Ramamoorthy, Harihara

Preferred name

Alternative Name

Ramamoorthy, H.

Main Affiliation

School of Engineering

harihara.ra@kmitl.ac.th

Now showing 1 - 10 of 28

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Plasmon-mediated energy relaxation in graphene
(2015-12-28)
Ferry, D. K.
;
Somphonsane, R.
;
Ramamoorthy, H.
;
Bird, J. P.
Energy relaxation of hot carriers in graphene is studied at low temperatures, where the loss rate may differ significantly from that predicted for electron-phonon interactions. We show here that plasmons, important in the relaxation of energetic carriers in bulk semiconductors, can also provide a pathway for energy relaxation in transport experiments in graphene. We obtain a total loss rate to plasmons that results in energy relaxation times whose dependence on temperature and density closely matches that found experimentally.
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CVD Synthesis of MoS2 Using a Direct MoO2 Precursor: A Study on the Effects of Growth Temperature on Precursor Diffusion and Morphology Evolutions
(2023-07-01)
Somphonsane, Ratchanok
;
Chiawchan, Tinna
;
Bootsa-ard, Waraporn
;
Ramamoorthy, Harihara
In this study, the influence of growth temperature variation on the synthesis of MoS<inf>2</inf> using a direct MoO<inf>2</inf> precursor was investigated. The research showed that the growth temperature had a strong impact on the resulting morphologies. Below 650 °C, no nucleation or growth of MoS<inf>2</inf> occurred. The optimal growth temperature for producing continuous MoS<inf>2</inf> films without intermediate-state formation was approximately 760 °C. However, when the growth temperatures exceeded 800 °C, a transition from pure MoS<inf>2</inf> to predominantly intermediate states was observed. This was attributed to enhanced diffusion of the precursor at higher temperatures, which reduced the local S:Mo ratio. The diffusion equation was analyzed, showing how the diffusion coefficient, diffusion length, and concentration gradients varied with temperature, consistent with the experimental observations. This study also investigated the impact of increasing the MoO<inf>2</inf> precursor amount, resulting in the formation of multilayer MoS<inf>2</inf> domains at the outermost growth zones. These findings provide valuable insights into the growth criteria for the effective synthesis of clean and large-area MoS<inf>2</inf>, thereby facilitating its application in semiconductors and related industries.
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Spin Seebeck effect and large spin conversion in amorphous Fe2TiSb/polycrystalline Y3Fe5O12 thin films
(2024-05-30)
Wongjom, Poramed
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Wongjom, Chalothon
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Pongophas, Ekkarat
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Infahsaeng, Yingyot
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Maiaugree, Wasan
This study investigates spin current generation in a Fe<inf>2</inf>TiSb/Y<inf>3</inf>Fe<inf>5</inf>O<inf>12</inf> multi-layer thin film as prepared via the magnetron sputtering method. Comprehensive characterization techniques are employed to assess film properties, including X-ray diffraction, energy-dispersive X-ray spectroscopy, Scanning electron microscopy, and Vibrating sample magnetometer. The Y<inf>3</inf>Fe<inf>5</inf>O<inf>12</inf> material exhibits a polycrystalline ferromagnetic insulator behavior, while the 20 nm-thick Fe<inf>2</inf>TiSb film displays small ferromagnetic metal properties with an amorphous structure. Spin current analysis utilizes the longitudinal spin Seebeck effect configuration, considering magnetic field and temperature dependencies and the results show that spin conversion within the Fe<inf>2</inf>TiSb/Y<inf>3</inf>Fe<inf>5</inf>O<inf>12</inf> structure is influenced by both the spin Seebeck effect and the anomalous Nernst effect, resulting in an overall spin signal enhancement. The spin Seebeck coefficient of Fe<inf>2</inf>TiSb/Y<inf>3</inf>Fe<inf>5</inf>O<inf>12</inf> was approximately 0.103 μV/K within a magnetic field of 300 mT.
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Probing charge trapping and joule heating in graphene field-effect transistors by transient pulsing
(2017-07-24)
Ramamoorthy, H.
;
Somphonsane, R.
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Radice, J.
;
He, G.
;
Nathawat, J.
We use pulsed electrical studies to investigate the various processes that limit the current carrying capacity of graphene high frequency transistors. By investigating the transient response of these devices over a time scale that spans some twelve orders of magnitude, we identify the presence of four distinct processes that degrade the current: (1) charge injection into deep traps within the interior of the oxide; (2) Joule heating of the transistor substrate by hot carriers in the graphene channel; (3) equilibration of interfacial-state filling in response to voltage transients, and; (4) leakage of captured charge from the deep traps, once the pulsed voltage is removed. The time scale associated with these processes ranges from nanoseconds to hours, with process (1) being the fastest and process (4) the slowest. By pulsing the transistors on time intervals as short as a few nanoseconds, we therefore demonstrate how it is possible to obtain output characteristics from them that are essentially free from the influence of these different mechanisms. Under such conditions, the hot-carrier drift velocity is shown to saturate at the large values expected for intrinsic graphene. Beyond graphene, this approach of pulsed characterization of transistor performance should be broadly applicable to studies of other two-dimensional semiconductors, including transition-metal dichalcogenides, black phosphorous, silicene, and topological insulators.
