Solution Processable Nanostructures for Molecular Electronics

PhDIn molecular electronics, the building material (traditionally elemental semiconductor) is replaced by single molecules or a nanoscale collection of molecules. Key to molecular electronics is the ability to precisely embed molecules into a nano device/structure and to manipulate large numbers of functional devices so they can be built in parallel, with each nano-device precisely located on the electrodes. In this work, the assembly of organic and inorganic nanostructures dispersed in aqueous solutions has been controlled via chemical functionalisation. By combining this bottom-up assembly strategy with traditional top-down lithographic apporaches, the properties of these nanostructures have been investigated via a range of different techniques. The high degree of control on the molecular design through chemical synthesis and the scalability by self-assembly make this approach of interest in the field of molecular electronics. In this regard, this dissertation presents a solution-based assembly method for producing molecular transport junctions employing metallic single-walled carbon nanotubes as nanoelectrodes. On solid substrates, electrical and electronic properties have been investigated by Conducting Atomic Force Microscopy (C-AFM). Furthermore, different strategies for asymmetric junction formation have been explored towards the development of a potential nanoscale Schottky diode. Moreover, various patterning techniques based on shadow evaporation and AFM probe scratching have been investigated for the assembly of 1-D nanostructures. Nanostructures dispersed in solution were organised onto surfaces by means of dielectrophoretic assembly, and their electronic properties was then measured by the means of a probing station. In addition to the aforementioned organic nanostructures, we also report on the dispersion of boron nitride nanotubes (BNNT) by DNA wrapping, followed by the formation of nano-hybrids of boron nitride nanotubes and carbon nanotubes. Previously, researchers have adopted BNNT as a 2D dielectric layer. The work inspires me to adopt boron nitride nanotubes as 1D dielectric materials. The techniques developed in this thesis are of interest for fundamental studies of electron transport in molecules and nanostructures. Addtionally, the approaches developed in this work may facilitate the advancement of new technologies for electronics, including, but not limited to, future circuits based on single-wall carbon/boron nitride nanotubes with specific functionality

Single Walled Carbon Nanotubes Assembly: Nanohybrids toward Photodetection and Junction Engineering.

PhD ThesesBy synergistically combining the individual properties of more than one nanoscale component, novel features of hybrid structure assemblies represent a key motivation for making future functional nanomaterials. In this thesis, the successful construction of a multiplexed photo-responsive chip from DNA-wrapped single walled carbon nanotubes (DNA-CNTs) and DNA-CNT templated inorganic-organic hybrid structures is first demonstrated. The effective assembly of the hybrids was characterized by atomic force microscopy (AFM) and the corresponding device performance as well as the key mechanisms behind were investigated. Then a facile approach for the fabrication of end-to-end SWCNT junctions exploiting oligonucleotides as molecular linkers is presented. The assembled junctions show clear stimuli-responsive features stemming from the designed sequences of oligonucleotides; this grants the SWCNTs the ability to self-assemble and disassemble under specific conditions in aqueous solutions. The junction formation was confirmed by Atomic Force Microscopy (AFM) and time-dependent fluorescence analysis. Moreover, an efficient strategy to sort DNA-wrapped SWCNTs (DNA-CNTs) by length via a gel electrophoresis technique was developed (confirmed by AFM). In addition to the application of oligonucleotides, the use of diazonium salts not only as a molecular linker but also the major reactive agent for CNT junction formation was also explored. In conclusion, by integrating DNA-CNTs with other active components, we have achieved the assembly for organic-inorganic nanohybrids of multiplexed photo-sensing capabilities and the assembly of reconfigurable SWCNT junctions with stimuli-responsive features. Moreover, the facile and efficient strategies developed in our work can contribute to the controlled assembly of CNT based functional nanohybrids

