We have studied the zero magnetic field resistivity, ρ, of unique high-mobility two-dimensional electron systems in silicon. At very low electron density, n s (but higher than some sample-dependent critical value, n cr ∼ 10 11 cm −2 ), conventional weak localization is overpowered by a sharp drop of ρ by an order of magnitude with decreasing temperature below ∼ 1 − 2 K. No further evidence for electron localization is seen down to at least 20 mK. For n s < n cr , the sample is insulating. The resistance is empirically found to scale with temperature both below and above n cr with a single parameter which approaches zero at n s = n cr suggesting a metal/insulator phase transition.

We have studied the temperature dependence of resistivity, p, for a two-dimensional electron system in silicon at low electron densities n, 10 cm, near the metal-insulator transition. The resistivity was empirically found to scale with a single parameter Tp, which approaches zero at some critical electron density n, and increases as a power To oc~n,n,~with P = 1.6 6 0.1 both in metallic (n,) n,) and insulating (n, (n) regions. This dependence was found to be sample independent. We have also studied the diagonal resistivity at Landau-level filling factor v =-,where the system is known to be in a true metallic state at high magnetic field and in an insulating state at low magnetic field. The temperature dependencies of resistivity at B = 0 and at v =were found to be identical. These behaviors suggest a true metal-insulator transition in the two-dimensional electron system in silicon at B = 0, in contrast with the well-known scaling theory.

Random and alternating fluorene/carbazole (F/Cz) copolymers with various carbazole contents (20−50 mol %) have been designed and synthesized for use as the hole-transporting as well as light-emitting layer in blue light-emitting diodes (LEDs). DSC analysis has indicated the complete suppression of the crystallizability of these polymers by the introduction of 3,6-carbazole linkages into the polymer backbone, which also results in changes in their optical properties. The absorption maximum has been blue-shifted with an increase in the carbazole content due to the interruption in the main chain conjugation. Meanwhile, the photoluminescent properties have been influenced by the sequence distribution of the fluorene segments as well as the carbazole content. The emission maxima and vibronic features of the alternating copolymers have changed with carbazole content, reflecting the differences in the electronic structures of the repeat units. However, in the case of the random copolymers, the emission spectra remain almost unchanged and are similar to poly(9,9-dioctylfluorene) (PF), despite the fact that the carbazole content increases up to 33 mol %. This feature has been attributed to the existence of longer fluorene segments in the random copolymers, which would be expected to have lower energy gaps, and thus effectively collect excitons from other parts of the polymer backbone. Consequently, the light emitted from these energy traps is similar to that from PF. Electrochemical studies indicate that the introduction of carbazole units effectively raises the HOMO energy levels, thereby facilitating hole injection. Controlling the carbazole content between 20 and 33 mol % results in copolymers with stable and reversible p-doping and n-doping processes. A test for a LED device from P(F3- alt -Cz) indicates that the F/Cz copolymers could be a good candidate for blue light-emitting and hole-transporting materials.

We report the synthesis of novel fluorene-based π-conjugated polymers and the investigation of their electroluminescent properties in organic light-emitting devices. We also report on the photo- and electroluminescence of materials of the same class whose synthesis was recently published. The alternated incorporation of phenylene or thiophene moieties in fluorene-based π-conjugated polymers leads to the tunability of the electroluminescent properties. The spectral emission varies from blue to green or yellow, depending on the composition of the copolymers. To enhance the luminous efficiency of the devices, the hole injection and hole transport into the polymer were improved by insertion of an insulating buffer layer and the incorporation of efficient hole transport material in the polymer. The insertion of a charge injection layer such as LiF and a hole transport layer such as N,N‘-diphenyl-N,N‘-bis(3-methylphenyl)-1,1-biphenyl-4,4‘-diamine significantly improves the electroluminescence efficiency of the diode from 4.5 to 125 cd/m2.

We report on a collective low temperature metal-insulator transition which develops in a dilute 2D electron system in Si at zero magnetic field, below a critical carrier density ris 10'^ cm-^. In the insulator phase, the dc conduction is thermally activated and exhibits a sharp threshold as a function of electric field. The collective insulator state at zero field shows many of the features attributed to the pinned Wigner solid. We have also observed a trend from a collective to a single-particle insulator state with decreasing electron density and/or increasing disorder. PACS numbers: 73.40.Qv, 71.55.Jv, 71.30.+h In the limit of zero temperature, the dilute twodimensional electron (2DE) system is expected to become an insulator in the presence of disorder [1]. Various theoretical models for this insulating state have been proposed ranging from Anderson's single-particle localization (SPL) [1] to collective phases like the pinned Wigner solid (WS) [2], and the pinned charge density wave (CDW) [3]. Pioneering work [4] on low mobility Si samples revealed features indicative of a collective glassy state in high magnetic fields. The search for an electron ordered state in a quantizing magnetic field has recently led to the experimental observation of metal-insulator (MI) transitions in high mobility samples: GaAs-AlGaAs [5] heterostructures and Si-MOSFET's [6][7][8]. In these samples, reentrant MI transitions in a magnetic field were observed around fractional (mainly 1/5 in GaAs) and integer (1 and 2 in Si) filling factors. However, the realization of the "true" Wigner solid at zero magnetic field in GaAs would require an unrealistically dilute electron system since the critical density for quantum melting at T -0 [9] (also known as "cold melting") is very low, i.e., Ticm ~ 3 X 10^ cm~^. In contrast, the cold melting density in (100) Si should be at least 20 x larger, due to the larger effective mass (m* = 0.197ne) and lower dielectric constant {n = 7.7) which increases the ratio of the electron-electron interaction energy [{nnsY^'^e^/K\ to the kinetic energy {nrish'^/2m*) [8]. Therefore, a zerofield collective electron solid (ES) is more likely to be observed in high mobility Si inversion layers. There has not been, to our knowledge, any observation of the collective ES (and, in particular, the pinned Wigner solid) and its quantum melting at zero magnetic field in 2D. In our earlier work [6,8], there is an indication, however, that the insulating phase does not disappear as iif -> 0 but rather shifts to lower density and has features similar to those of the reentrant MI transitions in a magnetic field.In this Letter, we present experimental evidence for the collective character of the low temperature metalinsulator transition at zero field in Si MOSFET's with weak disorder. The data show a trend from a collective to a single-particle insulator state with decreasing density and/or increasing disorder. We compare our data on nonlinear dc transport, activation energy, and threshold electric field with the available...