CHARGE TRANSPORT ALONG MOLECULAR WIRES

Transcription

1 CHARGE TRANSPORT ALONG MOLECULAR WIRES Aleksey Kocherzhenko DelftChemTech Faculty Colloquium / March 9, 2009 DelftChemTech: THREADMILL:

2 The Long Way to a Conventional Computer

3 2 (which could be even longer)

4 3 and the Self-Assembling Molecular Computer

5 Defects? No problem! 4 J. R. Heath, et al. Science, 280, 1716 (1998)

6 Molecular wires are interesting: electronics, in it s quest for higher integration densities, is approaching molecular dimensions; bottom-up approaches open up new opportunities for creating self-assembling nanoelectronic devices; there is a possibility to tune the conductivity and other properties of molecules to desired specifications; processing polymers is cheaper than processing inorganic semiconductors. Why molecular wires, not small molecules? Also those! 5

7 Conductivity characterization techniques Contact techniques: molecules between electrodes, electrode and STM tip, etc. Drawback: measured conductivity / charge carrier mobility is to a large extent determined by contacts. Contactless techniques are required to study intramolecular charge transport. 6

8 Pulse radiolysis time-resolved microwave conductivity (PR-TRMC): a contactless technique Close to uniform concentration of Bz + and e - produced in O 2 saturated benzene solution of polymer (P n ) Creation of charge carriers by pulse radiolysis (2 50 ns pulse of 3 MeV electrons) Detection by TRMC Microwaves are absorbed by mobile charge carriers ΔP P = AΔσ 7

9 TRMC experimental results O 2 -saturated solution of polymers (P) in benzene (Bz) 1 3 Irradiated with short (10 ns) pulse of 3 MeV electrons (e - ) Close to uniform concentration of Bz + and e - 2 T R 1 2 O 2 + e = O 2 Bz + + P = Bz + P + M C 3 P + + O 2 = P + O 2 Mobility independent of the chain length 8

10 Conducting polymers: Ladder-type PPP µ h ~ 600 cm 2 V 1 s 1 ( Silicon : µ Si h 450 cm 2 V 1 s 1 ) µ = v dr E ; σ = enµ 9 P. Prins, et al. Phys. Rev. Lett., 96, (2006)

11 Conducting polymers: Zn-porphyrin wires µ h 0.1 cm 2 V 1 s 1 Is the difference in the mobility only quantitative, or also qualitative? 10 F. C. Grozema, et al. JACS, 129, (2007)

12 Possible charge transport mechanisms Two extreme cases: Band-like transport typically occurs for systems with: - delocalized charges; - high charge carrier mobilities; - large electronic couplings. Hopping transport typically occurs for systems with: - localized charges; - lower charge carrier mobilities; - smaller electronic couplings. It is not always easy to distinguish between the above two mechanisms. In real systems a mixture of the two often occurs. 11 F. C. Grozema; Siebbeles, L. D. A. Int. Rev. Phys. Chem. 27, 87 (2008)

13 Why is the mechanism of charge transport important? Polymers with different mechanisms of charge transport will not only show very different charge carrier mobilities, but also a very different dependence of the mobility on various parameters: polymer chain length, temperature, external electric field frequency, 12

14 Relative stability Thermodynamic parameter: is it energetically favorable for the charge to localize on one unit or to delocalize over many? 100% 0% or 50% 50% (with corresponding geometries) +1, delocalized ( ) 0.5Oxidized Δ = E 0.5Oxidized ( ) +1, localized ( ) Neutral E Oxidized ( ) 13

15 Charge transport simulations Polymers consist of a very large number of atoms ab initio quantum-chemical calculations require a lot of time (calculation time N 4, N - number of atoms) To reduce calculation time, we can split the problem into two steps: calculate parameters of repeat units (same for all identical repeat units) and interconnects between them (same for all pairs of adjacent repeat units) calculate transport along the polymer chain in terms of the calculated parameters (neglecting the details of charge transport within a repeat unit) 14

