Probe-sample Distance Research Articles

High-Speed Near-Surface Tracking for Fast Atomic Force Microscope Scan Switching Based on the Squeeze Film Damping Effect

Atomic force microscope (AFM) is a crucial metrology tool in the semiconductor industry. However, the bottleneck of the AFM technique is its low efficiency, primarily owing to the time-consuming scanning point switching. The traditional method takes approximately one minute for a single switching. To shield the probe and sample, the switching should first lift the probe a considerable distance, move it to the next point, and finally engage the probe slowly to the sample surface. We propose a near-surface tracking method for use in fast scanning point switching. The probe-sample distance was sensed by the squeeze air film damping effect, and the probe was kept at a small constant height during the switching via feedback control. The method can decrease the lift and engage distance from approximately 1 mm to 5 μm, which was two order of reduction; thereby shortening the single switching time from 45 s to 6.1 s, which is an 87% reduction. Generally, the fast switching method can improve the measurement efficiency of the existing AFM working in the air, but it can not be used in liquid and vacuum. Furthermore, the proposed squeeze film damping effect based distance sensing method can be applied in various fields, such as vibration sensing and temperature drift measurement.

Acoustic monitoring humidity effects on the formation of water bridges at sharp contacts

Shear-force Acoustic Near-field Microscopy (SANM) and Whispering Gallery Acoustic Sensing (WGAS) are used to evaluate the role of humidity on the formation and rupture of water meniscus bridges that form between a laterally oscillating tapered probe and a flat substrate. SANM monitors the near-field acoustic wave emitted by the meniscus, while WGAS monitors acoustically the probe’s amplitude of oscillation, as a function of the probe-sample distance. In the instances reported here, the SANM/WGAS measurements are performed in frequency modulation modality, which allows to discriminate the elastic and inelastic components of the probe-fluid-substrate interaction. The response signal traces, acquired when the probe first approaches and then retracts from the sample at 56% relative humidity (or below), revealed absence of hysteresis; this result persists when performing the same experiment a few days later. However, as the humidity increases (different trials from 56% to 67% are included) a notorious hysteresis is observed between the approach and retraction traces. The results indicate that, as the probe approaches the substrate, a higher humidity promotes an increase of stochastic instabilities in the probe-fluid-substrate interactions (reflected in higher damping effects on the probe, and instabilities in the probe-sample distance feedback control). However, the elastic response of the water meniscus prevails over damping effect, as revealed by the greater acoustic emission despite a decrease in the probe’s amplitude. These measurements are relevant to investigations on the dynamic behavior of fluids near hydrophobic/philic substrates, and wetting properties of solids and nanotribology phenomena in general.

Quantifying Force and Energy in Single-Molecule Metalation.

An atomic force microscope is used to determine the attractive interaction at the verge of adding a Ag atom from the probe to a single free-base phthalocyanine molecule adsorbed on Ag(111). The experimentally extracted energy for the spontaneous atom transfer can be compared to the energy profile determined by density functional theory using the nudged-elastic-band method at a defined probe-sample distance.

In situ acoustic characterization of a locally reacting porous material by means of PU measurement and model fitting

Reliable data on acoustical properties of materials are crucial for the design of a desired acoustic environment as well as to obtain accurate results from acoustic simulations. Although the acoustical properties of materials can be obtained via laboratory measurements, situations where in situ measurements are needed are often encountered. However, in situ measurement methods presented so far are limited by their poor portability or inaccuracies in the low-frequency range. In this work, we propose a characterization method that combines an in situ pressure-velocity (PU) measurement with a model fitting procedure using the Delany-Bazley-Miki impedance model for porous materials. The method uses an optimization routine to find the best match of measured and modelled reflection coefficient values within a given frequency range for the optimization parameters: flow resistivity, panel thickness, and probe-sample distance. The optimal parameter values allow, in turn, calculating the porous panel’s reflection coefficients for a broad frequency range including frequencies below the lower bounds of the optimization frequency range. The sensitivity of the method to panel width, lower bound of fitting frequency range, and to excluding parasitic reflections by time windowing is studied. The study shows that the proposed method provides characterization results in good agreement with reference data for panels of dimensions larger than 1800 mm and that the method is robust for reduction of one dimension of the panel down to 300 mm. It also shows that the model fitting accuracy is best when the frequency range of analysis is restricted to 1000–5000 Hz.

