Browsing by Author "Chabal, Yves J."
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Item Aqueous-Derived Thin Films and their Interfacial Interactions with Semiconductor Surfaces: A Spectroscopic Study(2018-12) Thomas, Milana Cherie; 0000-0002-0173-1142 (Thomas, MC); Chabal, Yves J.Metal oxide systems are well known for their high dielectric constants, which are important for advanced microelectronics applications. The microelectronics industry currently employs vacuum-based techniques, such as chemical vapor deposition (CVD), to deposit metal oxide films. These vapor-phase deposition techniques suffer due to their slow deposition rates and their use of expensive equipment. Additionally, these processes sometimes require the use of harmful source gases and/or generate corrosive by-products. On the other hand, solution-processed thin films fabricated by spin-coating are advantageous because the process is simple, low cost, and scalable. Aqueous solution deposition is particularly attractive because it offers a green alternative to vapor-phase deposition and has been shown to produce uniform thin films by spin coating on hydrophilic silicon surfaces. However, it has been shown that silicon’s native oxide can degrade device performance due to its electronic interfacial states. In addition, aqueousderived thin films suffer from poor electrical performance due to mobile water and hydroxyl protons, often requiring very high temperature anneals to mitigate. Such anneals compromise the interface between the film and the silicon substrate, hence the electrical performance. One effective method to control the interface, and thus improve device performance, is to functionalize the semiconductor surface using wet chemistry. Here, we address the concerns of aqueous thin film deposition and present a method for alleviating the issues associated with current silicon-silicon oxide devices. We use wet chemical functionalization to graft selfassembled monolayers (SAMs) onto oxide-free silicon, then spin-coat an aqueous thin film on top of the SAM layer. The chemical stability of the SAM and the changes that occur at the interfaces between the Si/SAM/film stack during film deposition and dehydration are monitored by in situ Fourier transform infrared spectroscopy (FTIR) and ex situ X-ray photoelectron spectroscopy (XPS). The modification of the Si/SAM interface is studied as a function of annealing temperature, with electrical measurements used as a metric to quantify the effectiveness of the SAM layer to alleviate issues of interfacial defects observed for films on silicon oxide. The results are presented in three parts: (1) a dehydration study of aqueous-derived thin films deposited on silicon oxide, (2) the synthesis of a novel SAM interfacial layer tailored to accommodate aqueous, Al-based precursors and (3) a study to quantify the effectiveness, if any, on the SAM interfacial layer through electrical characterization methods. In the first part, we investigate the mechanism for dehydration of aqueous thin films and present a method to enhance the removal of water from the films. Using in situ FTIR, we find that the addition of a protective capping layer can enhance the dehydration of the thin film and prevent water reabsorption for a period of up to 14 days. In the second part, we present hydrosilylation methods to graft SAMs onto oxide-free silicon surfaces. The results show that it is possible to covalently attach the SAMs to silicon, evidenced by the formation of Si-C (detected by XPS) at the interface between the Si and the SAM. Four phosphonic acid-terminated SAMs are prepared and contact angle measurements are used as a metric for evaluating which can best accommodate aqueous spin-coater solutions. To conclude, we investigate the interface between the SAM layer and an aluminum-based thin film derived from aqueous precursor solutions. Current-voltage and capacitance-voltage measurements are used to quantify the effectiveness of the SAM layer.Item Biphenyl-Bridged Wrinkled Mesoporous Silica Nanoparticles for Radioactive Iodine Capture(Cambridge Univ Press, 