Frequency Modulation Atomic Force Microscopy Research Articles

Manipulating Si(100) at 5 K using qPlus frequency modulated atomic force microscopy: Role of defects and dynamics in the mechanical switching of atoms

We use small-amplitude qPlus frequency modulated atomic force microscopy (FM-AFM), at 5 K, to investigate the atomic-scale mechanical stability of the Si(100) surface. By operating at zero applied bias the effect of tunneling electrons is eliminated, demonstrating that surface manipulation can be performed by solely mechanical means. Striking differences in surface response are observed between different regions of the surface, most likely due to variations in strain associated with the presence of surface defects. We investigate the variation in local energy surface by ab initio simulation, and comment on the dynamics observed during force spectroscopy.

Wideband phase-locked loop circuit with real-time phase correction for frequency modulation atomic force microscopy

We have developed a wideband phase-locked loop (PLL) circuit with real-time phase correction for high-speed and accurate force measurements by frequency modulation atomic force microscopy (FM-AFM) in liquid. A high-speed operation of FM-AFM requires the use of a high frequency cantilever which, however, increases frequency-dependent phase delay caused by the signal delay within the cantilever excitation loop. Such phase delay leads to an error in the force measurements by FM-AFM especially with a low Q factor. Here, we present a method to compensate this phase delay in real time. Combined with a wideband PLL using a subtraction-based phase comparator, the method allows to perform an accurate and high-speed force measurement by FM-AFM. We demonstrate the improved performance by applying the developed PLL to three-dimensional force measurements at a mica/water interface.

Modification of a commercial atomic force microscopy for low-noise, high-resolution frequency-modulation imaging in liquid environment

A key issue for high-resolution frequency-modulation atomic force microscopy imaging in liquids is minimizing the frequency noise, which requires a detailed analysis of the corresponding noise contributions. In this paper, we present a detailed description for modifying a commercial atomic force microscope (Bruker MultiMode V with Nanoscope V controller), aiming at atomic-resolution frequency-modulation imaging in ambient and in liquid environment. Care was taken to maintain the AFMs original stability and ease of operation. The new system builds upon an optimized light source, a new photodiode and an entirely new amplifier. Moreover, we introduce a home-built liquid cell and sample holder as well as a temperature-stabilized isolation chamber dedicated to low-noise imaging in liquids. The success of these modifications is measured by the reduction in the deflection sensor noise density from initially 100 fm/√Hz to around 10 fm/√Hz after modification. The performance of our instrument is demonstrated by atomically resolved images of calcite taken under liquid conditions.

Comparison of photothermal and piezoacoustic excitation methods for frequency and phase modulation atomic force microscopy in liquid environments

In attempting to perform frequency modulation atomic force microscopy (FM-AFM) in liquids, a non-flat phase transfer function in the self-excitation system prevents proper tracking of the cantilever natural frequency. This results in frequency-and-phase modulation atomic force microscopy (FPM-AFM) which lies in between phase modulation atomic force microscopy (PM-AFM) and FM-AFM. We derive the theory necessary to recover the conservative force and damping in such a situation, where standard FM-AFM theory no longer applies. Although our recovery procedure applies to all cantilever excitation methods in principle, its practical implementation may be difficult, or even impossible, if the cantilever is driven piezoacoustically. Specifically, we contrast the piezoacoustic excitation method to the photothermal method in the context of force spectroscopy of hydration structures at the mica-water interface. The results clearly demonstrate that photothermal excitation is superior to piezoacoustic excitation, as it allows for accurate quantitative interpretation of the acquired data.

Angled long tip to tuning fork probes for atomic force microscopy in various environments

We expand the range of applications of a tuning fork probe (TFP) in frequency-modulation atomic force microscopy (FM-AFM) by attaching a long metal tip at a certain angle. By the combined flexure of the metal tip and the tuning fork prong, this TFP can change the direction of the detectable force by switching the resonance frequency, which has not been realized with conventional TFPs with short tips. The oscillatory behavior of the tip apex of the TFP is predicted by computer simulations and is experimentally confirmed with scanning electron microscope. FM-AFM operations using this TFP are performed in various environments, i.e., in ultrahigh vacuum, air, and water. FM-AFM images obtained at an atomic step of highly oriented pyrolytic graphite in air show a clear difference depending on the excitation frequency. It is also revealed that the higher order flexural modes of this TFP are advantageous for FM-AFM in water due to the reduction in the degree of hydrodynamic damping.

