We studied fluorescence intermittency (blinking) in pairs of colloidally grown nanocrystal quantum dots (NQDs) and found that the fluorescence trajectories of dots separated by up to ∼1 μm are correlated. Blinking rate enhancement was observed when nearby NQDs were in opposite emitting states. Models of fluorescence blinking in colloidal quantum dots typically invoke particle charging to explain bright and dark periods in the fluorescence trajectory. Likewise, the phenomenon of fluorescence blinking correlation observed in this study is explained by an interdot Coulomb interaction established by ejection of one or more photoinduced charges. Our results suggest that blinking can be controlled, ultimately leading to switchable nanoscale emitters.
Blue light-emitting diodes (LED's), utilizing InGaN-based multi-quantum well (MQW) active regions deposited by organometallic chemical vapor epitaxy (OMVPE), are one of the fundamental building-blocks for current solid-state lighting applications. Studies [1,2] have previously been conducted to explore the optical and physical properties of the active MQW's over a variety of different OMVPE growth conditions. However, the conclusions of these papers have often been contradictory, possibly due to a limited data set or lack of understanding of the fundamental fluid dynamics and gas-phase chemistry that occurs during the deposition process.Multi-quantum well structures grown over a range of pressures from typical low-pressure production processes at 200 Torr, up to near-atmospheric growth conditions at 700 Torr, have been investigated in this study. At all growth pressures, clear trends of gas-phase chemical reactions are observed for increased gas residence times (lower gas speeds from the injector flange and lower rotation rates) and increased V/III ratios (higher NH 3 flows).Confocal microscopy, excitation-dependent PL (PLE), and time-resolved photoluminescence (TRPL) have been employed on these MQW structures to investigate the carrier lifetime characteristics. Confocal emission images show spatially-separated bright and dark regions. The bright regions are red-shifted in wavelength relative to the dark regions, suggesting microscopic spatial localization of high indium content regions. As the growth pressure and gas residence times are reduced, a larger difference in band-gap between bright and dark regions, longer lifetimes, and higher average PL intensities can be obtained, indicating that higher optical quality material can be realized. Optimized MQW's grown at high pressure exhibit higher PLE slope intensities and IQE characteristics than lower pressure samples. Results on simple LED structures indicate that the improvement in MQW optical quality at high pressures translates to higher output power at a 110 A/cm 2 injection current density. 505
Confocal laser scanning microscopy and time-resolved photoluminescence (TRPL) spectroscopy were used to study blue-emitting InGaN/GaN multiple quantum wells. Spatial and spectral variations of photoluminescence (PL) were observed over submicron-scale regions. Spectral measurements showed that the bright regions have a higher PL intensity as well as smaller peak energy than the dark regions. Correlations among the bright region-dark region PL peak energy difference, the average PL intensity, the PL FWHM, the bright region PL intensity, and the extent of PL intensity fluctuation were observed. As the energy difference increased, the average PL intensity, the PL FWHM, and the bright region PL intensity increased, with a higher degree of areal PL intensity fluctuations. TRPL measurements and calculations showed that the effective PL lifetime at bright regions was longer than that at dark regions, and bright region lifetime increases as energy difference increases, possibly as a result of stronger confinement.
Confocal laser scanning microscopy (CLSM) and time-resolved photoluminescence (TRPL) spectroscopy were used to study blueemitting indium gallium nitride (InGaN)/gallium nitride (GaN) multi-quantum wells grown on c-plane sapphire substrate by metal-organic chemical vapor deposition under different growth conditions. Spatial and spectral variations of photoluminescence (PL) were observed in sub-micrometer scale. Spectrum measurement showed that the bright regions have higher PL intensity as well as smaller PL peak energy than the dark regions. The brightness of CLSM images is related with the PL peak energy difference between bright region and dark region: the larger the energy difference is, the brighter the CLSM image is. TRPL measurement and calculation showed that the effective PL lifetime at bright regions is longer than that at dark regions, and brightness of CLSM images increases with the increment of bright region effective PL lifetime.
Scanning confocal microscopy is used to study blueemitting Indium Gallium Nitride (InGaN)/Gallium Nitride (GaN) multi-quantum wells grown by metal-organic chemical vapor deposition under different growth conditions. Sub-micrometer scale spatial and spectral variation of photoluminescence (PL) has been observed. Spectrum measurement shows the PL peak in bright region is red-shifted comparing with that in dark region, and that the peak intensity of bright region is at least twice as strong as that of dark region. Images show defect luminescence features which are about 500 nm in diameter and have PL peak at around 550 nm. Experiments show that reducing In/Ga ratio, increasing growth pressure and increasing NH 3 flow rate can all increase the localization effect and result in the increase of sample average PL intensity. Moreover, average PL intensity increases with the increasing of bandgap difference and PL peak intensity difference between bright and dark regions in PL.
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