Tip Sample Research Articles

Synthesis and Chemoresistive Properties of Single-Layer MXene Ti2CTx

As part of the study, we have developed a method for obtaining a single-layer Ti2CTx MXene by the interaction of Ti2AlC with a mixture of hydrochloric acid and sodium fluoride followed by delamination using a tetramethylammonium hydroxide solution and ultrasonic exposure. The obtained stable aqueous dispersion of Ti2CTx has been applied by microplotter printing onto a specialized sensor chip, which has been dried at a temperature of 150°C under reduced pressure. The coating has been studied using modern physicochemical methods of analysis. According to the data of X-ray spectral elemental microanalysis, the ratio n(Ti) : n(F + Cl) = 2 : (0.82–0.85), n(F) : n(Cl) ≈ 6 : 4; aluminum impurity does not exceed 1.5–2.0%. Data have been obtained on the local electrophysical properties of the Ti2CTx coating: on the value of the electron work function from the surface of the material, the distribution of charge carriers, and the capacitance gradient of the “probe tip–sample microregion” capacitor. For the first time, at an operating temperature of 30°C, extremely high chemoresistive responses of the Ti2CTx receptor layer to the content of 1 and 5% oxygen in nitrogen have been determined, which amounted to 8.6 and >276, respectively.

Principles and Applications of Liquid‐Environment Atomic Force Microscopy

AbstractLiquid environment is essential for the occurrence of various vital processes in fields of chemistry, biology, mechanics, and so on, underscoring the importance of understanding the solid–liquid interfaces. In the past decades, liquid‐environment atomic force microscopy (LE‐AFM) has played a nonsubstitutable role in studying solid–liquid interfaces because of its sub‐nanometer spatial resolution, video‐level imaging rates as well as piconewton‐level force measuring capabilities. Various state‐of‐the‐art developments and exciting applications are made recently. In this review, following a brief discussion on the development history of LE‐AFM, its working principles and operating modes with an emphasis on challenges posed by liquid environments and the corresponding solutions are introduced. As tip–sample interactions set the basis of all kinds of AFM, the interactions between the tip and the sample in liquid, including long‐range DLVO forces with quantitative description and short‐range non‐DLVO, whose physical origination is, by contrast, still in ambiguous, are elaborated. The applications of LE‐AFM are then summarized in four aspects, including high‐resolution imaging, high‐speed imaging, tribological characterization, and force spectroscopy‐based characterization. Finally, the perspectives on the future developing direction of LE‐AFM are shared.

Males of Aedes aegypti show different clock gene expression profiles in the presence of conspecific females

BackgroundThe study of behavioral and physiological traits in mosquitoes has been mainly focused on females since males are not hematophagous and thus do not transfer the parasites that cause diseases in human populations. However, the performance of male mosquitoes is key for the expansion of populations and the perpetuation of mosquito species. Pre-copulatory communication between males and females is the initial and essential step for the success of copulation and studying the male facet of this interaction provides fertile ground for the improvement of vector control strategies. Like in most animals, reproduction, feeding, and oviposition are closely associated with locomotor activity in mosquitoes. Rhythmic cycles of locomotor activity have been previously described in Aedes aegypti, and in females, they are known to be altered by blood-feeding and arbovirus infection. In previous work, we found that males in the presence of females significantly change their locomotor activity profiles, with a shift in the phase of the activity peak. Here, we investigated whether this shift is associated with changes in the expression level of three central circadian clock genes.MethodsReal-time PCR reactions were performed for the gene period, cycle, and cryptochrome 2 in samples of heads, antennae, and abdominal tips of solitary males and males in the presence of females. Assays with antennae-ablated males were also performed, asking whether this is an essential organ mediating the communication and the variation in activity profiles.ResultsThe gene period showed a conserved expression pattern in all tissues and conditions, while the other two genes varied according to the male condition. A remarking pattern was observed in cry2, where the difference between the amplitude of expression at the beginning of photophase and the expression peak in the scotophase was greater when males were in the presence of females. Antennae ablation in males did not have a significant effect on the expression profiles, suggesting that female recognition may involve other senses besides hearing and olfaction.ConclusionOur results suggest that the expression of gene cryptochrome 2 varies in association with the interaction between males and females.Graphical abstract

A cantilever-based, ultrahigh-vacuum, low-temperature scanning probe instrument for multidimensional scanning force microscopy.

