Electronic Transport in Molecular Scale Devices · Molecular NanoElectronics Laboratory The 3rd Korea-US Nano Forum, Seoul, April 3-4, 2006 6 Determination of Working Molecular Devices

MMolecular olecular NNanoanoEElectronicslectronicsLaboratoryLaboratoryThe 3rd Korea-US Nano Forum, Seoul, April 3-4, 2006

1

Takhee LeeTakhee Lee, ,

TaeTae--Wook Wook Kim, Kim, HyunwookHyunwook Song,Song, GunukGunuk Wang Wang

Department of Materials Science and EngineeringDepartment of Materials Science and Engineering

Gwangju Institute of Science and Technology (GIST), KoreaGwangju Institute of Science and Technology (GIST), Korea

Electronic Transport Electronic Transport in Molecular Scale Devicesin Molecular Scale Devices


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I. Electronic Transport of Micro & NanoVia-hole Structures for Molecular

Electronic Devices

Tae-Wook Kim, Gunuk Wang, Hyunwook Song


3

Formation of Alkanethiol Self-Assembled Monolayers

• A spontaneous chemisorption processRS-H + Au → RS-Au + 0.5 H2

• Chemical bond formation of functional end groups of molecules with the substrate surface

• Intermolecular interactions between the backbones

Reed & Tour, Am. Sci. June 2000

• Octanethiol (C8) - CH3(CH2)7SH / length = 13.3Å• Dodecanethiol (C12) - CH3(CH2)11SH / length = 18.2Å• Hexadecanethiol (C16) - CH3(CH2)15SH / length = 23.2Å

Standard alkanethiol moleculeS

C

H

C8C12

C16

Conjugated molecule: Oligo(phenylene ethynylene) (OPE) OPE provided by Dr. Changjin Lee

(KRICT)Self-assembled monolayer (SAM)


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Design and Fabrication of molecular electronic devices

1 cm

One unit Contain 16 diesEach die contains 20 devicesTotal devices are 320 in a unit2.5 mm

Unit Die

190 µm

Mask patternMask pattern

SEM / AFM / Optical imagesSEM / AFM / Optical images

Devices After Top electrode

Schematic of device

Fabrication of Planar-type Via-Hole Molecular Devices

Devices

Silicon wafer

Ti/AuSiO

Au

4” wafer(Collaboration with ETRI, Dr. HyoyoungLee)

1 cm piece AFM image of a device

Die

2 µm

Optical lithography: ~ 2 µmPlan: E-beam lithography: ~ 50 nm

TW Kim, HW Song, GW Wang


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Yield of Microscale Via-Hole Molecular Devices

Simmons tunneling fitting results for all working devices

6057

201

C16C12C8

1.5%8.2%87.4%2.9%100%

8411031174439213440

WorkingOpenshort

Fab. failure

# of fabricated

device

-1.0 -0.5 0.0 0.5 1.0

10-8

10-7

10-6

10-5

10-4

10-3

10-2

C16

C12

Cur

rent

[ A

]

Voltage [ V ]

C8

0.87±0.050.52±0.042.67±0.285.2±4.7C160.83±0.040.72±0.041.26±0.082000±400C120.87±0.160.76±0.091.29±0.4978000±46000C8β (Å-1)αФB (eV)J at 1 V (A/cm2)Alkanethiol

Device yield (micro-via hole molecular devices)

1. Yield of molecular devices ?2. How to define working molecular devices ?

Representative devices determined by Gaussian fitting of J !


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Determination of Working Molecular Devices

Gaussian fitting ( 3σ: 99.7 % )

-2 -1 0 1 2 3 4 5 6 7 80

4

8

12

16

20

24

28

32 <x> = 4.92 σ = 0.30

# of

Dev

ice

Log10 (Current Density @ 1 V (A/cm2)) 2.0 2.5 3.0 3.5 4.0 4.5 5.00

2

4

6

8

10

12

14

16<x> = 3.06σ = 0.35

# of

Dev

ice

Log10 (Current Density @ 1 V (A/cm2)) -1 0 1 2 3

0

2

4

6

8

10

12

14

16

<x> = 0.56σ = 1.21

# of

Dev

ice

Log10 (Current Density @ 1 V (A/cm2))

