- Nano Express
- Open Access
Graphite/InP and graphite/GaN Schottky barriers with electrophoretically deposited Pd or Pt nanoparticles for hydrogen detection
© Zdansky.; licensee Springer. 2012
- Received: 31 May 2012
- Accepted: 14 July 2012
- Published: 23 July 2012
Large attention has been devoted worldwide to the investigation of hydrogen sensors based on various Schottky diodes. We prepared graphite semimetal Schottky contacts on polished n-InP and n-GaN wafers partly covered with nanoparticles of catalytic metals Pd or Pt by applying colloidal graphite. Metal nanoparticles were deposited electrophoretically from colloids prepared beforehand. Deposited nanoparticles were imaged by scanning electron microscopy, atomic force microscopy, and scanning tunneling microscopy on the as-made and annealed-in-vacuum samples. Current–voltage characteristics of prepared Schottky diodes had very high rectification ratios, better than 107 at 1 V. It was shown that the barrier heights of these diodes were equal to the difference between the electron affinity of InP or GaN and the electron work function of the metal Pd or Pt (Schottky-Mott limit). That was a good precondition for the high sensitivity of the diodes to hydrogen, and indeed, high sensitivity to hydrogen, with the detection limit better than 1 ppm, was proved.
- Hydrogen detection
There has been dealing with hydrogen at many places in industry, medicine, and research and recently also for driving automotive vehicles. Hydrogen sensors in such places are needed for safety reasons because hydrogen, which cannot be detected with human senses, has a wide flammable range (4% to 75%) and its easy leakage into the environment forms a dangerous explosive of high power. Another good usage is in a device for detecting leaks in a high-vacuum apparatus. In addition, hydrogen sensors are used inside of various machineries to measure hydrogen concentration, like in various engines using hydrogen fuel. Of course, the last case requires hydrogen sensors which are stable also at high temperatures. Hydrogen monitoring is essential also in various industrial processes where hydrogen can appear via unwanted reactions with water .
Traditional hydrogen detectors are large and expensive, have a slow response, and require much maintenance. Hydrogen sensors based on semiconductor technology are of lower cost, smaller size, faster response, and long-term reliability. There are many different types of hydrogen sensors, which are commercially available or in development. Favored are semiconductor sensor chips to be easily integrated into electronic networks.
InP-based sensors can well operate at room temperature, and sensors based on more expensive GaN are well suited for operations at high temperatures. It is known that high-quality Schottky barriers including an effective catalytic metal like palladium (Pd) or platinum (Pt), prepared on n-type InP or n-type GaN, can detect hydrogen with high sensitivity and fast response . Catalytic metals dissociate hydrogen molecules (H2) to atomic hydrogen (H) which is adsorbed on the semiconductor–metal interface and changes the Schottky barrier height. The concentration of the adsorbed hydrogen is proportional to the hydrogen concentration in the surrounding atmosphere; thus, the barrier height returns to the original value when hydrogen gas is removed. That is different from the case when hydrogen gas permanently changes the Schottky barrier height like in the case of Er on a p-Si Schottky diode . It is generally assumed that hydrogen detection is performed in a way that the adsorbed atomic hydrogen forms a dipole layer that reduces the Schottky barrier height . However, the mechanism of the dipole layer formation by hydrogen adsorption has not yet been fully clarified. A controversial explanation says that the adsorbed hydrogen is polarized by the electric field of the Schottky barrier .
In this paper, the electrophoretic deposition (EPD) of nanoparticles (NPs) of the catalytic metals Pd and Pt and printing colloidal graphite on n-InP and n-GaN to form Schottky barriers highly sensitive to hydrogen is reported. The paper extends our previous studies published recently [4–12].
Pure chemicals for the preparation of catalytic metal NPs in colloid solutions were purchased from Sigma-Aldrich Corporation (St. Louis, MO, USA). One-side-polished wafers of n-InP and n-GaN were purchased from Wafer Technology (Milton Keynes, UK) and Kyma Technologies (Raleigh, NC, USA), respectively. Aqua colloid graphite of Agar Scientific (Stansted, UK) for printing Schottky contacts was purchased from Christine Gropl (Tulln, Austria).
A wafer of InP or GaN was provided with ohmic contact on the unpolished side and placed on the negative electrode (cathode) in the electrophoretic cell. The other electrode (anode) was plane-parallel with the wafer, 1 mm apart from the polished side. The cell was filled with 1 ml of prepared colloid solution with catalytic metal NPs. The EPD was performed with a negative voltage of 100 V applied on the wafer for various time periods in the range of 15 min to 4 h. The voltage was keyed with 10 Hz of frequency and 1:1 duty cycle. The layers of deposited NPs were imaged with SEM, atomic force microscopy (AFM), and scanning tunneling microscopy (STM).
The I V characteristics and current transients of the diodes were not changed when they were measured several months after their fabrication. Recovery transient of the diodes for switching from hydrogen to air flow consisted of two exponentials. The first exponential was fast with the time constant independent of the hydrogen concentration. The second exponential was larger and slower for Pd NPs than for Pt NPs. It can be explained by the release of hydrogen from the crystal lattice of Pd . The size and shape of Pd or Pt NPs after EPD may affect the sensitivity and response times of hydrogen-sensitive diodes. We schedule studying these effects in the near future.
Colloid solutions with Pd and Pt nanoparticles were prepared, and the nanoparticles were electrophoretically deposited on n-InP or n-GaN wafers. Nanoparticle layers were investigated by SEM, AFM, and STM. Schottky barriers were made on surfaces with layers of Pd and Pt nanoparticles by colloidal graphite. Prepared diodes show excellent rectification with Schottky barrier heights virtually equal to Schottky-Mott limits - a good preposition for high sensitivity to hydrogen. Indeed, it was proved that they act as very sensitive and temporally stable hydrogen sensors. Fabrication of such sensors is simple, inexpensive, and giving more sensitive devices when compared with commonly used methods. I believe that the advantage is in the protection of Pd or Pt NPs by AOT reverse micelles against chemical reactions with atoms on the semiconductor surface leading to the formation of unwanted interface states causing Fermi level pinning .
KZ is an emeritus scientist in the Institute of Photonics and Electronics, Academy of Sciences of the Czech Republic. He obtained his Ph.D. degree in solid state physics in 1961. His current research interests include preparation of new semiconductor nanomaterials and investigation of their electromagnetic, optical, and chemical-physical properties.
The author thanks Zdenek Sroubek for the stimulating discussions, Jan Vanis and Ondrej Cernohorsky for providing the microscopic measurements, and Marie Hamplova and Miloslav Fruhauf for the technical help. The work was financially supported by Academy of Sciences of the Czech Republic grant KAN401220801 in the program Nanotechnology for Society, by EU COST Action MP0805 grant OC10021 of the Ministry of Education, Czech Republic, and by grant M100671201 in the program Promoting bilateral international cooperation in the Academy of Sciences of the Czech Republic.
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