Rationalizing the Band Gap Tunability of Semiconductors via Electronic Structure Calculations

The polymorphs of titanium dioxide and various diamond-like semiconductor materials are promising candidates in photovoltaic solar cell applications. Several of these polymorphs have been studied with experimental and computational methods, which often aim at tuning the electronic structure, particularly the band gap value of the crystalline solid. Prior studies report that the addition of a substituent into the structure of titanium dioxide decreases its band gap value, but the reasons for this are unknown. Possible explanations for the change in band gap involve the substituent atom\u27s crystal radius, electronegativity, and ionization energy. Understanding the cause of these changes will provide great insight in designing new materials. The hypothesis of this work is that a substituent atom\u27s crystal radius has a greater impact on the change in band gap of titanium dioxide than the substituent atom\u27s electronegativity. In an aim to test this hypothesis, atoms of differing chemical properties were selected for substitution into the rutile and anatase polymorphs of titanium dioxide. The electronic structure (band structure and density of states) of the substituted systems was calculated using the full-potential linearized-augmented plane wave approach of density functional theory in the WIEN2k software. Upon calculating the electronic structure, the relationships between the band gap value and various chemical properties were evaluated

Structural Characterization of the Caveolin Scaffolding Domain in Association with Cholesterol-Rich Membranes

Members of the caveolin protein family are implicated in the formation of caveolae and play important roles in a number of signaling pathways and in the regulation of various proteins. We employ complementary spectroscopic methods to study the structure of the caveolin scaffolding domain (CSD) in caveolin-1 fragments, while bound to cholesterol-rich membranes. This key domain is thought to be involved in multiple critical functions that include protein recognition, oligomerization, and cholesterol binding. In our membrane-bound peptides, residues within the flanking intramembrane domain (IMD) are found to adopt an α-helical structure, consistent with its commonly believed helical hairpin conformation. Intriguingly, in these same peptides, we observe a β-stranded conformation for residues in the CSD, contrasting with earlier reports, which commonly do not reflect β-structure. Our experimental data based on solid-state NMR, CD, and FTIR are found to be consistent with computational analyses of the secondary structure preference of the primary sequence. We discuss how our structural data of membrane binding Cav fragments may match certain general features of cholesterol-binding domains and could be consistent with the role for CSD in protein recognition and homo-oligomerization

Optical Nonlinearity in Cu2CdSnS4 and α/β-Cu2ZnSiS4: Diamond-like Semiconductors with High Laser-Damage Thresholds

Cu2CdSnS4 and α/β-Cu2ZnSiS4 meet several criteria for promising nonlinear optical materials for use in the infrared (IR) region. Both are air-stable, crystallize in noncentrosymmetric space groups, and possess high thermal stabilities. Cu2CdSnS4 and α/β-Cu2ZnSiS4 display wide ranges of optical transparency, 1.4–25 and 0.7–25 μm, respectively, and have relatively large second-order nonlinearity as well as phase matchability for wide regions in the IR. The laser-damage threshold (LDT) for Cu2CdSnS4 is 0.2 GW/cm2, whereas α/β-Cu2ZnSiS4 has a LDT of 2.0 GW/cm2 for picosecond near-IR excitation. Both compounds also exhibit efficient third-order nonlinearity. Electronic structure calculations provide insight into the variation in properties

Polymorphism and second harmonic generation in a novel diamond-like semiconductor: Li2MnSnS4

© 2015 Elsevier Inc. All rights reserved. High-temperature, solid-state synthesis in the Li2MnSnS4 system led to the discovery of two new polymorphic compounds that were analyzed using single crystal X-ray diffraction. The α-polymorph crystallizes in Pna21 with the lithium cobalt (II) silicate, Li2CoSiO4, structure type, where Z=4, R1=0.0349 and wR2=0.0514 for all data. The β-polymorph possesses the wurtz-kesterite structure type, crystallizing in Pn with Z=2, R1=0.0423, and wR2=0.0901 for all data. Rietveld refinement of synchrotron X-ray powder diffraction was utilized to quantify the phase fractions of the polymorphs in the reaction products. The α/β-Li2MnSnS4 mixture exhibits an absorption edge of ∼2.6-3.0 eV, a wide region of optical transparency in the mid- to far-IR, and moderate SHG activity over the fundamental range of 1.1-2.1 μm. Calculations using density functional theory indicate that the ground state energies and electronic structures for α- and β-Li2MnSnS4, as well as the hypothetical polymorph, γ-Li2MnSnS4 with the wurtz-stannite structure type, are highly similar

