A qPlus sensor is a self-sensing cantilever based on a quartz tuning fork. It supplements the traditional cantilevers made of silicon. The qPlus sensors enable atomic force microscopy (AFM) and scanning tunneling microscopy (STM) to take place in parallel.
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The qPlus sensor travels over the surface of a sample; three properties of these quartz-based sensors enable AFM to be significantly simplified. These include self-sensing, the ability to demonstrate small variation with temperature, and stiffness which approaches the ideal stiffness of cantilevers. These properties are afforded by the quartz material of the qPlus sensor.
Consequently, the qPlus sensor produces a large stiffness that allows a small amplitude operation combined with a large size that allows single-crystal probe tips to be mounted and a self-sensing piezoelectric detection mechanism.
The Basis of qPlus Sensing: Building on AFM and STM
Both AFM and STM enable the characterization and modification of surfaces and none of the structures. STM is commonly used in the biological sciences to character eyes and alter objects at the atomic scale. However, the limiting characteristic of AFM is its limitation to conductive samples, as the tunneling current that flows between the probe on the sample surface is the feedback signal detected.
This conductive limitation was overcome by AFM, which enables the sensing of force acting between the tip and the sample. Therefore, this forced-based approach can be adapted to any sample independent of their ability to conduct electricity.
Consequently, AFM techniques have wider-ranging applications in biological research as well as physics and chemistry. More specifically, non-contact-AFM has been developed into a powerful tool for imaging with atomic resolution, performing single-atom manipulation, and determining chemical sensitivity across various surfaces.
The Scientist F J Giessibl introduced the qPlus sensor, which enables the simultaneous detection of the tunneling current and forces with small oscillation amplitude. The introduction of qPlus sensors substantially increases the sensitivity to tunneling current signals compared to traditional silicone-based cantilevers.
As such, this new approach to AFM enabled new ways of characterizing surfaces and nanostructures on the atomic scale. As such, qPlus sensors are increasingly used for non-contact measurements.
How Does qPlus Sensing Work?
qPlus sensor-based AFM and STM microscopy involves using a tuning fork; one prong is fixed and the other, which possesses a metal tip at the end, is free to oscillate. The changes in the resonant frequency of the unfixed prong directly reflect the degree of interaction of the sensor with the sample surface. Simultaneously, the presence of a metallic tip allows the average tunneling current, which flows between the tip and sample when the voltage is applied to be detected.
The force acting between the tip and the sample surface could be determined from frequency modulation; however, reliable estimations of the measured force are dependent on several factors. Among them, appropriate calibration of the mechanical properties – most notably, stiffness – is important
Quartz tuning forks were implemented in AFM to serve as an actuator and sensor for the tip-sampling interactions, which remove the need for optics and allow low oscillation amplitude to be detected.
However, the process of gluing a metallic tip to the quartz tuning fork, along with the interactions with the sample surface, induced a break of quartz tuning fork symmetry, producing low scanning speed and resolution. The qPlus sensor enables an efficient solution for these problems enables fast- scanning possibilities with easy interpretation.
The Latest in qPlus Sensing
Recent research has demonstrated that it is possible to image biological samples in a liquid medium using stiff qPlus sensors. In this setting, qPlus sensing overcomes the limit of AFM due to the fact that samples are non-transparent and may change their optical properties over time. This is due to the ability of qPlus sensors to be fitted with several types of tips, such as metal tips used in tip-enhanced Raman spectroscopy (TERS) and scanning near-field optical microscopy (SNOM).
Research from the same group has also produced a new generation of qPlus-based AFM, which pushes the sensitivity and resolution of SPM to the classical limit, allowing for direct imaging of the hydrogen atoms present in water molecules.
Using this principle, this research group has integrated nitrogen-vacancy (NV)-based sensing technology into an SPM based on qPlus sensing, resulting in a scanning quantum sensing microscope.
Due to the ultra-high stability of the qPlus sensor, it can function at small amplitude, approximately 100 pm at a close tip surface distance of ~1nm. At this distance, good coherence and resolution are maintained. This form of shallow NV enables the team to map the electric field from a metal tip with a spatial resolution of ~10nm and very high sensitivity.
Future work will see this technique applied to determine the local charge, dielectric response, and polarisation of materials at a microscopic level.
Challenges In qPlus Sensing
Despite the wide use and commercialization, qPlus sensors have several limitations, which include the fact that they are still produced by hand and suffer from a reduced resonance frequency and quality factor when compared to qPlus sensors without a tip (bare). This is due to the load induced by the large and heavy tungsten tip fixed to the free prong using epoxy. In addition, it is impossible to determine the exact mass added and the position of the tip on the free prong, alongside the amount of epoxy, used to fix the tip.
The imprecision results in a large spread of mechanical properties from one fabricated sensor to the next, making it difficult to calibrate the stiffness of the qPlus sensor.
Although qPlus sensors have enabled sub-molecular resolution imaging to take place as well as manipulation on the atomic and molecular scale, recent developments in non-contact AFM techniques using qPlus sensors have enabled quantitative interpretation of atomic forces to take place.
However, the exact value of the stiffness of the sensor is critically important to convert the experimentally measured frequency shift to the interaction forces as well as calculate other quantities such as energy dissipation. Since stiffness cannot be known exactly for a standard qPlus sensor, these quantitative measurements are imprecise, have large associated errors, and demonstrate variation from one sensor to the next.
This issue has recently motivated research into techniques for calibration of the qPlus sensor stiffness, also known as spring constant. However, this has proven difficult due to the imprecise nature of the fabrication method itself – i.e., the large tip mass, variation in tip position on the quartz tuning fork, on the variability in the amount and nature of the epoxy used to glue the tip.
Many companies today provide low- temperature STM and AFM that make use of qPlus sensors. This bolsters their resolution, increasing it to the atomic scale with greater precision and versatility. However, future work is centered on acquiring quantitative AFM measurements by overcoming the knowledge gaps associated with tip stiffness. In addition, researchers are working to optimize the fabrication technique and address current cross-talk challenges that occur during simultaneous current and force measurements.
Continue reading: qPlus Sensors in Atomic Force Microscopy
References and Further Reading
Berger J, Svec M, Müller M, Ledinský M, Fejfar A, Jelínek P, Majzik Z. (2013) Characterization of the mechanical properties of qPlus sensors. Beilstein J Nanotechnol https://doi.org/10.3762/bjnano.4.1
Labidi H, Kupsta M, Huff T, et al. (2015) New fabrication technique for highly sensitive qPlus sensor with well-defined spring constant. Ultramicroscopy.158:33-7. https://www.sciencedirect.com/science/article/pii/S0304399115001424
Bian K, Zheng W, Zeng X, et al. (2021) Nanoscale electric-field imaging based on a quantum sensor and its charge-state control under ambient condition. Nat Commun. 12(1):2457. https://www.nature.com/articles/s41467-021-22709-9
Pürckhauer K, Weymouth AJ, Pfeffer K, Kullmann L, Mulvihill E, Krahn MP, Müller DJ, Giessibl FJ. Imaging in Biologically-Relevant Environments with AFM Using Stiff qPlus Sensors. Sci Rep. https://dx.doi.org/10.1038%2Fs41598-018-27608-6
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