Directed Biomolecular Assembly of Functional Nanodevices

One of the objectives of nanotechnology is to develop ways to build functional nanoscale devices from nanostructures. Whether these nanodevices will constitute the basis for new technologies rests on the ability to precisely manipulate the nanostructures in such a way that large numbers of functional devices can be built in parallel, with each nanodevice precisely located and addressed. In this work nanostructures dispersed in solution are organized onto surfaces by means of molecular-scale directed assembly. This technique combines top down high resolution lithographic patterning to bottom up self-assembly: specific molecular interactions take place at locations precisely defined by lithography, resulting in the parallel assembly of an arbitrarily large number of devices into complex and precisely ordered arrangements. While different molecules are used in this study, DNA plays a key role throughout the work due to the specificity of its interactions, its programmability and outstanding chemical flexibility. Two approaches are developed to direct the assembly of nanostructures on a surface. The first involves the patterning and selective functionalization of metallic nanodots that are used as anchors for the attachment of DNA molecules, proteins, DNA nanostructures and single-wall carbon nanotube (SWCNT) segments wrapped by DNA. Different strategies are explored to maximize the yield of the desired assembly. This platform also allows the monitoring of DNA-protein interactions with single molecule resolution, which has many potential biomedical applications. In the second approach, lithographic patterning is used to define regions of high surface energy that promote the binding of DNA origami and SWCNT segments. The high patterning resolution again allows for single nanostructure manipulation. This method facilitates the assembly of SWCNT field effect transistors from DNA-wrapped SWCNT segments. The formation of multi-component nano-objects in solution, by directing the linkage of properly functionalized nanostructures, is also studied. The products of these reactions are suitable for surface placement with the developed directed assembly techniques, thereby resulting in a hierarchical directed assembly process. Among others, the synthesis of SWCNT-dsDNA heterostructures is described. These hybrid objects can be used to electrically probe dsDNA using the SWCNTs as electrodes, by assembling solid state devices by means of the directed assembly methods, and also by conductive AFM. The results of some electrical measurements of double stranded DNA are discussed. The techniques developed in this thesis are directly applicable to fundamental studies of electron transport in molecules and other nanostructures, but they also have utility in other fields, such as chemistry and biology, where single molecule resolution is required. In addition, the approaches developed in this work may facilitate the advancement of new electronics technologies, including, but not limited to, future circuits based on single-wall carbon nanotubes with specific electronic properties

Fusing synthetic biology with nanotechnology: Integrating proteins into carbon nanotube field-effect transistors

Proteins are nature’s own nanomachines. Crafted through years of evolution, they are optimised to perform a range of cellular functions. To translate this into a useful nanotechnological application, proteins can be integrated into fundamental electronic devices known as carbon nanotube field-effect transistors (NT-FETs). I do this by engineering in non-natural amino acid p-azido-L-phenylalanine (AzF), which can be activated by UV light to covalently bind the carbon nanotube channel of an NT-FET. This creates an intimate environment for signal transduction, whereby an external biochemical signal (e.g., a chemical reaction, or incoming charge density from a protein-protein interaction) is transduced into an electrical signal. Potential applications for this will be dependent on the protein interfaced, but this thesis will consider two key themes: biosensing and optoelectronic gating. Chapter 3 builds on previous research by the Jones and Palma collaboration to develop a biosensor for antibiotic resistance (ABR). I do this by covalently integrating BLIP-II (Beta-Lactamase Inhibitory Protein II) to an NT-FET, transducing binding events with ABR biomarkers, the class A β-lactamases. NT-FETs were functionalised with defined BLIP-IIAzF variants to sample different orientations of analytes TEM-1 and KPC-2 β-lactamase. The distinct electrical signals generated correlated to the unique electrostatic surface being sampled, providing evidence for electrostatic gating. Chapter 4 builds on the experimental results from Chapter 3 to consider whether the BLIP-IIAzF—NT-FET interface can be effectively modelled to predict AzF mutation site success in mediating proximal analyte sensing. Using molecular dynamics, data on AzF side chain rotamer propensity was extracted, and in silico modelling was performed to assess the possible binding orientations of BLIP-IIAzF variants at the NT-FET interface. Distance and electrostatic potential of incoming β-lactamases were measured and showed correlation to the electrostatic gating observed in Chapter 3. Chapter 5 was devised in collaboration with the Bobrinetskiy lab, as I looked to exploit nature’s own light-responsive elements by covalently integrating sfGFP (superfolder Green Fluorescent Protein) into an NT-FET platform. By defining sfGFP orientation through two distinct AzF anchor sites, light was shown to induce optoelectronic memory and optoelectronic gating. Further novelty was discovered as water regenerated the optoelectronic gating response after six months of protein dehydration

