Thesis: S.B., Massachusetts Institute of Technology, Department of Materials Science and Engineering, 2019 / Cataloged from PDF version of thesis. / Includes bibliographical references (pages 33-34). / Quantum dots (QDs) are nanometer-sized crystallites of inorganic semiconductors with tunable optoelectronic properties, which has led to a variety of real-world applications beginning in the 1980s, ranging including electronic displays, solar cells, and quantum computers [(Lee, SID), (Tang, Nature Mater.), (Puri, Phys Rev. B)]. However, most high-quality QD materials explored to date have been limited for large-scale application due to toxicity concerns or difficult-to-scale preparative methods. This thesis explores the synthesis and properties of colloidal nanocrystals composed of the non-toxic semiconductor copper indium sulfide (CulnS₂). We report an improved core nanoparticle synthesis with unique compositional control, a rationally-designed precursor for the synthesis of high-quality CulnS₂/ZnS nanocomposites, and describe the dependence on the photophysical properties of CulnS₂/ZnS on core CulnS₂ elemental composition. / by Trevor Walton Romero. / S.B. / S.B. Massachusetts Institute of Technology, Department of Materials Science and Engineering

This dissertation details the use of 93Nb (γ = 10.405 MHz/T, I = 9/2) and 77Se (γ = 8.13 MHz/T, I = 1/2) nuclear magnetic resonance (NMR) to investigate the phase transitions of the layered transition metal dichalcogenide (TMD) niobium diselenide (2H-NbSe2). 2H-NbSe2 has a trigonal prismatic structure and exhibits a charge density wave (CDW) transition at TCDW = 33.5 K and a superconducting (SC) transition with Tc = 7.2 K. The present experiments were undertaken with the external field (H0) first parallel, and then perpendicular to the crystallographic axis (c-axis). Single crystals of 2H-NbSe2 were probed in the temperature range of 0.35 K – 100 K and at frequencies and fields ranging from 19 MHz – 135 MHz and 1.8 T – 17.5 T respectively to investigate the normal, CDW and superconducting states. A value of Tc = 7.04 K was measured in situ by cooling the sample below Tc and measuring detuning. A value of Tc = 6.2 K was measured in a sample grown from the same batch using a SQUID magnetometer. 93Nb NMR spectral line shape and Knight shift were used to detect the CDW phase. The full 93Nb spectrum was field-swept and observed to possess line broadening and asymmetry which gain intensity the further the respective transition is from the central line. Pre-transitional broadening was detected in both orientations, beginning at a temperature of 60 K for the central transition and as high as 80 K for the first lower-frequency satellite transition. The broadening continues below TCDW and ends below 15 K. These results are believed to be evidence of a discommensurate CDW phase. Spin-lattice relaxation rate (T1) measurements are used to directly probe the electronic density of states (DOS) and investigate the SC gap. The T1 data of both nuclei reveal Korringa behavior above Tc, no visible Hebel-Slichter peak just below Tc, and a linear crossover to further Korringa behavior for T << Tc in both orientations. Both orientations were accurately fit to a two-gap function and 1/T1T in the H0⊥c direction displays a two-step transition, suggesting two superconducting gaps are present. A field-dependence of T1 in the SC state was detected in both sample orientations. In both the H0∥c and H0⊥c directions, 〖1/T〗_1∝H due to the Zeeman contribution of the Volovik effect. The Doppler effect contribution from the vortex supercurrents was absent from the relaxation, which suggests 2H-NbSe2 is a two-gap s-wave superconductor. / A Dissertation submitted to the Program in Materials Science and Engineering in partial fulfillment of the requirements for the degree of Doctor of Philosophy. / Fall Semester 2017. / November 14, 2017. / Charge Density Wave, Condensed Matter Physics, NbSe2, NMR, Superconductivity, Transition Metal Dichalcogenide / Includes bibliographical references. / Arneil Reyes, Professor Co-Directing Dissertation; Irinel Chiorescu, Professor Co-Directing Dissertation; Adrian Barbu, University Representative; Eric Hellstrom, Committee Member; Theo Siegrist, Committee Member.

