AVS2013 Session NS-TuP: Nanometer-scale Science and Technology Poster Session

Fabrication of templates for creating nanoscale structures becomes an important subject for innovation of nanoscale devices. The authors proposed new types of nanoscale structures such as a linked-crater structure and highly-oriented line structure on Al surface by unique combination of chemical and electrochemical processes. In this study, the authors investigate the optical functionality of nanostructures by photoluminescence (PL) measurements. The nanoscale linked-crater structure was fabricated on an Al surface by treatment with Semi Clean, alkali surfactant, and successive electrochemical anodization in 0.37 N H₂SO₄ solution creating a nanoscale finer linked-crater structure on the Al surface. The PL spectra of non-ripple and ripple patterns were observed by using Horiba Jobin-Yvon Spex Fluorolog-3 fluorimeter. In the case of non-ripple pattern, the PL spectrum showed that the main peak appeared around 400 nm with a long tail slope linearly downwards that stretched up to 550 nm regions. In the case of the ripple pattern, the PL spectrum exhibited a pattern composed of several ripple peaks in 350-550 nm range. The samples that exhibited rippled PL pattern were prepared under the optimized fabrication conditions : anodization current 7-9 mA for 40 min for the sample size of 10 mm×20 mm. The peak positions were invariant with the change of excitation wavelength through 300-420nm. Thus, the ripple pattern was originated from the nanostructure origin. The absorbance measurement was also conducted. The ripple absorbance peaks were observed and overlapped partially the rippled emission peaks. The absorbance peak positions were invariant with the change of excitation wavelength.

Non ripple-shaped emission peaks were observed in the case of the samples prepared under deviated conditions. The fabricated nanostructured surfaces were identified as alumina by Fourier transforms infrared spectroscopy (FT-IR). The FT-IR measurement showed that the difference between the ripple and non-ripple PL patterns might be attributed to the peak profiles and the intensity of Al-O mode. In the ripple emission and non ripple pattern, the intensity of Al-O mode at 1170 cm^-1 was different between two patterns. The lifetimes and the corresponding relative amplitudes were obtained by time-correlated single photon counting (TCSPC). In the case of ripple pattern, lifetimes and the relative amplitudes were definite values. On the other hand, in the non-ripple pattern, lifetimes and relative amplitudes were deviated from those values.

This work was aided by MEXT-supported Program for the Strategic Research Foundation at Private Universities.

In this report, we develop a synthesizing method for the gallium oxide and gallium nitride nanoparticle using non-thermal plasma. For gallium source, the gallium is evaporated by induction heating Nitrogen and oxygen source for nanoparticle synthesis are supplied from inductively coupled plasma with N2 and O2 gas. The synthesized nanoparticles are analyzed using field-emission scanning microscope(FESEM), transmission electron microscope(TEM) and x-ray photoelectron spectroscopy(XPS). The synthesized particles are investigated and discussed in wide range of experiment conditions such as flow rate, pressure and RF power.

References

1. L. Li, E. Auer, M. Liao, X. Fang, T. Zhai,U. K. Gautam, A. Lugstein, Y. Koide, Y. Bando and D. Golberg, Nanoscale, 3, 1120, 2011.

