In this work, nanocomposite films of indium tin oxide (ITO) nanowires in a PMMA matrix were obtained by tape casting. The electrical, optical and morphological properties of films were studied as a function of the amount of wires inserted in the composite, and it was used 1, 2, 5, and 10 wt %. Results confirmed that films transmittance decreases as the concentration of wires increases, attaining a minimum transmittance of 55% for 10 wt % of filler. On the other hand, the electrical resistance of composites was found to decrease by increasing the filler amount and the dc characterization indicate that percolation occurs for about 5 wt % of wires. The morphological studies carried out by TEM were considered to be in good agreement with the electrical results and confirm that for 5% of filler, the ITO nanostructures are in contact with one another inside the polymer. Moreover, we made computational simulation of 1D structures in a general matrix and it was found that percolation should occur for about 12 wt %. Although computational results indicate higher amount of wires necessary for percolation than we found experimentally, both results illustrate that using one-dimensional nanostructures as filler in composites enables obtaining percolation for a smaller amount of filler than when using, for instance, nanoparticles. Therefore, the simple processing technique employed here can be used to obtain transparent and conductive composites with several useful applications.
The joint density of states for the transition 1s to the first subband of an electron bound to a donor impurity in a compensated n-type quantum well is calculated in a semiclassical model which is valid in the low-concentration regime. A macroscopic electrostatic potential due to charged particles is obtained and its effect on the transition is analyzed.Recently two of us (E.A.A.S. and I.C.C.L.) calculated the impurity-electron density of states (DOS) and the onsite electric field distribution on donors inside an n-type quantum well (QW) using a Monte Carlo simulation. ' We have obtained, among other results, the average density of neutral donors inside the QW ( Fig. 9 in Ref. 1) which appears as a result of the compensation by acceptor impurities. In those calculations we have used a semiclassical model in such a way that ionized impurities interact among each other as point charges. These interactions, summed up with the confining potential of the QW, were taken into account both in the DOS and in the density of neutral donors. A question, however, might be raised: since there is an electric charge distribution inside the well due to the complete ionization of acceptors and partial ionization of donors (the compensation is k =N"/N& (1), an additional potential (macroscopic) appears inside the QW. What is its effect, alone, on the energy of the bound electron and on the edge of the first subband? In the present work we answer this question using a very simple model. We assume a collection of XD donor impurities and N"acceptors inside the QW. The energy of an electron bound to a donor was first calculated by Bastard and his approach has been improved in subsequent works. This energy depends. on z; (distance to the center of the QW in the growth direction) increasing as we approach the interfaces with the wider gap material. We neglect the overlap between the wave functions of electrons bound to different donors. Furthermore, we neglect microscopic interaction between point charges, contrary to Ref. 1. Our impurities are then isolated ones except by the macroscopic field introduced by charge distribution. This field will have, then, only the z component. Since all acceptors are ionized, we have, inside the well, a constant density of negative charge: keNp (z)=-L where L is the well width. Since the deeper inside the well the lower the energy of the bound state is, the neutral donors will occupy the central region of the well. Due to charge neutrality, the density of positive charges 1S where z = -(1k) L m is the half-width of the distribution of the neutral donors charge. Of course this p+(z) differs from the corresponding one resulting from Fig. 9 in Ref. 1 where disorder in the neighborhood of each impurity site has been taken into account in the Monte Carlo simulation. Poisson's equation can now be integrated to yield the electrostatic potential 2~ND y(z) = [ -( [z~-z ) e(~z~z )+kz ], xL 12 513
%e present a calculation of the density of states (DOS} of electrons bound to donor impurities in a lightly doped and compensated quantum well. %e use a quasiclassical treatment suitable for low compensation in which we apply the dipo1e model. If the fu11 electron-electro~Coulomb interactions were taken into account, instead of the short-range dipole interaction, a Coulomb gap should appear around the Fermi level. But this model is able to describe the small peak of unoccupied states that appear, as a result of compensation, in the high-energy side of the DOS. In this work we show how the additional peak varies with the well width, impurity concentration, and compensation in the small-impurity-concentration limit.It is well established by now that a gap in the density of states (DOS) occurs at the Fermi level in a lightly doped and compensated semiconductor. In the past Efros, Van Lien, and Shklovskii' have calculated the DOS, using a numerical simulation method due to Baranovskii et al.to obtain the ground state of this system, in the so-called classical impurity band model. In fact, when the impurity concentration is very small the electrons are in the completely localized regime. The overlap of the wave functions representing the one-electron ground state is negligible as the average distance between impurities is much greater than the localization length. In that case the donors and acceptors can be assumed as point charges.At suSciently low temperatures a11 acceptors are ionized. If n; represents the occupancy of a donor i, r;" stands for the distance between that donor and an acceptor p; the system can be described by the classical Hamiltonian:For low compensation the ionized donors are those near an acceptor, resulting in unoccupied electron states with higher energies than those occupied at neutral donors.It turns out that a simple way to treat that problem analytically is the dipole model: each acceptor ionizes its nearest-neighbor donor and the pair does not perturb the states of neutral donors as its potential rapidly decays. A bandwidth appears in this additional peak at the DOS located at e /kr, where r is the distance between atoms in the pair, corresponding to the unoccupied "dipole states. " The spread in energy is due to randomness in the dipole moment. Using this simplified picture, a two-peaked DOS comes out: a deltalike peak due to states in neutral donors and a separated broad peak of the unoccupied states in the donors forming dipoles with ionized acceptors. Actually, Auctuations of the electrostatic potential at the neutral donors are responsible for a considerable broadening of the delta peak. However the DOS does not change qualitatively and indeed the dipole model was shown" to give very good results in the caIculation of the electric field distribution.In this work we extend the dipole model to treat a lightly doped and compensated semiconductor heterostructure: the Ga, Al As/GaAs quantum well (QW). We assume that hydrogenlike donor and acceptor impurities occur in the small gap layer o...
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