We present a novel method for nanometer resolution subsurface imaging. When a sample of atomic force microscope (AFM) is vertically vibrated at ultrasonic frequencies much higher than the cantilever resonance, the tip cannot vibrate but it is cyclically indented into the sample. By modulating the amplitude of ultrasonic vibration, subsurface features are imaged from the cantilever deflection vibration at the modulation frequency. By adding low-frequency lateral vibration to the ultrasonic vibration, subsurface features with different shear rigidity are imaged from the torsional vibration of cantilever. Thus controlling the direction of vibration forces, we can discriminate subsurface features of different elastic properties.For the development of nanometer scale electronic and mechanical devices, there is an increasing need for nanometer resolution imaging method of subsurface features (groups of ions, clusters, lattice defects, crystal grains, etc.). Some relating methods have been proposed in scanning force microscopy (SFM) where the tipl'2 or the sample3,a is vibrated to modulate the force. The response to the force modulation is measured to image ion implanted layers,l embedded wires,2 carbon fiber and epoxy composites,3 and Langmuir-Blodgett films.a These methods are characterized, by a tip mounting spring with a spring constant comparable to that of the sample. It is sometimes different from the usual AFM requirement for the spring constant to be as small as possible.s In this letter we propose an alternative imaging method, ultrasonic force microscopy (UFM) that employs a tip mounting cantilever much softer than the tip-sample contact rigidity. We vibrate the sample at frequencies much higher than the resonant frequency of the cantilever6 and measure the deflection and/or torsional vibration of the cantilever. It gives nanometer resolution elastic or subsurface images, and moreoveq discriminates features of different elastic properties, by controlling the direction of vibration forces. We present a general imaging scheme extending our preliminary work,7 and an analysis to compare the elastic contrast of the force modulation mode3'a and the UFM. Then, it is verified by imaging two different subsurface features in a highly oriented pyrolytic graphite (HOPG) sample.We model the AFM with springs and the mass of tip cantilever rn as illustrated in Fig. 1. First, the cantilever is displaced by z" from its free position due to a static repulsive force. When the sample is vibrated at a frequency F lower than the cantilever resonant frequency Fs, the cantilgver is also vibrated following the sample vibration. The tip-sample contact rigidity is expressed as a spring constant s, as a slope u)Also at of the force-displacement relation.5 If s is approximated by a linear spring, the peak{o-peak cantilever vibration amplitude is given by a/zV :2211fu, where a is the sample vibration amplitude and /r is the cantilever spring constant. The amplitude V does not significantly depend upon the spring constant ratio K:t/s repre...

A new method is proposed to detect ultrasonic vibration of the samples in the Atomic Force Microscope (AFM) using nonlinearity in the tip-sample interaction force curve F(z). Small amplitude ultrasonic vibration less than 0.2 nm is detected as an average displacement of a cantilever. This Ultrasonic Force Mode (UFM) of operation is advantageous in detecting ultrasonic vibration with frequencies up to the GHz range, using an AFM cantilever with a resonant frequency below 100 kHz. It was found that a strong repulsive force is acting after an ultrasonic amplitude threshold of the is crossed, with the amplitude of this threshold depending upon the average force applied to the tip.

Two-dimensional (2D) compounds provide unique building blocks for novel layered devices and hybrid photonic structures. However, large surface-to-volume ratio in thin films enhances the significance of surface interactions and charging effects requiring new understanding. Here we use micro-photoluminescence (PL) and ultrasonic force microscopy to explore the influence of the dielectric environment on optical properties of a few monolayer MoS2 films. PL spectra for MoS2 films deposited on SiO2 substrates are found to vary widely. This film-to-film variation is suppressed by additional capping of MoS2 with SiO2 and SixNy, improving mechanical coupling of MoS2 with surrounding dielectrics. We show that the observed PL non-uniformities are related to strong variation in the local electron charging of MoS2 films. In completely encapsulated films, negative charging is enhanced leading to uniform optical properties. Observed great sensitivity of optical characteristics of 2D films to surface interactions has important implications for optoelectronics applications of layered materials.

