(M = Au, Pt) thin films prepared by pulsed laser deposition

(M = Au, Pt) thin films prepared by pulsed laser deposition

AGNIESZKA KOPIA, WOJCIECH MAZIARZ
Structural characterization and properties
of M-WO3 (M = Au, Pt) thin films prepared
by pulsed laser deposition
INTRODUCTION
A sensor for detecting and measuring gas concentrations should be
characterized by appropriately high sensitivity, selectivity, a short
response time and stability. These requirements led to a fast
development of research on semiconductor-based gas sensors
[1, 2]. The tungsten trioxide is the competition to others oxides
compositions regarding low costs of production and high
sensitivity in the presence of H2S, NOx, SO2. However, sensitivity
of the sensor, its stability and working time highly depend on
microstructure, thickness of film, grain size and grade of specific
surface [3, 4]. M. Bendahan et al. [5] showed in their investigations, that using magnetron sputtering it is possible to obtain
WO3 films sensitive to O3, however, the sensitivity depends on
working temperature. WO3, Ag-WO3, Au-WO3 and Pt-WO3 films
produced by means of magnetron sputtering method were investigated in the area of sensitivity and selectivity by M. Stankova et
al. [6]. Authors showed that WO3 films coated with z 3÷4 nm
layer of Au or Ag are sensitive to presence of H2S in CO2 at
temperature T = 260°C, but not reacting when SO2 is present in
CO2. F. Mitsuga et al. [7] produced WO3 films using laser ablation
method and showed that the highest sensitivity to the presence of
NOx presented films in the temperature 200°C. Investigations into
the sensitivity of WO3, Au-WO3 and Pt-WO3 films produced with
PLD method were done by H. Kawasaki et al. [8]. Authors
observed sensitivity four times higher for Au-WO3 and Pt-WO3
films regarding pure WO3 at the temperature 300°C. In all the
cases investigated WO3 films characterized high sensitivity to
various gases at different temperature. These films properties
allowed the temperature modulation and direct WO3 films to
detection of specified gas to be applied. In our previous
investigation [9] we described optimal production conditions of
nanocrystalline WO3 films by means of laser ablation using
Nd-YAG laser. Now we report the influence of the structure on the
gas sensing properties in thin films M-WO3 (M = Au, Pt).
ξ = 45°. Substrate and target were parallel. The deposition
conditions were: a frequency of f = 10 Hz, energy density on the
target ε = 7.8 J/cm2, substrate was heated at 650C, deposition
time t = 30 min. Deposition films of the Au and Pt were on the
thin films after PLD process using Sputter Coated K575X. The
crystalline structure of the target and M-WO3 thin films were
examined by means of the XRD (PANanalytical EMPYREAN DY
1061) with Cu K radiation in Bragg-Brentano and grazing
geometry  = 1°. The surface morphology of the films was
observed by AFM (Veeco DIMENSION ICON-PT). The structure
of the films was observed by TEM (JEOL JEM CX200). The
resistance R of the M-WO3 thin films, in air, CO and NO2
atmosphere, was measured using the two terminal resistance
methods.
RESULTS AND DISCUSSION
First, target phase composition and morphology were examined.
A diffraction pattern of a target is presented in Figure 1. Only
peaks originating from the WO3 phase were identified on the basis
of the card number 04-007-1277 JCPDS. The microstructure from
a laser beam passage is presented in Figure 2. Any cracks and
melted places were not observed, which can suggested the wrong
process parameters.
The impact of process parameters of PLD on the structure of
WO3 layers had been discussed before [9]. The next step was to
produce a WO3 layer with a thin Au and Pt layers. The X-ray
diffraction phase analysis in Grazing geometry (α = 1°) identified
the presence of the WO3 phase (Fig. 3). The identification was
based on the card No. 00-043-1035 JCPDS. Additional peak
generated by gold (04-004-5106 JCPDS) was identified in AuWO3 X-ray patterns (Fig. 3a), while a peak generated by platinum
(01-071-3756 JCPDS) was identified in Pt-WO3 thin films
(Fig. 3b). On the basis of the Williamson-Hall plot it was found out
EXPERIMENTAL STUDY
Thin films were elaborated by PLD technique. Targets were
initially prepared by compacting powders of WO3 under a pressure
of 140 MPa during 5 min. Then the pellets were sintered
at T = 1200°C for 2 hours. The WO3 thin films were deposited on
[100] oriented Si substrates using laser ablation system Nd-YAG
laser Continuum Powerlite DLS (maximum energy 2 J,  = 266 nm,
pulsed duration  = 8 ns) with Neocera chamber. The characteristics of the deposition system were: a target-substrate distance
of 70 mm, the oxygen pressure in the deposition camera –
P = 5 Pa. The laser beam hits the target at an incidence angle
______________________________
Dr hab. inż. Agnieszka Kopia ([email protected]) – AGH-University of
Science and Technology, Faculty of Metals Engineering and Industrial
Computer Science, Dr inż. Wojciech Maziarz – AGH-University of Science
and Technology, Faculty of Computer Science, Electronics and
Telecommunications
Fig. 1. X-ray diffraction patterns of target WO3
Fig. 1. Dyfraktogram rentgenowski powierzchni tarczy WO3
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Fig. 2. SEM micrograph of target WO3 surface
Rys. 2. Mikrofotografia SEM powierzchni tarczy WO3
Fig. 5 AFM three-dimensional images of 500 nm scans for: a) Au-WO3
and b) Pt-WO3
Rys. 5. Mikrofotografia powierzchni wykonana za pomocą AFM, skan
500 nm dla warstw: a) Au-WO3, b) Pt-WO3
Fig. 3. X-ray diffraction patterns for: a) Au-WO3, b) Pt-WO3 in
Graizing α = 1°
Rys. 3. Dyfraktogram rentgenowski warstw: a) Au-WO3, b) Pt-WO3 przy
SKP α = 1°
Fig. 4 Williamson-Hall plot for Pt-WO3 thin film
Rys. 4. Wykres Williamson-Hall dla warstwy Pt-WO3
that the crystallites of WO3 were of 23±3 nm in size (Fig. 4). AFM
examinations showed a very well developed surface of Pt-WO3 and
Au-WO3 layers (Fig. 5). The size of grains on the surface of these
layers fitted within the range from 20 to 30 nm.
The observations of microstructure thin films on cross-sections were
performed by means of transmission electron microscopy (Fig. 6, 7).
Fig. 6 TEM cross sectional image of Pt-WO3 thin film: a) bright field
and b) dark field
Rys. 6 Mikrofotografia TEM przekroju warstwy Pt-WO3 w: a) polu
jasnym, b) polu ciemnym
Observations were performed both in the dark and light field.
The thickness of the WO3 layer and of the platinum layer was
determined on the basis of microphotographs. The obtained layers
were of 180÷200 nm thick and they were characterised by columnshaped structure of 20÷50 nm wide columns grow from
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the substrate up. At the interface of the silicon substrate and the
WO3 layer, one can see 4 nm thick, light and amorphous SiO2
layer. The presence of this layer hinders directional epitaxial
growth films with the substrate orientation. The TEM
microphotographs of the layers cross-section show a platinum
layer. The Pt layer sputtered onto the WO3 layer is 15 nm thick.
During sample preparation for TEM examinations, this layer got
separated from the WO3 layer (Fig. 6). The presence of the Au
layer on the surface of WO3 layers was confirmed by XRD
examinations. However, no clear Au layer was observed during
TEM examinations (Fig. 7).
Next step was to measure resistance of Pt-WO3 and Au-WO3
layers in CO and NO2 atmospheres. Before measurements of
electric properties, the layers surfaces were cleaned of adsorbed
gases. The layers were annealed at the temperature of 450C for
12 h. Some problems with signal instability have been encountered
in Au-WO3 layer what was probably caused by the contact
phenomena. The correct results were obtained for the Pt-WO3
layer (Fig. 8, 9). The results of resistance for Pt-WO3 in CO and
NO2 gases at temperature T = 250, 300°C are presented in Figures
8 and 9. The resistance of the layer increased at the moment of
introducing NO2 into the atmosphere (Fig. 8). Inversely the
resistance of the layer decreased at the moment of introducing CO
atmosphere (Fig. 9).
In the case of reduction of gases, for examples CO, reaction
between CO particles and oxygen ions adsorbed on the surface
thin films (O–(ads.)) takes place. As the results of this reaction the
CO2 particles and free electrons (1) are formed [10]. The effect of
this reaction is decrease in resistance in thin films, which is
observed in Figure 9.

