The dielectric response of Molecular Wires

Julio Gómez

Laboratorio de Nuevas Microscopías.

Departamento de Física de la Materia Condensada C-III UniversidadAutónoma de Madrid

Nanowires overview

State of the art in DNA DC-conductivity

•Superconductor (1K)

•Good conductor at RT

Kasumov et al. Science 291, 280 (2001)

Fink et al. Nature 398, 407 (1999)Kasumov et al. Science 291, 280 (2001)

•Semi conductorPorath et al. Nature 403, 635 (2000)Cai et al. Appl. Phys. Lett 77 3105 (2000)Rakitin et al. Phys. Rew. Lett. 291, 280 (2001)

•InsulatorEverybody before Fink et al ANDDe Pablo et al. Phys. Rew. Lett. 85, 4992 (2000)

R. Reifenberger et al. “The handbook of Nanostructured Materials and Nanotechnology”.

Breakjunctions

Carbonnanotube

Organicmolecule

Quantum wire V2O5

nanofiberDNA

Composition Au, Cu, Ag... C C, H, O.... AsGa/AlGaAs,Au, V, O C, H, O, N,

P...

Geometry ? Tubular Chemicallydefined Planar 2D Stripe Double helix

Wide Atomic 1-40 nm 1 nm Several nm 6 nm 1 nm

Lenght Nanometers 1-2microns nanometers nanometers Up to tens of

micronsUp to tens of

micronsElectricalconnections Easy Difficult Problematic Easy Difficult Problematic

Fabrication Mechanicalcontact

Dischargearch Reaction tube Lithography Reaction

tubeReaction

tube

Fabrication atgreat scale Difficult Easy Easy Difficult Easy Easy

Conductionmechanism

Quasiballistic ? ? Ballistic ? ?

SWNT

Breakjunctions

Carbonnanotube

Organicmolecule

Quantum wire V2O5

nanofiberDNA

Composition Au, Cu, Ag... C C, H, O.... AsGa/AlGaAs,Au, V, O C, H, O, N,

P...

Geometry ? Tubular Chemicallydefined Planar 2D Stripe Double helix

Wide Atomic 1-40 nm 1 nm Several nm 6 nm 1 nm

Lenght Nanometers 1-2microns nanometers nanometers Up to tens of

micronsUp to tens of

micronsElectricalconnections Easy Difficult Problematic Easy Difficult Problematic

Fabrication Mechanicalcontact

Dischargearch Reaction tube Lithography Reaction

tubeReaction

tube

Composition Au, Cu, Ag... C C, H, O.... AsGa/AlGaAs,Au, V, O C, H, O, N,

P...

Geometry ? Tubular Chemicallydefined Planar 2D Stripe Double helix

Wide Atomic 1-40 nm 1 nm Several nm 6 nm 1 nm

Lenght Nanometers 1-2microns nanometers nanometers Up to tens of

micronsUp to tens of

micronsElectricalconnections Easy Difficult Problematic Easy Difficult Problematic

Fabrication Mechanicalcontact

Dischargearch Reaction tube Lithography Reaction

tubeReaction

tube

Fabrication atgreat scale Difficult Easy Easy Difficult Easy Easy

Conductionmechanism

Quasiballistic ? ? Ballistic ? ?

SWNT

Should the DNA be a conductor ?

Isosurfaces of charge

Ab initio theoretical calculation with SIESTA program Ordejón et al. Phys.Rev. B. 53 R10441 (1996).

λ-DNADNA poly(G)-poly(C): G-G-G-G-G-G... C-C-C-C-C-C...

The theoretical calculation agrees with our experiments

1.4µm Electrode

Electrode

Insulating substrate

Making contacts to molecular wires

390nm

Gold source

4 µm

Tungsten wire Sample

Contact experiment on a single DNA molecule

Gold

Mica substrate

DNA λ

10 V

Metallized SFM tip

Δ Z

= 2

7 n

m

1.2µm

00.0Current meter

pA

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 ~ 1x1010 µ Ω · cm.