2. H. Yang, R. Shi, J. Yu, R. Liu, R. Zhang, H. Zhao, L. Zhang and H. Zheng, J. Phys. Chern. C 113 (52), 21548, 2009 .

*: partially funded by CONACyT-Mexico

As-grown light emitting self-assembled Ge nanocrystals (Ge-NCs) embedded in a SiO₂ matrix were produced by a sequential deposition process of SiO₂/Ge/SiO₂ layers employing reactive radio frequency sputtering technique. Obtained Ge-NCs shown a crystallographic phase whose proportion, size, quality and specific orientation are determined by the oxygen partial pressure. Photoluminescence (PL) spectra indicate that the size distribution of Ge-NCs is reduced and centered at around 8 nm when higher oxygen partial pressure is employed, their sizes are consistent with estimates from PL measurements. After vacuum annealing it is observed the elimination of an instable high pressure tetragonal phase of germanium present in as-grown samples. In addition, the PL peaks shifted to higher energies indicating the formation of Ge-NCs probably from Ge dispersed within SiO₂ matrix. It was also found that the PL intensity increases drastically after annealing process. The strong size dependence of the PL spectra indicates that the observed PL originates from the recombination of electron-hole pairs confined in Ge-NCs. This samples exhibit photoresponse in near infrared range.

^† : partially funded by CONACyT-Mexico.

Silicon nanocrystals (Si-NC) embedded within a SiO₂ matrix deposited on p type Si(111) substrates have been successfully produced through a reactive RF sputtering technique employing the ratio of oxygen to argon as parameter to modulate the Si-NC size distribution [1] . The Si-NC shown photoluminescence emission with characteristics dependent on its size. Characterization of the transversal I vs V response present a rectifying-like behavior. The spectral photoresponse curves shown a maximum around 1100 nm with a FWMH dependent on the Si-NC size distribution. The intensity of the spectral photoresponse shown a dependence on the electrical polarization applied on the contacts. The zero of the I vs V curves does not change with the illumination of the sample indicating the absence of a photovoltaic driving force. Results are analyzed taking in account the size and distribution of Si-NC in the SiO₂ matrix.

[1]. E. Mota-Pineda et al. Journal of Applied Physics 108, 094323(2010)

*: The partial financial support of ICyT-DF and CONACyT-Mex is acknowledged

Fabrication of templates for creating nanoscale structures becomes an important subject for innovation of nanoscale devices. Authors have succeeded in fabricating unique nanostructures such as a line and crater structures on Al substrate surfaces using chemical treatments combining anodization. Polyaniline(PA) line pattern was fabricated on Si surface using these nanostructures as a template. The purpose of this study was to fabricate PA patterns on the Al template and investigate the possibility of optical devices. Nanoscopic polymerization of PA was performed nanoscopically in confined nanoscale structures fabricated on an aluminum surfaces by a chemical polymerization method. Polymerization of PA was carried out using as an oxidizing agent, ammonium peroxodisulfate (APS). Solution from 1 to 2 μl contains aniline monomer, APS and HCl dropped on the nanostructures as a droplet utilizing micropipette and extended on the surface. PA patterns were successfully fabricated by this technique. The authors also conducted transfer of PA patterns formed on the nanostructured alumina to a Si wafer by a nano-contact method. A Si substrate was attached to the PA patterned Al substrate and was pressed for several ten seconds to transfer the PA pattern to the Si substrate. Then the Al template was removed and the Si substrate was dried naturally.

Photoluminescence (PL) spectra of the line-typed nanostructured Al surface and PA patterned Al surfaces were investigated by using Horiba Jobin-Yvon Spex Fluorolog-3 fluorimeter. Ripple-patterned PL peaks were observed for the line-typed nanostructured Al surface. PA line pattern was fabricated by dropping 1 μl of PA on the line-patterned Al surface. The PL peak at 480 nm originated from the PA line overlapped the ripple pattern. For the densely deposited sample prepared by 10 μl PA solution, a PL peak at 480 nm due to PA was clearly observed. PL lifetimes were measured by time-correlated single photon counting (TCSPC). Average PL lifetimes of PA deposited line-patterned Al surface were shorter than the original Al line-pattern. I-V characteristics of PA on the nanostructured Al surface and PA pattern-transferred Si wafer were obtained. Step-like curves were obtained in the PA pattern that the dots were linearly oriented. Negative resistance was observed in the PA pattern-transferred Si wafer.

