Rb doping

Doping of alkali-metals into CNTs has been examined [11]. The X-ray powder diffraction (XRD) patterns of the K- or Rb-doped CNTs show that alkali-metals are intercalated between the CNT layers. The hexagonal unit cell is essentially the same as that of the stage-1 alkali-metal intercalated graphite ACg (A=K, Rb). For a sample doped with Rb, the observed lattice parameter of the perpendicular... 

Figure 149 PL and EL spectra of the multi-layer structure shown in the inset. The principal maximum is characteristic of rubrene (Rb) doped in the thin (3nm) layer of Alq3 [10% Rb Alq3], Adapted from Ref. 565.

There are two definitive tests for superconductivity zero electrical resistance and expulsion of magnetic flux. Both were demonstrated in the first KaCeo experiments, in thin films and powder samples, respectively.[He91 ] Initial efforts to observe superconductivity in Rb-doped films were unsuccessful, but the exposure of Ceo powder to Rb vapor, followed by appropriate heat treatment, did produce superconducting samples with a transition temperature of 28 K.[Ro91, Ho91a ]... 

Note added in proof During these studies we learned that Rosseinsky et al. 11) found a superconducting transition at 28 K in a Rb-doped Cgo sample. 

The results of converting all of the a(P) data in Fig. 1 point-by-point to T (a) are shown in Fig. 2. We interpolated between the T (P) data points of Spam et al. (2, 4) to obtain at the corresponding P s. This procedure goes beyond e approximation of linear TJ(P) and a(P) at low P but is not rigorously correa because the values were measured near 0 K whereas the P-de-pendent a walues were measured at 300 K. Also plotted in Fig. 2 are Fleming s T a) data at 1 bar for seven different compounds. The T (P) and relative accuracy of the T (a) correlations for K- and Rb-doped compounds because both T (P) measurements were performed in the same laboratory, as were both a(P) measurements. On the other hand, the absolute relation of the two pressure-dependent data sets to Fleming s 1-bar data is subject to... 

We now consider films doped to the AsQso phase. Raman spectra were obtained for K3C , (T, = 18 K) and Kb C (T, = 24 K) (Fig. 3). The striking feamre of these spectra is their simplicity. Only three modes are observed, the two A modes and the lowest frequency mode. Further, there is no dependence of the spectra on alkali dopant, indicating no A.-C60 mode is observed above 100 cm Spectra taken on a variety of K- and Rb-doped samples indicate that the Ag mode frequency in AjC o is 1445 2 cm . The FWHM of this mode is 9 cm , which is 6 cm less than the value for the pristine material. This reduction is likely due to the stationary orientation of the C<60 molecules on the time scale of the Raman scattering process, and is similar to the reduction observed in pristine as the molecules freeze at low temperatures (17). This result is consistent with recent NMR and x-ray difiraction results that indicate the molecules are jumping between symmetry-equivalent orientations in the K3C(,o material at room temperature (13). Finally, we point out that the lowest frequency Hg mode appears to remain, although it is significantly broadened. 

K3C50 and RbgC samples used in this study are shown in Fig. 1. The transition temperatures of these K- and Rb-doped materials are 19 and 29 K, respectively, and the low temperature shielding fractions are approximately 100% for both samples. 

Rb NMR shieldings have been shown experimentally to depend linearly on the distance of the Rb" ion to its nearest neighbor shell of halide ions in the family of Rb-doped alkali halides, and theoretical calculations bear out the experimental linear trends. The shielding derivatives with respect to the ionic distance are positive and are very similar, largely independent of the identity of the halide ion for the distances that naturally occur in the alkali halide lattices. At much shorter distances, we would not expect this to be the case. 

Several physical parametCTs are summarized in Tables 10.1 and 10.2. The resistivity decreases monotonicaUy with decreasing tempaature in the K-doped (or Rb-doped) Qo single crystal (no report has appeared on a single crystal of A3C ) with nominal composition of A3Q0 (Figure 10.11). ig 19 g K for A = K (/ = 14.25 A) and... 

Among the alkali metals, Li, Na, K, Rb, and Cs and their alloys have been used as exohedral dopants for Cgo [25, 26], with one electron typically transferred per alkali metal dopant. Although the metal atom diffusion rates appear to be considerably lower, some success has also been achieved with the intercalation of alkaline earth dopants, such as Ca, Sr, and Ba [27, 28, 29], where two electrons per metal atom M are transferred to the Cgo molecules for low concentrations of metal atoms, and less than two electrons per alkaline earth ion for high metal atom concentrations. Since the alkaline earth ions are smaller than the corresponding alkali metals in the same row of the periodic table, the crystal structures formed with alkaline earth doping are often different from those for the alkali metal dopants. Except for the alkali metal and alkaline earth intercalation compounds, few intercalation compounds have been investigated for their physical properties. 

