Article pubs.acs.org/JPCC
Calorimetric Measurements of Surface Energy of Amorphous HfO2 Nanoparticles Produced by Gas Phase Condensation Geetu Sharma, Sergey V. Ushakov, Hui Li, Ricardo H. R. Castro, and Alexandra Navrotsky* Peter A. Rock Thermochemistry Laboratory, NEAT-ORU, University of California Davis, Davis, California 95616, United States S Supporting Information *
ABSTRACT: Thermodynamics of nanomaterials is strongly influenced by the energetic contribution from atoms located at the interfaces. Therefore, accurately assessing the surface energy of nanomaterials is essential for calculating and predicting thermodynamic properties. In the present work, surface energy of amorphous hafnium oxide (am-HfO2) nanoparticles was measured using independent calorimetric techniques. am-HfO2 nanoparticles were synthesized by condensation from a gas phase generated through laser evaporation of bulk HfO2 targets at 0.1 Torr oxygen pressure. Their surface energy was directly measured using hightemperature oxide melt solution calorimetry, differential scanning calorimetry, and water adsorption calorimetry. The measured surface energies using the above techniques, respectively, are 0.76 ± 0.12, 0.47 ± 0.2, and 0.59 ± 0.1 J/m2. The differences among the surface energy values are about 0.3 J/m2, which is generally within the experimental uncertainties and different assumptions for each technique. The surface energy of the amorphous phase is substantially smaller than that of crystalline phases, as seen previously for other oxides. Thus, the amorphous phase may be thermodynamically favored when small particles are produced and retained.
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INTRODUCTION During the past decades, hafnium oxide HfO2 has attracted interest because of its unique and superior properties such as large heat of formation, high neutron absorption coefficient, relatively high dielectric constant (k ≥ 16) with optical bandgap larger than 5.7 eV and high breakdown field (3.9−6.7 MV/cm), and low thermal conductivity.1−5 In order to reduce standby power and improve performance in metal oxide semiconductor field-effect transistors, low gate leakage current and aggressive scaling are required.6 In general, amorphous high-k materials are preferred over crystalline phases due to isotropic dielectric constant and minimal interfacial defects. Most importantly, such amorphous materials also lack the parasitic conduction paths that occur at the grain boundaries. 7,8 However, one of the requirements for applications of amorphous high-k materials is to limit crystallization during processing stages.9 From a thermodynamic perspective, the stability against crystallization is related to the transformation free energy, which at the nanoscale is directly associated with the surface energetics. Because amorphous phases typically show low surface energies, amorphous nanoparticles should show enhanced stability as compared to crystalline nanoparticles, as observed for ZrO2 for instance.10 In the present work, amorphous HfO2 nanoparticles were synthesized by condensation from a gas phase generated through laser evaporation of bulk HfO2 targets in a controlled oxygen environment. We synthesized amorphous HfO2 nanoparticles in a water-free environment to minimize any © XXXX American Chemical Society
adsorption of water molecules or gases from the outside atmosphere. Subsequently, the surface energy of amorphous HfO2 nanoparticles was measured using various calorimetric techniques: differential scanning calorimetry, water adsorption calorimetry, and high-temperature oxide melt solution calorimetry.
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EXPERIMENTAL PROCEDURES Synthesis. Amorphous anhydrous HfO2 nanoparticles were synthesized by gas phase condensation using a custom-made chamber designed for unloading inside a glovebox, which is described elsewhere in detail.11,12 First, the HfO2 target (CERAC Incorporated, HfO2 white, 99.9% pure, ∼17 mm in diameter, ∼5 mm thick tablets) was placed onto the sample holder and the synthesis chamber was tightened properly. Prior to evaporation, the synthesis chamber was evacuated below 9 × 10−3 Torr using an oil-free scroll pump and subsequently filled with oxygen gas (99.9%) to 0.1 Torr. The CW-CO2 laser beam was set to 40 W and focused on the target rotating at 6 rpm. The evaporation was performed for ∼1 h with ∼1 min pause every 2−3 min to adjust the chamber pressure. Nanoparticles formed during laser evaporation were condensed on a stainless steel shim located about 50 mm from the target. After the completion of the evaporation condensation process, the entire chamber was backfilled with argon and transferred to an ArReceived: February 8, 2017 Revised: April 18, 2017 Published: May 2, 2017 A
DOI: 10.1021/acs.jpcc.7b01262 J. Phys. Chem. C XXXX, XXX, XXX−XXX
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The Journal of Physical Chemistry C filled glovebox where the powders were scraped off the steel shim and stored in a vial inside the glovebox for further measurements. The water content of the Ar-glovebox was maintained at
