LANGMUIR–BLODGETT FILMS

Introduction

Langmuir–Blodgett (LB) films, one of the earliest examples of smart materials,
provide the opportunity to control the structure of thin films at the molecular
level. In this method, a single layer of molecules is first organized on a liquid
surface, usually water, before being transferred onto a solid support to form a
thin film whose thickness is a constituent molecule. If the process is repeated,
multilayered films can be prepared. The layer of molecules on a liquid surface is
termed a Langmuir monolayer, and after transfer it is called a Langmuir–Blodgett
film. The ability to control the thickness of an organic film at the molecular level
and to control the placement of molecules in multilayer assemblies forms the basis
of many “smart” applications of LB films. Applications include electronics, optics,
microlithography, and chemical sensors, as well as biosensors and biochemical
probes. Many comprehensive review articles and books exist on the subject of LB
films (1–4).

History

The technique is named after Irving Langmuir and Katharine Blodgett, re-
searchers at the General Electric Co. in the first half of the twentieth century.
Langmuir, who was awarded the Nobel prize for chemistry in 1932 for his studies
of surface chemistry, used floating monolayers to learn about the nature of inter-
molecular forces. In the course of his studies, Langmuir developed several new
techniques that are still used today in studying monomolecular films. Together
with Langmuir, Katharine Blodgett refined the method of transferring the floating
monolayer onto solid supports. In addition to treating many practical details of LB

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methods, publications and patents by Langmuir and Blodgett cover a number of
potential applications, including controlling the reflectivity of glass, a step gauge
for optically measuring the thickness of thin films, submicrometer mechanical
filters, and biosensing.

Although the methods are commonly named after Langmuir and Blodgett,

numerous observations and experiments on floating organic films predate them
(1,5,6). The practice of pouring oil on water to calm choppy seas has been known
for centuries. Benjamin Franklin, however, who was aware of the effect, is credited
with the earliest scientific account of spreading oil on water, which he reported
in a paper presented to the Royal Society and published in 1774. An extract from
that paper reads as follows (1):

At length being at Clapham where there is, on the common, a large
pond, which I observed to be one day very rough with the wind, I fetched
out a cruet of oil, and dropped a little of it on the water. I saw it spread
itself with surprising swiftness upon the surface

. . .

In these experiments, one circumstance struck me with particular sur-
prise. This was the sudden, wide and forcible spreading of a drop of oil
on the face of water

. . . it spreads instantly, many feet around, becom-

ing so thin as to produce the prismatic colors, for a considerable space,
and beyond them so much thinner as to be invisible, except in its effect
of smoothing the waves at a much greater distance. It seems as if a
mutual repulsion between its particles took place as soon as it touched
the water

. . .

In the late 1800s, Lord Rayleigh became interested in surface layers on water.

When Langmuir determined that certain films on water indeed had the thickness
of only one molecule, it confirmed an earlier suggestion by Rayleigh. After pub-
lishing on the way overlayers on water influenced surface tension, he received a
communication from Agnes Pockels who had quantified that the surface tension
of water depended on the surface coverage of “contaminant.” Building on Pock-
els’s method of measurement, Rayleigh went on to estimate the size of an olive oil
“molecule.”

Modern studies of floating monolayers and LB films fall largely into two ar-

eas. The first area includes detailed fundamental studies of the physical nature
and structure of Langmuir monolayers and LB films. The second involves appli-
cations that take advantage of thin films that have controlled thickness and com-
position. Much of the current work on applications derives inspiration from the
pioneering work of Hans Kuhn in the 1960s (7). Kuhn used LB methods to control
the position and orientation of functional molecules within complex assemblies, an
elegant early example of what is currently being called “supramolecular assembly.”
To acknowledge Kuhn’s contributions, some authors now use the term Langmuir–
Blodgett–Kuhn (LBK) films for transferred films of functional molecules.

Equipment

References 1 and 2 both discuss common equipment. Essentially, all LB film work
begins with the Langmuir–Blodgett trough (alternatively called Langmuir trough

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LANGMUIR–BLODGETT FILMS

649

Dipping well

Water subphase

LB trough

Barrier

Langmuir monolayer

Wilhelmy
balance

Wilhelmy
plate

Fig. 1.

