Changes in optical properties of TDO BAM BCC Langmuir films during a phase transition
ABSTRACT The isotherms of water molecule adsorption and the spectra of absorption, diffusion reflection, and polarization of reflected
light for hyperfine Langmuir films that were fabricated based on liquid crystals are investigated. Singularities in the reflection
spectra at the temperature of the structural phase transition (∼70°C) are revealed. Some reasonable assumptions on the nature
of the phase transition are made.
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ABSTRACT: Electric and electro-optical properties of a smectic liquid crystal (LC) compound formed by rod-like molecules have been investigated in the thin film sandwich geometry typically used in solid-state technology. Thin (~10–30 nm) Langmuir–Blodgett (LB) films of the compound (tetradecyl homologue (n=14) of the series p-oxybenzylidene-p′-amino-2-methylbutyl(α-cyano)cinnamate) were transferred onto glasses covered with a transparent electrode and supplied with a top electrode evaporated in vacuum. In contrast to the LC bulk, thin films of the compound manifest only a solid crystalline phase throughout the whole temperature range (20–110 °C) investigated. Ferroelectric properties of the films (covering linear electro-optic effect, pyroeffect and polarization switching) have been observed in the whole temperature range studied.Thin Solid Films 01/2008; 516(24):8905-8908. · 1.87 Impact Factor
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ISSN 0027?1349, Moscow University Physics Bulletin, 2011, Vol. 66, No. 1, pp. 50–53. © Allerton Press, Inc., 2011.
Original Russian Text © V.B. Zaitsev, N.L. Levshin, S.V. Khlybov, S.G. Yudin, 2011, published in Vestnik Moskovskogo Universiteta. Fizika, 2011, No. 1, pp. 49–52.
Methods for obtaining Langmuir films from mate?
rials that are capable of existing in a mesophase liquid
crystal phase have been developed in the last several
years. The problem concerning variations in the prop?
erties of these substances in the process of obtaining
hyperfine layers based on the Langmuir technique
remains open. Thus, such films need a thorough anal?
ysis. Several papers devoted to the investigation of
Langmuir–Blodgett (LB) films fabricated based on
the materials that form smectic liquid crystals have
been published [1–3]. The limited body of the experi?
mental data does not allow making any conclusion on
how the properties of a liquid crystal mesophase
change in the process of fabricating hyperfine films of
these materials deposited on a substrate. The study of
their optical properties at different temperatures
within the range including the phase transition tem?
perature can give rather valuable information. In the
present study, we investigated the temperature depen?
dences of the spectra of absorption and diffusion
reflection of Langmuir films made of para?tetradecy?
namate (TDO BAM BCC), which is capable of exist?
ing as a liquid crystal in a bulk phase. In addition, the
method of adsorption isotherms, which is highly suit?
able for determining structural transformations of fine
films, was applied [4, 5].
The specimens under analysis were films that were
obtained using the Langmuir–Blodgett method based
on the Schiff compound of TDO BAM BCC. In bulk
phases, this compound is a ferroelectric liquid crystal
(smectic C) in the temperature range of 54–70°C.
Spontaneous polarization is Ps ≈ 10–9 C cm–2. The
chemical formula of TDO BAM BCC is C14H29O–
(CH)3C2H5. The asterisk marks the carbon atom that
provides the chiral structure of the substance. A dipole
momentum inclined with respect to the director is cre?
ated by the CN group.
The films were produced using a TDO BAM BCC
solution in chloroform with the concentration of (1–3)
10–2 wt %. The studied films with a thickness of
30 monolayers were transferred from the aqueous sur?
face to quartz glass substrates using the Langmuir–
Scheffer method (horizontal lifting). The thickness of
the films on the substrate was about 20 nm. A method
proposed in previous studies consisting in the measure?
ment of the isotherms of molecule adsorption from a
gaseous phase at different temperatures was used for
recording phase transitions in Langmuir films .
The absorption spectra were investigated using a
UV?3600 double?beam spectrophotometer (Shi?
mazdu) with an operating wavelength range of 185–
3300 nm and a ±0.2 nm accuracy of setting the wave?
length for the visible range for UV and visible ranges.
The spectra of diffusion reflection and polarization
of reflected light were studied using an LS?55 spec?
trometer (Perkin Elmer). The spectrometer was oper?
ated in the spectral range of 200–900 nm with a spec?
tral slit width from 2.5 to 20 nm.
Polarization filters in the path of the incident and
recorded light made it possible to investigate the spec?
tra in polarized light.
To study structural transformations in an LB film,
the spectral studies were carried out at different tem?
peratures in the range of 17°C–110°C. For this, a spe?
