Advances in Ma terials Physics and Che mist ry, 2012, 2, 84-88
doi:10.4236/ampc.2012.24B023 Published Online December 2012 (htt p://
Copyright © 2012 SciRes. AMPC
Laser Deposition of Tetrasulfonated Phthalocyanine
Layers for Gas Sensors
Premysl Fitl1, Martin Vrnata1, Dusan Kopecky1, Jan Vlcek1, Jitka Skodova1,
Jaroslav Hofmann1, Vladimir Myslik2
1De par t ment of Physics a n d Measurement, Institute of Chemical Technology, Tech ni cka 5, P rague 6, CZ - 166 28, Czech Republ ic
2Department of Solid-St ate Eng in eer in g , Institute of Chemical Technology, Technicka 5, Prague 6, CZ - 166 2 8, C z ech Republic
Received 2012
Thin layers of nickel and copper tetrasulfonated phthalocyanines (NiPcTS and CuPcTS) were prepared by Matrix Assisted Pulsed
Laser Evapo rat io n method . Th e depo sit ions were carr ied ou t with KrF exci mer laser (en er g y densit y of laser r adiation EL = 0.1 t o 0.5 from dimethylsulfoxide matrix. For both materials the ablation threshold EL-th was determined. The following properties of
deposited layers were characterized: a) chemical composition (FTIR spectra); b) morphology (SEM and AFM portraits); c) imped-
ance of gas senso rs b ased on NiPcTS and CuP cTS layers i n the pres ence o f two anal ytes - hydrogen and ozone. The prepared sensors
exhibit response to 1000 ppm of hydrogen and 100 ppb of ozone even at laboratory temperatur e.
Keywords: Matrix Assisted Pulsed Laser Evaporation; Substituted Phthalocyanines; Gas S ensors; Impedance Measurement
1. Introduction
Matrix Assisted Pulsed Laser Evaporation (MAPLE), intro-
duced by Piqué et al. in 1999, is one of the experimental laser
deposition methods used for deposition of thin uniform films o f
organic [1] and even biological materials [2].
MAPLE is ch aracterized b y indirect con tact o f the laser radi-
ation with depo sited material. Th e target us ed for MAPLE con-
sists of two substances; each has a different function during the
deposition process. The first one is the deposited material itself,
the second is a matrix, which has majority representation (ap-
proximately 95% of target) and has usually a character of low
molecular weight volatile solvent of the deposited material.
Both substances are mixed together and frozen to liquid nitro-
gen temperature to suppress sublimation of the matrix at low
temperature. In case of correct setting of the deposition condi-
tions, all the energy of laser pulse is absorbed by matrix. The
matrix has two functions: i) it protects the environment of a
fragile deposited material from high-energy laser radiation and
ii) serv es as an en erg y trans mitter fro m ele ctro magneti c radiation
to kinetic energy of the molecular oscillation motion, which
causes abl ation of a depo sited material to a plasma state and it s
subsequent deposition. Selection of a suitable matrix is there-
fore essential for successful and non-destructive deposition of a
The mechanism of MAPLE deposition is shown in Figure 1.
Frozen target is placed i n a vacuum and impinged by pulses of
laser radiation, whose energy is absorbed preferentially by the
matrix, which leads to local overheating of the frozen target,
followed by abrupt release of the matrix so-called surface
ablation of target. Through collective collisions, matrix mole-
cules pull the molecules of the deposited material and impart
them sufficient kinetic energy required to overcome the tar-
get-substrate distance. Large molecules of the deposited ma-
terial have lower vapor pressure than volatile small molecules
of matrix and therefore they are less often pumped away by
vacuum system; so the substrate is gradually covered with a
thin layer of deposited material with minimum content of ma-
trix molecules.
The most significant group of chemical gas sensors operates
on the basis of ability of thin semiconductive layers (=sensitive
layers) to chemisorb various gaseous analytes on their surfaces
with subsequent exchange of electrons between analyte and
sensitive layer [ 3]. Wh ile red u cing gas es po sses ab ility to act as
electron donors, the oxidizing ones are electron acceptors, i.e.
they extract el ectron s from the sen sitive layer. At present , some
classes of organic substances (conducting polymers [4], com-
plexes of organic ligands with metallic cation [5] et c.) are inten-
sively invest igated as prosp ective materials for sensitive layers.
In general , the s uitab le materials can be ch aracterized as o rgan-
ic molecules containing conjugated system of double bonds,
Figure 1. Principle of MAPLE method.
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where π-electrons form delocalised and highly polarizable sys-
tem which exhibits ability to enter reversible interaction with
the an alyte.
