J. M. Perez

University of North Texas, Denton, Texas, United States

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Publications (44)82.31 Total impact

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    ABSTRACT: We study oxidative pit formation in pristine, hydrogenated, and dehydrogenated monolayer graphene (MLG), bilayer graphene (BLG) and trilayer graphene (TLG). Graphene samples are produced by mechanical exfoliation of highly oriented pyrolytic graphite (HOPG) onto SiO2 substrates. Etching is carried out by exposing samples to O2 gas at 450-700 °C. Using atomic force microscopy, we observe that pre-heating pristine MLG in vacuum at 590 °C increases the onset temperature for pit formation to values comparable to those in HOPG. We attribute this decrease in reactivity to an increase in adhesion between the MLG and substrate. In hydrogenated MLG and BLG, we observe a significant decrease in the onset temperature for pit formation. Dehydrogenation of these materials results in a decrease in the density of pits. We attribute the decrease in onset temperature to H-related defects in their sp3-bonded structure. In contrast, hydrogenated TLG and thicker-layer samples show no significant change in pit formation. We propose that this is because they are not transformed into an sp3-bonded structure by hydrogenation.
    Applied Surface Science 01/2013; 264:853-863. DOI:10.1016/j.apsusc.2012.10.161 · 2.54 Impact Factor
  • Y Mo, J D Jones, A Neogi, Y Fujita, J M Perez
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    ABSTRACT: We compare the effects of O2, CO2, N2, H2, and Ar residual gas exposure on the field emission (FE) properties of ZnO nanorods. In contrast to carbon nanotubes and Mo metal microtips, we find that O2 and CO2 exposures do not significantly degrade the FE properties of ZnO nanorods. However, N2 exposure significantly degrades the FE properties. We propose that this could be due to the dissociation of N2 into atomic nitrogen species and the reaction of such species with ZnO. H2 and Ar exposures are not observed to significantly degrade the FE properties.
    Journal of Nanoscience and Nanotechnology 04/2011; 11(4):3630-4. DOI:10.1166/jnn.2011.3759 · 1.34 Impact Factor
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    ABSTRACT: We propose a new mechanism for the hydrogenation of mono-, bi-, and tri-layer graphene samples using an H2 plasma. We find that hydrogenation occurs as a result of electron irradiation of H2O adsorbates on the sample rather than H species from within the plasma. We propose that the mechanism is electron-impact fragmentation of the H2O adsorbates occurring naturally above and below the sample. The stability of the hydrogenation increases with the incident electron energy, allowing for hydrogenated samples that are stable at temperatures > 200 ^oC. We also observe fully hydrogenated bi- and tri-layer graphene, which may be evidence for new materials, diamane and triamane. Diamane, a two atom thick layer of hydrogenated diamond, is predicted to have a band gap of 3.12 eV and be stronger than graphane, hydrogenated graphene.
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    ABSTRACT: We have investigated the possibility of hypothetical alternative carbon allotropes using computational methodologies using Gaussian and VASP molecular simulation programs. We investigate the possible existence of carbon based balls, nanotubes and sheets composed of hexagonal rings, cyclobutane rings or pentagonal rings. The possibility of the existence of a hypothetical allotrope is determined by the convergence of the given allotrope under geometric optimization. The theories used to compute such convergence are Hartree-Fock theory and density functional theory. The theoretical Raman spectra of each allotrope can also be computed using Gaussian. The results concerning the reality of the substances under investigation are inconclusive except for a C24 ball, which has been shown to converge to graphene and is therefore an unstable molecule.
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    ABSTRACT: Graphene samples exfoliated from highly ordered pyrolytic graphite are deposited using the standard scotch-tape method on 300nm thick SiO2 covered and slightly conductive Si substrates. Devices with 4 silver electrode pads on the graphene samples for Hall effect measurements are made with simple evaporation procedures by using transmission electron microscopy grids as masks. At room temperature, we measure the Hall effect of mono- and multi-layer graphene before and after plasma hydrogenation. During plasma hydrogenation, the sample substrates are biased at +150 V to attract electrons in the plasma for hydrogenation and push away ions in the plasma avoiding possible damage to the graphene. We also measure the Hall effect after annealing the samples at 200 ^oC and vacuum of 10-6 torr for an hour. Micro-Raman is employed to monitor the quality and change of the graphene at each process step. We compare the Hall effect results for pristine, hydrogenated, and annealed mono- and multi- layer graphene samples.
