- Email: [email protected]
The production of chemically converted graphenes from graphite fluoride
CARBON
1425
5 0 ( 20 1 2 ) 1 4 2 2–14 4 4
The production of chemically converted graphenes from graphite fluoride Athanasios B. Bourlinos
a,b
, Klara Safarova b, Karolina Siskova b, Radek Zborˇil
b,*
a
Physics Department, University of Ioannina, GR-45110 Ioannina, Greece Regional Centre of Advanced Technologies and Materials, Faculty of Science, Department of Physical Chemistry, Palacky University in Olomouc, 77146 Olomouc, Czech Republic
b
A R T I C L E I N F O
A B S T R A C T
Article history:
The liquid-phase extraction of graphene fluoride by exfoliation of graphite fluoride in
Received 3 June 2011
dimethylformamide and subsequent reduction with triethylsilane or zinc particles yields
Accepted 7 October 2011
the corresponding chemically converted graphene, namely reduced graphene fluoride.
6 October Available online 14 October2011 2011
The formation of either graphene fluoride (fluorographene) or graphene monolayers in the course of the reaction was demonstrated by several microscopy techniques. Fluorine elimination after reduction was verified by elemental analysis and infrared/Raman spectroscopy. Ó 2011 Elsevier Ltd. All rights reserved.
Chemically converted graphenes (CCGs) are often described in the literature as reduced graphene oxide [1]. Typically, graphite oxide (GO) is exfoliated in water to afford colloidal graphene oxide. In a follow-up step, the dispersed layers are converted into graphenes using a variety of mild reducing agents. While numerous reports deal with this well established protocol, however, only limited work refers to analogous processes utilizing graphite fluoride (GF) in a similar manner, e.g. exfoliation in a solvent and subsequent reduction. GF is another covalent derivative of graphite [2] that merits further attention as a potential source of graphene through intermediate graphene fluoride. Likewise the case with GO, CCGs from GF could be envisaged as being reduced graphene fluoride. In a previous study we have demonstrated that the liquid-phase exfoliation of GF in sulfolane results in dispersed graphene fluoride that upon halide exchange and disproportionation reactions involving potassium iodide (KI) is converted into graphene [3]. Herein we explore new reaction pathways towards CCGs from GF. In particular, we present alternative preparations of graphenes based on the reduction of solvent-extracted graphene fluoride directly from GF. To this aim, we use dimethylformamide (DMF) as solvent and Et3Si-H or Zn particles as reducing agents. In a standard procedure, 750 mg purchased GF (Aldrich, CF0.5) was suspended in 150 mL DMF under sonication (2 h) in an ultrasound bath operating at 130 W. DMF is considered a good solvent for the direct dispersion of various carbon allotropes by sonication, including carbon nanotubes, carbon cones, graphene/graphene oxide and nanodiamonds. The suspension was left in rest for 2–4 days prior collecting the
supernatant clear colloid. The colloid had a pale grayish hue and gave an intense Tyndall effect with a laser pointer. The graphene fluoride production yield was estimated at 2–3%. In case of hydride reduction, 2 g Et3Si-H (triethylsilane, Alfa Aesar) were added in 50 mL colloid and the solution was refluxed for a day under stirring. The black solid formed was collected by centrifugation at 5000 rpm for 10 min, washed with DMF and re-suspended by sonication in few mL DMF for further characterizations. In case of metal reduction, 200 mg Zn fine powder (BDH Chemicals) was suspended in 50 mL colloid and the mixture was refluxed for a day under stirring. After reaction completion, the mixture was centrifuged at 5000 rpm for 10 min and the solid was suspended in 15 mL water. Following, Zn was dissolved by the dropwise addition of concentrated HCl. The remaining black solid was centrifuged at 5000 rpm for 10 min, washed well with water and re-suspended by sonication in few mL water for further characterizations. The as-derived CCGs are thereafter denoted as RGF (from reduced graphene fluoride). In the first step, the liquid-phase exfoliation of GF in DMF provides colloidal graphene fluoride. According to this versatile technique [4], thick multilayer plates are suspended in a liquid medium in order to solvent-extract single layers by mild sonication. Following, prolonged sedimentation removes any insoluble particulates or thick plates from solution. Finally, reduction of the dispersed layers by Et3Si-H or Zn particles yields the corresponding graphenes (codenamed RGF). The silane resembles NaBH4 employed for the reduction of graphite oxide/graphene oxide into graphite/graphene [5,6]. Generally, the reductive defluorination of perfluorocycloalkanes is considered to occur by transfer of electrons from a donor to the
* Corresponding author: Fax: +420 58 563 4958. E-mail addresses: [email protected] (R. Zborˇil). 0008-6223/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.carbon.2011.10.012
1426
CARBON
5 0 ( 2 0 1 2 ) 1 4 2 2 –1 4 4 4
Fig. 1 – SEM (top) and TEM (bottom) images of folded graphene fluoride sheets overlapping to each other.
