A limiting mean free path was considered in order to better understand the temperature and wire diameter dependence of the resistivity and Seebeck coefficient of bismuth microwire and nanowire samples. The mean free path limited mobility was numerically calculated from experimentally measured mobility in a bulk bismuth sample, and the electron and hole mobilities were dramatically decreased to a 10 μm mean free path. Therefore, the temperature dependence of resistivity in very thin wire was quite different from that of a bulk sample, which had a positive temperature coefficient. The calculations showed that the temperature coefficient decreased gradually with decreasing mean free path, and the coefficient became negative for a mean free path of less than 1 μm at about 150 K. The Seebeck coefficient was also calculated, but showed only a weak dependence on mean free path compared with the resistivity. Experimental comparisons were made to previous measurements of bismuth microwire or nanowire samples, and the temperature and wire diameter dependencies of the resistivity and Seebeck coefficient were qualitatively and quantitatively in very good agreement. Therefore, the temperature dependencies of nanowire samples over 850 nm in diameter were well described using the mean free path limitation.
We present a mean free path limitation model to describe the temperature dependence of both resistivity and Seebeck coefficient for bismuth nanowire. Since the mobility of carriers for bismuth nanowire was limited due to dominant collision at wire boundary, the effective mobility for each carrier was estimated using cyclotron mass, appropriate band structure, and temperature dependence of Fermi energy from 4 to 300 K. Then, the resistivity and the Seebeck coefficient were calculated by using carrier density reported for bulk single crystal. In addition, an individual single-crystal bismuth nanowire sample (725 nm diameter and 2.37 mm length) grown into a quartz template was prepared to estimate the model, and the measurements were also performed. The temperature dependences of not only resistivity, but also Seebeck coefficient were quantitatively and qualitatively in very good agreement in the whole temperature region by using its crystal orientation measured from Laue measurement. We conclude that the mean free path limitation model proposed made us understand the temperature dependences of single-crystal bismuth nanowire without a finite size effect.
Butylammonium chloride crystals annealed by slowly scanning through the phase transitions many times have been shown to exhibit a simple thermogram, having a single phase transition at 241 K (Tt,.) on heating from ca. 100 K up to the melting temperature T, (487 K). Another additional thermal treatment was made on the annealed crystals, then measurements of 'H NMR spin-lattice relaxation times T , , T , , and the second moment M, were measured over t h e s a m e temperature range. These results showed that the cationic axial reorientation mainly contributes to 'H T , around room temperature, while the cationic self-diffusion plays an important role in t h e high-temperature range near T , . AC electrical conductivity measurements on single crystals revealed that ionic conduction takes place in t h e 2D layers of the room-temperature phase (rotator phase) which have a lamellartype double-layer structure. The observed T , and T l p data were explained well by applying the theory of 20 diffusion by MacGillivray and Sholl in the low vacancy concentration limit. The average jump times for the
Bismuth nanowires with lengths of over 1 mm length and diameters of the order of nanometers have been fabricated by high-pressure injection into a quartz template. The temperature dependences of the Seebeck coefficient and resistivity were simultaneously measured over the temperature range of 77–300 K. The Seebeck coefficient and resistivity at 300 K were estimated to be approximately −57 μV/K and 1.31 μΩ m, respectively. We also estimated the mobilities of electrons and holes to determine their temperature dependences. We found that the temperature dependence of the resistivity can be accounted for by the reduction in the mobility.
The temperature dependences of 'H spin-lattice relaxation times T 1 at the Larmor frequencies of 10.5, 16.0,20.0, and 45.5 MHz, and of IH NMR second moments M 2 were determined for (pyH)AuCI 4 and (pyH)AuBr4' where pyH + indicates a pyridinium ion. The small M 2 data less than 1 G 2 obtained above ca. 380 K indicated that the cations perform reorientational motion rapidly enough about its pseudohexad C;;axis existing at the center of the cation and perpendicular to its plane. Below 140 K, both complexes yielded rigid lattice M 2 values of the cation. It is interesting features for these complexes that motional narrowing for the NMR line occurs over an extremely wide range of temperature, the log T, vs. T-' plots are asymmetric about the lH T, minimum with a gentler gradient on the low temperature side, and the minima are extraordinarily long. These unusual results were interpreted by assuming the C;; reorientation of the cations having electric dipoles among nonequivalent in-plane orientations. Three potential barriers to the C;; reorientation were determined as 21.8, 17.2,and 13.5 kJ mol-I for (pyH) AuCI 4, and 22.2, 17.4,and 13.7 kJ mol~I for (pyH)AuBr4' The fade-out phenomenon of 35C1 NQR signals observed for (pyH)AuCI 4 at ca. 230 K when the sample temperature was lowered is also discussed, by referring to the motion of the pyridinium cations.
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