Combined dynamic scanning tunneling microscopy and frequency modulation atomic force microscopy investigations on polythiophene chains on graphite with a tuning fork sensor

Polythiophene molecules adsorbed on a highly oriented pyrolytic graphite surface were studied by combined dynamic scanning tunneling microscopy (STM) and frequency modulation atomic force microscopy (FM-AFM) with a quartz tuning fork sensor operating in Qplus mode and equipped with a Pt/Ir tip. Upon completing a careful sub-angström oscillation amplitude calibration of the probe, experiments were conducted in an ultra high vacuum at room temperature. By selecting the tip/surface distance regulation parameter, one can select the type of simultaneous information obtained in an area. For distance regulation based on the mean tunneling current, dynamic STM images together with maps of tip/surface force gradient were obtained. FM-AFM images with maps of the tunneling current were also acquired when the distance regulation was based on the frequency shift. Comparison between these images reveals interesting features. For example the tip which operates in STM mode with ultra low current (<10 pA) generates different interaction forces above molecules or graphite. Changes in energy dissipation processes as small as tens of millielectronvolts per cycle were recorded when the tip oscillates above the polymer or on the graphite surface. Hence data demonstrates that a stiff piezoelectric tuning fork of several kilonewtons/meters working as an AFM/STM probe with sub-angström amplitude can characterize weakly adsorbed molecules.

Imaging of Transverse Electron Transfer through a DNA Molecule by Simultaneous Scanning Tunneling and Frequency-Modulation Atomic Force Microscopy

Simultaneous scanning tunneling and frequency-modulation atomic force microscopy (STM/FM-AFM) was applied to a DNA molecule on a Cu(111) surface to analyze transverse electron transfer which is related to the electron transfer and the electrical contact in DNA-based devices. In particular, we demonstrated correlation mapping of the electron transfer parameters by visualizing specific combinations of the decay constant β and the contact conductance G. The results clearly revealed that electron transfer across the DNA strand varies spatially in the DNA molecule. This spatial variation is probably correlated with deformation of DNA on the surface. From quantitative analysis, the decay constant β was estimated to be 3.3 nm(-1), which is consistent with previously reported values for π-conjugated molecules.

Direct Submolecular Scale Imaging of Mesoscale Molecular Order in Supported Dipalmitoylphosphatidylcholine Bilayers

Supported dipalmitoylphosphatidylcholine (DPPC) bilayers are widely used membrane systems in biophysical and biochemical studies. Previously, short-range positional and orientational order of lipid headgroups of supported DPPC bilayers was observed at room temperature using low deflection noise frequency modulation atomic force microscopy (FM-AFM). While this ordering was supported by X-ray diffraction studies, it conflicted with diffusion coefficient measurements of gel-phase bilayers determined from fluorescence photobleaching experiments. In this work, we have directly imaged mica-supported DPPC bilayers with submolecular resolution over scan ranges up to 146 nm using low deflection noise FM-AFM. Both orientational and positional molecular ordering were observed in the mesoscale, indicative of crystalline order. We discuss these results in relation to previous biophysical studies and propose that the mica support induces mesoscopic crystalline order of the DPPC bilayer at room temperature. This study also demonstrates the recent advance in the scan range of submolecular scale AFM imaging.

Reduction of frequency noise and frequency shift by phase shifting elements in frequency modulation atomic force microscopy

We recently reported the analysis of the frequency noise in the frequency modulation atomic force microscopy (FM-AFM) both in high-Q and low-Q environments [Rev. Sci. Instrum. 80, 043708 (2009)]. We showed in the paper that the oscillator noise, the frequency fluctuation of the oscillator, becomes prominent in the modulation frequency lower than f(0)∕2Q, where f(0) and Q are the resonance frequency and Q-factor. The magnitude of the oscillator noise is determined by the slope of the phase versus frequency curve of the cantilever at f(0). However, in actual FM-AFM in liquids, the phase versus frequency curve may not be always ideal because of the existence of various phase shifting elements (PSEs). For example, the spurious resonance peaks caused by the acoustic excitation and a band-pass filter in the self-oscillation loop increase the slope of the phase versus frequency curve. Due to those PSEs, the effective Q-factor is often increased from the intrinsic Q-factor of the cantilever. In this article, the frequency noise in the FM-AFM system with the PSEs in the self-oscillation loop is analyzed to show that the oscillator noise is reduced by the increase of the effective Q-factor. It is also shown that the oscillation frequency deviates from the resonance frequency due to the increase of the effective Q-factor, thereby causing the reduction in the frequency shift signal with the same factor. Therefore the increase of the effective Q-factor does not affect the signal-to-noise ratio in the frequency shift measurement, but it does affect the quantitativeness of the measured force in the FM-AFM. Furthermore, the reduction of the frequency noise and frequency shift by the increase of the effective Q-factor were confirmed by the experiments.

