Ultrahigh Concentration Hydrogen Doping into TiO₂.

Area of Science:

• Materials Science
• Solid State Physics
• Surface Science

Background:

• Rutile titanium dioxide (TiO₂) is a widely studied material with applications in catalysis and electronics.
• Controlling hydrogen incorporation in TiO₂ is crucial for tuning its electronic and optical properties.
• Previous studies were limited in achieving high hydrogen concentrations in TiO₂.

Purpose of the Study:

• To investigate ultrahigh concentration doping of hydrogen into rutile TiO₂ (100) single crystals.
• To understand the effects of low-temperature hydrogen irradiation on TiO₂.
• To characterize the resulting electronic properties and defect structures.

Main Methods:

• Low-energy hydrogen ion beam irradiation at low temperature (50 K).
• In situ nuclear reaction analysis (NRA) for hydrogen concentration measurement.
• In situ transport and photoemission measurements.
• Density functional theory (DFT) calculations.

Main Results:

• Achieved ultrahigh hydrogen doping concentration up to H_1.2TiO₂, significantly higher than at room temperature.
• Observed a large lattice expansion, indicating hydrogen occupation of interstitial sites.
• Identified that excess hydrogen generates a deeper, metastable in-gap state (IGS).
• Observed unusual electrical transport properties after postannealing.

Conclusions:

• Low-temperature irradiation is an effective method for ultrahigh hydrogen doping in rutile TiO₂.
• The generated metastable IGS, possibly due to double interstitial hydrogens, significantly impacts electrical properties.
• This work provides insights into defect engineering in TiO₂ for potential electronic applications.