Browsing by Author "Doty, Matthew F."
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Item Determining the band alignment of TbAs:GaAs and TbAs:In0.53Ga0.47As(American Institute of Physics, 2015-09-10) Bomberger, Cory C.; Vanderhoef, Laura R.; Rahman, Abdur; Shah, Deesha; Chase, D. Bruce; Taylor, Antoinette J.; Azad, Abul K.; Doty, Matthew F.; Zide, Joshua M. O.; Cory C. Bomberger, Laura R. Vanderhoef, Abdur Rahman, Deesha Shah, D. Bruce Chase, Antoinette J. Taylor, Abul K. Azad, Matthew F. Doty, and Joshua M. O. Zide; Bomberger, Cory C.; Vanderhoef, Laura R.; Chase, D. Bruce; Doty, Matthew F.; Zide, Joshua M. O.We propose and systematically justify a band structure for TbAs nanoparticles in GaAs and In0.53Ga0.47As host matrices. Fluence-dependent optical-pump terahertz-probe measurements suggest the TbAs nanoparticles have a band gap and provide information on the carrier dynamics, which are determined by the band alignment. Spectrophotometry measurements provide the energy of optical transitions in the nanocomposite systems and reveal a large blue shift in the absorption energy when the host matrix is changed from In0.53Ga0.47As to GaAs. Finally, Hall data provides the approximate Fermi level in each system. From this data, we deduce that the TbAs:GaAs system forms a type I (straddling) heterojunction and the TbAs:In0.53Ga0.47As system forms a type II (staggered) heterojunction.Item Diamagnetic and paramagnetic shifts in self-assembled InAs lateral quantum dot molecules(American Physical Society, 2015-05-19) Zhou, Xinran; Royo, Miquel; Liu, Weiwen; Lee, Jihoon H.; Salamo, Gregory J.; Climente, Juan I.; Doty, Matthew F.; Xinran Zhou, Miquel Royo, Weiwen Liu, Jihoon H. Lee, Gregory J. Salamo, Juan I. Climente, and Matthew F. Doty; Doty, Matthew F.We uncover the underlying physics that explains the energy shifts of discrete states of individual InAs lateral quantum dot molecules (LQDMs) as a function of magnetic fields applied in the Faraday geometry. We observe that ground states of the LQDM exhibit a diamagnetic shift while excited states exhibit a paramagnetic shift. We explain the physical origin of the transition between these two behaviors by analyzing the molecular exciton states with effective mass calculations. We find that charge carriers in delocalized molecular states can become localized in single QDs with increasing magnetic field. We further show that the net effects of broken symmetry of the molecule and Coulomb correlation lead to the paramagnetic response.Item Effects of composition and thermal treatment on VOC-limiting defects in single-crystalline Cu2ZnSnSe4 solar cells(Progress in Photovoltaics, 2021-11-19) Lloyd, Michael A.; Ma, Xiangyu; Kuba, Austin G.; McCandless, Brian E.; Doty, Matthew F.; Birkmire, RobertSingle-crystalline Cu2ZnSnSe4 (CZTSe) solar cells with open circuit voltages reaching 500 mV are achieved through a combination of composition control and a low-temperature thermal ordering treatment. A comparison of the device results for Cu-poor CZTSe with Cu/Zn + Sn ratios of 0.77 and 0.86 is presented with and without the implementation of a 130°C absorber annealing treatment. An increase in bandgap energy is observed via external quantum efficiency measurements with both the decrease in Cu content and the implementation of the order anneal, the latter of which also leads to a decrease in Urbach energy. Defect characterization performed with admittance spectroscopy on devices is demonstrated as insufficient because of low-temperature current barriers. Photoluminescence (PL) on crystal surfaces however enables a qualitative comparison of the defect landscape between crystal compositions and annealing treatments. Both a highly compensated and a lightly doped defect model are used to fit PL as a function of laser fluence to identify defects contributing to each observed recombination channel. The PL signatures attributed to the ZnSn defect become unresolvable with a decrease in Cu/Zn + Sn ratio from 0.86 to 0.77. Furthermore, a decrease in Cu–Zn disorder is observed upon the implementation of the annealing treatment through both a comparison of potential fluctuation depths and of both PL models.Item An image analysis method for quantifying precision and disorder in nanofabricated photonic structures(Nanotechnology, 2022-11-28) Carfagno, Henry; Garcia, Pedro David; Doty, Matthew F.Disorder is an essential parameter in photonic systems and devices, influencing phenomena such as the robustness of topological photonic states and the Anderson localization of modes in waveguides. We develop and demonstrate a method for both analyzing and visualizing positional, size, and shape disorder in periodic structures such as photonic crystals. This analysis method shows selectivity for disorder type and sensitivity to disorder down to less than 1%. We show that the method can be applied to more complex shapes such as those used in topological photonics. The method provides a powerful tool for process development and quality control, including