In this chapter, we present a detailed theoretical perspective of magnetic properties of graphene along with recent experimental validations. We extend our discussion towards the general two-dimensional materials beyond graphene in the light of their unprecedented magnetic properties.
2022
ACS
Light-Induced Hypoxia in Carbon Quantum Dots and Ultrahigh Photocatalytic Efficiency
Sanjit Mondal, Soumya Ranjan Das, Lipipuspa Sahoo, and 2 more authors
Carbon quantum dots (CQDs) represent a class of carbon materials exhibiting photoresponse and many potential applications. Here, we present a unique property that dissolved CQDs capture large amounts of molecular oxygen from the air, the quantity of which can be controlled by light irradiation. The O2 content can be varied between a remarkable 1 wt % of the CQDs in the dark to nearly half of it under illumination, in a reversible manner. Moreover, O2 depletion enhances away from the air–solution interface as the nearby CQDs quickly regain them from the air, creating a pronounced concentration gradient in the solution. We elucidate the role of the CQD functional groups and show that excitons generated under light are responsible for their tunable adsorbed-oxygen content. Because of O2 enrichment, the photocatalytic efficiency of the CQDs toward oxidation of benzylamines in the air is the same as under oxygen flow and far higher than the existing photocatalysts. The findings should encourage the development of a new class of oxygen-enricher materials and air as a sustainable oxidant in chemical transformations.
2021
Springer
Edge State Induced Spintronic Properties of Graphene Nanoribbons: A Theoretical Perspective
Soumya Ranjan Das, and Sudipta Dutta
In Carbon Nanomaterial Electronics: Devices and Applications , Feb 2021
Low-dimensional carbon-based nanomaterials have generated enormous interest in the scientific community due to their quantum confinement induced novel electronic, magnetic, optical, thermal, mechanical, and chemical properties. The synthesis of two-dimensional graphene has provided a fertile experimental platform for studying these exotic properties and harnessing them for promising carbon-based nanoelectronic devices. This chapter reviews the edge-induced spintronic properties of nanographene ribbons (GNR) from a theoretical perspective and discusses their possible applications in nanoscale devices. The presence of edges bears a crucial impact on the low-energy spectrum of the itinerant Dirac electrons in graphene. Nanoribbons with zigzag edges (ZGNR) possess robust localized edge states near the Fermi energy that induces ferrimagnetic spin polarization along the zigzag edge. In contrast, such localized edge states are absent in nanoribbons with armchair edges (AGNR). We discuss how applying a transverse electric field to ZGNRs, or chemical modification of its edges, can break the spin degeneracy and lead to a half-metallic state which shows spin polarization of the current or the spin-filtering behavior that is very crucial for spintronic device applications. The presence of edges also endows GNRs with peculiar transport properties characterized by the absence of Anderson localization, making them ideal for ultra-low power electronics. Our review highlights the edge states’ role as the origin of GNR’s diverse physical and chemical properties. It hence has a significant bearing on the future realization of carbon nanomaterial electronics.
2020
Nature Publishing
Ab-Initio Investigation of Preferential Triangular Self-Formation of Oxide Heterostructures of Monolayer WSe2
Soumya Ranjan Das, Katsunori Wakabayashi, Kazuhito Tsukagoshi, and 1 more author
Triangular growth patterns of pristine two-dimensional (2D) transition metal dichalcogenides (TMDs) are ubiquitous in experiments. Here, we use first-principles calculations to investigate the growth of triangular shaped oxide islands upon layer-by-layer controlled oxidation in monolayer and few-layer \{}hbox {WSe}_{2}\ WSe 2 systems. Pristine 2D TMDs with a trigonal prismatic geometry prefer the triangular growth morphology due to structural stability arising from the edge chalcogen atoms along its three sides. Our ab-initio energetics and thermodynamic study show that, since the Se atoms are more susceptible to oxygen replacement, the preferential oxidation happens along the Se zigzag lines, producing triangular islands of transition metal oxides. The thermodynamic stability arising from the preferential triangular self-formation of TMD based oxide heterostructures and their electronic properties opens a new avenue for their exploration in advanced electronic and optoelectronic devices.
AIP
Inter-Layer Interaction Induced Electronic Properties in Partially Oxidized Transition Metal Dichalcogenides
Soumya Ranjan Das, and Sudipta Dutta
In DAE Solid State Physics Symposium 2019, AIP Conference Proceedings , Dec 2020
We perform first-principles calculations to investigate the electronic properties of two dimensional (2D) self- assembled heterojunctions of partially oxidized mono- and bi-layer transition metal dichalcogenides (TMDs). Pristine 2D TMDs are direct band gap semiconductors. Selective oxidation leads to a broad spectrum of electronic properties, ranging from indirect band gap semiconductor to semi-metallic behavior. We tune the interlayer interaction in bilayer TMDs by selective oxidation of either top or bottom surface of the top layer with a partially oxidized bottom layer. In particular, we observe that the intrinsic inversion symmetry of the bilayer TMDs gets broken when the bottom surface of the top layer is fully oxidized. Moreover, interlayer charge transfer results in an unprecedented metallic state with completely decoupled non-degenerate bands near the Fermi energy, arising exclusively from either top or bottom layer. Our study reveals a huge possibility for these dichalcogenide based heterojunctions to be exploited in advanced optoelectronic and valleytronic device applications.
2019
ACS
Spin Filtering and Rectification in Lateral Heterostructures of Zigzag-Edge BC3 and Graphene Nanoribbons: Implications for Switching and Memory Devices
Heterostructures of different atomically thin nanomaterials provide an ideal platform of admixing their unique properties to achieve new functionalities. Here, we study the lateral heterostructures of graphene and BC3 nanoribbons with zigzag edge terminations within first-principles calculations. These heterostructures show localization of one kind of spin along the open graphene zigzag edge, whereas the other edge does not show any spin polarization. Therefore, the systems stabilize in the ferromagnetic ground state, inducing net magnetization and broken time reversal symmetry in each valley. Owing to this asymmetric spin distribution, the system shows spin-polarized current that is confirmed by further first-principles-based quantum transport calculations. The conduction behavior can be tuned further by changing the polarity of the side-gates in either sides of the ribbon edges. We observe higher spin polarization of current under forward gate bias but lower spin polarization of current under reverse gate bias. This functionality provides a unique way of spin filtering and rectification by changing the electrode polarity, which can find major applications in switching and memory storage devices.
2018
ACS
Layer-by-Layer Oxidation Induced Electronic Properties in Transition-Metal Dichalcogenides
Soumya Ranjan Das, Katsunori Wakabayashi, Mahito Yamamoto, and 2 more authors
Recent progress in transition-metal dichalcogenides has opened up new possibilities for atomically thin nanomaterial based electronic device applications. Here we investigate atomic-scale self-assembled heterojunction modulated by layer-by-layer controlled oxidation in monolayer and few-layer dichalcogenide systems and their electronic properties within a first-principles framework. Pristine dichalcogenide systems exhibit semiconducting behavior. We observe reduction of the band gap for partial oxidation of the top layer. However, complete oxidation of the top layer makes the system metallic, owing to the charge transfer from the pristine to the oxidized layer, as observed in recent experiments. When the bottom layer gets partially oxidized with fully oxidized top layers, the system shows unprecedented semimetallic behavior. The appearance of valence band maximum and conduction band minimum at different k-points can introduce valley polarization. Therefore, our study shows controlled oxidation induced varying electronic properties in dichalcogenide based heterojunctions that can be exploited for advanced electronic, optoelectronic, and valleytronic device applications.
