Non-contact Scanning Probe Microscopy Research Articles

Analysis and improvement of positioning reliability and accuracy of theta pipette configuration for scanning ion conductance microscopy

Scanning ion conductance microscopy (SICM) as an emerging non-contact scanning probe microscopy technique and featuring its strong in-situ detectability for soft and viscous samples, is increasingly used in biomedical and materials related studies. In SICM measurements, employing theta pipette as SICM probe to scan sample is an effective method to extend the applications of SICM for multi-parameter measurement. There are two crucial but still unclear issues that influence the reliability and accuracy of the usage of theta pipette in the SICM measurements, which are the safe feedback threshold and the horizontal measurement offset. In this work, aiming at the theta pipette configuration of SICM, we systematically investigated the two issues of the theta pipette by both finite element method (FEM) simulation and SICM experiments. The FEM analysis results show that the safe feedback threshold of the one side barrel of the theta pipette is above 99.5%, and the horizontal measurement offset is ~0.53 times of the inner radius of the probe tip. Based on this, we proposed an improved scanning method used by the theta pipette to solve the reliability and accuracy problems caused by the feedback threshold too close to the reference current (100%) and the measurement offset error at the tip radius level. Then through testing the polydimethylsiloxane (PDMS) samples with different embossed patterns with the improved method of SICM, we can conclude that the improved method can enhance the scanning reliability by adding the double barrels approaching process and increase the positioning accuracy by compensating an offset distance. The theoretical analysis and the improved scanning method in this work demonstrate more property and usage details of the theta pipette, and further improve the reliability and accuracy of the diversified multifunctional applications of the theta pipette for SICM to meet the increasingly complex and precise research needs.

A microchip microcontroller-based transducer controller for non-contact scanning probe microscopy with phase-locked loop, amplitude, and Q control.

An inexpensive yet versatile transducer controller for non-contact scanning probe microscopy (SPM) based on a PIC32 microcontroller from Microchip Technology, Inc is described. In addition to feedback control using the amplitude or phase of the signal from the non-contact transducer, the controller includes a phase-locked loop for frequency-shift feedback, as well as fixed-amplitude, quality factor (Q) control, and self-excitation modes. Apart from the input amplifiers, output buffers, and the Q-control circuit, all other functions of the controller are instantiated in software on the microchip, enabling rapid changes in operating parameters if needed. The controller communicates with a host personal computer via a simple serial connection. The controller has been tested with a quartz tuning-fork transducer but can be used with any oscillating non-contact transducer.

Noncontact tip-enhanced Raman spectroscopy for nanomaterials and biomedical applications.

Tip-enhanced Raman spectroscopy (TERS) has been established as one the most efficient analytical techniques for probing vibrational states with nanoscale resolution. While TERS may be a source of unique information about chemical structure and interactions, it has a limited use for materials with rough or sticky surfaces. Development of the TERS approach utilizing a non-contact scanning probe microscopy mode can significantly extend the number of applications. Here we demonstrate a proof of the concept and feasibility of a non-contact TERS approach and test it on various materials. Our experiments show that non-contact TERS can provide 10 nm spatial resolution and a Raman signal enhancement factor of 105, making it very promising for chemical imaging of materials with high aspect ratio surface patterns and biomaterials.

Large-scale and non-contact surface topography measurement using scanning ion conductance microscopy and sub-aperture stitching technique

In this paper, we propose a large-scale and non-contact surface topography measurement method using a non-contact scanning probe microscopy (SPM) technique, scanning ion conductance microscopy (SICM), combined with the sub-aperture stitching technique. The phase correlation techniques were first applied to the three-dimensional (3D) images measured by the SICM to acquire an initially coarse stitching position. Then the tip–tilt compensated sub-aperture stitching algorithm is utilized to eliminate tilts and translations among adjacent images and expand the lateral measuring range of the existing hopping mode SICM system. This SICM and the stitching based method has been used to measure some large-scale samples (micrometer to millimeter scale) in a non-contact, quantitative and high resolution way. Simulation and experimental results on these samples verify the feasibility of this method and the effectiveness of the stitching algorithm. A measuring range of 1.08 mm × 0.55 mm and a lateral resolution of 100 nm or even higher were obtained in these experiments. Compared with atomic force microscopy (AFM), the non-contact feature of the proposed method ensures less damage to the surface topography. The non-optical feature makes the data stitching simpler than the existing optical microscopic methods, which need consider how to compensate the vignetting effect caused by the inhomogeneity of light.