3σ

Log (J) for C8 Log (J) for C12 Log (J) for C16

2

2

2)(

21)( σ

πσ

><−−

⋅=xx

exf

Summary of Log(J)

-1 0 1 2 3 4 5 60

5

10

15

20

25

30

35 C8 C12 C16

# of

Dev

ice

Log10 (Current Density @ 1 V (A/cm2))

C16 C12

C8

m σ3+mσ3−m99.7%

)(xfy =


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II. Molecular Electronic Junctions Studied by Conducting Atomic Force

Microscopy

Hyunwook Song


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Charge Transport Study by CAFM

Au(20nm)/Cr(20nm) coated AFM Probe

Au

I

AuChemisorbed

probePhysical contact area

PSIA, XE-100 modelambient AFM

HP 4145B

C8 OPE


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-1.0 -0.5 0.0 0.5 1.0

1E-4

1E-3

0.01

0.1

1

1 nN5 nN10 nN20 nN30 nN

I (nA

)

V (V)

I(V) for C12 (1 to 30 nN)

• Larger current with larger loading force• J: C8 > C12 > C16• Error bar: sample-to-sample variation

Length (Å)12 14 16 18 20 22 24

10-1

100

101

102

103

104

105

30 nN25 nN20 nN15 nN10 nN5 nN2 nN1 nN

C16

C12

C8

Cur

rent

den

sity

(A/c

m2 )

Loading force Jo (A/cm2)1 nN 1.25×108

2 nN 1.11×108

5 nN 1.37×108

10 nN 1.45×108

15 nN 1.30×108

20 nN 8.03×107

25 nN 7.63×107

30 nN 1.25×108

J @ 1 V (1 to 30 nN)

R ∝ exp (βd) Exponential dependence

Force-and Length-Dependent Tunneling


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Through-bond vs. Through-space

d

Tunneling distance in “through space” = d – dcctanθ + dcc

θdcctanθdcc

JACS, 119, 11910 (1997)

M

cc

t 0 tb M

cos

s0 tb M cc ts cc

1 s

exp( )

! exp( ( tan )) exp( )( )! !

Ld

N

I I L

nI L Nd Ndn N N

θ

β

β θ β=

= − +

− − × −−∑

J. Phys. Chem. B, 106, 7469 (2002)

The dominant transport mechanism of alkanethiol SAM: “through-bond” tunnelingWhen tilted considerably, chain-to-chain coupling: “through-space tunneling” appears

Au

(dt- δ)

CAFM Tip


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20 30 40 50 60 70-14

-12

-10

-8

-6

-4

-2

C8

Tilt angle θ (degree)

ln(J/

J o)

0 10 20 30

50

60

70

Net force (nN)94786245

Applied loading force (nN)

Tilt

angl

e (d

egre

e)

0.6

0.8

1.0

1.2

Sepa

ratio

n be

twee

n co

ntac

ts (n

m)

N=1

N=2

Experiment

C8: Thru-bond + Thru-space (Single hopping: N = 1)

N~1 (N ≤ LMcos θ/dcc)


12

20 30 40 50 60 70-18

-16

-14

-12

-10

-8

-6

-4


ln(J/

J o)

0 10 20 3040

50

60



Tilt

angl

e (d

egre

e)

1.2

1.4

1.6

Sepa

ratio

n be

twee

n co

ntac

ts (n

m)

N=1

N=2

C12: Thru-bond + Thru-space (Double hopping: N = 2)



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20 30 40 50 60 70-24

-20

-16

-12

-8

-4

C16


ln(J/

J o)

0 10 20 30

45

50

55



Tilt

angl

e (d

egre

e)

1.6

1.7

1.8

1.9

2.0

2.1

Sepa

ratio

n be

twee

n co

ntac

ts (n

m)

N=1

N=2

N=3

C16: Thru-bond + Thru-space (Triple hopping: N = 3)



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Conclusions

• Electronic transport study by CAFM- Tip-loading force effects on molecular structural properties- Thru-bond vs. thru-space transport

• Fabrication and characterization of micro-via hole molecular devices- Detail yield study ~ 1.5 % (out of > 13,000 devices measured)

• Electronic properties of various nanoscale building blocks

Documents

Electronic Transport in Molecular Scale Devices · Molecular NanoElectronics Laboratory The 3rd Korea-US Nano Forum, Seoul, April 3-4, 2006 6 Determination of Working Molecular Devices