Fig. 3 – SEM (top) and TEM (bottom) images of RGF sheets derived by hydride reduction. The inset SAED pattern shows a single hexagon reflection.
Fig. 2 – AFM image (top) and height profile (bottom) of a single-layer graphene fluoride. Fig. 4 – AFM image (top) and height profile (bottom) of a single-layer RGF derived by hydride reduction. r* orbital of tertiary carbon–fluorine bonds to give radical anions [7]. The radical anions which are formed lose fluorine to begin a cascade process that can lead to complete defluorina-
tion of the substrate into a neat polyaromatic system [7]. On the other hand, Zn fine powder is widely exploited for the
CARBON
5 0 ( 20 1 2 ) 1 4 2 2–14 4 4
1427
Fig. 5 – TEM images of RGF derived by metal reduction depicting several overlapped thin sheets (left) along with the portrait of a single sheet (right). The inset SAED pattern shows that the sheets are crystalline. Zn has been removed by mild acid treatment. de-halogenation of vicinal haloalkanes into alkenes [8] according to the reaction scheme: X–C–C–X þ Zn ! –C@C– þ ZnX2 ðX : halideÞ In this respect, graphene fluoride can be regarded a vicinal perfluoroalkane whose complete de-fluorination may result in an extended conjugated system of alternating single and double bonds, similar to graphene. In our case, this reaction is feasible mainly due to the accessibility of the zinc surface by the dispersed layers in solution. The metal can be then removed by mild acid treatment. The presence of graphene fluoride in the stock colloid was demonstrated by the scanning electron microscopy (SEM), transmission electron microscopy (TEM) and atomic force microscopy (AFM) techniques. In previous TEM studies over the pristine GF we had observed thick micro-platelets typical of a layered solid [3]. On the other hand, the SEM and TEM images of the exfoliated graphene fluoride shown here (Fig. 1) display folded thin sheets of large lateral dimensions (submicron to several microns) overlapping to each other. In addition, the sheets in the TEM portrait appear with a rather rough surface texture thank to a high degree of fluorination. Generally, unmodified graphenes appear with a flat and smooth surface; however, dramatic covalent functionalization thereof may induce such surface roughness [9]. The experimental thickness of the sheets was assessed at 0.92 nm by AFM (Fig. 2 and Fig. S1 in Supplementary material). This value is close to the theoretical thickness of graphene fluoride (0.65 nm) [3]. In line with TEM, AFM reveals a rough surface texture as well. This roughness may eventually contribute to the slightly higher thickness measured for graphene fluoride. The SEM and TEM images of the RGF sheets derived by hydride reduction are shown in Fig. 3. Once again, we observe thin layers with lateral sizes in the micron region. The sheets appear crystalline as they display a single hexagon reflection in their selected area electron diffraction (SAED) pattern (see inset). A specific fingerprint of single-layer crystals is that in the diffraction pattern the intensity of the hexagon reflections fades as moving from the center to the periphery, e.g. the intensity of the inner reflections is always stronger than that of the outer ones [10]. Accordingly, the present SAED pat-
