molecular electronics where builders meet chiselersramu/msnt505/lec_notes/kuila...sample preparation...

1

Molecular Electronics

• Self-assembly of molecules on metal and semiconductor surfaces– New possibilities for nanoscale devices– Eliminates machinery required to manipulate

objects with nm resolution

• Nanowires as interconnects for interfacing nanoscale devices to the microelectronic systems

•100 x 10-6 m (100 µm)

• 10 x 10-6 m (10 µm)

• 1 x 10-6 m (1 µm)

•100 x 10-9 m (100 nm)

• 10 x 10-9 m (10 nm)

• 1 x 10-9 m (1 nm)• 1 x 10-10 m (1 Å)

Transistor based Devices , 1960

Visible LightIntegrated Circuits, 1990

---Predicted Scaling Barrier---Mesoscopic PhysicsBiomolecules, Molecular AssembliesMoleculesAtoms

Scale

BuildingUp

(Chemists)

Scaling Down(Engineers)

Where Builders Meet Chiselers

Nanostructures

• Nanotechnology is still very much in infant stages

• Characterization of the nanoscale sytems is necessary– Knowledge of electrostatic interaction can

provide a powerful insight into electronic properties

“Plenty of Room at the Bottom”

• R. Metzger “ Electrical Rectification by a Molecule: The Advent of UnimolecularElectronic Devices” Acc. Chem. Res. 1999, 32, 950-957

2

Roughness analysis- unannealed gold- area II

Profile along the grains

Roughness of Au/Ti/Si Substrates

• It depends on preparation.– ~ 2:00 nm ( e-beam evaporation at Purdue)– H2-Flame annealing reduces roughness to

0.7-0.8 nm

• Please see also the hand-out distributed today (4/21).

Sample Preparation

Scan Size: 1.7 µm X 1.7 µm

500 nm

1) Au substrates are flame-annealed and cleaned. This procedure produces large flat Au(111) grains.

2) The surface potentials of the bare Au substrates are measured prior to SAM deposition.

3) SAMs are then grown on the annealed Au substrates.

4) The surface potential of the SAMs are measured using EFM techniques (discussed above). The surface potential measurements are referenced to a bare Au reference sample.

Non-Contact Scans of BM Coated Au

Howell 00

3

• Au substrates flame-annealed to produce large flat Au (111) grains

• SAMs prepared by placing Au (111) in ~ 1 mM solution of organic thiols for 12-18 hrs., followed by rinsing with solvent and drying in air or in a dry box

• SAMs characterized by ellipsometry (thickness) and RAIR (orientation, etc.) techniques

Preparation and Characterization of SAMs

DDT ODT

Alkanethiols

(Dodecanethiol) (Octadecylthiol)

Symmetric Non-Symmetric

XYLTMXYL

(Xylyldithiol) (Tetramethyl-xylyl-dithiol)

BM(Benzyl mercaptan)

Molecules Under Investigation

PMBM

(Pentamethylbenzylmercaptan )

Characterization of SAMs

• G. Whitesides• D. Allara; R. Nuzzo

• Hand-out [ Reflectance Absorption IR Spectroscopy (RAIRS) or IR-Reflectance Absorption Spectroscopy (IR-RAS)]

4

References

• See the cross references on I-V studies in our book chapter– Heath & Ratner ( Physics Today, 2003)

• Scientific American, 2000 and 2001• Mark Reed, James Tour, Charles Leiber• IBM papers• HP papers ( Stan Williams)

Normal Vibrations & Vibrational Spectroscopy

• A non-linear molecule has 3N-6 normal vibrations ( or normal modes of vibration) – N is the # of atoms

• A linear molecule – 3N-5 normal vibrations• A fundamental transition will be IR active, if the

excited normal mode belongs to the same representation as any one or several of the Cartesian coordinates

• For Raman, the integral containing polarizabilitytensor has to be non zero.

• Ref. F. A. Cotton; Ch-10: “Chemical Applications of Group Theory” Second Edition, Wiley-InterScience, New York [ Relevant pages distributed as handouts ( 4/21)]

Vibrational Spectra

• Assignments for Vibrational Spectra of Seven Hundred Benzene Derivatives by G. Varsanyi ( John Wiley & Sons, New York)

• Hand-outs

Vibrational Spectroscopy ( IR) of Molecules on Metal Surfaces

• Chemisorption –may involve major rearrangement of the bonding pattern

• Metal-Surface Selection rules – high electron mobility of electrons (dielectric behavior) has an important influence as the electrons are able to screen centers of charge in electric fields– Vibrational modes with a component of dynamic dipole

moment perpendicular to the surface can be observed

Ref. F. M. Hoffman, “Infrared Reflection-Absorption Spectroscopy of Adsorbed Molecules” Surf. Sci. Rep. 1983, 3, 107-192.

