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Pulickel M. Ajayan

Bio: Pulickel M. Ajayan is an academic researcher from Rice University. The author has contributed to research in topic(s): Carbon nanotube & Graphene. The author has an hindex of 176, co-authored 1223 publication(s) receiving 136241 citation(s). Previous affiliations of Pulickel M. Ajayan include University of Hawaii at Manoa & University of Florida.
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Abstract: Hydrogenated diamond has been regarded as a promising material in electronic device applications, especially in field-effect transistors (FETs). However, the quality of diamond hydrogenation has not yet been established, nor has the specific orientation that would provide the optimum hydrogen coverage. In addition, most theoretical work in the literature use models with 100% hydrogenated diamond surfaces to study electronic properties, which could be unreachable experimentally. In this work, we have carried out a detailed study using fully atomistic reactive molecular dynamics (MD) simulations on low indices diamond surfaces i.e. (0 0 1), (0 1 3), (1 1 0), (1 1 3), and (1 1 1) to evaluate the quality and hydrogenation thresholds on different diamond surfaces and their possible effects on electronic properties. Our simulation results indicate that the 100% surface hydrogenation on these surfaces is hard to achieve because of the steric repulsion between the terminated hydrogen atoms. Among all the considered surfaces, the (0 0 1), (1 1 0), and (1 1 3) surfaces incorporate a larger number of hydrogen atoms and passivate the surface dangling bonds. Our results on hydrogen stability also suggest that these surfaces with optimum hydrogen coverage are robust under extreme conditions and could provide homogeneous p-type surface conductivity on the diamond surfaces, a key requirement for high-field, high-frequency device applications.

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S. S. Mishra1, S. S. Mishra2, Anurag Bajpai1, Thakur Prasad Yadav2  +10 moreInstitutions (6)
Abstract: The present study reports the evolution of microstructure and magnetic properties of Cu2MnAl1-xGax Heusler alloys. The L21 phase, which remained stable up to a Ga substitution of x=0.3, transformed into mixed phases, i.e., hexagonal close packed (HCP) and complex cubic structure (CCS) on further addition of Ga. The non-monotonic increase in the lattice constant indicates that some amount of Ga retains its monovalent state. The magnetic phase showed a transition from ferromagnetic to paramagnetic state with increasing Ga concentration. The re-entrant temperature (TR) decreased from 8.2 K to 5.2 K as Ga content increased from 5 at% to 10 at%. The strength of magnetic exchange-coupling also reduced with an increase in Ga content. Gaussian process regression (GPR) was used to estimate the lattice parameter using the ionic radii and Pauling electronegativity of the constituents. The modeling approach showed high accuracy and stability, providing new insights into future alloy development.

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Mengmeng Fan1, Mengmeng Fan2, Jiewu Cui3, Jiewu Cui1  +11 moreInstitutions (6)
Abstract: N-doped carbon-based single-atom catalysts (NC-SACs) are widely researched in various electrochemical reactions due to high metal atom utilization and catalytic activity. The catalytic activity of NC-SACs originates from the coordinating structure between single metal site (M) and the doped nitrogen (N) in carbon matrix by forming M-Nx-C structure (1≤ x≤ 4). The M-N4-C structure is widely considered to be the most stable and effective catalytic site. However, there is no in-depth research for the “x” modulation in Pt-Nx-C structure and the corresponding catalytic properties. Herein, atomically dispersed Pt on N-doped carbon (Pt-NC) with Pt-Nx-C structure (1≤ x≤ 4), as a research model, is fabricated by a ZIF-8 template and applied to catalytic oxygen reduction. Different carbonization temperatures are used to control N loss, and then modulate the N coordination of Pt-Nx-C structure. The Pt-NC has the predictable low half-wave potential (E1/2) of 0.72 V vs RHE compared to the Pt/C 20% of 0.81V due to low Pt content. Remarkably, the Pt-NC shows a high onset potential (1.10 V vs RHE, determined for j = -0.1 mA cm2) and a high current density of 5.2 mA cm−2, more positive and higher than that of Pt/C 20% (0.96 V) and 4.9 mA cm−2, respectively. As the structural characterization and DFT simulation confirmed, the reducing Pt-N coordination number induces low valence of Pt atoms and low free energy of oxygen reduction, which is responsible for the improved catalytic activity. Furthermore, the Pt-NC shows high mass activity (172 times higher than that of Pt/C 20%), better stability and methanol crossover resistance.

