Arc-Discharge Synthesis of Iron Encapsulated in Carbon Nanoparticles for Biomedical Applications

The objective of the present work is to improve the protection against the oxidation that usually appears in core@shell nanoparticles. Spherical iron nanoparticles coated with a carbon shell were obtained by a modified arc-discharge reactor, which permits controlling the diameter of the iron core and the carbon shell of the particles. Oxidized iron nanoparticles involve a loss of the magnetic characteristics and also changes in the chemical properties. Our nanoparticles show superparamagnetic behavior and high magnetic saturation owing to the high purity α-Fe of core and to the high core sealing, provided by the carbon shell. A liquid iron precursor was injected in the plasma spot dragged by an inert gas flow. A fixed arc-discharge current of 40 A was used to secure a stable discharge, and several samples were produced at different conditions. Transmission electron microscopy indicated an iron core diameter between 5 and 9 nm. Selected area electron diffraction provided evidences of a highly crystalline and dense iron core. The magnetic properties were studied up to 5 K temperature using a superconducting quantum interference device. The results reveal a superparamagnetic behaviour, a narrow size distribution (σg=1.22), and an average diameter of 6 nm for nanoparticles having a blocking temperature near 40 K

Reliability aspects of ferroelectric TiN/Hf0.5Zr0.5O2/Ge capacitors grown by plasma assisted atomic oxygen deposition

The reliability of Hf0.5Zr0.5O2 (HZO) metal-ferroelectric-semiconductor capacitors grown by plasma-assisted atomic oxygen deposition on Ge substrates is investigated with an emphasis on the influence of crystallization annealing. The capacitors show very weak wake-up and imprint effects, allowing reliable operation in excess of 10 years, which is attributed partly to the clean, oxide-free Ge/HZO bottom interface. The weak temperature dependence and the observed asymmetries between polarization up and down states and between positive and negative coercive voltage shifts lead to the conclusion that imprint is controlled by carrier injection at the top electrode interface. The latter mechanism is associated with trapping at interfacial oxygen-vacancy defects. On the other hand, using ultrafast (millisecond) flash annealing improves the leakage current by at least an order of magnitude and the endurance by a factor of 3 compared to conventional rapid thermal annealing, which makes them suitable for low power nonvolatile memory applications where (ultra)thin HZO is an essential requirement. © 2020 Author(s)

Cu vapor-assisted formation of nanostructured Mo2C electrocatalysts via direct chemical conversion of Mo surface for efficient hydrogen evolution reaction applications

In order to fulfill the demands for sustainable hydrogen production, novel, more efficient and inexpensive (precious metal-free) catalysts are thoroughly investigated. In the present study, a nanostructured Mo carbide film is prepared, using a chemical vapor deposition (CVD) process in Cu vapor, which promotes CH4 decomposition and the rapid carbonization of a commercial Mo foil. Structural X-ray diffraction (XRD) analysis reveals the orthorhombic crystal structure of the film. The Mo carbide film is tested as an electrocatalyst for the hydrogen evolution reaction (HER) in acidic media. The interconnection between the carbide and the underlying Mo foil leads to an enhanced electrocatalytic activity (Tafel slope − 65 mV/dev, overpotential at 10 mA/cm2 – 330 mV), followed by excellent durability after 1000 cycles. Ultraviolet photoemission spectroscopy (UPS) provides clear evidences verifying the enhanced activity on the carbonized Mo surface. Furthermore, an increase in the carbon precursor flow favors the simultaneous growth of graphene/Mo2C heterostrustructure that forms a vertical stack and exhibits even greater electrocatalytic properties. Thus, the heterostructure possesses Tafel slope and overpotential of 56 mV/dec, at n = 212–218 mV, and 270 mV, respectively, which approach values of commercially available Pt catalysts. © 2020 Elsevier B.V

Depletion induced depolarization field in Hf1-xZrxO2metal-ferroelectric-semiconductor capacitors on germanium

Germanium Metal-Ferroelectric-Semiconductor (MFS) capacitors based on ferroelectric Hf1-xZrxO2 (HZO) with clean, oxide free Ge/HZO interfaces emerge as an interesting layer structure for the fabrication of ferroelectric field effect transistor (FeFET) non-volatile memory devices. It is shown that, at low temperature (<160 K), a semiconductor depletion forms in Ge near the interface, resulting in an increase in coercive voltage by about 2 V, accompanied by a distortion of the ferroelectric hysteresis with subloop asymmetric behavior, which becomes more severe at higher frequencies of measurement. At higher temperatures, the Ge surface near the ferroelectric is easily inverted due to the low energy gap of Ge, providing sufficient screening of the polarization charge by minority free carriers, in which case, nearly ideal, symmetric hysteresis curves are recovered. The depolarization field is experimentally extracted from the coercive voltage and the capacitance measurements, is found to be ∼2.2 MV/cm in the low temperature range, comparable to the coercive field, then rapidly decreases at higher temperatures, and effectively diminishes at room temperature. This makes Ge MFSs good candidates for FeFETs for low voltage non-volatile memory with improved reliability. © 2020 Author(s)

Very large remanent polarization in ferroelectric Hf1-xZrxO2 grown on Ge substrates by plasma assisted atomic oxygen deposition

Plasma assisted atomic oxygen deposition was used to grow polycrystalline ferroelectric Hf1-xZrxO2 (x = 0.5-0.7) on technologically important (100) Germanium substrates showing sharp crystalline interfaces free of interfacial amorphous layers and strong evidence for the presence of a predominately orthorhombic phase. The electrical properties, evaluated using metal-ferroelectric-semiconductor (MFS) capacitors, show symmetric and robust ferroelectric hysteresis with weak or no wake-up effects. The MFS capacitors with x = 0.58 show very large remanent polarization up to 34.4 μC/cm2 or 30.6 μC/cm2 after correction for leakage and parasitics, combined with good endurance reaching 105 cycles at a cycling field of 2.3 MV/cm. The results show good prospects for the fabrication of Ge ferroelectric field effect transistors (FeFETs) for use in 1 T FeFET embedded nonvolatile memory cells with improved endurance. © 2019 Author(s)

The Role of Interface Defect States in n- and p-Type Ge Metal–Ferroelectric–Semiconductor Structures with Hf0.5Zr0.5O2 Ferroelectric

The discovery of ferroelectricity in doped HfO2 represents an excellent opportunity to overcome the obstacles in manufacturing reliable ferroelectric field effect transistors (FeFET) for nonvolatile memory applications, considering that HfO2 is compatible with Si and Ge and it is already used in semiconductor industry. The presence of interface defects may have detrimental effects on the operation of FeFETs, so their role is systematically investigated in this study in correlation with the substrate doping. Metal–ferroelectric–semiconductor (MFS) structures are fabricated by depositing Hf0.5Zr0.5O2 (HZO) layers on n-type Ge substrate. Their electric properties are compared with those of MFS structures obtained by depositing HZO on p-type Ge, to study the influence of the doping. It is found that, although the ferroelectric properties of HZO are similar, the capacitance and impedance of the MFS structures behave differently. For n-Ge, the occupation probability of a large number of low-lying interface defect acceptor states, charges the interface negatively which adversely affects the C–V response of the MFS, albeit without harming the ferroelectric (P–V) hysteresis. Although the interface defects do not harm ferroelectricity, they could inhibit inversion in p-type Ge or accumulation in n-type Ge so they should be taken into account when designing Ge FeFET devices. © 2021 Wiley-VCH Gmb