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DOI: 10.1002/adma.200800401

Enhanced Thermal Conductivity in a Hybrid GraphiteNanoplatelet – Carbon Nanotube Filler for EpoxyComposites**

By Aiping Yu, Palanisamy Ramesh, Xiaobo Sun, Elena Bekyarova, Mikhail E. Itkis, and

Robert C Haddon*

The increased heat generated in high density electronics has

intensified the search for advanced thermal interface materials

(TIMs) and prompted fundamental studies at the nanoscale

level to develop filler materials with enhanced thermal

performance.[1–4] Single-walled carbon nanotubes (SWNTs)

considerably improve the heat transport in polymer compo-

sites as a result of their one-dimensional (1D) structure, high

thermal conductivity and high aspect ratio.[5–12] Recently,

two-dimensional (2D) nanostructures such as graphite nano-

platelets (GNPs), have emerged as a promising filler in

polymer matrices[13–19] and it has been shown that they provide

even higher thermal conductivity enhancement than

SWNTs.[16] In this study we combine 1D-SWNTs and

2D-GNPs to prepare a series of hybrid graphitic nanofillers

and we observe a synergistic effect between the GNPs and

SWNTs in the enhancement of the thermal conductivity of

epoxy composites to the point that at certain filler loadings the

hybrid composition outperforms composites utilizing pure

GNP or SWNT fillers. The increased thermal conductivity is

ascribed to the formation of a more efficient percolating

nanoparticle network with significantly reduced thermal

interface resistances.

The idea of using a hybrid filler comprised of two or more

traditional filler materials has already been explored in the

literature and it has been demonstrated that improved

composite performance can be achieved by combining the

advantages of each filler.[20,21] Commercially available thermal

greases and adhesives often utilize several components to

achieve the desired combination of thermal and electrical

conductivities, viscosity and low coefficient of thermal

expansion. In our study, we utilize two different nanostruc-

tured graphitic fillers for incorporation into epoxy resin:

purified SWNTs and graphite nanoplatelets (GNPs) comprised

[*] Prof. R. C Haddon, A. Yu, Dr. P. Ramesh, Dr. X. Sun, Dr. E. Bekyarova,Dr. M. E ItkisCenter for Nanoscale Science and EngineeringDepartments of Chemistry and Chemical & Environmental EngineeringUniversity of California – RiversideRiverside, California 92521 (USA)E-mail: haddon@ucr.edu

[**] We acknowledge the financial support from DOD/DMEA underaward # H94003-06-20604 and # H94003-08-2-0803 and technicalhelp in graphite exfoliation from Yasir Khalid Ali and Kimberly Worsley.Supporting Information is available online fromWiley InterScience orfrom the authors.

� 2008 WILEY-VCH Verlag Gmb

of few graphene layer Gn, where n� 4. The SWNT component

of the hybrid filler is electric arc produced purified SWNTswith

a typical length of 0.3–1.0mm and an average diameter of

1.4 nm. The purification process[22] leaves the SWNTs ends and

side-walls functionalized with carboxylic acid groups and this

facilitates their homogeneous dispersion into the polymer

matrix. In addition, the epoxy curing process is accompanied

by a cross-linking reaction between the carboxylic acid groups

of the SWNTs and the epoxy groups of the polymer,[23] thus

improving the integration of SWNTs into the polymer matrix.

GNPs are typically prepared by intercalation and exfoliation

of graphite;[24–29] and by control of the exfoliation conditions

we were able to obtain GNPs comprised of 2 to 8 graphene

layers with a lateral dimension of 200–1000 nm and an aspect

ratio in the range of 50 to 300.[16] This was achieved by thermal

shock exfoliation of natural graphite flakes at 800 8C[25,26]

followed by high shear mixing and sonication in order to

separate the exfoliated graphite flakes into nanoplatelets.[16] A

series of composites were prepared with a hybrid filler loading

between 5wt % and 40wt % in the epoxy (EPON 682/

EPIKURE) matrix. The ratio of SWNTs and GNPs in the

hybrid filler was varied in order to study their efficiency in

enhancing the thermal conductivity of the composite. The

thermal and electrical conductivity measurements were

performed using composite disks with a diameter of 2.54 cm

and thickness of 4–12mm. A detailed composite preparation

procedure was reported in our previous publications,[12,16] and

it is described briefly in the experimental section.