tern pinpoints a single-layer graphene. Indeed, the experimental thickness of the reduced sheets was assessed at 0.48 nm by AFM (Fig. 4 and Fig. S2 in Supplementary material). This value is very close to the theoretical thickness of graphene (0.35 nm) [1]. In addition, it seems that fluorine removal restores the surface to better clarity. The energy dispersive X-ray (EDX) elemental profile of RGF clearly supports fluorine elimination after reduction, as compared to GF (Fig. S3, Supplementary material). In this same context, the infrared (IR) spectrum of GF layers exhibits a characteristic band at 1219 cm1 due to the C–F stretching vibration that is suppressed in RGF (Fig. S4, Supplementary material). The conversion of GF to RGF also causes radical changes in the Raman spectra (Fig. S5, Supplementary material). While GF shows no Raman peaks in the region 1000–1800 cm1 [11], the Raman signal of RGF consisted of two peaks. The peaks position is very similar to those reported for D band (ca. 1320 cm1) and G band (ca. 1590 cm1) of reduced graphene oxide [6]. An alternative to the hydride reduction is the de-halogenation of graphene fluoride by Zn particles. Fig. 5 shows representative TEM images of large and crystalline RGF thin sheets derived by metal reduction (Zn has been removed by mild acid treatment). The inset SAED pattern suggests the presence of multi-layers (graphite-like halo rings near the periphery) in addition to monolayers (single hexagon reflection near the center). Fluorine elimination after reduction was supported by EDX analysis (Fig. S6, Supplementary material). As mentioned earlier, the reduction of graphene fluoride by Zn is possible only through direct contact of the dispersed layers with the zinc surface in solution. This type of interaction becomes clear in the TEM/SEM images of some intermediate hybrids showing RGF thin sheets surface covered by zinc particles (Fig. S7, Supplementary material). De-halogenation could be possibly expanded to other reactive metals, including zero-valent iron. In conclusion, it has been presented new alternatives to prepare CCGs from GF. For this purpose, GF is dispersed in DMF with the aid of ultrasound. This results in exfoliated graphene fluoride in solution. The dispersed monolayers are then chemically reduced to afford crystalline graphenes. This approach is complementary to the wet chemistry of graphenes from GO.
1428
CARBON
5 0 ( 2 0 1 2 ) 1 4 2 2 –1 4 4 4
Acknowledgement The work was supported by the Operational Program Research and Development for Innovations-European Social Fund (CZ.1.05/2.1.00/03.0058).
Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.carbon.2011.10.012.
[4] [5]
[6]
[7] [8]
R E F E R E N C E S
[9] [1] Park S, Ruoff RS. Chemical methods for the production of graphenes. Nature Nanotechnol 2009;4:217–24. [2] Kita Y, Watanabe N, Fujii Y. Chemical composition and crystal-structure of graphite fluoride. J Am Chem Soc 1979;101:3832–41. [3] Zborˇil R, Karlicky´ F, Bourlinos AB, Steriotis TA, Stubos AK, Georgakilas V, et al. Graphene fluoride: a stable
[10]
[11]
stoichiometric graphene derivative and its chemical conversion to graphene. Small 2010;6:2885–91. Coleman JN. Liquid-phase exfoliation of nanotubes and graphene. Adv Funct Mater 2009;19:3680–95. Bourlinos AB, Gournis D, Petridis D, Szabo´ T, Szeri A, De´ka´ny I. Graphite oxide: chemical reduction to graphite and surface modification with primary aliphatic amines and amino acids. Langmuir 2003;19:6050–5. Gengler RYN, Veligura A, Enotiadis A, Diamanti EK, Gournis D, Jo´zsa C, et al. Large-yield preparation of high-electronicquality graphene by a Langmuir–Schaefer approach. Small 2010;6:35–9. Sandford G. Perfluoroalkanes. Tetrahedron 2003;59:437–54. Totten LA, Jans U, Roberts AL. Alkyl bromides as mechanistic probes of reductive dehalogenation: reactions of vicinal dibromide stereoisomers with zerovalent metals. Environ Sci Technol 2001;35:2268–74. Georgakilas V, Bourlinos AB, Zborˇil R, Steriotis TA, Dallas P, Stubos AK, et al. Organic functionalisation of graphenes. Chem Commun 2010;46:1766–8. Catheline A, Valle´s C, Drummond C, Ortolani L, Morandi V, Marcaccio M, et al. Graphene solutions. Chem Commun 2011;47:5470–2. Nair RR, Ren W, Jalil R, Riaz I, Kravets VG, Britnell L, et al. Fluorographene: a two-dimensional counterpart of teflon. Small 2010;6:2877–84.
