DSpace at VNU: Reconstruction and electronic properties of interface between carbon nanotubes and ferromagnetic Co electrodes

Applied Mechanics and Materials Vols 229-231 (2012) pp 183-187 © (2012) Trans Tech Publications, Switzerland doi:10.4028/www.scientific.net/AMM.229-231.183 Online: 2012-11-29 Reconstruction and Electronic Properties of Interface between Carbon Nanotubes and Ferromagnetic Co Electrodes Nguyen Tien Cuong1,2,a, Mohd Ambri Mohamed1, Nobuo Otsuka1, and Dam Hieu Chi1,2,b School of Materials Science, Japan Advanced Institute of Science and Technology, 1-1 Asahidai, Nomi, Ishikawa, 923-1292, Japan Faculty of Physics, Hanoi University of Science, VNU-Hanoi, 334 Nguyen Trai, Thanh Xuan, Hanoi, Vietnam a cuongnt@jaist.ac.jp , bdam@jaist.ac.jp Keywords: Carbon nanotubes, CNTs, Ferromagnetic electrodes, Co electrodes, Fisrt-principle calculatons, Reconstruction of interface between CNTs and ferromagnetic electrodes Abstract: The reconstruction and electronic structures of the interfaces between single wall carbon nanotubes and ferromagnetic Co electrodes were studied in the framework of density functional theory The obtained results revealed that there is a strong interaction between carbon nanotubes and Co electrodes At the interface region, Top layers of Co surface have been significantly reconstructed The nature of chemical bonds at the Co-C interface is covalent bonding The increase of the electron density occurs mainly at the interface where a substantial concentration of electron accumulates in Co-C bonds A small amount of charge transfer from Co electrode to carbon nanotube junction was found In addition, the spin polarization of Co atoms at the interface region has been suppressed due to the interaction with a carbon nanotube It implies that the spin transport through this layer is low, which results in the small hysteretic magneto-resistance of carbon nanotube devices Introduction Carbon nanotubes (CNTs) are molecular-scale tubes of graphitic carbon with many novel properties originating from the small size and cylindrical structure CNTs exhibit long electron mean-free path [1,2] and weak spin orbit coupling, so the spin diffusion length is expected to be extremely long [3] In addition, it has a higher carrier velocity (~106 ms-1) with a potentially very long spin relaxation time (spin lifetime, τs), compared to purely metallic systems Thus, they are considered to be a potential candidate for a material used in spintronic devices Actually, field-effect transistors (FETs) [4] and spin valve device [5] using CNTs have been demonstrated Their performance is, however, still much lower than expected One of the most important issues is weak electric contact between CNTs and electrodes In the fabrication process of FETs, the method for disposition of CNTs in the channel of FETs is one of the most important issues to be established There are two major methods in fabricating CNTFET which are postdispersion [6] and predispersion [7] In postdispersion method, CNT is dispersed in between the source and drain electrodes fabricated beforehand Although this method is the most popular and easiest, weak electric contact between CNTs and electrodes, and contamination of dispersion medium in CNTs are unavoidable The other method is predispersion, where CNTs are first dispersed on the substrate, followed by formation of electrodes on the CNTs by a lithography method In this method, electric contact between CNTs and electrodes can be greatly improved However, the contamination problem, still remains where some chemical residue remains on the CNTs after lithography The direct growth method, where CNTs are directly grown and bridging the electrodes, is an ideal method for solving existing issues Recently, we have successfully fabricated the CNT-FETs by direct synthesis of single-walled nanotubes (SWNTs) from ferromagnetic electrodes [8-10] The devices showed ambipolar operation with very small carrier injection barriers However, they showed spin-dependent transport property with small hysteretic magneto-resistance (MR=1.8%) at low temperatures [9] Theoretically, the key issues that All rights reserved No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of Trans Tech Publications, www.ttp.net (ID: 132.239.1.230, University of California, San Diego, La Jolla, USA-03/06/15,23:29:43) 184 Mechanical and Electrical Technology IV remain to