Figure 1a shows the thermal conductivity (k) of

GNP-SWNT/epoxy composites as a function of the GNP

fraction in the hybrid filler at a hybrid filler loading of 10wt %.

The epoxy composites prepared with a 10wt % loading

of the individual fillers gave thermal conductivities of

kSWNT¼ 0.85W m�1 K�1 for the SWNT filled composite[12]

and kGNP¼ 1.49W m�1 K�1 for the GNP-filled composite.[16]

In the case of a hybrid filler (HYB), the thermal conductivity is

expected to increase monotonically as the fraction of the more

efficient GNP filler increases, in accord with the rule of

mixtures. However, the experimental data show a pronounced

maximum of kHYB¼ 1.75W m�1 K�1 at a GNP:SWNT filler

ratio of �3:1 (7.5wt % GNPs and 2.5wt % SWNTs in epoxy).

Thus the hybrid filler demonstrates a strong synergistic effect

and surpasses the performance of the individual SWNT and

GNP fillers. This synergistic behavior is quite remarkable

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GNPXSWNT10-X

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Figure 1. a) Thermal and b) electrical conductivities of epoxy compositesprepared with hybrid GNPxSWNT10-X filler as a function of GNP fillerpercentage (x); the total filler loading is maintained at 10wt %. Dashed linein (a) corresponds to the change of thermal conductivity expected from therule of mixtures. Dotted line in (b) is provided as a guide to the data.Triangles and squares in (a) and (b) correspond to two independent sets ofcomposite sample preparation.

Figure 2. a) SEM and b) TEM images of the cross-section of GNP-SWNThybrid filler/epoxy composite. Note that SWNTs are bridging adjacentgraphite nanoplatelets and SWNTs ends are extended along the nanopla-telet surfaces. c) Schematic representation of GNP-SWNT network inpolymer matrix. (see Supporting Information for SEM images of othercompositions).

because the substitution of 25wt % of the GNPs with the less

efficient SWNT filler should lead to a decrease in the overall

thermal conductivity assuming the rule of mixtures is obeyed

(dashed line in Fig. 1a).

Figure 1b shows the electrical conductivity of the composites

(total filler loading of 10wt %) as a function of the

GNP-SWNT composition. The electrical conductivity (s) of

theGNP composite (s¼ 0.3 S cm�1) is two orders of magnitude

higher than the conductivity of the SWNT-filled composite

(s �0.003 S cm�1), which suggests that the 2D-GNP material

provides a more efficient percolating network[30] compared to

the 1D-SWNTs. In the case of composites prepared with a

hybrid filler the electrical conductivity shows non-monotonic

behavior as a function of the GNP fraction (Fig. 1b) with a

minimum in the vicinity of a GNP:SWNT filler ratio of 1:3,

which contrasts with the maximum observed in the thermal

conductivity data. Thus the introduction of SWNTs into the

GNP filler detracts from the electrical transport properties,

whereas the thermal transport is augmented at certain

compositions.

Figure 2 shows SEM and TEM images of epoxy composite

with a hybrid filler loading of GNP:SWNT of 3:1 (7.5wt % of

GNPs and 2.5wt % of SWNTs), which corresponds to the

maximum observed in thermal conductivity (Fig. 1a). The

Adv. Mater. 2008, 20, 4740–4744 � 2008 WILEY-VCH Verl

images show complex nanostructures with multiple SWNTs

bridging adjacent GNPs (see Supporting Information for SEM

images of other compositions). A simple estimate taking into

account the average particle size of the GNPs (diameter

�350 nm, thickness �2 nm), SWNTs (bundle diameter �5 nm

and length�0.5mm), and densities: GNP (2.26 g cm�3), SWNT

(1.4 g cm�3), epoxy (1.17 g cm�3) gives a proportion of �10–20

SWNT bundles per GNP and a mean distance between

adjacent nanoplatelets of <50 nm, which is in reasonable

accord with the microscopy (Fig. 2a and b).