2019-02-11) Brown, Alexander T.; Lin, Jason; Thomas, Milana C.; Chabal, Yves J.; Balkus, Kenneth J.; 0000-0003-0291-2081 (Brown, AT); Brown, Alexander T.; Lin, Jason; Thomas, Milana C.; Chabal, Yves J.; Balkus, Kenneth J.The capture of volatile radioactive iodine-129 is an important process for nuclear fission. Biphenyl-bridged wrinkled mesoporous silica shows similar performance for iodine sequestration to commercial Ag-mordenite and avoids the use of expensive silver The biphenyl-wrinkled mesoporous silica nanoparticles function as a scaffold for biphenyl groups and also as a fluorescent indicator for the loading of iodine. The nanoparticles have a surface area of 973 m²/g and the biphenyl molecules form an electron charge-transfer complex with iodine. Iodine was loaded into the biphenyl-bridged wrinkled mesoporous silica (BUMS) at 19 ± 0.2 % loading by mass.Item Capture of Organic Iodides from Nuclear Waste by Metal-Organic Framework-Based Molecular Traps(Nature Publishing Group) Li, Baiyan; Dong, Xinglong; Wang, Hao; Ma, Dingxuan; Tan, Kui; Jensen, Stephanie; Deibert, Benjamin J.; Butler, Joseph; Cure, Jeremy; Shi, Zhan; Thonhauser, Timo; Chabal, Yves J.; Han, Yu; Li, Jing; Tan, Kui; Butler, Joseph; Cure, Jeremy; Chabal, Yves J.Effective capture of radioactive organic iodides from nuclear waste remains a significant challenge due to the drawbacks of current adsorbents such as low uptake capacity, high cost, and non-recyclability. We report here a general approach to overcome this challenge by creating radioactive organic iodide molecular traps through functionalization of metal-organic framework materials with tertiary amine-binding sites. The molecular trap exhibits a high CH₃I saturation uptake capacity of 71 wt% at 150 ⁰C, which is more than 340% higher than the industrial adsorbent Ag⁰@MOR under identical conditions. These functionalized metal-organic frameworks also serve as good adsorbents at low temperatures. Furthermore, the resulting adsorbent can be recycled multiple times without loss of capacity, making recyclability a reality. In combination with its chemical and thermal stability, high capture efficiency and low cost, the adsorbent demonstrates promise for industrial radioactive organic iodides capture from nuclear waste. The capture mechanism was investigated by experimental and theoretical methods.Item Characterization of Ru Thin-Film Conductivity upon Atomic Layer Deposition on H-Passivated Si(111)Roodenko, Ecatherina (Katy); Park, S. K.; Kwon, Jinhee; Wielunski, L.; Chabal, Yves J.; 0000 0000 4239 3958 (Chabal, YJ); 89624105 (Chabal, YJ)The sheet resistance measured by a four-probe technique is compared to the resistivity data derived from the optical response of thin ruthenium films grown on hydrogen-passivated Si(111) surfaces by atomic-layer deposition using cyclopentadienyl ethylruthenium dicarbonyl, Ru(Cp)(CO)2Et and O 2 as gas reactant. The Drude-Landauer theory is applied to evaluate the spectroscopic ellipsometry response and the DC resistivity evaluated by 4-point probe measurements. Results indicate that thin Ru films (below ∼5nm) deposited on Si exhibit a higher sheet resistance than similarly grown Ru films on TiN. This is explained by an island-growth mechanism at the initial stages of Ru deposition that greatly diminishes the film conductivity before the formation of a continuous film. © 2012 American Institute of Physics.Item Chemical Modification Mechanisms in Hybrid Hafnium Oxo-Methacrylate Nanocluster Photoresists for Extreme Ultraviolet Patterning(American Chemical Society) Mattson, Eric C.; Cabrera, Yasiel; Rupich, Sara M.; Wang, Yuxuan; Oyekan, Kolade A.; Mustard, T. J.; Halls, M. D.; Bechtel, H. A.; Martin, M. C.; Chabal, Yves J.; Mattson, Eric C.; Cabrera, Yasiel; Rupich, Sara M.; Wang, Yuxuan; Oyekan, Kolade A.; Chabal, Yves J.The potential implementation of extreme ultraviolet (EUV) lithography into next generation device processing is bringing urgency to identify resist materials that optimize EUV lithographic performance. Inorganic/organic hybrid nanoparticles or clusters constitute a promising new