Raphael Grüter,1 Pascal Behr,1,2 Michael Gabi,2 Jérôme Polesel Maris,3 János Vörös,1 Tomaso Zambelli1
1Laboratory of Biosensors and Bioelectronics, D-ITET, ETH Zurich, Switzerland
2Cytosurge GmbH, Zurich, Switzerland
3Commissariat à l’énergie atomique, SACLAY, France mailto:[email protected]
The FluidFM [1] combines AFM technology with nanofluidics. A channel incorporated directly in the cantilever and an aperture at the apex of the tip allows local liquid dispensing of soluble molecules in air and in liquid. Therefore, the FluidFM is the adapted tool for local chemistry with high resolution.
Especially for synthesis and modification of nanowires, it provides new opportunities.
Nanowires have attracted great interest in biological sensing research. The high contact area between analyst and nanowire leads to a high sensitivity. A novel protocol for local synthesis and functionalization by FluidFM technology is presented thanks to the opportunity of filling the microchannel with specific soluble molecules. Both, metal and conductive polymer nanowires can be deposited in liquid and at desired position of the substrate. Because of the dimension of the aperture at the apex of the tip, the FluidFM can later be used for individual functionalization of closely packed nanowires with specific receptor molecules.
Additionally, the FluidFM can be used as an electrochemical lithographic tool. A non-conducting organic coating can be locally electrografted on a conducting substrate by incorporation of a counter electrode into the connected fluidic circuit and applying a potential between counter electrode and conducting substrate.
Acknowledgements
The authors acknowledge the financial support from the Swiss federal program Nano-Tera.ch.
References
[1] A. Meister, M. Gabi, P. Behr, P. Studer, J. Vörös, P. Niedermann, J. Bitterli, J. Polesel-Maris, M.
Liley, H. Heinzelmann, T. Zambelli, Nano Letters 9 (2009) 2501.
Figures
Figure 1: Different geometries of microchanneled cantilever tips. a) Pyramidal tip with an aperture of 200 nm at the apex. b) Cylindrical tip with embedded pyramid for high-resolution scanning.
Figure 2: Deposition (in air) of Au-colloids (5nm) onto a PEI-coated Si-wafer.
Epitaxial growth of highly mismatched GaInAs layers on nanoporous GaAs substrates Jan Grym1, Dušan Nohavica1,2, Petar Gladkov1,2, Eduard Hulicius2, Jiří Pangrác2
1Institute of Photonics and Electronics, ASCR, Prague, Czech Republic
2Institute of Physics ASCR, Prague, Czech Republic [email protected]
Pore formation in AIIIBV materials was reported in a number of recent papers [1-4]. On the contrary, their technological applications in epitaxial growth have not been thoroughly investigated yet.
Epitaxial growth has always been a marriage of convenience between deposited layer and substrate. In the simplest case, both layer and substrate are of the same material, providing a homoepitaxial match.
Frequently, it is impractical to use the same material for both the layer and the substrate since certain large single crystals are not available, are expensive, or their properties are inappropriate for the intended application. Different strategies were suggested to achieve high quality single-crystal thin films grown on a lattice mismatched substrate. Significant development in defect density reduction in semiconductor materials has been accomplished using epitaxial lateral overgrowth techniques [5].
Recently, semiconductor epitaxial growth has progressed to pseudomorphic, lattice mismatched systems where a small amount of strain is accommodated in very thin layers [6].
A new approach extending the critical layer thickness in highly mismatched heterostructures is nanoheteroepitaxy [7]. Nanoheteroepitaxy exploits the three-dimensional stress relief mechanisms that are available in nanoscale objects and applies this property to reduce the strain energy in lattice mismatched heterojunctions. While in conventional planar structures the epilayer can only deform vertically, in a nanopatterned substrate a selectively growing epilayer can deform vertically and laterally, and the strain energy decreases exponentially with the distance from the growth interface [8].
We take advantage of a novel concept of the epitaxial growth of largely lattice mismatched layers on nanoporous substrates [9]. It is essential that the substrate takes over most of the strain of the layer at the initial growth stage. We report on the preparation of nanoporous GaAs substrates and on the growth of GaInAs epitaxial layers by the liquid phase epitaxy and metal organic vapour phase epitaxy (MOVPE) on these nanoporous substrates.