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In order to rationalize the synergistic enhancement of the

thermal conductivity in the case of hybridGNP/SWNTfiller we

first consider factors limiting the thermal conductivity of the

composites in the cases of individual fillers. It is now well

established that the thermal conductivity enhancement in

SWNT-filled polymers does not reach the theoretically

predicted value due to the presence of thermal interface

resistance at the SWNT/polymer or SWNT/SWNT bound-

aries.[31–34] It has been suggested that for a SWNT embedded in

a polymer matrix the formation of a percolating network for

heat flow is suppressed due to the presence of a thin polymer

layer which separates the SWNTs in the network and precludes

direct SWNT-SWNT phonon transfer.[33,34] Even in case of a

direct contact, the geometry of the junctions between two

crossed SWNTs leads to a point contact with a very small

surface area (�10�14 cm2). Thus the interaction between

crossed SWNTs is very weak and the surrounding polymer

matrix provides the dominant contribution to the heat flow

between SWNTs.[34] This interruption of heat flow along the

SWNT network due to the involvement of the polymer matrix

results in the introduction of a large thermal interface

resistance due to the phonon mismatch which is further

enhanced by the high curvature of the SWNT surface.[2,35]

The GNP filler provides stronger enhancement of the

thermal conductivity in comparison with the SWNTs (Fig. 1a)

and this is attributed to the following factors: i) the flat surface

of the graphite nanoplatelets enhances the GNP-polymer

matrix interaction, and ii) GNP rigidity allows for better

preservation of their high aspect ratio in comparison with the

more flexible SWNTs.[16] On the other hand, in a 3D-matrix

the most probable contact geometry between rigid GNPs

is also point-type with a small contact area (�10�14 cm2) as

in the case of SWNTs; thus direct phonon transport across

the GNP junctions is also very weak and heat transfer

would require involvement of the polymer matrix resulting in

a high thermal interface resistance and suppression of

percolation.

In the case of hybrid fillers (Fig. 2a and b) the 1D-SWNTs

are bridging adjacent 2D-nanoplatelets and provide additional

channels for the heat flow bypassing the polymer matrix. It can

be noticed in the SEM and TEM images (Fig. 2a and b) that the

end fragments of some of the bridging SWNTs are aligned

along the GNP surfaces by the van der Waals attraction

between the graphitic structures and because the full length

extension of the flexible SWNTs is limited in space by the

GNPs; such a hybrid structure is presented schematically in

Figure 2c (see Fig. S-3 in Supporting Information). In

comparison with individual SWNT or GNP fillers the 0D

point contact geometry along the filler network is substituted

by a 1D linear contact with significantly increased area of

interface junctions within the hybrid filler network. This leads

to a decreased thermal interface resistance and may be

considered as the major reason for the observed synergistic

effect of the 1D and 2D hybrid fillers.

