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Du M.H.,Materials Science and Technology Division
Journal of Materials Chemistry A | Year: 2014

Halide perovskites have recently been shown to exhibit excellent carrier transport properties. Density functional calculations are performed to study the electronic structure, dielectric properties, and defect properties of β-CH3NH3PbI3. The results show that Pb chemistry plays an important role in a wide range of material properties, i.e., small effective masses, enhanced Born effective charges and lattice polarization, and the suppression of the formation of deep defect levels, all of which contribute to the exceptionally good carrier transport properties observed in CH3NH3PbI3. Defect calculations show that, among native point defects (including vacancies, interstitials, and antisites), only iodine vacancy is a low-energy deep trap and non-radiative recombination centre. Alloying iodide with chloride reduces the lattice constant of the iodide and significantly increases the formation energy of interstitial defects, which explains the observed substantial increase in carrier diffusion length in mixed halide CH3NH3PbI2Cl compared to that in CH3NH3PbI3. © 2014 the Partner Organisations. Source


Cooper V.R.,Materials Science and Technology Division
Physical Review B - Condensed Matter and Materials Physics | Year: 2010

In this Rapid Communication, an exchange functional which is compatible with the nonlocal Rutgers-Chalmers correlation functional [van der Waals density functional (vdW-DF)] is presented. This functional, when employed with vdW-DF, demonstrates remarkable improvements on intermolecular separation distances while further improving the accuracy of vdW-DF interaction energies. The key to the success of this three-parameter functional is its reduction in short-range exchange repulsion through matching to the gradient expansion approximation in the slowly varying/high-density limit while recovering the large reduced gradient, s, limit set in the revised Perdew-Burke-Ernzerhof (revPBE) exchange functional. This augmented exchange functional could be a solution to long-standing issues of vdW-DF lending to further applicability of density-functional theory to the study of relatively large, dispersion bound (van der Waals) complexes. © 2010 The American Physical Society. Source


Du M.H.,Materials Science and Technology Division
Journal of Materials Chemistry C | Year: 2014

Mn4+ is known to activate red emission in many materials. However, the existing Mn4+ activated red phosphors have relatively long emission wavelengths and are therefore inefficient for general lighting purposes. Density functional calculations are performed on a large number of Mn4+ doped materials with diverse crystal structures to understand how material properties of different hosts affect the emission energy of the Mn4+ dopant. The results show that weak Mn4+-ligand hybridization generally leads to higher Mn4+ emission energies. Host materials allowing long Mn-ligand distance and/or significant distortion of bond angles around the Mn octahedral site are shown to have higher emission energies. Several new oxide host materials are found for Mn4+. Their emission energies are found to be higher than those currently known for Mn 4+ doped oxides and should be closer to that of Y2O 3:Eu3+, which is the current commercial red phosphor for fluorescent lighting. © 2014 the Partner Organisations. Source


Du M.H.,Materials Science and Technology Division
Journal of Materials Chemistry C | Year: 2014

Heavy 6p and 5p ions in groups IIIB, IVB, and VB (Tl, Pb, Bi, In, Sn, Sb) are multivalent ions, which act as electron and hole traps and radiative recombination centers in many wide band gap materials. In this paper, Tl + as a prototypical ns2 ion (ns2 ions here refer to 6p and 5p ions with outer electronic configurations of ns2) is studied as a luminescent center in alkali halides. Density functional calculations reveal the chemical trend that determines the luminescence mechanism in ns2-ion activated alkali halides. The activator-halogen hybridization strength and the ionicity of the host material strongly affect the positions of the activator levels relative to the valence and conduction band edges. This determines whether the radiative recombination occurs within the activator ion or involves the hole polaron, or the Vk center. Strategies for exploring different combinations of host materials and activators for desired luminescence mechanisms are discussed. The insight obtained in this work will help the search and the design of more efficient scintillators and phosphors. This journal is © the Partner Organisations 2014. Source