Junction Barrier Schottky (JBS) Rectifier Interface Engineering Facilitated by Two-Dimensional (2D) Dopant Imaging

The shielding cell architecture of a buried grid (BG) Junction Barrier Schottky (JBS) diode consisting of multiple consecutive p+-implanted stripes below the metal/semiconductor interface has been observed by performing non-contact Scanning Probe Microscopy (SPM) and Secondary Electron Potential Contrast (SEPC) measurements on the cross-section of the device. We have demonstrated that these techniques succeeded in mapping the two-dimensional carrier distribution inside the active area of the device, however with different resolution and quantification possibilities.

Resolving Atomic Connectivity in Graphene Nanostructure Junctions.

We report on the structural characterization of junctions between atomically well-defined graphene nanoribbons (GNRs) by means of low-temperature, noncontact scanning probe microscopy. We show that the combination of simultaneously acquired frequency shift and tunneling current maps with tight binding (TB) simulations allows a comprehensive characterization of the atomic connectivity in the GNR junctions. The proposed approach can be generally applied to the investigation of graphene nanomaterials and their interconnections and is thus expected to become an important tool in the development of graphene-based circuitry.

Two-Dimensional Carrier Profiling on Lightly Doped n-Type 4H-SiC Epitaxially Grown Layers

Electronically active dopant profiles of epitaxially grown n-type 4H-SiC calibration layer structures with concentrations ranging from 3.1015 cm-3 to 1·1019 cm-3 have been investigated by non-contact Scanning Probe Microscopy (SPM) methods. We have shown that Kelvin Probe Force Microscopy (KPFM) and Electrostatic Force Microscopy (EFM) are capable of resolving two-dimensional carrier maps in the low doping concentration regime with nanoscale spatial resolution. Furthermore, different information depths of this wide band gap semiconductor material could be assessed due to the inherent properties of each profiling method. We additionally observed a resolution enhancement under laser illumination which we explain by reduced band-bending conditions. To gauge our SPM signals, we utilized epitaxially grown layers which were calibrated, in terms of dopant concentration, by C-V measurements.

Wetting properties of glycerol on silicon, native SiO2, and bulk SiO2 by scanning polarization force microscopy

Scanning polarization force microscopy, a relatively new non-contact scanning probe microscopy technique, was applied in order to investigate the properties of liquid surfaces (droplets), such as: topography, microscopic contact angle θ, surface potential energy P(e), spreading coefficient S, and disjoining pressure П. Investigations were carried out on glycerol droplets deposited on surfaces of bare silicon, silicon covered with native oxide, and bulk silicon oxide. Contact angle values were determined from directly measured topography profiles of micro- and nanodroplets. Values of surface potential energy, spreading coefficient, and disjoining pressure were calculated based on a model of the dependence of contact angle on droplet height. The results of these experiments offer valuable insights into the mechanisms of wetting phenomena at the microscopic scale.

A hybrid analog-digital phase-locked loop for frequency mode non-contact scanning probe microscopy

Non-contact scanning probe microscopy (SPM) has developed into a powerful technique to image many different properties of samples. The conventional method involves monitoring the amplitude, phase, or frequency of a cantilever oscillating at or near its resonant frequency as it is scanned across the surface of a sample. For high Q factor cantilevers, monitoring the resonant frequency is the preferred method in order to obtain reasonable scan times. This can be done by using a phase-locked-loop (PLL). PLLs can be obtained as commercial integrated circuits, but these do not have the frequency resolution required for SPM. To increase the resolution, all-digital PLLs requiring sophisticated digital signal processors or field programmable gate arrays have also been implemented. We describe here a hybrid analog/digital PLL where most of the components are implemented using discrete analog integrated circuits, but the frequency resolution is provided by a direct digital synthesis chip controlled by a simple peripheral interface controller (PIC) microcontroller. The PLL has excellent frequency resolution and noise, and can be controlled and read by a computer via a universal serial bus connection.

Scanning polarization force microscopy investigation of contact angle and disjoining pressure of glycerol and sulfuric acid on highly oriented pyrolytic graphite and aluminum

For liquid droplets of sub-micrometer dimensions, the study of wetting properties (quantified by contact angle, disjoining pressure, spreading coefficient, etc.) is possible using the relatively new technique known as scanning polarization force microscopy (SPFM). This non-contact scanning probe microscopy technique was successfully implemented in our laboratory in order to study the wetting properties of glycerol and sulfuric acid on the surface of highly oriented pyrolytic graphite (HOPG) and glycerol on aluminum film deposited on mica. An AC polarization bias of 3 V at 3 kHz frequency was applied between a conductive atomic force microscope tip and the substrate. The resulting polarization force was measured with high accuracy, allowing non-contact topography profile measurements of liquid micro- and nanodroplets. The dependence of the contact angle on droplet height was determined in order to calculate the values of the spreading coefficient and the disjoining pressure between the liquid and substrates. The calculated potential energies give disjoining pressure values of ∼0.4 atm for glycerol on HOPG, ∼0.47 atm for glycerol on aluminum and ∼13 atm for H2SO4 on HOPG. In the case of H2SO4 on HOPG the strength of the force appears to be thirty times bigger than that for glycerol on HOPG and aluminum.

