Atomically resolved force microscopy images of complex surface unit cells: Ultrathin alumina film on NiAl(110)

Abstract

In this Brief Report we present atomically resolved images of the ultrathin alumina film on NiAl͑110͒. For the first time detailed images of the complex microstructure for both reflection domains have been obtained by frequency modulation dynamic force microscopy using a very stable, custom built, dual mode scanning force and scanning tunneling microscope. Measurements have been performed under ultrahigh vacuum conditions at 5 K with a quartz tuning fork as a force sensor. The high spatial resolution allows to derive 28 atomic positions in real space in the surface unit cell by simple graphical analysis. This has been successful without application of filtering or correlation methods, emphasizing the potential of this force microscopy method on complex oxide surfaces. With respect to topographic height even quantitative agreement with theory could be achieved, here shown for selected structural elements within the unit cell. Furthermore, deeper insight into a wavelike morphological feature could be obtained. Consistency with a published density-functional theory model and connections to other data from the literature are discussed.Ultrathin alumina overlayers on certain low index faces of metal or alloy single crystals, originally developed to mimic properties of bulk aluminum oxide in surface studies, where charging hampered analysis with electron techniques, have long become a research field in their own right. This is still due to the full set of electron analysis techniques applied to them, but besides this the thin films can be linked to technological problems regarding interfaces and insulating interlayers in miniaturized electronic and magnetic devices as well as protective coatings and heterogeneous catalysis. Of academic interest are fundamental physical and chemical properties of these wide band-gap insulators, in particular the relation between structure and functions such as adsorption as well as transport and interface properties. One example for such a film is the 5 Å thin ordered alumina overlayer which can be grown by selective oxidation on ͑110͒ surfaces ͑CsCl structure͒ of the ordered intermetallic NiAl. 1 Despite its large band gap of about 6.7 eV 2 it allows electron flow due to its limited thickness. Thorough low-energy electron diffraction ͑LEED͒, photoemission spectroscopy, Auger electron spectroscopy, electron-energy-loss spectroscopy, angle-resolved ultraviolet photoelectron spectroscopy, ion scattering spectroscopy, and scanning tunneling microscopy ͑STM͒ and scanning tunneling spectroscopy studies 1-10 have been performed in the framework of model catalysis over the last two decades. Together with a recent STM and density-functional theory ͑DFT͒ study this led to an atomistic model of the film that matches many of the experimental findings. 11 The accumulated data on this film in the literature give so far evidence for a two double-layer structure free of nickel with alternating aluminum and oxygen layers. It is oxygen terminated toward the vacuum with the next lower aluminum layer bein...

Higher-order eigenmodes of qPlus sensors for high resolution dynamic atomic force microscopy

“…The "qPlus sensor," a sensor built from a quartz tuning fork where one prong is fixed to some substrate and the other to which a tip is attached serves as a self-sensing cantilever, 1-4 has raised increased interest in the past years. While the first applications of qPlus showed an increased spatial resolution 5,6 and simultaneous scanning tunneling microscopy ͑STM͒ and atomic force microscopy ͑AFM͒ operation, 6,7 later applications demonstrated the capability to measure the forces that act in atomic manipulation on a piconewton scale, 8 the threedimensional distribution of short range chemical forces on graphite, 9 surface properties of oxides, 10 the measurement of single electronic charges on single atoms, 11 and the resolving of the full structure of an organic molecule that is weakly adsorbed to a surface. 12 While all these experiments were performed at the fundamental flexural eigenmode of the qPlus sensor, recently 300 kHz operation at the second flexural eigenmode on a modified qPlus sensor 13 was reported at Omicron Nanotechnology.…”

Imaging of individual adatoms on oxide surfaces by dynamic force microscopy

“…Here O S and Al S sites at the surface cannot only be distinguished by their number ͑O S :28,Al S :24͒, but also by their lateral and vertical arrangement. 9 Hence the imaged protrusions are clearly identified as surface oxygen sites. The ⌬f set point used to acquire Fig.…”

“…It accurately reproduces the film topography and allows clear identification of the surface oxygen sublattice at high lateral resolution. 9,10 With this, adsorption sites of single Au adatoms can be determined with substrate site precision and in principle this can be done for any imageable adsorbed species. Considering the capabilities of FM-DFM on bulk insulators 11,12 this will enable adatom studies on oxides with spatial resolution equivalent or even surpassing that of scanning tunneling microscopy ͑STM͒.…”

“…1͑a͒ which could be shown to represent the topography of the terminating oxygen layer of the alumina film on NiAl͑110͒ with high accuracy. 9,10 For the determination of individual adsorption sites the position and chemical nature of surface species needs to be clearly identifiable from the obtained images. Here O S and Al S sites at the surface cannot only be distinguished by their number ͑O S :28,Al S :24͒, but also by their lateral and vertical arrangement.…”

Towards an atomic level understanding of niobia based catalysts and catalysis by combining the science of catalysis with surface science

“…Its structure and properties have been the object of intense studies across the world. Finally, Scanning Tunneling Microscopy (STM) and Atomic Force Microscopy (AFM) (Simon et al 2008) images with atomic resolution have been published, and density functional calculations (Kresse et al 2005) have allowed us to understand the film at the atomic level including its defect structure, which turns out to be particularly relevant when it comes to anchoring of metal nanoparticles (Schmid et al 2006). Figure 7 shows atomic resolution of STM and AFM images together with the structural model as deduced from Density Functional Theory (DFT) calculations at areas of the film that do not contain defects.…”