Thermal conductivity of nanocrystalline carbon films studied by pulsed photothermal reflectance M. Shakerzadeh B.K. Tay a,b,*
a,b
, M.K. Samani a, N. Khosravian a, E.H.T. Teo c, M. Bosman d,
a
School of Electrical and Electronic Engineering, Nanyang Technological University, Nanyang Avenue, Singapore 639798, Singapore CINTRA-CNRS/NTU/THALES, UMI 3288, Research Techno Plaza, 50 Nanyang Drive, Border X Block, Level 6, Singapore 637553, Singapore c Temasek Laboratories@NTU, 50 Nanyang Avenue, Singapore 639798, Singapore d A*STAR Institute of Materials Research and Engineering (IMRE), 3 Research Link, Singapore 117602, Singapore b
A R T I C L E I N F O
A B S T R A C T
Article history:
The effect of nanocrystals with preferred orientation on the thermal conductivity of carbon
Received 3 March 2011
films is studied. During graphitization, the presence of biaxial compressive stress results in
Accepted 8 October 2011 Available online 19 October2011 2011 6 October
the formation of preferred orientation in the microstructure of graphitic nanocrystals if the corresponding activation energy is supplied. This formation of preferred orientation leads to the orientation of graphitic basal planes perpendicular to the substrate. Due to the high thermal conductivity of graphite in the basal planes, there is a significant increase in thermal conductivity of textured nanocrystalline films compared to amorphous film. Ó 2011 Elsevier Ltd. All rights reserved.
Graphite has among the highest (1910 W/m K) thermal conductivity in the direction parallel to basal planes while
possess four orders of magnitude lower conductivity in the perpendicular direction. On the other hand, amorphous car-
* Corresponding author at: School of Electrical and Electronic Engineering, Nanyang Technological University, Nanyang Avenue, Singapore 639798, Singapore. Fax: +65 6792 0415. E-mail address: [email protected] (B.K. Tay). 0008-6223/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.carbon.2011.10.015
1425
5 0 ( 20 1 2 ) 1 4 2 2–14 4 4
The production of chemically converted graphenes from graphite fluoride Athanasios B. Bourlinos
a,b
, Klara Safarova b, Karolina Siskova b, Radek Zborˇil
b,*
a
Physics Department, University of Ioannina, GR-45110 Ioannina, Greece Regional Centre of Advanced Technologies and Materials, Faculty of Science, Department of Physical Chemistry, Palacky University in Olomouc, 77146 Olomouc, Czech Republic
b
A R T I C L E I N F O
A B S T R A C T
Article history:
The liquid-phase extraction of graphene fluoride by exfoliation of graphite fluoride in
Received 3 June 2011
dimethylformamide and subsequent reduction with triethylsilane or zinc particles yields
Accepted 7 October 2011
the corresponding chemically converted graphene, namely reduced graphene fluoride.
6 October Available online 14 October2011 2011
The formation of either graphene fluoride (fluorographene) or graphene monolayers in the course of the reaction was demonstrated by several microscopy techniques. Fluorine elimination after reduction was verified by elemental analysis and infrared/Raman spectroscopy. Ó 2011 Elsevier Ltd. All rights reserved.
