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Dive into the research topics where Akinola D. Oyedele is active.

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Featured researches published by Akinola D. Oyedele.


Journal of the American Chemical Society | 2017

PdSe2: Pentagonal Two-Dimensional Layers with High Air Stability for Electronics

Akinola D. Oyedele; Shize Yang; Liangbo Liang; Alexander A. Puretzky; Kai Wang; Jingjie Zhang; Peng Yu; Pushpa Raj Pudasaini; Avik W. Ghosh; Zheng Liu; Christopher M. Rouleau; Bobby G. Sumpter; Matthew F. Chisholm; Wu Zhou; Philip D. Rack; David B. Geohegan; Kai Xiao

Most studied two-dimensional (2D) materials exhibit isotropic behavior due to high lattice symmetry; however, lower-symmetry 2D materials such as phosphorene and other elemental 2D materials exhibit very interesting anisotropic properties. In this work, we report the atomic structure, electronic properties, and vibrational modes of few-layered PdSe2 exfoliated from bulk crystals, a pentagonal 2D layered noble transition metal dichalcogenide with a puckered morphology that is air-stable. Micro-absorption optical spectroscopy and first-principles calculations reveal a wide band gap variation in this material from 0 (bulk) to 1.3 eV (monolayer). The Raman-active vibrational modes of PdSe2 were identified using polarized Raman spectroscopy, and a strong interlayer interaction was revealed from large, thickness-dependent Raman peak shifts, agreeing with first-principles Raman simulations. Field-effect transistors made from the few-layer PdSe2 display tunable ambipolar charge carrier conduction with a high electron field-effect mobility of ∼158 cm2 V-1 s-1, indicating the promise of this anisotropic, air-stable, pentagonal 2D material for 2D electronics.


Nano Letters | 2016

Tailoring Vacancies Far Beyond Intrinsic Levels Changes the Carrier Type and Optical Response in Monolayer MoSe2−x Crystals

Masoud Mahjouri-Samani; Liangbo Liang; Akinola D. Oyedele; Yong-Sung Kim; Mengkun Tian; Nicholas C.P. Cross; Kai Wang; Ming-Wei Lin; Abdelaziz Boulesbaa; Christopher M. Rouleau; Alexander A. Puretzky; Kai Xiao; Mina Yoon; Gyula Eres; Gerd Duscher; Bobby G. Sumpter; David B. Geohegan

Defect engineering has been a critical step in controlling the transport characteristics of electronic devices, and the ability to create, tune, and annihilate defects is essential to enable the range of next-generation devices. Whereas defect formation has been well-demonstrated in three-dimensional semiconductors, similar exploration of the heterogeneity in atomically thin two-dimensional semiconductors and the link between their atomic structures, defects, and properties has not yet been extensively studied. Here, we demonstrate the growth of MoSe2-x single crystals with selenium (Se) vacancies far beyond intrinsic levels, up to ∼20%, that exhibit a remarkable transition in electrical transport properties from n- to p-type character with increasing Se vacancy concentration. A new defect-activated phonon band at ∼250 cm(-1) appears, and the A1g Raman characteristic mode at 240 cm(-1) softens toward ∼230 cm(-1) which serves as a fingerprint of vacancy concentration in the crystals. We show that post-selenization using pulsed laser evaporated Se atoms can repair Se-vacant sites to nearly recover the properties of the pristine crystals. First-principles calculations reveal the underlying mechanisms for the corresponding vacancy-induced electrical and optical transitions.


Nanotechnology | 2017

High performance top-gated multilayer WSe2 field effect transistors

Pushpa Raj Pudasaini; Michael G. Stanford; Akinola D. Oyedele; Anthony T. Wong; Anna N. Hoffman; Dayrl P. Briggs; Kai Xiao; D. Mandrus; Thomas Ward; Philip D. Rack

