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The thermal properties of amorphous and crystalline phases in chalcogenide phase change materials (PCM) play a key role in device performance for non-volatile random-access memory. Here, we report the nanothermal morphology of amorphous and crystalline phases in laser pulsed GeTe and Ge2Sb2Te5 thin films by scanning thermal microscopy (SThM). By SThM measurements and quantitative finite element analysis simulations of two film thicknesses, the PCM thermal conductivities and thermal boundary conductances between the PCM and SThM probe are independently estimated for the amorphous and crystalline phase of each stoichiometry.

Recently, multiple theoretical and experimental studies have been published regarding the properties of stacked two-dimensional (2D) layers forming a twisted heterostructure. This field (known as twistronics) shows that properties of 2D materials can be modified to a great degree, including bandgap modulation and creating superconductive structures. Given the versatility that these structures have, many exciting engineering is being applied to them resulting in promising properties. In this study, we investigated a heterostructure composed of two twisted graphene bi-layers with a small angle between them (1.1º), where an atomic reconstruction is induced changing the lattice symmetry and creating a Moiré pattern. The electrical and mechanical properties of the 2D nanostructure are affected by this symmetry reconstruction, generating relaxation-induced strain gradients. We compared nanomechanical mapping via Ultrasonic Force Microscopy (UFM) and electromechanical response probed by Piezoresponse Force Microscopy (PFM) and Electrical Heterodyne Force Microscopy (E-HFM). These allowed us to assign Moiré patterns of the heterostructure to the particular crystallographic arrangements and to quantify the local Young’s modulus variation between single and double domain walls. Moreover, by measuring these domain walls specifically with PFM, it is possible to extract evidence of non-uniform strain in stretched triangular domains in the Moiré pattern. The phase images from the E-HFM allow us to observe a fast time-domain nanoelectromechanical relaxation in the order of picoseconds with nanoscale lateral resolution.

To date, there has been limited reporting on the fabrication and properties of macroscopic sheet assemblies (specifically buckypapers) composed of carbon/boron nitride core–shell heteronanotubes (MWCNT@BNNT) or boron nitride nanotubes (BNNTs). Herein we report the synthesis of MWCNT@BNNTs via a facile method involving Atmospheric Pressure Chemical Vapor Deposition (APCVD) and the safe h-BN precursor ammonia borane. These MWCNT@BNNTs were used as sacrificial templates for BNNT synthesis by thermal oxidation of the core carbon. Buckypaper fabrication was facilitated by facile sonication and filtration steps. To test the thermal conductivity properties of these new buckypapers, in the interest of thermal management applications, we have developed a novel technique of advanced scanning thermal microscopy (SThM) that we call piercing SThM (pSThM). Our measurements show a 14% increase in thermal conductivity of the MWCNT@BNNT buckypaper relative to a control multiwalled carbon nanotube (MWCNT) buckypaper. Meanwhile, our BNNT buckypaper exhibited approximately half the thermal conductivity of the MWCNT control, which we attribute to the turbostratic quality of our BNNTs. To the best of our knowledge, this work achieves the first thermal conductivity measurement of a MWCNT@BNNT buckypaper and of a BNNT buckypaper composed of BNNTs not synthesized by high energy techniques.

Nanomechanical measurements of minimally twisted van der Waals materials remained elusive despite their fundamental importance for device realisation. Here, we use Ultrasonic Force Microscopy (UFM) to locally quantify the variation of out-of-plane Young's modulus in minimally twisted double bilayer graphene (TDBG). We reveal a softening of the Young's modulus by 7% and 17% along single and double domain walls, respectively. Our experimental results are confirmed by force-field relaxation models. This study highlights the strong tunability of nanomechanical properties in engineered twisted materials, and paves the way for future applications of designer 2D nanomechanical systems.

