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Recent years witnessed a significant volume of research for understanding the simple physics in photodetection and its applications in energy and sustainability. Photodetectors are useful in variety of applications from consumer electronic devices to industrial and environmental monitoring, flame and gas sensing, space communication, proximity sensors, missile technology etc to name a few. The possibility of engineerable band gap brought semiconducting TMDC MoS2 as one of the prominent among the 2D layered material family for photodetection and gas sensing applications. In my presentation, I will explain the structural and optical features of the MoS2/AlN heterostructure device fabricated through pulsed laser ablation. A few layers of MoS2 were grown on commercially procured and AlN templated Si wafer in an evacuated PLD chamber by ablating locally prepared MoS2 target with KrF excimer laser. Both photovoltaic and photothermal responses are to be analyzed to account for the observed intensity-modulated and wavelength-dependent photoswitching in MoS2/AlN heterostructures. The self-powered response obtained in the device was attributed to the large difference in band gap values of the complementary materials in the binary system, their band alignment, and band bending. Raman spectrum analysis, XPS, XRR, photodetection, etc will be presented to reveal the physical features and structural characteristics of the heterostructure. A detailed account of the photodetection and switching mechanism will also be discussed. 
Acknowledgment
Pius acknowledges the research grant TARE (TAR/2020/000241) by Science and Engineering Research Board (SERB), DST, Govt. of India to carry out research at Material Research Centre, Indian Institute of Science Bangalore.
Reference
[1] Clark, Jenny, and Guglielmo Lanzani. Nature Photonics 4, no. 7 (2010): 438-446.
[2] Fang, Huajing, Cheng Zheng, Liangliang Wu, Yi Li, Jian Cai, Mingxiang Hu, Xiaosheng Fang, Rong Ma, Qing Wang, and Hong Wang. Advanced Functional Materials 29, no. 20 (2019): 1809013.
[3]Lu, Xiaowei, Peng Jiang, and Xinhe Bao. Nature Communications 10, no. 1 (2019): 1-7.
[4][Singh Deependra Kumar, Rohit Pant, Arun Malla Chowdhury, Basanta Roul, Karuna Kar Nanda, and Saluru Baba Krupanidhi. ACS Applied Electronic Materials 2, no. 4 (2020): 944-953.


MATERIALS FOR HUMANITY (MH 21)

Photoswitching and self-powered response in PLD grown MoS2/AlN heterostructure photodetector

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MH 21 is arranged around 6 themes that focus attention on pressing needs that should be addressed to enhance the well-being of communities and provide a forum for presenting breakthrough discoveries.

To date, many different techniques have been developed to fulfill the increasing demand for 3D representation of information, e.g. in optical security, and virtual reality applications. While holography provides both amplitude and phase information for the reconstruction of the 3D light field, it requires coherent laser sources and suffers from speckles. Here, we developed a simple color-filtering microlens to control both the propagating phase and colors simultaneously, working with incoherent white light sources. Fabricated by two-photon polymerization (2PP) 3D laser printing, our color-filtering microlenses can achieve colorful focal spots at different “floating heights”, when illuminated with incoherent white light source. Meanwhile, it appears like a color prints consisting of bright colorful spots when inspected directly under common microscopy. By arranging the floating colorful spots in 3D space, we designed two metasurfaces, showing the radially and spirally unfolded “optical fireworks” when continuously tunning the focus of microscope. Due to the flexibility in controlling the propagating phase, spectrum, and “floating heights”, our color-filtering microlenses could find broad applications in passive optical security labels and conformal metasurfaces.

