METAL/CARBON-NANOTUBE INTERFACE EFFECT ON ELECTRONIC TRANSPORT

Transcription

1 METAL/CARBON-NANOTUBE INTERFACE EFFECT ON ELECTRONIC TRANSPORT S. Krompiewski Institute of Molecular Physics, Polish Academy of Sciences, M. Smoluchowskiego 17, Poznań, Poland

2 OUTLINE 1. Introductory remarks. 2. CNT/metal -electrode junction. 3. Giant magnetoresistance (GMR) in CNTs. 4. CNTs at magnetic fields. 5. Conclusions

3 C 1s 2 2s 2 2p 2 C r h sp 2 hybrid + free p z Metallic vs. semiconducting behaviour n m = 3 i, (i = integer) The wrapping vector R= (n,m) on the graphite sheet (equals to the circumference of the nanotube) determines the chirality. E.g.: R= (n, m) armchair, R=(n, 0) zigzag

4 Strong vs. weak confinement CB (T = 75 mk, P C = 0.15) Kondo features at V=0, (T = 75 mk, P C = 0.6) Interference patterns (T = 1.2 K, P C = 0.9) Nygård 2001 G max =4 e 2 / h G RT = G max P C /(2-P C ) 0 P C 1 (transmission probability)

5 Low resistive Comparison of the two-terminal resistance R at room temperature of CNT devices which were contacted with different metals: (a) Ti, (b) Au and (c) Pd. (e 2 /h) -1 = 25.8 kω Babic, Schönenberger 2004

6 Pd 0.3 Ni 0.7 contacts Sahoo et al. Appl.Phys.Lett 05 1/(e 2 /h)

7 Quality of contacts Geometry of M-SWCNT interfaces (end-contacted, side-contacted, embedded) First theoretical papers suggested that metal/carbon-based structures form bad high resistive contacts because of a mismatch of the involved Fermi vectors [Tesroff 99]. This conclusion was next refined and proven to be applicable only to graphene, but not to the CNTs [Delaney 99, Anantram 00] in general. In fact, it is well known that k vector conservation rules are obeyed only for directions where the entire system is translationally invariant (or at least, the metal /CNT contact extends over several unit cells), so in practice any amount of disorder would result in relaxing these rules. SK Xue et al.

8 4π/3a 0 π k F Al 2π/3a 0 π k F Au

9 Binding energies and wetting properties of M-SWCNT interfaces (Maiti, Chem, Phys. Lett.2004) Binding energy of a metal single atom to a SWCNT: E b (Au) < E b (Pd) < E b (Pt) For adlayers (films) the metal-metal binding within the metal film was found to be much stronger than that between the film and graphite. This, coupled, with the fact that Pt has substantially higher cohesive energy, led to the result that binding between Pt layer and graphite is actually smaller than that between Pd film and graphite. Existence of a critical cluster size such that metal nanoparticles smaller than such size will efficiently wet the graphite surface, while bigger particles will coalesce into even bigger clusters forming a weaker contact. Such critical cluster size was predicted to be smaller for Pt than for Pd Au Pd Pt

10 High transparency contacts (quasi ballistic regime) Liang, Nature 2001 L = 530 nm L = 220 nm

11 Krompiewski, et al. PRB 2002 L = 220 nm L = 530 nm

12 Methodology and α = L, R stand for left- and right-hand sides, σ denotes the spin and µ α = E F ± ev/2.

13 GMR = 1-R / R R field

14 K.Tsukagoshi, B. Alphenaar,.. Nature 1999 GMR ca. 9%, MWCNT + Co contacts s A single multi-walled carbon nanotube electrically contacted by Co. s s a, Scanning electron microscope image of the device, near the Co/MWNT junction. b Schematic diagram of the device

15 GMR = (R AP -R PA )/R AP = 2 P 1 P 2 /(1+P 1 P 2 ) For Co P = 34% expected GMR Julliere = 21 % Spin-polarization reduces as: exp(-l/l s ) measured GMR exp = 9% P reduced For L= 250 nm, one finds L s ~ 130nm

16 MWCNT + Ni 0.7 Pd 0.3 contacts Sahoo et al.. Appl Phys Lett GMR 8 % 6 %

17 Fcc (111) 0.22 P = (N N ) / ( N + N ) P= 50% P= 0% surface DOS E Energy band lineup: I II Electrode SWCNT ε ε E N= E E F

18 SWCNT 30nm-(8,8) A B A B C C NOS A B C # atom

19

20 SWCNT (8,8), length = 30 nm 10 G[e 2 /h], GMR G P 2 1 G AP GMR Ε Ideal SWCNT, G/spin

21 Minimal geometrical model of the DWCNT S. K. et al.. PRB 2004 View of the carbon nanotube sandwiched between two fcc(111) leads and detail of the contact region. What is shown consists of a few ferromagnetic electrode atoms with the nanotube forming the so-called extended molecule. The other parts of the electrodes (not shown) are infinite in all the directions.

22 angle between the π orbitals, d relative distance, δ = 0.45 Å, a = 3.34 Å (S. Roche 2001) SK, phys. stat. sol. (b), 2005 t int (i,j) = -(t/8) cos(θ ij ) exp [(d ij -a)/δ]

23 3.0 P=50%, 5 P= 50%, L outer = 39, L inner = 38, (3,3)@(8,8) G [e 2 /h], GMR G P G AP GMR G [e 2 /h], GMR E [ t ] E[ t ] F F F F

24 On-site (Anderson) disorder ε i ε i + ξ i, ξ i [-W/2, W/2] MWCNT SWCNT + disorder

25 Magnetic Field Peierls substitution t t exp[i (2π/Φ 0 )ξ] x y B parallel ξ = (Φ/C h ) x B perpendicular B[C h /(2π)] 2 y/ x [cos(2πx/c h )- cos(2π(x+ x)/c h )], x 0 ξ = B [C h /(2π)] y sin(2πx/c h ), x=0 Φ 0 =h/e, Φ=B π (C h /2π) 2, C h =a n 2 +m 2 +mn Zeeman splitting = ±gµ B B/2

26 G [e 2 / h] P= 0%, L = 41, SWCNT(8,8) B parallel, s= B parallel, s= B parallel, total Φ/Φ 0

27 4.0 G [e 2 /h] n= n=16 n= Parallel field Φ/Φ 0 E F B 0 B=0

28 Ajiki-Ando Theory Light cone approximation : ( ) 3 1 1, 0, 3 1 (0) ) ( 0, 2 ) ( 3 2 ), ( ; ), ( ) ( Φ Φ = = Φ Φ = Φ Φ = Φ Φ Φ = Φ + Φ = ± ± ν κ ν π κ κ n for E E t E n C n k n t k E h n band gap n is a subband index, ν= 0 for matal ±1 for semiconductor Experimental confirmations: Coskun et al., Science 2004; Zaric et al., Science 2004

29 CONCLUSIONS 1. Depending on the interface (increasing contact transparency): Coulomb, LL, Kondo, ballistic regimes 2. GMR: Ideally quasi periodic with a period scaling as 1/CNT-length critically dependent on weather or not the inner tubes of the MWCNTs are contacted to the electrodes, MWCNT outer SWCNT+disorder 3. Magnetic field: At parallel magnetic field, clear Aharonov-Bohm oscillations are observed also in the presence of electrodes (characteristic features of the A-A theory survive) The doping effect from the electrodes shows up.