RICERCA ITALIANA     

Growth and properties of semiconducting-oxide based quasi one-dimensional nanocrystals

Università degli Studi di Lecce

Abstract

The present proposal concerns a base-research project whose main activities and objectives aim at: (i) developing new synthesis/nano-fabrication techniques for the self-assembly growth of quasi one-dimensional (1D) nanocrystals of high-gap metal oxide (MOX) semiconductors; (ii) understanding their fundamental growth mechanisms; and (iii) studying the effects of low dimensionality and gas-surface interactions on their electronic, vibrational and optical properties. In particular the project intend to investigate and develop self-assembly methods of crystallographic-oriented (epitaxial) and random (non epitaxial) free-standing quasi 1D nanocrystals based on chemical vapour deposition (CVD) processes or on the innovative combination of CVD methods and advanced nano-fabrication tools, like focused ion beam nano-fabrication. Both metal catalyst assisted growth, the so-called vapour-liquid-solid mechanism, and catalyst-free vapour crystallisation will be employed. Besides the growth/nanofabrication activity, the research will address for the first time thermo/fluid-dynamic and atomistic modelling of quasi 1D nanocrystal self-assembly. The potentialities of the proposed self-assembly growth/nano-fabrication technologies will be specifically focused on the realization of both epitaxial 1D nanocrystals (nanorods), 1D hetero-structures and ordered nanorod arrays, as well as of large ensembles of long non-epitaxial 1D systems (nanowires and nanobelts) deposited over wide substrate >>>

Principal Investigator

Nicola LOVERGINE Università degli Studi di LECCE

Research Objectives

The project aims at: (i) developing new synthesis/nano-fabrication techniques for the self-assembly growth of quasi one-dimensional (1D) nanocrystals of high-gap metal oxide (MOX) semiconductors; (ii) understanding their growth mechanisms; and (iii) studying the effects of low dimensionality on their electronic, vibrational and optical properties. The project wants to investigate and develop self-assembly of both crystallographic-oriented (epitaxial) and random (non epitaxial) free-standing quasi 1D nanocrystals based on chemical vapour deposition (CVD) processes or on the combination of CVD and nano-fabrication tools. The potentialities of the proposed technologies will be focused on the realization of both epitaxial 1D nanocrystals (nanorods), 1D hetero-structures and ordered arrays, as well as of large ensembles of long non-epitaxial 1D systems (nanowires and nanobelts) uniformly deposited over wide substrate areas. Long-term applications comprise UV opto-electronics, photonics and novel optical gas sensors. We intend to investigate the nanorod and nanowire/nanobelt properties, with special attention to the effect of reduced dimensionality and quantum confinement on electronic and phononic states. Besides providing experimental data on the physical properties of MOX-based quasi 1D nanocrystals, the characterization of morphological, structural and optical properties of as-grown quasi 1D nanocrystal structures will provide the ultimate feedback to assess the reliability of >>>

First Results

The list of intermediate results expected at completion of FASE I is the following:

- Functional validation of a well defined set of precursors for the CVD growth of high quality nanorods and nanobelts of high gap MOX semiconductors
- Definition of substrate treatment procedures and metal catalyst preparation specifications before growth
- Definition of CVD growth parameter values (temperatures, gas flows, pressures, etc), growth procedures for the VLSE growth of highly oriented ZnO, MgO and ZnMgO epitaxial nanorods
- Definition of CVD growth parameter values (temperatures, gas flows, pressures, etc), growth procedures for the growth of high quality ZnO, and SnO2 nanowires/nanorods on large substrate areas
- Complete quantitative description of CVD reactors thermo/fluid-dynamic conditions and cross-checking of predictions with experimental results
- Quantitative morphological and structural definitions of nanorod/nanobelt as function of growth conditions and comparison with 1D nucleation description in terms of atomistic models
- Completion of the experimental set up for resonant Raman scattering measurements
- Completion of the experimental set up for visible PL characterisation of high gap MOX
- Demonstration of quantum confined effects on the electronic and vibrational properties of nanorods/nanowires structures and comparison between experimental energy levels and theoretical calculations
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Timescale

24 months

National and international background

Quasi one-dimensional (1D) semiconductor nanocrystals represent the forefront of today's solid state physics and technology. These systems, having two of their dimensions comparable to the wavelengths of the electronic or phononic wavefunctions, are expected to show a variety of quantum confinement effects, such as density of state singularities, molecular-like states extending over large distances, high luminescence efficiencies and lower lasing threshold. Also, their high surface-to-volume ratio allows to exploit the role of surface states (and their ambient-driven changes) in determining carrier optical excitation/recombination phenomena.
Despite these attractive properties, until very recently, not too many studies were performed on 1D semiconductor systems, whilst two-dimensional structures (quantum wells) have been under study already for more than two decades and quasi 0-dimensional (0D) objects (quantum dots) have been in the focus of researchers for nearly a decade. The main reason for such discrepancy resides in the difficulty of fabricating quasi 1D nanocrystals. In the recent past these structures have been generally grown on and stabilised by the surface of a 3Dimensional (3D) substrate, with their major dimension running in the surface plane [1], with the consequence, however, that interactions with the 3D substrate can be quite strong and may dominate the 1D effects. Free standing quasi 1D nanocrystals having negligible interaction with the substrate >>>