Multi-metallic nanoparticles constitute a new class of materials offering the opportunity to tune the properties via the composition, atomic ordering and size. In particular, supported bimetallic nanoparticles have generated intense interest in catalysis and electrocatalysis. However, traditional synthesis methods often lack precise control, yielding a mixture of monometallic and bimetallic particles with various compositions. Here we report a general strategy for synthesizing supported bimetallic nanoparticles by atomic layer deposition, where monometallic nanoparticle formation is avoided by selectively growing the secondary metal on the primary metal nanoparticle but not on the support; meanwhile, the size, composition and structure of the bimetallic nanoparticles are precisely controlled by tailoring the precursor pulse sequence. Such exquisite control is clearly demonstrated through in situ Fourier transform infrared spectroscopy of CO chemisorption by mapping the gradual atomic-scale evolution in the surface composition, and further confirmed using aberration-corrected scanning transmission electron microscopy.

Hydrothermal multiwall closed carbon nanotubes are shown to contain an encapsulated multiphase aqueous fluid, thus offering an attractive test platform for unique in situ nanofluidic experiments in the vacuum of a transmission electron microscope. The excellent wettability of the graphitic inner tube walls by the aqueous liquid and the mobility of this liquid in the nanotube channels are observed. Complex interface dynamic behavior is induced by means of electron irradiation. Strong atomic-scale interactions between the entrapped liquid phase and the wetted terminated graphite layers are revealed by means of high-resolution electron microscopy. The documented phenomena in this study demonstrate the potential of implementing such tubes in future nanofluidic devices.

Sequential infiltration synthesis (SIS) has been recently demonstrated to increase the etch resistance of optical, e-beam, and block copolymer lithography resists for sub-50 nm pattern transfer. Although SIS can dramatically enhance pattern transfer relevant to device applications, the complex processes involved in SIS are not clearly understood. Fundamental knowledge of the chemistry underlying SIS is necessary to ensure a high degree of perfection in large-scale lithography. To this end, we performed in situ Fourier transform infrared (FTIR) spectroscopic measurements during the SIS of Al2O3 using trimethylaluminum (TMA) and H2O into poly­(methyl methacrylate) (PMMA). The FTIR results revealed that TMA reacts quickly with PMMA to form an unstable complex. The subsequent conversion of this intermediate complex into stable AlO linkages is slow and must compete with rapid TMA desorption. We support this interpretation of the FTIR data using density functional theory to calculate plausible structures for the unstable TMA–PMMA complex and the covalently linked species. As a consequence of this two-step reaction between TMA and PMMA, the detailed history of the TMA exposure becomes critical to achieving reliable patterns in SIS lithography. We demonstrate this using scanning electron microscopy to image the patterns resulting from SIS treatment of block copolymer films under different TMA exposure conditions. This better understanding of the SIS reaction dynamics should improve reliability in SIS lithography as well as other SIS applications.

Atomic layer deposition (ALD) of Al2O3 using trimethylaluminum (TMA) and water on Pd nanoparticles (NPs) was studied by combining in situ quartz crystal microbalance (QCM) measurements, in situ quadrupole mass spectrometry (QMS), and transmission electron microscopy (TEM) with density functional theory (DFT) calculations. TEM images of the ALD Al2O3 overcoated Pd showed conformal Al2O3 films on the Pd NPs as expected for ALD. However, hydrogen detected by in situ QMS during the water pulses suggested that the ALD Al2O3 films on the Pd NPs were porous rather than being continuous coatings. Additional in situ QCM and QMS measurements indicated that Al2O3 ALD on Pd NPs proceeds by a self-poisoning, self-cleaning process. To evaluate this possibility, DFT calculations were performed on Pd(111) and Pd(211) as idealized Pd NP surfaces. These calculations determined that the TMA and water reactions are thermodynamically favored on the stepped Pd(211) surface, consistent with previous observations. Furthermore, the DFT studies identified methylaluminum (AlCH3*, where the asterisk designates a surface species) as the most stable intermediate on Pd surfaces following the TMA exposures, and that AlCH3* transforms into Al(OH)3* species during the subsequent water pulse. The gas phase products observed using in situ QMS support this TMA dissociation/hydration mechanism. Taken together, the DFT and experimental results suggest a process in which the Pd surface becomes poisoned by adsorbed CH3* species during the TMA exposures that prevent the formation of a complete monolayer of adsorbed Al species. During the subsequent H2O exposures, the Pd surface is cleaned of CH3* species, and the net result is a porous Al2O3 film. This porous structure can retain the catalytic activity of the Pd NPs by providing reagent gases with access to the Pd surface sites, suggesting a promising route to stabilize active Pd catalysts.