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SBPMat

BRAZIL-MRS

2nd Brazilian MRS Meeting

October 26-29, 2003



Symposium A:
Current trends in Nanostructured

Materials and Systems II


Symposium Organizers:____________________________________________________


Fernando Lázaro Freire Jr. (PUC-Rio)


Guillermo Solórzano (PUC-Rio, SBPMat)
Israel Baumvol (UFRGS)
Wander Vasconcelos (UFMG)





iNVITED PRESENTATIONS







A-I1

Self-assembling systems and surface patterning strategies for Nanotechnologies

Giovanni Marletta, Dipartimento di Scienze Chimiche - University of Catania - Viale A.Doria 6 - I-95125 Catania – Italy
The need for very large scale planar integration of different functions in multifunctional devices including electronic devices, arrays of biosensors, highly integrated optoelectronic devices, etc.., is driving a growing effort towards techniques for surface patterning. Two basically different philosophies of patterning have been extensively developed, respectively exploiting self-organization properties of matter (bottom-up approach), and radiation-based lithography (top-down approach).

In the present paper we report a number of case studies dealing with the use of techniques as Langmuir-Blodgett deposition (LB), SPM tip-writing and focused-ion beam activation of surfaces to obtain micro- and nanoscale spatially resolved structures. In particular, we will report specific cases of unusual self-structuring of LB-deposited monolayers on solid surfaces, of tip-induced chemical patterning of organic monolayers on silicon, and spontaneous protein aggregation on ion-irradiated polymer dots. The formation of various structures, as variously shaped domains, stripes, fibers and dots, will be related to the molecular structure of the employed compounds, the deposition parameters, and the substrate composition and properties obtained by Near Field Microscopies (NFM), X-ray Photoelectron Spectroscopy (XPS), Imaging ToF-SIMS, Dynamic Contact angle measurements, Surface Enhanced Raman Spectroscopy (SERS) and theoretical simulations. The perspective of the employed techniques in the context of the developing patterning technologies will be given with respect to the state-of-art of the field.



A-I2

Teaching “Old” Materials “New” Tricks: Site- and Shape-Specific Nanopatterning of Functional Nanostructures

Vinayak P. Dravid, Professor, Materials Science & Engineering, Director, NUANCE Center

Northwestern University, Evanston, IL 60208 USA


At Northwestern, we are working towards designing the intricate architecture of functional nanostructures, as well as using them as building blocks for device systems for sensing, diagnostics and therapeutics. Embedded in this scheme are several nanopatterning approaches, some are based on the original invention of Dip-Pen Nanolithography (DPN) developed at Northwestern. The original DPN approach is modified to pattern, at the nanoscale, templates for inorganic and organic-inorganic complexes of arbitrary shape/size on arbitrary substrates, thus extending the efficacy and elegance of DPN.

The talk will outline sol-based precursor “inks” as an enabling approach to pattern and characterize magnetic, electronic, chemical- and optical active nanostructures at the nanoscale. Success is already evident for magnetic oxides, inorganic mesoporous structures, ferroelectrics and optically-active nanostructures.The real need for characterizing structure/crystallography/chemistry, as well as unambiguous measurement of their local properties, will be emphasized. The prospects for patterning at single-molecule resolution, especially for bioactive molecules, both by themselves and as templates for inorganics, will also be discussed.




A-I3

MOLECULAR ORIGINS OF BIO(IN)COMPATIBILITY : MATERIALS DESIGN ISSUES AND CHALLENGES AT THE NANOSCALE

C. Ortiz, Department of Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, MA USA

To produce truly bioinert or bioactive surfaces a detailed knowledge of the nanoscale morphology, mechanical properties, physical and chemical interactions with the biological environment is needed. Using the powerful technique of high resolution force spectroscopy (HRFS), we have measured the intermolecular interaction forces between a nanosized probe tip covalently bound with human serum albumin (HSA), the most abundant blood plasma protein in the human body, and various well-defined chemically modified surfaces including; COO- and CH3-terminated alkanethiol self-assembling monolayers and end-grafted poly(ethylene oxide) mushrooms. By varying the solution conditions and using molecular-level theoretical models this net interaction was deconvoluted into the individual constituent interactions including; electrostatic double layer, configurational entropy, H-bonding, and van der Waals forces. A series of chemically advantageous molecular attributes was formulated and based on this information, new glycocalyx-mimetic oligosaccharide-functionalized surfaces were chosen and studied using HRFS.