class of materials, with high EUV sensitivity from the core and tunable chemistry through the coordinating ligands. Development of a thorough mechanistic understanding of the solubility switching reactions in these materials is an essential first step toward their implementation in patterning applications but remains challenging due to the complexity of their structures, limitations in EUV sources, and lack of rigorous in situ characterization. Here, we report a mechanistic investigation of the solubility switching reactions in hybrid clusters comprising a small HfOx core capped with a methacrylic acid ligand shell (HfMAA). We show that EUV-induced reactions can be studied by performing in situ infrared (IR) spectroscopy of electron-irradiated films using a variable energy electron gun. Combining additional ex situ metrology, we track the chemical evolution of the material at each stage of a typical resist processing sequence. For instance, we find that a cross-linking reaction initiated by decarboxylation of the methacrylate ligands under electron irradiation constitutes the main solubility switching mechanism, although there are also chemical changes imparted by a typical post application bake (PAB) step alone. Lastly, synchrotron-based IR microspectroscopy measurements of EUV-irradiated HfMAA films enable a comparison of reactions induced by EUV vs electron beam irradiation of the same resist material, yielding important insight into the use of electron beam irradiation as an experimental model for EUV exposure.Item Cobalt and Iron Segregation and Nitride Formation from Nitrogen Plasma Treatment of CoFeB Surfaces(American Institute of Physics Inc) Mattson, Eric C.; Michalak, D. J.; Veyan, Jean Francois; Chabal, Yves J.; 0000-0002-3743-5521 (Veyan, JF); 0000-0002-6435-0347 (Chabal, YJ); Mattson, Eric C.; Veyan, Jean Francois; Chabal, Yves J.Cobalt-iron-boron (CoFeB) thin films are the industry standard for ferromagnetic layers in magnetic tunnel junction devices and are closely related to the relevant surfaces of CoFe-based catalysts. Identifying and understanding the composition of their surfaces under relevant processing conditions is therefore critical. Here we report fundamental studies on the interaction of nitrogen plasma with CoFeB surfaces using infrared spectroscopy, x-ray photoemission spectroscopy, and low energy ion scattering. We find that, upon exposure to nitrogen plasma, clean CoFeB surfaces spontaneously reorganize to form an overlayer comprised of Fe2N3 and BN, with the Co atoms moved well below the surface through a chemically driven process. Subsequent annealing to 400 °C removes nitrogen, resulting in a Fe-rich termination of the surface region. © 2016 Author(s).Item Colored Porous Silicon as Support for Plasmonic NanoparticlesLublow, M.; Kubala, S.; Veyan, Jean-Francois; Chabal, Yves J.; 0000 0000 4239 3958 (Chabal, YJ); 89624105 (Chabal, YJ)Colored nanoporous silicon thin films were employed as dielectric spacing layers for the enhancement of localized surface plasmon (LSP) polaritons. Upon formation of Au nanoparticles (Au-NPs) on these layers, a visible color change is observed due to multiple LSP resonance excitations. Far-field effects were assessed by angle-resolved reflectometry. Resonance enhancements, particularly for s-polarized light, account for the observed color change and are discussed in terms of effective medium and Mie scattering theory. Enhancements of the electric field strengths in the near-field and of the absorption in the substrate were deduced from finite difference time domain calculations and exceed considerably those of the non-porous Au-NP/Si interface. First results of improved photoelectrocatalytic hydrogen evolution at these interfaces are discussed. Samples were prepared by varied procedures of metal assisted etching and dry etching with XeF2. Structural and chemical properties were investigated by scanning electron and atomic force microscopy as well as energy dispersive x-ray analysis.Item Design Approaches for Enhancing Photovoltaic Performance of Silicon Solar Cells Sensitized by Proximal