Solution-processed optoelectronic materials offer a route to low cost photodetectors, large area solar cells, and integrated optical sources. While significant progress has been reported in organic and polymer spin-cast optoelectronics, colloidal quantum dots offer a distinct further advantage -- the convenient tuning of absorption onset via the quantum size effect. Electronic transport has recently been enhanced in size effect tuned colloidal quantum dot films using ligand exchange, resulting in ultrasensitive photodetectors in both visible and infrared wavelengths. Solid-film ligand exchange, however, generally results in rough film morphologies that are incompatible with high uniformity image sensors. Here, we report a new route to visible-wavelength spin-cast lead sulfide (PbS) nanocrystal photoconductive photodetectors with a sub 1% roughness, compared to the ~10% roughness obtained using previously reported approaches. The new procedure yields devices that exhibit 10 A/W responsivities and reveals an added significant advantage: when illumination conditions change, the photodetectors respond with a single time constant of 20 ms. This compares very favorably to the multi second and multi-time-constant response of previously reported PbS-nanocrystal photoconductive photodetectors.

Solution-processed optoelectronic materials offer a route to low cost photodetectors, large area solar cells, and integrated optical sources. While significant progress has been reported in organic and polymer spin-cast optoelectronics, colloidal quantum dots offer a distinct further advantage -- the convenient tuning of absorption onset via the quantum size effect. Electronic transport has recently been enhanced in size effect tuned colloidal quantum dot films using ligand exchange, resulting in ultrasensitive photodetectors in both visible and infrared wavelengths. Solid-film ligand exchange, however, generally results in rough film morphologies that are incompatible with high uniformity image sensors. Here, we report a new route to visible-wavelength spin-cast lead sulfide (PbS) nanocrystal photoconductive photodetectors with a sub 1% roughness, compared to the ~10% roughness obtained using previously reported approaches. The new procedure yields devices that exhibit 10 A/W responsivities and reveals an added significant advantage: when illumination conditions change, the photodetectors respond with a single time constant of 20 ms. This compares very favorably to the multi second and multi-time-constant response of previously reported PbS-nanocrystal photoconductive photodetectors.

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This thesis is concerned with the characterisation, fabrication and testing of devices capable of generating and detecting terahertz (THz) radiation. Such devices are based on semiconductor photoconductors grown under low temperature (LT) conditions using the technique of Molecular Beam Epitaxy (MBE). The absorption of a pulsed or continuous wave (CW) signal by these structures in conjunction with the presence of an electric field generates photocurrent, which is fed into an antenna structure fabricated on the surface of the active layers. As a result of such a sequence, a THz signal is generated and radiated from the substrate side into free space. Therefore, the efficiency of the devices is determined by the characteristics of the photoconductors and the geometry of the designed antenna structures. The desired material characteristics are high absorption at the corresponding illumination wavelength, high dark resistivity, high electron mobility and sub picosecond carrier lifetime. The determination of these characteristics for all the structures grown in this work composes the characterisation part of the thesis. The fabrication part comprises of the design of several antenna structures with various geometrical characteristics, while the testing part consists of their evaluation as THz sources and detectors in a time-domain spectroscopy (TDS) system under pulsed excitation. To date, THz devices based on low temperature grown GaAs (LT-GaAs) photoconductors have been reported to be the most efficient. However, their operational wavelength, at 800 nm, requires very expensive and complex components spurring interests in solutions consisting of devices operating at longer wavelengths, where cheaper and simpler components exist. The most desirable and practical operational wavelength is the telecommunication one at 1.55 μm. Thus, the biggest challenge is the development of efficient devices operating at this illumination wavelength. In this work, devices operating at the very important wavelength of 1.55 μm as well as at the wavelengths of 1 μm and 800 nm are presented. The key findings for the long wavelength devices (1.55 μm) demonstrate photoconductors with ultrafast carrier lifetimes (~ 120 fs), high resistivity (> 105 Ω / sq), high mobility (> 1000 cm2 / Vs) and system responses with spectral range up to 3 THz and power-to-noise ratio of 60 dB. These characteristics are among the best ever reported for such material systems, making them efficient THz devices for various optoelectronic applications, especially for telecommunication laser-driven CW THz systems.

