Matching Items (52)
Description
In this dissertation, in-situ X-ray and ultraviolet photoemission spectroscopy have been employed to study the interface chemistry and electronic structure of potential high-k gate stack materials. In these gate stack materials, HfO2 and La2O3 are selected as high-k dielectrics, VO2 and ZnO serve as potential channel layer materials. The gate stack structures have been prepared using a reactive electron beam system and a plasma enhanced atomic layer deposition system. Three interrelated issues represent the central themes of the research: 1) the interface band alignment, 2) candidate high-k materials, and 3) band bending, internal electric fields, and charge transfer. 1) The most highlighted issue is the band alignment of specific high-k structures. Band alignment relationships were deduced by analysis of XPS and UPS spectra for three different structures: a) HfO2/VO2/SiO2/Si, b) HfO2-La2O3/ZnO/SiO2/Si, and c) HfO2/VO2/ HfO2/SiO2/Si. The valence band offset of HfO2/VO2, ZnO/SiO2 and HfO2/SiO2 are determined to be 3.4 ± 0.1, 1.5 ± 0.1, and 0.7 ± 0.1 eV. The valence band offset between HfO2-La2O3 and ZnO was almost negligible. Two band alignment models, the electron affinity model and the charge neutrality level model, are discussed. The results show the charge neutrality model is preferred to describe these structures. 2) High-k candidate materials were studied through comparison of pure Hf oxide, pure La oxide, and alloyed Hf-La oxide films. An issue with the application of pure HfO2 is crystallization which may increase the leakage current in gate stack structures. An issue with the application of pure La2O3 is the presence of carbon contamination in the film. Our study shows that the alloyed Hf-La oxide films exhibit an amorphous structure along with reduced carbon contamination. 3) Band bending and internal electric fields in the gate stack structure were observed by XPS and UPS and indicate the charge transfer during the growth and process. The oxygen plasma may induce excess oxygen species with negative charges, which could be removed by He plasma treatment. The final HfO2 capping layer deposition may reduce the internal potential inside the structures. The band structure was approaching to a flat band condition.
ContributorsZhu, Chiyu (Author) / Nemanich, Robert (Thesis advisor) / Chamberlin, Ralph (Committee member) / Chen, Tingyong (Committee member) / Ponce, Fernando (Committee member) / Smith, David (Committee member) / Arizona State University (Publisher)
Created2012
Description
Molecular dynamics simulations were used to study properties of water at the interface with nanometer-size solutes. We simulated nonpolar attractive Kihara cavities given by a Lennard-Jones potential shifted by a core radius. The dipolar response of the hydration layer to a uniform electric field substantially exceeds that of the bulk. For strongly attractive solutes, the collective dynamics of the hydration layer become slow compared to bulk water, as the solute size is increased. The statistics of electric field fluctuations at the solute center are Gaussian and tend toward the dielectric continuum limit with increasing solute size. A dipolar probe placed at the center of the solute is sensitive neither to the polarity excess nor to the slowed dynamics of the hydration layer. A point dipole was introduced close to the solute-water interface to further study the statistics of electric field fluctuations generated by the water. For small dipole magnitudes, the free energy surface is single-welled, with approximately Gaussian statistics. When the dipole is increased, the free energy surface becomes double-welled, before landing in an excited state, characterized again by a single-welled surface. The intermediate region is fairly broad and is characterized by electrostatic fluctuations significantly in excess of the prediction of linear response. We simulated a solute having the geometry of C180 fullerene, with dipoles introduced on each carbon. For small dipole moments, the solvent response follows the results seen for a single dipole; but for larger dipole magnitudes, the fluctuations of the solute-solvent energy pass through a second maximum. The juxtaposition of the two transitions leads to an approximately cubic scaling of the chemical potential with the dipole strengh. Umbrella sampling techniques were used to generate free energy surfaces of the electric potential fluctuations at the heme iron in Cytochrome B562. The results were unfortunately inconclusive, as the ionic background was not effectively represented in the finite-size system.