5

RAIRS or IR-RAS

• Grazing angle incidence necessary to have more interaction of light w the surfcae

• Signal is quite weak; need a lot of scans– Signals proportional to ( # of scans)1/2

• You do not see all the peaks as in regular-IR• Remember that normal IR ( solution, solid, or

gas) is quite strong [ using KBr windows or ATR)

• We have both RAIR and ATR accessories at IfM

DDT ODT

Alkanethiols



XYLTMXYL




PMBM


1112

1260 15

96

0

0.002

0.004

0.006

0.008

0.01

0.012

1000 1200 1400 1600 1800 2000 2200

Wavenumber (cm-1)

Abs

orba

nce

RAIR Spectrum of Benzylthiol on Au

6

RAIR Spectrum of Xylyl-dithiol (XYL) SAM on Au 4-Pyridinethiol Derivatives (4-PySHD) and 4-PySHD coordinated to MTPP

N

(CH2)nSH

When n = 0; 4-Pyridinethiol

n = 0, 1, 2, etc.

M

N

N N

N

N

(CH2)n-SAu

M = Zn, Co, Ni, Mn

n = 0, 1, 2,3Preliminary ESP Measurements4-PySH 30 ± 50mV 4-PySH-CoTPP 130± 50mV

RAIR Spectrum of PySH–CoTPP SAM on Au Primer: A Cell Up Close

Cell membrane: Lipids (structure), Proteins (gateways)

bR

Cell wall: Rigid, Permeable

PurdueNanoscience

7

Percent CoveragePurdueNanoscience

HOPG30% Coverage

Au40% Coverage

Crittenden & Reifenberger

Suggested Papers for Reading

• K. Vijayamohanan & M. Aslam “ Applications of Self-Assembled Monolayers for Biomolecular Electronics” Appl. Biochem. Biotech., 2001, 96, 25-39.

• J. F. Fang et al. “Self-Assembled Rigid Monolayers of 4’-Substituted-4-mercaptobiphenyls on Gold and Silver Surfaces” Langmuir, 2001, 17, 95-106.

Nanostructures

• Nanotechnology is still very much in infant stages

• Characterization of the nanoscale sytems is necessary– Knowledge of electrostatic interaction can

provide a powerful insight into electronic properties

• AFM is capable of measuring piconewtonforces with nm resolution

Experimental Set-Up

Photo DiodeLaser Topo Lock-In EFM Lock-In

Ref Ref

Feedback Control

Piezotube

Sample

AFM Tip

+

Out Out

InIn

Piezo Vibrator

1ω

oω

To Computer

The EFM lock-in measures the amplitude of the ω 1 component: ]Vy)(x,[VVdzdC

Amp tipsoω1 −−=

t)sin(ωV 1o

TipV

t)sin(ωV oamp

8

PurdueNanoscience Experimental Set-Up



Nano-Scale Charge Transfer in Au/Organic Interfaces

+ + + +

Au Substrate

Molecule Dipoles

AFM Tip

Debasish Kuila

Langmuir, 2002, 18, 5120-25

+

Contact potential difference (CPD)

• CPD exists when crystalline objects are placed in intimate contact to form a junction– Results from the equilibrium of both the

temperature and the chemical potential throughout the junction

-600 -400 -200 0 200 400 600

0

100

200

300

400

500

600

700

800

900

340 mV

Tip over Ni

Tip over Au

Ele

ctro

stat

ic F

orce

Mag

nit

ud

e (A

. U.)

Tip Voltage (mV)

PurdueNanoscience Elimination of the Electrostatic Force

cpdtip VV =

When two metals are in contact, their Fermi levels will coincide due to thermodynamic equilibrium. By connecting this system to a bias voltage source, the electrostatic potential can be eliminated.

Contact Potential Difference Test2FE1FE

Tip Sample

{ }}1φ 2φ eVcpd

21 φφ −=

1FE2FE

{

}{ 2φ1φ

tipV

VACE

Howell 00

9

Macroscopic Kelvin Probe

• A device that measures the CPD between a sample and a reference electrode ( w known WF, Work Function)

• Two electrodes composed of different metals to form a parallel plate capacitor– Diameters > separation of the plates

connected in series w a current meter and a voltage source

Atomic Force Microscopy (AFM)• Measures forces by detecting the motion of a

spring like probe known as cantilever– Long thin micro-machined beams of Si with a

base containing a tiny tip attached at its end ( radius ~ 10 nm)

• High lateral resolution of force measurements is due to the small diameter of the tip’s apex.

