M. Bala Murali Krishna

Bio: M. Bala Murali Krishna is an academic researcher from Okinawa Institute of Science and Technology. The author has contributed to research in topics: Graphene & Absorption (electromagnetic radiation). The author has an hindex of 11, co-authored 28 publications receiving 612 citations. Previous affiliations of M. Bala Murali Krishna include Indian Institute of Technology, Jodhpur & University of Hyderabad.

Synthesis and structural, spectroscopic and nonlinear optical measurements of graphene oxide and its composites with metal and metal free porphyrins

Abstract: In this paper we present the structure and spectroscopic, photophysical and nonlinear optical studies of covalently functionalized novel graphene oxide–[Cu, Zn, Sn, H2 (metal free), VO] porphyrin composites. The composites were characterized by Field Enhanced Scanning Electron Microscopy (FE-SEM), micro-Raman, optical absorption, Fourier transform infrared (FT-IR), steady state and time resolved fluorescence spectroscopic techniques. The composites exhibit strong fluorescence quenching, suggesting strong electronic interactions between the porphyrin and graphene oxide molecules. Nonlinear optical absorption (NLA) studies of graphene oxide–porphyrin composites were investigated using the Z-scan technique at 532 and 800 nm with nanosecond (ns) and femtosecond (fs) laser pulses. Composites show strong two-photon absorption (TPA) as well as excited state absorption (ESA) leading to reverse saturable absorption (RSA) behaviour in the ns regime and saturable absorption (SA) behaviour was observed in fs regime. The metal free porphyrin–graphene oxide (GO) composite shows significant nonlinear absorption behaviour as well as highest fluorescence quenching behaviour compared to other GO–porphyrin composites. We further observed the enhanced figure of merit (FOM) values for composites in comparison with individual molecules.

Imaging the motion of electrons across semiconductor heterojunctions

Abstract: The flow of photoexcited electrons in a type-II heterostructure can be imaged with energy, spatial and temporal resolution. Technological progress since the late twentieth century has centred on semiconductor devices, such as transistors, diodes and solar cells1,2,3,4,5,6,7,8. At the heart of these devices is the internal motion of electrons through semiconductor materials due to applied electric fields3,9 or by the excitation of photocarriers2,4,5,8. Imaging the motion of these electrons would provide unprecedented insight into this important phenomenon, but requires high spatial and temporal resolution. Current studies of electron dynamics in semiconductors are generally limited by the spatial resolution of optical probes, or by the temporal resolution of electronic probes. Here, by combining femtosecond pump–probe techniques with spectroscopic photoemission electron microscopy10,11,12,13, we imaged the motion of photoexcited electrons from high-energy to low-energy states in a type-II 2D InSe/GaAs heterostructure. At the instant of photoexcitation, energy-resolved photoelectron images revealed a highly non-equilibrium distribution of photocarriers in space and energy. Thereafter, in response to the out-of-equilibrium photocarriers, we observed the spatial redistribution of charges, thus forming internal electric fields, bending the semiconductor bands, and finally impeding further charge transfer. By assembling images taken at different time-delays, we produced a movie lasting a few trillionths of a second of the electron-transfer process in the photoexcited type-II heterostructure—a fundamental phenomenon in semiconductor devices such as solar cells. Quantitative analysis and theoretical modelling of spatial variations in the movie provide insight into future solar cells, 2D materials and other semiconductor devices.

Nonlinear optical properties of covalently linked graphene-metal porphyrin composite materials

Abstract: The nonlinear optical (NLO) and optical limiting (OL) properties of covalently linked graphene-porphyrin composite materials were investigated using Z-scan technique in nanosecond regime. We observed enhanced NLO and OL properties of graphene-porphyrin composites in comparison to the individual graphene and porphyrins. The improved OL property of composites is attributed to energy transfer between porphyrin and graphene, which improved excited state absorption and nonlinear scattering. Nonlinear optical susceptibilities χ(3) of graphene and graphene-porphyrin composites were, in the order of 10−12 esu, measured using degenerate four wave mixing technique in nanosecond regime. High values of excited state absorption and two-photon absorption were observed for the composites.

Ultrafast Charge Transfer and Enhanced Absorption in MoS2–Organic van der Waals Heterojunctions Using Plasmonic Metasurfaces

Abstract: Hybrid organic–inorganic heterostructures are attracting tremendous attention for optoelectronic applications due to their low-cost processing and high performance in devices. In particular, van der Waals p–n heterojunctions formed between inorganic two-dimensional (2D) materials and organic semiconductors are of interest due to the quantum confinement effects of 2D materials and the synthetic control of the physical properties of organic semiconductors, enabling a high degree of tunable optoelectronic properties for the heterostructure. However, for photovoltaic applications, hybrid 2D–organic heterojunctions have demonstrated low power conversion efficiencies due to the limited absorption from constraints on the physical thickness of each layer. Here, we investigate the ultrafast charge transfer dynamics between an organic polymer:fullerene blend and 2D n-type MoS2 using transient pump–probe reflectometry. We employ plasmonic metasurfaces to enhance the absorption and charge photogeneration within the p...

