Synthesis of hafnium nanoparticles and hafnium nanoparticle films by gas condensation and energetic deposition

In this work we study the fabrication and characterization of hafnium nanoparticles and hafnium nanoparticle thin films. Hafnium nanoparticles were grown in vacuum by magnetron-sputtering inert-gas condensation. The as deposited nanoparticles have a hexagonal close-packed crystal structure, they possess truncated hexagonal biprism shape and are prone to surface oxidation when exposed to ambient air forming core–shell Hf/HfO2 structures. Hafnium nanoparticle thin films were formed through energetic nanoparticle deposition. This technique allows for the control of the energy of charged nanoparticles during vacuum deposition. The structural and nanomechanical properties of the nanoparticle thin films were investigated as a function of the kinetic energy of the nanoparticles. The results reveal that by proper adjustment of the nanoparticle energy, hexagonal close-packed porous nanoparticle thin films with good mechanical properties can be formed, without any additional treatment. It is shown that these films can be patterned on the substrate in sub-micrometer dimensions using conventional lithography while their porosity can be well controlled. The fabrication and experimental characterization of hafnium nanoparticles is reported for the first time in the literature.

where L is crystal grain size, λ = 1.5418 Å is the wavelength of Cu Kα radiation, and βD is the FWHM of the peak at angle 2θ.
In order to calculate the FWHM, we used the X-ray pattern of the biggest Hf NPs (16 nm × 15 nm), which has more intense and narrow peaks ( Figure 3). The FWHM was estimated from fitting the X-ray peaks with a pseudo-Voigt function. For the Hf crystal diameter determination we used the most intense and narrow (10−10) Hf peak with βD = 0.73° and the crystal grain size was calculated equal to L = 11 nm.

Experiment for testing the charge of Hf NPs
We have tested the charge of Hf nanoparticles with the following experiment ( Figure S1): Initially, we recorded the deposition rate on the substrate using a voltage Vs = 0 V, then gradually we increased the applied voltage while simultaneously recording the deposition rate. From the data it appears that increasing the applied voltage leads to a reduction of the recorded deposition rate, regardless of the sign of the applied voltage. From these data, it appears that the vast majority of the produced nanoparticles are charged, either positively or negatively, since the trajectory of the nanoparticles is affected when voltage is applied. The applied voltage, depending on the polarity, leads to attraction of clusters of one sign and deflection of clusters with opposite sign. As voltage is increased, the negatively charged NPs beam is more and more focused towards the sample holder, and thus not monitored by the quartz crystal monitor (QCM).
At the same time the positively charged nanoparticles beam is deflected by the sample holder and thus not reaching the QCM ( Figure S1b).

Nanomechanical properties
The nanomechanical properties of the NTFs were determined by nanoindentation.

Nanoidentation testing was performed with a Hysitron TriboLab Nanomechanical Test
Instrument, which allows the application of loads from 1 to 30.000 μN and records the displacement as a function of applied loads with high resolutions of load (1 nN) and displacement (0.04 nm). In all tests, ten indents with a spacing of 50 μm were averaged.
Nanoindentation tests were performed using the displacement-control protocol. The durations of S4 loading and unloading are 20 s, and the holding time at the maximum load is 2 s. All nanoindentation measurements have been performed, with a standard three-sided pyramidal Berkovich probe. Hardness (H) and elastic modulus (E) values were extracted from the experimental data (load-displacement curves) using the Oliver-Pharr method.

Method of Ramakrishnan-Arunachalam
In this method the porous solid is considered a continuous medium with randomly distributed pores and the effect of porosity p on the elastic modulus E is described through the equation: where E 0 is the elastic modulus of the solid without pores, E p is the elastic modulus of the porous solid, b E = 2-3ν 0 is a quantity that depends on the Poisson ratio of the solid ν 0 without pores. For determination of the elastic modulus of the Hf film without pores we have prepared a hafnium film with thickness of 120 nm on SiO 2, using radio-frequency (RF) magnetron sputtering. The elastic modulus of this film was determined through nanoidentation and was found to be E = 94.5 GPa. The load-displacement curve of the RF-sputtered Hf film is shown in Figure S3 Figure S3: Nanoidentation load-displacement curve of a ca. 120 nm RF-sputtered Hf thin film on a Si substrate.