• Interaction between the tip and the sample cause the cantilever beam to deflect– different forces (Magnetic, van der Waals,

electrostatic, adhesion) can be measured simultaneously

Measurement of Electrostatic Interaction

• A conducting tip is biased with a controlled voltage– Modifies the tip-sample potential difference which causes

a deflection of the cantilever

• Controlling the tip-sample potential difference, the electrostatic force emanating from a sample’s surface can be measured as a function of position

• EFM ( Electrostatic Force microscope) – a modified AFM

References:1. Stephen W. Howell, Ph.D. Thesis, Purdue U, May 20012. Langmuir, 2002, 18, 5120-25

Nano-Scale Charge Transfer in Au/Organic Interfaces

+ + + +

Au Substrate

Molecule Dipoles

AFM Tip

Debasish Kuila

Louisiana Tech University

Langmuir, 2002, 18, 5120-25

+

10

Electrostatic Surface Potential (ESP) of Organic Thiols on Au using AFM

ESP Measurements

• SAMs of Aliphatic and Aromatic thiols

• ESPs of Symmetric vs. Non-symmetric Systems

• Theoretical Calculations (Preliminary)

• Summary

Electrostatic Surface Potential (What and Why)

Self-Assembled Monolayer of Molecules on Au

+ + + + + + + +- - - - - - - -

E

V

VSAM (wrt Au)

0Au

=

Why Measure Surface Potential

• Insight into electronic properties of SAMs

• A diagnostic feature for the molecule (s)

• Better models to I-V

• Potential Chemical Sensors

• Potential Chemical FETs for nanoelectronic devices

Experimental Set-Up

Photo DiodeLaser Topo Lock-In EFM Lock-In

Ref Ref

Feedback Control

Piezotube

Sample

AFM Tip

+

Out Out

InIn

Piezo Vibrator

1ω

oω

To Computer



t)sin(ωV 1o

TipV

t)sin(ωV oamp

Measuring the Electrostatic Force

Force Detector

y)(x,VS

Voltage Control

)ω,2ω,(ωF 11otot

t)sin(ωVV 1oTip +

Vibrator

The electrostatic forces acting on the cantilever due to the tip-sample capacitance is:

The potential difference between the tip and substrate is:

t))sin(V(Vy)(x,VV 1oTipS ω+−=

t)sin(V]Vy)(x,[VdzdC

1oTipS ω−−Howell 00

11

-400 -200 0 200 400 600

0

100

200

300

400

500

Mag

nit

ud

e of

Ele

ctro

stat

ic F

orce

(a.

u.)

Tip Voltage (mV)

Experimental Procedure

Null Voltage

V1

EFM Probe

Sample

Howell 00

Au

VSAM

VAu

EFM Probe

Electrostatic Surface Potential of Molecules

Au

VTip

VTip

Howell 00

PurdueNanoscience

Evac

LUMO

HOMO

EF

Evac

φM

Molecule Metal

qVbi

Energy Structure of a Molecule Bonded to a Metal

EFo

qVmol

qVmol is the potential of the molecule wrt the metal.Howell 00

-600 -400 -200 0 200 400 600

0

100

200

300

400

500

600

700

800

900

340 mV

Tip over Ni

Tip over Au

Ele

ctro

stat

ic F

orce

Mag

nit

ude

(A. U

.)

Tip Voltage (mV)

Elimination of the Electrostatic Force

cpdtip VV =

When two metals are in contact, their Fermi levels will coincide due to thermodynamic equilibrium. By connecting this system to a bias voltage source, the electrostatic potential can be eliminated.

Contact Potential Difference Test2FE1FE

Tip Sample

{ }}1φ 2φ eVcpd

21 φφ −=

1FE2FE

{

}{ 2φ1φ

tipV

VACE

Howell 00

12

φSAMφSAM

Tip Over Au Inferred surface potential Between SAM and Au

Comparing the Electrostatic Surface Potential of SAMs and Au Reference Samples

Since the work function of the tip is the same for both measurements, the surface potential of the SAM coated Au can be referenced to the bare Au substrate.

gap

EF

tip Au

φtip φAu

}eV1

Tip Over SAM/Au

gap

EF

tip Au

φtip φAu

eV2

SAM gap

EF

Au Au

φAu

eVSAM

SAM

φAu

φtip=eV1+ φAu φtip=eV2+ φSAM VSAM = (V1-V2)= -(φAu-φSAM)/e

Howell 00

PurdueNanoscience

Evac

EFo

LUMO

HOMO

EF

Evac

φm

Isolated Molecule Isolated Metal

φmolI. P.

qVbi

qVbi = φm - φmol

Energy Structure for Isolated Systems

Howell 00

Au

VSAM

VAu

EFM Probe

Electrostatic Surface Potential of Molecules

Au

VTip

VTip

Howell 00

Sample Preparation

Scan Size: 1.7 µm X 1.7 µm

500 nm

1) Au substrates are flame-annealed and cleaned. This procedure produces large flat Au(111) grains.

2) The surface potentials of the bare Au substrates are measured prior to SAM deposition.

3) SAMs are then grown on the annealed Au substrates.