  • MRS Online Proceeding Library 01/2011; 509. DOI:10.1557/PROC-509-137
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    ABSTRACT: We report that hydrogenation of mono-, bi-, and trilayer graphene samples via exposure to H <sub>2</sub> plasma occurs as a result of electron irradiation of H <sub>2</sub> O adsorbates on the samples, rather than H species in the plasma as reported by [Elias etal, Science 323, 610 (2009)]. We propose that the hydrogenation mechanism is electron-impact fragmentation of H <sub>2</sub> O adsorbates into H <sup>+</sup> ions. At incident electron energies >60 eV , we observe hydrogenation that is significantly more stable at temperatures >200 ° C than previously reported.
    Applied Physics Letters 01/2011; 97(23-97):233104 - 233104-3. DOI:10.1063/1.3524517 · 3.52 Impact Factor
  • J.M. Perez, R.E. Stallcup, I.A. Akwani
    MRS Online Proceeding Library 01/2011; 558. DOI:10.1557/PROC-558-589
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    ABSTRACT: We report the formation of graphane and partially hydrogenated graphene by electron irradiation of graphene having chemisorbed H2O and NH3. Graphane is formed by the irradiation of graphene that is mechanically exfoliated onto SiO2 substrates in air at a relative humidity of 50%. Partially hydrogenated graphene is formed by the irradiation of graphene that is heated to remove adsorbates and then exposed to H2O or NH3 gases. We propose that hydrogenation is due to H+ ions and H radicals produced by the fragmentation of H2O and NH3 adsorbates by impact with backscattered and secondary electrons. This effect could be used as a technique for writing graphane and partially hydrogenated graphene nanostructures on graphene using electron-beam lithography.
    Carbon 07/2010; 48(8-48):2335-2340. DOI:10.1016/j.carbon.2010.03.010 · 6.16 Impact Factor
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    K M Lee, A Neogi, J M Perez, T Y Choi
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    ABSTRACT: A focused-ion beam (FIB) and a nanomanipulator provide a novel way to selectively control and obtain a few layers of graphene. Because of its weak van der Waals force in the interlayer of graphite, the nanomanipulator could easily exfoliate a graphitic thin layer with no wrinkles on the surface from a highly oriented pyrolitic graphite (HOPG) by applying a shear force which exceeds the static interlayer shear force. Subsequently, a few layers of graphene were successfully obtained by applying a uniform shear force from a detached graphitic thin layer that had been transferred to a pre-determined site on an oxide wafer. The required shear force for clean cleavage of a graphitic thin layer was then estimated based upon experimental data. Raman scattering analysis was used to confirm the number of placed graphene layers and the placement of a few layers of graphene was projected to have about five atomic layers.
    Nanotechnology 04/2010; 21(20):205303. DOI:10.1088/0957-4484/21/20/205303 · 3.67 Impact Factor
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    J. D. Jones, P. A. Ecton, Y. Mo, J. M. Perez
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    ABSTRACT: Abstract unavailable.
    Applied Physics Letters 12/2009; 95(24):246101-246101-2. DOI:10.1063/1.3272954 · 3.52 Impact Factor
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    ABSTRACT: We report the formation of pits having widths of approximately 10 nm and a density of 2.5 X 10(11)/cm(2) on epitaxial diamond (100) films. The pits are formed by etching the films using atomic hydrogen at a substrate temperature of approximately 500 C. Exposure to oxygen followed by etching with atomic hydrogen forms additional pits. We propose that the high-density pits are formed due to etching that occurs both perpendicular and parallel to the surface. (c) 2007 Elsevier B.V All rights reserved.
    Diamond and Related Materials 09/2007; 16(9):1727-1731. DOI:10.1016/j.diamond.2007.06.001 · 1.57 Impact Factor
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    ABSTRACT: The semiconducting silicides offer significant potential for use in optoelectronic devices. Full implementation of the materials, however, requires the ability to tailor the energy gap and band structure to permit the synthesis of heterojunctions. One promising approach is to alloy the silicides with Ge. As part of an investigation into the synthesis of semiconducting silicide heterostructures, a series of β- Fe ( Si <sub>1-x</sub> Ge <sub>x</sub>)<sub>2</sub> epilayer samples, with nominal alloy content in the range 0≤x≤0.15 , have been prepared by molecular beam epitaxy on Si(100). We present results of the epitaxial and crystalline quality of the films, as determined by reflection high-energy electron diffraction, Rutherford backscattering spectroscopy, and double crystal x-ray diffraction, and of the band gap dependence on the alloy composition, as determined by Fourier transform infrared spectroscopy. We observe a reduction in band gap with increasing Ge content, in agreement with previous theoretical predictions [Tani etal, J. Solid State Chem. 169, 19 (2002)]. However we also observe Ge segregation in β- Fe ( Si <sub>1-x</sub> Ge <sub>x</sub>)<sub>2</sub> epilayers when x≥0.04 .