Single-spin qubit magnetic spectroscopy of two-dimensional superconductivity

A single-spin qubit placed near the surface of a conductor acquires an additional contribution to its $1/T_1$ relaxation rate due to magnetic noise created by electric current fluctuations in the material. We analyze this technique as a wireless probe of superconductivity in atomically thin two dimensional materials. At temperatures $T \lesssim T_c$, the dominant contribution to the qubit relaxation rate is due to transverse electric current fluctuations arising from quasiparticle excitations. We demonstrate that this method enables detection of metal-to-superconductor transitions, as well as investigation of the symmetry of the superconducting gap function, through the noise scaling with temperature. We show that scaling of the noise with sample-probe distance provides a window into the non-local quasi-static conductivity of superconductors, both clean and disordered. At low temperatures the quasiparticle fluctuations get suppressed, yet the noise can be substantial due to resonant contributions from collective longitudinal modes, such as plasmons in monolayers and Josephson plasmons in bilayers. Potential experimental implications are discussed.

Characterizing two-dimensional superconductivity via nanoscale noise magnetometry with single-spin qubits

We propose nanoscale magnetometry via isolated single-spin qubits as a probe of superconductivity in two-dimensional materials. We characterize the magnetic field noise at the qubit location, arising from current and spin fluctuations in the sample and leading to measurable polarization decay of the qubit. We show that the noise due to transverse current fluctuations studied as a function of temperature and sample-probe distance can be used to extract useful information about the transition to a superconducting phase and the pairing symmetry of the superconductor. Surprisingly, at low temperatures, the dominant contribution to the magnetic noise arises from longitudinal current fluctuations and can be used to probe collective modes such as monolayer plasmons and bilayer Josephson plasmons. We also characterize the noise due to spin fluctuations, which allows probing the spin structure of the pairing wave function. Our results provide a non-invasive route to probe the rich physics of two-dimensional superconductors.

Passive near-field optical scanning imaging based on semiconductor nanowire/tapered microfiber probe

<sec>In this paper, we propose a passive near-field scanning imaging system by using the structure of cadmium sulfide (CdS) nanowire/tapered microfiber probe, which combines the near-field scanning structure and the nanowire/microfiber coupling technology. In the passive near-field scanning imaging system, a passive nanoprobe is adopted to detect the intensity change of the reflected light field on the sample surface, which not only retains the advantage of the nanoprobe for the strong restriction of the reflected light on the sample surface, but also reduces the interference of strong excitation light during detection. Through the high efficiently evanescent field coupling between the CdS nanowire and the tapered microfiber, the collected light signal is transmitted to the photodetector in the far field, and finally the imaging of the target sample morphology can be realized.</sec><sec>At first, the light field model of the nanowire/tapered microfiber probe structure is verified by the finite element analysis method. The calculated collection efficiency from the sample to the probe is about 4.65‰ and the transmission efficiency from the nanowire to the tapered microfiber is about 74.47%. The collection efficiency is improved by an order of magnitude compared with traditional metal-coated near-field probe. In the experiments, a scanning step of 20 nm and a probe-sample distance of 230 nm are selected. The nanowire/tapered microfiber probe and traditional tapered fiber probe are both used to measure the widths of different CdSe nanoribbons samples, and the atomic force microscopy measurement is used as the benchmark to calculate their measurement error, which is increased about 3 times. By changing the angle <i>θ</i> between the probe and the sample, it is found that the resolution obtained using the designed nanowire/microfiber probe is always higher than only using the tapered microfiber probe. Comparing with the tapered microfiber probe scheme, the measurement error is reduced to a value less than 7.2%.</sec><sec>In addition, compared with the active luminescence probe scheme, this passive near-field scanning scheme reduces the preparation complexity of the optical probe and the detection structure complexity of the optical system. The large microscopic illumination area can avoid the influence of the small laser spot size on imaging, and the imaging range is determined only by the travel distance of the linear stage. Therefore, our work may provide an attractive approach for developing new near-field scanning microscopy systems in the future.</sec>

Research on the Adaptive Sensitivity Scanning Method for Ion Conductance Microscopy with High Efficiency and Reliability.