The Weyl semimetal requires the breaking of either the time-reversal symmetry (TRS) or the lattice inversion symmetry. When the TRS and inversion symmetry coexist, a pair of degenerate Weyl points may exist, leading to the related Dirac semimetal phase. In other words, a Dirac semimetallic state can be regarded as two copies of Weyl semimetal states. In this dissertation, we study tellurium based compounds like the Weyl semimetal candidate MoTe2 and the Dirac semimetal candidate PtTe2 within the transition metal dichalcogenides family. Firstly, we report a systematic study on the Hall-effect of the semi-metallic state of bulk MoTe2, which was recently claimed to be a candidate for a novel type of Weyl semi-metallic state. The temperature (T) dependence of the carrier densities and of their mobilities, as estimated from a numerical analysis based on the isotropic two-carrier model, indicates that its exceedingly large and non-saturating magnetoresistance may be attributed to a near perfect compensation between the densities of electrons and holes at low temperatures. A sudden increase in hole density, with a concomitant rapid increase in the electron mobility below T ∼ 40 K, leads to comparable densities of electrons and holes at low temperatures suggesting a possible electronic phase-transition around this temperature. Secondly, the electronic structure of semi-metallic transition-metal dichalcogenides, such as WTe2 and orthorhombic γ−MoTe2, are claimed to contain pairs of Weyl points or linearly touching electron and hole pockets associated with a non-trivial Chern number. For this reason, these compounds were recently claimed to conform to a new class, deemed type-II, of Weyl semi-metallic systems. A series of angle resolved photoemission experiments (ARPES) claim a broad agreement with these predictions detecting, for example, topological Fermi arcs at the surface of these crystals. We synthesized single-crystals of semi-metallic MoTe2 through a Te flux method to validate these predictions through measurements of its bulk Fermi surface (FS) via quantum oscillatory phenomena. We find that the superconducting transition temperature of γ−MoTe2 depends on disorder as quantified by the ratio between the room- and low-temperature resistivities, suggesting the possibility of an unconventional superconducting pairing symmetry. Similarly to WTe2, the magnetoresistivity of γ−MoTe2 does not saturate at high magnetic fields and can easily surpass 106 %. Remarkably, the analysis of the de Haas-van Alphen (dHvA) signal superimposed onto the magnetic torque, indicates that the geometry of its FS is markedly distinct from the calculated one. The dHvA signal also reveals that the FS is affected by the Zeeman-effect precluding the extraction of the Berry-phase. A direct comparison between the previous ARPES studies and density-functional-theory (DFT) calculations reveals a disagreement in the position of the valence bands relative to the Fermi level εF . Here, we show that a shift of the DFT valence bands relative to εF , in order to match the ARPES observations, and of the DFT electron bands to explain some of the observed dHvA frequencies, leads to a good agreement between the calculations and the angular dependence of the FS cross-sectional areas observed experimentally. However, this relative displacement between electron- and hole-bands eliminates their crossings and, therefore, the Weyl type-II points predicted for γ−MoTe2. Finally, we investigate the electronic structure and transport properties in single crystals of the semi-metallic platinum ditelluride (PtTe2), recently claimed to be a novel type-II Dirac semimetal, via a methodology similar to that applied to γ−MoTe2, i.e. the temperature and angular dependence of the SdH and dHvA effects. Our high-quality PtTe2 crystal displays a large non-saturating magnetoresistance under magnetic field up to 61 T. The dHvA oscillation and SdH effect reveal several high and low frequencies suggesting a rather complex Fermi surface. We also find evidence for a non-trivial Berry phase. The crystal quality improved considerably under subsequent annealing at high-temperatures leading to the observation of linear in field magnetoresistivity. Combined with effective masses in the order of ∼ 0.1 free electron mass, these results further suggest that PtTe2 displays bulk Dirac-like bands. / A Dissertation submitted to the Department of Physics in partial fulfillment of the requirements for the degree of Doctor of Philosophy. / Fall Semester 2017. / October 16, 2017. / fermi surfaces, semimetals, topological materials / Includes bibliographical references. / Luis Balicas, Professor Co-Directing Dissertation; Nicholas Bonesteel, Professor Co-Directing Dissertation; Petru Andrei, University Representative; Peng Xiong, Committee Member; Horst Wahl, Committee Member.

