AVS2009 Session NS-TuP: Nanometer-scale Science and Technology Poster Session

Nanoimprint lithography (NIL) is a very useful technique to make nanostructure devices with low cost and high throughput. So far, we reported room-temperature NIL (RT-NIL) using hydrogen silsesquioxane (HSQ) as a resin. However, the issue of a higher imprinting pressure than those of thermal NIL and UV NIL still remain to be addressed in NIL using spin-coated HSQ as the replication material. We attribute these disadvantages to evaporation of the solvent contained into HSQ during the spin-coating.

To overcome the above problem, we reported a Nanoimprint technique using liquid-phase HSQ as a replication material alternative to HSQ resin formed by conventional spin-coating. A low imprinting pressure and HSQ residual layer with less than 10 nm thick were achieved by using liquid-phase HSQ. But the HSQ-coated substrate is required to heat up around 90 ^oC to evaporate the solvent contained in the HSQ resin while the pressure was held. And a large area imprinting was difficult because the organic solvent in the HSQ evaporated only through the side-gap between SiO₂/Si mold and HSQ coated Si substrate. To address these problems, we used Poly(dimethyl siloxane) (PDMS) mold instead of SiO₂/Si mold. Because PDMS is a porous gel, the organic solvent in the HSQ solution can be evaporated through PDMS mold.

The new imprint technique using liquid-phase HSQ with PDMS mold is as follows. First, the PDMS mold was fabricated by spin-coating a mixture of base resin and hardener resin (Dow Corning Co. Sylgard 184) on the SiO₂/Si master mold. Following, PDMS coated substrate was baked at 70 ^oC for 15 min to cure the PDMS. After curing of the PDMS, the PDMS mold was demolded from the SiO₂/Si master mold. Next, HSQ solution was dropped onto a Si substrate. Following, the PDMS was pressed onto the Si substrate with 1MPa pressure. And then, the organic solvent in the HSQ solution was evaporated through the pores of PDMS. The imprinting temperature, pressure and time were RT, 1MPa, and 10 min, respectively. In this experiment, we used two kinds of HSQ, one is the caged HSQ (Dow Corning Co. Fox-16) and another is the ladder HSQ (Tokyo Ohka Kogyo Co. OCD T-12). Finally, the PDMS mold was removed from the Si substrate, and then HSQ patterns were successfully replicated on a 4 inch wafer in full area.

MoO is an interesting material for application in solid state microbatteries,electronic information displays, and optical memory and sensor devices. MoO₃ exhibits several polymorphs and, hence, the controlled growth and structure of MoO₃ are highly important. The successful efforts to synthesize metastable hexagonal phase (h–MoO₃) have been very limited, partly due to the complications in stabilizing the metastable-phase as compared to the thermodynamically stable a -MoO₃. As such, the physicochemical properties of the metastable h–MoO₃ phase are largely unknown, in spite of the fact that metastable structures often demonstrate new or enhanced activity when compared to thermodynamically stable phases. Here, we demonstrate a simple low-temperature chemical method to produce metastable h-MoO₃ nanorods and their excellent structural characteristics. Hexagonal MoO₃ samples were prepared via the precipitation of molybdenum oxide from an ammonium paramolybdate solution, by the addition of nitric acid. The structure of h-MoO₃ nano-rods was examined in detail using high-resolution scanning electron microscopy (HR-SEM) and high-resolution transmission electron microscopy (HR-TEM). A drop of the nanorods diluted in ethanol was added onto a carbon-coated TEM grid, and allowed to evaporate for analysis on a FEI Tecnai TF20 (200kV) equipped with a STEM unit, high-angle annular dark-field (HAADF) detector and X-Twin lenses. The SEM data reveal that the nano-rod h-MoO₃ crystals have the shape of straight hexagonal rods with an aspect ratio ~60. The HR-TEM results confirm the hexagonal structure of the MoO₃ nanocrystals. Computations of the observed TEM data along with x-ray diffraction pattern further confirm the stability of the nano-structure of h-MoO₃ rods. The results obtained are presented and discussed.