In contrast, the hybrid network does not provide synergistic

enhancement to the electrical conductivity (Fig. 1b). This may

www.advmat.de � 2008 WILEY-VCH Verlag GmbH &

be due to the presence of a thin (few nm) layer of polymer (as

described above), which prevents the direct contact between

the SWNTs and GNPs (Fig. S-3 Supporting Information) and

introduces a scattering layer for the phonon transport as well as

an insulating layer in the tunneling barrier for electrical

transport.[33,34] According to theoretical modeling,[33,34] an

increase in the thickness of the polymer layer from 0 to 10 nm

does not affect significantly the heat transport, however such

an increase of the width of the tunneling barrier would

effectively eliminate the electrical transport. Because of their

rigid shape the GNPs are brought into intimate contact in

the highly viscous epoxy matrix during processing; thus the

interparticle insulating barrier that is formed between the

GNPs may be thinner than in the case of the more flexible

SWNTs. This can explain the higher electrical conductivity of

the GNP-filled composites (Fig. 1b) in comparison with

composites prepared with SWNT or SWNT/GNP hybrid

fillers. Another factor that may mask the potential synergistic

behavior in the electrical conductivity is the large difference in

the electrical conductivities of the composites with individual

fillers: s(GNP)/s(SWNT) �100 (Fig. 1b). In comparison the

ratio of the thermal conductivities of composites withGNP and

SWNTs is k (GNP)/k (SWNT) �1.7 (Fig. 1a and b), thus the

sharp drop of the electrical conductivity associated with the

decreased GNP fraction (Fig. 1b) is much more difficult to

compensate than in the case of the thermal conductivity.

Lastly, the electrical conductivity is suppressed due to the

presence of Schottky barriers at the GNP/SWNT junction,

which depend on the type of SWNTs. A high contact resistance

(200 kOhm), exists even in case of junctions between two

metallic SWNTs, which is more than an order of magnitude

higher than the intrinsic resistance of SWNTs, while contact

between metallic and semiconducting SWNTs provides the

most resistive junction (>10 MOhm).[36] Thus it is to be

expected that there will be a significant Schottky barrier

between the GNPs and the SWNTs – particularly for the GNP

to semiconducting SWNT contacts. Heat (phonon) transport is

expected to be unaffected by the presence of Schottky barriers.

Figure 3a presents the thermal conductivity of epoxy

composites as a function of hybrid filler loading at GNP:SWNT

weight ratio of 3:1. The strength of the synergistic effect can be

represented as the ratio (kHYB� kGNP)/kGNP (%), and is shown

in Figure 3b as a function of the filler loading. In the 10wt% to

20wt % loading range the synergistic effect exceeds 20% and

the thermal conductivity of the composite with a hybrid filler

reaches k¼ 3.35W m�1 K�1, the highest value achieved for

epoxy composites with a filler loading of less than 20wt%. The

hybridmaterial provides randomly orientedGNPs and SWNTs

in the 3D polymer matrix and the thermal enhancement is

isotropic. At higher loadings (greater than 20wt %), we

observed a decrease of the hybrid filler efficiency with a

crossover in the range of 25–30wt %, and beyond 30wt % the

thermal conductivity provided by theGNP filler became higher

than that of the hybrid filler. We suggest that at high hybrid

loading the GNP concentration increases to the point that an

extended network of GNP conducting pathways is created and

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400

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Figure 3. a) Thermal conductivity of epoxy composites with GNP-SWNThybrid filler (red circles, GNP:SWNT¼ 3:1) and GNP filler (black squares)as a function of the filler loading. b) Synergistic effect (kHYB� kGNP)/kGNP(%) associated with the hybrid filler as a function of the filler loading,where kHYB and kGNP are thermal conductivities of hybrid and GNP fillers,respectively; (c) thermal conductivity enhancement of epoxy compositesfor SWNT, GNP and GNP-SWNT hybrid filler at 10wt % loading incomparison with carbon black (CB).

