Electronic and Structural Investigation of Nanocrystal Thin Films Tuned via Surface Chemistry

Monodisperse colloidal nanocrystals (NCs) provide an opportunity to access physical properties that cannot be realized in bulk materials, simply by tuning the particle size or shape. These NCs form the basis of an artificial periodic table that can be used as building blocks to engineer a new class of solid-state materials with emergent properties. The monodispersity offers a structural advantage for assembling NCs into an ordered superlattice, in addition to a narrow distribution of band energies which in principle promote more efficient transport when the NCs are electronically coupled in a thin film solid after undergoing surface chemistry treatments. However, previous methods for NC assembly have been limiting in their scalability, and while there has been much work in general on the effects of different ligand surface chemistries on semiconductor NC solids, little work has been done to controllably tune the Fermi level and quantify its position in order to promote better device engineering. Herein, we investigate dip-coating as a method by which to scale up NC superlattice assembly. We demonstrate large-area ordering on wafer-scale for both single component and binary nanocrystal superlattices with a diverse set of NC materials and binary crystal geometries. We confirm the extent over which these films are ordered via GISAXS, TEM, and SEM characterization. In the remainder of this work, we study the electronic effects of different ligand chemistry treatments of the NCs. We show that a sequential two step surface treatment can offer increased control over the tuning of the Fermi level and we quantify its positioning and band edge energies relative to vacuum level via a pairing of temperature dependent Seebeck measurements, cyclic voltammetry, and absorption spectroscopy. This provides a reference by which NC devices can be more precisely engineered. Furthermore, we apply that the AC magnetic field Hall effect measurement to a series of common ligand treatments used for making NC devices such as solar cells and field effect transistors to better understand their relative electronic transport properties. We demonstrate this method can be used to determine the hall mobility in these generally high resistivity, low mobility films

Chemically triggered formation of two-dimensional epitaxial quantum dot superlattices

Two dimensional superlattices of epitaxially connected quantum dots enable size-quantization effects to be combined with high charge carrier mobilities, an essential prerequisite for highly performing QD devices based on charge transport. Here, we demonstrate that surface active additives known to restore nanocrystal stoichiometry can trigger the formation of epitaxial superlattices of PbSe and PbS quantum dots. More specifically, we show that both chalcogen-adding (sodium sulfide) and lead oleate displacing (amines) additives induce small area epitaxial superlattices of PbSe quantum dots. In the latter case, the amine basicity is a sensitive handle to tune the superlattice symmetry, with strong and weak bases yielding pseudohexagonal or quasi-square lattices, respectively. Through density functional theory calculations and in situ titrations monitored by nuclear magnetic resonance spectroscopy, we link this observation to the concomitantly different coordination enthalpy and ligand displacement potency of the amine. Next to that, an initial similar to 10% reduction of the initial ligand density prior to monolayer formation and addition of a mild, lead oleate displacing chemical trigger such as aniline proved key to induce square superlattices with long-range, square micrometer order; an effect that is the more pronounced the larger the quantum dots. Because the approach applies to PbS quantum dots as well, we conclude that it offers a reproducible and rational method for the formation of highly ordered epitaxial quantum dot superlattices

Balancing charge carriertransport in a quantum dot P-N junction toward hysteresis-free high-performance solar cells

In a quantum dot solar cell (QDSC) that has an inverted structure, the QD layers form two different junctions between the electron transport layer (ETL) and the other semiconducting QD layer. Recent work on an inverted-structure QDSC has revealed that the junction between the QD layers is the dominant junction, rather than the junction between the ETL and the QD layers, which is in contrast to the conventional wisdom. However, to date, there have been a lack of systematic studies on the role and importance of the QD heterojunction structure on the behavior of the solar cell and the resulting device performance. In this study, we have systematically controlled the structure of the QD junction to balance charge transport, which demonstrates that the position of the junction has a significant effect on the hysteresis effect, fill factor, and solar cell performance and is attributed to balanced charge transport

Inorganic Nanocrystals And Their Applications In Hybrid 0D:2D Material Optoelectronics

