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(1 - 11 of 11)
- Title
- Experiments in Graphene and Plasmonics.
- Creator
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Smith, Christian, Ishigami, Masa, Peale, Robert, Mucciolo, Eduardo, Chanda, Debashis, University of Central Florida
- Abstract / Description
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Graphene nanoribbons, graphene based optical sensors, and grating based plasmonics are explored experimentally. Graphene nanoribbons exhibit highly insulating states that may allow for graphene based digital applications. We investigate the sensitivity of these states to local charged impurities in ultra high vacuum. We look into the possibility of isolating two-dimensional films of H-BN and BSCCO, and test for any interesting phenomena. We also assess graphene's applicability for optical...
Show moreGraphene nanoribbons, graphene based optical sensors, and grating based plasmonics are explored experimentally. Graphene nanoribbons exhibit highly insulating states that may allow for graphene based digital applications. We investigate the sensitivity of these states to local charged impurities in ultra high vacuum. We look into the possibility of isolating two-dimensional films of H-BN and BSCCO, and test for any interesting phenomena. We also assess graphene's applicability for optical sensing by implementing a new style of spectral detector. Utilizing surface plasmon excitations nearby a graphene field-effect transistor we are able to produce a detector with wavelength sensitivity and selectivity in the visible range. Finally, we study another plasmonic phenomenon, and observe the resonant enhancement of diffraction into a symmetry-prohibited order in silver gratings.
Show less - Date Issued
- 2014
- Identifier
- CFE0005887, ucf:50874
- Format
- Document (PDF)
- PURL
- http://purl.flvc.org/ucf/fd/CFE0005887
- Title
- Selective electro-magnetic absorbers based on metal-dielectric-metal thin-film cavities.
- Creator
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Nath, Janardan, Peale, Robert, Ishigami, Masa, Chernyak, Leonid, Vodopyanov, Konstantin, University of Central Florida
- Abstract / Description
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Efficient absorption of light is required for a large number of applications such as thermo-photovoltaics,thermal imaging, bio-sensing, thermal emitters, astronomy, and stealth technology. Strong light absorbers found in nature with high intrinsic losses such as carbon black, metal-black, and carbon nano-tubes etc. are bulky, not design-tunable and are hard to pattern for micro- and nano- devices. We developed thin-film, high performance absorbers in the visible, near-, mid-, long-wave - and...
Show moreEfficient absorption of light is required for a large number of applications such as thermo-photovoltaics,thermal imaging, bio-sensing, thermal emitters, astronomy, and stealth technology. Strong light absorbers found in nature with high intrinsic losses such as carbon black, metal-black, and carbon nano-tubes etc. are bulky, not design-tunable and are hard to pattern for micro- and nano- devices. We developed thin-film, high performance absorbers in the visible, near-, mid-, long-wave - and far-IR region based on a 3 layer metal-dielectric-metal (MDM) structure.We fabricated a 3-layerMDMabsorber with large band-widths in the visible and near IR spectral range without any lithographic patterning. This was the first demonstration in the optical range of the Salisbury Screen, which was originally invented for radar absorption. A Fabry-Perotcavity model depending on the thickness of the dielectric, but also the effective permittivity of the semi-transparent top metal gives calculated spectra that agree well with experiment.Secondly, we fabricated long-wave IR and far-IR MDM absorbers comprising surface patterns of periodic metal squares on the dielectric layer. Strong absorption in multiple bands were obtained, and these depended weakly on polarization and angle of incidence. Though such absorbers had been extensively studied by electrodynamic simulations and experiment in the visible to far- R regions, there existed no analytic model that could accurately predict the wavelengths of the multiple resonances. We developed a theoretical model for these absorbers based on standingwave resonances, which accurately predicts resonance wavelengths for experiment and simulation for the first time. Unlike metamaterial theories our model does not depend on the periodicity of the squares but only on their lateral dimension and the thickness of the dielectric. This feature is confirmed by synchrotron-based IR spectral imaging microscopy of single isolated squares.
Show less - Date Issued
- 2015
- Identifier
- CFE0005851, ucf:50907
- Format
- Document (PDF)
- PURL
- http://purl.flvc.org/ucf/fd/CFE0005851
- Title
- Theoretical and Computational Studies of the electronic, Structural, Vibrational, and Thermodynamic Properties of Transition Metal Nanoparticles.
