Terahertz chemical microscopy (TCM) was applied to visualize the distribution of the work function shift of catalytic metals under hydrogen gas. TCM measures the chemical potential on the surface of a SiO2/Si/sapphire sensing plate without any contact with the plate. By controlling the bias voltage between an electrode on the SiO2 surface and the Si layer, the relationship between the voltage and the THz amplitude from the sensing plate can be obtained. As a demonstration, two types of structures were fabricated on the sensing plate, and the work function shifts due to catalytic reactions were visualized.
© 2012 OSA
In recent years, the use of sustainable energy, including solar power, wind power, and fuel cell energy, has become crucial because of the ongoing depletion of fossil energy sources. Hydrogen energy shows promise as one of these potential sustainable resources because it is essentially zero emission, and an inexhaustible supply of hydrogen gas can be produced from the ocean. However, hydrogen gas explodes easily when the hydrogen volume concentration in air exceeds 4.65 vol.%. Safer use of hydrogen energy requires the development of highly sensitive hydrogen sensors and the detection of hydrogen gas leaks at an earlier stage.
The field effect transistor type of hydrogen sensor with a gate electrode made of a catalytic metal film is promising because of its low energy consumption and rapid response compared to resistive gas sensors [1–3]. When hydrogen gas is adsorbed and dissociated on the catalytic metal, the chemical potential of the metal shifts, which changes the threshold voltage of the transistor. To develop these gas sensors, the work function shifts of catalytic metals under hydrogen gas exposure must be measured . The Kelvin probe microscope  is a useful tool for obtaining the distribution of the work function on the metal surface. It is a scanning probe type of microscope that uses a conductive cantilever for scanning. Therefore, it can evaluate only the surface side of the gate electrode.
On the other hand, a laser terahertz (THz) emission and detection technique  was recognized as a useful tool for evaluating the local field in semiconductor devices such as large scale integration circuits [7–10]. This type of THz technique can realize higher spatial resolution than standard THz imaging systems because the spatial resolution is not determined by the wavelength of the THz pulse but by that of the femtosecond laser, which is typically around 790 nm. Our group demonstrated the detection of the work function shift in a catalytic metal by fabricating a Pd/SiO2/Si layered structured and measuring it using the laser terahertz emission technique. However, the laser irradiates the sample from the metal side, and the system cannot be used for a thick Pd sample or when the Pd surface is covered by another layer, because the laser was reflected and/or absorbed before photons reached the Si layer.
THz chemical microscopy (TCM) was recently proposed and developed using the laser THz emission technique in order to visualize the chemical potential distribution on SiO2/Si films on a transparent substrate [11–14]. In this type of system, the laser is introduced from the substrate side of the plate; consequently, the chemical potential on the SiO2 surface can be measured.
In this letter, we applied TCM to visualize and evaluate the chemical potential shift of catalytic metals and the insulator layer. As a demonstration, two types of structures were fabricated on the sensing plate, and the potential distributions were visualized.
Figure 1(a) shows a schematic diagram of the sensing plate that was used as the glass slide in TCM. It was prepared by depositing a Si thin film on a sapphire substrate and forming a SiO2 insulator layer on the Si layer by thermal oxidation. The thickness of the Si and SiO2 thin films were 150 nm and 275 nm, respectively. The size of the plate was about 150 mm × 150 mm. Similar structures have been reported elsewhere . Figure 1(b) shows a schematic of the band diagram of the sensing plate. Because of defects around the boundary between the Si thin film and the SiO2 layer, a depletion layer was formed, which gives rise to a local electric field around the boundary. When femtosecond laser pulses having sufficient photon energy compared to the band gap of the Si were focused and introduced to the Si layer from the substrate side of the sensing plate, the carriers were excited and accelerated by the local field of the Si layer. According to Maxwell’s law, electromagnetic pulses can be generated and radiated to free space by modulating the carriers. The electric field of the electromagnetic pulses at the far field, ETHz, is proportional to the time derivative of the current density J produced by carrier modulation. It can be described as15]. Therefore, the changes in the chemical and electric potentials at the surface of the sensing plate can be measured as changes in the amplitude of the electromagnetic pulses. The frequency component of the electromagnetic pulses depends on the width of the laser pulse and/or the relaxation time of the excited carriers; it is generally in the THz frequency region.