16 The model i = 1,10 ε 1 ε 3i 1 ε 3i J 3i 1, 3i J 3i, 3i+1 Saturated hydrocarbon chains ε 32 ε 33 ε 34 Molecular site (Type 1) ε 3i+1 Molecular site (Type 2) Approximation: molecular sites have no internal structure 15

17 Band-like transport simulation Simultaneously: Quantum-mechanically propagate charge on polymer chain # ε 1 J 1,2 0 0 & % J 1,2 ε 2 J 2,3 0 ( % ( H = % 0 J 2,3 ε 3 0 ( % ( % ( $ % ε 34 '( i Ψ( t) t = HΨ( t) Classically propagate geometry (relevant coordinates) J( θ) = J( 0)cosθ θ( t + Δt) = θ( t) 1 2τ rot k B T ' ( V tor θ ) * Δt ± Δt τ rot 16 P. Prins, et al. Mol. Simulations. 32, 695 (2006)

18 Band-like transport simulation algorithm Sample θ(t = 0) from Boltzman distribution Not yet N times Δx 2 ( t) H = ε i a i + a i + J i,j i Ψ( t) eq ( ) = Φ where n0 ε i ( t) = ε ph,fl + Δε fluct ( t) = HΨ( t) t J i, j ( t) = Acos[ θ( t) Ψ( t) ] Ψ 0 P(θ) = [ ( ) k B T] ( ) k B T exp V tor θ [ ] exp V tor θ dθ i i j a i + a j N times V tor (θ) (mev) θ θ (degrees) Average over N values of Δx 2 (t) θ( t + Δt) = θ( t) µ 1D ( ω) = eω 2 2k B T µ 1D DC 1 2τ rot k B T ( ) Δx 2 t = ed k B T, ' ( V tor θ ( ) where ) * Δt ± Δt τ rot cos( ω t)d t D = Δx2 ( t) 2t 17

19 The charge transport mechanism in ladder-type PPP is band-like µ h ~ 600 cm 2 V 1 s 1 18

20 Hopping transport The charge can be described classically 19

21 Hopping transport simulation Marcus hopping rate: Γ ij = 2π J ij 2 1 ( ) 2 & 4πλk B T exp E E + λ j i ( 4λk B T ' ( ) + * + Γ i, i 1 Γ i, i+1 E i 2 E i 1 E i E i+1 E i+2 Δx 2 ( t) µ 1D ( ω) 20 Parameters: E i J ij λ equilibrium energy of charge localization site i electronic coupling between sites i and j reorganization energy, characterizes distortion of the molecular geometry when a charge is placed on a site

22 The charge transport mechanism in Zn-porphyrin based wires is hopping µ h 0.1 cm 2 V 1 s 1 21

23 22 Hopping transport mobility is extremely sensitive to disorder in the polymer and in its environment

24 Disorder can be engineered Ladder-like double strand structures formed upon addition of bidentate ligand (Bipy=4,4 -bipyridyl) to benzene solution. 23

25 Experimental results: ladder-like structure without Bipy with Bipy µ = cm 2 /(V s) µ = 0.93 cm 2 /(V s) 24

26 Conclusions Molecular electronics is feasible Conjugated polymers are promising and versatile materials for electronic applications Charge transfer properties of conjugated polymers can be controlled supramolecularly: - either by just changing their molecular conformation (changing solvent, preparing aligned films) - or by changing the disorder in their environment 25

27 Acknowledgements University of Oxford: - Prof. Dr. Harry Anderson - Dr. Shane MacDonnell - Dr. Coralie Houarner-Rassin - Giuseppe Sforazzini - Johannes Sprafke University of Mons Hainaut: - Prof. Dr. David Beljonne - Michael Wykes Belarusian State University: - Prof. Dr. Nikolai A. Poklonski Delft University of Technology: - Dr. Paulette Prins - Sameer Patwardhan Funding: - European Union 26

28 THANK YOU FOR YOUR ATTENTION! CHARGE TRANSPORT ALONG MOLECULAR WIRES Aleksey Kocherzhenko DelftChemTech: THREADMILL: Delft University of Technology, Faculty of Applied Sciences DelftChemTech, Opto-electronic Materials Section Julianalaan 136, 2628 BL, Delft, The Netherlands