Color-filtering Microlenses for colorful 3D display

MoS2 is a promising layered semiconducting material, suitable for electronic and optoelectronic applications due to its remarkable properties such as inherent bandgap and high mobility. However, excessive reverse bias leakage current in mechanically exfoliated MoS2 devices remains a challenge in developing electronic devices. In this work, we studied the mechanism determining reverse bias leakage current behavior in MoS2|ZnO n-n+ isotype heterostructure diodes, fabricated using multilayer MoS2 flakes obtained from two different sources, namely natural and synthetic crystals. These devices are characterized by using temperature dependent J-V characteristics in the temperature range between 145 K to 300 K as shown in Figure 1(a) and (b) for devices made on natural crystals (I-a-N), and Figure 1(c) and (d) for synthetic crystals (I-b-S), respectively. The analysis of temperature dependent J-V characteristics shows that leakage is governed by Poole- Frenkel mechanism. The linear behavior of the plot (b) and (d) confirms that the Poole-Frankel mechanism is operative. This provides evidence that the barrier at the MoS2|ZnO interface is lowered on application of field. The barrier at the interface controls the transport barrier in the structure. The possible role of defects and the band bending at the interface will be discussed. Figure 1: Poole-Frankel analysis of the reverse bias leakage current. (a) and (c) depicts the leakage currents in the device I-a-N (natural MoS2 flake) and I-b-S (synthetic MoS2 flake) devices, respectively. Figure 1 (b) and (d) represents the plot of ln(J/Er) vs. Er(1/2) for I-a-N and I-b-S devices, respectively.

Analysis of leakage current mechanism in isotype MoS2|ZnO heterostructure diodes

Magnetic skyrmions are nanoscale swirling like magnetization textures with topological protection which can be stabilized in ferromagnetic systems with Dzyaloshinskii-Moriya interaction1. Currently, skyrmion are being explored for promising candidate for spintronic based memory and logic applications with low power consumption, fast processing and high density. Ultrafast skyrmion dynamics in GHz-band offers potential for high frequency applications. Recent studies on skyrmion dynamics reveal that there are three types of skyrmion resonant modes: clockwise (CW), counter-clockwise (CCW) gyrotropic modes and breathing mode[2]. In this work, we present a systematic investigation on the microwave properties of the skyrmions in concentric and eccentric ring structures using micromagnetic simulations MuMax3 3. We have varied the inner diameter of concentric ring between 50 to 135 nm and a frequency shift of 2.2 GHz is observed as shown in Fig. 1(a). The frequency of skyrmion resonance mode increases when the diameter of the inner circle is increased. This can be attributed to the fact that the skyrmion experiences edge repulsion from the periphery of both inner and outer circles of the ring geometry. We have observed that the skyrmion is annihilated as the repulsion from the edges suppresses the topological stability of the skyrmion.The skyrmion dynamics are found to be extremely sensitive to the edge repulsions and a remarkably large frequency shift of 2 GHz of the skyrmion resonance modes is observed by simply varying the position of a skyrmion in an eccentric ring structure shown in Fig 1(b). It offers a potential route for two (binary) distinct frequency states by simply driving the skyrmion in different positions of the eccentric ring. Such tunable skyrmion modes can be achieved by driving a skyrmion between different locations using an oscillating magnetic field or current (spin torque effect). Such widely tunable resonant skyrmion modes may have potential implications for the realization of on-chip passive microwave devices such as filter and isolators.

Binary switching of gyrotropic modes of skyrmion in a nano-ring structure

Magnetic multilayer films are known to host a variety of novel magnetic configurations such as chiral domain walls and topological magnetic skyrmions with potential nano-electronic applications. However, characterizing these material systems typically involves a combination of several magnetometry, microscopy, and spectroscopy experiments, and some of these techniques can be time-consuming and expensive. Therefore, to rapidly characterize these material systems, it is crucial to maximize the information extracted from results of experiments that are more accessible, such as domain images obtained from magnetic force microscopy. We show that deep convolutional neural networks can extract magnetic parameters such as the exchange interaction, Dzyaloshinskii-Moriya interaction, and uniaxial anisotropy from images of domain configurations. Experimentally realistic training and validation data were generated through micromagnetic simulations. The trained models were consistently able to reach R2 values greater than 0.7 on validation data. By inspecting the intermediate feature maps of the neural network, we find that the network is able to learn features such as domain boundaries. Testing the models on actual experimental data yield values that were consistent with our knowledge of the material systems. Our work thus demonstrates the utility of developing machine models trained on simulation data as a means to accelerate the characterization of magnetic systems.