Cantilever-based atomic force microscopy (AFM) performed under ambient conditions has become an important tool to characterize new material systems as well as devices. Current instruments permit robust scanning over large areas, atomic-scale lateral resolution, and the characterization of various sample properties using multifrequency and multimodal AFM operation modes. Research of new quantum materials and devices, however, often requires low temperatures and ultrahigh vacuum (UHV) conditions and, more specifically, AFM instrumentation providing atomic resolution. For this, AFM instrumentation based on a tuning fork force sensor became increasingly popular. In comparison to microfabricated cantilevers, the more macroscopic tuning forks, however, lack sensitivity, which limits the measurement bandwidth. Moreover, multimodal and multifrequency techniques, such as those available in cantilever-based AFM carried out under ambient conditions, are challenging to implement. In this article, we describe a cantilever-based low-temperature UHV AFM setup that allows one to transfer the versatile AFM techniques developed for ambient conditions to UHV and low-temperature conditions. We demonstrate that such a cantilever-based AFM offers experimental flexibility by permitting multimodal or multifrequency operations with superior force derivative sensitivities and bandwidths. Our instrument has a sub-picometer gap stability and can simultaneously map not only vertical and lateral forces with atomic-scale resolution, but also perform rapid overview scans with the tip kept at larger tip–sample distances for robust imaging.

Soil nutrients and plant diversity affect ectomycorrhizal fungal community structure and functional traits across three subalpine coniferous forests.

The symbiotic relationship between ectomycorrhizal fungi (EMF) and the roots of host plants is significantly important in regulating the health and stability of ecosystems, especially of those such as the climate warming affected subalpine forest ecosystems. Therefore, from the coniferous forest systems located in the Southern Qinghai-Tibetan Plateau, root tips from three forest tree species: Pinus wallichiana, Abies spectabilis and Picea spinulosa, were collected to look for the local causes of EMF community composition and diversity patterns. The EMF colonization rate, diversity and taxonomic community structure were determined by morphotyping and sanger sequencing of the fungal ITS gene from the root tip samples. Soil exploration types were identified based on the morphologies of the ectomycorrhizas, coupled with soil properties analysis and plant diversity survey. Contrasting patterns of EMF community and functional diversity were found across the studied three forests types dominated by different coniferous tree species. In terms of associations between soil and EMF properties, the total phosphorus (TP) and nitrate (NO3−) contents in soil negatively correlated with the colonization rate and the Shannon diversity index of EMF in contrast to the positive relationship between TP and EMF richness. The soil total nitrogen (TN), ammonium (NH4+) and plant diversity together caused 57.6% of the total variations in the EMF taxonomic community structure at the three investigated forest systems. Whereas based on the soil exploration types alone, NH4+ and TN explained 74.2% of variance in the EMF community structures. Overall, the findings of this study leverage our understanding of EMF dynamics and local influencing factors in coniferous forests dominated by different tree species within the subalpine climatic zone.

Ectomycorrhizospheric Microbiome Assembly Rules of Quercus mongolica in the Habitat of SongRong (Tricholoma matsutake) and the Effect of Neighboring Plants

Host plants are known to determine the distribution and development of ectomycorrhizal fungi such as Tricholoma matsutake; however, we found that the fruit body distribution of T. matsutake was different in Quercus mongolica pure or mixed forests. To clarify the fungal and other microbial composition rules of host plants, ectomycorrhizal root tip samples of Q. mongolica mixed with different plants were selected for study. By using high-throughput sequencing, we obtained 5229 fungal and 38,834 bacterial amplicon sequence variants (ASVs) as determined by internally transcribed spacer ribosomal RNA (ITS rRNA) and 16S ribosomal RNA (16S rRNA) sequencing via the Illumina NovaSeq platform. Among the neighboring plants, there were no significant differences in fungal or bacterial alpha diversity, but there was a significant difference (p < 0.05) in ectomycorrhizal alpha diversity. The fungal, bacterial and ectomycorrhizal fungal communities in the ectomycorrhizosphere of Q. mongolica all showed differences in beta diversity and species composition. In addition, the physical and chemical properties of the soil and the relationships among species could affect the relative abundance of fungi, bacteria and ectomycorrhizal fungi, but the soil microbial pool had little effect on microbial composition. Using PICRUSt2, some significantly up-regulated (p < 0.05) metabolic functions in ectomycorrrhizospheric microbial communities were predicted, which would be an interesting research field for ectomycorrhizal microecology.