Nanocrystalline Quantum Dots(2017-07-24) Shafiq, Natis; 0000 0000 4239 3958 (Chabal, YJ); Chabal, Yves J.Energy transfer (ET) based sensitization of silicon (Si) using proximal nanocrystal quantum dots (NQDs) has been studied extensively in recent years as a means to develop thin and flexible Si based solar cells. The driving force for this research activity is a reduction in materials cost. To date, the main method for determining the role of ET in sensitizing Si has been optical spectroscopic studies. The quantitative contribution from two modes of ET (namely, nonradiative and radiative) has been reported using time-resolved photoluminescence (TRPL) spectroscopy coupled with extensive theoretical modelling. Thus, optical techniques have established the potential for utilizing ET based sensitization of Si as a feasible way to develop novel NQD-Si hybrid solar cells. However, the ultimate measure of the efficiency of ET-based mechanisms is the generation of electron-hole pairs by the impinging photons. It is therefore important to perform electrical measurements. However, only a couple of studies have attempted electrical quantification of ET modes. A few studies have focused on photocurrent measurements, without considering industrially relevant photovoltaic (PV) systems. Therefore, there is a need to develop a systematic approach for the electrical quantification of ET-generated charges and to help engineer new PV architectures optimized for harnessing the full advantages of ET mechanisms. Within this context, the work presented in this dissertation aims to develop an experimental testing protocol that can be applied to different PV structures for quantifying ET contributions from electrical measurements. We fabricated bulk Si solar cells (SCs) as a test structure and utilized CdSe/ZnS NQDs for ET based sensitization. The NQD-bulk Si hybrid devices showed ~30% PV enhancement after NQD deposition. We measured external quantum efficiency (EQE) of these devices to quantify ET-generated charges. Reflectance measurements were also performed to decouple contributions of intrinsic optical effects (i.e., anti-reflection) from NQD mediated ET processes. Our analysis indicates that the contribution of ET-generated charges cannot be detected by EQE measurements. Instead, changes in the optical properties (i.e., anti-reflection property) due to the NQD layer are found to be the primary source of the photocurrent enhancement. Based on this finding, we propose to minimize bulk Si absorption by using an ultrathin (~300 nm) Si PV architecture which should enable measurements of ET-generated charges. We describe an optimized process flow for fabricating such ultrathin Si devices. The devices fabricated by this method behave like photo-detectors and show enhanced sensitivity under 1 Sun AM1.5G illumination. The geometry and process flow of these devices make it possible to incorporate NQDs for sensitization. Overall, this dissertation provides a protocol for the quantification of ET-generated charges and documents an optimized process flow for the development of an ultrathin Si solar cells.Item Diffusion of Small Molecules in Metal Organic Framework MaterialsCanepa, Pieremanuele; Nijem, Nour; Chabal, Yves J.; Thonhauser, T.; 0000 0000 4239 3958 (Chabal, YJ); 89624105 (Chabal, YJ)Ab initio simulations are combined with in situ infrared spectroscopy to unveil the molecular transport of H-2, CO2, and H2O in the metal organic framework MOF-74-Mg. Our study uncovers-at the atomistic level-the major factors governing the transport mechanism of these small molecules. In particular, we identify four key diffusion mechanisms and calculate the corresponding diffusion barriers, which are nicely confirmed by time-resolved infrared experiments. We also answer a long-standing question about the existence of secondary adsorption sites for the guest molecules, and we show how those sites affect the macroscopic diffusion properties. Our findings are important to gain a fundamental understanding of the diffusion processes in these nanoporous materials, with direct implications for the