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M Shakerzadeh,
G C Loh,
N Xu,
W L Chow,
C W Tan,
C Lu,
R C C Yap,
D Tan,
S H Tsang, E H T Teo,
B K Tay
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ABSTRACT: Formation of nanocrystals with preferred orientation within the amorphous carbon matrix has attracted lots of theoretical and experimental attentions recently. Interesting properties of this films, easy fabrication methods and practical problems associated with the growth of other carbon nanomaterials such as carbon nanotubes (CNTs) and graphene gives this new class of carbon nanostructure a potential to be considered as a replacement for some applications such as thermal management at nanoscale and interconnects. In this short review paper, the fabrication techniques and associated formation mechanisms of these nanostructured films have been discussed. Besides, electrical and thermal properties of these nanostructured films have been compared with CNTs and graphene.
Advanced Materials 05/2012; 24(30):4112-23. · 13.88 Impact Factor
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ABSTRACT: The pillared-graphene architecture is a conceivable way of conjoining graphene nanoribbons and carbon nanotubes (CNTs) in nanoelectronics. Especially promising is its capability to dissipate thermal energy in thermal management applications. However, the thermal boundary resistance (Kapitza resistance) at the graphene nanoribbon-CNT interface is a phonon barricade and a bottleneck for efficacious heat extraction. Parallel to strain studies on thermal conductance, this work is a first report on the effects of mechanical strain on the interfacial phonon dynamics in the pillared-graphene nanostructure (PGN). Molecular dynamics simulations are employed to derive the changes in phononics as axial, torsional, and compound strains of various degrees are applied on the PGN. The pillar lattice structure behaves dissimilarly to the different types of strains. In-plane transverse optical mode softening as induced by torsional strain is more effective than LO softening (triggered by tension) in minimizing the thermal boundary resistance. Essentially, it is shown that there is a strong relationship between strained PGN pillar lattice structure, interfacial phononics, and thermal boundary resistance.
Journal of Applied Physics 01/2012; 111(1):013515-013515-6. · 2.17 Impact Factor
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ABSTRACT: The pillared-graphene architecture proves to be a plausible attempt at integrating both graphene and carbon nanotubes in nanoelectronics. The similitude of both material compositions reduces mismatching of lattice vibrational spectra at their interfaces, thereby enhancing capacity as a thermal sink to extract heat. Unlike previous work, this study centers on the interpillar phononics in these pillared-graphene nanostructures (PGN). Classical molecular dynamics simulations are performed to emulate the phonon transport in PGN. It is evinced that intertube interaction diminishes the nanotube thermal conduction. The simulations show that long-wavelength out-of-plane modes contribute significantly to thermal conduction. A bidirectional mode propagation mechanism is proposed and believed to be indirectly responsible for the reduced thermal flux in PGN. Finally, parity analyses of three-phonon scattering selection rules further substantiate the notion of a dual-scattering nature of flexural modes.
Journal of Applied Physics 10/2011; 110(8):083502-083502-6. · 2.17 Impact Factor
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ABSTRACT: Tears in any material act as barriers to phonon transport. In this study, molecular dynamics simulations are employed to investigate thermal transport around tears in graphene. Specifically, thermal boundary conductance across different tear orientations and lengths is computed. Analysis of vibrational density of states suggests that long-wavelength acoustic phonons within the spectrum range 0–700 cm<sup>-1</sup> are vital to thermal transport across the tears. Different phonon scattering phenomena are observed for both tear orientations. It is proposed that the dissimilitude of the scattering processes encountered by phonons carrying energy around the tears to the opposite end explains why thermal transport is generally more efficient for longitudinal tears in our simulations.
