Chemical imaging of insulators by STM

Abstract

Nanostructures of CaF 2 and CaF 1 on Si͑111͒ are used to demonstrate a chemical imaging method for insulators. Chemical sensitivity is achieved in scanning tunneling microscopy via a sharp drop of the tunneling current for bias voltages below the conduction-band minimum. This imaging method has a spatial resolution of better than 1 nm and distinguishes different oxidation states. A resonance is found in (dI/dV)/(I/V) at the conduction-band minimum that enables an accurate determination of its position. We observe enhancements by up to a factor of 5 and absolute values in the range of 20-50, compared to 1 for an Ohmic metal. A minimal model is given, explaining the resonance in terms of tunneling across a thin insulator film. These methods should be generally applicable for determining local Schottky barriers and band offsets in nanostructures and for chemically selective imaging of insulators and wide-gap semiconductors. ͓S0163-1829͑99͒07215-X͔ I. CHEMICAL IMAGING IN SCANNING TUNNELING MICROSCOPY "STM…

Selective adsorption of metallocenes on clean and chemically modified Si(111) surfaces

“…10 s annealing at 830°C is followed to form a Si-Ca-F interface layer with CaF 1 stoichiometry, where the Ca atoms bond to Si. 7 By growing 1-2 ML CaF 2 , the CaF 2 stripes are formed at the upper step edges in regular arrays, covering the CaF 1 /Si͑111͒ interface. 7,8 STM images are taken typically with a sample voltage of ϩ4 V and tunneling current of 0.4 nA.…”

“…7 By growing 1-2 ML CaF 2 , the CaF 2 stripes are formed at the upper step edges in regular arrays, covering the CaF 1 /Si͑111͒ interface. 7,8 STM images are taken typically with a sample voltage of ϩ4 V and tunneling current of 0.4 nA. The positive sample voltage of 4 V leads to electron tunneling from the tip directly into the conduction band of CaF 2 .…”

“…For chemical modification of the clean Si͑111͒ surface, we choose to grow a CaF 2 layer because of the close match of the lattice constant between deposit and substrate as well as the insulating nature of CaF 2 . 7,8 The CaF 2 growth is conducted at a substrate temperature of 610°C at a growth rate of ϳ20 Å/min. 10 s annealing at 830°C is followed to form a Si-Ca-F interface layer with CaF 1 stoichiometry, where the Ca atoms bond to Si.…”

“…The accumulated knowledge now puts us in a position to devise a template containing precisely arranged stripes with alternating local chemical reactivity with a periodicity of 10-20 nm. 7 We expect the metallocene molecules to be rejected in regions covered by CaF 2 while being accepted in places with CaF 1 . The width of the CaF 2 stripes can be finely adjusted in the range of a few nm, whereas the order of the structures is self-organized during the growth.…”

“…The factors controlling this effect have been discussed in previous studies. 7,8 After the substrate is exposed to nickelocene vapor at room temperature followed by a brief high temperature annealing, the upper step edge is decorated by a string of adsorbates guided by the steps. Voids in the CaF 2 layer expose the underlying CaF 1 /Si͑111͒ interface.…”