Due to the unique feature of tunable size and surface chemistry, the optical and electronic properties of quantum dots (QDs) can be controlled, which helps to circumvent the challenges to turning these versatile materials into high-performance optoelectronic devices. However, a major bottleneck has prevented further development of devices with superior performance based on QDs, especially for small band QDs. It is the lack of a good understanding of surface chemistry on the electronic properties of QDs that limits their application.

In this thesis, we have precisely controlled the surface chemistry of lead selenide (PbSe) QDs by replacing initial oleate with a group of thiophenol-based organic molecules. Consequently, we are able to successfully engineer the carrier type within PbSe QDs by varying the substitutional group on these thiophenol-based molecules.

Field-effect transistors employing ligand-exchanged QDs are fabricated to study the charge transport properties and trap states within QD films. The results show that ligands with electron-donating substitutional groups give n-channel transistors while electron-withdrawing groups tend to yield p-channel transistors. X-ray photoelectron spectroscopy results confirm that the Fermi level tends to swing n-type for QDs with ligands containing electron-donating groups and p-type for withdrawing groups. This series of ligands shift valence band energy values spanning 2.0 eV for 6.5 nm QDs, which, by far, is the largest energy shift achieved through ligand exchange for PbSe QDs. Models of (111) plane PbSe slabs attached with different ligands are studied by First-principles calculations. The calculation shows a similar trend of band energy shift for semi-infinite 2D slabs passivated with different ligands. These ligands were also applied to 7.3 nm PbSe QDs to fabricate photodetectors used in the extended short-wavelength infrared region (2-3 μm). Preliminary results show that the adjustment of band energies and doping types have great impacts on the charge transport of these devices.

In summary, we have developed a surface modification technique for tailoring the band energy and the doping type of QD films. Although the resulting films are not very conductive, the adjustable energy level with a 2.0 eV window enables these ligands applied for directing charge flow in QD optoelectronic devices.

Colloidal quantum dot (CQD) solids represent a class of materials that allows one to control the optical and electronic properties due to their unique size-dependent properties with special electronic and optoelectronic device applications. Unfortunately, integration of these materials into high performing devices such as transistors and solar cells have been challenging due to: 1) uncontrolled environmental stability 2) lack of accurate control over charge carrier type and mobility 3) poor device operational stability and 4) limited experimental methods to probe the density of states in these materials in order to understand fundamental electronic and optical properties. In this thesis, we demonstrate the ability to stabilize and improve the environmental stability of these materials with amorphous Al2O3 (a-alumina). More importantly, we can accurately engineer the carrier type and mobility by varying the thickness of the alumina. Through a combination of small, compact inorganic ligands and the ability to passivate surface electronic traps, air-stable, high electron mobility PbSe QD field-effect transistors (FET) are obtained.

We then show that we can also improve transistor device operational stability through an in-vacuo ligand exchange with H2S gas introduced in an atomic layer deposition (ALD) chamber. We find that this method is universal when volatile ligands are used. Possible mechanisms for device instability will be discussed such as proton migration and trap passivation. Using an optimized film preparation, this work will be the first demonstration of a QD FET with an electron mobility greater than 10 cm2 V-1 s-1 that is also operationally stable.

Finally, we introduce a unique transmission spectroscopy technique of field-effect transistors to electrostatically probe induced charge carriers in PbSe QD films. With this technique we resolve occupation of quantized states of the quantum dots rather than the matrix or interfacial states. This platform is used to test fundamental transport models as it relates to disordered semiconductors such as QDs. From this technique, we can draw important conclusions about charge transport at room temperature. This novel experimental method can be extended to other experimental setups such as photoluminescence and photoconductivity in order to understand how to rationally improve the electronic properties of QD films.

Solar driven water splitting has the potential to be a true

terawatt-scale solution to the current clean energy crisis. The broad goal

of this thesis is to outline and demonstrate two novel device architectures

for high efficiency tandem water splitting.

Initially focus was dedicated towards the bottlenecks encountered in

fabricating standalone high efficiency p-type dye sensitized solar cells

(DSCs). Synthesis and characterization of 5 novel nanocrystalline potential

hole conducting oxides will be discussed.

Special emphasis will be devoted to Cr-based spinels, which have the

potential to constitute a new class of wide band gap, p-type oxides with

substantially deeper valence bands than all current existing p-type oxides

in literature.Finally, hole injection from CdSe quantum dots into the VB of

zinc chromate (ZnCr2O4) will be demonstrated with photovoltages as high as

350mV using an polysuflide-based redox couple, making it one of the largest

photovoltages of all p-type oxides using this redox couple.

