Simultaneous Topography Imaging Research Articles

Boxcar Averaging Scanning Nonlinear Dielectric Microscopy.

Scanning nonlinear dielectric microscopy (SNDM) is a near-field microwave-based scanning probe microscopy method with a wide variety of applications, especially in the fields of dielectrics and semiconductors. This microscopy method has often been combined with contact-mode atomic force microscopy (AFM) for simultaneous topography imaging and contact force regulation. The combination SNDM with intermittent contact AFM is also beneficial for imaging a sample prone to damage and using a sharp microscopy tip for improving spatial resolution. However, SNDM with intermittent contact AFM can suffer from a lower signal-to-noise (S/N) ratio than that with contact-mode AFM because of the shorter contact time for a given measurement time. In order to improve the S/N ratio, we apply boxcar averaging based signal acquisition suitable for SNDM with intermittent contact AFM. We develop a theory for the S/N ratio of SNDM and experimentally demonstrate the enhancement of the S/N ratio in SNDM combined with peak-force tapping (a trademark of Bruker) AFM. In addition, we apply the proposed method to the carrier concentration distribution imaging of atomically thin van der Waals semiconductors. The proposed method clearly visualizes an anomalous electron doping effect on few-layer Nb-doped MoS2. The proposed method is also applicable to other scanning near-field microwave microscopes combined with peak-force tapping AFM such as scanning microwave impedance microscopy. Our results indicate the possibility of simultaneous nanoscale topographic, electrical, and mechanical imaging even on delicate samples.

Simultaneous topography imaging and broadband nanomechanical mapping on atomic force microscope

In this paper, an approach is proposed to achieve simultaneous imaging and broadband nanomechanical mapping of soft materials in air by using an atomic force microscope. Simultaneous imaging and nanomechanical mapping are needed, for example, to correlate the morphological and mechanical evolutions of the sample during dynamic phenomena such as the cell endocytosis process. Current techniques for nanomechanical mapping, however, are only capable of capturing static elasticity of the material, or the material viscoelasticity in a narrow frequency band around the resonant frequency(ies) of the cantilever used, not competent for broadband nanomechanical mapping, nor acquiring topography image of the sample simultaneously. These limitations are addressed in this work by enabling the augmentation of an excitation force stimuli of rich frequency spectrum for nanomechanical mapping in the imaging process. Kalman-filtering technique is exploited to decouple and split the mixed signals for imaging and mapping, respectively. Then the sample indentation generated is quantified online via a system-inversion method, and the effects of the indentation generated and the topography tracking error on the topography quantification are taken into account. Moreover, a data-driven feedforward-feedback control is utilized to track the sample topography. The proposed approach is illustrated through experimental implementation on a polydimethylsiloxane sample with a pre-fabricated pattern.

Utilizing intentional internal resonance to achieve multi-harmonic atomic force microscopy

During dynamic atomic force microscopy (AFM), the deflection of a scanning cantilever generates multiple frequency terms due to the nonlinear nature of AFM tip–sample interactions. Even though each frequency term is reasonably expected to encode information about the sample, only the fundamental frequency term is typically decoded to provide topographic mapping of the measured surface. One of main reasons for discarding higher harmonic signals is their low signal-to-noise ratio. Here, we introduce a new design concept for multi-harmonic AFM, exploiting intentional nonlinear internal resonance for the enhancement of higher harmonics. The nonlinear internal resonance, triggered by the non-smooth tip–sample dynamic interactions, results in nonlinear energy transfers from the directly excited fundamental bending mode to the higher-frequency mode and, hence, enhancement of the higher harmonic of the measured response. It is verified through detailed theoretical and experimental study that this AFM design can robustly incorporate the required internal resonance and enable high-frequency AFM measurements. Measurements on an inhomogeneous polymer specimen demonstrate the efficacy of the proposed design, namely that the higher harmonic of the measured response is capable of enhanced simultaneous topography imaging and compositional mapping, exhibiting less crosstalk with an abrupt height change.

Fabrication and characterization of a K⁺-selective nanoelectrode and simultaneous imaging of topography and local K⁺ flux using scanning electrochemical microscopy.

A nanopipette containing a solution of bis(benzo-15-crown-5) dissolved in 1,6-dichlorohexane was used as an ion-selective electrode (ISE) to probe K(+) for shear force-based constant-distance scanning electrochemical microscopy (SECM). In a previous study, the ISE responded only at low K(+) concentrations ([K(+)] < 1 mM), due to the depletion of the bis(benzo-15-crown-5) at the oil/water interface at high K(+) concentrations and the unstable response of the tip at the oil/water interface for shear force and current detection. In the present study, a nanopipette reshaped by heating and with the hydrophobic layer removed was used as the ISE. This modified ISE enabled a rapid response to changes in K(+) flux at a physiological concentration of K(+) and allowed SECM imaging on a nanometer scale. The fabricated nano-ISE was used as a probe for shear force-based SECM. Topography and K(+) flux images were obtained simultaneously at a polycarbonate membrane filter with 5 μm pores and human embryonic kidney 293 cells (HEK293). Several areas containing a K(+) flux larger than the surrounding areas were found in the SECM images of the HEK293 cells, which indicated the existence of K(+) channels.