Ordered nano-structure of a stamped self-organized protein layer on a HOPG surface using a HFB carrier

A groundbreaking method for ordered molecular layer preparation on a solid surface employing the drop-stamp method has been developed by us taking advantage of the characteristics of the HFB molecule as a self-organizer/adsorption carrier. It is a smart method which can be used to prepare a self-organized protein layer on a solid surface without unspecific adsorption or defects. In our previous report, we clarified the self-organizing nature of HFB-tagged protein molecules on a surface of a solution droplet. In this report, a protein layer was prepared on a HOPG surface by using the drop-stamp method with a maltose binding protein (MBP)-tagged HFBII molecule. The structure of the stamped protein layer was investigated using frequency modulation atomic force microscopy (FM-AFM) in a liquid condition. The FM-AFM images show that the drop-stamp method can prepare an ordered protein layer on a solid surface smartly. The drop-stamp method using a HFB carrier is a practical method which can be used to prepare an ordered protein layer on a solid substrate surface without unspecific adsorption defects.

Role of water in atomic resolution AFM in solutions

We use computer modelling to investigate the mechanism of atomic-scale corrugation in frequency modulation atomic force microscopy imaging of inorganic surfaces in solution. Molecular dynamics simulations demonstrate that the forces acting on a model microscope tip result from the direct interaction between a tip and a surface, and forces entirely due to the water structure around both tip and surface. The observed force is a balance between largely repulsive potential energy changes as the tip approaches and the entropic gain when water is sterically prevented from occupying sites near the tip and surface. Only extremely sharp tips are likely to measure direct tip-surface interactions. An investigation into the dynamics of water confined between tip and surface shows that water diffusion can be slowed by at least two orders of magnitude compared to its rate in bulk solution.

Highly Sensitive Electrostatic Force Detection Using Small Amplitude Frequency-Modulation Atomic Force Microscopy in the Second Flexural Mode

Highly Sensitive Electrostatic Force Detection Using Small Amplitude Frequency-Modulation Atomic Force Microscopy in the Second Flexural Mode

Nanoscale potential measurements in liquid by frequency modulation atomic force microscopy

We have developed a method for local potential measurements in liquid using frequency modulation atomic force microscopy. In this method, local potential is calculated from the first and second harmonic vibrations of a cantilever induced by applying an ac bias voltage between a tip and a sample. The use of an ac bias voltage with a relatively high frequency prevents uncontrolled electrochemical reactions and redistribution of ions and water. The nanoscale resolution of the method is demonstrated by imaging potential distribution of a dodecylamine thin film deposited on a graphite surface in 1 mM NaCl solution.

Aqueous Solution Structure over α-Al2O3(011̅2) Probed by Frequency-Modulation Atomic Force Microscopy

An α-Al2O3(011̅2) wafer was immersed in an aqueous KCl solution of 1 mol L−1 and observed with a frequency-modulation atomic force microscope. The tip−surface force was precisely determined as a function of the tip−surface distance. The force−distance relationship contained oscillations accompanied with an exponentially decayed, electric double layer force. The force oscillations were ascribed to liquid water layers confined over the Al2O3 surface.

Atomic Scale Analysis of Ultrathin SiO2 Films Prepared on TiO2(100) Surfaces

Rutile titanium dioxide (TiO2) (100) surfaces covered by silicon oxide (SiO2) ultrathin films were examined by X-ray photoelectron spectroscopy (XPS), low energy electron diffraction (LEED), and frequency modulation atomic force microscopy (FM-AFM) techniques. The SiO2 films were fabricated on TiO2 crystals by annealing in a quartz case which was used as a SiO2 source. The amount of Si increased with annealing time, and a (3 × 4) LEED pattern was observed on surfaces with the XPS peak intensity ratio of Si 2p to O 1s of TiO2 larger than 0.023. FM-AFM observation in pure water showed that the (3 × 4) surface consists of atomically flat terraces. Within the terraces, rows which extend in the [001] direction were observed. Every fourth row appeared brighter, consistent with the ×4 periodicity in the [010] direction observed in LEED. Models where rutile SiO2 units are accumulated on the TiO2(100) surface via rutile Ti1−xSixO2 units are consistent with the results.