Local dielectric spectroscopy of near-surface glassy polymer dynamics

A noncontact scanning probe microscopy method was used to probe local near-surface dielectric susceptibility and dielectric relaxation in polyvinyl acetate near the glass transition. Dielectric spectra were measured from 10(-4) to 10(2) Hz as a function of temperature. The measurements probed a 20 nm thick layer below the free surface of a bulk film. A significant change in the fragility index and moderate narrowing of the distribution of relaxation times were found in the near-surface layer. In contrast to results for ultrathin films confined on or between metallic electrodes, no reduction in the dielectric strength was found, inconsistent with the immobilization of slower modes.

Local dielectric spectroscopy of polymer films

A noncontact scanning probe microscopy method of probing local frequency-dependent dielectric spectroscopy is described. Dielectric spectra can be measured with this technique from 0.01to100Hz, in nanometer-scale near-surface regions of materials. The technique is applied to polymer films (polyvinyl acetate), in order to determine if polymer glassy dynamics are altered near a free surface. A small reduction in glass transition temperature and a moderate narrowing of the distribution of relaxation times are found within 20nm of a free surface.

Dielectrophoretic Force Microscopy of Aqueous Interfaces

A novel scanning probe microscopy technique has allowed dielectrophoretic force imaging with nanoscale spatial resolution. Dielectrophoresis (DEP) traditionally describes the mobility of polarizable particles in inhomogeneous alternating current (ac) electric fields. Integrating DEP with atomic force microscopy allows for noncontact imaging with the image contrast related to the local electric polarizability. By tuning the ac frequency, dielectric spectroscopy can be performed at solid/liquid interfaces with high spatial resolution. In studies of cells, the frequency-dependent dielectrophoretic force is sensitive to biologically relevant electrical properties, including local membrane capacitance and ion mobility. Consequently, dielectrophoretic force microscopy is well suited for in vitro noncontact scanning probe microscopy of biological systems.

비접촉 원자간력 현미경의 탐침 캔틸레버 진동 특성 및 측정 성능 평가

This paper presents the vibration analysis and the performance evaluation of cantilevers with probing tips for non-contact scanning probe microscopy. One of the current issues of the scanning probe microscopy technology is to increase the measurement speed, which is closely tied with the dynamic characteristics of cantilevers. The primary concern in this research is to investigate the relation between the maximum possible speed of non-contact scanning probe microscopy and the dynamic characteristics of cantilevers. First, the finite element analysis is made for the vibration characteristics of various cantilevers in use. The computed natural frequencies of the cantilevers are in good agreement with measured ones. Then, each cantilever is tested with topographic measurement for a standard sample with the scanning speed changed. The performances of cantilevers are analyzed along with the natural frequencies of cantilevers. Experiments are also performed to test the effects of how to attach cantilevers in the piezo-electric actuator. Finally, measurement sensitivity has been analyzed to enhance the performance of scanning probe microscopy.

Electronic Properties of Modified Surfaces Using Contact and Non-Contact Scanning Probe Microscopy Techniques and SECM.

ABSTRACTElectrochemical experiments with tetracyanoquinodimethane (TCNQ) modified electrodes in contact with aqueous copper containing electrolytes leads to the incorporation and expulsion of copper ions. This process occurs concomitantly with nucleation and growth processes and significant crystal fragmentation to produce particles with dimensions of the order of 10's of nanometres. During reduction of TCNQ and intercalation of copper ions, different phases of the semiconducting compound CuTCNQ are formed.[1,2] The preparation of both conducting and insulating substrates coated with electroactive TCNQ and CuTCNQ particles of variable size have been made by dip and spin coating procedures. Results suggest that the phase and hence electronic properties of CuTCNQ is dependent on the size of particles that decorate the electrode surface. Combining atomic force microscope (AFM) based methods that interrogate the morphological and electronic properties of nanometre sized particles with use of a scanning electrochemical microscope (SECM) is a new advance in materials characterisation that has proved highly valuable in understanding the highly complex behaviour of these semi-conducting particles.