usability of MOFs in gas sequestration and storage applications. DOI: 10.1103/PhysRevLett.110.026102Item Effective Sensing of RDX via Instant and Selective Detection of Ketone VaporsHu, Zhichao; Tan, Kui; Lustig, William P.; Wang, Hao; Zhao, Yonggang; Zheng, Chong; Banerjee, Debasis; Emge, Thomas J.; Chabal, Yves J.; Li, Jing; 0000 0000 4239 3958 (Chabal, YJ); 89624105 (Chabal, YJ)Two new luminescent metal-organic frameworks (LMOFs) were synthesized and examined for use as sensory materials. Very fast and effective sensing of RDX was achieved by vapor detection of a cyclic ketone used as a solvent in the production of plastic explosives. The effects of porosity and electronic structure of the LMOFs on their sensing performance were evaluated. We demonstrate that the optimization of these two factors of an LMOF can significantly improve its sensitivity and selectivity. We also elucidate the importance of both electron and energy transfer processes on the fluorescence response of a sensory material.Item Energy Transfer from Colloidal Nanocrystals into Si Substrates Studied via Photoluminescence Photon Counts and Decay Kinetics(2013-08-16) Nguyen, H. M.; Seitz, Oliver; Gartstein, Yuri N.; Chabal, Yves J.; Malko, Anton V.; 0000 0001 2678 9765 (Malko, AV); Nguyen, H. M.; Seitz, Oliver; Gartstein, Yuri N.; Chabal, Yves J.; Malko, Anton V.We use time-resolved photoluminescence (PL) kinetics and PL intensity measurements to study the decay of photoexcitations in colloidal CdSe/ZnS nanocrystals grafted on SiO₂ - Si substrates with a wide range of the SiO₂ spacer layer thicknesses. The salient features of experimental observations are found to be in good agreement with theoretical expectations within the framework of modification of spontaneous decay of electric-dipole excitons by their environment. Analysis of the experimental data reveals that energy transfer (ET) from nanocrystals into Si is a major enabler of substantial variations in decay rates, where we quantitatively distinguish contributions from nonradiative and radiative ET channels. We demonstrate that time-resolved PL kinetics provides a more direct assessment of ET, while PL intensity measurements are also affected by the specifics of the generation and emission processes.Item Energy Transfer from Colloidal Nanocrystals to Strongly Absorbing Perovskites(Royal Society of Chemistry, 2018-06-01) Cabrera, Yasiel; Rupich, Sara M.; Shaw, Ryan; Anand, Benoy; Villa, Manuel de Anda; Rahman, Rezwanur; Dangerfield, Aaron; Gartstein, Yuri N.; Malko, Anton V.; Chabal, Yves J.; 0000-0002-6435-0347 (Chabal, YJ); Cabrera, Yasiel; Rupich, Sara M.; Shaw, Ryan; Anand, Benoy; Villa, Manuel de Anda; Rahman, Rezwanur; Dangerfield, Aaron; Gartstein, Yuri N.; Malko, Anton V.; Chabal, Yves J.Integration of colloidal nanocrystal quantum dots (NQDs) with strongly absorbing semiconductors offers the possibility of developing optoelectronic and photonic devices with new functionalities. We examine the process of energy transfer (ET) from photoactive CdSe/ZnS core/shell NQDs into lead-halide perovskite polycrystalline films as a function of distance from the perovskite surface using time-resolved photoluminescence (TRPL) spectroscopy. We demonstrate near-field electromagnetic coupling between vastly dissimilar excitation in two materials that can reach an efficiency of 99% at room temperature. Our experimental results, combined with electrodynamics modeling, reveal the leading role of non-radiative ET at close distances, augmented by the waveguide emission coupling and light reabsorption at separations >10 nm. These results open the way to combining materials with different dimensionalities to achieve novel nanoscale architectures with improved photovoltaic and light emitting functionalities.Item Film Formation Mechanisms and Interfacial Interactions Derived from In-Situ Fourier-Transform Infrared Spectroscopy and Ex-Situ XPS(2018-08) Dangerfield, Aaron M.; 0000-0001-9700-8583 (Dangerfield, AM); Chabal, Yves J.There is a trend toward the miniaturization of devices for a multitude of industries (microelectronic, energy