# Dynamic Change of the Work Function of Plasma
Electrode Materials due to Hydrogen Plasma exposure

Mamiko Sasao, Juan-Pablo Broude, Gilles Cartry, Jean-Marc Layet, Haruhisa Nakano, Mayuko Nishiwaki, Motoi Wada

► To cite this version:

Mamiko Sasao, Juan-Pablo Broude, Gilles Cartry, Jean-Marc Layet, Haruhisa Nakano, et al. Dynamic Change of the Work Function of Plasma Electrode Materials due to Hydrogen Plasma exposure. 20th International Conference on Ion Sources, Sep 2023, Victoria (B.C.), Canada. pp.012028, 10.1088/1742-6596/2743/1/012028. [https://doi.org/10.1088/1742-6596/2743/1/012028](https://doi.org/10.1088/1742-6596/2743/1/012028)

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# Dynamic Change of the Work Function of Plasma Electrode Materials due to Hydrogen Plasma exposure

To cite this article: Mamiko Sasao et al 2024 J. Phys.: Conf. Ser. 2743 012028

View the [article online](#) for updates and enhancements.

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# Dynamic Change of the Work Function of Plasma Electrode Materials due to Hydrogen Plasma exposure

Mamiko SASAO¹, Juan-Pablo BROUDE², Gilles CARTRY², Jean-Marc LAYET², Haruhisa NAKANO³, Mayuko NISHIWAKI⁴, Motoi WADA⁴

1. Organization for Research Initiatives and Development, Doshisha University, Kyoto 602-8580, Japan

2. Aix-Marseille Université, CNRS, PIIM, UMR7345, F-13013 Marseille, France

3. National Institute for Fusion Science, 322-6 Oroshi-cho, Toki 509-5292, Japan.

4. Graduate School of Science and Engineering, Doshisha University, Kyotanabe, 610-0394, Kyoto, Japan.

Corresponding author : msasao@mail.doshisha.ac.jp

Abstract. It has been known that the work function of plasma electrode (PE) surfaces such as Cs-covered Mo and C12A7-electride can be decreased when the surfaces are immersed in hydrogen (or deuterium) plasma. We discovered that the work function of the C12A7-electride changed right after the termination of plasma exposure with the time constant of a few minutes. This phenomenon was not observed on the surface of clean metals (Mo or Ta). By extrapolating the dependence of work function against time to determine the value at the plasma termination, the work function during plasma exposure was estimated to be 2.4 – 2.5 eV, which can be compared with the value of the electride exhibiting metallic conductivity.

## 1. Introduction

The work function of plasma electrodes (PE) in negative hydrogen ion sources plays an important role to increase the H/D⁻ yields, and cesium (Cs) is currently used to decrease the work function. It has been known that a half monolayer (or less) Cs coverage on a Mo surface realizes the minimum work function, but it can be further decreased when the surface is immersed in a hydrogen plasma [1]. In addition, decreases of caesiated surface after hydrogen discharge treatment was observed [2].

Hydrogen doping on a complex surface has attracted theoretical studies. For example, influence of the adsorption of H on Cs/W (110) was studied by performing DFT based total energy calculations, showing the adsorption energies of both H and Cs decrease in the presence of the other [3].

Recently, a new material, C12A7-electride [4, 5] has been studied as one of the promising candidate PE materials for Cs-free negative hydrogen ion sources [6 - 8], and the dynamical work function changes of this material after hydrogen plasma treatment was reported [9, 10]. It was found that positive or negative electrical bias applied with respect to the plasma affected the changes [10].

<figure><img src="image_4.png" /></figure>
In order to clarify the mechanism that changes negative ion yields and the work function of C12A7-electride due to plasma exposure, a series of experiments were carried out using a surface-produced negative ion measurement system and a photoelectron yield spectroscopy system installed on the diffusion plasma reactor at PIIM Aix-Marseille University [6, 11, 12]. Dynamic changes in negative ion yield and energy distribution were reported, and H⁺/D⁺ and H⁰/D⁰ bombardment effects were clearly seen in the H/D⁻ yields [6, 13]. In the present paper, we report unique phenomena of rapid and dynamic time change of the work-function of C12A7-electride, of the time constant of few minutes observed after the plasma turn-off.

