Universidad Autónoma de MadridFacultad de Ciencias

Supramolecular Organization inSupramolecular Organization in Thin Films

Tomás Torres

AREAS DE TRABAJO EN MATERIALES MOLECULARES

-Sí t i d M lé l P lí- Síntesis de Moléculas y Polímeros

- Organización a Nivel Supramolecular

- Organización y/o Preparación de Fases condensadas

C t i ió d l Si t E t di d i d d- Caracterización de los Sistemas, y Estudio de propiedades

- Preparación de Dispositivos y Aplicaciones

-

Thin filmsThin-films

Thin-film deposition is any technique for depositing a thin film of material onto a substrate or onto previously deposited layer.p y p y

"Thin" is a relative term, but most deposition techniques allow layer thickness to be controlled within a few tens of nanometers, and somethickness to be controlled within a few tens of nanometers, and some (molecular beam epitaxy) allow single layers of atoms to be deposited at a time.

It is useful in the manufacture of optics (for reflective or anti-reflective coatings, for instance), electronics (layers of insulators, semiconductors and conductors form integrated circuits) packagingsemiconductors, and conductors form integrated circuits), packaging(aluminum-coated film).

i i h i f ll i b d i d diDeposition techniques fall into two broad categories, depending on whether the process is primarily chemical (CVD) or physical (PVD).

Chemical vapor deposition (CVD)

It is a chemical process used to produce high-purity, high-performance solid materials. The process is often used in the semiconductor industry to produce thi fil I t i l CVD th fthin films. In a typical CVD process, the wafer(substrate) is exposed to one or more volatile precursors, which react and/or decompose on the substrate surface to produce the desired deposit. F tl l til b d t l d d Frequently, volatile by-products are also produced, which are removed by gas flow through the reaction chamber.

Microfabrication processes widely use CVD to deposit materials in various forms, including: monocrystalline, polycrystalline, amorphous, and epitaxial These materials include: silicon carbon epitaxial. These materials include: silicon, carbon fiber, carbon nanofibers, filaments, carbon nanotubes, SiO2, silicon-germanium, tungsten, silicon carbide, silicon nitride, , titanium nitride, and various high k dielectrics The CVD process is also various high-k dielectrics. The CVD process is also used to produce synthetic diamonds

Techniques for the preparation and depositionon surfaces of molecular nanostructures

Nanotechnological applications require immobilization on a surface for many of theobjectives that these molecules are directed towards. Therefore, to achieve a suitableorganization one must consider not only interactions between the molecules themselvesbut also those with the surface which play a role of varying importance according to thebut also those with the surface, which play a role of varying importance according to thesystem.

Different techniques are used for the preparation and deposition on surfaces of molecularnanostructures:nanostructures:

-Solution and Thermal Evaporation on surfaces-The Layer-by-Layer Approachy y y pp- Langmuir–Blodgett (LB) films- Self-Assembled Monolayers (SAMs)

Solution and Thermal Evaporation on surfaces

The Layer-by-Layer Approach Recently much attention has been devoted to the development of

molecular assemblies of thin film based on successive deposition ofalternate layers of anionic and cationic polyelectrolytes includingalternate layers of anionic and cationic polyelectrolytes includingsynthetic polymers, proteins and nucleic acids, as an alternativeprocedure to the Langmuir–Blodgett Technique.

The layer-by-layer deposition technique relies on the electrostatic forceof attraction as an origin of the strong adhesion between the anionic andcationic layers. Therefore, the combination of polymers and biomaterialswhich have the same sign of electric charges and electrically neutralwhich have the same sign of electric charges and electrically neutralpolymers cannot be used in this procedure.

However, it may be possible to construct the layer-by-layer structure by However, it may be possible to construct the layer by layer structure by means of polymers and biomaterials which have biological interactions such as protein–ligand, antibody–antigen and lectin–saccharide bindings. This would extend the scope of the layer-by-layer deposition technique in constructing the thin film assemblies composed of proteins and otherin constructing the thin film assemblies composed of proteins and other biomolecules, because non-ionic polymers and even polymeric materials of the same polarity can be built into the same assemblies simultaneously through the biological interactions.

