Erik Jonsson School of Engineering and Computer Science
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Browsing Erik Jonsson School of Engineering and Computer Science by Author "0000 0000 4239 3958 (Chabal, YJ)"
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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 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 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 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 Novel Binder-Free Electrode Materials for Supercapacitors Utilizing High Surface Area Carbon Nanofibers Derived from Immiscible Polymer Blends of PBI/6FDA-DAM:DABA(Royal Society of Chemistry, 2018-06-01) Abeykoon, Nimali C.; Garcia, Velia; Jayawickramage, Rangana A.; Perera, Wijayantha; Cure, Jeremy; Chabal, Yves J.; Balkus, Kenneth J.; Ferraris, John P.; 0000 0000 4239 3958 (Chabal, YJ); 0000-0002-3225-0093 (Ferraris, JP); Abeykoon, Nimali C.; Garcia, Velia; Jayawickramage, Rangana A.; Perera, Wijayantha; Cure, Jeremy; Chabal, Yves J.; Balkus, Kenneth J.; Ferraris, John P.Carbon nanofibers with high surface area have become promising electrode materials for supercapacitors because of their importance in increasing energy density. In this study, a high free volume polymer, 6FDA-DAM:DABA (6FDD) was blended with polybenzimidazole (PBI) in different ratios to obtain different compositions of PBI/6FDD immiscible polymer blends. Freestanding nanofiber mats were obtained via electrospinning using blend precursors dissolved in N,N-dimethylacetamide (DMAc). Subsequently, carbonization, followed by CO₂ activation at 1000 °C was applied to convert the fiber mats into porous carbon nanofibers (CNFs). The addition of 6FDD shows significant effects on the microstructure and enhancement of the surface area of the CNFs. The obtained CNFs show specific surface area as high as 3010 m² g⁻¹ with pore sizes comparable to those of the electrolyte ions (PYR₁₄TFSI). This provides good electrolyte accessibility to the pore of the carbon materials resulting in enhanced energy density compared to the CNFs obtained from pure PBI. Electrodes derived from PBI:6FDD (70 : 30) exhibited outstanding supercapacitor performance in coin cells with a specific capacitance of 142 F g⁻¹ at the scan rate of 10 mV s⁻¹ and energy density of 67.5 W h kg⁻¹ at 1 A g⁻¹ (58 W h kg⁻¹ at 10 A g⁻¹) thus demonstrating promising electrochemical performance for high performance energy storage system.Item Selectivity of Metal Oxide Atomic Layer Deposition on Hydrogen Terminated and Oxidized Si(001)-(2x1) Surface(A V S Amer Inst Physics, 2014-02-10) Longo, Roberto C.; McDonnell, Stephen; Dick, D.; Wallace, Robert M.; Chabal, Yves J.; Owen, James H. G.; Ballard, Josh B.; Randall, John N.; Cho, Kyeongjae; 0000 0000 4239 3958 (Chabal, YJ); 89624105 (Chabal, YJ)In this work, the authors used density-functional theory methods and x-ray photoelectron spectroscopy to study the chemical composition and growth rate of HfO₂, Al₂O₃, and TiO₂ thin films grown by in-situ atomic layer deposition on both oxidized and hydrogen-terminated Si(001) surfaces. The growth rate of all films is found to be lower on hydrogen-terminated Si with respect to the oxidized Si surface. However, the degree of selectivity is found to be dependent of the deposition material. TiO₂ is found to be highly selective with depositions on the hydrogen terminated silicon having growth rates up to 180 times lower than those on oxidized Si, while similar depositions of HfO₂ and Al₂O₃ resulted in growth rates more than half that on oxidized silicon. By means of density-functional theory methods, the authors elucidate the origin of the different growth rates obtained for the three different precursors, from both energetic and kinetic points of view.Item Structure Matters: Correlating Temperature Dependent Electrical Transport Through Alkyl Monolayers With Vibrational And Photoelectron SpectroscopiesShpaisman, Hagay; Seitz, Oliver; Yaffe, Omer; Roodenko, Ecatherina (Katy); Scheres, Luc; Zuilhof, Han; Chabal, Yves J.; Sueyoshi, Tomoki; Kera, Satoshi; Ueno, Nobuo; Vilan, Ayelet; Cahen, David; 0000 0000 4239 3958 (Chabal, YJ); 89624105 (Chabal, YJ)Freezing out of molecular motion and increased molecular tilt enhance the efficiency of electron transport through alkyl chain monolayers that are directly chemically bound to oxide-free Si. As a result, the current across such monolayers