Showing papers by "Joshua E. Goldberger published in 2014"

Improving the stability and optical properties of germanane via one-step covalent methyl-termination

Abstract: Germanane is a two-dimensional material that represents a promising alternative to graphene for applications in electronics and optoelectronics. Here, Jiang and colleagues demonstrate that germanane’s optical properties can be improved by methyl-termination instead of hydrogen.

Tunable gaps and enhanced mobilities in strain-engineered silicane

Abstract: The recent demonstration of single-atom thick, sp3-hybridized group 14 analogues of graphene enables the creation of materials with electronic structures that are manipulated by the nature of the covalently bound substituents above and below the sheet. These analogues can be electronically derived from isolated (111) layers of the bulk diamond lattice. Here, we perform systematic Density Functional Theory calculations to understand how the band dispersions, effective masses, and band gaps change as the bulk silicon (111) layers are continuously separated from each other until they are electronically isolated, and then passivated with hydrogen. High-level calculations based on HSE06 hybrid functionals were performed on each endpoint to compare directly with experimental values. We find that the change in the electronic structure due to variations in the Si-H bond length, Si-Si-Si bond angle, and most significantly the Si-Si bond length can tune the nature of the band gap from indirect to direct with dramatic effects on the transport properties. First-principles calculations of the phonon-limited electron mobility predict a value of 464 cm2/Vs for relaxed indirect band gap Si-H monolayers at room temperature. However, for 1.6% tensile strain, the band gap becomes direct, which increases the mobility significantly (8 551 cm2/Vs at 4% tensile strain). In total, this analysis of Si-based monolayers suggests that strain can change the nature of the band gap from indirect to direct and increase the electron mobility more than 18-fold.

A first principles method to simulate electron mobilities in 2D materials

Abstract: We examine the predictive capabilities of first-principles theoretical methods to calculate the phonon- and impurity-limited electron mobilities for a number of technologically relevant two-dimensional materials in comparison to experiment. The studied systems include perfect graphene, graphane, germanane and MoS2, as well as graphene with vacancies, and hydrogen, gold, and platinum adsorbates. We find good agreement with experiments for the mobilities of graphene (μ=2×10 5 cm 2 V �1 s �1 ) and graphane (μ=166cm 2 V �1 s �1 ) at room temperature. For monolayer MoS2 we obtain μ=225cm 2 V �1 s �1 . This value is higher than what is observed experimentally (0.5–200cm 2 V �1 s �1 ) but is on the same order of magnitude as other recent theoretical results. For bulk MoS2 we obtain μ=48cm 2 V �1 s �1 . We obtain a very high mobility of 18200cm 2 V �1 s �1 for single-layer germanane. The calculated reduction in mobility from the different impurities compares well to measurements where experimental data are available, demonstrating that the proposed method has good predictive capabilities and can be very useful for validation and materials design.

Mechanism of the pH-Controlled Self-Assembly of Nanofibers from Peptide Amphiphiles.

Abstract: Stimuli-responsive, self-assembling nanomaterials hold a great promise to revolutionize medicine and technology. However, current discovery is slow and often serendipitous. Here we report a multiscale modeling study to elucidate the pH-controlled self-assembly of nanofibers from the peptide amphiphiles, palmitoyl-I-A3E4-NH2. The coarse-grained simulations revealed the formation of random-coil based spherical micelles at strong electrostatic repulsion. However, at weak or no electrostatic repulsion, the micelles merge into a nanofiber driven by the β-sheet formation between the peptide segments. The all-atom constant pH molecular dynamics revealed a cooperative transition between random coil and β-sheet in the pH range 6-7, matching the CD data. Interestingly, although the bulk pKa is more than one unit below the transition pH, consistent with the titration data, the highest pKa's coincide with the transition pH, suggesting that the latter may be tuned by modulating the pKa's of a few solvent-buried Glu side chains. Together, these data offer, to our best knowledge, the first multiresolution and quantitative view of the pH-dependent self-assembly of nanofibers. The novel protocols and insights gained are expected to advance the computer-aided design and discovery of pH-responsive nanomaterials.

The structure and amorphization of germanane

Abstract: Germanane, a direct band gap, high electron mobility germanium graphane analogue, has attracted considerable interest for optoelectronics. Here, temperature dependent pair distribution function analysis shows, upon annealing, the two-dimensional germanane framework begins to amorphize. The amorphization onset is independent of the existence of halogens or other impurities.

