Complementary in-situ probes for. Graphene and CNT CVD. Department of Engineering

Transcription

1 Complementary in-situ probes for Graphene and CNT CVD Department of Engineering

2 In-situ nanometrology mean free path of e - scattering in gas: λ e = 1/nσ = kt/pσ for electron energy of 20keV, and p=1mbar λ e 1 cm TecnaiF20, kV, differential pumping environmental TEM sample ASU e - (KE <1500 ev): small mean free path in solid λ e ~ 1 nm XPS characterises (sub)surface high pressure XPS FHI beamline at Bessy in-situ XRD to probe bulk effects Rossendorf Beamline at ESRF extensive ex-situ calibrations Complementary approach of point and integral probing

3 1CNT vs graphene CVD 2 CNT CVD on conductive support

4 1 CNT vs graphene CVD minimum: CNT CVD requires nanoparticle catalyst which dictates CNT size distribution interesting comparison to graphene CVD eg based on thick polycrystalline catalyst films

5 Reshaping of solid catalyst particles capillary-driven surface-diffusion is dominant transport mechanism μ r =(2γVcosΘ)/r μ R =(2γV)/R time-scale of restructuring is determined by reduced Ni diffusion through the stepped Ni-C interface region Moseler et al., ACS Nano 4, 7587 (2010)

6 Fe/SiO 2 XPS during C 2 H 2 exposure SiOx+0.25 nm Fe Fe active in reduced state Heating up in vacuum NH 3 exposure (0.8 mbar) O 2 exposure ( mbar) NH 3 exposure (0.5 mbar) C 2 H 2 exposure (~ mbar) Another C 2 H 2 exposure (~up to mbar) Hofmann et al., J. Phys. Chem. C 113, 1648 (2009) Hofmann et al., Nano Lett. 7, 602 (2007)

7 The carbide question

8 Fe/SiO 2 Fe/Al XRD 2 during O 3 XRD C 2 during H 2 exposure C 2 H 2 exposure (% α-fe % γ-fe) (% α-fe >> % γ-fe) (100 % γ -Fe) (α-fe + γ-fe) (α-fe + γ-fe + Fe 3 C) (100 % γ -Fe) alpha (bcc) to gamma (fcc) phase transition for Fe nano-particles Bulk Fe 3 C can form, but not necessity for CNT growth Fe 3 C appears to form from alpha Fe

9 2 CNT CVD on conductive support

10 Support interactions Fe on SiO x /Al 2 O 3 SiO 2 Al 2 O 3 (10nm) on SiO2 UHV system (10-10 mbar base p) Fe (~0.5 nm) deposited in-situ, vacuum anneal ~580C, C 2 H 2 8*10-5 mbar (PE)CVD system (10-7 mbar base p) air exposed Fe (~0.3 nm), NH3 anneal ~500C, C2H mbar Microwave CVD system (10-3 mbar base p) air exposed Fe (~0.5 nm), CH4/H2 600C, 60W plasma power (Zhong et al.) 100nm 500nm Furnace (atmospheric p) air exposed Fe (~0.7 nm), Ar/H2 anneal ~750C, C2H2

11 Oxides Wardle et al. no catalytic C 2 H 2 dissociation on Al 2 O 3 but After H 2 Exposure Zr3d zirconia catalysed CNT CVD During CNT growth C 2 H 2 :H 2 1:5 525 C Cool down in Vacuum 200 C Binding Energy [ev] (sub)oxide acts as CNT catalyst Mattevi et al., J. Phys. Chem. C 112, (2008) Steiner et al., JACS 131, (2009)

12 FeTa Metals as conductive support much more complex problem reactions with carbon gas (+contamination), alloys with catalyst Strong dependence on process conditions Bayer et al, J. Phys. Chem. C 115, 4359 (2011)

13 FeTa Strong dependence on process conditions Bayer et al, J. Phys. Chem. C 115, 4359 (2011)

14 FeTa Fe & Ta form surface alloy Fe promotes oxidation of Ta Ta promotes reduction of Fe small residual oxygen/water amount enough to oxidise Ta Bayer et al, J. Phys. Chem. C 115, 4359 (2011)

15 FeTa part II Ta bimetallic co-catalyst Fe-Ta optimum Fe-Ta ratio Si Si

16 FeTa part II Solid state reducing agent (unlike FeCu and sim. where reducing gas is required)

17 Co silicide support nitrides or silicides offer conductivity and stability CoSi 2 is a good candidate as a support material: Low resistivity High chemical & structural stability Fully ULSI/CMOS-compatible Fe nanoparticles become stabilized on the CoSi 2 surface by an interfacial reaction thin SiO 2 top barrier

18 High density mats Ability to grow forests with area density of >1.3x10 13 cm -2 density (cm-2) 1E+14 1E+13 1E+12 1E+11 Zhong 2010 Zhong goal Esconjauregui Hata fully compact forrest de-wetting limit Yamazaki 1E+10 Rogers/ TFT, horizontal Yokoyama 1E diameter (nm)

19 Conclusions 1+2 Many of the posed questions do not have a simple, single answer

20 Acknowledgements Dorothy Hodgkin Fellowship A. D. Gamalski, R. Weatherup, P. Kidambi, C. T. Wirth, B. Bayer, M. Fouquet, S. Esconjauregui, J. Robertson C. Ducati Department of Materials Science and Metallurgy, University of Cambridge R. Blume, M. Haevecker, A. Knob-Gericke, R. Schloegl Fritz Haber Institute of the Max Planck Society, Berlin R. Sharma NIST J. Tersoff IBM C. Baetz Rossendorf ESRF, Grenoble R. Dunin-Borkowski Center for Electron Nanoscopy, Technical University of Denmark C. Cepek, A. Goldoni CNR TASC,Trieste, Italy S. Steiner, B.L. Wardle MIT D. Stokes FEI Michael Moseler Fraunhofer-Institute for Mechanics of Materials, Freiburg, Germany and all the people I forgot to mention

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