Nanotechnology 2 vo 2007gebeshuber/NT_bilder/Vorlesung...Chemical Synthesis of Artificial Nanostructures From Structural Control to Designed Properties and Functions 2.2. Molecular

Nanotechnology2nd lecture

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Last time we had:

• Introduction to Nanotechnology• Part A Nanostructures, Micro/Nanofabrication, and

Micro/Nanodevices2. Nanomaterials Synthesis and Applications: Molecule- h Based Devices. 2.1. Chemical Approaches to Nanostructured Materials

From Molecular Building Blocks to Nanostructures


Plan for today: Nanoscaled Biomolecules: Nucleic Acids and ProteinsChemical Synthesis of Artificial NanostructuresFrom Structural Control to Designed Properties and Functions

2.2. Molecular Switches and Logic Gates

From Macroscopic to Molecular SwitchesDigital Processing and Molecular Logic GatesMolecular AND, NOT, and OR GatesCombinational Logic at the Molecular LevelIntermolecular Communication

2.3. Solid State DevicesFrom Functional Solutions to Electroactive and Photoactive SolidsLangmuir–Blodgett FilmsSelf-Assembled Monolayers

Nanogaps and Nanowires

2.4. Conclusions and Outlook

Chemical approaches tonanostructured materials

Part A: Nanostructures, micro-/nanofabrication and micro-/nanodevices

Nanomaterials synthesis and applications: molecule based devices



1. Chemical approaches to nanostructured materials

• The top-down approach to engineeredbuilding blocks is extremely powerful andcan deliver effectively and reproduciblymicroscaled objects.

• This strategy becomes increasinglychallenging, however, as the dimensionsof the target structures approach thenanoscale.



• Nature efficiently builds nanostructuresby relying on chemical approaches.

• Tiny molecular building blocks areassembled with a remarkable degree ofstructural control in a variety ofnanoscaled materials with definedshapes, properties, and functions.

From molecular building blocks tonanostructures




• Nanoscaled macromolecules play afundamental role in biological processes.

• Nucleic acids ensure the transmissionand expression of genetic information.

• Nature can build a huge number of closelyrelated nanostructures relying only on 4building blocks (A, C, T, G).

Nanoscaled biomolecules:nucleic acids (1)




• e.g.: 100 nucleotide repeating units • 4100 polynucleotide sequences possible• Precise structural control @ the

nanometer level

Nanoscaled biomolecules:nucleic acids (2)




A polynucleotidestrand (a)incorporatesalternate phosphateand sugar residuesjoined by covalentbonds. Each sugarcarries one of fourheterocyclic bases.




(b) noncovalentinteractions betweencomplementarybases in twoindependentpolynucleotidestrands encouragethe formation ofnanoscaled doublehelices (c).




• Proteins are also built joining simplemolecular building blocks, the aminoacids, by strong covalent bonds.

• 20 different amino acids, backbone ofrobust [C-N] and [C-C] bonds.

• Genetic code.

Nanoscaled biomolecules:proteins (1)




• e.g.: polymer with 100 amino acids • 20100 polypeptide sequences possible• Nature can assemble an enormous

number of different biomolecules relyingon the same fabrication strategy and arelatively small number of building blocks.





• Secondary and tertiary structure of theprotein

• α−helices: tubular nanostructures, 0.5nmwide, 2nm long

• β−sheets: 2 to 15 parallel or antiparallelpolypeptide chains form nanoscaledsheets, average dimension 3*2nm2






A polypeptide strand (a) incorporates amino acid residues differing in their sidechains and joined by covalent bonds. Hydrogen bonding interactions curl a singlepolypeptide strand into a helical arrangement (α-helix) (b) or lock pairs of strandsinto nanoscaled sheets (β-sheets) (c).




• Modern chemical synthesis has evolvedconsiderably over the past few decades.

• Experimental procedures to join molecularcomponents with structural control atthe picometer level are available.

Chemical synthesis of artificialnanostructures (1/3)




Furthermore, the tremendous progress ofcrystallographic and spectroscopictechniques has provided efficient andreliable tools to probe directly thestructural features of artificial inorganicand organic compounds.





• Mimicking Nature: bipyridines as smallbuilding blocks, covalent bonds,secondary structure via noncovalentactions – double helical arrangement,wrapping around Cu centers.

• diameter 0.6nm, length 3nm.





