Thin Films •  Crystalline, microcrystalline, amorphous, monolayer, multilayers

•  Important in a wide range of technologies: Protective coatings Filters, mirrors, lenses Microelectronic devices Batteries Electrode surfaces Photoelectric devices Photography Lithography Information storage Displays

•  Properties depend on: Composition Thickness (thinness) Surface area : volume Surface structure vs. bulk Surface properties vs. bulk

Chemical Electrochemical Physical Thermal Oxidation Cathodic deposition Sputtering Chemical Vapor Deposition Electroless deposition Evaporation Langmuir-Blodgett Films Anodic oxidation Molecular Beam Epitaxy Self-Assembled Monolayers

Critical Parameters for Thin Films

•  Compositional control

•  Uniform thickness, 5 to 50,000 Å layer thickness

•  Precise reproducibility

•  Control of deposition/growth rate

•  Impurity concentration < 10–9; cleanliness of growth system

•  Interfaces of multilayered films: abrupt changes of composition or dopant concentration

•  Large area, > 100 cm2

•  High degree of perfection for crystalline films: defects degrade device performance

Physical Methods of Thin Film Preparation

Thermal Vacuum Evaporation •  High vacuum, < 10–6 Torr

•  Sublimation of metal source: heat or e-beam

•  Gaseous M deposits on surroundings and substrate

•  Substrates: glass, metals, insulators, alkali halides

•  Sources: Metals, alloys, salts

•  Containers for M: W, Ta, Mo or Pt crucibles

•  p.221-2

Physical Methods of Thin Film Preparation

Cathode Sputtering

M M M Ar+

•  Bell Jar, 10–1 to 10–2 Torr Ar

•  Potential of several kV creates +ve rare gas ions, accelerate to cathode

•  Dislodges (“sputters”) M, condenses on surroundings

•  Composite multimetallic layers from mutlitarget sputtering

•  p.221

Electrochemical Preparation of Thin Films

•  Cathodic Deposition Two metal electrodes dipped into an electrolyte solution Externally applied field Metal ions from solution deposited on cathode as thin film Anode metal slowly dissolves Conducting (metallic) substrate

•  Electroless Deposition Spontaneous No applied field Common reducing agent: NaPO2H2

Ni films most common

•  Limitation of above methods: metallic films only

Electrochemical Preparation of Thin Films

•  Anodic Oxidation Formation of oxide films on metal surfaces (e.g. Al, Ta, Nb, Ti, Zr) Metal is anode, dipped into electrolyte (salt or acid) solution Oxide ions attracted to anode, forms thin oxide layer

e.g. Pt | H3PO4, H2O | Al

Al Al3+ + 3e- Anode

PO43- + 2e- PO3

3- + O2- Cathode

2Al3+ + 3O2- Al2O3 Anode Overall electrochemistry: 2Al + 3PO4

3- Al2O3 + PO33-

Equilibrium layer thickness depends on field strength

Langmuir Films •  Surfactant molecules at water-air interface are generally highly disordered

•  As packing density increases, molecules become more ordered

•  Simple way to induce highly ordered packing: the Langmuir trough

Langmuir Films

•  Compression of the surfactant layer by moving barrier

•  2D “flatland” states of matter: 2D gas, 2D liquid, 2D solid

Langmuir-Blodgett (LB) Films •  Transfer of a compressed Langmuir film to a solid surface •  Hydrophilic heads interact with hydrophilic surface (e.g. glass)

•  Langmuir film continually pressed against glass as it’s withdrawn

•  Building up a molecular crystal layer by layer

Disadvantages •  Restricted to amphiphilic molecules

•  Formation and manipulation of Langmuir film at the liquid-gas interface

•  Weak van der Waals interactions

•  Possible thermal or mechanical decomposition of layers

Advantages •  Well-established, well-studied area

•  Extensive number of examples to date

•  Readily forms ordered multilayered thin films

•  Simple design

•  Ambient conditions

•  LB films mimic the lipid bilayers of cell membranes

•  Model system for studying processes that occur at cell surfaces, e.g. immune responses

•  Passive: protective coatings, lithographic masks

•  Active: insulator layer in electronic devices (MIS junction), nonlinear optical materials, optical molecular switches

•  C8H17-PhN=NPh-O-C12H24Py+

·[(TCNQ)2]–

•  Photoinduced trans-cis isomerization

•  365 nm forward, 436 nm reverse

•  20% increase in conductivity upon trans-cis isomerization

Self-Assembly Molecular Self-Assembly

•  Spontaneous organization of molecules into stable, structurally well-defined aggregates •  e.g. protein folding and aggregation; base pairs in DNA; supramolecular frameworks

Self-Assembled Monolayers •  Molecule from solution or vapor with a surface-active head group •  (Covalent) binding of the head group to the surface, decrease in surface free energy •  Lateral ordering of the molecules by close-packing with neighboring molecules via weak, multiple, reversible interactions •  Ambient conditions, no need for clean room or ultrahigh vacuum •  Simpler to generate than LB films; wide variety of ligands and substrates

Self-Assembled Monolayers (SAMs)