Functional nanomaterials have garnered great interest as candidates for use in next-generational optoelectronics such as solar photovoltaics, light-emitting diodes, and photodetectors. Among these low-dimensional materials, hybrid devices employing both 0D and 2D materials are of interest due to exploitation of the favorable characteristics of each component, and performances superior to standalone counterparts are achievable. This thesis is divided into two parts, as follows. The first two chapters will introduce lowdimensional materials and their favorable characteristics; our work on the formation of ligand-exchanged nanocrystal thin films purified by gel-permeation chromatography will also be discussed. In the second component, the formation and study of two hybrid nanocrystal/epitaxial graphene optoelectronic devices will be presented. My work on a standalone epitaxial graphene/silicon carbide ultraviolet photodetector will also be described

Noble-Transition Alloy Absorbers for Near-Infrared Hot-Carrier Optoelectronics

Optoelectronics is the field of technology concerned with the study and application of electronic devices that source, detect and control light. Here we focus on the optical communications field which relies on optical fiber systems to carry signals to their destinations operating in the near-infrared range. To improve the performance of current optical fiber systems, one of the paths is to develop better near-infrared photodetectors. The current group of materials used for near-infrared photodetection relies in the III-V semiconductor family. Although their spectral photosensitivity correlates well with the near-infrared, response time performance and electronic circuit integration remain limited for this class of material. Complementary metal-oxide-semiconductor-Si photonics technology can be coupled with metal interface to form a Schottky barrier extending the silicon detection range to near-infrared. Above-equilibrium “hot” carrier generation in metals is a promising route to convert photons into electrical charge for optoelectronics. However, metals which offer both hot-carrier generation in the near-infrared and sufficient carrier lifetimes remain elusive. The aim of this thesis is to contribute to the development of a novel class of materials for near-infrared optoelectronic applications. Early progress in hot-carrier generation showed that one can tune optical and electronic properties of noble metals by alloying. The performance of these noble-metals alloys relied however on visible light application. Transition metals have a band structure much more favorable for hot-carrier generation in the near-infrared. However, due to the electron-electron scattering rates, oxidation states, and broad, weak or absence plasmon resonance, they have not gained attention in this field. Prior to this thesis, no noble-transition alloy for hot-carrier generation had been reported. Here, it is shown that a noble-transition alloy, AuxPd1-x, outperforms its constituent metals concerning generation and lifetime of hot carriers when excited in the near infrared. We show that at optical fiber wavelengths (e.g., 1550 nm) Au50Pd50 provides a 20-fold increase in the number of ~0.8 eV hot holes, compared to Au, and a 3-fold increase in the carrier lifetime, compared to Pd. In addition, we show that to keep their properties, these alloys should not be exposed to high temperatures (450 oC) during fabrication steps or application