- Creator
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Sadatshafaie, Ghazal, Rahman, Talat, Stolbov, Sergey, Ishigami, Masa, Masunov, Artem, University of Central Florida
- Abstract / Description
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The main objective of this dissertation is to provide better understanding of the atomic configurations, electronic structure, vibrational properties, and thermodynamics of transition metal nanoparticles and evaluate the intrinsic (i.e. size and shape) and extrinsic (i.e. ligands, adsorbates, and support) effects on the aforementioned properties through a simulational approach. The presented research provides insight into better understanding of the morphological changes of the nanoparticles...
Show moreThe main objective of this dissertation is to provide better understanding of the atomic configurations, electronic structure, vibrational properties, and thermodynamics of transition metal nanoparticles and evaluate the intrinsic (i.e. size and shape) and extrinsic (i.e. ligands, adsorbates, and support) effects on the aforementioned properties through a simulational approach. The presented research provides insight into better understanding of the morphological changes of the nanoparticles that are brought about by the intrinsic factors as well as the extrinsic ones. The preference of certain ligands to stabilize specific sizes of nanoparticles is investigated. The intrinsic and extrinsic effects on the electronic structure of the nanoparticles are presented. The physical and chemical properties of the nanoparticles are evaluated through better understanding of the above effects on the experimentally observed properties as well as the applied techniques. The unexpected experimental results are tested and interpreted by deconvolution of the affecting factors. The application of Debye model to nanoparticles is tested and its shortcomings at nanoscale are discussed. Predictions which can provide insight into intelligent choice of candidates to cater to certain properties are provided. The results of this thesis can be used in the future in design and engineering of functionalized materials. We use ab initio calculations based on Density Functional Theory (DFT) to obtain information about the energetics, atomic configuration, and electronic structure of the nanoparticles. Ab initio Molecular Dynamics (MD) is used to study the evolution of the structures of the nanoparticles. To calculate vibrational frequencies, the finite displacement method is employed.
Show less - Date Issued
- 2015
- Identifier
- CFE0006385, ucf:51536
- Format
- Document (PDF)
- PURL
- http://purl.flvc.org/ucf/fd/CFE0006385
- Title
- Solving Constraint Satisfaction Problems with Matrix Product States.
- Creator
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Pelton, Sabine, Mucciolo, Eduardo, Ishigami, Masa, Leuenberger, Michael, University of Central Florida
- Abstract / Description
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In the past decade, Matrix Product State (MPS) algorithms have emerged as an efficient method of modeling some many-body quantum spin systems. Since spin system Hamiltonians can be considered constraint satisfaction problems (CSPs), it follows that MPS should provide a versatile framework for studying a variety of general CSPs. In this thesis, we apply MPS to two types of CSP. First, use MPS to simulate adiabatic quantum computation (AQC), where the target Hamiltonians are instances of a...
Show moreIn the past decade, Matrix Product State (MPS) algorithms have emerged as an efficient method of modeling some many-body quantum spin systems. Since spin system Hamiltonians can be considered constraint satisfaction problems (CSPs), it follows that MPS should provide a versatile framework for studying a variety of general CSPs. In this thesis, we apply MPS to two types of CSP. First, use MPS to simulate adiabatic quantum computation (AQC), where the target Hamiltonians are instances of a fully connected, random Ising spin glass. Results of the simulations help shed light on why AQC fails for some optimization problems. We then present the novel application of a modified MPS algorithm to classical Boolean satisfiability problems, specifically k-SAT and max k-SAT. By construction, the algorithm also counts solutions to a given Boolean formula (\#-SAT). For easy satisfiable instances, the method is more expensive than other existing algorithms; however, for hard and unsatisfiable instances, the method succeeds in finding satisfying assignments where other algorithms fail to converge.
Show less - Date Issued
- 2017
- Identifier
- CFE0006902, ucf:51713
- Format
- Document (PDF)
- PURL
- http://purl.flvc.org/ucf/fd/CFE0006902
- Title
- Computational Approach to Electrocatalysis.