be addressed is understanding of the interface between SWNT and metal electrodes [11] Furthermore, the contact with the electrodes sensitively influences the transport properties of SWNTs The nature of atomic bonding at interface could affect scattering in the electron transport Therefore, it is necessary to investigate the reconstruction and electronic structures of the interfaces between SWNTs and metal electrodes, in the atomic scale In this work, the first-principles density functional calculations were performed to clarify the nature of contact between ferromagnetic Co electrodes and SWNTs The obtained results have provided valuable information for experimental studies Model and Calculation Method In our experiments, Co electrodes were annealed at 9000C At the temperature, the Co usually exists in the faced center cube (FCC) structure [8] Thus, FCC Co crystal is chosen as an initial model for building the Co surface The periodic system including FCC Co (100) electrode contacted SWNTs junctions which were terminated by hydrogen atoms, as shown in figure 1, were used for first-principle density functional calculations For the electrodes, the large area of the Co surface of 5x5 unit cells was chosen in order to minimize the effect of the interactions between SWNT and its neighbors Two bottom layers of Co surface are fixed and other Co atoms and SWNT are relaxed for investigating the possibility of the reconstruction at the Co/SWNTs interface For the junctions, both of (5,5) metallic SWNT and (8,0) semiconducting SWNT were examined Fig.1 Configuration of the system: Co(100) – 5x5 surface and H-terminated SWNT All first-principles calculations were performed using the software package OpenMX which was based on a linear combination of pseudo-atomic orbital method [12] The exchange-correlation energy GGA-PBE functional was used In the treatment of pseudo-potentials, the 3s, 3p semicore states of Co atoms were treated as valence states Double-valence and polarization orbitals were included as basis set, which are s2p2d2f1 for Co, s2p2d1 for C and s2p1 for H The cutoff radius for the pseudo-atomic orbitals was chosen based on the comparison of the calculated parameters and experimental parameters Applied Mechanics and Materials Vols 229-231 185 Results and Discussions Fig shows the reconstructions of contact between Co electrode and SWNT junction in both (5,5) and (8,0) SWNTs (a) (b) Fig.2 Reconstructions of contact between Co(100)-5x5 electrode and (a) metallic (5,5) SWNT and (b) semiconducting (8,0) SWNT One can see that Co atoms in the top layer of the Co surface are significantly displaced The distance between Co surface and SWNTs are around 1.7 Å for (5,5) case and 1.75 Å for (8,0) case These results indicate the strong interaction between Co electrode and SWNT junction It implies that the contact affects on the electronic properties of interface region between Co electrode and SWNTs in the CNTFET devices To clarify the electronic properties of interface region between Co surface and SWNTs, changes in the electron-density and spin-density have been analyzed Figure shows the differences between the electron-density of Co(100)-SWNTs and sum of the electron densities of isolated Co(100) surface and of isolated SWNTs [∆ρ= ρCo-SWNT - (ρCo + ρSWNT)] We can see that the changes in the electron density occurred mainly at the interface region between Co surface and SWNTs, where a substantial concentration of electron accumulates in Co-C bonds These results reveal the covalent bonding nature of Co surface and SWNTs (a) (b) Fig.3 Electron density difference between Co(100) – x surface and (a) metalic(5,5) SWNT, (b) semiconducting (8,0) SWNT at 0.01 (a.u) isosurface value Electron density flows from the orange regions into yellow regions Co electrode is ferromagnetic Thus, Co atoms in Co(100) surface have spin-polarization The interaction of Co atoms with SWNTs can affect the spin polarization of the Co atoms Figure shows the difference between the spin-density of Co(100)- SWNTs and sum of the spin densities of isolated Co(100) surface and of isolated SWNTs [∆ρspin=ρCo-SWNT-spin - (ρCo-spin + ρSWNT-spin)] 186 Mechanical and Electrical Technology IV Fig Spin-density difference between Co(100) – x surface and (a) metalic(5,5) SWNT, (b) semiconducting (8,0) SWNT at 0.01 (a.u) isosurface value Spin-density flows from the orange regions into yellow