Molecular Electronics

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

2
Where Builders Meet Chiselers
Scale
Scaling Down (Engineers)

• 100 x 10-6 m (100 mm)
• 10 x 10-6 m (10 mm)
• 1 x 10-6 m (1 mm)
• 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
Light Integrated Circuits, 1990 ---Predicted
Scaling Barrier--- Mesoscopic Physics Biomolecule
s, Molecular Assemblies Molecules Atoms
Building Up (Chemists)
3
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

4
Plenty of Room at the Bottom

• R. Metzger Electrical Rectification by a
  Molecule The Advent of Unimolecular Electronic
  Devices Acc. Chem. Res. 1999, 32, 950-957

5
Roughness analysis- unannealed gold- area II
6
Profile along the grains
7
Roughness of Au/Ti/Si Substrates

• It depends on preparation.
• 200 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).

8
Sample Preparation
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
500 nm
Scan Size 1.7 ?m X 1.7 ?m
Howell 00
9
Preparation and Characterization of SAMs

• 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

10
Alkanethiols
DDT
ODT
(Dodecanethiol)
(Octadecylthiol)
11
Molecules Under Investigation
Symmetric
Non-Symmetric
BM
PMBM
TMXYL
XYL
(Benzyl mercaptan)
(Pentamethylbenzyl mercaptan )
(Tetramethyl-xylyl-dithiol)
(Xylyldithiol)
12
Characterization of SAMs

• G. Whitesides
• D. Allara R. Nuzzo
• Hand-out Reflectance Absorption IR Spectroscopy
  (RAIRS) or IR-Reflectance Absorption Spectroscopy
  (IR-RAS)

13
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)

14
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 polarizability
  tensor 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)

15
Vibrational Spectra

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

16
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-Absorptio
  n Spectroscopy of Adsorbed Molecules Surf. Sci.
  Rep. 1983, 3, 107-192.

17
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

18
Alkanethiols
DDT
ODT
(Dodecanethiol)
(Octadecylthiol)
19
Molecules Under Investigation
Symmetric
Non-Symmetric
BM
PMBM
TMXYL
XYL
(Benzyl mercaptan)
(Pentamethylbenzyl mercaptan )
(Tetramethyl-xylyl-dithiol)
(Xylyldithiol)
20
RAIR Spectrum of Benzylthiol on Au
21
RAIR Spectrum of Xylyl-dithiol (XYL) SAM on Au
22
4-Pyridinethiol Derivatives (4-PySHD) and
4-PySHD coordinated to MTPP
23
RAIR Spectrum of PySHCoTPP SAM on Au
24
Primer A Cell Up Close
Cell wall
Rigid, Permeable
Cell membrane
Lipids (structure), Proteins (gateways)
bR
25
Percent Coverage
HOPG 30 Coverage
Au 40 Coverage
Crittenden Reifenberger
26
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.

27
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 piconewton forces
  with nm resolution

28
Experimental Set-Up
In
In
Laser
Photo Diode
Topo Lock-In
EFM Lock-In
Ref
Ref
Out
Out
Piezo Vibrator
AFM Tip
To Computer
Feedback Control
Sample
Piezotube

The EFM lock-in measures the amplitude of the ?1
component
29
Experimental Set-Up
The EFM lock-in measures the amplitude of the ?1
component
30
Nano-Scale Charge Transfer in Au/Organic
Interfaces
Debasish Kuila
AFM Tip
Molecule Dipoles





Au Substrate
Langmuir, 2002, 18, 5120-25
31
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

32
Elimination of the Electrostatic Force
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.
Sample
Tip
Contact Potential Difference Test
Howell 00
33
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 gt separation of the plates connected in
  series w a current meter and a voltage source

34
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 tips 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

35
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 samples
  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
  2001
• 2. Langmuir, 2002, 18, 5120-25

36
Nano-Scale Charge Transfer in Au/Organic
Interfaces
Debasish Kuila Louisiana Tech University
AFM Tip
Molecule Dipoles





Au Substrate
Langmuir, 2002, 18, 5120-25
37
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

38
Electrostatic Surface Potential (What and Why)
VSAM (wrt Au)
Self-Assembled Monolayer of Molecules on Au

V
0
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

39
Experimental Set-Up
In
In
Laser
Photo Diode
Topo Lock-In
EFM Lock-In
Ref
Ref
Out
Out
Piezo Vibrator
AFM Tip
To Computer
Feedback Control
Sample
Piezotube

