Engineering of Semiconductor Nanocrystals for Light Emitting Applications

Abstract

Semiconductor nanocrystals are rapidly spreading into the display and lighting markets. Compared with liquid crystal and organic LED displays, nanocrystalline quantum dots (QDs) provide highly saturated colors, wide color gamut, resolution, rapid response time, optical efficiency, durability and low cost. This remarkable progress has been made possible by the rapid advances in the synthesis of colloidal QDs and by the progress in understanding the intriguing new physics exhibited by these nanoparticles. In this review, we provide support to the idea that suitably engineered core/graded-shell QDs exhibit exceptionally favorable optical properties, photoluminescence and optical gain, while keeping the synthesis facile and producing QDs well suited for light emitting applications. Solid-state laser emitters can greatly profit from QDs as efficient gain materials. Progress towards fabricating low threshold, solution processed DFB lasers that are optically pumped using one- and two-photon absorption is reviewed. In the field of display technologies, the exploitation of the exceptional photoluminescence properties of QDs for LCD backlighting has already advanced to commercial levels. The next big challenge is to develop the electroluminescence properties of QD to a similar state. We present an overview of QLED devices and of the great perspectives for next generation display and lighting technologies.

Bridging Energetics and Dynamics of Exciton Trapping in Core–Shell Quantum Dots

“…Incidentally, the Raman characterization of analogous series evidenced a peculiar behavior above a 3ML shell thickness. 42 For the CdSe/CdS we ascribed this to the formation of a 3ML thick CdSeS alloy at the core/shell interface, whereas for the CdSe/CdZnS we did not observe alloy formation and inferred that an abrupt interface is formed when CdZnS is grown onto CdSe QDs. Table SI_2.…”

The “Surface Optical” Phonon in CdSe Nanocrystals

“…[18][19][20][21][22][23] Closer examination of Raman spectra obtained with a high signal-to-noise ratio reveals an additional, low-frequency shoulder on the LO phonon. This shoulder is seen quite consistently in nanocrystals with different sizes and shapes and with different surface chemistries, 6,[24][25][26][27][28][29][30][31][32][33][34][35][36] and also in many semiconductor materials other than CdSe. It was assigned as a "surface optical" (SO) phonon by comparison with predictions based on dielectric continuum theory for the optical phonon modes of ionic crystals.…”

“…37,38 Although alternative explanations involving only the dispersion of the LO phonons have been presented, 39 the SO assignment seems to have been widely accepted and there have been a number of efforts to correlate the frequency and/or intensity of this mode with surface properties of core-shell nanocrystals such as shell thickness and degree of alloying at the surface. 6,26,27,33,40 However, the phonon mode assignments seem tenuous given that surface modification often affects the LO phonon about as much as the "SO". 6,[26][27][28]35 The frequencies of the surface modes of a small spherical nanocrystal are given by dielectric continuum theory as 37 …”

Electron–Phonon Coupling in CdSe/CdS Core/Shell Quantum Dots

“…11,31 The phonons of CdSe/CdS nanocrystals have been studied by a number of groups using both direct electronic spectroscopy (emission/emission excitation) 2,7,32 and Raman spectroscopy. 1,10,15,22 Low-temperature electronic spectroscopy on single nanocrystals reveals resolved phonon modes whose intensities give the electron-phonon coupling strengths for the 1S-1S excitonic transition. Raman spectroscopy has been used mainly to examine alloying at the CdSe/CdS interface and the existence of lattice strain in core and/or shell resulting from the lattice mismatch.…”

“…The frequency of the CdS LO phonon has been reported to increase by anywhere from 6 to 24 cm -1 upon increasing the shell thickness in CdSe/CdS core/shell structures. 1,10,15,22 Most studies on CdSe/CdS core/shells also report that the CdSe LO fundamental shifts to higher frequencies by 3-7 cm -1 between bare CdSe cores and thick CdS shells. Most of the previously reported Raman spectra for CdSe/CdS core/shell structures show significant intensity in the CdS overtone, but in all cases these spectra were excited >3500 cm -1 above the lowest exciton.…”