Atomic Force Microscopy Reveals the Structure and Dynamics of the Cell Cortex

Recent studies have shown that processes such as eukaryotic cell-cell interactions, differentiation and tissue development are controlled by mechanical signals. These mechanical stimuli come from the outside of the cells and induce remodeling of the cytoskeleton in the cell interior. Unfortunately, no dynamic data at high spatial resolution could be acquired so far on the cytoskeleton of live cells, and the mechanical heterogeneity at the subcellular level remains unknown. In particular, the cell cortex is a major determinant of the cell mechanics but its spatial arrangement is poorly understood, and the dynamic behavior of its elements could only be inferred through indirect methods. Here we demonstrate that simultaneous topography imaging and mechanical mapping of live cells under physiological conditions at high resolution and low forces is possible using atomic force microscopy. We applied our methods to perform direct imaging of the cell membrane actin cortex, reaching a resolution inferior to 100nm and a maximal 10s image acquisition rate. The cell cortex is structurally, mechanically and dynamically heterogeneous at the subcellular level, and its fastest rearrangement time was in the 10s range. Our resolution enabled direct sizing the sub-membrane actin meshwork, confirming estimates from electron microscopy and molecular diffusion studies. Furthermore, we can attribute dynamic parameters to actin meshworks of various architecture and estimate the forces that they can exert on neighboring cells.

A New AFM-Based Technique to Detect the NFT Protrusion on HAMR Head

Heat assisted magnetic recording (HAMR) is anticipated to increase the areal density in hard disk drives to multiple Tb/in2. However, the newly introduced protrusions caused by additional heat sources must be taken into account in head-disk spacing control. The light absorption by the near field transducer (NFT) and the heat dissipation into the slider through the laser delivery system can result in the temperature rise well over a hundred degrees Celsius. In response to such a big temperature change, an instantaneous protrusion at the NFT region is expected. This protrusion has a direct impact on the head-magnetic spacing (HMS) and therefore affects writing performance. Due to its small lateral size below 1 micrometer, the NFT protrusion is difficult to probe. In this paper, we introduce a new technique, which is based on atomic force microscopy (AFM) and allows for a simultaneous topography imaging and NFT protrusion mapping on the air bearing surface (ABS) with a nano-scale resolution. The measured NFT protrusion profile of a nonflying head indicates the local min-fly point at the location predicted by our simulation results. A further quantitative analysis proves the dependence of NFT protrusion on the laser power, which agrees very well with modeling and can offer a good guidance to the head-disk interface management during recording.

Simultaneous Noncontact Topography and Electrochemical Imaging by SECM/SICM Featuring Ion Current Feedback Regulation

We described a hybrid system of scanning electrochemical microscopy (SECM) and scanning ion conductance microscopy (SICM) with ion current feedback nanopositioning control for simultaneous imaging of noncontact topography and spatial distribution of electrochemical species. A nanopipette/nanoring electrode probe provided submicrometer resolution of the electrochemical measurement on surfaces with complex topology. The SECM/SICM probe had an aperture radius of 220 nm. The inner and outer radii of the SECM Au nanoring electrode were 330 and 550 nm, respectively. Characterization of the probe was performed with scanning electron microscopy (SEM), cyclic voltammetry (CV), and approach curve measurements. SECM/SICM was applied to simultaneous imaging of topography and electrochemical responses of enzymes (horse radish peroxidase (HRP) and glucose oxidase (GOD)) and single live cells (A6 cells, superior cervical ganglion (SCG) cells, and cardiac myocytes). The measurements revealed the distribution of activity of the enzyme spots on uneven surfaces with submicrometer resolution. SECM/SICM acquired high resolution topographic images of cells together with the map of electrochemical signals. This combined technique was also applied to the evaluation of the permeation property of electroactive species through cellular membranes.

