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Growth and Characterization of Potassium-Doped Superfulleride Thin Films

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Growth and Characterization of Potassium-Doped Superfulleride Thin Films Growth and characterization of potassium- doped superfulleride thin films Cite as: Journal of Vacuum Science & Technology A 16, 2395 (1998); https://doi.org/10.1116/1.581358 Submitted: 10 September 1997 . Accepted: 09 January 1998 . Published Online: 09 July 1998 Nathan Swami, Yujian You, Mark E. Thompson, and Bruce E. Koel ARTICLES YOU MAY BE INTERESTED IN Electrical properties of K-doped superfulleride thin films Journal of Applied Physics 85, 3696 (1999); https://doi.org/10.1063/1.369734 Color-tunable organic light-emitting devices Applied Physics Letters 69, 2959 (1996); https://doi.org/10.1063/1.117743 Photophysics of Pt-porphyrin electrophosphorescent devices emitting in the near infrared Applied Physics Letters 90, 213503 (2007); https://doi.org/10.1063/1.2740113 Journal of Vacuum Science & Technology A 16, 2395 (1998); https://doi.org/10.1116/1.581358 16, 2395 © 1998 American Vacuum Society. Growth and characterization of potassium-doped superfulleride thin films Nathan Swami, Yujian You, Mark E. Thompson, and Bruce E. Koela) Department of Chemistry, and Department of Materials Science, University of Southern California, Los Angeles, California 90089-0482 ~Received 10 September 1997; accepted 9 January 1998! Growth conditions for the formation of thin films ~100–300 Å! of potassium-doped superfullerides ~KxC60, x.6! are examined. Thin films of these compounds are formed by depositing C60 onto a potassium precovered single crystal quartz substrate maintained at 200 K or lower, in a proportion of K:C60.12:1, followed by annealing the surface to the K-sublimation temperature ~300 K!. In situ measurements of electrical and optical properties are used to identify the compounds. The formation of superfullerides is confirmed by C60 doping of these phases to check for the formation of insulating K6C60 with a characteristic absorption spectrum. The absorption spectrum of the superfullerides shows distinct features corresponding to the filling of the t1g band. The presence of two superfulleride phases is suggested, a near-metallic superfulleride KxC60 (x'11.2) and a more insulating KxC60 (x'8–9). © 1998 American Vacuum Society. @S0734-2101~98!01104-X# I. INTRODUCTION tron transfer from alkali metal to C60 and the related changes in film stochiometry and conductivity have been Alkali-metal-doped fulleride compounds have generated explained.18,19 Our work presented here uses the growth 1,2 great interest in the past due to the discovery of supercon- techniques described by Jiang et al.13 in a high vacuum 3 4 ductivity in K3C60 (Tc518 K), Cs3C60 (Tc540 K), and chamber with instrumentation similar to that employed by 5 mixed fulleride phases. These compounds were formed by Wilson et al.16 to report the first determinations of electrical half filling of the t1u ~LUMO derived! band of C60. Despite resistivity and optical absorption spectra of these superful- of the high electron affinity of C60, it was contended that leride compounds. The compounds are identified by correlat- charge transfer saturation apparently occurred at six elec- ing resistivity measurements with distinct features in the ab- trons per C molecule for the alkali metal-doped 60 sorption spectrum corresponding to the filling of the t1u and compounds,6 and early attempts were unsuccessful7,8 in fill- t1g bands of C60. We suggest the growth of two superful- ing the t ~LUMO11 derived! band and forming superful- 1g leride phases occurs, a near metallic KxC60 (x'11.2) phase lerides ~K C , x.6!. The deposition in these experiments x 60 and a more insulating KxC60 (x'8 – 9) phase. was carried out by evaporating potassium on C60 films at room temperature. The occupation of the t1g band was, how- ever, obtained for alkaline earth metal-doped fullerides.9,10 II. EXPERIMENTAL METHODS More recently, Benning et al.11 and Jiang et al.12 altered the growth conditions by depositing potassium first on a liquid The deposition and growth of potassium-doped fulleride nitrogen-cooled substrate, followed by C60 deposition. The compounds was carried out in a high vacuum chamber, as occupation of the t1g band was then observed for the K- shown schematically in Fig. 1, with a base pressure of 5 12–14 27 doped superfullerides in photoemission spectroscopy. 