Langmuir Blodgett (LB) filmsLangmuir–Blodgett (LB) films

La Técnica de Langmuir-Blodgett

Método para la preparación de sólidos organizados en películas finas estructuradas en capas de espesor molecular

Película de Langmuir Película de LB

AIREAIRE

AGUA SUSTRATO SOLIDO

S 1 Th l l f i i di l d i l il i l h ill i hStep 1. The molecule of interest is dissolved in a volatile organic solvent that will not react with ordissolve into the subphase. A quantity of this solution is placed on the surface of the subphase, andas the solvent evaporates, the molecules spread forming a two-dimensional gas.

Step 2. A moveable barrier then compresses the surface of the subphase, allowing control of thesurface area available to the floating layer. Generally, the state of the monolayer on the watersurface is monitored by measuring the surface pressuresurface is monitored by measuring the surface pressure

Compression IsothermCompression IsothermCollapse pressure

Collapse point

Bidimensional solidSuperficial pressure

Bidimensional liquid

Bidimensional gas

Area per molecule

S 3 L i Bl d (LB) fil d b f i L i fil lidStep 3. Langmuir–Blodgett (LB) films are prepared by transferring Langmuir films onto a solidsubstrate by vertical or horizontal lifting. Multilayers are prepared by repeated (periodic) dipping ofthe substrate in/onto compressed films on appropriate solutions.

The traditional means of forming an organic monolayer film is to spread an insolublecompound on an aqueous subphase, compress the film mechanically with a barrier untilthe molecules are densely packed and oriented approximately normal to the surface, andy p pp y ,then to transfer this monolayer, if desired, to a solid substrate by dipping.

Langmuir Trough

Talham, Chem. Rev., 2004, 104, 5479

The molecules for LB Films formationThe molecules for LB Films formation

Polar head Neutral CH3-(CH2)n-COOH (-COOR, CONH2, OH, NH2,…)Charged: COO-, R4N+, HPO4

2-, SO4-,…

Hydrophobic chain Aliphatic (van der waals interactions)Aromatics (- interactions)

LB Films DrawbacksLB Films Drawbacks

They are only metastable and tend to relax into more stable structural forms. Surfaceproperties of LB films are most easily studied after the film has been transferred to aproperties of LB films are most easily studied after the film has been transferred to asolid substrate, a procedure that may be complicated by changes in the structure of themonolayer during the transfer process.

They are not normally chemically bonded to the substrate and hence are not robust.LB monolayers can often be removed from a substrate simply by rinsing with eitheraqueous or non-aqueous solventsaqueous or non aqueous solvents.

One is restricted to compounds that form LB films on water and that can betransferred intact to a substrate. It is, in particular, difficult to generate surfacesexposing polar functional groups at the monolayer-air interface by this technique.

It is also difficult to form highly crystalline monolayers since they tend to be brittleand crack easily upon compression or during transfer.

S lf A bl d M l (SAM )Self-Assembled Monolayers (SAMs)

Adapted from de M Angeles HerranzAdapted from de M. Angeles HerranzUniversidad Complutense de Madrid

Self-assembled monolayers (SAMs) provide a convenient, flexible, andi l t ith hi h t t il th i t f i l ti f t lsimple system with which to tailor the interfacial properties of metals,

metal oxides, and semiconductors. SAMs are organic assembliesformed by the adsorption of molecular constituents onto thesurface of solids or in regular arrays on the surface of liquids (in thecase of mercury); the adsorbates organize spontaneously into crystalline(or semicrystalline) structures.( y )

Love, Estroff, Kriebel, Nuzzo, Whitesides, Chem. Rev., 2005, 105, 1103

Combinations of Headgroups and SubstratesUsed in Forming SAMsUsed in Forming SAMs

SubstratesAdsorbed CompoundsAdsorbed Compounds SubstratesAdsorbed CompoundsAdsorbed Compounds

Sulfur CompoundsSulfur Compounds(Ag, Cu, Pt, Hg, Fe, -Fe2O3, Au, GaAs, InP)

Glass and Aluminum OxidesGlass and Aluminum Oxides(alkylsiloxanes)

Pl tiPl tiCopperCopper

(sulfur compounds)

Fatty AcidsFatty Acids(Al2O3, Ag, AgO, CuO)

PlatinumPlatinum(thiophenol and derivatives)

GoldGold

(sulfur compounds)

SiliconSilicon(alkyl derivatives)

Organic Silicon CompoundsOrganic Silicon Compounds(Silicon oxide, Aluminum oxide, Quartz, Glass,

Zinc selenide, Germanium oxide, Gold)

(sulfur compounds)

SilverSilver(sulfur compounds)

MercuryMercury(sulfur compounds)

( p )

Why Is Gold the Standard?Why Is Gold the Standard?