Nanoparticles have been extensively studied in the past few decades due to the possibilities they offer in applications ranging from medicine to energy generation. A new class of ultra-small noble metal nanoparticles consisting of tens to hundreds of atoms, commonly known as clusters or nanoclusters, have drawn interest of the research community recently due to their unique optical, electronic and structural properties. Over the past few years, advances have been made in the synthesis of atomically precise noble metal clusters (for example, silver and gold) with distinct optical properties. Their ultra-small size distinguishes them from conventional plasmonic nanoparticles and the properties are very sensitive to the slight variation in the compositon of the cluster, i.e. the number of the metal atoms and/or the nature of the ligands. These clusters are interesting because of their potential applications in field such as sensing, imaging, catalysis, clean energy, photonics, etc. as well as they provide fundamental insight into the evolution of the optical and electronic properties of these clusters. In this project, we explored the strategies to synthesize luminescent metallic clusters of gold and silver and to promote their solubility and stability in aqueous and biological medium. We focused particularly on the thiolate protected clusters due to the higher affinity of gold and silver to sulfur. Lipoic acid (Thioctic acid) is a bio-molecule with a cyclic disulfide ring, which also acts as a chelating ligand. Due to the higher binding affinity of the cyclic disulfide ring to nanocrystal surface, lipoic acid and chemically modified lipoic acid molecules have been widely reported for the synthesis and functionalization of inorganic nanocrystals. Here, we describe the use of bidentate lipoic acid ligands in the one phase growth of luminescent gold and silver nanoclusters. In addition, we have synthesized a new set of monothiol ligands containing PEG and zwitterion for the functionalization of fluorescent clusters. Chapter 1 introduces the fundamental properties of metallic clusters and the origin of these properties from electronic and structural point of view. The optical properties of ultra-small nanocrystals (<2 nm) in comparison to the plasmonic particles is described. In addition, the variation of optical and structural properties from one metal to another as well as one ligand to another is also compared. Chapter 2 describes the synthesis of ultra-small size gold clusters with different optical emission (ranging from blue to red) using photo-activated LA-PEG ligands. The influence of various factors on the growth of the clusters is also studied. Optical properties of the clusters were studied by UV-visible absorption, PL emission and excitation and time resolved fluorescence spectroscopy. XPS and DOSY NMR were used to characterize the oxidation states and sizes of these clusters. The photo-chemical transformation of LA-PEG ligands to thiols and the effect of various experimental parameters such as solvent, oxygen, ligand functional group and effect of acid are described in chapter 3. Thiol yield percentage was quantified using ellman assay. Chapter 4 describes the one phase aqueous synthesis of Ag29 clusters capped with bidentate dihydrolipoic acid (DHLA). We also describe the drastic enhancement of the PL intensity upon gold doping of the Ag29 clusters. Optical properties along with the size characterization by electrospray ionization mass spectrometry is also described. We further describe the growth of these clusters using DHLA-PEG molecules. Chapter 5 describes the synthesis of highly fluorescent Au25-xAgx clusters stabilized with two types of ligands (triphenylphosphine and thiols). We designed a set of monothiolate ligands appended with PEG and zwitterionic moieties. This approach allows to prepare water soluble and stable metallic clusters with enhanced photoluminescence and well defined optical properties. Chapter 6 is the overall summary of our findings and prospects and outlook. / A Dissertation submitted to the Department of Chemistry and Biochemistry in partial fulfillment of the requirements for the degree of Doctor of Philosophy. / Fall Semester 2017. / November 15, 2017. / Includes bibliographical references. / Hedi Mattoussi, Professor Directing Dissertation; Peng Xiong, University Representative; Igor V. Alabugin, Committee Member; Michael Shatruk, Committee Member; Geoffrey F. Strouse, Committee Member.

There has been considerable amount of interest in the ion-exchange properties of layered zirconium phosphates. This interest has been renewed due to potential applications in the remediation of nuclear waste. They are believed to be preferred to the conventional ion-exchange materials due to their increased stability under acid conditions. The ability of the material to withstand low pH is crucial as the legacy waste pools have very low pH. It has been well documented that substituting metals with different radii into material’s framework can alter the ion-exchange properties of the material due to differences in the crystal structure and the interactions between the framework and the non-framework ions. The work presented here focuses on the synthesis and characterisation of a series of α-zirconium phosphate materials doped with trivalent cations (Y, Fe, Ce) of which the results indicate that complete solid solutions were not formed for iron-zirconium phosphate and cerium-zirconium phosphate, whereas solubility limits exist for yttrium-zirconium phosphate. The new yttrium-zirconium phosphates obtained were characterised by analytical techniques such as XRD, XRF, SEM/EDAX, MAS-NMR and FT-IR. The structural refinement of these materials was carried out and a study of the doping effect was also done to conclude that yttrium substitution into the zirconium phosphate framework did not follow Vegard’s law. Finally, ion exchanges of the inactive ions of Cs, Sr and Co commonly found in nuclear waste was carried out using single ion exchange and competitive exchange experiments with Na, Mg and Ca as interfering ions.

