INFRARED OPTICS APPLICATIONS OF THE THIN POLYANILINE
EMERALDINE BASE FILMS
Edward Bormashenko*a, Roman Pogreba, Semion Sutovskia, Alexander
Shulzingera, Izaksson Gregory,b Abraham Katzirb
a
The College of Judea and Samaria, Ariel, 44837, Israel
b
Tel-Aviv University, School of Physics and Astronomy, Tel Aviv,
66978, Israel
Abstract
Use of polyaniline emeraldine base films as antireflection coating for middle and far
IR optics elements was studied. Optical properties of PANI coated ZnSe plates were
studied using FTIR spectrometer. It was shown that PANI coating allows significant
decrease of Fresnel losses in broad IR band (1.5 - 25 μm). Laser irradiation damage
threshold of PANI coating was studied. Microhardness of coated ZnSe is established
as satisfactory.
Keywords: Polyaniline,
microhardness
infrared,
ZnSe,
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spectrum,
antireflection,
coating,
1. Introduction
The optical properties (including the precise measurement of refractive index)
of polyaniline and its derivatives have been the subject of much investigation recently
[1-3]. IR spectra of polyaniline its solutions and related compounds were studied
thoroughly [4-8]. It could be recognized from the analysis of the spectral data that
PANI demonstrates high transparency in broad middle and far infrared bands, this
fact makes it suitable for coating of IR optics elements. In addition to that PANI
solutions allow production of thin films with thickness, which could be controlled
with high accuracy necessary for optical coating. All materials used in the field of IR
optics (ZnSe, ZnS, Ge, AgCl, AgBr) suffer from the same disadvantage: high Fresnel
losses, dictated by a high refraction index [9-10]. Usually this problem is surmounted
by applying of antireflection coating to the surface of IR optics element [9-10]. Such
coating in addition to antireflection effect and high transparency in broad IR band has
to demonstrate good adhesion to the material to be coated, satisfactory surface
hardness, high laser radiation damage threshold and thermal stability of optical
constants. The best possible coating in use today is diamond coating, which raises
prices of IR windows and lenses significantly. We supposed that PANI coating could
supply all above-listed properties. Very few reports were devoted to the study of the
interaction of PANI derivatives with strong IR laser radiation [11-13], at the same
time extremely high optical damage threshold measured at wavelength 1064 nm,
established as 10GW/cm2 for polyethoxyaniline is signed in [11].
The most abundant material used for IR optical purposes is ZnSe, so we
concentrated our efforts in the development of PANI coating of ZnSe IR windows at
this stage of our investigation. PANI is only partially dissolved in dymethylsulfoxide
(DMSO) [14], but we chose (DMSO) as a solvent for PANI emeraldine base, because
Se demonstrates weak solubility in DMSO [15], so some etching effect is possible at
the interface ZnSe/PANI DMSO solution, which could provide good adhesion of
PANI film to the surface of the ZnSe window to be coated.
2. Experimental
Polyaniline emeraldine base and dimethylsulfoxide were supplied by SigmaAldrich Israel Ltd. ZnSe plates were supplied by Eksma Ltd, parameters of the plates:
transmission band: 0.6-22 μm, refractive index in the band 8- 13 μm: n = 2.417-2.385,
flatness λ/60, parallelism 3 arcmin.
Saturated PANI solutions in DMSO were prepared .PANI EB films with a
thickness 100-2000 nm were applied to ZnSe substrates by spin-casting process.
Films were exposed to vacuum evaporation. Then part of PANI coated ZnSe surface
was polished, in order to compare optical parameters of coated and uncoated ZnSe
substrates. Optical quality of obtained EB PANI coating was controlled using Linnik
interferometer combined with optical microscope. Absorbance spectrum of coated
and uncoated ZnSe substrates in the middle and far IR bands was established using
Bruker Vector 22 FTIR spectrometer, and the same in the near IR band with Varian
Cary 500 spectrophotometer. Then films were exposed to strong IR radiation
produced by Apollo Inc. CO2 laser. Comparative surface hardness of coated and
original ZnSe surfaces was measured with Buehler 2100 microhardness tester.
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3. Results and discussion
Presence of solvent traces in obtained PANI EB films applied to ZnSe
substrates is crucial for the optical quality of coating (solvent traces produce
additional “dips” in absorption spectrum). We developed optical bench, which
allowed “real time” control of the process of vacuum evaporation of coated ZnSe
substrates described in Fig.1. Absorption spectra of PANI EB coating were taken
when vacuumed using FTIR spectrometer Bruker Vector 22. Evaporation of the
solvent has been accompanied with the changes in the absorption spectra (see Fig.2).