Single-photon atomic force microscopy

In the last few years, an array of novel technologies, especially the big family of scanning probe microscopy, now often integrated with other powerful imaging tools such as laser confocal microscopy and total internal reflection fluorescence microscopy, have been widely applied in the investigation of biomolecular interactions and dynamics. But it is still a great challenge to directly monitor the dynamics of biomolecular interactions with high spatial and temporal resolution in living cells. An innovative method termed "single-photon atomic force microscopy" (SP-AFM), superior to existing techniques in tracing biomolecular interactions and dynamics in vivo, was proposed on the basis of the combination of atomic force microscopy with the technologies of carbon nanotubes and single-photon detection. As a unique tool, SP-AFM, capable of simultaneous topography imaging and molecular identification at the subnanometer level by synchronous acquisitions and analyses of the surface topography and fluorescent optical signals while scanning the sample, could play a very important role in exploring biomolecular interactions and dynamics in living cells or in a complicated biomolecular background.

Phase modulation atomic force microscopy in constant excitation mode capable of simultaneous imaging of topography and energy dissipation

The authors have developed phase modulation atomic force microscopy in constant excitation mode capable of simultaneously imaging the topography and energy dissipation of a sample surface in a liquid. This setup utilizes a fast, low-cost sample-and-hold technique to analyze the oscillation signals of a cantilever. The proposed circuitry allows us to measure the local energy dissipated by the tip-sample interaction during imaging. The energy dissipation image exhibits a material-specific contrast for a polymer-blend film.

Focussed ion beam machined cantilever aperture probes for near‐field optical imaging

Near-field optical probe is the key element of a near-field scanning optical microscopy (NSOM) system. The key innovation in the first two NSOM experiments (Pohl et al., 1984; Lewis et al., 1984) is the fabrications of a sub-wavelength optical aperture at the apex of a sharply pointed transparent probe tip with a thin metal coating. This paper discusses the routine use of focussed ion beam (FIB) to micro-machine NSOM aperture probes from the commercial silicon nitride cantilevered atomic force microscopy probes. Two FIB micro-machining approaches are used to form a nanoaperture of controllable size and shape at the apex of the tip. The FIB side slicing produces a silicon nitride aperture on the flat-end tips with controllable sizes varying from 120 nm to 30 nm. The FIB head-on drilling creates holes on the aluminium-coated tips with sizes down to 50 nm. Nanoapertures in C and bow tie shapes can also be patterned using the FIB head-on milling method to possibly enhance the optical transmission. A transmission-collection NSOM system is constructed from a commercial atomic force microscopy to characterize the optical resolution of FIB-micro-machined aperture tips. The optical resolution of 78 nm is demonstrated by an aperture probe fabricated by FIB head-on drilling. Simultaneous topography imaging can also be realized using the same probe. By mapping the optical near-field from a bow-tie aperture, optical resolution as small as 59 nm is achieved by an aperture probe fabricated by the FIB side slicing method. Overall, high resolution and reliable optical imaging of routinely FIB-micro-machined aperture probes are demonstrated.

Combined scanning electrochemical atomic force microscopy for tapping mode imaging

With the integration of submicro- and nanoelectrodes into atomic force microscopy (AFM) tips using microfabrication techniques, an elegant approach combining scanning electrochemical microscopy (SECM) with atomic force microscopy has recently been demonstrated. Simultaneous imaging of topography and electrochemistry at a sample surface in AFM tapping mode with integrated SECM–AFM cantilevers oscillated at or near their resonance frequency is shown. In contrast to contact mode AFM imaging frictional forces at the sample surface are minimized. Hence, topographical and electrochemical information of soft surfaces (e.g., biological species) can be obtained.

Tip–sample distance feedback control in a scanning evanescent microwave probe for nonlinear dielectric imaging

We implemented tip–sample distance control in a scanning evanescent microwave probe for nonlinear dielectric microscopy. With the analytic expression of the tip–sample capacitance as a function of tip–sample distance, we can quantitatively regulate the tip–sample separation and independently measure the dielectric nonlinearity by application of an ac bias voltage. Simultaneous imaging of topography and ferroelectric domains has been demonstrated on periodically poled LiNbO3 single crystals.

Near-field scanning optical microscopy imaging of individual threading dislocations on relaxed GexSi1−x films

We have identified individual threading dislocations on relaxed, compositionally graded GexSi1−x structures through simultaneous imaging of topography and photoresponse. The threading dislocations show shallow depressions in surface morphology and a 5%–10% reduction in photoresponse compared with the defect-free regions. The average spatial extent of the reduced photoresponse is (0.66±0.16) μm, which is larger than the associated morphology. This study further demonstrates the application of near-field scanning optical microscopy to characterization of electrically active defects. The spatial resolution is ten times higher than conventional optical techniques.

Kelvin probe force microscopy

Measurements of the contact potential difference between different materials have been performed for the first time using scanning force microscopy. The instrument has a high resolution for both the contact potential difference (better than 0.1 mV) and the lateral dimension (&amp;lt;50 nm) and allows the simultaneous imaging of topography and contact potential difference. Images of gold, platinum, and palladium surfaces, taken in air, show a large contrast in the contact potential difference and demonstrate the basic concept.