4) The surface potential of the SAMs are measured using EFM techniques (discussed above). The surface potential measurements are referenced to a bare Au reference sample.

Non-Contact Scans of BM Coated Au

Howell 00

13

DDT ODT

Molecules for Initial Studies


Howell100 ± 20 mV 230 ± 30 mV

DDT ODT

Theoretical Calculations (preliminary)


100 ± 20 mV 230 ± 30 mV

MRS Proceedings, 2000, # D9.38

SAM on a Au-substrate ESPs of Alkanethiols ( lit. & Purdue results)

14


XYL TMXYL




PMBM(Pentamethylbenzylmercaptan )

1112

1260 15

96

0

0.002

0.004

0.006

0.008

0.01

0.012

1000 1200 1400 1600 1800 2000 2200

Wavenumber (cm-1)

Abs

orba

nce

RAIR Spectrum of Benzylthiol on Au

-1.0 -0.5 0.0 0.5 1.0

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

XYL+Au

Au

Mag

nit

ud

e of

EL

ectr

osta

tic

For

ce (

a.u

.)

Tip Voltage (V)

Electrostatic Surface Potential Measurements of Symmetric and Non-Symmetric Molecules

-800 -600 -400 -200 0 200 400 600 800

0

50

100

150

200

2501/2 ΞΨΛ+Αυ

Au

Mag

nit

ud

e of

Ele

ctro

stat

ic F

orc

e (a

. u

.)

Tip Voltage (mV)


Howell

BM + Au


XYL TMXYL




PMBM(Pentamethylbenzylmercaptan )

50 ± 30 mV16 ± 70 mV 235 ± 50 mV

150 ± 50 mV

15

Chemisorption of Xylyldithiol on Au

Au onGlass

Xylyldithiol on Gold

Benzyl Mercaptan on Gold ( LANL2DZ basis set) Theoretical Calculations

• Currently underway in collaboration with Prof. Ramachandran

16

• Measured ESPs of molecules w.r.t. bare Au

• ESPs of alkanethiols increase with chain length and the trend is similar to that reported in the literature

• Charge-transfer at the interface appears to be small and is dominated by the molecular structure

• Non-symmetric aromatic thiols have higher ESPs than

symmetric ones

• Theoretical work is underway to understand the

magnitude of these differences

Summary ESPs of Phenylthiols

SH SH SH

MPT BPT TPT-0.38 V -0.76 V -0.72 V

Book chapter & cross references

ESPs of TMXYL and the Charge-Transfer Complex

20±70 mV

-140±25 mV 30 ± 60 mV

• MRS Proceedings, 2000, #D9.38

• Langmuir, 2002, 18, 5120-25

• Encyclopedia of Nanoscience and Nanotechnology, “Nanoscale Charge Transfer in Metal-Molecule Heterostructures” 2004, Vol 1., pp. 683-698.– www.dekker.com

Additional Information

17

ESP of N-terminal Peptides with different lengths

•

Gold substrate

NH

S S

O=CO

CH2O=C

OCH2

NH

S S

+

-

Alpha helical peptide

N terminal

C terminal

Few hundred mV –veESP

Chem. Phys. Lett1999, 315, 1-6

Book chapter & cross references

Molecule/Metal Heterostructure as a Sensor

Au

-+

+-

Symmetry: Small Net Dipole

Simple Physical Interpretation Based on Symmetry

Au

+

-

Non-Symmetry: Large Net Dipole

Mirror Plane Mirror Plane

Howell 2000

Experimental Set-Up



18

RAIR Spectrum of Xylyl-dithiol SAM on Au


XYL TMXYL


BT

(Benzyl mercaptan)

PurdueNanoscience Molecules Under Investigation

PMBT


50 ± 30 mV 16 ± 70 mV 235 ± 50 mV 150 ± 50 mV

Nonanedithiol

-0.08

-0.06

-0.04

-0.02

0

0.02

0.04

0.06

-1.5 -1 -0.5 0 0.5 1 1.5

Voltage (V)

Cu

rren

t (A

)

pad 1

pad2

pad 3

pad 3 meas 2

pad 4

S. Kadathur and D.B.Janes

I-V Measurements on NonanedithiolInterdigitated Au Fingers to Build Nano-

sensors

Pads for Probing Interdigited fingers

5.568µm

4.176µm

2.784µm

2.083µm

1.382µm

# of Square(*10^-3)

Space between Fingers

Choi, Janes, Santanam & Andres

19

Acknowledgements

Support : DARPA/ARO, Indiana 21st Century Program

• S. Howell

• H. McNally

• B. Kasibhatla

• C. Kubaik

• D. Janes

• R. Reifenberger

• S. Datta

• T. Rakshit

• P. Damle

• P. Das

RAIR Spectrum of Xylyl-dithiol SAM on Au

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