HfO2 Nanoparticles Research Articles

Tracking of Short Distance Transport Pathways in Biological Tissues by Ultra-Small Nanoparticles.

In this work, ultra-small europium-doped HfO2 nanoparticles were infiltrated into native wood and used as trackers for studying penetrability and diffusion pathways in the hierarchical wood structure. The high electron density, laser induced luminescence, and crystallinity of these particles allowed for a complementary detection of the particles in the cellular tissue. Confocal Raman microscopy and high-resolution synchrotron scanning wide-angle X-ray scattering (WAXS) measurements were used to detect the infiltrated particles in the native wood cell walls. This approach allows for simultaneously obtaining chemical information of the probed biological tissue and the spatial distribution of the integrated particles. The in-depth information about particle distribution in the complex wood structure can be used for revealing transport pathways in plant tissues, but also for gaining better understanding of modification treatments of plant scaffolds aiming at novel functionalized materials.

Nanoscale High-k Dielectrics for Junctionless Nanowire Transistor for Drain Current Analysis

Hafnium oxide (HfO2) nanoparticles were prepared by a chemical precipitation method and its physical and electrical properties were investigated. HfO2 thin films were prepared via a dip-coating method using the synthesized HfO2 nanoparticles. The crystallinity and resistivity of both prepared thin films were analysed by x-ray diffraction and impedance analyser. The crystalline size was found to be 8 nm and 13 nm for HfO2 and SiO2 thin films, respectively. The electrical (bulk) resistance was calculated by fitting the complex impedance plane using z-view software, and was found to be 2.15 × 107 Ω for HfO2, whereas it was found to be 3.6 × 105 Ω for SiO2 thin films. Junctionless nanowire transistors were implemented using HfO2 dielectric and performance was compared to SiO2 gate dielectric. The results revealed that nano-sized HfO2 proves to be a better gate dielectric material than conventional gate dielectrics. Thus, the nano-sized HfO2 based junctionless transistor device is more suitable for future low power applications.

How Ligands Affect Resistive Switching in Solution-Processed HfO2 Nanoparticle Assemblies.

Advancement of resistive random access memory (ReRAM) requires fully understanding the various complex, defect-mediated transport mechanisms to further improve performance. Although thin-film oxide materials have been extensively studied, the switching properties of nanoparticle assemblies remain underexplored due to difficulties in fabricating ordered structures. Here, we employ a simple flow coating method for the facile deposition of highly ordered HfO2 nanoparticle nanoribbon assemblies. The resistive switching character of nanoribbons was determined to correlate directly with the organic capping layer length of their constituting HfO2 nanoparticles, using oleic acid, dodecanoic acid, and undecenoic acid as model nanoparticle ligands. Through a systematic comparison of the forming process, operating set/reset voltages, and resistance states, we demonstrate a tunable resistive switching response by varying the ligand type, thus providing a base correlation for solution-processed ReRAM device fabrication.

Study of the high-temperature oxidation resistance mechanism of magnetron sputtered Hf7B23Si17C4N45 film

The microstructure evolution and high temperature oxidation mechanism of a hard, amorphous, and optically transparent Hf7B23Si17C4N45 film was studied by x-ray diffraction and transmission electron microscopy. The Hf7B23Si17C4N45 films were deposited by reactive pulse dc magnetron sputtering and annealed in air at temperatures from 1100 to 1500 °C. All annealed films were found to have a two-layered structure composed of the original amorphous and homogeneous layer followed by a nanocomposite oxidized surface layer. The top nanocomposite layer consists of an amorphous SiOx-based matrix and a population of HfO2 nanoparticles with two distinct sublayers. The first sublayer is next to the original amorphous layer and has a dense population of small HfO2 nanoparticles (up to several nanometers) followed by a surface sublayer with coarsened and dispersed HfO2 nanoparticles (up to several tens nm). The HfO2 nanoparticles in the bottom sublayer form by a nucleation and growth process whereas the ones in the surface sublayer coarsen via Ostwald ripening. An estimate of the activation energy for oxygen diffusion through the oxidized layer produced a value around 3.43 eV attesting to the high oxidation resistance of the film. The oxidation resistance mechanism is attributed to the precipitation of HfO2 nanoparticles within a dense SiOx-based matrix and quartz SiO2 in front of the base layer interface that can act as a barrier to heat transfer and O diffusion.

