DSpace Repository :: Browsing by Author "Kung, Patrick"

Browsing by Author "Kung, Patrick"

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Analytical evaluation of effective solar sail attitude control using metasurfaces for deep space exploration
(University of Alabama Libraries, 2018) Ullery, Dylan; Kim, Seongsin; Kung, Patrick; University of Alabama Tuscaloosa
We examine the theoretical implications of incorporating metasurfaces on solar sails, and the effect they can have on the forces and torques applied to the sail. This would enable a significant enhancement over state-of-the- art attitude control by demonstrating a novel, propellant-free and low mass approach to induce a roll torque on the sail, which is a current limitation in present state-of-the-art technology. We do so by utilizing anomalous optical reflections from the metasurfaces to generate a net in-plane lateral force. This can lead to a net torque along the roll axis of the sail, in addition to the other spatial movements exhibited by the sail from solar radiation pressure. We characterize this net lateral force as a function of incidence angle. In addition, the influence of the phase gradients and anomalous conversion efficiency characteristics of the metasurfaces are independently considered. The optimum incidence angle that corresponded with the maximum net lateral-to-normal force ratio was found to be -30° for a metasurface exhibiting 75% anomalous conversion efficiency with a phase gradient of 0.71k0. Upon comparison with the force and torque plots from current reflection control devices such as those that were utilized on IKAROS, the anomalously reflecting metasurfaces offer a considerable increase in torque along the roll axis. This is important because up until this point, roll control has been a particularly difficult aspect of solar sail attitude control to manipulate without the use of reaction wheels or propellant. The torque along the roll axis can reach values of torque as high as 358 μN m under ideal conditions.
Artificial Intelligent Aided Terahertz Technology and Applications
(University of Alabama Libraries, 2023) Gungordu, Muhammed Zeki; Kim, Seongsin M
Terahertz (THz) technologies are at the forefront of emerging technologies for future applications, including bio-chemical sensing and imaging, non-destructive testing, biomedicine, security inspection, materials science, future 6G/7G communications, and more. Although terahertz techniques can be utilized in a wide range of areas, some limitations, such as high water absorption, limited spatial resolution, high costs, and complex analysis, prevent their widespread development. We purposed to develop and hasten the intricate design of THz metamaterials and analysis of THz spectra and image data in this research. At that point, combining Artificial Intelligence (AI) and THz technology was used to improve the generality and robustness of models that analyze and improve the performance of THz spectra and image data. We first developed THz spectroscopic images for biomedical applications such as distinguishing cancerous and healthy cells by independent component analysis-aided THz imaging. However, a characteristic of THz radiation, such as its strong water absorption, makes it challenging to reconstruct clear images of tissues in vivo or reconstructed THz images. By utilizing the ICA decomposition, THz spectroscopic images with high sensitivity of frequency dependence were achieved by significantly improving contrast and differentiation of the tumor region of the phantom cancerous cell. In the following study, combining artificial intelligence (AI) techniques with THz-TDS, we have demonstrated a method to obtain the conductivity of nanowire-based conducting thin films in a significantly effective, steady, and rapid manner. The training of neural networks has been simplified by utilizing time-domain waveforms rather than frequency-domain spectrums as input data. According to our neural network models, the calculated and predicted conductivity matched successfully.At last, the design of the stereo-metamaterial-based THz polarizers study was focused on in a rapid and efficient way. We actualized using the power of AI for rapid and high-efficiency inverse design of THz stereo-metamaterial polarizers and accelerated the analysis of the desired device. A tandem neural network (TNN) with a weighted loss function was successfully displayed to inversely design the SMM-based THz polarizer device from a desired ellipticity angle spectrum instead of the structure's reflection and phase spectra, to obtain the corresponding structural parameters for the first time.
