[Paper Review] The Effect of Annealing Temperature on Statistical Properties of $WO_3$ Surface

G. R. Jafari, Abbas Ali Saberi+3 more|ArXiv.org|Apr 8, 2007

Surface Roughness and Optical Measurements1 references4 citations

TL;DR

This study investigates how annealing temperature affects the statistical morphology of WO₃ thin films using atomic force microscopy (AFM) and two analysis methods: structure function and level crossing. It identifies 400 °C as the optimal annealing temperature, where surface roughness and effective area peak (2× the as-deposited area) without compositional change, indicating a phase transition from amorphous to crystalline structure that enhances gas sensing and catalytic performance.

ABSTRACT

We have studied the effect of annealing temperature on the statistical properties of $WO_3$ surface using atomic force microscopy techniques (AFM). We have applied both level crossing and structure function methods. Level crossing analysis indicates an optimum annealing temperature of around 400$^oC$ at which the effective area of the $WO_3$ thin film is maximum, whereas composition of the surface remains stoichiometric. The complexity of the height fluctuation of surfaces was characterized by roughness, roughness exponent and lateral size of surface features. We have found that there is a phase transition at around 400$^oC$ from one set to two sets of roughness parameters. This happens due to microstructural changes from amorphous to crystalline structure in the samples that has been already found experimentally.

Motivation & Objective

• To determine how annealing temperature influences the statistical surface properties of WO₃ thin films.
• To identify the optimal annealing temperature that maximizes surface area and roughness for enhanced gas sensing and catalytic performance.
• To apply AFM-based statistical methods—structure function and level crossing—to extract morphological parameters efficiently without complex modeling.
• To correlate microstructural changes (amorphous to crystalline) with measurable statistical surface properties.

Proposed method

• AFM imaging was performed on WO₃ thin films annealed at 200–500 °C to extract surface height data.
• Structure function analysis was used to compute roughness exponent (α), roughness (σ), and correlation length (Cs* and Cl*) to characterize surface heterogeneity.
• Level crossing analysis quantified the total number of positive-slope crossings (N⁺_tot) to estimate the effective surface area.
• The normalized N⁺_tot was fitted to quadratic functions to identify the annealing temperature with maximum effective area.
• XPS and UV-Vis spectroscopy confirmed stoichiometric composition and optical properties remained unchanged across annealing temperatures.
• Statistical parameters were compared across annealing temperatures to detect phase transition points.

Experimental results

Research questions

• RQ1 At what annealing temperature is the effective surface area of WO₃ thin films maximized?
• RQ2 How does annealing temperature influence the roughness exponent and lateral size of surface features in WO₃ films?
• RQ3 Does a phase transition from amorphous to crystalline structure in WO₃ correspond to a change in statistical surface properties?
• RQ4 Can level crossing and structure function analyses reliably predict optimal annealing conditions without extensive experimental validation?
• RQ5 Is the optimal annealing temperature for surface morphology consistent with preserved stoichiometry and composition?

Key findings

• The effective surface area of WO₃ films reaches a maximum at 400 °C, approximately 2.00 times larger than the as-deposited film (27 °C), as determined by level crossing analysis.
• A distinct phase transition occurs at 400 °C, marked by a shift from one set to two sets of roughness parameters, indicating a structural change from amorphous to crystalline phase.
• The roughness (σ) increases from 0.91 nm at 27 °C to 17.00 nm at 400 °C, peaking at this temperature before declining at higher temperatures.
• The correlation length (Cs* and Cl*) increases with annealing temperature, indicating larger lateral features, with values rising from 60 nm (27 °C) to 300 nm (400 °C) and further to 1400 nm (500 °C).
• The roughness exponent (α) changes from 0.15 (27 °C) to 0.40–0.15 at 400 °C, indicating a shift in surface growth dynamics post-phase transition.
• XPS analysis confirmed no compositional changes or formation of Magneli phases up to 500 °C, validating that the optimal performance at 400 °C is due to morphological rather than chemical changes.