The dielectric response of Molecular Wires

Transcription

1 The dielectric response of Molecular Wires Julio Gómez Laboratorio de Nuevas Microscopías. Departamento de Física de la Materia Condensada C-III Universidad Autónoma de Madrid

2 Nanowires overview Breakjuncti ons Carbon nanotube Organic molecule Quantum wire V 2 O 5 nanofiber DNA SWNT R. Reifenberger et al. The handbook of Nanostructured AsGa/AlGaAs, Materials and Nanotechnology. C, H, O, N, State of the art in DNA DC-conductivity Superconductor (1K) Good conductor at RT Semi conductor Insulator Kasumov et al. Science 291, 280 (2001) Fink et al. Nature 398, 407 (1999) Kasumov et al. Science 291, 280 (2001) Porath et al. Nature 403, 635 (2000) Cai et al. Appl. Phys. Lett (2000) Rakitin et al. Phys. Rew. Lett. 291, 280 (2001) Everybody before Fink et al AND De Pablo et al. Phys. Rew. Lett. 85, 4992 (2000)

3 Should the DNA be a conductor? Ab initio theoretical calculation with SIESTA program Ordejón et al. Phys. Rev. B. 53 R10441 (1996). Isosurfaces of charge DNA poly(g)-poly(c): G-G-G-G-G-G... C-C-C-C-C-C... λ-dna The theoretical calculation agrees with our experiments

4 Making contacts to molecular wires Gold source 1.4µm Electrode Insulating substrate Tungsten wire Sample Electrode 4 µm 390nm

5 Contact experiment on a single DNA molecule 10 V 00.0 pa Current meter Gold Metallized SFM tip Δ Z = 27 nm DNA λ 100 nm 1.2µm Mica substrate The minimum length is about 50 nm. 12 V are applied with a resolution < 1 pa. R = 12 TΩ. R * =171 G Ω/nm ρ ~ 1x10 10 µ Ω cm. 100 nm P. J. de Pablo et al. Phys. Rev. Lett.. 12, 573 (2000) Suplemento de El País

6 { Electrostatic force in SFM: a non-intrusive method F k V R C Resistance and Capacitance V I A set Amplitude A elect A set A elect V df/dz >0 tip = 0 V tip 0 Δω ω 0 1 2k df dz e ω elect ω 0 Frequency Δz z piezo displacement

7 Experimental Frequency shift Metallic SFM tip V V=0V Z Amplitude ω frequency Conducting surface Z Aset df/dz >0 Vtip = 0 Vtip 0 V=8V Aelect ω electω 0 Δz Frequency z piezo displacement Δω 1 dfe ω 0 2k dz

8 Visualization of electrical networks of SWNT nm 2 400nm V tip = 0 V V tip = 2 V Electrically connected molecules shine due electrostatic force P. J. de Pablo et al. Appl. Phys. Lett. (2001).

9 Applying electrostatic to DNA. 100nm Nanotubes and DNA co-adsorbed on mica Co-adsorption of SWNT and DNA to compare both electrostatic signals Insulating substrates used: Mica SiO 2 Glass Electrodes metal used: Gold Silver Chromium

10 Comparing electrostatic signals of DNA and SWNT. single DNA molecule 100nm SWNT 100nm Tip-sample bias: 0 V Tip-sample bias: 3 V

11 Contacting DNA molecule with SWNT nm single DNAmolecule SWNT 100nm Tip-sample bias: 0 V 100nm Tip-sample bias: 1.3 V

12 Electrostatic experiments on V 2 O 5 fibers. J. Muster et al. Adv. Mat. 12, 420 (2000) Semiconductors 1,5 nm Resistances in the range of 100 MΩ (1000 times bigger than SWNT) 10nm Topography Current Simultaneously acquired using Jumping Mode V A 240nm 240nm

13 Electrostatic enhancement on V 2 O 5 fibers. 290nm 290nm V=0 V=2V The same effect is observed in spite of the higher resistance of the fibers. The electrostatic is general purpose method, which can be applied to any nanowire.

14 DNA electrical properties without any electrical contact. SFM tip SFM tip Conducting molecule on a dielectric substrate ε >> 1 Insulating molecule on a dielectric substrate ε 1 S. Gómez-Moñivas et al.. Appl.Phys Lett.. In press.

15 3D modes in SFM. Classical SFM images represent a magnitude as a function of the geometrical position x,y : f(x,y) Z Normal force, Adhesion force Y (SLOW SCAN) Y (SLOW SCAN) Y (SLOW SCAN) X (FAST SCAN) X (FAST SCAN) X (FAST SCAN). 3D MODES allows to measure images as a function of non-geometrical variables: x 3 (x 1,x 2 ) X 3 (Signal to be measured) Current Z Amplitude Δω X 2 (slow scan) Bias Z Bias Z X 1 (fast scan) 0 V Lateral displacement

16 Using Phase Lock Loop (PLL) to measure the dielectric response. Using a PLL the system is kept at is resonance frequency. The frequency shift introduced by the electrostatic force is now monitored Z V Metallic SFM tip y x z s Z s Insulating substrate.

17 Comparation of Δω for nanotubes and DNA 130nm x Δω is registered in the line X, with a SWNT and a DNA molecule. V tip = 6 V ΔωRes (KHz) -20.0nm nanotube 160nm x X(nm) DNA Z

18 Conclusions and Acknowledgements Electrostatic methods allow to visualize electrically connected nanowires networks: connected molecules appear to shine When applying this method to adsorbed DNA molecules no contrast is observed: DNA does not shine On the basis of our experimental evidences we conclude that adsorbed DNA is an insulator SFM C. Gómez-Mavarro P.J. de Pablo F. Moreno-Herrero J. Colchero A.M. Baró Software R. Fernández I. Horcas Theory S. Gómez-Moñivas J.J. Sáenz J.M. Soler E. Artacho V 2 O 5 Y.Fan M. Burghart SWNT W. Maser A.M. Benito M.T. Martínez