Recently, carbon materials are attracting researcher’s attention for the development of next-generation devices. In the present study, the authors focused on grapheme film fabrication on non-flat surfaces. In the present work, the authors selected carbon drawing rather than the other methods such as taping method because carbon drawing is applicable to fabricate grapheme film on the non-flat surface. The aim of present study is to fabricate grapheme films on chemically and electrochemically processed nanostructured Al surface by carbon drawing.

Carbon drawing is a nanofabrication method by gently pushing bulk graphite on the surface and drawing it. In the present study, the authors used HOPG for carbon drawing which was performed on a line-typed nanostructured Al substrate. The line-typed nanostructured Al template was fabricated by chemically and electrochemically processes. In the present study, carbon drawing was performed in the vertical direction to the paralleled lines on the line-typed Al template. Fabricated structures after carbon drawing were analyzed and characterized by atomic force microscopy/current imaging tunneling spectroscopy (AFM/CITS), dynamic force microscopy (DFM), optical microscopy with differential interference contrast (DIC) and laser Raman spectroscopy. The DFM image demonstrated that triangular structures of 1 μm were created along the line structure and the triangular structures with a different thickness overlapped each other. The image in the large area of this sample obtained by optical microscopy with differential interference contrast (DIC) also showed that films with a different thickness overlapped each other. In the DFM image, terrace-like structures also appeared on the aluminum surface. Some characteristics peaks in the Raman spectrum were observed for this sample. A peak at 1600cm^-1 can be assigned to G band due to carbon. A peak at 1360cm^-1 can be assigned to D band. A peak at 2700cm^-1 can be assigned to G’ band. The current image in the AFM/CITS measurements showed that the current flows on the overall area. In point-contact measurements, current flows differently at the different points. Quantum conductivity such as step-type was observed. At the different area, wrinkles of thick films were formed on the surface in the AFM image. High conductivity was observed in the hill area whereas the valley area shows low conductivity. In the point-contact measurements, current flows on the overall area. Step-like conductivity was obtained in the I-V characteristics by point contact measurements.

This work was aided by MEXT-supported Program for the Strategic Research Foundation at Private Universities.

Carbon Nanotubes (CNTs) have attracted particular attention because of their unique properties in various application fields. Especially, CNTs are strong candidate for the anisotropic opt-material in the future optical devices. Some unique optical properties of CNTs have been reported, such as polarization dependent absorption of vertically aligned (VA) short length SWNTs[1], and black body behavior of VA long length SWNTs[2].

There are few reports for the optical properties of CNT forests with controlled structures. We focused on the early growth stage of the CNT forests whose heights are near the light wavelength. In this paper, we investigated optical properties of CNTs having various lengths with controlled structure parameters, such as vertical alignment.

Fe/Al or Fe/AlO multi-layered catalyst films were prepared on Si substrates by magnetron sputtering method. CNT forests were grown on the substrates with the catalyst films by thermal CVD method from C₂H₂ source gas at 730 °C. The CNT growth height was controlled precisely at the order of micro meter by adjusting synthesis period using electronic valves with electric circuits. Optical properties of CNTs were observed by a reflectance spectroscope (HITACHI U-3900).

Two different CNT forest samples were prepared by controlling CVD condition, one is highly oriented VA CNTs (Sample high-VA) with the average diameter of about 15 nm and the other is relatively low-oriented CNT forest (Sample low-VA) with the average diameter of about 13 nm. The average growth height of Sample high-VA and low-VA were controlled at about 1, 2, and 3 μm. Their growth densities of sample high-VA and low-VA were almost same at 10¹¹/cm², which were estimated from the cross-sectional SEM images. From the diffuse reflectance spectra, it was found that the CNT forests had low reflectivity with the diffusion reflectance below 5%. From specular reflectance, it was found that the specular reflectance was reduced with increase of forest height for both of sample high-VA and low-VA. It is noted that specular reflectance of sample high-VA was lower than sample low-VA at the same growth height. Absorption coefficients per 1μm calculated from the specular reflectance of sample low-VA were lower than that of sample high-VA.