The pore etching was carried out in an electrochemical cell containing a fluoride-iodide aqueous electrolyte (H2O-HF-KI) using a configuration equivalent to four electrodes. A home-made potentiostat/galvanostat was computer-controlled and allowed to register all process variables. (100)- oriented GaAs:Si substrates with a carrier concentration of 2x1018 cm-3 were used for the pore preparation. The layers of Ga0.8In0.2As were grown in AIXTRON 200 MOVPE apparatus. The porous structures before and after the epitaxial overgrowth were observed by scanning electron microscopy (SEM) and atomic force microscopy (AFM). The composition of the grown layer was determined by the electron microprobe with wavelength-dispersive spectrometer and correlated with the results of low temperature photoluminescence (PL) spectroscopy. The surface morphology of the layers was observed by Nomarski differential interference contrast microscopy (NDICM).
Figure 1 shows a cross-section of a GaAs substrate with a porous layer of 5 μm in thickness, which was overgrown by 0.7 μm thick layer of Ga0.8In0.2As. A comparison of the surface morphology of the epitaxial layer grown on porous and nonporous substrates observed by NDICM is shown in figure 2. While a typical cross-hatching pattern corresponding to a large misfit between the substrate and the layer is observed on the nonporous substrate, a random pattern is observed on the porous substrate. This observation, together with the results of low temperature PL measurements indicate that the porous substrate gives rise to the decrease in the density of extended defects at the heterointerface.
The work was supported by the project P108/10/0253 of the Czech Science Foundation.
References
[1] Föll, H., et al., Advanced Materials, 3 (2003) 183-198.
[2] Santinacci, L. and Djenizian, T., Comptes Rendus Chimie, 9 (2008) 964-983.
[3] Ulin, V. and Konnikov, S., Semiconductors, 7 (2007) 832-844.
[4] Ulin, V. and Konnikov, S., Semiconductors, 7 (2007) 845-854.
[5] Nishinaga, T., Journal of Crystal Growth, 237-239 (2002) 1410-1417.
[6] Atkinson, A., Jain, S.C., and Harker, A.H., Journal of Applied Physics, 5 (1995) 1907-1913.
[7] Zubia, D., et al., Journal of Vacuum Science & Technology B, 6 (2000) 3514-3520.
[8] Luryi, S. and Suhir, E., Applied Physics Letters, 3 (1986) 140-142.
[9] Sitnikova, A.A., et al., Semiconductors, 39 (2005) 523-527.
Figures
Figure 1: SEM micrographs of .the cross section of porous GaAs substrate overgrown by 700 nm thick layer of GaInAs. The sample was cleaved along two perpendicular directions shown on the upper and lower panel respectively. The right panel shows the same with a higher magnification.
Figure 2: Nomarski contrast optical micrograph of the InGaAs layer grown on standard GaAs substrate (left panel) and porous GaAs substrate (right panel).
Hydrogen Sensors Based on Electrophoretically Deposited Pd Nanoparticles onto InP Jan Grym1, Olga Procházková1, Roman Yatskiv1, Kateřina Piksová2
1Institute of Photonics and Electronics, ASCR, Prague, Czech Republic
2Faculty of Nuclear Science and Physical Engineering, CTU in Prague, Czech Republic [email protected]
Metal nanoparticles (MNPs) form a bridge between bulk materials and atomic or molecular structures.
Bulk metals show constant size independent physical properties while the properties of MNPs are driven by their size, shape, and inter-particle distance. Surface properties are crucial because the number of surface atoms becomes significant as the MNP reaches the nanoscale limit [1]. III-V semiconductors have established their position in electronic devices thanks to their unique properties.
As compared to silicon, they offer higher operating speeds, lower power consumption, or higher light emission efficiency. However, to fully exploit their properties, there is one key point remaining to be solved. III-V semiconductor structures suffer from a high density of surface/interface states causing so called Fermi level pinning [2]. The Fermi level pinning leads to low Schottky barrier heights (SBH) on n- type III-Vs, which are metal independent when prepared by standard evaporation techniques [3]. In this paper we report on the preparation of Schottky barriers on InP substrates with increased SBHs by the electrophoretic deposition of palladium NPs. We also demonstrate their application in hydrogen sensors.
