qPlus Sensor Research Articles

Graphite, graphene on SiC, and graphene nanoribbons: Calculated images with a numerical FM-AFM.

Background: Characterization at the atomic scale is becoming an achievable task for FM-AFM users equipped, for example, with a qPlus sensor. Nevertheless, calculations are necessary to fully interpret experimental images in some specific cases. In this context, we developed a numerical AFM (n-AFM) able to be used in different modes and under different usage conditions.Results: Here, we tackled FM-AFM image calculations of three types of graphitic structures, namely a graphite surface, a graphene sheet on a silicon carbide substrate with a Si-terminated surface, and finally, a graphene nanoribbon. We compared static structures, meaning that all the tip and sample atoms are kept frozen in their equilibrium position, with dynamic systems, obtained with a molecular dynamics module allowing all the atoms to move freely during the probe oscillations.Conclusion: We found a very good agreement with experimental graphite and graphene images. The imaging process for the deposited nanoribbon demonstrates the stability of our n-AFM to image a non-perfectly planar substrate exhibiting a geometrical step as well as a material step.

Simultaneous current, force and dissipation measurements on the Si(111) 7×7 surface with an optimized qPlus AFM/STM technique.

We present the results of simultaneous scanning-tunneling and frequency-modulated dynamic atomic force microscopy measurements with a qPlus setup. The qPlus sensor is a purely electrical sensor based on a quartz tuning fork. If both the tunneling current and the force signal are to be measured at the tip, a cross-talk of the tunneling current with the force signal can easily occur. The origin and general features of the capacitive cross-talk will be discussed in detail in this contribution. Furthermore, we describe an experimental setup that improves the level of decoupling between the tunneling-current and the deflection signal. The efficiency of this experimental setup is demonstrated through topography and site-specific force/tunneling-spectroscopy measurements on the Si(111) 7×7 surface. The results show an excellent agreement with previously reported data measured by optical interferometric deflection.

QPlus magnetic force microscopy in frequency-modulation mode with millihertz resolution

Magnetic force microscopy (MFM) allows one to image the domain structure of ferromagnetic samples by probing the dipole forces between a magnetic probe tip and a magnetic sample. The magnetic domain structure of the sample depends on the alignment of the individual atomic magnetic moments. It is desirable to be able to image both individual atoms and domain structures with a single probe. However, the force gradients of the interactions responsible for atomic contrast and those causing domain contrast are orders of magnitude apart, ranging from up to 100 Nm−1 for atomic interactions down to 0.0001 Nm−1 for magnetic dipole interactions. Here, we show that this gap can be bridged with a qPlus sensor, with a stiffness of 1800 Nm−1 (optimized for atomic interaction), which is sensitive enough to measure millihertz frequency contrast caused by magnetic dipole–dipole interactions. Thus we have succeeded in establishing a sensing technique that performs scanning tunneling microscopy, atomic force microscopy and MFM with a single probe.

Effect of the tip state during qPlus noncontact atomic force microscopy of Si(100) at 5 K: Probing the probe.

Background: Noncontact atomic force microscopy (NC-AFM) now regularly produces atomic-resolution images on a wide range of surfaces, and has demonstrated the capability for atomic manipulation solely using chemical forces. Nonetheless, the role of the tip apex in both imaging and manipulation remains poorly understood and is an active area of research both experimentally and theoretically. Recent work employing specially functionalised tips has provided additional impetus to elucidating the role of the tip apex in the observed contrast.Results: We present an analysis of the influence of the tip apex during imaging of the Si(100) substrate in ultra-high vacuum (UHV) at 5 K using a qPlus sensor for noncontact atomic force microscopy (NC-AFM). Data demonstrating stable imaging with a range of tip apexes, each with a characteristic imaging signature, have been acquired. By imaging at close to zero applied bias we eliminate the influence of tunnel current on the force between tip and surface, and also the tunnel-current-induced excitation of silicon dimers, which is a key issue in scanning probe studies of Si(100).Conclusion: A wide range of novel imaging mechanisms are demonstrated on the Si(100) surface, which can only be explained by variations in the precise structural configuration at the apex of the tip. Such images provide a valuable resource for theoreticians working on the development of realistic tip structures for NC-AFM simulations. Force spectroscopy measurements show that the tip termination critically affects both the short-range force and dissipated energy.