Biofunctionalized metal nanoparticles provide a sensitive platform for the analysis and detection of molecular recognitions in biological system. The remarkably sensitive optoelectronic properties of metal nanoparticles make nanoparticle-based materials a powerful tool to study fundamental biorecognition processes. Here we present a new and versatile method for coupling underivatized carbohydrates to gold nanoparticles (Au NPs) via the photochemically induced reaction of perfluorophenylazide (PFPA). A colorimetric model system based on the carbohydrate-modified Au NPs was furthermore constructed for the sensitive detection of carbohydrate-protein interactions. Treating mono-, and di-mannose-modified Au NPs with Concanavalin A (Con A) lead to a distinct color change of the Au NP solution, which was directly associated with the surface plasmon resonance shift upon nanoparticle aggregation. The coupling chemistry is highly efficient and is applicable to a wide range of carbohydrate structures.

Polymer based organic solar cells have made rapid progress after the advent of bulk-heterojunction concept consisting of an interpenetrating network of electron donor and acceptor material. Since slow growth of active layer allows to improve the cell performance, some approaches have been already used to reduce the film growth rate such as boiling point modification using solvent mixture, slow drying or solvent annealing by reducing spin coating time which controls the solvent annealing time.

In this work, we investigate the role of spin coating frequency combined with solution concentration at fixed active layer thickness on performance of polymer solar cells based on regioregular poly(3-hexylthiophene) (RR-P3HT):[6,6]-phenyl-C61–butyric acid methyl ester(PCBM) system. The thickness of active layer is standardized by controlling the solution viscosity with solution concentration. It is found that the spin coating frequency is associated with solvent evaporation time and highly affects the degree of interpenetration of the crystalline P3HT and the highest power conversion efficiency was obtained with the combination of the possible lowest spin coating frequency and solution concentration under AM 1.5 G spectral illumination of 100 mWcm^-2. The structural and optical properties of active layer are characterized by various analysis methods such as X-ray diffraction (XRD), ultraviolet-visible absorption spectroscopy and atomic force microscopy (AFM).

Recently, the organic solar cell has been receiving lots of attention as a clean and safe energy source instead of the fossil fuel due to its lower manufacturing cost and environment-friendly energy conversion capability. Several research groups have claimed the organic solar cells with ~5 % conversion efficiency. However, they still have low efficiency and the unstable because organic materials are easily oxidized by humidity and UV light under the atmosphere environment.

In this work, we propose an inverted-type organic/inorganic hybrid solar cell using ITO/ZnO/P3HT:PCBM/Au (indium tin oxide/zinc oxide/poly(3-hexylthiophene) : [6,6]-phenyl C₆₁ butyric acid methyl ester/Au) structure to enhance the efficiency and the stability of the organic solar cell. ZnO thin film is used as buffer layer to prevent the interface of organic layer by UV light from the oxidation and to reduce the energy barrier for easily transferring electron between ITO electrode and the LUMO level of the organic acceptor. Also, ZnO film blocks the injection of the hole from the P3HT to ITO for no charge carrier recombination. It is observed that the power conversion efficiency is significantly dependent on the thickness of ZnO thin film, which is deposited with magnetron sputtering system. The power conversion efficiency of 2.6 % under AM 1.5 G spectral illumination of 100 mWcm^-2 is obtained when the thickness of ZnO thin film and P3HT:PCBM is optimized. In addition, Au used as back electrode to solve the problem of Al, which is susceptible to reactions with oxygen and water, and to control the work function between HOMO level of P3HT and the energy level of metal electrode. From the periodically testing result for long days, we proved that our solar cell which isn’t encapsulated has a longer stability than typical organic solar cell fabricated by the same condition under ambient.

