Showing papers by "Joshua E. Goldberger published in 2017"

Buckled two-dimensional Xene sheets

Abstract: Silicene, germanene and stanene are part of a monoelemental class of two-dimensional (2D) crystals termed 2D-Xenes (X = Si, Ge, Sn and so on) which, together with their ligand-functionalized derivatives referred to as Xanes, are comprised of group IVA atoms arranged in a honeycomb lattice - similar to graphene but with varying degrees of buckling. Their electronic structure ranges from trivial insulators, to semiconductors with tunable gaps, to semi-metallic, depending on the substrate, chemical functionalization and strain. More than a dozen different topological insulator states are predicted to emerge, including the quantum spin Hall state at room temperature, which, if realized, would enable new classes of nanoelectronic and spintronic devices, such as the topological field-effect transistor. The electronic structure can be tuned, for example, by changing the group IVA element, the degree of spin-orbit coupling, the functionalization chemistry or the substrate, making the 2D-Xene systems promising multifunctional 2D materials for nanotechnology. This Perspective highlights the current state of the art and future opportunities in the manipulation and stability of these materials, their functions and applications, and novel device concepts.

EuSn2As2: an exfoliatable magnetic layered Zintl–Klemm phase

Abstract: Despite the immense interest in magnetic, topological, and magnetoelectronic phenomena in isolated layers of 2D materials and their vertically stacked heterostructures, there remains a dearth of 2D magnetic materials that can be readily exfoliated into single layers. Here, we synthesize EuSn2As2, the first exfoliatable, layered magnetic Zintl–Klemm phase that features van der Waals bonding between neighboring layers. In these crystals, Eu2+ 4f7 spins magnetically order at 24 K and are coupled ferromagnetically within each layer and antiferromagnetically, with some potential canting, across adjacent layers. Magnetic ground state DFT calculations and temperature-dependent transport measurements on single crystals together establish that EuSn2As2 is semimetallic. Finally, we show that this material can be thinned down to a few layers having steps corresponding to SnAsEuAsSn layers via micromechanical exfoliation. This work opens up further explorations into the design, synthesis, and layer-dependent magnetic properties of exfoliatable, lanthanide-based magnetic van der Waals Zintl phases.

Optical properties and Raman-active phonon modes of two-dimensional honeycomb Zintl phases

Abstract: Intermetallic Zintl phases whose structures consist of honeycomb layers of main group elements often serve as useful precursors for two-dimensional group 14 graphane derivatives. Here, we probe how the Raman spectra and band gaps evolve in these precursor intermetallic phases before they transform into their van der Waals counterparts. Through polarization-dependent Raman experiments on seven different group 14 and 15 Zintl phases with different stacking motifs, we show that the in-plane and out-of-plane Raman frequencies mostly depend on the reduced mass and the in-plane bond lengths of the elements constituting the honeycomb framework. From optical absorption measurements, we show that the honeycomb framework comprised of single elements is metallic, but when the framework is comprised of alternating elements in a BN-like fashion, such as in NaSnP, KSnAs and KSnSb, a band gap is opened up ranging from 0.54 eV for KSnSb to 0.95 eV for NaSnP. This study allows for the determination of the structure and stacking motifs for this emerging class of 2D layered metallic and semiconducting materials.

Group-13 and group-15 doping of germanane.

Abstract: 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 CaGe2 phase, and remain intact in the lattice after the topotactic deintercalation, using HCl, to form GeH. After deintercalation, a maximum of 1.1% As and 2.3% Ga can be substituted into the germanium lattice. Electronic transport properties of single flakes show that incorporation of dopants leads to a reduction of resistance of more than three orders of magnitude in H2O-containing atmosphere after As doping. After doping with Ga, the reduction is more than six orders of magnitude, but with significant hysteretic behavior, indicative of water-activation of dopants on the surface. Only Ga-doped germanane remains activated under vacuum, and also exhibits minimal hysteretic behavior while the sheet resistance is reduced by more than four orders of magnitude. These Ga- and As-doped germanane materials start to oxidize after one to four days in ambient atmosphere. Overall, this work demonstrates that extrinsic doping with Ga is a viable pathway towards accessing stable electronic behavior in graphane analogues of germanium.

Balancing the intermolecular forces in peptide amphiphiles for controlling self-assembly transitions.

Abstract: While the influence of alkyl chain length and headgroup size on self-assembly behaviour has been well-established for simple surfactants, the rational control over the pH- and concentration-dependent self-assembly behaviour in stimuli responsive peptides remains an elusive goal. Here, we show that different amphiphilic peptides can have similar self-assembly phase diagrams, providing the relative strengths of the attractive and repulsive forces are balanced. Using palmitoyl-YYAAEEEEK(DO3A:Gd)-NH2 and palmitoyl-YAAEEEEK(DO3A:Gd)-NH2 as controls, we show that reducing hydrophobic attractive forces through fewer methylene groups in the alkyl chain will lead to a similar self-assembly phase diagram as increasing the electrostatic repulsive forces via the addition of a glutamic acid residue. These changes allow creation of self-assembled MRI vehicles with slightly different micelle and nanofiber diameters but with minimal changes in the spin-lattice T1 relaxivity. These findings reveal a powerful strategy to design self-assembled vehicles with different sizes but with similar self-assembly profiles.

Electron Diffraction of Germanane

Abstract: Differences in the physical phenomena exhibited by two-dimensional materials, compared with bulk materials, is the driver of significant interest in monolayers and bilayers of van der Waals layered materials. The ability to tune the properties and electronic structure of these layered materials via chemical functionalization opens numerous opportunities for novel applications and devices. For example, hydrogen terminated graphene, or graphane, has been extensively studied and used for a variety of applications. However, other two-dimensional materials that exhibit direct band gaps and high carrier mobilities are desired. Germanane, a group IV analogue of graphane, has recently been synthesized and exhibits a larger and direct band gap and higher electron mobility than that of bulk germanium [1-3]. However, while the electronic properties have been studied, the structure of the material remains largely unexplored. We have used electron diffraction in a transmission electron microscope (TEM) to investigate the structure of germanane on the nanometer scale.