Direct measurements of anisotropic thermal transport in 2D materials and heterostructures

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 conductivities of 2DMs. We use x-SThM for measurements of the -InSe nanolayers with in-plane and cross-plane conductivities of 2.16 Wm-1K-1 and 0.89 Wm-1K-1, respectively [3], 2DM perovskites for advanced solar cells [2] and superlattices of MoS2 interspersed with nanolayered Sb2Te3 [4] where the extremely low in-plane thermal conductivity of 0.7±0.1Wm−1K−1 lead to record values of thermoelectric figure of merit ZT of 2.08 ± 0.37 at room temperature

Thermal transport in enhanced thermoelectric performance high FOM Sb2Te3/MoS2 heterostructure

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 structures

From molecular-scale electrical double layer structure to 3D nano-rheology properties of solid electrolyte interphase

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

Quantitative scanning thermal microscopy studies of the influence of interfaces and heat transport anisotropy in 2D materials

Since the discovery of graphene, the physical properties of layered and two-dimensional (2D) materials have been widely investigated to enable their applications in real-world devices. In particular, the heat transport in these materials is largely influenced not only by the nature of the measured material itself but also by the interaction with the substrates and the resulting 2D-3D materials interfaces. Given the nanoscale dimensions involved, matching effective thermophysical measurement techniques are required. Here, we focus on how nanoscale features affect the thermal transport of three different layered materials (graphene, γ-InSe, and perovskite), conducting the measurements with Scanning Thermal Microscopy (SThM). This characterization technique provides us with analogous lateral and thickness resolution to measure the heat transport anisotropy of the layered nanostructures and the effects of the substrate and the interfaces within the diffusive thermal transport approximation. We use the sample preparation method of beam-exit cross-sectional nano-polishing (BEXP), which provides a low-angle cut with a nearly atomically-flat wedge-like surface accessible for the SThM study. We then study the thickness-dependent variation of heat transport of each material deriving their anisotropic thermal conductivities. By investigating three materials with different thermal transport and anisotropic degree we can compare the interface effects of high (Si) and low (SiO2) thermal conductive substrates. We notice that the interfacial thermal conductivity of the substrate and the top layer material plays a vital role in the overall heat transport of the structure, which can be observed by the SThM-measured profiles of thermal resistance. With a further analytical model of the experimental results, we are able to extract the degree of anisotropy of the thermal conductivities and estimate the limits of the interfacial thermal resistivity in the material-substrate junction

Correlation of nanoscale electromechanical and mechanical properties of twisted double bi-layer graphene via UFM, PFM, and E-HFM

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

Determinar el grado de correlacion entre la voluntad de aprender y los puntajes de la prueba de aptitud academica de los alumnos de primer ano de la carrera de Ingenieria Comercial de la Universidad de Talca.

126 p.La investigacion pretendia determinar el grado de correlacion que existe entre la voluntad de aprender y los puntajes de la prueba de aptitud academica. Para ello fue necesario la determinacion, mediante un analisis factorial, de la relacion entre las variables y posteriormente, determinar los factores con los cuales se desarrollaron cada uno de los objetivos. Para realizarla dividimos en dos grandes arreas llamadas Motivacion y Estrategias de Aprendizaje el estudio, para poder trabajar en forma especifica con cada uno de ellos. Los primeros cinco objetivos especificados en la parte II de esta investigacion los modelos presentan una gran importancia, ya que fue posible determinar cuales son las caracteri­sticas que se definen en los alumnos de las dos menciones y las estrategias de aprendizaje y motivacion, asi como tambien, las diferencias entre ellas. Para desarrollar una relacion entre la Prueba de Aptitud Academica y la encuesta realizada fue necesario determinar un i­ndice en cada una de las arreas, que permitiera trabajar con un modelo aun mas reducido para poder correlacionar el instrumento con los puntajes de la prueba. Aqui obtuvimos como conclusion final que estos puntajes no tienen relacion directa con la voluntad de aprender del alumno de primer año de la Universidad de Talca es decir, la relacion que existe es tan baja, que no se afectan entre si­ por lo que la voluntad en querer aprender durante el primer año de carrera no depende de los puntajes obtenidos en la prueba de aptitud academica. Su unica excepcion fue la prueba de aptitud matematica, la cual entrega una correlacion inversa apoyada por un test de prueba que demuestra su significacion

Anisotropic thermal transport using xSThM studies in 2D-3D heterostructures and composites

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 Evangeli, Alex J. Robson, Alexandros El Sachat, Linda Haenel, M. Isabel Alonso,○ Miquel Garriga,○ Benjamin J. Robinson, Michael Oehme, Jörg Schulz, Francesc Alzina, Clivia Sotomayor Torres, Oleg V. Kolosov, Nanoscale, 24, 10829 (2021)

Sub-wavelength focusing of mid-IR light using metal/diamond/metal campanile probe for ultra-broadband SPM

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