Device Applications of Solution Processed MIR Semiconductor Nanocrystal Thin Films

<p>Colloidal semiconductor nanocrystals (NCs) with bandgaps less than 1 eV allow the development of mid wave infrared (MIR) sensitive detectors that exploit the benefits of colloidal materials, primarily bandgap selection and solution deposition. Additionally, the electrical behaviour of these films can be examined for characteristics that can increase the functionality of NC based detectors. The production of devices that are designed to be competitive as ultra-low-cost, room temperature MIR detectors, operating with photonic, rather than thermal detection is detailed. The evolution of the colloidal synthesis, spray deposition methods, substrate materials and post deposition treatments used here lead to highly robust and high performing devices. These devices demonstrate a “colour” sensitivity down to 300 nm in the MIR (≈10 % of scale), with superior responsivities for this class of device, up to 0.9 AW⁻¹, and competitive specific detectivity up to 8 × 10⁹ Jones at 200 Hz and 300 K. Furthermore, these devices utilise a cheap and robust substrate material that allows operation after deformation up to 45 ° without degradation over many cycles. These devices offer a template for ultra-low-cost MIR detectors with performance that rivals microbolometers but with better measurement speed and spectral sensitivity. As such these devices showcase the key advantages of using colloidal NCs in MIR applications. Planar and fully air processed thin film devices that demonstrate photo-induced memristive behaviour and can be used as a transistors, photode-tectors or memory devices are investigated. Following long term (60 h) air exposure, unpackaged NC films develop reliable memristive characteristics in tandem with temperature, gate and photoresponse. On/off ratios of more than 50 are achieved and the devices show long term stability, producing repeatable metrics over days of measurement. The on/off behaviour is shown to be dependent on previous charge flow and carrier density, implying memristive rather than switching behaviour. These observations are described within a long term trap filling model. This work represents an advance in the integration of NC films into electronic devices, which may lead to the development of multi-functional electronic components. Building on the previous work the steps taken to move from a planar device, that works well in controlled conditions, to a multi-pixel sensor that can demonstrate MIR video imaging at room temperature in a noisy environment are shown. This is achieved with a 15 pixel detector that consists only of a polymer substrate and solution patterned NC pixels. This device can detect a 373 K object with the device at 298 K in a noisy environment. This performance is enabled by photogain at 5 V bias that reaches a maximum External Quantum Efficiency (EQE) of 1940 ± 290 % for a pixel with a 3.3 µm bandgap. Through the use of four separate bandgaps it is shown that “multicolour” thermal imaging systems can deliver another layer of information, on top of intensity, to the user. The behaviour of the system is examined under use and it is shown that the photoconductive device behaves as expected with regards to bias, and that trap enabled gain is sensitive to total incident flux, more than the spectral energy distribution of the target. Finally, it is shown that solution patterned QD fabrication methods can deliver electrical reproducibility between pixels that is sufficient to allow an imaging plane of multiple pixels. The somewhat neglected tin chalcogenide semiconductor nanocrystals are investigated and inverse MIR detection at room temperature is demonstrated with planar, solution and airprocessed PbSnTe and SnTe QD devices. The detection mechanism is shown to be mediated by an interaction between MIR radiation and the vibrational stretches of adsorbed hydroxyl species at the oxdised NC surface. Devices are shown to possess mAW⁻¹ responsivity via a reduction in film conductance due to MIR radiation and, unlike classic MIR photoconductors, are unaffected by visible wavelengths. As such these devices offer the possibility of MIR thermal imaging that has an intrinsic solution to the blinding caused by higher energy light sources. In summary, it is shown that semiconductor NCs with an all ambient fully solution processed deposition and ligand exchange procedure can be used to create simple, robust and cheap devices that are beginning to demonstrate metrics on par with current commercial thermal detector systems. It is also shown that these devices can under certain circumstances demonstrate novel behaviours that offer the prospects of enhanced or novel functionality.</p>

Device Applications of Solution Processed MIR Semiconductor Nanocrystal Thin Films