Proteins are commonly found molecules in biological systems: our fingernails, hair, skin, blood, muscle, and eyes are all made of protein. Many diseases simply arise because a protein is not folded properly. Therefore, knowledge of protein structure is considered a prerequisite to understanding protein function and, by extension, a cornerstone for drug design and for the development of therapeutic agents. Protein crystallography is a tool that allows structural biologists to discern protein structures to the highest degree of detail possible in three dimensions. The recording of x-ray diffraction data from the protein crystal is a central part of protein crystallography. As such, an important challenge in protein crystallography research is to design x-ray detectors to accurately determine the structures of proteins. This research presents the design and evaluation of a solid-state large area at panel detector for protein crystallography based on an amorphous selenium (a-Se) x-ray sensitive photoconductor operating in avalanche mode integrated with an amorphous silicon (a-Si:H) charge storage and readout pixel. The advantages of the proposed detector over the existing imaging plate (IP) and charge coupled device (CCD) detectors are large area, high dynamic range coupled to single x-ray detection capability, fast readout, high spatial resolution, and inexpensive manufacturing process. The requirement of high dynamic range is crucial for protein crystallography since both weak and strong diffraction spots need to be imaged. The main disadvantage of a-Si:H thin film transistor (TFT) array is its high electronic noise which prohibits quantum noise limited operation for the weak diffraction spots. To overcome the problem, the x-ray to charge conversion gain of a-Se is increased by using its internal avalanche multiplication gain. Since the detector can be made approximately the same size as the diffraction pattern, it eliminates the need for image demagnification. The readout time of the detector is usually within the ms range, so it is appropriate for crystallographic application. The optimal detector parameters (such as, detector size, pixel size, thickness of a-Se layer), and operating parameters (such as, electric field across the a-Se layer) are determined based on the requirements for protein crystallography. A complete model of detective quantum efficiency (DQE) of the detector is developed to predict and optimize the performance of the detector. The performance of the detector is evaluated in terms of readout time (< 1 s), dynamic range (~10^5), and sensitivity (~ 1 x-ray photon), thus validating the detector's efficacy for protein crystallography. The design of an in-house a-Si:H TFT pixel array for integration with an avalanche a-Se layer is detailed. Results obtained using single pixel are promising and highlight the feasibility of a-Si:H pixels coupled with avalanche a-Se layer for protein crystallography application.

Proteins are commonly found molecules in biological systems: our fingernails, hair, skin, blood, muscle, and eyes are all made of protein. Many diseases simply arise because a protein is not folded properly. Therefore, knowledge of protein structure is considered a prerequisite to understanding protein function and, by extension, a cornerstone for drug design and for the development of therapeutic agents. Protein crystallography is a tool that allows structural biologists to discern protein structures to the highest degree of detail possible in three dimensions. The recording of x-ray diffraction data from the protein crystal is a central part of protein crystallography. As such, an important challenge in protein crystallography research is to design x-ray detectors to accurately determine the structures of proteins. This research presents the design and evaluation of a solid-state large area at panel detector for protein crystallography based on an amorphous selenium (a-Se) x-ray sensitive photoconductor operating in avalanche mode integrated with an amorphous silicon (a-Si:H) charge storage and readout pixel. The advantages of the proposed detector over the existing imaging plate (IP) and charge coupled device (CCD) detectors are large area, high dynamic range coupled to single x-ray detection capability, fast readout, high spatial resolution, and inexpensive manufacturing process. The requirement of high dynamic range is crucial for protein crystallography since both weak and strong diffraction spots need to be imaged. The main disadvantage of a-Si:H thin film transistor (TFT) array is its high electronic noise which prohibits quantum noise limited operation for the weak diffraction spots. To overcome the problem, the x-ray to charge conversion gain of a-Se is increased by using its internal avalanche multiplication gain. Since the detector can be made approximately the same size as the diffraction pattern, it eliminates the need for image demagnification. The readout time of the detector is usually within the ms range, so it is appropriate for crystallographic application. The optimal detector parameters (such as, detector size, pixel size, thickness of a-Se layer), and operating parameters (such as, electric field across the a-Se layer) are determined based on the requirements for protein crystallography. A complete model of detective quantum efficiency (DQE) of the detector is developed to predict and optimize the performance of the detector. The performance of the detector is evaluated in terms of readout time (< 1 s), dynamic range (~10^5), and sensitivity (~ 1 x-ray photon), thus validating the detector's efficacy for protein crystallography. The design of an in-house a-Si:H TFT pixel array for integration with an avalanche a-Se layer is detailed. Results obtained using single pixel are promising and highlight the feasibility of a-Si:H pixels coupled with avalanche a-Se layer for protein crystallography application.

<p>The phenomenon of image blurring on laser-printed or electro-photocopied paper has been discovered since the 1980s. In the 1990s, the problem was confirmed to be associated with the undesired surface conduction along the unique photoconductive polymer surface during the photoconduction process. Other than this, little progress has been made in investigating this phenomenon, due to the limited experimental techniques.</p> <p>In this thesis, the electrical properties of a commercially available photoconductor as a result of Corona charging were studied. Various techniques including vacuum deposition and step-function impedance spectroscopy were employed, to overcome the nature of the photoconductor that prevented the use of conventional techniques such as AC impedance spectroscopy. Negative dielectric constant (NDC) has been prevalently discovered at a broad range of frequencies (below 1Hz and up to 1 MHz) and it was questioned in the form of a physically-impossible inductor. This precipitous sign switch of dielectric constant is found in various areas ranging from physics, chemistry, biology to electronics. The magnitude of the NDC decreased drastically with the decrease of electric field frequency. The system obeyed the proposed free-carrier plasma model with a resonance frequency at MHz level.</p> <p>Commercially available polymeric photoconducting materials showing NDC at extremely low frequency are expected to provide unusual scattering to electromagnetic waves and therefore demonstrate profound implications with reduced cost. It has paved the way for many applications such as inductors in integrated chips without bulky coils and provides an insight into a possible revolution in electronics and photonics.</p> / Doctor of Philosophy (PhD)