Materials and Manufacturing Processes, 21: 275–278, 2006
Copyright © Taylor & Francis Group, LLC
ISSN: 1042-6914 print/1532-2475 online
DOI: 10.1080/10426910500464651
Metal Oxide Nanoparticles as Novel Gate Materials
for Field-Effect Gas Sensors
S. Roy
1
, A. Salomonsson
1
, A. Lloyd Spetz
1
, C. Aulin
2
, P.-O. Käll
2
, L. Ojamäe
2
,
M. Strand
3
, and M. Sanati
3
1
S-SENCE and Division of Applied Physics, Linköping University, Linköping, Sweden
2
Physical and Inorganic Chemistry, Linköping University, Linköping, Sweden
3
Division of Chemistry, Växjö University, Växjö, Sweden
Oxide nanoparticle layers have shown interesting behavior as gate materials for high temperature (typically at 300–400
C) metal-insulator-silicon
carbide (MISiC) capacitive sensors. Distinct shifts in the depletion region of the C-V (capacitance-voltage) characteristics could be observed while
switching between different oxidizing and reducing gas ambients (air, O
2
,H
2
, NH
3
, CO, NO
x
,C
3
H
6
). Shifts were also noticed in the accumulation
region of the C-V curves, which can be attributed to the change in resistivity of the gate material. Sensor response patterns have been found to
depend on operating temperature.
Keywords Accumulation region; Adsorption; Capacitance-voltage (C-V); Depletion region; Field-effect; Gas sensors; Gate material; High
frequency; High temperature; Interface; Metal-insulator-semiconductor (MISIC); Nano particles; Ruthenium oxide; Silicon carbide; Transient
response.
1. Introduction
Metal oxide nanoparticles, owing to their very high
surface specific area and versatile catalytic properties, are
exceedingly promising as gas-sensitive materials. When
used as resistive sensing element, oxide nanoparticle layers
exhibit high sensitivity (down to ppb level) and enhanced
selectivity to the target gas [1]. In this project, we explored
the potential of catalytically active oxide nanoparticles to
be used as gate material in field-effect sensor devices.
Two major objectives are: a) to improve the selectivity of
the SiC-based FET sensors by tailoring the dimension and
surface chemistry of the nanoparticles and b) to improve
the high temperature stability, which is often a tricky
issue for the field-effect sensors because of restructuring
of the metal gate [2]. Both semiconducting (e.g., RuO
2
,
Co
3
O
4
and insulating (e.g., -Al
2
O
3
and SiO
2
oxides are
being investigated. In the case of insulating oxides, the
nanoparticles are loaded with catalytically active metals
(e.g., Pt). The semiconducting oxide nanoparticles are
being synthesized by wet chemical procedure while aerosol
technology is adopted to deposit the layers of insulating
oxides.
This article reports on the gas-sensing behavior of RuO
2
nanoparticle layer, acting as the gate of MISiC capacitors.
Both static and transient response characteristics have
been obtained at different temperatures in various gas
ambients. Temperature-dependent response patterns have
been analyzed.
Received June 22, 2005; Accepted November 9, 2005
Address correspondence to S. Roy, Department of Mechanical and
Materials Engineering, Florida International University, 10555 West
Flagler Street, Miami, FL 33174, USA; Fax: 305-348-1932; E-mail:
roys@fiu.edu
2. Experimental
2.1. Methods for Synthesizing RuO
2
RuO
2
nanoparticles were synthesized by wet chemical
procedure. A couple of methods were adopted in order to
obtain particles of various sizes.
2.1.1. Synthesis of rutile-phase RuO
2
nanocrystals using
a strong organic alkali in an alcoholic solution.
Ruthenium trichloride hydrate, RuCl
3
· H
2
O
x
, (0.2036 g)
was dissolved into an alcoholic solvent, containing ethanol
(10 mL) and 2-propanol (10 mL).
Tetrabuthylammonium hydroxide, [CH
3
(CH
2
)
3
]
4
NOH,
TBAH, (2 mL) was added to the solution. The initially
formed precipitate was dissolved during stirring. The
precipitate system was left for 4 h at 90
C and later separated
from the solvent by centrifuging. The precipitates were
washed several times with distilled water, treated with an
oxidizing agent by dropwise adding H
2
O
2
(30%), and then
dried. After drying, the precipitate was heated in an oven at
400
C for 4 h, yielding a black powder [3].
2.1.2. Synthesis of rutile-phase RuO
2
nanocrystals by an
aqueous solution gel route. RuCl
3
(0.2074 g) was mixed
with an aqueous solution of citric acid, C
6
H
8
O
7
· H
2
O,
(0.6304 g) (molar ratio Cit : RuCl
3
3 : 1) under stirring. The
suspension was heated to 40
C and, subsequently, H
2
O
2
(30%) was added. Only small portions of H
2
O
2
were added
each time because the reaction that takes place is highly
exothermic. Upon the addition of 120 equivalents H
2
O
2
(∼11 mL), a clear dark-red solution was obtained with a pH
of 1.9. Refluxing at 90
C for 3 h dropped pH to 1.5 and the
color changed from dark red to dark green. The solution
was poured into a vessel and the water was evaporated in
a furnace at 60
C. A brown gel was formed, which was
heat-treated in an oven at 300
C for 4h [4].
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