of qPlus Sensor in Atomic Force Microscopy
The qPlus sensor is useful for conducting Scanning Tunnel Microscopy (STM) and AFM simultaneously and provides subatomic level resolution, which is higher quality than STM. They are used in AFM that deals with normal forces and are characterized by their stiffness, eigenfrequency, and sensitivity.
Their stiffness is the function of their beam’s width, thickness, and length, and their eigenfrequency is dependent upon their stiffness and effective mass.
The sensor’s probe tip comprises iridium, silicon, tungsten, and other metals and is attached to one of these metals with conductive glue. The sensor’s rigid characteristics allow mounting individual crystal tips from nickel (II) oxide (NiO) and other cleavable materials by splitting the tips in situ or cleaning with large voltage pulses.
Generally, the qPlus sensor’s tip is made up of metals. It is characterized by the carbon monoxide front atom identification (COFI) method at the atomic scale.
In this method, a CO molecule probes the tip’s front end; the molecule is attached to a surface where it exposes its oxygen atom to the tip and stays upright. In this way, the tip images organic molecules on the atomic scale in high resolution.
Current Research and Applications of qPlus Sensor
Research applications of the qPlus based AFM range from subatomic scale spatial imaging, spin-dependent forces’ spectroscopy, and imaging, atomic manipulation force measurement to atomic imaging of graphene and graphene oxides.
The qPlus sensor is well suited for AFM at (ultra) low temperatures and ambient, liquid, and vacuum environments. It supports operating AFM and STM in parallel and has spatial resolution expanded up to the subatomic range, which is a higher resolution than that of STM.
Their primary element is the force detector, which comprises a deflection sensor and a tip. In the qPlus sensor, molecular imaging with a carbon monoxide (CO) based tip has emerged a new scope, which is reviewed by Pavliček and Gross (2017) and Gross, et al., (2018). This AFM-based molecular analysis gives insights into the geometry and occurrence, such as alkyl chains’ connectivity and length, aromaticity, heterocycles’ locations, and heterocycles’ types.
Future of qPlus Sensor in Atomic Force Microscopy
For a qPlus sensor-based AFM, quantification of the small chemical force that acts in between the sample and the qPlus sensor’s tip is challenging. There are four emerging challenges regarding the sample-tip force. The first is that the jump-to-contact process in the method affects the soft spring, a force sensor
Secondly, the sample-tip forces are simultaneously both attractive and repulsive, and thus are not monotonous, keeping them from achieving a feedback loop. Thirdly the short-range forces comprising possibly small magnitude are under layered by the long-range forces, which results in poor atomic imaging.
Finally, the force measurement in the pN and nN range is more challenging than the current measurement in the same range.
The future of qPlus sensor-based AFM combines inelastic electron tunneling spectroscopy (IETS) and AFM. It explores the technology in vacuum at the high magnetic field and ultralow (mK) temperature conditions.
Since the signal intensities of IETS varies widely for several tips, IETS’ tip is evaluated with carbon monoxide front atom identification (COFI). This method allows repetitive checking and poking of the tip just to confirm the tip is a single atom. Although research on qPlus based STM/AFM studies in vacuum at liquid helium temperatures are currently happening, they need to be explored as they extend the sensor’s operation up to the mK range.
Continue reading: How Can Sensors Help to Improve AFM?
References and Further Reading
Canale, L., Laborieux, A., Mogane, A., Jubin, L., Comtet, J., & Lainé, A. (2018). MicroMegascope. Nanotechnology. doi:10.1088/1361-6528/aacbad
Emmrich, M., Huber, F., Pielmeier, F., Welker, J., Hofmann, T., Schneiderbauer, M., & Meuer, D. (2015). Subatomic resolution force microscopy reveals internal structure and adsorption sites of small iron clusters. Science. doi:10.1126/science.aaa5329
Extance, A. (2018). How atomic imaging is being pushed to its limit. Nature. doi:10.1038/d41586-018-03305-2
Giessibl, F. (2019). The qPlus sensor, a powerful core for the atomic force microscope. Rev Sci Instrum. doi:10.1063/1.5052264
Gross, L., Schuler, B., Pavliček, N., Fatayer, S., Majzik, Z., & Moll, N. (2018). Atomic Force Microscopy for Molecular Structure Elucidation. Chemie. doi:10.1002/anie.201703509
Hofmann, T., Welker, J., & Giessibl, F. (2010). Preparation of light-atom tips for scanning probe microscopy by explosive delamination. Journal of Vacuum Science & Technology B. doi:10.1116/1.3294706
Klimov, N., Jung, S., Zhu, S., Li, T., Wright, C., & Solares, S. (2012). Electromechanical Properties of Graphene Drumheads. Science. doi:10.1126/science.1220335
Pavliček, N., & Gross, L. (2017). Generation, manipulation and characterization of molecules by atomic force microscopy. Nature Reviews Chemistry. doi:10.1038/s41570-016-0005
Song, Y., Otte, A., Kuk, Y., Hu, Y., Torrance, D., & First, P. (2010). High-resolution tunnelling spectroscopy of a graphene quartet. Nature. doi:10.1038/nature09330
Wutscher, T., & Giessibl, F. (2011). Note: In situ cleavage of crystallographic oriented tips for scanning probe microscopy. Review of Scientific Instruments. doi:10.1063/1.3549628