310 Torr. C60 of 99.5% purity was vacuum sublimed Herein we draw on these earlier studies that concerned the from a tantalum crucible at a rate of 15–25 Å/min onto growth of few monolayers thick superfulleride films and single-crystal quartz substrates polished on both sides. The demonstrate the growth of 100–300 Å superfulleride films substrate was mounted onto a sample holder made of by correlating the electrical and optical properties of these oxygen-free copper of dimensions 53630.3 in3, which compounds with their electronic structure.11–14 could be cooled using liquid nitrogen to 200 K and radia- Optical absorption is an important in-situ tool for charac- tively heated to 400 K. Potassium doping was carried out 15 terizing these thin films. Bulk C60 is a molecular solid, and using 40 mm SAES K getters at a rate of 6–10 Å/min. The hence the bands in the absorption spectra of C60 thin films distance between the substrate and the evaporating source can be correlated to electronic bands derived directly from was 12 in. Deposition rates were determined using a water- the molecular states of the C60 molecule. Further, due to the cooled quartz microbalance ~Inficon XTM/2! positioned similarity in the absorption spectra of various alkali-metal- close to the substrate. About 100–300 Å of the phases of doped C60 compounds, the peaks of the spectra may be as- interest were grown, where the thickness of the films was signed to electronic transitions in the host fullerene mol- determined primarily by the quartz microbalance, but this ecule, rather than those of the dopant.16 Thus, the peaks in was confirmed by ex situ ellipsometry ~Rudolph Technology the absorption spectra of C60,K3C60, and K6C60 have been Auto EL!. Ex situ x-ray diffraction was performed on the C60 15–17 assigned in previous publications, and the extent of elec- films using Cu Ka radiation from a rotating anode source and a highly oriented pyrolytic graphite ~HOPG! monochro- a!Electronic mail: [email protected] mator and analyzer. 2395 J. Vac. Sci. Technol. A 16„4…, Jul/Aug 1998 0734-2101/98/16„4…/2395/5/$15.00 ©1998 American Vacuum Society 2395 2396 Swami et al.: Potassium doped superfulleride thin films 2396 FIG. 2. X-ray scan of a 300 Å C60 film on single-crystal quartz showing the major fcc peaks. FIG. 1. Schematic of the vacuum deposition system used for synthesis and characterization of superfulleride thin films. films as has been done earlier.7 No charge transfer occured beyond the formation of K6C60 under these conditions. The Due to the air sensitivity of the K-doped fullerides, in situ resistivity decrease at longer K deposition times has been optical and electrical measurements were used to identify the attributed to unreacted metallic potassium depositing on the 7 phases. The thin films were grown on substrates that con- surface and contributing to a resistance decrease. Working tained four 1000 Å thick silver pads ~evaporated on the on the premise that formation of higher fullerides is limited quartz substrate prior to film deposition! that can be used as by the relatively low potassium concentrations that can be electrodes in four-point resistivity measurements of the achieved at room temperature prior to compound formation, films. For optical measurements, the chamber was equipped potassium was deposited on the quartz surface cooled to 200 with a sapphire window ~transparent from 250 to 1500 nm!. K, followed by C60 deposition in a proportion such that The absorption measurements utilized a 200 W Hg-Xe lamp K:C60.12:1. The surface was then annealed to 300 K for as a source and the optics ~scanning monochrometer, focus- about two hours, yielding a phase labeled as KxC60 which ing lenses, and mirrors! produced a 231in2 beam of 300– has a near metallic resistivity, as shown in Fig. 3. Metallic 700 nm light onto the quartz substrate. Two solar cells, potassium conductivity could not be detected until the depo- mounted on the sample holder, were used as detectors. UV- sition of some C60 on the surface, which suggests that potas- visible absorption spectra of the phases of interest were de- sium grows on the quartz surface as islands rather than as a termined in the 300–700 nm wavelength range by depositing uniform layer. A possible growth model for the fulleride the thin films on a portion of the substrate such that one cell films is that C60 and K interdiffuse to form fulleride com- detected light passing only through the substrate, while the pounds that grow as polycrystalline films atop the quartz other cell detected light passing through the phase of interest substrate. Annealing to 300 K promotes interdiffusion and and the substrate. The absorbance was determined from the brings about desorption of excess potassium from the sur- currents of the two solar cells. The instrumentation was set face. The latter is evident from the observation that for an- up such that resistivity and optical measurements could be nealing temperatures lower than the K-sublimation tempera- obtained simultaneously as a function of doping. ture ~300–325 K!, the low resistivity of metallic potassium dominates the resistivity of the thin film. The presence of metallic potassium under the above conditions is also sup- III. RESULTS AND DISCUSSION ported by the UV-visible absorption spectra. We assume that Powder x-ray diffraction ~XRD! was used to determine our fulleride thin films are polycrystalline since we have ob- the quality of C60 films grown in our chamber.
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