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Earth-abundant sustainable inorganic thin-film solar cells, independent of precious elements, pivot on a marginal material phase space targeting specific compounds. Advanced materials characterization efforts are necessary to expose the roles of microstructure, chemistry, and interfaces. Herein, the earth-abundant solar cell device, Cu[subscript 2]ZnSnS[subscript (4–x)]Se[subscript x], is reported, which shows a high abundance of secondary phases compared to similarly grown Cu[subscript 2]ZnSnSe[subscript 4].

Contributors
Aguiar, Jeffery A., Patel, Maulik, Aoki, Toshihiro, et al.
Created Date
2016-02-02

Transition metal trichalcogenides form a class of layered materials with strong in-plane anisotropy. For example, titanium trisulfide (TiS[subscript 3]) whiskers are made out of weakly interacting TiS[subscript 3] layers, where each layer is made of weakly interacting quasi-one-dimensional chains extending along the b axis. Here we establish the unusual vibrational properties of TiS[subscript 3] both experimentally and theoretically. Unlike other two-dimensional systems, the Raman active peaks of TiS[subscript 3] have only out-of-plane vibrational modes, and interestingly some of these vibrations involve unique rigid-chain vibrations and S–S molecular oscillations. High-pressure Raman studies further reveal that the A[subscript g][superscript S–S] S-S molecular ...

Contributors
Wu, Kedi, Torun, Engin, Sahin, Hasan, et al.
Created Date
2016-09-22

We present two-dimensional Mg(OH)[subscript 2] sheets and their vertical heterojunctions with CVD-MoS[subscript 2] for the first time as flexible 2D insulators with anomalous lattice vibration and chemical and physical properties. New hydrothermal crystal growth technique enabled isolation of environmentally stable monolayer Mg(OH)[subscript 2] sheets. Raman spectroscopy and vibrational calculations reveal that the lattice vibrations of Mg(OH)[subscript 2] have fundamentally different signature peaks and dimensionality effects compared to other 2D material systems known to date. Sub-wavelength electron energy-loss spectroscopy measurements and theoretical calculations show that Mg(OH)[subscript 2] is a 6 eV direct-gap insulator in 2D, and its optical band gap displays ...

Contributors
Tuna, Aslihan, Wu, Kedi, Sahin, Hasan, et al.
Created Date
2016-02-05

Vibrational spectroscopy in the electron microscope would be transformative in the study of biological samples, provided that radiation damage could be prevented. However, electron beams typically create high-energy excitations that severely accelerate sample degradation. Here this major difficulty is overcome using an ‘aloof’ electron beam, positioned tens of nanometres away from the sample: high-energy excitations are suppressed, while vibrational modes of energies <1 eV can be ‘safely’ investigated. To demonstrate the potential of aloof spectroscopy, we record electron energy loss spectra from biogenic guanine crystals in their native state, resolving their characteristic C–H, N–H and C=O vibrational signatures with no ...

Contributors
Rez, Peter, Aoki, Toshihiro, March, Katia, et al.
Created Date
2016-03-10

The emission properties of GeSn heterostructure pin diodes have been investigated. The devices contain thick (400–600 nm) Ge [subscript 1− y] Sn [subscript y] i-layers spanning a broad compositional range below and above the crossover Sn concentration y [subscript c] where the Ge [subscript 1− y] Sn [subscript y] alloy becomes a direct-gap material. These results are made possible by an optimized device architecture containing a single defected interface thereby mitigating the deleterious effects of mismatch-induced defects. The observed emission intensities as a function of composition show the contributions from two separate trends: an increase in direct gap emission as ...

Contributors
Gallagher, J. D., Senaratne, Charutha Lasitha, Sims, Patrick, et al.
Created Date
2015-03-02

The development of non-volatile logic through direct coupling of spontaneous ferroelectric polarization with semiconductor charge carriers is nontrivial, with many issues, including epitaxial ferroelectric growth, demonstration of ferroelectric switching and measurable semiconductor modulation. Here we report a true ferroelectric field effect—carrier density modulation in an underlying Ge(001) substrate by switching of the ferroelectric polarization in epitaxial c-axis-oriented BaTiO[subscript 3] grown by molecular beam epitaxy. Using the density functional theory, we demonstrate that switching of BaTiO[subscript 3] polarization results in a large electric potential change in Ge. Aberration-corrected electron microscopy confirms BaTiO[subscript 3] tetragonality and the absence of any low-permittivity interlayer ...

Contributors
Ponath, Patrick, Fredrickson, Kurt, Posadas, Agham B., et al.
Created Date
2015-01-01

The Ni/NiO core/shell structure is one of the most efficient co-catalysts for solar water splitting when coupled with suitable semiconducting oxides. It has been shown that pretreated Ni/NiO core/shell structures are more active than pure Ni metal, pure NiO or mixed dispersion of Ni metal and NiO nanoparticles. However, Ni/NiO core/shell structures on TiO2 are only able to generate H2 but not O2 in aqueous water. The nature of the hydrogen evolution reaction in these systems was investigated by correlating photochemical H2 production with atomic resolution structure determined with aberration corrected electron microscopy. It was found that the core/shell structure ...

Contributors
Crozier, Peter, Zhang, Liuxian, Aoki, Toshihiro, et al.
Created Date
2015

Novel hydride chemistries are employed to deposit light-emitting Ge [subscript 1- y] Sn [subscript y] alloys with y ≤ 0.1 by Ultra-High Vacuum Chemical Vapor Deposition (UHV-CVD) on Ge-buffered Si wafers. The properties of the resultant materials are systematically compared with similar alloys grown directly on Si wafers. The fundamental difference between the two systems is a fivefold (and higher) decrease in lattice mismatch between film and virtual substrate, allowing direct integration of bulk-like crystals with planar surfaces and relatively low dislocation densities. For y ≤ 0.06, the CVD precursors used were digermane Ge [subscript 2]H[subscript 6] and deuterated stannane ...

Contributors
Senaratne, Charutha Lasitha, Gallagher, J. D., Jiang, Liying, et al.
Created Date
2014-10-07

The compositional dependence of the lowest direct and indirect band gaps in Ge[subscript 1−y]Sn[subscript y] alloys has been determined from room-temperature photoluminescence measurements. This technique is particularly attractive for a comparison of the two transitions because distinct features in the spectra can be associated with the direct and indirect gaps. However, detailed modeling of these room temperature spectra is required to extract the band gap values with the high accuracy required to determine the Sn concentration y[subscript c] at which the alloy becomes a direct gap semiconductor. For the direct gap, this is accomplished using a microscopic model that allows ...

Contributors
Jiang, L., Gallagher, J. D., Senaratne, Charutha Lasitha, et al.
Created Date
2014-11-01