Charge transfer in Carbon Nanotubes (?)

Transcription

1 Charge transfer in Carbon Nanotubes (?) ESDG 13 Feb 2013 Robert Bell, Arash Mostofi

2 d in the literature. es of over-idealised actions present in ron transport in dispersion interactions which is assumed to render intertube conduction weak, thus severely inhibiting electron transport through macroscopic networks of nanotubes. Little accurate study of this problem and techniques to combat it have been performed to date, largely a consequence of unfavourably-scaling ab initio techniques and the large system sizes required for the study of realistic nanotube structures. Recent developments in local orbital and linear scaling density functional theory (DFT) now permit this study and forms the basis of this work [1]. hydrated nanotube The system We calculate the charge transfer and electron transport through a 68.48Å length (10,0) nanotube segment with increasing numbers of water molecules adsorbed onto a central region of the nanotube. Charge transfers were calculated via both Mulliken analysis and Natural Bond Orbital (NBO) analysis [4]; the latter is less sensitive to choice of basis set improving reliability. Presented here are preliminary results. Transport PBE DFT+D Charge transfer

3 hydrated nanotube The system We calculate the charge transfer and electron transport through a 68.48Å length (10,0) nanotube segment with increasing numbers of water molecules adsorbed onto a central region of the nanotube. Charge transfers were 2 calculated via both Mulliken analysis and Natural Bond Orbital (NBO) analysis [4]; the latter is less sensitive to choice of basis set improving reliability. Presented here are preliminary results. H O molecules can be adsorbed on the nanotubes and act like electron donors Transport Charge transfer The electronic properties of [nanotubes] can be deeply modified by [...] minute quantities of H2O. PBE DFT+D PRB (2000)

4 PRB (2000) Charge transfer model (?)

5 PRB (2000) Charge transfer model (?) +

6 PRB (2000) Charge transfer model (?) Mulliken analysis to assign ionic partial + charges Water net positive CNT net negative

7 LUMO E Eg~1eV HOMO CNT H2O

8 LUMO 0.01 e E Eg~1eV HOMO CNT H2O

9 E Eg~1eV 0.01 e LUMO Expected observations: Strong interaction Modified bandstructure Delocalised electrons Issues: Energy penalty Poor state alignment Scattering effect HOMO CNT H2O

10 E Eg~1eV 0.01 e LUMO Expected observations: Strong interaction Modified bandstructure Delocalised electrons Issues: Energy penalty Nanotechnology 13 (2002) Poor state alignment Scattering effect HOMO CNT H2O

11 E Eg~1eV 0.01 e LUMO Expected observations: Strong interaction Modified bandstructure Delocalised electrons Issues: Energy penalty Nanotechnology 13 (2002) Poor state alignment Scattering effect HOMO CNT H2O

12 E Eg~1eV 0.01 e LUMO Expected observations: Strong interaction Modified bandstructure Delocalised electrons Issues: Energy penalty App. Phys. Lett (2006) Poor state alignment Scattering effect HOMO CNT H2O

13 E Eg~1eV 0.01 e LUMO Expected observations: Strong interaction Modified bandstructure Delocalised electrons Issues: Energy penalty App. Phys. Lett (2006) Poor state alignment Scattering effect HOMO CNT H2O

14 E Eg~1eV 0.01 e LUMO Expected observations: Strong interaction Modified bandstructure Delocalised electrons Issues: Energy penalty App. Phys. Lett (2006) Poor state alignment Scattering effect HOMO CNT H2O

15 67.5 Angstrom (z) = = Z n(~r) dx dy (1) Z h n CNT+H2O(~r) n CNT(~r) n H2O(~r)i dx dy. (2)

16 Single water molecule: dipole away Δλ (10 3 e/å) dqmull ~ 0.01e DFT z (Å)

17 Δλ (10 3 e/å) DFT Δλ (10 3 e/å) DFT z (Å) z (Å) Δλ (10 3 e/å) DFT Δλ (10 3 e/å) DFT z (Å) z (Å)

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20 +q q V loc (~r )= q ~r ~r + h e i~g ~r + V loc (~g )=q g 2 q ~r ~r i e i~g ~r g 2 No electrons added Exact long-range interaction Parameter free

21 Single water molecule: dipole away Δλ (10 3 e/å) z (Å) DFT model

22 Single water molecule: dipole away log 10 (Δλ (e/å)) z (Å) DFT model

23 Δλ (10 3 e/å) DFT model Δλ (10 3 e/å) DFT model z (Å) z (Å) Δλ (10 3 e/å) DFT model Δλ (10 3 e/å) DFT model z (Å) z (Å)

24 3 2 Dipole away Δλ x (10 3 e/å) x (Å)

25 Δλ x (10 3 e/å) Dipole away Dipole toward x (Å)

26 Summary Mulliken analysis has been consistently misinterpreted in the literature Electrostatic effects may be isolated through the use of classical charges There exists no charge transfer between water and CNTs

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29 E b (mev) d C O (Å)

30 E b (mev) x10-3 e d C O (Å)

31 E b (mev) x10-3 e d C O (Å)

32 E b (mev) x10-3 e d C O (Å)

33 E b (mev) x10-3 e d C O (Å)

34 E b (mev) mev d C O (Å)

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37 4.3 Angstrom

38 4.3 Angstrom

39 4.3 Angstrom

40 4.3 Angstrom ~ 40 mev

41 4.3 Angstrom ~ 40 mev