Scanning Thermal Microscopy (SThM) uses micromachined thermal sensors integrated in a force sensing cantilever with a nanoscale tip can be highly useful for exploration of thermal management of nanoscale semiconductor devices. As well as mapping of surface and subsurface properties of related materials. Whereas SThM is capable to image externally generated heat with nanoscale resolution, its ability to map and measure thermal conductivity of materials has been mainly limited to polymers or similar materials possessing low thermal conductivity in the range from 0.1 to 1 Wm -1 K -1 , with lateral resolution on the order of 1 µm.In this paper we use linked experimental and theoretical approaches to analyse thermal performance and sensitivity of the micromachined SThM probes in order to expand their applicability to a broader range of nanostructures from polymers to semiconductors and metals. We develop physical models of interlinked thermal and electrical phenomena in these probes and their interaction with the sample on the mesoscopic length scale of few tens of nm and then validate these models using experimental measurements of the real probes, which provided the basis for analysing SThM performance in exploration of nanostructures. Our study then highlights critical features of these probes, namely, the geometrical location of the thermal sensor with respect to the probe apex, thermal conductance of the probe to the support base, heat conduction to the surrounding gas, and the thermal conductivity of tip material adjacent to the apex. It is furthermore allows us to propose a novel design of the SThM probe that incorporates a multiwall carbon nanotube (CNT) or similar high thermal conductivity graphene sheet material with longitudinal dimensions on micrometre length scale positioned near the probe apex that can provide contact areas with the sample on the order of few tens of nm. The new sensor is predicted to provide greatly improved spatial resolution to thermal properties of nanostructures, as well as to expand the sensitivity of the SThM probe to materials with heat conductivity values up to 100-1000 Wm -1 K -1 .

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We present measurements of the nanoscale elastic and viscoelastic properties of samples of poly(methylmetacrylate) (PMMA)/rubber nanocomposites. For these studies we have used a new technique based on atomic force microscopy (AFM) with ultrasonic excitation, heterodyne force microscopy (HFM), which provides a means of testing the viscoelastic response of polymeric materials locally (in tip-probed regions) at MHz frequencies. Phase-HFM contrast distinguishes local differences in the dynamic response of PMMA/rubber composites. Comparison of HFM with other AFM-based techniques (ultrasonic force microscopy, friction force microscopy and force modulation microscopy), while imaging the same surface region, emphasizes the unique capabilities of HFM for these kinds of studies, and reveals key nanostructural characteristics of the composites. Some of the toughening particles appear to be broken down, with areas of PMMA detached from the surrounding matrix.

Increasing concerns regarding the sustainability of lithium sources, due to their limited availability and consequent expected price increase, have raised awareness of the importance of developing alternative energy-storage candidates that can sustain the ever-growing energy demand. Furthermore, limitations on the availability of the transition metals used in the manufacturing of cathode materials, together with questionable mining practices, are driving development towards more sustainable elements. Given the uniformly high abundance and cost-effectiveness of sodium, as well as its very suitable redox potential (close to that of lithium), sodium-ion battery technology offers tremendous potential to be a counterpart to lithium-ion batteries (LIBs) in different application scenarios, such as stationary energy storage and low-cost vehicles. This potential is reflected by the major investments that are being made by industry in a wide variety of markets and in diverse material combinations. Despite the associated advantages of being a drop-in replacement for LIBs, there are remarkable differences in the physicochemical properties between sodium and lithium that give rise to different behaviours, for example, different coordination preferences in compounds, desolvation energies, or solubility of the solid–electrolyte interphase inorganic salt components. This demands a more detailed study of the underlying physical and chemical processes occurring in sodium-ion batteries and allows great scope for groundbreaking advances in the field, from lab-scale to scale-up. This roadmap provides an extensive review by experts in academia and industry of the current state of the art in 2021 and the different research directions and strategies currently underway to improve the performance of sodium-ion batteries. The aim is to provide an opinion with respect to the current challenges and opportunities, from the fundamental properties to the practical applications of this technology.