Acoustic measurements for nanoscale test instruments

Acoustic methods have been used successfully for microscale contact nondestructive evaluation, but until very recently, were not available for the nanometer scale. Noncontact submicrometer resolution ultrasonic microscopy and scanning probe microscopy (SPM) operate at higher acoustic modes and have great potential to be qualitative but face data interpretation difficulties. Nanomechanical test instruments offer nanometer scale quantitative characterization. These tools coupled with in situ SPM-type imaging and simultaneous acoustic response monitoring open new instrumentation horizons for micro/nano fracture mechanics. Both active and passive acoustic methods can be utilized to characterize a large variety of substrates and coatings. Examples of AE monitoring of nanoindentation/scratch on data storage media will be presented. Plastic deformation induced events can be separated from contact friction events by identifying AE signatures. Active ultrasonic methods can be utilized for nanoscale characterization of tribological surfaces. A friction coefficient reduction of 20% was observed on an ultrasonically excited surface during nanoscratch testing and was further investigated using post-scratch SPM-type imaging. A synergy of localized ultrasonic monitoring and nanoindentation technique can lead to the development of new and promising instrumentation for characterizing in vivo/vitro biological tissues at the molecular level. Examples on evaluating ultrasonically transmitted signal through biological samples will be discussed.

Backside probing of flip–chip circuits using electrostatic force sampling

A non-contact scanning probe microscopy technique for measuring high-frequency voltage waveforms from the backside of a flip-chip mounted integrated circuit is presented. The signals on interconnects on the frontside of the die are accessed by mechanical thinning and focused ion beam milling through the backside of the die. A scanning force microscopy micromachined probe is placed inside the focused ion beam hole so that it is in close proximity to the circuit measurement point. Internal circuit voltage waveforms are measured by using the scanning probe in a non-contact mode and sensing the local electrostatic force on the tip of the probe. The instrument currently has a 3 GHz bandwidth and a capacitive loading on the test point of less than 1 fF. The output waveforms from ring oscillator flip-chip test circuits are measured.

Nanometer Scale Dielectric Fluctuations at the Glass Transition

Using non-contact scanning probe microscopy (SPM) techniques, dielectric properties were studied on 50 nanometer length scales in poly-vinyl-acetate (PVAc) films in the vicinity of the glass transition. Low frequency (1/f) noise observed in the measurements, was shown to arise from thermal fluctuations of the electric polarization. Anomalous variations observed in the noise spectrum provide direct evidence for cooperative nano-regions with heterogeneous kinetics. The cooperative length scale was determined. Heterogeneity was long-lived only well below the glass transition for faster than average processes.

Atomic force measurement of low-frequency dielectric noise

Using noncontact scanning probe microscopy techniques, dielectric properties were studied on 50-nm-length scales in poly-vinyl-acetate (PVAc) and poly-methyl-methacrylate films. Low-frequency (1/f ) fluctuations observed in the measurements, peaked in intensity near the glass transition temperature in PVAc. The noise is shown to arise from thermal dielectric polarization fluctuations. Analysis of this noise provides a noninvasive method of probing equilibrium nanometer-scale dynamical processes in dielectric materials and devices.

Non-contact scanning probe microscopy with sub-piconewton force sensitivity

Aiming at detecting and visualizing weak interaction forces between biological macromolecules in non-contact mode, we have refined scanning probe microscopy to have approximately 100-fold higher sensitivity than conventional atomic force microscopy (AFM). The essential features of this system are that the force sensitivity is in the sub-piconewton range and the gap distance between interacting stylus and the sample surfaces can be controlled with nanometer accuracy. We have achieved these technical improvements by using flexible handmade cantilevers with the spring constant of approximately 0.1 pN nm −1. Thermal bending motions have been reduced to less than 1 nm in root-mean-square amplitude by exerting feedback forces with a laser radiation pressure. The performance of this newly developed scanning probe microscopy was tested by measuring electrostatic repulsive forces in solution. Intermolecular forces in the sub-piconewton range were resolved at controlled gaps in the nanometer range between an amino-silanized stylus and an amino-silanized glass surface. Force curves as a function of gap distance coincided well with the theory of electricity. Debye lengths calculated at various ionic strengths were in good agreement with the Debye-Hückel theory. Furthermore, non-contact two-dimensional images mapped with electrostatic forces within the piconewton range could also be obtained. This system should prove useful not only for constructing the surface topographies of soft biomaterials but also for mapping the surfaces with intermolecular forces which are closely related to their functions.