The EFM lock-in measures the amplitude of the ?1
component
40
Measuring the Electrostatic Force
Force Detector
Vibrator
Voltage Control
The potential difference between the tip and
substrate is
The electrostatic forces acting on the cantilever
due to the tip-sample capacitance is
Howell 00
41
Experimental Procedure
EFM Probe
Null Voltage
V1
Sample
Howell 00
42
Electrostatic Surface Potential of Molecules
EFM Probe
VTip
VSAM
VTip
VAu
Au
Au
Howell 00
43
Energy Structure of a Molecule Bonded to a Metal
Molecule
Metal
Evac
qVmol
Evac
qVbi
?M
LUMO
EFo
EF
HOMO
qVmol is the potential of the molecule wrt the
metal.
Howell 00
44
Elimination of the Electrostatic Force
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.
Sample
Tip
Contact Potential Difference Test
Howell 00
45
Comparing the Electrostatic Surface Potential of
SAMs and Au Reference Samples
Inferred surface potential Between SAM and Au
Tip Over Au
Tip Over SAM/Au
Au
gap
Au
gap
SAM
tip
tip
Au
gap
SAM
Au
eV2

eV1
eVSAM
?tip
?tip
?Au
?Au
?Au
?SAM
?Au
?SAM
EF
EF
EF
?tipeV1 ?Au
?tipeV2 ?SAM
VSAM (V1-V2) -(?Au-?SAM)/e
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.
Howell 00
46
Energy Structure for Isolated Systems
Isolated Molecule
Isolated Metal
Evac
Evac
?mol
LUMO
?m
I. P.
EFo
qVbi
EF
HOMO
qVbi ?m - ?mol
Howell 00
47
Electrostatic Surface Potential of Molecules
EFM Probe
VTip
VSAM
VTip
VAu
Au
Au
Howell 00
48
Sample Preparation
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
500 nm
Scan Size 1.7 ?m X 1.7 ?m
Howell 00
49
Molecules for Initial Studies
DDT
ODT
(Dodecanethiol)
(Octadecylthiol)
100 ? 20 mV
230 ? 30 mV
Howell
50
Theoretical Calculations (preliminary)
DDT
ODT
(Dodecanethiol)
(Octadecylthiol)
100 ? 20 mV
230 ? 30 mV
MRS Proceedings, 2000, D9.38
51
SAM on a Au-substrate
52
ESPs of Alkanethiols ( lit. Purdue results)
53
Molecules Under Investigation
Symmetric
Non-Symmetric
BM
PMBM
TMXYL
XYL
(Benzyl mercaptan)
(Pentamethylbenzyl mercaptan )
(Tetramethyl-xylyl-dithiol)
(Xylyldithiol)
54
RAIR Spectrum of Benzylthiol on Au
55
Electrostatic Surface Potential Measurements of
Symmetric and Non-Symmetric Molecules
Non-Symmetric
Symmetric
BM Au
Howell
56
Molecules Under Investigation
Symmetric
Non-Symmetric
BM
PMBM
TMXYL
XYL
(Benzyl mercaptan)
(Pentamethylbenzyl mercaptan )
(Tetramethyl-xylyl-dithiol)
(Xylyldithiol)
50 ? 30 mV
150 ? 50 mV
16 ? 70 mV
235 ? 50 mV
57
Chemisorption of Xylyldithiol on Au
Au on Glass
58
Xylyldithiol on Gold
59
Benzyl Mercaptan on Gold ( LANL2DZ basis set)
60
Theoretical Calculations

• Currently underway in collaboration with Prof.
  Ramachandran

61
Summary

• 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

62
ESPs of Phenylthiols
-0.38 V -0.76 V -0.72 V
Book chapter cross references
63
ESPs of TMXYL and the Charge-Transfer Complex
2070 mV
30 60 mV
-14025 mV
64
Additional Information

• 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

65
ESP of N-terminal Peptides with different lengths

Few hundred mV ve ESP
Chem. Phys. Lett 1999, 315, 1-6
Book chapter cross references
66
Molecule/Metal Heterostructure as a Sensor
67
Simple Physical Interpretation Based on Symmetry
Symmetry Small Net Dipole
Non-Symmetry Large Net Dipole
-


-
Au
Mirror Plane
Mirror Plane
Howell 2000
68
Experimental Set-Up
The EFM lock-in measures the amplitude of the ?1
component
69
RAIR Spectrum of Xylyl-dithiol SAM on Au
70
Molecules Under Investigation
Symmetric
Non-Symmetric
XYL
TMXYL
BT
PMBT
(Xylyldithiol)
(Tetramethyl-xylyl-dithiol)
(Benzyl mercaptan)
(Pentamethylbenzyl mercaptan )

50 ? 30 mV
16 ? 70 mV
235 ? 50 mV
150 ? 50 mV
71
I-V Measurements on Nonanedithiol
S. Kadathur and D.B.Janes
72
Interdigitated Au Fingers to Build Nano-sensors
Space between Fingers of Square (10-3)
2µm 1.38
3µm 2.08
4µm 2.78
6µm 4.17
8µm 5.56
Choi, Janes, Santanam Andres
Interdigited fingers
Pads for Probing
73
Acknowledgements

• S. Howell
• H. McNally
• B. Kasibhatla
• C. Kubaik
• D. Janes

• R. Reifenberger
• S. Datta
• T. Rakshit
• P. Damle
• P. Das

Support DARPA/ARO, Indiana 21st Century Program
74
RAIR Spectrum of Xylyl-dithiol SAM on Au