Very strong line of water (0.2% of water is present in DMSO) located between 3400
and 3700 cm-1, “dips” at 3000 and 2910 cm-1 inherent for the vibrations of CH3
group, very strong “dip” at 1033 cm-1 inherent for S=O group of DMSO disappear
under vacuum evaporation. The termination of such changing is indicative of the net
removal of the solvent from the PANI EB film. It could be recognized from the final
spectrum of the evaporated film (Fig. 2) that PANI EB coating is highly transparent
in the broad middle and far IR bands 2.0-6 μm Absorption spectrum of evaporated
ZnSe substrate coated with PANI EB film measured with near IR spectrophotometer
leads to the conclusion that the window of high transparency extends to 1.0 μm (the
same results concerning near IR spectrum of PANI were reported by F.Fitrilawati,
M.O. Tija [11]), so the results of the spectral measurements when combined suggest
that thin PANI EB layers produce highly transparent coating in the range 1.0-6.0 μm.
This fact makes them suitable for various IR optics applications.
It has to be signed that thin PANI EB layers (up to 200 nm) provided good
optical qualities of the coated ZnSe subtrates. Images obtained using Linnik
interferometer-microscope revealed the high homogeneity of the optical constants of
the coating (see Fig. 3). On the other hand coatings with comparatively high thickness
(more than 1000 nm) didn’t provide satisfactory optical quality.
Resistance of the coating layer to the powerful IR radiation is crucial for its IR
optics applications. In order to test the optical quality of PANI EB coating under
strong IR radiation we used optical bench presented at Fig.4 Radiation produced by
CO2 laser was directed on the PAN EB coated ZnSe plate at small incident angle. IR
radiation detector A measured the power transmitted trough the sample It, and
detector B indicated power of the reflected beam Ir. The total incident power Ii = It +Ir
(when absorption in the sample is neglected), and transmission coefficient R is given
as R = It/Ii. Fig. 5 presents dependence R(Ii*), where Ii* - power density of the incident
beam. It can be recognized that PANI EB coated ZnSe plates demonstrate stable
transmission coefficient (R = 0,75-0.76) up to very high power densities as high as 3
W/mm2. It could be recognized from the absorbance spectrum, presented at Fig.2, that
10.6 μm – wavelength of CO2 laser – located in the “problematic” part of PANI EB
spectrum, absorption at this wavelength is not minimal, this makes the results of the
test all the more important for further IR optics applications of PANI EB coating
layers.
Another parameter important for IR optics applications is hardness of the
coating layer. Microhardness of PANI EB layer (150 nm thickness) applied to ZnSe
substrate was established as 90 HV (load 10g), comparatively to 120 HV measured
for uncoated ZnSe substrate. Despite some decrease of the microhardness of the
substrate when coated it could be concluded that PANI EB layers form surface layers
with satisfactory mechanical properties.
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4. Conclusions
Thin PANI EB demonstrated layers demonstrated their suitability as a coating
of IR optics elements. The broad window of transparency in the IR band, high
stability under strong IR radiation, good surface hardness make them attractive for
different IR optics applications, such as anti-reflecting coating of IR windows, lenses,
etc.
Acknowledgements
This work was supported by the Israel Ministry of Science, Culture and Sport
(Project No. 1461-2-00) and Israel Ministry of Absorption The authors are thankful to
Mr. Avigdor Sheshnev for his assistance in the analysis of the spectral data.
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IR detector
ZnSe
windows
w
hermetic
cell
vacuum
pump
PANI layer
under vacuum
evaporation
FTIR
PANI coated
ZnSe plate
Fig.1. Optical bench, intended for controlling of the vacuum evaporation of
PANI films
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Fig.2. Changes in the PANI EB solution spectrum under vacuum
evaporation
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Fig. 3. Image of the surface of EB PANI coated (dark field) and
uncoated (light field) ZnSe substrate obtained with Linnik
interferometer-microscope (magnification 500, wavelength –
λ = 526 nm). The thickness of the film is established as λ/4 ~ 130 nm)
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IR powermeter B
mirror
CO2 laser
PANI coated
ZnSe plate
IR powermeter A
Fig. 4. Optical bench used for the exposure of PANI EB
coated substrates to the strong continuous IR radiation
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Fig. 5. Transmission coefficient measured at different power
densities
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