Nonlinear Optical Phenomena in Emerging Low-dimensional Materials

As digital information technologies continue to evolve at much faster rates than the growth of Si-based processors, the encroachment of light-based technologies into computing seems inevitable. With the advent of lasers, photonic crystals, and optical diodes, photonic computing has made significant strides in information technology over the past 30 years. This continuing integration of light into all-optical computing, optoelectronic components, and emerging optogenetic technologies demands the ability to control and manipulate light in a predictable fashion, or by design. Of particular interest, is the passive control and manipulation of light in all-optical switches, photonic diodes, and optical limiting which can be achieved by leveraging intrinsic non-linear optical properties of low dimensional materials. The reverse saturable absorption in fullerenes has been widely used to realize excellent passive optical limiters for the visible region up to 650 nm. However, there is still a need for passive optical switches and limiters with a low limiting threshold (\u3c0.5 J/cm2) and higher damage limits. The electronic structure of fullerenes can be modified either through doping or by the encapsulation of endohedral clusters to achieve exotic quantum states of matter such as superconductivity. Building on this ability, we discuss in Chapter 2 that the encapsulation of Sc3N, Lu3N or Y3N in C80 alters the HOMO-LUMO gap and leads to passive optical switches with a significantly low limiting threshold (0.3 J/cm2) and a wider operation window (average pulse energy \u3e0.3 mJ in the ns regime). In addition to extraordinary and strongly anisotropic electronic properties, two dimensional (2D) materials such as graphene and boron nitride, exhibit strong light-matter interactions despite their atomic thickness. The nonlinear light-matter interactions in 2D materials are well suited for several applications in photonics and optoelectronics, such as ultrafast optical switching and optical diodes. Unlike most 2D materials that display nonlinear saturable absorption or increased light transmission at higher fluences, hexagonal boron nitride nanoplatelets (BNNPs) exhibit enhanced opaqueness with increasing light fluence. A two-photon absorption (2PA) process was previously proposed to explain the intrinsic non-linear absorption in BNNPs at 1064 nm or 1.16 eV (Kumbhakar et al., Advanced Optical Materials, vol. 3, pp. 828, 2015); which is counter-intuitive because a 2PA process at 1.16 eV cannot excite electrons across the wide band gap of BNNPs (~5.75 eV). Here, through a systematic study of the non-linear properties of BNNPs we uncover a notoriously rare non-linear phenomenon, viz., five-photon absorption (5PA) at 1064 nm for low laser input fluences (below 0.6 J/cm2) that irreversibly transforms to a 2PA for higher laser input fluences (above 0.6 J/cm2). Our detailed experimental and theoretical findings delineated in Chapter 3 provide compelling evidence that the high laser fluence generates defects in BNNPs (e.g., oxygen/carbon doping), which support a 2PA process by inducing new electronic states within the wide band gap of BNNPs. MXenes comprise a new class of two-dimensional (2D) transition metal carbides, nitrides, and carbonitrides that exhibit unique light-matter interactions. Recently, 2D Ti3C2Tx (Tx represents functional groups such as –OH and –F) was found to exhibit nonlinear saturable absorption (SA) or increased transmittance at higher light fluences that is useful for mode locking in fiber-based femtosecond lasers. However, the fundamental origin and thickness-dependence of SA behavior in MXenes remains to be understood. We fabricated 2D Ti3C2Tx thin films of different thicknesses using an interfacial film formation technique to systematically study their nonlinear optical properties. Using the open aperture Z-scan method, we find that the SA behavior in Ti3C2Tx MXene arises from plasmon-induced increase in the ground state absorption at photon energies above the threshold for free carrier oscillations. The saturation fluence and modulation depth of Ti3C2TxMXene was observed to be dependent on the film thickness. Unlike other 2D materials, Ti3C2Tx was found to show higher threshold for light-induced damage with up to 50% increase in nonlinear transmittance. Lastly, building on the SA behavior of Ti3C2Tx MXenes, we demonstrate in Chapter 4 a Ti3C2Tx MXene-based photonic diode that breaks time-reversal symmetry to achieve non-reciprocal transmission of nanosecond laser pulses. Finally, in Chapter 5, we discuss the equilibrium and non-equilibrium free carrier dynamics in a 16 nm thick Ti3C2Tx film. High (~2 x 1021 cm-3) intrinsic charge carrier density and relatively high (~34 cm2/Vs) mobility of carriers within individual nanoplates (that comprise the Ti3C2Tx film) result in an exceptionally large (~ 46 000 cm-1) absorption in the THz range, implying the potential use of Ti3C2Tx for THz detection. We also demonstrate that Ti3C2Tx conductivity and THz transmission can be manipulated by photoexcitation, as absorption of near-infrared 800 nm pulses is found to cause transient suppression of the conductivity that recovers over hundreds of picoseconds. The possibility of controlling THz transmission and conductivity via photoexcitation makes 2D MXenes suggests a promising material for application in THz modulation devices and variable electromagnetic shielding

Optical and Structural Studies of Shape-Controlled Semiconductor Nanocrystals and Their Self-Assembled Solids

