AVS2010 Session NS-TuP: Nanometer-Scale Science and Technology Poster Session

Dip Pen Nanolithography^® (DPN^®) is an established method of nanofabrication in which materials are deposited onto a surface via a sharp tip. DPN enables controlled deposition of a variety of materials with nanoscale registry onto various substrates. Recent advances in DPN technology has resulted in the ability to directly print larger, biologically relevant materials on to a variety of surfaces under ambient conditions.

A novel method for the construction of hydrogel patterns has been developed. Hydrogels are of great interest to tissue engineers and other biomedical researchers because of the versatility of PEG chemistry and excellent biocompatibility. Also the mechanical and swelling properties of PEG hydrogels can be easily tuned by controlling the degree of cross-linking and choosing the appropriate molecular weight. Patterning of hydrogels in submicron scale with defined mechanical properties is highly desirable as a scaffold for tissue engineering and in vitro cell culture studies.

We report a novel method for generation of hydrogel patterns at subcellular scales. Hydrogel precursors are directly deposited at defined location and then polymerized to form hydrogels. This method allows for rapid fabrication of high resolution patterns. We used a simple desktop nanolithography platform (NLP 2000™, NanoInk, Inc.) for the deposition of the hydrogel precursors. The NLP 2000 consists of a stacked 3axis stage system with a travel range of 40 mm and a resolution of 25 nm. A high resolution optical microscope is available for monitoring the printing process. A custom fabricated array of cantilever based writing tools (M-Type, 12-pen, NanoInk, Inc.) were used to transport the hydrogel precursors on to the surface. Controlling the environmental conditions during the printing process allows for the transfer of defined volumes of hydrogel precursors. At 37°C, hydrogel domains of 6 μm were printed while at 25 °C 1.5 μm domains were printed. Patterning areas of 1 mm² with domains of less than 5 μm can be easily achieved in less than 30 min. AFM result confirmed the size and homogeneity of the printed hydrogel patterns.

Si_1-xGe_x and Si_1-yC_y alloys have attracted much attention from the perspective of the fabrication of novel devices, e.g., resonant tunneling diodes with strained alloys [1] and hetero bipolar transistors with double quantum wells [2]. It has been known that the lattice constants of these alloys are approximately proportional to their compositional ratios in accordance with Vegard’s law [3]. Further, it has been reported that the band-gap of Si_1-xGe_x, other than the lattice constant, changes with its composition [4]. In this study, we draw attention to the dependence of the dielectric constant on the composition of Si_1-xGe_x and Si_1-yC_y as well as the dependence of other physical quantities such as lattice constants and band-gaps. We explore the band discontinuity and the spatial modulation of dielectric constants for the Si_1-xGe_x/Si_1-yC_y superlattices as novel device structures, using first-principles ground-state calculations in external electric fields [5, 6].

We have adopted the cubic supercells containing 8 atoms for the Si_1-xGe_x and Si_1-yC_y bulk models. It has been shown that the lattice constants of Si_1-xGe_x and Si_1-yC_y alloys increase and decrease linearly with their compositions, respectively, obeying the Vegard’s law. In contrast, the nonlinearity with the composition is found for the band gap and the dielectric constants. In our presentation we discuss the origin of the onset of the nonlinearity and report the band and dielectric discontinuities of the Si_1-xGe_x/Si_1-yC_y superlattices from an atomic scale point of view.