Schematic of a Langmuir trough and its components.

or Langmuir film balance), which, save for modern materials and electronics, is
similar in design and function to the early apparatuses of Pockels and of Langmuir.
The trough that contains the subphase is normally about 1 cm deep. The surface
area can range from about 100 cm

2

for smaller troughs to more than 1 m

2

for larger

systems, designed primarily for transferring large amounts of material. Modern
troughs are generally inert and hydrophobic, for example, made from or coated
with Teflon or a similar polymer. Moveable barriers that can skim the surface of the
subphase permit controlling the surface area available to the floating monolayer.
The simplest approach is a bar that moves across the top of a rectangular trough,
as shown in Figure 1. For transfer onto solid supports, troughs contain a dipping
well—a section that is deep enough to allow immersing the substrate into the
subphase—and a sample holder that can vertically displace the substrate. A photo
of a complete LB film system is shown in Figure 2.

The rectangular trough that has a moveable barrier is the most common

design, but other systems are also widely used. Some variations include circular
troughs, systems that use a continuous flexible belt for compression, moving-wall
troughs, and troughs designed for continuous deposition or alternate layer depo-
sition. All systems are equipped with a mechanism for monitoring the state of the
monolayer on the water surface during the course of experiments, usually, by mea-
suring the surface pressure using either a Wilhelmy plate or Langmuir balance.

Film-Forming Molecules and Polymers

The classic “LB active” molecules are fatty acids such as stearic acid (1). The
long-chain examples, n-octadecanoic (stearic), n-eicosanoic (arachidic), and n-
docosanoic (behenic) acids constitute the best-studied class of Langmuir mono-
layers and LB films; work dates back to the original papers by Langmuir and

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LANGMUIR–BLODGETT FILMS

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Fig. 2.

Photograph of a modern Langmuir–Blodgett system. Courtesy of KSV Instru-

ments.

Blodgett. These molecules can be used to demonstrate most of the principles in-
volved in floating monolayers. However, Langmuir monolayers can be prepared
from many variations of straight-chain molecules, polymerizable molecules, poly-
mers, and conductive materials (1,2,4,8).

Fatty Acids.

Fatty acids possess clearly defined hydrophilic (carboxylic

acid) head groups and hydrophobic (linear alkane) tails. The amphiphilic nature
of these molecules leads to a preferred orientation at the air–water interface,
where the hydrophobic tails extend from the water surface. Other film-forming
molecules normally possess similar hydrophobic and hydrophilic parts. For exam-
ple, replacing the carboxylic acid by other functional head groups leads to alky-
lamines, amides, nitriles, esters, alcohols, and phosphonic acids, all of which form
Langmuir monolayers. In addition to the head groups, the hydrophobic tail can be
modified. Double bonds, triple bonds, and aromatic groups can normally be incor-
porated into the hydrophobic tail, and the film-forming properties are retained. In
addition to hydrocarbon amphiphiles, molecules that have fluorocarbon tails also
form Langmuir monolayers.

Phospholipids and Related Biomolecules.

The structural features of

phospholipids that lead to naturally occurring bilayer structures, such as those
found in biological membranes, also make them good film-forming amphiphiles.

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651

The double hydrocarbon chains can be saturated, or they can contain double
bonds. The state of the monolayer, whether it is expanded or condensed, at a
given temperature depends on the length and nature of the hydrocarbon chains.
The polar head groups vary, but all contain phosphate groups that are anionic
at neutral pH. Phosphatidic acids, phosphatidylethanolamines (cephalins), phos-
phatidylcholines (lecithins), and phosphatidylserines are some of the many types
of phospholipids studied as Langmuir monolayers. Other components of biologi-
cal membranes also form Langmuir monolayers. Derivatives of sterols (such as
cholesterol), naturally occurring pigments (porphyrins and carotenes) (9), cellu-
lose (10), vitamin B

12

(11), and proteins have been incorporated into monolayers

as single-component films or as mixtures. Commonly, fatty acid molecules are em-
ployed in mixtures to stabilize monolayers of compounds that may not form stable
monolayers alone.

Polymers.

