The multi-frequency method, recently introduced in atomic force microscopy (AFM), has shown remarkable enhancement of sensitivity and resolution of microscopy with a variety of heterogeneous materials. Under ambient conditions, Kelvin probe force microscopy (KPFM) is commonly carried out using only the first flexural eigenmode of the micro-cantilever probe. Here we report a resonant multi-frequency method for KPFM in air. To implement this method, the first eigenmode of the cantilever probe is used for topography imaging, whereas the second one is used to measure the local contact potential difference in the two-pass mode with the tip lifted. By introducing an additional feedback controller, a multi-frequency KPFM (MF-KPFM) is developed upon a commercial AFM. The performance of MF-KPFM, including the feedback controller, sensitivity and noise, lift height of the cantilever and lateral resolution, is evaluated and optimized. The capabilities of MF-KPFM are demonstrated by characterizing a charge pattern on a polymer electret. The results show that the lateral resolution of KPFM in air can be improved by the resonant multi-frequency method.
A bimodal magnetic force microscopy (MFM) that uses an external magnetic field for the detection and imaging of magnetic thin films is developed. By applying the external modulation magnetic field, the vibration of a cantilever probe is excited by its magnetic tip at its higher eigenmode. Using magnetic nanoparticle samples, the capacity of the technique which allows single-pass imaging of topography and magnetic forces is demonstrated. For the detection of magnetic properties of thin film materials, its signal-to-noise ratio and sensitivity are demonstrated to be superior to conventional MFM in lift mode. The secondary resonance MFM technique provides a promising tool for the characterization of nanoscale magnetic properties of various materials, especially of magnetic thin films with weak magnetism.
Tabia is one of the most widely used construction materials in ancient China. In this work, the tabia samples from three coastal defense fort sites at Zhejiang Province, China, we analyzed to determine their component composition and physiochemical properties. The results show that these tabia samples exhibited high compressive strength, which could be as high as 4–9 MPa. Furthermore, the systematic analysis demonstrated that the high compressive strength was related to the following factors: 1) a suitable mass ratio of sand, lime, and clay; 2) an appropriate sand particle size ratio; and 3) the formation of hydrated calcium silicate (C-S-H). Moreover, sticky rice was also detected in the tabia samples. These findings are fundamentally important, which could be beneficial to further study of the craftsmanship of the coastal defense forts and could further provide essential guidance for the protection and restoration of the tabia relics.
Given the stray capacitance between the probe and sample surface, electrostatic force microscopy (EFM) suffers from the probe averaging effect of electrostatic signals for measuring nanoscale potential distributions. A method for reconstructing an EFM image is presented by using the step response function (SRF) as the system transfer function. The SRF is constructed numerically by conducting finite element method simulations and reconsidering both the probe shape and tip-sample distance. The deconvolution of the probe averaging effect for the electrostatic image is demonstrated using an elaborated sample of graphene ribbons that are used as nanoscale surface potential steps. The lateral resolution of the electrostatic image is improved via deconvolution. The results present a powerful tool for explaining the EFM image to reduce the probe averaging effect effectively, especially for the sample with nanoscale potential steps.
Recently, great advances had been made by using scanning probe microscopy (SPM) to quantify the relative permittivity of thin film materials on a nanometer scale. The imaging techniques of permittivity for thin film materials with SPM, especially for photoelectric materials, have not been fully researched until now. Here, we presented a method to image permittivity of thin film materials by using a scanning capacitance microscope (SCM). This method combined the quantitative measurement by using SCM with the capacitance gradient–distance fitting curve to obtain the two-dimensional (2D) permittivity image at room temperature under atmospheric conditions. For the demonstration, a 2D permittivity image of film of molybdenum oxide (MoO3), a kind of photoelectric material, was acquired. From the image, it could be found that the average values of permittivity of MoO3 film and of MoO3 film-doped NaCl were about 8.0 and 9.5, respectively. The experimental results were quantitatively consistent with other experimental results of the same material. The reported technique here could provide a novel method for imaging the relative permittivity with nanometer resolution and be helpful for the study of photoelectric materials.
Tabia is one of the most widely used construction materials in ancient China. In this work, the tabia samples from three coastal defense fort sites at Zhejiang Province, China, we analyzed to determine their component composition and physiochemical properties. The results show that these tabia samples exhibited high compressive strength, which could be as high as 4-9 MPa. Furthermore, the systematic analysis demonstrated that the high compressive strength was related to the following factors: 1) a suitable mass ratio of sand, lime, and clay; 2) an appropriate sand particle size ratio; and 3) the formation of hydrated calcium silicate (C-S-H). Moreover, sticky rice was also detected in the tabia samples. These findings are fundamentally important, which could be beneficial to further study of the craftsmanship of the coastal defense forts and could further provide essential guidance for the protection and restoration of the tabia relics.
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