In this paper, high performance top-gated WSe2 field effect transistor (FET) devices are demonstrated via a two-step remote plasma assisted ALD process. High-quality, low-leakage aluminum oxide (Al2O3) gate dielectric layers are deposited onto the WSe2 channel using a remote plasma assisted ALD process with an ultrathin (∼1 nm) titanium buffer layer. The first few nanometers (∼2 nm) of the Al2O3 dielectric film is deposited at relatively low temperature (i.e. 50 °C) and remainder of the film is deposited at 150 °C to ensure the conformal coating of Al2O3 on the WSe2 surface. Additionally, an ultra-thin titanium buffer layer is introduced at the WSe2 channel surface prior to ALD process to mitigate oxygen plasma induced doping effects. Excellent device characteristics with current on-off ratio in excess of 106 and a field effect mobility as high as 70.1 cm2 V-1 s-1 are achieved in a few-layer WSe2 FET device with a 30 nm Al2O3 top-gate dielectric. With further investigation and careful optimization, this method can play an important role for the realization of high performance top gated FETs for future optoelectronic device applications.


Nano Research | 2018

High-performance multilayer WSe2 field-effect transistors with carrier type control

Pushpa Raj Pudasaini; Akinola D. Oyedele; Cheng Zhang; Michael G. Stanford; Nicholas Cross; Anthony T. Wong; Anna N. Hoffman; Kai Xiao; Gerd Duscher; D. Mandrus; Thomas Ward; Philip D. Rack

In this study, high-performance multilayer WSe2 field-effect transistor (FET) devices with carrier type control are demonstrated via thickness modulation and a remote oxygen plasma surface treatment. Carrier type control in multilayer WSe2 FET devices with Cr/Au contacts is initially demonstrated by modulating the WSe2 thickness. The carrier type evolves with increasing WSe2 channel thickness, being p-type, ambipolar, and n-type at thicknesses <3, ∼4, and >5 nm, respectively. The thickness-dependent carrier type is attributed to changes in the bandgap of WSe2 as a function of the thickness and the carrier band offsets relative to the metal contacts. Furthermore, we present a strong hole carrier doping effect via remote oxygen plasma treatment. It non-degenerately converts n-type characteristics into p-type and enhances field-effect hole mobility by three orders of magnitude. This work demonstrates progress towards the realization of high-performance multilayer WSe2 FETs with carrier type control, potentially extendable to other transition metal dichalcogenides, for future electronic and optoelectronic applications.


ACS Nano | 2018

Photocarrier Transfer across Monolayer MoS2–MoSe2 Lateral Heterojunctions

Matthew Z. Bellus; Masoud Mahjouri-Samani; Samuel D. Lane; Akinola D. Oyedele; Xufan Li; Alexander A. Puretzky; David B. Geohegan; Kai Xiao; Hui Zhao

In-plane heterojuctions formed from two monolayer semiconductors represent the finest control of electrons in condensed matter and have attracted significant interest. Various device studies have shown the effectiveness of such structures to control electronic processes, illustrating their potentials for electronic and optoelectronic applications. However, information about the physical mechanisms of charge carrier transfer across the junctions is still rare, mainly due to the lack of adequate experimental techniques. Here we show that transient absorption measurements with high spatial and temporal resolution can be used to directly monitor such transfer processes. We studied MoS2-MoSe2 in-plane heterostructures fabricated by chemical vapor deposition and lithographic patterning followed by laser-generated vapor sulfurization. Transient absorption measurements in reflection geometry revealed evidence of exciton transfer from MoS2 to MoSe2. By comparing the experimental data with a simulation, we extracted an exciton transfer velocity of 104 m s-1. These results provide valuable information for understanding and controlling in-plane carrier transfer in two-dimensional lateral heterostructures for their electronic and optoelectronic applications.


ACS Applied Materials & Interfaces | 2018

Ion Migration Studies in Exfoliated 2D Molybdenum Oxide via Ionic Liquid Gating for Neuromorphic Device Applications

Cheng Zhang; Pushpa Raj Pudasaini; Akinola D. Oyedele; Anton V. Ievlev; Liubin Xu; Amanda Haglund; Joo Hyon Noh; Anthony T. Wong; Kai Xiao; Thomas Ward; David Mandrus; Haixuan Xu; Olga S. Ovchinnikova; Philip D. Rack