[1] P. Han et al., J. Crystal Growth, 209, 315 (2000)

[2] D. C. Houghton et al., Phys. Rev. Lett 78 2441 (1997)

[3] W. Windl et al., Phys. Rev. B 57 2431 (1998)

[4] J. Weber et al., Phys. Rev. B 40 5683 (1989)

[5] J. Nakamura et al., J. Appl. Phys. 99, 054309 (2006); Appl. Phys. Lett. 89, 053118 (2006)

[6] S. Wakui et al., J. Vac. Sci. Technol. B 26, 1579 (2008)

The synthesis and characterization of spinel M_xCo_3-xO₄ (M = Mn, Fe, Ni, Cu) nanoparticles was undertaken. Cobalt oxide-based nanoparticles doped with transition metals have a wide variety of potential applications, ranging from new material coatings for concentrating solar power applications to performing as cathode catalysts in fuel cells and metal-air batteries. We have recently demonstrated that M_xCo_3-xO₄ nanoparticles can be prepared using simple, low temperature solution precipitation methods, and that the final size and morphology of the nanoparticle depends on the extent of doping. These materials have been characterized by powder X-ray diffraction (P XRD), high-resolution transmission electron microscopy (HR-TEM), N₂ adsorption-desorption, and thermal gravimetric analysis/differential thermal analysis ( TGA/DTA). The synthesis, characterization, as well as some potential applications of these materials, will be presented.

This work is supported by the Department of Energy, Office of Basic Energy Science and the United States Department of Energy under contract number DE-AC04-94AL85000. Sandia is a multi-program laboratory operated by Sandia Corporation, a Lockheed-Martin Company, for the United States Department of Energy.

Polymer-nanoparticles are technologically important composite systems, gaining increasing interest from many different fields of science in the last decade. Many procedures were introduced for synthesis of gold and other metallic NPs in solution and their integration to polymer films afterwards. But a more efficient approach is to synthesize nanoparticles directly inside the polymer matrix. Irradiation of polymer-metal precursor mixtures with energetic light is a practical method for this purpose. In this study, we present in-situ synthesis of gold nanoparticles within poly(methylmethacrylate) (PMMA) films by UV irradiation.

An advantage of synthesizing gold NPs within polymer films is the opportunity of photo-patterning. Films having patterns made of regions with and without gold NPs can be produced, using masks designed to cut off the UV radiation at desired places. Such patterned films were investigated with secondary electron microscope (SEM) and dark regions between irradiated regions and masked regions were observed. These dark regions are speculated to be “ion free regions”, where gold ions diffuse through irradiated regions during UV irradiation. These regions of about 10 μm width, suggests a very large distance for gold ions to diffuse through a rigid matrix like PMMA, which is very interesting. Supporting evidence for the existence and the properties of these regions was obtained from fluorescence studies with Rhodamine 6G molecule and x-ray electron spectroscopy (XPS). In XPS the gold content gradient shows, the border of the irradiated region contains more gold than the middle of the irradiated region which supports the diffusion of gold ions from masked regions through the irradiated regions.

Additionally, the electrical properties of PMMA with and without gold nanoparticles were investigated using XPS, while applying external bias to PMMA films with and without gold nanoparticles to probe the charging properties of the films. We observe an enhancement of conductivity of PMMA films containing gold nanoparticles.

Controlled positioning of metallic nanoparticles to assembly nanostructure has drawn interests in the field of nano-architecture, because of their electronic and optical properties. To achieve this, scientists have adopted varies materials and methods for “bottom-up” and “top-down” fabrication of nanostructures. DNA nowadays is more than just a carrier for genetic codes, but it has also been used as novel materials for building nano-architectures. In this study, we aim to assembly the long, periodic single-stranded DNA nanotemplate (ssDNA) on the silicon-based substrate covalently and couple the supramolecules into a functional bioactive system. The preparation of ssDNA nanotemplates is based on the aptameric recognition of tumor marker: platelet-derived growth factors and rolling circle amplification (RCA) technology. Because of the periodic, repeated sequence with secondary structures on the ssDNA nanotemplate, the supramolecules, including gold nanoparticles and peroxidase enzyme will be incorporated on the specific sites of the ssDNA nanotemplate based on the Watson-Crick base-pairing strategy in a programmable way. The distance between the gold nanoparticles and peroxidase enzyme can be controlled by manipulated the sequence on each repeat. In addition, the distance can also be increased merely by thermal treatment (around 80 °C) to open up the secondary structure on the ssDNA nanotemplate. Gold nanoparticles and peroxidase were periodically allocated precisely on each repeating sequence. The property of the peroxidase was affected by the gold nanoparticles and demonstrated by the luminescence measurement. In this study, we attempt to incorporate the gold nanoparticles on the ssDNA nanotemplate through hierarchical self-organization. The gold nanoparticle chains integrated with ssDNA nanotemplate were confirmed and visualized by the atomic force microscopy. We anticipate this study will pave a way for assembling novel bioactive materials with metallic nanoparticles for the development of modern electronic devices.