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Signatures of hot carriers and hot phonons in the re-entrant metallic and semiconducting states of Moiré-gapped graphene
(2023-12-01)
Nathawat, Jubin
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Mansaray, Ishiaka
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Sakanashi, Kohei
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Wada, Naoto
;
Randle, Michael D.
Stacking of graphene with hexagonal boron nitride (h-BN) can dramatically modify its bands from their usual linear form, opening a series of narrow minigaps that are separated by wider minibands. While the resulting spectrum offers strong potential for use in functional (opto)electronic devices, a proper understanding of the dynamics of hot carriers in these bands is a prerequisite for such applications. In this work, we therefore apply a strategy of rapid electrical pulsing to drive carriers in graphene/h-BN heterostructures deep into the dissipative limit of strong electron-phonon coupling. By using electrical gating to move the chemical potential through the “Moiré bands”, we demonstrate a cyclical evolution between metallic and semiconducting states. This behavior is captured in a self-consistent model of non-equilibrium transport that considers the competition of electrically driven inter-band tunneling and hot-carrier scattering by strongly non-equilibrium phonons. Overall, our results demonstrate how a treatment of the dynamics of both hot carriers and hot phonons is essential to understanding the properties of functional graphene superlattices.
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Universal scaling of weak localization in graphene due to bias-induced dispersion decoherence
(2020-12-01)
Somphonsane, R.
;
Ramamoorthy, H.
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He, G.
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Nathawat, J.
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Yin, S.
The differential conductance of graphene is shown to exhibit a zero-bias anomaly at low temperatures, arising from a suppression of the quantum corrections due to weak localization and electron interactions. A simple rescaling of these data, free of any adjustable parameters, shows that this anomaly exhibits a universal, temperature- (T) independent form. According to this, the differential conductance is approximately constant at small voltages (V < k<inf>B</inf>T/e), while at larger voltages it increases logarithmically with the applied bias. For theoretical insight into the origins of this behaviour, which is inconsistent with electron heating, we formulate a model for weak-localization in the presence of nonequilibrium transport. According to this model, the applied voltage causes unavoidable dispersion decoherence, which arises as diffusing electron partial waves, with a spread of energies defined by the value of the applied voltage, gradually decohere with one another as they diffuse through the system. The decoherence yields a universal scaling of the conductance as a function of eV/k<inf>B</inf>T, with a logarithmic variation for eV/k<inf>B</inf>T > 1, variations in accordance with the results of experiment. Our theoretical description of nonequilibrium transport in the presence of this source of decoherence exhibits strong similarities with the results of experiment, including the aforementioned rescaling of the conductance and its logarithmic variation as a function of the applied voltage.
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Versatile, Low-Cost, and Portable 2D Material Transfer Setup with a Facile and Highly Efficient DIY Inert-Atmosphere Glove Compartment Option
(2021-07-20)
Buapan, Kanokwan
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Somphonsane, Ratchanok
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Chiawchan, Tinna
;
Ramamoorthy, Harihara