Polymeric monolayers are formed either by spreading pre-

formed polymers or by polymerizing reactive monomers on the water surface.
A large variety of preformed polymers has been studied, including polyacrylates
and polymethacrylates, poly(vinyl butyral), poly(vinyl methyl ether), poly(vinyl
acetate), poly(vinyl fluoride), poly(vinylidene fluoride), (12), silicone copolymers,
maleic anhydride copolymers, and polypeptides (1). Monomers that have been suc-
cessfully polymerized at the air–water interface include derivatives of aniline (13),
vinyl alcohol (14), and styrene (15). In addition to the interesting chemistry and
the two-dimensional structural control, polymer LB films are much more robust
than films formed from straight-chain amphiphiles.

Langmuir Monolayer Formation

To form a Langmuir monolayer, the molecule of interest is dissolved in a volatile
organic solvent (frequently chloroform or hexane) that will not react with or dis-
solve in the subphase (1,2,4). A quantity of this solution is placed on the surface
of the subphase, and as the solvent evaporates, the surfactant molecules spread
and alter the surface pressure of the water surface. A barrier designed to measure
this surface pressure (

), relative to that of the pure subphase, is the principle

behind the Langmuir balance. Alternatively, the surface pressure is measured as
the difference between the surface tension (

γ ) of the monolayer and that of the

pure subphase (

γ

0

),

 = γ

0

− γ . A common method for measuring surface tension

involves using a Wilhelmy plate, usually a piece of platinum or paper that is wet-
ted by the subphase, suspended from a balance. As the monolayer is compressed
by using the moveable barrier to reduce the surface area, the surface pressure
increases. A plot of the surface pressure versus surface area is called a pressure
versus area isotherm (or

–A isotherm). Isotherms are normally plotted in terms

of area/molecule, and the units of surface pressure are mN/m.

A representative pressure versus area isotherm is shown in Figure 3. As the

pressure increases, the two-dimensional monolayer goes through different phases
that have some analogy to three-dimensional gas, liquid, and solid states. If the
area/molecule is sufficiently high, the floating film is in a two-dimensional gas
phase where the surfactant molecules are not interacting. As the monolayer is
compressed, the pressure rises, signaling a change in phase. The molecules begin

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Surface pressure, mN/m

−10

0

10

20

30

40

50

60

70

Mean molecular area (MMA), nm

2

/molecule

0.1

0.15

0.20

0.25

0.30

0.35

0.40

0.45

0.50

Fig. 3.

A representative pressure vs. area isotherm. The cartoons represent idealized gas-

analogous, liquid-expanded, and liquid-condensed phases of the monolayer. Two phases
often coexist at a given temperature and surface pressure.

to interact, forming a two-dimensional phase, called a liquid-expanded (LE) phase,
which is analogous to a three-dimensional liquid. Upon further compression, the
pressure begins to rise more steeply, as the liquid-expanded phase gives way to a
condensed phase (or a series of condensed phases). This transition, analogous to
a liquid–solid transition in three dimensions, does not normally result in a true
two-dimensional solid. Rather, condensed phases tend to be liquid crystalline in
nature and are called liquid-condensed (LC) phases. Condensed phases have low
compressibility, so the slope of the

–A curve becomes quite steep. As the pressure

is increased further, the monolayer eventually collapses under the pressure and
either slides over upon itself or folds under into the subphase. When two phases
such as gas and LE or LE and LC phases are in equilibrium, the surface pressure
plateaus across a range of mean molecular areas. An image of two phases in
equilibrium is shown in Figure 4.

The mean molecular area of an amphiphile is determined foremost by the

size of the hydrophilic head group and its interactions with counterions in the
subphase and with other head groups within the plane. Once these interactions
are at an energetic minimum, the alkyl chains pack to maximize both van der
Waals interactions and alkane density. By extrapolating the steepest part of the
curve before the collapse at zero pressure, a minimum cross-sectional area per
molecule can be found. This approach was one of the original methods used in
attempts to measure the size of a molecule.

Isotherms differ from one molecule to another, and they also differ as a

function of temperature. Monolayers, like three-dimensional substances, undergo
temperature-dependent phase transitions. At any given temperature, the mono-
layer may pass through several phases during compression, or only a couple. The
various states of monolayers can be detected in a number of ways, including surface
potential measurements, optical spectroscopy, and optical imaging. One imaging

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653

Fig. 4.

