S. Ghosh

University of California, Riverside, Riverside, CA, United States

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Publications (12)17.6 Total impact

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    ABSTRACT: Device downscaling leads to higher chip power densities. A possible approach for heat removal from the localized hot spots is incorporation to chips of materials with high thermal conductivity. Recently, graphene and few-layer graphene (FLG) were proposed for heat removal owing to their superior thermal conductivity [1]. To evaluate the feasibility of this approach we simulated numerically heat propagation in SOI-based chip with and without graphene layers. It was found that incorporation of graphene or FLG can lead to substantial reduction of the hot spot's temperature [2]. The obtained results and are important for the design of graphene heat spreaders and interconnects [3]. [4pt] [1] A.A. Balandin, et al., Nano Lett., 8, (2008); S. Ghosh, et al., Appl. Phys. Lett., 92, (2008) [0pt] [2] S. Subrina, et al., Electron Dev. Lett., December (2009) [0pt] [3] A.A. Balandin, ``New materials can keep chips cool,'' IEEE Spectrum, October 2009
    03/2010;
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    ABSTRACT: We review the results of our experimental investigation of heat conduction in suspended graphene and offer a theoretical interpretation of its extremely high thermal conductivity. The direct measurements of the thermal conductivity of graphene were performed using a non-contact optical technique and special calibration procedure with bulk graphite. The measured values were in the range of ~3000–5300 W mK−1 near room temperature and depended on the lateral dimensions of graphene flakes. We explain the enhanced thermal conductivity of graphene as compared to that of bulk graphite basal planes by the two-dimensional nature of heat conduction in graphene over the whole range of phonon frequencies. Our calculations show that the intrinsic Umklapp-limited thermal conductivity of graphene grows with the increasing dimensions of graphene flakes and can exceed that of bulk graphite when the flake size is on the order of a few micrometers. The detailed theory, which includes the phonon-mode-dependent Gruneisen parameter and takes into account phonon scattering on graphene edges and point defects, gives numerical results that are in excellent agreement with the measurements for suspended graphene. Superior thermal properties of graphene are beneficial for all proposed graphene device applications.
    New Journal of Physics 09/2009; 11(9):095012. · 4.06 Impact Factor
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    ABSTRACT: The authors proposed a simple model for the lattice thermal conductivity of graphene in the framework of Klemens approximation. The Gruneisen parameters were introduced separately for the longitudinal and transverse phonon branches through averaging over phonon modes obtained from the first principles. The calculations show that Umklapp-limited thermal conductivity of graphene grows with the increasing linear dimensions of graphene flakes and can exceed that of the basal planes of bulk graphite when the flake size is on the order of a few micrometers. The obtained results are in agreement with experimental data and reflect the two-dimensional nature of phonon transport in graphene.
    Applied Physics Letters 01/2009; 94(20):203103. · 3.52 Impact Factor
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    ABSTRACT: The authors report results of investigation of thermal conductivity of nanocrystalline yttria-stabilized zirconia. The optically transparent pore-free bulk samples were prepared via the spark plasma sintering process to ensure homogeneity. Thermal conductivity K was measured by two different techniques. It was found that the pore-free nanostructured bulk zirconia is an excellent thermal insulator with the room-temperature K similar to 1.7-2.0 W/m K. It was also shown that the "phonon-hopping" model can accurately describe specifics of K dependence on temperature and the grain size. The obtained results are important for optimization of zirconia properties for specific applications in advanced electronics and coatings.
    Journal of Applied Physics 01/2009; 106(11):-. · 2.21 Impact Factor
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    ABSTRACT: The authors reported on investigation of the thermal conductivity of graphene suspended across trenches in Si/SiO2 wafer. The measurements were performed using a noncontact technique based on micro-Raman spectroscopy. The amount of power dissipated in graphene and corresponding temperature rise were determined from the spectral position and integrated intensity of graphene’s G mode. The extremely high thermal conductivity in the range of ∼ 3080–5150 W/m K and phonon mean free path of ∼ 775 nm near room temperature were extracted for a set of graphene flakes. The obtained results suggest graphene’s applications as thermal management material in future nanoelectronic circuits.
    Applied Physics Letters 04/2008; 92(15):151911-151911-3. · 3.52 Impact Factor