Gold is easy to obtain, both as a thin film and as a colloid. It isstraightforward to prepare thin films of gold Although expensive andstraightforward to prepare thin films of gold. Although expensive andnot essential to most studies of SAMs, single crystals are availablecommercially.G ld i ti ll t tt b bi ti f lith hiGold is exceptionally easy to pattern by a combination of lithographic

tools and chemical etchants.

Gold is a reasonably inert metal: it does not oxidize at temperaturesbelow its melting point; it does not react with atmospheric O2; it does notreact with most chemicals. These properties make it possible to handlep p pand manipulate samples under atmospheric conditions. Gold bindsthiols with a high affinity, and it does not undergo any unusual reactionswith them.with them.

Why Is Gold the Standard?

Gold is compatible with cells, that is, cells can adhere and function on gold surfaces without evidence of toxicity. SAMs formed from thiols on gold are stable for periods of days to weeks when in contact with the complex liquid media required. p q q

Thin films of gold are common substrates used for a number ofThin films of gold are common substrates used for a number of existing spectroscopies and analytical techniques. This characteristic is particularly useful for applications of SAMs as interfaces for studies in biology for cell studiesin biology for cell studies.

Preparation of SAMsPreparation of SAMs

• The gold substrate can be obtained either by using crystal substratesor by evaporation of thin Au films on flat supports, typically glass oror by evaporation of thin Au films on flat supports, typically glass orsilicon.• Typically concentrations of 1-2 mM are used.• Several different solvents are employed although ethanol is the most• Several different solvents are employed although ethanol is the mostconvenient: not toxic, cheap, easy to obtained with high purity and, ableto solvate very different structures.

• Even though a self-assembled monolayer forms very rapidly on thesubstrate, it is necessary to use adsorption times of 15h or more toobtain well-ordered, defect-free SAMs.• After the adsorption meticulous rinsing and drying are highlyimportant.• The monolayers obtained are thermodynamically stable andmechanically robust.

Multilayer Films Go 3-D

A combination of metal-ligand coordination and ligand-ligand aromatic interactionsleads to laterally stabilized assembly of wirelike chains (purple spheres and goldcrosses)crosses)

Altman, Zenkina, Evmenenko, Dutta, van der Boom, J. Am. Chem. Soc., 2008, 130, 5040

Structure of SAMs

Au

S(CH2)n‐X

25-40 º25-40 º

Characterization of SAMsCharacterization of SAMs

SAMs can be characterized by a range techniques including:

– Ellipsometry and contact angle determinationEllipsometry and contact angle determination– Transmision Electron Microscopy (TEM)– Scanning Tunneling Microscopy (STM)– Atomic Force Microscopy (AFM)– Reflection Adsorption Infrared Spectroscopy(RAIRS)(RAIRS)– X-ray photoelectron spectroscopy (XPS)– Electrochemistryec oc e s y

Reflection Adsorption Infrared Spectroscopy (RAIRS)p p py ( )

S

SS

SO

S O

O

OS

S

SSn SS

OS

SSn

a (n=0), b (n=3), c (n=4), d (n=5)

0.2-C-O-C-

21-C-7

-C=O

0.2-C-O-C-

21-C-7

-C=O

0 1

21-C-7

18-C-6sity

(a.u

)

0 1

21-C-7

18-C-6sity

(a.u

)

0.1 18-C-6

C S

Inte

ns0.1 18-C-6

C S

Inte

ns

15-C-5 -C-S15-C-5 -C-S

3200 3000 2800 2000 1600 1200 800

Wavenumbers (cm-1)3200 3000 2800 2000 1600 1200 800

Wavenumbers (cm-1)

Materials Assembly: Self-Assembled Monolayers (SAMs)