this dominates the contribution of the SWNT bridges. In our

previous study we observed a decreased efficiency of the

SWNT filler at loadings greater than 5wt %,[12,16] which was

associated with the bending of the SWNTs and decreased

effective aspect ratio,[11] as well as increased viscosity of the

SWNT-filled epoxy which reduced the processability of the

composites and negatively affected the SWNT-polymer matrix

bonding.[12] At high hybrid filler loading (>20wt %) we

observed inhomogeneity of the filler distribution in the

polymer matrix (see Supporting Information) which may

contribute to the suppressed synergistic effect. Thus the

optimum range of application for the GNP-SWNT hybrid filler

Adv. Mater. 2008, 20, 4740–4744 � 2008 WILEY-VCH Verl

is 10–20wt % and this loading provides thermal conductivity

which would otherwise require 60–70wt % loading of

conventional fillers.[2] Most likely, the optimum ratio of

GNP and SWNT components in the hybrid filler for thermal

conductivity enhancement varies with the total filler loading

and can shift the point of crossover towards higher loading, but

these details of the filler performance are deferred for later

study. For comparison, we prepared epoxy composites filled

with carbon black (CB), which is one of the most commonly

used fillers for composites materials. Figure 3c shows that the

thermal conductivity enhancements (k� k0)/k0 achieved with

the GNP-SWNT hybrid filler (775%) exceeds, by more than a

factor of 10, the performance of carbon black at the same

loading.

Currently, the major limitation to the efficiency of high

aspect ratio carbon-based nanofillers (SWNTs and GNPs) and

their underperformance in comparison with the theoretical

predictions originate from the high thermal interface

resistance along the nanofiller percolating network and at

the interface with the polymer matrix.[31–34,37–39] This thermal

interface resistance can be reduced and the performance of the

hybrid GNP-SWNT filler can be significantly improved by

careful engineering of the chemical functionalities onGNP and

SWNT surfaces to decrease the thermal interface resistance as

theoretically predicted for SWNT composites.[40]

In conclusion, by combining 1D-SWNT and 2D-graphitic

nanoplatelet fillers we achieved a synergistic effect in the

thermal conductivity enhancement of epoxy composites. We

posit that this synergism originates from the bridging of planar

nanoplatelets by the flexible SWNTs which lead to a decreased

thermal interface resistance along the (2D-1D) hybrid filler

network due to the extended area of the SWNT-GNP junctions.

The hybrid filler reported herein provides the highest efficiency

in the thermal conductivity enhancement of composites amongst

all reported fillers and can be utilized at low filler loading which

is important for decreasing the viscosity and improving the

processability of thermal interface materials.

Experimental

Sample Preparation: The electric arc produced and purified SWNTmaterial (P3-SWNT, average SWNT diameter �1.4 nm, length of0.5–1mm, bundle diameter of 4–5 nm) was obtained from CarbonSolutions, Inc. Exfoliated graphite was prepared by the acidintercalation and thermal exfoliation of natural graphite flakes(500mm,AsburyGraphiteMills Inc.). The exfoliated graphite particleswere subjected to high shear mixing for 30 minutes followed by bathsonication for 24 h (sonic power 270W) in acetone to obtain graphitenanoplatelets (GNPs). [12] For a typical hybrid sample preparation(filler loading of 10wt % and GNP:SWNT weight ratio of 3:1), 0.833 gexfoliated graphite and 0.278 g SWNTs were dispersed in 300mLacetone through 30min shearmixing and 24 h sonication. Subsequently7.94 g epoxy (pre-polymer, diglycidyl ether of bisphenol F, EPON 862)was added and shear mixed for another 30min. The acetone wasremoved from the dispersion in air and 2.06 g curing agent(diethyltoluenediamine, EPI-KURE)was added. The resulting gel-likematerial was loaded in a custom-mademold, and cured at 80 8C, 100 8C,and 150 8C for 2h at each temperature in a vacuum oven to prepare the

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disks (diameter 2.54 cm, thickness 4 to 12mm). For comparisonpurposes, 1.11 g of carbon black (CB, Vulcan XC72R, 20 nm, CabotCorp.), SWNTs and GNPs were separately used to prepare compositeswith 10wt % loading following the procedure described above.

