Non-conducting Samples Research Articles

Adaptive block imaging based on compressive sensing in AFM.

Atomic force microscopy (AFM) is a kind of high-precision instrument to measure the surface morphology of various conductive or nonconductive samples. However, obtaining a high-resolution image with standard AFM scanning requires more time. Using block compressive sensing (BCS) is an effective approach to achieve rapid AFM imaging. But, the routine BCS-AFM imaging is difficult to balance the image quality of each local area. It is easy to lead to excessive sampling in some flat areas, resulting in time-consuming. At the same time, there is a lack of sampling in some areas with significant details, resulting in poor imaging quality. Thus, an innovative adaptive BCS-AFM imaging method is proposed. The overlapped block is used to eliminate blocking artifacts. Characteristic parameters (GTV, Lu, and SD) are used to predict the local morphological characteristics of the samples. Back propagation neural network is employed to acquire the appropriate sampling rate of each sub-block. Sampling points are obtained by pre-scanning and adaptive supplementary scanning. Afterward, all sub-block images are reconstructed using the TVAL3 algorithm. Each sample is capable of achieving uniform, excellent image quality. Image visual effects and evaluation indicators (PSNR and SSIM) are employed for the purpose of evaluating and analyzing the imaging effects of samples. Compared with two nonadaptive and two other adaptive imaging schemes, our proposed scheme has the characteristics of a high degree of automation, uniformly high-quality imaging, and rapid imaging speed. HIGHLIGHTS: The proposed adaptive BCS method can address the issues of uneven image quality and slow imaging speed in AFM. The appropriate sampling rate of each sub-block of the sample can be obtained by BP neural network. The introduction of GTV, Lu, and SD can effectively reveal the morphological features of AFM images. Seven samples with different morphology are used to test the performance of the proposed adaptive algorithm. Practical experiments are carried out with two samples to verify the feasibility of the proposed adaptive algorithm.

Signal-to-noise ratio versus field strength for small surface coils.

The increasing signal-to-noise ratio (SNR) is the main reason to use ultrahigh field MRI. Here, we investigate the dependence of the SNR on the magnetic field strength, especially for small animal applications, where small surface coils are used and coil noise cannot be ignored. Measurements were performed at five field strengths from 3 to 14.1 T, using 2.2-cm surface coils with an identical coil design for transmit and receive on two water samples with and without salt. SNR was measured in a series of spoiled gradient echo images with varying flip angle and corrected for saturation based on a series of flip angle and T1 measurements. Furthermore, the noise figure of the receive chain was determined and eliminated to remove instrument dependence. Finally, the coil sensitivity was determined based on the principle of reciprocity to obtain a measure for ultimate SNR. Before coil sensitivity correction, the SNR increase in nonconductive samples is highly supralinear with B0 1.6-2.7, depending on distance to the coil, while in the conductive sample, the growth is smaller, being around linear close to the surface coil and increasing up to a B0 2.0 dependence when moving away from the coil. After sensitivity correction, the SNR increase is independent of loading with B0 2.1. This study confirms the supralinear increase of SNR with increasing field strengths. Compared with most human measurements with larger coil sizes, smaller surface coils, as mainly used in animal studies, have a higher contribution of coil noise and thus a different behavior of SNR at high fields.

Experimental evaluation of selectively laser sintered polyamide 12 surface treatment for direct electrodeposition

The paper deals with the problem of electrodeposition of metallic coatings on previously nonconductive materials. In connection with rapid prototyping of 3D printing an easy and effective method for electroplating nonconductive materials should be found. In this case polyamide 12 (PA12) has been chosen for a base material and copper (Cu) for the metallic coating. Samples prepared by selective laser sintering (SLS) differing in roughness and pretreatment were used. Graphite was sprayed on the samples to ensure their conductivity. Cu coating was deposited electrolytically from an acidic copper electrolyte. Coating roughness was measured and compared with the roughness of bare samples. The thickness of the deposited layer was calculated, and the data were then compared with destructive measurements on a scanning electron microscope. To test the adhesion of the Cu coating to the substrate, the bending test was used. Cu coating was successfully deposited on previously nonconductive samples. Their conductivity was easily obtained by spraying them with graphite. The results indicate that the use of vaporization prior to Cu coating deposition leads to better coating properties according to the tests performed.

Reduction of surface charging effects in laser ablation ionisation mass spectrometry through gold coating.

