Scanning Tunneling Microscopy
Scanning tunneling microscopy (STM) images a conductive surface by rastering a sharp tip close to it and recording the tunneling current (or the height needed to hold the current constant). The images reflect the local electronic density of states of both sample and tip, so the atomic structure and chemistry of the tip apex strongly affect resolution, stability and contrast. Scanning tunneling spectroscopy, in which the bias is swept at a fixed position, records current-voltage curves that can fingerprint adsorbed molecules. Related: Heteropolyacids, Negative differential resistance.
Heteropolyacid-functionalized tips (2002)
Song, Kitchin and Barteau attached H3PW12O40 molecules to a mechanically formed Pt/Ir tip by dipping it in a dilute aqueous solution and drying it. Attachment was confirmed by the appearance of the molecule’s NDR peak (about -1.15 V) in spectra taken on bare graphite. The modified tips gave atomic resolution images of graphite (2.46 Å) and molecular resolution images of H6P2W18O62 monolayers, which the authors attribute to tunneling through a single terminal W=O oxygen. About 20% of graphite images with these tips showed commensurate hexagonal Moiré superstructures (70.4 Å and 14.97 Å periods) that were never seen with bare tips, suggesting that the discrete electronic levels of the oxide cluster make the tip more sensitive to electronic perturbations of the surface, as had been reported for C60-functionalized tips (Song et al. 2002). Summary: song2002.
Sizing supported nanoparticles on TiO2(110) (2003)
Constant current STM at 300 K was used to follow Au and Mo nanoparticles on TiO2(110)-(1x2) through annealing. Particle location, area, equivalent diameter, fitted-ellipse axes, height and volume were extracted with Matlab routines that remove atomic steps by subtracting a background obtained from morphological opening and then threshold the particles; the authors note the results are uncorrected for tip convolution, that heights are underestimated, and that different background choices changed the measured properties by 10 to 20%. Integrated particle volumes showed the total Mo volume was roughly conserved up to about 800 K, and fitted ellipses revealed anisotropic Mo particles about one TiO2 row spacing (1.5 nm versus 1.33 nm) wide (Kitchin et al. 2003). Summary: kitchin2003a.
Growth of Ni films on Pt(111) (2003)
STM of Ni on Pt(111) near room temperature showed single-layer islands nucleating at step edges with sharp Ni-Pt step boundaries, multilayer growth before the first layer completes, and dislocation networks and superlattices attributed to the roughly 11% bond length mismatch. Annealing to 640 K, or depositing at 640 K, gave smooth surfaces with a long-range threefold line network, alongside a drop in the Ni/Pt Auger ratio that indicated Ni diffusion into the crystal (Kitchin et al. 2003). Summary: kitchin2003.
Imaging Pd sintering on TiO2(001) to explain sensor response (2003)
McCormick and coworkers used room-temperature constant-current STM (about +2 V, 0.1 nA) to follow thermally induced sintering of Pd on TiO2(001), with typical particle sizes growing from 1 to 6 nm as deposited to 5 to 12 nm after a 573 K anneal and 10 to 23 nm after 773 K. The STM sizes were paired with four-point probe resistivity measurements to correlate particle size with oxygen sensing performance (McCormick et al. 2003). Summary: mccormick2003.
Imaging isolated atoms in surface alloys (2009)
Low-temperature STM was used to image isolated Pd atoms in Cu(111) and Au(111) and the hydrogen islands that formed by spillover on Pd/Cu(111) (Tierney et al. 2009). Summary: tierney2009.