ContributorsFriesen, Allan Dwayne (Author) / Matyushov, Dmitry V (Thesis advisor) / Angell, C Austen (Thesis advisor) / Beckstein, Oliver (Committee member) / Mujica, Vladimiro (Committee member) / Arizona State University (Publisher)
Created2012
Description
In mesoscopic physics, conductance fluctuations are a quantum interference phenomenon that comes from the phase interference of electron wave functions scattered by the impurity disorder. During the past few decades, conductance fluctuations have been studied in various materials including metals, semiconductors and graphene. Since the patterns of conductance fluctuations is related to the distributions and configurations of the impurity scatterers, each sample has its unique pattern of fluctuations, which is considered as a sample fingerprint. Thus, research on conductance fluctuations attracts attention worldwide for its importance in both fundamental physics and potential technical applications. Since early experimental measurements of conductance fluctuations showed that the amplitudes of the fluctuations are on order of a universal value (e2/h), theorists proposed the hypothesis of ergodicity, e.g. the amplitudes of the conductance fluctuations by varying impurity configurations is the same as that from varying the Fermi energy or varying the magnetic field. They also proposed the principle of universality; e.g., that the observed fluctuations would appear the same in all materials. Recently, transport experiments in graphene reveal a deviation of fluctuation amplitudes from those expected from ergodicity.
Thus, in my thesis work, I have carried out numerical research on the conductance fluctuations in GaAs nanowires and graphene nanoribbons in order to examine whether or not the theoretical principles of universality and ergodicity hold. Finite difference methods are employed to study the conductance fluctuations in GaAs nanowires, but an atomic basis tight-binding model is used in calculations of graphene nanoribbons. Both short-range disorder and long-range disorder are considered in the simulations of graphene. A stabilized recursive scattering matrix technique is used to calculate the conductance. In particular, the dependence of the observed fluctuations on the amplitude of the disorder has been investigated. Finally, the root-mean-square values of the amplitude of conductance fluctuations are calculated as a basis with which to draw the appropriate conclusions. The results for Fermi energy sweeps and magnetic field sweeps are compared and effects of magnetic fields on the conductance fluctuations of Fermi energy sweeps are discussed for both GaAs nanowires and graphene nanoribbons.
Thus, in my thesis work, I have carried out numerical research on the conductance fluctuations in GaAs nanowires and graphene nanoribbons in order to examine whether or not the theoretical principles of universality and ergodicity hold. Finite difference methods are employed to study the conductance fluctuations in GaAs nanowires, but an atomic basis tight-binding model is used in calculations of graphene nanoribbons. Both short-range disorder and long-range disorder are considered in the simulations of graphene. A stabilized recursive scattering matrix technique is used to calculate the conductance. In particular, the dependence of the observed fluctuations on the amplitude of the disorder has been investigated. Finally, the root-mean-square values of the amplitude of conductance fluctuations are calculated as a basis with which to draw the appropriate conclusions. The results for Fermi energy sweeps and magnetic field sweeps are compared and effects of magnetic fields on the conductance fluctuations of Fermi energy sweeps are discussed for both GaAs nanowires and graphene nanoribbons.
ContributorsLiu, Bobo (Author) / Ferry, David K. (Thesis advisor) / Akis, Richard (Committee member) / Saraniti, Marco (Committee member) / Goryll, Michael (Committee member) / Arizona State University (Publisher)
Created2015
Description
A theoretical study of a three-dimensional (3D) N/S interface with arbitrary spin
polarization and interface geometry is presented. The 3D model gives the same intrinsic
spin polarization and superconducting gap dependence as the 1D model. This
demonstrates that the 1D model can be use to t 3D data.
Using this model, a Heusler alloy is investigated. Andreev reflection measurements
show that the spin polarization is 80% in samples sputtered on unheated MgO(100)
substrates and annealed at high temperatures. However, the spin polarization is
considerably smaller in samples deposited on heated substrates.
Ferromagnetic FexSix alloys have been proposed as potential spin injectors into
silicon with a substantial spin polarization. Andreev Reflection Spectroscopy (ARS) is
utilized to determine the spin polarization of both amorphous and crystalline Fe65Si35
alloys. The amorphous phase has a significantly higher spin polarization than that of
the crystalline phase.
In this thesis, (1111) Fe SmO0:82F0:18FeAs and Pb superconductors are used to
measure the spin polarization of a highly spin-polarized material, La0:67Sr0:33MnO3.