Scanning ion conductance microscopy (SICM) is a type of in situ measurement technology for noncontact detection of samples in electrolytes with nanoscale resolution and has been used increasingly in biomedical and electrochemical fields in recent years. However, there is an inherent contradiction in the technique that makes SICM's sensitivity and accuracy difficult to balance. Higher sensitivity allows for faster probe speeds and higher scanning reliability but leads to lower accuracy, and vice versa. To resolve this problem, an adaptive sensitivity scanning method is proposed here that is designed to increase SICM's imaging efficiency without reducing its scanning reliability and accuracy. In the proposed scanning method, the sensitivity is automatically switched via the bias voltage based on the probe-sample distance. When the probe is located far away from the sample, the probe then predetects the sample position rapidly with high sensitivity. When the sample has been sensed in the high-sensitivity phase, the probe then detects the sample with low sensitivity. The basic theory and the feasibility of the alterable sensitivity detection strategy is also studied using the finite element method (FEM) and by performing experiments in this work. Finally, through testing of the standard silicon and polydimethylsiloxane (PDMS) samples, the proposed method is shown to increase SICM imaging efficiency significantly by up to 5 times relative to the conventional hopping mode without sacrificing the scanning accuracy and reliability.

Development of a low-cost modular VIS/NIR spectroscopy for predicting soluble solid content of banana

Soluble solids content (SSC) is one of the most important parameters of banana associated with taste and consumer acceptance. NIR spectroscopy has been applied for nondestructive determination of SSC, but limited studies were conducted for a low-cost and modular VIS/NIR spectroscopy. This study was conducted to develop a calibration model to predict SSC in bananas using a modular type of VIS/NIR spectroscopy in the range of 350-1000 nm by varying distances of fiber optic probe to samples. Two varieties of bananas, namely Musa acuminata × balbisiana and Musa acuminata ‘Lady Finger’ were used. Partial least square regression (PLSR) was used to build a calibration model and to predict SSC of bananas. Normalization, baseline correction, standard normal variate (SNV), and multiple scattered (MSC) correlation were used for spectra preprocessing. The research showed that using 2 cm probe-sample distance and SNV method resulted in the best model with the coefficient correlation of calibration and prediction of 0.95 and 0.87, respectively. This study proved that probe-sample distances affected the efficiency of the model for VIS/NIR spectroscopy. This work concluded that the low-cost modular VIS/NIR spectroscopy is a promising tool for SSC measurement.

A High Accuracy Ion Conductance Imaging Method Based on the Approach Curve Spectrum

Scanning ion conductance microscopy (SICM), as an emerging non-contact in situ topography measurement tool with nano resolution, has been increasingly used in recent years in biomedicine, electrochemistry and materials science. In the conventional measurement method of SICM, the sample topography is constructed according to the position of the probe at the feedback threshold of the ion current. Nevertheless, for different structures, a constant threshold cannot maintain a constant probe-sample distance. This phenomenon makes the measurement topography inconsistent with the real sample surface. In order to solve this problem and improve the measurement accuracy of SICM, a new ion conductance imaging method based on the approach curve spectrum is proposed in this work. In the new method, the local feature around the measurement point is firstly evaluated according to the change rate of ion current. Secondly, based on the local feature, the corresponding approach curve is searched from the prior approach curve spectrum to accurately evaluate the distance between the probe and the sample. Finally, the sample topography is constructed by the probe position subtracting the probe-sample distance. In this paper, we verify the feasibility of the new imaging method by combining finite element theory and experiments. To examine the measurement accuracy, the standard strip silicon and cylindrical polydimethylsiloxane (PDMS) samples are tested, and the improved imaging method can obtain the topography closer to the real samples and reduce the volumetric measurement error by 5.4%. The implementation of the new imaging method will further promote SICM application in related research fields.

Quantum plasmonic control of trions in a picocavity with monolayer WS2.