O ads.
 CO  CO 2  e 
before the analysis. This shows that layers surfaces get
regenerated. The stronger signals for both gases were obtained at
higher temperature T = 300°C.
(1)
The reaction time for both gases in Pt-WO3 is several seconds.
When NO2 or CO inflow is cut off and the chamber is blown
through with the air, the resistance returns to the value observed
Fig. 7. TEM cross sectional image of Au-WO3 thin film: a) bright field
and b) dark field
Rys. 7. Mikrofotografia TEM przekroju warstwy Au-WO3 w: a) polu
jasnym, b) polu ciemnym
Fig. 8. Resistance of Pt-WO3 thin film in NO2 atmosphere at
temperature: a) 250°C, b) 300°C
Rys. 8. Rezystancja cienkiej warstwy Pt-WO3 w atmosferze NO2
w temperaturze a) 250°C, b) 300°C
Fig. 9. Resistance of Pt-WO3 thin film in CO atmosphere at
temperature a) 250°C, b) 300°C
Rys. 9 Rezystancja cienkiej warstwy Pt-WO3 w atmosferze, CO
w temperaturz:e a) 250°C, b) 300°C
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CONCLUSIONS
The M-WO3 (M-Au, Pt) thin films were produced by PLD method
for gas sensor application. Structure characterisation was the first
step. The X-ray diffraction phase analysis in grazing geometry
identified the presence of the WO3, Au and Pt phases, respectively.
On the basis of the Williamson-Hall plot we calculated crystallites
size of WO3 which were of 23±3 nm in size. Highly developed
layer surface was observed in AFM microscope. The TEM crosssection observation of the layers showed column-shaped structure
of 20÷50 nm in the width.
The resistance measurements R of the Pt-WO3 thin film, in air,
CO and NO2 atmospheres showed stronger signals for both gases
(NO2, CO) at higher temperature T = 300°C.
So fine crystalline structure, highly developed layer surface and
the presence of platinum on the surface showed that the film is
sensitive to CO and NO2 and the layers surfaces get regenerated.
ACKNOWLEGMENTS
This work was financially supported by the Ministry of Science
and Higher Education through the projects No. 11.11.110.936.
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