Journal of Applied Physics 05/2011; · 2.17 Impact Factor
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ABSTRACT: Carbon films were prepared using a filtered cathodic vacuum arc deposition system operated with a substrate bias varying linearly with time during growth. Ion energies were in the range between 95 and 620 eV. Alternating dark, high density (sp(3) rich) bands and light, low density (sp(2) rich) bands were observed using cross-sectional transmission electron microscopy, corresponding to abrupt transitions between materials with densities of approximately 3.1 and 2.6 g cm(-3). No intermediate densities were observed in the samples. The low density bands show strong preferred orientation with graphitic sheets aligned normal to the film. After annealing, the low density bands became more oriented and the thinner high density layers were converted to low density material. In molecular dynamics modelling of film growth, temperature activated structural rearrangements occurring over long timescales ([Formula: see text] ps) caused the transition from sp(3) rich to oriented sp(2) rich structure. Once this oriented growth was initiated, the sputtering yield decreased and channelling was observed. However, we conclude that sputtering and channelling events, while they occur, are not the cause of the transition to the oriented structure.
Journal of Physics Condensed Matter 06/2009; 21(22):225003. · 2.55 Impact Factor
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ABSTRACT: We demonstrate that when, and only when, the biaxial stress is increased above a critical value of 6+/-1 GPa during the growth of a carbon film at room temperature, tetrahedral amorphous carbon is formed. This confirms that the stress present during the formation of an amorphous carbon film determines its sp;{3} bonding fraction. In the vicinity of the critical stress, a highly oriented graphitelike material is formed which exhibits low electrical resistance and provides Ohmic contacts to silicon. Atomistic simulations reveal that the structural transitions are thermodynamically driven and not the result of dynamical effects.
Physical Review Letters 05/2008; 100(17):176101. · 7.37 Impact Factor
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ABSTRACT: At room temperature, we observe the self assembly of nanoclusters in an amorphous matrix using a vacuum deposition technique. Self-assembled ZnO nanoclusters embedded in hard diamond-like amorphous carbon thin films, deposited by high vacuum Filtered Cathodic Vacuum Arc (FCVA) technique at room temperature without post-processing, have been observed. A selective self assembly of metal and oxygen ions in a 3-element plasma was observed. XPS distinctly showed presence of ZnO and DLC-mixture in 5, 7 and 10 at.% Zn (in target) films while maintaining high sp3 content. This in turn improved the Young's modulus value of the ZnO nanoclusters embedded in DLC film (~ 220 GPa) compared to bulk ZnO (~ 110 GPa). Films with ZnO detected were observed to exhibit absorption edge at 377 nm monochromatic UV light emissions. This corresponded to a band gap value of about 3.30 eV. The emission with greatest intensity (after normalization) was detected from 10 at.% Zn (in target) film where presence of ZnO nanoclusters (~ 40 nm) in DLC matrix were confirmed by TEM. This showed that well-defined crystalline ZnO nanoclusters contributed to strong PL signal. Strong monochromatic emissions detected hinted that no defect states were present.
Diamond and Related Materials.
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ABSTRACT: The effect of plasma parameters on the nanostructures formed during physical vapor deposition growth of carbon films has been studied. It was shown that the formation and nature of nanostructures strongly depend on plasma density and ion energy during the deposition. High plasma density results in formation of nanocrystals with preferred orientation even at low negative substrate bias (300 V) while at low plasma density higher substrate bias (500 V) is required for the nanocrystals. Moreover, at the same plasma density the nature of the nanostructures strongly depends on the ion energy. At higher ion energies, carbon nanotubes are formed in the microstructure while at lower ion energies, graphitic nanostructures are more stable. It was also found that prior to the formation of preferred orientation, an amorphous layer is formed at the silicon/carbon interface. Through electron energy loss spectroscopy, it is shown that the structure of this layer strongly depends on ion energy during the deposition.
Carbon. 49(5):1733-1744.
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ABSTRACT: Experimental and analytical techniques are introduced for the quantitative, nanoscale mapping of chemical bonding information in carbon-based materials. With these techniques, the spatial orientation of graphitic crystallites in tetrahedrally bonded amorphous carbon was imaged. Simultaneously, the percentage of sp2- and sp3-bonded carbon could be mapped quantitatively, all with spatial resolution of just a few nanometers. Two electron energy-loss spectroscopy (EELS) techniques were compared: low-loss mapping of the plasmon energy and core-loss mapping of the carbon ionization edge. The recently developed EELS acquisition routine of binned gain averaging was applied, together with multivariate statistical analysis, providing a robust method for obtaining real-space, two-dimensional bonding maps.
Carbon.