## 2. Experiments

### 2.1. Measurement System

The plasma was generated at 2 Pa by inductive coupling from an external antenna using an 800W, 13.56 MHz RF power source. The sample holder was inserted by a UHV transporter, and fixed at the center of the chamber, which was separated from the plasma driver region by a metallic mesh, facing towards a Hiden-EQP300 mass spectrometer equipped with an energy filter. The sample could be biased negatively by an external DC power supply, and could be heated by a resistive heater embedded inside the sample holder, and the sample temperature was monitored by a thermocouple. Without external heating, the sample temperature was maintained below 60 degree because the impinging ion flux was low enough. The sample holder can be rotated by 180 degree to face toward the photoelectron-emission yield spectroscopy (PYS) system to measure the work function. The PYS setup consists of a 300 W Xenon lamp combined with a Zolix monochromator to generate a monochromatic light with the wavelength width about 10 nm. The photon beam was focused to form a 3-mm-diameter spot on the sample. The light wavelength was scanned from 200 to 600nm (6 to 2 eV), and for each wavelength the sample current was acquired by a Keithley 616 electrometer and sent to a computer through an amplifier giving the so-called PYS curve [9,13]. The scan took about 170 seconds, which can be diminished by scanning a narrower wavelength range. Without light, the sample leakage current was about 30 fA and was subtracted from PYS curves.

### 2.2. Determination of Work function

A Fowler plot, which was based on a theoretical description developed for the temperature effect on the photoelectric sensitivity of a clean metal near the threshold, is widely used to determine a surface work function, $\phi_w$ [14]. Here photoelectron current is measured by scanning mono-energetic photons, $h\nu$. A simplified relation, such as the following, is often used.

$$Y \propto T^2 \mu^2 \propto (h\nu - \phi_w)^2 \qquad \text{Eq. (1)}$$

where Y is the photoelectric yield (PY), i.e. photoelectron current at $h\nu$ divided by the relative light intensity at $h\nu$, T is the surface temperature, and

$$\mu = \frac{h\nu - \phi_w}{k_B T} \qquad \text{Eq. (2)}$$

This relation can be used when $\mu > 5$.

However, Lange et al. has shown that the photoelectric yield should be fitted to a power law with variable exponent, n, in case of the presence of an adsorbate on a metal surface [15].

$$Y \propto (h\nu - \phi_w)^n \qquad \text{Eq. (3)}$$

They concluded that the apparent discrepancies between the Kelvin work function measurement and that of photoelectric measurement for oxygen-covered magnesium was resolved through this data treatment.

Considering that the electron density of the C12A7 sample surface may be much less than that of metals, and that some possible impurity adsorbent such as oxygen still remains, a least-square fitting
procedure has been carried out to determine the best-fit exponent “n” in Eq. (3), for several PYS curves in the present work. One example is shown in Fig. 1. The exponent ranges 2.4 – 2.8 when the $\phi_w$ value ranges 2.4 eV – 3 eV.

<figure><img src="image_5.png" /><figcaption>Fig. 1. An example of photoelectric yield spectroscopy curve (PYS). (a). The photoelectric yield (Blue) and that after subtraction of the background (red). (b) The exponent of photoelectric yield was determined from PY in 370 – 430 nm by least-squares fitting as shown in a red solid line. The exponent was 2.7 ± 0.6, and the $\phi_w$ was 2.7 ± 0.1 eV.</figcaption></figure>

## 2.3. *Dynamic change of photoelectron currents and work functions.*

A high-quality H/D' production surface of a C12A7-electride is usually realized by baking and/or immersing in an Ar or H/D plasma [8, 9, 10]. During this experimental campaign, we measured not only the energy spectra of negative ions emitted perpendicularly from the surface, but also the PYS curves one after the other [16].