- Creator
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Dhakal, Nagendra, Stolbov, Sergey, Rahman, Talat, Ishigami, Masa, Masunov, Artem, University of Central Florida
- Abstract / Description
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The main objective of this work is to understand the theoretical basis of the working principle of the Hydrogen fuel cell. We seek the physical basis of the Rational Design Technique, the smart way of preselecting materials from the material-pool, implemented in our study anticipating highly promising electrocatalysts for promoting the conversion of chemical energy stored in hydrogen molecules into the electrical energy. It needs the understanding of the relationship among the compositions of...
Show moreThe main objective of this work is to understand the theoretical basis of the working principle of the Hydrogen fuel cell. We seek the physical basis of the Rational Design Technique, the smart way of preselecting materials from the material-pool, implemented in our study anticipating highly promising electrocatalysts for promoting the conversion of chemical energy stored in hydrogen molecules into the electrical energy. It needs the understanding of the relationship among the compositions of the materials under consideration, their electronic structure and catalytic activities. We performed the first principle DFT calculations to achieve the goal.Our work is focused first on the issues in hydrogen oxidation reaction taking place in anode compartment of the cell. Next comes up with the issues with Oxygen Reduction Reaction taking place in cathode compartment. Finally, we focus on mechanisms underlying binding of small molecules on substrates.Platinum perfectly catalyzes hydrogen oxidation reaction on the hydrogen fuel cell anodes. However, it has at least two drawbacks: a) it is too expensive; b) it has a low tolerance to CO poisoning. Pt-Ru bi-functional catalysts are more tolerant to CO, but they are still very expensive. In this work, we performed first-principle studies of stability and reactivity of M/W (110) structures, where M = Pd, Ru, Au monolayers. All three systems are found to be stable: formation energy of MLs is significantly higher than cohesive energy of the M-elements. The calculated binding energies of H, H2, OH, CO, and H2O were used to obtain the reaction free energies. Analysis of the free energies suggests that Au-W bonding does not activate sufficiently Au monolayer, whereas Ru/W (110) is still too reactive for the CO removal. Meanwhile, Pd/W (110) is found to catalyze hydrogen oxidation and at the same time to be highly tolerant to the CO poisoning. The latter finding is explained by the fact that CO binds much weaker to Pd on W (110) than to Pt, while the OH binding is strong enough to ensure CO oxidation. The obtained results are traced to the electronic structure of the systems.Oxygen Reduction Reaction (ORR) is the heart core reaction in fuel cells, Proton Exchange Membrane Fuel cell and DEMFC. However, the reaction is not so obvious and need suitable electrocatalyst. Pt or Pt-based catalysts are found to be the best catalyst so far. But, its cost and shortage make it not feasible economically. Moreover, lower onset potential (maximal electrode potential at which the reaction can proceed) of such catalysts is offering another limitation to fuel cell performance. Research has been conducted in many directions for lowering the cost by replacing the Pt with some other elements of lower cost or reducing the Pt-load in the material; and even more finding the material performing better than Pt. In this paper, we've tried to understand the ORR mechanism and look for the material that could be potential option to Pt. Our calculations suggest that for monolayer of Pt on 5 layered slab of Nb or Mo the onset potential is the same as for Pt, while cost of these systems are much lower than that of Pt. Presence of water changes the reaction rate very minimum. Rational design method facilitates the research of selecting the appropriate catalyst and saves time and effort significantly. The result shows that the d-band center model is not accurate to describe the reactivity of the catalyst.For decades, adsorbates' binding energy (????) has been used as an indicator of the adsorbate-substrate bond strength (??????). Thus, although one can compute accurately any ?? models to gauge bond-strength are developed and applied to rationalize and anticipate ????'s because that is a key aspect in the rational search for efficient catalysts. Yet bond-strength alone fails to predict ???? trends. Therefore, quantifying and understanding the difference between ???? and ?????? is essential to catalysts design. Indeed, the adsorbate-substrate bond formation perturbs the substrate's electronic charge density, which reduces ???? by the energy attached to such perturbation: ??????????. Here, with the example of carbon monoxide adsorption on metal-doped graphene, we show that ?????????? may exceed 1 eV and render an unusual situation: although the EB of CO to the Au-doped graphene indicates that binding does not happen, we find evidence of a strong bond between CO and the substrate. Thus, in this case, the large ?????????? totally disrupt the equivalency between ?????? and ???? we also propose a method to compute ?????????? that bypasses dealing with an excited electronic state of the system.
