Roughness Control of Layer-by-Layer and Alternative Spray Films from Congo Red and PAH via Laser Light Irradiation

doi:10.4236/msa.2012.38078

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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.

Email: [email protected]

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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