Monitoring and controlling the neutral and charged excitons (trions) in two-dimensional (2D) materials are essential for the development of high-performance devices. However, nanoscale control is challenging because of diffraction-limited spatial resolution of conventional far-field techniques. Here, we extend the classical tip-enhanced photoluminescence based on tip-substrate nanocavity to quantum regime and demonstrate controlled nano-optical imaging, namely, tip-enhanced quantum plasmonics. In addition to improving the spatial resolution, we use the scanning probe to control the optoelectronic response of monolayer WS2 by varying the neutral/charged exciton ratio via charge tunneling in Au-Ag picocavity. We observe trion "hot spots" generated by varying the picometer-scale probe-sample distance and show the effects of weak and strong coupling, which depend on the spatial location. Our experimental results are in agreement with simulations and open an unprecedented view of a new range of quantum plasmonic phenomena with 2D materials that will help to design new quantum optoelectronic devices.

Carrier distribution imaging using ∂C/∂z-mode scanning nonlinear dielectric microscopy.

Scanning nonlinear dielectric microscopy (SNDM) can be used to visualize the carrier distribution in semiconductors with high sensitivity and spatial resolution. We recently proposed a complementary method named ∂C/∂z-SNDM that avoids the problem of contrast reversal. This paper describes a methodology for calculating the signal intensity in ∂C/∂z-SNDM using examples. For the simulation, the capacitance of a conductive-probe metal/oxide/semiconductor model was calculated and then the response signal for various probe-sample distances was analyzed. The simulation results confirm that the ∂C/∂z-SNDM signal intensity increases monotonically with dopant concentration, avoiding contrast reversal. Moreover, in addition to the fundamental (1ω) signal, higher-harmonic (2ω, 3ω) signals have sufficient intensities to be detected. The results suggest that the detection sensitivity for low dopant concentrations can be improved by conducting the measurement under an appropriate dc bias.

Measurement of propagation of ultrafast surface plasmon polariton pulses using dual-probe scanning near-field optical microscopy.

We report the construction of diagnostics for ultrafast surface plasmon polariton (SPP) pulses that evolve spatiotemporally in femtosecond and nanometer scales. We constructed two types of scanning near-field optical microscopes (SNOMs) and verified that the temporal waveform of ultrafast SPP pulses can be measured by combining spectral interferometry (SI). In the illumination-collection (I-C) mode SNOM, which uses a single fiber probe to excite samples and collect optical responses, a lock-in detection scheme using a lock-in camera detects SI fringes even for extremely weak signal light pulses. With this I-C SI-SNOM scheme, we measured the complex plasmon response functions of gold (Au) nanorods on Ge2Sb2Te5 thin film, both in the crystal and amorphous phases. For a dual-probe SNOM (DSNOM), a dual-band modulation technique was introduced to independently control the probe-sample and probe-probe distances. With the DSNOM and by employing femtosecond SPP pulse excitation, we successfully measured the temporal waveform of an ultrafast SPP pulse that is propagating on an Au thin layer.

Nonlinear Reconstruction of Multilayer Media in Scanning Microwave Microscopy

Scanning microwave microscopy (SMM) is a nanoscale characterization technique widely used to image electrical or magnetic properties of materials at microwave frequencies. Nevertheless, it is challenging to reconstruct the properties of materials from measured SMM signals, and it is even difficult to retrieve multilayer media directly from measured signals. This paper proposes a nonlinear inversion scheme to quantitatively reconstruct multilayer media. In the forward solver, hybrid boundary integral-finite element method is used to reduce the computational cost. In the inverse problems, capacitance or conductance derivative signals at different probe–sample distances are used to retrieve multilayer media. Reconstructed results show that the proposed approach is able to reconstruct both permittivity and conductivity distributions under noisy environment in 3-D samples. Most importantly, it is found that the resolution has been significantly improved in the retrieved images compared with capacitance or conductance signals. To the best of our knowledge, it is the first time that the multilayer media in 3-D media is reconstructed in SMM.