Show less - Date Issued
- 2017
- Identifier
- CFE0006583, ucf:51336
- Format
- Document (PDF)
- PURL
- http://purl.flvc.org/ucf/fd/CFE0006583
- Title
- Electronic transport properties of carbon nanotubes: the impact of atomic charged impurities.
- Creator
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Tsuchikawa, Ryuichi, Ishigami, Masa, Mucciolo, Eduardo, Peale, Robert, Masunov, Artem, University of Central Florida
- Abstract / Description
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Even changing one atom in nanoscale materials is expected to alter their properties due to their small physical sizes. Such sensitivity can be utilized to modify materials' properties from bottom up and is essential for the utility of nanoscale materials. As such, the impact of extrinsic atomic adsorbates was measured on pristine graphene and a network of carbon nanotubes using atomic hydrogen, cesium atoms, and dye molecules. In order to further quantify such an atomic influence, the...
Show moreEven changing one atom in nanoscale materials is expected to alter their properties due to their small physical sizes. Such sensitivity can be utilized to modify materials' properties from bottom up and is essential for the utility of nanoscale materials. As such, the impact of extrinsic atomic adsorbates was measured on pristine graphene and a network of carbon nanotubes using atomic hydrogen, cesium atoms, and dye molecules. In order to further quantify such an atomic influence, the resistance induced by a single potassium atom on metallic and semiconducting carbon nanotubes was measured for the first time. Carbon nanotubes are sensitive to adsorbates due to their high surface-to-volume ratio. The resistance arising from the presence of extrinsic impurity atoms depends on the types of nanotubes. Metallic carbon nanotubes are resilient to a long-ranged, Coulomb-like potential, whereas semiconducting carbon nanotubes are susceptible to these impurities. The difference in the scattering strength originates from the chirality of carbon nanotubes, which defines their unique electronic properties. This difference had not directly measured experimentally because of the issue of contact resistance, the difficulty of chirality identification, and the uncertainty in the number of impurity atoms introduced on carbon nanotubes.We synthesized atomically clean, long ((>)100 ?m) carbon nanotubes, and their chirality was identified by Rayleigh scattering spectroscopy. We introduced potassium atoms on the nanotubes to impose a long-range, Coulomb potential and measured the change in resistivity, excluding the contact resistance, by plotting the resistance as a function of the carbon nanotube length. The flux of potassium atoms coming onto the nanotubes was monitored by quartz crystal microbalance, and the scattering strength of a single potassium atom was deduced from the change in resistivity and the density of potassium atoms on the nanotubes. We found that the scattering strength of potassium atoms on semiconducting nanotubes depends on the charge carrier type (holes or electrons). Metallic nanotubes were found to be less affected by the presence of potassium atoms than semiconducting nanotubes, but the scattering strength showed a large dependence on Fermi energy. These experimental results were compared to theoretical simulations, and we found a good agreement with the experiments. Our findings provide crucial information for the application of carbon nanotubes for electronic devices, such as transistors and sensors.
Show less - Date Issued
- 2015
- Identifier
- CFE0005729, ucf:50078
- Format
- Document (PDF)
- PURL
- http://purl.flvc.org/ucf/fd/CFE0005729
- Title
- Mid-infrared plasmonics.
- Creator
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Khalilzadeh Rezaie, Farnood, Peale, Robert, Ishigami, Masa, Schoenfeld, Winston, Buchwald, Walter, Abdolvand, Reza, University of Central Florida
- Abstract / Description
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This dissertation reports investigations into materials for, and applications of, infrared surface plasmon polaritons (SPP). SPPs are inhomogeneous electromagnetic waves that are bound to the surface of a conductor. Tight confinement of electromagnetic energy, the primary virtue of SPPs for so-called (")plasmonic(") applications, requires plasma frequencies for the conductor near the intended infrared operational frequencies. This requires carrier concentrations that are much less than those...