increases as the temperature decreases from room temperature to similar to 80 K, i.e., opposite to thermally activated transport such as hopping or semiconductor transport. The 30-fold change for transport through an 18-carbon long alkyl monolayer is several times the resistance change for actual metals over this range. FTIR vibrational spectroscopic measurements indicate that cooling increases the packing density and reduces the motional freedom of the alkyl chains by first stretching the chains and then gradually tilting the adsorbed molecules away from the surface normal. Ultraviolet photoelectron spectroscopy shows drastic sharpening of the valence band structure as the temperature decreases, which we ascribe to decreased electron-phonon coupling. Although conformational changes are typical in soft molecular systems, in molecular electronics they are rarely observed experimentally or considered theoretically. Our findings, though, indicate that the molecular conformational changes are a prominent feature, which imply behavior that differs qualitatively from that described by models of electronic transport through inorganic mesoscopic solids. © 2012 The Royal Society of ChemistryItem Thermal Atomic Layer Etching of Silica and Alumina Thin Films Using Trimethylaluminum with Hydrogen Fluoride or Fluoroform(American Chemical Society) Rahman, Razwanur; Mattson, Eric C.; Klesko, Joseph P.; Dangerfield, Aaron; Rivillon-Amy, S.; Smith, D. C.; Hausmann, D.; Chabal, Yves J.; 0000 0000 4239 3958 (Chabal, YJ); 0000-0002-8109-4787 (Rahman, R); 0000-0002-0755-2583 (Mattson, EC); 0000-0003-3989-8009 (Klesko, JP); 0000-0002-6435-0347 (Chabal, YJ); Rahman, Razwanur; Mattson, Eric C.; Klesko, Joseph P.; Dangerfield, Aaron; Chabal, Yves J.Thermal atomic layer etching (ALE) is an emerging technique that involves the sequential removal of monolayers of a film by alternating self-limiting reactions, some of which generate volatile products. Although traditional ALE processes rely on the use of plasma, several thermal ALE processes have recently been developed using hydrogen fluoride (HF) with precursors such as trimethylaluminum (TMA) or tin acetylacetonate. While HF is currently the most effective reagent for ALE, its potential hazards and corrosive nature have motivated searches for alternative chemicals. Herein, we investigate the feasibility of using fluoroform (CHF₃) with TMA for the thermal ALE of SiO₂ and Al₂O₃ surfaces and compare it to the established TMA/HF process. A fundamental mechanistic understanding is derived by combining in situ Fourier transform infrared spectroscopy, ex situ X-ray photoemission spectroscopy, ex situ low-energy ion scattering, and ex situ spectroscopic ellipsometry. Specifically, we determine the role of TMA, the dependence of the etch rate on precursor gas pressure, and the formation of a residual fluoride layer. Although CHF₃ reacts with TMA-treated oxide surfaces, etching is hindered by the concurrent deposition of a fluorine-containing layer, which makes it unfavorable for etching. Moreover, since fluorine contamination can be deleterious to device performance and its presence in thin films is an inherent problem for established ALE processes using HF, we present a novel method to remove the residual fluorine accumulated during the ALE process by exposure to water vapor. XPS analysis herein reveals that an Al₂O₃ film etched using TMA/HF at 325 °C contains 25.4 at. % fluorine in the surface region. In situ exposure of this film to water vapor at 325 °C results in 90% removal of the fluorine. This simple approach for fluorine removal can easily be applied to ALE-treated films to mitigate contamination and retain surface stoichiometry. ©2018 American Chemical Society.Item Understanding and Controlling Water Stability of MOF-74(Royal Society of Chemistry, 2018-06-01) Zuluaga, S.; Fuentes-Fernandez, Erika M. A.; Tan, Kui; Xu, F.; Li, J.; Chabal, Yves J.; Thonhauser, T.; 0000 0000 4239 3958 (Chabal, YJ); Fuentes-Fernandez, Erika M. A.; Tan, Kui; Chabal, Yves J.Metal organic framework (MOF) materials in general, and MOF-74 in particular, have promising properties for many technologically important processes. However, their instability under humid conditions severely restricts practical use. We show that this instability and the accompanying reduction of the CO2 uptake capacity of MOF-74 under humid conditions originate in the water