Materials Sciences and Applications, 2012, 3, 552-556
http://dx.doi.org/10.4236/msa.2012.38078 Published Online August 2012 (http://www.SciRP.org/journal/msa)
Roughness Control of Layer-by-Layer and Alternative
Spray Films from Congo Red and PAH via Laser
Light Irradiation
Gleidson Cardoso, Romário J. da Silva, Rafael R. G. Maciel, Nara C. de Souza, Josmary R. Silva
Grupo de Materiais Nanoestruturados, Universidade Federal de Mato Grosso, Barra do Garças, Brazil.
Received May 10th, 2012; revised June 17th, 2012; accepted July 12th, 2012
ABSTRACT
Films from congo red (CR) alternated with poly(allylamine hydrochloride), PAH, were prepared by layer-by-layer and
alternative spray techniques. In order to investigate the change of roughness induced by laser light irradiation (532 nm),
both kinds of films were characterized by using UV-visible spectroscopy and atomic force microscopy (AFM). At dif-
ferent irradiation times, layer-by-layer, LbL, films showed small changes in the roughness and irregular behavior,
whereas spray films exhibited higher and a regular decreasing of roughness with increasing irradiation time. The higher
roughness of spray films as compared with the LbL ones was attributed to different formation mechanisms of the films.
The decreasing of the roughness as a function of the irradiation time (exhibited by the spray films) was associated to
surface relaxation due to the interplay between photoisomerization of congo red dye and the heating of the sample dur-
ing the laser light irradiation. The results suggested that the alternative spray technique is the best choose to control of
roughness of the films by using light irradiation.
Keywords: Congo Red; Poly(Allylamine Hydrochloride); Layer-by-Layer Films; Roughness Control; Laser Light
Irradiation
1. Introduction
Molecules able to exhibit geometric isomerization (change
of geometric shape in space) by the action of light, such
as azobenzene and their derivatives are promising
because they have characteristics suitable for application
in memory devices, optical switches and modulators [1].
Moreover, they make an important role in processes at
the nanoscale transduction of biological signals [2].
Azobenzene derivate compounds have been widely stud-
ied for their trans-cis-trans photoisomerization property,
which may lead to optical storage and formation of sur-
face-relief grades. When azobenzene derivated molecules
are excited from conformational state trans to cis by the
action of light, and then returned to the state trans [3,4],
they adopt the orientation in which its electric dipole
moment is perpendicular to the direction of polarization
light. Because of this, after some time of light irradiation,
which occur during the trans photoisomerization cycles
trans  cis  trans, comes an excess of molecules
oriented perpendicular to the axis of the dipole electric
field optical excitation, providing the optical biref-
ringence and dichroism in films [4]. Another important
property of azobenzene derivatives, which also comes
from the molecular photoisomerization, is the change of
surface morphology of films [5] producing surface relief
grades. This process is among the most interesting effects
associated with the photoisomerization [1,6]. Nanostruc-
tured films of azobenzene derivatives have been pre-
pared by using various techniques, such as layer-by-layer
[7], Langmuir-Blodgett [4], and spin-coating [8]. In
special, the LbL technique is based on the spontaneous
adsorption of charged species from a solution onto a
solid substrate. The paradigm of LbL is the adsorption
based on ionic interaction [9,10]. A technique proposed
recently is the spray self-assembly introduced by Decher
et al. [11]. In this technique, layers of material are alter-
nated by spraying their solutions by controlled time [12].
Despite several studies on films of derivatives of
azobenzene, we are not aware of any comparison of the
effect of laser irradiation on LbL and simple spray films.
In this work we have proposed an alternative, very sim-
ple and low cost spray technique. We have investigated
the effect of laser irradiation on roughness of layer-
by-layer and films prepared by alternative spray tech-
nique, alternating poly (allylamine hydrochloride), PAH,
and congo red dye, CR. We have noted that the LbL

Roughness Control of Layer-by-Layer and Alternative Spray Films from Congo Red and PAH
via Laser Light Irradiation
553
films exhibited lower roughness than the spray ones;
however, the spray showed more regular change behavior
of roughness as a function of light irradiation and, con-
sequently, allowed a good roughness control.
2. Materials and Methods
Films were adsorbed on BK7 optical glass slides (36 mm
× 14 mm × 1 mm), which were rendered hydrophilic in a
3:7 mixture of hydrogen peroxide (H2O2)/concentrated
sulfuric acid (H2SO4). The slides were rinsed and then
further cleaned in a solution containing pure water, H2O2,
and ammonium hydroxide (NH4OH) in a ratio of 5:1:1
(v/v). BK7 slides were chosen because of its negligible
absorbance in the visible region and its finely polished
surface [13]. Congo red (CR) and poly(allylamine hydro-
chloride) (PAH) were purchased from AMRESCO Inc.
and Sigma-Aldrich, respectively. Layer-by-layer films
were assembled by the alternated immersion of the BK7
substrate in the PAH and CR solutions for 3 min each at
room temperature. After each deposition step, the formed
layer was drying under an air flow. The concentration of
the dipping solutions (CR and PAH) for the films pre-
paration was set at 0.5 g/L and the pH was adjusted to 8
to the solutions of CR (adding NH4OH) and 4 to the
solutions of PAH by adding HCl.
2.1. Alternative Spray Technique
Alternative spray films were fabricated by using simple
glass flasks, such as small fragrance bottle, with volume
capacity of 10 mL, as showed in Figure 1. The BK7 sub-
strate was kept vertically and perpendicular to the spray
flux. The waiting time for drying between one layer and
the next one was 5 min. The spray droplets from the slip
surface by gravitational effect forming a thin film.
In order to achieve uniformity in wetting of the surface
with the flasks used to prepare the spray films, it was
performed a plot of average diameter of wetting area as a
function of distance between spray nozzle and film sur-
face, as shown in Figure 1(b). From this result, it was
fixed 5 cm in distance for all experiments.
2.2. Characterization
The growth of LbL and alternative spray films were
monitored using UV-visible spectroscopy with a Ther-
molab Genesys 10 spectrophotometer. The surface mor-
phology of the films was studied with a NanoSurf In-
struments atomic force microscope EasyScan II in the
tapping mode (256 × 256 pixels), under ambient con-
ditions. A sample area of 40 μm × 40 μm was scanned
and an image was acquired. The film roughnesses were
determined using NanoSurf Instruments software. Ir-
radiations of films were carried out by impinging a line-
arly polarized laser beam at 532 nm from a diode laser
(BW & Tek Inc.). The used power was 5.0 mW with a
spot of ca. 1.8 mm in diameter.
3. Results and Discussion
The CR molecules absorb in the visible region, which
makes the UV-vis spectroscopy technique a powerful tool
in the investigation of adsorption kinetics during the
growth of films. Figure 2 shows an inset with the spectra
of CR solution and PAH/CR LbL and alternative spray
films. The spectra found in our study are essentially the
same as those found in the literature for the CR [14].
There is not observed spectrum shift as the solid phase
UV-vis is compared to spectra of the solution phase,
(a) (b)
Figure 1. (a) Glass flasks used to prepare the spray films and (b) plot of average diameter of wetting spot as a function of
distance between spray nozzle and film surface. The inset (negative photo) depicts a complete wetting area. The solid line is a
uide to the eyes. g