The vertical Si nanowires were grown on Si(100) substrate using a low-pressure chemical vapor deposition(LPCVD) system. The square pattern of AAO was obtained by combining pre-patterning Al thin film surface using focused ion beam (FIB) with anodizing process. The pore size and its period of the square matrix were approximately 50nm and 100nm, respectively. To determine the role of the indent depth on Al surface in AAO formation, FIB doses were varied from 1x10¹⁷ ions/point to 8x10¹⁷ ions/points. We found that a minimum indent depth is about 16nm. The minimum indent depth seems to be required to localize electric fields in a downward direction thereby yielding an indented pattern after AAO formation. In addition, we found that the higher voltage in anodizing process yields better uniformity in pore shapes. The Au nanoparticles as the catalyst for epitaxial growth of Si nanowires were deposited at the bottom of the AAO template on Si substrate using a electroless deposition with a mixuture of a solution containing Au ions and 1% HF. The average diameter of Au nanoparticles in each pore was 65nm.

Polycrystalline Fe-3%Si alloy was chosen as a model system for studying sputter induced topography modulations due to its properties: each grain, ranging few µm in size and of different crystalline orientation, behaves as a single-crystal surface that sputters independently of the surrounding grains. Thus, it is possible to experimentally include a large amount of grains of different crystallographic orientations in the same experimental run. Additionally, the samples were doped with a small amount of surface active elements to verify their role on the formation of self-assembled structures.

Samples of Fe-3%Si alloy were polished to achieve a smooth starting surface and subjected to Ar+ ion beam irradiation of different energy, angle and ion dose to obtain a complete set of experimental data on ion induced morphology changes. Results show that the surface exhibits grain-orientation dependent patterns, ranging from well-defined ripple structures to terraces, pits, or pyramidal structures. The characteristic length of these structures is in the order of a few hundred nanometres. Preliminary results show that surface active elements, such as Se or Sn, have a determining role on the formation of ripple-like patterns, causing the ripples to develop facet-like characteristics.

The samples were characterized using a multitechnique approach: Field Emission Scanning Electron Microscope (FE-SEM, Jeol JSM 6500-F) was used for the determination of different structures that formed in individual grains and to visualize a larger area of the sample, while Atomic Force Microscope (AFM, Veeco) gave detailed information on corrugation, individual ripple wavelengths, facet angles, pit depths and other morphological details.

First, 4-nitrobenzenthiol (NBT) SAMs were irradiated with visible light through photomask. The SAMs were formed on gold substrates by immersing the substrate in ethanol solution of NBT (1 mM) at room temperature for 24 hours. Photochemical changes were detected as surface potential shift using Kelvin-probe force microscopy. The surface potential of the irradiated regions was positively shifted by approximately 10 mV. This potential change suggests some photochemical changes induced by the light irradiation on the SAM. Water contact angle measurement and X-ray photoelectron spectroscopy (XPS) were also applied to the SAMs which are irradiated with the light through plain glass plate in these cases. Water contact angle increased from about 45 degrees to about 50 degrees after visible light irradiation. Proof of the photoinduced reduction was not detected by the XPS measurement probably because X-ray irradiation also promoted reduction of the nitrophenyl groups. These results suggest some changes induced on the surfaces by the light irradiation.

[1] M. Naoki, S. Takahiro, O. Masatoshi and U. Isamu, Chemistry Letters, 24, 145 (1994).

Size-selected Pt nanoparticles were synthesized by inverse micelle encapsulation on PS-PVP diblock copolymers. The thermal stability of these Pt nanoparticles deposited on TiO₂(110) has been investigated by scanning tunneling microscopy. Our micelle-based nanoparticles were found to be very stable against agglomeration and they preserved their initial size (~3.2 nm) and hexagonal arrangement up to an annealing temperature of at least 1000°C [1]. Furthermore, the original spherical shape of these particles was found to change upon annealing above 1000°C, leading to the formation of 3D faceted particles. Atomic desorption and cluster coarsening was observed at 1060°C. Our study also revealed that strong nanoparticle/support interactions are present in this system, resulting in the formation of TiO₂ nanostripes at high temperature with widths and a 2D spatial arrangement pre-determined by the presence and distribution of the Pt nanoparticles on the substrate.

