- Nano Express
- Open Access
Palladium nanoparticles on InP for hydrogen detection
- Ondrej Cernohorsky†1, 2Email author,
- Karel Zdansky†2,
- Jiri Zavadil2,
- Pavel Kacerovsky2 and
- Katerina Piksova1
© Cernohorsky et al; licensee Springer. 2011
- Received: 9 November 2010
- Accepted: 2 June 2011
- Published: 2 June 2011
Layers of palladium (Pd) nanoparticles on indium phosphide (InP) were prepared by electrophoretic deposition from the colloid solution of Pd nanoparticles. Layers prepared by an opposite polarity of deposition showed different physical and morphological properties. Particles in solution are separated and, after deposition onto the InP surface, they form small aggregates. The size of the aggregates is dependent on the time of deposition. If the aggregates are small, the layer has no lateral conductance. Forward and reverse I-V characteristics showed a high rectification ratio with a high Schottky barrier height. The response of the structure on the presence of hydrogen was monitored.
- Schottky Barrier
- Schottky Barrier Height
- Electrophoretic Deposition
- Indium Phosphide
- Negative Deposition
An energetic barrier called the Schottky barrier is formed on the interface of a metal and semiconductor. This barrier shows a rectifying effect like classical PN diodes. This structure could be exploited as a hydrogen sensor, where the sensing mechanism consists in the change of the barrier height by the presence of hydrogen on the interface. A palladium (Pd) was used here for its ability to dissociate hydrogen molecules to single atoms. This fact is further enhanced by the nanoparticle form of Pd because of its high surface-to-volume ratio. An n-type indium phosphide (InP) played the role of the semiconductor here. Electrophoretic deposition of Pd nanoparticles from their colloid solution appeared to be the most convenient method for the interface preparation in the sense of the Schottky barrier height and, therefore, also for the sensitivity of the sensor .
The Schottky barrier height, Φ B, is theoretically given (in the case of n-type semiconductors) by the relation Φ B = Φ M - χS, where Φ M is the work function of metal and χ S is the electronic affinity of the semiconductor. In practice, the Schottky barrier is lower than the difference Φ M - χ S (in the case of n-type semiconductors). This is caused by a Fermi level pinning explained by disorder-induced gap states - electronic states in a bandgap given by a disorder of atoms on the interface . These gap states come from an imperfect metal/semiconductor interface. We can partly eliminate these unwanted gap states by methods of fabrication of the interface. It was found that the sensor has lower sensitivity when a metal is deposited by high energetic means (e.g., thermal evaporation, e-gun) . A more convenient method of interface fabrication is the electrophoretic deposition of metal nanoparticles. The height of Schottky barrier prepared by electrophoretic deposition was 0.85 eV. This height is higher when compared to 829 meV reported in Ref. .
Preparation of Pd nanoparticles
Pd nanoparticles were prepared in reverse micelles by a reduction of palladium(II) chloride by hydrazine according to Wu et al.. Palladium(II) chloride, 0.05 M, and 1 M hydrazine water solutions were prepared. Equal amounts ruled by parameter ω 0, defined as a ratio of molar concentration of H2O and AOT in final isooctane solution, ω 0 = [H2O]/[AOT], were added to two equal amounts of 0.1 M of AOT/isooctane solution. AOT (sodium 1,4-bis(2-ethylhexoxy)-1,4-dioxobutane-2-sulfonate) plays a role of surfactant here. Parameter ω 0 controls the size of the nanoparticles. Here, ω 0 = 5.
In the end, these AOT/isooctane solutions, the first with palladium salt, the second with hydrazine, were mixed. The solution of Pd nanoparticles was monitored by a scanning electron microscope (SEM) (JSM-7500F, JEOL Ltd., Tokyo, Japan). The size of the particles was 10 nm with less than 10% dispersion.
Preparation of layers of Pd nanoparticles on InP
The n-type InP wafers were purchased from Wafer Technology Ltd. (Milton Keynes). The crystallographic orientation of the wafers was 100, the carrier concentration was ≤1016 cm-3, and the E.P.D was ≤1 × 105. The full-area ohmic contact on one side of the wafer was made by an obtrusion of molten Gallium and then by the application of a conductive silver colloid paint.
Electrophoretic deposition was performed in the cell composed of two electrodes; the first is at the bottom of an insulating (Teflon) bottle, and the second is on the top. On the bottom electrode, the InP wafer was mounted by a conductive silver colloid paint, and the colloid solution was placed between the electrodes (the distance between the electrodes was 2 mm). Both polarities of deposition were realized; the negative deposition (ND), where negative voltage was applied to the bottom electrode, and the positive deposition (PD) with the positive voltage to the bottom electrode. The applied voltage was from 30 to 100 V for various times.
After the deposition, the wafer was rinsed with isopropyl alcohol in order to remove residual AOT/isooctane solution, and small contacts by colloid graphite paint were created. Current-voltage characteristics were measured using a Keithley Source-Measure Unit 237 (Keithley, Cleveland, OH, USA).
An AFM profile measurement of the PD layer showed that nanoclusters on the surface are separated, and the distance between them is about 100 nm. This fact explains a significant lateral resistance when the distance between clusters is too large to enable quantum-mechanical tunneling. The size of aggregates grows with deposition time. The particles probably settle selectively in the vicinity of the previously deposited particles because of the higher gradient of the electric field. When the deposition time is long enough, e.g., 18 h, the layer turns to be laterally conductive.
Colloid solutions of the Pd nanoparticles of size 10 nm were prepared, and electrophoretic deposition of these nanoparticles onto the InP wafer was performed. The PD layers had more Pd on the semiconductor surface while the ND layers had a cleaner surface without a surplus of AOT molecules, and the surface is not fully covered. The Schottky barrier height of the ND layers was 0.85 eV. When compared to Ref. , the height of the Schottky barrier of the structure presented in this work was about 0.26 eV higher, and the sensing capabilities were more than 100 times better (0.1 forward voltage, 0.1% H2). We suppose the main improvement consist in using the ND deposition instead of the PD reported in Ref.  and in tuning the deposition time to prepare the appropriate porous layer. The colloid graphite also has an indispensable influence on sensing quality due to its porosity for hydrogen molecules.
The work has been financially supported by grant KAN401220801 of the Academy of Sciences of the Czech Republic, as well as grant no. 102/09/1037 of the Czech Science Foundation.
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