A-I4

PATTERNED ASSEMBLY OF EPITAXIAL SEMICONDUCTOR NANOSTRUCTURES

R. Hull; S. Atha, J.L. Gray, M. Kammler, A. Portavoce, N. Singh, D.M. Elzey, Dept. of Mat. Sci., Univ. of Virginia, Charlottesville, VA 22904, USA; F.M. Ross, IBM Yorktown Heights Research Center, NY, USA; X. Luo, G. Snider, Dept of Elec. Eng., Notre Dame Univ., IN, USA; J.A. Floro, Sandia National Laboratories, NM, USA.
Substantial progress has been described in the literature with respect to the synthesis of ordered semiconductor quantum dot (QD) arrays with single spatial frequencies, through control of elastic interactions during epitaxial growth. For most nano-electronic architectures, more complex patterns are required. For example, in the quantum cellular automata concept, QD quadruplets are charged in bistable configurations, with quadruplet cells arranged in more complex patterns to enable the functions of digital logic. We will describe two experimental approaches for achieving accurate surface placement of quantum dots in the epitaxial Ge(Si)/Si system: 1) Creation of localized topography, chemistry and strain on the substrate surface using focused ion beams for seeding of QD nucleation. 2) Synthesis of QD quadruplet molecules, where molecules self-assemble through cooperative nucleation around nano-scaled surface pits formed during epitaxial growth. Combined focused ion beam / electron beam lithography methods for coupling these QD architectures into nano-electronic test structures will also be presented.


A-I5

Mapping 3D alloying of Ge:Si (001) domes

G.Medeiros-Ribeiro, S. Kycia; LNLS, Campinas, SP; A. Malachias, R. Magalhães-Paniago; Depto. de Física, UFMG, Belo Horizonte, MG; T. I. Kamins, and R. Stanley Williams; Hewlett-Packard Labs, Palo Alto, CA
The issue of Si diffusion into Ge islands grown on Si(100) has been addressed by several independent studies with electron microscopy and x-ray techniques. Alloying is a significant factor in determining the shape and size distribution of an island ensemble. All experiment evidences support the existence of a distinct SiGe vertical composition variation, with most of the Si concentrated at the base of the island. Different growth conditions produce distinct lateral profiles. Hence, the assessment of lateral composition profiles is important to both identify the dominant growth mechanisms and model the confining potential of quantum dots. We report Grazing Incidence Angle X-ray Scattering (GIAXS) measurements on an ensemble of Ge domes with a uniform size distribution. We determined the (average) three-dimensional composition of the domes from an analysis of the anomalous scattering reciprocal space intensity maps near the Ge K absorption edge by first segmenting the domes into a stack of layers and then mapping the lateral concentration of Ge in each layer. Finally, we performed selective-etching experiments and qualitatively confirmed our proposed structural model. The significance of the analysis and procedure presented is key for the realistic modeling of quantum dot confining potentials, as this work presents a solution for imputing lateral composition profiles with its associated strain. This work was funded by FAPESP (contract 98/14757-4) and HP Brazil.