Measurements: Scanning electron microscopy (SEM) images wereobtained with a Philips XL30-FEG instrument operating at 10 kV. Anumber of samples were cut from different locations of the compositedisk by razor blade and used for SEM analysis. Other SEM samplepreparation techniques included machining, polishing and etchingwere also used, however the razor blade cutting provided the mostrepresentative imaging of the composite samples. High-resolutionTEM analysis was performed with Technai12 instrument operating at120 kV accelerating voltage. TEM samples with a thickness of�200 nmwere cut from the composite disks using microtome. Thermalconductivity of disk shaped composite samples with a 2.54 cm diameterwas measured using a FOX50 (Laser Comp. Inc.), steady-state heatflowmeasurement apparatus, employing a dual thicknessmeasurementcycle in order to eliminate the thermal contact resistance of the sample;a value of k0¼ 0.201Wm�1 K�1 was obtained for neat epoxy. Electricalconductivity was measured by a commercial instrument (Signatone,S-302-4) with four in-line probes. For a specific sample, the testing wasperformed 10 times at different positions of the disk surface and thefinal value was averaged. Correction was made for the finite diskgeometry taking into account the disk diameter (2.54 cm), the probe tipspacing (1.59mm) and the thickness of the disk (4 to 12mm). [41, 42]Four probe electrical conductivity measurements were also performedby the van der Pauw technique by placing four narrow lines of silverpaint at the edge of the disk (908 angle between adjacent contacts) inorder to produce 2D current flow. [43, 44] The results from the two setsof measurements were comparable.

Received: February 8, 2008Revised: May 19, 2008

Published online: October 23, 2008

[1] D. G. Cahill, W. K. Ford, K. E. Goodson, G. D. Mahan, A. Majumdar,

H. Maris, R. Merlin, S. R. Phillpot, J. Appl. Phys. 2003, 93, 793.

[2] R. S. Prasher, J.-Y. Chang, I. Sauciuc, S. Narasimhan, D. Chau, G.

Chrysler, A. Myers, S. Prstic, C. Hu, Intel Technol. J. 2005, 9, 285.

[3] P. K. Schelling, L. Shi, K. E. Goodson, Mater. Today 2005, 8, 30.

[4] R. Prasher, Proc. IEEE 2006, 94, 1571.

[5] P. Kim, L. Shi, A. Majumdar, P. L. McEuen, Phys. Rev. Lett. 2001, 87,

215502.

[6] M. J. Biercuk, M. C. Llaguno, M. Radosavljevic, J. K. Hyun, A. T.

Johnson, J. E. Fischer, Appl. Phys. Lett. 2002, 80, 2767.

[7] E. S. Choi, J. S. Brooks, D. L. Eaton, M. S. Al-Haik,M. Y. Hussaini, H.

Garmestani, D. Li, K. Dahmen, J. Appl. Phys. 2003, 94, 6034.

[8] C. H. Liu, H. Huang, Y.Wu, S. S. Fan,Appl. Phys. Lett. 2004, 84, 4248.

[9] E. T. Thostenson, C. Y. Li, T.-W. Chou, Compos. Sci. Technol. 2005,

65, 491.

[10] C. Yu, L. Shi, Z. Yao, D. Li, A. Majumdar, Nano Lett. 2005, 5, 1842.

[11] P. C. Song, C. H. Liu, S. S. Fan, Appl. Phys. Lett. 2006, 88, 153111.

[12] A. Yu, M. E. Itkis, E. Bekyarova, R. C. Haddon, Appl. Phys. Lett.

2006, 89, 133102.

www.advmat.de � 2008 WILEY-VCH Verlag GmbH &

[13] M.-T. Hung, O. Choi, Y. S. Ju, H. T. Hahn, Appl. Phys. Lett. 2006, 89,

023117.

[14] A. Mills, M. Farid, J. R. Selman, S. Al-Hallaj,Appl. Therm. Eng. 2006,

26, 1652.

[15] H. Fukushima, L. T. Drzal, B. P. Rook, M. J. Rich, J. Therm. Anal.

Calorim. 2006, 85, 235.

[16] A. Yu, P. Ramesh, M. E. Itkis, E. Bekyarova, R. C. Haddon, J. Phys.

Chem. C. 2007, 111, 7565.

[17] S. Shaikh, K. Lafdi, R. Ponnappan, J. Appl. Phys. 2007, 101, 064302.

[18] B. Debelak, K. Lafdi, Carbon 2007, 45, 1727.