The formation of an electric double layer in ionic liquid (IL) can electrostatically induce charge carriers and/or intercalate ions in and out of the lattice which can trigger a large change of the electronic, optical, and magnetic properties of materials and even modify the crystal structure. We present a systematic study of ionic liquid gating of exfoliated 2D molybdenum trioxide (MoO3) devices and correlate the resultant electrical properties to the electrochemical doping via ion migration during the IL biasing process. A nearly 9 orders of magnitude modulation of the MoO3 conductivity is obtained for the two types of ionic liquids that are investigated. In addition, notably rapid on/off switching was realized through a lithium-containing ionic liquid whereas much slower modulation was induced via oxygen extraction/intercalation. Time of flight-secondary ion mass spectrometry confirms the Li intercalation. Density functional theory (DFT) calculations have been carried out to examine the underlying metallization mechanism. Results of short-pulse tests show the potential of these MoO3 devices as neuromorphic computing elements due to their synaptic plasticity.


ACS Applied Materials & Interfaces | 2018

Atmospheric and Long-term Aging Effects on the Electrical Properties of Variable Thickness WSe2 Transistors

Anna N. Hoffman; Michael G. Stanford; Cheng Zhang; Ilia N. Ivanov; Akinola D. Oyedele; Maria Gabriela Sales; Stephen McDonnell; Michael R. Koehler; David Mandrus; Liangbo Liang; Bobby G. Sumpter; Kai Xiao; Philip D. Rack

Atmospheric and long-term aging effects on electrical properties of WSe2 transistors with various thicknesses are examined. Although countless published studies report electrical properties of transition-metal dichalcogenide materials, many are not attentive to testing environment or to age of samples, which we have found significantly impacts results. Our as-fabricated exfoliated WSe2 pristine devices are predominantly n-type, which is attributed to selenium vacancies. Transfer characteristics of as-fabricated devices measured in air then vacuum reveal physisorbed atmospheric molecules significantly reduced n-type conduction in air. First-principles calculations suggest this short-term reversible atmospheric effect can be attributed primarily to physisorbed H2O on pristine WSe2, which is easily removed from the pristine surface in vacuum due to the low adsorption energy. Devices aged in air for over 300 h demonstrate irreversibly increased p-type conduction and decreased n-type conduction. Additionally, they develop an extended time constant for recovery of the atmospheric adsorbents effect. Short-term atmospheric aging (up to approximately 900 h) is attributed to O2 and H2O molecules physisorbed to selenium vacancies where electron transfer from the bulk and adsorbed binding energies are higher than the H2O-pristine WSe2. The residual/permanent aging component is attributed to electron trapping molecular O2 and isoelectronic O chemisorption at selenium vacancies, which also passivates the near-conduction band gap state, p-doping the material, with very high binding energy. All effects demonstrated have the expected thickness dependence, namely, thinner devices are more sensitive to atmospheric and long-term aging effects.


Advanced Functional Materials | 2017

High Conduction Hopping Behavior Induced in Transition Metal Dichalcogenides by Percolating Defect Networks: Toward Atomically Thin Circuits

Michael G. Stanford; Pushpa Raj Pudasaini; Elisabeth T. Gallmeier; Nicholas Cross; Liangbo Liang; Akinola D. Oyedele; Gerd Duscher; Masoud Mahjouri-Samani; Kai Wang; Kai Xiao; David B. Geohegan; Alex Belianinov; Bobby G. Sumpter; Philip D. Rack


JOM | 2016

Hierarchical Model for the Analysis of Scattering Data of Complex Materials

Akinola D. Oyedele; Nicholas W. McNutt; Orlando Rios; David J. Keffer


Carbon | 2018

The growth and assembly of organic molecules and inorganic 2D materials on graphene for van der Waals heterostructures

Akinola D. Oyedele; Christopher M. Rouleau; David B. Geohegan; Kai Xiao

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Kai Xiao

Oak Ridge National Laboratory

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Liangbo Liang

Oak Ridge National Laboratory

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Bobby G. Sumpter

Oak Ridge National Laboratory

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David B. Geohegan

Oak Ridge National Laboratory

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Pushpa Raj Pudasaini

University of Texas at San Antonio

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Alexander A. Puretzky

Oak Ridge National Laboratory

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