Atom probe tomography study of wide bandgap semiconductor materials
(University of Alabama Libraries, 2014) Dawahre Olivieri, Nabil Farah; Kung, Patrick; University of Alabama Tuscaloosa
This dissertation focuses on developing atom probe tomography (APT) for semiconductors. APT is quickly gaining interest in the field of material characterization because of its unique ability to provide 3D nanoscale studies. APT has been widely used in metals and conductive materials but design changes in the tool in recent years have made atom probe a suitable tool for semiconductor analysis. Because research in atom probe tomography of semiconductors is still in its infancy, it is still unclear whether this characterization method is suitable for semiconductor and how the added knowledge can be different than other accessible tools. This work will utilize APT as a characterization tool for wide bandgap semiconductors, specifically zinc oxide (ZnO) nanowires and GaN-based epitaxial sample. Wide bandgap semiconductor materials have attracted considerable attention in recent years because of the practical applications such as green and blue light emitting and laser diodes, solid-state lighting, photovoltaics, RF and microwave electronics, and gas sensors. Although silicon has remained the industry standard for many of these applications, its limitations have made way into the research of wide bandgap semiconductor materials, such as zinc oxide (ZnO) and gallium nitride (GaN). Because of their large direct bandgap, these materials show excellent promise in the field of optoelectronics, high frequency, high temperature and high power applications. First, we understand the behavior of the material to achieve field evaporation under APT conditions and the mechanisms behind, as well as ways to overcome the different artifacts introduced during sample preparation and data collection. Following this understanding, we can begin to apply APT to device structures to understand the effects of radiation on materials at the atomic scale, as well as the cluster formation of some of the elements along the material. At the conclusion of this dissertation, APT will deliver the results necessary to maximizing device efficiency as well as build the pathway for future APT analysis.
Breast cancer biomarker detection through the photoluminescence of epitaxial monolayer MoS2 flakes
(Nature Portfolio, 2020) Catalan-Gomez, Sergio; Briones, Maria; Cortijo-Campos, Sandra; Garcia-Mendiola, Tania; de Andres, Alicia; Garg, Sourav; Kung, Patrick; Lorenzo, Encarnacion; Luis Pau, Jose; Redondo-Cubero, Andres; Autonomous University of Madrid; Consejo Superior de Investigaciones Cientificas (CSIC); CSIC - Instituto de Ciencia de Materiales de Madrid (ICMM); University of Alabama Tuscaloosa
In this work we report on the characterization and biological functionalization of 2D MoS2 flakes, epitaxially grown on sapphire, to develop an optical biosensor for the breast cancer biomarker miRNA21. The MoS2 flakes were modified with a thiolated DNA probe complementary to the target biomarker. Based on the photoluminescence of MoS2, the hybridization events were analyzed for the target (miRNA21c) and the control non-complementary sequence (miRNA21nc). A specific redshift was observed for the hybridization with miRNA21c, but not for the control, demonstrating the biomarker recognition via PL. The homogeneity of these MoS2 platforms was verified with microscopic maps. The detailed spectroscopic analysis of the spectra reveals changes in the trion to excitation ratio, being the redshift after the hybridization ascribed to both peaks. The results demonstrate the benefits of optical biosensors based on MoS2 monolayer for future commercial devices.
Building practical apertureless scanning near-field microscopy
(University of Alabama Libraries, 2017) Gungordu, Muhammed Zeki; Kim, Seongsin; University of Alabama Tuscaloosa
The fundamental objective of this study is to establish a functional, practical apertureless type scanning near-field optical microscope, and to figure out the working mechanism behind it. Whereas a far-field microscope can measure the propagating field’s components, this gives us little information about the features of the sample. The resolution is limited to about half of the wavelength of the illuminating light. On the other hand, the a-SNOM system enables achieving non-propagating components of the field, which provides more details about the sample’s features. It is really difficult to measure because the amplitude of this field decays exponentially when the tip is moved away from the sample. The sharpness of the tip is the only limitation for resolution of the a-SNOM system. Consequently, the sharp tips are achieved by using electrochemical etching, and these tips are used to detect near-field signal. Separating the weak a-SNOM system signals from the undesired background signal, the higher demodulation background suppression is utilized by lock-in detection.
