Chemical Approaches to Nanostructured Materials

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Nov 15, 2013 (3 years and 10 months ago)

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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




Transducer

Input 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



Transducer

Input stimulus

Output response

Chemical

Electrical

Optical

Chemical

Electrical

Optical

Change 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 [A
0
]
to co
-
conformer [B
+
]



Co
-
conformer [A
0
] 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
[B
0
] 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 [A
0
] 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 drain

SiO
2

gate insulator

C
60

Molecule

Single C
60

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