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    ABSTRACT: The authors report on a new method for the synthesis of graphene, a mono-layer of carbon atoms arranged in a honey comb lattice, and the assessment of the properties of obtained graphene layers using micro-Raman characterisation. Graphene was produced by a high pressure-high temperature (HPHT) growth process from the natural graphitic source material by utilising the molten Fe-Ni catalysts for dissolution of carbon. The resulting large-area graphene flakes were transferred to the silicon-silicon oxide substrates for the spectroscopic micro-Raman and scanning electron microscopy inspection. The analysis of the G peak, D, T + D and 2D bands in the Raman spectra under the 488 nm laser excitation indicate that the HPHT technique is capable of producing high-quality large-area single-layer graphene with a low defect density. The disorder-induced D peak ~1359 cm<sup>-1</sup> while very strong in the initial graphitic material is completely absent in the graphene layers. The proposed method may lead to a more reliable graphene synthesis and facilitate its purification and chemical doping.
    Micro & Nano Letters 04/2008; · 0.85 Impact Factor
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    ABSTRACT: We report on a new method for graphene synthesis and assessment of the properties of the resulting large-area graphene layers. Graphene was produced by the high pressure - high temperature growth from the natural graphitic source by utilizing the molten Fe-Ni catalysts for dissolution of carbon. The resulting large-area graphene flakes were transferred to the silicon - silicon oxide substrates for the spectroscopic micro-Raman and scanning electron microscopy inspection. The analysis of the G peak, D, T+D and 2D bands in the Raman spectra under the 488-nm laser excitation indicate that the high pressure - high temperature technique is capable of producing the high-quality large-area single-layer graphene with a low defect density. The proposed method may lead to a more reliable graphene synthesis and facilitate its purification and chemical doping.
    03/2008;
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    ABSTRACT: We report on the first measurement of the thermal conductivity of a suspended single layer graphene. The measurements were performed using a non-contact optical technique. The near room-temperature values of the thermal conductivity in the range ~ 4840 to 5300 W/mK were extracted for a single-layer graphene. The extremely high value of the thermal conductivity suggests that graphene can outperform carbon nanotubes in heat conduction.
    03/2008;
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    Journal of Nanoelectronics and Optoelectronics 03/2008; 3(1). · 0.48 Impact Factor
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    ABSTRACT: The authors report on the experimental investigation of the thermal conductivity of nitrogenated ultrananocrystalline diamond UNCD films on silicon. For better accuracy, the thermal conductivity was measured by using two different approaches: the 3 method and transient "hot disk" technique. The temperature dependence of the thermal conductivity of the nitrogenated UNCD films was compared to that of undoped UNCD films and microcrystalline diamond MCD films on silicon. It was shown that the temperature dependence of the thermal conductivity of UNCD films, which is substantially different from that for MCD films, can be adequately described by the phonon-hopping model. The room-temperature thermal conductivity of UNCD is 8.6– 16.6 W / m K and decreases with the addition of nitrogen. The obtained results shed light on the nature of thermal conduction in partially disordered nanostructured materials and can be used for estimating the thermal resistance of doped UNCD films. © 2008 American Institute of Physics.
    Journal of Applied Physics 01/2008; 103. · 2.21 Impact Factor
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    ABSTRACT: Graphene, a recently discovered form of carbon, revealed many unique properties, including extremely high electron mobility of ~15000 cm2/Vs at room temperature (RT). We have experimentally studied the thermal conductivity of graphene suspended over a trench in silicon (Si) wafer. It was found for a given set of samples that RT thermal conductivity of graphene is in the range ~ 3080 - 5150 W/mK. The giant thermal conductivity and demonstrated graphene - Si integration suggest that graphene can become superior material for thermal management of Si nanoelectronic circuits.
    01/2008;
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    ABSTRACT: We review the results of our experimental and theoretical investigation of heat conduction in suspended graphene layers. Through direct measurements using a noncontact optical technique, we established that the thermal conductivity of the suspended graphene flakes is extremely high, and exceeds that of diamond and carbon nanotubes. By invoking Klemens' theoretical model, we explained the physical mechanisms behind such unusual thermal conduction in two-dimensional graphene layers. Our detailed theory, which includes the phonon-mode dependent Gruneisen parameter and takes into account phonon scattering on graphene edges and point defects, gives results in excellent agreement with the measurements. Superior thermal properties of graphene are beneficial for all proposed graphene device applications.
    Fullerenes Nanotubes and Carbon Nanostructures 18:474-486. · 0.76 Impact Factor