UHV-STM Study of Oxidation on Flat and Stepped Ni Surfaces

Introduction

The most recent experimental addition to the Sibener Group vacuum work on metals has been Scanning Tunneling Microscopy. Previous work done with Helium Atom Scattering (HAS), Auger Electron Spectroscopy (AES) and Electron Energy Loss Spectroscopy (EELS) on Ni(977) and Ni(111) has raised many questions that can only be answered with local probe analysis.



Experiments

1. Ni(111) oxidation
We want to understand how oxide islands form on Ni(111) as a function of exposure and local electron injection. For the first 15 L of oxygen exposure, oxygen forms a p(2x2) overlayer on the Ni(111) at a 1/4 ML. With increased oxygen exposure, the chemisorbed oxygen islands are rapidly transformed into oxide regions which then interact to saturate to a 3 ML oxide. We intend on imaging each step of the oxidation process in order to derive a mechanism of the interaction between chemisorbed regions and then also how oxide islands interact to thicken.

Oxide growth was also found to be stimulated by the presence of electrons. With STM, electrons can be locally injected at selected energies to the surface. The subsequent oxide formation (after more oxygen exposure) will be imaged to compare against electron free oxide growth to determine what role

2. Ni(977) step doubling
Surface reconstruction: Kinetics experiments involving Helium Atom Scattering (HAS) on Ni(977) have been performed in this group. The surface undergoes a morphological phase transition from single to double steps in the presence of trace oxygen between 375 and 470 K. The goal of the STM study is to resolve the step interaction in real time and derive a mechanism for the transition in surface structure. The image below shows an array of single steps imaged at T=500 K. Details of this work can be found in references 89, 90, and 92.

3. O/Ni(977) phase diagram
We have studied the effect of an extended array of steps on the two-dimensional phase behavior of chemisorbed oxygen overlayers on a vicinal nickel surface using low energy electron diffraction (LEED), Auger electron spectroscopy (AES), and scanning tunneling microscopy (STM). Phase behavior of oxygen on the vicinal Ni(977) surface was examined and compared with that for oxygen adsorbed on the flat Ni(111) surface.

4. Monte Carlo simulations of Ni(977) step doubling
In order to make effective use of the extreme density of nanoscale elements that form spontaneously in self-assembling architectures, one must address the associated issue of minimizing defect creation during the formation of such structures. In this experiment we examine the competing roles that nucleation kinetics and two-dimensional growth processes play in nanostructure formation and defect minimization. We employ oxygen-induced step doubling of vicinal Ni(977) surfaces as our physical system, using elevated temperature scanning tunnelling microscopy and Monte Carlo simulations to extract the desired details of interface evolution.


UHV-STM Machine

Schematic of Top-View of UHV-STM
The experimental system was designed to accomodate many surface experiments on metals and semiconductors. The entire assembly is floated for STM operation by laminar flow vibration isolators. The UHV is maintained with a 220 L/s ion pump along with TSP and cryoshroud. A load lock assembly allows for transfer of samples and is pumped by a 55 L/s turbo molecular drag. Samples can be inserted into vacuum and prepared using electron bombardment or resistive heating. Surface cleanliness and order is checked with a 4 grid LEED that also serves for Auger electron spectroscopy by retarding field. The STM is an Omicron Micro STM mounted on a viton stack driven by Topometrix electronics and software. A radiant heater assembly is mounted above the microscope for elevated temperature imaging. A W filament is mounted in a water cooled shroud that can be positioned very close to the back of the sample being imaged.

References

81. "Proximity heater for the elevated temperature in situ vacuum scanning tunneling microscopy of metal surfaces"
T.P. Pearl and S.J. Sibener, Rev. Sci. Instrum. 71 124-127 (2000) Abstract

89. "Oxygen driven reconstruction dynamics of Ni(977) measured by time-lapse scanning tunneling microscopy"
T.P. Pearl and S.J. Sibener, J. Chem. Phys. 115 1916-1927 (2001) Abstract

90. "Spatial and temporal dynamics of individual step merging events on Ni(977) measured by scanning tunneling microscopy"
T.P. Pearl and S.J. Sibener, J. Phys. Chem. B 105 6300-6306 (2001) [Cover Story] Abstract

91. "Step-modified phase diagram for chemisorbed oxygen on nickel"
T.P. Pearl, S.B. Darling, and S.J. Sibener, Surf. Sci. 491 140-148 (2001) Abstract

92. "Mechanism and energetics for step merging on a metallic surface captured with scanning tunneling microscopy"
T.P. Pearl and S.J. Sibener, Surf. Sci. Lett. 496 L29-L34 (2002) Abstract

93. "In search of nano-perfection: Experiment and Monte Carlo simulation of nucleation-controlled step doubling"
Yi Wang, T.P. Pearl, S.B. Darling, J.L. Gimmell, and S.J. Sibener, J. Appl. Phys. 91 10081-10087 (2002) Abstract

STM/SPM Links

Companies
TopoMetrix Home Page
OMICRON Vakuumphysik GmbH - Instruments for Surface Science

Resources
An Introduction to Surface Chemistry

Research Groups
Fritz-Haber-Institut der MPG
Janice Reutt-Robey at UMD-College Park
Bob Hamers Research Group
Miquel Salmeron STM/AFM Group
IBM-Don Eigler
Center for Atomic-scale Materials Physics at Aarhus
Swartzentruber's STM Lab
IAP-Vienna - Surface Physics
Wilson Ho Group
Paul Weiss Group
STM at UMKC
Max G. Lagally Group
Lyding Group


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last modified: 01-Feb-02