Structural Sorting and Oxygen Doping of Semiconducting Single-Walled Carbon Nanotubes

Existing growth methods produce single-walled carbon nanotubes (SWCNTs) with a range of structures and electronic properties, but many potential applications require pure nanotube samples. Density gradient ultracentrifugation (DGU) has recently emerged as a technique for sorting as-grown mixtures of single-walled nanotubes into their distinct ( n,m ) structural forms, but this approach has been limited to samples containing only a small number of nanotube structures, and has often required repeated DGU processing. For the first time, it has been shown that the use of tailored nonlinear density gradient ultracentrifugation (NDGU) can significantly improve DGU separations. This new sorting process readily separated highly polydisperse samples of SWCNTs grown by the HiPco method in a single step to give fractions enriched in any of ten different ( n,m ) species. In addition, minor variants of the method allowed separation of the minor-image isomers (enantiomers) of seven ( n,m ) species. Optimization of this new approach was aided by the development of instrumentation that spectroscopically mapped nanotube contents inside undisturbed centrifuge tubes. Besides, sorted nanotube samples enabled the discovery of novel oxygen-doped SWCNTs with remarkable photophysical properties. Modified nanotube samples were produced using mild oxidation of SWCNTs with ozone followed by a photochemical conversion step that induced well-defined changes in emissive properties. As demonstrated for a set of ten separated SWCNT ( n,m ) structures, chemically altered nanotubes possess slightly lower band gap energies with correspondingly longer photoluminescence wavelengths. Treated samples showed distinct, structure-specific near-infrared fluorescence at wavelengths 10 to 15% longer than the pristine semiconducting SWCNTs. Quantum chemical modeling suggests that dopant sites harvest light energy absorbed in undoped nanotube regions by trapping mobile excitons. The oxygen-doped SWCNTs are much easier to detect and image in biological specimen than pristine SWCNTs because they give stronger near-IR emission and do not absorb at the shifted emission wavelength. This novel modification of SWCNT properties may lead to new optical and electronic applications, as it provides a way to change optical band gaps in whole nanotubes or in selected sections

Synthesis, Characterization and Chemical Functionalization of Nitrogen Doped Carbon Nanotubes for the Application in Gas- and Bio-Sensors

In this work, a chemiresistor-type sensing platform based on aligned arrays of nitrogen-doped multi-walled carbon nanotubes (N-MWCNTs) was developed. Our N-MWCNT based sensors can be made on both rigid and flexible substrates; they are small, have low power consumption and are suitable for highly efficient and reliable detection of different biomolecules and gases, at room temperature. The performance of these sensors was demonstrated for avian influenza virus (AIV) subtype H5N1 DNA sequences and toxic gases NO and NH3 at low concentrations. In our study, chemical vapor deposition (CVD) method was applied to synthesize vertically aligned nitrogen doped carbon nanotube arrays on a large area (> 1 cm2) on Si/SiO2 substrate using Fe/Al2O3 layer as a catalyst and a mixture of ethanol and acetonitrile as a C/N source. Especially, the diameter, length, nitrogen-doping concentration and morphology of the nanotubes were controllably tailored by adjusting the thickness of catalyst film, reaction duration and temperature as well as the amount of nitrogen-containing precursor. For integrating N-MWCNTs into chemiresistor devices, we developed a direct contact printing method for a dry, controllable and uniform transferring and positioning of the CVD-grown vertical nanotubes onto well-defined areas of various rigid and flexible substrates. After horizontally aligned N-MWCNT arrays were formed on a target substrate, interdigitated metallic microelectrodes with an interspacing of 3 µm were deposited perpendicular to the nanotube alignment direction to fabricate chemiresistor devices for biomolecule and gas sensing. This way, well-aligned nanotubes were laid across the Au/Cr interdigitated electrode fingers, had a strong adhesion with the electrodes and served as conducting channels bridging the electrodes. The N-MWCNT based chemiresistor device was applied as a label-free DNA sensor for a highly sensitive and fast detection of AIV subtype H5N1 DNA sequences. For this, the nanotubes were functionalized with probe DNA, which was non-covalently attached to sidewalls of the N-MWCNTs via π-π interaction. Such functionalized sensors were applied to quantitatively detect complementary DNA target with concentration ranging from 20 pM to 2 nM after 15 min incubation at room temperature. The sensors showed no response to non-complementary DNA target for concentrations up to 2 µM showing an excellent selectivity. Investigations on the efficient gas sensing of N-MWCNT-based chemiresistor of reducing/ oxidizing gases NH3 and NO were also reported in this work. The aim was to assess the possibility for N-MWCNTs to be applied as innovative sensing materials for room temperature gas sensing. N-MWCNTs with varying doping levels (N/C ratio of 5.6 to 9.3at%) were used as sensing materials and exposed to NH3 (1.5-1000 ppm) and NO (50-1000 ppm) for exploring and comparing their sensing performance. This study offered an effective route for further modification of CNTs according to various sensing application. Finally, our investigations showed a high potential of the developed N-MWCNT-based sensing platform for various applications ranging from environmental monitoring to point-of-care medical diagnostics

