Axis dependent conduction polarity in the air-stable semiconductor, PdSe2.

Axis-dependent conduction polarity (ADCP) is a unique electronic phenomena in which the charge polarity of carrier conduction can differ from p-type to n-type depending on the direction of travel through the crystal. Most materials that exhibit ADCP are metals, and very few semiconducting materials exhibit this effect. Here, we establish that PdSe2, a ∼0.5 eV band gap semiconductor that is air- and water-stable, exhibits ADCP, through the growth and characterization of the transport properties of crystals with extrinsic p- and n-type doping levels of Ir and Sb, respectively, in the 1016-1018 cm-3 range. Electron doped PdSe2 exhibits p-type conduction in the cross-plane direction and n-type conduction along the in-plane directions above an onset temperature of 100-200 K that varies with doping level. Lightly p-doped samples show p-type thermopower in all directions at low temperatures, but above ∼360 K the in-plane thermopower turns negative. Density functional theory calculations indicate that the origin of ADCP arises from the complementary effective mass anisotropies in the valence and conduction bands in this material, which facilitate hole transport in the cross-plane direction, and electron transport along the in-plane directions. ADCP occurs at temperatures with sufficient thermal population of both carrier types to overcome the extrinsic doping levels to exploit the effective mass anisotropy. In total, the development of this stable semiconductor in which thermally or optically excited holes and electrons inherently migrate along different directions opens up numerous potential applications in a multitude of technologies.

The Chemical Design Principles for Axis-Dependent Conduction Polarity.

The recent discovery that specific materials can simultaneously exhibit n-type conduction and p-type conduction along different directions of the single crystal has the potential to impact a broad range of electronic and energy-harvesting technologies. Here, we establish the chemical design principles for creating materials with this behavior. First, we define the single-carrier and multicarrier mechanisms for axis-dependent conduction polarity and their identifying band structure fingerprints. We show using first-principles predictions that the AMX (A = Ca, Sr, Ba; M = Cu, Ag, Au; X = P, As, Sb) compounds consisting of MX honeycomb layers separated by A cations can exhibit p-type conduction in-plane and n-type conduction cross-plane via either mechanism depending on the doping level. We build up the band structure of BaCuAs using a molecular orbital approach to illustrate the structural origins of the two different mechanisms for axis-dependent conduction polarity. In total, this work shows this phenomenon can be quite prevalent in layered materials and reveals how to identify prospective materials.