Both materials yield the same intrinsic spin polarization, therefore, Fe-superconductors
can be used in ARS. Based on the behavior of the differential conductance for highly
spin polarized LSMO and small polarization of Au, it can be concluded that the Fe-Sc
is not a triplet superconductor.
Zero bias anomaly (ZBA), in point contact Andreev reflection (PCAR), has been
utilized as a characteristic feature to reveal many novel physics. Complexities at a
normal metal/superconducting interface often cause nonessential ZBA-like features,
which may be mistaken as ZBA. In this work, it is shown that an extrinsic ZBA,
which is due to the contact resistance, cannot be suppressed by a highly spin-polarized
current while a nonessential ZBA cannot be affected the contact resistance.
Finally, Cu/Cu multilayer GMR structures were fabricated and the GMR% measured
at 300 K and 4.5 K gave responses of 63% and 115% respectively. Not only
do the GMR structures have a large enhancement of resistance, but by applying an
external magnetic eld it is shown that, unlike most materials, the spin polarization
can be tuned to values of 0.386 to 0.415 from H = 0 kOe to H = 15 kOe.
polarization and interface geometry is presented. The 3D model gives the same intrinsic
spin polarization and superconducting gap dependence as the 1D model. This
demonstrates that the 1D model can be use to t 3D data.
Using this model, a Heusler alloy is investigated. Andreev reflection measurements
show that the spin polarization is 80% in samples sputtered on unheated MgO(100)
substrates and annealed at high temperatures. However, the spin polarization is
considerably smaller in samples deposited on heated substrates.
Ferromagnetic FexSix alloys have been proposed as potential spin injectors into
silicon with a substantial spin polarization. Andreev Reflection Spectroscopy (ARS) is
utilized to determine the spin polarization of both amorphous and crystalline Fe65Si35
alloys. The amorphous phase has a significantly higher spin polarization than that of
the crystalline phase.
In this thesis, (1111) Fe SmO0:82F0:18FeAs and Pb superconductors are used to
measure the spin polarization of a highly spin-polarized material, La0:67Sr0:33MnO3.
Both materials yield the same intrinsic spin polarization, therefore, Fe-superconductors
can be used in ARS. Based on the behavior of the differential conductance for highly
spin polarized LSMO and small polarization of Au, it can be concluded that the Fe-Sc
is not a triplet superconductor.
Zero bias anomaly (ZBA), in point contact Andreev reflection (PCAR), has been
utilized as a characteristic feature to reveal many novel physics. Complexities at a
normal metal/superconducting interface often cause nonessential ZBA-like features,
which may be mistaken as ZBA. In this work, it is shown that an extrinsic ZBA,
which is due to the contact resistance, cannot be suppressed by a highly spin-polarized
current while a nonessential ZBA cannot be affected the contact resistance.
Finally, Cu/Cu multilayer GMR structures were fabricated and the GMR% measured
at 300 K and 4.5 K gave responses of 63% and 115% respectively. Not only
do the GMR structures have a large enhancement of resistance, but by applying an
external magnetic eld it is shown that, unlike most materials, the spin polarization
can be tuned to values of 0.386 to 0.415 from H = 0 kOe to H = 15 kOe.
ContributorsGifford, Jessica Anna (Author) / Chen, Tingyong (Thesis advisor) / Bennett, Peter (Committee member) / Nemanich, Robert (Committee member) / Tsen, Kong-Thon (Committee member) / Arizona State University (Publisher)
Created2015
Description
In recent years, there has been increased interest in the Indium Gallium Nitride (InGaN) material system for photovoltaic (PV) applications. The InGaN alloy system has demonstrated high performance for high frequency power devices, as well as for optical light emitters. This material system is also promising for photovoltaic applications due to broad range of bandgaps of InxGa1-xN alloys from 0.65 eV (InN) to 3.42 eV (GaN), which covers most of the electromagnetic spectrum from ultraviolet to infrared wavelengths. InGaN’s high absorption coefficient, radiation resistance and thermal stability (operating with temperature > 450 ℃) makes it a suitable PV candidate for hybrid concentrating solar thermal systems as well as other high temperature applications. This work proposed a high efficiency InGaN-based 2J tandem cell for high temperature (450 ℃) and concentration (200 X) hybrid concentrated solar thermal (CSP) application via numerical simulation. In order to address the polarization and band-offset issues for GaN/InGaN hetero-solar cells, band-engineering techniques are adopted and a simple interlayer is proposed at the hetero-interface rather than an Indium composition grading layer which is not practical in fabrication. The base absorber thickness and doping has been optimized for 1J cell performance and current matching has been achieved for 2J tandem cell design. The simulations also suggest that the issue of crystalline quality (i.e. short SRH lifetime) of the nitride material system to date is a crucial factor limiting the performance of the designed 2J cell at high temperature. Three pathways to achieve ~25% efficiency have been proposed under 450 ℃ and 200 X. An anti-reflection coating (ARC) for the InGaN solar cell optical management has been designed. Finally, effective mobility model for quantum well solar cells has been developed for efficient quasi-bulk simulation.