•  Complete reaction of –SiCl3 groups, confirmed by XPS, IR

•  Careful control of amount of water in solution (~ 0.15 mg/100 mL)

•  No H2O: incomplete SAMs

•  Excess H2O: polymerization of molecules in solution

•  Si/SiO2 substrate suitable for some applications (microfabrication, processing directly on Si/SO2)

Alkanethiol SAMs on Au

•  Thermal evaporation of Au on polished Si wafer, glass slide or mica

•  Ti or Cr adhesion under-layer

•  Spontaneous adsorption of alkanethiols [X(CH2)nSH] on Au

•  S-Au bond (~ 44 kcal/mol), van der Waals interaction between alkyl chains (~2 kcal/mol per CH2)

•  All-trans alkyl chain, ~30° angle for n-alkylthiolates on Au with ≥ 8 CH2 groups

•  Organic and inorganic functional groups on end of alkyl chain: hydrocarbons, fluorocarbons, acids, esters, alcholols, nitriles, amines, …

•  RSH more soluble, faster reaction than RSSR or RSR

•  Coinage metal - coated substrates: Cu, Ag, Au

Self-Assembled Multilayers •  Defects in first layer propagate

to next layer and expand

•  All order lost by tenth layer

•  Need a self-healing layer (e.g. siloxane)

SiO2

Si

CH Cl2

Si

CH Cl2

Si

CH Cl2

N

NOH

N

N

SiO2

SiSi Si

+Cl2CH

N - +Cl2CH

N - +Cl2CH

N -

SiO2

SiSi SiSi O Cl23 8

Si Si Si Si

+Cl2CH

- +Cl2CH

- +Cl2CH

-N N N

J. Am. Chem. Soc. 1996, 118, 8034

•  Ellipsometry for thickness; XPS, UPS, XRR, IR, UV for elements, functionality; contact angle for hydrophilicity/hydrophobicity

•  High temporal stability

•  High density of aligned chromophores

•  Strong NLO susceptibility: SHG (λ à λ/2)

•  Improved over poled polymers and acentric LB films

•  Little known about detailed 3D structure: no bulk analog of these materials exist

Zirconium Phosphate (α-ZrP)

•  Zr(IV)OCl2 + H3PO4 Zr(O3POH)2⋅H2O •  Can replace H+ with Na+, then other

cations: metals, clusters, organics, …

•  Zirconium phosphonate: RPO3H2 or H2O3-PR-PO3H2 in place of H3PO4

•  Can use other metals in place of Zr

J. Am. Chem. Soc. 1994, 116, 8817

•  X(CH2)n-PO3H2 anchoring agent; X = HS for Au substrate, X = (MeO)3Si for Si substrate

•  Inorganic layered material for stability and paving defects •  4 hours per cycle for M4+ or M3+; 5 to 10 min for M2+

•  Zn2+: 19 Å increase in thickness per layer, cf. 20.5 Å for bulk

Alcohol Intercalation into α-ZrP

•  R-OH alcohols interact weakly with α-ZrP, cannot intercalate (even in boiling pure solvent)

•  Must pre-expand the layers with Na+

•  Ethanol can then intercalate:

α-ZrP → ZrNaH(PO4)2·5H2O + ROH + H+ → Zr(HPO4)2·ROH + Na+ + 5H2O

•  Organic bilayer hydrophobic region •  Longer or branched chain, go through ethanol phase first •  Layer separation increases 2.07 Å for each trans C •  Low stability, lose guests in air or by rinsing with polar solvent •  Easy to exchange for larger guests (acac, pentanedione)

2.07 Å/C

1.10 Å/C

bilayer

monolayer

1.27 Å/C for all-trans alkyl chain ⇒ θ = 54°

Fullerene SAMs

J. Am. Chem. Soc. 1994, 116, 8817

Exfoliation of Inorganic Layers

Photolithography

Limits of Photolithography •  Contact printing: mask visually aligned to previous pattern on wafer and pressed against wafer •  1973, projection lithography, 1:1, increased mask lifetime, reduction in defect density •  Si wafer size grew (3” - late 1970’s, 4” - early 1980’s, 5” - mid 1980’s, 6” - late 1980’s, 8” - 1990’s, 12” - 2000’s); Step and repeat: 1 to 2 cm2 exposed at a time •  Minimum resolution:

R = k1λ/NA Where: λ is the wavelength of exposing radiation

k1 is 0.4 to 1, “process-dependent” NA = numerical aperture: refractive index of the surrounding medium × sin(θ)

•  Depth of focus: DOF = k2λ/(NA)2

•  For smallest features, need small R, large DOF •  k1, k2 limited (lens sizes increased or focal length decreased); increasing NA for smaller R decreases DOF by a factor of 1/(NA)2

•  Research for photolithography focused largely on decreasing λ, using suitable photoresist, light source and lens materials

Chem. Rev. 1999, 99, 1801

Photolithography “Deep-UV”: shorter wavelengths (250 nm and below)

X-ray Lithography 4 to 20 Å wavelength

Low-brightness of electron-impact X-ray sources Synchrotron radiation, but requires stable masks and commercially available resists

