ICMCTF2014 Session F1: Nanomaterials, Nanofabrication, and Diagnostics

There has been an increased interest in group IV nanoparticles (NPs) for a variety of applications including photovoltaics, lithium ion batteries, and bio-imaging. The properties of these quantum-confined NPs are governed by their size as well as the surface passivating layer. Si NPs, 3-7 nm in size, were synthesized in a tubular, capacitively-coupled, radio-frequency SiH₄/Ar plasma at pressures ranging from 5-8 Torr. The H-terminated surface of the as-synthesized Si NPs is highly reactive, and requires surface passivation to prevent oxidation. We have developed a single-step synthesis and in-flight surface passivation technique wherein we use a dual-plasma setup, which consists of a second capacitively-coupled C₂H₂ plasma, downstream from the SiH₄/Ar synthesis plasma. The Si NPs can be coated with amorphous carbon (a-C) to obtain core-shell nanostructures, which a thin SiC interface between Si and a-C. These core-shell NPs are transported by flow into a surface analysis chamber, which is equipped with in situ attenuated total reflection Fourier transform infrared and photoluminescence spectroscopy setups to determine the surface composition and the optical band gap of the NPs, respectively. The NPs are also extensively characterized using ex situ x-ray diffraction, Raman spectroscopy, and transmission electron microscopy (TEM). The thickness of the coating determined from TEM is ~2-4 nm. We have also studied the effect of varying the C₂H₂ plasma parameters on the structure and composition of the a-C coating and the SiC interface This plasma synthesis and passivation method has been extended to other group IV NPs such as Ge, which are less likely to have a carbide interface.

Copper oxide nanoparticles are increasingly used in various applications such as solar energy transformation, magnetic phase transitions, gas sensors, catalysts, superconductors, and nanomedicines. The worldwide quest for a clean, renewable and economical source of energy has encouraged an extensive research in the field of solar cells. Solar cell technology, as a sustainable source of energy, has enjoyed a tremendous growth in recent years and production of solar cells increases at an annual average of ~ 40% [1].Copper oxide compounds as a p-type semi-conductors provide a unique possibility to tune the optical and electronic properties from insulating to metallic conduction, from band gap energies of 2.1ev to the infrared at 1.4ev, i.e. right into the middle of the maximum efficiency for solar-cell applications. With the decrease of the crystallite size of the copper oxide particles to the nanoparticles range, it exhibits unique physical and chemical properties from those of their bulk materials and thereby enhances its performance in the currently existing applications. Metal oxide nanoparticles have special physiochemical properties arising from the quantum size effect and a high specific surface area which may be different from their bulk counterparts [2]. It is well-known that inorganic nanocrystals are a benchmark model for nanotechnology, given that the tunability of optical properties and stabilization of specific phases are uniquely possible at the nanoscale .

We have easily prepared nanoparticles of copper oxides by a cost effective hydrothermal process using coppersulphate penthydrate as the precursor. The shape and size as well as the phase of the copper oxide can be engineered by altering the pH, reaction temperature, and time. The following SEM Images are taken for copper oxides formed at different reaction temperatures and time, using different bases (NaOH and NH₄OH).Different morphology of nanostructures are visualized like pollen grains, flakes and rods. The structural, phase identification, molecular vibrational modes, optical properties and morphology was determined by using XRD, Raman Spectroscopy, PL,UV-Visible spectroscopy and SEM.

REFERENCE:

[1] A.Jager-Waldau, Solar Energy, 667, 77 (2004)

[2] A Chowdhuri, V Gupta and K Sreenivas, “Response speed of SnO2-based H2S gas sensors with CuO Nanoparticles”, Applied physics Lett. 84, 1180 (2004)

In recent years, there has been considerable interest paid to semiconductor oxides to replace noble metals (Pt, Pd, ...) for gas sensor applications [1-2]. Zinc oxide (ZnO) as a n-type semiconductor and lanthanum cobalt based oxides (LaCoO₃) as a p-type semiconductor have attracted the interest of many scientists and have been the subject of intensive investigations as gas sensors mainly because of their high electrical conductivity, excellent catalytic activity and chemical stability.

In principle, electrical conductivity of a p-type semiconductor increases (or decreases) when oxidizing (or reducing) gases are adsorbed on their surface (the opposite for n-type semiconductors).The performance of the active layers in a number of modern devices and especially as gas sensors is strongly linked to their specific surface area that can be tuned by controlling their morphologies.