Characteristics of thz carrier dynamics in gan thin film and zno nanowires by temperature dependent terahertz time domain spectroscopy measurement
(University of Alabama Libraries, 2012) Balci, Soner; Kim, Seongsin; University of Alabama Tuscaloosa
We present a comprehensive study of the characteristics of carrier dynamics using temperature dependent Terahertz Time Domain Spectroscopy. By utilizing this technique in combination with numerical calculations, the complex refractive index, dielectric function, and conductivity of n-GaN, undoped ZnO NWs, and Al-doped ZnO NWs were obtained. The unique temperature dependent behaviors of major material parameters were studied at THz frequencies, including plasma frequency, relaxation time, carrier concentration and mobility. Frequency and temperature dependent carrier dynamics were subsequently analyzed in these materials through the use of the Drude and the Drude-Smith models.
Chemical Vapor Deposition of Bismuth Ferrite-Based Multiferroics for Device Applications
(University of Alabama Libraries, 2022) Acharya, Mahendra; Gupta, Arunava; University of Alabama Tuscaloosa
Epitaxial bismuth ferrite and substituted bismuth ferrite films have attracted significant attention due to their potential application in energy-efficient memory and logic devices. However, bismuth ferrite suffers from processing and application issues that can be addressed by improving and optimizing synthetic techniques. Chemical vapor deposition (CVD) is one of the suitable techniques for the high-volume manufacturing of bismuth ferrite. CVD can quickly produce conformal coating over a large area and still produce a superior-quality epitaxial layer. However, development in the CVD of bismuth ferrite has been slow and fragmentary. Very limited works have grown good-quality epitaxial bismuth ferrite films with robust ferroelectric properties using CVD. In this dissertation, a systematic study has been carried out to understand the problems underlying the synthesis of bismuth ferrite by CVD. Moreover, the crucial role of stoichiometry and lattice misfit strain in controlling ferroelectric switching has been elucidated, and the effect of vapor flow dynamics in controlling ferroelectric domain orientation and, subsequently, ferroelectric switching properties has been explained.The importance of the miniaturization of devices and the need for high-density, high-speed, and energy-efficient material systems for memory and logic applications has been mounting. Hence, understanding the application of the finite-size effect in CVD-grown bismuth ferrite is critical. The finite-size effect in CVD-grown bismuth ferrite has been validated by confirming the application of the Kay-Dunn law in the thickness scaling of switching voltage in the material. The insufficiency of thickness scaling to lower the switching voltage of the material to the desired range prompted us to explore the efficacy of samarium substitution in lowering the switching voltage of bismuth ferrite. A 50% reduction in switching voltage has been achieved by samarium substitution at the bismuth site.
Design, simulation, fabrication, and characteristics of terahertz metamaterial devices
(University of Alabama Libraries, 2012) Butler, Lee; Kim, Seongsin; University of Alabama Tuscaloosa
In recent years metamaterials have been extensively researched and show strong potential to improve many devices. Metamaterials offer significant advantages over conventional materials because their properties depend mainly on geometrical design rather than composition. This important property allows metamaterials to be geometrically scaled such as to operate in any desired spectral range. Specifically, implementation of metamaterials into devices targeted to operate in the terahertz regime could greatly improve fields such as chemistry, biology, security, and medicine. In this work, terahertz metamaterials have been studied and novel devices have been designed and demonstrated. In particular, this work is focused mainly on metamaterial structures designed to absorb incident radiation. These absorber devices show promise for use in areas such as imaging and interference reduction applications. Both narrow-band and broadband absorber devices have been designed using simulations and fabricated using standard photolithography and electron deposition techniques.