In femtosecond Laser Ablation Ionisation Mass Spectrometry (fs-LIMS) short laser pulses are used to ablate, atomise, and ionise solid sample material shot-by-shot. When ablating non-conductive samples electric charging of the surface can occur. Depending on the geometry of the instrument, the surface charge can influence the spread of the ablation plume and reduce spectral quality. Methods to reduce surface charging were investigated using a non-conductive geological sample and a miniature fs-LIMS system with a co-linear ablation geometry. Pausing five seconds between consecutive laser bursts fired on non-coated material improved the spectral quality by giving surface charges more time to dissipate. However, best mass spectrometric results were achieved after the sample was sputter coated with a thin gold layer, as a conductive sample surface hinders charge build-up. Consequently, gold coating allowed operation of the laser system at much higher laser pulse energies improving sensitivity and reliability. It also removed the need to pause between laser bursts, speeding up the measurement acquisition.

Conductive cross-section preparation of non-conductive painting micro-samples for SEM analysis

Scanning electron microscopy (SEM) is a common method for the analysis of painting micro-samples. The high resolution of this technique offers precise surface analysis and can be coupled with an energy-dispersive spectrometer for the acquisition of the elemental composition. For light microscopy and SEM analysis, the painting micro-samples are commonly prepared as cross-sections, where the micro-sample positioned on the side is embedded in a resin. Therefore, the sequence of its layers is exposed after the cross-section is polished. In common cases outside of cultural heritage, a conductive layer is applied on the polished side, but in this field, the measurements are mostly done in low-vacuum SEM (LV-SEM). Although the charging effect is reduced in LV-SEM, it can still occur, and can hardly be prevented even with carbon tape or paint. This work presents two conductive cross-section preparation methods for non-conductive samples, which reduce charging effects without impairing the sample integrity.

Direct visualization of the drug release process of non-conductive polymeric implants via molecular imaging

Long-acting parenteral (LAP) implant has garnered the attraction as a drug delivery technique in recent years. Understanding the drug release process is critical for the study of underlying release mechanism. In this paper, we present a novel application of matrix-assisted laser desorption/ionization-mass spectrometry imaging (MADLI-MSI) for the direct visualization of the drug release process from non-conductive polymeric based LAP implants at molecular level. Custom-made sample holders were designed for LAP sample introduction in place of traditional conductive glass slides. The main technical obstacles of applying MALDI-MSI to study non-conductive materials are surface conductivity which can lead to charge build-up. In order to obtain homogeneous imaging of non-conductive sample surfaces, we developed a new sample surface treatment procedure, which is a critical control step to ensure the data reliability and accuracy in understanding kinetics of drug release process of LAP. Overall, this is the first comprehensive report of a sample preparation methodology tailored for imaging LAP at molecular level, allowing for the direct chemical identification and 2D mapping of an active pharmaceutical ingredient (API) distribution during LAP release process. Furthermore, this work has established the foundation to apply MALDI-MSI to the understanding of LAP implant formulation homogeneity, chemical composition, and degradation. More importantly, this work enabled the extension of MALDI-MSI technique to study a wide range of non-conductive materials.

Understanding the Effects of Graphene Coating on the Electrostatic Field at the Tip of an Atom Probe Tomography Specimen

Abstract As a three-dimensional characterization method, atom probe tomography can provide key information that other methods cannot offer. Conductive coatings have proved to be an effective way for biological samples, and nonconductive samples in general, to be analyzed using voltage-pulsed atom probe tomography. In this study, we analyzed the effects of graphene coating on an electrically conductive material and were able to confirm the detection of carbon atoms. We compare quantitative electrostatic field metrics for a single-coated and a multi-coated specimen and measure both a reduced voltage after graphene coating and lowered charge-state ratios for different ion species, suggesting a lowered evaporation field related to the graphene coating. This information will be instructive for future studies on graphene-coated, nonconductive biological specimens.

Performance enhancement of a scanning electron microscope using a deep convolutional neural network

We report noise reduction and image enhancement in scanning electron microscope (SEM) imaging while maintaining a fast scan rate during imaging, using a deep convolutional neural network (D-CNN). SEM images of non-conducting samples without a conducting coating always suffer from charging phenomenon, giving rise to SEM images with low contrast or anomalous contrast and permanent damage to the sample. One of the ways to avoid this effect is to use fast scan mode, which suppresses the charging effect fairly well. Unfortunately, this also introduces noise and gives blurred images. The D-CNN has been used to predict relatively noise-free images as obtained from a slow scan from a noisy, fast scan image. The predicted images from D-CNN have the sharpness of images obtained from a slow scan rate while reducing the charging effect. We show that by using the proposed method it is possible to increase the scanning rate by a factor of about seven with an output of image quality comparable to that of slow scan mode. We present experimental results in support of the proposed method.

