FUNPROB · Functional semiconductor nanowire probes

Periodic Report Summary 1 - FUNPROB (Functional semiconductor nanowire probes)

Project summary and objectives Scanning probe microscopy (SPM) is an established technique for the characterisation of materials and structures at the nanoscale and is increasingly bridging traditional disciplines including the biosciences. As such, it is of fundamental and practical interest across the sciences and industry. The geometry of the tip is the critical factor which determines the resolution of an SPM sensor. In an attempt to achieve ultra-high spatial resolution, carbon nanotubes have been widely investigated and shown some promise. However, it is extremely difficult to control the properties of these tubes, especially the electrical behaviour and growth geometry. Equally, their manipulation is a daunting task. We propose to use III-V semiconductor nanowires as functioning sensors at the apex of scanning probes. These structures can be directly grown on the substrates or SPM cantilevers with controllable properties at the nanometre scale. Using these nanowires offers excellent new avenues for the integration of established semiconducting devices onto the tip of a scanning probe. This will improve the sensitivity and functionality of scanning probe methods. An example of potential applications for such a novel probe is the detection of viruses based on their electrical response which can be coupled to that of nanowires under appropriate conditions. Within the framework of this project, we will combine the complementary expertise of various internationally leading institutions for the creation of integrated individual semiconductor nanowire SPM probes exhibiting enhanced functionalities. Objectives • To controllably grow III-V NW and to characterise individual III-V NWs including their electrical, mechanical, optical and thermal properties. This should include the characterisation of NWs with dielectric barriers which could form the basis of future nano memory arrays. • To undertake fundamental investigation of the growth of III-V NWs on complex substrates. This will include amorphous, patterned, oxidised and etched substrates. In particular, the growth of wires on micro pillars and from recessed holes. The investigation will include the morphological, crystallographic, chemical, electrical and optical properties of the NWs. • To model the growth mechanism with the view to identifying and implementing the optimal growth conditions which will enable the international team to fine tune the properties of the NW so produced. • To develop theoretical models that will not only facilitate the interpretation of the data but also optimise both the growth and the fabrication of the SPM probe. • To fabricate a versatile ultra-high resolution thermal probe capable of simultaneously recording optical and electrical properties. This takes advantage of the very small size of NWs (a few nanometres in diameter) and their well-defined structure (i.e. high aspect ratio). • To address their integration into a scanning probe using purposely designed substrates. Although nanowires have already demonstrated exceptional sensitivity, they have not yet been successfully integrated in a SPM probe configuration because of the on-going difficulty to manipulate them. • Integration in a scanning probe will reduce mechanical stress exerted on the nanowires, therefore rendering the sensors more robust and expanding the range of applications. • To enable the electrical detection of individual viruses based on the known phenomena of the variation their conductivity with adsorption. This will investigate the chemical and biological environment likely to facilitate immobilisation of antibodies onto the nanowire probe, to enhance the electrical response of a virus under thermal and optical excitation Work performed since the beginning of the project, The research effort was devoted to carrying out the main three scientific work packages, Growth and modelling (WP1), Characterisation (WP2) and Fabrication (WP3), The main results achieved so far The main results achieved over this reporting period are directly relevant to the scientific work programme: Growth and modelling (WP1), Characterisation (WP2) and Fabrication (WP3). The highlights of the key results are now given: Growth and modelling (WP1) and characterisation (WP2) (1) Au catalyst free of epitaxial growth of coherent and high phase purity (zincblende structure) GaAs nanowires on Si(111) substrates was demonstrated and the Vapor−aporsubstrates was demonstrated an model and its variant proposed, where the phase purity was attributed due to a lower surface energy of liquid Ga compare to that of Au-Ga alloys. (2) Elastic relaxation and plastic deformation in nanostructures on lattice mismatched substrates were investigated to derive analytical expressions of elastic strain and dislocation energies by minimising the total energy for a given nanostructure geometry. (3) For the first time Arrays of (Ga,Mn)As nanowires on a GaAs (100) substrate were obtained using molecular beam epitaxy. Epitaxial growth of GaAs nanowire arrays on tilted faces of linear mesa was demonstrated. (4) Growth and modelling of non-stationary supersaturation and growth interfaces in “vapour-liquid-solid” nanowires. A comprehensive theoretical analysis of catalyzed nanowire growth was performed using the material balance in a droplet within one monolayer growth cycle. (5) Theoretical and experimental studies of Be doping of InP nanowires. The growth kinetic modelling is still on-going. (6) Synthesis and modelling of the Au-catalyzed CdTe nanowires on Mo foils by close-sublimation vapour-liquid-solid technique were performed to demonstrate size dependence. (7) DFT calculations on SNOx systems were developed to determine of the degree of the tin oxidation in such systems. DFT calculations of phonons in GaAs with zinc blende and wurtzite structures were also undertaken and contrasted with Raman data to confirm the assignments of the peaks in the spectra to transfer optical and longitudinal optical phonon modes in wurtzite structure in forming GaAs nanowires. (8) The synthesis and electro-optical studies of GaAsP nanowires photovoltaic properties was carried out. (9) Various nanowires based on GaN systems were grown and systematically characterised to (i) describe the relevance of size distribution of the physical properties and the associate theoretical validation; (ii) establish a scaling thermodynamic model for self-induced nanowires and (iii) investigate the growth rate of self-induced nanowires to develop a theoretical model of diffusion-induced nanowire growth and self-induced growth. (10) The elastic relaxation, plastic deformation, critical dimensions, photoluminescence and lasing in core-shell InGaAs/GaAs nanopillar nanolasers on silicon was demonstrated. (11) Active feedback loop between the nanowire growth, characterization and modelling has allowed to refine the understanding of the growth kinetics and to improve the control of the nanowire optical and structural properties. Fabrication (WP3) & characterisation (WP2) (12) The photovoltaic properties of GaAsP nanowires and core-shell wires contacted using a transparent top contact electrode were performed to extract the associated electrical and electro-optical properties. (13) Fabrication of modified SPM tips, nanolithography, nanosurgery and manipulation with surface nano-objects by modification of Si cantilevers with carbon nanowires and using modified tips for nanolithography, nanosurgery of erythrocytes and manipulation of colloidal particles with enhanced resolution and precision. (14) A platform for the nanoscale characterisation of materials using atomic microscopy, thermal microscopy and variants was developed to include advanced prefabrication, SThM characterisation, mapping of nanomechanical proprieties and advanced sample preparation for interface characterisation at the nanoscale. Modelling on the thermal energy transport and interface phenomena are currently in hand. (15) A method for the formation of single GaAs semiconductor nanowhiskers and their assemblies on the tip of a tungsten needle by molecular-beam epitaxy was proposed. (16) A method based on focused ion beam to attach carbon nanotubes to scanning probe tips for thermal and topographical measurements was developed and higher performance (thermal and spatial resolution) demonstrated. (17) The SPM integration has also compared the performance of nanowires and carbon nanotubes as heat transport media. CNT was shown to exhibit a superior behaviour and lead to topographical and thermal resolution better than 20 and 30 nm, respectively. Dissemination and knowledge transfer: 21 peer review journal papers were published during the second part of this interim period. A significant progress has been made in the dissemination and knowledge transfer of the results achieved over period 1 of this programme, i.e. 31 peer-review papers were reported and a large number of conference papers produced. It should be noted that only a small sample is given in this report. More than 20 invited lectures were delivered, with 5 symposia organised with a direct contribution from members of the FUNPROB consortium.

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