Tutorial Session 1: 24 March 2008 (9.00 am - 10.30 am)
Materials Chemistry for 1D Semiconductor Nanostructures
Lionel VAYSSIERES
Editor-in-chief
International Journal of Nanotechnology
Born in 1968, he obtained a MSc. in Physical Chemistry in 1991 and a Ph.D. in Inorganic Chemistry in November 1995 from the Université Pierre et Marie Curie in Paris, France for his research work on the Interfacial and thermodynamic growth control of metal oxide nanoparticles in aqueous solutions. Thereafter, he joined Uppsala University, Sweden as a postdoctoral researcher for the Swedish Materials Consortium on Clusters and Ultrafine Particles to extend his concepts and develop purpose-built metal oxide nanomaterials for photoelectrochemical applications as well as to characterize their electronic structure by x-ray spectroscopies at synchrotron radiation facilities.:He has been invited as a visiting scientist: at the department of Chemical Engineering at the University of Texas at Austin, USA on nanocomposite metallic oxides for biosensors, at the UNESCO Centre for Macromolecules & Materials and at the department of Biochemistry, at Stellenbosch University, South Africa on bio-nanocomposite materials, at the Glenn T. Seaborg Center, Chemical Sciences Division, at Lawrence Berkeley National Laboratory, USA on actinide nanomaterials, at the Texas Materials Institute on metal oxide-based nanomaterials for optical, magnetic, and energy storage and conversion devices, and at the Ecole Polytechnique Fédérale de Lausanne (EPFL), Switzerland on metal oxide oriented arrays for photocatalytic devices, at the University of Queensland in Brisbane, Australia, at the Nanyang Technological University in Singapore and at iThemba laboratories in Cape Town, South Africa.
He has (co-)authored about 50 refereed publications, 3 ISI highly cited papers (first author) for the last 10 years, single author 2003 paper #1 in the Top 10 hot papers published in the last 2 years in Chemistry (Jul-Aug 05), #2 in the Top 3 hot papers published in the last 2 years in Materials Science (Sep-Dec 05) and #3 (May-June 05) in major international journals, refereed proceedings and book series, which have generated over 1400 citations. He has been interviewed by ISI as well as by ScienceWatch in 2006 for a single authored 2003 paper cited over 300 times. Two other first and corresponding author 2001 papers have already been cited over 200 times. He has presented over 135 lectures at universities, research institutes and international conferences as well as acting as chairman, program committee and advisory member at major international conferences and projects worldwide.
He is currently a senior researcher at the International Center for Young Scientists, National Institute for Materials Science (NIMS) in Tsukuba, Japan; a R&D consultant; a guest scientist at the Chemical Sciences Division and the Advanced Light Source at Lawrence Berkeley National Laboratory, USA. He is also the founder and editor-in-chief of a new ISI journal dedicated to reviews in nanotechnology and related fields, the International Journal of Nanotechnology published by Inderscience Ltd and a referee for over 45 international journals.
Tutorial 1 Synopsis
Materials chemistry has emerged as one of the most consistent fabrication tools for the rational delivery of high purity functional nanomaterials, engineered from molecular to microscopic scale at low cost and large scale. An overview of the major achievements and latest advances of a recently developed growth concept and low temperature aqueous synthesis method, for the fabrication of purpose-built large bandgap metal oxide semiconductor materials and oriented nano-arrays is presented. Indeed, the design and large scale fabrication of ordered arrays consisting of advanced and well-defined building blocks such as quantum dots, nanorods, or nanowires is essential to the creation of new devices based on nanoscience. A concept as well as a growth model and a thin film technique were developed by the presenter to contribute to such challenge. Such ideas and synthesis method led to the creation of a new generation of materials built directly onto substrates from aqueous solutions with designed morphology and orientation, which is in better adequacy with their applications. Such nanomaterials are growing directly onto substrates by heteronucleation from the hydrolysis condensation of metal salt precursors from aqueous solutions. Although such a bottom-up technique allows the generation of anisotropic and oriented building block of various length scales and on various types of substrates (polycrystalline, single crystalline or amorphous), it is carried out without template, surfactant, applied field, or undercoating. Therefore high purity, low cost, and large scale fabrication of advanced nanomaterials is achievable. In addition, a direct growth (contact) between one-dimensional building blocks and their substrate is essential to take full advantage of oriented nanorods, that is, a direct path (free of grain boundaries) for electron for instance. Such conformation is a particular interest to develop more efficient devices such as gas sensor, photovoltaic, and photocatalytic