The Metalorganic Chemical Vapor Deposition (MOCVD), already promising for its potentialities in the scaling down of phase change memories (PCM, PRAM), was adopted for the Au-catalyzed self assembly of NanoWires (NWs) formed by Ge-Sb-Te alloys on SiO₂/Si substrates; the employed metalorganic precursors were tetrakisdimethylaminogermanium, trisdimethylaminoantimony and diisopropyltelluride; the process gas was N₂.

The NW properties were studied by X-ray Diffraction (XRD), Total Reflection X-ray Fluorescence (TXRF) and Scanning Electron Microscopy (SEM) observations. Local measurements were carried out on single nanostructures by High Resolution and Analytical Transmission Electron Microscopy (TEM).

Ge-Sb-Te (GST) NWs exhibited a mean diameter distribution centered on 35 nm and a length up to 1 μm. Two types of GST NWs were identified: i) “big” NWs, featured by mean cross size > 50 nm and aspect ratio (AR = length/mean width) distribution centered around 5 and ii) “small” NWs, featured by mean cross size < 50 nm and AR distribution centered around 12. Measurements indicated that GST NWs are compatible with both the Ge₁Sb₂Te₄ and Ge₁Sb₄Te₇ compositions and crystallographic phases.

The typical diameter of the GeTe NWs resulted to be around 50 nm and their length up to 4 μm. The structural measurements showed that both the cubic and rhombohedral crystalline phases of GeTe are present in the obtained NWs, whereas the compositional analyses yielded a Ge₃₆Te₆₄ composition. In particular, TEM observations revealed that the fcc wires are single crystals, 110 oriented.

An important area of biomedical nanotechnology is based on the interaction of living systems with inorganic and organic materials at the nanoscale. Silica nanospheres (NS) are attractive biomaterials because of their advantages as readily functionalized transport and imaging devices: the porous amorphous structure of silica colloid allows small molecule storage; the surface of silica can be modified easily with trimethoxysilyl reagents; silica has low biotoxicity and good biocompatibility. Silica nanospheres potentially have multiple biomedical applications as imaging agents, targeted drug delivery agents or gene transferring motherships. A simple method to fabricate hollow silica nanospheres with 100 nm or 200 nm diameters has been developed and tested. Amino polystyrene beads were used as templates and a 5-10 nm thick silica gel coating was formed by the sol-gel reaction. After removing template by calcinations, porous dehydrated silica gel nanoshells of uniform size were obtained. The porous structure of silica shell wall was characterized by transmission electron microscopy measurements, while particle size and zetapotentials of the particles suspended in aqueous solution were characterized by dynamic light scattering. The surfaces of the NS have been functionalized with folic acid in order to specifically target cancer cells. Folic acid, also known as vitamin B₉ or Folate, is essential for the synthesis of nucleotide bases and binds with high affinity to Folate receptors, which are frequently over-expressed in tumor cells and epithelial lineaged tumors such as ovarian carcinomas. With the use of confocol and two-photon microscopy, it was found that as the amount of folate on the surface of the NS was increased, a higher amount of NS endocytose into HeLa cancer cells, a cervical cancer cell line. Cytotoxicity studies will quantify the effectiveness of using folate coated siica shells for enhancing endocytosis of chemotherapy drugs in cell lines and in animal studies.