Among organocomplexes, both phthalocyanines and substi-
tuted phthalocyanines are known to be excellent materials for
gas sensing [ 6,7]. However, when one selects proper method
for depositing sensitive layer, there is a significant difference
between them: while phthalocyanines are almost insoluble in all
solvents, some of their substituted derivatives exhibit a good
solubility in low molecular solvents. Due to this fact substituted
phthalocyanines (unlike non-substituted ones) can be deposited
by MAPLE method.
The presented paper deals with preparation of gas sensor
sensitive layers based on tetrasulfonated phthalocyanines (NiPcTS
and CuPcTS). The depositions were carried out from dimethyl-
sulfoxide matrix by MAPLE method, providing tool for gentle,
non-destructive and easily adjustable grown of organic mate-
rials. Responses of prepared sensors to hydrogen and ozone are
also p resented.
2. Experimental
2.1. Deposition of NiPcTS a nd C uPcTS thin Layers
by MAPLE Method
In our experiments MAPLE instrumentation was carried out as
follows: Powder of NiPcTS or CuPcTS (Sigma-Aldrich) was
diluted in dimethylsulfoxide matrix to obtain solution containing
0.2 weight%. Dimethylsulfoxide matrix was recently proved to
be proper for depositions with KrF excimer laser [8]. After
sonification the resulting solution was filtered and then frozen
by liquid nitrogen. The freezing process proceeded in a tubular
mould so as to produce targets for MAPLE in the form of tab-
lets (approx. 40 mm in diameter and 10 mm thick). Then the
deposition conditions were set (KrF excimer laser operating at
248 nm; energy density of laser radiation EL ranging from 0.1
to 0.5; repetition rate of laser pulses frep = 10 Hz, pulse
duration 15 ns; residual pressure in the deposition chamber 10-4
Pa; working atmosphere during depositions was 3 Pa of nitro-
gen; target -subst r ate dist ance 35 mm).
2.2. Characterization of Chemical Composition and
Morphology of the Layers
Chemical composition of the deposited layers was analyzed
from the IR sp ectra scanned b y the Attenu ated Total Reflecti on
Fourier Transform Infrared spectroscopy (ATR FTIR). The
spectra were scanned using a BRUKER IFS 66 V device (di-
amond crystal) in the interval of wavenumbers from 600 to
1800 cm-1covering finger-print of MePcTS molecules.
The surface morphology of the samples deposited on po-
lished silicon wafer was acquired using Atomic Force Micro-
scopy (AFM). The AFM images were taken on Veeco Digital
Instruments CP II apparatus. For sample characterization, ‘Tap-
ping mode’ rather than ‘Contact mode’ was chosen to minimize
damage to the sample surfaces. A Veeco oxide-sharpened sili-
con probe RTESPA-CP attached to a flexible microcantilever
was used at its resonant frequency of 300 kHz. The image res-
olution was 256×256 pixels. Layers morphology was further
characterized by Scanning Electron Microscopy (SEM) with
JEOL JSM-7500F instrument.
2.3. Measuring of Gas Sensor Response
In ord er to test gas sensing propert ies the layers were depo sited
onto alumina sensors substrates (2.0 x 2.5 mm2) equipped with
interdigital electrodes (Figure 2). The sensor impedance was
measured in puresynthetic air (as a reference atmosphere)
and in synthetic air containing 1000 ppm of hydrogen or 100
ppb of ozone respectively. The impedance measurements were
performed with a HP4192LF impedance analyser with fre-
quency of testing signal from 15 Hz to 10 MHz and amplitude
remained constant at 100 mV. The obtained data were
represented as Nyquist diagrams - i.e. they are plotted as im-
aginary part of complex impedance vs. real part of complex
impedance with frequency of testing signal as a parameter.
From Nyquist diagrams the so called phase-angle sensitivity Spa
[deg.] o f sensors was evaluated. The detailed definiton of Spa is
in [9,10] and s ee also F igure 3.
Figure 2. Sensor substrate - front side with interdigital electrodes
(lef t); back side with resis tance hea ting (right).
Figure 3. A typical Nyquist diagram of gas sensor with MePcTS
sensitive layer in synthetic air (reference) and measured gas (1000
ppm of hydrogen). In this example phase-angle sensitivity Spa is
evaluated as a difference of sensor impedance arguments (θ angle)
for 1 MHz frequency of testing signal.
