We study the evolution of the interband optical excitation spectra of remotely doped GaAs/ Al 0X33 Ga 0X67 As quantum wells (QWs) with excess electron density using photoluminescence excitation (PLE) and electroreflectance (ER) spectroscopies. At the lowest electron densities the PLE spectra resemble those of undoped QWs, with a discrete peak due to the 1s neutral exciton (X) and a higher energy band due to the 2s/continuum. At very dilute excess electron densities the excitons bind a single excess electron to form a trion or negatively charged exciton (X À ), producing a second peak in the PLE a few meV below that due to X. With increasing density the X À peak shifts to higher energy, tracking the position of the k-vector conserving Fermi edge transition observed on the highenergy side of the photoluminescence band, to form the single threshold seen at high carrier density. The peak originating from X at low density shifts more rapidly to higher energy with increasing density and weakens, although it can be resolved to densities of up to 10 11 cm À2 . On the other hand the 2s/continuum threshold disappears at the lowest excess electron densities.

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We examined the electrical conduction through single-molecular junctions comprising of anthracenedithiol molecule coupled to two gold electrodes having 1,0,1 , 1,1,0 and 1,1,1 crystallographic orientations. Owing to this jellium model, we evaluated the values of current and conductance using non-equilibrium Green's functions combined with extended Huckel theory. This data was further interpreted in terms of transmission spectra, density of states and their molecular orbital analysis for zero bias. We evinced the oscillating conductance in all three cases, due to the oscillation of orbital energy relative to Fermi level. Our detailed analysis suggested that electrode orientation can tune the molecule-electrode coupling and hence conduction. Anthracene molecular junction with 1,1,0 orientation displayed favourable conduction, when compared to the other two orientations, thus can provide us an insight while designing futuristic molecular electronic devices.

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