Explain the surface-layer insensitivity of TEEY in ZnO–MgO multilayers

Explain why the intrinsic maximum total electron emission yield of the 16/16* and 16*/16 ZnO–MgO heterostructures is nearly identical despite their different surface-layer compositions and surface-layer thicknesses exceeding 2 nm.

Background

The study examines ZnO–MgO heterostructures with alternating atomic-layer-deposited layers and evaluates how composition, microstructure, layer thickness, and surface termination affect total electron emission yield (TEEY). Because TEEY is strongly influenced by surface chemistry, including electron production, electron transport, and work function, changing the terminating layer from ZnO to MgO would ordinarily be expected to alter the measured emission yield.

The 16/16* and 16*/16 samples have different terminating materials—ZnO and MgO, respectively—and each terminating layer is thicker than 2 nm. Nevertheless, their measured intrinsic maximum TEEY values are almost identical. The paper explicitly identifies this observation as unexpected and presently unexplained, leaving the physical mechanism responsible for the apparent insensitivity to surface-layer composition unresolved.

References

Interestingly, the intrinsic maximum TEEY values measured for the 16/16* and 16/16* heterostructures are almost identical, even though the thickness of the top surface layer (ZnO or MgO) exceeds 2 nm. Since TEEY is strongly dependent on surface chemistry (which controls electron emission, transport, and the work function, as highlighted by the effect of carbon contamination in the as-received measurements) this lack of sensitivity to the surface layer composition is unexpected and not explained at this time.

— Tuning the secondary electron yield and electronic conductivity of MgZnO multilayers synthesized by Atomic layer deposition  (2609.36919 - Lafarie et al., 29 Sep 2026) in Section b) TEEY analyses, Results