Roughness Control of Layer-by-Layer and Alternative Spray Films from Congo Red and PAH
via Laser Light Irradiation
554
Figure 2. Absorbance at 500 nm versus number of bilayers
for PAH/CR LbL and cycles of alternative spray films onto
BK7 glass. The solid line is just a guide for the eyes. The
inset shows the spectra of CR solution and CR alternated
with PAH on films form.
indicating that the molecular H or J-aggregation does not
occur for these systems [15].
As shown in Figure 2, a linear increase in the absorb-
ance at 500 nm was observed, suggesting that the same
amount of material was adsorbed in each deposition step
(each layer). Moreover, the linear film growth of the first
bilayer indicates that for this kind of LbL or alternative
spray films, the influence of the substrate on the growth
film is negligible [13,16]. The absorbance is proportional
to the amount adsorbed [16] and a direct comparison
between the techniques shows that although the growth
of film is approximately linear for both, the film obtained
with the alternative spray technique grows more rapidly
than that obtained by the conventional LbL technique
(Figure 2). So, this technique can provide thicker films
with less cycles of deposition. This can be interesting for
some applications that require a more effective coating,
such as surface relief gratings.
Figure 3 shows AFM images of 15-bilayer LbL films
and 15 complete cycles of alternative spray films from
PAH/CR in which the top layer is CR. The images were
taken in a scan window of 40 μm × 40 μm. A clear
structure rod-shaped lying on the surface can be identi-
fied in the analysis of morphology [17-20]. This kind of
structures in spray films is larger than that of LbL films.
The surface morphology of alternative spray films
change when the films are irradiated, while the change is
less observable in LbL films. This change can be more
easily observed in Figure 4 that shows how the rough-
ness changes when the LbL or alternative spray films are
irradiated for different times. For LbL films the rough-
ness remains about 80 nm, whereas for alternative spray
films, the roughness decreases with irradiation time
Spray LbL
0 min0 min
10 min10 min
20 min20 min
30 min30 min
Figure 3. AFM images of PAH/CR alternative spray and
LbL films with CR atop. The images were taken with a scan
window of 40 μm × 40 μm. The surface morphology was
investigated as a function of the irradiation time.
Figure 4. Roughness vs. irradiation times for the PAH/CR
LbL and alternative spray films.

Roughness Control of Layer-by-Layer and Alternative Spray Films from Congo Red and PAH
via Laser Light Irradiation
555
about 50%.
The large difference in roughness between the LbL
and spray films can be explained by considering the for-
mation process of each kind of film. LbL films are
formed by the spontaneous adsorption of molecules from
solution onto the substrate [9] or PAH layer that is ac-
companied of drying process, whereas in the alternative
spray films, a large amount of molecules staying in
contact for more time during the drying process (5 min
for each layer). Decher showed that the thickness in-
crease as function of time [11].
In addition LbL films have a smaller roughness and a
more stable structure under irradiation because of the
spontaneous adsorption of molecules which produces a
higher molecular packing. In the case of alternative spray
films, the lower packing concert with the higher rough-
ness becomes possible controlling the roughness through
the laser irradiation. This can occur though relaxing of
the surface structure caused by the interplay between
photoisomerization and heating of the films.
4. Conclusion
Layer-by-layer and alternative spray films were success-
fully prepared from congo red dye alternated with PAH.
The comparison between the two systems revealed that
the roughness of the LbL films is smaller than those
exhibited by the spray ones. This was attributed to dif-
ference in structure of the two kinds of films due to for-
mation mechanisms. It was found that only the spray
films displayed regular roughness changes of their
surface under laser irradiation which was associated to
lower packing concert with the higher roughness. These
results suggested that this alternative spray technique
provide films thicker and with higher roughness that may
be a good alternative to LbL one when is required more
material adsorbed and a roughness control for device
applications.
5. Acknowledgements
This work was supported by CNPq and CAPES (Brazil).
R. J. da Silva and R. R. G. Maciel thank Capes (nbioNet)
and CNPq for the scholarship.
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