Phase-Specific EELS-Derived Optical Onsets and Photoluminescence in α-MoO3 and Orthorhombic γ-Mo4O11

Abstract

Reported optical-gap-related values for molybdenum oxides show substantial dispersion partly because spatially averaged optical measurements may include contributions from different MoOx phases within the probed region, and partly because the resulting spectra are interpreted using nonequivalent analytical procedures. Here, we combine local micro-Raman spectroscopy, site-specific transmission electron microscopy/selected-area electron diffraction (TEM/SAED), low-loss electron energy-loss spectroscopy, and room-temperature photoluminescence to correlate phase assignment and optical response in α-MoO3 and orthorhombic γ-Mo4O11 domains produced under the same synthesis conditions. Phase-assigned α-MoO3 regions exhibit local low-loss Electron Energy Loss Spectroscopy (EELS)-derived optical onsets of 3.35–3.63 eV, whereas orthorhombic γ-Mo4O11 regions yield values of 3.18–3.37 eV. Because the same onset-extraction procedure is applied to both phases, these measurements provide an internally consistent, phase-specific comparison that does not require a priori direct or indirect Tauc exponent. In addition, γ-Mo4O11 displays a relatively narrow photoluminescence emission near 2.6 eV that is absent or strongly suppressed in α-MoO3. These results provide phase-specific and internally consistent local optical-onset reference ranges for two structurally identified phases within the polymorphic MoOx system, helping to distinguish genuine phase-dependent behavior from discrepancies arising from phase coexistence, spatial averaging, and nonequivalent analysis methods. The similarity of their high-energy optical response may be related to the extended MoO3-like octahedral blocks present in γ-Mo4O11, although this remains a structure-based hypothesis requiring further theoretical and atomic-scale investigation.