All-Oxide-Based Highly Transparent Photonic Synapse for Neuromorphic Computing

Kumar, Mohit,Abbas, Sohail,Kim, Joondong American Chemical Society 2018 ACS APPLIED MATERIALS & INTERFACES Vol.10 No.40

<P>The neuromorphic system processes enormous information even with very low energy consumption, which practically can be achieved with photonic artificial synapse. Herein, a photonic artificial synapse is demonstrated based on an all-oxide highly transparent device. The device consists of conformally grown In<SUB>2</SUB>O<SUB>3</SUB>/ZnO thin films on a fluorine-doped tin oxide/glass substrate. The device showed a loop opening in current-voltage characteristics, which was attributed to charge trapping/detrapping. Ultraviolet illumination-induced versatile features such as short-term/long-term plasticity and paired-pulse facilitation were truly confirmed. Further, photonic potentiation and electrical habituation were implemented. This study paves the way to develop a device in which current can be modulated under the action of optical stimuli, serving as a fundamental step toward the realization of low-cost synaptic behavior.</P> [FIG OMISSION]</BR>

Morphology-controlled synthesis of 3D flower-like NiWO₄ microstructure via surfactant-free wet chemical method

Kumar, Mohit,Lee, Young Hun,Kim, Min Seob,Jeong, Dong In,Kang, Bong Kyun,Yoon, Dae Ho Elsevier 2018 JOURNAL OF ALLOYS AND COMPOUNDS Vol.753 No.-

<P><B>Abstract</B></P> <P>Three-dimensional (3D) flower-like controllable microstructures of nickel tungsten oxide (NiWO<SUB>4</SUB>) were synthesized by using a surfactant free wet chemical method at low temperature (60 °C). The uniform morphologies of NiWO<SUB>4</SUB> were molded by adjusting the temperature, stirring rate, and dosage of hydrazine in aqueous solution. We reported immediate precipitation of the 3D NiWO<SUB>4</SUB> microflowers (MFs) by introducing hydrazine monohydrate (N<SUB>2</SUB>H<SUB>4</SUB>·H<SUB>2</SUB>O). This approach resulted in a better yield and shortened the reaction time. In this work, we investigated the probable mechanism behind the formation of flower-like microstructures. Furthermore, the flower-like microstructure was assessed for its electrochemical properties towards non-enzymatic glucose sensing by addition of 50 μL of a glucose solution (50 μM) in an electrolyte (0.1 M NaOH) using glassy carbon electrode.</P> <P><B>Highlights</B></P> <P> <UL> <LI> 3D NiWO<SUB>4</SUB> MFs was successfully synthesized through a wet chemical method. </LI> <LI> 3D flower-like morphology was transformed by introducing hydrazine. </LI> <LI> 3D NiWO<SUB>4</SUB> MFs exhibits good electrochemical performance in terms of sensing glucose. </LI> </UL> </P>

A non-volatile “programmable” transparent multilevel ultra-violet perovskite photodetector

Kumar, Mohit,Kim, Hong-Sik,Park, Dae Young,Jeong, Mun Seok,Kim, Joondong The Royal Society of Chemistry 2018 Nanoscale Vol.10 No.24

<P>Due to their outstanding physical properties, perovskite materials are considered to be promising semiconductors for next-generation optoelectronics. However, these materials are often unstable under an ambient atmosphere and ultra-violet illumination. Therefore, the construction of an air-stable visible light transparent perovskite-based ultra-violet photodetector is still highly challenging. In this study, we go beyond the conventional operation of photodetectors by utilizing the undesired hysteresis loop in the typical current-voltage characteristics of perovskites and design a (C4H9NH3)2PbBr4-based high-performance visible transparent programmable ultra-violet photodetector. The photodetector shows multiple operating levels and can switch from one level to another with a short electric pulse. The photodetector exhibits a fast response time of ∼2 ms, good responsivity of ∼32 mA W<SUP>−1</SUP> and detectivity of 8.5 × 10<SUP>8</SUP> Jones with a low working voltage of 0.5 V. Moreover, the photodetector shows long-term stability, and the optoelectronic performance is retained under ambient conditions. This breakthrough in the controlled tunable features opens a new avenue for the development of multipurpose transparent optoelectronic devices.</P>

Flexoelectric effect driven colossal triboelectricity with multilayer graphene

Kumar Mohit,Lim Jaeseong,Park Ji-Yong,서형택 한국물리학회 2021 Current Applied Physics Vol.32 No.-