Finite element method analysis of whispering gallery acoustic sensing

Whispering Gallery Acoustic Sensing (WGAS) has recently been introduced as a sensing feedback mechanism to control the probe-sample separation distance in scanning probe microscopy that uses a quartz tuning fork as a sensor (QTF-SPM) [1]. WGAS exploits the SPM supporting frame as a resonant acoustic cavity to monitor the nanometer-sized amplitude of the QTF oscillations. Optimal WGAS sensitivity depends on attaining an exact match between the cavity’s frequency peak response and the TF resonance frequency. However, two aspects play against this objective: i) the unpredictable variability of the TF resonance frequency (upon attaching a SPM-probe to one of its tines), and ii) cavities of arbitrary geometry tend to display complicated (multiple peaks) frequency response, making difficult to identify which cavity dimension control which peak. Practical matching frequency procedures are needed then to operate the Shear-force Acoustic Near-field Microscopy (SANM) more efficiently. As a first step, here we undertake finite-element method (FEM) analysis to find out cavities of simple frequency response and, ideally, easy frequency tuning ability. Based on previous results we focus our studies in analyzing the frequency response of conical cavities within a range around the 32 kHz operating frequency. To first validate our numerical simulation studies, we reproduce the experimental results obtained from a specific conical cavity. Then we proceed to simulate the response of cavities of slightly different geometries, and investigate the dependence on the young modulus, poison ratio, and slight changes in dimensions. This initial success encourages to undertake studies of cavities having more sophisticated geometries.

Instrumentation-level improvements in shear-force near-field acoustic microscopy

The recently introduced Shear-force Near-field Acoustic Microscopy (SANM) brings a new sensing mechanism to the scanning probe microscopy family. SANM’s ability to simultaneously monitor, in real time, several physical sample’s responses presents some challenges for ensuring optimal operation; namely, avoid “cross-talk” among the multiple signals, address the compensation of thermal drift to ensure reproducibility of the measurements, and measuring the typical low-level signals obtained from nanometer-sized tested regions. Here, several improvements relevant to SANM, but valid for SPM in general, are addressed. i) The probe’s coarse approach is performed via stepper motors, which are controlled either by a computer software interface, or simply by a user-friendly RF remote control. ii) The inherent mechanical drift of the stage is evaluated first (by monitoring the feedback voltage that acts on the sample to maintain the probe-sample distance constant), and then automatically compensated (via linear interpolation) in the immediate subsequent probe’s approach/retraction measurements. iii) To determine the absolute position of the substrate relative to the probe, the probe-substrate contact current is measured with circuitry properly shielded from eventual electrical ground loops. Buffer amplifiers are used to drive the quartz tuning fork (that holds the probe) and to measure the AC and DC tunneling currents between the tip and sample. iv) To improve the signal to noise ratio, a Kalman filter is implemented into the SANM’s field-programmable gate array board, which processes the signals in real time. Finally, v) to simplify the operation of the microscope, an intuitive LabVIEW host program is developed to control the whole system and offer the user a visualization of the data in real time.

