Guozhong Cao. Nanostructures & Nanomaterials: Synthesis, Properties & Applications

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216

Nanostructures and Nanomaterials

Fig.

5.24.

The more conventional vertical deposition method

for

the formation

films on substrates.

(dipping or down stroke). A monolayer usually will be transferred during

retraction when the substrate surface is hydrophilic, and the hydrophilic

head groups interact with the surface. However, if the substrate surface is

hydrophobic, the monolayer will be transferred in the immersion, and the

hydrophobic alkyl chains interact with the surface. If the deposition

process starts with a hydrophilic substrate, it becomes hydrophobic after

the first monolayer transfer, and thus the second monolayer will be trans-

ferred in the immersion. Multiple layer films can be synthesized just by

repeating the process. Figure

5.25

shows the film thickness proportionally

increased with the number of layers.’

Another method to build

multilayer structure is the horizontal lift-

ing, also referred to as Schaefer’s method. Schaefer’s method is useful for

the deposition of very rigid films. In this method as sketched in Fig.

5.26,

a compressed monolayer is first formed at the water and air interface,

a flat substrate is placed horizontally on the monolayer film. When the

substrate is lifted and separated from the water surface, the monolayer is

transferred onto the substrate.

Thermal stability and order-disorder transition are two important

issues for any practical applications of

films. Although a lot of

research has been done in the past two decades, many issues remain

unsolved and our understanding on the structures and stability

films

is still very limited.

Self-assembly and

technique offer the possibility of design and

the construction of stable organic superlattices. For example,

can be

applied to assemble electron donor and electron acceptor groups, sepa-

rated by well-defined distances

that can exchange electrons following

optical excitation. This may allow the construction, for example, of an

Two-Dimensional Nanostructures: Thin Films

217

Layer

number

Fig.

5.25.

Film thickness proportionally increased with the number of layers.

[N.

Tillman,

Ulman, and

T.L.

Penner,

Langmuir

101

(1989).]

Fig.

5.26.

Schaefer’s method useful for the deposition of very rigid films,

which, a com-

pressed monolayer is first formed at the water and air interface, a flat substrate

placed

horizontally on the monolayer film.

218

Nanostructures and Nanomaterials

electronic shift register memory based on molecular electron transfer

reactions.

5.1

Electrochemical Deposition

Electrochemical deposition or electrodeposition is a very well established

thin film growth method. In the previous chapter, we discussed the growth

of nanowires using this method, and some hndamentals of the process have

been introduced already. In this section, our focus will be on the deposition

of films. The key parameters in the electrodeposition of elemental films can

be conveniently grouped into thermodynamic and kinetic considerations.

discussed in the previous chapter, the electrochemical potential of a

metal electrode,

is given by the Nernst equation:

niF

lnai

(5.38)

where

is the standard electrode potential, or the potential difference

between the electrode and the solution, when the activity,

of the ions is

unity,

the Faraday's constant,

R,,

the gas constant and

temperature.

The Nernst equation represents an equilibrium state. When the electro-

chemical potential is deviated from its equilibrium value by, for example,

applying an external electric field, either reduction (leading to deposition

of solid) or oxidation (dissolution of solid) reaction will take place on the

surface or metal electrode

till

a new equilibrium state is reached. The dif-

ference in potential is referred to as the over-potential or over-voltage.

careful control of over-potential is very important to avoid electrolysis of

solvent or deposition of impurity phase. In addition, the interactions of the

solute ion

Mm+

with the solvent, or with complex-forming ligands should

be considered. These interactions and other factors such as the ionic strength

of the solution must be carefully controlled. Besides thermodynamics, there

are many kinetic factors that influence the deposition of elemental films.

The rate

the electron transfer reaction, i.e. the oxidation-reduction kinet-

ics, influences the nature and morphology of the deposit. The nucleation

rate of crystals is a hnction of the over potential,'16 and also influences the

nature of the deposit. In the case of a diffkion-limited deposition, the rate

of mass transport of solute species to the electrode surface has great effect

on the rate of deposition that can be achieved. Electrolyte agitation can lessen

the difhsion layer thickness and favor rapid deposition, but maximum stable

growth is generally produced in solutions

relatively high solute activity,

high diffusion coefficient (low solution viscosity), and low growth velocity.'

Dissociation kinetics of solvated or complex ions influences the metal ion

Two-Dimensional Nanostructures:

Thin

Films

219

activity at the electrode surface and may limit the deposition rate that can

be achieved for desired deposition morphology.

Electrodeposition of alloys and compounds is far more complex."8-'20

In alloy and compound electrodeposition, the equilibrium potentials of the

alloy or compound components, the activities of the ions in solution, and

the stability of the resultant deposit all are important thermodynamic con-

siderations. For a compound

MnNm, the conditions necessary to obtain the

simultaneous deposition of two different kinds of ions at the cathode is:

7-p

(5.39)

where

and

are the respective equilibrium potentials of

and

are the over potentials required for electrodepositing

and

respectively. Considering the fact that the activities of the metals

and

in the compound or alloy are determined by their concentrations in the

solution and by the thermodynamic stability of the deposit, and often vary

during deposition, it is very difficult to control deposit stoichiometry. In

addition, control of ionic strength and solute concentration are important

for uniform deposition.