dissociation reaction H2O → OH + H at the metal centers. After this dissociation, the OH groups coordinate to the metal centers, explaining the reduction in the MOF's CO2 uptake capacity. This reduction thus strongly depends on the catalytic activity of MOF-74 towards the water dissociation reaction. We further show that - while the water molecules themselves only have a negligible effect on the crystal structure of MOF-74 - the OH and H products of the dissociation reaction significantly weaken the MOF framework and lead to the observed crystal structure breakdown. With this knowledge, we propose a way to suppress this particular reaction by modifying the MOF-74 structure to increase the water dissociation energy barrier and thus control the stability of the system under humid conditions.Item Water Interactions In Metal Organic Frameworks(Royal Society of Chemistry) Tan, Kui; Nijem, Nour; Gao, Yuzhi; Zuluaga, S.; Li, J.; Thonhauser, T.; Chabal, Yves J.; 0000 0000 4239 3958 (Chabal, YJ); 89624105 (Chabal, YJ); A-5998-2011 (Chabal, YJ)Metal organic frameworks (MOFs) have a strong potential for gas adsorption and separation such as H2 and CH4 storage, and CO2 capture. However, their instability in the presence of water vapor (many MOFs are hygroscopic) is one of the key issues that limit their large-scale application. Previous studies of water adsorption in MOFs have mainly relied on isotherm measurements that provide useful parameters such as adsorption uptake and isosteric heat of adsorption. The structural stability of MOFs in water vapor was also evaluated by powder X-ray diffraction measurements (PXRD). However, more studies are required to unravel the water interaction or reaction mechanisms within MOFs, which would be beneficial for the development of more robust frameworks. This review highlight focuses on water adsorption in two representative MOFs: M(bdc)(ted)0.5 [M = Cu2+, Zn2+, Ni2+, Co2+; bdc = 1,4-benzenedicarboxylate; ted = triethylenediamine] with saturated metal centers and MOF-74 [M2(dobdc), M = Mg2+, Zn2+, Ni2+, Co2+ and dobdc = 2,5-dihydroxybenzenedicarboxylic acid] with unsaturated metal centers. It shows how vibrational spectroscopy combined with van der Waals density functional (vdW-DF) calculations makes it possible to elucidate the details of water reaction in MOFs. The results presented in this highlight suggest that the reactivity and initial decomposition pathway of MOFs in water vapor critically depend on their structure and the specific metal cation in the building units. Water interaction with a hydrophobic MOF, in this case FMOF-1, is also reviewed. This information provides a framework for understanding water interactions or reactions within different types of MOFs.Item Wet chemical surface functionalization of oxide-free silicon(Elsevier Limited, 2012-09-13) Thissen, Peter; Seitz, Oliver; Chabal, Yves J.; 0000 0000 4239 3958 (Chabal, YJ); 89624105 (Chabal, YJ)Silicon is by far the most important semiconductor material in the microelectronic industry mostly due to the high quality of the Si/SiO2 interface. Consequently, applications requiring chemical functionalization of Si substrates have focused on molecular grafting of SiO2 surfaces. Unfortunately, there are practical problems affecting homogeneity and stability of many organic layers grafted on silicon oxide (SiO2), such as silanes and phosphonates, related to polymerization and hydrolysis of Si-O-Si and Si-O-P bonds. These issues have stimulated efforts in grafting functional molecules on oxide-free Si surfaces, mostly with wet chemical processes. This review focuses therefore directly on wet-chemical surface functionalization of oxide-free Si surfaces, starting from H-terminated Si surfaces. The main preparation methods of oxide-free H-terminated Si and their stability are first summarized. Functionalization is then classified into indirect substitution of H-termination by functional organic molecules, such as hydrosilylation, and direct substitution by other atoms (e.g. halogens) or small functional groups (e.g. OH, NH2) that can be used for further reaction. An emphasis is placed on a recently discovered method to produce a nanopattern of functional groups on otherwise oxide-free, H-terminated and atomically flat Si(111) surfaces. Such model surfaces are particularly interesting because they make it possible to derive fundamental knowledge of surface chemical reactions.