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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 109 records · Page 6
All sourcesNASA NTRSDOE OSTI
DOE OSTI2013

Improved value for the silicon free exciton binding energy

The free exciton binding energy is a key parameter in silicon material and device physics. In particular, it provides the necessary link between the energy threshold for valence to conduction band optical absorption and the bandgap determining electronic properties. The long accepted low temperature binding energy value of 14.7 ± 0.4 meV is reassessed taking advantage of developments subsequent to its original determination, leading to the conclusion that this value is definitely an underestimate. Using three largely independent experimental data sets, an improved low temperature value of 15.01 ± 0.06 meV is deduced, in good agreement with the most comprehensive theoretical calculations to date.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Observation of intravalley phonon scattering of 2s excitons in MoSe 2 and WSe 2 monolayers

Abstract We present a high-resolution resonance Raman study of hBN encapsulated MoSe 2 and WSe 2 monolayers at 4 K using excitation energies from 1.6 eV to 2.25 eV. We report resonances with the WSe 2 A2s and MoSe 2 A2s and B2s excited Rydberg states despite their low oscillator strength. When resonant with the 2s states we identify new Raman peaks which are associated with intravalley scattering between different Rydberg states via optical phonons. By calibrating the Raman scattering efficiency and separately constraining the electric dipole matrix elements, we reveal that the scattering rates for k = 0 optical phonons are comparable for both 1s and 2s states despite differences in the envelope functions. We also observe multiple new dispersive Raman peaks including a peak at the WSe 2 A2s resonance that demonstrates non-linear dispersion and peak-splitting behavior that suggests the dispersion relations for dark excitonic states at energies near the 2s state are extremely complex.

Materials Science↗
DOE OSTI2018

Nanoscale excitonic photovoltaic mechanism in ferroelectric BiFeO 3 thin films [Nanoscale excitonic mechanism underlying the bulk photovoltaic effect in ferroelectric BiFeO 3 thin films]

We report an electrode-free photovoltaic experiment in epitaxial BiFeO 3 thin films where the picosecond optical absorption arising from carrier dynamics and piezoelectric lattice distortion due to the photovoltaic field are correlated at nanoscale. The data strongly suggest that the photovoltaic effect in phase-pure BiFeO 3 originates from diffusion of charge-neutral excitons and their subsequent dissociation localized at sample interfaces. This is in stark contrast to the belief that carrier separation is uniform within the sample due to the lack of center of symmetry in BiFeO 3 . As a result, this finding is important for formulating strategies in designing practical photovoltaic ferroelectric devices.

36 MATERIALS SCIENCE↗
DOE OSTI2017

Magnetic field tuning of an excitonic insulator between the weak and strong coupling regimes in quantum limit graphite [Tunable excitonic insulator in quantum limit graphite]

Here, the excitonic insulator phase has long been predicted to form in proximity to a band gap opening in the underlying band structure. The character of the pairing is conjectured to crossover from weak (BCS-like) to strong coupling (BEC-like) as the underlying band structure is tuned from the metallic to the insulating side of the gap opening. Here we report the high-magnetic field phase diagram of graphite to exhibit just such a crossover. By way of comprehensive angle-resolved magnetoresistance measurements, we demonstrate that the underlying band gap opening occurs inside the magnetic field-induced phase, paving the way for a systematic study of the BCS-BEC-like crossover by means of conventional condensed matter probes.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Efficient Blue Phosphorescent OLEDs with Improved Stability and Color Purity through Judicious Triplet Exciton Management

Abstract Highly efficient and stable blue phosphorescent organic light‐emitting diodes are achieved by employing a step‐wise graded doping of platinum(II) 9‐(pyridin‐2‐yl)‐2‐(9‐(pyridin‐2‐yl)‐9 H ‐carbazol‐2‐yloxy)‐9 H ‐carbazole (PtNON) in a device setting. A device employing PtNON demonstrates a high peak external quantum efficiency (EQE) of 17.4% with an estimated LT 70 lifetime of over 1330 h at a brightness of 1000 cd m −2 . PtNON is then investigated as a “triplet sensitizer” in an alternating donor–acceptor doped emissive layer to further improve the device emission color purity by carefully managing an efficient Förster resonant energy transfer from PtNON to 2,5,8,11‐tetra‐ tert ‐butylperylene as a selected acceptor material. Thus, such OLED devices demonstrate an EQE of 16.9% with color coordinates of (0.16, 0.25) and an estimated luminance (LT 70 ) lifetime of 628 h at a high brightness of 1000 cd m −2 .

Klimes, Kody↗