An oligobipyridine strandcan be synthesized joiningfive bipyridine subunits bycovalent bonds. Thetetrahedral coordination ofpairs of bipyridine ligandsby Cu(I) ions encouragesthe assembly of twooligobipyridine strands intoa double helicalarrangement.




• Mimicking Nature: using amino acids asthe small building blocks make„macrocycles“ with a covalent backbone.

• Result: nanotube with 0.8nm innerdiameter, walls maintained in position via8 hydrogen bonds per macrocycle.

Chemical synthesis of artificialnanostructures (4)




Cyclic oligopeptides can be synthesized joining eight aminoacid residues by covalent bonds. The resulting macrocyclesself-assemble into nanoscaled tube-like arrays.




Chemical synthesis of certain structures isnice, but what about their function? Can wecontrol this?




• The high degree of structural control isaccompanied by the possibility ofdesigning specific properties into thetarget nanostructures.

• Creating functional machines!Electroactive, photoactive.

From structural control to designedproperties and functions




• Studies show: organic and inorganiccompounds can exchange electrons withmacroscopic electrodes. It is possible totune the absorption and emission ofphotons at a molecular level.

• Engineer chromophoric andfluorophoric functional groups withdefined absorption and emissionproperties!

From structural control to designedproperties and functions




From structural control to designed propertiesand functions: the molecular motor




• Material: [2]rotaxane, synthesis requires12 synthetic steps

• electrocactive unit• Light-induced start of shuttle shuttle

walks along chain. Shuttle walks backafter diffusion of the molecular oxygen inthe solution.

From structural control to designed propertiesand functions: the molecular motor


Nanomaterials synthesis and applications: molecule based devices2. Molecular switches and logic gates

Molecular switches and logic gates



Logic gates

• A logic gate is the elementary building block of adigital circuit. Most logic gates have two inputsand one output.

• Using combinations of logic gates, complexoperations can be performed.

• A digital integrated circuit consists of an array oflogic gates



From macroscopic to molecular switches (1)

• The major challenge in the quest for nanoswitches, isthe identification of reliable design criteria andoperating principles for these innovative andfascinating devices.

• Certain organic molecules adjust their structural andelectronic properties when stimulated with chemical,electrical, or optical inputs.



From macroscopic to molecular switches (2)

• Overall, these nanostructures transduce input stimulations intodetectable outputs and, appropriately, are called molecularswitches.

• The chemical system returns to the original state when the inputsignal is turned off.

• The output of a molecular switch can be a chemical, electrical,and/or optical signal that varies in intensity with the interconversionprocess.



Digital processing and molecular logic gates

• The three basic AND, NOT, and OR operatorscombine binary inputs into binary outputsfollowing precise logic protocols.

• NOT operator: 01, 10 (inverter)• OR operator: (1,1)1, (0,1)1, (1,0)1, (0,0)

0• AND operator: (1,1)1, (0,1)0, (1,0)0, (0,0)0




• The output of one gate can be connected to one of theinputs of another operator.

• NAND gate: connecting the output of an AND gate withthe input of a NOT gate

• NOR gate: connecting the output of an OR gate with theinput of a NOT gate.

• The NAND and NOR operations are termed universalfunctions because any conceivable logic operation canbe implemented relying only on one of these two gates




• The logic gates of conventional microprocessorsare assembled interconnecting transistors, andtheir input and output signals are electrical.

• But the concepts of binary logic can be extendedto chemical, mechanical, optical, pneumatic, orany other type of signal.




AND, NOT and OR operations can bereproduced with fluorescent molecules, in whichthe flurescence quantum yield depends on theconcentration of (an) certain ion(s).



Molecular NOT gate

The fluorescence intensity of thispyrazoline derivative is high whenthe concentration of H+ is low,and vice versa.



Molecular OR gate

The fluorescence intensity of thisanthracene derivative is highwhen the concentration of Na+

and/or K+ is high. The emission islow when both concentrations arelow.



Molecular AND gate

The fluorescence intensity of theanthracene is high only when theconcentrations of H+ and Na+ arehigh. The emission is low in theother three cases.



Combinatorial logic at the molecular level

• The molecular AND, NOT, and OR gates have stimulated the designof related chemical systems able to execute the three basic logicoperations and simple combinations of them.

• The implementation of logic operations at the molecular level is notlimited to the use of chemical inputs. For example, electrical signalsand reversible redox processes can be exploited to modulate theoutput of a molecular switch.