Comparison of force sensors for atomic force microscopy based on quartz tuning forks and length-extensional resonators

The force sensor is key to the performance of atomic force microscopy (AFM). Nowadays, most AFMs use micro-machined force sensors made from silicon, but piezoelectric quartz sensors are applied at an increasing rate, mainly in vacuum. These self sensing force sensors allow a relatively easy upgrade of a scanning tunneling microscope to a combined scanning tunneling/atomic force microscope. Two fundamentally different types of quartz sensors have achieved atomic resolution: the 'needle sensor' that is based on a length extensional resonator and the 'qPlus sensor' that is based on a tuning fork. Here, we calculate and measure the noise characteristics of these sensors. We find four noise sources: deflection detector noise, thermal noise, oscillator noise and thermal drift noise. We calculate the effect of these noise sources as a factor of sensor stiffness, bandwidth and oscillation amplitude. We find that for self sensing quartz sensors, the deflection detector noise is independent of sensor stiffness, while the remaining three noise sources increase strongly with sensor stiffness. Deflection detector noise increases with bandwidth to the power of 1.5, while thermal noise and oscillator noise are proportional to the square root of the bandwidth. Thermal drift noise, however, is inversely proportional to bandwidth. The first three noise sources are inversely proportional to amplitude while thermal drift noise is independent of the amplitude. Thus, we show that the earlier finding that quoted optimal signal-to-noise ratio for oscillation amplitudes similar to the range of the forces is still correct when considering all four frequency noise contributions. Finally, we suggest how the signal-to-noise ratio of the sensors can be further improved and briefly discuss the challenges of mounting tips.

Atomic force microscopy at ambient and liquid conditions with stiff sensors and small amplitudes

We report on atomic force microscopy (AFM) in ambient and liquid environments with the qPlus sensor, a force sensor based on a quartz tuning fork with an all-electrical deflection measurement scheme. Small amplitudes, stiff sensors with bulk diamond tips and high Q values in air and liquid allow to obtain high resolution images. The noise sources in air and liquid are analyzed and compared for standard silicon cantilevers and qPlus sensors. First, epitaxial graphene was imaged in air, showing atomic steps with 3 Å height and ridges. As a second sample system, measurements on calcite (CaCO(3)) in liquids were performed in water and polyethylenglycol (PEG). We demonstrate high resolution images of steps in PEG on calcite and nanolithography processes, in particular with frequency-modulation AFM the controlled dissolution of calcite monolayers.

Higher-order eigenmodes of qPlus sensors for high resolution dynamic atomic force microscopy

The time response of tuning-fork based sensors can be improved by operating them at higher eigenmodes because a measurement takes at least one oscillation cycle in dynamic force microscopy and the oscillation period of the second eigenmode is only about one sixth of the fundamental mode. Here we study the higher-order eigenmodes of quartz qPlus sensors [Bettac et al., Nanotechnology 20, 264009 (2009); Giessibl and Reichling, Nanotechnology 16, S118 (2005); Giessibl, Appl. Phys. Lett. 76, 1470 (2000); and Giessibl, Appl. Phys. Lett. 73, 3956 (1998)], their equivalent stiffness, and piezoelectric sensitivity, while paying special attention to the influence of the mass and rotary inertia of the sensing tip which is attached to the end of the qPlus quartz cantilever. A combination of theoretical modeling and scanning laser Doppler vibrometry is used to study the eigenmodes of qPlus sensors with tungsten tips. We find that the geometry of tungsten tips can greatly influence the shape, equivalent stiffness, and piezoelectric sensitivity of the second eigenmode of the quartz cantilever. At a critical tip length it is possible to theoretically achieve infinite equivalent stiffness and infinite piezoelectric sensitivity when the tip becomes a perfect node of vibration and beyond this critical tip length the second eigenmode loses its vibration node and the trajectory of the tip reverses with respect to the beam curvature. The findings have major implications for optimizing tip geometry for high-resolution imaging with qPlus sensors using higher eigenmodes.