Particle Beams •  Electron beam lithography: 1 to 100 keV electrons (de Broglie wavelength = 8 pm at 25 keV) focused and scanned over substrate

Electrons collide with atoms of resist (PMMA), forming radicals Image blurred by:

e–-e– repulsion in beam scattering of electrons backscattering of electrons

•  Ion-beam lithography: accelerated beam of ions (H+, He+ or Ga+) Energy loss for ions more efficient, leading to less scattering than for electrons High brightness sources, high-speed deflection systems, stable masks and lenses

•  Above two particle beam methods are performed in serial

Scanning Probe Scanning probe lithography - (very) serial, but may be good for formation of masters

Nanochemistry •  Nanoscience: nanochemistry, nanophysics

•  Nanomaterials: nanocrystals, nanoparticles, nanoclusters, nanotubes

•  Nanoscale structures with interesting materials properties or function: nanotechnology, nanoengineering

•  Existing technologies: top(bulk)-down approach

•  Nanoscience: bottom(molecule)-up approach

•  Deals with the synthesis rather than engineering of nanoscale materials

•  Extended in 0, 1, 2 or 3 dimensions

Adv. Mater. 1992, 4, 612

Nanochemistry: Quantum Confinement

Sci. Amer. 1990, 124

Semiconductor Nanocrystals

JACS 1993, 115, 8706

•  New behavior, properties between molecular and bulk

•  CdE (E = S, Se, Te) semiconductor nanocrystals

•  Synthesis: Cd(Me)2 in TOP (tri-n-octylphosphine) + Se in TOP added to TOP-oxide at 300°C

•  Aliquots extracted to watch absorption spectrum (peak widths) and adjust temperature → “Ostwald ripening”

•  Can produce monodisperse samples of many sizes from single experiment

•  Macroscopic quantities of monodisperse particles of regular core and TOPO cap structure, 12Å to 115Å

•  Slightly prolate, aspect ratio 1.1 to 1.3

•  TEM, PXRD: wurtzite structure (hcp S2-, Zn4+ in half Td sites)

•  Spatial confinement of electronic and vibrational excitations

•  Widening HOMO-LUMO gap with decreasing crystallite size

12Å: 410 nm (indigo)

35Å: 540 nm (green)

100Å: broad peak

1.2 nm

Eg ~ 3.0 eV

3.5 nm

Eg ~ 2.3 eV

10 nm

Eg ~ 1.8 eV

Luminescent Silicon

JACS 1993, 115, 8706; Adv. Mater. 1997, 9, 783

•  Si is abundant, cheaper than III-V or IV-VI materials

•  Invoking visible light emission from Si for optoelectronics

•  Ease of integration into Si-based technology

•  Si is an indirect material: interband transitions require phonons

•  Quantum-confined Si nanoscale objects lead to quantum-size effects: UV-excited visible luminescence

•  Blue shift with decreasing size of Si nanostructure

JACS 1993, 115, 8706; Adv. Mater. 1997, 9, 783

•  Cylindrical pore enlargement: Anodic oxidative etching in dilute HF

•  1D quantum wires (2D quantum confinement)

•  Actual morphology and SiO2 content depends on dopant level and etching conditions

Ultrasonic dispersion in organic solvent

•  Si nanocluster

•  Quantum dots (3D quantum confinement)

•  Photoluminescent

Luminescent Silicon

Amorphous Silicon •  a-Si: dangling bonds create interband trap states that fill holes in

VB (p-type) or are filled by electrons in CB (n-type) (Fig. 4.15)

•  SiH4 + RF plasma in CVD chamber à a-Si:H

•  Passivated by H, PECVD allows large area

•  Clean band gap, can be n- or p-doped

•  Can be formed at low temperature, thus variety of substrates

•  Lower electron mobility, but can make thinner (1% compared to c-Si), flexible film, roll-to-roll processing

•  Photocopiers, calculator solar cells, large area solar cells (each thin layer tuned for λ), large-area electronics

•  p.217-9

Molecular Machines: Rotaxanes

•  Stimulus (s) of switching can be chemical, electrochemical or light

Acc. Chem. Res. 1998, 31, 405

Pseudorotaxanes

Rotaxanes

Catenanes

Molecular Machines: Rotaxanes

•  Dumbbell has two different recognition sites (benzidine and biphenol) •  Cylcophane unit resides mostly on the more π-electron rich benzidine site •  Electrochemical switching: oxidation of dumbbell to radical cation repels cationic cyclophane •  Chemical switching: protonation with trifluoroacetic acid •  Molecular-based logic gates

Soft Lithography: Microcontact

Printing

Chem. Rev. 1999, 99, 1823

Rogers & coworkers J. Vac. Sci. Technol. B

2002, 20, 2853

Electrophoretic Displays •  Electronic paper (~ 100 µm thick), reflects light •  No refresh necessary, holds image without power •  Grayscale at present; color prototypes only; slow refresh rate •  1 µm diameter TiO2 in oil + surfactant + charging agent •  Electrophoretic migration to electrode of opposite charge •  White if particles at top (scattering), black if at bottom (black dye in oil)