Standards Development

Scope

This technical specification offers an overview of the measurement of the work function (WF) using ultraviolet photoelectron spectroscopy (UPS). It covers the selection of an appropriate spectrometer for WF measurement, calibration of the UPS energy scale, optimization of sample bias, determination of the secondary electron cutoff, and consideration of associated uncertainties. The RRT study for WF measurement utilizes samples that are both electrically conductive and flat across the observed area, including examples such as a gold (Au) film on a silicon wafer and highly-oriented pyrolytic graphite (HOPG).

This technical specification provides guidelines for measuring the work function (WF) using ultraviolet photoelectron spectroscopy (UPS). The scope is limited to electrically conductive and uniformly flat samples. This specification covers the following key aspects: (a) an overview of WF measurement, (b) sample preparation, (c) spectrometer settings, (d) sample bias optimization, (e) data analysis, (f) associated uncertainties, (g) results from round-robin tests (RRT), and (h) references. The RRT results confirm the consistency and reliability of WF measurements on gold (Au) films on silicon wafers and highly-oriented pyrolytic graphite (HOPG) across multiple laboratories.

Purpose

The work function (WF) of a material is defined as the potential difference for electrons between the Fermi level and the maximum potential just outside a specified surface, representing the minimum energy required to release an electron from the material. Measuring the WF is crucial, as it directly influences the emission and injection behaviour of charged particles at solid interfaces. This property plays a significant role in various applications such as electronic devices, catalysis, and energy conversion systems.

Given that the WF is inherently a surface-specific property, it is highly sensitive to factors such as crystal orientation and surface treatment conditions. The WF value can therefore vary significantly depending on these surface characteristics. This variability underscores the need for reliable and standardized measurement methods that ensure accurate and consistent WF data across different research and industrial settings.

Among the numerous techniques available for measuring WF—such as thermionic emission, field emission, photoelectric effect, and Kelvin probe, photoelectron spectroscopy (specifically ultraviolet photoelectron spectroscopy, or UPS) is considered the most widely adopted. However, to achieve reliable and comparable results across laboratories, a standardized procedure for measuring WF using UPS is essential. The Versailles Project on Advanced Materials and Standards (VAMAS TWA2) conducted an interlaboratory study to address these challenges and establish robust measurement protocols. The study found that a gold (Au) film exhibits a WF of 5.40 ± 0.13 eV, while highly-oriented pyrolytic graphite (HOPG) has a WF of 4.62 ± 0.16 eV, both measured using UPS. Along with their defined surface preparation protocols, calibrating and validating spectrometer settings provide reliable WF measurements. Such standardized approaches are essential not only for academic research but also for the advancement of practical technologies relying on WF data.