Download Chemical Approaches to Nanostructured Materials

Chemical Approaches to Nanostructured Materials
Springer Handbook of Nanotechnology (2004): Ch. 2
Chemical Approaches to Nanostructured Materials
Springer Handbook of Nanotechnology (2004): Ch. 2
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Conventional device fabrication
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Bottom-up approach of nature
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Relies on assembly of macroscopic building blocks with specific
configurations
Increasingly difficult for nanosized features with sub-nanometer
precision
Relies on chemical approaches
Small components are connected to produce larger components
Molecular building blocks can be assembled with well defined shapes,
properties and functions
Nanoscaled Biomolecules
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Nucleic Acids
Proteins
Chemical Synthesis of Artificial Nanostructures
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Borrow from nature’s design - Biomimetics
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Use nature’s building blocks
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Covalent scaffolding
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Noncovalent interactions to define threedimensional arrangement and overall shape
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DNA: covalent backbone (polynucleotides) bound
together with  and hydrogen bonds between
base-pairs
Chemical synthesis and analytical techniques
enable tailored molecules with control at picoscale
Synthetic double helix. Five bipyridine subunits joined by
covalent bonds for form oligobipyridine strand. Double
helix forms in presense of inorganic cation
Nanoscale tube-like arrays
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Amino acid residues used as building blocks
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Multiple synthesis steps to form covalent [C-N] bonds
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Circular cavity (macrocycles) piled on top of one another and held
together by H-bonds
Amino Acids
Molecular switches and logic gates
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Basic operation of switch
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Motivation of molecular switch
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Input stimulations change physical state of switch
to produce a specific output
Small scale
Power of chemical synthesis
Major challenges for nanoswitches
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Reliable design
Operating principles
Output response
Input stimulus
Transducer
Organic molecules as switches
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Organic molecules change structural and electronic properties when stimulated with chemical, electrical, or
optical input
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Change in properties of molecule is accompanied by electrochemical or spectroscopic response
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Changes in properties are often reversible
Output response
Input stimulus
Transducer
Chemical
Electrical
Optical
Change in structural and
electronic properties
Chemical
Electrical
Optical
Changes in absorbance, fluorescence, pH, redox potential
Most molecular switches rely on chemical input and spectroscopic output
Binary Logic
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Logic threshold established for each signal in a switch, which defines 0
and 1 digits
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Encoded bits manipulated by switch to execute logic functions
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Basic logic operators
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More complex gate constructed by combining basic operators
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NOT: One input, One output; Inverts signal
AND: Two input signals, One output
OR: Two input signals, One output
NAND
NOR
Universal functions - any logic operation can be constructed from these two
Molecular switches respond to a variety of stimulations producing a
variety of specific outputs, which can be exploited to implement logic
functions
Molecular Gates
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Fluorescent molecules in mixture of methanol and water
Emission intensity of molecule depends on concentration of H+, K+, and Na+ ions
Complexation of cations inside azacrown receptor alters efficiency of photoinduced
electron transfer thereby enhancing or repressing the fluorescence
Pyrazoline
Anthracene derivatives
A.P. de Silva, H.Q.N. Gunaratne, C.P. McCoy: A molecular photoionic AND gate
based on fluorescent signaling, Nature 364 (1993) 42-44
Limitations of molecular switches
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Combining individual molecular switches is difficult
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Different molecule has to be designed, synthesized, and analyzed for each new function
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Degree of complexity achievable for single molecule is limited
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Methods to transmit binary data between distinct molecular switches need to be identified
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Molecular switches operated in solution and organic solvents are difficult to integrate into
practical devices
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Logic operations of chemical systems rely on bulk addressing
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Macroscopic collection of individual switches is required for digital processing
Solid State Devices
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Development requires method for transferring switching mechanism from solution to solid
state
Borrow design and fabrication strategies from conventional electronics
Lithography + Surface chemistry = Self assembly of patterned organic layers on inorganic
supports
Approaches
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Langmuir-Blodget Films: amphiphilic molecules deposited on solid support
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SAMs: self assembly of organic molecules on gold nanoscaled electrodes
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Requires collection of molecules
Nanogaps and Nanowires: unimolecular devices
Solid State Switch based on Langmuir-Blodgett films
Science 18 August 2000:
Vol. 289. no. 5482, pp. 1172 - 1175
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LB film is sandwiched between poly-Si and metal electrodes
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Basic of device: voltage-driven circumrotation of co-conformer [A0]
to co-conformer [B+]
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Co-conformer [A0] represents both the ground-state structure of the
[2]catenane and the "switch open" state of the device.
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When the [2]catenane is oxidized (by applying a voltage pulse of -2
V), the TTF groups (green) are ionized and experience a Coulomb
repulsion with the tetracationic cyclophane (blue), resulting in the
circumrotation of the ring and the formation of co-conformer [B+].
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When the voltage is reduced to a near-zero bias, the co-conformer
[B0] is formed, and this represents the "switch closed" state of the
device. Partial reduction of the cyclophane (voltage pulse of +2 V) is
necessary to regenerate the [A0] co-conformer.
Application of voltage pulse changes conductive state of molecule
Molecular device formed with SAMs
Science 19 November 1999:
Vol. 286. no. 5444, pp. 1550 - 1552
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Nanopore etched through nitride membrane
Au-SAM-Au junction formed in pore area
Molecular layer of ~1000 SAMs
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As voltage is applied, SAM molecule under-goes one-electron
reduction that provides a conductive state (Q=-1)
Further increase of voltage cause another one-electron
reduction to form a dianion insulating state (Q=-2)
Nanogaps and Nanowires
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Nanogaps and nanowires enable transition from devices relying on
collection of molecules to single molecule devices
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Challenge of miniaturizing contacting electrodes to nanoscale
C60 Molecule
Au source
Au drain
Bridge junction with other molecules
SiO2 gate insulator
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Metal ion complex
Silicon gate
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DNA molecule
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Carbon nanotube
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Graphene
Single C60 transistor: Nature 407 (2000), 57-60
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Gold strip patterned with e-beam
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Electromigration creates 1nm gap in Au
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Current between S and D adjusted by changing
gate bias
Single Molecule Transistor
Divanadium molecule
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Metal electrode patterned with e-beam lithography
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Electromigration induced junction
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Au electrode with ~ 1nm gap
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Gap bridged by single divanadium molecule
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Trapping molecule between two metal electrodes is a
challenge, the process of which has been described as
a lucky occurrence
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Molecule need suitable terminations that reliably bind it
chemically to the the electrodes, bridging the gap
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Transport across junction: single electron tunneling
Nature (2002) 417, 725-729
Final remarks
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Use chemical synthesis to mimic nature’s approach to nanostructured materials
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Artificial nanoscaled molecules can be assembled piece by piece with high structural control
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Electroactive and photoactive fragments can be incorporated into single molecule
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Use of both covalent and noncovalent bonds enables unique molecular geometries
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Electroactive and photoactive molecules have been used to demonstrate simple logic operations
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Major challenges for advancing molecular electronics
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Mastering the operating principles of molecular-scales devices
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Develop fabrication strategies to incorporate molecules into reliable device architecture