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Daniel P Hashim,
Narayanan T Narayanan,
Jose M Romo-Herrera, David A Cullen,
Myung Gwan Hahm,
Peter Lezzi,
Joseph R Suttle,
Doug Kelkhoff,
E Muñoz-Sandoval,
Sabyasachi Ganguli,
Ajit K Roy,
David J Smith,
Robert Vajtai,
Bobby G Sumpter,
Vincent Meunier,
Humberto Terrones,
Mauricio Terrones,
Pulickel M Ajayan
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ABSTRACT: The establishment of covalent junctions between carbon nanotubes (CNTs) and the modification of their straight tubular morphology are two strategies needed to successfully synthesize nanotube-based three-dimensional (3D) frameworks exhibiting superior material properties. Engineering such 3D structures in scalable synthetic processes still remains a challenge. This work pioneers the bulk synthesis of 3D macroscale nanotube elastic solids directly via a boron-doping strategy during chemical vapour deposition, which influences the formation of atomic-scale "elbow" junctions and nanotube covalent interconnections. Detailed elemental analysis revealed that the "elbow" junctions are preferred sites for excess boron atoms, indicating the role of boron and curvature in the junction formation mechanism, in agreement with our first principle theoretical calculations. Exploiting this material's ultra-light weight, super-hydrophobicity, high porosity, thermal stability, and mechanical flexibility, the strongly oleophilic sponge-like solids are demonstrated as unique reusable sorbent scaffolds able to efficiently remove oil from contaminated seawater even after repeated use.
Scientific Reports 01/2012; 2:363.
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ABSTRACT: The microstructure, composition and electrostatic fields of near-lattice-matched Al1-xInxN/AlN/GaN heterostructures and devices have been characterized using a variety of electron microscopy techniques. A thin parastic Ga-rich layer was often observed immediately above the AlN spacer and was attributed to GaN nucleation during specimen cooling. Mapping of electrostatic potential profiles across a GaN/Al0.85In0.15N/AlN/ GaN heterostructure using off-axis electron holography showed polarization-induced fields of 7.5MV/cm within the AlN layer, and 2.2MV/cm within the Ga-rich layer. A two-dimensional electron gas with a density of ∼9.8×1012 cm–2 was observed in the underlying GaN layer located very close to the AlN/GaN interface. Contact inclusions were observed extending into the AlInN and GaN layers along mixed-type threading dislocations under the source and drain regions of HFET devices. The density and size of the contact inclusions was determined by the annealing temperature (© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
physica status solidi (c) 07/2010; 7(10):2436 - 2439.
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ABSTRACT: The effects of polarization charge on the electrostatic potential distribution across the heterostructure of a AlGaN/GaN high electron mobility transistor device have been investigated. Simulations were performed using a full-band cellular Monte Carlo simulator, which included electronic dispersion and the phonon spectra. Quantum effects were taken into account using the effective potential method. Experimental extraction of potential profiles across the device was carried out using off-axis electron holography. Based on comparison to simulations, the differences between the theoretical predictions and experimental results could be explained, thereby providing better understanding of device operation.
Journal of Applied Physics 04/2010; · 2.17 Impact Factor
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ABSTRACT: Here we report the synthesis of single-walled carbon nanotube bundles by chemical vapor deposition in the presence of electron donor elements (N, P, and Si). In order to introduce each dopant into the graphitic carbon lattice, different precursors containing the doping elements (benzylamine, pyrazine, triphenylphosphine, and methoxytrimethylsilane) were added at various concentrations into ethanol/ferrocene solutions. The synthesized nanotubes and byproduct were characterized by electron microscopy and Raman spectroscopy. Our results reveal intrinsic structural and electronic differences for the N-, P-, and Si- doped nanotubes. These tubes can now be tested for the fabrication of electronic nanodevices, and their performance can be observed.