The disposal of nuclear waste is highly regulated and the disposal option will be dependent on the radionuclide content of the waste. The encapsulation of nuclear waste to prevent migration of radionuclides into the environment and as a safe means of long term storage and disposal can be achieved using ordinary Portland cement (OPC) and various additives such as blast furnace slag (BFS) or pulverised fly ash (PFA). Treated radioactive wastes in this manner are characterised by good thermal, chemical, physical stability and compressive strength. In addition the alkaline chemistry of concrete renders most radionuclides highly insoluble. The ultimate destination of some of these encapsulated wastes is in a Deep Geological Facility (GDF), where for many years the wastes will remain inert to their environment. In the longer-term the environmental conditions will change and the inertness of these waste forms could be affected from the seepage of water into the facility along with microbial activity. The diffusivity or leaching behaviour of cement encapsulated radioactive waste is crucial to ensure the overall safety of a storage/disposal system. The research presented in this thesis evaluates the diffusivity of strontium, caesium and cobalt when added as inactive forms to BFS:OPC and PFA:OPC formulation as their chlorides and for strontium when added as chloride and carbonate. The cylindrical cement paste samples (CPS) having diameter of 3.2 cm and height 5.3 cm were immersed in re-circulating test solutions consisting of de-ionised water, concentrated Sellafield pore water (CSPW), diluted Sellafield pore water (DSPW) and bacterial inoculated water, John Innes Soil Solution (JISS). Strontium carbonate was selected to determine the influence of a water insoluble compound on diffusivity of the cation. Freshly cured and aged BFS:OPC samples were also studied to evaluate the impact of carbonation on cation diffusivity. Chloride salts were used, as these would be benign to microorganisms, i.e. would not stimulate or support growth unlike nitrate or sulphate anions. The outcome of this study indicate that the make-up water composition affected the segregation of inherent and added cations in the cement paste samples and also both the bleed water volume and physical characteristics of the cement paste samples. Strontium when added as a soluble salt to the make-up water influenced the rate of diffusivity. Depending on the type of formulation (BFS:OPC, PFA:OPC), a direct correlation was observed between diffusivity of Sr2+ and total amount of Ca2+ present in the CPS. The rate of diffusivity and the depth of cation diffusion was significantly higher in 3% SrCl2 PFA:OPC having lower concentration of Ca2+ compared to its BFS counterpart. The concentration of the added salt to the make-up water also affected the diffusivity. The difference in the diffusivity was observed between closed and open diffusivity system. The solubility limits were not a factor in open circuit which was comparable with the pH values; contrary to the closed circuits. The concentration of cations and anions in the test solution influenced strontium and caesium diffusivity. The diffusivity of sulphate was influenced by the nature of the cation added to the make-up water. Strontium had the greatest effect on lowering the diffusion primarily due to the formation of sparingly soluble strontium sulphate. The pH values of the circulating JISS test solutions from all the contaminated cement samples were lower in comparison with control, which was comparable with viable population in the circulating system. There was no significant viable population measured in the JISS from control CPS. The JISS test solution composition retard strontium diffusivity but accelerated caesium diffusion in comparison with distilled water values, this retardation could be due to the inherent sulphate content (≈8600 ppb) of the JISS test solution. This work provides fundamental understanding of the physic-chemical factors influencing the diffusivity of cations from BFS:OPC and PFA:OPC formulations. The scheme i.e. closed circuit recirculation adopted in this research would be more fitting of the real situation i.e. stagnation followed by percolation and therefore diffusivity of ions will be greatly influenced by the test solution chemistry and composition.

Thesis: S.B., Massachusetts Institute of Technology, Department of Materials Science and Engineering, 2018. / Cataloged from PDF version of thesis. / Includes bibliographical references (page 30). / Lithium-ion batteries are widespread in use due to their thermal stability and high energy density. The most common design uses an organic electrolyte and lithium-cobalt electrode. While safe under typical operating conditions, the use of an organic electrolyte subjects the battery user to certain risks; in particular, Li-ion liquid batteries are explosive when exposed to air and subject to thermal runoff, making them highly sensitive to any physical damage. The use of cobalt also poses a moral concern, as the mining and sourcing of cobalt is geographically restricted and most commonly sourced from countries that have a history of foreign exploitation and child labor. An all solid state battery is suggested as a possible alternative battery that reduces operation risks and maintains similar performance characteristics. Lithium-lanthanum-zirconium oxide is presented as a suitable electrolyte replacement. Coupled with cobalt-free electrodes, this battery design would provide a safer, more responsible battery. / by Erick Ivan Hernandez Alvarez. / S.B.