QPlus: atomic force microscopy on single-crystal insulators with small oscillationamplitudes at 5 K

Based on a proven low temperature scanning tunneling microscope (STM) platform, wehave integrated a QPlus sensor, which employs a quartz tuning fork for force detection innon-contact atomic force microscopy (AFM). For combined STM operation, this sensor haskey advantages over conventional sensors. For quantitative force spectroscopy oninsulating thin films or semiconductors, decoupling of the tunneling current and thepiezo-electrically induced AFM signal is important. In addition, extremely low signalsrequire the first amplification stage to be very close to the sensor, i.e. to be compatiblewith low temperatures. We present atomic resolution imaging on single-crystalNaCl(100) with oscillation amplitudes below 100 pm (peak-to-peak) and operation athigher flexural modes in constant frequency shift (df) imaging feedback. We alsopresent atomic resolution measurements on MgO(100) and Au(111), and firstevaluation measurements of the QPlus sensor in Kelvin probe microscopy on Si(111)7 × 7.

Higher‐harmonic atomic force microscopy

AbstractTo maximize the spatial resolution of atomic force microscopy (AFM), it is helpful to isolate the spurious short‐range components from the plethora of force contributions acting between AFM tips and samples. It is shown theoretically and experimentally that higher‐harmonic AFM, a technique that utilizes the anharmonic deflection contributions that arise when a cantilever oscillates in the highly nonlinear force field of a sample, is a suitable technique for maximizing spatial resolution. The qPlus sensor, a quartz cantilever that utilizes the piezoelectric effect for deflection detection, is particularly well suited for higher harmonic AFM because the physics of piezoelectric detection inherently magnifies the signal level of higher harmonics. Experimental results for W on graphite (0001), Si (111)‐(7 × 7), NiO (100) and CaF2(111) are presented. Copyright © 2006 John Wiley & Sons, Ltd.

Investigating atomic details of theCaF2(111) surface with a qPlus sensor

The (111) surface of CaF2 has been intensively studied with large-amplitude frequency-modulation atomicforce microscopy, and the atomic contrast formation is now well understood. Ithas been shown that the apparent contrast patterns obtained with a polar tipstrongly depend on the tip terminating ion, and three sub-lattices of anions andcations can be imaged. Here, we study the details of atomic contrast formation onCaF2(111) with small-amplitude force microscopy utilizing the qPlus sensor that has beenshown to provide the utmost resolution at high scanning stability. Step edgesresulting from cleaving crystals in situ in the ultra-high vacuum appear as very sharpstructures, and on flat terraces the atomic corrugation is seen in high clarityeven for large area scans. The atomic structure is also not lost when scanningacross triple layer step edges. High-resolution scans of small surface areas yieldcontrast features of anion- and cation sub-lattices with unprecedented resolution.These contrast patterns are related to previously reported theoretical results.

Stability considerations and implementation of cantilevers allowing dynamic forcemicroscopy with optimal resolution: the qPlus sensor

In frequency modulation atomic force microscopy, the stiffness, quality factor andoscillation amplitude of the cantilever are important parameters. While the first atomicresolution results were obtained with amplitudes of a few hundred ångstrom, ithas subsequently been shown that smaller amplitudes should result in a bettersignal-to-noise ratio and an increased sensitivity to the short-range components ofthe tip–sample interaction. Stable oscillation at small amplitudes is possible ifthe product of stiffness and amplitude and the energy stored in the oscillatingcantilever are large enough. For small amplitudes, stability can be achieved by usingstiff cantilevers. Here, we discuss the physical requirements for small amplitudeoperation and present design criteria and technical details of the qPlus sensor, aself-sensing cantilever with large stiffness that allows small amplitude operation.

Evaluation of a force sensor based on a quartz tuning fork for operation at low temperatures and ultrahigh vacuum

The noise performance of the force sensor is crucial for optimizing the resolution in non-contact atomic force microscopy. Sensing forces in vacuum and low temperatures is even more demanding than at ambient conditions. Here we analyze the noise performance and the sensitivity of a force sensor based on a quartz tuning fork of which one of the prongs is fixed (qPlus sensor). The noise characteristic of the qPlus sensor, optical and piezoresistive detection schemes at room temperature are compared and the qPlus sensor is investigated at low temperatures. The frequency variation of quartz tuning forks as a function of temperature is experimentally determined for the temperature range from 4 to 300 K.