Estimation of magnetic parameters from domain images with convolutional neural networks

Demand for Artificial Intelligence (AI) has been growing due to their usefulness in commercial electronics and in tackling global issues, such as a pandemic. However, conventional von Neumann devices that use AI require extensive programming and power. Thus, significant attention is dedicated to the brain-inspired neuromorphic computing (NC) as a more power-efficient alternative and a suitable platform for AI. NC involves a network of interconnected synapses and neurons that mimic their biological counterparts. In this work, we present a DW-based artificial neuron that automatically resets without applying current.
We simulated a synthetic antiferromagnetic (SAF) DW device where the free layer (CoFeB) is driven by spin-transfer-torque (STT), achieving the integrate function. In the absence of current, the antiferromagnetic coupling (AFC) between the free and pinned layers pushes the DW back towards the starting position. This happens automatically, and is indicative of the leakage function. The integrate and leakage functions are also achieved experimentally, suggesting that an SAF-based neuron is viable for NC.

Synthetic antiferromagnetic domain wall device as artificial neuron for neuromorphic computing

Descriptor based predictive model can be efficient in identifying an optimal carbon-based catalyst for oxygen evolution reaction (OER). Here, we correlate the O atom adsorption strength with the OER activity of graphene nanoribbon systems and define the energy parameters to identify the overpotential (ɳ). The π electron-based descriptor can predict the catalytic activity of the graphene surfaces. Machine learning algorithms like Multiple Linear Regression (MLR), Random Forest Regression (RFR) and Support Vector Regression (SVR) are trained on the data generated by density functional theory to predict the energy parameter (ΔGO-ΔGOH) and overpotential. An optimal active site for OER using proposed SVR model is identified with the overpotential (0.29 V) and then validate through DFT calculations. Also, to generalize the study we used SVR model on nitrogen doped graphene system to predict the site-specific activity towards OER. Such a combined approach can be extended to estimate the site-specific OER activity of different carbon catalysts at a dramatically reduced computational cost.

Energy Parameter and Electronic Descriptor for Carbon Based Catalyst Predicted using QM/ML

Either experimental trial and error or density functional theory (DFT) calculations for screening electrocatalysts remain comparatively expensive and scale poorly with system size. Here, we develop a simple but accurate binding-energy prediction machine-learning model to accelerate the hydrogen evolution reaction (HER) catalyst screening. Based on around 20 000 DFT-calculated HER binding-energy datasets, we develop a simple machine-learning model with only four features based on the optimized finger mapping method. Our method predicts H binding energies with a mean absolute error (MAE) of 0.14 eV. It is much lower than that from the regression of the Voronoi tessellations-based crystal structure modeling method (0.27 eV) and crystal graph convolutional neural networks (0.19 eV), which requires hundred features or expensive training cost. Moreover, for a small dataset, our explainable model has superior performance to the deep neural network models.

Binding-energy prediction machine-learning models for accelerating HER catalyst screening

Defects including the pores and cracks significantly degrade the properties of as-built parts. The molten pool flow, particularly the keyhole effect, plays a key role in the defects that are difficult to observe through experiments in the selective laser melting process. In this study, we derived a new formulation of evaporation for metal alloys from thermodynamics considering the chemical compositions, and the formation of the keyhole was simulated using the Finite Volume Method (FVM) with the Volume of Fraction method to track the gas-liquid interface, where the multi-reflection of the laser within the keyhole and powder bed was incorporated using a ray-tracing algorithm. The geometry features of the simulated keyhole and melting pool showed a good agreement with the experimental observation by ultrahigh-speed X-ray imaging in Argonne National Lab. To understand the physical mechanisms of the keyhole, the laser energy distribution on the keyhole surface, and the driving forces (e.g. Marangoni force, recoil pressure) on the keyhole surface were further analyzed. The laser scanning simulations on a bare plate under external magnetic fields indicate that the Lorentz force can smooth the fluid flow field in the molten pool and increase the ratio of equiaxed grains in the solidication front.