The latter half will be devoted towards fabricating high efficiency

BiVO4 photoanodes for oxygen evolution. The first strategy utilizes the

enhanced reactivity of the (004) facet of BiVO4, wherein a textured

nanostructured Mo:BiVO4 photoanode shows near unity carrier separation and

record catalytic efficiency towards oxygen evolution for a standalone BiVO4

photoanode. These photoanodes increased the electron diffusion length to ~

200nm. The second involves tackling the lack of photoresponse at low biases

(~0.6V vs RHE) via phosphate treatments of undoped BiVO4. Using appropriate

phosphate precursor treatments, photocurrent densities in excess of 3 mA/cm2

were achieved at 0.6V vs RHE and greater than 4 mA/cm2 at 1.23V vs RHE

towards sulfite oxidation. It was found that the electron diffusion length

using these treatments could be further increased to ~ 300nm which is one of

the highest reported for all known metal oxide semiconductors.

This thesis can be used as a guideline for the fabrication of a broad range

of device architectures to achieve low-cost, high efficiency

solar-to-hydrogen conversion efficiencies. This work is aimed at improving

the existing library of high work function p-type oxides as well as setting

the benchmark and improving photoanode-based PEC via a rational approach

towards improving the electronic properties of BiVO4 photoanodes.

Iron pyrite (cubic FeS2) is a non-toxic, earth abundant semiconductor possessing a set of excellent optical/electronic properties for serving as an absorber layer in PV devices. Additionally, pyrite is a very efficient hydroxyl radical generator via Fenton chemistry and has shown promise in oxidative protein and DNA foot-printing application. The main focus of this thesis is on fabricating phase and elementally pure iron pyrite thin films using a solution-based approach that employs hydrazine as a solvent. A precursor ink is formed at room temperature by mixing elemental iron and sulfur in anhydrous hydrazine and then deposited on Mo-coated glass substrates, via spin coating, to yield amorphous iron sulfide films that are then annealed in H2S (340 o C) and sulfur gas (≤ 500 o C) to form uniform, polycrystalline and phase pure pyrite films with densely packed grains. This approach is likely to yield the most elementally pure pyrite thin films made to date, through a very simple and scalable process. The ink has shown to be very sensitive to environmental conditions and has a very short shelf life (~1 day). Additionally, the film microstructure is greatly influenced by the S:Fe concentration ratio that when tuned to 3:1, yielded uniform, robust and optically flat iron sulfide thin films with an optimal thickness (~320 nm) for PV application. The results however were not reproducible, mainly due to failure in applying multiple layers without compromising film morphology. Thinner (< 100 nm) iron sulfide films, on the other hand, are reproducibly produced, but are too thin to be employed in PV devices. Direct annealing in sulfur gas at 475 o C for 4 hours, bypassing the > 12 hour H2S annealing step, yielded phase pure pyrite films, with good morphology, at lower processing time and annealing temperatures (< 500 o C). The latter part of this thesis regards the use of pyrite nano-crystals in conjunction with high surface area polymer laminates for protein foot-printing application in collaboration with the Brenowitz lab at the Albert Einstein College of Medicine and the Khine lab at the University of California, Irvine. A thin film of pyrite nano-crystals is spray deposited (Video in supplementary) onto a shape memory polymer that is then thermally treated with a heat gun, causing the sheet to retract and stiffen as the nanocrystalline layer crumples and integrates into the polyolefin, forming a mechanically robust and highly reactive laminate of pyrite nano-crystals. Micro-wells are thermoformed into the laminate under negative pressure. ·OH dose-oxidation response relationship were established via varying the H2O2 concentration and reaction time. The flexibility, cost effectiveness and scalability of this platform enables integration into macro-structural analysis systems. Pyrite shrink laminates and hydrazine ink films were characterized by Scanning Electron Microscopy (SEM), X-Ray Diffraction (XRD), and Raman Spectroscopy. Drop deposition oxidation experiments and MALDI-TOF "Matrix Assisted Laser Desorption/Ionization-Time of Flight" Mass Spectroscopy of protein aliquots reacted on PSWL were conducted in the Brenowitz lab at the department of biochemistry at the Albert Einstein College of Medicine in New York.