Pd nanoparticles with the diameters of 7 and 10 nm were prepared in isooctane colloid solution by the reverse micelle technique [4]. The electrophoretic deposition from the colloid solution took place in a cell with two parallel electrodes. The upper electrode was made from a high-purity graphite, the lower electrode was formed by an InP substrate of n-type conductivity with the background concentration of about 6×1015 cm-3. Pulsed DC voltage with a duty cycle of 50 % was applied for a selected period of time to deposit a Pd nanolayer.
We discuss the influence of (i) the final substrate surface treatment, (ii) the properties of the deposited colloid solution, (iii) the elecrophoretic deposition conditions (time, electrode polarity, applied voltage), and (iv) the post-deposition treatment of the layers (chemical treatment in peroxide and annealing at elevated temperatures) on the morphology of the deposited layers, their electrical properties, and their sensitivity towards hydrogen.
Layers of nanoparticles were observed in JEOL JSM 7500F scanning electron microscope and by AFM.
Selected layers were contacted by the spots of a conductive silver or graphite colloid paint. These structures were further characterized by the measurement of current-voltage characteristics and their detection towards hydrogen was tested in a cell with a through-flow gas system.
The coverage of the surface strongly depends on the applied voltage. The higher the voltage, the better the coverage and the smaller the size of deposited clusters. Figure 1 shows the morphology of the layers deposited at 100 V for 1 hour and 18 hours respectively. The high values of SBH reaching 0.9 eV – in comparison with thermally evaporated Pd reaching 0.45 eV only – indicate a very low degree of Fermi level pinning. This is further proved by the hydrogen detection measurement. The best results reached for a mixture of hydrogen (20 %) and nitrogen gases show an increase of current by six orders of magnitude (see Figure 2). The hydrogen molecules are absorbed and dissociated at Pd surface, atomic hydrogen rapidly diffuses to the Pd/InP interface, where the dipole layer develops. Subsequently, the Schottky barrier height decreases and the electric current increases.
The work was supported by the projects 102/09/1037 of the Czech Science Foundation and grant KJB200670901 of the ASCR.
References
[1] Hossam, H., Journal of Physics D: Applied Physics, 23 (2007) 7173-7186.
[2] Hasegawa, H. and Akazawa, M., Applied Surface Science, 24 (2008) 8005-8015.
[3] Hasegawa, H., Solid-State Electronics, 10 (1997) 1441-1450.
[4] Chen, D.-H., Wang, C.-C., and Huang, T.-C., Journal of Colloid and Interface Science, 1 (1999) 123-129.
Figures
Figure 1: SEM images of Pd NPs deposited onto the InP substrate with the applied voltage of 100 V with different deposition times: 1 hour (left panel) and 18 hours (right panel).
Figure 2: Current-transient characteristics of Pd-InP Schottky diodes exposed to hydrogen/nitrogen gas mixture.
Samples 004 to 007 were deposited at a different applied voltage ranging from 30 V to 100 V.
Reducing zinc oxide in rubber industry use through the development of mixed metal oxide nanoparticles
Manuel Guzmán, Núria Agulló, Salvador Borrós
Grup d’Enginyeria de Materials (GEMAT), Institut Químic de Sarrià-Universitat Ramon Llull, Via Augusta 390, Barcelona, Spain
Zinc oxide is a widely used compound in rubber industry due to the excellent properties that shows as activator for sulphur vulcanisation. The tire industry remains the largest single market for ZnO, consuming more than half of the total worldwide demand of 1,200,000 metric tons1. Traditionally, ZnO is used in rubber formulations in concentrations of 3–8 parts per hundred rubber (phr).
Despite its superior characteristics, there is an increased concern about the environmental effects that zinc oxide causes and over the years lower levels of zinc have been tried in order to decrease its impact and to minimise the production costs. Different approaches have been considered for reducing zinc levels. Between all the alternatives proposed, the use of nano–sized ZnO particles with high surface area seems to be promising. However, it was found that the use of more active forms of zinc oxide did not substantially reduce further the minimum zinc content that can be achieved with conventional zinc oxide although the dispersion of high surface area ZnO during mixing was found to be significantly better, which could enable low levels of this zinc oxide to be used in industry with more confidence2. There have been a number of studies comparing different metal oxides as vulcanisation activators in order to find substitutes for zinc oxide. Several metal oxides have been used, CaO, MgO, CdO, CuO, PbO and NiO. Among them, MgO is the most promising candidate since it is a non heavy metal oxide that provokes the breakdown of the accelerator to be faster than when ZnO is used and it is able to form active sulphurating agents. However the crosslink level achieved is lower than that obtained with zinc oxide, which has limited its industrial application.