X-ray detection capabilities of plastic scintillators incorporated with hafnium oxide nanoparticles surface-modified with phenyl propionic acid

We synthesized plastic scintillators incorporated with HfO2 nanoparticles as detectors for X-ray synchrotron radiation. Nanoparticles with sizes of less than 10 nm were synthesized with the subcritical hydrothermal method. The detection efficiency of high-energy X-ray photons improved by up to 3.3 times because of the addition of the nanoparticles. Nanosecond time resolution was successfully achieved for all the scintillators. These results indicate that this method is applicable for the preparation of plastic scintillators to detect X-ray synchrotron radiation.

Ion induced crystallization and grain growth of hafnium oxide nano-particles in thin-films deposited by radio frequency magnetron sputtering

We report on the swift heavy ion (SHI) irradiation induced crystallization and grain growth of HfO2 nanoparticles (NPs) within the HfO2 thin-films deposited by radio frequency (RF) magnetron sputtering technique. As grown films consisted of amorphous clusters of non-spherical HfO2 NPs. These amorphous clusters are transformed to crystalline grains under 100 MeV Ag ion irradiation. These crystallites are found to be spherical in shape and are well dispersed within the films. The average size of these crystallites is found to increase with fluence. Pristine and irradiated films have been characterized by high resolution transmission electron microscopy (HRTEM), selected area electron diffraction (SAED), grazing incident x-ray diffraction (GIXRD) and photo luminescence (PL) measurements. The PL measurements suggested the existence of different types of oxygen related defects in pristine and irradiated samples. The observed results on crystallization and grain growth under the influence of SHI are explained within the framework of thermal spike model. The results are expected to provide useful information for understanding the electronic excitation induced crystallization of nanoparticles and can lead to useful applications in electronic and photonic devices.

A novel approach to low-temperature synthesis of cubic HfO2 nanostructures and their cytotoxicity

The development of a strategy to stabilise the cubic phase of HfO2 at lower temperatures is necessary for the emergence of unique properties that are not realised in the thermodynamically stable monoclinic phase. A very high temperature (>2600 °C) is required to produce the cubic phase of HfO2, whereas the monoclinic phase is stable at low temperature. Here, a novel rapid synthesis strategy was designed to develop highly crystalline, pure cubic-phase HfO2 nanoparticles (size <10 nm) using microwave irradiation. Furthermore, the as-prepared nanoparticles were converted to different morphologies (spherical nanoparticles and nanoplates) without compromising the cubic phase by employing a post-hydrothermal treatment in the presence of surface modifiers. The cytotoxicities and proliferative profiles of the synthesised cubic HfO2 nanostructures were investigated over the MCF-7 breast cancer cell line, along with caspase-3/7 activities. The low-temperature phase stabilisation was significantly attributed to surface imperfections (defects and deformations) induced in the crystal lattice by the desirable presence of Na2S·xH2O and NaOH. Our work provides unprecedented insight into the stabilisation of nanoscale cubic-phase HfO2 in ambient environments; the method could be extended to other challenging phases of nanomaterials.

Optical characterization of polar HfO2 nanoparticles in the mid- and far-infrared

Monoclinic HfO2 nanoparticles with nominal diameters of 9, 30, and 45 nm are characterized using transmission and reflection spectroscopy from the mid- to far-infrared. Phonon modes are identified in the measured spectra and agree with previously reported values in HfO2 thin films and density functional perturbation theory calculations. An anomaly in both reflection and transmission is observed at 556 cm−1 that is not attributed to the optical phonon modes. Numerical models predict a localized surface phonon polariton mode near this frequency, and we attribute the anomaly to coupling to this surface mode. The results of this work suggest that HfO2 nanoparticles could enable engineered optical properties in new mid- and far-infrared materials and devices.

