DNA-templated semiconductor nanowires

Abstract

This thesis presents theoretical and experimental studies regarding the nucleation, growth, morphology, optical and electronic properties of DNAtemplated compound semiconductor nanowires, emphasising the comparison of experimental results, theoretical calculations and numerical simulations. First, following a general introduction to the unique properties of lowdimensional semiconductor systems and their applications in biological labelling, chemical and biological sensing, and next-generation optoelectronic devices, the discussion centres on the challenges of growing semiconductor one-dimensional nanostructures (1DNS) and the opportunities naturally occurring 1D materials provide to circumvent these limitations and guide their formation. Among those naturally occurring materials, DNA stands out, as it is biopolymer only 2.0 nm in diameter and available in precise sub-nanometre lengths. Then, a review of several relevant characterisation techniques to study DNA and semiconductor 1DNS is presented, discussing their principles, applicability and limitations for qualitatively and quantitatively assessing their properties. Afterwards, an evaluation of different DNA sources as possible templates for the directed growth of semiconductor 1DNS is presented based on their morphology and length distributions, obtained by the development of a reliable and reproducible methodology to prepare, image and analyse atomic force microscopy (AFM) samples at the single-molecule level. Next, the morphology of individual DNA-templated CdS is studied using atomic force microscopy, demonstrating that the density of the inorganic material on the template can be controlled by varying the concentrations and molar ratios of the precursors, obtaining fully-covered densely-packed granular nanostructures with narrow size distributions. Additionally, a theoretical model of nucleation and growth is presented, indicating that this narrow size distribution can be understood as the result of an instantaneous nucleation process spaced at regular intervals along the DNA templates. Moreover, using these optimised synthesis parameters, pure and Cu-doped DNA-templated ZnxCd1–xS are prepared, confirming with AFM and TEM the formation of fully-covered densely-packed nanostructures. The formation of homogeneously alloyed nanostructures and the presence of Cu(I) in the doped samples are verified using X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy. Notably, ultravioletvisible (UV-Vis) and photoluminescence (PL) spectroscopy demonstrate that their optical properties can be precisely controlled by modifying the molar ratios of the Zn/Cd precursors. Likewise, following the preparation of DNA-templated CdS and SnS, the formation of fully-covered densely-packed nanostructures is verified with AFM and TEM, in addition to their composition via XRD and XPS, and optical properties using UV-Vis and PL spectroscopy. Furthermore, the conduciii tivity of supramolecular rope-like arrangements is probed using scanning conductance microscopy (SCM), obtaining an indication of the conductive nature of the nanostructures. Finally, the conductance of devices made of nanowire networks drop-cast onto interdigitated microelectrodes is examined by measuring their current-voltage (I-V) curves.