The optical setup for TCM was similar to that for conventional laser THz emission microscopy. Femtosecond laser pulses were focused and introduced to the sensing plate by an objective lens at an incident angle of 45°. The radiated THz pulses were collimated and focused on the THz detector by a pair of off-axis paraboloidal mirrors. The central wavelength of the femtosecond laser was 780 nm, and the repetition rate was 82 MHz. The average laser power used to illuminate the sensing plate was about 120 mW. A bow-tie type of photoconductive antenna made from the low-temperature grown GaAs thin film was used as the THz detector. Laser pulses were also introduced to the photoconductive antenna in order to adjust the detection timing of the THz pulses. In our experiments, the detection timing was fixed at where the peak amplitude was observed. Note that we confirmed that the peak position of the THz pulse radiated from the each corner of the sensing plate was observed at the same timing. The chemical potential distribution can be visualized by scanning the surface of the substrate side of the sensing plate using the laser. The spot size of the laser, which determines the spatial resolution of TCM, was about 1 mm in diameter for our measurements.
3. Results and discussion
To evaluate the dependence of the electric potential change on the THz pulses, a Ti thin film was deposited on the SiO2 surface of the sensing plate, and a voltage was applied between this film and the Si layer in the sensing plate, as shown in Fig. 2(a) . A silver paste was used to connect the electric wires to the Si layer and Ti thin film. Figure 2(b) indicates the peak amplitude of the THz pulses as a function of the bias voltage. The peak THz amplitude increased with increasing bias voltage and gradually saturated. On the other hand, the peak THz amplitude decreased with decreasing bias voltage, and the amplitude became negative after reaching zero at a bias voltage of −5.3 V. These changes in amplitude demonstrate that the energy band was bent as the bias voltage changed and became turned in the opposite direction at bias voltages below −5.3 V. Although the bias voltage and the THz pulse amplitude did not exhibit a linear relationship, this result indicates that the electric potential shift at the surface of the sensing plate could be evaluated using the change in the THz pulse amplitude.
Figure 3(a) shows a schematic of the test sensing plate. The plate had four areas with different structures on the SiO2 surface: the Ti layer, Pt/Ti layers, Pt layer, and Pt layer covered by epoxy resin. The Ti and Pt layers were each 50 nm thick. The epoxy resin was used to protect the Pt surface from exposure to hydrogen gas. The Ti single layer was electrically grounded using silver paste and an aluminum wire. Figure 3(b) shows a TCM image of the test sensing plate in a N2 gas atmosphere. The THz amplitude was enhanced where the metal layers were deposited. Figure 3(c) shows a differential image comparing TCM images obtained in the N2 gas and a 1% H2 gas atmosphere. Although all the layers were in electrical contact, the THz amplitude was enhanced only in the region where the single Pt layer existed. This result indicates that the hydrogen gas was adsorbed and dissociated at the surface of the Pt layer, and the chemical potential of the Pt surface was shifted. However, the shifts in the chemical potential in each region were independent of each other, which could not be explained by the bulk model, in which the Fermi levels remain flat when the two metals are in contact and the chemical potential, therefore the peak amplitude of THz pulse, of each region should be the same value. This result also suggests that an electropotential difference occurs in the Pt layers covered and uncovered by epoxy resin under exposure to H2 gas, which is consistent with the behavior of a device reported elsewhere .
Figures 4(a) and 4(b) show another type of sensing plate consisting of Pt/Nafion® layers and a differential image comparing TCM images obtained in air and in a 1% H2 atmosphere, respectively. Nafion® is a conventional proton-conductive membrane; therefore, H2 gas is dissociated at the Pt thin films and recombined after transmission inside the Nafion® film. An inhomogeneous reaction of the H2 gas was observed by TCM. This result suggests that proton transmission in the Nafion® film was inhomogeneous because the film was non-uniform, and that TCM can be used to evaluate proton-conductive films that are used in hydrogen sensors and fuel cells.
To achieve higher spatial resolution than the image in Fig. 3 and Fig. 4, the combination of the beam expander and the infinity corrected objective lens were introduced to the TCM. Figure 5 shows the TCM image at the boundary between the SiO2 region and the Pt on SiO2 region and its cross-section. By assuming the Gaussian beam profile of the excited laser, the spatial resolution could be estimated to be about 57 μm. The observation of fine structures in the sensing plates with the Nafion® films is underway.
TCM was used to investigate the chemical potential shift of catalytic metals. The THz amplitude from the sensing plate could be controlled by changing the bias voltage between the metals on the plate surface and the Si layer. We fabricated two types of test sensing plate using Pt thin films as the catalytic metal. The results suggest that TCM is a useful tool for evaluating the reactions of catalytic metals used in gas sensors and/or fuel cells.
This study was partially supported by the Industry-Academia Collaborative R&D of the Japan Science and Technology Agency (JST).
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