ICMCTF2013 Session F1-1: Nanomaterials, Nanofabrication, and Diagnostics

The tremendous interest in various nanostructures is stimulated by the multifunctional characteristics of such materials and the possibility to tune their properties controlling the size, morphology, chemical and phase composition, and surface states. Laser ablation in liquid (LAL) has attracted a lot of attention as one of synthetic techniques to prepare new attractive nanomaterials, with the ability to control both product chemistry and morphology in many systems. Recently, a number of novel nanostructures prepared via this approach have been reported, with diverse chemistries and morphologies. Here, we expand the technique, which was normally limited to the ambient pressure (1 atm), to higher (up to subcritical or even supercritical) pressures, in order to study the effect of pressure and prepare novel nanomaterials with unique properties.

The ablated target and liquid medium used in this study were Zn plates and deionized water, respectively. A nanosecond Nd:YAG laser with the wavelength of 532 nm was applied to irradiate Zn targets placed in a high-pressure cell with the inner volume of approximately 20 ml. The repetition pulse rate was 10 Hz, and the laser power was approximately 30 mJ/pulse. No heating, except for locally induced by laser pulses in pressurized water, was applied, while the pressure was varied from 0.1 MPa (1 atm) to ~30 MPa.

X-ray diffraction (XRD) measurements indicated that the resulted nanoparticles were ZnO. Their average size was approximately 10-20 nm depending on medium pressure, which was confirmed by both transmission electron microscopy observations and XRD measurements. The nanoparticles prepared at higher pressures demonstrated smaller sizes and more homogeneous size distribution. Photoluminescence spectroscopy was performed by using a 266-nm pulsed laser as the excitation source. All collected spectra exhibited a relatively sharp emission peak in the UV range and a broad emission band at ~570 nm. With the increase of medium pressure, the position of the former was shifting to narrower wavelengths (higher energies), while the relative intensity of the latter (at ~570 nm) was gradually increasing. Interestingly, a discontinuity in the visible emission enhancement of the product was observed near ~22 MPa, which is believed to be associated with the critical pressure of water (22.1 MPa).

In order to create novel nanosolders that bond at lower processing temperatures, it is important to understand the fundamentals of bonding between metallic nanoprticles. Next generation solders can potentially avoid issues with processing, reliability and manufacturing that plague conventional solders. We present the results of large scale molecular dynamics simulations aimed at revealing the mechanisms responsible for reactions at nanoscale metallic interfaces. Specifically, we study the bonding of Ag/Ag and Ag/Cu nanoparticles of varying radii at a range of temperatures. The specifics of the bonding process and the stable final states, both above and below the bulk eutectic temperature, will be compared to high resolution experiments conducted with an aberation correct STEM.

In this report, well-dispersed, stable pyrite NC inks were successfully produced by beads-milling technique and smooth pyrite thin films less than 50 nm were produced by coating the dispersed pyrite NC inks on various substrates. Such a work has not been reported up to date and will be interesting to the field.

Initially, 50 μm zirconia beads and a surfactant in a solvent were utilized for grinding and dispersing the pyrite NC clusters comprising pyrite NCs with sizes of 10 to 20 nm. The dynamic light scattering (DLS) was used to measure particle size distributions over time during the milling process. The NCs’ size, shape, and dispersive condition after beads milling were observed by transmission electron microscopy (TEM). X-ray diffractometer (XRD) was employed both before and after beads milling process to check any structure and crystallinity changes of pyrite NCs.

Pyrite thin films less than 50 nm were then spin-coated using the well-dispersed NCs inks. In order to obtain high quality pyrite thin films, post annealing treatment at 400 °C were conducted. The grazing incidence XRD (GIXRD) patterns were applied to show the intensity and FWHM of pyrite peaks. The Hall measurements revealed the semiconducting character of the thin films. The photoelectron emission and Tauc plot will be utilized to determine the band diagram of pyrite thin films.

