Random Lasing with Systematic Threshold Behavior in Films of CdSe/CdS Core/Thick-Shell Colloidal Quantum Dots (original) (raw)
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The Impact of Core/Shell Sizes on the Optical Gain Characteristics of CdSe/CdS Quantum Dots
ACS Nano
Colloidal quantum dots (QDs) are highly attractive as the active material for optical amplifiers and lasers. Here, we address the relation between the structure of CdSe/CdS core/shell QDs, the material gain they can deliver and the threshold needed to attain net stimulated emission by optical pumping. Based on an initial gain model, we predict that reducing the thickness of the CdS shell grown around a given CdSe core will increase the maximal material gain, while increasing the shell thickness will lower the gain threshold. We assess this trade-off by means of transient absorption spectroscopy. Our results confirm that thinshell QDs exhibit the highest material gain. In quantitative agreement with the model, core and shell sizes hugely impact on the material gain, which ranges from 2800 cm −1 for large core/thin shell QDs to less than 250 cm −1 for small core/thick shell QDs. On the other hand, the significant threshold reduction expected for thick-shell QDs is absent. We relate this discrepancy between model
Composition-controlled optical properties of colloidal CdSe quantum dots
Applied Surface Science, 2014
A strategy with respect to band gap engineering by controlling the composition of CdSe quantum dots (QDs) is reported. After the CdSe QDs are prepared, their compositions can be effectively manipulated from 1:1 (Cd:Se) CdSe QDs to Cd-rich and then to Se-rich QDs. To obtain Cd-rich CdSe QDs, Cd was deposited on equimolar CdSe QDs. Further deposition of Se on Cd-rich CdSe QDs produced Se-rich CdSe QDs. The compositions (Cd:Se) of the as-prepared CdSe quantum dots were acquired by Energy-dispersive X-ray spectroscopy (EDX). By changing the composition, the overall optical properties of the CdSe QDs can be manipulated. It was found that as the composition of the QDs changes from 1:1 (Cd:Se) CdSe to Cd-rich and then Se-rich CdSe, the band gap decreases along with a red shift of UV-vis absorption edges and photoluminescence (PL) peaks. The quantum yield also decreases with surface composition from 1:1 (Cd:Se) CdSe QDs to Cd-rich and then to Se-rich, largely due to the changes in the surface state. Because of the involvement of the surface defect or trapping state, the carrier life time increased from the 1:1 (Cd:Se) CdSe QDs to the Cd-rich to the Se-rich CdSe QDs. We have shown that the optical properties of CdSe QDs can be controlled by manipulating the composition of the surface atoms. This strategy can potentially be extended to other semiconductor nanocrystals to modify their properties.
Nanoscale Research Letters, 2007
Picosecond time-resolved photoluminescence measurements were performed on CdSe core and CdSe/ZnS core/shell colloidal quantum dots (QDs). Photoluminescence (PL) emission is observed to originate from intrinsic (1 U and (1 L bright states with lifetimes of 60 and 450 ps, respectively, and from a long living component with nanosecond lifetimes. The latter is attribuited to the emission from surface states (ss) approximately 16 and 13 meV below the (1 L state for core and core/shell QDs, respectively. We show that in the temperature range between 15 and 70 K the three recombination processes compete and they are thermally populated through different pathways ((1 L f (1 U and ss f (1 L ).
ExcitonâExciton Interaction and Optical Gain in Colloidal CdSe/CdS Dot/Rod Nanocrystals
Advanced Materials, 2009
Semiconductor colloidal nanocrystals have been proposed as optically-active media for solution-processable optoelectronic devices, because they combine inexpensive, wet-chemistry synthesis with high photoluminescence quantum yield, large oscillator strength and size tuneability of optical transitions. Key to the success of nanocrystal-based devices is the possibility to design and consistently synthesize nanocrystals with desired properties. Size uniformity can be usually controlled within less than 5% uncertainty; surface capping, passivation and core/shell structures can lead to photoluminescence quantum yields exceeding 50%, optical gain and lasing. A new frontier in nanocrystal design has appeared with heterostructures allowing spatial separation of electron and hole wavefunctions, like in type-II CdSe/CdTe core/shell nanocrystals, through staggered conduction and valence band offsets. Charge separation inside nanocrystals is useful in photodetector and photovoltaic devices, quantum optics and low-threshold lasers. Exciton nonlinearities also depend on the degree of separation of electron and hole wavefunction. In type-II heterostructures, it has been demonstrated that charge separation can lead to a large repulsive exciton-exciton interaction. The resulting blueshift of the exciton-to-biexciton transition suppresses to a large extent resonant re-absorption of stimulated emission from single-exciton states, allowing net optical gain and lasing at excitations corresponding to less than one electron-hole pair per nanocrystal. In this regime, losses inherent to multiexciton recombinations are avoided, resulting in optical gain with a much longer lifetime, an essential step towards the demonstration of lasing under continuous wave operation.
