ICMCTF2011 Session F1-2: Nanomaterials, Nanofabrication, and Diagnostics

The molybdenum trioxide (MoO₃) nanowires were prepared by thermal chemical vapor deposition through a two-step evaporation process and distributed on a p-type Si (100) substrate from molybdenum trioxide powders at 900^oC. These obtained nanowires were deposited the platinum nanoparticles by atomic layer deposition (ALD), the morphology, crystal and alcohol gas sensing properties of undoped and platinum doped MoO­₃ nanowires were investigated. The MoO₃ nanowires were rod-like shape and the diameter were approximately 10-500 nm and several hundreds of nanometers in length. Gas sensors base on updoped, 0.5 wt% and 1.0 wt% platinum-doped MoO₃ were fabricated. The MoO­­₃ nanowire gas sensor showed a reversible response to alcohol gas at an operating temperature of RT-250^oC. The sensor response increased with increasing Pt concentration. The results demonstrated that Pd doping improved the sensor response and lowered the operating temperature at which the sensor response was maximized.

Silicene, which is a two-dimensional sheet of silicon, is increasingly attracting interests owing to the success of graphene - its carbon counterpart. Similarly, massless fermions may occur as suggested in a recent theoretical study [1]. Here, we demonstrate that a two-dimensional, ordered Si layer can be prepared through surface segregation on a metallic film grown epitaxially on Si(111).

Zirconium diboride (ZrB₂) is a metallic ceramics with a high melting point. Thin films with a high purity and crystallinity can be grown on Si(111) through thermal decomposition of Zr(BH₄)₄ molecules under optimized conditions [2]. We have recently obtained predominantly single-crystalline films [3] with an epitaxial relationship of ZrB₂(0001)//Si(111) and ZrB₂[1-100]//Si[11-2]. Natural oxides formed on the film upon exposure to air could be removed by heating in ultrahigh vacuum. The oxide-free film surface shows a ZrB₂(0001)-(2×2) reconstruction. Scanning Tunneling Microscopy (STM) imaging on this surface shows that the atomically-flat ZrB₂(0001) terraces are uniformly covered with periodic stripes of two types of (2×2) domains having domain boundaries along ZrB₂<11-20> directions and domain width equivalent to four to five (2×2) half unit cells. The observation of Zr-related surface states by Angle-Resolved Ultraviolet Photoelectron Spectroscopy (ARUPS) proofs that the top-most hexagonal close-packed Zr layer remains intact in spite of the (2×2) reconstruction [3]. Both Zr- and Si-related electronic states follow the (2×2) symmetry showing that the electronic subsystems are not independent from each other. Surface-sensitive core-level photoelectron spectroscopy performed using a photon energy of 130 eV identifies Si atoms in different chemical states that are either in contact with Zr atoms or not. Further details of the relationship between the electronic structure of this epitaxial Si layer on metallic ZrB₂(0001) thin film and the atomistic structure will be discussed.

References:

[1] S. Cahangirov, M. Topsakal, E. Aktuerk, H. Sahin, and S. Ciraci, Phys. Rev. Lett. 102, 236804 (2009).

[2] C. W. Hu, A. V. G. Chizmeshya, J. Tolle, J. Kouvetakis, and I. S. T. Tsong, J. Cryst. Growth 267, 554 (2004).

Iron-carbon and iron-carbon based alloys , which are of great importance from both fundamental and applied points of view, have triggered a wealth of publications which are most often devoted to studies of crystalline alloys with nanosized, ultra-fine or micron-sized grains. A full knowledge of the Fe-C system requires further to investigate and characterize amorphous Fe-C alloys which are known to exist in a wide concentration range. Sputtered Fe_1-xC_x amorphous films were for instance found to exist for a carbon content x ranging between 0.19 and 0.50. These amorphous alloys can be prepared either as thin films or as powders, for instance when they are synthesized by mechanical alloying. The Fe-C amorphous alloys are interesting in their own for their peculiar structure, which depends on the carbon content, and for their intrinsic physical properties. Further, they are sources of new metastable carbides which are formed during their crystallization. The present work addresses the characterization of the carbon-richest Fe_1-xC_x films (x > 0.32) with a special attention dedicated to Fe_0.55C_0.45. Electron probe microanalysis was used to determine the carbon content. As-sputtered amorphous films were characterized by Mössbauer spectrometry, X-ray and/or electron diffraction. Their thermal stability was studied by differential scanning calorimetry. The products of crystallization were identified both from Mössbauer spectra and from X-ray diffraction patterns. Crystallization was followed in situ using a hot stage transmission electron microscopy. For any carbon content in the range 0.32-0.50, the Mössbauer hyperfine parameters are observed to remain constant while the main crystallization product is identified to be the Fe₇C₃ carbide. These results indicate that, for x > 0.32, the amorphous films are composed of two phases: one Fe-rich metallic Fe-C phase, with a carbon content close to 0.3, and an amorphous carbon phase deprived of iron. The composition of the metallic phase remains essentially constant when the carbon content increases. Then, it is the increase of the fraction of the amorphous carbon phase which accomodates the carbon content increase. According to the carbon content, amorphous Fe-C films may be looked at as nanocomposites formed by two components, a metallic amorphous one and an amorphous carbon one, whose proportions can be controlled by a choice of the carbon content.

In refrigeration system, the heat transfer performance of evaporator is decreased when the fins covered with frost. The defrosting process is time and energy consumptions. The modification of superhydrophobic thin film on the aluminum fin surface could prevent the condensate water drop staying on the surface and so reduce the frost deposition. In order to modify the aluminum surface to be superhydrophobic, a thin film of micro/nano structure was coated by sol-gel process. The Tetraethoxysilane(TEOS) and Methyltrimethoxysilane (METMS) were employed and mixed with micro/nano silica particles to function like the lotus leaf. Those silane couple agents also provided the bonding force between aluminum and silica particles. The water contact angle of original untreated aluminum surface is 85 degrees. The best one after modification can reach 163 degrees. The SEM images tell that the more nano particles used, the more roughness the surface was, which the contact angle enhancement can be explained by Cassie-Baxter model. In order to understand how the wettability affects the frost deposition, the aluminum plate samples were placed in the refrigerator cold chamber and hot water was used to speed up the frosting process. After a period of time the samples were weighed by digital scale. The results show that the higher contact angle the surface had, the less frost deposition on surface happened. Compare to untreated aluminum plate, the frost deposition is only half when the contact angle is 163 degrees. For the long term (7 hours) frosting running, the frost reduction ability is decreased. Because when the superhydrophobic film was covered with frost, the high contact angle state cannot persist.

Cr₂AlC has recently been studied as a MAX phase coating due to its excellent thermo-mechanical properties resulting from its inherent nano-laminated structure. In the present paper the correlation between plasma characteristics during sputtering and the resulting coating properties will be shown. A sintered Cr₂AlC powder target was used for high power impulse magnetron sputtering (HIPIMS) or conventional DC magnetron sputtering. HIPIMS sputtering was performed at different pressures and at different pulse power levels; the effect of process parameters on the plasma was investigated by a quadrupole mass spectrometer. Plasma composition as well as amount and energies of single and double charged ions were analyzed. Moreover, the structure, the morphology and selected properties of the coatings were investigated. The Cr₂AlC coatings were single MAX phase; coating morphology and internal stress state were varied by using different bias voltages. Depending on the process parameters, the Cr₂AlC MAX phase coatings showed high adhesion in scratch test and very good plastic performance.