Development of a Photonic Link for Enhancements in Quantum Applications
(University of Alabama Libraries, 2024) Moseley, Carson; Kung, Patrick
Quantum computing leverages the laws of quantum mechanics to process information and solve complex problems at levels that classical computers can't attain. However, its hardware implementation remains challenging due to environmental constraints. To address these concerns, this work explores the use of a photonic link to alleviate the thermal load associated with traditional cryogenic qubit control methods. The development and testing of two separate photonic links is covered, as well as the design and validation of a system to address nitrogen-vacancy centers in diamond
Development of fabrication and characterization techniques for inorganic, organic and hybrid semiconductor devices
(University of Alabama Libraries, 2015) Rivera, Elmer G.; Kung, Patrick; University of Alabama Tuscaloosa
Different semiconducting types are applied in various fields of the semiconductor industry: organic, inorganic and hybrid. Each of these semiconducting types of materials have their own strengths as well as their weakness. Inorganic materials possess low absorption and high carrier mobility while organic materials possess high absorption and low carrier mobility. Inorganic/organic hybrid semiconducting devices take advantage of the mixing of these two types of semiconductors. By building a heterojunction with inorganic and organic materials, the advantages of each individual material is passes onto this new hybrid while cancelling out the disadvantages. In this master thesis, the fabrication procedure and characterization techniques are studied for inorganic, organic and hybrid semiconducting devices. For the inorganic materials, fabrication was performed in the MicroFabrication Facility in order to properly achieve small features in the micrometer range. Device processing was performed to achieve a high-electron mobility transistor using AlGaN/GaN and AlInN/GaN heterostructures. The fabrication procedure involved the defining of features through photolithography, ion mill etching, and electron-beam evaporation. Electrical characterization was performed on both heterostructures to make a comparison. The organic device studied was a photoconductor using the conducting polymer P3HT and an optical and electro-optic comparison was made with the addition of MWCNT into the polymer matrix. A hybrid pn-junction diode was fabricated using P3HT and electrical measurements were performed and analyzed through an equivalent circuit to characterize and compare it to a P3HT:MWCNT active layer for the pn-junction.
Electronic transport in two-dimensional transition metal dichalcogenide MoS₂ under photoexcitation
(University of Alabama Libraries, 2019) Nguyen, Phuong Xuan; Kim, Seongsin; University of Alabama Tuscaloosa
We investigate the formation and transport behaviors of photo-excited carriers in two dimensional transition metal dichalcogenide MoS2 under intense polarized continuous-wave excitation. By opto-electrical measurement and contactless terahertz time-domain spectroscopy, involving a dc electric field and a weak broadband terahertz as probe fields, respectively, we characterize the material's electrical and photo-responses. Circularly polarized light plays an important role in studying the valley-specific behaviors by breaking time reversal symmetry and creating a net valley polarization. In the THz TDS approach, we obtained important parameters like carrier concentrations, and carrier scattering times through fitting with a modified Drude model. Especially, the formation of negatively charged trion, a quasiparticle consisting of two electrons and one hole in MoS2, and the steady-state electron-trion dynamics made significant contributions to the material photoconductivities in the THz regime. On the other hand, under a dc electric probe field, where the trion and exciton formations are suppressed, we look at the valley Hall effect. This anomalous Hall voltage is a result of the intrinsic strong spin-orbit coupling and the lack of inversion symmetry in 2D MoS2, in addition to time reversal breaking. The valley Hall effect and its corresponding photo-conductivities can be quantitatively explained by the quantum kinetic Liouville equation, using the massive Dirac Hamiltonian model of MoS2. The analytical expressions for the longitudinal and Hall conductivity are obtained by the density matrix approach underrotating wave approximation. A good understanding of carrier formation and transport parameters are instrumental for the development of TMD-based optoelectronics, spin- and valley-tronics in the future.