In this paper, we investigate the feasibility of ZnO and double substituted lanthanum cobalt (La_1-x-ySr_xAg_yCoO_3-α)coatings with different morphologies (dense, nano-tree and nano-wire) by reactive magnetron sputtering. After a short description of the experimental devices used for the deposition stage and the hydrocarbon sensing bench, a first part will be dedicated to the chemical, microstructural and structural characterization (SEM, XRD,…) of coatings in relation with their deposition parameters. Finally, the performance as dodecane-sensors of these coatings will be discussed depending on dodecane concentrations and sensitive surface’s temperature and they will be compared together.

We report the synthesis of hexagonal-YMn_1-xFe_xO₃ (x=0, 0.1) thin films deposited on (111)Pt/(0001)sapphire and (111)Pt/(111)MgO substrates by pulsed laser deposition. X-ray diffraction confirms the c-axis orientation of the deposited thin films. The thickness of the films obtained from X-ray reflectivity measurement is found to be around 200 nm. The stoichiometric composition of the thin films is confirmed by energy dispersive X-ray spectroscopy. The dielectric and magnetic properties of the as-deposited films were studied. The dielectric response with temperature indicates an anomaly in the dielectric constant ε and loss tangent tan δ in the vicinity of 150 K, well above the bulk Neel temperature T_N~70 K. This anomaly in ε and tan δ is ascribed to the strain resulting from the lattice mismatch between substrate and YMn_1-xFe_xO₃ (x=0, 0.1) thin films. The magnetization hysteresis loops indicate high coercivity in YMn_0.9Fe_0.1O₃ as compared to YMnO₃ thin film. Our results indicate that it is possible to enhance the multiferroic properties by Fe-substitution in YMnO₃ thin films.

Metallic glasses have exciting potential for structural, chemical, and magnetic applications with the sizes ranging from micrometer to centimeter, but the fabrication and characterization down to nanoscale remains an important challenge. Progress has been hindered by the lack of bottom-up methodologies to produce amorphous nanostructures. We showed the self-organized amorphous nanowires that are formed on the fracture surfaces of bulk metallic glasses [1]. However, it is difficult to control their morphologies because they were formed during the instantaneous fracture process. Recently, the gas atomization, which is a conventional technique in powder metallurgy, is adapted for the formation of metallic glass nanofibers. This approach is able to produce a large quantity of the nanowires with the diameters of 50-2000 nm in range. Experiments performed with different conditions and alloy compositions confirm that the key mechanism of the nanowire formation is the spinnability that increases exponentially when the melt stream is supercooled from the liquid state [2].

[1] K. S. Nakayama et al., Nano Lett. 8, 516 (2008).

[2] K. S. Nakayama et al., Nano Lett. 12, 2404 (2012).

The nanomechanical properties of Pt thin films grown on Si (100) using atomic layer deposition (ALD) were investigated using nanoindentation. Recently, atomic layer deposition (ALD) has successfully demonstrated the capability to deposit ultrathin films of platinum (Pt). Using methylcyclopentadienyltrimethylplatinum (MeCpPtMe₃) as chemical platinum precursor and oxygen (O₂) as the oxidizing agent, the ALD synthesis of Pt can be achieved with high conformity and excellent film uniformity. The ALD process window for Pt films was experimentally established in the temperature range between 270-320 ºC, where the sheet conductance was constant over that temperature range, indicating stable ALD Pt film growth rate. ALD growth of Pt films exhibits very poor nucleation and adhesion characteristics on bare Si surfaces when the native oxide was removed by 2% HF etch. Pt adhesion improves for thermally oxidized Si wafers and for Si wafers covered with native oxide. Thin films and coatings of the noble metal platinum (Pt) find numerous applications in microelectronics and catalysis due to their excellent electrical properties and chemical stability. For example, platinum is used as electrode at high temperature in oxidative and reductive environments and Platinum is applicable as a gate metal with high-k dielectrics in metal-oxide-semiconductor field effect transistors (MOSFETs) because of its high work function. Furthermore Pt is also widely used in fuel cells due to its high catalytic activity and in chemical engineering Pt coatings are utilized as catalyst to enhance a multitude of chemical reactions. Aside from the jewelry industry major applications of Pt films are found in the automotive industry, where it is used in catalytic converters for emission control and in spark plug coatings. Three Pt films deposited with 800, 900, and 1000 ALD cycles were tested for the structural and mechanical properties. Additionally, the 900 ALD cycles sample was further annealed at 450 °C in forming gas for grain boundary passivation for 30 minutes. Cross-sectional transmission electron microscopy (TEM), X-ray diffraction (XRD), and atomic force microscopy (AFM) were employed to characterize the films’ surface structure and morphology. Nanoindentation technique was used to evaluate the hardness and modulus of the Pt films of various film thicknesses. The results indicate that the films depict comparable hardness and modulus results, however the 800 and 1000 ALD cycles films experienced significant amount of pile-up whereas the 900 ALD cycles forming gas annealed sample resulted in a small pile-up.