Synthesis, Characterization and Chemical Functionalization of Nitrogen Doped Carbon Nanotubes for the Application in Gas- and Bio-Sensors

In this work, a chemiresistor-type sensing platform based on aligned arrays of nitrogen-doped multi-walled carbon nanotubes (N-MWCNTs) was developed. Our N-MWCNT based sensors can be made on both rigid and flexible substrates; they are small, have low power consumption and are suitable for highly efficient and reliable detection of different biomolecules and gases, at room temperature. The performance of these sensors was demonstrated for avian influenza virus (AIV) subtype H5N1 DNA sequences and toxic gases NO and NH3 at low concentrations. In our study, chemical vapor deposition (CVD) method was applied to synthesize vertically aligned nitrogen doped carbon nanotube arrays on a large area (> 1 cm2) on Si/SiO2 substrate using Fe/Al2O3 layer as a catalyst and a mixture of ethanol and acetonitrile as a C/N source. Especially, the diameter, length, nitrogen-doping concentration and morphology of the nanotubes were controllably tailored by adjusting the thickness of catalyst film, reaction duration and temperature as well as the amount of nitrogen-containing precursor. For integrating N-MWCNTs into chemiresistor devices, we developed a direct contact printing method for a dry, controllable and uniform transferring and positioning of the CVD-grown vertical nanotubes onto well-defined areas of various rigid and flexible substrates. After horizontally aligned N-MWCNT arrays were formed on a target substrate, interdigitated metallic microelectrodes with an interspacing of 3 µm were deposited perpendicular to the nanotube alignment direction to fabricate chemiresistor devices for biomolecule and gas sensing. This way, well-aligned nanotubes were laid across the Au/Cr interdigitated electrode fingers, had a strong adhesion with the electrodes and served as conducting channels bridging the electrodes. The N-MWCNT based chemiresistor device was applied as a label-free DNA sensor for a highly sensitive and fast detection of AIV subtype H5N1 DNA sequences. For this, the nanotubes were functionalized with probe DNA, which was non-covalently attached to sidewalls of the N-MWCNTs via π-π interaction. Such functionalized sensors were applied to quantitatively detect complementary DNA target with concentration ranging from 20 pM to 2 nM after 15 min incubation at room temperature. The sensors showed no response to non-complementary DNA target for concentrations up to 2 µM showing an excellent selectivity. Investigations on the efficient gas sensing of N-MWCNT-based chemiresistor of reducing/ oxidizing gases NH3 and NO were also reported in this work. The aim was to assess the possibility for N-MWCNTs to be applied as innovative sensing materials for room temperature gas sensing. N-MWCNTs with varying doping levels (N/C ratio of 5.6 to 9.3at%) were used as sensing materials and exposed to NH3 (1.5-1000 ppm) and NO (50-1000 ppm) for exploring and comparing their sensing performance. This study offered an effective route for further modification of CNTs according to various sensing application. Finally, our investigations showed a high potential of the developed N-MWCNT-based sensing platform for various applications ranging from environmental monitoring to point-of-care medical diagnostics