Catheter Tip or Direct Urine Culture – Choosing the Better Specimen for Biofilm Detection.

Objectives: To assess the better specimen for biofilm detection among catheter tip and direct urine culture and to evaluate the effect of age on biofilm forming capacity of organisms isolated. Study Design: Cross Sectional Descriptive study. Setting: Lahore General Hospital and Department of Microbiology, Post Graduate Medical Institute. Period: August 2018 to February 2019. Material & Methods: Convenient sampling technique was used to collect 75 Catheter tips and corresponding Urine samples from catheterized patients. Catheter tips were cultured by Brun-Buisson technique and Quantitative method was used for culture. The isolates were identified using standard operating procedures and the MDR isolates recovered were subjected to Microtiter plate assay to determine their biofilm forming capacity. Results: The mean age of catheterization was calculated to be 55.8 years. Statistically, similar number of isolates were recovered from catheter tip and urine sample. However, significantly lesser number of urine samples were found positive for growth (p<0.05). MTP assay of catheter tip and urine sample revealed maximum isolates exhibit strong biofilm forming capacity (56.1% vs 47.6%) while minimum number of organisms display no biofilm formation (1.8% vs 2.4%). Catheter tips and urine culture both detect biofilm forming capacity of isolates similarly (p> 0.05). Maximum biofilm formation (100%) is seen in extremes of age. Conclusion: Less number of urine samples were found positive for growth compared to catheter tips but there is no significant difference in detecting biofilm by catheter tip and urine sample.

A piezoelectric rotatable magnetic force microscope system in a 10T cryogen-free superconducting magnet.

We constructed a piezoelectric rotatable magnetic force microscope (MFM) that works in a 10T cryogen-free superconducting magnet. The piezoelectric tube is deformed tangentially and drives a bearing under the inertial drive principle so the MFM head can obtain rotary movement. Due to the novel piezoelectric design, the MFM can be hung underneath the heat sink via a soft spring, and it can be rotated in a cryogen-free superconducting magnet so that the direction of the magnetic field can be changed from 0° to 90° continuously. The system functions in magnetic fields of up to 10T in any direction relative to the tip-sample geometry. This is the first piezoelectric rotatable MFM ever reported. Using this homemade rotatable MFM, we imaged the structure of magnetic tracks on a commercial videotape. When the magnetic field angle changes from 0° to 90°, the magnetic moments on the tape and probe tip also rotate. A magnetic field strength of 0.8T parallel to the sample surface is required to fully rotate the magnetic moment of the tip we used, but 0.8T is not enough to fully rotate the magnetic moment of the sample. The piezoelectric rotatable MFM is expected to be widely used to study the anisotropy of magnetic materials due to its superiority in obtaining the same high field in and out of plane (compared with a vector magnet) as well as in maintaining the same scan area precisely (compared with a mechanical rotatable MFM, especially for atomic-scale scan areas).

Hyperelastic Microcantilever AFM: Efficient Detection Mechanism Based on Principal Parametric Resonance

The impetus of writing this paper is to propose an efficient detection mechanism to scan the surface profile of a micro-sample using cantilever-based atomic force microscopy (AFM), operating in non-contact mode. In order to implement this scheme, the principal parametric resonance characteristics of the resonator are employed, benefiting from the bifurcation-based sensing mechanism. It is assumed that the microcantilever is made from a hyperelastic material, providing large deformation under small excitation amplitude. A nonlinear strain energy function is proposed to capture the elastic energy stored in the flexible component of the device. The tip–sample interaction is modeled based on the van der Waals non-contact force. The nonlinear equation governing the AFM’s dynamics is established using the extended Hamilton’s principle, obeying the Euler–Bernoulli beam theory. As a result, the vibration behavior of the system is introduced by a nonlinear equation having a time-dependent boundary condition. To capture the steady-state numerical response of the system, a developed Galerkin method is utilized to discretize the partial differential equation to a set of nonlinear ordinary differential equations (ODE) that are solved by the combination of shooting and arc-length continuation method. The output reveals that while the resonator is set to be operating near twice the fundamental natural frequency, the response amplitude undergoes a significant drop to the trivial stable branch as the sample’s profile experiences depression in the order of the picometer. According to the performed sensitivity analysis, the proposed working principle based on principal parametric resonance is recommended to design AFMs with ultra-high detection resolution for surface profile scanning.