Converting mechanical deformation from surrounding environment into detectable electrical signals remains one of the most attractive fields due to its potential applications in sustainable energy harvesting, self-powered sensors, and others. Presently, deformation energy is harvested by generating voltage/current through bending/twisting of piezoelectric materials, but its recyclability is limited in number. In contrast, polarization is generated in all known insulators/semiconductors due to elastic strain gradient, which offers unique electromechanical coupling and in turn, could generate significant potential differences to drive charge transfer. Here, we demonstrate that extremely high direct current with density of 28 × 106 A m 2 is generated without need of any external power supply by applying pointed force using conductive-atomic force microscope (cAFM) tip on multilayer graphene/substrate (SiO2, Si, glass). Further, the ramp-dependent time-resolved current is measured at a localized point, which indicates that pointed force-induced flexoelectric potential differences are the main driving factor to utilize mechanoelectrical coupling and in turn generate high current density. This research work provides a new strategy to utilize the flexoelectric effect to utilize electromechanical coupling to generate giant energy harvesting, which will have a potential impact on the various multiple fields including smart devices, materials, and even a fundamental understanding of physics.

Highly stable functionalized cuprous oxide nanoparticles for photocatalytic degradation of methylene blue

Kumar, Mohit,Das, Rupasree Ragini,Samal, Monica,Yun, Kyusik Elsevier 2018 Materials chemistry and physics Vol.218 No.-

<P><B>Abstract</B></P> <P>We report the synthesis and characterization of Cu<SUB>2</SUB>O nanoparticles (NPs) in the presence of the coordinating ligands like <SMALL>L</SMALL>-glutamic acid (Glu-Cu<SUB>2</SUB>O) and <SMALL>D</SMALL>-glucosamine (GlcN-Cu<SUB>2</SUB>O). Even if –NH<SUB>2</SUB> group is common to both the ligands, <SMALL>L</SMALL>-glutamic acid is a straight chain compound containing additional two –COOH groups, whereas, glucosamine is a cyclic compound with additional four –OH groups and an ether (-O-) linkage. Thus the coordinating ligands are different in the nature of the functional groups as well as their molecular skeletons leading to cubic morphology (∼350 nm of average edge length) for <SMALL>L</SMALL>-glutamic acid coordinated Cu<SUB>2</SUB>O and spherical morphology (∼250 nm of average diameter) for glucosamine one as observed by Field Effective Scanning Electron Microscopy (FE-SEM) and Transmission Electron Microscopy (TEM) images. The observed band gap of Cu<SUB>2</SUB>O of 1.89 eV is decreased to 1.84 ev for both the ligated structures. Both the ligated cuprous oxide (Cu<SUB>2</SUB>O) nanoparticles (NPs) were used for the photocatalytic degradation of methylene Blue (MB). The spherical GlcN-Cu<SUB>2</SUB>O showed 98% degradation of MB in 105 min and after 3 cycles of operation, the photocatalytic activity was enough for the 93% degradation of the dye, whereas, the cubic Gu-Cu<SUB>2</SUB>O could degrade MB up to 97% in 135 min and after three cycles, only 86% of the dye was degraded. This indicates, the molecular skeleton and functional groups on the ligand control the shape and size of the Cu<SUB>2</SUB>O nanoparticles as well as the photocatalytic efficiency and stability. It is presumed, the cyclic glucosamine ligand can protect the Cu<SUB>2</SUB>O nanoparticles from erosion during the degradation process.</P> <P><B>Highlights</B></P> <P> <UL> <LI> Different structure of functionalized Cu<SUB>2</SUB>O was synthesized. </LI> <LI> NH<SUB>2</SUB> group is common to both the ligands. </LI> <LI> Photocatalytic dye degradation was done. </LI> <LI> Synthesized nanomaterials were highly stable up to third cycle. </LI> </UL> </P>

Growth of Wafer-Scale ReS₂ with “Tunable” Geometry toward Electron Field-Emission Application

Kumar, Mohit,Park, Dae Young,Singh, Ranveer,Jeong, Mun Seok,Som, Tapobrata,Kim, Joondong American Chemical Society 2019 ACS APPLIED MATERIALS & INTERFACES Vol.11 No.39

<P>Despite high potential, the promise of 2D materials has not been realized practically because of limits of tiny grown size and difficult manipulation of the active spot. The utilization of 2D layers is the ultimate approach, which should be supported by large-scale production. In this very first report, we demonstrate the wafer-scale production of ReS<SUB>2</SUB> using the conventional sputtering method. The controllability of ReS<SUB>2</SUB> geometry has been investigated to form typical thin films or vertically aligned layers that are further applied for field emission. The vertically aligned ReS<SUB>2</SUB> layers exhibit ultralow turn-on electric field (0.6 V μm<SUP>-1</SUP>) with the current density (0.6 mA cm<SUP>-2</SUP>) and significantly low threshold electric field (0.8 V μm<SUP>-1</SUP>), respectively, along with outstanding emission stability. The results are attributed to weakly coupled ReS<SUB>2</SUB> layers and the high geometrical field enhancement factor (∼1.08 × 10<SUP>5</SUP>). Further, Kelvin probe force microscopy measurements confirm that lowering the work function is not solely responsible to achieve the ultralow operative field. Moreover, finite element simulation suggests that not only the length, width, and separation of the nanostructures but also the local slope plays an important role in suppressing screening effects.</P> [FIG OMISSION]</BR>