Show moreThis dissertation reports investigations into materials for, and applications of, infrared surface plasmon polaritons (SPP). SPPs are inhomogeneous electromagnetic waves that are bound to the surface of a conductor. Tight confinement of electromagnetic energy, the primary virtue of SPPs for so-called (")plasmonic(") applications, requires plasma frequencies for the conductor near the intended infrared operational frequencies. This requires carrier concentrations that are much less than those of usual metals such as gold and silver. I have investigated the optical properties and SPP excitation resonances of two materials having infrared plasma frequencies, namely the semimetal bismuth and the transparent conducting fluorine-doped tin-oxide (FTO). The complex permittivity spectra for evaporated films of Bi were found to be distinctly different than earlier reports for crystal or polycrystalline films, and SPP excitation resonances on Bi-coated gratings were found to be disappointingly broad. Permittivity spectra for chemical spray deposited FTO were obtained to long-wave IR wavelengths for the first time, and nano-crystalline FTO-coated silicon lamellar gratings show remarkable conformity. SPP excitation resonances for FTO are more promising than for Bi. Thus, FTO appears to be a promising SPP host for infrared plasmonics, e.g. a planer waveguide plasmonic spectral sensor, whose design was elaborated and investigated as part of my research and which requires SPP-host coating on deep vertical side walls of a trench-like analyte interaction region. Additionally, FTO may serve as a useful conducting oxide for a near-IR plasmonic spectral imager that I have investigated theoretically.
Show less - Date Issued
- 2015
- Identifier
- CFE0006222, ucf:51080
- Format
- Document (PDF)
- PURL
- http://purl.flvc.org/ucf/fd/CFE0006222
- Title
- Charge and Spin Transport in Low-Dimensional Materials.
- Creator
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Ahmadi, Amin, Mucciolo, Eduardo, Del Barco, Enrique, Ishigami, Masa, Guo, Jing, University of Central Florida
- Abstract / Description
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My research has been focused on two main areas. First, electronic transports in chiral carbonnanotubes in the presence of charged adatoms. To study such systems we employed recursiveGreens function technique to evaluate the conductance using the Landauer formula. Comparingwith the experimental data, we determined the effective amplitude and the range of scatteringpotentials. In addition, using a similar approach we explained qualitatively an unusual conductancefeature in a metallic carbon...
Show moreMy research has been focused on two main areas. First, electronic transports in chiral carbonnanotubes in the presence of charged adatoms. To study such systems we employed recursiveGreens function technique to evaluate the conductance using the Landauer formula. Comparingwith the experimental data, we determined the effective amplitude and the range of scatteringpotentials. In addition, using a similar approach we explained qualitatively an unusual conductancefeature in a metallic carbon nanotube. The second part of my study was concerned to the dynamicalspin injection and spin currents in low-dimensional materials. We have developed an atomisticmodel to express the injected spin current in terms of the systems Greens function. The newformulation provides a framework to study the spin injection and relaxation of a system with anarbitrary structure.
Show less - Date Issued
- 2017
- Identifier
- CFE0006550, ucf:51343
- Format
- Document (PDF)
- PURL
- http://purl.flvc.org/ucf/fd/CFE0006550
- Title
- Electrostatic control over temperature-dependent tunneling across single-molecule junctions.
- Creator
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Rodriguez Garrigues, Alvar, Del Barco, Enrique, Flitsiyan, Elena, Ishigami, Masa, Hernandez, Eloy, University of Central Florida
- Abstract / Description
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The aim of the present dissertation is to improve the understanding and methodology of temperature-dependent tunnel conduction through individual molecules by single-electron transport spectroscopy. New advances in electrochemistry present individual molecular diodes as a realistic option for the implementation on molecular circuits thanks to their high current rectification ratios. Therefore, a major requisite in this field is to understand and control the conduction behaviors for a large...