ContributorsFang, Yi, Ph.D (Author) / Vasileska, Dragica (Thesis advisor) / Goodnick, Stephen (Thesis advisor) / Ponce, Fernando (Committee member) / Nemanich, Robert (Committee member) / Arizona State University (Publisher)
Created2017
Description
A driving force for studies of water, alcohols, and amides is the determination of the role of hydrogen bonding. Hydrogen bonds can break and reform, consequently creating supramolecular structures. Understanding the role supramolecular structures play in the dynamics of monohydroxyl alcohols is important to understanding hydrogen bonding in more complex systems such as proteins. Since many monohydroxyl alcohols are good glass formers, dielectric spectroscopy in the supercooled regime is used to gather information about the dynamics of these liquids. Application of high external fields will reversibly alter the polarization responses of the material from the linear response. This results in nonlinear dielectric effects (NDE) such as field induced suppression (saturation) and enhancement of amplitudes (chemical effects) as well as shifts in the time constants toward slower (entropy) and faster (energy absorption) dynamics.
The first part of this thesis describes the nonlinear dielectric experiments on monohydroxyl alcohols, with an emphasis on the time dependence of NDEs. For the first time, time-dependent experiments on monoalcohols were done, the results showed that NDEs occur on the Debye time scale. Furthermore, physical vapor deposition (PVD) is used to modify the supramolecular structure of 4-methyl-3-heptanol. Upon deposition the film cannot form the ring like structures, which are preferred in the bulk material. The as deposited film shows an enhancement of the dielectric peak by a factor of approximately 11 when compared to the bulk material. The conversion from the as deposited material back to the near bulk material was found to occur on the Debye timescale.
The second part of this thesis focuses on the question of what is governing the field induced changes seen in the liquids studied. Here a complete set of high field experiments on highly polar propylene carbonate derivatives were performed. It was demonstrated that these materials exhibit a Debye-like peak and using a combination of Adam-Gibbs and Fröhlich’s definition of entropy, proposed by Johari [G.P. Johari, J. Chem. Phys 138, 154503 (2013)], cannot solely be used to describe a frustration of dynamics. It is important to note that although these material exhibit a Debye like peak, the behavior is much different than monoalcohols.
The first part of this thesis describes the nonlinear dielectric experiments on monohydroxyl alcohols, with an emphasis on the time dependence of NDEs. For the first time, time-dependent experiments on monoalcohols were done, the results showed that NDEs occur on the Debye time scale. Furthermore, physical vapor deposition (PVD) is used to modify the supramolecular structure of 4-methyl-3-heptanol. Upon deposition the film cannot form the ring like structures, which are preferred in the bulk material. The as deposited film shows an enhancement of the dielectric peak by a factor of approximately 11 when compared to the bulk material. The conversion from the as deposited material back to the near bulk material was found to occur on the Debye timescale.
The second part of this thesis focuses on the question of what is governing the field induced changes seen in the liquids studied. Here a complete set of high field experiments on highly polar propylene carbonate derivatives were performed. It was demonstrated that these materials exhibit a Debye-like peak and using a combination of Adam-Gibbs and Fröhlich’s definition of entropy, proposed by Johari [G.P. Johari, J. Chem. Phys 138, 154503 (2013)], cannot solely be used to describe a frustration of dynamics. It is important to note that although these material exhibit a Debye like peak, the behavior is much different than monoalcohols.