In this article, a new approach to overcome the problems between ZnO and MgO is presented. It consists in the development of a new activator based in the mixture of both mixed oxides at nanoscale to take advantage of the behaviour of both zinc and magnesium oxides as nanoparticles. The new activator is nanometer–sized mixed metal oxide particles of zinc and magnesium (Zn1–xMgxO) with very precise stoichiometry prepared employing a polymer–based method. In this accelerator, magnesium is incorporated into the ZnO structure and this inclusion and its size are expected to show a better performance taking advantage of the behaviour of both ZnO and MgO in sulphur vulcanisation.
Basically, the method consists on the preparation of a polymer/metal salt complex that is water-soluble, its purification by precipitation/redissolution cycles and finally the calcination of the dried purified complex to give nanosized crystals3. The polymer used to form the polymer/metal salt complex is poly [acrylic acid]. Magnesium nitrate hexahydrate, and zinc nitrate hexahydrate are the starting materials.
Dynamic Light Scattering was employed to measure the particle size of the Zn1-xMgxO particles, which was found to be is in the range of 100 to 175 nm with a narrow distribution as seen in Figure 1. No apparent dependence of the particle size with the magnesium content was found.
X–Ray diffraction was employed to characterise the crystal structure of the mixed metal oxide particles.
Figure 2 shows the X-ray diffraction patterns of the oxide products obtained in the different syntheses.
The patterns of the pure ZnO are indexed according to the known hexagonal phase (zincite)4, and that of MgO is indexed according to its cubic phase (periclase)5. In ¡Error! No se encuentra el origen de la referencia., the vertical line corresponds to the standard reflections of the ZnO phase and the dashed vertical lines are the standard reflections of the MgO phase which planes are indicated with a star. The scans showed a weak (200) peak of the MgO phase, which demonstrates that the material is mostly present in the hexagonal phase (zincite) and that magnesium has been incorporated into the ZnO structure. In addition, the lattice constant c of the synthesized nanoparticles has decreased in comparison to the corresponding ZnO phase. As magnesium replaces zinc in the hexagonal phase, due to the smaller radius of Mg2+ than that of Zn2+, there is a shrinking of the lattice constant along the c–
axis and a displacement of the diffraction peaks to higher angles. These findings indicate Mg2+ ions replace the Zn2+ ions into the zincite.
The model compound vulcanisation (MCV) approach with squalene as a model molecule for natural rubber and N-Cyclohexylbenzothiazole-2-sulphenamide (CBS) as accelerator has been used to study the role of the mixed metal oxide along the reaction. The results obtained with Zn1-xMgxO nanoparticles
as activator for sulphenamide accelerated sulphur vulcanisation have shown when Zn1-xMgxO nanoparticles are used it is possible to take advantage of the behaviour of both ZnO and MgO in sulphur vulcanisation. It has been seen that the reactions that take place during the scorch time, the breakdown of the accelerator and the formation of MBT occur faster, which could be due to the presence of magnesium into the zinc oxide structure.
Nevertheless, the crosslink degree achieved is higher than those obtained with zinc oxide nanoparticles.
It is worth noting that mixed metal oxide nanoparticles of zinc and magnesium lead to around a 30 % higher crosslink degree than the one obtained with standard zinc oxide. This effect can be partly attributed to the small particle size of the Zn1-xMgxO since Bhowmick et al.6-7 found that ZnO nanoparticles (30-70 nm) increased the crosslink degree by 15 % compared with standard ZnO. On the other hand, the fact that, even with bigger sizes, higher amounts of crosslinked products are formed suggests that Zn1-xMgxO nanoparticles are more active and more effective transporting sulphur into the hydrocarbon chain than ZnO nanoparticles. Therfore, Zn1-xMgxO nanoparticles not only overcome the disadvantages of the use of a mixture of ZnO and MgO reported previously in the literature8, which shows a crosslink degree similar to the one obtained with magnesium oxide, but a better performance is achieved.
References
[1] Walter, J., Tire Technology International, March (2009) 18.
[2] Chapman, A. V., Johnson, T., Kautschuk Gummi Kunststoffe 58 (2005) 358.
[3] Lu, G., Lieberwirth, I., Wegner, G., Journal of the American Chemical Society 128 (2006) 15445.
[4] JCPDS Card No 36-1451.