XNOR gate

The charge-transfer absorbance of this complex is high when the voltage inputaddressing the tetrathiafulvalene (TTF) unit is low and that stimulating the bipyridinium(BIPY) units is high and vice versa. If a positive logic convention is applied to the TTFinput and to the absorbance output (low = 0, high = 1) while a negative logic conventionis applied to the BIPY input (low = 0, high = 1), the signal transduction translates into thetruth table of a XNOR circuit.



Intermolecular communication

The logic function of a given circuit can beadjusted altering the number and type ofbasic gates and their interconnectionprotocol.



Intermolecular communication

• However, the connection of the input andoutput terminals of independent molecularAND, NOT, and OR operators would offerthe possibility of assembling anycombinational logic circuit from threebasic building blocks.

• Modular approach very powerful.• Problem: which wires to use.

Solid state devices

From functional solutions to electro-active and photoactive solids

• The molecular switches, as presentedbefore, are operated exclusively insolution and remain far from potentialapplications in information technology atthis stage.

• The integration of liquid componentsand volatile organic solvents in practicaldigital devices is hard to envisage.


• The development of miniaturizedmolecule-based devices requires theidentification of methods to transfer theswitching mechanisms developed insolution to the solid state.

• Nanometer-thick organic films on thesurfaces of micro-scaled or nano-scaledelectrodes are what we want to build.


Two main approaches for the deposition oforganized molecular arrays on inorganicsupports have emerged so far:– In one instance, amphiphilic molecular building blocks

are compressed into organized monolayers atair/water interfaces. The resulting films can betransferred on supporting solids employing theLangmuir–Blodgett technique.

– Alternatively, certain molecules can be designed toadsorb spontaneously on the surfaces of compatiblesolids from liquid or vapor phases. The result is theself-assembly of organic layers on inorganicsupports.

Solid state devices

Langmuir–Blodgett Films

• A LB-film is a packed monolayer at the air/waterinterface.

• Hydrophilic (hydros = water; philic = loving)molecules dissolve well in water (a polar solvent),whereas hydrophobic (waterminding) moleculesdissolve best in an apolar solvent.

• Molecules that consist of both a hydrophobic anda hydrophilic part are called amphiphiles (amphi= both) and are the building blocks forLangmuir–Blodgett Films.

Langmuir–Blodgett Films• The compression of the

amphiphilic dication with amoving barrier results in theformation of a packedmonolayer at the air/waterinterface.

• The lifting of the electrodepre-immersed in the aqueoussubphase encourages thetransfer of part of themonolayer on the solidsupport electrodes.

Inset: lipid bilayer of cells

• The lipid bilayer is the outer membrane of manybiological cells.

• It is comprised of a special type of lipid. Themembrane contains numerous proteins andsugars.

Amphiphile used for LB-films

• This amphiphile has a hydrophobichexadecyl tail (top) attached to ahydrophilic bipyridinium dication (bottom).

• It dissolves in chloroform and methanol.

Formation of an LB-Film• Disorganized amphiphiles are floating on the

water surface, after the organic solvent hasevaporated.

• The molecular building blocks are compressedinto a monolayer with the aid of a moving barrierand build an organized monolayer at the air/waterinterface.

• The slow lifting of the solid support drags themonolayer away from the aqueous subphase.

• The result is a surface coverage of ca.4×1010mol*cm−2, and a molecular area ofca.40Å2.

• This film can become the functional componentsof molecule-based devices.

Building block ofmolecule based devices

This electroactiveand photoactiveamphiphile containshydrophobicferrocene and pyrenetails and a hydrophilicbipyridinium head.

LB-film of amphiphilesand arachidic acid

• A mixed monolayer ofthese amphiphiles andarachidic acid, which istransferred with the LBtechnique to the surfaceof a transparent goldelectrode.

• The coated electrodecan be integrated in aconventionalelectrochemical cell.

Fabricate molecule based devices

• A unidirectional flow of electronsacross this monolayer/electrode junctionis established under the influence oflight.

• The ability to transfer electroactivemonolayers from air/water interfaces toelectrode surfaces can be exploited tofabricate molecule-based electronicdevices.

Electrode / molecular monolayer /electrode junction

Arrays of interconnected electrode / monolayer/ electrode tunneling junctions can beassembled combining the Langmuir–Blodgetttechnique with electron beam evaporation.

[2]rotaxane as building block• Monolayers of the [2]rotaxane

(amphiphilic character) aretransferred with the LM-technique to aluminum /aluminum oxide electrodes of apatterned silicon chip.