ContributorsYoung-Gonzales, Amanda R (Author) / Richert, Ranko (Thesis advisor) / Angell, Charles (Committee member) / Wolf, George (Committee member) / Arizona State University (Publisher)
Created2019
Description
Chemical Vapor Deposition (CVD) is the most widely used method to grow large-scale single layer graphene. However, a systematic experimental study of the relationship between growth parameters and graphene film morphology, especially in the industrially preferred cold wall CVD, has not been undertaken previously. This research endeavored to address this and provide comprehensive insight into the growth physics of graphene on supported solid and liquid Cu films using cold wall CVD.
A multi-chamber UHV system was customized and transformed into a cold wall CVD system to perform experiments. The versatile growth process was completely custom-automated by controlling the process parameters with LabVIEW. Graphene growth was explored on solid electrodeposited, recrystallized and thin sputter deposited Cu films as well as on liquid Cu supported on W/Mo refractory substrates under ambient pressure using Ar, H₂ and CH₄ mixtures.
The results indicate that graphene grown on Cu films using cold wall CVD follows a classical two-dimensional nucleation and growth mechanism. The nucleation density decreases and average size of graphene crystallites increases with increasing dilution of the CH₄/H₂ mixture by Ar, decrease in total flow rate and decrease in CH₄:H₂ ratio at a fixed substrate temperature and chamber pressure. Thus, the resulting morphological changes correspond with those that would be expected if the precursor deposition rate was varied at a fixed substrate temperature for physical deposition using thermal evaporation. The evolution of graphene crystallite boundary morphology with decreasing effective C deposition rate indicates the effect of edge diffusion of C atoms along the crystallite boundaries, in addition to H₂ etching, on graphene crystallite shape.
The roles of temperature gradient, chamber pressure and rapid thermal heating in C precursor-rich environment on graphene growth morphology on thin sputtered Cu films were explained. The growth mechanisms of graphene on substrates annealed under reducing and non-reducing environment were explained from the scaling functions of graphene island size distribution in the pre-coalescence regime. It is anticipated that applying the pre-coalescence size distribution method presented in this work to other 2D material systems may be useful for elucidating atomistic mechanisms of film growth that are otherwise difficult to obtain.
A multi-chamber UHV system was customized and transformed into a cold wall CVD system to perform experiments. The versatile growth process was completely custom-automated by controlling the process parameters with LabVIEW. Graphene growth was explored on solid electrodeposited, recrystallized and thin sputter deposited Cu films as well as on liquid Cu supported on W/Mo refractory substrates under ambient pressure using Ar, H₂ and CH₄ mixtures.
The results indicate that graphene grown on Cu films using cold wall CVD follows a classical two-dimensional nucleation and growth mechanism. The nucleation density decreases and average size of graphene crystallites increases with increasing dilution of the CH₄/H₂ mixture by Ar, decrease in total flow rate and decrease in CH₄:H₂ ratio at a fixed substrate temperature and chamber pressure. Thus, the resulting morphological changes correspond with those that would be expected if the precursor deposition rate was varied at a fixed substrate temperature for physical deposition using thermal evaporation. The evolution of graphene crystallite boundary morphology with decreasing effective C deposition rate indicates the effect of edge diffusion of C atoms along the crystallite boundaries, in addition to H₂ etching, on graphene crystallite shape.
The roles of temperature gradient, chamber pressure and rapid thermal heating in C precursor-rich environment on graphene growth morphology on thin sputtered Cu films were explained. The growth mechanisms of graphene on substrates annealed under reducing and non-reducing environment were explained from the scaling functions of graphene island size distribution in the pre-coalescence regime. It is anticipated that applying the pre-coalescence size distribution method presented in this work to other 2D material systems may be useful for elucidating atomistic mechanisms of film growth that are otherwise difficult to obtain.
ContributorsDas, Shantanu, Ph.D (Author) / Drucker, Jeff (Thesis advisor) / Alford, Terry (Committee member) / Chen, Tingyong (Committee member) / Arizona State University (Publisher)
Created2018
Description
The structure of glass has been the subject of many studies, however some
details remained to be resolved. With the advancement of microscopic
imaging techniques and the successful synthesis of two-dimensional materials,
images of two-dimensional glasses (bilayers of silica) are now available,
confirming that this glass structure closely follows the continuous random
network model. These images provide complete in-plane structural information
such as ring correlations, and intermediate range order and with computer
refinement contain indirect information such as angular distributions, and
tilting.