ACS Nano 03/2010; 4(3):1696-702. · 10.77 Impact Factor
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ABSTRACT: Off-axis electron holography in the transmission electron microscope is a powerful interferometric technique that enables electrostatic and magnetic fields to be imaged and quantified with spatial resolution often approaching the nanometer scale. Here, we demonstrate the capabilities of the technique for phase quantification at the nanoscale by briefly reviewing some of our recent studies of nanostructured materials. Examples that are described include determination of the electrostatic potential profiles associated with doped Si- and GaAs-based semiconductor devices, measurement of hole accumulation in Ge quantum dots, mapping of polarization fields in III-nitride heterostructures, and observation of the remanent states and reversal mechanisms of lithographically patterned magnetic nanorings. Some issues associated with sample preparation for doped semiconductor heterostructures are also briefly discussed.
Ultramicroscopy 02/2010; · 2.47 Impact Factor
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ABSTRACT: We found that multiwalled carbon nanotubes (MWNTs) can be opened longitudinally by intercalation of lithium and ammonia followed by exfoliation. Intercalation of open-ended tubes and exfoliation with acid treatment and abrupt heating provided the best results. The resulting material consists of: (i) multilayered flat graphitic structures (nanoribbons), (ii) partially open MWNTs, and (iii) graphene flakes. We called the completely unwrapped nanotubes ex-MWNTs, and their large number of edge atoms makes them attractive for many applications.
Nano Letters 05/2009; 9(4):1527-33. · 13.20 Impact Factor
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ABSTRACT: Off-axis electron holography has been used to measure the built-in potential profile across an Al <sub>0.85</sub> In <sub>0.15</sub> N / AlN / GaN high electron mobility transistor heterostructure. Profile measurements indicated a polarization-induced electric field of 6.9 MV/cm within the AlN layer. A two-dimensional electron gas with a density of ∼2.1×10<sup>13</sup> cm <sup>-2</sup> was located in the GaN layer at ∼0.8 nm away from the AlN/GaN interface in reasonable agreement with the reported simulations. Electron microscopy confirmed that the Al <sub>0.85</sub> In <sub>0.15</sub> N layer was uniform and that Al <sub>0.85</sub> In <sub>0.15</sub> N / AlN and AlN/GaN interfaces were abrupt and well defined.
Applied Physics Letters 04/2009; · 3.84 Impact Factor
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ABSTRACT: A detailed characterization, using high resolution electron microscopy/microanalysis (SEM, TEM, HRTEM, and EDX), reveals tubular carbon nanostructures exhibiting complex and fascinating morphologies. The materials were obtained by sulfur-assisted chemical vapor deposition. It is demonstrated that S not only acts on the catalyst, but also can be detected in the carbon lattice of the nanostructures. The experimental data presented here confirms the critical role of S, which is responsible for inducing curvature and therefore influencing the final carbon nanostructure morphology. In particular, different types of covalent Y-junctions of CNTs and even sea urchin-like nanostructures were produced and their experimental conditions are listed and discussed.
Advanced Functional Materials. 01/2009; 19:1193–1199.
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ABSTRACT: The surface amorphization and ion implantation in AlGaN-based high electron mobility transistor (HEMT) model structures caused by ionized gallium during focused-ion-beam milling have been investigated. The extent of Ga <sup>+</sup> surface implantation likely to occur during deposition of the surface Pt protective layer was simulated for 30, 5, and 2 keV ion beams. Electron-transparent cross sections of AlGaN/GaN and AlGaN/AlN/GaN HEMT structures were then prepared for electron microscope observation using a dual-beam focused-ion-beam instrument operated at different beam energies. Experimental studies revealed that the upper 9 nm of the AlGaN layer had been amorphized during Pt deposition. Nanoprobe x-ray microanalysis confirmed intermixing with Pt as well as implantation of Ga ions into the upper regions of the foil. Deposition of the first few hundred nanometers of Pt using an electron beam, rather than the usual Ga <sup>+</sup> beam, enabled surface damage and ion implantation to be completely avoided. Sidewall damage for specially prepared cross sections was assessed from bright-field and high-angle annular-dark-field images. For final membrane thinning at 30, 5, and 2 keV, the thicknesses of visibly damaged layers were approximately 20, 8, and 4 nm, respectively, roughly twice as large as predicted by simulations.