6 citations

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Peter Serles1, Taib Arif1, Anand B. Puthirath2, Shwetank Yadav1  +12 moreInstitutions (4)
Abstract: Two-dimensional (2D) materials are known to have low-friction interfaces by reducing the energy dissipated by sliding contacts. While this is often attributed to van der Waals (vdW) bonding of 2D m...

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Xu Zhang1, Xiang Zhang2, Pulickel M. Ajayan2, Jianguo Wen3  +1 moreInstitutions (3)
16 Nov 2021-Small
Abstract: Annihilation of vacancy clusters in monolayer molybdenum diselenide (MoSe2 ) under electron beam irradiation is reported. In situ high-resolution transmission electron microscopy observation reveals that the annihilation is achieved by diffusion of vacancies to the free edge near the vacancy clusters. Monte Carlo simulations confirm that it is energetically favorable for the vacancies to locate at the free edge. By computing the minimum energy path for the annihilation of one vacancy cluster as a case study, it is further shown that electron beam irradiation and pre-stress in the suspended MoSe2 monolayer are necessary for the vacancies to overcome the energy barriers for diffusion. The findings suggest a new mechanism of vacancy healing in 2D materials and broaden the capability of electron beam for defect engineering of 2D materials, a promising way of tuning their properties for engineering applications.

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Wu Qi1, Wu Qi2, Wenrui Wang2, Wenrui Wang1  +8 moreInstitutions (3)
Abstract: CoCrFeNi high entropy alloy (HEA) has attracted extensive attention due to its excellent corrosion resistance, but the low strength limits its engineering application prospects. In order to develop CoCrFeNi based HEAs with high strength, ductility and corrosion resistance, the effects of Zr content on the microstructure, mechanical properties and corrosion resistance of heterogeneous CoCrFeNiZrx (x = 0, 0.25, 0.5 and 1) HEAs were investigated in this work. The results indicate that the increase of Zr content can significantly affect the phase stability of the alloy, and promote the formation of intermetallic compounds (Ni7Zr2 and/or Laves phase) and the transformation of solid solution from face-centered cubic (FCC) structure (x = 0, 0.25 and 0.5) to body-centered cubic (BCC) structure (x = 1). Reasonable control of the Zr content can endow the alloy excellent comprehensive properties. Especially, for CoCrFeNiZr0.25 alloy, composed of FCC matrix and a small amount of Ni7Zr2 phases, the yield strength (∼655 MPa) is increased by nearly four times higher than that of Zr-free alloy, and it also has good ductility (fracture stain > 50%). Meanwhile, the corrosion resistance of CoCrFeNiZr0.25 alloy is better than that of SS304. The EIS results show that the addition of Zr reduces the stability of the passive film on the alloy, which can be related to the content of the beneficial oxide in the passive film and the thickness of the passive film through XPS analysis. Moreover, the work functions of different phases in CoCrFeNiZrx alloys were obtained by first-principles calculations, which further confirmed the selective corrosion mechanism of the CoCrFeNiZrx alloy combining the experimental results.

Journal ArticleDOI
Dapeng Zhu1, Dapeng Zhu2, Weiwei Liu1, Rongzhi Zhao2  +8 moreInstitutions (2)
Abstract: Cerium oxide possesses intrinsic hydrophobic properties ascribed to the unique electronic structure. However, the relationship between the crystal structure and hydrophobicity of cerium oxide has not been systematically studied. Herein, it is experimentally and theoretically demonstrated that the water contact angle (105.9°) of the (111) surface is higher than that (91.7°) of the (220) surface, associated with the lower surface free energy (28.44 mN/m) of (111) surface than that (38.48 mN/m) of (220) surface. Furthermore, cerium oxide films with (111)-terminated surface are annealed at 300 °C and 600 °C for 1 h, respectively. The lattice constant increases (5.4594 A 300 °C > the as-deposited), leading to the increased water contact angle (96.7°