Brewster angle micrograph of stearylamine on a water/glycerol subphase, showing

contrast between liquid-expanded (darker regions) and liquid-compressed (lighter regions)
phases. Photo courtesy of C. Mingotaud.

method is fluorescence microscopy, which involves adding to the subphase a fluo-
rescent probe molecule that will be either included or excluded from the surface
layer, depending on the state of the monolayer. Imaging the surface yields the
structure of domains of phases in equilibrium. Brewster angle microscopy (BAM)
is an imaging technique that does not depend on a probe molecule. Laser light
is reflected from the subphase surface and is detected by a CCD camera. The
light is reflected at the Brewster angle of water, the minimum angle where light
is totally internally reflected. As the monolayer is compressed, its refractive in-
dex changes, leading to reflection of the laser light. Different phases will have
different optical properties, leading to clear images of the state of the monolayer
(Fig. 4). Grazing incidence x-ray diffraction (gixd) has been used to identify the
structures of condensed phases. Highly collimated x-rays from a synchrotron
source sample the monolayer at a low angle of incidence. The in-plane packing
and tilt angles of the alkyl tails of many fatty acid phases have been determined
in this way.

Langmuir–Blodgett Films

According to one reference (16), the “resulting degree of control over film thickness
and molecular architecture (using LB deposition) is unsurpassed by any other
deposition technique.” The term Langmuir–Blodgett film traditionally refers to
Langmuir monolayers that have been vertically transferred from the water sub-
phase onto a solid support such as glass, silicon, mica, or quartz. Vertical depo-
sition is the most common method of LB transfer; however, horizontal lifting of
Langmuir monolayers onto solid supports is also possible through the Langmuir–
Blodgett–Schaefer method.

Monolayer or Multilayer Transfer.

Highly hydrophilic or highly hy-

drophobic substrates are desirable for preparing LB films. Rigid Langmuir mono-
layers are difficult to transfer, so some flexibility in the monolayer is necessary.
When hydrophobic, the substrate originates above the water surface. After the
monolayer has been spread and compressed to the desired transfer pressure, the
substrate is dipped vertically through the monolayer; transfer is by hydrophobic

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LANGMUIR–BLODGETT FILMS

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Barriers

(a)

(b)

Fig. 5.

Schematic of Langmuir–Blodgett transfers. The gray circles represent the hy-

drophilic head groups, and the thin black lines represent the alkyl chains. The molecules
have been spread on an aqueous subphase. (a) A hydrophilic substrate is being withdrawn
from the dipping well and is attracting the hydrophilic head groups that drive the film
transfer. (b) A hydrophobic substrate is being pushed down through the monolayer. The
alkyl chains are transferred, and the monolayer is pulled off the surface by hydrophobic
interactions.

interactions between the alkyl chains and the surface. A hydrophilic substrate is
submerged in the aqueous subphase before spreading and compressing the mono-
layer film. After the monolayer is stabilized, the substrate is withdrawn from the
subphase, and the hydrophilic interactions drive the transfer (Fig. 5).

Three common film architectures can result from vertical deposition (Fig. 6).

In X-type LB films, a Langmuir monolayer is consistently transferred onto a hy-
drophobic substrate so that it maintains head–tail interactions. In Z-type LB films,
the monolayer is transferred onto a hydrophilic substrate that also forms head–
tail interactions. X- and Z-type films are not common but can be prepared on a
specially designed trough. However, some amphiphiles prefer this type of interac-
tion, and upon regular dipping these structures form spontaneously. Y-type mul-
tilayers are most common, can be prepared on either hydrophilic or hydrophobic
substrates, and are typically the most stable due to the strength of the head–head
and tail–tail interactions. Even when films of some amphiphiles have been de-
liberately transferred by an X or Z method, the spacing between the hydrophilic

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655

Z-type transfer

X-type transfer

Y-type transfer

on hydrophilic

substrate

Alternating transfer

on hydrophobic

substrate

Fig. 6.