For the growth of films by electrodeposition, a few practical concerns

deserve a brief discussion here:

(1)

Though aqueous solutions are often used, nonaqueous solvent or

molten salts are also used. Electrolysis

of water is one of the main rea-

sons that nonaqueous solvent or molten salts are used.

(2) The electrical conductivity of the deposit must be high enough to

permit the deposition of successive layers. The electrodeposition is

therefore applied only for the growth of metal, semiconductors and

conductive polymer films.

(3)

Deposition can be accomplished at constant current or constant poten-

tial, or by other means, such as involving pulsed current or voltage.

(4)

Post treatment may be employed to improve the characteristics of the

deposits.

5.11.

Sol-Gel

Films

Sol-gel processing is widely used in the synthesis of inorganic and

organic-inorganic hybrid materials and capable of producing nanoparti-

cles, nanorods, thin films and monolith. In the previous chapters, we

discussed the fabrication of nanoparticles and nanorods using sol-gel pro-

cessing.

general introduction to sol-gel processing was presented in

Chapter

For more detailed information, the readers are recommended to

excellent books by Brinker and Scherer,12' Pierre,'

and Wright and

220

Nanostructures and Nanomaterials

S~mmerdijk.'~~ Sol-gel methods for oxide coatings were reviewed by

Francis.124 Here we will focus

our

discussion only on the hdamentals and

methods of the formation of sol-gel thin films. Prior to sol-gel transition or

gelation, sol is a highly diluted suspension of nanoclusters in a solvent, and

typically sol-gel films are made by coating sols onto substrates. Although

some two-dozen methods are available for applying liquid coatings to

substrates, the best choice depends on several factors including solution

viscosity, desired coating thickness and coating speed.'25 Most commonly

used methods for sol-gel film deposition are spin- and dip-coating~,'~~

though spray and ultrasonically pulverized spray were also u~ed.'~*

In dip-coating, a substrate is immersed in a solution and withdrawn at

a constant speed.

the substrate is withdrawn upward, a layer of solution

is entrained, and a combination of viscous drag and gravitational forces

determines the film thickness,

cl(pg)

TUO

(5.40)

where is the viscosity,

the withdrawal speed,

the density of the coating

sol, and

is a constant. Figure

5.27

illustrates various stages of the dip-coat-

ing process.I3' It should be noted that the equation does not account for the

evaporation of solvent and continuous condensation between nanoclusters

dispersed in the sol as illustrated in Fig.

5.28.'32

However, the relationship

between the thickness and the coating variables is the same and supported by

IMMERSION

DEPOSITION

DRAINAQE

EVAPORATION

CONTINUOUS

Fig.

5.27.

Stages

the dip-coating process: (a-e) batch and

continuous.

[L.E.

Scriven,

Better Ceramics Through Chemistry

III,

eds.

C.J.

Brinker,

D.E.

Clark, and

D.R.

Ulrich,

The Materials Research Society, Pittsburgh,

PA,

717,

1988.1

Two-Dimensional Nanostructures: Thin Films

Fig.

5.28.

Schematic showing the competing processes

evaporation

solvent and con-

tinuous condensation between nanoclusters dispersed in the

sol

during dip-coating.

[C.J.

Brinker and

A.J.

Hurd,

Phys.

111

(Fr.)

123

994).]

the experimental results,'33 but the proportionality constant is different. The

thickness of a dip-coated film is commonly in the range of

5&500nm,'34

though a thinner film of

-8

per coating was also re~0rted.I~~

Spin-coating is used routinely in microelectronics to deposit photore-

sists and specialty polymers and has been well ~tudied.'~~.'~~ A typical

spin coating consists of four stages: delivery of solution or sol onto the

substrate center, spin-up, spin-off and evaporation (overlaps with all

stages). After delivering the liquid to the substrate, centrifugal forces drive

the liquid across the substrate (spin-up). The excess liquid leaves the sub-

strate during spin off. When flow in the thin coating is no longer possible,

evaporation takes over to further reduce the film thickness. A uniform

film can be obtained when the viscosity of the liquid is not dependent on

shear rate (i.e. Newtonian) and the evaporation rate is independent of posi-

tion. The thickness of a spin-coated film,

is given by'38

(5.41)

where

is the mass of volatile solvent per unit volume,

pAo

its initial

volume,

the angular velocity,

-q

the liquid viscosity, and

the evapora-

tion rate, which is related to the mass transfer coefficient. It is clear from

the equation that the film thickness can be controlled by adjusting the

solution properties and the deposition conditions.