Physical and optical properties of HfO2 NPs – Synthesis and characterization in finding its feasibility in opto-electronic devices

HfO2 nanoparticles (HfO2 NPs) with different precursors (NaOH, H2O, ethanol) were synthesized by a simple co-precipitation method. FTIR and EDX spectroscopy analysis shows the Stoichiometries composition of HfO2 NPs. X-ray diffraction pattern analysis revealed that the as prepared and calcined HfO2 NPs are respectively amorphous and polycrystalline nature. TEM analysis confirms that the morphology of the calcined HfO2 NPs is spherical in shape with less agglomeration. The crystallite size was evaluated to be 51nm, 37nm and 30nm respectively, for the precursors NaOH, H2O and ethanol used for HfO2 NPs. SEM analysis shows spherical, rock and sponge like surface morphology respectively, for the precursors NaOH, H2O and ethanol used for HfO2 NPs. UV–visible spectroscopy analysis revealed that the optical band gap energy of NaOH, H2O and ethanol precursors used HfO2 NPs were respectively 5.50eV, 5.52eV and 5.50eV. The observed optical properties indicated the feasibility of utilizing these NPs as anti reflection layers in solar cells and for the construction of poultry farms to save energy. The estimated dielectric constant value indicated that these NPs could be used to prepare dielectric layers in thin film transistor.

Structural and optical properties of phase transformation Barium doped HfO2 nanorods—Synthesis and characterization

Barium doped HfO2 nanoparticles (Ba-HfO2 NPs) were synthesized by a simple co-precipitation method using HfCl4 and (Ba (OH)2·H2O) as precursors. Stoichiometries compositions were analyzed by Fourier transform infrared spectroscopy and Energy Dispersive X-Ray spectroscopy. X-Ray diffraction pattern analysis of Ba doped HfO2 NPs revealed the occurrence of structural transformation from monoclinic phase to orthorhombic phase. TEM analysis confirms the formation of nanorods having the aspect ratio (length/diameter) of ∼1.99 with less agglomeration which could be used for sensor applications. UV–vis spectroscopy analysis revealed that the optical energy band gap of as prepared NPs is 5.4eV and calcined NPs is 5.1eV.

Investigations on new carbon-based nanohybrids combining carbon nanotubes, HfO2 and ZnO nanoparticles

In this work, we present two types of hybrid materials. The first hybrid material is a combination of carbon nanotubes (CNT) with HfO2 nanoparticles. The latter constituent on its own exhibits unusual visible photoluminescence, which is in stark contrast to non-luminescent HfO2 in the bulk form. The small size of HfO2 nanoparticles, 2.6 nm in average, suggests surface-defect related origin of the observed photoluminescence. The other hybrid material is CNT-ZnO with the embedded ZnO nanoparticles ranging from 50nm to 100nm in size. ZnO represents a direct bandgap semiconductor renowned as highly-luminescent in a broad spectral range. The visible region is attributed to luminescence involving deep-level defects, and hence depends on the synthesis conditions. In this study we compare the morphology of the two hybrid materials with transmission electron microscopy. We further compare the photoluminescence properties and the influence of the CNT coupling on enhancing or suppressing defect related emissions. Finally, we present a novel hybrid material CNT-HfO2 capable of producing a photocurrent under zero bias.