Changes in Optical Properties of TDO BAM BCC Langmuir Films
during a Phase Transition
V. B. Zaitseva, N. L. Levshina, S. V. Khlybova, and S. G. Yudinb
a Faculty of Physics, Moscow State University, Moscow, 119991 Russia
b Institute of Crystallography, Russian Academy of Sciences, Moscow, 119333 Russia
e?mail: email@example.com, firstname.lastname@example.org
Received September 28, 2010; in final form, October 20, 2010
Abstract—The isotherms of water molecule adsorption and the spectra of absorption, diffusion reflection,
and polarization of reflected light for hyperfine Langmuir films that were fabricated based on liquid crystals
are investigated. Singularities in the reflection spectra at the temperature of the structural phase transition
(~70°C) are revealed. Some reasonable assumptions on the nature of the phase transition are made.
Keywords: Langmuir–Blodgett films, smectic liquid crystals, adsorption
MOSCOW UNIVERSITY PHYSICS BULLETIN Vol. 66 No. 1 2011
CHANGES IN OPTICAL PROPERTIES OF TDO BAM BCC LANGMUIR FILMS51
cial thermal attachment was designed. The heating
element (an oven) had a special window for the radia?
tion to pass through. On the opposite side of the heater
just opposite the window a thermosensor (a thermo?
couple) was place symmetrically with respect to the
LB film. The accuracy of the temperature specifica?
tion in the experiment was ±1°C.
RESULTS AND DISCUSSION
The behavior of a TDO BAM BCC monolayer on
an aqueous surface was first studied: the dependence
of the surface pressure on the area per one molecule (a
π?A?isotherm) was measured at temperatures of 21–
28.5°C. The averaged monolayer thickness (based on
the area per one molecule calculated using compres?
sion curves) was from 2.97 nm to 2.74 nm at tempera?
ture variations from 21 to 28.5°C. After the films were
transferred to the quartz substrate, the thickness of one
mono layer was 0.6–0.7 nm (based on the data of
capacitive measurements). This fact shows that the
molecules of the Langmuir film transferred to the sub?
strate are inclined to the substrate plane (at angles
from 11° to 15°). Such positioning of the molecules is
similar to that in the layers of smectic C. According to
the data of , the film was in a polar phase in the tem?
perature range of 20–110°C. No phase transitions
were recorded using electrophysical methods.
The study of the isotherms of water molecule
absorption at different temperatures on the film with a
thickness of ten monolayers showed that the absorption
capacity of the specimen first decreased in the temper?
ature range of 20–65°C, then sharply increased at T ~
70°C, and again decreased at temperatures above 85°C
(Fig. 1). An initial decrease in adsorption activity with
an increase in temperature is typical for any solid sur?
faces. An increase in the number of adsorbed mole?
cules Na at temperatures of 70–90°C points to a struc?
tural phase transition that takes place in this tempera?
ture range [4, 5].
In the UV and visible ranges, the absorption spec?
trum of the films deposited on the substrate has three
characteristic bands with a maxima in the vicinity of
227 nm, 291nm, and 422 nm (see the inset in Fig. 2).
When the temperature increases in the range from
20°C to 100°C, the 422 nm band does not undergo
noticeable changes, while the intensity of the 227 nm
and 291 nm bands smoothly decreases (see Fig. 2).
Singularities in the temperature range of possible
phase transitions in the films are absent in the corre?
sponding dependences. The energy of the light quanta
203040 506070 8090
Fig. 1. The number of adsorbed molecules Na as a function
of temperature for a ten monolayer?thick film.
300 400λ, nm
5060 70 8090
Fig. 2. Temperature dependences of the intensity of TDO BAM BCC absorption bands with the maximum at 227 nm (1) and
291 nm (2). The inset shows the TDO BAM BCC absorption spectrum.
MOSCOW UNIVERSITY PHYSICS BULLETIN Vol. 66 No. 1 2011
ZAITSEV et al.
in the absorption bands corresponds to electron exci?
tations of TDO BAM BCC molecules. Since the TDO
BAM BCC molecules are bound by weak Van der
Waals forces, phase transitions in the film are struc?
tural and they do not affect the energy of electron tran?
Next, we obtained the spectra of the diffusion
reflection of light at the surfaces of the analyzed films
at different temperatures. The diffusion reflection was
chosen to minimize the effect of the radiation
reflected at the polished surface of the quartz sub?
strate. The spectra of the vertically polarized (perpen?
dicular to the incidence plane) and the horizontally
polarized (in the incidence plane) components of dif?
fusively reflected radiation during the incidence of
natural light onto the specimen were measured sepa?
rately. The general appearance of the smoothed reflec?
tion spectra is shown in Fig. 3; the maximum of the
diffusion scattering corresponds to a wavelength of
about 400–420 nm.
The next stage of our investigation was to study the
temperature dependences of the spectra of the polar?
ized components and the total intensity of the diffu?
sively reflected light. Figure 4 exemplifies the tempera?
ture dependences of the total intensity of vertically? and
horizontally?polarized components of the reflected
light at the wavelengths of 420 nm and 580 nm.