In the process of creating a sol-gel coating, the removal of solvent or dry-

ing of the coating proceeds simultaneously with continuous condensation

222

Nanostructures and Nanomaterials

and solidification of the gel network. The competing processes lead to cap-

illary pressure and stresses induced by constrained shrinkage, which result

in the collapse of the porous gel structure, and may also lead to the forma-

tion of cracks in the resultant films. The drying rate plays a very important

role in the development

stress and formation of cracks particularly in the

later stages and depends on the rate at which solvent or volatile components

difise to the free surface of the coating and the rate at which the vapor is

transported away in the gas.

Stress develops during drying of a solidified coating due to constrained

shrinkage. Solvent loss after solidification is a common source of stress in

solvent-cast polymer coatings and Croll defines such a stress as139,140:

(5.35)

where

E(a)

is a nonlinear elastic modulus anci

the Poisson's ratio of the

coating,

and

are the volume fractions of solvent at solidification and

residual after drying, respectively. The relationship shows that solvent con-

tent at solidification should be minimized to lower the stress in the coating.

In the formation of sol-gel coating, it is very important to limit the con-

densation reaction rate during the removal of solvent upon drying,

that

the volume fraction of solvent at solidification is kept small. To relieve

stresses, the material can relax internally by molecular motion or it can

deform. Internal relaxation slows as the material approaches an elastic

solid and deformation is restricted by adherence to the substrate. Since the

stress-free state shrinks during solidification and adherence to the substrate

confines shrinkage in the coating to the thickness direction, in-plane ten-

sile stresses result. Cracking is another form of stress relief. For sol-gel

coatings, the formation of cracks limits the coating thickness commonly

less than

micron. A critical coating thickness,

T,,

has been defined.14'p*42

bti,

Aa2

(5.36)

where

is the Young's modulus of the film,

is a dimenionless propor-

tionality constant, and

the energy required to form two new crack

surfaces. The concept of critical thickness is supported by experimental

reports. For example, a critical thickness

600

nm was reported in Ceria

sol-gel films, and cracks formed above this thi~kness.'~~

It should also been noted that sol-gel coatings are commonly porous

and amorphous. For many applications, subsequent heat treatment is

required to achieve full densification and convert amorphous to crys-

talline. Mismatch of thermal expansion coefficients of sol-gel coatings

and substrates is another important source

stress, and a residual stress

in sol-gel coatings can be as high as 350MPa.'44

Two-Dimensional Nanostructures: Thin Films

223

Organic-inorganic hybrids are a new type of materials, which are not

present in nature, and sol-gel is the obliged route to synthesize them.i221145

The organic and inorganic components can interpenetrate each other on

a nanometer scale. Depending

the interaction between organic and

inorganic components, hybrids are divided into two classes: (i) hybrids

consisting of organic molecules, oligomers or low molecular weight poly-

mers embedded in an inorganic matrix to which they are held by weak

hydrogen bond or van der Waals force and (ii) in those, the organic and

inorganic components are bonded to each other by strong covalent or par-

tially covalent chemical bonds. The organic component can significantly

modify the mechanical properties of the inorganic c~mponent.'~~ The

porosity can also be controlled as well as the hydrophilic and hydropho-

bic ba1an~e.I~~ Hybrids with new ~ptical'~~>'~~ or electrical'50 properties

can be tailored. Some hybrids can display new electrochemical reactions

as well as special chemical or biochemical rea~tivity.'~'''~~

Porosity is another important property of sol-gel film. Although for

many applications, heat-treatment at elevated temperatures is employed to

remove the porosity, the inherited porosity enables sol-gel film for many

applications such as matrix of catalyst, host of sensing organic or biocom-

ponents, electrode in solar cells. Porosity itself also renders other unique

physical properties such as low dielectric constant, low thermal conductiv-

ity, etc. Organic molecules such as surfactants and diblock polymers have

been used to form templates in the synthesis of ordered mesoporous mate-

rials, which will be another subject of discussion in the next chapter.

There are many other chemical solution deposition (CSD) methods.

Fundamentals discussed above are generally applicable to other CSD meth-

ods. For example, the competing processes during drying, the development

of stresses and the formation of cracks are similar to that in sol-gel films.

5.1

Summary

A variety of methods for thin film deposition has been summarized and

briefly discussed. Although all the methods can make films with thickness

less than 100nm, they do offer varied degree of control of thickness and

surface smoothness. Both MBE and ALD offer the most precise control of

deposition at the single atomic level, and the best quality of the grown film.

However, they suffer from the complicated deposition instrumentation and

slow growth rate. Self-assembly is another method offering a single atomic

level control; however, it is in general limited to the fabrication of organic

or inorganic-organic hybrid thin films.

224

Nanostructures and Nanomaterials

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Guozhong Cao. Nanostructures &amp; Nanomaterials: Synthesis, Properties &amp; Applications

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Guozhong Cao. Nanostructures & Nanomaterials: Synthesis, Properties & Applications