Low‐Threshold Lasing from Copper‐Doped CdSe Colloidal Quantum Wells
Laser & Photonics Reviews, 2021
Transition metal doped colloidal nanomaterials (TMDCNMs) have recently attracted attention as promising nano-emitters due to dopant-induced properties. However, despite ample investigations on the steady-state and dynamic spectroscopy of TMDCNMs, experimental understandings of their performance in stimulated emission regimes are still elusive. Here, the optical gain properties of copper-doped CdSe colloidal quantum wells (CQWs) are systemically studied with a wide range of dopant concentration for the first time. This work demonstrates that the amplified spontaneous emission (ASE) threshold in copper-doped CQWs is a competing result between the biexciton formation, which is preferred to achieve population inversion, and the hole trapping which stymies the population inversion. An optimum amount of copper dopants enables the lowest ASE threshold of ≈7 µJ cm −2 , about 8-fold reduction from that in undoped CQWs (≈58 µJ cm −2) under sub-nanosecond pulse excitation. Finally, a copper-doped CQW film embedded in a vertical cavity surface-emitting laser (VCSEL) structure yields an ultralow lasing threshold of 4.1 µJ cm −2. Exploiting optical gain from TMDCNMs may help to further boost the performance of colloidal-based lasers.
Nano letters, 2011
We report a systematic study of photoluminescence (PL) intensity and lifetime fluctuations in individual CdSe/CdS core/shell nanocrystal quantum dots (NQDs) as a function of shell thickness. We show that while at low pump intensities PL blinking in thin-shell (4-7 monolayers, MLs) NQDs can be described by random switching between two states of high (ON) and low (OFF) emissivities, it changes to the regime with a continuous distribution of ON intensity levels at high pump powers. A similar behavior is observed in samples with a medium shell thickness (10-12 MLs) without, however, the PL intensity ever switching to a complete "OFF" state and maintaining ca. 30% emissivity ("gray" state). Further, our data indicate that highly stable, blinking-free PL of thick-shell (15-19 MLs) NQDs ("giant" or g-NQDs) is characterized by nearly perfect Poisson statistics, corresponding to a narrow, shot-noise limited PL intensity distribution. Interestingly, in this case ...
Optical Properties of CdTe Nanocrystal Quantum Dots, Grown in the Presence of Cd0 Nanoparticles
Journal of Physical Chemistry C, 2007
Using metallic nanoislands, we demonstrate the localized plasmonic control and modification of the spontaneous emission from closely-packed nanocrystal emitters, leading to significant changes in their collective emission characteristics tuned spectrally and spatially by plasmon coupling. Using randomly-distributed silver nanoislands, we show that the emission linewidth of proximal CdSe/ZnS core-shell quantum dots is reduced by 22% and their peak emission wavelength is shifted by 14nm, while their ensemble photoluminescence is enhanced via radiative energy transfer by 21.6 and 15.1 times compared to the control groups of CdSe/ZnS nanocrystals with identical nano-silver but no dielectric spacer and the same nanocrystals alone, respectively.
Colloidal Semiconductor Quantum Dots with Tunable Surface Composition
Nano Letters, 2012
Colloidal CdS quantum dots (QDs) were synthesized with tunable surface composition. Surface stoichiometry was controlled by applying reactive secondary phosphine sulfide precursors in a layer-by-layer approach. The surface composition was observed to greatly affect photoluminescence properties. Band edge emission was quenched in sulfur terminated CdS QDs and fully recovered when QDs were cadmium terminated. Calculations suggest that electronic states inside the band gap arising from surface sulfur atoms could trap charges, thus inhibiting radiative recombination and facilitating nonradiative relaxation.