CARBON NANOSTRUCTURES-QUANTUM DOT HYBRIDS: SELF-ASSEMBLY AND PHOTO-PHYSICAL INVESTIGATIONS OF SINGLE-MOLECULE HETEROSTRUCTURES

PhDThe possibility of integrating materials with different properties into heterostructures is crucial in the field of nanotechnology and can lead to new functionalities and emergent behaviour at the interfaces. In this regard, whereas semiconductor quantum dots (QDs) are tuneable emitters and efficient broadband light harvesting systems for new generation photovoltaic devices and light-emitting diodes, carbon nanomaterials are ideal scaffolds to collect and transport charges for device implementation. Therefore, the combination of carbon nanomaterials and QDs into novel nanohybrid structures has drawn interdisciplinary attention for a wide range of applications including photovoltaics, photocatalysis, sensing, bioimaging, and quantum information processing. In this thesis, the assembly, via covalent approaches, of semiconductor quantum dots with carbon-based nanomaterials in solution and at the single-molecule level is reported. First, a controlled assembly strategy for the formation of carbon nanotube-quantum dot nanohybrids is presented, where the terminal ends of individual single-walled carbon nanotubes (SWCNTs) were selectively functionalised with single semiconductor quantum dots. This was followed by a further study of these heterostructures, where different bridging linkers were used to control the electronic coupling between the two nanomoieties. Notably, the assembly, in environmentally friendly and biocompatible aqueous solution, was controlled towards the formation of monofunctionalized SWCNT-QD structures. Additionally, photo-physical investigations in solution and at the single-molecule level allowed us to cast light on the electronic coupling between the two components of the heterostructures. We further developed a covalent assembly strategy for the formation of semiconductor quantum dot-graphene hybrids, and we explored the application of these nanohybrids in a solar cell device. Atomic force microscopy was used to image the nanostructures and allowed us to identify the morphology of the nanohybrids investigated, while photoluminescence studies were employed to assess the light induced processes at the interface. Finally, we present an approach to investigate the chemical groups present at the edges of graphene pre-patterned nanogaps - generated by electroburning - where selective reactions for specific chemical groups carboxyl groups (COOH), aldehyde groups (CHO) and hydroxyl groups (OH)) were carried out towards the attachment of QDs, allowing to indirectly locate and identify, via AFM, the chemical groups for the specific reaction performed. By and large, the strategies developed in this work contribute to the tailored fabrication of nanohybrid materials with single-particle control, an important feature in the design of novel QD-based optoelectronic and light-energy conversion devices

Carbon Nanotubes for Electronics and Energy

Ever since their discovery, carbon nanotubes have been touted as a new material for the future and a correspondingly lengthy list of possible applications are often cited in the literature. This excitement for carbon nanotubes is a result of their richly varying physical, electronic and optical properties, where it is possible to have single, double and multiple carbon walls with each wall potentially being either semiconducting or metallic and possessing unique optical transitions covering the ultraviolet to infrared spectral range. However, to date the realization of many of the proposed applications has been hindered by exactly the characteristic that made carbon nanotubes so attractive in the first place, namely the inherent inhomogeneity and varying properties of as-prepared or grown material. In order to become a true advanced material of the future, methods to prepare carbon nanotubes with defined length, wall number, diameter, electronic and optical property are necessary. Additionally, such methods to sort carbon nanotubes must afford high purity levels, be amenable to large-scale preparation and be compatible with subsequent integration into device architectures. In this work these issues are addressed with the use of gel based sorting techniques, which with the use of an automated gel permeation system allows for the routine preparation of milligram quantities of metallic and semiconducting carbon nanotubes, chirality pure single walled carbon nanotubes and even double walled carbon nanotubes sorted by their outer-wall electronic type. Having developed techniques to prepare large quantities, methodologies to control the order and orientation of this 1 D nanomaterial on the macro scale are developed. Inks of carbon nanotubes with liquid crystal concentrations and aligned films thereof are developed and this newfound control over the electronic and structural property opened the door for energy related applications. For example the use of thin films as the transparent electrodes in silicon:carbon nanotube solar cells or as the light harvesting layer in combination with fullerenes with the goal of creating an all carbon solar cell. Likewise on the few nanotube level the unique optical transitions of different nanotube chiralities are used in the fabrication of nanoscale photosensitive elements