analyzing the precision of E-beam lithography before patterns are transferred; quantifying the precision limits of lithography, deposition, or etch processes; and studying the intentional displacement of individual objects within otherwise periodic arrays.Item Impact of Different Surface Ligands on the Optical Properties of PbS Quantum Dot Solids(MDPI (Multidisciplinary Digital Publishing Institute), 2015-04-21) Xu, Fan; Gerlein, Luis Felipe; Ma, Xin; Haughn, Chelsea R.; Doty, Matthew F.; Cloutier, Sylvain G.; Fan Xu, Luis Felipe Gerlein, Xin Ma, Chelsea R. Haughn, Matthew F. Doty and Sylvain G. Cloutier; Xu, Fan; Ma, Xin; Doty, Matthew F.; Cloutier, Sylvain G.The engineering of quantum dot solids with low defect concentrations and efficient carrier transport through a ligand strategy is crucial to achieve efficient quantum dot (QD) optoelectronic devices. Here, we study the consequences of various surface ligand treatments on the light emission properties of PbS quantum dot films using 1,3-benzenedithiol (1,3-BDT), 1,2-ethanedithiol (EDT), mercaptocarboxylic acids (MPA) and ammonium sulfide ((NH4)2S). We first investigate the influence of different ligand treatments on the inter-dot separation, which mainly determines the conductivity of the QD films. Then, through a combination of photoluminescence and transient photoluminescence characterization, we demonstrate that the radiative and non-radiative recombination mechanisms in the quantum dot films depend critically on the length and chemical structure of the surface ligands.Item Impact of Different Surface Ligands on the Optical Properties of PbS Quantum Dot Solids(MDPI AG, 2015-04-21) Xu, Fan; Gerlein, Luis Felipe; Ma, Xin; Haughn, Chelsea R.; Doty, Matthew F.; Cloutier, Sylvain G.; Fan Xu, Luis Felipe Gerlein, Xin Ma, Chelsea R. Haughn, Matthew F. Doty and Sylvain G. Cloutier; Xu, Fan; Ma, Xin; Haughn, Chelsea R.; Doty, Matthew F.; Cloutier, Sylvain G.The engineering of quantum dot solids with low defect concentrations and efficient carrier transport through a ligand strategy is crucial to achieve efficient quantum dot (QD) optoelectronic devices. Here, we study the consequences of various surface ligand treatments on the light emission properties of PbS quantum dot films using 1,3-benzenedithiol (1,3-BDT), 1,2-ethanedithiol (EDT), mercaptocarboxylic acids (MPA) and ammonium sulfide ((NH4)2S). We first investigate the influence of different ligand treatments on the inter-dot separation, which mainly determines the conductivity of the QD films. Then, through a combination of photoluminescence and transient photoluminescence characterization, we demonstrate that the radiative and non-radiative recombination mechanisms in the quantum dot films depend critically on the length and chemical structure of the surface ligands.Item Inverse Designed Couplers for Use in Gallium Arsenide Photonics(ACS Photonics, 2023-05-17) Carfagno, Henry; Guidry, Melissa A.; Yang, Joshua; McCabe, Lauren; Zide, Joshua M. O.; Vučković, Jelena; Doty, Matthew F.Highly efficient photonic couplers are a necessary component of a scalable platform to couple quantum emitters into quantum fiber networks. We inverse-designed couplers for use in gallium arsenide membrane-based photonics that are compatible with indium arsenide quantum dots, one of the highest quality quantum light sources available. We fabricated and tested at least 4 instances of devices following 11 different designs. All inverse-designed structures outperformed the traditional grating outcoupler in a single-mode optical fiber optical setup. Using a novel sleeve and bulk fabrication method allowed for a smaller allowable minimum feature size constraint in the inverse design optimization protocol. Employing this new design constraint improved the average device transmission efficiency from 17.4% to 27.5%. The use of broadband optimization criteria did not result in statically significant improvement in actual bandwidth, but did decrease the variance in the measured bandwidth, suggesting a more robust design.Item Role of Semiconductor Nanostructures in Photon Upconversion Applications(ACS Applied Optical Materials, 2023-04-28) Cleveland, Jill M.; Welsch, Tory A.; Chase, D. Bruce; Doty, Matthew F.Photon upconversion, a process in which multiple low-energy photons are absorbed and re-emitted as higher-energy photons, has recently received a significant amount of attention due to its potential utility across a wide range of optical applications. Traditionally, two types of materials have been used for photon upconversion applications: lanthanide-doped nanocrystals and triplet–triplet annihilation molecules. While these systems have demonstrated good upconversion efficiencies, they both suffer from some limitations, particularly in spectral utilization. In this review, we will highlight the ways semiconductor nanocrystals have been integrated into existing upconverison platforms to address their limitations and improve their usability for some specific upconversion