Dynamics of Crystalline and Amorphous Polytetrafluoroethylene Studied by Multiple Quantum NMR

1993 ◽  
Vol 321 ◽  
Author(s):  
David A. Lathrop ◽  
Karen K. Gleason
Keyword(s):  
Dipolar Coupling ◽  
Polymer Dynamics ◽  
Multiple Quantum ◽  
Temperature Dependent ◽  
Crystalline Sample ◽  
Ptfe Sample ◽  
Coupling Strengths ◽  
First Order Transition ◽  
Thermal Histories ◽  
A New Technique

ABSTRACTWe report a new technique for probing polymer dynamics through the refocussing of multiple quantum (MQ) nuclear magnetic resonance (NMR) coherences. The MQ-NMR experiment follows the correlated behavior of multiple spin-1/2 nuclei interacting through dipolar couplings. Motion which modulates the dipolar coupling strengths on the same time scale as the experiment (∼1 to 20 kHz) alters the intensity of the observed coherences. Temperature dependent 19F data are presented on polytetrafluoroethylene samples of varying crystallinity. For the as-polymerized 98% crystalline PTFE sample, a sharp increase in MQ coherence refocussing occurs, centered at -298 K. The 64% crystalline melt-quenched sample shows a increase at the same temperature but which has a lower intensity. Thus, the ∼298 K peak is most associated with motion in the crystalline phase. This temperature is intermediate between the two first order transition at 293 and 303 K. Oscillations in the refocussed fractions are observed from 208 to 230 K for the 98% crystalline sample, while this ratio is constant over the same temperature range for the 64% crystalline sample. These oscillations may be associated with paracrystalline defects found only in the first sample. Thus, the MQ refocussing experiment is able to clearly differentiate between polymer samples which have different thermal histories. The sharpness of the MQ refocussing features and their variations in magnitude, shape, and sign with temperature are signatures of the molecular level details of the underlying dynamics which produce them.

Temperature Dependence of Photoresponse in p-Type GaAs/AlGaAs Multiple Quantum Wells: Theory and Experiment

1999 ◽  
Vol 607 ◽  
Author(s):  
F. Szmulowicz ◽  
A. Shen ◽  
H. C. Liu ◽  
G. J. Brown ◽  
Z. R. Wasilewski ◽  
...  
Keyword(s):  
Quantum Wells ◽  
Temperature Dependence ◽  
Bound States ◽  
Light Hole ◽  
Multiple Quantum ◽  
Temperature Dependent ◽  
Long Wavelength ◽  
Hole Level ◽  
Polarization Characteristics ◽  
P Type

AbstractThis paper describes a study of the photoresponse of long-wavelength (LWIR) and mid-infrared (MWIR) p-type GaAs/AlGaAs quantum well infrared photodetectors (QWIPs) as a function of temperature and QWIP parameters. Using an 8x8 envelope-function model (EFA), we designed and calculated the optical absorption of several bound-to-continuum (BC) structures, with the optimum designs corresponding to the second light hole level (LH2) coincident with the top of the well. For the temperature-dependent study, one non-optimized LWIR and one optimized MWIR samples were grown by MBE and their photoresponse and absorption characteristics measured to test the theory. The theory shows that the placement of the LH2 resonance at the top of the well for the optimized sample and the presence of light-hole-like quasi-bound states within the heavy-hole continuum for the nonoptimized sample account for their markedly different thermal and polarization characteristics. In particular, the theory predicts that, for the LWIR sample, the LH-like quasi-bound states should lead to an increased Ppolarized photoresponse as a function of temperature. Our temperature dependent photoresponse measurements corroborate most of the theoretical findings with respect to the long-wavelength threshold, shape, and polarization and temperature dependence of the spectra.

