No abstract

The need for the conformal deposition of TiO 2 thin films in device fabrication has motivated a search for thermally robust titania precursors with noncorrosive byproducts. Alkylamido-cyclopentadienyl precursors are attractive because they are readily oxidized, yet stable, and afford environmentally mild byproducts. We have explored the deposition of TiO 2 films on OH-terminated SiO 2 surfaces by in situ Fourier transform infrared spectroscopy using a novel titanium precursor [(EtCp)-Ti(NMe 2 ) 3 (1), Et = CH 2 CH 3 ] with either ozone or water. This precursor initially reacts with surface hydroxyl groups at ≥150 °C through the loss of its NMe 2 groups. However, once the precursor is chemisorbed, its subsequent reactivities toward ozone and water are very different. There is a clear reaction with ozone, characterized by the formation of monodentate formate and/or chelate bidentate carbonate surface species; in contrast, there is no detectable reaction with water. For the ozone-based ALD process, the surface formate/carbonate species react with the NMe 2 groups during the subsequent pulse of 1, forming TiOTi bonds. Ligand exchange is observed within the 250−300 °C ALD window. X-ray photoelectron spectroscopy confirms the deposition of stoichiometric TiO 2 films with no detectable impurities. For the water-based process, ligand exchange is not observed. Once 1 is adsorbed, there is no spectroscopic evidence for further reaction. However, there is still TiO 2 deposition under typical ALD conditions. Co-adsorption experiments with controlled vapor pressures of water and 1 indicate that deposition arises solely from 1/water gas-phase reactions. This striking lack of reactivity between chemisorbed 1 and water is attributed to the electronic and steric effects of the EtCp group and facilitates the observation of gas-phase reactions.

Coadsorption of multicomponents in metal-organic framework (MOF) materials can lead to a number of cooperative effects, such as modification of adsorption sites or during transport. In this work, we explore the incorporation of NH and HO into MOFs preloaded with small molecules such as CO, CO, and SO. We find that NH (or HO) first displaces a certain amount of preadsorbed molecules in the outer portion of MOF crystallites, and then substantially hinders diffusion. Combining in situ spectroscopy with first-principles calculations, we show that hydrogen bonding between NH (or HO) is responsible for an increase of a factor of 7 and 8 in diffusion barrier of CO and CO through the MOF channels. Understanding such cooperative effects is important for designing new strategies to enhance adsorption in nanoporous materials.

The mechanical properties of graphene-cellulose (GC) nanocomposites are investigated using molecular dynamic (MD) simulations in this work. The influences of graphene concentrations, aspect ratios, and agglomeration on elastic constants and interfacial properties are reported. A polymer consistent force field (pcff) was used in the analysis. The GC nanocomposites system underwent NVT (constant number of atoms, volume, and temperature) and NPT (constant number of atoms, pressure, and temperature) ensemble with an applied uniform strain during the MD simulations. The stress-strain responses were evaluated for both randomly dispersed and stacked GC unit cell in order to study the effects of graphene concentrations, aspect ratio, and agglomeration on Young's modulus. The results indicate that Young's modulus of neat cellulose may be enhanced by incorporating graphene in the GC nanocomposites. It is observed that dispersed graphene shows a comparatively higher Young's modulus than the same with agglomerated graphene. The cohesive and pullout forces versus displacement data are reported under normal and shear modes. It is seen that both cohesive and pullout forces are enhanced for GC specimens with higher graphene aspect ratios due to enlarged surface/interfacial area. The MD simulation results show reasonable agreement with available experimental data.

Amorphous epoxy is considered for investigating the role of graphene in enhancing elastic stiffness of polymers. Graphene is incorporated in the amorphous epoxy in order to develop graphene-epoxy systems. The mechanical properties of crosslinked graphene-epoxy (G-Ep) nanocomposites have been investigated using molecular mechanics (MM) and molecular dynamics (MD) simulations. The influences of graphene nanoplatelet weight concentrations, aspect ratios, and dispersion on elastic constants were studied. Both randomly oriented and stacked graphene-epoxy nanocomposites were considered. A polymer consistent force field (pcff) was used in the analysis. The G-Ep nanocomposites system underwent MD equilibration followed by uniform deformation. The stress-strain responses were evaluated in order to determine Young's modulus. MM simulation was also used to calculate the Young's modulus and shear modulus at 0 K. The results from MD and MM simulation showed reasonable improvement in Young's modulus and shear modulus for G-Ep system in comparison to neat epoxy resin. The graphene concentrations in the range of 1%-3% and graphene with high aspect ratio are seen to improve the Young's modulus by 82% approximately. The results from the simulations were compared with the results from micromechanics based analysis and nanoindentation tests. It was observed from both the atomistic scale simulation and nanoindentation tests that incorporation of graphene in neat epoxy at low weight concentration improves the elastic properties. Using similar MD scheme, it was also seen that the dispersed graphene-epoxy system possesses enhanced in-plane elastic modulus compared to the agglomerated graphene-epoxy system.

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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 (CHF 3 ) with TMA for the thermal ALE of SiO 2 and Al 2 O 3 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 CHF 3 reacts with TMA-treated oxide surfaces, etching is hindered by the concurrent deposition of a fluorinecontaining 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 Al 2 O 3 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.

Quasi-optical (QO) methods of dielectric spectroscopy are well established in the millimeter and submillimeter frequency bands. These methods exploit standing wave structure in the sample produced by a transmitted Gaussian beam to achieve accurate, low-noise measurement of the complex permittivity of the sample [e.g., J. A. Scales and M. Batzle, Appl. Phys. Lett. 88, 062906 (2006); R. N. Clarke and C. B. Rosenberg, J. Phys. E 15, 9 (1982); T. M. Hirovnen, P. Vainikainen, A. Lozowski, and A. V. Raisanen, IEEE Trans. Instrum. Meas. 45, 780 (1996)]. In effect the sample itself becomes a low-Q cavity. On the other hand, for optically thin samples (films of thickness much less than a wavelength) or extremely low loss samples (loss tangents below 10(-5)) the QO approach tends to break down due to loss of signal. In such a case it is useful to put the sample in a high-Q cavity and measure the perturbation of the cavity modes. Provided that the average mode frequency divided by the shift in mode frequency is less than the Q (quality factor) of the mode, then the perturbation should be resolvable. Cavity perturbation techniques are not new, but there are technological difficulties in working in the millimeter/submillimeter wave region. In this paper we will show applications of cavity perturbation to the dielectric characterization of semi-conductor thin films of the type used in the manufacture of photovoltaics in the 100 and 350 GHz range. We measured the complex optical constants of hot-wire chemical deposition grown 1-μm thick amorphous silicon (a-Si:H) film on borosilicate glass substrate. The real part of the refractive index and dielectric constant of the glass-substrate varies from frequency-independent to linearly frequency-dependent. We also see power-law behavior of the frequency-dependent optical conductivity from 316 GHz (9.48 cm(-1)) down to 104 GHz (3.12 cm(-1)).