regions We can see that, the changes in the spin-density occur mainly at the top layer of Co surface It means that the spin polarization of Co atoms at the interface region have been suppressed due to the interaction with SWNTs Thus, the spin transport through this layer is expected to be low This result explains why the metal-CNT devices experimentally showed spin-dependent transport property with small hysteretic magneto-resistance at low temperature In addition, the Co atoms at the interface region are significantly relaxed, which also affects on the spin polarization of these atoms These results suggest that the spin transport properties of devices can be improved by controlling the structural surface of Co electrode and the contact region between Co and SWNT in the CNT directed growth process Conclusions In summary, the reconstruction and electronic structures of the interfaces between single wall carbon nanotubes and ferromagnetic Co electrodes have been investigated using density functional theory calculations Result revealed that there is strong interaction between Co(100) and SWNT At the interface region, the Co-C bonds have covalent natures Top layers of Co surface have been significantly reconstructed The electron density occur mainly at the interface region between Co surface and SWNTs, where a substantial concentration of electrons accumulates in Co-C bonds It is found that a small amount of charge transfers from Co surface to SWNT In addition, the spin polarization of Co atoms at the interface region has been suppressed due to the interaction with SWNTs Thus, the spin transport through this layer becomes low, which results in the small hysteretic magneto-resistance of CNT devices Acknowledgments Nguyen Tien Cuong would like to thank the Vietnamese Government for a 322 program scholarship We thank Dr Pham Tien Lam, Dr Nguyen Thanh Cuong for their discussions and useful comments about the manuscript Applied Mechanics and Materials Vols 229-231 187 References [1] Bachtold A, Fuhrer M S, Plyasunov S, Forero M, Anderson E H, Zettl A and McEuen P L Phys Rev Lett 84 (2000) 6082 [2] Yao Z, Kane C L and Dekker C Phys Rev Lett 84 (2000) 2936 [3] Dresselhaus M, Dresselhaus G and Avouris Ph 2001 Carbon Nanotubes: Synthesis, Structure, Properties and Applications (Berlin: Springer) [4] S J Tans, M H Devoret, H Dai, A Thess, R E Smalley, L J.Geerligs and C Dekker, Nature 386 (1997) 474-477 [5] K Tsukagoshi, B W Alphenaar and H Ago, Nature 401 (1999) 572 [6] M Shiraishi, T Tkenobu, T Iwai, Y Iwasa, H Kataura and M Ata, Chem Phys Lett 394, 110-113 (2004) [7] Y Ohno, S Iwatsuki, T Hiraoka, T Okazaki, S Kishimoto, K Maezawa, H Shinohara and T Mizutani, Jpn J Appl Phys Part 42 4116 (2003) [8] Mohd Ambri Mohamed, Mohd Asyadi Azam, Eiji Shikoh and Akihiko Fujiwara, Jpn J Appl Phys 49 (2010) 02BD08 [9] Mohd Ambri Mohamed, Nobuhito Inami, Eiji Shikoh, Yoshiyuki Yamamoto, Hidenobu Hori and Akihiko Fujiwara, Sci Technol Adv Mater 9, (2008) 02519 [10] Nobuhito Inami, Mohd Ambri Mohamed, Eiji Shikoh and Akihiko Fujiwara, Appl Phys Lett 92, (2008) 243115 [11] W J M Naber, S Faez and W G van der Wiel, Organic spintronics, J Phys D: Appl Phys 40, (2007) R205 [12] Information on http://www.openmx-square.org/ Mechanical and Electrical Technology IV 10.4028/www.scientific.net/AMM.229-231 Reconstruction and Electronic Properties of Interface between Carbon Nanotubes and Ferromagnetic Co Electrodes 10.4028/www.scientific.net/AMM.229-231.183 DOI References [1] Bachtold A, Fuhrer M S, Plyasunov S, Forero M, Anderson E H, Zettl A and McEuen P L Phys Rev Lett 84 (2000) 6082 http://dx.doi.org/10.1103/PhysRevLett.84.6082 [4] S J Tans, M H Devoret, H Dai, A Thess, R E Smalley, L J Geerligs and C Dekker, Nature 386 (1997) 474-477 http://dx.doi.org/10.1038/386474a0 ... mainly at the interface region between Co surface and SWNTs, where a substantial concentration of electron accumulates in Co- C bonds These results reveal the covalent bonding nature of Co surface and. .. the reconstruction and electronic structures of the interfaces between single wall carbon nanotubes and ferromagnetic Co electrodes have been investigated using density functional theory calculations... transport properties of SWNTs The nature of atomic bonding at interface could affect scattering in the electron transport Therefore, it is necessary to investigate the reconstruction and electronic