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3. Results and Discussion
3.1. Determination of Ablation Threshold
The depositions made by KrF laser in the range of energy den-
sity of laser radiation from 0.1 to 0.6 J∙cm -2 can be assembled
into a dependence of growth rate on laser fluence, known as the
ablati on cu rve (Figure 4). On basis of these curves, the ablation
thresholds EL-th were determined to be EL-th ~ 0.2 J∙cm -2 for
NiPcTS and EL-th ~ 0.3 J∙cm -2 for CuPcTS. Ablation threshold
is a parameter important from the practical point of view, as it
corresponds to energy density sufficient for effective layer
grown on one side, while there is no excessive photo- and heat -
stress of deposited material on the other side.
3.2. FTIR Spectra of Source Substances and
Deposited Layers
Infrared spectra o f sou r ce s ubstances were compared to thos e of
deposited materials (Figures 5(a),(b) and Figures 6(a), (b) in
order to evaluate the degree of material decomposition during
MAPLE pr ocess. Bo th mater ials wer e deposit ed at ener gy den si-
ties corresponding to their ablation threshold. Analyzing spectra
of sou rce material s (Figures 5(a) and 6(a)) one may notice that
absorbtion bands are rather wide. This phenomenon can be
attributed to occurrence of traces of mono-, di- and tri- sulfo-
nated phthalocyanines in commercially distributed metal tetra-
sulfonated phthalocyanines (MePcTS). Nevertheless, it is ap-
parent, that in both cases the transfer of material by MAPLE
method was nondestructive, as the positions and in most cases
also amplitudes of absorbtion maxima are retained when com-
paring Figure 5(a) with (b) or Figures 6(a) with (b). An over-
view of absorption bands of phthalocyanines is summarized in
Figure 4. Ablation curves for NiPcTS (top) and CuPcTS (botto m).
3.3. Layers Morphology
The layers deposited at ablation thresholds were also studied by
SEM (Figure 7) and AFM (Figure 8) methods. The portraits
resulting from these methods reveal that the structure of
MAPLE deposit ed layer is rath er segmented with lar ge relative
surface; these properties are favourable for applications in gas
sensing (the detection process is localized on the surface of
sensitive layer).
Figure 5. FTIR spectrum of NiPcTS: source material (a); layer
deposited at EL-th = 0. 2 J∙cm-2 (b).
Figure 6. FTIR spectrum of CuPcTS: source material (a); layer
deposited at EL-th = 0. 3 J∙cm-2 (b).
Figure 7. SEM portraits of NiPcTS (left) and CuPcTS (right).
Copyright © 2012 SciRes. AMPC
3.4. Impedance Measurements and Phase-Angle
The NiPcTS and CuPcTS layers were deposited to sensor sub-
strates, t he impedance o f obtained structures was measu red and
phase-angle (Spa) sensitivity to 1000 ppm of hydrogen and 100
ppb of ozone evaluat ed. The results are summarized i n sequen-
tion of Figures 9-12.
The sensors exhibit the highest phase-angle sensitivity (Spa
ranging from 5 to 12 deg) at approx. 500 kHz frequency of
measuring signal in all cases. CuPcTS sensors have also sec-
ondary maximum in the vicinity of 100 Hz. As for temperature
dependence of Spa - the sensors were tested at operating tem-
peratu res 25 - 90°C, becau se at higher temperatu res reaction o f
MePcTS with atmospheric oxygen can start. There was also
found certain low-temperature sensitivity - i. e. measu rable sen-
sor response at 25°C.
Figure 8. AFM portrait of NiPcTS.
Figure 9. Phase-angle sensitivity of NiPcTS to 1000 ppm of hydro-
Figure 1 0. Phase-an gl e se n s it ivity of NiPc TS to 100 ppb o f ozone .
Figure 11. Phase-angle sensitivity of CuPcTS to 1000 ppm of hy-
Figure 1 2. Phase-angle sensitivity of CuPcT S to 100 pp b of ozone.
4. Conclusions
Tetrasulfonated metal phthalocyanines were deposited by
MAPLE method from dimethylsulfoxide matrix. It was proved
that for energy density of laser radiation corresponding to abla-
tion threshold the molecular structure of MePcTS remained
preserved. The pr epared l ayers have po rous str ucture with large
relative su rface -properties suitable for sensor applications. The
sensors based on deposited layers were successfully tested for
detection of hydrogen and ozone; low temperature sensitivity
was observed, hence these sensors are able to operate at labor-
atory temperature .
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5. Acknowledgements
This work was supported by Grant Agency of the Czech Re-
public (GAČR) projects No. P108/11/1298 and P108/12/P802
and also financial support from specific university research
(MSMT No. 21/2012) .
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