Mechanism of Contrast Formation in Atomic Force Microscopy in Water

We use computer modeling to investigate the mechanism of atomic-scale corrugation in frequency-modulation atomic force microscopy imaging of inorganic surfaces in solution. Molecular dynamics simulations demonstrate that the forces acting on a microscope tip result from the direct interaction between a tip and a surface, and forces entirely due to the water structure around both tip and surface. The observed force depends on a tip structure and is a balance between largely repulsive potential energy changes as the tip approaches and the entropic gain when water is sterically prevented from occupying sites near the tip and surface.

Langasite as a piezoelectric material for near-field microscopy resonant cantilevers

Quartz length-extension resonators have already been used to obtain atomically-resolved images by frequency-modulation atomic force microscopy. Other piezoelectric materials such as gallium orthophosphate (GaPO(4)), langatate (LGT), and langasite (LGS) could be appropriate for this application. In this paper, the advantages of langasite crystal are presented and the fabrication of similar microsensors in langasite temperature-compensated cuts by chemical etching is proved. A monolithic length extension resonator, with a tip at its end, is obtained which constitutes a real advantage in regard to the existing quartz devices.

Simultaneous observation of surface topography and elasticity at atomic scale by multifrequency frequency modulation atomic force microscopy

The authors integrated the frequency modulation (FM) technique into multifrequency atomic force microscopy (AFM). Based on theoretical considerations, simultaneous excitation of the cantilever oscillation at the first and second flexural modes allows us to acquire the surface topography and surface elasticity simultaneously. The authors performed multifrequency FM-AFM observation using a tungsten-coated silicon cantilever on a Ge(001) surface exhibiting a dimer structure at room temperature. The topography and the elasticity of the surface were successfully obtained at the atomic scale. The authors found that the dimer atoms around a missing dimer defect have higher elasticity than the other dimer atoms. This suggests that stiffer atomic bonding of the dimer atoms occurred as a result of the additional tensile strain field from the defect. Therefore, the multifrequency FM-AFM described in the present study is expected to be useful for the investigation of the surface elasticity at the atomic scale.

A procedure to determine the optimum imaging parameters for atomic/molecular resolution frequency modulation atomic force microscopy

We propose a general procedure to determine the optimum imaging parameters (spring constant and oscillation amplitude) to obtain the optimum resolution in frequency modulation atomic force microscopy. We calculated the effective signal-to-noise ratio for various spring constants and oscillation amplitudes, based on the measurement of frequency shift and energy dissipation versus tip-sample distance curves, to find the optimum. We applied this procedure for imaging a lead phthalocyanine (PbPc) thin film on a MoS(2)(0001) substrate, and found that the optimum parameters were about 5 N/m and 20 nm, respectively. An improved signal-to-noise ratio was attained in a preliminary experiment using parameters which were close to the calculated optimum.

Atomic force microscopy imaging using a tip-on-chip: Opening the door to integrated near field nanotools

We describe in detail how atomic force microscopy (AFM) images can be routinely achieved with macroscopic silicon-based chips integrating mesoscopic tips, paving the way for the development of new near field devices combining AFM imaging with any kind of functionality integrated on a chip. The chips have been glued at the end of the free prong of 100 kHz quartz tuning forks mounted in Qplus configuration. Numerical simulations by modal analysis have been carried out to clarify the nature of the vibration modes observed in the experimental spectra. It is shown that two low frequency modes can be used to drive the system and scan the surface with a great stability in amplitude modulation as well as in frequency modulation AFM under ultrahigh vacuum. The AFM capabilities are demonstrated through a series of examples including phase and dissipation contrast imaging, force spectroscopy measurements, and investigations of soft samples in weak interaction with the substrate. The lateral resolution with the tips grown by focused ion beam deposition already matches the one achieved in standard amplitude modulation mode AFM experiments.