One STEP Synthesis of Pure Cubic and Monoclinic HfO2 Nanoparticles: Effects of Temperature and Ambient on the Photoluminescent Properties
Authors: Rauwel Protima , Galeckas Augustinas , Rauwel Erwan 
Published: February 2020

Hafnium dioxide is a wide band-gap, high-k material that has already been integrated into micro-electronic devices. The pure cubic HfO2 phase has been highly investigated because of its higher permittivity (>25). Numerous investigations were performed on the incorporation of an appropriate cation in the aim of stabilizing the cubic or higher symmetry structure. However, the addition of divalent or trivalent dopants induces the modification of the electronic properties of HfO2. Here, we employed a one-pot synthesis approach to produce undoped cubic and monoclinic HfO2 nanoparticles by choice of solvent alone.
The average size of these nanoparticles from transmission electron microscopy studies was estimated to be around 2.6nm. We present a study of the morphology and microstructure and also demonstrate the presence of a strong visible photoluminescence linked to the nanosize of the particles and intrinsic defects. Furthermore, the synthesis in equivalent conditions of these two phases of HfO2 provides means for direct comparison of the chemical composition and electronic structures of the two polymorphs. This has therefore allowed us to experimentally elucidate similarities and differences in the valence band, band gap states, and conduction band of these pure phases seconded by first principles calculations within the density functional theory.
In this work, we have employed synthesis processes that allow the direct precipitation of two HfO2 polymorphs, i.e., monoclinic and cubic. The synthesis of HfO2 in the nanoparticles is of fundamental importance as size reduction and shape engineering allow modification and tuning of the fundamental properties.2 Monoclinic and cubic HfO2 as small as 2.6nm in diameter were synthesized via sol-gel process.3,4 The synthesis procedures were carried out in a glove box using hafnium (IV) tert-butoxide (Hf(OtBu)4) mixed into benzyl alcohol or benzylamine, respectively. The mixture was transferred into a stainless steel autoclave, carefully sealed and then heated in a furnace at 250ºC for 2 days.
The structural properties of these nanoparticles were studied by XRD, TGA, HRTEM and XPS. HRTEM micrographs reveal high crystallinity of the defect free nanoparticles. The equivalent synthesis conditions of the two polymorphs also provided means for direct comparison of their optical absorption and emission properties, which are found to exhibit different behaviors as a function of temperature and ambient environment. The origin of the strong visible luminescence observed from both cubic and monoclinic HfO2 is discussed in terms of surface-defects and nanosize of the particles.5 The comparative analysis of the PL responses of the two polymorphs suggests that emission at 3.1eV originates from the oxygen vacancy related defects. The comparative analysis of the absorption and emission properties of the two polymorphs points towards intrinsic and extrinsic nature of this anomalous luminescence greatly enhanced by nanosize of the particles. The consistency of PL intensity variation with temperature in the case of HfO2cubic nanoparticles offers potential for applications in the field of thermal imaging and optoelectronics.

Acknowledgements
Financial support from Marie Curie (PERG05-GA-2009-249243), the Research Council Norway project 176740/130 and the European Social Fund's Doctoral Studies and Internationalization Program “DoRa” Activity 6 under project 1.2.0201.08-0001 in Estonia is acknowledged
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Figure: HRTEM micrographs of cubic HfO2 nanoparticles. PL spectra of cubic and monoclinic HfO2 nanoparticles at RT in air ambient. Reprinted with permission. Copyright 2012 AIP Publishing LLC. The integrated PL intensity as a function of temperature of cubic and monoclinic HfO2.