News Article
Site: http://www.nrl.navy.mil/media/news-releases/

Scientists at the U.S. Naval Research Laboratory (NRL) have reported the first observation of spin precession of spin currents flowing in a silicon nanowire (NW) transport channel, and determined spin lifetimes and corresponding spin diffusion lengths in these nanoscale spintronic devices. The spin currents were electrically injected and detected using ferromagnetic metal contacts with a tunnel barrier consisting of single layer graphene between the metal and silicon NW. False color atomic force microscopy image of a silicon nanowire with the four contacts used in the spin measurements. The ferromagnetic metal / graphene tunnel barrier contacts used to inject and detect spin appear as blue, the gold ohmic reference contacts appear as yellow, and the green line is the silicon nanowire transport channel. The bright dot on the end of the nanowire is the gold nanoparticle used to seed the nanowire growth. (Photo: U.S. Naval Research Laboratory) False color atomic force microscopy image of a silicon nanowire with the four contacts used in the spin measurements. The ferromagnetic metal / graphene tunnel barrier contacts used to inject and detect spin appear as blue, the gold ohmic reference contacts appear as yellow, and the green line is the silicon nanowire transport channel. The bright dot on the end of the nanowire is the gold nanoparticle used to seed the nanowire growth. The NRL research team observed spin precession (the Hanle effect) for both the spin-polarized charge near the contact interface and for pure spin currents flowing in the NW channel. The latter unambiguously shows that spins have been injected and transported in the Si NW. The use of graphene as the tunnel barrier provides a low-resistance area product contact and clean magnetic switching characteristics, because it smoothly bridges the NW and minimizes complicated magnetic domains that otherwise compromise the magnetic behavior. The team's discovery is an essential step toward the realization of highly scaled semiconductor spintronic devices. The research results are reported in the 19 June 2015 issue of Nature Communications (DOI 10.1038/ncomms8541). Semiconductor nanowires provide an avenue to further reduce the ever-shrinking dimensions of transistors. Including electron spin as an additional state variable offers new prospects for information processing, enabling future non-volatile, reprogrammable devices beyond the current semiconductor technology roadmap. Silicon is an ideal host for such a spin-based technology because its intrinsic properties promote spin transport, explains principal investigator Dr. Olaf van't Erve. Realization of spin-based Si NW devices requires efficient electrical spin injection and detection, which depend critically on the interface resistance between a ferromagnetic metal contact and the NW. This is especially problematic with semiconducting NWs because of the exceedingly small contact area, which can be of order 100 nm2. Researchers have shown standard oxide tunnel barriers to provide good spin injection into planar Si structures, but such contacts grown on NWs are often too resistive to yield reliable and consistent results. The NRL team developed and used a graphene tunnel barrier contact that produces excellent spin injection and also satisfies several key technical criteria: it provides a low resistance-area product, a highly uniform tunnel layer with well-controlled thickness, clean magnetic switching characteristics for the magnetic contacts, and compatibility with both the ferromagnetic metal and silicon NW. Using intrinsic 2D layers such as graphene or hexagonal boron nitride as tunnel contacts on nanowires offers many advantages over conventional materials deposited by vapor deposition (such as Al O or MgO), enabling a path to highly scaled electronic and spintronic devices. The use of multilayer rather than single layer graphene in such structures may provide much higher values of the tunnel spin polarization because of band structure derived spin filtering effects predicted for selected ferromagnetic metal / multi-layer graphene structures. This increase would further improve the performance of nanowire spintronic devices by providing higher signal to noise ratios and corresponding operating speeds, advancing the techological applications of nanowire devices. The NRL research team includes Dr. Olaf van't Erve, Dr. Adam Friedman, Dr. Connie Li, and Dr. Berend Jonker from the Materials Science and Technology Division, and Dr. Jeremy Robinson from the Electronics Science and Technology Division. About the U.S. Naval Research Laboratory The U.S. Naval Research Laboratory provides the advanced scientific capabilities required to bolster our country's position of global naval leadership. The Laboratory, with a total complement of approximately 2,500 personnel, is located in southwest Washington, D.C., with other major sites at the Stennis Space Center, Miss., and Monterey, Calif. NRL has served the Navy and the nation for over 90 years and continues to advance research further than you can imagine. For more information, visit the NRL website or join the conversation on Twitter, Facebook, and YouTube.

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