It is seen from Fig. 4 that the temperature depen?
dences of the light diffusively reflected at different
wavelengths are similar. In the temperature range from
60 to 90°C, one can observe a sharp decrease in inten?
sity (with a minimum near 70°C) of the reflected light
in the chosen direction within a broad wavelength
The spatial distribution of the diffusively reflected
radiation flow, its intensity, and its other properties are
different for different special cases; they depend on the
illumination conditions, the properties of the reflect?
ing medium, the presence of irregularities in it, and
the structure of these irregularities. Thus, for example,
at substantial variations in the film structure the angle
distribution of the scattered light can change and the
Lambert distribution law can be violated. In addition,
variations in the angular distribution of molecular
dipole moments in the film can substantially affect the
polarization characteristics of the scattered light.
Since, as follows from Fig. 3, the light that was dif?
fusively reflected at the film turned out to be partially
polarized (predominantly perpendicular to the inci?
dence plane), we investigated the degree of polariza?
tion of the reflected light. The degree of polarization of
diffusively reflected light was calculated based on the
spectral data for the vertically and horizontally polar?
ized components. Figure 5 shows our calculations of
the temperature dependence of the degree of polariza?
tion for the light that was diffusively reflected at the
film under analysis at the wavelength of 580 nm
obtained using the equation
Fig. 3. The spectra of vertically polarized (1) and horizon?
tally polarized (2) components in reflected light at a tem?
perature of 24°C.
Fig. 4. Temperature dependences of the intensity of diffu?
sively reflected light for two wavelengths, 420 nm (1) and
580 nm (2).
Polarization degree P
30 50 70
Fig. 5. The degree of polarization of the reflected light at
the wavelength of 580 nm as a function of temperature.
MOSCOW UNIVERSITY PHYSICS BULLETIN Vol. 66 No. 1 2011
CHANGES IN OPTICAL PROPERTIES OF TDO BAM BCC LANGMUIR FILMS53
P = .
It should be noted that the initial degree of polar?
ization of the diffusively reflected light at room tem?
perature depended on the experimental geometry (as
is the case with partial polarization during reflection at
a dielectric film).
It is apparent from the figure that the degree of
polarization of the light reflected at an LB liquid crys?
tal film sharply varies within the temperature range
from 60 to 90°C; the maximum was reached at 70°C,
where a structural phase transition was revealed.
The authors of  discovered a polar state of a
Langmuir film in a broad temperature range of 50–
105°C, with the film polarization slightly increasing
with an increase in temperature. The latter effect can
be related to several reasons. First, as is known,
directly after deposition on a substrate at room tem?
perature an LB film can be in a state which is not the
most favorable from the thermodynamic point of view.
When the temperature rises and the internal energy of
the film increases as compared with the energy of
binding with the substrate, one can observe one or sev?
eral successive structural transformation of the film,
yielding a more thermodynamically stable state. In
this case, the additional ordering of dipole moments of
the molecules entering the film and, as a result, its
increased polarization are possible. Second, we can?
not exclude the possibility that the polarization
increase is explained by an increase in the dissociation
of molecules entering the film and the motion of the
formed ions under the effect of the external field.
Thus, in the case of TDO BAM BCC we are likely
to deal with a phase transition belonging to one of the
two following types, namely a “disorder–order"?type
transition or a “shift”?type transition (or both). The
polar state with the dipole moments in each unit cell
has to reflect light with different polarization in a dif?
ferent manner. The observed effects of variations in the
intensity and the degree of reflected light polarization
can be explained by the phase transitions in the tem?
perature range from 50 to 100°C observed in the ana?
Since the main structural feature of liquid crystals
is the existence of orientation order, it is natural that
all their properties are determined, to various extents,
by the degree of orientation ordering. Restructuring
due to phase transformations yields variations in the
anisotropic properties of a film . This seems to
result in changes in the spatial distribution of the signal
diffusively reflected at the film and in sharp variation
in the degree of its polarization.
It is interesting to note that, as was mentioned
above, the authors of  investigated films that were
similar to those considered in the present study. Elec?
trophysical and electrooptical methods did not reveal
any phase transitions in these films in the temperature
range from 20°C to 100°C. By using adsorption and
optical techniques we discovered signs of structural
transformations in the films at a temperature of about
70°C. Since the nature of the revealed phase transition
is not yet clear, it is not surprising that some film char?
acteristics do not change in the process of phase trans?
Thus, we have obtained the spectra of optical
absorption and diffusion reflection of light for TDO
BAM BCC films at wavelengths from 200 to 800 nm in
the temperature range of 20–100°C. A structural
phase transition in the film at 70°C and a sharp
decrease in the diffusion reflection coefficient near
this temperature were revealed. The reflected light is
partially polarized. The temperature dependence of
the degree of reflected light polarization was studied.
A sharp increase and then a decrease in the degree of
polarization of the light that was diffusively reflected
from the films was observed at the phase?transition
The study was supported by the Russian Founda?
tion for Basic Research, project no. 09?08?00362?a.
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