Ramamoorthy, Harihara

Preferred name

Alternative Name

Ramamoorthy, H.

Main Affiliation

School of Engineering

harihara.ra@kmitl.ac.th

Now showing 1 - 10 of 30

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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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Spin Seebeck effect and large spin conversion in amorphous Fe2TiSb/polycrystalline Y3Fe5O12 thin films
(2024-05-30)
Wongjom, Poramed
;
Wongjom, Chalothon
;
Pongophas, Ekkarat
;
Infahsaeng, Yingyot
;
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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In-situ current annealing of graphene-metal contacts
(2018-12-19)
Ramamoorthy, Harihara
;
Somphonsane, Ratchanok
We study the effect of current-induced annealing on contact resistance between graphene and metal contacts, demonstrating that this technique may be used as an effective in-situ annealing procedure to improve the graphene-metal contact resistance which has long been an issue in the characterization of graphene-based devices. By studying as many as 30 devices with varying sample sizes and geometry, we are able to reduce the overall resistance systematically to around 400 Ω⋅μm, which is competitive with the best values obtained in the literature to treat this problem. We also demonstrate the effectiveness of current annealing in desorbing contaminants from the surface of the graphene layer, simultaneously shifting the charge-neutrality point to zero back-gate voltage, thus allowing the tuning of carrier density on both the electron and hole sides of the Dirac spectrum.
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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
;
Mansaray, Ishiaka
;
Sakanashi, Kohei
;
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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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
;
Somphonsane, Ratchanok
;
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
;
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.
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Investigating the shunting effect in a Fe/Co ferromagnetic metal hybrid structure and its impact on the spin Seebeck effect
(2024-03-01)
Phumying, Santi
;
Wongjom, Chalothon
;
Pongophas, Ekkarat
;
Infahsaeng, Yingyot
;
Maiaugree, Wasan
The generation of spin voltage by heat, known as the spin Seebeck effect (SSE), involves the injection of spin current from a ferromagnetic to a normal metal. In this study, the shunting effect in SSE is investigated within a hybrid structure consisting of iron (Fe) and cobalt (Co) films deposited on a Si-wafer substrate using thermal evaporation [Si/Fe(500 nm)/Co(10 nm)]. Spin voltage measurements performed in the in-plane configuration revealed a voltage reversal in the Co film and Fe film. However, in the hybrid structure (Si/Fe/Co), the voltage signal exhibited consistent directionality. This intriguing observation hints at a potential shunting effect, wherein the voltage influence from the Fe layer contributes to the Co film. Consequently, it is deduced that a significant shunting effect occurs when the resistivity of Fe is approximately three orders of magnitude lower than that of the Co film. This insight sheds light on the intricate dynamics of spin thermoelectric applications, emphasizing the role of material properties in optimizing performance.
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Negative differential conductance & hot-carrier avalanching in monolayer WS2 FETs
(2017-12-01)
He, G.
;
Nathawat, J.
;
Kwan, C. P.
;
Ramamoorthy, H.
;
Somphonsane, R.
The high field phenomena of inter-valley transfer and avalanching breakdown have long been exploited in devices based on conventional semiconductors. In this Article, we demonstrate the manifestation of these effects in atomically-thin WS<inf>2</inf> field-effect transistors. The negative differential conductance exhibits all of the features familiar from discussions of this phenomenon in bulk semiconductors, including hysteresis in the transistor characteristics and increased noise that is indicative of travelling high-field domains. It is also found to be sensitive to thermal annealing, a result that we attribute to the influence of strain on the energy separation of the different valleys involved in hot-electron transfer. This idea is supported by the results of ensemble Monte Carlo simulations, which highlight the sensitivity of the negative differential conductance to the equilibrium populations of the different valleys. At high drain currents (>10 μA/μm) avalanching breakdown is also observed, and is attributed to trap-assisted inverse Auger scattering. This mechanism is not normally relevant in conventional semiconductors, but is possible in WS<inf>2</inf> due to the narrow width of its energy bands. The various results presented here suggest that WS<inf>2</inf> exhibits strong potential for use in hot-electron devices, including compact high-frequency sources and photonic detectors.

Ramamoorthy, Harihara

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