Helium Ion Microscopy Research Articles

Silicone Mold Accuracy in Polyurethane Vacuum Casting

AbstractVacuum casting of polyurethane (PUR) in silicone molds is used industrially for the production of prototypes and small series as well as in various noncommercial research areas. This includes the reproduction of archeological findings, biological samples or electronic devices. In this study, the authors investigate the molding accuracy of different commercial silicones and PUR casting resins both on microscopic and macroscopic scales. For this, they used a variety of master models to generate silicone rubber casting molds. The resulting PUR castings are investigated by helium ion microscopy, confocal laser scanning microscopy, and optical 3D‐scanning.

Sensitive and Low-Power Metal Oxide Gas Sensors with a Low-Cost Microelectromechanical Heater.

In this study, a simple and cost-effective metal oxide semiconductor (MOS) gas sensor, which can be fabricated utilizing only two photolithography steps, was designed and developed through the planar microelectromechanical systems (MEMS) technique. Ball-milled porous tin dioxide nanoparticle clusters were precisely drop-coated onto the integrated microheater region and subsequently characterized using a helium ion microscope (HIM). The spatial suspension of the silicon nitride platform over the silicon substrate provides superior thermal isolation and thus dramatically reduces the power consumption of the microheater. The well-designed microheater exhibits excellent thermal uniformity, which was verified both computationally and experimentally. The as-fabricated sensors were tested for ethanol gas sensing at various operating temperatures with different concentrations. At the optimal work temperature of ∼400 °C, our gas sensors demonstrated a respectable sensitivity to 1 ppm ethanol, which is the lower detection limit to most commercial products. Moreover, stable performance over repetitive testing was observed. The innovative sensor developed here is a promising candidate for portable gas sensing devices and various other commercial applications.

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.

Bandgap recovery of monolayer MoS2 using defect engineering and chemical doping.

Two-dimensional transition metal dichalcogenide materials have created avenues for exciting physics with unique electronic and photonic applications. Among these materials, molybdenum disulfide is the most known due to extensive research in understanding its electronic and optical properties. In this paper, we report on the successful growth and modification of monolayer MoS2 (1L MoS2) by controlling carrier concentration and manipulating bandgap in order to improve the efficiency of light emission. Atomic size MoS2 vacancies were created using a Helium Ion Microscope, then the defect sites were doped with 2,3,5,6-tetrafluro7,7,8,8-tetracyanoquinodimethane (F4TCNQ). The carrier concentration in intrinsic (as-grown) and engineered 1L MoS2 was calculated using Mass Action model. The results are in a good agreement with Raman and photoluminescence spectroscopy as well as Kelvin probe force microscopy characterizations.

Comparative evaluation of MAX-Ti3AlC2 and MXene-Ti3C2 as affinity chromatographic materials for highly selective enrichment of phosphopeptides.

MAX and MXene have received considerable attention owing to their outstanding performance in fields like battery and catalysis. However, their possible biomedical applications have rarely been considered, especially the affinity chromatographic applications in proteomics. In this work, considering the large number of exposed metal sites, small binding potential resistance and fast mass transfer speed, layered ternary carbides MAX-Ti3AlC2 and MXene-Ti3C2 with a two-dimensional nanostructure were successfully explored for the first time as affinity chromatography stationary phases for the specific capture of phosphopeptides from complex biological samples. Helium ion microscopy, transmission electron microscopy, atomic force microscopy, X-ray diffraction spectra, X-ray photoelectron spectroscopy and zeta potential measurement results confirmed that the MXene-Ti3C2 was well exfoliated from the pristine MAX-Ti3AlC2. Ti3AlC2 showed better enrichment specificity than MXene-Ti3C2. The detection limit of Ti3AlC2 was as low as 5 fmol. Even when the molar ratio of BSA to β-casein tryptic digests increased to 1000 : 1, two characteristic phosphopeptides with a relatively clear background could be detected after enrichment. After five cycles of repeated use, the enrichment specificity of Ti3AlC2 still remains. Furthermore, 91 and 830 unique phosphopeptides from 23 and 525 phosphoproteins were identified from milk and BEL7402 cells, respectively. Among them, 27 and 170 phosphopeptides, 12 and 56 phosphoproteins identified from milk and BEL7402 cells were not detected with commercial TiO2 after three independent replicates, which have great potential in providing complementary coverage of phosphoproteome. This work opens up new applications of Ti3AlC2 and MXene-Ti3C2, and will play more important role for phosphorylated proteomics in biomedicine.