Transient Two-Layer Electroosmotic Flow and Heat Transfer of Power-Law Nanofluids in a Microchannel.

To achieve the optimum use and efficient thermal management of two-layer electroosmosis pumping systems in microdevices, this paper studies the transient hydrodynamical features in two-layer electroosmotic flow of power-law nanofluids in a slit microchannel and the corresponding heat transfer characteristics in the presence of viscous dissipation. The governing equations are established based on the Cauchy momentum equation, continuity equation, energy equation, and power-law nanofluid model, which are analytically solved in the limiting case of two-layer Newtonian fluid flow by means of Laplace transform and numerically solved for two-layer power-law nanofluid fluid flow. The transient mechanism of adopting conducting power-law nanofluid as a pumping force and that of pumping nonconducting power-law nanofluid are both discussed by presenting the two-layer velocity, flow rates, temperature, and Nusselt number at different power-law rheology, nanoparticle volume fraction, electrokinetic width and Brinkman number. The results demonstrate that shear thinning conducting nanofluid represents a promising tool to drive nonconducting samples, especially samples with shear thickening features. The increase in nanoparticle volume fraction promotes heat transfer performance, and the shear thickening feature of conducting nanofluid tends to suppress the effects of viscous dissipation and electrokinetic width on heat transfer.

THz-TDS Techniques of Thickness Measurements in Thin Shim Stock Films and Composite Materials

Terahertz wave (T-ray) scanning applications are one of the most promising tools for nondestructive evaluation. T-ray scanning applications use a T-ray technique to measure the thickness of both thin Shim stock films and GFRP (glass fiber-reinforced plastics) composites, of which the samples were selected because the T-ray method could penetrate the non-conducting samples. Notably, this method is nondestructive, making it useful for analyzing the characteristics of the materials. Thus, the T-ray thickness measurement can be found for both non-conducting Shim stock films and GFRP composites. In this work, a characterization procedure was conducted to analyze electromagnetic properties, such as the refractive index. The obtained estimates of the properties are in good agreement with the known data for poly methyl methacrylate (PMMA) for acquiring the refractive index. The T-ray technique was developed to measure the thickness of the thin Shim stock films and the GFRP composites. Our tests obtained good results on the thickness of the standard film samples, with the different thicknesses ranging from around 120 μm to 500 μm. In this study, the T-ray method was based on the reflection mode measurement, and the time-of-flight (TOF) and resonance frequencies were utilized to acquire the thickness measurements of the films and GFRP composites. The results showed that the thickness of the samples of frequency matched those obtained directly by time-of-flight (TOF) methods.

Scanning electron microscopy as a useful tool for the analysis of non-conductive materials

Scanning electron microscopy (SEM) in the analysis of non-conductive samples became one of the most important methods for the investigation of material properties. In this work, we used SEM microstructure analysis for the investigation of the origin of cracks in granite composites and also, we tested the porosity inside the regenerated carbon biowaste, potentially used as a clean source of carbon for the future applications in materials production. Additionally, the morphology and the chemical composition of small particles used for the moulding processes of plastics were also tested. The importance of the microstructure investigation was supported by Energy-Dispersive X-ray Spectroscopy (EDS) often used for the chemical composition evaluation of these non-conductive materials.

Subcycle contact-free nanoscopy of ultrafast interlayer transport in atomically thin heterostructures

Tunnelling is one of the most fundamental manifestations of quantum mechanics. The recent advent of lightwave-driven scanning tunnelling microscopy has revolutionized ultrafast nanoscience by directly resolving electron tunnelling in electrically conducting samples on the relevant ultrashort length- and timescales. Here, we introduce a complementary approach based on terahertz near-field microscopy to perform ultrafast nano-videography of tunnelling processes even in insulators. The central idea is to probe the evolution of the local polarizability of electron–hole pairs with evanescent terahertz fields, which we detect with subcycle temporal resolution. In a proof of concept, we resolve femtosecond interlayer transport in van der Waals heterobilayers and reveal pronounced variations of the local formation and annihilation of interlayer excitons on deeply subwavelength, nanometre scales. Such contact-free nanoscopy of tunnelling-induced dynamics should be universally applicable to conducting and non-conducting samples and reveal how ultrafast transport processes shape functionalities in a wide range of condensed matter systems.

