Download Adding the Molecule

MIAMI UNIVERSITY CENTER FOR NANOTECHNOLOGY
Fabrication of Single Digit Electrodes by E-Beam Lithography for Coupled Raman Spectroscopy and Electrical Conductivity Measurements of
Single Molecules
Fig 10 : This is chemical depiction of PDNC with one side
connected to a gold metal surface. This molecule will bond to the
surface perpendicularly or at a very small angle. This complex will
be used to bond the paddlewheel complexes at the axial positions.
Neil Smith, and Jan M. Yarrison-Rice, Physics Department, & Archana Jaiswal, Shouzhong Zou, Thomas Scott and Hongcai Zhou, Chemistry Department, Miami University
Bulk Measurements of PDNC
Closing the Gap
In order to assemble nanoscale devices based on molecular electronics, an understanding of the basic physics
and chemistry that occurs on a nanometer length scale must be gained. The objective of this research is measure
the conductivity of single molecule wires with a metal-metal bonds. Ultimately, other devices, such as diodes,
transistors and light activated switches, can be developed using single molecules.
v
Ga /In
Digit Design and Fabrication via E-Beam Lithography
Au Layer (~8 nm)
A
Al Layer (~10 nm)
Developed Resist
Exposed Resist
Prepared Silicon Wafer
Fig 8: Bulk measurements are performed by evaporating gold on to a glass slide
about 16 nm thick. The slide is then dipped into a solution containing the molecule
paradicyanobenzene (PDNC) to form a monolayer about 1 nm thick. The top layer is
evaporated through a mask. The top layer consists of either an Au layer or an Au/Al
layer. Each contact pad is round with a diameter of 0.5 mm, and is in contact with
about 5 x 1011 molecules. I-V characteristics are obtained by applying voltage to the
top and bottom layers of gold. The Raman spectra are collected through the back
scattering light.
PDNC (~1.5 nm)
Ga /In
Au Layer (~16 nm)
PMMA
Silicon Dioxide
Glass Slide
Fig 4: Contact pads connected with 30 μm
thick gold wires to package.
Silicon
Final Design
2.00E-07
(a)
3.00E-10
1.00E-07
-V
Current (A)
+V
Fig. 1 : E-Beam Lithography is a multi-step process. 1) Apply a layer of resist, about 300 nm by
spinning the resist onto the wafer 2) Expose the patterns through E-Beam lithography 3) Develop
Resist 4) Metal is then evaporated onto the wafer through a thermal evaporation method. Chromium
20 nm thick, Gold 180 nm thick, total thickness of 200 nm. 5) Lift-off procedure
4.00E-10
3.00E-07
Current (A)
Evaporate Metal
0.00E+00
-1.00E-07
2.00E-10
1.00E-10
0.00E+00
-2.00E-07
-1.00E-10
Fig 5: Through electrolysis, the 30 nm gaps can be
closed to 1 or 2 nm with Gold-Cyanide solution
-3.00E-07
-0.6
-0.4
-0.2
0
0.2
0.4
0.6
-2.00E-10
Voltage (V)
-2.5
-1
-0.5
0
0.5
1
1.5
2
2.5
Fig 10: I-V characteristics of Au/PDNC/Al/Au junction. This shows that the molecule
has rectification towards the positive bias.
γ
β
α
-1.5
Voltage (Volts)
Fig 9: I-V characteristics of Au/PDNC/Au junctions. The
I-V curve shows behavior found in many types of
molecular wires.
Adding the Molecule
-2
1
2
3
1
Fig. 2: GDSII design of digits with a 500 μm contact pads and a designed gap of 75 nm
2050
2100
2150
2200
2250
2300
Wavenumber (cm-1)
a)
Fig 3: Exposed digits with a width of
800 nm and a gap spacing of 30 nm
a) Fingers with contact pads, b)
Close-up of gap where the digits meet
b)
2
3
Fig 6: The molecule on the left is Di-Di-chloro-tetra(μ2-2-mercaptopyridine)di-rhenium(III) (abbreviated by Re2(mp)4Cl2) . The molecule on the right is
Di-chloro-tetra(μ2-2-mercaptopyridine)-di-platinum(III) (abbreviated by
Pt2(mp)4Cl2 ). These are the two molecule which are proposed to be placed
between the electrodes for the molecular transistor. Re2(mp)4Cl2 is a
quadruple bond while the Pt2(mp)4Cl2 is only a single bond.
800
1000
1200
1400
1600
1800
2000
2200
Wavenumber (cm-1)
Fig 11: Raman spectra of PDNC on Au: (1) before top electrode; (2) after 15 nm top Au layer; (3) after
top Al (8 nm) + Au (10 nm) layer deposition. Both the α and γ peaks come from the benzene ring of the
molecule while the β peak comes from the fluoresces of the glass slide which the sample was made on.
Fig 12: Focusing on the C=N junction, a peak is
observed at 2180 cm-1 (Raman line #1). Once a layer of
gold was added to the top of the molecule, the peak
shifted by 10 cm-1 to 2191 cm-1 (Raman line #2). The
peak was further shifted when an Au/Al layer was
added on top of the molecule to 2203 cm-1 (Raman Line
#3).
Summary
We have successfully fabricated Cr: Au electrodes on thermally oxidized silicon wafers with repeatable gaps of 35 nm using e-beam
lithography and thermal deposition.
Note: Because of
proximity effects, the 75
nm gaps closed to a
distance of 30 nm
Gold Digits
Gold Digits
Bulk measurements have been completed on similar molecules
•With an Au/ Al-Au contact pad, asymmetric I-V curves are observed with rectification toward the positive bias.
•With only an Au/Au contacts, a macular wire behavior is observed
Fig 7: This is a chemical depiction of the thiol-actate group directly connected
between gold electrodes on either side and an Re2(mp)4Cl2 molecule.
[Note: Raith 150 System: 300 nm PMMA on Silicon Wafer]
Through electro chemical synthesis, we try to close the gaps to a size of 2 or 3 nm.
A shift in the C=N Raman peak was observed when the C=N junction was made with metal
We gratefully Acknowledge the support of NFS through the NER-ECS-0403669 Grant
For more information contact Jan Yarrison-Rice at [email protected]