Synthesis and characterization of Ce-doped HfO2 nanoparticles in molten chlorides

Ce-doped HfO2 nanoparticles were successfully prepared in molten chlorides. Nanoparticles were obtained at 380 °C during 1 h in a LiCl/KCl molten flux. The Ce-doped HfO2 nanoparticles showed sizes below 7 nm and spherical shapes. Complete stabilization of cubic phase was achieved by adding 10 mol % of Ce. XRD, Raman and HRTEM techniques confirmed that the Ce-doped HfO2 nanoparticles have a cubic structure. Furthermore, this simple, efficient and high yield (>95%) method represents an alternative route for the synthesis of fluorite-type cubic phases and should be useful for doping or co-doping HfO2 with other cations.

Wide band gap of Strontium doped Hafnium oxide nanoparticles for opto-electronic device applications – Synthesis and characterization

Strontium doped HfO2 nanoparticles (Sr-HfO2 NPs) were synthesized by a simple co-precipitation method. FTIR and EDAX spectroscopy analysis shows stoichiometries compositions of HfO2 NPs are pure. X-Ray diffraction pattern analysis revealed that the calcined NPs shown the polycrystalline nature of the orthorhombic phase structure. TEM analysis revealed that the morphology of the calcined NPs was spherical in shape with less agglomeration and the crystallite size was about 30nm. UV–visible spectroscopy analysis revealed that the optical band gap energy of calcined NPs was 5.4eV.

Photocurrent generation in carbon nanotube/cubic-phase HfO2 nanoparticle hybrid nanocomposites.

A hybrid material consisting of nonfunctionalized multiwall carbon nanotubes (MWCNTs) and cubic-phase HfO2 nanoparticles (NPs) with an average diameter of 2.6 nm has been synthesized. Free standing HfO2 NPs present unusual optical properties and a strong photoluminescence emission in the visible region, originating from surface defects. Transmission electron microscopy studies show that these NPs decorate the MWCNTs on topological defect sites. The electronic structure of the C K-edge in the nanocomposites was probed by electron energy loss spectroscopy, highlighting the key role of the MWCNT growth defects in anchoring HfO2 NPs. A combined optical emission and absorption spectroscopy approach illustrated that, in contrast to HfO2 NPs, the metallic MWCNTs do not emit light but instead expose their discrete electronic structure in the absorption spectra. The hybrid material manifests characteristic absorption features with a gradual merger of the MWCNT π-plasmon resonance band with the intrinsic defect band and fundamental edge of HfO2. The photoluminescence of the nanocomposites indicates features attributed to combined effects of charge desaturation of HfO2 surface states and charge transfer to the MWCNTs with an overall reduction of radiative recombination. Finally, photocurrent generation under UV–vis illumination suggests that a HfO2 NP/MWCNT hybrid system can be used as a flexible nanodevice for light harvesting applications.

Electronic Structure Study And Dielectric Properties Of Amorphous ZrO2 And HfO2

ZrO2 and HfO2 powder samples were prepared by using the chemical precipitation method and subsequent annealing. Crystal structure, local electronic structure and dielectric constant of amorphous and crystalline powders of ZrO2 and HfO2 have been examined using the synchrotron X-ray diffraction, O K-edge X-ray absorption spectroscopy, Zr 3d and Hf 4f core-level X-ray photoelectron spectroscopy and temperature dependent dielectric measurements, respectively. Amorphous ZrO2 and HfO2 powders exhibit a local tetragonal structure, with mixed +3 and +4 valence states of Zr and Hf ions, and demonstrated the high dielectric performance. After the heat treatment, the tetragonal phase transforms into the monoclinic phase with dominant +4 valence state of Zr and Hf ions and the larger sized ZrO2 and HfO2 nanoparticles exhibited low dielectric constant. The manifestation of high dielectric constants in the amorphous ZrO2 and HfO2 samples is because of the hopping of electrons between the Zr +3 -Zr +4 and Hf +3 -Hf +4 networks.