Evaporation Model for Keyhole Dynamics During Additive Manufacturing of Metal

Historically, students of materials science and engineering were introduced to crystal chemical principles through examining physical models of simple crystals (typically metals or binary salts), latterly augmented by 3D visualisation of structures (the persistent interest in perovskites comes to mind). Both approaches have their strengths and weaknesses. Physical models cater well to tactile learners but are limited to simple lattices. Commonly, the concept of close packing is illustrated with large balls to demonstrate 2-layer hexagonal close packing (hcp) and 3-layer cubic close packing (ccp) with smaller balls occupying interstitial cavities (1). Such models are appreciated by students and feedback indicates improved appreciation of octahedral and tetrahedral co-ordination. A limitation of this approach is that interstitial sites are ‘hidden’ amongst the large balls, although this can be overcome through the use of transparent spheres (2). Ball and stick models remove some of these limitations, but are more appropriate for molecular chemistry, as they become rather intricate when dealing with ‘infinite’ lattices of even moderate complexity.

Computer-generated structure representations have the advantage of being readily scalable to larger classes, inexpensive and applicable to complex structures. Their use can also be integrated with crystal database extraction and introducing the basics of crystallographic information files (CIFs). However, the very ease of producing beautiful and dynamic drawings, especially with those packages that automatically locate and display the polyhedra can lead to a black-box exercise with poor educational outcomes. Nonetheless, such packages are de rigueur in crystal chemical research and students should be proficient in their use and the interpretation of topological relationships.

Precision polyhedral models bridge the gap between classical constructions from spheres or balls-and-sticks and contemporary computed representations. First, more complex structures can be assembled from polyhedral groupings. Second, the different topological connectivity between polyhedra (corner-, edge- face-sharing) becomes evident, as do changes in symmetry through displacive topological transformations. Third, the Wyckoff positions extracted from the CIFs can be readily identified, and finally, the physical models bear a striking resemblance to the 3D visualizations.

This video poster demonstrates the process and the value of exploiting close-packed kernels to explore the breadth of connectivity in several important structures. Using hemi-cuboctahedra the ccp kernel (a cuboctahedron) and the hcp kernel (a twinned cuboctahedron) are constructed. CIF for rutile (hcp), wurtzite (hcp), rhenia (ccp) and sphalerite (ccp) are drawn using 3D visualisation tools. Students then construct the polyhedral representations of these structures around the kernels.

Unfortunately, research concerning the synergistic benefits of the combined manipulation of physical and virtual models is sparse, but the anecdotal evidence is that a deeper appreciation of crystal chemical concepts results (3). No doubt students with different learning-strengths will benefit more-or-less from these complementary approaches, and for this reason, team learning where abstract thinkers produce the 3D visualisation and hands-on practitioners build the physical model can be especially instructive.

(1) Ma et al., Using Magnet-Embedded Silicone Balls to Construct Sable Models for Close-Packed Crystal structures, J. Che. Educ., 97, 4063-4068 (2020).
(2) Lenzer et al, Making it Clear: Exploring Crystal Structures by Constructing and Comparing See-Through Models. J. Chem. Educ. 2019, 96, 8, 1630–1639 (2019)
(3) Kelly, I Comparison of Virtual Models and Hands-on Models for Teaching Crystallography, (2001) Master Thesis, Purdue University


Precision Polyhedral Models for Crystal Chemical Instruction: Integrating Crystal Structure Databases and Enhancing the Appreciation of 3D Visualisation

ATLAS is a platform jointly developed by Nanyang Technological University (NTU) and Ludwig Maximilian University of Munich (LMU). It aims to facilitate global students and lifelong learners to have more flexible and personalized learning experiences. Its current largest deployment is an international graduate degree program in Materials Science, NTU. The initial-deployed courses were launched in 2016. The platform has delivered 14 courses to approximately 800 global graduate students during these years. Some of the features of ATLAS include dynamic reasoning maps, clickable transcript linked with video, diverse assessments including instant auto-marking MCQs, peer-review essay, and peer-review presentation. In the future, ATLAS is planned to facilitate cross referencing among courses, in order to provide more opportunities for personalized learning. Bases on students’ feedback, the flexibility, clear structure, and sufficient explanation of the concepts made them enjoy the courses. Further developments of ATLAS will enhance platform stability, formalize training of faculty in learning design and course conversion, regularize course operation and maintenance support, and improve the interaction between faculty and students by deploying hybrid learning and live tutorial sessions.

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