Representations of X-, Y-, and Z-type LB films. Z-type LB films have head–tail

interactions between layers and are transferred onto hydrophilic substrates. X-type LB
films also have head–tail interactions but are transferred onto hydrophobic substrates. Y-
type films can be transferred onto either hydrophilic or hydrophobic substrates but involve
head–head and tail–tail interactions between layers. Alternating films can be transferred
by using specially designed “double troughs.”

head groups shows packing similar to Y-type films, implying that some molecular
rearrangement occurs during or shortly after deposition. Ultimately, the molec-
ular structure of the amphiphile determines the preferred method of dipping;
multilayers can be built by repeating the vertical deposition procedure.

The quality of the transferred film is first indicated by the transfer ratio (

τ),

which is a measure of the change in the area of the floating monolayer versus the
area of the substrate coated by the transferred monolayer. A transfer ratio of unity
indicates that the monolayer transferred has the same area per molecule as it
occupied on the water surface. Assuming that the monolayer on the water surface
was stable and the monolayer was not reorganizing significantly during transfer,

τ

should be between 0.95 and 1.05 to be considered a successful transfer. A consistent
deviation from unity could imply a change in organization upon transfer, but if
the transfer ratio is irregular or significantly different from unity, the transferred
film is most likely of poor quality.

Characterization of Transferred Films.

Many analytical techniques are

used to study transferred films. The film characteristics typically of interest are
thickness, interlayer spacing, molecular orientation and packing, film coverage,
surface topology, chemical composition, and optical and magnetic properties. The
techniques used to study these parameters are well described in the literature
(1,2,4).

Film Thickness and Layered Structure.

X-ray diffraction is a reliable tech-

nique for probing interlayer spacing, and film thickness can be inferred from in-
terlayer spacing. This technique is very sensitive to long-range periodicity. Many
orders of 00l Bragg peaks, indicative of a well-defined layered architecture, are
often observed for LB films. Neutron diffraction is similar to x-ray diffraction in

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LANGMUIR–BLODGETT FILMS

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that both result in interference effects that can lead to an understanding of film
organization. Film thickness can be determined by ellipsometry, which compares
the amplitude and phase of s- and p-polarized light reflected from a film. Differ-
ences in amplitude and phase of the reflected light between the coated surface
and the untouched substrate can be related to the film thickness.

Molecular Orientation and Molecular Packing.

Grazing incidence x-ray

diffraction was mentioned earlier as a method for determining the arrangement
of molecules within a floating Langmuir monolayer. The same methods can be ap-
plied to transferred films. Transmission electron diffraction has also been observed
in LB films and used to determine molecular packing, although the organic films
are mostly unstable to the electron beam. Atomic force microscopy (qv) (afm)
at molecular scale resolution has also been used to observe the arrangement of
molecules in transferred monolayer and multilayer films (17).

Chemical Composition.

Standard spectroscopic methods, including ftir,

Raman scattering, and uv–visible absorption can be used to study the chemi-
cal makeup of films vibrational spectroscopy. These methods are often sensitive
enough to be applied even to monolayer films. In the Raman scattering, the signal
can be enhanced by preparing the LB film on a noble metal substrate, such as Au or
Ag, or by interacting the film with surface plasmons. For each of these techniques,
polarization studies can be used to determine the orientation of chromophores
within the transferred films. Solid-state nmr has also been applied to LB films.
Studies have included

1

H,

13

C, and

31

P magic-angle spinning experiments (18).

NMR is complicated by the small amount of materials available, so these stud-
ies are not routine. The elemental composition, and possibly atomic proportions,
within a film can be determined by x-ray photoelectron spectroscopy (xps), which
measures the energy of an expelled electron as the surface is bombarded by a
monochromatic x-radiation source. Binding energies are unique to each element
in the film, as well as to that element’s chemical environment and oxidation state.
The intensities of the xps peaks can be used to determine the relative ratios of
the elements present and can indicate the type of crystalline lattice formed. How-
ever, the intensities of the xps peaks are sensitive to many parameters, such as
the element’s electron escape depth, which can complicate determination of the
elemental ratios (19).

Imaging.

