RESUMO
Alane (AlH3 ) is a unique energetic material that has not found a broad practical use for over 70 years because it is difficult to synthesize directly from its elements. Using density functional theory, we examine the defect-mediated formation of alane monomers on Al(111) in a two-step process: (1)â dissociative adsorption of H2 and (2)â alane formation, which are both endothermic on a clean surface. Only with Ti dopant to facilitate H2 dissociation and vacancies to provide Al adatoms, both processes become exothermic. In agreement, in situ scanning tunneling microscopy showed that during H2 exposure, alane monomers and clusters form primarily in the vicinity of Al vacancies and Ti atoms. Moreover, ball milling of the Al samples with Ti (providing necessary defects) showed a 10 % conversion of Al into AlH3 or closely related species at 344â bar H2 , indicating that the predicted pathway may lead to the direct synthesis of alane from elements at pressures much lower than the 10(4) â bar expected from bulk thermodynamics.
Assuntos
Compostos de Alumínio/química , Alumínio/química , Técnicas de Química Sintética/métodos , Modelos Moleculares , Conformação Molecular , Propriedades de SuperfícieRESUMO
Sub-5 nm metallic hafnium diboride (HfB(2)) nanostructures were directly written onto Si(100)-2 × 1:H surfaces using ultrahigh vacuum scanning tunneling microscope (UHV-STM) electron beam induced deposition (EBID) of a carbon-free precursor molecule, tetrakis(tetrahydroborato)hafnium, Hf(BH(4))(4). Scanning tunneling spectroscopy data confirm the metallic nature of the HfB(2) nanostructures, which have been written down to lateral dimensions of â¼2.5 nm. To our knowledge, this is the first demonstration of sub-5 nm metallic nanostructures in an STM-EBID experiment.