The solid electrolyte interphase (SEI), a passivation layer formed on the battery electrode-electrolyte interface [1], defines fundamental battery properties - its capacity, cycle stability, and safety. While understanding the SEI formation holds keys to these, such studies are complicated by the diversity of interlinked surface reactions and complex nanoarchitecture of the anode active material and electrical double layer (EDL) [2]. Such nanoarchitecture predetermines the electrolyte supramolecular interactions, electrical charge, and ion transport, therefore, dominating the initial SEI formation. To date, the real space molecular arrangements of electrolyte solvents/anions inside the EDL and their effects on the SEI formation remain elusive [3]. In this work, we resolve this complex puzzle, using a novel solid-liquid interface characterization tool with a nanoscale spatial resolution for accessing the whole evolution process from initial molecular-scale EDL structures, toward nanoscale 3D SEI structures. We introduce in-situ electrochemical 3D nanorheology microscopy (3D-NRM) [4] combined with magnetic excitation molecular-level solvation force spectroscopy and molecular dynamics simulations to explore a matrix of two morphologically dissimilar but chemically identical surfaces of typical carbon electrode material (basal and edge graphene planes) and different solvent-electrolyte systems (strong and weakly solvating electrolytes, as well as ionic liquid electrolyte). These approaches allowed us to get direct insight into the atomistic pictures for the underlying influence of cation’s intercalation and solvation structures on the initial SEI formation.

To effectively realize the potential of two dimensional (2D) materials (2DMs) in heat management, power electronics, new semiconductor processors, and thermoelectric applications, it is essential to measure heat transport in 2DMs and their heterostructures. This task presents several formidable challenges: the measurements are to be done on nanometre-scale thick 2DMs with structures often consisting of flakes of only a few m across, with multiple interfaces, and with a highly anisotropic nature of heat conductance due to strong covalent bonds in atomic planes vs weak van der Waals (vdW) bound atomic layers. Here we report a pioneering approach for direct measurements of anisotropic thermal conductivity in 2DMs. The approach uses scanning thermal microscopy, SThM, that while sensitive to heat flow to a sample via nanoscale probe tip [1], on its own neither can quantify thermal conductivity due to generally unknown probe-sample thermal resistance nor can detect the anisotropy of the thermal conductivity. We, therefore, combine SThM with the measurements of 2DMs and their heterostructures at variable thickness by using dedicated Ar-ion cross-sectioning [2] to produce a low-angle wedge structure (inset in Fig.1a). By scanning SThM across such wedge we obtain in a single measurement (Fig 1b,c) heat conductance as a function of thickness. For low thicknesses (compared with the size of the probe-sample contact), the heat transport is predominantly normal to the layers, while at larger thicknesses it becomes three-dimensional, with such transition directly affected by the anisotropy of the thermal conductivity of the 2DM sample. Using a simple analytical Musychka-Spiece model [2], validated by the finite element analysis, we first find the dimensions of the SThM tip-sample thermal contact using the test wedge sample of a known material (e.g. isotropic SiO2 on Si) via the simple curve fitting (Fig 1d) and then find the absolute in-plane and cross-plane values of thermal con

Developing methods for efficient nanoscale probing of light-matter interaction, especially in the Mid-IR and THz spectral range, is essential for studying fundamental physical phenomena as well as chemical properties at micrometer to nanometer length scales. A highly efficient nanoscale focusing of visible and near-IR radiation light was reported recently using Au-SiO2-Au tapered gap campanile plasmon waveguide with an 830 nm wavelength that couples free space light into the nanoscale domain, enabling probing of materials in the visible and near-IR spectral range [1]. We expand this capability to the highly important mid-IR and THz range providing valuable information on local nanoscale chemistry and physical processes of materials and devices using a campanile shaped diamond tetragonal pyramid [2]. Our finite difference time domain (FDTD) simulation reveals that nanoscale focusing of mid-IR light is possible within the range of geometries and metal coatings including Au, Al and Cu. Here we report linked modeling and experimental results showing the confining efficiency of diamond pyramid in the mid-IR range (8-10 µm). Furthermore, we will demonstrate the integration of Au/diamond/Au light concentrator into a scanning probe microscope for performing sub-wavelength spectroscopy of various materials in both reflection and transmission geometries.

Van der Waals heterostructures are tremendously versatile designer materials whose functionality can be engineered to an extend that goes far beyond the properties of the individual materials the heterostructure consists of [1]. In particular, by twisting two graphene layers, it is possible to induce an atomic reconstruction in the two-dimensional stack, which leads to a dramatic modification of the lattice symmetry [2]. This has important repercussions on its mechanical and electro-mechanical properties [3,4]. Here we investigate the local mechanical properties of double bi-layer graphene twisted by an angle ~1.1°. To this end, we employ three force microscope techniques, Piezoresponse Force Microscopy, Ultrasonic Force Microscopy and Electric Heterodyne Force Microscopy, respectively. We demonstrate that these methods are reliable and effective to visualize the Moiré pattern, to evidence the presence of strain solitons [5], and – for the first time – to extract the local Youngs modulus in such systems. Our results bring on a comprehensive study of such complex structures and unlock critical understanding of these materials. References [1] Geim, A., Grigorieva, I., Nature, 499 (2013) 419–425. [2] Dai, S., Xiang, Y., Srolovitz, D. J., Nano Lett., 16, 9 (2016) 5923–5927. [3] De Sanctis, A., Mehew, J. D., et al., Nano Lett., 18, 12 (2018) 7919–7926. [4] Li, Y., Wang, Xet al., Adv. Mater., 33 (2021) 2105879. [5] Alden, J. S., Tsen, A. W., et al., PNAS, 110 (2013) 11256–11260.