Show moreThe aim of the present dissertation is to improve the understanding and methodology of temperature-dependent tunnel conduction through individual molecules by single-electron transport spectroscopy. New advances in electrochemistry present individual molecular diodes as a realistic option for the implementation on molecular circuits thanks to their high current rectification ratios. Therefore, a major requisite in this field is to understand and control the conduction behaviors for a large variety of conditions. This work focuses on the electric conduction through ferrocene-based molecules as a function of temperatures within a wide range of bias and gate voltages by means of three-terminal electromigrated-broken single-electron transistors (SETs).The results show that the temperature dependence of the current (from 80 to 260 K) depends strongly on the bias and gate voltages, with areas in where the current increases exponentially with temperature (at the Coulomb blockade regimes), and others where the increase of the temperature makes the current only to vary slightly (at resonance) or to decrease monotonically (at the charge degeneracy points). These different observed behaviors of the tunneling current with increasing temperatures can be well explained by a formal single-level coherent tunneling model where the temperature dependence relies on the thermal broadening of the Fermi distributions of the electrons in the leads. The model portraits the molecule as a localized electrostatic level capacitively coupled to the transistor leads, and the electrical conduction through the junction as coherent sequential tunneling.
Show less - Date Issued
- 2016
- Identifier
- CFE0006171, ucf:51132
- Format
- Document (PDF)
- PURL
- http://purl.flvc.org/ucf/fd/CFE0006171
- Title
- TERAHERTZ AND SUB-TERAHERTZ TUNABLE RESONANT DETECTORS BASED ON EXCITATION OF TWO DIMENSIONAL PLASMONS IN InGaAs/InP HEMTs.
- Creator
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Nader Esfahani, Nima, Peale, Robert, Ishigami, Masa, Del Barco, Enrique, Buchwald, Walter, University of Central Florida
- Abstract / Description
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Plasmons can be generated in the two dimensional electron gas (2DEG) of grating-gated high electron mobility transistors (HEMTs). The grating-gate serves dual purposes, namely to provide the required wavevector to compensate for the momentum mismatch between the free-space radiation and 2D-plasmons, and to tune the 2DEG sheet charge density. Since the plasmon frequency at a given wavevector depends on the sheet charge density, a gate bias can shift the plasmon resonance. In some cases,...
Show morePlasmons can be generated in the two dimensional electron gas (2DEG) of grating-gated high electron mobility transistors (HEMTs). The grating-gate serves dual purposes, namely to provide the required wavevector to compensate for the momentum mismatch between the free-space radiation and 2D-plasmons, and to tune the 2DEG sheet charge density. Since the plasmon frequency at a given wavevector depends on the sheet charge density, a gate bias can shift the plasmon resonance. In some cases, plasmon generation results in a resonant change in channel conductance which allows a properly designed grating-gated HEMT to be used as a voltage-tunable resonant detector or filter. Such devices may find applications as chip-scale tunable detectors in airborne multispectral detection and target tracking.Reported here are investigations of InGaAs/InP-based HEMT devices for potential tunable resonant sub-THz and THz detectors. The HEMTs were fabricated from a commercial double-quantum well HEMT wafer by depositing source, drain, and semi-transparent gate contacts using standard photolithography processes. Devices were fabricated with metalized transmission gratings with multiple periods and duty cycles. For sub-THz devices, grating period and duty cycle were chosen to be 9 ?m and 22%, respectively; while they were chosen to be 0.5 ?m and 80% for the THz device. The gratings were fabricated on top of the gate region with dimensions of 250 ?m (&)#215; 195 ?m.The resonant photoresponse of the larger grating-period HEMT was investigated in the sub-THz frequency range of around 100 GHz. The free space radiation was generated by an ultra-stable Backward Wave Oscillator (BWO) and utilized in either frequency modulation (FM), or amplitude modulation (AM) experiments. The photoresponse was measured at 4K sample temperature as the voltage drop across a load resistor connected to the drain while constant source-drain voltages of different values, VSD, were applied. The dependence of such optoelectrical effect to polarization of the incident light, and applied VSD is studied. The results of AM and FM measurements are compared and found to be in agreement with the calculations of the 2D-plasmon absorption theory, however, a nonlinear behavior is observed in the amplitude and the line-shape of the photoresponse for AM experiments. For detection application, the minimum noise-equivalent-power (NEP) of the detector was determined to be 235 and 113 pW/Hz1/2 for FM and AM experiments, respectively. The maximum responsivity of the detector was also estimated to be ~ 200 V/W for the two experiments. The far-IR transmission spectra of the device with nanometer scale period was measured at 4 K sample temperature for different applied gate voltages to investigate the excitation of 2D-plasmon modes. Such plasmon resonances were observed, but their gate bias dependence agreed poorly with expectations.