Two technologically important aspects for the applications of metal nanoparticles in the fields of catalysis, molecular electronics, and plasmonics are discussed here: (i) the influence of the nanoparticle synthesis method and support morphology on thermally-driven coarsening phenomena, (ii) the high-temperature growth of oxide nanowires with tunable width, orientation, and spacing seeded by micellar metal nanoparticles (NPs). Along those lines, a comparison of the mobility of evaporated Pt NPs supported on pristine TiO₂(110) and on polymer-modified TiO₂(110) and micelle-based Pt NPs is established. To gain additional insight into the NP coarsening mechanisms, simulations were conducted following the two most coarsening routes: (i) diffusion-coalescence, and (ii) Ostwald ripening. Some changes were introduced in these models to guarantee the correct temperature dependence and proper energetics in the diffusion model, and a new formulation of the critical radius in the Ostwald ripening model is introduced to satisfy the mass balance without the need to use very small time steps.

Iron doped titanium dioxide nanoparticles were synthesized using the sol-gel method. Solution was prepared in propanol, diluting Tween 80 surfactant (as pore directing agent), acetic acid and finally titanium tetrapropoxide (TTP); constantly stirred with a motor driven Teflon® palette. Doping iron was introduced by dissolving iron (III) chloride in propanol and adding to the solution; in the required amount to obtain 0.25; 0.5; 0.75; 1.0; 1.25 and 1.57 mol% iron with respect to titanium. Employed proportions of Tween80/PrOH/acetic acid/TTP were defined mostly following the values reported by Dionysiou´s group on pure TiO₂ [1]. We obtained nanoparticles with sizes around 9 nm; being larger for lower iron content and smaller for higher iron content, as observed by electron microscopy and calculated from the full width at half maximum (FWHM) of X ray diffraction peaks, using Scherrer’s formula. Raman spectroscopy and X ray diffraction measurements showed the presence of additional features, possibly related to a foreign phase into an almost pure anatase powder, increasing along with iron content.

[1] H. Choi, E. Stathatos, D.D. Dionysiou, Thin Solid Films 510 (2006) 107.

Aknowledgements: Authors want to thank the technical assistance of Gabriel Ramos Romo and Dr. Israel Ceja Andrade; as well as the financial support from UdeG (under program pro-SNI) and PROMEP (under project 103.5/07/2636).

CNT film field emitters (FFE) with a high current density up to a couple of 100 A/cm² and a high total current up to 100 mA are applicable to electron sources such as accelerators and high intensity X-ray sources for medical and security examinations. Based on our previous work of FFE, it turned out that one of the most crucial factors for high current and long time operation is to select appropriate CNT junction and substrate materials so as to maintain high thermal and electric conduction with high tensile strength. Our FFE was prepared by dispersing MWCNTs over a titanium film deposited on a metallic substrate by magnetron sputtering technique followed with rooting of MWCNTs into the titanium film at high temperature. In this study, use of titanium micropowders was attempted to sprinkle on the dispersed MWCNTs. This powder would allow to enhance reaction for better carbide formation. This paper describes the detailed procedures and the experiments to achieve the high emission current density from the FFE.

In this work we present the results of the pH effect on the synthesis of Sn0₂ by precipitation. This simple chemical route allowed us to obtain Sn0₂ nanocrystals whose grain size depends linearly on the pH, ranging from 3 to 8nm, from basic to acid solutions, respectively. The most important, microstructures were also fabricated by this simple method, mainly whiskers and long fibers. The crystalline quality and the morphology were confirmed by X-ray diffraction and electron microscopy. The samples showed high-energy optical absorption as observed by UV-Vis measurements.

Aknowledgements:

The technical support of Israel Gradilla and Francisco Ruiz are greatly acknowledged. This work was partially supported by PROMEP (projects 103.5/07/2636 and 103.5/08/2919) and COECYTJAL (project PS-2009-873).