    Journal of vacuum science & technology. B, Microelectronics and nanometer structures: processing, measurement, and phenomena: an official journal of the American Vacuum Society 06/2005; DOI:10.1116/1.1924607 · 1.36 Impact Factor
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    ABSTRACT: We report the preferential nucleation and synthesis of β‐FeSi2 nanostructures at pinned step bunches on the Si(111) surface. The nanostructures are synthesized by depositing Fe on Si at room temperature and subsequent annealing. The surface topography is studied using scanning tunneling microscopy and atomic force microscopy. The size, shape and orientation of the nanostructures indicate that the phase is the semiconducting β‐FeSi2 phase.
    Applied Physics Letters 05/2005; 86(22):223102-223102-3. DOI:10.1063/1.1940128 · 3.52 Impact Factor
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    ABSTRACT: FeSi2 is a direct band gap semiconductor with a band gap of approximately 0.8 eV having great potential for the realization of Si optoelectronic devices. We report atomic resolution ultrahigh vacuum (UHV) scanning tunneling microscopy (STM) studies of FeSi2 films grown on Si (111) substrates using reactive deposition epitaxy. Half a monolayer of Fe was deposited on Si (111) and then the sample was annealed for 5 minutes at a temperature of 500 C in UHV. UHV STM showed FeSi2 islands having a 2x2 hexagonal structure. Some surface roughening as a result of annealing was observed. Atomic force microscopy studies of the films carried out in air confirmed the island formation observed using UHV STM. This work was supported by the Texas Advanced Technology Program.
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    ABSTRACT: We compare the field emission (FE) properties of different types of nanostructured carbon films including carbon sheets, single and multi-walled carbon nanotubes, and spiraled and wavy multi-walled carbon nanotubes. The effects of residual gasses such as oxygen, hydrogen, and nitrogen on the FE properties of the materials will be examined in detail. We find that oxygen exposure generally degrades the FE properties, while hydrogen and nitrogen do not significantly affect the FE properties.
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    ABSTRACT: We compare the effects of residual gases on the field emission (FE) properties of sinle-walled and multi-walled carbon nanotubes, polycrystalline diamond films, and amorphous carbon films. The different films were exposed to oxygen, hydrogen and nitrogen residual gases at a pressure of 10-6 Torr for varying lengths of time under operating FE conditions. We observe that hydrogen and nitrogen exposure do not significantly affect the FE properties of all the samples. Oxygen exposure degrades the turn-on voltage of all the samples. The carbon nanotubes and diamond films are similarly affected by oxygen exposure. However, since nanotubes field emitt at lower voltages, they would be a better material for FE applications.
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    A. Wadhawan, D. Garrett, J. M. Perez
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    ABSTRACT: We report the effects of microwave irradiation on both unpurified and purified iron-catalyzed high-pressure disproportionation (HiPco)-grown single-walled carbon nanotubes (SWNTs) in ultrahigh vacuum. Under microwave irradiation, we observe that unpurified HiPco SWNTs quickly reach temperatures of approximately 1850 °C. As a result, H2, H2O, CO, CO2, and CH4 gases are observed, and the Fe catalyst nanoparticles melt and coalesce into larger crystallites approximately four times their original diameter. In contrast, carbon black and purified HiPco SWNTs heat up to temperatures of 500–650 °C. We propose that the significant heating of unpurified HiPco SWNTs is due to the Fe catalysts. © 2003 American Institute of Physics.
    Applied Physics Letters 09/2003; 83(13):2683-2685. DOI:10.1063/1.1615679 · 3.52 Impact Factor
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    Chemistry of Materials 09/2003; 15(21). DOI:10.1021/cm034530g · 8.54 Impact Factor
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    ABSTRACT: beta-FeSi2 shows promise as a Si based light emitter operating in ˜ 1.5mum wavelength range. However, there exists a number of concerns related to the nature of the bandstructure and the ability to dope the material. In order to determine the effect of Cr and Co dopants, temperature-dependent Hall effect measurements were taken on a series of beta-(Fe_1-xCr_x)Si2 , beta-(Fe_1-xCo_x)Si2 doped films, grown by MBE. The results show that Cr is a p-type dopant and Co is an n-type dopant in this material system. It is shown that at low temperature, conduction is dominated by defect band conduction, while at higher temperatures valence band conduction prevails. These samples show good conduction properties with typical resistivity of 1Omega-cm at 30K and .05 Omega-cm at 300K.