Validation and measurement of physiological stress and reproductive hormones in wolf hair and claws

AbstractThe use of keratinized tissues (e.g., hair, claws) to investigate physiological effects of environmental and anthropogenic stressors in free‐ranging wildlife populations has increased because these tissues retain steroid hormones during growth and are relatively easy to collect and store in the field. We measured reproductive and stress‐related steroid hormones in wolves (Canis lupus ligoni; n = 31) captured on Prince of Wales Island, Alaska, USA, during 1993–1994 and 2012–2014, representing periods of time when both wolf harvest and densities ranged from high to moderate. We validated enzyme immunoassay kits to measure steroid hormone concentrations in wolf guard hair, undercoat hair, and claw tip samples. Progesterone, testosterone, and cortisol were extracted and measured in the 3 keratinous tissues from wolves of different age class, sex, residency status, and collection periods. Within each tissue type, progesterone and testosterone were positively correlated (guard hair, r = 0.59, P = 0.003; undercoat hair, r = 0.55, P = 0.011; claws, r = 0.62, P ≤ 0.001) and cortisol concentrations were not related to either reproductive hormone. We were able to measure hormone concentrations in archived keratinous tissues collected up to 25 years earlier to assess stress and reproductive activity in historical samples. Our study validates a method for measuring steroid hormones in hair, and for the first time, continuously growing claws in wolves. Measurement of hormone concentrations in keratinous tissues may aid in the assessment of reproductive activity and physiological stress responses in wolf populations over long‐term time periods (i.e., decades) to enhance conservation efforts of an important apex predator.

Confined Vacuum Resonances as Artificial Atoms with Tunable Lifetime.

Atomically engineered artificial lattices are a useful tool for simulating complex quantum phenomena, but have so far been limited to the study of Hamiltonians where electron–electron interactions do not play a role. However, it is precisely the regime in which these interactions do matter where computational times lend simulations a critical advantage over numerical methods. Here, we propose a platform for constructing artificial matter that relies on the confinement of field-emission resonances, a class of vacuum-localized discretized electronic states. We use atom manipulation of surface vacancies in a chlorine-terminated Cu(100) surface to reveal square patches of the underlying metal, thereby creating atomically precise potential wells that host particle-in-a-box modes. By adjusting the dimensions of the confining potential, we can access states with different quantum numbers, making these patches attractive candidates as quantum dots or artificial atoms. We demonstrate that the lifetime of electrons in these engineered states can be extended and tuned through modification of the confining potential, either via atomic assembly or by changing the tip–sample distance. We also demonstrate control over a finite range of state filling, a parameter which plays a key role in the evolution of quantum many-body states. We model the transport through the localized state to disentangle and quantify the lifetime-limiting processes, illustrating the critical dependence of the electron lifetime on the properties of the underlying bulk band structure. The interplay with the bulk bands gives rise to negative differential resistance, leading to possible applications in engineering custom atomic-scale resonant tunnelling diodes, which exhibit similar current–voltage characteristics.

Mechanical energy dissipation of an oscillating cantilever close to a conductive substrate partly covered with thin mica films evaluated by frequency modulation atomic force microscopy

Mechanical energy stored in an oscillating cantilever in frequency modulation atomic force microscopy (FM-AFM) was dissipated through nonconservative interactions between a sample and a tip on the cantilever. The energy dissipation (D dis) was measured using FM-AFM with a metal-coated tip for a metal-coated Si substrate partly covered with thin mica films. At tip–sample separations where electrostatic force was dominant under a bias voltage, Joule heat was generated owing to the tip oscillation, responsible for D dis. From analysis of D dis and the frequency shift of the cantilever, electric resistance responsible for the Joule heat was estimated to be of the order of GΩ. The great values of the resistance were discussed in the terms of surface scattering of charges moved by the oscillating tip and the dielectric energy loss in the mica films. Measurement of the energy dissipation exhibited potential to probe the local surface electronic properties in non-contact.

Extraction algorithm for longitudinal and transverse mechanical information of AFM

The atomic force microscope (AFM) can measure nanoscale morphology and mechanical properties and has a wide range of applications. The traditional method for measuring the mechanical properties of a sample does so for the longitudinal and transverse properties separately, ignoring the coupling between them. In this paper, a data processing and multidimensional mechanical information extraction algorithm for the composite mode of peak force tapping and torsional resonance is proposed. On the basis of a tip–sample interaction model for the AFM, longitudinal peak force data are used to decouple amplitude and phase data of transverse torsional resonance, accurately identify the tip–sample longitudinal contact force in each peak force cycle, and synchronously obtain the corresponding characteristic images of the transverse amplitude and phase. Experimental results show that the measured longitudinal mechanical characteristics are consistent with the transverse amplitude and phase characteristics, which verifies the effectiveness of the method. Thus, a new method is provided for the measurement of multidimensional mechanical characteristics using the AFM.