The Scientific Method as a Scaffold to Enhance Communication Skills in Chemistry

Scientific success in the field of chemistry depends upon the mastery of a wide range of soft skills, most notably scientific writing and speaking. However, training for scientific communication is typically limited at the undergraduate level, where students struggle to express themselves in a clear and logical manner. The underlying issue is deeper than basic technical skills; rather, it is a problem of students\u27 unawareness of a fundamental and strategic framework for writing and speaking with a purpose. The methodology has been implemented for individual mentorship and in our regional summer research program to deliver a blueprint of thought and reasoning that endows students with the confidence and skills to become more effective communicators. Our didactic process intertwines undergraduate research with the scientific method and is partitioned into six steps, referred to as phases , to allow for focused and deep thinking on the essential components of the scientific method. The phases are designed to challenge the student in their zone of proximal development so they learn to extract and ultimately comprehend the elements of the scientific method through focused written and oral assignments. Students then compile their newly acquired knowledge to create a compelling and logical story, using their persuasive written and oral presentations to complete a research proposal, final report, and formal 20 min presentation. We find that such an approach delivers the necessary guidance to promote the logical framework that improves writing and speaking skills. Over the past decade, we have witnessed both qualitative and quantitative gains in the students\u27 confidence in their abilities and skills (developed by this process), preparing them for future careers as young scientists

Optical Nonlinearity in Cu₂CdSnS₄ and α/β-Cu₂ZnSiS₄: Diamond-like Semiconductors with High Laser-Damage Thresholds

Cu₂CdSnS₄ and α/β-Cu₂ZnSiS₄ meet several criteria for promising nonlinear optical materials for use in the infrared (IR) region. Both are air-stable, crystallize in noncentrosymmetric space groups, and possess high thermal stabilities. Cu₂CdSnS₄ and α/β-Cu₂ZnSiS₄ display wide ranges of optical transparency, 1.4–25 and 0.7–25 μm, respectively, and have relatively large second-order nonlinearity as well as phase matchability for wide regions in the IR. The laser-damage threshold (LDT) for Cu₂CdSnS₄ is 0.2 GW/cm², whereas α/β-Cu₂ZnSiS₄ has a LDT of 2.0 GW/cm² for picosecond near-IR excitation. Both compounds also exhibit efficient third-order nonlinearity. Electronic structure calculations provide insight into the variation in properties

Optical Nonlinearity in Cu₂CdSnS₄ and α/β-Cu₂ZnSiS₄: Diamond-like Semiconductors with High Laser-Damage Thresholds

Cu₂CdSnS₄ and α/β-Cu₂ZnSiS₄ meet several criteria for promising nonlinear optical materials for use in the infrared (IR) region. Both are air-stable, crystallize in noncentrosymmetric space groups, and possess high thermal stabilities. Cu₂CdSnS₄ and α/β-Cu₂ZnSiS₄ display wide ranges of optical transparency, 1.4–25 and 0.7–25 μm, respectively, and have relatively large second-order nonlinearity as well as phase matchability for wide regions in the IR. The laser-damage threshold (LDT) for Cu₂CdSnS₄ is 0.2 GW/cm², whereas α/β-Cu₂ZnSiS₄ has a LDT of 2.0 GW/cm² for picosecond near-IR excitation. Both compounds also exhibit efficient third-order nonlinearity. Electronic structure calculations provide insight into the variation in properties

Direct observation of a nonheme iron(IV)–oxo complex that mediates aromatic C–F hydroxylation

The synthesis of a pentadentate ligand with strategically designed fluorinated arene groups in the second coordination sphere of a nonheme iron center is reported. The oxidatively resistant fluorine substituents allow for the trapping and characterization of an FeIV(O) complex at −20 °C. Upon warming of the FeIV(O) complex, an unprecedented arene C–F hydroxylation reaction occurs. Computational studies support the finding that substrate orientation is a critical factor in the observed reactivity. This work not only gives rare direct evidence for the participation of an FeIV(O) species in arene hydroxylation but also provides the first example of a high-valent iron–oxo complex that mediates aromatic C–F hydroxylation

Polymorphism and Second Harmonic Generation in a Novel Diamond-like Semiconductor: Li2MnSnS4

High-temperature, solid-state synthesis in the Li2MnSnS4 system led to the discovery of two new polymorphic compounds that were analyzed using single crystal X-ray diffraction. The α-polymorph crystallizes in Pna21 with the lithium cobalt (II) silicate, Li2CoSiO4, structure type, where Z=4, R1=0.0349 and wR2=0.0514 for all data. The β-polymorph possesses the wurtz-kesterite structure type, crystallizing in Pn with Z=2, R1=0.0423, and wR2=0.0901 for all data. Rietveld refinement of synchrotron X-ray powder diffraction was utilized to quantify the phase fractions of the polymorphs in the reaction products. The α/β-Li2MnSnS4 mixture exhibits an absorption edge of ∼2.6–3.0 eV, a wide region of optical transparency in the mid- to far-IR, and moderate SHG activity over the fundamental range of 1.1–2.1 μm. Calculations using density functional theory indicate that the ground state energies and electronic structures for α- and β-Li2MnSnS4, as well as the hypothetical polymorph, γ-Li2MnSnS4 with the wurtz-stannite structure type, are highly similar