Optical Properties of Quasi One-Dimensional Synthetic Metals.
Abstract
The electronic properties of Hg(,2.82)AsF(,6) and (SNBr(,0.4))(,x), two quasi one-dimensional synthetic metals, and trans-(CH)(,x), a quasi one-dimensional semiconductor that can become a metal when doped, were studied experimentally by their reflectivity and those of trans-(CH)(,x), by its photovoltaic effect. For Hg(,2.82)AsF(,6), the only structure in the reflectance of light (0.5-4.2 eV) polarized parallel to either of two sets of chains (mutually perpendicular) is a metallic plasma edge near 2.9 eV, well-fit by the Drude formula with an effective electron density equivalent to a single set of chains. Non-parallel reflection from anisotropic crystal faces shows no metallic reflectance for light polarized perpendicular to both sets of chains. There is a high electronic anisotropy at optical frequencies, similar to the anisotropies in the structure and the dc conductivity. For (SNBr(,0.4))(,x), the reflectance of light (0.6-2.5 eV) polarized parallel to the (SN)(,x) chains shows structure below a reflectance edge near 1.6 eV. A Drude fit to the high energy data gives the same electron density along the chains as in (SN)(,x), but a greater electron scattering time. The perpendicular reflectance is small and constant. The (SN)(,x) chains remain intact during the bromination of (SN)(,x), but the electronic anisotropy of the material increases. Small changes in the Fermi surface may lead to the large increase in dc conductivity. Trans-(CH)(,x) has one (pi)-electron per CH unit, has an overall (pi)-electron bandwidth of (TURN)10 eV, and could be metallic if all C-C bond lengths were equal and correlation sufficiently unimportant. Previous observations of a small, activated dc conductivity and infrared transparency indicate it is, instead, semiconducting. For partially oriented trans-(CH)(,x), the parallel reflectance of light (0.1-3.0 eV) is a large, broad peak centered near 2 eV; the perpendicular reflectance is low except for a small peak near 1.7 eV. The anisotropy in the reflectance shows a large anisotropy in the (pi)-electron states. The parallel reflectance peak is due to an optical transition with an oscillator strength the same as that of the whole (pi) -electron system. This transition has a threshold at 1 1/2 eV, with a peak at 2 eV that suggests a broadened one -dimensional divergence due to the large anisotropy. Trans-(CH)(,x) forms depletion layers as if it were uniform with a net acceptor density of 1.5-4 x 10('18)cm(' -3). The trans-(CH)(,x) photovoltaic effect, measured with photoelectrochemical cells and heterojunctions with CdS and ZnS, has a threshold near 1 1/2 eV with a continued rise in quantum efficiency to at least 2.4 eV. The quantum efficiency of the photoelectrochemical cell exceeds 1% at 2.4 eV. The quantum efficiency of trans-(CH)(,x):CdS heterojunctions has a complex dependence on reverse bias and photon energy and has a small dependence on the trans -(CH)(,x) absorption coefficient above 1.5 eV. The photocarriers in trans-(CH)(,x) have a diffusion length of (TURN)1000(ANGSTROM) and probably are not excitons, but charge carriers that may be of low mobility and subject to geminate recombination. Some trans-(CH)(,x):CdS heterojunctions show a response to light of 0.9 eV that depends on prior illumination. Details indicate a trapping effect in the trans-(CH)(,x) or, possibly, the CdS. The direct generation of change carrier pairs in trans-(CH)(,x) by photons with energies of 1 1/2 eV and more shows the optical transition to be an interband transition across a gap of 1 1/2 eV in a system well modeled as quasi one-dimensional with a Peierls distortion.
- Publication:
-
Ph.D. Thesis
- Pub Date:
- 1981
- Bibcode:
- 1981PhDT........17P
- Keywords:
-
- Physics: Condensed Matter;
- Arsenic Compounds;
- Iron Compounds;
- Mercury Compounds;
- Organic Compounds;
- Semiconductors (Materials);
- Synthetic Metals;
- Anisotropy;
- Direct Current;
- Electrical Resistivity;
- Optical Properties;
- Photovoltaic Effect;
- Polarization (Charge Separation);
- Reflectance;
- Solid-State Physics