applications. A thermodynamic model demonstrates that acidity and ionic strength act on protonation-deprotonation equilibria of surface hydroxylated groups and thus on the electrostatic charge. The resulting change in interfacial chemical composition induces a decrease of the interfacial tension as stated by the Gibbs adsorption equation. The surface contribution to the free enthalpy of formation of nanoparticles from aqueous solutions is substantially lowered, allowing the surface area of the system to increase and therefore the creation of thermodynamically stable dispersion of metal oxide nanoparticles[1]. Such equilibrium is reversible and thus, the average particle size is indeed directly related to the precipitation conditions. The nanoparticle size can be stabilized and controlled over an order of magnitude by precipitating the oxide nanoparticles far from their PZC, that is, at condition where the interface is electrostatically saturated, for instance at high pH and high ionic strength for spinel iron oxides. An empirical equation is given to account for the particle size. The stability condition is satisfied when the interfacial tension gamma is close to zero. Such theoretical concepts have been successfully applied to develop a new generation of functional materials, the so-called purpose-built materials[2]. Indeed, when thermodynamic stabilization is applied to heterogeneous nucleation rather than homogeneous nucleation, not only the size of spherical nanoparticles can be controlled but also the shape and the orientation of anisotropic building blocks onto various substrates can be tailored to build smart nanostructures[3]. Indeed, the fabrication of highly oriented crystalline arrays of large physical area consisting of nanorods of iron oxides[4], iron-chromium sesqui-oxide nanocomposites[4a], iron oxyhydroxides and iron metal[4b] have been successfully designed by following such ideas. In addition, ZnO nanorods and nanowires[5], microrods[5a], microtubes[5b] along with other morphologies[3], nanowires and nanoparticles of manganese oxides[6] and arrays consisting of sub-monolayers of non-aggregated mesoparticles of a-Cr2O3[7] have also been obtained by such a method. More recently, ordered arrays of c-axis elongated nanorods of rutile SnO2[8] with squared cross section have also been successfully fabricated following the same concept and aqueous thin film processing method. Such purpose-built nanomaterials have been designed for instance to improve the photovoltaic[9] and gas sensing[9a] properties of large band gap semiconductors[9b]. Better fundamental understandings of the electronic structure[10] and orbital symmetry contribution of II-VI semiconductor nanomaterials[10a] as well as quantum confinement effects in TiO2 quantum dots and a-Fe2O3 ultrafine nanorods[10b] have also been demonstrated with such materials.
Tutorial Session 2: 24 March 2008 (11.00 am - 12.30 pm)
Scanning Probe Microscopy: Fundamentals and Applications
Federico ROSEI
Professor, Canada Research Chair in Nanostructured Organic and Inorganic Materials
INRS-EMT, University of Quebec, Canada
Federico Rosei received a Laurea degree (1996) and a PhD (2001) in Physics from the University of Rome La Sapienza, working on the growth and characterization of semiconductor nanostructures. He then worked as a Post-Doctoral Research Associate and Marie Curie Fellow at the Center for Atomic Scale Materials Physics in Aarhus (Denmark) from the end of 2000 to April 2002, investigating the adsorption properties of complex organic molecules at metal surfaces. He then joined the faculty at INRS- Energie, Materiaux et Telecommunications, Universite du Quebec as Assistant Professor in May 2002. Two years later, he was promoted to Associate Professor, with tenure. He is recipient of a Strategic FQRNT Fellowship for New Professors from the Province of Quebec and holds the Canada Research Chair in Nanostructured Organic and Inorganic Materials since October 2003. Dr. Rosei's research interests focus on the properties of nanostructured materials, and how to control their size, shape, composition, stability and positioning when grown on suitable substrates. He has extensive experience in fabricating, processing and characterizing inorganic, organic and biocompatible nanomaterials. He has co-authored 50 articles in prestigious international journals, has given over 60 Invited, Keynote and Plenary Lectures at international conferences and over 80 colloquia and seminars at Universities, Government Laboratories and Industrial Laboratories. Presently he leads a group of about 12 young scientists, composed of 8 graduate students and 4 post-doctoral fellows. He is a referee for the US NSF and DOE, the European Commission, the European Science Foundation, FQRNT and NSERC in Canada, A-STAR in Singapore and over 25 international journals (including Science, Nature Materials, Angew Chemie, Phys Rev Lett, J Am Chem Soc, Appl Phys Lett, etc.). He devotes a significant portion of his time to mentoring young scientists, and has co-authored the book 'Survival Skills for Scientists' published in July 2006 by World Scientific. More information is available on his website, www.nanofemtolab.qc.ca.