"Double-W-shaped" temperature dependence of emission linewidth in an InGaN/GaN multiple quantum well structure with intense phase separation

2020 ◽  
Vol 10 (1) ◽  
pp. 140-144
Author(s):  
Changfu Li ◽  
Mingsheng Xu ◽  
Ziwu Ji ◽  
Kaiju Shi ◽  
Hongbin Li ◽  
...  
Keyword(s):  
Phase Separation ◽  
Temperature Dependence ◽  
Multiple Quantum Well ◽  
Excitation Power ◽  
Multiple Quantum ◽  
Temperature Dependent ◽  
Two Phase ◽  
Phase Structures ◽  
Temperature Dependences ◽  
Lower Excitation

The temperature dependence of the spectra of photoluminescence (PL) from a blue InGaN/GaN multiplequantum-well (MQW) structure is investigated at lower excitation power. Two emission peaks, related to InGaN and assigned to In-rich quasi-quantum dots (QDs) and InGaN-matrix in the full PL spectrum, were observed. Upon increasing the temperature, both PL peak linewidths exhibited "double-W-shaped" (narrowing–broadening–narrowing–broadening–narrowing–broadening) temperature dependence. Combined with the observed features of the temperature dependences of the PL intensities, the temperature-dependent behaviors in this case can be interpreted as the relaxation and thermalization of carriers inside respective phase structures and the transfer of carriers between two phase structures, because of the strong phase separation and significant component fluctuation in the InGaN well layers.

PHOTOCONDUCTIVITY, PHOTOREFLECTANCE AND PHOTOLUMINESCENCE OF GaAs-AlAs MULTIPLE QUANTUM WELLS

1985 ◽  
Vol 56 ◽  
Author(s):  
H. NEFF ◽  
K. J. BACHMANN ◽  
W. D. LAIDIG
Keyword(s):  
Quantum Well ◽  
Quantum Wells ◽  
Heavy Hole ◽  
Multiple Quantum Well ◽  
Light Hole ◽  
Multiple Quantum ◽  
Indirect Transition ◽  
Temperature Dependent ◽  
Direct Inter ◽  
Phonon Absorption

AbstractEmploying temperature dependent photoconductivity, photoluminescence and photoreflectivity measurements, we have analyzed a GaAs-AlAs multiple quantum well. The above optical techniques clearly resolve the fundamental inter-subband transitions, including heavy hole-light hole splittings. At T < 60K an anomalously high photoconductivity was discovered below the direct inter-subband transitions and is attributed tentatively to the presence of extrinsic interface states within the bandgap. For T > l00K the fundamental indirect transition was discovered and associated with LO (L) - phonon absorption.

scholarly journals Photoreflectance Spectroscopy Characterization of Ge/Si0.16Ge0.84Multiple Quantum Wells on Ge Virtual Substrate

2013 ◽  
Vol 2013 ◽  
pp. 1-6 ◽  
Author(s):  
Hung-Pin Hsu ◽  
Pong-Hong Yang ◽  
Jeng-Kuang Huang ◽  
Po-Hung Wu ◽  
Ying-Sheng Huang ◽  
...  
Keyword(s):  
Quantum Wells ◽  
Interband Transition ◽  
Multiple Quantum Well ◽  
Chemical Vapor ◽  
Detailed Comparison ◽  
Multiple Quantum ◽  
Temperature Dependent ◽  
Wide Range ◽  
Quantum Confined

We report a detailed characterization of a Ge/Si0.16Ge0.84multiple quantum well (MQW) structure on Ge-on-Si virtual substrate (VS) grown by ultrahigh vacuum chemical vapor deposition by using temperature-dependent photoreflectance (PR) in the temperature range from 10 to 300 K. The PR spectra revealed a wide range of optical transitions from the MQW region as well as transitions corresponding to the light-hole and heavy-hole splitting energies of Ge-on-Si VS. A detailed comparison of PR spectral line shape fits and theoretical calculation led to the identification of various quantum-confined interband transitions. The temperature-dependent PR spectra of Ge/Si0.16Ge0.84MQW were analyzed using Varshni and Bose-Einstein expressions. The parameters that describe the temperature variations of various quantum-confined interband transition energies were evaluated and discussed.

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