• The molecular area is ca.130Å2.

• The current/voltage signatureof these electrode / monolayer /electrode junctions can berecorded grounding the topelectrode and scanning thepotential of the bottomelectrode.

Characteristics of [2]rotaxanebased device

• If the applied potential is below −0.7V: bipyridinium-centeredLUMOs mediate the tunnelling of electrons leads to a currentenhancement.

• Also, modest increase in current when potential above 0.7V(tunneling due to participation of the phenoxy-centered HOMOs inthe tunneling process.

• After a single positive voltage pulse, however, no current can bedetected anymore if the potential is returned to negative values.

• This positive potential scan suppresses irreversibly the conductingability of the electrode/molecule/electrode junction.

LUMOs, HOMOs• HOMO and LUMO are acronyms for highest

occupied molecular orbital and lowest unoccupiedmolecular orbital, respectively.

• The energy level difference of the two (HOMO-LUMO) can sometimes serve as a measure of theexcitability of the molecule: the smaller theenergy, the easier it will be excited.

Simple logic operations with the[2]rotaxane device

• The two bottom electrodes can be stimulatedwith voltage inputs while measuring a currentoutput at the common top electrode.

• When at least one of the two inputs is high (0 V),the output is low (< 0.7nA). When both inputs arelow (−2V), the output is high (ca. 4nA).

Simple logic operations with the[2]rotaxane device

If a negative logic convention is applied to thevoltage inputs (low = 1, high = 0) and a positivelogic convention is applied to the current output(low = 0, high = 1), the signal transductionbehaviour translates into the truth table of anAND gate.

[2]catenane as building block

• Organic solutions of the hexafluorophosphate salt of this[2]catenane and six equivalents of the sodium salt ofdimyristoylphosphatidic acid can be co-spread on thewater surface of a Langmuir trough.

• The molecular area is ca. 125Å2.• Monolayers are transferred to the surfaces of the bottom

n-doped silicon/silicon dioxide electrodes of a patternedsilicon chip.

• Subsequent assembly of a top titanium/aluminumelectrode affords electrode/monolayer/electrode arrays.

Characteristics of [2]catenanebased device

• Alternating positive and negative voltage pulses can switchreversibly the junction resistance between high and lowvalues.

• This intriguing behavior is a result of the redox and dynamicproperties of the [2]catenane.

• The voltage pulses applied to the bottom electrode of theelectrode/monolayer/junction oxidize and reduce thetetrathiafulvalene unit inducing the interconversion between theoxidized and reduced forms.

• The difference in the stereoelectronic properties of these twostates translates into distinct current/voltage signatures.

Solid state devices

Self-assembled monolayers• LB-films are one way to

produce molecularmonolayers.

• An alternative strategy tocoat electrodes withmolecular layers relieson the ability of certaincompounds to adsorbspontaneously on solidsupports from liquid orvapor phases.

© http://www.ehcc.kyoto-u.ac.jp/laboratory/material/images/image_2.jpg

Thiols

• Thiols (formerly known asmercaptans) are compoundswhich contain the thiol group–SH attached to a carbon atom.

Self-assembled monolayers

• The affinity of certain sulfurated functional groups forgold can be exploited to encourage the self-assembly oforganic molecules on microscaled and nanoscaledelectrodes.

• The thiol-gold bond is believed to be a covalent bond.

© http://www.chem.wvu.edu/hfinklea/dbrevnov/monolayer.jpg

That´s it for today.

Plan for the next lectureHier noch was einfügen, sams und ??

3.1 Structure of Carbon Nanotubes– 3.1.1 Single-Wall Nanotubes– 3.1.2 Multiwall Nanotubes

3.2 Synthesis of Carbon Nanotubes– 3.2.1 Solid Carbon Source-Based Production Techniques for

Carbon Nanotubes– 3.2.2 Gaseous Carbon Source-Based Production Techniques for

Carbon Nanotubes– 3.2.3 Miscellaneous Techniques– 3.2.4 Synthesis of Aligned Carbon Nanotubes

3.3 Growth Mechanisms of Carbon Nanotubes– 3.3.1 Catalyst-Free Growth– 3.3.2 Catalytically Activated Growth

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Nanotechnology 2 vo 2007gebeshuber/NT_bilder/Vorlesung...Chemical Synthesis of Artificial Nanostructures From Structural Control to Designed Properties and Functions 2.2. Molecular