P. J. de Pablo et al. Phys. Rev. Lett.. 12, 573 (2000) Suplemento de “El País” 26-08-01

100 nm

100 nm

Electrostatic force in SFM: a non-intrusive method

Aelect

Δz z piezo displacement

Aset

Am

plit

ud

e

ω elect

Aelect

dF/dz >0 V

tip = 0

Vtip

0

ω 0

Aset

Frequency

≠

{

R

V

F

C

I

dzdF

ke

≈

Δ21

0ωω

V

Resistance and Capacitance

k

Experimental Frequency shift

V=0VZ

ω

Conducting surface

MetallicSFM tip

dzdF

ke

≈

Δ21

0ωωV=8VA

elect

Δz z piezo displacement

Aset

Am

plit

ud

e

ω elect

Aelect

dF/dz >0 V

tip = 0

Vtip

0

ω 0

Aset

Frequency

≠

frequency

Z

Amplitude

V

Visualization of electrical networks of SWNT

Electrically connected molecules ‘shine’ due electrostatic force

Vtip= 2 VVtip= 0 V

400nm

1

2

3

400nm

1

2

3

P. J. de Pablo et al. Appl. Phys. Lett. (2001).

Applying electrostatic to DNA.

Nanotubes and DNAco-adsorbed on mica

100nm

•Co-adsorption of SWNT and DNA tocompare both electrostatic signals

•Insulating substrates used:•Mica•SiO2

•Glass

•Electrodes metal used:•Gold•Silver•Chromium

Comparing electrostatic signals of DNA and SWNT.

100nm100nm

single DNA molecule

Tip-sample bias: 0 V

SWNT

Tip-sample bias: 3 V

100nm 100nm

Contacting DNA molecule with SWNT

single DNAmolecule

SWNT

400nm

1

2

3

Tip-sample bias: 0 V Tip-sample bias: 1.3 V

Electrostatic experiments on V2O5 fibers.

10nm

1,5 nm

•Semiconductors

•Resistances in the range of 100 MΩ(1000 times bigger than SWNT)

J. Muster et al. Adv. Mat. 12, 420 (2000)

240nm240nm

AV

Topography CurrentSimultaneously acquiredusing Jumping Mode

Electrostatic enhancement on V2O5 fibers.

290nm290nm

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.

DNA electrical properties without any electricalcontact.

S. Gómez-Moñivas et al.. Appl.Phys Lett.. In press.

SFM tip SFM tip

Conducting molecule ona dielectric substrate

ε >> 1

Insulating molecule ona dielectric substrate

ε ∼ 1

3D modes in SFM.

0 V

Y (SLOW SCAN)

X (FAST SCAN)

Z

Classical SFM images represent a magnitude as a function of the geometricalposition x,y : f(x,y)

. 3D MODES allows to measure images as a function of non-geometrical variables: x3(x1,x2)

X1 (fast scan)

X3 (Signal tobe measured)

Y (SLOW SCAN)

X (FAST SCAN)

Normal force,Adhesion force

Bias

Y (SLOW SCAN)

X (FAST SCAN)

X2 (slow scan)

CurrentZ

Bias

Amplitude

Z

Lateraldisplacement

Δω

Z

Z

s

•Using a PLL the system is kept at is resonance frequency.

•The frequency shift introduced by the electrostatic force is nowmonitored

Using Phase Lock Loop (PLL) tomeasure the dielectric response.

MetallicSFM tip

Insulating substrate.

VZ

y

x

s

z

160nm

-20.0nm

Δω is registered in the line X,with a SWNT and a DNA

molecule.

Comparation of Δω for nanotubes and DNA

130nm

Z

x

0 200 400 600 800-2.34-2.32-2.30-2.28-2.26-2.24-2.22-2.20-2.18

Δω Res(KHz)

X(nm)

Vtip= 6 V

nanotube

DNA

x

Conclusions and Acknowledgements

•Electrostatic methods allow to visualize electricallyconnected nanowires networks: connected moleculesappear to ‘shine’•When applying this method to adsorbed DNAmolecules no contrast is observed: DNA does not ‘shine’•On the basis of our experimental evidences weconclude that adsorbed DNA is an insulatorSFM

•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

V2O5

Y.Fan

M. Burghart

SWNT

W. Maser

A.M. Benito

M.T. Martínez