Nanoporous Activated Carbon Functionalized Graphene Sensors for Selective Detection of Ethanol

Graphene surface functionalization enables the facile detection of low concentration volatile organic compounds (VOCs). Herein, activated carbon functionalization of chemical vapour deposition (CVD) graphene is shown to enhance ethanol gas sensing in CVD graphene by three orders of magnitude. The presence of activated carbon was confirmed via Helium Ion Microscopy, and Raman spectroscopy and porosity were confirmed via Transmission Electron Microscopy (TEM). Unlike CVD graphene, the activated carbon functionalized graphene field effect transistor (aCF-GFET) had a sensitivity of 50 ppb with performance improving with increase in temperature. The extreme activated carbon-induced sensitivity is attributed to the catalytic activity and oxidation of the adsorbed ethanol molecules, consequently inducing higher graphene-ethanol charge transfer and Van der Waals bonding. The dependence of this charge transfer on the applied tuning voltage was investigated using the charge neutrality point disparity (CNPD) method, which shows an increase in CNPD values with increasing concentration, confirming the activated carbon-induced oxidation of adsorbed ethanol molecules.

Time-Resolved Focused Ion Beam Microscopy: Modeling, Estimation Methods, and Analyses

In a focused ion beam (FIB) microscope, source particles interact with a small volume of a sample to generate secondary electrons that are detected, pixel by pixel, to produce a micrograph. Randomness of the number of incident particles causes excess variation in the micrograph, beyond the variation in the underlying particle-sample interaction. We recently demonstrated that joint processing of multiple time-resolved measurements from a single pixel can mitigate this effect of source shot noise in helium ion microscopy. This paper is focused on establishing a rigorous framework for understanding the potential for this approach. It introduces idealized continuous- and discrete-time abstractions of FIB microscopy with direct electron detection and estimation-theoretic limits of imaging performance under these measurement models. Novel estimators for use with continuous-time measurements are introduced and analyzed, and estimators for use with discrete-time measurements are analyzed and shown to approach their continuous-time counterparts as time resolution is increased. Simulated FIB microscopy results are consistent with theoretical analyses and demonstrate that substantial improvements over conventional FIB microscopy image formation are made possible by time-resolved measurement.

Mechanics of free-standing inorganic and molecular 2D materials.

The discovery of graphene has triggered a great interest in inorganic as well as molecular two-dimensional (2D) materials. In this review, we summarize recent progress in the mechanical characterization of free-standing 2D materials, such as graphene, hexagonal boron nitride (hBN), transition metal-dichalcogenides, MXenes, black phosphor, carbon nanomembranes (CNMs), 2D polymers, 2D metal organic frameworks (MOFs) and covalent organic frameworks (COFs). Elastic, fracture, bending and interfacial properties of these materials have been determined using a variety of experimental techniques including atomic force microscopy based nanoindentation, in situ tensile/fracture testing, bulge testing, Raman spectroscopy, Brillouin light scattering and buckling-based metrology. Additionally, we address recent advances of 2D materials in a variety of mechanical applications, including resonators, microphones and nanoelectromechanical sensors. With the emphasis on progress and challenges in the mechanical characterization of inorganic and molecular 2D materials, we expect a continuous growth of interest and more systematic experimental work on the mechanics of such ultrathin nanomaterials.

Tritrichomonas foetus: New structures by high-resolution scanning helium ion microscopy

Tritrichomonas foetus: New structures by high-resolution scanning helium ion microscopy

Scanning transmission imaging in the helium ion microscope using a microchannel plate with a delay line detector.

A detection system based on a microchannel plate with a delay line readout structure has been developed to perform scanning transmission ion microscopy (STIM) in the helium ion microscope (HIM). This system is an improvement over other existing approaches since it combines the information of the scanning beam position on the sample with the position (scattering angle) and time of the transmission events. Various imaging modes, such as bright field and dark field or the direct image of the transmitted signal, can be created by post-processing the collected STIM data. Furthermore, the detector has high spatial and temporal resolution, is sensitive to both ions and neutral particles over a wide energy range, and shows robustness against ion beam-induced damage. A special in-vacuum movable support gives the possibility of moving the detector vertically, placing the detector closer to the sample for the detection of high-angle scattering events, or moving it down to increase the angular resolution and distance for time-of-flight measurements. With this new system, we show composition-dependent contrast for amorphous materials and the contrast difference between small-angle and high-angle scattering signals. We also detect channeling-related contrast on polycrystalline silicon, thallium chloride nanocrystals, and single-crystalline silicon by comparing the signal transmitted at different directions for the same data set.