Imaging of the film is sometimes possible by using a trans-

mission electron microscope (tem). Films are either transferred directly onto a
tem-appropriate surface, such as a Cu grid, or are removed from the original
substrate by, for example, submerging the film in highly diluted hydrofluoric acid
and lifting it onto a grid surface for imaging by tem. Scanning probe microscopy,
including atomic force microscopy (afm) and scanning tunneling microscopy
(stm), can be used to obtain direct nanometer-scale images of surfaces without
significant film degradation, which plagues some other imaging techniques (17)
(see M

ICROSCOPY

).

Applications

A wide range of potential applications exists for materials formed by the LB tech-
nique. A few examples include coatings, chemical or gas sensors, conducting LB
films, NLO and SHG materials, and magnetic films.

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657

LB coatings have been used as photoresists. In the area of ultrathin (single

molecule) photoresists, spin-coated polymers tend to lack the consistency in struc-
ture and thickness necessary for these applications. However, preformed polymer
LB films or polymerized monomeric LB films are extremely uniform, and can
be used in these applications. Photo-cross-linking of poly(p-phenylene) and pho-
topolymerization of

ω-tricosenoic acid and methacrylic acid LB films have been

investigated (20,21). Etching resulted in patterns that had resolutions of ca 60
nm. (see L

ITHOGRAPHIC

R

ESISTS

)

Chemical or gas sensors have been prepared by the LB technique. Sensing LB

films often comprise chromionophores that have two functionally different groups
on the same molecule. The ionophore component senses the presence of specific
ions, and the chromophore component translates information from the ionophore
into an optical signal. Gas sensors can translate oxidation or reduction by an
atmospheric gas into an optical signal. Porphyrin, porphine, and phthalocyanine
LB films have been studied for gas sensing of NH

3

, I

2

, O

2

, CO, and NO

x

(22–

25). However, chemical sensing does not have to trigger an optical change. LB
films of redox materials such as polythiophene and polypyrrole, ferrocene, TTF,
and metallo porphyrins, as well as fullerenes and Schiff bases, have been used to
modify electrodes for electrochemical sensing of a guest molecule or ion (26).

Conductivity has been studied in organic LB films based on porphyrins, ph-

thalocyanines, and charge transfer complexes of derivatives of the electron donor,
tetrathiafulvalene (TTF), and electron acceptor, TCNQ (2). Partially filled con-
duction bands can be achieved by forming films of nonstoichiometric mixtures of
the donors and acceptors or by using donors and acceptors that have incomplete
electron transfer, and electrical conduction is possible. Because LB films are not
completely crystalline in the plane of the film, electrical conductivities are aver-
ages across crystalline domains (2). Films of TTF derivatives have been success-
fully prepared whose in-plane dc conductivities ranged from 10

− 3

to 10

1

S cm

− 1

(27). LB films of conducting polymers, such as poly- and oligo(arylarenes)
have also been fabricated (see E

LECTRICALLY

A

CTIVE

P

OLYMERS

). LB films have

been used as active and inactive layers in heterostructured electronic devices,
including diodes and field effect transistors (28–30). Fullerene LB films have been
investigated for their photovoltaic and charge transfer properties (31).

A material that can interact with electromagnetic radiation to generate new

waves that have a different phase, frequency, or other propagative character-
istics is called a nonlinear optical (NLO) material. Some NLO materials react
in second-order processes that lead, for example, to frequency-doubling of laser
radiation and electro-optic modulation of light (see E

LECTROOPTICAL

A

PPLICATIONS

).

Materials with such NLO behavior are called second-harmonic-generation (SHG)
materials. To be suitable for NLO applications, the material should be typically
highly conjugated and have an asymmetrical charge distribution along the pri-
mary molecular axis. The solid-state structure must also be noncentrosymmetric.
The LB technique can enable the control of molecular architecture to build noncen-
trosymmetric crystalline environments. Nonlinear optical materials are prepared
by either LB transfer of asymmetrical molecules, noncentrosymmetric transfers,
such as X- or Z-type transfers, or by transfer of alternating films of NLO active
and passive amphiphiles (1,2,4). Additionally, diluting an NLO active molecule in
a fatty acid matrix can lead to noncentrosymmetric aggregation that can enhance

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LANGMUIR–BLODGETT FILMS

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NLO or SHG effects. In a related application, LB films have been used as thin film
optical waveguides (2).