Colloidal nanocrystals are prominent candidates to displace current electronic active layers in solid-state device technologies and offer a body of physics which diverges from those of bulk materials and discreet molecules. Realizing the potential of colloidal nanocrystals may transform the costs and performance of common technologies, but understanding of the relationship between particle size, shape, uniformity, and composition and outputs like physical properties or device performance is often incomplete. This work uses the controlled synthesis of anisotropic colloidal nanocrystals to implement characterization techniques including X-ray diffraction and simulation, which allows an ensemble-level description of particle structure, as well as polarized and time-resolved spectroscopy, which demonstrates subtle synthetic control over the properties of quantum-mechanical wavefunctions. Time- and temperature-resolved optical spectroscopy is employed to analyze the behavior of nanocrystal samples under more realistic device operating conditions and to determine the structure/property relationships that underpin improved performance. Highly-uniform samples of colloidal nanocrystals are self-assembled into large-area thin films. Discussion of self-assembly is placed within the context the fundamentals of self-assembly processes and the roadmap to high-performance devices based upon colloidal nanocrystals. X-ray diffraction and microscopic analysis are performed to analyze and qualify the structure of self-assembled films. These measurement techniques provide figures of merit for nanocrystal assemblies including the sample crystallinity and purity, surface coverage, homogeneity. Diffraction analysis is further used to measure alignment of nanocrystal assemblies with respect to a substrate and the orientation of individual particles within assemblies. Monodisperse anisotropic building blocks encode the unique optoelectronic properties of isolated nanocrystals into solid state materials with long-range structural orientation

Controlling the Co-Precipitation of Colloidal PbSe Quantum Dots and Their Device Performance in a Diode Architecture

In this work, colloidal PbSe quantum dots (QDs) are shown to be synthesized in a controlled manner by changing the growth time and reaction temperature in a microwave synthesis method. It is explained why quantum dots make such a unique material to create electronic devices and the specific benefits that PbSe QDs portray including its large Exciton Bohr Radius and multiple exciton generation phenomena. Across all synthesis batches the particle morphology was found to be spherical and size variation was kept between 9-15%. X-ray diffraction of each batch sample showed the same set of consistent peaks with matching relative intensities to JCPDS standard #00-006-0354. High resolution transmission electron microscopy also showed crystallographic planes with three angstrom separation corresponding to the (200) face of PbSe. UV-Vis-NIR data yielded the location of each batch’s first excitonic absorption peak and when combined with size distribution data, a correlation was created between average particle diameter and wavelength to be compared to other data in literature. It was also shown that the material could be implemented into a diode architecture after several device iterations. The performance of each is documented via I-V curves and material was not used unless it was ligand exchanged to increase conductivity. Excellent, repeatable rectifying behavior is obtained in the last device architecture and in future work synthesized material will be incorporated into these diodes and operated in the rectifying region to serve as high resolution radiation sensors

Design and Development of Planar Antennas and Dielectric Devices for use at W Band Frequencies

The main topic examined in this thesis is the development of planar antennas for observations at W band frequencies. A large portion of this analysis is based on the specific development of devices as part of an ESA research package entitled “New Technology High Efficiency Horn Antennas for CMB Experiments and Far-Infrared Astronomy”. The development of W band focal plane pixels (planar antenna with small lenses) was part of the research in this work package. Several planar antenna designs are modelled and optimised in CST Studio Microwave Suite, a commercially available computer modelling software used extensively in order to predict the device performance around 100 GHz. The final designs were manufactured and their beam patterns were measured in the Vector Network Analyser (VNA) setup. The main body of this research consisted of the development and analysis of patch antennas. The design of back-fed and side-fed patch antennas are modelled in the CST work environment, manufactured in-house and measured with the VNA. A number of lenslet (small lenses for each planar antenna in the array) designs constructed from High Density Polyethylene (HDPE) are developed as part of this ESA contract in order to develop a lens array for a planar antenna array. A particular focus was put on reducing the potential crosstalk between neighbouring pixels and optimising the lens shape. The lenslets examined included a hemisphere, a cylindrical and a plano-convex lens. A novel truncated plano-convex lens was also analysed for the task of reducing crosstalk between neighbouring pixels. Plano-convex lenses with cleaved sides (referred to as a truncated lens) were manufactured and tested with the VNA. The crosstalk signal level caused by these lenslets between neighbouring pixels were considered and measured. Additional topics developed include the analysis of a multi-moded terahertz horn antenna measured in an experimental setup in Cardiff University using GBMA (Gaussian Beam Mode Analysis). This 2.7 – 5 THz pyramidal horn antenna couples to Transition Edge Sensor detectors (TESs) and was placed in a cryogenic chamber to measure the farfield pattern. The antenna was illuminated by a terahertz source through a window in the cryostat. A GBMA model was extended to include truncation of the beam at this window in order verify no loss of signal and to ensure all power propagated though this window