Anaerobic benzene mineralization by natural microbial communities from Niger Delta

The Niger Delta is one of the most damaged ecosystems in the world, mainly due to petroleum contamination by oil exploration accidents. We investigated the natural attenuation potential of Niger Delta subsurface sediment samples for anaerobic hydrocarbon degradation using benzene as a model compound under iron-reducing, sulfate-reducing, and methanogenic conditions. Benzene was slowly mineralized under methanogenic and iron-reducing conditions using nitrilotriacetic acid (NTA)-Fe(III), or poorly crystalline Fe(III) oxyhydroxides as electron acceptors, analyzed by measurement of 13CO2 produced from added 13C-labelled benzene. Highest mineralization rates were observed in microcosms amended with Fe(III) oxyhydroxides. The microbial communities of benzene-mineralizing enrichment cultures were characterized by next-generation sequencing of the genes coding for 16S rRNA and methyl coenzyme M reductase A (mcrA). Abundant phylotypes were affiliated to Betaproteobacteriales, Ignavibacteriales, Desulfuromonadales, and Methanosarcinales of the genera Methanosarcina and Methanothrix, illustrating that the enriched benzene-mineralizing communities were diverse and may contain more than a single benzene degrader. The diversity of the microbial communities was furthermore confirmed by scanning helium-ion microscopy which revealed the presence of various rod-shaped as well as filamentous microbial morphotypes.

High resolution microscopy to evaluate the efficiency of surface sterilization of Zea Mays seeds.

Surface sterilization of seeds is a key step in providing microorganisms-free seeds for numerous applications like understanding the role of seed-borne microorganisms in plant development, studying microbial cells-plant interactions by inoculating model microorganisms in a simplified system or selective cultivation of seed endobionts. However applying efficient treatment for surface sterilization of seeds without affecting the plant growth is not an easy task. In this study we aimed to provide an efficient surface sterilization treatment for maize seeds using i) hydrogen peroxide (HP), ii) sodium hypochlorite (SH) and iii) ethanol-sodium hypochlorite (EtOH-SH) under stirring (st) and vacuum-stirring (va-st) conditions. We used fluorescence microscopy and ultra-high resolution Helium Ion Microscopy (HIM) as powerful imaging approaches in combination with macroscopic techniques to visualize, quantify and evaluate the efficiency of seed sterilization, quality of root germination, seedlings and root hair development as well as the presence or absence of microorganisms on the root surface. Our results showed a strong reduction in microbial cell numbers of 4 orders of magnitude after the EtOH-SH treatments. Moreover, seeds exposed to EtOH-SH treatments displayed the lowest percentage of microbial growth (50%) and the highest percentage of germinated seeds (100%) compared to other sterilization treatments. HIM imaging proved the absence of microbial cells on the roots grown from seeds exposed to EtOH-SH treatments. Moreover, root hair development seemed not to be affected by any of the sterilization treatments. Our findings demonstrated that EtOH-SH treatments are significantly reducing the abundance of microbial cells from the surface of maize seeds and can be used with high confidence in future studies.

Imaging and milling resolution of light ion beams from helium ion microscopy and FIBs driven by liquid metal alloy ion sources.

While the application of focused ion beam (FIB) techniques has become a well-established technique in research and development for patterning and prototyping on the nanometer scale, there is still a large underused potential with respect to the usage of ion species other than gallium. Light ions in the range of m = 1–28 u (hydrogen to silicon) are of increasing interest due to the available high beam resolution in the nanometer range and their special chemical and physical behavior in the substrate. In this work, helium and neon ion beams from a helium ion microscope are compared with ion beams such as lithium, beryllium, boron, and silicon, obtained from a mass-separated FIB using a liquid metal alloy ion source (LMAIS) with respect to the imaging and milling resolution, as well as the current stability. Simulations were carried out to investigate whether the experimentally smallest ion-milled trenches are limited by the size of the collision cascade. While He+ offers, experimentally and in simulations, the smallest minimum trench width, light ion species such as Li+ or Be+ from a LMAIS offer higher milling rates and ion currents while outperforming the milling resolution of Ne+ from a gas field ion source. The comparison allows one to select the best possible ion species for the specific demands in terms of resolution, beam current, and volume to be drilled.