A “green methodology” approach to the synthesis of HfO2 nanoparticles

HfO2 nanoparticles were successfully synthesized by a green chemistry method, which consist of mechanically activated metathesis and molten salts, using LiCl/KCl. This methodology is useful to prepare cubic phase at low-temperature (380°C) without using any doping element, which is an outstanding advantage over existing techniques. Following this green route easily allows obtaining HfO2 nanoparticles; showing irregular and spherical like morphologies with a diameter in the range of 8–20nm by TEM observations and crystallite size of 6.5 and 14.7nm calculated from Scherer's equation for cubic and monoclinic phases respectively. The temperature and reaction time parameters played a very important role for obtaining monoclinic or cubic phase during molten salt stage.

Microstructures and Stabilization Mechanisms of Nanocrystalline Iron-Chromium Alloys with Hafnium Addition

The low thermal stability of nanocrystalline metals severely limits their applications at high temperatures. In this study, we investigate the nanocrystalline stabilization mechanisms for Fe-14Cr alloys with 1, 2, and 4 at. pct Hf addition at 1173 K (900 °C). Microstructural characterizations using aberration-corrected scanning transmission electron microscopy and energy-dispersive X-ray spectroscopy reveal high density of HfO2 nanoparticles with sizes of ~4 nm dispersed throughout the ferritic matrix. This indicates that kinetic stabilization by HfO2 nanoparticle pinning is primarily responsible for the observed high thermal stability. In addition, some Hf and Cr segregation on grain boundaries is observed in the Fe-14Cr-4Hf, suggesting the existence of thermodynamic stabilization at high Hf content. Second-phase precipitations such as hafnium carbide, M23C6, and Fe-Cr-Hf intermetallic phase are also found in the Fe-14Cr-4Hf, but their large sizes and inter-spacing suggest that their contribution to stabilization is minimal.

One Step Synthesis of Pure Cubic and Monoclinic HfO2 Nanoparticles: Effects of Temperature and Ambient on the Photoluminescent Properties

We report on the one-pot synthesis and characterization of pure cubic and monoclinic phase HfO2 nanoparticles. An average particles size of 2.6 nm was estimated from transmission electron microscopy studies. The equivalent synthesis conditions of the two polymorphs provided means for direct comparison of their optical absorption and emission properties, which are found to exhibit different behaviors as functions of temperature and ambient. 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. The comparative analysis of the PL responses of the two polymorphs suggests that emission at 3.1eV originates from the oxygen vacancy related defects.

Dielectric properties of laser-ablated HfO2 particles

Dielectric properties of hafnium dioxide (HfO2) nanoparticles obtained by pulsed laser ablation have been studied by impedance spectroscopy in a frequency range from 50 to 105 Hz. It is established that the dielectric permittivity of obtained particles depends on the degree of porosity of the ablated material, As the porosity decreases, both real (ɛ′) and imaginary (ɛ″) components of the complex dielectric permittivity tend to increase. According to analysis in the framework of the Bruggeman effective medium approximation, the static dielectric permittivity of obtained HfO2 nanoparticles amounts to ɛ′ = 24 ± 2.

Performance enhancement of Gd–Ba–Cu–O high temperature superconducting bulks by BaHfO3 as pinning centers

Since the potential widespread applications of RE–Ba–Cu–O superconducting bulks, it is crucial to enhance their superconducting performances. Binary oxide HfO2 nanoparticles were added into Gd–Ba–Cu–O superconducting matrix before the melt textured growth process. It was found that the HfO2 particles reacted with surrounding matrix and consequently formed BaHfO3 inclusions maintaining nanoscale without significant ripening during the melt growth process. The superconducting transition temperature TC is almost not affected by HfO2 addition, and interestingly, the TC of 0.8mol% HfO2 doped sample increases slightly by about 0.1K in comparison with the undoped one. Meanwhile, the critical current density JC was improved over the whole magnetic field range. These BaHfO3 particles discovered near the liquid phase (Ba–Cu–O, Cu–O, etc.) should be the origin for the improvement of flux trapping capability and critical current density in this system.