Chemically converted graphenes (CCGs) are often described in the literature as reduced graphene oxide [1]. Typically, graphite oxide (GO) is exfoliated in water to afford colloidal graphene oxide. In a follow-up step, the dispersed layers are converted into graphenes using a variety of mild reducing agents. While numerous reports deal with this well established protocol, however, only limited work refers to analogous processes utilizing graphite fluoride (GF) in a similar manner, e.g. exfoliation in a solvent and subsequent reduction. GF is another covalent derivative of graphite [2] that merits further attention as a potential source of graphene through intermediate graphene fluoride. Likewise the case with GO, CCGs from GF could be envisaged as being reduced graphene fluoride. In a previous study we have demonstrated that the liquid-phase exfoliation of GF in sulfolane results in dispersed graphene fluoride that upon halide exchange and disproportionation reactions involving potassium iodide (KI) is converted into graphene [3]. Herein we explore new reaction pathways towards CCGs from GF. In particular, we present alternative preparations of graphenes based on the reduction of solvent-extracted graphene fluoride directly from GF. To this aim, we use dimethylformamide (DMF) as solvent and Et3Si-H or Zn particles as reducing agents. In a standard procedure, 750 mg purchased GF (Aldrich, CF0.5) was suspended in 150 mL DMF under sonication (2 h) in an ultrasound bath operating at 130 W. DMF is considered a good solvent for the direct dispersion of various carbon allotropes by sonication, including carbon nanotubes, carbon cones, graphene/graphene oxide and nanodiamonds. The suspension was left in rest for 2–4 days prior collecting the
supernatant clear colloid. The colloid had a pale grayish hue and gave an intense Tyndall effect with a laser pointer. The graphene fluoride production yield was estimated at 2–3%. In case of hydride reduction, 2 g Et3Si-H (triethylsilane, Alfa Aesar) were added in 50 mL colloid and the solution was refluxed for a day under stirring. The black solid formed was collected by centrifugation at 5000 rpm for 10 min, washed with DMF and re-suspended by sonication in few mL DMF for further characterizations. In case of metal reduction, 200 mg Zn fine powder (BDH Chemicals) was suspended in 50 mL colloid and the mixture was refluxed for a day under stirring. After reaction completion, the mixture was centrifuged at 5000 rpm for 10 min and the solid was suspended in 15 mL water. Following, Zn was dissolved by the dropwise addition of concentrated HCl. The remaining black solid was centrifuged at 5000 rpm for 10 min, washed well with water and re-suspended by sonication in few mL water for further characterizations. The as-derived CCGs are thereafter denoted as RGF (from reduced graphene fluoride). In the first step, the liquid-phase exfoliation of GF in DMF provides colloidal graphene fluoride. According to this versatile technique [4], thick multilayer plates are suspended in a liquid medium in order to solvent-extract single layers by mild sonication. Following, prolonged sedimentation removes any insoluble particulates or thick plates from solution. Finally, reduction of the dispersed layers by Et3Si-H or Zn particles yields the corresponding graphenes (codenamed RGF). The silane resembles NaBH4 employed for the reduction of graphite oxide/graphene oxide into graphite/graphene [5,6]. Generally, the reductive defluorination of perfluorocycloalkanes is considered to occur by transfer of electrons from a donor to the
* Corresponding author: Fax: +420 58 563 4958. E-mail addresses: [email protected] (R. Zborˇil). 0008-6223/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.carbon.2011.10.012
1426
CARBON
5 0 ( 2 0 1 2 ) 1 4 2 2 –1 4 4 4
Fig. 1 – SEM (top) and TEM (bottom) images of folded graphene fluoride sheets overlapping to each other.
Fig. 3 – SEM (top) and TEM (bottom) images of RGF sheets derived by hydride reduction. The inset SAED pattern shows a single hexagon reflection.