Show less - Date Issued
- 2014
- Identifier
- CFE0005386, ucf:50461
- Format
- Document (PDF)
- PURL
- http://purl.flvc.org/ucf/fd/CFE0005386
- Title
- Dynamical spin injection in graphene.
- Creator
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Singh, Simranjeet, Del Barco, Enrique, Mucciolo, Eduardo, Ishigami, Masa, Hill, Stephen, University of Central Florida
- Abstract / Description
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Within the exciting current trend to explore novel low-dimensional systems, the possibility to injectpure spin currents in graphene and other two-dimensional crystals has attracted considerableattention in the past few years. The theoretical prediction of large spin relaxation times and experimentally observed mesoscopic-scale spin diffusion lengths places graphene as a promising base system for future spintronics devices. This is due to the unique characteristics intrinsic to the two...
Show moreWithin the exciting current trend to explore novel low-dimensional systems, the possibility to injectpure spin currents in graphene and other two-dimensional crystals has attracted considerableattention in the past few years. The theoretical prediction of large spin relaxation times and experimentally observed mesoscopic-scale spin diffusion lengths places graphene as a promising base system for future spintronics devices. This is due to the unique characteristics intrinsic to the two-dimensional lattice of carbon atoms forming graphene, such as the lack of nuclear spins and weak spin-orbit coupling of the charge carriers. Interestingly for some spintronic applications, the latter can be chemically and physically engineered, with large induced spin-orbit couplings found in functionalized graphene sheets. Understanding spin injection, spin current and spin dynamics in graphene is of a great interest, both from the fundamental and applied points of view.This thesis presents an experimental study of dynamical generation of spin currents in macroscopic graphene sheets by means of spin pumping from the precessing magnetization of an adjacent ferromagnet. The spin pumping characteristics are studied by means of ferromagnetic resonance (FMR) measurements in Permalloy/graphene (Py/Gr) bilayers. Changes in the FMR linewidth induced by the presence of graphene (when compared to studies with only Py films) correspond to an increase in the Gilbert damping in the ferromagnetic layer (proportional to the FMR linewidth) and interpreted as a consequence of spin pumping at the Py/Gr interface driven by the Py magnetization dynamics (i.e., magnetic induced by the microwave stimulus). FMR experiments are performed on different FM/Gr interfaces, completing a set of studies designed to systematically identify and eliminate damping enhancement arising from processes other than spin pumping. Remarkably, a substantial enhancement of the Gilbert damping observed in Py/Gr strips with graphene protruding a few micrometers from the strip sides is univocally associated to spin pumping at the quasi-onedimensional interface between the Py strip edges and graphene. This increase in the FMR linewidth compares with observations in other bilayer systems, in where thick (thicker than the spin diffusionlength) layers of heavy metals with strong spin-orbit coupling are employed as the non-magneticlayer, indicating that spin relaxation in chemically grown graphene must be greatly enhanced in order to account for the losses of angular momentum lost by the ferromagnet. The fundamentalimplications of the results presented in this thesis point to a non-trivial nature of the spin pumping mechanism owing to the two-dimensionality of the non-magnetic layer (i.e., graphene).In addition, a spintronics device designed to interconvert charge and spin currents has been designed. A high-frequency microwave irradiation lock-in modulation technique is employed todetect the small electrical voltages generated by the inverse spin Hall effect (ISHE). As a proofof principle, a successful spin-charge interconversion in Py/Pt-based devices is experimentally demonstrated in this thesis. The challenges associated with the spin-charge interconversion in twodimensional devices are discussed and systematically addressed, and a potential device geometry for measuring the ISHE in Py/Gr-based systems is provided.
Show less - Date Issued
- 2014
- Identifier
- CFE0005552, ucf:50284
- Format
- Document (PDF)
- PURL
- http://purl.flvc.org/ucf/fd/CFE0005552