Journal of Applied Physics 12/2008; · 2.17 Impact Factor
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ABSTRACT: We report the production of a modified carbon by heat treating bean husk (Phaseolus vulgaris) at 270 degrees C in Ar, followed by chemical activation using HNO(3). The material was studied using thermogravimetric analysis (TGA), infrared spectroscopy (IRS), high-resolution transmission electron microscopy (HRTEM), elemental mapping, energy dispersive X-ray spectroscopy (EDX), X-ray powder diffraction and scanning electron microscopy (SEM). Cd(2+) sorption studies with this material were carried out at different concentrations. It was found that cadmium (II) is effectively removed by the modified material obtained from bean husk (180 mg/g). The sorption mechanism is discussed in terms of the activated surface properties. A relationship between the oxygen content and sorption was found in this novel material. Commercial activated carbon (AC) (F400) was used for comparison.
Water Research 08/2008; 42(13):3473-9. · 4.86 Impact Factor
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Eduardo Cruz-Silva, David A Cullen,
Lin Gu,
Jose Manuel Romo-Herrera,
Emilio Muñoz-Sandoval,
Florentino López-Urías,
Bobby G Sumpter,
Vincent Meunier,
Jean-Christophe Charlier,
David J Smith,
Humberto Terrones,
Mauricio Terrones
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ABSTRACT: Arrays of multiwalled carbon nanotubes doped with phosphorus (P) and nitrogen (N) are synthesized using a solution of ferrocene, triphenyl-phosphine, and benzylamine in conjunction with spray pyrolysis. We demonstrate that iron phosphide (Fe(3)P) nanoparticles act as catalysts during nanotube growth, leading to the formation of novel PN-doped multiwalled carbon nanotubes. The samples were examined by high resolution electron microscopy and microanalysis techniques, and their chemical stability was explored by means of thermogravimetric analysis in the presence of oxygen. The PN-doped structures reveal important morphology and chemical changes when compared to N-doped nanotubes. These types of heterodoped nanotubes are predicted to offer many new opportunities in the fabrication of fast-response chemical sensors.
ACS Nano 04/2008; 2(3):441-8. · 10.77 Impact Factor
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Angewandte Chemie International Edition 02/2008; 47(16):2948-53. · 13.45 Impact Factor
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ABSTRACT: Carbon nanotube growth in the presence of nitrogen has been the subject of much experimental scrutiny, sparking intense debate about the role of nitrogen in the formation of diverse structural features, including shortened length, reduced diameters, and bamboo-like multilayered nanotubules. In this paper, the origin of these features is elucidated using a combination of experimental and theoretical techniques, showing that N acts as a surfactant during growth. N doping enhances the formation of smaller diameter tubes. It can also promote tube closure which includes a relatively large amount of N atoms into the tube lattice, leading to bamboo-like structures. Our findings demonstrate that the mechanism is independent of the tube chirality and suggest a simple procedure for controlling the growth of bamboo-like nanotube morphologies.
ACS Nano 12/2007; 1(4):369-75. · 10.77 Impact Factor
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ABSTRACT: This paper reviews some of our recent studies of III-nitride high electron mobility transistor (HEMT) devices using advanced electron microscopy methods. Sample preparation protocols have been developed that can routinely provide thin, electron-transparent specimen regions of uniform thickness extending across entire HEMT devices. The use of focused-ion-beam (FIB) thinning facilitated access to specific device regions, although structural damage and imaging artifacts can result unless suitable precautions are taken during milling to minimize ion-beam damage. The extent of gallium-ion sidewall implantation during FIB milling, and surface damage caused by deposition of Pt protective layers, have been assessed. As-processed device structures have been examined by conventional diffraction contrast imaging as well as high-resolution phase contrast imaging, while nanospectroscopy and nanoscale elemental mapping have been used to measure local variations in chemical composition. Annealing of Ti/Al/Ni/Au ohmic contacts for AlInN/AlN/GaN devices lead to the formation of TiN contact inclusions that are invariably located at mixed-type threading dislocations originating from the underlying GaN layers. Some preliminary observations of device structures after extended periods of operation and after device failure have also been made. The technique of off-axis electron holography has been used to quantify two-dimensional electrostatic fields within cross-sectioned devices with nanometer-scale resolution. Polarization fields of 6.9 MV/cm and a two-dimensional electron gas of ∼2.1 × 1013/cm2 have been measured for an AlInN/AlN/GaN heterostructure. Methods suitable for in situ biasing of HEMT samples during electron holography observations have also been explored.
Microelectronics Reliability.
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