Fundamental and applied studies of organic photovoltaic systems
(University of Alabama Libraries, 2014) Hill, Caleb M.; Pan, Shanlin; University of Alabama Tuscaloosa
Presented here are applied and fundamental studies of model organic photovoltaic (OPV) systems. Graphene oxide (GO) nanosheets were investigated as a potential electron acceptor in bulk heterojunction organic solar cells which employed poly[3-hexylthiophene] (P3HT) as an electron donor. GO nanosheets were transferred into organic solution through a surfactant-assisted phase transfer method. Electron transfer from P3HT to GO in solutions and thin films was established through fluorescence spectroscopy. Bulk heterojunction solar cells containing P3HT, P3HT-GO, and P3HT-phenyl-C₆₁-butyric acid methyl ester (PCBM, a prototypical elector acceptor employed in polymer solar cells) were constructed and evaluated. Single molecule fluorescence spectroscopy was employed to study charge transfer between conjugated polymers and TiO₂ at the single molecule level. The fluorescence of individual chains of the conjugated polymer poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV) at TiO₂ surfaces was shown to exhibit increased intermittent (on/off "blinking") behavior compared to molecules on glass substrates. Single molecule fluorescence excitation anisotropy measurements showed the conformation of the polymer molecules did not differ appreciably between glass and TiO₂ substrates. The similarities in molecular conformation suggest that the observed differences in blinking activity are due to charge transfer between MEH-PPV and TiO₂, which provides additional pathways between states of high and low fluorescence quantum efficiency. The electrodeposition of individual Ag nanoparticles (NPs), which can be used to enhance light harvesting in organic photovoltaic systems, was studied in situ via dark field scattering (DFS) microscopy. The scattering at the surface of an indium tin oxide (ITO) working electrode was measured during a potential sweep. Utilizing Mie scattering theory and high resolution scanning electron microscopy (SEM), the scattering data were used to calculate current-potential curves depicting the electrodeposition of individual Ag NPs. The oxidation of individual presynthesized and electrodeposited Ag NPs was also investigated using fluorescence and DFS microscopies.
Fundamental understanding of the growth, doping and characterization of aligned zno nanowires
(University of Alabama Libraries, 2014) Shen, Gang; Kung, Patrick; University of Alabama Tuscaloosa
Zinc oxide (ZnO) is a II-VI semiconductor whose wide direct bandgap (3.37 eV) and large exciton binding energy (60 meV) make it compelling for optoelectronic devices such as light emitting diodes, lasers, photodetectors, solar cells, and mechanical energy harvesting devices. One dimensional structures of ZnO (nanowires) have become significant due to their unique physical properties arising from quantum confinement, and they are ideal for studying transport mechanisms in one-dimensional systems. In this doctoral research work, ZnO nanowire (NW) arrays were synthesized on sapphire substrates through carbo-thermal reduction of ZnO powders, and the effects of growth parameters on the properties of ZnO NW arrays were studied by scanning and transmission electron microscopy, X-ray diffraction, photoluminescence and Raman spectroscopy. Based on the phonon mode selection rules in wurtzite ZnO, confocal Raman spectroscopy was used to assess the alignment of ZnO NWs in an array, thereby complementing X-ray diffraction. Al doped ZnO NW arrays were achieved by mixing Al powder into the ZnO and graphite source mixture, and the presence of Al was confirmed by Energy-dispersive X-ray spectroscopy. The incorporation of Al had the effects of lowering the electrical resistivity, slightly deteriorating crystal quality and suppressing defect related green emission. Two models of ZnO NW growth were developed by establishing the relationship between NW length and diameter for undoped and Al doped ZnO NWs separately. The growth of undoped ZnO NWs followed the diffusion-induced model which was characterized by thin wires being longer than thick wires, while the growth of Al doped ZnO was controlled by Gibbs-Thomson effect which was characterized by thin wires being shorter than thin wires. Local electrode atom probe analysis of ZnO NWs was carried out to study the crystal stoichiometry and Al incorporation. Undoped ZnO NWs were found to be high purity with no detectable impurities. Possible Al incorporation related peaks were observed in the mass spectrum of Al doped ZnO NWs. Further work on Al doped ZnO LEAP analysis is needed to better understand the Al dopant.