applications. Additionally, we will discuss the recent development of upconversion platforms based entirely on semiconductor nanostructures. These systems rely on the size-, shape-, and composition-dependent optical properties of semiconductors to design upconverting materials with the necessary electronic structure for a specific application. We discuss the current status of these hybrid and pure semiconductor-based upconverters and suggest future directions for further improving their upconversion performance.Item Separating the Effects of Nonorthogonal Variables on the Hot-Injection Synthesis of Core/Thick-Shell (“Giant”) CdTe/CdS Quantum Dots(Chemistry of Materials, 2022-11-22) Welsch, Tory A.; Cleveland, Jill M.; Chase, D. Bruce; Doty, Matthew F.“Giant” core/thick-shell quantum dot (QD) nanostructures are of interest due to their unusual optical properties and importance as components of more advanced heterostructures tailored to achieve increasingly complex optical functions. However, reliable one-step seeded growth of these structures poses a significant challenge: one must balance multiple competing reaction processes to find the growth regime that realizes spherical shells of both the target size and high crystalline quality. Adjusting synthesis conditions in thicker-shelled reactions is further complicated by multiple nonorthogonal variables that impact the reaction mechanism. These variables include the reaction volume, reaction concentration, and oleic acid (ligand) concentration. Here, we investigate the seeded growth of core/thick-shell CdTe/CdS QDs by adapting a “flash” shelling method. We systematically vary three key reaction parameters (particle concentration, oleic acid:Cd ratio, and Cd–S:CdTe core ratio) over 30+ different thick-shelling reactions to elucidate the separate and intersecting impacts of these parameters on shell growth. Our analysis of the resulting particle quality reveals that the particle concentration of the reaction plays a critical role in the shell growth mechanism. We find the impact of oleic acid to be dependent on the particle concentration for a given shell thickness. We also find that the optimal conditions shift when targeting increasingly thick shells. The results demonstrate the importance of testing and controlling for synthesis variables across a multidimensional parameter space. We develop and present general experimental design criteria to help guide efficient development of new seeded growth reactions that enable reliable synthesis of thick-shelled nanostructures.Item A sleeve and bulk method for fabrication of photonic structures with features on multiple length scales(Nanotechnology, 2022-09-21) Carfagno, Henry; McCabe, Lauren; Zide, Joshua; Doty, Matthew F.Traditional photonic structures such as photonic crystals utilize a) large arrays of small features with the same size and pitch and b) a small number of larger features such as diffraction outcouplers. In conventional nanofabrication, separate lithography and etch steps are used for small and large features in order to employ process parameters that lead to optimal pattern transfer and side-wall profiles for each feature-size category, thereby overcoming challenges associated with RIE lag. This approach cannot be scaled to more complex photonic structures such as those emerging from inverse design protocols. Those structures include features with a large range of sizes such that no distinction between small and large can be made. We develop a sleeve and bulk etch protocol that can be employed to simultaneously pattern features over a wide range of sizes while preserving the desired pattern transfer fidelity and sidewall profiles. This approach reduces the time required to develop a robust process flow, simplifies the fabrication of devices with wider ranges of feature sizes, and enables the fabrication of devices with increasingly complex structure.Item Spin currents with unusual spin orientations in noncollinear Weyl antiferromagnetic Mn3Sn(Physical Review Materials, 2023-03-10) Wang, Xinhao; Hossain, Mohammad Tomal; Thapaliya, T. R.; Khadka, Durga; Lendinez, Sergi; Chen, Hang; Doty, Matthew F.; Jungfleisch, M. Benjamin; Huang, S. X.; Fan, Xin; Xiao, John Q.There are intensive efforts to search for mechanisms that lead to spin-orbit torque with unusual spin orientation, particularly out-of-plane spin orientation which can efficiently switch perpendicular magnetizations. Such a phenomenon has been observed in materials with low structural symmetry, ferromagnetic materials, and antiferromagnets with noncollinear spin structures. Here, we demonstrate the observation of, in addition to out-of-plane spin orientation, spin orientation along the charge current direction in Mn3Sn, a noncollinear antiferromagnet and Weyl semimetal. The mechanism arises from noncollinear spin structure with spin-orbit coupling and it can be viewed as spin rotation around the octupole moment, the lowest order of cluster multipole moment pertaining to the Mn3Sn crystal group.