harvesting/collecting, sensing, etc.) to decrease production costs, increase yields and to optimize efficiencies. As these device dimensions decrease, the interfaces formed in these multicomponent systems are becoming increasingly important, to the point of dominating overall performance. This dissertation addresses the interfacial evolution and mechanisms of film formation for a variety of systems relevant to the microelectronics and energy harvesting industries. In the first example, the pretreatment of a silicon surface with an aluminum-containing precursor is evaluated for low temperature growth of silicon nitride; a film necessary for many steps in transistor fabrication. In the second example, the passivation of quantum dots by various metal oxides is explored for applications in photovoltaic energy harvesting. In the final example, the interface between silicon and a novel tin-oxo based photoresist is controlled to yield desirable film removal properties, while the mechanism for patterning of this resist is also explored.Item HIF-1α-PDK1 Axis-Induced Active Glycolysis Plays an Essential Role in Macrophage Migratory Capacity(American Physical Society) Anand, Benoy; Sampat, Siddharth; Danilov, E. O.; Peng, Weina; Rupich, Sara M.; Chabal, Yves J.; Gartstein, Yuri N.; Malko, Anton V.; 0000 0001 1969 6683 (Gartstein, YN); 0000 0001 2678 9765 (Malko, AV); 170647442 (Gartstein, YN); Anand, Benoy; Sampat, Siddharth; Peng, Weina; Rupich, Sara M.; Chabal, Yves J.; Gartstein, Yuri N.; Malko, Anton V.Ultrafast transient pump-probe measurements of thin CH₃NH₃PbI₃ perovskite films over a wide spectral range from 350 to 800 nm reveal a family of photoinduced bleach (PB) and absorption (PA) features unequivocally pointing to the fundamentally multiband character of the underlying electronic structure. Excitation pump-energy dependent kinetics of three long-lived PB peaks at 1.65, 2.55, and 3.15 eV along with a broad PA band shows the involvement of band-edge thermalized carriers in all transitions and at least four, possibly more, electronic bands. The evolution of the transient signatures is described in terms of the redistribution of the conserved oscillator strength of the whole system. The multiband perspective opens up different directions for understanding and controlling photoexcitations in hybrid perovskites.Item In Situ Studies of the Surface Chemistry Reactions Involved in Gas-Phase Deposition and Etching of Thin Dielectric Films(2018-08) Pena Orduna, Luis Fabian; 0000-0001-6021-4703 (Peña Orduña, LF); Chabal, Yves J.In this dissertation, key aspects of the surface chemistry associated with gas phase deposition and etching are discussed. Atomic layer deposition (ALD) is a gas-phase deposition technique primarily known for its superior self-limiting binary process that affords precise control, uniform and conformal thin film growth. Despite the extensive work done with ALD, the mechanisms behind nucleation and steady state growth remain unclear for many ALD processes. Additionally, in an effort to meet today's device integration requirements, e.g., scaling down nanostructures and thermal budget restrictions during film deposition, thermal ALD processes requiring high temperatures (>300 C) are now being forced out of production due to adverse thermally induced side effects, e.g., device degradation. To address this challenge and promote reactivity at low temperatures (<300 C), the surface reactivity of the substrate is increased by creating defects, dangling bonds or reactive sites using a plasma enhanced process, consequently lowering the thermal budget requirement for film deposition. However, this enhancement adds substantial complications due to complex surface reactions, which demands fundamental studies to sort out and to understand the mechanisms involved. Similarly, efforts have been undertaken to investigate gas-phase cleaning, passivation and characterization of transition metal substrates. It is well known that alcohols and acids are capable of reducing the native oxide on transition metals, e.g., cuprous oxide (Cu₂O), at relatively low temperatures (<325 C). However, the surface functionalization of the final surface after oxide reduction with organic agents has yet to be determined, particularly under industrially relevant processing conditions. The main challenge hindering a fundamental understanding of the surface science is the lack of in situ characterization sensitive enough to detect adsorbates, essential in developing reaction mechanisms. To this end, the investigations presented in this dissertation are devoted to the elucidation of a fundamental understanding of the surface chemistry. They combine in situ Fourier-transform infrared spectroscopy (FTIR) using both transmission and refection geometries to investigate the surface reaction mechanisms involved during growth of plasma enhanced ALD of silicon nitride and the etching of Cu₂O thin dielectric films at temperatures <300 C.Item Influence of Growth Temperature on Bulk and Surface Defects in Hybrid Lead Halide Perovskite Films(Royal Society of Chemistry, 2015-12-14) Peng, Weina; Anand, Benoy; Liu, Lihong; Sampat, Siddharth; Bearden, Brandon E.; Malko, Anton V.; Chabal, Yves J.The rapid development of perovskite solar cells has focused its attention on defects in perovskites, which are gradually realized to strongly control the device performance. A fundamental understanding is therefore needed for further improvement in this field. Recent efforts have mainly focused on minimizing the surface defects and grain boundaries in thin films. Using time-resolved photoluminescence spectroscopy, we show that bulk defects in perovskite samples prepared using vapor assisted solution process (VASP) play a key role in addition to surface and grain boundary defects. The defect state density of samples prepared at 150 °C (~10¹⁷ cm⁻³) increases by 5 fold at 175 °C even though the average grains size increases slightly, ruling out grain boundary defects as the main mechanism for the observed differences in PL properties upon annealing. Upon surface passivation using water molecules, the PL intensity and lifetime of samples prepared at 200 °C are only partially improved, remaining significantly lower than those prepared at 150 °C. Thus, the present study indicates that the majority of these defect states observed at elevated growth temperatures originates from bulk defects and underscores the importance to control the formation of bulk defects together with grain boundary and surface defects to further improve the optoelectronic properties of perovskites.Item Interaction of Acid Gases SO₂ and NO₂ with Coordinatively Unsaturated Metal Organic Frameworks: M-MOF-74 (M = Zn, Mg, Ni, Co)(Amer Chemical Soc, 2017-05-01) Tan, Kui; Zuluaga, Sebastian; Wang, Hao; Canepa, Pieremanuele; Soliman, Karim; Cure, Jeremy; Li, Jing; Thonhauser, Timo; Chabal, Yves J.; 0000-0002-5167-7295 (Tan, K); 0000-0002-6435-0347 (Chabal, YJ); Tan, Kui; Cure, Jeremy; Chabal, Yves J.In situ infrared spectroscopy and ab initio density functional theory (DFT) calculations are combined to study the interaction of the corrosive gases SO₂ and NO₂ with metal organic frameworks M-MOF-74 (M = Zn, Mg, Ni, Co). We find that NO₂ dissociatively adsorbs into MOF-74 compounds, forming NO and NO₃̅. The mechanism is unraveled by considering the Zn-MOF-74 system, for which DFT calculations show that a strong NO₂-Zn bonding interaction induces a significant weakening of the N-O bond, facilitating the decomposition of the NO₂ molecules. In contrast, SO₂ is only molecularly adsorbed into MOF-74 with high binding energy (>90 kJ/mol for Mg-MOF-74 and >70 for Zn-MOF-74). This work gives insight into poisoning issues by minor components of flue gases in metal organic frameworks materials.Item Investigation of LiAlH 4-THF Formation by Direct Hydrogenation of Catalyzed Al and LiHLacina, D.; Yang, L.; Chopra, Irinder; Muckerman, J.; Chabal, Yves J.; Graetz, J.; 0000 0000 4239 3958 (Chabal, YJ); 89624105 (Chabal, YJ)The formation of LiAlH 4-THF by direct hydrogenation of Al and LiH in tetrahydrofuran (THF) was investigated using spectroscopic and computational methods. The molecular structures and free energies of