Progress and development in multiple-probe scanning probe microscopes

Scanning probe microscopes (SPM) is one of the important tools for micro/nano-scale morphological characterization, physical property measurement and micro/nano operation. However, the traditional SPM has only a single probe and its function is limited. The emergence of multiple-probe scanning probe microscopes (MP-SPM) greatly expands the application of SPM. Characterizing the electrical transport properties of materials is very important for materials and microelectronics, because it can effectively guide synthetization of high-quality materials and fabrication of high-performance devices. Traditionally, directly measuring the conductivity of nanomaterials is performed using the nanofabricated electrodes connected to the nanomaterial of interest and the monolithic micro-four-point probes. However, photoresist and organic solvents may be left on the surface of the nanomaterial during nanofabrication, which will affect the intrinsic measurement of electrical transport properties. As for the monolithic micro-four-point probes, probe spacing is quite large due to the limitation of the manufacturing process and it will not change any more once they have been manufactured, which limits its flexibility. MP-SPMs multiple probes can act as measuring electrodes with precise localization, and the probe-sample distance can be controlled precisely, providing a means of flexible and non-destructive detection of micro/nanoscale electrical transport properties of samples. Micro/nano-manipulation systems help to manufacture microelectronic and photonic devices, renewable energy apparatus and biomedical diagnosis chip, etc. Typical micro/nano-manipulation systems are established based on optical tweezer, magnetic tweezer, scanning electron microscopy (SEM) and SPM. However, they all have limitations. Optical tweezer may damage the sample due to the continuously irradiated lasers. Magnetic tweezer has a poor ability to control the object quantificationally, and low resolution. The system based on SEM is restricted to a vacuum working environment. On the surface of the sample, traditional single-probe SPM can perform nano-manipulation, such as push and pull, but it is limited to two-dimensional space, and three-dimensional manipulation might be a great challenge. In addition, due to the lack of real-time visual feedback during the manipulation process, it always works at a time-consuming mode, such as scanning-manipulating-scanning. Although some researchers have designed augmented reality, local scanning and other methods to improve the efficiency of manipulation, the efficiency of manipulation is still low due to the serial procedure of single probe SPM. MP-SPM have been developed to provide multiple probes which can be controlled independently, and achieve complex nano-manipulation by cooperation among the probes. Furthermore, MP-SPM can image with a certain probe; and the imaging results, serving as priori/feedback information, can guide other probes manipulation, thereby it improves the efficiency and accuracy of micro/nano-manipulation. In this paper, the basic structure of MP-SPM, including the structure of multiple-probe scanning tunneling microscopy and multiple-probe atomic force microscopy, is presented. Furthermore, we also introduce the methods to decrease the distance between the probes and calibrate the multiple probes position and the principle of measuring the resistivity of materials using multiple-probe technology. Then we summarize the recent application advances of the measurement of electrical transport properties, the micro/nano-manipulation, the parallel imaging/ manipulation, and the mechanical properties at the micro/nano scale etc. Finally, we discuss the frontier development, the opportunities as well as the challenges of MP-SPM.

Influence of the Magnetic Moment of the Probe of a Magnetic Resonance Force Microscope on the Spin-Wave Resonance Spectra

Abstract—The ferromagnetic resonance in an array of permalloy microstrips 3000 × 500 × 30 nm3 in size, which are ordered on a rectangular 3.5 × 6 μm2 lattice, is studied using magnetic resonance force microscopy with strong probe–sample interaction. This interaction induces intricate modifications of the observed spectra, which are manifested both in line splitting and in changes in shape. The dependences of the observed spectra on the sample–probe distance and the orientation of the magnetic moment of the probe are analyzed.

Halbach Effect at the Nanoscale from Chiral Spin Textures

Mallinson's idea that some spin textures in planar magnetic structures could produce an enhancement of the magnetic flux on one side of the plane at the expense of the other gave rise to permanent magnet configurations known as Halbach magnet arrays. Applications range from wiggler magnets in particle accelerators and free electron lasers to motors and magnetic levitation trains, but exploiting Halbach arrays in micro- or nanoscale spintronics devices requires solving the problem of fabrication and field metrology below a 100 μm size. In this work, we show that a Halbach configuration of moments can be obtained over areas as small as 1 μm × 1 μm in sputtered thin films with Néel-type domain walls of unique domain wall chirality, and we measure their stray field at a controlled probe-sample distance of 12.0 ± 0.5 nm. Because here chirality is determined by the interfacial Dyzaloshinkii-Moriya interaction, the field attenuation and amplification is an intrinsic property of this film, allowing for flexibility of design based on an appropriate definition of magnetic domains. Skyrmions (<100 nm wide) illustrate the smallest kind of such structures, for which our measurement of stray magnetic fields and mapping of the spin structure shows they funnel the field toward one specific side of the film given by the sign of the Dyzaloshinkii-Moriya interaction parameter D.

Magnetic Resonance Force Microscopy of a Permalloy Microstrip Array

The ferromagnetic resonance in an array of permalloy microstrips 3000 × 500 × 30 nm in size ordered on a rectangular grid 3.5 × 6 μm in size has been investigated by magnetic resonance force microscopy. The dependences of magnetic resonance force microscopy spectra of a sample on the probe–sample distance are analyzed. The possibility of detection of a ferromagnetic resonance spectrum of a single microstrip is demonstrated.