A surface work function measurement technique utilizing constant deflected grazing electron trajectories: Oxygen uptake on Cu(001)

Abstract

We report on the application of a novel nondestructive in-vacuum technique for relative work function measurements, employing a grazing incidence electron deflection above a sample with a planar surface. Two deflected electron beam detectors are used as a position sensitive detector to control feedback to the sample potential as the sample work function changes. With feedback the sample potential exactly follows the surface sample-size averaged work function variation, so that the deflected beam trajectory remains stable. We also discuss methods to optimize the initial electron trajectories for this method, so as to minimize unwanted effects such as from uncontrolled external magnetic fields. As the electron beam does not impinge on the surface in this new technique electron induced desorption, ionization, dissociation, and/or decomposition is not induced at the interface. Importantly also the technique allows for free access to the surfaces enabling simultaneous deposition/evaporation and/or application of other surface characterization methods. We demonstrate its application in concurrent measurements of helium atom reflectivity and work function changes taking place during molecular oxygen exposure of a Cu(001) surface. A work function measurement sensitivity and stability is demonstrated at ∼10 mV at a sampling rate of 1 Hz and after application of an ∼7 s smoothing routine. In comparison to the helium atom reflectivity measurements, the work function measurements are more sensitive to the initial O uptake, and less so to the final coverage variations and possible surface reordering at higher O coverages.

Low-energy electron reflectivity of graphene on copper and other substrates

“…3; we refer to the experimental locations of such minima as expt E 1 . We reference these values to the vacuum level position measured in the same spectra, 30,31,32,33 (as further discussed in Section III), our measured values for the work function difference  between the bare metal surface (an oxidized Cu surface in the present case) and the graphene-covered surface, measured values for the work function of the bare metal surface M  from the literature, 34,35,36,37 and the difference    M which corresponds to an experimental value for the work function of graphene on the metal surface. this type of spectrum arises from two layers of graphene on the surface (as identified by prior authors and also as consistent with the analysis of Section III).…”

“…There are many possible sources for these discrepancies: (i) computed work functions from VASP, using GGA, typically underestimate work functions for clean metal surfaces by about 0.3 eV, 45,46 although the corresponding errors for graphene on metal surfaces are not presently known; (ii) for the case of the oxidized Cu(001) surface in Table I the M  value is an estimate, formed by combining values from Refs. [34] and [36]; (iii) the  values in Table I rely on vacuum onsets measured at different points in a LEEM image, and up to a few tenths of an eV error can occur in this difference due to details of the LEEM lens alignment. Unlike the situation for the onsets of the NFE band (second columns of Tables I and II), where the prior experimental values are quite well known and we could simply make a correction to our theoretical values, the sources(s) of the discrepancies between the work function values are less obvious, and hence we do not attempt any sort of correction to account for them.…”

“…3 Table I. Subsequent columns of Table I list measured values from photoemission for the onset of a nearly-free-electron (NFE) band in the substrate 30,31,32,33 (as further discussed in Section III), our measured values for the work function difference  between the bare metal surface (an oxidized Cu surface in the present case) and the graphene-covered surface, measured values for the work function of the bare metal surface M  from the literature, 34,35,36,37 and the difference    M which corresponds to an experimental value for the work function of graphene on the metal surface.…”

“…Our computed work function (difference between vacuum level and Fermi energy) for graphene on Cu(111) is 3.95 eV, listed in the final column of Table II. We compare that value to experiment, as listed in the final column of Table I where we take the difference between the measured bare metal work functions 34,35,36,37 and our values for the difference between the bare metal vacuum level and the vacuum level for graphene on the metal. Comparing the values in the final columns of Tables I and II, we find good agreement for the case of Cu(111) and Ir(111) surfaces, whereas discrepancies are apparent for the other surfaces.…”

Effective Contact Potential of Thin Film Metal-Insulator Nanostructures and Its Role in Self-Powered Nanofilm X-ray Sensors

“…An electric field is generated 9 or X-ray PES, 10 while different methods have been developed to measure the CPD based on Kelvin probe microscopy, 11,12 ionizing radiation, 13,14 or employing grazing incidence electron reflection. 15 It is known that work function depends on many factors, including material crystallization, 16 water uptake, 13 and on surface conditions, 17 tensile, or compressive surface tension, 18 and it is increased by thermal or natural oxidation processes. 19,20 Furthermore, MoO 3 deposition on different metals showed an increase in the metal/insulator WF because of redistribution of electronic states.…”