Numerical Simulation and Experimental Study of Capacitive Imaging Technique as a Nondestructive Testing Method

It was recently demonstrated that a coplanar capacitive sensor could be applied to the evaluation of materials without the disadvantages associated with the other techniques. This technique effectively detects changes in the dielectric properties of the materials due to, for instance, imperfections or variations in the internal structure, by moving a set of simple electrodes on the surface of the specimen. An AC voltage is applied to one or more electrodes and signals are detected by others. This is a promising inspection method for imaging the interior structure of the numerous materials, without the necessity to be in contact with the surface of the sample. In this paper, finite element (FE) modeling was employed to simulate the electric field distribution from a coplanar capacitive sensor and the way it interacts with a nonconducting sample. Physical experiments with a prototype capacitive sensor were also performed on a Plexiglas sample with subsurface defects, to assess the imaging performance of the sensor. A good qualitative agreement was observed between the numerical simulation and experimental result.

Bio-imaging with the helium-ion microscope: A review.

Scanning helium-ion microscopy (HIM) is an imaging technique with sub-nanometre resolution and is a powerful tool to resolve some of the tiniest structures in biology. In many aspects, the HIM resembles a field-emission scanning electron microscope (FE-SEM), but the use of helium ions rather than electrons provides several advantages, including higher surface sensitivity, larger depth of field, and a straightforward charge-compensating electron flood gun, which enables imaging of non-conductive samples, rendering HIM a promising high-resolution imaging technique for biological samples. Starting with studies focused on medical research, the last decade has seen some particularly spectacular high-resolution images in studies focused on plants, microbiology, virology, and geomicrobiology. However, HIM is not just an imaging technique. The ability to use the instrument for milling biological objects as small as viruses offers unique opportunities which are not possible with more conventional focused ion beams, such as gallium. Several pioneering technical developments, such as methods to couple secondary ion mass spectrometry (SIMS) or ionoluminescence with the HIM, also offer the possibility for new and exciting research on biological materials. In this review, we present a comprehensive overview of almost all currently published literature which has demonstrated the application of HIM for imaging of biological specimens. We also discuss some technical features of this unique type of instrument and highlight some of the new advances which will likely become more widely used in the years to come.

Experimental setups for XPS measurements beyond the instrumental lateral resolution limit

The lateral resolution of an X‐ray photoelectron spectroscopy instrument, which is equipped with a focused X‐ray beam, is limited by the nominal X‐ray beam diameter and the long tail intensity distribution of the X‐ray beam. The long tail intensity distribution of the X‐ray beam impedes to perform a measurement with good lateral resolution and low detection limits at the same time. Two experimental setups are described which allow examining sample structures that are smaller than the X‐ray beam dimensions. The first method uses differential sample charging on partly nonconductive samples by low‐energy electron flooding. The spectra of the nonconductive sample areas are shifted towards lower binding energy. That way, the surface composition of conductive and nonconductive sample areas are estimated independently. The second method utilizes the rather limited dimensions of the energy analyser acceptance volume. Here, only the sample is placed inside the energy analyser acceptance volume. That way, signals from the illuminated sample contribute exclusively to the measured photoelectrons intensity, independent from the sample size.

Conducting polyaniline prepared in the solutions of formic acid: Does functionalization with carboxyl groups occur?

Polyaniline is a conducting polymer with an application potential in the field of biomedical engineering. By employing FTIR spectroscopy and conductivity measurements, it has been shown that the oxidation at stoichiometric peroxydisulfate-to-aniline mole ratio 1.25 in the solutions of formic acid in the range 0-10 M provides samples of a moderate conductivity of the orders 0.01-0.1 S cm-1. They consist of polyaniline and aniline oligomers as typical of the aniline oxidation in weak acids. The detailed investigation of the infrared spectra indicates a partial ring-carboxylation of polyaniline at high acid concentrations. The extent of structural defects is higher for a series prepared at over-stoichiometric peroxydisulfate-to-aniline mole ratio 2.5, which provided only non-conducting samples. The reference sample series represented by poly(aniline-co-o-aminobenzoic acid) was also prepared and is used in the discussion of the infrared spectra.