Electroactive SAMsElectroactive SAMs

2.0E-05

2.5E-05

2.0E-05

2.5E-05

5.0E-06

1.0E-05

1.5E-05

I/A

5.0E-06

1.0E-05

1.5E-05

I/A

O SHO

O

O

-1.0E-05

-5.0E-06

0.0E+00

E = 30 mV-1.0E-05

-5.0E-06

0.0E+00

E = 30 mV

OO

-2.0E-05

-1.5E-05

-900 -800 -700 -600 -500 -400 -300 -200 -100 0

E/mV

E 30 mV

-2.0E-05

-1.5E-05

-900 -800 -700 -600 -500 -400 -300 -200 -100 0

E/mV

E 30 mV

E = E0 + (RT/ln ([ox]/[red])E/mVE/mV

Echegoyen, Diederich,et al., Chem. Mater., 2000, 14, 1721

El t ti i t d SAMElectroactive species trapped on SAMs15

0

5

10

Curr

ent/

A

-0.94-1.34

-0.4 -0.6 -0.8 -1.0 -1.2 -1.4 -1.6-10

-5

C

P t ti l/V A /A +O O

O OO O

Potential/V vs Ag/Ag+

A ASS S

Au (111)Au Au

Zhang, Palkar, Fragoso, Prados, de Mendoza, Echegoyen Chem. Mater., 2005, 8,Zhang, Palkar, Fragoso, Prados, de Mendoza, Echegoyen Chem. Mater., 2005, 8,2063

Applications of SAMs

In biochemistry and biology: Model substrates to investigatebiological membranes and cellular interactions.biological membranes and cellular interactions. In crystallography: substrates for crystallization and to favor thenucleation of orientated crystals or to align crystals. In metallurgy: corrosion investigations and surface protection In metallurgy: corrosion investigations and surface protection,covering of organic materials wih metals. In molecular electronics for the preparation of different types ofdevices: rectifiers transistors molecular wires or switches ordevices: rectifiers, transistors, molecular wires or switches, orto carry out conductance measurements between two metallicreservoirs.

I l t h i t f th i ti ti f t bl t t In electrochemistry for the investigation of systems able to generatephotocurrent or for the development of sensors.

SAMs for NanoelectronicsSAMs for Nanoelectronics

The SAMAZO immobilized between a transparent Au surface and a Hgdrop electrode. As a result of the light-triggered isomerization betweenp g ggthe rodlike trans isomer (left) and the more compact cis isomer (right),the distance between both electrodes varies, thus providing both anoptoelectronic switch and an optomechanical cargo lifter.optoelectronic switch and an optomechanical cargo lifter.

Ferri,, Samori, et al. Mayor, Rampi, Angew. Chem. Int. Ed., 2008, 47, 3407

SAMs for Nanoelectronics

To demonstrate and exploit thiscollective effect, a metal–molecule–metal junction was used based on ametal junction was used, based on aHg top electrode. Hg was chosenbecause it is a metal featuring a

li t li id f Th j ticompliant liquid surface. The junctionincorporates the SAMAZOchemisorbed on a 25-nm-thicksemitransparent Au(111) electrode ona quartz support and a Hg dropcoated with a SAM of dodecanethiolas the top electrode.

Ferri, Elbing, Pace, Dickey, Zharnikov, Samori, Mayor, Rampi, Angew. Chem. Int. Ed.,, g, , y, , , y , p , g ,2008, 47, 3407

Gold Nanoparticles

Daniel, Astruc, Chem. Rev., 2004, 104, 293

Diagram of hybrid (QD + hth l i )

pyridine-QD Phthalocyanine

QDTiO2

dye(QDs+phthalocyanine) cells

FF

F

T

O

T

Odipping QD-sensitized electrodes in a proper

dye solution

e-e- Cascade structure in charge transfer!

4.2 eV

e-

e

2ndsensitizer

4.2 eV

e-

e

2ndsensitizer

gSchematic representation of an hybrid solar cell based on a quantum dot – phthalocyanine combination

meso-TiO

hole conductor

FTO

h+

1stsensitizer

sensitizer

meso-TiO

hole conductor

FTO

h+

1stsensitizer

sensitizer dot phthalocyanine combination

TiO2

7.6 eV

h+TiO2

7.6 eV

h+