FIXED POINT OF α − ψ−CONTRACTIVE MULTIFUNCTION IN FUZZY METRIC SPACES

Mohit Kumar,Ritu Arora 한국전산응용수학회 2017 Journal of applied mathematics & informatics Vol.35 No.3

Recently Samet, Vetro and Vetro introduced the notion of α− ψ−contractive type mappings and initiated some fixed point theorems in complete metric spaces. The notion of α∗ − ψ−contractive multifunctions and initiated some fixed point results by Hasanzade Asl et. al. [8]. In this paper, we introduced the notion of α∗ − ψ−contractive multifunctions in a fuzzy metric space and gave fixed point results for these multifunctions in complete fuzzy metric spaces. We also obtain a fixed point results for self-maps in complete fuzzy metric spaces satisfying contractive condition.

Vertically aligned crystalline SnS layers-based NIR photodetector governed by pyro-phototronic effect

Kumar, Mohit,Patel, Malkeshkumar,Kim, JoohnSheok,Kim, Joondong,Kim, Byung Soo Elsevier 2018 Materials letters Vol.213 No.-

<P><B>Abstract</B></P> <P>Vertically aligned crystalline SnS layers were applied for the high-performing near-infrared (NIR) photodetector. Sputtering method was used to align the vertical SnS layers with an interlayer space of 0.56 nm on a large-scale Si substrate. Raman and XPS analyses confirmed the quality of SnS layers. Under NIR illumination, the SnS/Si photodetector showed the ultra-fast photoresponses (rise time of ∼12 μs and fall time of ∼42 μs) without an external bias. Due to the effect of the pyro-phototronic potential, high-performing photodetectivity was achieved to be 4.2 × 10<SUP>14</SUP> Jones. This excellent performance was attributed to the photon-induced pyroelectric effect in the vertically grown SnS layers. This novel approach of 2-dimensitonal material will open a new possibility to design ultra-performing photoelectric devices, including photodetectors and solar energy devices.</P> <P><B>Highlights</B></P> <P> <UL> <LI> Vertically aligned SnS layers were applied for the high-performing photodetector. </LI> <LI> Sputtering method is effective to grow crystalline SnS for large area. </LI> <LI> Pyro-phototronic potential induces the extremely enhanced photodetectivity. </LI> <LI> Fast photoresponses were achieved (rise time of ∼12 μs and fall time of ∼42 μs). </LI> <LI> Functional use of 2-dimensional material would enhance photoelectric performances. </LI> </UL> </P>

Optoelectronic Coupled Memorizing Behavior of Perovskite-based Memristor

Mohit Kumar,Dae Young Park,Hong-Sik Kim,Mun Seok Jeong,Joondong Kim 한국진공학회 2018 한국진공학회 학술발표회초록집 Vol.2018 No.8

High-performing ultrafast transparent photodetector governed by the pyro-phototronic effect

Kumar, Mohit,Patel, Malkeshkumar,Nguyen, Thanh Tai,Kim, Joondong,Yi, Junsin Royal Society of Chemistry 2018 Nanoscale Vol.10 No.15

<P>In this work we utilized the advantage of the photo-induced pyroelectric effect - known as “Pyro-phototronic” - to design a self-powered, ultrafast, transparent ultraviolet (UV, 365 nm) photodetector. The device architecture contains an UV absorbing pyroelectric ZnO layer sandwiched between hole-selective V2O5 and a bottom ITO electrode. In addition, the device shows a high optical transmittance, >70%, in the entire visible region. The photo current of the device was enhanced from 19 to 42 μA under pulsed UV light illumination (<I>λ</I> = 365 nm, 4 mW cm<SUP>−2</SUP>) by exploiting the pyro-phototronic potential. In addition, the photodetector demonstrated ultrafast responses of ∼4 μs for the rise time and ∼16 μs for the fall time. Further, a high photoresponsivity of ∼36.34 mA W<SUP>−1</SUP> and excellent photodetectivity of ∼6.04 × 10<SUP>14</SUP> Jones, with an enhancement of 725% in both due to the pyroelectric potential, were measured. This novel approach will open a new path to design transparent and ultrafast devices, as well as on the flexible substrates, for future optoelectronic applications.</P>