MPI Stuttgart

Max-Planck-Institut fuer FestkoerperforschungStuttgart, Germany

R. Sordan, K. Balasubramanian, M. Burghard

WP2(A): Single fullerene transistorassisted single spin detection

MPI Stuttgart

Method for contacting the moleculesUltra-small sandwich junctions in crossed-wire geometry

Schematic device structure: advantage:

• ferromagnetic metals can be used as contact(s)

disadvantages:

• cross section of junction: ~40 x 40nm2

dilution of molecules

• shielding of electric field (back gate)

bottom

top

How to attach the molecules?

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Molecule deposition onto bottom nanowireLangmuir-Blodgett (LB) monolayers

H2O

H2O

compression

C60 in toluene

transfer onto substrate

area per C60: ~1nm2

required area on LB trough:

~100cm2

~1016

molecules(~10µg)

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LB monolayers of C60Horizontally deposited onto Si/SiO2 wafer

low surface pressure

high surface pressure

14µm

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Molecule deposition onto bottom nanowireElectrochemical Method

approach no.1: Pure C60

Pt pseudo- reference electrode

Pt counterelectrode

probe needle electrolyte solution

deposition conditions:

-0.9V vs. Pt90 - 120sec

+N(CH2CH2CH2CH3)

4BF4-

CH2Cl2

TBA-BF4:

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Electrodeposited C60 molecules

major problem: low adhesion of top wire

(6 samples tested)

continuous films obtainedfor thickness > 2nm

electropolymerisation(?)

6,5 7,0 7,5 8,0 8,50

5

10

15

20

Initial After ECM

Hei

gh

t [n

m]

Length [m]

Height increase~ 2nm

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Electrodeposition of moleculesonto bottom nanowire

approach no.2: C60 in matrix

SH

SHmatrix = benzene-1,3-

dithiol

SH

SHSH

SH

S S

S

S S

oxidation

( -H+, -e- )

• conditions: +0.7V vs. Pt in toluene/acetonitrile (40 - 90sec); C60 : matrix = 1 : 10.

• continuous films for thickness >4nm

• film structure and extent of C60 inclusion to be determined

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electrodeposition

Fabrication of crossed-wire junctions

2nd e-beam lithography

Bottom wire defined by e-beam lithography, Ti/AuPd (1/12nm)

n+-Si

SiO 2 (1

00nm)

Top wire, Ti/AuPd (0.5/15nm)

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5 m

Crossed-wire junction

bottom wire

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Electrical behaviour of pure matrix sandwich junctions

gap widths: 0.4-0.6V

-1,0 -0,5 0,0 0,5 1,0-10

-5

0

5

10

I [

nA

]

V [ V ]

T = 2KB = 0T

-1,0 -0,5 0,0 0,5 1,0

-0,2

-0,1

0,0

0,1

0,2

I [

A ]

V [ V ]

T = 2KB = 0T

type I : “ohmic” (thinner films) type II : with gap (thicker films)

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-0,4 -0,2 0,0 0,2 0,4

-0,4

-0,2

0,0

0,2

0,4

0,0

0,5

1,0

1,5

2,0

I [

nA

]

V [ V ]

g d =

dI

/ dV

[ n

S ]

T = 2 KB = 0 T

I/V characteristics of C60/matrix- sandwich junctions

Sample no.1 (of 10)

~0.3V

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I/V characteristics of C60/matrix- sandwich junctions

-0,4 -0,2 0,0 0,2 0,4

-80

-60

-40

-20

0

20

40

60

80

10

100

I [

nA

]

V [ V ]

T = 2 KB = 0 T

g d =

dI

/ dV

[ n

S ]

Sample no.2 (of 10)

~50mV

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Features in the I/V of C60/matrix- junctionsC60 in nanogap(1)

(1): H. Park et al., Nature 407 (2000), 57. (2): Y. Noguchi et al., Thin Solid Films 438-439 (2003), 369.

• 5meV excitation from nano-mechanical oscillation

• features with small separation:

internal vibrational modes(several 10meV)

C60 embedded in polymer film (macroscopic junction

area)(2)

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Investigation of gate dependence for C60/matrix-sandwich junctions

-0,4 -0,2 0,0 0,2 0,4

-80

-60

-40

-20

0

20

40

60

80

VG = 15 V

VG = 0 V

VG = -15 V

I [

nA

]

V [ V ]

T=2K

Sample no.2 (of 10)

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Investigation of B field-dependence for C60/matrix-sandwich junctions

-0,5 0,0 0,5

10

100

T = 2 K B = 0 T B = 5 T B = 10 T

V [ V ]

g d

= dI

/ dV

[ n

S ]

Sample no.2 (of 10)

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Mn12-cluster for comparison

• synthesis

• electrodeposition: matrix method

• deposition: self-assemblySpin ground state:

S=10

AuPd

AuPd

[Mn12O12(O2CMe)16(H2O)

4]

S

COO

NH2ROOC

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I/V traces of Mn12 sandwich junction for increasing B field

-0,3 -0,2 -0,1 0,1 0,2 0,3

-0,2

-0,1

0,1

0,2 T = 2 K B = 0 T B = 1 T B = 5 T B = 10 T

I [

nA ]

V [ V ]

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Next steps

cobalt or dysprosium

bottom

top

• Sandwich junctions with ferromagnetic metal (top) contact

• Incorporation of N@C60 molecules

Top electrode (e.g., Co)

AuPd bottom electode

effect on features in I/V

spin polarised tunneling

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Nano-gaps via electromigration-induced breaking of nanowires

ion beam etching

V2O5 nanowire

Ti/AuPd (1/6 nm) layer

SiO2 (100 nm) layer

n+-Si substrate

1

3

2

100 nm

cross-section of ~100nm2 (7nm x 15nm) gap size 1nm for breaking at 4K (RRT< M)

after breaking via electro-migration:

advantages:

• allows for single molecule contacts

• possibly smaller gate shielding

disadvantage:

• possible with ferromagnetic metal wires ?

MPI Stuttgart

Electrical behaviour of pure matrix sandwich junctions

B = 0 T, 2 T, 5 T, 10 T, -10 TT = 2 K

type I - “ohmic”