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Register a new userOut-of-Plane Resistance Switching of 2D Bi2O2Se at Nanoscale
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2D bismuth oxyselenide (Bi2O2Se) with high electron mobility shows great potential for nanoelectronics. Although in-plane properties of Bi2O2Se have been widely studied, its out-ofplane electrical transport behavior remains elusive, despite its importance in fabricating devices with new functionality and high integration density. Here, we study the out-of-plane electrical properties of 2D Bi2O2Se at nanoscale by conductive atomic force microscope. We find that hillocks with tunable heights and sizes are formed on Bi2O2Se after applying vertical electrical field. Intriguingly, such hillocks are conductive in vertical direction, resulting in a previously unknown out-of-plane resistance switching in thick Bi2O2Se flakes while ohmic conductive characteristic in thin ones. Furthermore, we observe the transformation from bipolar to stable unipolar conduction in thick Bi2O2Se flake possessing such hillocks, suggesting its potential to function as a selector in vertical devices. Our work reveals unique out-of-plane transport behavior of 2D Bi2O2Se, providing the basis for fabricating vertical devices based on this emerging 2D material.
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For thin networked materials, which are spatial discrete structures constructed by continuum components, a paradox on the effective thickness defined by the in-plane and out-of-plane stiffnesses is found, i.e. the effective thickness is not a constant but varies with loading modes. To reveal the mechanism underneath the paradox, we have established a micromechanical framework to investigate the deformation mechanism and predict the stiffness matrix of the networked materials. It is revealed that the networked materials can carry in-plane loads by axial stretching/compression of the components in the networks and resist out-of-plane loading by bending and torsion of the components. The bending deformation of components has a corresponding relation to the axial stretching/compression through the effective thickness, as the continuum plates do, while the torsion deformation has no relation to the axial stretching/compression. The isolated torsion deformation breaks the classical stiffness relation between the in-plane stiffness and the out-of-plane stiffness, which can even be further distorted by the stiffness threshold effect in randomly networked materials. Accordingly, a new formula is summarized to describe the anomalous stiffness relation. This network model can also apply in atomic scale 2D nanomaterials when combining with the molecular structural mechanics model. This work gives an insight into the understanding of the mechanical properties of discrete materials/structures ranging from atomic scale to macro scale.
Hybrid organic-inorganic perovskites have emerged as very promising materials for photonic applications, thanks to the great synthetic versatility that allows to tune their optical properties. In the two-dimensional (2D) crystalline form, these materials behave as multiple quantum-well heterostructures with stable excitonic resonances up to room temperature. In this work strong light-matter coupling in 2D perovskite single-crystal flakes is observed, and the polarization-dependent exciton-polariton response is used to disclose new excitonic features. For the first time, an out-of-plane component of the excitons is observed, unexpected for such 2D systems and completely absent in other layered materials, such as transition-metal dichalcogenides. By comparing different hybrid perovskites with the same inorganic layer but different organic interlayers, it is shown how the nature of the organic ligands controllably affects the out-of-plane exciton-photon coupling. Such vertical dipole coupling is particularly sought in those systems, e.g. plasmonic nanocavities, in which the direction of the field is usually orthogonal to the material sheet. Organic interlayers are shown to affect also the strong birefringence associated to the layered structure, which is exploited in this work to completely rotate the linear polarization degree in only few microns of propagation, akin to what happens in metamaterials.
Two-dimensional (2D) materials and heterostructures have recently gained wide attention due to potential applications in optoelectronic devices. However, the optical properties of the heterojunction have not been properly characterized due to the limited spatial resolution, requiring nano-optical characterization beyond the diffraction limit. Here, we investigate the lateral monolayer MoS2-WS2 heterostructure using tip-enhanced photoluminescence (TEPL) spectroscopy on a non-metallic substrate with picoscale tip-sample distance control. By placing a plasmonic Au-coated Ag tip at the heterojunction, we observed more than three orders of magnitude photoluminescence (PL) enhancement due to the classical near-field mechanism and charge transfer across the junction. The picoscale precision of the distance-dependent TEPL measurements allowed for investigating the classical and quantum tunneling regimes above and below the ~320 pm tip-sample distance, respectively. Quantum plasmonic effects usually limit the maximum signal enhancement due to the near-field depletion at the tip. We demonstrate a more complex behavior at the 2D lateral heterojunction, where hot electron tunneling leads to the quenching of the PL of MoS2, while simultaneously increasing the PL of WS2. Our simulations show agreement with the experiments, revealing the range of parameters and enhancement factors corresponding to various regimes. The controllable photoresponse of the lateral junction can be used in novel nanodevices.
The pn junction is a fundamental electrical component in modern electronics and optoelectronics. Currently, there is a great deal of interest in the two-dimensional (2D) pn junction. Although many experiments have demonstrated the working principle, there is a lack of fundamental understanding of its basic properties and expected performances, in particular when the device is driven out of equilibrium. To fill the current gap in understanding, we investigate the electrostatics and electronic transport of 2D lateral pn junctions. To do so we implement a physics-based simulator that selfconsistently solves the 2D Poissons equation coupled to the drift-diffusion and continuity equations. Notably, the simulator takes into account the strong influence of the out of plane electric field through the surrounding dielectric, capturing the weak screening of charge carriers. Supported by simulations, we propose a Shockley-like equation for the ideal current voltage characteristics, in full analogy to the bulk junction after defining an effective depletion layer (EDL). We also discuss the impact of recombination generation processes inside the EDL, which actually produce a significant deviation with respect to the ideal behavior, consistently with experimental data. Moreover, we analyze the capacitances and conductance of the 2D lateral pn junction. Based on its equivalent circuit we investigate its cut-off frequency targeting RF applications. To gain deeper insight into the role played by material dimensionality, we benchmark the performances of single-layer MoS2 (2D) lateral pn junctions against those of the Si (3D) junction. Finally, a practical discussion on the short length 2D junction case together with the expected impact of interface states has been provided. Given the available list of 2D materials, this work opens the door to a wider exploration of material dependent performances.
This study explores the potentialities of Scanning Thermal Microscopy (SThM) technique as a tool for measuring thermal transporting properties of carbon-derived materials issued from thermal conversion of organic polymers, such as the most commonly known polyimide (PI), Kapton. For quantitative measurements, the Null Point SThM (NP-SThM) technique is used in order to avoid unwanted effects as the parasitic heat flows through the air and the probe cantilever. Kapton HN films were pyrolysed in an inert atmosphere at temperatures up to 1200{deg}C to produce carbon-based residues with varying degree of conversion to free sp2 disordered carbon. The thermal conductivity of carbon materials ranges from 0.2 to 2 Wm-1K-1 depending on the temperature of the carbonization process (varied between 500{deg}C and 1200{deg}C). In order to validate the applicability of NP-SThM approach to these materials, the results were compared to those obtained with the three more traditional techniques, namely photo-thermal radiometry, flash laser analysis and micro-Raman thermometry. It was found that NP SThM data are in excellent agreement with previous work using more traditional techniques. We used the NP-SThM technique to differentiate structural heterogeneities or imperfections at the surface of the pyrolysed Kapton on the basis of measured local thermal conductivity.
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