With a band gap of 6.1 eV [1], AlN appears to be an ideal material for the development of deep ultraviolet, vacuum UV, and extreme detectors [2] The 6.1 eV band gap permits the visible background to be intrinsically suppressed and the detectors to operate at room temperature. Results of a LED based on aluminium nitride with an emission wavelength of 210 nm (which is the shortest ever reported) have been recently published [3]. The estimated efficiency of this AlN LED is still lower than for commercial LED but it could be greatly improved by the introduction of carrier confinement structure such as quantum dots. With quantum dots, restrictions due to material properties are shifted or lifted [4] and thus AlN nanostructures could be used successfully in the applications previously mentioned.

In this work, ultrathin AlN films were grown by plasma enhanced chemical vapor deposition (PECVD). The precursors used were nitrogen (N₂) as plasma source and nitride precursor and trimethylaluminium (TMA) as aluminium precursor diluted in Ar. A particular attention has been paid to control the deposition time using a shutter above the substrate holder. The surface morphology of the films has been observed by AFM (Atomic force microscopy) in tapping mode under ambient atmosphere at room temperature, in order to characterize the size and surface density of nano-islands on the surface. To verify the crystalline development and preferential orientations relative to the substrate, X-ray diffraction measurements are performed. Spectroscopic ellipsometry (SE) measurements were used to determine the optical properties of the samples and to extract the optical band-gap energy (Eg).

The presence of the shutter greatly improved both cristallinity and chemical composition of the films. Deposition at different time allowed proposing a growth mechanism with a favorable time range to obtain isolated nano-islands. The evolution of Eg versus the deposition time showed that the quantum confinement theory was followed.

[1] J. Li, K.B. Nam, M.L. Nakarmi, J.Y. Lin, H.X. Jiang P. Carrier, S.-H. Wei, Appl. Phys. Lett. 83, 5163 (2003)

[2] J. Lia, Z.Y. Fan, R. Dahal, M.L Nakarmi, J.Y. Lin, H.X. Jiangb, App. Phys. Letters 89, 213510 (2006)

[3] Y. Taniyasu, M. Kasu, T. Makimoto, Nature 441 (7091), 325 (2006)

[4] J. Stangl, V. Holy, G. Bauer, Rev. Mod. Phys. 76, 725 (2004)

Indium-doped ZnO nanostructures are regarded as promising candidates for transparent conductors, gas sensors, and photodetectors due to their good physical properties. In this work, we report the structure, microstructure, luminescence and electrical property of ZnO nanowires doped with indium (In) and, effect of In concentration on these properties. Indium doped ZnO nanowires have been grown on Si substrate by Chemical Vapor Deposition (CVD) method, at relatively low temperature (550 °C). A thin ZnO layer with (0002) preferred orientation was deposited on the Si substrate as seed layer by sputtering, which facilitates the growth of well-aligned ZnO nanowires. The average diameter and length of these nanowires varies from 50-180 nm and 10-15 μm, respectively. These nanowires are single crystals having (0001) growth direction. Series of experiments were carried out to estimate the possible solubility of In in ZnO. The maximum solubility of In in ZnO, that has been achieved by this growth process is 3.4 at. %. Photoluminescence (PL) study shows both UV emission and visible luminescence. Careful observation indicates a red shift in the UV emission in the PL pattern and the intensity of the green emission is increased with increasing indium content. Effect of indium concentration on the electrical properties of these nano-wires has also been investigated.

Graphene oxide holds promise as a carbon-based nanomaterial that can be produced inexpensively in large quantities. However, its structural and electrical properties remain far from those of the graphene sheets obtained by mechanical exfoliation or by chemical vapor deposition – unless efficient reduction methods that preserve the integrity of the parent carbon-network structure are found. Here, we use molecular dynamics and density functional theory calculations to show that the oxygen from the main functional groups present on graphene oxide sheets is removed by the reducing action of carbon monoxide; the energy barriers for reduction by CO are very small and easily overcome

at low temperatures. Infrared and Raman spectroscopy experiments confirm the reduction in CO atmosphere, and also reveal a strong tendency for CO to heal vacancies in the carbon network. Our results show that reduced graphene oxide with superior properties can be obtained through reduction in CO atmosphere.