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 2006. / Includes bibliographical references (p. 164-171). / The dramatic advances in Si/SiO2-based microelectronic processing witnessed over the past several decades can largely be attributed to relatively material-independent device scaling. However, with physical and economic limitations to the continued scaling of such devices appearing on the horizon, it is likely that near-term advances will come from the integration of novel and previously underrepresented materials. One of the most promising ways to enhance performance comes from the integration of judiciously chosen lattice-mismatched materials with Si. However, the integration of such structures typically poses significant technical challenges. The work contained in this thesis seeks to address several of these important issues, primarily through the use of relaxed, graded SiGe buffers on Si (i.e. Vx[Si1-xGex]/Si). Several new phenomena in relaxed graded SiGe buffers are developed in this thesis. A rise in threading dislocation density was observed in high-Ge content relaxed graded SiGe layers grown at relatively high temperatures, which was attributed to dislocation nucleation. This observation is contrary to conventional graded buffer theory in which high growth temperatures are expected to result in reduced threading dislocation densities (TDDs). / (cont.) Additionally, a coupling effect between the effective strain and the growth rate was observed, as evidenced by increased TDD values at reduced growth rates. This observation is attributed to reduced growth rates allowing more time for the surface to evolve (i.e. roughen) during growth, thereby trapping mobile dislocations and necessitating the nucleation of additional dislocations to continue relaxing the structure. Also detailed in this thesis is the creation of two novel CMOS-compatible platforms for high-power applications: strained-silicon on silicon (SSOS) and strained-silicon on silicon-germanium on silicon (SGOS). SSOS substrate has an epitaxially-defined, tensilely strained silicon (-Si) layer directly on bulk silicon wafer without an intermediate SiGe or oxide layer. SSOS is a homochemical heterojunction, i.e. a heterojunction defined by strain state only and not by an accompanying compositional change, and therefore in principle SSOS may ease metal-oxide-semiconductor (MOS) -Si fabrication as SiGe is absent from the structure. SGOS has an epitaxially-defined SiGe layer between the strained silicon channel and the Si substrate, which is likely necessary to prevent excessive off-state leakage in MOS devices due to overlap of the source-drain contacts and the interfacial misfit array. / (cont.) The thesis concludes with a study of utilizing buried -Si layers for improving the fabrication of SSOI substrate via the hydrogen induced layer exfoliation process. Previous work involving tensile -Si.4Geo.6 layers in relaxed Ge/Vx[SiixGex/Si demonstrated that significant hydrogen gettering via the formation of strain-relieving platelets occurred within the tensile -Sio.4Ge.6 layers, leading to an overall increase in layer transfer efficiency for GOI substrate fabrication. Buried tensile -Si layers in relaxed SilGex for SSOI fabrication, however, demonstrate markedly different hydrogen gettering behavior that is dependent on a combination of both the degree of tensile strain as well the amount of damage present in the adjacent Si.xGex alloy. It was determined that a tensile strain level of approximately 1.6% in Si (corresponding to a Sio.6Ge.4-based donor structure) was needed to create sufficient engineered damage to overcome the implantation damage in the adjacent Sio.6Ge.4 layers and result in enhanced layer exfoliation. Lastly, an advanced Sio.6Geo.4-based structure which incorporated -Si layers as transfer, hydrogen gettering, and etchstop layers was demonstrated. Such a structure may prove useful for the reuse of significant portions of the original SSOI donor structure, thereby potentially speeding commercial adoption of the SSOI platform. / by David Michael Isaacson. / Ph.D.

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 2007. / This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections. / Includes bibliographical references. / The optical logic unit cell is the photonic analog to transistor-transistor logic in electronic devices. Active devices such as InP-based semiconductor optical amplifiers (SOA) emitting at 1550 nm are vertically integrated with passive waveguides using the asymmetric twin waveguide technique and the SOAs are placed in a Mach-Zehnder interferometer (MZI) configuration. By sending in high-intensity pulses, the gain characteristics, phase-shifting, and refractive indices of the SOA can be altered, creating constructive or deconstructive interference at the MZI output. Boolean logic and wavelength conversion can be achieved using this technique, building blocks for optical switching and signal regeneration. The fabrication of these devices is complex and the fabrication of two generations of devices is described in this thesis, including optimization of the mask design, photolithography, etching, and backside processing techniques. Testing and characterization of the active and passive components is also reported, confirming gain and emission at 1550 nm for the SOAs, as well as verifying evanescent coupling between the active and passive waveguides. In addition to the vertical integration of photonic waveguides, Esaki tunnel junctions are investigated for vertical electronic integration. Quantum dot formation and growth via molecular beam epitaxy is investigated for emission at the technologically important wavelength of 1310 nm. The effect of indium incorporation on tunnel junctions is investigated. The tunnel junctions are used to epitaxially link multiple quantum dot active regions in series and lasers are designed, fabricated, and tested. / by Ryan Daniel Williams. / Ph.D.

Thesis (B.S.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 1988. / Includes bibliographical references. / by Robert I. Park. / B.S.