Further insight into the mechanisms underlying the enhanced thermal stability of our micellar nanoparticles was obtained via a comparative study with UHV-evaporated Pt nanoparticles supported on pristine and polymer-coated TiO₂(110) surfaces.

A method to fabricate accurate nanostructures was developed using supperlattices and selective etching technique [1]. A probe shape characterizer for atomic force microscope (AFM) was developed for reliable measurement of AFM image and ISO standardization [2]. Comb-shape lines and spaces (10 nm to 50 nm) were fabricated using Si/SiO₂ superlattice. Line width and space distances were determined from the crossectional image of transmission electron microscope. Edge radii of these structures were less than 1.5 nm, and line width and line distance were fabricated in the accuracy less than 1.5 nm. The comb-shape lines and spaces were used to determine the aspect ratios of the AFM probe shape [1]. Aspect ratios of cone-like probe from tip apex to the bottom can be determined from apparent depth in the AFM image obtained from 8 kinds of different space widths. In case of the CNT probe, diameter of cylinder-like probe can be estimated from trajectory of the AFM probe on space structure. Advantage of characterizing probe shape using comb-shape pattern is that uncertainty can be defined from the shape of the reference material. Thus, reconstructed AFM image is reliable and applicable to characterize the precise morphology of nano-materials [3] and nano-structures.

References

[1] Hiroshi. Itoh, Toshiyuki. Fujimoto, and Shingo. Ichimura, Rev. Sci. Instrum. 77, 103704, (2006).

[2] Daisuke Fujita, Hiroshi Itoh, Shingo Ichimura and Tomizo Kurosawa, Nanotechnology 18, 084002 (2007).

[3] Hiroshi Itoh, Chunmei Wang, Yoshikazu Homma, Jielin Sun, Jun Hu, Shingo Ichimura, JJAP 47, 6128 (2008).

Zinc oxide is a very widely studied nanomaterial due to its potential applications in nanoscale optoelectronic and photovoltaic devices. A variety of ZnO quasi-one-dimensional morphologies have been synthesized by different methods. Often applications of the ZnO nanostructures are closed related to their morphology providing motivation to study the relationship between the morphology and growth technique.

Using a mixture of ZnO, Eu₂O₃ and C powder as source material, ZnO with a novel morphology referred to as nanoleaves was fabricated using thermal evaporation. The structures and morphologies of the as-synthesized samples were characterized by x-ray diffraction, field-emission scanning electron microscopy, high-resolution transmission electron microscopy and energy dispersive x-ray spectroscopy. The ZnO nanoleaves are wurtzite single crystals that have an appearance of fern-like leaves w ith symmetrical paired teeth on opposite sides. For some of the nanoleaves, there are long needles that grow from the end of some of the teeth and forming parallel arrays. The growth mechanism, kinetically driven process and photoluminescence of the ZnO nanoleaves will be discussed.

In this study, we report carbon nanotube (CNT) embedded 3-D carbon electrodes by using UV lithography on CNT embedded SU8 (photosensitive negative tone epoxy) and subsequent carbonization. Since the final electrodes consist of CNTs embedded in pyrolyzed carbon, they contain high electrical conductivity to contribute to increasing charging/discharging speed and chemical, mechanical stability in the electrolyte environment. Also, the small amount of CNTs in polymer does not interfere much with UV dose in the photolithography process, maintaining the high aspect ratio 3-D micropatterning capability.

[1] G.T. Teixidor, R.B. Zaouk, B.Y. Park, M.J. Madou, "Fabrication and characterization of threedimensional carbon electrodes for lithium-ion batteries", Journal of power sources 183 (2008) 730-740.