A qPlus-based scanning probe microscope compatible with optical measurements.

We design and develop a scanning probe microscope (SPM) system based on the qPlus sensor for atomic-scale optical experiments. The microscope operates under ultrahigh vacuum and low temperature (6.2K). In order to obtain high efficiency of light excitation and collection, two front lenses with high numerical apertures (N.A. = 0.38) driven by compact nano-positioners are directly integrated on the scanner head without degrading its mechanical and thermal stability. The electric noise floor of the background current is 5 fA/Hz1/2, and the maximum vibrational noise of the tip height is below 200 fm/Hz1/2. The drift of the tip-sample spacing is smaller than 0.1 pm/min. Such a rigid scanner head yields small background noise (oscillation amplitude of ∼2pm without excitation) and high quality factor (Q factor up to 140 000) for the qPlus sensor. Atomic-resolution imaging and inelastic electron tunneling spectroscopy are obtained under the scanning tunneling microscope mode on the Au(111) surface. The hydrogen-bonding structure of two-dimensional (2D) ice on the Au(111) surface is clearly resolved under the atomic force microscope (AFM) mode with a CO-terminated tip. Finally, the electroluminescence spectrum from a plasmonic AFM tip is demonstrated, which paves the way for future photon-assisted SPM experiments.

Very-high-frequency probes for atomic force microscopy with silicon optomechanics

Atomic force microscopy (AFM) has been consistently supporting nanosciences and nanotechnologies for over 30 years and is used in many fields from condensed matter physics to biology. It enables the measurement of very weak forces at the nanoscale, thus elucidating the interactions at play in fundamental processes. Here, we leverage the combined benefits of micro/nanoelectromechanical systems and cavity optomechanics to fabricate a sensor for dynamic mode AFM at a frequency above 100 MHz. This frequency is two decades above the fastest commercial AFM probes, suggesting an opportunity for measuring forces at timescales unexplored thus far. The fabrication is achieved using very-large-scale integration technologies derived from photonic silicon circuits. The probe’s optomechanical ring cavity is coupled to a 1.55 μm laser light and features a 130 MHz mechanical resonance mode with a quality factor of 900 in air. A limit of detection in the displacement of 3 × 10−16 m/√Hz is obtained, enabling the detection of the Brownian motion of the probe and paving the way for force sensing experiments in the dynamic mode with a working vibration amplitude in the picometer range. When inserted in a custom AFM instrument embodiment, this optomechanical sensor demonstrates the capacity to perform force-distance measurements and to maintain a constant interaction strength between the tip and sample, an essential requirement for AFM applications. Experiments indeed show a stable closed-loop operation with a setpoint of 4 nN/nm for an unprecedented subpicometer vibration amplitude, where the tip–sample interaction is mediated by a stretched water meniscus.

Characterizing the Ectomycorrhizal Fungal Community of Whitebark Pine in Interior British Columbia: Mature Trees, Natural Regeneration and Planted Seedlings

Whitebark pine (Pinus albicaulis Engelm.; WBP) is an endangered subalpine tree species and requires associations with ectomycorrhizal fungi (ECMF) for survival and growth. Despite this obligate dependence, there are gaps in the identification of ECMF that associate with WBP. In addition, ECMF rarely feature in assessments of recovery actions and little is known about the relationship between ECMF and the insects and pathogens affecting WBP. We used next-generation sequencing to characterize ECMF occurring in soil and mycorrhizal root tip samples from naturally occurring mature WBP trees and seedlings as well as planted WBP seedlings in the Columbia Mountains of Interior British Columbia, Canada. ECMF data was paired with data on tree age, tree health and soil conditions. Thirty-three species and twenty-one genera of ECMF were identified with medium or high confidence from mycorrhizal root tip samples. Major groups were: generalist ascomycetes [Cenococcum, Meliniomyces (=Hyaloscypha)], Atheliales (Piloderma, Amphinema, Tylospora), non-ascomycetous generalists (e.g., Amphinema), associates of high-elevation conifers (species of Cortinarius, Russula) and Suilloids (Suillus, Rhizopogon). Differences in WBP ECMF with other, drier and southerly regions that have been studied previously, were consistent with a distinct forest type and an endemism hypothesis. Soil at the planting site and planted seedlings hosted a reduced ECMF community or were non-ectomycorrhizal, which can be explained by site factors and is expected to affect seedling survival. ECMF composition on mature trees was correlated with tree health, which may have implications for WBPs resistance to pathogens and signals that ECMF are affected by the decline of their host. Understanding the ecology of WBP ECMF and their relationship with tree performance is essential for WBP recovery efforts.