The correlation between the negative ion yield and the work function was reported at ICPIG 2023 [16]. As to PYS curves, we found that both the threshold $h\nu$ and photoelectron current were changing after the termination of the D₂ plasma, as shown in Fig. 2a. Behaviors the PYS curves of Mo and Ta surfaces were measured as the references. They did not change after the plasma termination (Fig. 2b,2c).

In order to record the time dependent change of work function and extrapolate towards the value of the plasma phase, a monochromatic light of 380 nm, 400 nm, or 420 nm was injected, and the photoelectric current changes after the D2 plasma termination of 150 W and 800 W. There was a time interval of 17–20 seconds between the plasma termination of the plasma and the start-up of the photoelectric measurement, and the time jitter was corrected. As can be seen in Fig. 3a, a rapid decay of photoelectric current at each wavelength was observed. Assuming Eq. (3) and the exponent “n” of 2.7, the work function was calculated from the photoelectric currents of 3 photon energy by least-squares fitting, as can be seen in Fig. 3b. For both samples, the work function values gradually increased, and the extrapolation back to the plasma termination point yielded the value 2.4–2.5 eV, which is the value reported as the one exhibiting metallic conductivity [3,4].

## 3. Discussions and Summary

Photoelectric current from C12A7-electride decreases rapidly after the termination of plasma exposure for a time constant of 1–3 min. Changes of the work function were obtained from the photoelectric currents of 3 photon energy by least-squares fitting.
<figure><img src="image_6.png" /><figcaption>Fig. 2 Changes of photoelectric current wave length scan after the termination of 150 W, D₂ plasma at 2 Pa, from C12A7-electride (a), Tantalum (b) and Molybdenum (c) surfaces. The black line (a) is the photoelectric current scan before the plasma exposure.</figcaption></figure>

<figure><img src="image_7.png" /><figcaption>Fig.3 (a)Photoelectric yield changes after termination of 800 W D₂ plasma (upper) and 150 W plasma (lower) for the injection of monochromatic light of 380 nm, 400 nm, and 420 nm. (b) Changes of work function calculated by least-squares fitting to the photoelectric yields shown in (a). Error bars are fitting errors of the least-squares method.</figcaption></figure>
Work function values were gradually increasing, but by extrapolating to the plasma termination time, values are approaching to about 2.4–2.5 eV, which is the value reported as the one exhibiting metallic conductivity [4, 5].

Possible interpretation of work function lowering in a H/D plasma is an effect of hydrogen atoms ($H^0$ or $D^0$). (a) They can react with impurities on the C12A7-electride surface, such as oxygen, $O_2$, $O^+$, and/or $OH^-$, remove them, and a clean electride surface may appear. (b) Hydrogen atoms on the top surface might change the property similarly to the case of adsorption on Cs/W(110) [3]. Another interpretation is the intrinsic change of C12A7-electride. Sushko et al. theoretically investigated the geometric structures and electron density distributions of C12A7:($O^2_2$), C12A7:($H^+_4$), C12A7:($H^+_3(e^+)$) and C12A7:($e^+_4$) [17], but the theoretical potential energy formed in the H-doped C12A7-electride was predicted much below the cage conduction band. We found the reversion of improved work function is a process of the time constant of few minutes. In order to understand the hydrogen plasma exposure effect, experimental observation of reversion under various circumstances are waited.

## ACKNOWLEDGMENTS

The C12A7-Electride was supplied from Asahi Glass Co. LTD. (AGC), and deep discussion with Naomichi Miyakawa, Satoru Watanabe, and Yoshitake Toda of AGC are greatly acknowledged. The authors acknowledge support and advice from Hideo Hosono. The project received funding of the Grant-in-Aid for Scientific Research of JSPS (19H01883 and 23H01158), and funding from the Initiative of Excellence of Aix-Marseille University—A*Midex, Investissements d'Avenir Program AMX-19-IET-013. This project has also been supported by the Région Sud PACA through the Appel 'a Projets Recherche 2021—Volet exploratoire—Project SARDINE.

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