Recently reported phenomenon of “geometrical thermoelectricity” (GTE) that allows to tune a local Seebeck coefficient in a two-dimensional material (2DM) solely by changing device geometry [1], allows to manufacture thermoelectric (TE) devices such as nanoscale thermocouples [2] and heat management structures from a single material eliminating the need of thermoelectric junctions and multiple materials. It was suggested that this effect that is profound in the nanoscale constrictions have it origin in the modification of the energy dependent majority carriers mean-free path (MFP) in the nano-constriction [1]. The Seebeck coefficient in such a constriction is reduced by up to orders of magnitude, resulting in two opposing TE junctions between the 2DM in the constriction and the unaffected material on the either side of the constriction qualitatively explaining the phenomenon and the experimental results using nanoscale thermal probe [1,2]. At the same time, major questions remained on the a) role of the assymmetry of the constriction (e.g. tapered vs rectangular junctions), b) polarity of charge carriers in GTE phenomenon and c) effect of the tip of the nanoscale probe with previous studies performed on the symmetric graphene devices deposited on the SiO2/Si substrate that is known to provide a significant and not well defined doping of the graphene layer. Fig 1. STGM map of the encapculated graphene device. Here we explore outstanding questions of GTE charge and geometry symmetry using a recently developed scanning thermal gate microscopy (STGM) approach where a heated nanoscale scanning probe moves across sample surface while potential difference at the edges of the sample is measured producing STGM “maps” linked to a local Seebeck coefficient variation [3]. We created a monolayer graphene structure sandwiched between two hBN layers on SiO2/Si substrate serving as a back gate that has four nanoscale constriction junctions of approximately 100 nm width – (i) triangu

Thermal conductivity is a crucial parameter defining the thermal transport as well as thermophysical properties of materials in thermoelectric, manufacturing and processing applications of materials where heat transport plays a major role. To address a current challenge of measuring these properties locally, in the areas of few tens or hundreds nanometres, we used a novel approach of cross-sectional scanning thermal microrcopy, or SThM, (xSThM). In this method, we first create a fine low angle wedge of the studied material via precision Ar ion cross-sectional polishing [1] and then measure a thermal conductance via SThM with each measurement point providing thermal conductance of the material with different thickness. Furthermore, an analytical model is then used to extract not only anisotropic values of thermal conductivity but also determines the effect of interfacial thermal resistance between the substrate and complex structured materials (heterostructure and composite structures). This technique thus facilitates a direct measurement of thermal conductance as a function of thickness in 2D-3D based heterostructures (Sb2Te3/MoS2) and composites structures (Sb2Te3/AgSbTe2). The thickness and number of layers of MoS2 was optimized to achieve extremely lower values of thermal conductivity (0.7  0.1 Wm-1K-1) along with higher values of power factor ((4.97 0.39) ×10-3 Wm-1K-2) leading to high values of ZT of 2.08  0.37 at room temperature. Similarly, the concentration of Ag in Sb2Te3/AgSbTe2 is optimized for highest values of ZT. A major enhancement in the value of TE performance was observed due to the effective majority carriers filtering and phonon scattering at the potential barrier present due to multiple interfaces. The current methodology provides an efficient tool for quantifying the thermal transport in thin films and 2D materials, and hence is useful in establishing the thermal transport in such complex structures. References: [1] Jean Spièce, Charalambos