In this work, we deposited well-aligned CuSi helices on Cu substrates via an oblique angle ion-beam deposition technique. The electrochemical performance of these helices is compared with that of the non-aligned columnar CuSi thin films. Galvanostatic half-cell measurements showed that the well-aligned Cu-Si helices based thin film anodes exhibited much longer cycle lives with a moderate capacity due to the particularities in its structure and its morphology: The structural analysis revealed that depending on the differences in thin film nucleation and growth mechanisms, helical shaped thin films exhibited higher amounts of nano-sized and amorphous particles in them, which may have improved the cycleability of the anodes. Moreover, the morphological observation showed the importance of the helices formation in the thin film anode performance. Such a helical shaped thin film relaxes the stresses that may otherwise develop along the electrode and enhances the cyclic properties of these anodes because they increase the contact area of the thin film with Li, decrease the polarization and enhance the mechanical tolerance against the volumetric changes due to the homogenously distributed nanosized interspaces available among them.

In this work, we deposited well-aligned CuSi helices on Cu substrates via an oblique angle ion-beam deposition technique. The electrochemical performance of these helices is compared with that of the non-aligned columnar CuSi thin films. Galvanostatic half-cell measurements showed that the well-aligned Cu-Si helices based thin film anodes exhibited much longer cycle lives with a moderate capacity due to the particularities in its structure and its morphology: The structural analysis revealed that depending on the differences in thin film nucleation and growth mechanisms, helical shaped thin films exhibited higher amounts of nano-sized and amorphous particles in them, which may have improved the cycleability of the anodes. Moreover, the morphological observation showed the importance of the helices formation in the thin film anode performance. Such a helical shaped thin film relaxes the stresses that may otherwise develop along the electrode and enhances the cyclic properties of these anodes because they increase the contact area of the thin film with Li, decrease the polarization and enhance the mechanical tolerance against the volumetric changes due to the homogenously distributed nanosized interspaces available among them.

During the last decades ferroelectrics crystals attract researchers’ attention in the field of fundamental solid state physics and applied material science [1,2]. The presence of phase transition stipulates high receptivity of ferroelectrics to the action of external fields, that makes them perspective for the use in micro-electromechanical systems, pyroelectric receivers, optical modulators, devices for recording and information storage. Research of processes of ionic conductivity testifies to possibility of application of ferroelectrics and related materials in the autonomous sources of electric current [3-5].

Dielectrics with high ionic conductivity are marked by the specific lines of crystalline structure. For migration of ions the presence in the structure of channels of conductivity is necessary, those vacant positions are connected for charge carriers. The structure of crystals of system Li₂O-GeO₂, which are perspective for creating new superionic materials, meets this condition. In particular, framework of grates of weakly polar ferroelectricity of lithium heptagermanate Li₂Ge₇O₁₅ is formed from germanium-oxygen polyhedral and contained through channels [7-10], in which cations Li⁺ are located. It is possible to expect that mobility of ions lithium to a great extent determines electric properties of Li₂Ge₇O₁₅ crystals [11-14].

It is known, that introduction of admixtures substantially affects the processes of electric polarization and charge transfer in dielectric crystals. Therefore alloying crystals of lithium heptagermanate can be offered as a method of control after ionic payment in the processes of polarization and conductivity. We will also mark very small spontaneous electric polarization of crystals Li₂Ge₇O₁₅, which allows assuming the presence of anomalous physical properties in nearby critical point.

Scientific and practical interest to the study of processes of electric polarization and charge transfer in ferroelectrics determines actuality of research of electric properties nominally clean and doped crystals Li₂Ge₇O₁₅.