Study of high–low KPFM on a pn-patterned Si surface

Comparative measurements between frequency modulation Kelvin probe force microscopy (FM-KPFM) using low frequency bias voltage and heterodyne FM-KPFM using high frequency bias voltage were performed on the surface potential measurement. A silicon substrate patterned with p- and n-type impurities was used as a quantitative sample. The multi-pass scanning method in the measurements of FM-KPFM and heterodyne FM-KPFM was used to eliminate the effect of the tip–sample distance dependence. The measured surface potentials become lower in the order of the p-type region, n-type region and n+-type region by both FM-KPFM and heterodyne FM-KPFM, which are in good agreement with the order of the work functions of the pn-patterned Si sample. We observed the difference in the surface potentials due to the surface band bending measured by FM-KPFM and heterodyne FM-KPFM. The difference is due to the fact that the charge transfer between the surface and bulk levels may or may not respond to AC bias voltage.

Quantifying nanoscale forces using machine learning in dynamic atomic force microscopy.

Dynamic atomic force microscopy (AFM) is a key platform that enables topological and nanomechanical characterization of novel materials. This is achieved by linking the nanoscale forces that exist between the AFM tip and the sample to specific mathematical functions through modeling. However, the main challenge in dynamic AFM is to quantify these nanoscale forces without the use of complex models that are routinely used to explain the physics of tip–sample interaction. Here, we make use of machine learning and data science to characterize tip–sample forces purely from experimental data with sub-microsecond resolution. Our machine learning approach is first trained on standard AFM models and then showcased experimentally on a polymer blend of polystyrene (PS) and low density polyethylene (LDPE) sample. Using this algorithm we probe the complex physics of tip–sample contact in polymers, estimate elasticity, and provide insight into energy dissipation during contact. Our study opens a new route in dynamic AFM characterization where machine learning can be combined with experimental methodologies to probe transient processes involved in phase transformation as well as complex chemical and biological phenomena in real-time.

Shortcomings of the Derjaguin–Muller–Toporov model in dynamic atomic force microscopy

Force–distance curves recorded by frequency modulated atomic force microscopy (FM AFM) provide insight into the tip–sample mechanics. For quantitative analysis, FM AFM is able to separate conservative from dissipative forces by simultaneously measuring amplitude–distance and frequency–distance curves. Here, we report on the conservative forces in the gentle tip–sample contact of mesoscopic tips at low Tabor parameters. We introduce an analytical expression for the frequency shift based on the Derjaguin–Muller–Toporov (DMT) contact model to simplify the comparison between the experiment and theory. From the analytical formulas, a scaling law between the tip radius and minimal frequency shift is found, which is supported by experimental data. Although excellent fits for full frequency–distance curves are possible, the resulting material properties do not match the accepted literature values. We suspect that these flaws are a consequence of the incomplete treatment of attractive forces and DMT’s strain-stiffness approximation, rendering DMT-based models inappropriate to measure material properties by dynamic AFM in gentle contact.

Microwave impedance microscopy and its application to quantum materials

Materials in which quantum mechanical effects lead to novel properties on the macroscopic scale are of interest both fundamentally and for applications. For such quantum materials, the ability to measure local conductivity and permittivity with nanoscale spatial resolution, minimal sample preparation, in a contact-free fashion and under a wide range of external conditions (such as temperature and magnetic field) is highly desirable. To this end, microwave impedance microscopy, a scanning probe technique that measures tip–sample admittance (inverse of impedance) in a non-contact geometry at microwave frequencies, has matured over the past decade to become a tool that can do just that. This Technical Review describes its fundamental working principles and practical implementations, discusses its application to a wide range of quantum materials, including correlated, topological and 2D van der Waals materials, and outlines future opportunities in expanding the capabilities of microwave impedance microscopy.