Nanostructures made of wurtzite ZnO, such as dots, nanobelts, nanowires, and nanocrystals, have recently attracted attention due to their proposed applications in electronic and optoelectronic devices. Here we present a comparative study on the properties of ZnO thin films containing nanocrystals and nanowires.

ZnO nanocrystalline (NC) films, obtained by the sol-gel process, were deposited from a precursor solution using zinc acetate dehydrate in ethyl alcohol. After a four-layer spin-coating on crystalline substrates, the films were transformed into nanocrystalline films by a thermal treatment at 370 ^oC during 3 hours. ZnO nanowires (NW) were electrochemically grown onto a ZnO seed layer sol-gel spin-coated using a conventional three electrode cell, with the substrate as the cathode, a Zn sheet as the counter electrode and a saturated calomel electrode (SCE) as the reference one. The electrolyte was an aqueous solution of the Zn⁺² precursor (1 mM zinc acetate) and a supporting electrolyte (0.1 M sodium acetate), saturated with bubbling oxygen. The electrodeposition was carried out at 70 ^oC under potentiostatic conditions at two different potential values (-0.900 and -1.000 V vs. SCE) and during 70 min. The initial pH was adjusted to 6.76. Both, NC and NW samples, were deposited onto crystalline quartz substrates covered by a Au or Ag electrode, and ready to use in a quartz crystal microbalance (QCM).

Samples microstructure was characterized by X-ray Diffraction (XRD), Scanning Electron Microscopy (SEM), and Optical Diffuse Reflectance Spectroscopy (DRS). XRD measurements show in both cases a typical diffraction pattern of ZnO wurtzite structure. SEM micrographs of NC films have a smooth surface, while the NW sample reveal the presence of nanowires with hexagonal section and diameters ranging from 40 to 100 nm. No specific structure was observed by the DRS measurements on the seed layer, probably due to the fact that these films were not thick enough as required by the technique. For the NW onto seed layer samples, the optical characterization reveals the presence of ZnO with bandgap energy between 3.22 eV (for the ones grown onto Au metallic contacts) and 3.29 eV (for the Ag metallic contacts).

A QCM placed in a vacuum chamber was used to measure the water adsorption of the samples. Water vapour was introduced through a leak valve while a capacitance manometer was used to measure the partial pressure of water in the range 10¹-10⁵ Pa. The mass of water adsorbed on the surface of the quartz crystal was calculated using the Sauerbrey equation. The NW drastic increase of the surface area was revealed through a higher amount of water adsorption.

ZnO is a promising material for ultraviolet (UV) and white light-emitting diode (LED) applications, because of its large exciton binding energy of 60 meV relative to the thermal energy of 25 meV, as well as its large band gap of 3.37 eV at room temperature. In the past several years, the fabrications and characterization of one-dimensional ZnO nanostructures have been extensively investigated for their applications, such as LEDs, gas sensors, field emission devices, nanolasers and photovoltaics. We have studied on application of vertically aligned ZnO nanorods grown by CVD to field emission devices. In this paper we report on surface-sulfurization of Al-doped ZnO nanorods and the effects on the field emission characteristics.

Al-doped ZnO nanorods are grown by low-pressure thermal CVD cooperated with 10Hz Nd-YAG pulsed laser ablation of Al2O3 target, which is developed by us. Precursors for CVD are Zn vapor and O2. Substrates are n+Si(111) wafers. ZnO nanorods are grown in two stages. In first stage of growth no laser ablation is used to grow aligned ZnO nanorods and in second growth stage laser ablation is used to grow Al-doped ZnO layer on ZnO nanorods grown in 1st stage. Concentration of Al in ZnO nanorods is controlled by laser power. Finally surface-sulfurization is performed in H2S atmosphere. It is revealed that the conditions of sulfurization have very complex effects on field emission characteristics. An example of the conditions are 39.6 Pa of H2S partial pressure, 550 C and 5 min. Field emission characteristics are measured in vacuum of 10^-4 Pa using 12.7mm diameter metal ball as anode electrode with separation of 160 micron meter.