[5] JCPDS Card No 45-0946.
[6] Sahoo, S., Kar, S., Ganguly, A., Maiti, M., Bhowmick, A. K., Polymers & Polymer Composites 16 (2008) 193.
[7] Sahoo, S., Kar, S., Ganguly, A., George, J.J., Bhowmick, A. K., Journal of Applied Polymer Science 105 (2007) 2407.
[8] Vega, B., Doctoral Thesis, Universitat Ramon Llul, Barcelona (2008).
Figures
Figure 1. Particle size measurement of a) MG1.2 and b) MG1.5.
Figure 2. XRD patterns of Zn1-xMgxO.
Towards alternative 2D polymers based on coordination polymers
Cristina Hermosa,a Almudena Gallego,a Oscar Castillo,b Salomé Delgado,a Julio Gómez-Herrero,c Félix Zamoraa
a Departamento de Química Inorgánica, Universidad Autónoma de Madrid, 28049, Madrid, Spain
b Departamento de Química Inorgánica, Facultad de Ciencia y Tecnología, Universidad del País Vasco, Apartado 644, E–48080 Bilbao, Spain
c Dpto. de Física de la Materia Condensada, Universidad Autónoma de Madrid, 28049 Madrid, Spain [email protected]
Coordination polymers are a class of compounds formed by two building-blocks, the metal units (metal ions or complexes) and the ligands (molecules or ions), connected by means of coordination bonds. A wide variety of architectures with a broad panel of properties can be achieved by suitable selection of the building-blocks. Large amount of work has been done towards potential applications of materials based on coordination polymers. Probably, the most studied being gas storage and gas separation [1].
However, little is known about their potential use as alternative materials of nanometric dimensions. We have recently pointed out that a particular type of 1D coordination polymers seems particularly suitable as molecular wires [2]. In addition, the development of the unique properties of graphene has originated a scientific “revolution” around this material. This scenario has motivated us to consider new alternatives for the isolation of 2D polymers, in other words, new 2D materials with one atom or molecule of thickness [3]. As for the preparation of graphene, one feasible route to achieve this end consists of delamination of a layered compound [4].
Herein, we present the synthesis and structure of a laminar Cu(I) coordination polymer [Cu(μ-pymS2)(μ- Cl)]n·nMeOH (Fig. 1). It was synthesized by diffusing a dipyrimidindisulfide (PymS2) MeOH:MeCN solution into a CuCl2·2H2O methanolic solution. Isolated red crystals of [Cu(μ-pymS2)(μ-Cl)]n·nMeOH exhibit semiconducting and luminescent properties. X-ray diffraction analyses evidenced that the sheets are weakly face-to-face stacked through pyrimidine ligand moieties (Fig 1a). The packing of these sheets allocate channels along the c axis which are occupied with disordered guest methanol molecules (Fig1b). The solvent molecules can be selectively exchanged by ie. water or ethanol molecules, resulting in a slight shift in the relative position of the layers. As evidenced by crystallographic data and interchange host-guest properties let us to envision a feasible compound exfoliation.
In fact, crystalline [Cu(μ-pymS2)(μ-Cl)]n can be exfoliated into colloidal sheets by micromechanical cleaving. Both hydrophilic and hydrophobic sites can be found in the layer structure, hence mica (hydrophilic) and HOPG (hydrophobic) surfaces were explored in order to adsorb the compound by casting deposition of previously sonicated and diluted sample suspensions. Atomic force microscopy techniques were employed to characterise deposited sheets of [Cu(μ-pymS2)(μ-Cl)]n on HOPG (Fig.2) and mica (Fig.3). Its is noticeable the morphological features of the sheets adsorbed on HOPG which angles resemble to those observed in the monocrystals of this material.
References
[1]C.Janiak, Dalton Transactions, 14 (2003) 2781
.
[2] L. Welte, A. Calzolari, R. di Felice, F. Zamora, J. Gómez-Herrero, Nature Nanotechnology, 5 (2010) 110.
[3] P. Amo-Ochoa, L. Welte, R. González-Prieto, P. J. Sanz Miguel, C. J. Gómez-García, S. Delgado, J.
Gómez-Herrero, F. Zamora, Chemical Communications, 19 (2010) 3262.
[4]AJ.Jacobson, In Comprehensive Supramolecular Chemistry, Vol 7, G.Alberti and T. Bein (Ed) (1996) 315.