A-I6

On the nature of Mn in semiconductors*

Adalberto Fazzio, Instituto de Física, Universidade de São Paulo, CP 66318, 05315-970, São Paulo-SP, Brazil
We have performed a systematic study of the nature of Mn in semiconductors, using total energy ab initio calculations within the density-functional theory. We will discuss results for Mn in GaAs and in Si, Ge and SiGe. Our results for Ga1-xMnxAs diluted magnetic semiconductors (DMS) unambiguously show that the effective Mn-Mn coupling is always ferromagnetic, thus non-RKKY, and it is intermediated by the antiferromagnetic coupling of each Mn spin to the holes. We then address the following question: can there be a MnxSi1-x ferromagnetic semiconductor, similarly to MnxGe1-x? Since the main difference between Mn in Si and Ge is the fact that it prefers to be interstitial in Si, whereas it is substitutional in Ge, we investigate the origin of this difference. For that, we study the properties of interstitial and substitutional Mn impurities in both Si and Ge bulk crystals, as well as in the alloy Si1-xGex. We conclude that the main reason behind the different behavior of Mn in Si and Ge has a chemical origin, indicating that a Ge-rich neighborhood will stabilize a substitutional Mn in the Si1-xGex. Assuming that in order to obtain a Si based DMS it is fundamental to have substitutional Mn impurities, we investigate the possibility of incorporating MnSi using some sort of non-equilibrium growth procedure. Studying a variety of interstitial and substitutional sites for a Mn impurity on the Si(100) bare and hydrogenated surface, we find a substitutional Mn surface site with a formation energy similar to the interstitial surface site, suggesting that it might be possible to grow, probably through low temperature MBE, MnxSi1-x samples with a high enough concentration of substitutional Mn.

*This work was done in collaboration with Antônio J. R. da Silva, Raimundo R. dos Santos, Luiz E. de Oliveira, Alex Antonelli and Gustavo M. Dalpian, supported by the Brazilian agencies FAPESP and CNPq.



A-I7

NANOPARTICLES-BASED CHEMICAL GAS SENSORS FOR OUTDOOR AIR QUALITY MONITORING MICROSTATIONS

M.-I. Baraton, SPCTS CNRS, University of Limoges, France; L. Merhari, CERAMEC R&D, Limoges, France
Due to the growing concern worldwide about the consequences of urban air pollution on public health, the official organizations in charge of environment protection are strengthening the regulations and have set the maximum authorized concentrations of pollutants at very low levels. Current cost-effective semiconductor sensors used to monitor indoor air quality are not suitable for outdoor air quality monitoring due to their insufficient sensitivity and to their cross-sensitivity to humidity.

We present here some of our consolidated results on the optimisation of chemical gas sensors based on metal oxide semiconductors. In order to increase sensitivity and selectivity while keeping fabrication at low cost, three major research directions have been considered: 1) the use of semiconductor nanoparticles in the fabrication of screen-printed gas sensors to increase the sensitivity; 2) the printing of sensitive layers on alumina tiles using a simple mechanical layer-by-layer technique to further increase the sensitivity; 3) the control and tailoring of surface chemistry of nanoparticles proved to be a critical step for further device optimisation. We will finally show how our prototype devices which are capable of detecting O3 and NOx concentrations down to 20 ppb and 50 ppb respectively, and CO concentrations down to 3ppm in air, can be used as sensing elements in novel and cost-effective air monitoring micro-stations. This work has been financially supported by the European Community.




A-I8

SURFACE CHARGE INDUCED NANOSCALE MANIPULATION OF MOLECULAR FILMS

Francisco Nart and Frederico Cunha, Instituto de Química de São Carlos, Universidade de São Paulo, Av. Trabalhador Saocarlense, 400 - 13566-690 - São Carlos, SP – Brazil
The adsorption mechanism of organic molecules on metallic surfaces depends upon several factors that include molecular structure, concentration, temperature and surface charge. The careful control of some of these variables allows one to manipulate the adsorption geometry of these molecules leading to dissimilar packing structures, which can be followed by in situ STM and vibrational spectroscopy. The construction of nanostructured materials following a layer-by-layer approach is the final objective of this work. The molecular probe of choice is a series of derivatives of Uracil. These molecules present a variety of possibilities in adsorption geometries and ready availability. We found that the packing depends on the surface charge on the electrode and on the atom substituted in position 5 of the Uracil base molecule. We used also a silver electrode to investigate the charge induced restructuring of Uracil based molecules by Raman Spectroscopy, since SERS measurements on gold were not possible.


A-I9


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