the various possible adducts (THF-AlH 3, THF-LiH and THF-LiAlH 4) present in a LiAlH 4/THF solution were calculated and the dominant species were determined to be contact ion pairs where three THF molecules coordinate the lithium. Raman and X-ray absorption spectroscopy were used to investigate the effect of different Ti precursors on the formation of Al-H species and LiAlH 4-THF and determine the optimal reaction conditions. A unique sample stage was developed from a microfluidic cell to evaluate the catalysts in situ. The effectiveness of two types of catalysts, titanium chloride (TiCl 3) and titanium butoxide (Ti(C 4H 9O) 4), and the catalyst concentration were evaluated under similar reaction conditions. Both catalysts were effective at facilitating hydrogenation, although TiCl 3 was more effective over the first few cycles with the greatest kinetic enhancement achieved with a low concentration of around 0.15 mol%. These results were qualitatively supported by infrared spectroscopy, which indicated that although a small amount of Ti is necessary for disassociating H 2, excess surface Ti (>0.1 ML) hinders the formation of Al-H species. © the Owner Societies 2012.Item Lowering the Density of Electronic Defects on Organic-Functionalized Si(100) Surfaces(Amer Inst Physics) Peng, Weina; DeBenedetti, William J. I.; Kim, Seonjae; Hines, Melissa A.; Chabal, Yves J.; 0000 0000 4239 3958 (Chabal, YJ); 89624105 (Chabal, YJ)The electrical quality of functionalized, oxide-free silicon surfaces is critical for chemical sensing, photovoltaics, and molecular electronics applications. In contrast to Si/SiO₂ interfaces, the density of interface states (D-it) cannot be reduced by high temperature annealing because organic layers decompose above 300⁰C. While a reasonable D-it is achieved on functionalized atomically flat Si(111) surfaces, it has been challenging to develop successful chemical treatments for the technologically relevant Si(100) surfaces. We demonstrate here that recent advances in the chemical preparation of quasi-atomically-flat, H-terminated Si(100) surfaces lead to a marked suppression of electronic states of functionalized surfaces. Using a non-invasive conductance-voltage method to study functionalized Si(100) surfaces with varying roughness, a D-it as low as 2.5 x 10¹¹ cm⁻² eV⁻¹ is obtained for the quasi-atomically-flat surfaces, in contrast to > 7 x 10¹¹ cm⁻² eV⁻¹ on atomically rough Si(100) surfaces. The interfacial quality of the organic/quasi-atomically-flat Si(100) interface is very close to that obtained on organic/atomically flat Si(111) surfaces, opening the door to applications previously thought to be restricted to Si(111).Item Mechanistic Studies of Atomic Layer Deposition and Thermal Atomic Layer Etching Processes of Various Oxide Thin Films(2018-12) Rahman, Rezwanur; 0000-0002-8109-4787 (Rahman, R); Chabal, Yves J.; Quevedo-López, Manuel A.Atomic layer deposition (ALD) and atomic layer etching (ALE) will be the key techniques for sub10 nm node technology. Establishing a mechanistic understanding of the underlying surface chemistry is crucial for the optimization ALD/ALE processes and their use in microelectronics device fabrication. However, non-traditional reactions occurring concurrently with ALD/ALE complicate the deposition/etching process. Herein, several mechanisms are investigated for the ALD and thermal ALE of various oxide thin films. The first study demonstrates how the coreactant can affect the deposition process for TiO2 ALD from a cyclopentadienyl-based precursor. The next study highlights an important, but mostly overlooked, phenomenon of precursor/substrate reactivity during an ALD process. The third study describes the ALD of Sc2O3 thin films using a novel precursor with ozone. The final study reveals the limitations of thermal ALE processes for Al2O3 and SiO2 thin films, and proposes possible solutions to mitigate contamination issues inherent to the etching of these films. These investigations emphasize the need for surfacesensitive characterization techniques to unravel the complexities of these ALD/ALE processes.