The interaction of nitrogen, oxygen, and hydrogen plasmas with spin‐coated arrays of colloidal cobalt–platinum particles was investigated with a large variety of microscopic and spectroscopic techniques. It could be demonstrated that the organic ligands of the nanoparticles can be completely removed. Yet, due to the short (∼1.6 nm) interparticle distances within the layers, strong degradation and sintering effects are observed after hydrogen and nitrogen plasma treatments. In the case of oxygen plasma, the shape and size of the individual particles are unaffected and can be preserved, even if a short hydrogen plasma is subsequently applied to reduce the particles back to their metallic state. Nevertheless, the mesoscopic order of the particle arrays is slightly decreased as observed by the breakup of larger ordered areas into smaller domains forming island–trench structures. Probing the surface chemistry of the particles with temperature programmed desorption, a rather complex surface chemistry is found to result from the plasma treatments. The first TPD spectrum after the cleaning process with oxygen and subsequent hydrogen plasmas reveals that the particles are loaded with adsorbed and implanted hydrogen. After removal of this hydrogen, subsequent TPD spectra using CO as a probe molecule, show broad signals between 190 and 360 K pointing to nonmetallic surface properties. While the platinum was found to be completely reduced, XPS measurements reveal a remaining fraction of oxidic cobalt species which are enriched at the surface. Thus, although the structure of the close‐packed Co–Pt nanoparticle arrays can be qualitatively preserved during plasma‐based ligand removal, the treatment leads to a complex materials system the chemical properties of which are influenced by the particle components, the substrate, and the plasma media.
A compact, versatile, and simple rf plasma source with capacitive coupling compatible to ultrahigh vacuum (UHV) requirements was designed and built to allow sequences of sample surface modification in plasma and surface preparation and analysis in vacuum without breaking the vacuum. The plasma source was operated at working pressures of less than 1 to a few millibars. Sample transfer to UHV was performed at pressures around 10−9mbar. For easy integration into an existing UHV setup, the sample recipient and transfer system were made to accept standard commercial sample holders. Preliminary experiments were performed by exposing monolayers of colloidal CoPt3 nanoparticles to oxygen and hydrogen plasmas. The structural and chemical effects of the plasma treatments were analyzed with scanning electron microscopy and x-ray photoelectron spectroscopy.
Miniaturization of electronic devices imposes challenges in terms of materials and production methods, and advances in the chemical vapor deposition ͑CVD͒ of metals are a key prerequisite toward reliable interconnects that are essential for their functionality. Electrically conducting ultrathin films of pure copper were grown on glass and silicon substrates starting at a temperature of 195°C. The growth kinetics does not exhibit any measurable nucleation time enabling early stage coalescence and high electrical conductivity. In situ monitoring of the CVD process using synchrotron-based mass spectrometry shows that the enhanced dehydrogenation of alcohols by copper II acetylacetonate precursor drives the Cu 0 deposition, which is kinetically favorable already at low temperature.The development of microelectronic devices faces several challenges regarding miniaturization and increased degree of integration, which is in part limited by the metal interconnects. 1 Copper remains a subject of intense development as an interconnect material 2-4 because of its remarkably low bulk electrical resistivity and resistance to electromigration. A highly conformal process, such as chemical vapor deposition ͑CVD͒, is particularly well adapted to overcome the deficiency in conformality of physical vapor deposition. However, CVD features are perceived to include complex nucleation kinetics on semiconducting surfaces and lack of morphology control for ultrathin films, in addition to the toxicity, limited availability, and laborious handling of successful precursors. These limitations are approached either by influencing the surface chemistry via the introduction of other families of CVD and atomic layer deposition ͑ALD͒ precursors 5 or by the development of deposition techniques, such as chemical fluid deposition ͑CFD͒. 6 Also, a two-step CVD process 4 was proposed to overcome the high readiness of copper atoms to diffuse on the surface, an effect which was demonstrated by ab initio molecular dynamics simulation. 3 Our recent efforts, taking advantage of pulsed liquid delivery ͓pulsed-spray evaporation-chemical vapor deposition ͑PSE-CVD͔͒ of the reactants, show that the use of alcohol as a unique coreactant is more efficient and attractive than the hydrogen reduction route for several transition metals. 7-10 Indeed, the utilization of alcohol as an additive in the CVD process was reported to enhance the growth kinetics, 11-13 yet the presence of hydrogen as a reducing agent was considered necessary. In contrast to CVD, the presence of hydrogen in addition to alcohol was not required to attain metallic thin films by CFD. 14 However, Cu-CFD requires the presence of catalytic surfaces ͑cobalt and nickel͒, and deposition temperatures are 100°C higher than those needed with H 2 reduction and thus less attractive.Our previous work on the growth of copper by CVD, which was performed using copper acetylacetonate ͓Cu͑acac͒ 2 ͔ and methanol, shows that smooth and polycrystalline copper films can be grown above 280°C. 9,15 In the present paper...
Among the multitude of colloidal nanoparticles, bimetallic systems have been increasingly attracting interest due their tunable physical (e.g. magnetism) and chemical (e.g. catalytic) properties. Oxidation-resistant magnetic bimetallic particles can be self-assembled into ordered layers to become the building blocks for a future generation of data storage devices [1] or enable the retrieval and recycling of catalytically active organic molecules in a chemical reactor.[2] On the other hand, the composition of such nanoparticles can be chosen in a way that the particles themselves show an increased activity and selectivity for catalytic processes [3,4] due to their bimetallic nature. Potential applications also include the growing field of biotechnology where functionalized magnetic nanoparticles could enable the field-guided localized delivery of pharmaceutical agents [5] or the hyperthermal treatment of tumors [6] or provide means for an efficient and highly sensitive separation of biomolecules. [7,6] Many characteristics of the particles, such as solubility, biocompatibility, adhesive properties and chemical functionality, which are crucial for most of these applications, are determined by the shell of ligand molecules bound to their surface during preparation. To selectively tune the chemical properties, ligand exchange strategies have been developed in the recent years allowing the addition of new functionalities to the nanoparticles. [8][9][10] As compared to monometallic particles, ligand exchange on bimetallic systems is more challenging as complications can arise due to different binding affinities between the ligand molecules and the two metals, possibly not only resulting in changes in particle diameter and size-distribution but also in stoichiometry and shape by selective dissolution of one metal component. Previous studies report only small changes in morphology and composition when for example, exchanging nonpolar ligands to polar ligands. [11][12][13][14][15][16] Little attention, however, has been paid to a systematic investigation of all possible effects taking place during the exchange process. Focusing on thiol ligands, which are particularly versatile and widespread in colloidal chemistry, we show for cobalt-platinum nanoparticles that the thiol ligands interact predominantly with one component resulting in composition and morphology changes after long exposure times. At the same time, we can show that the original amine ligands are not completely removed.The alloy particles chosen for the study represent a wellcharacterized [17][18][19][21][22] system with a narrow selectable size distribution in which the magnetic cobalt component is passivated against oxidation through the addition of platinum, resulting in particles with potential use in magnetic and catalytic applications. In their as-prepared state, these particles are enveloped by a binary ligand system of 1-adamantanecarboxylic acid (1-ACA) and hexadecylamine (HDA). Due to their strong tendency for binding to cobalt, [23] platinum [24]...
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