This dissertation reports the first work that integrates the actual atomic
coordinates obtained from such images with structural refinement to enhance
the extracted information from the experimental data.
The correlations in the ring structure of silica bilayers are studied
and it is shown that short-range and intermediate-range order exist in such networks.
Special boundary conditions for finite experimental samples are designed so atoms
in the bulk sense they are part of an infinite network.
It is shown that bilayers consist of two identical layers separated by a
symmetry plane and the tilted tetrahedra, two examples of
added value through the structural refinement.
Finally, the low-temperature properties of glasses in two dimensions
are studied. This dissertation presents a new approach to find possible
two-level systems in silica bilayers employing the tools of rigidity theory
in isostatic systems.
details remained to be resolved. With the advancement of microscopic
imaging techniques and the successful synthesis of two-dimensional materials,
images of two-dimensional glasses (bilayers of silica) are now available,
confirming that this glass structure closely follows the continuous random
network model. These images provide complete in-plane structural information
such as ring correlations, and intermediate range order and with computer
refinement contain indirect information such as angular distributions, and
tilting.
This dissertation reports the first work that integrates the actual atomic
coordinates obtained from such images with structural refinement to enhance
the extracted information from the experimental data.
The correlations in the ring structure of silica bilayers are studied
and it is shown that short-range and intermediate-range order exist in such networks.
Special boundary conditions for finite experimental samples are designed so atoms
in the bulk sense they are part of an infinite network.
It is shown that bilayers consist of two identical layers separated by a
symmetry plane and the tilted tetrahedra, two examples of
added value through the structural refinement.
Finally, the low-temperature properties of glasses in two dimensions
are studied. This dissertation presents a new approach to find possible
two-level systems in silica bilayers employing the tools of rigidity theory
in isostatic systems.
ContributorsSadjadi, Seyed Mahdi (Author) / Thorpe, Michael F (Thesis advisor) / Beckstein, Oliver (Committee member) / Schmidt, Kevin E (Committee member) / Treacy, Michael Mj (Committee member) / Arizona State University (Publisher)
Created2018
Description
Layer structured two dimensional (2D) semiconductors have gained much interest due to their intriguing optical and electronic properties induced by the unique van der Waals bonding between layers. The extraordinary success for graphene and transition metal dichalcogenides (TMDCs) has triggered a constant search for novel 2D semiconductors beyond them. Gallium chalcogenides, belonging to the group III-VI compounds, are a new class of 2D semiconductors that carry a variety of interesting properties including wide spectrum coverage of their bandgaps and thus are promising candidates for next generation electronic and optoelectronic devices. Pushing these materials toward applications requires more controllable synthesis methods and facile routes for engineering their properties on demand.
In this dissertation, vapor phase transport is used to synthesize layer structured gallium chalcogenide nanomaterials with highly controlled structure, morphology and properties, with particular emphasis on GaSe, GaTe and GaSeTe alloys. Multiple routes are used to manipulate the physical properties of these materials including strain engineering, defect engineering and phase engineering. First, 2D GaSe with controlled morphologies is synthesized on Si(111) substrates and the bandgap is significantly reduced from 2 eV to 1.7 eV due to lateral tensile strain. By applying vertical compressive strain using a diamond anvil cell, the band gap can be further reduced to 1.4 eV. Next, pseudo-1D GaTe nanomaterials with a monoclinic structure are synthesized on various substrates. The product exhibits highly anisotropic atomic structure and properties characterized by high-resolution transmission electron microscopy and angle resolved Raman and photoluminescence (PL) spectroscopy. Multiple sharp PL emissions below the bandgap are found due to defects localized at the edges and grain boundaries. Finally, layer structured GaSe1-xTex alloys across the full composition range are synthesized on GaAs(111) substrates. Results show that GaAs(111) substrate plays an essential role in stabilizing the metastable single-phase alloys within the miscibility gaps. A hexagonal to monoclinic phase crossover is observed as the Te content increases. The phase crossover features coexistence of both phases and isotropic to anisotropic structural transition.
Overall, this work provides insights into the controlled synthesis of gallium chalcogenides and opens up new opportunities towards optoelectronic applications that require tunable material properties.