Some

derivatives

of

poly(p-phenylenevinylene)

(qv)

coupled

with

poly(pyridine) derivatives behave as a light-emitting layer and an electron-
transport layer, respectively, (see L

IGHT

E

MITTING

D

IODES

) (32). Dye molecules

are pervasive in the LB literature. Azobenzenes have been studied in LB films
along with their trans–cis isomerization properties (33). Anthracene, pyrene,
and numerous heterocyclic long-chain derivatives have been studied in LB films.
Kuhn studied cyanine dyes as one of his original examples of energy transfer
molecules incorporated into multilayer films, and hemicyanine dyes are studied
for photoelectric conversion (34). Porphyrins and phthalocyanines, which are
important in biological energy transport systems, have been extensively studied
in LB films for their in-plane electrical conductivity and interplane charge
transfer properties (35,36). Quinolinium salts, which are electron acceptors, can
be bridged to electron donors (such as naphthalene), and it has been observed
that they operate as multifunctional dyes that exhibit SHG and photoelectric
conversion behavior (37). Electroluminescent devices have also been prepared
using LB films of quinolinium salts (38).

By incorporating functional inorganic layers into the polar part of LB bi-

layers, phenomena typical of an inorganic solid state have been engineered into
these normally organic assemblies (39). For example, LB films can be formed from
organophosphonic acids, and if metal ions are bound to the head groups, a contin-
uous inorganic lattice is formed in the polar part of the film. The inorganic lattice
energy greatly stabilizes the films. Although LB films are often unstable to heat
and solvents, mixed organic/inorganic metal phosphonate films can be thermally
cycled to more than 100

◦

C and are stable to both organic and aqueous conditions

(40,41). When the inorganic ions are Mn

2

+

or Cu

2

+

, these films—it has also been

shown—order magnetically. Another example of a magnetic LB film is a mixed
organic–inorganic film that incorporates a Prussian blue lattice.

Some LB films exhibit a temperature-dependent pyroelectric coefficient that

makes them candidates for thermal imaging applications (1). Both X- and Z-type
films and alternating Y-type films (Fig. 6) can be built with permanent
polarization, whose magnitude is often temperature-dependent. Their pyroelectric
coefficient p is small relative to standard semiconducting and ceramic materials,
but so is their permittivity

ε; this makes their ratio p/ε, the figure of merit, com-

parable to that as for currently employed inorganic materials. The advantage of
LB methods is that they can be used to prepare very thin, very uniform films.

BIBLIOGRAPHY

1. G. G. Roberts, ed., Langmuir–Blodgett Films, Plenum Press, New York, 1990.
2. M. C. Petty, Langmuir–Blodgett Films: An Introduction, Cambridge University Press,

New York, 1996.

3. G. J. Gaines, Insoluble Monolayers at Liquid–Gas Interfaces, Wiley-Interscience, New

York, 1966.

4. A. Ulman, An Introduction to Ultrathin Organic Films: From Langmuir–Blodgett to

Self-Assembly, Academic Press, Boston, 1991.

5. C. H. Giles, S. D. Forrester, and G. G. Roberts, Langmuir–Blodgett Films, Plenum Press,

New York, 1990, Chapt. “1”.

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659

6. C. H. Giles, Chem. Ind. (London) 45, 1616 (1969).
7. H. Kuhn, D. M¨obius, and H. B ¨

ucher, in A. Weissberger and B. W. Rossiter, eds.,

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C

HRISTINE

M. L

EE

Unilever Research US
D

ANIEL

R. T

ALHAM

University of Florida

LASER LIGHT SCATTERING.

See Volume 3.

LDPE.

See E

THYLENE

P

OLYMERS

, LDPE.

660

LITHOGRAPHIC RESISTS

Vol. 6

LIGHT-EMITTING DIODES.

See Volume 3.

LIGNIN.

See Volume 3.

LINEAR LOW DENSITY POLYETHYLENE.

See E

THYLENE

P

OLYMERS

, LLDPE.

LIQUID CRYSTALLINE POLYMERS, MAIN-CHAIN.

See Volume 3.

LIQUID CRYSTALLINE THERMOSETS.

See Volume 3.

LITERATURE OF POLYMERS.

See I

NFORMATION

R

ETRIEVAL

.