Journal ArticleDOI
Shuaishuai Gao1, Zuju Ma1, Chengwei Xiao2, Zhitao Cui2  +6 moreInstitutions (6)
Abstract: Confined metal clusters as sub-nanometer reactors for electrocatalytic N2 reduction reaction (eNRR) have received increasing attention due to the unique metal-metal interaction and higher activity than single-atom catalysts. Herein, the inspiration of the superior capacitance and unique microenvironment with regular surface cavities of the porous boron nitride (p-BN) nanosheets, we systematically studied the catalytic activity for NRR of transition-metal single-clusters in the triplet form (V3, Fe3, Mo3 and W3) confined in the surface cavities of the p-BN sheets by spin-polarized density functional theory (DFT) calculations. After a two-step screening strategy, Mo3@p-BN was found to have high catalytic activity and selectivity with a rather low limiting potential (–0.34 V) for the NRR. The anchored Mo3 single-cluster can be stably embedded on the surface cavities of the substrate preventing the diffusion of the active Mo atoms. More importantly, the Mo atoms in the Mo3 single-cluster would act as “cache” to accelerate electron transfer between active metal centers and nitrogen-containing intermediates via the intimate Mo-Mo interactions. The cooperation of Mo atoms can also provide a large number of occupied and unoccupied d orbitals to make the "donation–backdonation" mechanism more effective. This work not only provides a quite promising electrocatalyst for NRR, but also brings new insights into the rational design of triple-atom NRR catalysts.

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Abstract: A high-performance electro-absorption optical modulator based on the epsilon-near-zero (ENZ) effect is proposed. The structure consists of a waveguide with a silicon (Si) core over which a stack of graphene/HfO $_2$ /graphene/ITO/HfO $_2$ /graphene is grown, covered by a Si cladding. An external voltage is applied across the graphene layers to change the carrier concentration in the indium tin oxide (ITO) layers. Using a self-consistent theory, the required voltage to achieve the ENZ points in the ITO layers is calculated up to 3.42 V for an ITO thickness of 5 nm. The operation of the modulator is investigated using a three-dimensional finite-difference time-domain (FDTD) method, resulting in a modulation depth as high as 5.23 dB/ $\boldsymbol{\mu}$ m (5.36 dB/ $\boldsymbol{\mu }$ m) at a wavelength of 1.55 $\boldsymbol{\mu }$ m for the TE (TM) polarization, which ensures the polarization-insensitivity of our proposed modulator. It is also calculated that the insertion loss of the modulator is in the order of $\boldsymbol{ 2.5 \times 10^{-3}}$ dB/ $\boldsymbol{\mu }$ m that yields the figure of merit (FOM) of more than 1800. The outstanding features of our proposed modulator are mainly attributed to using the Si cladding layer instead of metal cladding. Furthermore, in contrast to the previously studied structures with metal electrodes, graphene layers significantly reduce the insertion loss.

Journal ArticleDOI
Yihan Chen1, Longxing Su1, Longxing Su2, Mingming Jiang3  +1 moreInstitutions (3)
Abstract: In this study, single crystal ZnO microwires (MW) with size of ∼5.4 mm × 30 μm are prepared through a chemical vapor deposition technique at high temperature (1200 °C). Subsequently, p-type conducting polyaniline (PANI) polymers with different conductivities are densely coated on part of the ZnO MW to construct organic/inorganic core-shell heterojunction photodetectors. The hetero-diodes reach an extremely high rectification ratio (I+3V/I-3V) of 749230, and a maximum rejection ratio (I350nm/Idark) of 3556 (at -3 V), indicating great potential as rectified switches. All the heterojunction devices exhibit strong response to ultraviolet (UV) radiation with high response speed. Moreover, the obvious photovoltaic behavior can also be obtained, allowing the device to operate as an independent and stable unit without externally supplied power. Under 0 V bias voltage, the self-powered photodetector shows a maximum responsivity of 0.56 mA W−1 and a rapid response speed of 0.11 ms/1.45 ms (rise time/decay time). Finally, a finite difference time domain (FDTD) simulation is performed to further demonstrate the interaction mechanism between the incident light field and the hexagonal cavity, which strongly supports the enhancement of the light absorption in whispering-gallery-mode resonator.

1 citations

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Author's H-index: 176

No. of papers from the Author in previous years