Recently, ZnO nanowires (NWs) have been extensively studied for solar cell and field-effect transistor applications because of their intrinsic n-type semiconductor property with a wide band gap of 3.36 eV and a large exciton binding energy of 60 meV[1]. Specially, the nanowire architecture in field-effect transistors provides many advantages against the conventional bulk substrate approach such as device size reduction, isolated channel, reduced leakage current, reduced control voltage, and reduced power consumption even to several nJ[2]. The ZnO nanowire based transistor can be extended to a non-volatile memory device by adopting ferroelectric materials for the gate dielectric layer. While Bi-based perovskite Bi_3.25La_0.75Ti₃O₁₂ (BLT) and Pb(Zr,Ti)O₃ (PZT) thin films have been extensively investigated for non-volatile ferroelectric random access memory (FRAM) devices, BLT based devices are reported to offer better fatigue resistant characteristic, low processing temperature, and large remnant polarization.

In this study, we show a process to make ZnO nanowires using electrospinning with the polymer ZnO nanocomposite and subsequent sintering in various temperatures and characterize them. Also, BLT is prepared by the metal organic deposition (MOD) process and is spincoated on a Pt(150nm)/Ti(50nm)/SiO₂/Si substrate, followed by sintering in different temperatures (550°C, 600°C, 650°C, and 700°C) to form crystalline perovskite structures [3]. And the ZnO nanowires are formed on the BLT ferroelectric gate and the electrodes for source and drain are defined using a UV lithography followed by metallization. We show non-volatile switching in ZnO NWs with the BLT ferroelectric gate. We also evaluate the field dependent conduction of ZnO NWs with the remnant field of ferroelectric thin films and show fatigue and retention free performance. These devices are useful not only for non-volatile memory devices but also for various biosensors and nanoelectromechanical systems.

[1] B. Sun, and H. Sirringhaus, Nano Lett., 5, 2408( 2005)

[2] X. Duan, Y. Huang, and C. M. Lieber, 2, Nano Letters, 487 ( 2002)

[3] Kyoung-Tae Kim, Chang-Il Kim, Yong-Kyu Yoon, “ Characteristics of La-substituted Bismuth Titanate Ferroelectric Nanofibers by Electrospinning ”AVS 55th Inter. Symp. Boston, USA (October 19-24, 2008)

Single-walled carbon nanotubes (SWCNTs) have been considered as a promising nanostructured material for the realization of future nanoelectronic devices because of their unique electrical properties such as the ballistic transportation of electrons or holes in SWCNTs. In this paper, we report deposition and alignment of SWCNTs using the dielectrophoresis (DEP) method and fabrication of single-walled carbon nanotube field-effect transistors (CNTFETs) with semiconductor materials as the source and drain materials. Ultra-purified HiPCO-grown single-walled carbon nanotubes (SWCNTs) from Carbon Nanotechnologies, Inc. (CNI) were used for the fabrication of CNTFETs. N-Methyl Pyrrolidone (NMP) was used to disperse SWCNTs in solutions. The dispersion of SWCNTs in the solvent was ultrasonically assisted, and then centrifuged. The degree of dispersion was examined by SEM. Dielectrophoresis (DEP) method was used to deposit, align, and assemble carbon nanotubes (CNTs) across the source and drain of CNTFETs to form the channel. Microfabrication techniques such as UV lithography and e-beam lithography were used to fabricate the CNTFETs. The gap between the source and drain varied from 800 nm to 3 um. Both metals such as gold and semiconductors such as bismuth telluride (Bi₂Te₃) were used as the source and drain materials for the CNTFETs. The drain-source current (I_DS) versus drain-source voltage (V_DS) and gate voltage (V_G) was characterized for the fabricated CNTFETs. The fabricated devices and measured electrical results will be reported in the Conference.