<p>Colloidal semiconductor nanocrystals (NCs) with bandgaps less than 1 eV allow the development of mid wave infrared (MIR) sensitive detectors that exploit the benefits of colloidal materials, primarily bandgap selection and solution deposition. Additionally, the electrical behaviour of these films can be examined for characteristics that can increase the functionality of NC based detectors. The production of devices that are designed to be competitive as ultra-low-cost, room temperature MIR detectors, operating with photonic, rather than thermal detection is detailed. The evolution of the colloidal synthesis, spray deposition methods, substrate materials and post deposition treatments used here lead to highly robust and high performing devices. These devices demonstrate a “colour” sensitivity down to 300 nm in the MIR (≈10 % of scale), with superior responsivities for this class of device, up to 0.9 AW⁻¹, and competitive specific detectivity up to 8 × 10⁹ Jones at 200 Hz and 300 K. Furthermore, these devices utilise a cheap and robust substrate material that allows operation after deformation up to 45 ° without degradation over many cycles. These devices offer a template for ultra-low-cost MIR detectors with performance that rivals microbolometers but with better measurement speed and spectral sensitivity. As such these devices showcase the key advantages of using colloidal NCs in MIR applications. Planar and fully air processed thin film devices that demonstrate photo-induced memristive behaviour and can be used as a transistors, photode-tectors or memory devices are investigated. Following long term (60 h) air exposure, unpackaged NC films develop reliable memristive characteristics in tandem with temperature, gate and photoresponse. On/off ratios of more than 50 are achieved and the devices show long term stability, producing repeatable metrics over days of measurement. The on/off behaviour is shown to be dependent on previous charge flow and carrier density, implying memristive rather than switching behaviour. These observations are described within a long term trap filling model. This work represents an advance in the integration of NC films into electronic devices, which may lead to the development of multi-functional electronic components. Building on the previous work the steps taken to move from a planar device, that works well in controlled conditions, to a multi-pixel sensor that can demonstrate MIR video imaging at room temperature in a noisy environment are shown. This is achieved with a 15 pixel detector that consists only of a polymer substrate and solution patterned NC pixels. This device can detect a 373 K object with the device at 298 K in a noisy environment. This performance is enabled by photogain at 5 V bias that reaches a maximum External Quantum Efficiency (EQE) of 1940 ± 290 % for a pixel with a 3.3 µm bandgap. Through the use of four separate bandgaps it is shown that “multicolour” thermal imaging systems can deliver another layer of information, on top of intensity, to the user. The behaviour of the system is examined under use and it is shown that the photoconductive device behaves as expected with regards to bias, and that trap enabled gain is sensitive to total incident flux, more than the spectral energy distribution of the target. Finally, it is shown that solution patterned QD fabrication methods can deliver electrical reproducibility between pixels that is sufficient to allow an imaging plane of multiple pixels. The somewhat neglected tin chalcogenide semiconductor nanocrystals are investigated and inverse MIR detection at room temperature is demonstrated with planar, solution and airprocessed PbSnTe and SnTe QD devices. The detection mechanism is shown to be mediated by an interaction between MIR radiation and the vibrational stretches of adsorbed hydroxyl species at the oxdised NC surface. Devices are shown to possess mAW⁻¹ responsivity via a reduction in film conductance due to MIR radiation and, unlike classic MIR photoconductors, are unaffected by visible wavelengths. As such these devices offer the possibility of MIR thermal imaging that has an intrinsic solution to the blinding caused by higher energy light sources. In summary, it is shown that semiconductor NCs with an all ambient fully solution processed deposition and ligand exchange procedure can be used to create simple, robust and cheap devices that are beginning to demonstrate metrics on par with current commercial thermal detector systems. It is also shown that these devices can under certain circumstances demonstrate novel behaviours that offer the prospects of enhanced or novel functionality.</p>

Adsorption and Photocatalytic Degradation of an Industrial Azo Dye Using Colloidal Semiconductor Nanocrystals

In this study, mercaptosuccinic acid capped CdSe nanocrystals was successfully synthesized by in-situ medium colloid and used as photocatalyst for the effective photodegradation of methylene blue from aqueous solution under visible light and sunlight irradiations.The particle size and the crystal structure of these nanocrystals were analyzed by different analytical techniques. Dye adsorption prior to photocatalysis using these nanomaterials was studied via the experimental quantification of kinetics and isotherms. These experimental data were modeled including the application of statistical physics theory to analyze the corresponding adsorption mechanism. A maximum adsorption capacity of 27.1 mg/g (80% dye removal) was observed in 10 min using an initial concentration of 30 mg/L. Statistical physics calculations indicated that the adsorption energy was lower than 40 kJ/mol. Itwas also established that the dye adsorption was associated to the electrostatic interactions and hydrogen bonding. Overall, the dye removal was a spontaneous, feasible process and exothermic. Adsorption properties of CdSe-MSA nanocrystals improved the dye photodegradation efficiency under visible light thus achieving up to 80% degradation in 60 min. The synergic effect of adsorption and photocatalytic degradation performance was mainly due to the surface area, small size (3.7 nm) and structural defects (selenium vacancies Se, interstitial of cadmium ICd), which enhanced the response of these nanomaterials inside the visible range for the photocatalytic activity. In summary, these nanocrystals are promising materials to be used in wastewater treatment under sun light for the removal of organic compounds like dyes.

Size-Dependent Properties of Solution-Processable Conductive MOF Nanocrystals

The diverse optical, magnetic, and electronic behaviors of most colloidal semiconductor nanocrystals emerge from materials with limited structural and elemental compositions. Conductive metal-organic frameworks (MOFs) possess rich compositions with complex architectures, but remain unexplored as nanocrystals, hindering their incorporation into scalable devices. Here, we report the controllable synthesis of conductive MOF nanoparticles based on Fe(1,2,3-trizolate)2. Sizes can be tuned as small as 5.5 nm, ensuring indefinite colloidal stability. These solution-processable MOFs can be analyzed by solution-state spectroscopy and electrochemistry and cast into conductive thin films with excellent uniformity. This unprecedented analysis of MOF materials reveals a strong size-dependence in optical and electronic behavior sensitive to the intrinsic porosity and guest-host interactions of MOFs. These results provide a radical departure from typical MOF characterization, enabling insight into physical properties otherwise impossible with bulk analogs, while offering a roadmap for the future of MOF nanoparticle synthesis and device fabrication.