Fig. 2 – AFM image (top) and height profile (bottom) of a single-layer graphene fluoride. Fig. 4 – AFM image (top) and height profile (bottom) of a single-layer RGF derived by hydride reduction. r* orbital of tertiary carbon–fluorine bonds to give radical anions [7]. The radical anions which are formed lose fluorine to begin a cascade process that can lead to complete defluorina-
tion of the substrate into a neat polyaromatic system [7]. On the other hand, Zn fine powder is widely exploited for the
CARBON
5 0 ( 20 1 2 ) 1 4 2 2–14 4 4
1427
Fig. 5 – TEM images of RGF derived by metal reduction depicting several overlapped thin sheets (left) along with the portrait of a single sheet (right). The inset SAED pattern shows that the sheets are crystalline. Zn has been removed by mild acid treatment. de-halogenation of vicinal haloalkanes into alkenes [8] according to the reaction scheme: X–C–C–X þ Zn ! –C@C– þ ZnX2 ðX : halideÞ In this respect, graphene fluoride can be regarded a vicinal perfluoroalkane whose complete de-fluorination may result in an extended conjugated system of alternating single and double bonds, similar to graphene. In our case, this reaction is feasible mainly due to the accessibility of the zinc surface by the dispersed layers in solution. The metal can be then removed by mild acid treatment. The presence of graphene fluoride in the stock colloid was demonstrated by the scanning electron microscopy (SEM), transmission electron microscopy (TEM) and atomic force microscopy (AFM) techniques. In previous TEM studies over the pristine GF we had observed thick micro-platelets typical of a layered solid [3]. On the other hand, the SEM and TEM images of the exfoliated graphene fluoride shown here (Fig. 1) display folded thin sheets of large lateral dimensions (submicron to several microns) overlapping to each other. In addition, the sheets in the TEM portrait appear with a rather rough surface texture thank to a high degree of fluorination. Generally, unmodified graphenes appear with a flat and smooth surface; however, dramatic covalent functionalization thereof may induce such surface roughness [9]. The experimental thickness of the sheets was assessed at 0.92 nm by AFM (Fig. 2 and Fig. S1 in Supplementary material). This value is close to the theoretical thickness of graphene fluoride (0.65 nm) [3]. In line with TEM, AFM reveals a rough surface texture as well. This roughness may eventually contribute to the slightly higher thickness measured for graphene fluoride. The SEM and TEM images of the RGF sheets derived by hydride reduction are shown in Fig. 3. Once again, we observe thin layers with lateral sizes in the micron region. The sheets appear crystalline as they display a single hexagon reflection in their selected area electron diffraction (SAED) pattern (see inset). A specific fingerprint of single-layer crystals is that in the diffraction pattern the intensity of the hexagon reflections fades as moving from the center to the periphery, e.g. the intensity of the inner reflections is always stronger than that of the outer ones [10]. Accordingly, the present SAED pat-
tern pinpoints a single-layer graphene. Indeed, the experimental thickness of the reduced sheets was assessed at 0.48 nm by AFM (Fig. 4 and Fig. S2 in Supplementary material). This value is very close to the theoretical thickness of graphene (0.35 nm) [1]. In addition, it seems that fluorine removal restores the surface to better clarity. The energy dispersive X-ray (EDX) elemental profile of RGF clearly supports fluorine elimination after reduction, as compared to GF (Fig. S3, Supplementary material). In this same context, the infrared (IR) spectrum of GF layers exhibits a characteristic band at 1219 cm1 due to the C–F stretching vibration that is suppressed in RGF (Fig. S4, Supplementary material). The conversion of GF to RGF also causes radical changes in the Raman spectra (Fig. S5, Supplementary material). While GF shows no Raman peaks in the region 1000–1800 cm1 [11], the Raman signal of RGF consisted of two peaks. The peaks position is very similar to those reported for D band (ca. 1320 cm1) and G band (ca. 1590 cm1) of reduced graphene oxide [6]. An alternative to the hydride reduction is the de-halogenation of graphene fluoride by Zn particles. Fig. 5 shows representative TEM images of large and crystalline RGF thin sheets derived by metal reduction (Zn has been removed by mild acid treatment). The inset SAED pattern suggests the presence of multi-layers (graphite-like halo rings near the periphery) in addition to monolayers (single hexagon reflection near the center). Fluorine elimination after reduction was supported by EDX analysis (Fig. S6, Supplementary material). As mentioned earlier, the reduction of graphene fluoride by Zn is possible only through direct contact of the dispersed layers with the zinc surface in solution. This type of interaction becomes clear in the TEM/SEM images of some intermediate hybrids showing RGF thin sheets surface covered by zinc particles (Fig. S7, Supplementary material). De-halogenation could be possibly expanded to other reactive metals, including zero-valent iron. In conclusion, it has been presented new alternatives to prepare CCGs from GF. For this purpose, GF is dispersed in DMF with the aid of ultrasound. This results in exfoliated graphene fluoride in solution. The dispersed monolayers are then chemically reduced to afford crystalline graphenes. This approach is complementary to the wet chemistry of graphenes from GO.
1428
CARBON
5 0 ( 2 0 1 2 ) 1 4 2 2 –1 4 4 4
Acknowledgement The work was supported by the Operational Program Research and Development for Innovations-European Social Fund (CZ.1.05/2.1.00/03.0058).
Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.carbon.2011.10.012.
[4] [5]
[6]
[7] [8]
R E F E R E N C E S
[9] [1] Park S, Ruoff RS. Chemical methods for the production of graphenes. Nature Nanotechnol 2009;4:217–24. [2] Kita Y, Watanabe N, Fujii Y. Chemical composition and crystal-structure of graphite fluoride. J Am Chem Soc 1979;101:3832–41. [3] Zborˇil R, Karlicky´ F, Bourlinos AB, Steriotis TA, Stubos AK, Georgakilas V, et al. Graphene fluoride: a stable
[10]
[11]
stoichiometric graphene derivative and its chemical conversion to graphene. Small 2010;6:2885–91. Coleman JN. Liquid-phase exfoliation of nanotubes and graphene. Adv Funct Mater 2009;19:3680–95. Bourlinos AB, Gournis D, Petridis D, Szabo´ T, Szeri A, De´ka´ny I. Graphite oxide: chemical reduction to graphite and surface modification with primary aliphatic amines and amino acids. Langmuir 2003;19:6050–5. Gengler RYN, Veligura A, Enotiadis A, Diamanti EK, Gournis D, Jo´zsa C, et al. Large-yield preparation of high-electronicquality graphene by a Langmuir–Schaefer approach. Small 2010;6:35–9. Sandford G. Perfluoroalkanes. Tetrahedron 2003;59:437–54. Totten LA, Jans U, Roberts AL. Alkyl bromides as mechanistic probes of reductive dehalogenation: reactions of vicinal dibromide stereoisomers with zerovalent metals. Environ Sci Technol 2001;35:2268–74. Georgakilas V, Bourlinos AB, Zborˇil R, Steriotis TA, Dallas P, Stubos AK, et al. Organic functionalisation of graphenes. Chem Commun 2010;46:1766–8. Catheline A, Valle´s C, Drummond C, Ortolani L, Morandi V, Marcaccio M, et al. Graphene solutions. Chem Commun 2011;47:5470–2. Nair RR, Ren W, Jalil R, Riaz I, Kravets VG, Britnell L, et al. Fluorographene: a two-dimensional counterpart of teflon. Small 2010;6:2877–84.
Thermal conductivity of nanocrystalline carbon films studied by pulsed photothermal reflectance M. Shakerzadeh B.K. Tay a,b,*
a,b
, M.K. Samani a, N. Khosravian a, E.H.T. Teo c, M. Bosman d,
a
School of Electrical and Electronic Engineering, Nanyang Technological University, Nanyang Avenue, Singapore 639798, Singapore CINTRA-CNRS/NTU/THALES, UMI 3288, Research Techno Plaza, 50 Nanyang Drive, Border X Block, Level 6, Singapore 637553, Singapore c Temasek Laboratories@NTU, 50 Nanyang Avenue, Singapore 639798, Singapore d A*STAR Institute of Materials Research and Engineering (IMRE), 3 Research Link, Singapore 117602, Singapore b
A R T I C L E I N F O
A B S T R A C T
Article history:
The effect of nanocrystals with preferred orientation on the thermal conductivity of carbon
Received 3 March 2011
films is studied. During graphitization, the presence of biaxial compressive stress results in
Accepted 8 October 2011 Available online 19 October2011 2011 6 October
the formation of preferred orientation in the microstructure of graphitic nanocrystals if the corresponding activation energy is supplied. This formation of preferred orientation leads to the orientation of graphitic basal planes perpendicular to the substrate. Due to the high thermal conductivity of graphite in the basal planes, there is a significant increase in thermal conductivity of textured nanocrystalline films compared to amorphous film. Ó 2011 Elsevier Ltd. All rights reserved.
Graphite has among the highest (1910 W/m K) thermal conductivity in the direction parallel to basal planes while
possess four orders of magnitude lower conductivity in the perpendicular direction. On the other hand, amorphous car-
* Corresponding author at: School of Electrical and Electronic Engineering, Nanyang Technological University, Nanyang Avenue, Singapore 639798, Singapore. Fax: +65 6792 0415. E-mail address: [email protected] (B.K. Tay). 0008-6223/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.carbon.2011.10.015