Ultrafast Intrinsic Photoresponse and Direct Evidence of Sub-gap States in Liquid Phase Exfoliated MoS2Thin Films

Abstract: Ultrafast Intrinsic Photoresponse and Direct Evidence of Sub-gap States in Liquid Phase Exfoliated MoS 2 Thin Films

An Essay on the Shaking Palsy

Abstract: PREFACE The advantages which have been derived from the caution with which hypothetical statements are admitted, are in no instance more obvious than in those sciences which more particularly belong to the healing art. It therefore is necessary, that some conciliatory explanation should be offered for the present publication: in which, it is acknowledged, that mere conjecture takes the place of experiment; and, that analogy is the substitute for anatomical examination, the only sure foundation for pathological knowledge. When, however, the nature of the subject, and the circumstances under which it has been here taken up, are considered, it is hoped that the offering of the following pages to the attention of the medical public, will not be severely censured. The disease, respecting which the present inquiry is made, is of a nature highly afflictive. Notwithstanding which, it has not yet obtained a place in the classification of nosologists; some have regarded its characteristic symptoms as distinct and different diseases, and others have given its name to diseases differing essentially from it; whilst the unhappy sufferer has considered it as an evil, from the domination of which he had no prospect of escape. The disease is of long duration: to connect, therefore, the symptoms which occur in its later stages with those which mark its commencement, requires a continuance of observation of the same case, or at least a correct history of its symptoms, even for several years. Of both these advantages the writer has had the opportunities of availing himself, and has hence been led particularly to observe several other cases in which the disease existed in different stages of its progress. By these repeated observations, he hoped that he had been led to a probable conjecture as to the nature of the malady, and that analogy had suggested such means as might be productive of relief, and perhaps even of cure, if employed before the disease had been too long established. He therefore considered it to be a duty to submit his opinions to the examination of others, even in their present state of immaturity and imperfection. To delay their publication did not, indeed, appear to be warrantable. The disease had escaped particular notice; and the task of ascertaining its nature and cause by anatomical investigation, did not seem likely to be taken up by those who, from their abilities and opportunities, were most likely to accomplish it. That these friends to humanity and medical science, who have already unveiled to us many of the morbid processes by which health and life is abridged, might be excited to extend their researches to this malady, was much desired; and it was hoped, that this might be procured by the publication of these remarks. Should the necessary information be thus obtained, the writer will repine at no censurewhich the precipitate publication of mere conjectural suggestions may incur: but shall think himself fully rewarded by having excited the attention of those, who may point out the most appropriate means of relieving a tedious and most distressing malady.

Dirac Semimetal in Three Dimensions

Abstract: We show that the pseudorelativistic physics of graphene near the Fermi level can be extended to three dimensional (3D) materials. Unlike in phase transitions from inversion symmetric topological to normal insulators, we show that particular space groups also allow 3D Dirac points as symmetry protected degeneracies. We provide criteria necessary to identify these groups and, as an example, present ab initio calculations of β-cristobalite BiO(2) which exhibits three Dirac points at the Fermi level. We find that β-cristobalite BiO(2) is metastable, so it can be physically realized as a 3D analog to graphene.

Low-dimensional catalysts for hydrogen evolution and CO2 reduction

Abstract: Low-dimensional materials and their hybrids have emerged as promising candidates for electrocatalytic and photocatalytic hydrogen evolution and CO2 conversion into useful molecules. Progress in synthetic methods for the production of catalysts coupled with a better understanding of the fundamental catalytic mechanisms has enabled the rational design of catalytic nanomaterials with improved performance and selectivity. In this Review, we analyse the state of the art in the implementation of low-dimensional nanomaterials and their van der Waals heterostructures for hydrogen evolution and CO2 reduction by electrocatalysis and photocatalysis. We explore the mechanisms involved in both reactions and the different strategies to further optimize the activity, efficiency and selectivity of low-dimensional catalysts. The electrochemical oxidation and reduction of water and carbon dioxide are associated with the release or storage of energy. This Review reports the latest developments in the design and use of low-dimensional materials and their van der Waals heterostructures for electrocatalytic and photocatalytic hydrogen evolution and CO2 conversion.

Ultrafast Optical-Pump Terahertz-Probe Spectroscopy of the Carrier Relaxation and Recombination Dynamics in Epitaxial Graphene

Abstract: The ultrafast relaxation and recombination dynamics of photogenerated electrons and holes in epitaxial graphene are studied using optical-pump Terahertz-probe spectroscopy. The conductivity in graphene at Terahertz frequencies depends on the carrier concentration as well as the carrier distribution in energy. Time-resolved studies of the conductivity can therefore be used to probe the dynamics associated with carrier intraband relaxation and interband recombination. We report the electron-hole recombination times in epitaxial graphene for the first time. Our results show that carrier cooling occurs on sub-picosecond time scales and that interband recombination times are carrier density dependent.