Hyperbaric growth of carbon fibers by laser chemical vapor deposition
(University of Alabama Libraries, 2019) Rife, Justin Lee; Thompson, Gregory B.; University of Alabama Tuscaloosa
Laser Chemical Vapor Deposition (LCVD) is a promising new processing technique by which freestanding structures, such as fibers, can be deposited. The deposition of carbon fibers by use of ethylene as a precursor gas can be easily achieved and has been investigated as a way to complement or even replace current carbon fiber production techniques. The properties of carbon fibers deposited from ethylene via LCVD have been investigated for low precursor pressures thus far. However, deposition rates for low precursor pressures are limited and rates that are orders of magnitude faster can be achieved by use of higher precursor pressures. No detailed studies on properties of fibers processed at these higher pressures have been conducted. This thesis fills this knowledge gap by exploring the relationships between processing conditions, growth behavior, microstructure and mechanical properties of carbon fibers deposited from ethylene at hyperbaric pressures. It is found that the fiber growth rates are limited by surface reaction kinetics at low temperatures, while they are mass transport limited or gas phase nucleation limited at high temperatures. When grown under mass transport limited conditions, fibers exhibit drastic changes in morphology and microstructure. The tensile strengths of the carbon fibers grown by LCVD are generally found to be poor due to the nature of graphitic carbon deposits. However, the Weibull modulus among the LCVD grown carbon fibers is found to be high. Trends in mechanical properties with processing conditions and microstructure are observed.
Impact of Substrate and Bright Resonances on Group Velocity in Metamaterial without Dark Resonator
(Nature Portfolio, 2015) Hokmabadi, Mohammad Parvinnezhad; Kim, Jy-Hyung; Rivera, Elmer; Kung, Patrick; Kim, Seongsin M.; University of Alabama Tuscaloosa
Manipulating the speed of light has never been more exciting since electromagnetic induced transparency and its classical analogs led to slow light. Here, we report the manipulation of light group velocity in a terahertz metamaterial without needing a dark resonator, but utilizing instead two concentric split-ring bright resonators (meta-atoms) exhibiting a bright Fano resonance in close vicinity of a bright Lorentzian resonance to create a narrowband transmittance. Unlike earlier reports, the bright Fano resonance does not stem from an asymmetry of meta-atoms or an interaction between them. Additionally, we develop a method to determine the metamaterial "effective thickness", which quantifies the influence of the substrate on the metamaterial response and has remained challenging to estimate so far. By doing so, very good agreement between simulated and measured group delays and velocities is accomplished. The proposed structure and method will be useful in designing optical buffers, delay lines, and ultra-sensitive sensors.
Investigation of Microwave Transducer for Linearity Dependence and Applications in Quantum Networking
(University of Alabama Libraries, 2022) Bolton, Summer Alexis; Kim, Seongsin M.; Kung, Patrick; University of Alabama Tuscaloosa
Quantum devices have the potential to revolutionize applications in computing, communications, and sensing; however, current state-of-art resources must operate at extremely low temperatures. There is an increasing interest in optical fiber transduction to microwave link with superconducting and Si qubits. The quality depends strongly on the characteristics of the photodiode, specifically uni-traveling carrier photodiodes (UTC-PDs) for high-frequency operation, yet most do not have the high-speed and high-linearity performance or the ability to handle ultracold temperatures. To address these low-temperature and high-frequency problems, an RF photonic microwave transducer is used to measure the frequency response and investigate the linearity dependence on frequency, bias voltage, and temperature. An electro-optical microwave transducer is created using the heterodyning beat method. A high-speed MUTC photodiode designed for cryogenic temperatures is tested as the transducer and characterized with a spectrum analyzer. The linearity of the device is also tested at bias voltages of 0 V and −5 V, frequencies of 3 GHz and 10 GHz, and temperatures of 300 K and 77 K. With a low bias voltage, the frequency response shows a decrease in power due to the increase of harmonic noise. The results show that the linearity does depend on frequency, bias voltage, and temperature. A higher reverse bias voltage showed the highest 1-dB compression point, while a bias voltage of 0 V and a frequency of 10 GHz showed the lowest power and the lowest 1-dB compression point. Our results should help contribute to the future design of highly linear cryogenic quantum links.
