Polygermyne: Germanium sheet polymers with efficient near-infrared luminescence
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Polygermyne: Germanium sheet polymers with efficient near-infrared luminescence Günther Vogg1, Martin S. Brandt1, Lex J.-P. Meyer1, Martin Stutzmann1, Zoltán Hajnal2, Bernadett Szücs2, Thomas Frauenheim2 1 Walter Schottky Institut, Technische Universität München, Am Coulombwall, D-85748 Garching, Germany. 2 Theoretische Physik, Universität Paderborn, Warburger Str. 100, D-33095 Paderborn, Germany.
ABSTRACT The structural, optical and electronic properties of the Ge sheet polymer poylgermyne are summarized. Prepared via topotactic transformation of Zintl-phase CaGe2, (GeH)n forms a layered crystal in a tr6 stacking sequence with a distance of 5.65 Å between adjacent layers. The photoluminescence at 1.3 eV is excited nearly resonantly with a Stokes shift of 0.2 eV. Together with band structure calculations this shows that polygermyne has a direct band gap.
INTRODUCTION The search for materials with a high luminescence efficiency on the basis of silicon has renewed the interest in the crystalline sheet polymer siloxene [1] and its derivatives. The layered crystal structure of this material based on the 3-fold coordination of Si backbone atoms leads to an electronic bandstructure with a direct bandgap and an intense luminescence with external quantum efficiencies of up to 10% [2-4]. Siloxene (Si2HOH)n as well as the oxygen-free counterpart polysilyne (SiH)n [5] are formed in a topotactic reaction from the layered Zintl-phase CaSi2 which contains extended puckered Si layers, similar to the {111} layers of crystalline Si and separated from each other by planar monolayers of calcium. Whereas Ca is removed during the deintercalation reaction in concentrated hydrochloric acid, the Si backbone remains unchanged since the fourth sp3-valence of each Si atom pointing out of the layers is stabilized by H- or partly by OH-groups, leading to the formation of polysilyne or siloxene, respectively. Similar sheet polymers based on germanium have only been described in the literature recently [6]. In this report we summarize the preparation and characterization of polygermyne (GeH)n which is obtained as a layered 2-dimensional Ge sheet polymer from epitaxially grown CaGe2. Polygermyne is found to be the direct counterpart of polysilyne exhibiting semiconducting properties and strong infrared photoluminescence.
GROWTH AND STRUCTURAL PROPERTIES Epitaxial CaGe2 films were grown by reactive deposition epitaxy (RDE) of calcium on crystalline Ge(111) substrates as described in [7]. The film thickness was about 1µm. For the preparation of polygermyne, the CaGe2 films were immersed into concentrated aqueous HCl for 12 hours at a temperature of -30°C, and then rinsed in acetone. X-ray diffraction (XRD) shows that the RDE growth of CaGe2 leads to epitaxial thin films with an even higher crystalline quality than similar CaSi2 films [7,8] which is also confirmed by scanning electron microscopy (SEM). G3.13.1
Figure 1. Scanning electron microscope side and top view of epitaxial CaGe2 films on Ge(111).
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Figure 1 shows micrographs of th
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