The Fermi surface geometrical origin of axis-dependent conduction polarity in layered materials.

Electronic materials generally exhibit a single isotropic majority carrier type, electrons or holes. Some superlattice and hexagonal materials exhibit opposite conduction polarities along in-plane and cross-plane directions due to multiple electron and hole bands. Here, we uncover a material genus with this behaviour that originates from the Fermi surface geometry of a single band.

Group-13 and group-15 doping of germanane.

Germanane, a hydrogen-terminated graphane analogue of germanium has generated interest as a potential 2D electronic material. However, the incorporation and retention of extrinsic dopant atoms in the lattice, to tune the electronic properties, remains a significant challenge. Here, we show that the group-13 element Ga and the group-15 element As, can be successfully doped into a precursor CaGe phase, and remain intact in the lattice after the topotactic deintercalation, using HCl, to form GeH.

NaSnAs: An Exfoliatable Layered van der Waals Zintl Phase.

The discovery of new families of exfoliatable 2D crystals that have diverse sets of electronic, optical, and spin-orbit coupling properties enables the realization of unique physical phenomena in these few-atom-thick building blocks and in proximity to other materials. Herein, using NaSnAs as a model system, we demonstrate that layered Zintl phases having the stoichiometry ATtPn (A = group 1 or 2 element, Tt = group 14 tetrel element, and Pn = group 15 pnictogen element) and feature networks separated by van der Waals gaps can be readily exfoliated with both mechanical and liquid-phase methods. We identified the symmetries of the Raman-active modes of the bulk crystals via polarized Raman spectroscopy.

Stabilizing metastable tetragonal HfO using a non-hydrolytic solution-phase route: ligand exchange as a means of controlling particle size.

There has been intense interest in stabilizing the tetragonal phase of HfO since it is predicted to outperform the thermodynamically stable lower-symmetry monoclinic phase for almost every application where HfO has found use by dint of its higher dielectric constant, bandgap, and hardness. However, the monoclinic phase is much more thermodynamically stable and the tetragonal phase of HfO is generally accessible only at temperatures above 1720 °C. Classical models comparing the competing influences of bulk free energy and specific surface energy predict that the tetragonal phase of HfO ought to be stable at ultra-small dimensions below 4 nm; however, these size regimes have been difficult to access in the absence of synthetic methods that yield well-defined and monodisperse nanocrystals with precise control over size.

Water activated doping and transport in multilayered germanane crystals.

The synthesis of germanane (GeH) has opened the door for covalently functionalizable 2D materials in electronics. Herein, we demonstrate that GeH can be electronically doped by incorporating stoichiometric equivalents of phosphorus dopant atoms into the CaGe2 precursor. The electronic properties of these doped materials show significant atmospheric sensitivity, and we observe a reduction in resistance by up to three orders of magnitude when doped samples are measured in water-containing atmospheres.

Covalently-controlled properties by design in group IV graphane analogues.

CONSPECTUS: The isolation of graphene has sparked a renaissance in the study of two-dimensional materials. This led to the discovery of new and unique phenomena such as extremely high carrier mobility, thermal conductivity, and mechanical strength not observed in the parent 3D structure. While the emergence of these phenomena has spurred widespread interest in graphene, the paradox between the high-mobility Fermi-Dirac electronic structure and the need for a sizable band gap has challenged its application in traditional semiconductor devices.

Ferroelastic domain organization and precursor control of size in solution-grown hafnium dioxide nanorods.

We demonstrate that the degree of branching of the alkyl (R) chain in a Hf(OR)4 precursor allows for control over the length of HfO2 nanocrystals grown by homocondensation of the metal alkoxide with a metal halide. An extended nonhydrolytic sol-gel synthesis has been developed that enables the growth of high aspect ratio monoclinic HfO2 nanorods that grow along the [100] direction. The solution-grown elongated HfO2 nanorods show remarkable organization of twin domains separated by (100) coherent twin boundaries along the length of the nanowires in a morphology reminiscent of shape memory alloys.