Scanning Gel Electrochemical Microscopy (SGECM): Lateral Physical Resolution by Current and Shear Force Feedback.

Scanning gel electrochemical microscopy (SGECM) is a novel technique measuring local electrochemistry based on a gel probe. The gel probe, which is fabricated by electrodeposition of hydrogel on a microdisk electrode, immobilizes the electrolyte, and constitutes a two-electrode system upon contact with the sample. The contact area determines the lateral physical resolution of the measurement, and considering the soft nature of the gel it is essential to be well analyzed. In this work, the lateral physical resolution of SGECM is quantitatively studied from two aspects: (1) marking single sampling points by locally oxidizing Ag to AgCl and measuring their size; (2) line scan over reference samples with periodic topography and composition. The gel probe is approached to the sample by either current or shear force feedback, and the physical resolution of them is compared. For the optimal gel probe based on 25 μm diameter Pt disk electrode of Rg ≈ 2, the lateral physical resolution of SGECM at contact position is ca. 50 μm for current feedback and ca. 63 μm for shear force feedback. More importantly, the lateral physical resolution of SGECM can be flexibly tuned in the range of 14-78 μm by pulling or pressing the gel probe after touching the sample. In general, current feedback is more sensitive to gel-sample contact than shear force feedback. But the latter is more versatile, which is also applicable to nonconductive samples.

Target Material Selection for Sputter Coating of SEM Samples

Abstract

Synergistic Effect of Fullerenes on the Laser-Induced Periodic Surface Structuring of Poly(3-Hexyl Thiophene).

Ordered and homogeneous laser-induced periodic surface structures (LIPSS) could be fabricated in poly(3-hexyl thiophene):[6,6]-phenyl C71-butyric acid methyl ester (P3HT:PC71BM) blends by using wavelengths in the ultraviolet (UV) range (266 nm). The absorption coefficient of PC71BM, which is maximum in its UV–Visible absorption spectrum around 266 nm, enhanced the overall absorption of the blend. In addition, PC71BM itself was capable of developing homogeneous LIPSS by laser irradiation at λlaser = 266 nm. Therefore, we proposed that the synergistic effect of PC71BM on the LIPSS formation in P3HT:PC71BM (1:1) was due to a templating effect for the LIPSS formation of the PC71BM itself, which added to the overall increment of the absorption of the blend. LIPSS formation at ambient conditions in this wavelength range led to chemical modification of both P3HT and PC71BM, which rendered to non-conducting samples. Irradiation in vacuum significantly reduced radiation damage, rendering to the characteristic electrical conductivity pattern observed in P3HT LIPSS samples irradiated in the visible range. This effect could be of potential interest in order to obtain LIPSS in low absorbing polymers.

Pulsed DC bias for the study of negative-ion production on surfaces of insulating materials in low pressure hydrogen plasmas

In this work, negative-ion production on the surface of a sample negatively DC biased in a hydrogen plasma is studied. The negative ions created under positive ion bombardment are accelerated toward the plasma, self-extracted, and detected according to their energy and mass, by a mass spectrometer placed in front of the sample. The use of a pulsed bias allows applying a quasi-DC bias on insulating material during a short period of time and offers the possibility to extend the measurement method to nonconductive samples. The pulsed-bias tests were performed first with highly oriented pyrolytic graphite (HOPG), a conductive material, to demonstrate the feasibility of the method. By changing the pulsed-bias frequency, it was possible to obtain HOPG material with different hydrogen surface coverages and hence different surface states leading to an increase of negative-ion production by up to 30%–50% as compared to the continuous bias case. To establish a protocol for insulating materials, charge accumulation on the surface during the bias pulse and influence of the bias duration and frequency were explored using microcrystalline diamond (MCD) thin layers. By using a pulse short enough (10 μs) at 1 kHz frequency, it has been possible to measure negative-ions on the MCD sample at a quasi-constant surface bias of 130 V, with only 1 V variation during the measurement. Negative-ion surface production on MCD has been studied in pulsed mode with surface temperature from room temperature to 800 °C. It is shown that pulsing the bias and increasing the temperature allows limiting defect creation on MCD which is favorable for negative-ion production. Consequently, at 400 °C, the yield on MCD in pulsed mode is one order of magnitude higher than the yield on HOPG in continuous mode at room temperature.