A Mechanistic Exploration of Natural Organic Matter Aggregation and Surface Complexation in Smectite Mesopores

Soil minerals and organic matter play critical roles in nutrient cycling and other life-essential biogeochemical processes, yet the structural and dynamical details of natural organic matter (NOM) film formation on smectites are not fully understood on the molecular scale. XRD of Suwannee River NOM-hectorite (a smectite clay) complexes shows that the humic and fulvic components of NOM bind predominantly at the external surfaces of packets of smectite platelets rather than in the interlayer slit pores, suggesting that the key behavior governing smectite-NOM interactions takes place in mesopores between smectite particles. New molecular dynamics modeling of a ∼110 Å H2O-saturated smectite mesopore at near-neutral pH shows that model NOM molecules initially form small clusters of 2-3 NOM molecules near the center of the pore fluid. Formation of these clusters is driven by the hydrophobic mechanism, where aromatic/aliphatic regions associate with one another to minimize their interactions with H2O, and charge-balancing cations associated with the deprotonated carboxylate sites are located only at the outer surface of these clusters. Despite hydrophobicity driving the initial clustering, NOM clusters are formed more quickly when high-charge-density cations like Ca2+ are present vs low-charge-density cations like Cs+, as the former cations more effectively minimize the electrostatic repulsions between the negatively charged NOM molecules. Once the small hydrophobicity-driven NOM clusters form, the simulations show that Ca2+ promotes the aggregation of NOM clusters through tetradentate Ca2+ bridges involving carboxylate groups on two different NOM clusters. Importantly, our studies indicate that Ca2+ plays a crucial role in binding the NOM clusters to the smectite surface, which occurs through multiple quaternary complexes (Ob)-H2O-Ca2+-COO-NOM. In contrast, Cs+ never forms any coordination or acts like bridges between NOM molecules nor as ion bridges to the smectite surface. Additionally, we observe the formation of a metastable superaggregate involving all 16 NOM molecules several times in a Ca2+-bearing mesopore fluid. Superaggregates are never observed in the simulations involving Cs+. The modeling results are fully consistent with helium ion microscope images of NOM-hectorite complexes suggesting that NOM surface films develop when preformed NOM clusters interact with smectite surfaces. Overall, the binding of NOM clusters to the outer surfaces of smectite particles and the formation of large NOM aggregates at neutral pH occur through cation bridging, and cation bridging only occurs when high-charge-density cations like Ca2+ are present.

Organic matter pores structure and evolution in shales based on the he ion microscopy (HIM): A case study from the Triassic Yanchang, Lower Silurian Longmaxi and Lower Cambrian Niutitang shales in China

To illustrate organic matter (OM) pores structure and evolution in shales with different maturities, the Triassic Yanchang shale (0.5% < Vitrinite reflectance (Ro) < 1.2%) in the Ordos Basin, the Lower Silurian Longmaxi shale (2.0% < Ro < 2.8%) and the Lower Cambrian Niutitang shale (3.0% < Ro < 4.2%) in the Yangtze Platform have been selected in this study. OM pores were analyzed by helium ion microscopy (HIM). The results show that OM pores are mainly developed in pyrobitumen. A large number of OM pores were developed in pyrobitumen of the Longmaxi shale samples. However, no OM pores were basically found in solid kerogen. OM pores number and size in pyrobitumen of the Yanchang shale samples are much smaller than that in the Longmaxi shale samples. OM pores are all basically undeveloped in pyrobitumen of the Niutitang shale samples. The structure that small size OM pores nested in large size OM pores is favorable to gas enrichment and seepage. Thermal maturity has more huge impact on OM pores structure than TOC content in shale. Low thermal maturity (Ro < 1.5%) will lead to insufficient thermal evolution of OM, resulting in small size and quantity of OM pores in shale. Excessive thermal maturity that ranges from 3.0% to 4.0% will leads to the disappearance of OM pores in pyrobitumen. Appropriate thermal maturity that ranges from 1.5% to 3.0% ensures a large number of OM pores in shale.