In conclusion, those anisotropic optical properties of CNT forests were considered caused by their unique anisotropic structural parameters of vertical orientation and diameters.

This work was supported by Japan Society for the Science (JSPS) KAKENHI (No. 24560050, Grant-in-Aid for Scientific Research (C).

[1] Y. Murakami et al., Carbon. 43(13) (2005) 2664–2676.

[2] K. Mizuno et al., PNAS 106 (2009) 6044-6047.

Artificial superhydrophobic surfaces have generated considerable attention in the last two decades due to their significant potential for industrial and scientific applications. We introduced silicone nanofilaments (SNF) which grow on a variety of technologically important substrates via a convenient gas or solvent phase process. Coated substrates exhibited superhydrophobic behavior with contact angles higher than 160°. The simple one step production process includes only low cost starting materials and is ecologically friendly because it does not contain any fluorine compounds. Therefore it is of high interest for industrial applications. Until recently a drawback of the process was the release of hydrochloric acid which restricts the coating to acid-resistant substrates. Therefore we developed a similar solvent phase coating procedure using different starting materials to reduce the acid formation tremendously. Acid sensitive substrates were coated with SNF and also Silicone Nanotubes (SNT) without any damages to the material. In particular modified surfaces covered with homogenous SNT showed extreme water repellent properties with contact angles over 175° and sliding angles below 4°. The nanotubes were up to several micrometers long with diameters between 80 - 100 nm and channel diameters between 10-20 nm (Fig.1). This new acid-reduced process extends the number of materials suitable for superhydrophobic coating. Furthermore a convenient gas phase process for the fabrication of SNT-modified surfaces was developed. The process includes chemical vapor deposition at room temperature. The structure of the tubes can be easily controlled by adjusting the relative humidity in the reaction chamber. Such surfaces showed also superhydrophobic properties with contact angle higher than 179° and sliding angles below 1°. To the best of our knowledge the production of silicone nanotubes is reported for the first time. This new structures opens the opportunities in the field of silicone chemistry and might be of interest for potential applications.

Fig. 1. (a) TEM images of the active region of an InN/InGaN/GaN dot-in-a-nanorods heterostructures. The (b) and (c) show the micro-photoluminescence on the InN/InGaN/GaN dot-in-a-nanorods heterostructures.

Direct Methanol Fuel Cell (DMFC) has been deemed as one of the many feasible candidates for the portable power supply of the next generation. However, the slow process of methanol oxidation has been the root problem in the development of DMFC. Platinum based alloys have been widely developed and commonly utilized as the catalyst in methanol oxidation. In another field of technology, metal-titanium oxide composite has been adopted in distillation for decades due to the improved photo- catalytic capability resulting from the prolongation of recombination time between electrons and holes thereof. In this paper, Pt- TiO₂ composite nanostructure was deposited on carbon cloth by a one- step photo-deposition process in the atmospheric condition. Various surface compositions of Pt-TiO₂ nano composites were prepared. Also, a comparative study was conducted aiming at assessing the enhancement on the oxidation current of methanol, the suppression of CO poison, and the durability in acidic surrounding among the aforesaid Pt-TiO₂ nano composites. Furthermore, the electrochemical characteristics of photo enhanced methanol oxidation were studied via cyclic voltammetry and chronoamperometry under various illumination conditions. More than 40% of enhancement on the oxidation current density was found on the Pt-TiO₂ modified carbon cloth under UV illumination. The photo induced hydrophilicity also contributed to the local mass transfer on the surface of the aforesaid composite catalyst during the durability test. Moreover, the durability test also suggests the dispersion of TiO₂ NPs and the adhesion between TiO₂ NPs and carbon fibers play crucial roles in the performance of DMFC. In summary, the preliminary results suggest, with proper design on the surface composition, the Pt-TiO₂ nano-composite exists a great potential to serve as the anodic catalyst in DMFC.