Synthesis and Stability of Two-Dimensional Ge/Sn Graphane Alloys

Abstract: There has been considerable interest in the germanium and tin graphane analogues due to their potential as optoelectronic building blocks, and novel topological materials. Here, we have synthesized for the first time alloyed germanium/tin graphane analogues from the topochemical deintercalation of CaGe2–2xSn2x (x = 0–0.09) in aqueous HCl. In these two-dimensional alloys, the germanium atom is terminated with hydrogen while tin is terminated with hydroxide. With greater tin incorporation, the band gap systematically shifts from 1.59 eV in GeH down to 1.38 eV for Ge0.91Sn0.09H0.91(OH)0.09, which allows for more sensitive photodetection at lower energies. In contrast to germanane’s oxidation resistance, the Ge and Sn atoms in these graphane alloys rapidly oxidize upon exposure to air. This work demonstrates the possibility of creating functional tin-incorporated group IV graphane analogues.

Li intercalation into 1D TiS2(en) chains.

Abstract: The intercalation of metal cations in 2D layered materials allows for the discovery of unique electronic, magnetic and correlated properties. We demonstrate that reversible Li intercalation is also achievable in the hybrid organic/inorganic dimensionally reduced 1D van der Waals solid TiS2(ethylenediamine). Upon intercalation, electrons are injected into the lattice as Ti4+ is reduced to Ti3+ leading to an order of magnitude decrease in electrical resistivity. This reversible intercalation process opens up new opportunities to fine-tune the physical properties in this emerging family of dimensionally reduced materials.

Epitaxial co-deposition growth of CaGe2 films by molecular beam epitaxy for large area germanane

Abstract: Two-dimensional crystals are an important class of materials for novel physics, chemistry, and engineering. Germanane (GeH), the germanium-based analogue of graphane (CH), is of particular interest due to its direct band gap and spin-orbit coupling. Here, we report the successful co-deposition growth of CaGe2 films on Ge(111) substrates by molecular beam epitaxy and their subsequent conversion to germanane by immersion in hydrochloric acid. We find that the growth of CaGe2 occurs within an adsorption-limited growth regime, which ensures stoichiometry of the film. We utilize in situ reflection high energy electron diffraction (RHEED) to explore the growth temperature window and find the best RHEED patterns at 750 °C. Finally, the CaGe2 films are immersed in hydrochloric acid to convert the films to germanane. Auger electron spectroscopy of the resulting film indicates the removal of Ca, and RHEED patterns indicate a single-crystal film with an in-plane orientation dictated by the underlying Ge(111) substrate. X-ray diffraction and Raman spectroscopy indicate that the resulting films are indeed germanane. Ex situ atomic force microscopy shows that the grain size of the germanane is on the order of a few micrometers, being primarily limited by terraces induced by the miscut of the Ge substrate. Thus, optimization of the substrate could lead to the long-term goal of large area germanane films.

Programming pH-triggered self-assembly transitions via isomerization of peptide sequence.

Abstract: While the ordering of amino acids in proteins and peptide-based materials is known to affect their folding patterns and supramolecular architectures, tailoring self-assembly behavior in stimuli responsive peptides by isomerizing a peptide sequence has not been extensively explored. Here, we show that changing the position of a single hydrophobic amino acid in short amphiphilic peptides can dramatically alter their pH-triggered self-assembly transitions. Using palmitoyl-IAAAEEEE-NH2 and palmitoyl-IAAAEEEEK(DO3A:Gd)-NH2 as controls, moving the Isoleucine away from the palmitoyl tail preferentially induces nanofiber formation over spherical micelles. Shifting the Isoleucine one residue away makes the transition pH more basic by 2 units. When in the third or fourth position, nanofibers are formed exclusively above 10 μM. We propose that moving the Isoleucine away from the tail enhances its ability to promote β-sheet formation instead of folding back into the palmitoyl core. These findings reveal a novel strat...

Rational Synthesis of Dimensionally Reduced TiS2 Phases

Abstract: Despite the fact that new dimensionally reduced hybrid organic–inorganic compounds have attracted considerable interest because of their unique optical and electronic properties, the rational synthesis of these new materials remains elusive. Here we systematically studied the relative influence of the major synthetic parameters, including temperature, ligand structure, and ligand-to-metal stoichiometry, on the preparation of dimensionally reduced TiS2. One-dimensional TiS2 phases tend to form at high ligand-to-metal ratios and relatively lower temperatures, while the two-dimensional parent lattices are preferred at higher temperatures. The organic ligand structure dictates the temperature window in which a dimensionally reduced phase can be accessed. Although a small change in ligand structure, such as changing ethylenediamine to propylenediamine, for example, will significantly influence the stability of these phases, it will only subtly change the electronic structure. Via the development of a systemati...

Li Intercalation into 1D TiS2(en) Chains.

Abstract: Hybrid organic/inorganic van der Waals solid TiS2(en) is prepared by solvothermal reaction of a sulfur solution in en mixed with a TiCl4 solution in PhCl (Teflon cup in evacuated Parr reactor, 200—220 °C, 5 d) followed by purification via repeated recrystallization of the gel-like product and centrifugation.