In this dissertation, vapor phase transport is used to synthesize layer structured gallium chalcogenide nanomaterials with highly controlled structure, morphology and properties, with particular emphasis on GaSe, GaTe and GaSeTe alloys. Multiple routes are used to manipulate the physical properties of these materials including strain engineering, defect engineering and phase engineering. First, 2D GaSe with controlled morphologies is synthesized on Si(111) substrates and the bandgap is significantly reduced from 2 eV to 1.7 eV due to lateral tensile strain. By applying vertical compressive strain using a diamond anvil cell, the band gap can be further reduced to 1.4 eV. Next, pseudo-1D GaTe nanomaterials with a monoclinic structure are synthesized on various substrates. The product exhibits highly anisotropic atomic structure and properties characterized by high-resolution transmission electron microscopy and angle resolved Raman and photoluminescence (PL) spectroscopy. Multiple sharp PL emissions below the bandgap are found due to defects localized at the edges and grain boundaries. Finally, layer structured GaSe1-xTex alloys across the full composition range are synthesized on GaAs(111) substrates. Results show that GaAs(111) substrate plays an essential role in stabilizing the metastable single-phase alloys within the miscibility gaps. A hexagonal to monoclinic phase crossover is observed as the Te content increases. The phase crossover features coexistence of both phases and isotropic to anisotropic structural transition.
Overall, this work provides insights into the controlled synthesis of gallium chalcogenides and opens up new opportunities towards optoelectronic applications that require tunable material properties.
ContributorsCai, Hui, Ph.D (Author) / Tongay, Sefaattin (Thesis advisor) / Dwyer, Christian (Committee member) / Zhuang, Houlong (Committee member) / Arizona State University (Publisher)
Created2018
Description
The rigidity of a material is the property that enables it to preserve its structure when deformed. In a rigid body, no internal motion is possible since the degrees of freedom of the system are limited to translations and rotations only. In the macroscopic scale, the rigidity and response of a material to external load can be studied using continuum elasticity theory. But when it comes to the microscopic scale, a simple yet powerful approach is to model the structure of the material and its interparticle interactions as a ball$-$and$-$spring network. This model allows a full description of rigidity in terms of the vibrational modes and the balance between degrees of freedom and constraints in the system.
In the present work, we aim to establish a microscopic description of rigidity in \emph{disordered} networks. The studied networks can be designed to have a specific number of degrees of freedom and/or elastic properties. We first look into the rigidity transition in three types of networks including randomly diluted triangular networks, stress diluted triangular networks and jammed networks. It appears that the rigidity and linear response of these three types of systems are significantly different. In particular, jammed networks display higher levels of self-organization and a non-zero bulk modulus near the transition point. This is a unique set of properties that have not been observed in any other types of disordered networks. We incorporate these properties into a new definition of jamming that requires a network to hold one extra constraint in excess of isostaticity and have a finite non-zero bulk modulus. We then follow this definition by using a tuning by pruning algorithm to build spring networks that have both these properties and show that they behave exactly like jammed networks. We finally step into designing new disordered materials with desired elastic properties and show how disordered auxetic materials with a fully convex geometry can be produced.
In the present work, we aim to establish a microscopic description of rigidity in \emph{disordered} networks. The studied networks can be designed to have a specific number of degrees of freedom and/or elastic properties. We first look into the rigidity transition in three types of networks including randomly diluted triangular networks, stress diluted triangular networks and jammed networks. It appears that the rigidity and linear response of these three types of systems are significantly different. In particular, jammed networks display higher levels of self-organization and a non-zero bulk modulus near the transition point. This is a unique set of properties that have not been observed in any other types of disordered networks. We incorporate these properties into a new definition of jamming that requires a network to hold one extra constraint in excess of isostaticity and have a finite non-zero bulk modulus. We then follow this definition by using a tuning by pruning algorithm to build spring networks that have both these properties and show that they behave exactly like jammed networks. We finally step into designing new disordered materials with desired elastic properties and show how disordered auxetic materials with a fully convex geometry can be produced.
ContributorsFaghir Hagh, Varda (Author) / Thorpe, Michael F. (Thesis advisor) / Beckstein, Oliver (Committee member) / Chamberlin, Ralph V. (Committee member) / Schmidt, kevin E. (Committee member) / Arizona State University (Publisher)
Created2018