Investigation of terahertz technology and applications
(University of Alabama Libraries, 2015) Kim, Juhyung; Kim, Seongsin; University of Alabama Tuscaloosa
Terahertz Technology promises great advancement in various field including military applications, biomedical industry and future communication as a convergence area between electronics fields and photonics fields. In this thesis, I present the development of Terahertz technology based on Terahertz Time Domain Spectroscopy and its methodologies that I had to learn to utilize the system and to improve the quality of data acquired. After the methodology is presented, there are three main topics investigated for the Terahertz technology applications. First, Flexible Metamaterial Perfect Absorber was fabricated and investigated as a new Terahertz responsive device. Second, human breast tissue cell lines and cancer cell lines are distinguished by analyzing spectroscopic data gotten from transmission mode of Terahertz Time-domain Spectroscopy. Third, after testing the Terahertz wave radiation from SiGe nanowires surface emitter, InP and InGaAs nanowires were tested as an alternative material. Especially, those were fabricated with a new trial of fabrication, the E-beam lithography, to obtain a regular vertical alignment and uniform distribution with desired diameter and length, instead of randomly grown nanowires. Polarization dependent measurement result will be demonstrated depending on the size and shape of these nanowires.
Nanostructure characterization, fabrication and devices of 1D & 2D ZNO and 2D MOS2
(University of Alabama Libraries, 2018) Waters, Joseph Lionel; Kung, Patrick; University of Alabama Tuscaloosa
One of the main complications in the synthesis of Zinc oxide (ZnO) nanowires (NWs), is the ability to reproduce well aligned wires. ZnO was studied due to its optoelectronic applications. Its simple crystal growth abilities, lead to potentially lower cost for ZnO based devices. Other semiconductors such as MoS2 in bulk form contain an indirect bandgap of 1.2 eV. As layers are removed, the materials band gap undergoes a shift and switches from indirect to direct bandgap for a single monolayer. This monolayer of MoS2 contains a bandgap of 1.8 eV, therefore the goal was to synthesis single layer MoS2 on various substrates. In this doctoral research wide bandgap ZnO and emerging MoS2 were studied individually. These novel semiconductors were then fabricated together to form heterostructures to enhance the functionality of ZnO and MoS2 by covering the UV (380 nm) to the visible region (650 nm). ZnO powders were reduced by carbo-thermal reduction and grown onto sapphire substrates to act as a ZnO NW scaffold. The material properties such as the crystalline phase of the hexagonal wurtzite ZnO were examined by SEM, TEM and complemented by optical characterizations. X-ray photoelectron spectroscopy determined the chemical species and lack of impurities present in the NW. Local electrode atom probe analysis of the crystal stoichiometry and concentration gradient of oxygen content from the center of the NW to the edge wall. A single ZnO NW was removed from the surface and a Schottky diode device was fabricated to determine the effects of UV illumination. Defects at the edge of the wire can lead to external growth of MoS2 and good bonding at the interface of the heterostructure. The triangular formation, thickness and edge effects at the grain boundaries were studied by SEM and AFM. The two main phonon modes in MoS2 are used to determine the number of layers present by Raman spectroscopy. TEM led to determination of the 2H phase of MoS2. When the 1D-2D hybrid structure is fabricated regions of molybdenum and sulfur on the ZnO NW were mapped by EDS on a SEM and TEM.