Metal-organic frameworks (MOFs) which consist of organic ligands linked together by metal ions belong to a relatively new class of porous materials.¹ Due to their potential applications in storage, separation, and heterogeneous catalysis, MOFs have attracted increasing attention. So far MOF research has been mainly focused on bulk structure of the material prepared in the form of a powder. Only very recently a new, and radically different, approach was undertaken by exploring the possibilities of MOF growth on surfaces in the form of thin films.^2,3 An important issue for further progress in optimizing growth of MOF crystals, and particularly MOF thin films, is control of their surface structure. Therefore, high-resolution imaging of MOF surfaces is of key importance, as has been previously recognized for zeolites which are analogous to MOFs.

The talk focuses on the AFM study of a MOF single crystal surface.⁴ By performing analysis both in air and under ultra high vacuum conditions the high-resolution AFM imaging of a MOF surface is demonstrated. The surface structure of a MOF crystal grown directly on the functionalized substrate is revealed and, moreover, this information is further utilized to optimize growth conditions. Finally, our study clearly demonstrates a much higher structural quality of MOF crystals grown on the substrate in comparison to the conventional powder growth method.

References

[1] O. M. Yaghi et al. 2003, Nature, 423, 705

[2] O. Shekah et al. 2007 J. Am. Chem. Soc. 129, 15118

[3] E. Biemmi et al. 2007 J. Am. Chem. Soc. 129, 8054

[4] K. Szelagowska-Kunstman et al. 2008 J. Am. Chem. Soc. 130, 14446

The broccoli-like TiO₂ film as working electrode for dye-sensitized solar cell (DSSC) was fabricated successfully by flat-flame chemical vapor deposition (FF-CVD) method we developed. The cell efficiency approaches 4.2% with the film thickness about 9~ 13 mm if the film was used without further surface modification. As we known, one source of the energy loss in DSSC is charge recombination occurring between electrolyte and working electrode. In order to reduce charge recombination, we supply a thin metal oxide coating on TiO₂ electrode by using surface modification with metal-containing salt solution as precursor. A higher band gap of the metal oxide semiconductor provides energy barrier to prevent electron transfer back to the oxidative species, and then reduce the charge recombination. However, although thicker metal oxide coating may reduces the rate of charge recombination, it may also reduce the rate of electron injection into the conduction band of TiO₂ working electrode and reduce the cell efficiency. Therefore, the thickness of the metal oxide coating needs to be optimized for the DSSC efficiency enhancement. In this research, we choose ZnO as coating oxide, since ZnO has slightly higher band gap (3.37eV) than TiO₂ (3.2eV). We vary the concentration of zinc acetate solution from 0.1 M to 0.001 M for coating different thicknesses of zinc oxide on TiO₂ working electrode. The crystalline quality and morphologies of surface modified TiO₂ electrodes were characterized by using XRD and FESEM. EDS and XPS were used to confirm the presence of zinc on the surface of TiO₂ electrode. The coating thicknesses were determined by high resolution transmission electron microscopy (HRTEM). The DSSCs using these working electrodes were measured under AM 1.5G 100 mW/cm² by Keithley 2400 sourcemeter. As a result, as the concentration of zinc oxide precursor solution decreases from 0.1 M to 0.001 M, the cell efficiency increases from 1.9% to 5.6%. The short circuit current density ( ) increases from 3.59 mA/cm² to 11.89 mA/cm² as the concentration decreases from 0.1 M to 0.001 M . Furthermore, we also found that the excess thickness of zinc oxide coating will block the electron injection, and results in lower efficiency after surface modification. The drop of efficiency from 4.2% to 1.9% is mainly attributed to the decrease of from 10.08 mA/cm² to 3.59 mA/cm². It reveals that appropriate thin zinc oxide coating is necessary in reducing charge recombination, while maintaining the high rate of electron injection to the conduction band of TiO₂ working electrode.