Tutorial 2 Synopsis
Scanning Probe Microscopes, in particular Scanning Tunneling Microscopy (STM) and Atomic Force Microscopy (AFM) have progressively become essential tools to analyze the properties of surfaces and nanostructures. Since its development about 25 years, the STM has permitted to study the structure of conducting surfaces with atomic scale resolution. Although it does not have a strong chemical sensitivity and in most cases it also lacks time resolution, the STM has proved to be a powerful tool in the study of adsorbate-surface interactions and of elementary surface processes in general [ , , , , , , ]. Not surprisingly, its inventors G. Binning and H. Röhrer (who also invented the AFM) were awarded the Nobel Prize in Physics in 1986. By means of high-resolution, fast-scanning scanning tunneling microscopy (STM) unprecedented new insight was recently achieved into a number of fundamental processes related to the interaction of complex molecules with surfaces such as molecular diffusion, bonding of adsorbates on surfaces and molecular self-assembly [4,5]. In addition to the normal imaging mode, the STM tip can also be employed to manipulate single atoms and molecules in a bottom-up fashion, collectively or one at a time [6]. In this way, molecule-induced surface restructuring processes can be revealed directly and nanostructures can be engineered with atomic precision to study surface quantum phenomena of fundamental interest. The AFM is perhaps even more powerful and its use by now more widespread than that of the STM, its parent technique. Although it rarely achieves atomic resolution, it is not limited to the study of conductive surfaces. Among its derivative techniques the most used ones are Magnetic Force Microscopy and Piezoresponse Force Microscopy. In this Tutorial Lecture I will initially describe the basic functioning principle of Scanning Probe Microscopes, focusing on STM and AFM. I will then give examples of how they are used for the advanced characterization of surfaces and nanostructures also in the context of applications.
Tutorial Session 3: 24 March 2008 (2.00 pm - 3.30 pm)
Electronic Transport in Low Dimensional Semiconductor Heterostructures
Harry E RUDA
Energenius Professor of Advanced Nanotechnology
University of Toronto, Canada
Harry Ruda received the B.Sc. degree in materials physics with honors from Imperial College, London University, England, in 1979, and the Ph.D. degree from Massachusets Institute of Technology, Cambridge, Massachusets, USA, in 1982, for work on growth and characterization of HgCdTe for infrared detectors. Following these studies, he accepted an IBM postdoctoral fellowship to work on defect calculations in GaAs and transport in low dimensional GaAlAs-based quantum heterostructures. In 1984 he joined 3M where his work focused on theoretical optical and transport properties of wide bandgap II-VI semiconductors, principally ZnSe-based. In 1989, Dr. Ruda joined the University of Toronto and now holds the position of Full Professor. He currently is also the Energenius Advanced Nanotechnology chair holder, and director of the Energenius Centre for Advanced Nanotechnology.
Tutorial 3 Synopsis
Vapour-Liquid-Solid (VLS) growth was used to fabricate high quality II-VI and III-V nanowires. We will discuss photoluminescence, conductivity and photoconductivity measurements on arrays of such nanowires [1], used as a means for optimizing their properties - indeed, we show how optimized nanowires dominated by band-edge related emissions can be achieved demonstrated [2]. Lithographic processing of such nanowires was used to study their electrical and optical properties [3, 4], and in particular single nanowires with peak responsivity as high as 22 A/W are demonstrated. Nanowire transistors with varying strength confinement will be discussed, as well as limitation for ultrafast (THz) electronics in such systems. We also discuss the unique polarization sensitive optical response of such nanowires [5] as well as plasmonic effects in such nanowires [6].