Nanostructure Characterization, Fabrication and Devices of 2D Mos2 and Mos2/WS2 Hetrostructures
(University of Alabama Libraries, 2021) Garg, Sourav; Kung, Patrick; University of Alabama Tuscaloosa
Sparked by the 2D graphene, advanced 2D transition metal dichalcogenides have captured enough attention due to their extraordinary properties and are promising enough for future high speed flexible electronic and optoelectronic devices. Among all the transition metal dichalcogenides, molybdenum disulphide (MoS2) and tungsten disulphide (WS2) are explored most extensively since the last few years because of their complementary nature to metallic graphene. These thin 2D materials are semiconducting in nature, and moreover, bandgap also changes from indirect to direct as these materials are thinned down from bulk to monolayer form. In this study, a stabilized and large area growth of MoS2 monolayers has been established on oxide and semiconducting substrates such as (0001) sapphire, (100) p-type SiO2/Si, GaN and Ga2O3 using low pressure chemical vapor deposition. The quality and crystalline nature of grown MoS2 is deeply investigated optically by micro-photoluminescence and micro-Raman spectroscopy. Topography and morphology are characterized by scanning electron and atomic force microscopy. The applications of as grown MoS2 monolayers have been studied by the fabrication of large area photodetector. Also, the gas sensing ability of MoS2 has been explored by using CO2 gas, and the minimum detection limit found is 200ppm. In-addition one step growth of ternary alloys Mo1-xWxS2 has been achieved by LPCVD. Different compositions of W in MoS2 have been investigated by micro-photoluminescence and micro-Raman spectroscopy. In-plane heterojunctions of atomic-thick (2D) semiconductors (MoS2/WS2) are novel structures that can potentially pave the way for efficient ultrathin and flexible optoelectronics, such as light sources and photovoltaics. Such heterostructures are very rare and not much is known about their characteristics. They can only be achieved through a synthetic growth process such as chemical vapor deposition (CVD). This is unlike vertical heterostructures, for which the materials can be mechanically stacked one layer on top of the other. Here, we report a one-step CVD growth of monolayer thick MoS2/WS2 in-plane heterostructures. We have characterized their morphological and optical properties using micro-Raman and photoluminescence spectroscopy. Kelvin probe force microscope was used to extract the contact potential difference profile across the MoS2/WS2 heterojunction boundary. The junction region of these heterostructures are observed to be a ternary alloy Mo1-xWxS2. Moreover, through the tip enhanced Raman spectroscopy (TERS), the minimum junction width is extracted out to be pixel limited 25nm. Also, some novel Raman modes are detected through TERS in MoS2, and WS2 monolayers, which were not elaborated before.
Pb_0.95 La_0.05 Zr_0.54 Ti_0.46 O_3 thin films for photovoltaic applications
(University of Alabama Libraries, 2012) Vasudevan Nampoori, Harshan; Kotru, Sushma; University of Alabama Tuscaloosa
Ferroelectrics have shown potential as a promising alternative material for future photovoltaic applications. Observance of high open circuit voltages in ferroelectric thin films, have generated considerable interest in the field of ferroelectric photovoltaic in recent years. The field of ferroelectric photovoltaic is evolving and not yet completely understood compared to the semiconductor based photovoltaic technology. This dissertation presents photovoltaic properties of ferroelectric Pb_0.95 La_0.05 Zr_0.54 Ti_0.46 O_3 thin films. The films were fabricated by solution based methods and spin coating technique. The post annealing process on these films was optimized to achieve the desired ferroelectric and dielectric properties. A measurement setup was established to study the PV characteristic of the devices. Dependence of current-voltage (I-V) behavior of the cells on parameters such as electrical poling, annealing temperature, nature of top electrodes, and intensity of illumination, was investigated. The photovoltaic response was shown to improve by using electrodes with low work functions. An electric circuit model was developed to simulate the behavior of a single ferroelectric photovoltaic cell and the dependence of open circuit voltage (Voc) and short circuit current (Isc) on light intensity.

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