Chemical Approaches to Nanostructured Materials

Springer Handbook of Nanotechnology (2004): Ch. 2

Chemical Approaches to Nanostructured Materials

• Conventional device fabrication – Relies on assembly of macroscopic building blocks with specific

configurations– Increasingly difficult for nanosized features with sub-nanometer

precision• Bottom-up approach of nature

– 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– Nucleic Acids– Proteins

Springer Handbook of Nanotechnology (2004): Ch. 2

Chemical Synthesis of Artificial Nanostructures

• Borrow from nature’s design - Biomimetics

• Use nature’s building blocks– Covalent scaffolding

– Noncovalent interactions to define three-dimensional arrangement and overall shape

– DNA: covalent backbone (polynucleotides) bound together with and hydrogen bonds between base-pairs

• Chemical synthesis and analytical techniques enable tailored molecules with control at pico-scale

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

• Amino acid residues used as building blocks

• Multiple synthesis steps to form covalent [C-N] bonds

• Circular cavity (macrocycles) piled on top of one another and held together by H-bonds

Amino Acids

Molecular switches and logic gates

• Basic operation of switch– Input stimulations change physical state of switch

to produce a specific output• Motivation of molecular switch

– Small scale– Power of chemical synthesis

• Major challenges for nanoswitches– Reliable design– Operating principles

TransducerInput stimulus Output response

Organic molecules as switches

• Organic molecules change structural and electronic properties when stimulated with chemical, electrical, or optical input

• Change in properties of molecule is accompanied by electrochemical or spectroscopic response

• Changes in properties are often reversible

TransducerInput stimulus Output response

ChemicalElectrical Optical

ChemicalElectrical OpticalChange in structural and

electronic properties

Changes in absorbance, fluorescence, pH, redox potential

Most molecular switches rely on chemical input and spectroscopic output

Binary Logic

• Logic threshold established for each signal in a switch, which defines 0 and 1 digits

• Encoded bits manipulated by switch to execute logic functions

• Basic logic operators – NOT: One input, One output; Inverts signal– AND: Two input signals, One output– OR: Two input signals, One output

• More complex gate constructed by combining basic operators– 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

• 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

• Combining individual molecular switches is difficult– Different molecule has to be designed, synthesized, and analyzed for each new function

– Degree of complexity achievable for single molecule is limited

• Methods to transmit binary data between distinct molecular switches need to be identified

• Molecular switches operated in solution and organic solvents are difficult to integrate into practical devices

• Logic operations of chemical systems rely on bulk addressing– Macroscopic collection of individual switches is required for digital processing

Solid State Devices

• 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• Langmuir-Blodget Films: amphiphilic molecules deposited on solid support• SAMs: self assembly of organic molecules on gold nanoscaled electrodes

– Requires collection of molecules• Nanogaps and Nanowires: unimolecular devices

Solid State Switch based on Langmuir-Blodgett films

• LB film is sandwiched between poly-Si and metal electrodes

• Basic of device: voltage-driven circumrotation of co-conformer [A0] to co-conformer [B+]

• Co-conformer [A0] represents both the ground-state structure of the [2]catenane and the "switch open" state of the device.

• 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+].

• 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.

Science 18 August 2000:Vol. 289. no. 5482, pp. 1172 - 1175

Application of voltage pulse changes conductive state of molecule

Molecular device formed with SAMs

• 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)

• Nanopore etched through nitride membrane• Au-SAM-Au junction formed in pore area• Molecular layer of ~1000 SAMs

Science 19 November 1999:Vol. 286. no. 5444, pp. 1550 - 1552

Nanogaps and Nanowires

• Nanogaps and nanowires enable transition from devices relying on collection of molecules to single molecule devices

• Challenge of miniaturizing contacting electrodes to nanoscale

Silicon gate

Au source Au drainSiO2 gate insulator

C60 Molecule

Single C60 transistor: Nature 407 (2000), 57-60

• Gold strip patterned with e-beam

• Electromigration creates 1nm gap in Au

• Current between S and D adjusted by changing gate bias

• Metal ion complex

• DNA molecule

• Carbon nanotube

• Graphene

Bridge junction with other molecules

Single Molecule Transistor

• Metal electrode patterned with e-beam lithography

• Electromigration induced junction

• Au electrode with ~ 1nm gap

• Gap bridged by single divanadium molecule

• Trapping molecule between two metal electrodes is a challenge, the process of which has been described as a lucky occurrence

• Molecule need suitable terminations that reliably bind it chemically to the the electrodes, bridging the gap

• Transport across junction: single electron tunneling

Divanadium molecule

Nature (2002) 417, 725-729

Final remarks

• Use chemical synthesis to mimic nature’s approach to nanostructured materials

• Artificial nanoscaled molecules can be assembled piece by piece with high structural control

– Electroactive and photoactive fragments can be incorporated into single molecule

• Use of both covalent and noncovalent bonds enables unique molecular geometries

• Electroactive and photoactive molecules have been used to demonstrate simple logic operations

• Major challenges for advancing molecular electronics

– Mastering the operating principles of molecular-scales devices

– Develop fabrication strategies to incorporate molecules into reliable device architecture