Research in van der Waals heterostructures has been rapidly progressing in the past decade, thanks to the art of sequential and deterministic placement of one two-dimensional (2D) material over another. The successful creation of heterostructures however has relied largely on expensive transfer systems that are not easily accessible to researchers. Although a few reports on low-cost systems have recently surfaced, the full functionality, portability features, and overall effectiveness of such systems are still being explored. In this work, we present an "all-in-one"low-cost transfer setup that is compact, lightweight, and portable and which can be quickly installed with a facile and do it yourself (DIY)-style anaerobic glovebox option that performs at par with commercial anaerobic systems. The "installable"glovebox option means the user has the convenience of quickly converting the working environment into an inert one when air-sensitive 2D materials are used. The lowest RH values obtained in our glovebox is <3%, and the O2 levels rapidly drop from 21% to less than 0.1% in just a few minutes of purging the chamber with inert gas. The transfer system is also equipped with a light-weight PID-controlled substrate heating option that can be easily assembled within just a few hours. We test the versatility of our low-cost system by the successful creation of hexagonal boron nitride (hBN)-encapsulated graphene and hBN-encapsulated molybdenum disulphide (MoS2) heterostructures using the hot pickup technique and graphene-hBN, MoS2-hBN, twisted MoS2, and twisted MoS2 on hBN stacks using the wetting technique, and a MoS2-hBN-graphene vertical tunneling heterostructure was formed using a combination approach. The effectiveness of the DIY glovebox is proven with the demonstration of extended stability of freshly exfoliated black phosphorous (BP) flakes, their encapsulation between thin hBN layers, and the formation of electrically contacted BP devices with a protective hBN top layer. At an overall price point of approximately 1000 $, the versatile setup presented here is expected to further contribute to the growth of research in 2D materials, in particular, for researchers initially faced with overcoming a huge entry-level threshold to work in the field of 2D materials and van der Waals heterostructures.
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Energy relaxation of hot carriers in graphene via plasmon interactions
(2016-03-01)
Ferry, D. K.
;
Somphonsane, R.
;
Ramamoorthy, H.
;
Bird, J. P.
Energy relaxation of hot carriers in graphene is studied theoretically and experimentally at low temperatures, where the loss rate may differ significantly from that predicted for electron–phonon interactions. We show here that plasmons, important in the relaxation of energetic carriers in bulk semiconductors, can also provide a pathway for energy relaxation in transport experiments in graphene. Reflecting the linear nature of graphene’s bands, we obtain a total loss rate to plasmons that is independent of carrier density. This results in energy relaxation times whose dependence on temperature and density closely matches that reported experimentally.
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Reversing hot-carrier energy-relaxation in graphene with a magnetic field
(2014-05-12)
Ramamoorthy, H.
;
Somphonsane, R.
;
He, G.
;
Ferry, D. K.
;
Ochiai, Y.
We investigate the influence of a perpendicular magnetic field on hot-carrier energy relaxation in bilayer graphene. Working in the regime of incipient Landau quantization, we find that the magnetic field influences the relaxation in a very different manner, dependent upon the position of the Fermi level relative to the Dirac point. While for carrier densities >10 <sup>12</sup> cm<sup>-2</sup> relaxation is slowed by the magnetic field, as the density of free carriers approaches zero it instead becomes quicker. We discuss this behavior in terms of the emergence of the zero-energy Landau level, and the role of charge puddling in graphene. © 2014 AIP Publishing LLC.
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Cost-Effective Experimental Setup for Studies of Spin Seebeck Effect and Electrical Transport in Thermoelectric Materials
(2020-06-01)
Wongjom, Poramed
;
Thongsamrit, Wannisa
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Ramamoorthy, Harihara
;
Chinwong, Suriya
;
Somphonsane, Ratchanok
In this article, we report on the design of a low-cost, accurate, and easy-to-implement roomerature experimental setup to comprehensively study the spin Seebeck effect (SSE) in ferrimagnetic insulators (FIs). Neodymium permanent magnets (NdPMs) are used to generate a fixed uniform magnetic field while the sample is staged in a custom-designed vacuum chamber containing thermal baths (for generating the required temperature gradient) and a complete 360° sample rotation mechanism (for studying the magnetic field angle dependence). Our experiments reveal excellent magnetic field uniformity (±1%) formed between the magnet poles, a highly accurate temperature gradient stability (±1%), and excellent agreement of the longitudinal SSE (LSSE) response for the Pt/YIG structure studied here with those reported in the literature. We also measure the anomalous Nernst effect (ANE) exhibited in a graphite sample, demonstrating the capability of our setup to accurately measure this parameter. Finally, we demonstrate that our setup can also be used to measure the conventional Seebeck effect (SE) and the electrical resistivity of commercially obtained Bi<inf>2</inf>Te<inf>3</inf> samples, making it a versatile tool for the broad characterization of thermoelectric materials.

Ramamoorthy, Harihara

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