Nanoparticle-Doped Hybrid Polyelectrolyte Microcapsules with Controlled Photoluminescence for Potential Bioimaging Applications

Fluorescent imaging is widely used in the diagnosis and tracking of the distribution, interaction, and transformation processes at molecular, cellular, and tissue levels. To be detectable, delivery systems should exhibit a strong and bright fluorescence. Quantum dots (QDs) are highly photostable fluorescent semiconductor nanocrystals with wide absorption spectra and narrow, size-tunable emission spectra, which make them suitable fluorescent nanolabels to be embedded into microparticles used as bioimaging and theranostic agents. The layer-by-layer deposition approach allows the entrapping of QDs, resulting in bright fluorescent microcapsules with tunable surface charge, size, rigidity, and functional properties. Here, we report on the engineering and validation of the structural and photoluminescent characteristics of nanoparticle-doped hybrid microcapsules assembled by the deposition of alternating oppositely charged polyelectrolytes, water-soluble PEGylated core/shell QDs with a cadmium selenide core and a zinc sulfide shell (CdSe/ZnS), and carboxylated magnetic nanoparticles (MNPs) onto calcium carbonate microtemplates. The results demonstrate the efficiency of the layer-by-layer approach to designing QD-, MNP-doped microcapsules with controlled photoluminescence properties, and pave the way for the further development of next-generation bioimaging agents based on hybrid materials for continuous fluorescence imaging.

Synthesis and Characterization of Tin and Germanium Based Semiconductor Nanocrystals

<p>Inorganic nanomaterials are being actively researched due to their unique physical and chemical properties. These materials can be used for a wide variety of applications and technologies which have stimulated research into the discovery, understanding and control of the morphology of materials at the nanoscale. Biologists have recently integrated biomaterials with semiconductor nanoparticles to expand their applications to include biosensing, bioimaging and therapeutic strategies. Since the water solubility of semiconductor nanoparticles is crucial for bioapplications, the fabrication of water-soluble semiconductor nanocrystals with tailored properties has become more significant. This thesis is focused on the solution phase synthesis of nanoparticles and nanowires containing the element tin. This includes tin nanoparticles, tingermanium alloy nanowires, tin sulphide nanoparticles and tin telluride nanoparticles. The aim of this research was to synthesize nanocrystals with tightly controlled size and shape for various applications,in particular for bioapplications. The properties, potential applications and crystal structure of target materials have been discussed in Chapter 1. The target materials synthesized by using chemical reaction in the presence of surfactant were characterized primarily by Transmission Electron Microscopy (TEM), Energy Dispersive X-ray Spectroscopy (EDX) and Selected Area Electron Diffraction (SAED). Powder X-ray Diffraction (XRD), Scanning Transmission Electron Microscopy (STEM), Scanning Electron Microscopy (SEM), Ultraviolet-Visible Microscopy Absorption (UV-VIS), Fourier Transform Infrared (FTIR), Photoluminescence (PL) and Diffuse Reflectance were also used extensively (Chapter 2). The third chapter of this thesis focuses on the the development of a facile and cheap route for the synthesis of tin nanoparticles by reducing a tin precursor in an organic solvent. The low-melting tin nanoparticles have been considered as a good catalyst for the growth of semiconductor nanowires. The fourth chapter in this thesis focuses on the development of a convenient synthesis of tin germanium alloy nanowires via solution-liquid-solid growth (SLS). Tin germanium alloy nanowires were synthesized through a self-catalyzed process in which the wires were grown from in situ made Sn droplets and Ge(Ph)₃Cl. The factors affecting morphology were ascertained and the growth direction, composition, local crystal structure and possible growth mechanism have been investigated. The fifth chapter in this thesis focuses on the development of a novel one-pot synthesis of water-soluble SnS nanoparticles. The synthesis of SnS nanoparticles involves the reaction of inorganic starting materials SnBr₂ and Na₂S in the presence of various ethanolamine derivatives in ethylene glycol. Optical studies of as synthesized SnS nanoparticle show size dependent effects in both absorbance and reflectivity. The sixth chapter in this thesis focuses on the development of a facile direct synthesis of water dispersible SnTe nanoparticles. The optical properties of prepared SnTe nanoparticles were determined. The final chapter in this thesis summarizes the main findings of this study and draws out recommendations for future work. In this study, some novel contributions have been made to produce facile one-pot synthesis of tin germanium nanowires and water soluble, size controlled tin chalcogenides nanoparticles. The main future work for tin germanium alloy naowires is to develop the method to produce nanowires without seed nanoparticles for optoelectronics applications. Further work is also needed to optimize the water synthesis of SnTe nanoparticles.</p>