The control of the phonon scattering and the understanding of thermal transport at nanoscale level poses a big challenge in upgrading the efficiencies of nanostructured devices and materials. Furthermore, the lack in analysis of the anisotropic and multi-layered materials restrict the implementation of exotic structures in manufacturing processes. Particularly, for the thermoelectric (TE) devices the understanding of underlying mechanisms of enhancement of phonon scattering paves new pathways for increasing ZTs and enabling efficient commercial applications. In the present study, our aim is to establish the role of multiple interfaces on the thermal transport in Sb2Te3/MoS2 superlattice. We use Scanning Thermal Microscopy (SThM) that provides a flexible approach that can be easily adapted to varying nanoscale structure and geometry of the multilayer sample. In particular, we use a novel approach of cross-sectional SThM (xSThM) that measures the heat transport in the nanoscale sample with the layer of interest is polished using Ar ion beam to produce a damage free nanoscale-flat wedge shaped structure [1]. With the typical wedge angle of about 3-50, two-dimensional xSThM scans map the thermal transport of the Sb2Te3/MoS2 multilayer sample layer as a function of it thickness, allowing to investigate specific contribution of the in-plane and out-of-plane thermal conductivity of the multi-layer samples. In our studies we demonstrate that due to the effective majority carrier filtering and phonon scattering at the potential barrier present at the multiple interfaces, a major enhancement in the value of TE power factor was observed. The present study is important not only for enhancing the TE performance but also helps to establish the efficient approach to quantifying the thermal transport in 2D-3D interfaces, multilayers, as well as in hybrid or buried nanostructures and hence overcome a critical bottleneck in understanding the thermal transport in these complex structu

The understanding of thermal transport at nanoscale level opens up new pathways in upgrading the efficiencies of nanostructured devices and materials. In addition, thin films with highly anisotropic thermal conductivities offer high potential for thermal management of the present day electronics [1]. Apart from deploying anisotropic materials in manufacturing processes, it is extremely difficult and challenging to measure and analyze even the basic thermophysical properties of materials. Scanning Thermal Microscopy (SThM) provides versatile approach for measurements of thermal conductivity of the materials and devices for various spatial geometries [2]. Whereas most of SThM studies are performed in ambient conditions in air and at room temperature (RT), these measurements suffer from spurious effects of the through-the-air heat transport and do not allow investigating the nanoscale thermal transport in the various temperatures. In the present study, we have performed SThM measurements of the thermal conductance in exfoliated InSe and graphene layers of thickness from few atomic layers to quasi bulk materials, in air as well as high vacuum (HV) of 10^-7 torr. The room temperature results for in air and HV measurements were compared and the effect of heat conductance in air were analyzed in detail. The HV measurements were performed both at room temperatures as well as at cryogenic temperatures to understand the thermal transport mechanisms. InSe is one of the novel van der Waals materials that promises high performance as future thermoelectrics, used often in combination with graphene electrodes. An analytical model was used to deconvolute the values of contact resistances and thermal resistances. The temperature dependent thermal conductance values for variable thickness of InSe samples gave insight into the thermal transport mechanism and possibility of its use as an active thermoelectric material. This new approach allowed us to measure anisotropic thermal conduct

The thermoelectric (TE) applications of graphene have generated great interest due to its extraordinary electronic and thermal properties, gate-controlled ambipolar behaviour, and competitive Seebeck coefficient (S). In recent studies, the effects of nanostructuring on the local variations of the Seebeck coefficient have been explored for bare graphene samples with patterned bow-tie constrictions[1], and mono- and bi-layer junctions[2]. These represent a new paradigm in the control of the TE properties on the nanoscale, enabling the development of single metal thermocouples[3] for temperature sensing and coolers for thermal load distribution and hot-spot removal with nanoscale dimensions. Here, we study the spatial distribution of the local Seebeck domains via the thermoelectric voltage (V_th )in encapsulated graphene devices with patterned constrictions. This novel approach explores two different strategies to improve the V_th signal and control of the S domains: (1) the enhancement of the Seebeck coefficient by encapsulation, gate carrier control, and temperature gradient control; and (2) the creation of local S domains by patterning constrictions of varying geometry and size as shown in figure 1(a). To study the response of the devices, maps of the thermovoltage with nanoscale resolution were created using scanning thermal gate microscopy (STGM), a novel SPM mode in which a hot tip is employed as the local heating source and scanned over the sample in open circuit configuration, thus creating thermovoltage signal that is proportional to the Seebeck coefficient. A schematic representation of the STGM is depicted in figure 1(b). The resulting thermovoltage maps were acquired for different gate voltages (V_BG ) and different temperature gradients between the tip and sample. In figure 1(c) and (d), V_th maps acquired for p- and n-doped graphene, respectively, are presented. Local TE junctions with an opposite gradient of Seebeck coefficient are formed across the devi