Porphyrin Molecules for ChargeBased Information Storage
Pratik Joshi
Schematic of Molecular Electronic Device
• Field started in 1974-Aviram and Ratner
• Molecule could act as diode if a molecule has donor
group on one side and acceptor group on other side
and a spacer group in between
• But how do we contact the molecules?
Ref: Jan M. van Ruitenbeek, Universiteit Leiden ,Single Molecules as Electronic Devices
Metal-molecule contacts: Anchoring groups
Standard Solution:
• Use Self assembled (SAM) Molecules and then
• Thiols are used as attachment to gold and
alcohols for attachment to Si and Ge surfaces
• alkanedithiols –archetypical insulators
• benzenedithiol - most simple aromatic molecule
which can be coupled to metal electrodes.
Ref: Juan Carlos,” Molecular Electronics: An Introduction to Theory and Experiment”,2010
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Porphyrins: Nature’s Pigment of Life
• Fast electron-transfer
reactions
• Stable radical cations
• Operate under real world
conditions
Ref: Dr. Ritu Shrivastava, ZettaCore Molecular Technology, Stanford Computer Systems Colloquium, 2005
“Engineerable” Molecular Properties
1. Charge Storage Molecule
• Composition determines charge
• density, size, isolation, voltage,
• stability (thermal and electrical)
2. Surface attachment group(tether or linker)
• Composition determines site of attachment,
stability (i.e. endurance), charge transfer rate,
charge retention
3. Molecules can be synthesized prior to
attachment or stepwise added on the surface
Ref: Werner G. Kuhr, The Electrochemical Society Interface, Spring 2004
Porphyrin-Based Molecular Architectures
Ref: Dr. Ritu Shrivastava, ZettaCore Molecular Technology, Stanford Computer Systems Colloquium, 2005
Design of the experiment
• The microelectrodes consisted of
25 mm gold wire sealed in soft
glass.
• The self-assembled monolayers
~SAMs! of the porphyrins were
formed by immersing the
microelectrode in a 2 mg/ml
solution of porphyrin for 20 min
and sonicating for an additional 1
min.
• All electrochemical
measurements were performed
Zn-tetraarylporphyrins bearing Son a locally constructed twoelectrode potentiostat with a 5 acetylthio-derivatized linkers
MHz bandwidth.
Ref: Kristian M. Roth et al, “Molecular approach toward information storage based on
the redox properties of porphyrins in self-assembled monolayers”, Journal of Vaccum
Science & Technology B, 2000
Oxidation and Reduction
•
•
•
mono-cation (Ep:0.73 V) and dication (Ep:1.05 V).
The efficiency by which the
porphyrins self-assemble is
demonstrated by integrating
the charge under each
oxidation wave in the
voltammogram.
porphyrin, can be repetitively
oxidized/reduced for several
thousand cycles
PM 0 SAM
Ref: Kristian M. Roth et al, “Molecular approach toward information storage based on
the redox properties of porphyrins in self-assembled monolayers”, Journal of Vaccum
Science & Technology B, 2000
Charge storage in Porphyrin SAMs
PM 1 SAM
• This experiment is conventionally
known as chronoamperometry.
• The potential step from 500 to 800
mV quantitatively oxidizes the
porphyrin monolayer.
1. Vapplied < 0.8 V- 00
2. 0.8V > Vapplied < 0.9 V - 01
3. Vapplied > 1V- 10
• Open Circuit potential (OCP)
method used to read the data
Ref: Kristian M. Roth et al, “Molecular approach toward information storage based on
the redox properties of porphyrins in self-assembled monolayers”, Journal of Vaccum
Science & Technology B, 2000
Scalability of molecular approach
PM 1 SAM
• Current versus time profile
sharpens considerably and the
instantaneous current increases.
• Thus, a smaller electrode area
results in less filtering of the
faradaic current
• Thus, miniaturization to submicron
dimensions should greatly
increase the read and write
speed.
Ref: Kristian M. Roth et al, “Molecular approach toward information storage based on the redox properties of
porphyrins in self-assembled monolayers”, Journal of Vaccum Science & Technology B, 2000
Charge storage
PM 0 SAM
PM 2 SAM
The charge retention time is a
function of many parameters
1.intrinsic stability of stored charge
2. For the PMn SAMs, addition of –
CH2– spacers between
the aryl substituent monotonically
increases the charge retention
time
Ref: Kristian M. Roth et al, “Molecular approach toward information storage based on
the redox properties of porphyrins in self-assembled monolayers”, Journal of Vaccum
Science & Technology B, 2000
Progress in Porphyrin Research
• Charge storage properties of molecules are well
understood, characterized and engineerable
• High charge density facilitates fabrication of sub100nm memory cells
• Molecular properties are intrinsic and engineered
into molecules
• Voltage, charge density, density of states, charge
transfer and charge retention properties accessible
via chemical synthesis
Challenges
• How the type of atom that is used for surface
attachment affects the electron transport properties?
• Electron transfer rates depend on packing density of
SAMs
• Could packing density of SAM be controlled
effectively?
• Can defective molecules be replaced to restore
the functional parameters the device? What is the
right balance between molecular economy and
needed redundancy for a given function?
• Fatigue after very large numbers of write/read cycles
Ref: Veena Mishra et al,” Porphyrin Architectures Tailored for Studies of Molecular
Information Storage”, 2004
“New technologies are like a new-born:
• Need to feed him enough for sure
• Hard to predict how much he may grow
• May develop unknown strange habits”
(Takasu, Rohm Corp., 1999 VLSI Symposium
Panel Discussion)
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