The interaction of C 60 with Si(111) and Co/Si(111)
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The interaction of C60 with Si(111) and Co/Si(111) M.A.K. Zilani, H. Xu, X.S. Wang, A.T.S. Wee* Department of Physics, National University of Singapore, 2 Science Drive 3, Singapore 117542
ABSTRACT We have studied the interaction of C60 with clean Si(111) and sub-monolayer Co covered Si(111) using scanning tunneling microscopy (STM) and X-ray photoelectron spectroscopy (XPS). Our STM results indicate that C60 has little mobility at room temperature (RT) on Co/Si(111). After annealing to 450 ºC, STM images show a regular arrangement of partially decomposed C60. XPS reveals a partial decomposition of C60 on Co/Si(111) at 520 ºC, and total decomposition to form a SiC-3x3 phase at 720 ºC. These results show that Co catalyses C60 decomposition resulting in the formation of the ordered SiC-3x3 phase ~200 ºC below that on clean Si(111).
INTRODUCTION Many studies using STM, XPS and other techniques have been done to investigate the interaction of C60 with metal [1, 2, 3] and semiconductor surfaces [4, 5, 6, 7, 8]. There are fewer studies that explore the interaction of C60 with semiconductor surfaces with metal overlayers [9]. Adsorbate mobility and decomposition temperature are two important issues that have been studied. On Au, Cu and Ag surfaces, C60 molecules diffuse and accumulate at the terrace edges at RT, while on Si(111) they remain chemisorbed to the initial deposition site even after annealing to 700 ºC [1, 2, 3, 4]. Some ordering of these molecules has been observed on Si(100)-2×1 at RT [10, 11]. At 270ºC on Ag/Si(111)-√3×√3R30º, C60 diffuses along the terrace edges and only very few molecules remain on the terrace [9]. The decomposition temperature of C60 depends on the nature and reactivity of the surface. C60 decomposes at relatively low temperatures (490ºC) on Ni(110) [14], whereas this temperature is much higher, 900ºC and 850ºC for Si(111) and Si(100) [13], respectively. The decomposition is followed by the formation of Si carbide and graphite sheets on Ni surfaces. The transition metal Co is expected to have similar properties to Ni. Here we present a STM and XPS investigation of C60 on Co-deposited Si(111) as a function of annealing temperature. EXPERIMENTAL A multi-chamber UHV system with separate growth and analysis chambers is used; the analysis chamber is equipped with an Omicron VT-STM, XPS and LEED, and the base pressure is 2×10-10 mbar [14]. The growth chamber contains the necessary sample preparation facilities and a C60 (>99.95%, Strem Chemicals, USA) evaporation source, comprising a heatable Ta boat with a thermocouple attached. 1 ML (monolayer) corresponds to 1 C60 molecule per square nanometer on the surface [4], and the deposition rate was calibrated by the STM images. The
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Si(111)-7×7 reconstructed surface was prepared by degassing the sample at 500 ºC overnight, then flashing to 1200 ºC and rapidly cooling to 900 ºC (at 60 ºC/s). An e-beam evaporator was used to deposit Co, and during deposition the base pressure was kept below 1×10-8 mbar. An Mg X-ray source (hv =1253.6
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