Quantitative investigation of surface structure and interatomic potential with impact-collision ion scattering spectroscopy

Helium ion beam interactions with materials have important implications for magnetic confinement fusion, material modification, and helium ion microscopy. These interactions depend on the precise physics of how helium ions channel into the materials, which can vary greatly based on the local crystalline orientation. In this work, we performed a dedicated experiment to investigate helium ion channeling in a well-characterized tungsten single crystal. Time-of-flight impact-collision ion scattering spectroscopy was used to obtain multi-angle maps of the backscattering intensity for 3 keV He+ → W(111). We found that the backscattering intensity profile arising from helium ion channeling could be well described by a shadow cone analysis. This analysis revealed that subsurface W atoms as deep as the ninth monolayer contributed to the backscattering intensity profile. Binary collision approximation simulations were performed with MARLOWE to model the experimental maps with sufficient accuracy to allow for quantitative comparisons using reliability factors. These quantitative comparisons were applied to investigate how the W lattice structure and He–W interatomic potential affect the multi-angle maps.

Helium ion microscope - secondary ion mass spectrometry for geological materials.

The helium ion microscope (HIM) is a focussed ion beam instrument with unprecedented spatial resolution for secondary electron imaging but has traditionally lacked microanalytical capabilities. With the addition of the secondary ion mass spectrometry (SIMS) attachment, the capabilities of the instrument have expanded to microanalysis of isotopes from Li up to hundreds of atomic mass units, effectively opening up the analysis of all natural and geological systems. However, the instrument has thus far been underutilised by the geosciences community, due in no small part to a lack of a thorough understanding of the quantitative capabilities of the instrument. Li represents an ideal element for an exploration of the instrument as a tool for geological samples, due to its importance for economic geology and a green economy, and the difficult nature of observing Li with traditional microanalytical techniques. Also Li represents a “best-case” scenario for isotopic measurements. Here we present details of sample preparation, instrument sensitivity, theoretical, and measured detection limits for both elemental and isotopic analysis as well as practicalities for geological sample analyses of Li alongside a discussion of potential geological use cases of the HIM–SIMS instrument.

The evolution of the structure, mechanical, and gas separation properties of P84 hollow fiber membranes from the polymer to the carbon stage

To provide insight into the formation process of carbon molecular sieve (CMS) hollow fiber membranes (HFMs), P84 co-polyimide HFMs were thermally treated at 200, 250, 300, 350, 400, 550, 650, and 800 °C. TG-MS was used to study the weight loss and released gases at all temperatures. In situ SEM, HIM (helium ion microscope) and in situ XRD were used to investigate the structural evolution of P84 HFMs. The structural evolution and changes in the mechanical properties and gas separation performance of P84 HFMs from the polymer to the carbon stages were divided into three stages (sub-Tg, intermediate, and carbon stages) by their Tg (322 °C) and Td (460 °C). The substructure of P84 HFMs began to collapse during the intermediate stage, which impeded the gas permeance of P84 CMS HFMs. At the carbon stage, the permeance of CO2, O2, N2, and CH4 decreased with the increased carbonization temperature; however, the H2 permeance reached a maximum at 650 °C due to the partial pyrolysis and structural reorganization of P84 CMS HFMs.

Liquid crystalline ordered arrays of gold nanoparticles in evaporative deposits

Gold nanoparticles assembly have been studied using two different evaporation techniques. To evaluate the influence CTAB molecules on self-assembly behavior 0.1 M CTAB suspension have been imaged via scanning helium ion microscopy (HIM) and then by SEM. By using immersion methode, dumbbell-like nanoparticles crystallizes over the substrate in island-like arrays showing no specific ordered structures whereas gold nanorods crystallizes in ordered arrays within the deposited islands. However, droplet drying showed aligned arrays of gold nanorods all over the coffee-stain ring. For purified suspensions, domain-like superstructures observed away from the outer-region of the coffee stain. These crystalline arrays grows into domain-like superstructures hydrodynamically.

Helium Ion Microscopy for Reduced Spin Orbit Torque Switching Currents.

Spin orbit torque driven switching is a favorable way to manipulate nanoscale magnetic objects for both memory and wireless communication devices. The critical current required to switch from one magnetic state to another depends on the geometry and the intrinsic properties of the materials used, which are difficult to control locally. Here, we demonstrate how focused helium ion beam irradiation can modulate the local magnetic anisotropy of a Co thin film at the microscopic scale. Real-time in situ characterization using the anomalous Hall effect showed up to an order of magnitude reduction of the magnetic anisotropy under irradiation, with multilevel switching demonstrated. The result is that spin-switching current densities, down to 800 kA cm-2, can be achieved on predetermined areas of the film, without the need for lithography. The ability to vary critical currents spatially has implications not only for storage elements but also neuromorphic and probabilistic computing.