Fig. 11. Composition dependence of the resistivity p x) for thick films of Cbo doped with Na, K, Rb, and Cs. Points indicate where exposure to the alkali-metal source was stopped and x-ray and ultraviolet photoemission spectra were acquired to determine the concentration x. The labels indicate the known fulleride phases at 300 K. The minima in p x) occur for stoichiometries corresponding to NaQCeo, K.iCeo and Cs,.,.Cfio[ll3].

Raman spectra have also been reported on ropes of SWCNTs doped with the alkali metals K and Rb and with the halogen Br2 [30]. It is found that the doping of CNTs with alkali metals and halogens yield Raman spectra that show spectral shifts of the modes near 1580 cm" associated with charge transfer. Upshifts in the mode frequencies are observed and are associated with the donation of electrons from the CNTs to the halogens in the case of acceptors, and downshifts are observed for electron charge transfer to the CNT from the alkali metal donors. These frequency shifts of the CNT Raman-active modes can in principle be u.sed to characterise the CNT-based intercalation compound for the amount of intercalate uptake that has occurred on the CNT wall. 

Figure 4.33. Measured concentration of Cr versus concentration of Cr added during film growth for / -FeSi2 films doped with Cr. ERDA heavy-ion RBS O standard RBS. (Bohne et al. 2000.)...

DF O Brien, C Giebler, RB Fletcher, AJ Cadby, LC Palilis, DG Lidzey, PA Lane, DDC Bradley, and W Blau, Electrophosphorescence from a doped polymer light emitting diode, Synth. Met., 116 379-383, 2001. 

The discovery of superconductivity [105] in alkali metal doped attracted the attention of a broad cross-section of the scientific community. The composition of the first alkali metal doped fullerene was determined to be [106,107]. In this compoimd, the transition to a superconducting state occurs at = 19.3 K [106]. In a short time, a large variety of alkali metal fullerides M Cgo have been synthesized and studied with respect to their solid-state properties [1, 106-114]. Thereby, the stoichiometries of as well as the alkali metal with M = Na, K, Rb, Cs have... 

Rb, Cs) can be obtained using vapor-transport techniques [115]. Thereby, a weighed amoimt of Cjq is treated with a large excess of the alkali metal at 225 °C under vacuum. This procedure leads to the saturation-doped products MgC g. To these fullerides a specific amount of Cjq is added to give the desired stoichiometry. The process of the formation of is completed after a subsequent annealing... 

Alkali metal doping of Cjq is also possible by solution-phase techniques [1,118-121]. K CgQ and Rb CgQ containing small fractions of the superconducting M3C50 phases were prepared by allowing toluene solutions of Cjq to react with the alkali metal [118, 119]. During the reaction, the alkali metal fullerides form a black precipitate. In another example, sonication of a solution of Cjq and excess potassium in TMEDA yields K3C5q(THF)j4 with a defined stoichiometry [104],... 

Cgo doped with K T < 8.1 K) [322] and Rb T < 23 K) [323] exhibit superconductivity on LB films, which was detected by the AC complex magnetic susceptibility or low magnetic field microwave absorption measurements. However, both the stmctural disorder inherent to the LB films and the low-dimensional nature of the thin-layer structure severely prohibit the observation of superconductivity by resistivity measurements. 

After Little s proposal, many researchers have pursued such an exciting system in vain. Even metallic behavior was rarely seen in doped organic polymers, gels, and actuators. As mentioned in Sect. 3.4.4, MCso with linearly polymerized Ceo" exhibited one-dimensional (M = Rb, Cs) or three-dimensional (M = K) metallic behavior [144]. Recently a doped poly aniline was reported to exhibit a metallic temperature dependence for a crystalline polymer chemical oxidation of monomers grew crystallite polyaniline [329] early doping studies on polypyrrole (PFg) and poly(3,4-ethylene-dioxythiophene)X (X = PFg, BF4, and CF3SO3) prepared by electrooxidation at low temperatures also showed a metallic temperature dependence below 10-20 K (Scheme 16) [330, 331]. 

Figure 4.10 Damage and doping profiles for the same set of random as-implanted 6H-SIC samples, (a) Comparison between RBS-C measured and MC-BCA simulated damage profiles. (From [73]. 1999 Elsevier B.V. Reprinted with permission.) (b) Comparison between SIMS measured and MC-BCA simulated chemical profiles. (From [72]. 2001 Material Science Forum. Reprinted with permission.)...

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