[19] S. Ghose, D. C. Working, J. W. Connell, J. G. Smith, K. A. Watson, D.

M. Delozier, Y. P. Sun, Y. Lin, High Perform. Polym. 2006, 18,

961.

[20] E. H. Weber, M. L. Clingerman, J. A. King, J. Appl. Polym. Sci. 2003,

88, 112.

[21] G. W. Lee, M. Park, J. Kim, J. I. Lee, H. G. Yoon, Composites Part A

2006, 37, 727.

[22] A. Yu, E. Bekyarova, M. E. Itkis, D. Fakhrutdinov, R. Webster, R. C.

Haddon, J. Am. Chem. Soc. 2006, 128, 9902.

[23] E. Bekyarova, E. T. Thostenson, A. Yu, M. E. Itkis, D. Fakhrutdinov,

T.-W. Chou, R. C. Haddon, J. Phys. Chem. C 2007, 111, 17865.

[24] D. D. L. Chung, J. Mater. Sci. 1987, 22, 4190.

[25] P. Ramesh, S. Sampath, Chem. Commun. 1999, 2221.

[26] P. Ramesh, S. Sampath, Anal. Chem. 2003, 75, 6949.

[27] G. H. Chen, D. J. Wu, W. U. Weng, C. L. Wu, Carbon 2003, 41, 619.

[28] M. Inagaki, R. Tashiro, Y. Washino, M. Toyoda, J. Phys. Chem. Solids

2004, 65, 133.

[29] A. Celzard, J. F. Mareche, G. Furdin, Prog. Mater. Sci. 2005, 50, 93.

[30] S. Stankovich, D. A. Dikin, G. H. B. Dommett, K. M. Kohlhaas, E. J.

Zimney, E. A. Stach, R. D. Piner, S. T. Nguyen, R. S. Ruoff, Nature

2006, 442, 282.

[31] S. T. Huxtable, D. G. Cahill, S. Shenogin, L. Xue, R. Ozisik, P. Barone,

M. L. Usrey, M. S. Strano, G. Siddons, M. Shim, P. Keblinski, Nat.

Mater. 2003, 2, 731.

[32] C. W. Nan, Z. Shi, Y. Lin, Chem. Phys. Lett. 2003, 375, 666.

[33] S. Shenogin, L. P. Xue, R. Ozisik, P. Keblinski, D. G. Cahill, J. Appl.

Phys. 2004, 95, 8136.

[34] N. Shenogina, S. Shenogin, L. Xue, P. Keblinski, Appl. Phys. Lett.

2005, 87, 133106.

[35] R. Prasher, Appl. Phys. Lett. 2007, 90, 143110.

[36] M. S. Fuhrer, J. Nygard, L. Shih, M. Forero, Y.-G. Yoon, M. S. C.

Mazzoni, H. J. Choi, J. Ihm, S. G. Louie, A. Zettl, P. L. McEuen,

Science 2000, 288, 494.

[37] C. W. Nan, R. Birringer, D. R. Clarke, H. Gleiter, J. Appl. Phys. 1997,

81, 6692.

[38] P. Keblinski, F. Cleri, Phys. Rev. B 2004, 69, 184201.

[39] M. B. Bryning, D. E. Milkie, M. F. Islam, J. M. Kikkawa, A. G. Yodh,

Appl. Phys. Lett. 2005, 87, 16 1909.

[40] S. Shenogin, A. Bodapati, L. Xue, R. Ozisik, P. Keblinski,Appl. Phys.

Lett. 2004, 85, 2229.

[41] F. M. Smits, Bell Syst. Tech. J. 1958, 37, 711.

[42] D. K. Schroder, Semiconductor Material and Device Characterization,

Wiley, NJ, USA 2006.

[43] L. J. van der Pauw, Philips Tech. Rev. 1958/59, 20, 220.

[44] C. Kasl, M. J. R. Hoch, Rev. Sci. Instrum. 2005, 76, 033907.

Co. KGaA, Weinheim Adv. Mater. 2008, 20, 4740–4744